Towards Construction of the “Periodic Table” of 1-Methylbenzotriazole

Abstract

Metal complexes of benzotriazole-type ligands continue to attract the intense interest of many inorganic chemistry groups around the world for a variety of reasons, including their aesthetically beautiful structures, physical properties and applications. 1-methylbenzotriazole (Mebta) is the N-substituted archetype of the parent 1H-benzotriazole. The first attempt to build a “periodic table” of Mebta, which includes its complexes with several metal ions, is described in this work. This, at first glance, trivial ligand has led to interesting results in terms of the chemistry, structures and properties of its metal complexes. This work reviews the to-date published coordination chemistry of Mebta with Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(I), Cu(II), Zn(II), Pd(II), Au(I) and {U^VIO₂}²⁺, with emphasis on their preparations, reactivity, structures and properties. Unpublished results from our group comprising other Co(II), Ni(II), Cu(II) and Zn(II) complexes, as well as Cd(II), Hg(II), Ag(I), In(III) and Sn(IV) ones are briefly reported. Mebta can also provide access to 1D and 3D heterometallic thiocyanato-bridged Co(II)/Hg(II) and Ni(II)/Hg(II) compounds. In almost all cases, Mebta behaves as a monodentate ligand with the nitrogen of position 3 of the azole ring as the donor atom. However, there are two copper complexes in which this molecule adopts a bidentate bridging coordination behavior. Our efforts to complete the “periodic table” of Mebta are continued.

1. Introduction

In the last 15 years or so, there has been a renaissance in the coordination chemistry of benzotriazoles (Figure 1, left) [1], a rather old family of ligands [2], for a variety of reasons (vide infra). The aim of this comprehensive review is to provide a synthetic and structural “flavor” of the coordination chemistry of 1-methylbenzotriazole (Mebta; Figure 1, right), the N-substituted archetype of 1H-benzotriazole (btaH). Over the past three decades, we have been trying to build a “periodic table” of Mebta (for a recent article in this journal, see ref. [3]), by preparing complexes with as many metal and metalloid ions as possible. The spaces (positions) in this “periodic table” contain ions, whose complexes with Mebta have been structurally characterized by single-crystal X-ray crystallography; thus, their composition and molecular and crystal structures are known without any doubt. Sporadically, Mebta complexes that have been characterized only by spectroscopic methods will be mentioned. Most of the complexes reported so far come from our group, hence the relatively large number of self-citations.

The emphasis of this review is on the preparation and molecular structures of the metal complexes; in a few cases, a brief note on their spectroscopic characteristics and physical properties will appear. Our intention is to avoid long (and most often boring) preparative descriptions and, for this reason, we shall present the syntheses using balanced chemical equations written in their molecular (and not ionic) format; this representation is correct only if there is one metal-containing product in the reaction (which is probably not always the case). The molecular structures shown are of two types. Some will be illustrated in a schematic way. Other structures have been created from the corresponding CIFs which have been deposited with the Cambridge Crystallographic Data Center (CCDC). For the description of the coordination modes of the ligands involved, the now well-established “Harris Notation” [4] will be used. In a few cases, and for clarity reasons, the ligation modes will be also represented in a schematic way with the coordination bonds drawn with bold lines. Since the description of structures and spectroscopic features/physical properties will be brief, we assume that the readers of this article have a satisfactory background in structural chemistry, spectroscopy and elementary molecular magnetism.

This review is divided into sections. Since Mebta is a ligand, Section 2 provides the reader with some historical information and the utility of ligands in modern inorganic chemistry. Section 3 describes the important role of benzotriazoles in various branches of chemistry with emphasis on their coordination chemistry. Section 4 gives some information concerning the organic, physical and theoretical chemistry of free Mebta. Section 2, Section 3 and Section 4 are the “hors d’oeuvre” of the review article. Section 5 and Section 6 will be the “main menu” of our scientific meal. Section 5 describes the so-far published coordination chemistry of Mebta; we have chosen to arrange our discussion according to the metal justifying the term “periodic table”. Section 6 is a brief overview of the unpublished results on the topic by our group. Finally, some preliminary conclusions and perspectives on the metal chemistry of Mebta will be presented.

An excellent review of recent advances in the coordination chemistry of benzotriazole-based ligands is available [1]; this has limited information about complicated N1- and N2-substituted benzotriazoles with substituents containing other donor groups, but without any citation of Mebta. An informative review on metal complexes of triazoles and tetrazoles also appeared in ref. [5], with a limited description of the coordinating properties of a few benzotriazoles but again with no mention of Mebta. Thus, this article appears to be the first review describing the coordination chemistry of Mebta.

2. Ligands in Coordination Chemistry

Metal ions and ligands are the necessary ingredients of coordination chemistry. Perhaps most inorganic chemists do not know that the word “ligand” and its concept were unknown to Alfred Werner [6]. The term “ligand” (Latin ligare, to bind) was introduced in 1917 by Alfred Stock to describe the groups bonded to boron, carbon and especially silicon. The word did not come into use among English-speaking chemists until the Second World War. Between the two world wars the term was used by Klement in a German journal [7], and by 1940 Tsuchida and Tumaki were using it often in descriptions of coordination complexes [8,9]. The popularity of Jannik Bjerrum’s PhD Thesis made the term “ligand” widely accepted by scientists publishing in English and American journals. It was officially introduced by IUPAC in the inorganic chemistry nomenclature in the late 1950s [10].

Coordination chemistry is a living branch of chemistry, continuously evolving and generating new concepts and challenges in bonding and structure. Ligands are at the core of inorganic, supramolecular and nanoscale chemistry, and the proper utilization of known ligands and the design of new ones is behind many exciting developments. Theoretical concepts related to them [11] are the chelate effect, the macrocyclic effect, the cryptate effect, the conformation of chelating rings, chirality, the isoelectronic and isolobal relationships and the reactivity of coordinated ligands, among others.

3. Benzotriazoles in Several Branches of Chemistry

1H-benzotriazole or more strictly 1H-benzo[1,2,3]triazole (btaH; R = R’ = H in Figure 1, left) and its derivatives belong to the class of heterocycles known as azoles; these ligands, especially 1H-imidazole, have been central “players” in the development of modern coordination and bioinorganic chemistry after 1970. Benzotriazoles contain two fused 6-(benzene) and 5-(1,2,3-triazole) rings. The parent molecule (btaH) exists in two tautomeric forms, the benzenoid form (Figure 2, left) and the quinonoid form (Figure 2, right); the former is more stable than the latter (by ca. 10 kcal mol⁻¹) accounting for its presence more than 99.9% at equilibrium. This difference has been attributed to the much higher dipole moment (ca. 4.5 D) of 1H-benzotriazole (benzenoid) than that of its 2H (quinonoid) tautomer (ca. 0.4 D). The result of this difference is that most derivatives appear as the N(1)-isomer [12]. If the steric bulk of the N-substituent increases, the proportion of the N(2)-isomer becomes higher [12]. The btaH molecule has weak acidic (pK_a = 8.2) and extremely weak basic (pK_b < 0) properties; due to these characteristics, it is soluble both in bases (e.g., aqueous Na₂CO₃) and acids (e.g., hydrochloric acid of various molarities), but it is insoluble in H₂O.

The popularity of btaH among organic chemists is tremendous [13,14,15,16], mainly thanks to the pioneering research of Alan Katritzky; when he passed away in 2014, he had published ca. 640 papers on the chemistry, properties and technological aspects of benzotriazoles. Today, this number exceeds 700! Among the properties that make btaH extremely useful in organic chemistry are its non-toxicity (in small quantities), low cost, very good solubility in a variety of organic solvents, easy synthesis and chemical (e.g., in the presence of H₂SO₄, NaOH, LiAlH₄, etc.) and thermal (its ring system is stable up to 400 °C) stability. Another important feature of synthetic utility is that btaH is both electron donating and electron attracting; the former characteristic results in acyl, aroyl, phosphonyl and sulfonyl derivatives, while the latter can stabilize carbanions. The benzotriazolate anion (bta⁻) is a good leaving group and it can be used in place of halogen substituents in many reactions, because the benzotriazolyl derivatives are more stable than their halogen analogues [15]. Concerning the derivatives of btaH, 1-hydroxybenzotriazole (btaOH; R = OH and R’ = H in Figure 1) is an extremely useful auxiliary in the synthesis of peptides [15].

In the related field of medicinal chemistry, benzotriazoles play an important role. They exhibit antimicrobial, antiparasitic, choleretic, cholesterol-lowering and even antitumor properties [17,18]. Transition-metal complexes of benzotriazoles are also candidates for the development of antitumor agents. Copper(II) complexes can be functional models of the copper-zinc-superoxide dismutase (Cu, Zn-SOD). According to the theory of Buettner and Oberley [19], SOD activity in tumor cells is lower than that in healthy cells. Complexes 1 and 2 (Figure 3) showed very low and very high SOD activities, respectively. Since these compounds differ only in the position of the substitution of the benzotriazole ring, it is obvious that the biological activity depends on the structural characteristics of the coordinated ligand. Complex 2 exhibits an in vitro SOD activity with a 0.06 μΜ IC₅₀ value, comparable with results reported for other SOD-mimicking agents. In accordance with these data, the evaluation of the in vitro cytotoxicity on several tumor lines showed that 2 is much better than 1 with IC₅₀ values in the 13–28 μΜ range.

Benzotriazole and its derivatives also attract the intense interest of environmental scientists. Polar benzotriazoles, e.g., btaH, btaOH and compounds substituted (Me-, Cl-, -NH₂ etc.) at position 5 of the benzene ring are characterized as emerging pollutants in aqueous media [20], where they exist in low concentrations; their presence in such systems arises from their uses (vide infra). Because of their high aqueous mobility, which is a consequence of their polarity, they have been detected in rivers, lakes, groundwater, snow, wastewater (treated and untreated) and, unfortunately, in drinking water. Their presence has also been confirmed in soil air, house dust, textiles, plants and human urine. Several techniques have been used for their separation from a variety of matrices, such as classical liquid–liquid extractions, microextraction methods and solid-phase extraction, the latter being the most important.

Benzotriazole chemistry is also related to many aspects of material science. Thus: (a) Benzotriazole-decorated graphene oxide is a valuable material for successful removal of U(VI) from nuclear waste water [21]. Using 5-methylbenzotriazole-modified graphene oxide, a high removal capacity of 264 mg/g for U(VI) was achieved at pH 3.5. (b) Several btaH derivatives have been successfully tested for energetic material applications [22,23,24]. These contain amino, azide and/or nitro substituents at the benzene ring, combined sometimes with nitro-containing 6-membered aromatic ring substituents on the azole system (Figure 4). Such nitrogen-rich compounds contain a large number of N-N and C-N bonds, and their energetic potential derives from the N₂ formation during combustion. For example [24], the compounds 5-amino-4-nitro-1H-benzo[1,2,3]triazole (left in Figure 4) and 5-azido-4-nitro-1H-benzo[1,2,3]triazole (middle in Figure 4) have densities of 1.61 and 1.62 g cm⁻³, detonation velocity values of 5.70 and 5.45 km s⁻¹ and detonation pressures of 13.4 and 12.4 Gpa, respectively, all comparable to the classical energetic material 2-methyl-1,3,5-trinitrobenzene (TNT). (c) Orthohydroxyphenyl benzotriazoles are among the best ultraviolent absorbers (stabilizers) and find use as additives in coatings, paints, adhesives, plastic materials, food packing films and personal care products. These compounds absorb the destructive UV radiation from sunlight and dissipate the energy in the form of heat generating a thermally excited ground-state species [25,26,27]. The mechanism of excited-state deactivation has been proposed to be due to an excited-state intramolecular proton transfer (ESIPT). The molecules contain a strong intramolecular H bond which is essential for their photostability; (d) 1H-benzotriazole and its polar derivatives are excellent corrosion inhibitors for metals (mainly Cu) and their alloys [28,29,30,31,32]. Procter and Gamble patented btaH as an inhibitor in 1947 [33]. For this reason, they find widespread applications in aircraft de-icing fluids, antifreeze liquids, detergents for household uses in dishwashing machines and in a variety of industrial systems (cooling, breaking, cutting, etc.). Most importantly, btaH and some of its derivatives, e.g., a mixture of 4-methylbenzotriazole and 5-methylbenzotriazole (with the market name tolyltriazole), are used in the treatment (for protection against further corrosion) of metal-based artifacts of archaeological and historical importance, the first report dating back in 1967 [34]. Today this mixture is effective for the storage of corroded objects and in the preservation of stabilized antiquities [35,36]. The exact mechanism of btaH action on copper materials has not yet been elucidated [29,31]. It is generally accepted [28] that the inhibition is achieved through the complexation of the deprotonated inhibitor molecules and the species in the metal surface (e.g., oxides), thus resulting in strong Cu-N bonds and in the formation of a compact surface film.

The physical chemistry (including theoretical studies) and spectroscopy of benzotriazoles have also attracted the intense interest of researchers [12,37,38,39,40,41,42,43,44,45]. Thus, their ¹H, ¹³C and ¹⁵N NMR spectra have been studied in depth [38,39,42]. Early theoretical studies provided an explanation for the great difference in stability between btaH and 1,2,3-triazole [12]. The electronic structures and relevant stabilities of N-substituted benzotriazoles were studied by UV photoelectron spectroscopy, in combination with high-level ab initio methods [40], and the N-substituent effects were shown to depend on the mode of attachment of the substituent to the benzotriazole skeleton. Surface-enhanced Raman spectroscopic studies of benzotriazoles on copper electrodes have been also performed to investigate the nature of the adsorbates at the metal/solution interface and address issues of corrosion inhibition by these compounds [43,44,45].

Benzotriazole and benzotriazole-based compounds have been used as primary ligands in coordination chemistry, albeit less than the triazole ligands. In general, these ligands are redox “innocent”. The parent compound and its C-substituted derivatives can be deprotonated and bridge up to three metal ions [1]. The to-date crystallographically established coordination modes of such bta⁻ ligands are illustrated in Figure 5. Benzotriazolate-type compounds are suitable ligands for the synthesis of coordination clusters, which often contain other chelating ancillary ligands, with aesthetically beautiful structures [46,47,48,49,50,51,52,53,54,55,56] and metal topologies, e.g., cyclic cages and kuratowski-type molecules. The deprotonated ligands can also act as linkers towards the construction of coordination polymers [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74]. A perusal of references [46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74] reveals that benzotriazole (and its C-substituted with no donor groups derivatives) confirms that these ligands form, in the solid state, complexes with a variety of s-, p-, d- and f-block metal ions. When additional donor groups are present on the azole (e.g., R = OH and R’ = H in Figure 1, left) or benzene (e.g., R = H and R’ = COOH) rings, the bridging capabilities of the resulting ligands increase, often leading to clusters [55,56] or polymers [60,61,62,63,64,65,66,67,69,70,71,72,73,74] which sometimes exhibit interesting physical (optical, magnetic, …) properties. Examples in cluster chemistry include compounds [MZn₄Cl₄(L)₆] (M^II = Zn, Fe, Co, Ni, Cu; L⁻ = the 5,6-dimethylbenzotriazolate anion) represented by the nonplanar K_3,3 graph [50], [Fe^II₃Zn₆Cl₆(L’)₁₂] (L’ = the 5,6-dimethoxybenzotriazolate anion) which exhibits the thermal spin-crossover behavior [49] and (DMFH)[NaHg^II₄(bta)₆I₄] which is photoluminescent [51]. Examples in the coordination polymer area include {[Cu(btaO)₂(MeOH)]}_n (btaO⁻ is the deprotonated 1-hydroxybenzotriazole; R = OH and R’ = H in Figure 1, left), which was the first molecular Cu^II soft magnet reported [74], and the 2D coordination polymer {[Zn(L″)(py)₂]}_n (L″ = the doubly deprotonated benzotriazole-5-carboxylic acid, R = H and R’ = COOH in Figure 1, left; py = pyridine) which is a highly selective and sensitive luminescent sensor for PO₄³⁻ and Cu²⁺ ions in aqueous media [70].

4. 1-Methylbenzotriazole: The “Free” Organic Compound

1-methylbenzotriazole (Mebta; Figure 1, right) is the archetype of N-substituted benzotriazoles with non-donor substituents. Mebta is a cream solid (almost white). It is moderately harmful to the respiratory system, dangerous if ingested and causes irritation upon contact with the skin and the eyes.

Mebta is synthesized by the methylation of btaH with MeI in MeOH under strongly alkaline conditions [75,76], but it is also available in the market. The compound is readily soluble in most organic solvents and insoluble in H₂O. It is readily dissolved in HCl, HBr, HI, HNO₃ and HClO₄ acids, from which the corresponding salts can be isolated. Its NMR δ values are as follows. ¹H NMR: 8.06 (d, H4); 7.86 (d, H7); 7.57 (t, H6); 7.42 (t, H5); 4.34 (s, H10). ¹³C[¹H] NMR: 127.6 (C6); 133.5 (C9); 145.3 (C8); 110.7 (C7); 124.0 (C5); 119.1 (C4); 34.2 (CH₃). These values (in ppm) are from our own data in d₆-DMSO and are in agreement with those reported in the literature [3,38,39,42,75,76]. The ¹⁵N NMR signals in the same solvent appear at δ-161.5 (N1), -1.1 (N2) and -41.0 (N3) ppm [39]. Recently, Marinakis’ group studied the IR, Raman, UV/Vis, ¹H and ¹³C NMR spectra of Mebta using experimental and computational methods [77]. DFT calculations using various functionals and basis sets were employed to model the experimental data. The experimental vibrational and NMR data were in excellent agreement with the theoretical predictions. The solvation of the compound in various solvents (MeOH, MeCN, DMF, DMSO) was also studied through UV/Vis spectroscopy experiments and time-dependent DFT calculations.

The crystal structure of Mebta was unknown. Very recently, Edelmann’s group investigated the reactivity of the stable N-heterocyclic nitrenium salt 1,3-dimethyl-1,2,3-benzotriazolium iodide, (Me)₂bta⁺ I⁻ (which is prepared by treatment of Mebta with an excess of MeI) towards nucleophilic reagents [78]. Strong nucleophiles, e.g., KH, potassium cyclopentadienide (Kcp), NaOMe, KO^tBu and NaSEt, all reacted with (Me)₂btaI leading to demethylation and back-formation of crystals of Mebta; the yields of the reactions were in the range 80–90% (Figure 6). The reactions with NaOMe and NaSEt require high temperatures (reflux), whereas those with the other nucleophiles proceed at 20 °C. As expected, the nine atoms of the condensed two-ring system are perfectly in a plane, confirming the aromaticity of Mebta. In agreement with this, the –CH₃ group bonded to N1 is twisted out of the plane by only 7 °C. The N1-N2-N3 angle is 109.4°. Due to resonance stabilization, the N-N bond lengths are in the range between the typical values of single and double bonds. The N1-N2 bond (1.36 Å) is longer than the N2-N3 (1.31 Å), implying a higher double-bond character for the latter.

5. The Published Coordination Chemistry of 1-methylbenzotriazole

5.1. Is Mebta a Boring Ligand?

At first glance, the answer to this question is “yes”. In the great majority of Mebta metal complexes (with one published exception, vide infra) the molecule behaves as a monodentate ligand, the donor atom being the N atom of position 3 of the azole ring (Figure 1 and Figure 7, left).

The almost exclusive N(3)-coordination of Mebta has been explained using ab initio-level (MP2) calculations in EtOH solution of the compound (EtOH is an often used solvent for the preparation of its metal complexes) [3]. The calculated negative neutral atomic charges are q_r = 0.31 |e| for N3 and q_r = −0.04 |e| for N2, strongly implying that N3 should be the preferable center for the attack of Mebta by metal ions. This terminal monodentate ligation has been related to the inability of Mebta to act as a corrosion inhibitor for copper and its alloys [45,79]. Thus, Mebta can be considered a typical aromatic monodentate N-donor, like pyridine. From another viewpoint, the answer to the titled question is “no”. The reasons are the unique steric and electronic properties of Mebta, and the recent discovery [80] (see also Section 6) that Mebta is capable of bridging two metal ions in a 2.1₂1₃0₁ (or simply 2.110) manner (Figure 7, right). Most complexes with N(3)-coordination of Mebta have different chemistry (composition, structures, properties) compared with those of the typical ligands with one N donor atom, even if the ancillary inorganic ligands (Cl⁻, Br⁻, NO₃⁻, N₃⁻, …) or counterions (ClO₄⁻, PF₆⁻, BF₄⁻, CF₃SO₃⁻, …) are the same! The Mebta complexes appear also different from those of the closely related 1-methyl-1,2,3-triazole (Meta, Figure 8), although the number of reported metal complexes of the latter is extremely small (vide infra).

5.2. An 1D Mn(II)/Mebta Coordination Polymer from the Use of the Dicyanamido Ligand

The dicyanamide ion, N(CN)₂⁻, is a popular inorganic ligand that often behaves as a bridging group leading to 1D, 2D and 3D coordination polymers with aesthetically beautiful molecular structures and interesting topological features. An aqueous Mn^II/N(CN)₂⁻ solution, prepared in situ from the reaction between Mn(ClO₄)₂∙6H₂O and Na{N(CN)₂}, was layered with a methanolic solution of two equivs. of Mebta; slow mixing of the solutions resulted in the isolation of needle-like colorless crystals of {[Mn{N(CN)₂}₂(Mebta)₂]}_n (3) in low to moderate yield (≤30%), Equation (1).

$$\begin{array}{c}\mathrm{n}\mathrm{Mn}({\mathrm{ClO}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+2\mathrm{n}\mathrm{Na}\{\mathrm{N}(\mathrm{CN}{)}_2\}+2\mathrm{n}\mathrm{Mebta}\stackrel{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}/{\mathrm{H}}_2\mathrm{O}}{\to }\{[\mathrm{Mn}\{\mathrm{N}(\mathrm{CN}{)}_2{\}}_2(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+2\mathrm{n}{\mathrm{NaClO}}_4+6\mathrm{n}{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} \mathbf{3}\end{array}$$

(1)

Complex 3 consists of linear 1D chains in which two neighboring Mn^II centers are bridged by two μ(1,5)-N(CN)₂⁻ (2.1₁1₅0₃) groups via the terminal nitrile nitrogen atoms; two N(3)-coordinated ancillary Mebta ligands complete the distorted octahedral coordination sphere at each metal ion which lies on an inversion center resulting in N-Mn-N angles of 180° (Figure 9). The Mn^II∙∙∙Mn^II distance is ~7.46 Å. Variable-temperature (2–300 K) dc magnetic susceptibility studies revealed a very weak antiferromagnetic exchange interaction between the S = 5/2 metal ions, typical for μ₂(1,5)-Ν(CN)₂⁻ ligands.

5.3. Fe(II) and Fe(III) Complexes

Complex {[Fe{N(CN)₂}₂(Mebta)₂]}_n (4) was prepared using the same approach as 3 in a slightly higher yield (~45%). The two complexes are isomorphous [81]. The Fe^II∙∙∙Fe^II distance is ~7.39 Å and a weak antiferromagnetic interaction was observed.

The 1:2 reaction between Fe(NO₃)₃∙9H₂O and Mebta in EtOH/triethyl orthoformate (TEOF) under reflux gave an orange solid, recrystallization of which from MeNO₂ produced crystals of [Fe₂O(NO₃)₄(Mebta)₄] (5) in a 55% yield [82], Equation (2). The dehydrating agent TEOF (vide infra) is necessary to remove most of the water contained in EtOH and in the iron(III) starting material, thus avoiding hydroxo precipitates. The produced HNO₃ would be expected to attack the oxo bridge and decompose the product. This does not happen because the protons produced from water could be neutralized by Mebta which does not react. In accordance with this, the yield never exceeds 55%, meaning that Mebta and an unknown iron(III) species remain in solution. If this assumption is correct, the formation of 5 can be represented by Equation (3).

$$\begin{array}{c}2\mathrm{Fe}({\mathrm{NO}}_3{)}_3·9{\mathrm{H}}_2\mathrm{O}+4\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}/\mathrm{T}\mathrm{E}\mathrm{O}\mathrm{F},\mathrm{T}}{\to }[{\mathrm{Fe}}_2\mathrm{O}({\mathrm{NO}}_3{)}_4(\mathrm{Mebta}{)}_4]+2{\mathrm{HNO}}_3+17{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{5}\end{array}$$

(2)

$$\begin{array}{c}2\mathrm{Fe}({\mathrm{NO}}_3{)}_3·9{\mathrm{H}}_2\mathrm{O}+6\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}/\mathrm{T}\mathrm{E}\mathrm{O}\mathrm{F},\mathrm{T}}{\to }[{\mathrm{Fe}}_2\mathrm{O}({\mathrm{NO}}_3{)}_4(\mathrm{Mebta}{)}_4]+2(\mathrm{MebtaH})({\mathrm{NO}}_3)+17{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{5}\end{array}$$

(3)

The crystal structure of 5 consists of isolated centrosymmetric dinuclear molecules [Fe₂O(NO₃)₄(Mebta)₄] (Figure 10). The two Fe^II centers are distorted octahedral bonded to the bridging oxo group, two N(3)-coordinated Mebta molecules, and one bidentate chelating (1.110) and one monodentate (1.100) nitrate group. The Fe^III∙∙∙Fe^III distance is 3.54 Å. The Fe-O(nitrate) bond trans to the oxo group (2.28 Å) is longer than its counterpart in the cis position (2.10 Å), a consequence of the powerful trans influence of the short Fe-μ₂(O) bond. The two bulky, planar Mebta ligands are trans to each other, thus minimizing steric interaction. The Mebta groups in syn positions are nearly parallel. There are intradimer stacking interactions between these ligands on the two sides of the molecule [82]. The ν_as(Fe-O-Fe) vibration appears at 851 cm⁻¹ in the IR spectrum of the complex. The ν_s(Fe-O-Fe) mode occurs at 384 cm⁻¹ in the FT-Raman spectrum and it shifts to 375 cm⁻¹ upon ¹⁸O substitution. Magnetic data indicate strongly antiferromagnetically coupled high-spin diiron(III) species. The ⁵⁷Fe-Mössbauer spectrum of 5 at 4.2 K in zero field consists of a single quadrupole doublet with δ = 0.50 mm s⁻¹ (vs. metallic Fe at room temperature) and ΔΕ_Q = 1.30 mm s⁻¹, values typical for an oxo-bridged octahedral complex in an O/N environment.

Changes in the inorganic starting material from Fe(NO₃)₃∙9H₂O to FeCl₃ and the solvent from EtOH to CHCl₃ affect the chemical and structural identity of the product. The 1:2 reaction of FeCl₃ and Mebta in refluxing CHCl₃ gave the yellow product [FeCl₃(Mebta)₂] (6) in typical yields of 55–60%, Equation (4) [83]. Excess of Mebta led again to complex 6.

$$\begin{array}{c}{\mathrm{FeCl}}_3+2\mathrm{Mebta}\stackrel{\mathrm{C}\mathrm{H}{\mathrm{C}\mathrm{l}}_3,\mathrm{T}}{\to }[{\mathrm{FeCl}}_3(\mathrm{Mebta}{)}_2]\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{6}\end{array}$$

(4)

The molecule (Figure 11) of 6 possesses a crystallographically imposed two-fold axis of symmetry passing through the Fe^III and a Cl atom. The metal ion is surrounded by three terminal chloro ligands and two nitrogen atoms from two planar N(3)-coordinated Mebta molecules. The five donor atoms define a slightly distorted trigonal bipyramid around Fe^III with the two Mebta nitrogens at the axial sites. The Fe-N bond distances (2.18 Å) are somewhat long, and this is due to atom–atom repulsions between the donor nitrogen atoms and the chloro groups (~3.10 Å). The crystal structure of 6 is stabilized by intermolecular π-π stacking interactions between the Mebta ligands, each [FeCl₃(Mebta)₂] molecule interacting with four of its neighbors.

The far-IR spectrum of 6 shows one strong terminal metal ion-chloro stretch, ν(Fe-Cl)_t, at 378 cm⁻¹ and one weaker Fe^III-N stretching mode, ν(Fe-N), at 196 cm⁻¹. A reasonable approximation of the molecular symmetry around Fe^III is D_3h, and thus one ν(Fe-Cl)_t (E’) and one ν(Fe-N) (A₂″) mode would be expected. Molar conductivity data (25 °C, 10⁻³ M) indicate that the complex is non-electrolyte in MeCN; surprisingly 6 is a 1:1 electrolyte in MeNO₂, probably suggesting the presence of [FeCl₂(Mebta)₄]⁺ and [FeCl₄]⁻ species in solution. The δ and ΔΕ_Q values in the zero-field ⁵⁷Fe-Mössbauer spectrum of 6 at room temperature are 0.24 mm s⁻¹ and 0.27 mm s⁻¹, respectively. The δ value is typical for high-spin 5-coordinate iron(III). The asymmetry in peak intensity is typical for complexes having a large positive zero-field splitting parameter, D. The complex has been tested as a homogeneous catalyst (MeCN under N₂) in the presence of the “green” H₂O₂ oxidant; it displays moderate to high catalytic activity towards the oxidation of several alkenes, cyclohexane and n-hexane [83]. The mechanism involves hydrogen abstraction and oxygen transfer in a variety of alkene substrates, resulting in good yields of epoxidation and allyl oxidation products. The products from the cyclohexane oxidation are cyclohexanol, cyclohexanone and 4-hexen-1-ol, and those from n-hexane are hexanones and hexanoles.

5.4. Tetrahedral and Octahedral Co(II) Complexes

Complexes [CoCl₂(Mebta)₂] (7), trans-[Co(NCS)₂(Mebta)₄] (8), trans-[Co(NCS)₂(MeOH)₂(Mebta)₂] (9) and cis-[Co(NO₃)₂(Mebta)₂] (10) were prepared by simple reactions of Co(II) starting materials (CoCl₂, Co(NCS)₂, Co(NO₃)₂∙6H₂O) and Mebta in alcohols in moderate to good yields [84]. The metal:ligand molar ratio does not affect the product identity. The reaction of Co(NCS)₂ with Mebta in alcohols is solvent-dependent. In MeOH, the product is compound 9, while the reaction in EtOH or heavier alcohols produces complex 8. This different behavior may be due to the better coordinating ability of MeOH. Another point of synthetic interest is that the concentration of H₂O in the Co(NO₃)₂∙6H₂O/Mebta reaction mixture in EtOH has a pronounced effect on the product identity. Only in H₂O-free medium [use of TEOF (vide infra) under heating] could compound 10 be isolated. In H₂O-containing ethanolic mixtures, the resulting product was trans-[Co(NO₃)₂(H₂O)₂(Mebta)₂] (10a) as evidenced by a poor quality X-ray structure. Since the reaction yields complexes 10 and 10a if carried out in H₂O-free or H₂O-containing EtOH, respectively, the former can give the latter by the addition of small amounts of water, Equation (5). Contributory factors to the ligand redistribution (cis→trans) may be the minimization of the total repulsion energy and hydrogen bonding.

cis-[Co(NO₃)₂(Mebta)₂] + 2 H₂O → trans-[Co(NO₃)₂(H₂O)₂(Mebta)₂]
 10                   10a

(5)

There are two crystallographically independent molecules in the structure of 7; one of them is shown in Figure 12. The structural characteristics of the two molecules are similar. The Co^II center is surrounded by two N atoms belonging to two 1.1₃0₁0₂ Mebta molecules (Figure 7) and two terminal chloro ligands in a slightly distorted tetrahedral geometry. The repulsion between the two chlorides is presumably responsible for the observed distortion. The Cl-Co-Cl plane makes angles of ~85° with each of the Mebta planes, while the dihedral angle between the two planar ligands is ~40°.

The octahedral molecules of 8 and 9 possess a crystallographically imposed center of symmetry. In 8 (Figure 13), the metal ion is coordinated to two isothiocyanato groups and four N(3)-bonded Mebta molecules. In complex 9 (Figure 14), the Co^II ion is coordinated by pairs of trans-related isothiocyanato groups, MeOH molecules and Mebta ligands. The Co-N and Co-O bond lengths in the two complexes are typical for high-spin electronic configurations. The NCS⁻ ions are coordinated in a slightly bent fashion (Co^II-N≡C = ~165°). Both methanolic oxygen atoms are involved in intermolecular H bonds with the acceptors being the sulphur atoms of isothiocyanato ligands of neighboring complex molecules.

The structure of 10 (Figure 15) consists of well-separated, severely distorted cis-[Co(NO₃)₂(Mebta)₂] octahedral molecules with two anisobidentate chelating nitrato groups and two monodentate Mebta ligands. The high spin Co^II center lies on a crystallographic two-fold axis (C₂), which bisects the N-Co-N angle. The Co-O bonds trans to N atoms (2.28 Å) are longer than those trans to O atoms (2.06 Å). This difference in bond lengths is attributed to the fact that in cis-bis(bidentate ligand)bis(monodentate ligand) metal complexes, one end of the bidentate ligands is associated with a smaller repulsion energy than the other end. The Co-N bond distance in 10 is noticeably shorter than the corresponding lengths in 8 and 9 (2.05 vs. ~2.18 Å); these distances in 10 are similar to those observed in the tetrahedral complex 7. This structural feature is reflected in the room-temperature effective magnetic moment, μ_eff, value of 10 (4.58 B.M.), which is in the region expected for tetrahedral Co(II) complexes. The explanation is the distorted 6-coordinate structure of C₂ symmetry for 10; the degeneracy of the Co^II ion is lifted [⁴T_1g(O_h)→⁴A, 2 ⁴B in C₂]. The new ground state, being an orbital singlet, gives rise to a magnetic moment closer to the spin-only value than that observed in an octahedral complex (~5 B.M.). The absence of a center of symmetry in 10 also gives a high-intensity d-d spectrum (contrary to normal octahedral complexes) since both p and d orbitals span common representations in C₂. The EPR spectra of 8–10 at various temperatures show typical rhombic g values. The low-temperature data and their temperature dependence are both indicative of a rather large and positive (>15 K) axial zero-field splitting. In the octahedral complex 10a, the Co^II ion is coordinated with pairs of trans-related monodentate nitrato, aqua and Mebta ligands.

5.5. Reactions of Ni(II) Thiocyanate and Nitrate with 1-methylbenzotriazole

The 1:2 reaction between Ni(SCN)₂ and Mebta in refluxing MeOH gave complex trans-[Ni(NCS)₂(MeOH)₂(Mebta)₂] (11) in yields higher than 70% [85]. The complex is isostructural (but not isomorphous) with its Co(II) analogue 9 (Figure 14).

The Ni(NO₃)₂∙6H₂O/Mebta reaction system proved to be synthetically and structurally interesting. In the 1:2 reaction mixture, the concentration of H₂O affects the product identity (Figure 16). Complex cis-[Ni(NO₃)₂(Mebta)₂] (12) could be isolated only in H₂O-free [use of TEOF (vide infra)] warm ethanolic or acetonic mixtures; the yield was ~70%. Employing a ca. 1 M H₂O concentration in Me₂CO and a ca. 10:1 H₂O:Ni^II molar ratio, the isolated blue-green solid [Ni(NO₃)(H₂O)₂(Mebta)₂](NO₃) (12a) was obtained in a 68% yield. Higher H₂O concentration (1.9 M) and H₂O:Ni^II molar ratio (20:1) gave the blue material trans-[Ni(H₂O)₄(Mebta)₂](NO₃)₂ (12b) in a 52% yield. Since the 1:2 reaction between nickel(II) nitrate and 1-methylbenzotriazole gives complexes 12 and 12a, 12b if carried out in H₂O-free or H₂O-containing Me₂CO, we suspected that 12 would react with increasing amounts of H₂O to give 12a and 12b; indeed, this turned out to be the case (Figure 16).

The distorted octahedral complex 12 (Figure 17) is isomorphous with its Co(II) analogue 10 (Figure 15). There are two anisobidentate chelating nitrato groups and two monodentate Mebta ligands, each pair in a cis position. The severe distortion of the octahedron is due to the restricted “bite” of the chelating nitrates, with an angle at the metal ion of ~61°.

A poor single-crystal X-ray data set for 12a revealed a distorted octahedral cation, with one bidentate chelating nitrato group, two cis H₂O molecules and two cis Mebta ligands; the 1+ charge of the cation is counter-balanced by a H-bonded ionic NO₃⁻. The cation of 12a can be considered as deriving from the molecule of 12 by replacing a bidentate chelating nitrato group of the latter with two cis aqua ligands in the former.

The cation of complex 12b (Figure 18) has a nearly regular octahedral {Ni^IIO₄N₂} coordination sphere, involving two centrosymmetrically related Mebta ligands and four H₂O molecules; the two nitrates are ionic. Two of the coordinated H₂O molecules are trigonal and two pseudotetrahedral. In the 3D supramolecular network, each NO₃⁻ is connected via H bonds with four coordinated H₂O molecules belonging to three different [Ni(H₂O)₄(Mebta)₂]²⁺ cations.

The Ni^II:Mebta reaction ratio is also important in the Ni(NO₃)₂∙6H₂O/Mebta system. Employment of a large excess of the ligand (Ni^II:Mebta = 1:12 and 1:15) in anhydrous Me₂CO, complex mer-[Ni(NO₃)₂(Mebta)₃] (13), as the mono acetone solvate, could be isolated in moderate yield, Equation (6). A chemically more correct representation of the reaction is shown in Equation (7), which emphasizes dehydration by TEOF; its capacity for H₂O removal increases with increasing temperature. The structure of 13 is shown in Figure 19. The Ni^II center is surrounded by three N and three O atoms in a distorted octahedral configuration. The six-coordinate molecule is the mer isomer. Both planar nitrates take part in coordination, one as a monodentate ligand and the other as a bidentate ligand. Complex 13 joins only a handful of structurally characterized Ni(II) complexes with both monodentate and bidentate chelating nitrato groups, most of which contain one tridentate chelating organic ligand.

$$\begin{array}{c}\mathrm{Ni}({\mathrm{NO}}_3{)}_2·6{\mathrm{H}}_2\mathrm{O}+3\mathrm{Mebta}\stackrel{{\mathrm{M}\mathrm{e}}_2\mathrm{C}\mathrm{O}/\mathrm{T}\mathrm{E}\mathrm{O}\mathrm{F},\mathrm{e}\mathrm{x}\mathrm{c}.\mathrm{M}\mathrm{e}\mathrm{b}\mathrm{t}\mathrm{a},\mathrm{T}}{\to }\mathit{mer}\text{-}[\mathrm{Ni}({\mathrm{NO}}_3{)}_2(\mathrm{Mebta}{)}_3]+6{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{13}\end{array}$$

(6)

$$\begin{array}{c}\mathrm{Ni}({\mathrm{NO}}_3{)}_2·6{\mathrm{H}}_2\mathrm{O}+3\mathrm{Mebta}+6\mathrm{HC}({\mathrm{OC}}_2{\mathrm{H}}_5{)}_3\stackrel{{\mathrm{M}\mathrm{e}}_2\mathrm{C}\mathrm{O},\mathrm{e}\mathrm{x}\mathrm{c}.\mathrm{M}\mathrm{e}\mathrm{b}\mathrm{t}\mathrm{a},\mathrm{T}}{\to }\mathit{mer}\text{-}[\mathrm{Ni}({\mathrm{NO}}_3{)}_2(\mathrm{Mebta}{)}_3]+6\mathrm{HCO}({\mathrm{OC}}_2{\mathrm{H}}_5)+12{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{13}\end{array}$$

(7)

5.6. Square Planar Pd(II) and Pt(II) Complexes

The square planar complexes cis-[PdCl₂(Mebta)₂] (14) and cis-[PtCl₂(Mebta)₂] (15) have been prepared by the reactions shown in Equations (8) and (9), respectively; yields of 45–65% were routinely obtained [86].

$$\begin{array}{c}({\mathrm{H}}_3\mathrm{O}{)}_2[{\mathrm{PdCl}}_4]+2\mathrm{Mebta}\stackrel{{\mathrm{H}}_2\mathrm{O}/{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}(1:1 \mathrm{v}/\mathrm{v}),80 °\mathrm{C}/2 \mathrm{h}}{\to }\mathit{cis}\text{-}[{\mathrm{PdCl}}_2(\mathrm{Mebta}{)}_2]+2\mathrm{HCl}+2{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{14}\end{array}$$

(8)

$$\begin{array}{c}{\mathrm{PtCl}}_2+2\mathrm{HCl}+{\mathrm{Na}}_2{\mathrm{CO}}_3+2\mathrm{Mebta}\stackrel{{\mathrm{H}}_2\mathrm{O}/{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}(1:1\mathrm{v}/\mathrm{v}),25°\mathrm{C}/24\mathrm{h}}{\to }\mathit{cis}\text{-}[{\mathrm{PtCl}}_2(\mathrm{Mebta}{)}_2]+2\mathrm{NaCl}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{15}\end{array}$$

(9)

No X-ray-quality single crystals of the products could be grown. The complexes were characterized by a variety of solid-state and solution physical (conductivity, TG/DTG, pXRD) and spectroscopic (IR, Raman, UV/VIS, ¹H NMR) techniques. Based on these combined studies, cis square planar structures were assigned for the two complexes Figure 20). In particular, the appearance of two ν(M-Cl)_t (A₁ and B₂ under C_2v symmetry) and two ν(M-N) (A₁ and B₂ under C_2v) in both the far-IR and low-frequency Raman spectra of the complexes implies a cis configuration (M = Pd, Pt). The cis structural type suggests that Cl⁻ has a greater trans effect compared to Mebta. Apparently, the benzene ring of Mebta near the donor site hampers the arrangement of four Mebta ligands around the metal ion in a square planar environment, and thus preparation of [M(Mebta)₄]Cl₂ ionic complexes is not possible.

5.7. The Rich Coordination Chemistry of Mebta with Cu(II)

Cu(II) is the most studied metal ion in the coordination chemistry of Mebta. Complex [Cu(Mebta)₄(H₂O)](ClO₄)₂ (16) was readily prepared by the 1:4 reaction of Cu(ClO₄)₂∙6H₂O with Mebta in EtOH in ca. 75% yield [87]. The metal coordination geometry in the [Cu(Mebta)₄(H₂O)]²⁺ cation (Figure 21) is well described as distorted square pyramidal with the aqua ligand occupying the apical position. Comparison between the structural and spectroscopic properties of 16 and those of the Cu^II site of Cu-Zn superoxide dismutase (the enzyme that catalyzes efficiently the dismutation of the harmful superoxide ion to dioxygen and hydrogen peroxide in almost all eukaryotic cells and a few prokaryotes through consecutive reduction and oxidation of the copper ion in the active site) shows that the former can be considered as a fairly good model of the latter. The solid-state and solution electronic spectrum of the complex displays a ligand field band at ~675 nm; this d-d absorption maximum is close to that observed for intact Cu^II₂Zn^II₂SOD (680 nm). The X-band EPR spectrum of a polycrystalline sample of 16 at 80 K is typical for axial-type copper(II) compounds with two g values (g_|| = 2.245, g_⊥ = 2.060) and g_||> g_⊥ > 2.03, indicating a d_x²_−y² ground state. The enzyme exhibits similar spectral parameters for copper(II) [g_|| = 2.271, g_⊥ = 2.083].

The CuCl₂/Mebta reaction system was proven extremely fertile [88,89]. A systematic investigation of this reaction system, involving the determination of the influence of the Cu^II:Mebta ratio, the nature of the solvent and the presence of non-coordinating counterions on the product identity, was carried out by our group [88,89]. As a consequence, complexes [Cu₂Cl₄(Mebta)₄] (17), [CuCl₂(Mebta)₂] (18), {[Cu₂Cl₄(Mebta)₂]}_n (19), [Cu₄OCl₆(Mebta)₄] (20) and [Cu₂Cl₂(Mebta)₆](ClO₄)₂ (21) were isolated and structurally characterized by single-crystal X-ray crystallography. The various synthetic procedures and transformations are conveniently summarized in the chemical Equations (10)–(17), where R is Me or Et.

$$\begin{array}{c}2{\mathrm{CuCl}}_2·2{\mathrm{H}}_2\mathrm{O}+4\mathrm{Mebta}\stackrel{\mathrm{R}\mathrm{O}\mathrm{H}}{\to }[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_4]+4{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{17}\end{array}$$

(10)

$$\begin{array}{c}[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_4]\stackrel{\mathrm{ROH}}{\rightleftarrows }2[{\mathrm{CuCl}}_2(\mathrm{Mebta}{)}_2]\\ \mathbf{17}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{18}\end{array}$$

(11)

$$\begin{array}{c}\mathrm{n}[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_4]\stackrel{105–205°\mathrm{C}{,\mathrm{N}}_2}{\to }\{[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+2\mathrm{n} \text{Mebta}\\ \mathbf{17}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{19}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\end{array}$$

(12)

$$\begin{array}{c}\mathrm{n}[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_4]\stackrel{\mathrm{EtOH}}{\rightleftarrows }\{[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+2\mathrm{n} \text{Mebta}\\ \mathbf{17}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{19}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\end{array}$$

(13)

$$\begin{array}{c}2\mathrm{n}{\mathrm{CuCl}}_2·2{\mathrm{H}}_2\mathrm{O}+2\mathrm{n}\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H},\mathrm{e}\mathrm{x}\mathrm{c}.{\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}}_2·{2\mathrm{H}}_2\mathrm{O}}{\to }\{[{\mathrm{Cu}}_2{\mathrm{Cl}}_4(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+4\mathrm{n}{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{19}\end{array}$$

(14)

$$\begin{array}{c}4{\mathrm{CuCl}}_2·2{\mathrm{H}}_2\mathrm{O}+4\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }[{\mathrm{Cu}}_4{\mathrm{OCl}}_6(\mathrm{Mebta}{)}_4]+2\mathrm{HCl}+7{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{20}\end{array}$$

(15)

$$\begin{array}{c}3{\mathrm{CuCl}}_2+\mathrm{CuO}+4\mathrm{Mebta}\stackrel{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H},\mathrm{T}}{\to }[{\mathrm{Cu}}_4{\mathrm{OCl}}_6(\mathrm{Mebta}{)}_4\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{20}\end{array}$$

(16)

$$\begin{array}{c}2{\mathrm{CuCl}}_2·2{\mathrm{H}}_2\mathrm{O}+6\mathrm{Mebta}+2{\mathrm{NaClO}}_4·{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{H}\mathrm{C}\mathrm{l}3\mathrm{N}}{\to }[{\mathrm{Cu}}_2{\mathrm{Cl}}_2(\mathrm{Mebta}{)}_6]({\mathrm{ClO}}_4{)}_2+2\mathrm{NaCl}+6{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{21}\end{array}$$

(17)

Equation (12) represents a solid-state reaction. Concerning Equation (17), our studies indicated that HCl 3N does not take part in the reaction mechanism and merely assists the dissolution of Mebta, in a non-organic solvent system.

The crystal structure of 17 consists of isolated dinuclear molecules [Cu₂Cl₄(Mebta)₄] with two bridging chloro groups; a terminal chloride and two trans monodentate Mebta ligands complete five-coordination at each metal ion (Figure 22). Due to the presence of a crystallographically imposed inversion center in the middle of the copper(II)∙∙∙copper(II) distance, the bridging {Cu₂Cl₂} unit is strictly planar. The geometry at each Cu^II center is square pyramidal with the apical site being occupied by the chloro group which is basal to the other Cu^II in the dimeric molecule. There appear to be intradimer π-π stacking interactions between the nearly parallel syn Mebta ligands on the two sides of the molecule. Variable-temperature magnetic susceptibility data indicate a moderate ferromagnetic exchange interaction between the Cu^II (S = ½) centers in the dimer and a weak interdimer antiferromagnetic interaction. An insight concerning the effect of structural parameters on the sign and magnitude of the magnetic exchange interactions in 17 and 21 (vide infra) was gained through Extended Huckel Molecular Orbital (EHMO) calculations performed on a model system. The calculations have permitted the establishment of a new criterion, holding for magnetostructural correlations in bis(μ-chloro)copper(II) dinuclear complexes [89].

The Cu^II center in the mononuclear compound 18 (Figure 23) is bonded to two N atoms of the (N3)-coordinated Mebta molecules and two terminal chloro ligands. The coordination geometry can be described as tetrahedrally distorted trans-square planar. The cis (93.6–95.0°) and trans (143.7–152.3°) angles of all donor atoms around Cu^II are intermediate between the expected values for tetrahedral (109.5°) and square planar (90°, 180°) coordination, respectively.

The crystal structure of 19 consists of linear, well-separated polymeric chains of Cu^II ions bridged asymmetrically by two chloro groups (Figure 24). A regular alternation of two crystallographically non-equivalent metal ions (Cu1, Cu2) occurs along the chain. Cu1 is bonded to four chloro groups in an almost perfect square–planar arrangement, whereas Cu2 has the trans-octahedral {Cu(μ-Cl)₄N₂} environment, the N atoms belonging to two trans N(3)-coordinated Mebta molecules. The symmetry elements of the molecular structure (two mirror planes, one two-fold crystallographic axis) are such that each metal ion is located on a crystallographic center of symmetry. The doubly bridged chain with two alternating coordination spheres observed in 19 is extremely rare in complexes of the empirical formula CuX₂L (X = Cl, Br; L = monodentate ligand). Magnetically, complex 19 behaves as consisting of ferromagnetic chains, which interact antiferromagnetically with the neighboring chains.

The crystal structure of 20 is composed of tetranuclear [Cu₄OCl₆(Mebta)₄] molecules (Figure 25). The molecule contains a tetrahedron of Cu^II ions which are held together by one μ₄-oxo (O²⁻) group and six μ-chloro ligands; each bridging Cl⁻ spans an edge of the tetrahedron. One monodentate N(3)-coordinated Mebta molecule is bound to a Cu^II ion. Thus, the core is {Cu^II₄(μ₄-O)(μ-Cl)₆}. The μ₄-O²⁻ group and two of the bridging chloro ligands lie on a crystallographic 2-fold axis. The Cu₄(μ₄-O) tetrahedron is regular, the Cu-O-Cu angles being 108.6 and 110.9°, and the Cu∙∙∙Cu non-bonding distances varying from 3.10 to 3.15 Å. Angles at the bridging chloro groups are ~81°. Each of the Cu^II centers is in a slightly trigonal bipyramidal coordination environment, with the μ₄-O²⁻ group and the Mebta N atom in the two axial positions. The six μ-Cl⁻ groups comprise a distorted octahedron around μ₄-O²⁻ ion. The O∙∙∙Cl distances are in the range 2.88–2.95 Å and the Cl∙∙∙Cl ones vary between 3.98 and 4.25 Å; the cis Cl∙∙∙O∙∙∙Cl angles have values between 86.5 and 93.5°, close to those expected for an octahedral configuration. The IR spectrum shows a strong band at 575 cm⁻¹, attributable to the asymmetric vibration (F₂ mode) of the {Cu₄(μ₄-O)}⁶⁺ unit.

The two Cu^II ions in the centrosymmetric cation [Cu₂Cl₂(Mebta)₆]²⁺ of 21 (Figure 26) are asymmetrically bridged by two chloro groups; three monodentate Mebta ligands complete five-coordination at each metal. The five donor atoms do not define a regular polyhedron around copper, and the coordination geometry can be described as either square pyramidal or trigonal bipyramidal. The cation of 21 is somewhat structurally similar to the molecule of 17, the terminal chloro ligands of the latter having been replaced by monodentate Mebta molecules in the former. Complex 21 exhibits a very similar magnetic behavior to that of 17.

Mainly for comparison reasons, a systematic investigation of the CuBr₂/Mebta reaction system was described [90]. Particular emphasis was placed on determining the influence of the Mebta:Cu^II ratio and the nature of the solvent on the identity of the products. Complex [CuBr₂(Mebta)₂(MeCN)] (22) was readily prepared by the 4:1, 3:1 and 2:1 Mebta to Cu^II reactions in MeCN in a very good yield (~75%). The 8:1, 6:1, 4:1 and 2:1 reactions in MeOH or EtOH at room temperature gave a brown solution from which was crystallized [CuBr₂(Mebta)₂] (23), also in good yield. Complex 23 was also isolated in pure form from the solid-state endothermic reaction represented by Equation (18). The H₂O concentration in the alcoholic reaction mixtures at high Mebta:Cu^II molar ratios gives a different product. Employing ca. 3 M H₂O concentration in MeOH (or EtOH 96%) and 8:1, 5:1 or 3.6:1 Mebta:Cu^II ratios, a mixture of light green crystals of [CuBr₂(Mebta)₃] (24) and 23 was obtained.

$$\begin{array}{c}[{\mathrm{CuBr}}_2(\mathrm{Mebta}{)}_2(\mathrm{MeCN})]\stackrel{88–107°\mathrm{C}{,\mathrm{N}}_2}{\to }[{\mathrm{CuBr}}_2(\mathrm{Mebta}{)}_2]+\mathrm{MeCN}\\ \mathbf{22}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{23}\hspace{1em}\hspace{1em}\end{array}$$

(18)

The five donor atoms in the mononuclear complex 22 (Figure 27) do not define a regular coordination polyhedron around Cu^II. However, it can be described as a very distorted trigonal bipyramid with two Mebta N3 atoms at the axial positions; the equatorial bond angles are 142, 114 and 104°. The linear MeCN group binds in a significantly nonlinear fashion (Cu-N≡C = 149°).

The geometry around the metal ion in the mononuclear complex 23 (Figure 28) can be described as tetrahedrally distorted trans-square planar. The cis (92.6 and 95°) and trans (143.1 and 154.6°) donor atom-Cu^II-donor atom angles are intermediate between the expected values for tetrahedral and square planar coordination, respectively. Complex 23 is isomorphous with its chloro counterpart 18.

The asymmetric unit of 24 contains two structurally similar, but crystallographically independent five-coordinate molecules [CuBr₂(Mebta)₃]; one of them is shown in Figure 29. The coordination sphere of Cu^II is composed of two terminal bromo ligands and three N(3)-coordinated Mebta molecules. The coordination geometry is described as distorted trigonal bipyramidal, in which the equatorial plane consists of two bromo ligands and the N atom of a Mebta ligand. Thus, the structure of 24 can be considered as being derived from that of 22 with the replacement of the MeCN ligand of the latter by one Mebta molecule.

Although we do not claim that the synthetic investigation of the CuX₂/Mebta (X = Cl, Br) reaction systems was complete, evidently, (a) for compounds [CuX₂(Mebta)₂] (18, 23), the molecular structures remain unchanged upon halide substitution, (b) complexes 17, 19 and 20 have no counterparts in CuBr₂/Mebta chemistry, and (c) the 1:3 bromo complex 24 does not have its analogue in CuCl₂/Mebta chemistry. Electronic factors may be responsible for (b) and (c).

The extension of the work that led to the 1D compounds 3 and 4 in Cu(II) chemistry gave compound {[Cu{N(CN)₂}₂(Mebta)₂]}_n (25) [81] in typical yields of ~40%. This complex (Figure 30) is isomorphous with 3 (Figure 9) and 4. This intrachain Cu^II∙∙∙Cu^II distance is ~7.34 Å. The crystal structure consists of interdigitated linear chains, where each Mebta ligand lies between two Mebta ligands of an adjacent chain. The centroid-centroid distance between close Mebta molecules belonging to neighboring chains is at the high limit of π-π stacking interactions (~3.80 Å). The complex shows a somewhat unusual, almost linear decrease of the χ_ΜΤ product from 0.38 cm³ K mol⁻¹ (1.52 B.M.) at 4.2 K. This behavior could not be modeled by a chain model.

Attempts to increase the dimensionality of 25 (1D → 2D or 3D) were made by using the tricyanomethanide, C(CN)₃⁻, ion. Complex {[Cu{C(CN)₃}(Mebta)₂]}_n (26) was isolated in a low yield (≤30%) through the reaction represented by Equation (19) [81]. However, just as observed in 25, the C(CN)₃⁻ ions bridge via only two of the three available nitrile N atoms, resulting in a 1D linear chain, analogous to those observed in 3, 4 and 25 (Figure 31). Thus, this ion behaves as a μ(1,5) (2.1₁1₅0₃) ligand. Each metal ion is located at a crystallographically imposed inversion center and exhibits a distorted octahedral coordination geometry; the Cu^II∙∙∙Cu^II distance is ~7.25 Å. The linear chains are packed in an interdigitated manner and the Mebta ligands of neighboring chains are ~3.72 Å apart. There is a very weak, antiferromagnetic intrachain Cu^II∙∙∙Cu^II interaction, typical of μ(1,5) C(CN)₃⁻ ligands. This tiny exchange implies essentially zero coupling (non-interacting Cu^II atoms), which can be explained by the fact that the unpaired spin on each Cu^II center is in a d_x²_-y² orbital, at 90° to the chain direction. The small decrease in χ_ΜΤ below 5 K could also be due to Zeeman-level depopulation effects in the 1 T field used for the measurements.

$$\begin{array}{c}\mathrm{n}\mathrm{Cu}({\mathrm{ClO}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+2\mathrm{n}\mathrm{K}\{\mathrm{C}(\mathrm{CN}{)}_3\}+2\mathrm{nMebta}\stackrel{{\mathrm{H}}_2\mathrm{O}/\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}{\to }\{[\mathrm{Cu}\{\mathrm{C}(\mathrm{CN}{)}_3\}(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+2\mathrm{n}\mathrm{KCl}+6\mathrm{n}{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{26}\end{array}$$

(19)

The azido group is a versatile ligand in coordination chemistry. It can coordinate to up four metal ions when acting as a monoatomic bridge (end-on, EO). The coordination of both terminal N atoms (EE) often leads to coordination polymers. The relatively rare EO/EE combination is capable of bridging up to six metal ions resulting in molecular clusters or polymeric compounds with aesthetically beautiful structures and interesting properties (mainly magnetic). The 1:2:4 reaction of Cu(ClO₄)₂∙6H₂O, NaN₃ and Mebta in DMF gave a dark olive-green solution, from which crystals of {[Cu(N₃)₂(Mebta)]}_n (27) were obtained in 35% yield [91]. The same compound can also be isolated using other Cu(II) starting materials, e.g., CuCl₂∙2H₂O, Cu(NO₃)₂∙3H₂O and Cu(O₃SCF₃)₂. The 1:2:1 stoichiometric reaction did not give the expected complex 27, but instead the Mebta-free 2D coordination polymer {[Cu₃(N₆)₆(DMF)₂]}_n built by EO azido groups. It is likely that there is competition between DMF and Mebta for Cu^II binding sites. When the organic N₃^—containing reagent Me₃Si(N₃) was used, the 3D coordination polymer {[Cu(N₃)₂]}_n was isolated in 40–60% yields depending on the Cu(II) source.

Complex 27 is a 1D coordination polymer. It can be described as consisting of two azido-linked, parallel linear chains (Figure 32). The bridging between the Cu^II atoms within each chain is achieved through two μ-(1,1) (or 2.20) EO N₃⁻ groups; one of these azido groups becomes μ₃-(1,1,1) (or 3.30) EO and links the metal ions of two neighboring chains giving rise to a 1D corrugated tape. Peripheral ligation about each Cu^II is provided by a N(3)-coordinated monodentate Mebta ligand. Thus, the highly elongated, octahedral coordination sphere of each metal ion is {Cu^IIN₅N’}, the five N atoms provided by five azido groups and the N’ atom originated from Mebta. The Cu^II∙∙∙Cu^II distance within each chain is 3.63 Å, and these separations between the neighboring parallel chains are 3.10 and 3.60 Å. The Mebta ligands are intramolecularly stacked in an onset, parallel manner; the centroid∙∙∙centroid distance between adjacent coordinated Mebta molecules is 3.63 Å. The packing of 27 shows the presence of very weak intermolecular interactions between neighboring double chains through π-π stacking forces between the phenyl groups of Mebta.

5.8. A Luminescent Copper(I) Polymer with a Unique Bridging Mode of Mebta

The up-to-now described metal complexes of Mebta contain the N(3)-coordinated ligand (1.1₃0₁0₂; Figure 7, left). The coordination polymer {[Cu^I₃I₃(Mebta)(MeCN)]}_n (28) [80] is unique because the molecule behaves as a bidentate bridging ligand (2.1₂1₃0₁; Figure 7, right). The reaction of CuI with an excess of Mebta in CH₂Cl₂ at room temperature for 24 h caused the precipitation of a yellow solid, recrystallization of which from MeCN gave light green crystals of the product, Equation (20). The reaction probably proceeds through the attack of MeCN on the intermediate {[Cu^I₂I₂(Mebta)]}_n (29) [92].

$$\begin{array}{c}3\mathrm{n}\mathrm{CuI}+\mathrm{n}\mathrm{Mebta}+\mathrm{n}\mathrm{MeCN}\stackrel{{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{l}}_2/\mathrm{M}\mathrm{e}\mathrm{C}\mathrm{N}}{\to }\{[{\mathrm{Cu}}_3{\mathrm{I}}_3(\mathrm{Mebta})(\mathrm{MeCN})]{\}}_{\mathrm{n}}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{28}\end{array}$$

(20)

The asymmetric unit of 28, illustrated in Figure 33, contains three crystallographically independent Cu^I atoms, one bidentate bridging Mebta group and one terminal MeCN molecule [80]. The three iodo ligands are μ₃, in such a manner that each iodide is at the apex and the three metal centers at the corner of the base of a trigonal pyramid. The Mebta ligand bridges Cu1 and Cu2, while the MeCN molecule completes the coordination sphere of Cu3. The coordination geometries are trigonal pyramidal for Cu1 and Cu2 and tetrahedral for Cu3. Thus, the complex consists of zigzag chains being a 1D coordination polymer. The chains are interconnected through π-π stacking interactions generating a 2D network. Solid 28 exhibited green light emission upon excitation with wavelengths below 475 nm, with a lifetime of 47 μs; the photophysical behavior was further elaborated using DFT calculations.

The single crystal X-ray data set for 29 was poor, but a tentative/preliminary structural solution gave the polymeric double-stranded stairlike ribbon motif shown in Figure 34. It is worth mentioning that the reaction solution that led to the isolation of 29 did not contain Mebta! The complex was formed as a byproduct from the 1:1:1 reaction of CuI, KI and benzotriazole (btaH; Figure 1, left) in MeOH under solvothermal conditions (140 °C, 72 h). The main product [92] was the 3D ionic polymer {(Me₂bta)₄[(Cu₁₀I₁₀)(Cu₆I₆)(Cu(bta)₂)]₃I⁻∙1.5I₂}_n, where (Me₂bta)⁺ is the 1,3-dimethyl-1,2,3-benzotriazolium cation (Figure 6).

5.9. Filling in the Empty Position of Zn(II)

In an attempt to fill in the empty Zn(II) position in the “Periodic Table” of Mebta, complexes [ZnBr₂(Mebta)₂] (30), [ZnI₂(Mebta)₂] (31), tet-[Zn(NO₃)₂(Mebta)₂] (32), oct-[Zn(NO₃)₂(Mebta)₂] (33) and [Zn(Mebta)₄](ClO₄)₂ (34) were prepared and structurally characterized [3]. The preparation of the compounds is summarized in Equations (21)–(24) (X = Br, I).

$$\begin{array}{c}{\mathrm{ZnX}}_2+2\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }[{\mathrm{ZnX}}_2(\mathrm{Mebta}{)}_2]\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{30},\mathbf{31}\end{array}$$

(21)

$$\begin{array}{c}\mathrm{Zn}({\mathrm{NO}}_3{)}_2·4{\mathrm{H}}_2\mathrm{O}+2\mathrm{Mebta}\stackrel{25°\mathrm{C},\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }\mathit{tet}\text{-}[\mathrm{Zn}({\mathrm{NO}}_3{)}_2(\mathrm{Mebta}{)}_2]+4{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{32}\end{array}$$

(22)

$$\begin{array}{c}\mathrm{Zn}({\mathrm{NO}}_3{)}_2·4{\mathrm{H}}_2\mathrm{O}+2\mathrm{Mebta}+4\mathrm{HC}({\mathrm{OC}}_2{\mathrm{H}}_5{)}_3\stackrel{55°\mathrm{C},\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }\mathit{oct}\text{-}[\mathrm{Zn}({\mathrm{NO}}_3{)}_2(\mathrm{Mebta}{)}_2]+4\mathrm{HCO}({\mathrm{OC}}_2{\mathrm{H}}_5)+2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{33}\end{array}$$

(23)

$$\begin{array}{c}\mathrm{Zn}({\mathrm{ClO}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+4\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }[\mathrm{Zn}(\mathrm{Mebta}{)}_4]({\mathrm{ClO}}_4{)}_2+6{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{34}\end{array}$$

(24)

Complexes 30 and 31 were obtained in moderate yields. Changes in the reaction ratio from 1:2 to 1:1 or 1:4 and the solvent from EtOH or MeCN led again to 30 and 31, suggesting that the 1:2 complexes are thermodynamically favored under the conditions employed. Like Ni(NO₃)₂∙6H₂O/Mebta (part 5.5), the 1:2 Zn(NO₃)₂∙4H₂O/Mebta reaction system is sensitive to the H₂O concentration but in a different manner. The 1:2 reaction of Zn(NO₃)₂∙4H₂O and Mebta at room temperature gave compound tet-[Zn(NO₃)₂(Mebta)₂] (32) in a ca. 50% yield, where “tet” denotes a tetrahedral geometry at the metal ion. The analogous reaction at 55 °C in the presence of an excess of the dehydrating agent TEOF (part 5.5) gave compound oct-[Zn(NO₃)₂(Mebta)₂] (33) in a ca. 60% yield. The abbreviation “oct” denotes an octahedral coordination sphere. It was hypothesized that in the presence of H₂O contained in zinc(II) nitrate and in EtOH, the octahedral species [Zn(NO₃)₂(Mebta)₂(H₂O)₂] (with monodentate nitrates) is formed in solution and the aqua ligands are removed during crystallization retaining the monodentate character of nitrates and leading to the tetrahedral complex 32. When H₂O is practically absent (use of TEOF under heating), the octahedral species [Zn(NO₃)₂(Mebta)₂] (with bidentate nitrato groups) is formed in solution and this is precipitated yielding 33. The use of ClO₄⁻ ions, which have little tendency for coordination, led to the cation complex 34 in a 45% yield.

Complexes 30 and 31 (Figure 35) are isostructural, but not isomorphous. The Zn^II atom is coordinated by two halo groups and two N atoms from two N(3)-coordinated monodentate Mebta ligands. In the tetrahedral molecules, the donor atom-Zn^II-donor atom bond angles are in the ranges 104–116 for 30 and 104–115° for 31. The Br-Zn^II-Br and I-Zn^II-I angles are the largest; inter ligand repulsions between the large and negatively charged halides seem to be responsible for this and the observed distortion from the regular tetrahedral coordination.

The Zn^II atom in tet-[Zn(NO₃)₂(Mebta)₂] (32) (Figure 36, up) is bonded to two monodentate Mebta ligands and two (practically) monodentate groups. The coordination geometry at the metal center is very distorted tetrahedral with the angles around Zn^II being in the range 88–118°. Complex oct-[Zn(NO₃)₂(Mebta)₂] (33) (Figure 36, bottom) is isomorphous with its Co(II) (10) and Ni(II) (12) analogues (vide supra). The metal ion sits on a crystallographic C₂ axis which bisects the N_Mebta-Zn^II-N_Mebta angle. The significant distortion of the octahedral stereochemistry is a consequence of the small bite angles (57°) of the bidentate chelating nitrato groups. The Zn^II-O_nitrato bonds trans to N_Mebta (2.39 Å) are longer than those trans to O (2.06 Å). As expected, the O-N-O angle involving the coordinated nitrato oxygen is the smallest (116°) of the three O-N-O angles. The Zn^II-N bond lengths in 33 are longer than the corresponding ones in 32, due to the higher coordination number of Zn^II in the former than in the latter (6 vs. 4).

In the cation of 34 (Figure 37), the Zn^II atom is coordinated to four monodentate Mebta ligands in a tetrahedral manner; the six N-Zn-N angles range between 108 and 111°. The ClO₄⁻ counterions are also tetrahedral.

A combination of π-π stacking interactions and non-classical H bonds gives rise to beautiful 3D architectures in the crystal structures of 30–34.

The molar conductivity value (Λ_Μ) of 34 in DMSO (25 °C) is 71 S cm² mol⁻¹ suggesting a 1:2 electrolyte [93], as expected from its ionic structure. Somewhat to our surprise, the Λ_Μ values of 30–33 (70–80 S cm² mol⁻¹) in the same solvent are also indicative of 1:2 electrolytes despite the fact that the compounds are neutral in the solid state! This is strong evidence for the decomposition of the complexes in solution, most probably through the reaction represented by Equation (25), where X = Br, I, NO₃. In agreement with the proposal, the Λ_Μ values of these complexes in MeNO₂ (a solvent with weak capacity for coordination) are between 10 and 15 S cm² mol⁻¹, values indicative of essentially non-electrolytes [93]. The validity of Equation (25) is further corroborated by the fact that the ¹H NMR [3,38,39,42,75,76] and ¹³C[¹H] NMR [3,38,42,75,76,77] spectra of the complexes in d₆-DMSO are identical with those of the free Mebta in the same solvent.

$$[{\mathrm{ZnX}}_2(\mathrm{Mebta}{)}_2]+x\mathrm{DMSO}\stackrel{\mathrm{D}\mathrm{M}\mathrm{S}\mathrm{O}}{\to }[\mathrm{Zn}(\mathrm{DMSO}{)}_{\mathrm{x}}{]}^{2+}+2{\mathrm{X}}^{-}+2\mathrm{Mebta}$$

(25)

5.10. A Two-Coordinate Au(I) Complex

Cross-coupling reactions for the synthesis of useful organic compounds catalyzed by transition-metal complexes are important in organic chemistry. Conjugated 1,3-diynes and polyyness are useful because they are often found in substances with biological activities and natural products. Complex [Au(Ph₃P)(Mebta)](OTf) (35), where TfO⁻ is the trifluoromethanesulfonate anion, was crystallized during systematic efforts for its use as a homogeneous catalyst for the synthesis of unsymmetrical 1,3-butadiynes from terminal alkynes and hypervalent iodine(III) reagents (Figure 38) [94].

The cation of 35 (Figure 39) consists of a two-coordinate Au^I atom with almost linear geometry. The metal ion forms bonds to one triphenylphosphine molecule and one monodentate Mebta ligand.

5.11. A Hexagonal Bipyramidal {U^VIO₂}²⁺/Mebta Complex

The {U^VIO₂}²⁺/benzotriazole chemistry is currently receiving attention because benzotriazole-decorated graphene oxide can be used for the removal of uranium(VI) in the nuclear industry [21]; see also Section 3. Prior to our work [95], few uranyl/azole complexes had been synthesized, and none was known with benzotriazoles as ligands.

The 1:2 reaction between UO₂(NO₃)₂∙6H₂O and Mebta in MeCN at room temperature gave a yellowish green solution, that upon storage at 20 °C produced crystals of [UO₂(NO₃)₂(Mebta)₂] (36) of the same color in 40% yield. The U^VI atom is at a crystallographically imposed inversion center. The metal is surrounded by two N and six O atoms in a hexagonal bipyramidal coordination environment. The two uranyl oxygen atoms occupy the axial positions. The four O atoms are from two bidentate chelating nitrato groups and the two N atoms are from two monodentate Mebta ligands; these six atoms are located in the almost planar equatorial plane. In the complex, the benzene rings are at the opposite side of the molecule. The dihedral angle between the planes of Mebta and the planes of the coordinate nitrato groups is 87°. The molecular structure of 36 is shown in Figure 40. H bonds and π-π stacking interactions stabilize the crystal structure. The IR spectrum of the compound exhibits a strong band at 944 cm⁻¹ assigned to the IR-active antisymmetric stretching vibration of the {O=U=O}²⁺ moiety (ν₃); this vibration is not visible in the Raman spectrum, in agreement with the centrosymmetric nature of the uranyl group. The Raman-active uranyl symmetric stretch (ν₁) appears at ~860 cm⁻¹.

6. Unpublished Work on the Coordination Chemistry of Mebta

The research described in Section 5 refers to published work. Our group continues to study the coordination chemistry of Mebta and at the time of writing, there are several unpublished results. In this section, we shall briefly comment on selected, structurally characterized metal complexes. All the to-date known such complexes are listed in

Table 1. Unpublished metal complexes of Mebta from our group.

A Ni(II) Complex
trans-[Ni(SCN)2(MeOH)2(Mebta)2] (37) a
A Trinuclear Cu(II) Cluster
[Cu3(OH)2(Mebta)10](BF4)4 (38)
Polymeric Dicyanoamido Co(II) and Zn(II) Complexes
{[Co{N(CN)2}2(Mebta)2]}n b (39)
{[Zn{N(CN)2}2(Mebta)2]}n b (40)
A Trinuclear Carboxylato-Bridged Zn(II) Cluster
[Zn3(O2CPh)6(Mebta)2] (41)
Three-Coordinate Silver(I) Complexes
[Ag(NO3)(Mebta)2] (42)
[Ag(CF3SO3)(Mebta)2] (43)
Cd(II) Complexes
[CdBr2(Mebta)2] (44)
[CdI2(Mebta)2] (45)
trans-[Cd(NO3)2(H2O)2(Mebta)2] (46)
{[CdBr2(Mebta)]}n b (47)
{[Cd3(SCN)6(Mebta)(H2O)]}n b (48)
{[Cd2.5(SCN)5(Mebta)4(H2O)]}n b (49)
{[Cd{N(CN)2}2(Mebta)2]}n b (50)
Hg(II) Complexes
{[HgCl2(Mebta)]}n b (51)
{[HgBr2(Mebta)]}n b (52)
[Hg2I4(Mebta)2] (53)
{[Hg2(SCN)4(Mebta)3]}n b (54)
In(III) Complexes
mer-[InCl3(H2O)(Mebta)2] (55)
mer-[InBr3(H2O)(Mebta)2] (56)
mer-[InCl3(H2O)2(Mebta)] (57)
mer-[In(Cl/Br)3(H2O)2(Mebta)] (58)
[InI3(Mebta)2] (59)
Sn(IV) Complexes
[(Me)2SnCl2(Mebta)2] (60)
(MebtaH)2[SnCl6] c (61)
(MebtaH)2[Sn(Cl0.55Br0.45)6] c (62)
Heterometallic Coordination Polymers
{[HgCo(SCN)4(Mebta)2]}n d (63)
{[HgCo(SCN)2Cl2(H2O)2(Mebta)2]}n b (64)
{[HgCo(SCN)2Br2(DMF)2(Mebta)2]}n b (65)
{[HgNi(SCN)2Cl2(MeOH)2(Mebta)2]}n b (66)

^a This complex is a polymorph of 11 (part 5.5). ^b 1D coordination polymers. ^c In these complex salts, the protonated Mebta molecules act as counterions. ^d 3D coordination polymer.

, while the molecular structures of some compounds are shown in Figure 41, Figure 42, Figure 43, Figure 44, Figure 45, Figure 46, Figure 47, Figure 48 and Figure 49.

With the exception of the Cu(I) complex 28 [80] (and possibly 29 [92]), where Mebta adopts the bridging ligation mode 2.1₂.1₃0₁ (Figure 7, right), in all the other described metal complexes in Section 5 Mebta behaves as a monodentate ligand coordinating through the N atom of the position 3 of the azole ring (1.1₃0₁0₂; Figure 7, left). The latter had also been predicted [3] through theoretical calculations at the ab initio level (MR2) in ethanolic solution (EtOH is the most often used solvent for the preparation of its metal complexes); see also Section 5 However, the crystal structures of 28 and 29 revealed a contradiction between theory and experiment. In an attempt to “force” the bidentate bridging behavior of Mebta, we performed reactions of Cu(II) starting materials with poorly coordinating anions and Mebta, with the metal ion in excess. Such a reaction between Cu(BF₄)₂∙6H₂O and Mebta in EtOH afforded the trinuclear cluster [Cu₃(OH)₂(Mebta)₁₀](BF₄)₄ (38) in low to moderate yields, Equation (26). The released protons might be expected to decompose the product by reacting with the hydroxo ligands. As in the case of the dinuclear oxo-bridged Fe(III) complex 5 (part 5.3), it is possible that the protons are neutralized by Mebta which has not reacted. If this assumption is correct, the stoichiometric reaction can be represented by Equation (27).

$$\begin{array}{c}3\mathrm{Cu}({\mathrm{BF}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+10\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }[{\mathrm{Cu}}_3(\mathrm{OH}{)}_2(\mathrm{Mebta}{)}_{10}]({\mathrm{BF}}_4{)}_4+2{\mathrm{HBF}}_4+16{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{38}\end{array}$$

(26)

$$\begin{array}{c}3\mathrm{Cu}({\mathrm{BF}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+12\mathrm{Mebta}\stackrel{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}{\to }[{\mathrm{Cu}}_3(\mathrm{OH}{)}_2(\mathrm{Mebta}{)}_{10}]({\mathrm{BF}}_4{)}_4+2(\mathrm{MebtaH})({\mathrm{BF}}_4)+16{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{38}\end{array}$$

(27)

In the centrosymmetric cation [Cu₃(OH)₂(Mebta)₁₀]⁴⁺ (Figure 41), the central six-coordinate Cu^II atom is doubly bridged to each 5-coordinate metal ion through a hydroxo atom and one 2.1₂1₃0₁ Mebta group. Two monodentate Mebta ligands complete the octahedral coordination sphere of the central copper and three, also monodentate ligands give each terminal copper a coordination number of five.

The silver(I) complexes [Ag(NO₃)(Mebta)₂] (42) and [Ag(CF₃SO₃)(Mebta)₂] (43) contain three monodentate ligands and the Ag^I center adopts a trigonal planar coordination geometry (Figure 42). The antimicrobial potency of the complexes, their inorganic starting materials [AgNO₃, Ag(CF₃SO₃)] and the free organic ligand (Mebta) was evaluated against Gram-negative and Gram-positive microbes. The silver(I) salts exhibit moderate properties, Mebta shows no activity, whereas the complexes exhibit their strongest activity towards the Gram-negative bacterial strain of P. aeruginosa.

The Cd(II) and Hg(II) chemistry of Mebta is also interesting (Figure 43, Figure 44 and Figure 45). For example, complex [Cd(NO₃)₂(H₂O)₂(Mebta)₂] (46) has an all-trans coordination environment, {[Cd{N(CN)₂}₂(Mebta)]}_n (50) is a six-coordinate 1D polymer structurally similar to 3 (Figure 9), 4 (part 5.3) and 25 (Figure 30), while [Hg₂I₄(Mebta)₂] (53) is a doubly iodo-bridged dimer with intramolecular π-π stacking interaction between the two Mebta ligands.

The best solvent for the synthesis and crystallization of In(III)/Mebta complexes was proven to be CHCl₃. The 1:2 reactions between InX₃ (X = Cl, Br, I) and Mebta in refluxing CHCl₃ gave complexes mer-[InCl₃(H₂O)(Mebta)₂] (55), mer-[InBr₃(H₂O)(Mebta)₂] (56) and mer-[InI₃(Mebta)₂] (59) in moderate to good yields. The chloro (Figure 46) and bromo complexes consist of octahedral molecules with the halogeno ligands in mer positions. The [InI₃(Mebta)₂] (59) molecules have a trigonal bipyramidal environment around In^III with the monodentate N(3)-coordinated Mebta ligands at axial positions (Figure 47).

The results from the 1:1 (and not 1:2 as in 55, 56 and 59) reactions of InX₃ (X = Br, I) in refluxing CHCl₃ were unexpected and surprising. The reaction of InBr₃ gave complex mer-[In(Cl/Br)₃(H₂O)₂(Mebta)] (58) with partial Cl⁻/Br⁻ occupancy, and that of InI₃ yielded the all-chloro complex mer-[InCl₃(H₂O)₂(Mebta)] (57); the molecular structure of the latter is shown in Figure 48. The 1:1 metal-to-ligand ratio is retained in the products while the halogeno ligands are again in mer positions. The only explanation for the presence of chloro ligands in 57 and 58 is that they have originated from the solvent used (CHCl₃) under refluxing conditions. Further experimental work and ab initio calculation are in progress in our laboratories to explain these unusual findings and propose mechanisms.

Exploiting (a) the Hard and Soft Acids and Bases (HSAB) model and (b) the tendency of Mebta (acting as ancillary ligand) to preferentially bind to divalent 3d-metal ions made possible the synthesis of heterometallic complexes. The “ingredients” were Hg(II) [a soft acid], the thiocyanato ligand (a soft base when coordinated through S and a “borderline” base when coordinated through N), Co(II) or Ni(II) [“borderline acids”] and occasionally chlorides or bromides. We have to date isolated and structurally characterized the 3D coordination polymer {[HgCo(SCN)₄(Mebta)₂]}_n (63), and the 1D chain polymers {[HgCo(SCN)₂Cl₂(H₂O)₂(Mebta)₂]}_n (64), {[HgCo(SCN)₂Br₂(DMF)₂(Mebta)₂]}_n (65) and {[HgNi(SCN)₂Cl₂(MeOH)₂(Mebta)₂]}_n (66). For example, complex 66 was prepared by the reaction represented by Equation (28). Its molecular structure (Figure 49) consists of chains comprising alternating {Ni^IIHg^II} units. Neighboring Hg^II and Ni^II atoms are bridged by one SCN⁻ ion, which coordinates to Ni^II through its N and to Hg^II through its S (as expected from HSAB considerations). Two monodentate Mebta ligands and two terminal MeOH molecules complete an octahedral environment for Ni^II, while two chloro groups are terminally bonded to Hg^II giving rise to tetrahedral coordination; thus, the coordination spheres are {Ni^IIN₂N’₂O₂} and {Hg^IIS₂Cl₂}. The preference of chlorides to coordinate with Hg^II may be explained by the great covalency of the Hg^II-Cl bond and the fact that such bonds already existed in the starting mercury(II) material (HgCl₂).

$$\begin{array}{c}\mathrm{n}\mathrm{Ni}({\mathrm{ClO}}_4{)}_2·6{\mathrm{H}}_2\mathrm{O}+\mathrm{n}{\mathrm{HgCl}}_2+2\mathrm{n}{\mathrm{NH}}_4\mathrm{SCN}+2\mathrm{n}\mathrm{Mebta}+2\mathrm{n}\mathrm{MeOH}\stackrel{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}{\to }\{[\mathrm{HgNi}(\mathrm{SCN}{)}_2{\mathrm{Cl}}_2(\mathrm{MeOH}{)}_2(\mathrm{Mebta}{)}_2]{\}}_{\mathrm{n}}+2{\mathrm{NH}}_4{\mathrm{ClO}}_4+6\mathrm{n}{\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{66}\end{array}$$

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7. Concluding Remarks in Brief and Prognosis for the Future

In this work, we have comprehensively reviewed the to-date published metal complexes of Mebta with emphasis on their synthetic chemistry and important structural features. We have also briefly commented on unpublished work from our group. We hope that we have convinced the readers that Mebta is an interesting ligand. Its chemistry is distinctly different from that of other typical monodentate aromatic N-donors, such as pyridines and quinolines. The recently discovered (and not expected) bidentate N(2), N(3)-bridging behavior in compounds 28 [80], possibly 29 [92] and 38 (unpublished work) presages new clusters and coordination polymers in which the metal ions will be bridged by Mebta ligands; such complexes may have interesting properties, e.g., magnetic ones. The to-date developed “periodic table” of Mebta is shown in Figure 50. There are many metal/metalloid positions empty, justifying the title of the review (“Towards Construction….1-methylbenzotriazole”). We can easily see that no s-, p- (except In^III) and 4f-metal complexes have been reported. Our preliminary experiments show that no lanthanide(III) [Ln^III] complexes of Mebta can be isolated. This may be due to the well-known inherent thermodynamic instability of the Ln^III-N bond, especially in media containing low concentrations of water, either from the solvent or the metal starting materials. We are working hard to fill in as many as possible empty positions in the table. If we are successful, a future review in “Inorganics” will have the title “Towards Completion of the ‘Periodic Table’ of 1-methylbenzotriazole”!

Figure 50 refers to metal ions whose complexes with Mebta have been characterized in the solid state. We have also made an effort to understand if the solid-state structures of the complexes are retained in solution. The technique used for the complexes with diamagnetic metal ions was NMR spectroscopy (¹H and ¹³C NMR), combined with the determination of molar conductivity values [3,86,95]. For solubility reasons, the solvent of choice was d₆-DMSO. The results have provided strong evidence that the complexes decompose completely in solution releasing the coordinated Mebta molecules; the coordination sphere of the metal ions is filled with molecules of the solvent, which has a strong donor capacity. The decomposition schemes for the Zn(II) complexes [3] 30–33 were illustrated in part 5.9, Equation (25). The decomposition schemes for the complexes 15 [86] and 36 [95] are represented by Equations (29) and (30), respectively (x = 5 or 6):

$$\begin{array}{c}\mathit{cis}\text{-}[{\mathrm{PtCl}}_2(\mathrm{Mebta}{)}_2]+2\mathrm{DMSO}\stackrel{\mathrm{D}\mathrm{M}\mathrm{S}\mathrm{O}}{\to }\mathit{cis}\text{-}[{\mathrm{PtCl}}_2(\mathrm{DMSO}{)}_2]+2\mathrm{Mebta}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{15}\end{array}$$

(29)

$$\begin{array}{c}[{\mathrm{UO}}_2({\mathrm{NO}}_3{)}_2(\mathrm{Mebta}{)}_2]+x\mathrm{DMSO}\to \left[{\mathrm{UO}}_2\right(\mathrm{DMSO}{)}_x{]}^{2+}+2{{\mathrm{NO}}_3}^{-}+2\mathrm{Mebta}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathbf{36}\end{array}$$

(30)

Finally, we wish to make a short comment on the closely related (to Mebta), but less bulky compound 1-methyl-1,2,3-triazole (Meta, Figure 8). Meta also has different electronic properties than Mebta due to the absence of the benzene ring. To our great surprise, the coordination chemistry of Meta has been studied much less [92,96,97] being confined to Ag(I) [96,97] and Cu(I) [92]. There has been no published work on the s-, p- and f-block metal complexes with this ligand. The Ag(I) complex of Meta with the weakly coordinating anion PF₆⁻, [Ag(Meta)₂](PF₆), is two-coordinate with a linear geometry [97], whereas 42 and 43 are three-coordinate (Figure 42). Notably, the addition of elemental iodine to [Ag(Meta)₂](PF₆) affords the corresponding linear iodonium complex [I(Meta)₂](PF₆) through the selective [N-Ag⁺-N] → [N-I⁺-N] cation exchange; we currently investigate if the reactions of 42 and 43 with I₂ can provide analogous iodonium species. On the contrary, {[Cu₂I₂(Meta)]}_n [92] seems to possess a similar molecular structure with its Mebta counterpart {[Cu₂I₂(Mebta)]}_n (29) [92]. We do believe that researchers should direct their efforts to investigate the coordination chemistry of Meta and compare it with that of Mebta.