Coordination Modes of Ortho-Substituted Benzoates Towards Divalent Copper Centres in the Presence of Diimines

Abstract

The coordination modes of several ortho-substituted benzoates towards the copper(II) centre are investigated. The coordination environment of the metal ion includes nitrogen atoms from 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) and occasionally oxygen atoms from coordinated water, ethanol molecules, or nitrate ions. The compounds are investigated by a variety of spectroscopic methods and by single-crystal X-ray diffraction. Although the reaction scheme involved equimolar amounts of the reactants, cationic dinuclear compounds with a metal/benzoate/diimine ratio of 2:3:2 have been realized, cationic in nature regardless of the counter anion used. Furthermore, the carboxylate moieties display a range of twisting relative to the orientation of the benzene ring to which they are attached.

1. Introduction

The divalent state of copper is probably the most studied among metal ions, owing to its energetic stability and structural versatility. The ion possesses a single lone electron in its valence d orbital set and is probably the most reactive Lewis acid, easily forming coordination compounds even with relatively weak Lewis bases. In the case of ligands that possess more than one lone pair of electrons available for coordination, either on the same or on different atoms, a wide variety of local coordination environments may be realized and overall structures of enhanced complexity may result, extending to 3D frameworks [1,2].

The electron distribution within such structures gives rise to either localized or dispersed electron populations, leading in turn to either unique or tunable electric, magnetic, and optical properties. Such properties have made copper compounds suitable candidates for practically every application related to modern life requirements, including catalysts for a wide variety of organic reactions [3], biomimicking processes [4,5], power sources in solid propellants [6], and ferromagnetic [7,8] or antiferromagnetic materials [9]. The availability of lone pairs on both oxygen atoms in a carboxylate ion provides the background for the adoption of several coordination modes of the COO⁻ moiety to metal centres, among which the bridging of neighbouring metal ions is not uncommon [10,11]. Dimer and oligomer formation prevails in such compounds and occasionally there exist short metal–metal interactions, while solvent molecules appear to participate in the coordination sphere of the metal ions [11]. The reaction medium, the ligand substituents, and the nature of the counterions are crucial factors in determining the overall structure of metal benzoates [12,13,14,15].

In the present study, o-Br and o-I benzoic acids were used, and their coordination towards Cu(II) was investigated in the presence of bipyridine and phenanthroline. The corresponding benzoates are symbolized as o-Y, where Y is the substituent at the ortho-position of the benzoic acid ring. The compounds synthesized could be grouped in accordance with the metal/acid/diimine ratio as neutral (1:2:1) and cationic (1:1:2 or 1:1:1), the latter being candidates for the formation of dimmers or oligomers. In the process, some of the compounds were crystallographically characterized as [Cu₂(o-Br)₃(bipy)₂] ^. NO₃ ^. 0.5(MeOH) ^. 0.5(H₂O), 1, [Cu₂(o-I)₃(bipy)₂] ^. NO₃ ^. 0.5(MeOH) ^. H₂O, 2 and [Cu₂(o-I)₃(phen)₂] ^. ClO₄ ^. 0.75(MeOH) ^. 0.75(H₂O), 3.

2. Experimental Section

2.1. Materials and Measurements

The copper salts, the benzoic acids, and the diimines were of the best quality available. No precaution was taken to completely dry the solvents used. FT ATR spectra of the compounds were recorded on a Thermo Scientific™ Nicolet™ iS20 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). A Jasco V-750 UV-Vis spectrometer (Jasco, Tokyo, Japan) was used for recording the electronic excitation spectra; 0.1 mM solutions were used and placed in 1 cm quartz cuvettes. Electrical conductivity measurements were carried out in 1 mM solutions of dichloromethane and methanol on a WTW conductometer with a type C cell and platinum electrodes.

2.2. X-Ray Crystallographic Details

Single-crystals of complexes 1–3 suitable for crystal structure analysis were obtained by slow evaporation of their mother liquids at RT. They were mounted at room temperature on a Bruker Kappa APEX2 diffractometer (Bruker, Billerica, MA, USA) equipped with a Triumph monochromator using Mo Kα (λ = 0.71073 Å, source operating at 50 kV and 30 mA) radiation. Unit cell dimensions were determined and refined by using the angular settings of at least 166 high-intensity reflections (>10σ(I)) in the range 11 < 2θ < 36°. Intensity data were recorded using φ and ω scans. All crystals presented no decay during data collection. The frames collected for each crystal were integrated with the Bruker SAINT Software package v 1.1 [16] using a narrow-frame algorithm. Data were corrected for absorption using the numerical method (SADABS) based on crystal dimensions [17]. The structure was solved using the SUPERFLIP package [18] incorporated in Crystals. Data refinement (full-matrix least-squares methods on F²) and all subsequent calculations were carried out using the Crystals version 14.61 build 7809 or 6236 program package [19]. All non-hydrogen non-disordered atoms were refined anisotropically. For the disordered atoms, their occupation factors under fixed isotropic thermal parameters were detected first. Afterwards, they were all refined with fixed occupation factors, isotropically in the case of compounds 1 and 3 (solvent molecules and counter ions or oxygen atoms of the counter ions) and anisotropically in the case of compound 2 (nitrate anions and solvent molecules).

Hydrogen atoms riding on non-disordered parent atoms were located from different Fourier maps and refined at idealized positions riding on the parent atoms with isotropic displacement parameters Uiso(H) = 1.2Ueq(C) or 1.5Ueq (methyl and -OH hydrogens) and at distances of 0.95 Å for C-H and 0.82 Å for O-H. Hydrogen atoms riding on disordered oxygen atoms of water and methanol solvent molecules were positioned geometrically to fulfil hydrogen bonding demands. Illustrations with 50% ellipsoids probability were drawn by CAMERON [20]. Crystallographic data for complexes 1–3 are presented in

Table 1. Crystal data and experimental details.

Chemical formula sum	C41.50H31Br3Cu2N5O10	C41.50H32Cu2I3N5O10.50	C45.75H32.50ClCu2I3N4O11.50
Chemical formula moiety	Cu2(C7H3O2Br)3(C10H8N2)2 . NO3 . 0.5(CH3OH). 0.5(H2O)	Cu2(C7H3O2I)3(C10H8N2)2 . NO3 . 0.5(CH3OH) . H2O	[Cu2(C7H3O2I)3(C10H8N2)2] . ClO4 . 0.75(CH3OH) . 0.75(H2O)
Mr	1126.54	1276.55	1365.54
Crystal system Space group	Triclinic Pī	Triclinic Pī	Triclinic Pī
Temperature (K)	295	295	295
a (Å) b (Å) c (Å)	14.933 (2) 16.939 (3) 19.285 (3)	14.911 (2) 17.044 (2) 19.322 (3)	10.7073 (6) 13.0072 (8) 18.2613 (10)
α (°) β (°) γ (°)	80.552 (4) 83.179 (5) 71.468 (4)	81.219 (4) 84.200 (4) 72.181 (4)	79.937 (2) 74.2319 (19) 75.857 (2)
V (Å3)	4551.0 (12)	4612.5 (11)	2357.3 (2)
Z	4	4	2
Radiation type	MoKα	MoKα	MoKα
µ (mm−1)	3.63	2.99	2.99
Crystal size (mm)	0.27 × 0.26 × 0.18	0.26 × 0.15 × 0.14	0.17 × 0.16 × 0.12
Data collection
Diffractometer	Bruker Kappa Apex2
Tmin, Tmax	0.39, 0.52	0.64, 0.66	0.62, 0.70
No. of reflections measured independent observed [I > 2.0σ(I)]	63,702 15,751 11,081	71,115 18,100 12,696	34,902 9026 7074
Rint	0.033	0.030	0.031
(sinθ/λ)max (Å−1)	0.595	0.619	0.616
Refinement
R[F2 > 2σ(F2)] wR(F2) S	0.049 0.067 1.00	0.048 0.090 1.00	0.051 0.093 1.00
No. of reflections	11,081	12,696	7074
No. of parameters	1106	1216	607
No. of restraints	24	40	22
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.82, −0.61	1.70, −1.19	1.30, −1.32

.

2.3. Synthesis of the Complexes

In a typical synthesis reaction, 1 mmol of the acid was dissolved in 10 mL of methanol and to the solution, 1.2 mL of 1 M NaOH in methanol was added. The solution was stirred at room temperature for 30 min. In a separate beaker, 1 mmol of a Cu²⁺ salt and 1 mmol of the appropriate diimine were added to 10 mL of methanol and the mixture was stirred at room temperature for 30 min. Following, the deprotonated acid solution was streamed into the metal–diimine solution and 20 mL of dichloromethane was added to facilitate precipitation of the inorganic salt produced. After stirring for 30 min at room temperature, the mixture was filtered and left to evaporate at room temperature, providing a microcrystalline or crystalline solid. The product yield of all reactions varied from 71 to 75% and was referred to as the metal ion.

3. Results and Discussion

The reaction between Cu(II) salts and carboxylates in the presence of diimines should not be considered a simple and straightforward interaction since the high Lewis acidity of the metal ion, the formation of chelate rings, and the generally shallow potential energy surfaces of the resulting compounds may give rise to a wide variety of local and overall structures.

3.1. Structure Elucidation

In the nitrate compounds there appear certain features indicating the presence of non-coordinating nitrate ions [21]. The couple of strong sharp bands at 728 and 770 cm⁻¹ and the very strong one at 1305 cm⁻¹ may be assigned to the infrared active vibrations of the ion possessing a relatively high symmetry, while the absence of a significant band above 1600 cm⁻¹ supports the non-coordinating nature of the ion since in this region there is expected to appear the N=O bond vibration. For the perchlorate compounds, the characteristic intense band attributed to non-coordinated perchlorate anion at 1080 cm⁻¹ was observed in every case.

The region where the carbonyl stretches are expected appears complicated as there is no strong band but rather a multiplet of medium strength bands ranging from 1580 to 1540 cm⁻¹. This is an indication of the presence of distinctively different carboxylate groups, something that may occur in dinuclear or oligonuclear complexes. Similar to the above are the findings in the infrared spectra of the perchlorate compounds with the distinct feature that the presence of the intense non-coordinating perchlorate band at around 1080 cm⁻¹ is present in all of them. Representative spectra are given in the Supplementary Material.

The spectra recorded present the typical intense band in the region below 300 nm that is dominated by intense and narrow π-π* bands accompanied by less intense and broader n-π* bands and charge transfer bands as evinced by analysis of the spectra line into Gaussian-type components (Figure 1). An analogous analysis was carried out on some of the low energy bands in the region above 650 nm. For the recording of these bands, 1.0 mM solutions were prepared and the analysis revealed at least four components. Due to the low symmetry of the chromophores, in “octahedral” complexes there is no expectation of realization of the Jahn–Teller split of the d-d transition but rather the presence of several closely lying bands. The same is true in the case of the “square pyramidal” compounds [22].

Due to the availability of the 1.0 mM solutions of the compounds, electric conductivity measurements were carried out on the grounds of available literature data for the characterization of compounds in various solvents at this concentration level [23]. The compounds revealed 1:1 electrolyte behaviour, with conductivity values lying closely but slightly outside the lower limit reported in the literature for dichloromethane and methanol solutions. This is an indication that the expected formula of [Cu(benzoate)(diamine)] cation is not the one present but instead either an equilibrium between the separate ingredients of the cation and a complex coordinated only with the anion or an association to form dimers or oligomer complexes is present.

3.2. X-Ray-Structures

In general, all three complexes 1, 2, and 3 are monocationic binuclear and contain two Cu(II) metal ions, two bipy ligands, and three o-substituted deprotonated carboxylic acids. One of the carboxylate ligands is coordinated as a bidentate μ_1,3–bridge between the copper atoms. The rest two are monodentately coordinated through one of the carboxylic oxygens bridging the Cu(II) cations. Each one of the bipy ligands is coordinated to one Cu(II) cation in a typical bidentate chelate fashion. In all benzoates, the carboxylate group is not parallel to the phenyl ring forming angles between 45° and 85°.

Compound 1 crystallizes in the triclinic Pī space group with Z = 4. The unit cell contains two pairs of symmetrically equivalent monocationic binuclear Cu(II) complexes formulated as [C₄₁H₂₈Br₃Cu₂N₄O₆]⁺ or better [Cu₂(o-Br)₃(bipy)₂]⁺, four nitrate counter anions, two methanol, and two water solvate molecules, all the latest placed on general positions filling the lattice voids. The nitrate anions, solvate methanol, and water molecules were all found disordered over two positions with equal occupation factors of 0.5. One of the total six bromine atoms was also found disordered with occupation factors of 0.8 and 0.2. Figure 2 presents the structures of complex 1a, (a), and 1b, (b) (both present in the asymmetric unit) of complex 1.

Table 2. Selected interatomic distances (Å) and bond angles (^o).

Complex 1a		Complex 1b
Cu1—Cu2	3.2376 (8)	Cu3—Cu4	3.1989 (8)
Cu1—O1	1.957 (3)	Cu3—O7	1.947 (3)
Cu1—O3	2.294 (3)	Cu3—O9	1.956 (3)
Cu1—O5	1.952 (3)	Cu3—O11	2.308 (3)
Cu1—N1	2.001 (4)	Cu3—N5	1.965 (3)
Cu1—N2	2.006 (3)	Cu3—N6	1.963 (3)
Cu2—O2	1.967 (3)	Cu4—O8	1.945 (3)
Cu2—O3	1.953 (3)	Cu4—O9	2.330 (3)
Cu2—O5	2.303 (3)	Cu4—O11	1.968 (3)
Cu2—N3	1.988 (4)	Cu4—N7	1.971 (3)
Cu2—N4	1.986 (3)	Cu4—N8	1.990 (3)
O1—Cu1—O3	87.34 (11)	O7—Cu3—O9	93.48 (11)
O1—Cu1—O5	93.44 (12)	O7—Cu3—O11	90.83 (11)
O3—Cu1—O5	76.11 (10)	O9—Cu3—O11	78.61 (11)
O1—Cu1—N1	171.85 (13)	O7—Cu3—N5	91.21 (13)
O3—Cu1—N1	94.88 (12)	O9—Cu3—N5	175.15 (13)
O5—Cu1—N1	94.70 (13)	O11—Cu3—N5	102.56 (13)
O1—Cu1—N2	91.15 (14)	O7—Cu3—N6	166.19 (13)
O3—Cu1—N2	116.50 (12)	O9—Cu3—N6	94.49 (13)
O5—Cu1—N2	166.80 (13)	O11—Cu3—N6	101.79 (12)
N1—Cu1—N2	80.84 (15)	N5—Cu3—N6	80.67 (15)
O2—Cu2—O3	91.37 (12)	O8—Cu4—O9	87.10 (11)
O2—Cu2—O5	90.27 (11)	O8—Cu4—O11	94.25 (11)
O3—Cu2—O5	75.88 (10)	O9—Cu4—O11	77.84 (11)
O2—Cu2—N3	172.71 (13)	O8—Cu4—N7	170.10 (13)
O3—Cu2—N3	95.85 (13)	O9—Cu4—N7	94.14 (12)
O5—Cu2—N3	90.54 (12)	O11—Cu4—N7	95.62 (13)
O2—Cu2—N4	91.47 (13)	O8—Cu4—N8	89.83 (13)
O3—Cu2—N4	170.31 (13)	O9—Cu4—N8	109.52 (12)
O5—Cu2—N4	113.37 (12)	O11—Cu4—N8	171.79 (12)
N3—Cu2—N4	81.56 (14)	N7—Cu4—N8	80.49 (14)

contains the interatomic distances and bond angles for complexes 1a and 1b. The two complexes in the asymmetric unit are generally similar but not identical. The distances between the copper ions in the binuclear complexes vary from 3.1989 (8) to 3.2376(8) Å. If Cu-O distances longer than 2.72 Å will be considered as non-bonding, then each Cu(II) cation is five coordinated with a CuO₃N₂ chromophore. The nitrogen-coordinated atoms come from the chelate bidentate bipy ligands with nearly equal bond distances for 1a and slightly different in 1b. Two of the oxygen atoms belong to the monodentate bridging deprotonated (o-Br) carboxylate ligands with bridging distances considerably different (ca Cu2—O3 1.953(3) and Cu1—O3 2.294(3) Å. The third oxygen comes from the μ_1,3 bridging bidentate (o-Br) carboxylate with oxygen atoms at nearly equal distances to the metal ions. Both copper cations in both 1a and 1b complexes adopt a tetragonal pyramidal coordination symmetry according to trigonality index τ₅ values (τ₅ = (φ₁ − φ₂)/60°, where φ₁ and φ₂ are the largest angles in the coordination sphere. The value τ₅= 0 denotes a perfect square pyramid while the value τ₅ = 1 denotes a perfect trigonal bipyramid) [24]. The values of τ₅ are 0.08 for Cu1 and 0.04 for Cu2 in complex 1a, while τ₅ = 0.15 for Cu3 and τ₅ = 0.03 for Cu4 in complex 1b. In all cases, the longer distanced O atom (O3 from Cu1 and O5 from Cu2 for 1a and O11 from Cu3 and O9 from Cu4 for 1b) lies on the apical position and the rest are closer to Cu(II); two oxygen and two nitrogen atoms form the basal plane.

The phenyl rings of the singly coordinated bridging benzoates lie almost parallel to the rings of the bipy ligands, giving rise to π-π interactions between them and adding rigidity to the whole structure. An extended hydrogen bonding net between the solvate methanol hydroxide groups and the water hydrogen atoms with the nitrate oxygen and the bromine atoms is also formed, giving stability to the crystal lattice.

Compound 2 crystallizes in the triclinic Pī space group with Z = 4. The unit cell is similar to that of complex 1 and again contains two pairs of equivalent monocationic binuclear complexes. The carboxylate anion used in this case was the (o-I). In complex 2, only one of the two solvate water molecules present per cell was disordered. This water molecule, as well as the solvate methanol molecule, were both disordered over two positions with equal occupation factors. Two of the nitrate anions were found disordered over two positions with equal occupation factors. The third nitrate anion had the nitrogen atom fully occupied and all three oxygen atoms equally disordered over two positions each. One of the six iodine atoms was also disordered over two positions with occupation factors of 0.8 and 0.2. The description of the structure is similar to that of complex 1. The structure of complex 2a (one of the two complexes present in the asymmetric unit) is presented in Figure 3a, while the second complex 2b is presented in Figure 3b. Interatomic distances and bond angles for complexes 2a and 2b are presented in

Table 3. Selected interatomic distances (Å) and bond angles (º).

Complex 2a		Complex 2b
Cu1—Cu2	3.2327 (11)	Cu3—Cu4	3.2183 (11)
Cu1—O1	2.011 (4)	Cu3—O7	1.960 (4)
Cu1—O3	2.315 (4)	Cu3—O9	2.350 (4)
Cu1—O5	1.964 (4)	Cu3—O11	2.112 (5)
Cu1—N1	1.954 (5)	Cu3—N5	1.992 (5)
Cu1—N2	1.954 (5)	Cu3—N6	1.985 (5)
Cu2—O2	1.947 (4)	Cu4—O8	1.953 (4)
Cu2—O3	1.966 (4)	Cu4—O9	1.975 (4)
Cu2—O5	2.327 (4)	Cu4—O11	2.294 (5)
Cu2—N3	1.989 (5)	Cu4—N7	1.996 (5)
Cu2—N4	1.992 (5)	Cu4—N8	1.958 (6)
O1—Cu1—O3	90.39 (16)	O7—Cu3—O9	86.52 (17)
O1—Cu1—O5	90.92 (18)	O7—Cu3—O11	95.73 (17)
O3—Cu1—O5	77.48 (16)	O9—Cu3—O11	76.54 (16)
O1—Cu1—N1	172.4 (2)	O7—Cu3—N5	170.5 (2)
O3—Cu1—N1	90.12 (19)	O9—Cu3—N5	94.02 (19)
O5—Cu1—N1	96.6 (2)	O11—Cu3—N5	93.6 (2)
O1—Cu1—N2	91.9 (2)	O7—Cu3—N6	89.68 (19)
O3—Cu1—N2	113.04 (19)	O9—Cu3—N6	109.45 (18)
O5—Cu1—N2	169.1 (2)	O11—Cu3—N6	172.22 (19)
N1—Cu1—N2	80.9 (2)	N5—Cu3—N6	81.2 (2)
O2—Cu2—O3	92.52 (18)	O8—Cu4—O9	93.85 (18)
O2—Cu2—O5	86.65 (17)	O8—Cu4—O11	93.09 (18)
O3—Cu2—O5	77.15 (16)	O9—Cu4—O11	80.54 (17)
O2—Cu2—N3	172.5 (2)	O8—Cu4—N7	165.7 (2)
O3—Cu2—N3	95.0 (2)	O9—Cu4—N7	93.6 (2)
O5—Cu2—N3	95.72 (19)	O11—Cu4—N7	100.2 (2)
O2—Cu2—N4	92.2 (2)	O8—Cu4—N8	91.3 (2)
O3—Cu2—N4	167.6 (2)	O9—Cu4—N8	174.8 (2)
O5—Cu2—N4	114.61 (19)	O11—Cu4—N8	100.09 (19)
N3—Cu2—N4	80.3 (2)	N7—Cu4—N8	81.2 (2)

.

The values of τ₅ were calculated as follows: for Cu1, τ₅ = 0.06; for Cu2, τ₅ = 0.08; for Cu3, τ₅ = 0.03; and for Cu4, τ₅ = 0.15. As all values are very close to zero. A square pyramidal geometry can be proposed for all four Cu(II) cations included in the two monocationic complexes in the asymmetric unit of complex 2. The four square pyramids formed have O5 and O3 oxygen atoms on the apical positions of complex 2a, and O9 and O11 on the apical positions of complex 2b. In both complexes, π-π interactions from the phenyl rings of the singly coordinated carboxylates to one of the pyridine rings of bipy arise, giving stability to the structure formed. Centroid-to-centroid distances were measured to expand by between 3.66 and 3.86 Å.

The formed crystal lattice gains extra stability from the hydrogen bonding net extended from all water and methanol hydroxide groups to the nitrate oxygen and the halogen atoms.

Compound 3 crystallizes in the triclinic Pī space group with Z = 2. In the asymmetric unit of 3, one monocationic Cu(II) complex, one disordered perchlorate counter anion, one and a half disordered solvate methanols, and one and a half disordered solvate water molecules are present. For the perchlorate anion, three oxygen atoms were found disordered over two positions with equal occupation factors. The chlorine atom and one of the oxygen atoms were fully occupied. The methanol and water solvate molecules were found disordered over two positions, each with occupation factors of 0.5 and 0.25. The main monocationic binuclear complex unit with the general formula [Cu₂(o-I)₃(bipy)₂]⁺ is similar to those previously described. Both Cu(II) cations are again five coordinated with square pyramidal geometry and a CuO₃N₂ chromophore. Figure 4 presents the structure of complex 3, and selected geometric parameters are presented in

Table 4. Selected interatomic distances (Å) and bond angles (^o) in complex 3.

Cu1—Cu2	3.1884 (9)	N1—Cu1—O5	98.13 (17)
Cu1—N1	2.003 (5)	N2—Cu1—O5	104.07 (16)
Cu1—N2	2.009 (5)	O1—Cu1—O5	94.79 (16)
Cu1—O1	1.955 (4)	O3—Cu1—O5	77.96 (16)
Cu1—O3	1.939 (4)	N3—Cu2—N4	82.4 (2)
Cu1—O5	2.267 (4)	N3—Cu2—O2	88.74 (19)
Cu2—N3	2.000 (5)	N4—Cu2—O2	171.06 (19)
Cu2—N4	1.993 (5)	N3—Cu2—O3	110.85 (16)
Cu2—O2	1.950 (4)	N4—Cu2—O3	92.82 (18)
Cu2—O3	2.354 (4)	O2—Cu2—O3	89.36 (16)
Cu2—O5	1.950 (4)	N3—Cu2—O5	173.09 (18)
N1—Cu1—N2	82.3 (2)	N4—Cu2—O5	95.08 (18)
N1—Cu1—O1	166.32 (18)	O2—Cu2—O5	93.86 (17)
N2—Cu1—O1	90.20 (18)	O3—Cu2—O5	75.62 (15)
N1—Cu1—O3	94.72 (19)	N1—Cu1—O5	98.13 (17)
N2—Cu1—O3	176.61 (18)	N2—Cu1—O5	104.07 (16)
O1—Cu1—O3	92.33 (17)	O1—Cu1—O5	94.79 (16)
		O3—Cu1—O5	77.96 (16)

.

The τ₅ values calculated for the Cu1 and Cu2 coordination environment were 0.17 and 0.03, respectively, giving evidence to the geometry already mentioned. The basal plane of the pyramid around Cu1 is formed from O1, O3, N1, and N2 and on the apical position lies the longer-distanced O5. Respectively, O5, O2, N3, and N4 form the basal plane of the square pyramid around Cu2, and O3 lies in the apical position.

The disordered solvate methanol and water molecules form a hydrogen bonding net interacting with each other and with the oxygen atoms of the disordered perchlorate anion, giving stability to the lattice formed.

All distances and bite angles in all complexes were found to be similar to those found in analogous complexes already published [25].

3.3. Computational Verification

In view of the unique coordination behaviour of the carboxylate ions observed by crystallography, we carried out a series of simple semi-empirical quantum chemical calculations by means of the MOPAC2016 program [26], utilizing the PM7 Hamiltonian implemented within it [27]. At first, the benzoic acid was considered and its ground state structure was optimized. The backbone of the molecule was used to represent a monodentate carboxylate, with the acidic H atom acting as the coordinated metal. Applying the rigid rotor approach, the twisting angle of the carboxylate group relative to the benzene ring varied at 15 degrees intervals and at each point, calculation of the Hessian matrix was performed. The obtained difference is the two carboxylate-centred vibrations revealed a maximum deviation of 13 cm⁻¹ relative to the value of the perfectly planar orientation. The benzoate anion was also optimized and its structure was used to account for the symmetrically chelating carboxylate. Following a series of single-point computations as above, a variation of the Δν between the two carboxylate-centred vibrations of 19 cm⁻¹ was obtained. Following the crystallographic investigation, three single-point calculations were carried out on the configurations observed in compound 2. In every case, the fragment was assigned a charge of −1 and the Hessian matrix calculation was carried out. The calculated asymmetric stretch revealed a spread of 70 cm⁻¹ between the three units while for the symmetric one, a spread of 15 cm⁻¹ was calculated.

Although qualitative, the above results are in accordance with the observed features of the infrared spectra of the compounds, namely the appearance of more than one medium-intensity band in the region where the characteristic intense vibration of the carboxylic group is expected. This is an indication of the presence of more than one carboxylic moiety within the formula and a distinctively different environment for each of them, which is crystallographically identified as a variation in the twisting of the carboxylate group relative to the benzene ring of the acids.

4. Conclusions

The coordination of ortho-benzoates towards Cu(II) centres is as facile as expected due to the high Lewis acidity of the metal ion and the presence of α-diimines gives rise to the formation of dinuclear ionic compounds. Apart from the expected stoichiometries, complexes with a Cu₂(diimine)₂(benzoate)₃ cation and non-coordinating nitrate or perchlorate anion have been realised. The resulting non-rigidity of the overall structures is evident from the slight but distinct peculiarities in the spectra line shapes both in the infrared spectra recorded in the solid state and visible spectra recorded in the solution. In the infrared spectra, a splitting and dispersion of the characteristic carboxylate bands is evident along with a diminishing in their intensity, while in the low energy visible region, the presence of several d-d bands is revealed by spectra deconvolution, indicating the presence of non-identical local metal environments.