Bis [4,4′-(1,3-Phenylenebis(azanylylidene))-bis(3,6-di-tert-butyl-2-oxycyclohexa-2,5-dien-1-one)-bis(dimethylsulfoxide)nickel(II)]

Abstract

A new cage-like dimeric nickel(II) complex Ni₂L₂(DMSO)₄ based on a ditopic redox-active hydroxy-para-iminobenzoquinone type ligand LH₂ (L is 4,4′-(1,3-phenylene-bis(azaneylylidene))-bis(3,6-di-tert-butyl-2-oxycyclohexa-2,5-dien-1-one dianion) was synthesized in DMSO at 120 °C. The molecular structure of the synthesized compound was determined by X-ray diffraction analysis. The complex Ni₂L₂(DMSO)₄ is almost insoluble in all organic solvents, probably due to the presence of a large number of intermolecular contacts in its structure. The electronic spectrum and thermal stability of the crystalline compound have been studied.

1. Introduction

The use of polytopic ligands in coordination chemistry allows for polynuclear complexes to be obtained [1,2,3,4,5], as well as metal–organic frameworks (MOFs) [6,7,8,9,10,11,12,13,14,15,16,17] and metal–organic cages (MOCs) [18,19,20,21,22,23,24,25,26]. The great variety and uniqueness of the applications of these systems contribute to the search for and creation of new linkers (polytopic ligands) with the required structural, spectral, chemical, electrochemical, and other properties. MOCs have their own characteristics and advantages compared to MOFs. In particular, they are soluble in various media while retaining their pore structure [27]. MOCs are capable of accommodating small molecules in a free cavity, whose properties vary depending on the rigidity of the organic linkers. A promising type of polytopic ligand that is capable of the formation of the above compounds is anilates. They include derivatives of the 2,5-dihydroxy-1,4-benzoquinone containing various substituents at the 3 and 6 positions [6,7,12,14,28,29,30,31,32,33,34]. The most important factor that determines the potential of this type of ligand is its redox activity. This provides compounds based on the ligand with additional promising characteristics [28,30,35,36,37,38,39]. We recently synthesized new extended anilate ligands, which are essentially the closest analogues of 2,5-dihydroxy-1,4-benzoquinone [5]. They contain two monohydroxy-para-iminoquinone functions linked by different bridges and are also redox-active. Based on them, dinuclear mercury(II) complexes were prepared [5]. The possibility of forming cadmium(II) coordination polymers [15] or nickel(II)- and magnesium(II) cages [23] using an extended anilate ligand with a para-phenylene bridge has also been demonstrated. In the present work, we report the first example of the preparation of a caged compound based on an extended anilate ligand constructed using a meta-phenylene linker.

2. Results and Discussion

The dimeric nickel(II) complex Ni₂L₂(DMSO)₄ is formed as a result of the reaction between nickel(II) acetate and ditopic redox-active hydroxy-para-iminobenzoquinone ligand LH₂ in a DMSO solution (Scheme 1). The reaction proceeds at 120 °C for 20 min and is accompanied by a color change to deep violet. Subsequent incubation of the reaction mixture at 50 °C for several hours allows for a quantitative precipitation of the final reaction product as a fine, deep colored powder. Heating the reaction mixture at 120 °C for a long time (more than 20 min) leads to the degradation of the organic ligand due to the hydrolysis of the C=N imine bonds. The synthesized compound is resistant to oxygen in the air and to moisture and has a low solubility in most of the organic solvents.

The composition of Ni₂L₂(DMSO)₄ was determined through elemental analysis and spectroscopic methods (IR and solid-phase UV–Vis spectroscopy). Its thermal stability is studied by thermogravimetry. The IR spectrum of the described complex (see Supplementary Materials, Figure S1) shows a set of strong absorption bands in the range of 1570–1670 cm⁻¹ that characterize the stretching vibrations in the system of C=O, C=N, and C=C bonds of para-iminobenzoquinone ligands [15,23,40,41,42,43].

A slow cooling (12 h) to room temperature of the reaction mixture, which was initially heated to 120 °C, yields several crystals of the complex as a solvate with two molecules of DMSO, suitable for X-ray diffraction analysis. The compound Ni₂L₂(DMSO)₄·2DMSO crystallizes in the triclinic P-1 space group. The crystal cell contains one unique molecule of the complex, which lies on the inversion center, and two solvated DMSO molecules. The molecular structure of Ni₂L₂(DMSO)₄·2DMSO is shown in Figure 1, while Figure 2 presents a fragment of its crystal packing. Selected bond length values and selected angles of the described compound are summarized in the caption of Figure 1, as well as in Table S2 (see Supplementary Materials). In general, the structure of Ni₂L₂(DMSO)₄·2DMSO is quite close to the structures of related magnesium(II) and nickel(II) dimers, in which two oxy-para-iminobenzoquinone fragments are linked by a para-phenylene bridge [23]. The dimer is formed by binding an oxy-para-iminobenzoquinone moiety of each of the two ligands to one of the two nickel atoms, forming five-membered chelate cycles of NiOCCO. In summary, the Ni₂L₂(DMSO)₄·2DMSO complex is a metal–organic cage with an internal cavity. The dihedral angle between the oxy-para-iminobenzoquinone moieties bonded to one metal atom is 51.97°. The meta-phenylene bridges of both ligands are located almost in one plane. The distance between the carbon atoms C(16)-C(16A) of the two ligands is 8.157 Å, and the nickel atoms Ni(1) and Ni(1A) are 13.546 Å from each other. However, due to geometrical features, the available cavity volume inside the cell of Ni₂L₂(DMSO)₄·2DMSO is very small, smaller than for the related dimeric nickel complex [23].

Nickel atoms in the dimer Ni₂L₂(DMSO)₄·2DMSO have a slightly distorted octahedral coordination geometry. The oxygen atoms, O(6) of one of the DMSO molecules and O(3A), occupy an apical position in the octahedron, while the other atoms—O(1), O(2), O(4A), and O(5)—form its base. The O(6)-Ni(1)-O(3A) angle (177.12(13)°) is quite close to 180°. The distribution of the bond lengths of the organic ligand is in full agreement with the structure of the para-iminobenzoquinone, as was previously observed for related compounds [15,23,42]. The values of the bond lengths also almost coincide with those of nickel(II) and magnesium(II) complexes based on an extended anilate ligand with a para-phenylene bridge [23]. The Ni-O bonds in chelate cycles are not equivalent. Thus, the distances Ni(1)-O(1) and Ni(1)-O(4A) (2.013(3) and 2.017(3) Å) are shorter and correspond to covalent bonds (the sum of Ni and O covalent radii is 2.11 Å [44]). The distances Ni(1)-O(2) and Ni(1)-O(3A) (2.077(3) and 2.086(3) Å) are slightly longer. However, this difference in bond lengths of 0.06 Å is very small compared to that of the recently reported para-iminobenzoquinone tin(IV) complexes, in which the described distances differ by 0.4–0.5 Å [42]. This indicates a more substantive charge delocalization in NiOCCO chelate cycles. There is an alternation of double (1.352(7)–1.387(6) Å) and single (1.443(7)–1.499(6) Å) C-C bonds corresponding to the para-iminobenzoquinone structure in the six-membered carbon cycles. The meta-phenylene bridge is characterized by approximately equal C-C bond lengths, indicating its aromatic nature.

The crystal packing of the Ni₂L₂(DMSO)₄·2DMSO is characterized by the presence of multiple intermolecular contacts between the hydrogen atoms of the tert-butyl groups and the π-system of the meta-phenylene rings, as well as between the oxygen atoms and methyl groups of the DMSO of neighboring dimer molecules. This is probably the reason for the poor solubility of the described complex in most organic solvents, as in the case of the related magnesium(II) and nickel(II) dimers [23].

The thermal stability of the Ni₂L₂(DMSO)₄ is studied via thermogravimetric analysis (Figure 3). The fine-crystalline sample isolated after the reaction mixture was kept at 50 °C was subjected to thermogravimetric study. The first step in the TG curve proceeds in the temperature range 115–180 °C and corresponds to the elimination of four DMSO molecules. The mass loss in this case is ~18%. The absence of a low-temperature loss step for the two solvent molecules confirms the formation of a non-solvated product under the indicated reaction conditions. Thus, a fine-crystalline sample of Ni₂L₂(DMSO)₄ does not contain solvate DMSO molecules, in contrast to crystals suitable for SC XRD. This fact is also confirmed by elemental analysis data for a fine-crystalline sample of the complex. The coordinated DMSO molecules are removed at lower temperatures than similar DMF molecules in a related dimeric nickel(II) complex containing a para-phenylene bridge in the extended anilate ligand [23] (the mass loss step corresponds to the decoordination and solvent loss occurred at 130–200 °C). This is probably explained by the weaker coordination of DMSO molecules on the nickel atom, as well as by the DMSO’s tendency to decompose starting from 150 °C. As the temperature was further increased to 217–265 °C, the second stage of mass loss (16.3%) was observed on the TG curve. This is associated with the partial dealkylation of the anilate ligand. Half of the tert-butyl substituents were removed in the form of isobutylene. The subsequent heating is accompanied by a complete decomposition of the complex.

Due to the low solubility of the dimer, we were unable to study its solution using UV spectroscopy. The absorption spectrum in the UV and the visible ranges of Ni₂L₂(DMSO)₄ was recorded in mineral oil and is presented in Figure 4. In general, the electronic absorption spectrum is similar to that recorded for the related dimeric nickel complex [23]. It contains an absorption band at 300 nm and a broad band at 450–800 nm, which are responsible for its deep violet color.

3. Materials and Methods

3.1. General Information

Commercial reagents (dimethylsulfoxide, Ni(CH₃COO)₂·4H₂O) were purchased from Aldrich (St. Louis, MO, USA). Solvents were purified using standard methods [45]. Ligand LH₂ was synthesized according to a known procedure [46]. The elemental analysis was performed on a Vario el Cube instrument (Okehampton, UK). IR spectra were recorded on an FSM-1201 Fourier transform spectrometer in Nujol mulls (range 4000–400 cm⁻¹) in a KBr cell. The solid-phase UV–vis spectrum of Nujol mull was registered on an SF-2000 spectrophotometer (range: 220–1100 nm). TG analysis was conducted using a Mettler Toledo TGA/DSC3+ instrument from 40 to 700 °C with a PCA pan and heat rate of 5 °C/min under a N₂ atmosphere.

3.2. Synthesis of Ni₂L₂(DMSO)₄

The mixture of solid Ni(CH₃COO)₂·4H₂O (0.0187 g, 0.075 mmol) and LH₂ (0.0409 g, 0.075 mmol) were placed in a glass vial, and dimethylsulfoxide (2 mL) was added. The reaction mixture was heated at 120 °C for 20 min, then slowly cooled to 50 °C and kept at 50 °C for 7 h. This resulted in the almost complete isolation of the product as a fine, crystalline, dark violet precipitate. The solid was separated by filtration, washed with two portions of DMSO (5 mL each) and dried in air (yield 91%). An elemental analysis of Ni₂L₂(DMSO)₄ was conducted. The calculations (%) performed for the C₇₆H₁₀₈N₄O₁₂S₄Ni₂ are as follows: C 60.24; H 7.18. The obtained results were as follows (%): C 60.05, H 7.32. IR (Nujol, KBr) cm⁻¹: 1673 (w), 1644 (w), 1585 (vs), 1573 (vs), 1552 (s), 1536 (s), 1484 (s), 1388 (m), 1357 (s), 1343 (s), 1315 (s), 1274 (w), 1263 (m), 1221 (m), 1187 (m), 1055 (s), 1001 (s), 958 (s), 911 (m), 897 (m), 881 (m), 823 (w), 792 (w), 782 (w), 748 (w), 697 (m), 669 (w), 653 (w), 621 (w), 602 (m), 562 (w), 540 (w), 528 (w), 501 (w), 480 (w).

3.3. Single-Crystal X-ray Diffraction Analysis

The SC XRD data for Ni₂L₂(DMSO)₄·2DMSO were obtained using the X-ray beamline of the Belok station at the Kurchatov Synchrotron Radiation Source of the Kurchatov Institute Russian Research Center (Moscow, Russia) in the ϕ scan mode using a Rayonix SX165 CCD detector at 100 K [47]. The primary indexing, refinement of unit cell parameters, reflection integration, and intensity absorption correction were applied using the XDS program package [48]. The structure was solved using direct methods and refined using full-matrix least-squares on F² with anisotropic thermal parameters for all non-hydrogen atoms [49,50]. The hydrogen atoms were placed in calculated positions and refined using a riding model with dependent isotropic displacement parameters [U_iso(H) = 1.5U_eq(C) for methyl groups and U_iso(H) = 1.2U_eq(C) for all other H-atoms. Due to the disorder of one tert-butyl group over two positions with site occupancies of 0.632(11) and 0.368(11), SHELXL SADI instruction was applied. All calculations were carried out using the SHELXTL program suite [51] and OLEX² X-ray data visualization program package [52]. The crystallographic details are presented in Table S1 (see Supplementary Materials).

Crystal data for Ni₂L₂(DMSO)₄·2DMSO were as follows: C₇₆H₁₀₈N₄Ni₂O₁₂S₄∙2C₂H₆OS, M = 1671.57; triclinic, P-1; a = 9.761(5) Å; b = 11.583(8) Å; c = 20.092(5) Å; α = 104.946(5)°; β = 90.46(2)°; γ = 98.28(2)°; V = 2169.5(19) ų; Z = 1, d_calc = 1.279 g/cm³. A violet plate single crystal with the dimensions 0.12 × 0.10 × 0.03 mm was selected for measurement and the intensities of 32,787 reflections were collected (μ = 0.860 mm^–1, θ_max = 30.95°). After merging the equivalence reflections and absorption corrections, 9856 independent reflections (R_int = 0.1000) were used for the structure solution and refinement. Final R factors were as follows: R₁ = 0.0742, wR₂ = 0.1870 [for 9856 reflections with I > 2σ(I)]; R₁ = 0.1247, wR₂ = 0.2221 (for all reflections), S = 1.030. The largest difference peak and hole was 1.018 and −0.767 e/ų, respectively.