8,8′-Dichloro-2,2,2′,2′-tetraethyl-4,4′-bibenzo[1,3,6,2]dioxazastannocinylidene

Abstract

Upon the reaction of glyoxal-bis(2-hydroxy-5-chlorophenyl)imine LH₂ with diethyltin dichloride in the presence of a base (Et₃N) in DMSO, the 1D-coordination polymer 1 was obtained, in which formally the L’(SnEt₂)₂ fragment acts as a monomeric unit. It was found that during the reaction, the initial ligand L undergoes transformation in the tin atom’s coordination sphere. This transformation results in the formation of a new ditopic 1,4-bis((5-chloro-2-oxidophenyl)imino)but-2-ene-2,3-bis(olate) ligand L’. The structure of the resulting complex 1 was examined by single-crystal X-ray diffraction analysis, elemental analysis, IR, and UV spectroscopy.

1. Introduction

This work focuses on the study of coordinating ability of glyoxal-bis(2-hydroxy-5-chlorophenyl)imine LH₂ [1], which is a redox-active, potentially tetradentate ligand. Metal complexes based on substituted glyoxal-bis(2-hydroxyphenyl)imines [2,3,4,5,6,7] and, to a much greater extent, their closest analogue, N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phenylenediamine [8,9,10,11], have been actively studied in recent years. Such compounds exhibit remarkable optical and electrochemical properties [4,8,9,10,11], which makes them promising for applications as photonic materials and energy storage devices. Furthermore, several compelling examples demonstrate the efficacy of such derivatives as catalysts for the oxidation of primary alcohols [12], the disproportionation of 1,2-diphenylhydrazine [13], and the polymerization of lactides [14]. Despite the fact that substituted glyoxal-bis(2-hydroxyphenyl)imines were synthesized more than 40 years ago [1,2,3,15], only a few examples of their complexes have been reported in the literature to date. These include compounds of uranium(VI) [2,5,6], nickel(II) [3], tin(IV) [4], lead(IV) [4], and vanadium(IV) [7]. The previously studied tin(IV) and lead(IV) complexes [4] based on glyoxal-bis(2-hydroxy-3,5-di-tert-butylphenyl)imine contained a tetradentate-coordinated redox-active ligand in a dianionic state and were characterized by redox-amphoteric properties, as well as intense absorption in the near-IR region. The replacement of substituents in the phenolic ring of the ligand is expected to change the optical or electrochemical characteristics of the complexes, or their solubility, depending on the desired objective. Therefore, we attempted to synthesize a tin(IV) complex based on glyoxal-bis(2-hydroxy-5-chlorophenyl)imine (LH₂).

2. Results and Discussion

It was found that the synthesized substituted glyoxal-bis(2-hydroxyphenyl)imines are isolated in the cyclic benzoxazine-benzoxazine form (Scheme 1a) [1,6,15]. An exception is the ligand bearing tert-butyl substituents at the 3- and 5-positions of the phenolic ring, which was successfully isolated in the glyoxal-bis-imine form [3], similar to that shown in Scheme 1b. However, in solution, this glyoxal-bis-imine undergoes intramolecular cyclization [7,16,17].

Studies of uranium compounds have shown that the complexation process involves the opening of the benzoxazine cycles of the ligands and conversion to the imine form [6].

A procedure described in reference [6] was tested for the synthesis of the tin complex based on LH₂. It involves the direct interaction of the ligand and the metal salt in boiling methanol. However, the reaction between LH₂ and Et₂SnCl₂ proceeded only in the presence of triethylamine. It results in the formation of a microcrystalline, nearly insoluble dark violet product that could not be identified. During the reaction in DMSO (Scheme 2), the solution color changes from colorless to dark blue, and the subsequent addition of methanol leads to the slow formation of a small amount of crystals of complex 1, suitable for X-ray diffraction analysis.

According to the SC XRD data, the complex 1 crystallizes in triclinic P-1 space group as solvate 1·2DMSO and forms a 1D-coordination polymer, where formally the L’(SnEt₂)₂ fragment acts as a monomeric unit (Figure 1). The initial ligand LH₂ undergoes a transformation during the reaction, involving not only ring opening in the initial benzoxazine-benzoxazine form but also subsequent hydrolysis and condensation. This process leads to the formation of a new ditopic ligand L’, which is coordinated by two tin atoms. Thus, the ligand L’ consists of two O-Ph-N moieties joined by a C-C(-O)=C(-O)-C linker. The main geometric parameters of complex 1 are given in Table S2 (see Supplementary Materials). Each of the Sn⁴⁺ cations in L’(SnEt₂)₂ fragment is bound to two oxygen atoms and one nitrogen atom of the ligand and two carbon atoms of two ethyl groups (Figure 1). However, the metal centers in 1 are not equivalent, participate differently in the binding of the units to each other, and have different coordination environments. Thus, from the crystal structure point of view, the monomeric unit of 1 appears as the [L’(SnEt₂)₂]₂ fragment (Figure 1). The chains pack in a linear fashion, stabilized by intermolecular contacts between the ligands of adjacent chains. These short contacts (3.45–3.58 Å) involve the chlorine atoms and carbon atoms of the C–C(–O)=C(–O)–C linker. Furthermore, multiple interactions between L’ and DMSO solvate molecules are also observed (see Supplementary Materials, Figure S2).

The Sn(1) and Sn(4) cations additionally coordinate two oxygen atoms of the neighboring ligand C-C(-O)=C(-O)-C linker, forming a flat SnOSnO metallocycle (Figure 1). The formal coordination number of these cations is seven; the coordination polyhedron is a distorted pentagonal bipyramid with Et- carbon atoms in the apical positions, and based on oxygen and nitrogen atoms of the ligands. The lengths of the covalent bonds Sn(1)-O(1) 2.195(3) Å, Sn(1)-O(2) 2.229(3) Å, Sn(4)-O(7) 2.249(3) Å, and Sn(4)-O(8) 2.164(3) Å are significantly shorter than the coordination bonds Sn(1)-O(2’) 2.392(3) Å, Sn(1)-O(3’) 2.581(3) Å, Sn(4)-O(6’) 2.584(3) Å, and Sn(4)-O(7’) 2.378(3) Å. In turn, the Sn(2) and Sn(3) cations additionally coordinate only one oxygen atom of the neighboring ligand O-Ph-N fragment (Figure 1). The formal coordination number of Sn(2) and Sn(3) is six, and the coordination environment can be described as distorted octahedral. The lengths of the Sn(2)-O(5) 2.560(3) Å and Sn(3)-O(4) 2.689(3) Å coordination bonds significantly exceed the lengths of the Sn(2)-O(3) 2.274(3) Å, Sn(2)-O(4) 2.135(3) Å, Sn(3)-O(5) 2.156(3) Å, and Sn(3)-O(6) 2.280(3) Å covalent bonds. It is interesting to note that the Sn-N distances for six-coordinate metal centers (2.178(3), 2.189(3) Å) are noticeably shorter than for seven-coordinate ones (2.293(3), 2.296(3) Å). It is also worth noting that for all metal centers, the Sn-O (from O-Ph-N fragment) bond lengths (2.135(3)–2.195(3) Å) are slightly shorter than the Sn-O (from linker) distances (2.229(3)–2.280(3) Å). The Sn-C_Et covalent bond lengths for all cations in the monomeric unit of 1 lie within a narrow range of 2.120(5)–2.137(5) Å.

The values of the single C–O and C–N bonds in the O–Ph–N fragments are 1.307(6)–1.348(5) Å and 1.397(5)–1.400(6) Å, respectively. The distribution of C-C bond lengths in six-membered rings of the O–Ph–N moieties bound to the Sn(1) and Sn(4) atoms indicates partial alternation: the C-C distances lie in the wide range 1.369(7)–1.435(6) Å. In contrast, the C–C distances (1.381(6)–1.409(6) Å) in the C(15)–C(20) and C(29)–C(34) rings of the O–Ph–N fragments at the Sn(2) and Sn(3) atoms are equalized, indicating their aromatic nature. It should be noted that the C-O bond lengths for bridged O atoms (from O-Ph-N fragment) (1.340(5), 1.348(5) Å) are significantly longer than for terminal ones (1.307(6), 1.316(5) Å). Complex 1 is a rare example of tin compounds featuring an ONO ligand scaffold with this specific bond length distribution. The structurally closest known tin derivatives containing the aforementioned ONO fragment are characterized by similar bond length values [18,19]. The electron density delocalization is observed in the C-C(-O)=C(-O)-C linkers of the ligands: while single C-C bonds are significantly shortened (1.406(6)–1.439(6) Å) compared to 1.54 Å, double C=C bonds are noticeably elongated (1.406(6), 1.407 (7) Å) compared to 1.34 Å. The ligands in 1 are non-planar: the dihedral angles between the median planes of the O-Ph-N fragments are 159.8 and 167.8°.

The observed transformation of the ligand L into L’ could have occurred either within the coordination sphere of the tin atom or directly in solution before the interaction of LH₂ with the metal salt. An additional experiment was carried out in an NMR tube, in which only LH₂ and Et₃N were initially present in the deuterated DMSO solution. It was found that even after heating the reaction mixture for 2 h at 60 °C, no changes occurred in the ¹H NMR spectrum (see Supplementary Materials, Figures S3 and S4). That is, LH₂ remains in the benzoxazine-benzoxazine form even in the presence of a base, and the transformation proceeds only upon the addition of diethyltin dichloride. The proposed mechanism of the ligand transformation is shown in Scheme S1 (see Supplementary Materials). We assume that this process must involve the stages of hydrolysis of the initial ligand L and condensation of the fragments formed as a result of hydrolysis. Therefore, the L’(SnEt₂)₂ unit is formed. A related process involving the hydrolysis of the initial ligand was previously observed for a cobalt complex based on the similar ligand-N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)ethylenediamine [20].

Dark colored crystals of complex 1 exhibit a deep navy-blue color in transmitted light and display a golden luster in reflected light. Figure 2 shows photographs of the crystals (left) and a mechanically grinded sample (right), illustrating the difference in their coloration.

Due to the insolubility of the coordination polymer 1, we were unable to study its solution using UV spectroscopy. The absorption spectrum in the UV and the visible ranges of 1 was recorded in mineral oil and is presented in Figure 3. In the near-UV region, intense absorption bands with maxima at 250 and 380 nm are observed, which are assigned to σ→π* and π→π* transitions. The red and near-IR region of the spectrum features a broad absorption band ranging from 450 to 1000 nm with a maximum at 695 nm, attributed to an intraligand charge transfer.

3. Materials and Methods

3.1. General Information

Solvents were purified using standard methods [21]. The following commercially available reagents were used: 2-amino-4-chlorophenol, glyoxal (40% w/w aq.), Et₂SnCl₂, Et₃N. Benzoxazino-benzoxazine LH₂ was synthesized according to a known procedure [6], and its characteristics (NMR and IR spectra, elemental analysis data) are in full agreement with the literature data [1]. The elemental analysis was performed on a Vario el Cube instrument (Elementar, Okehampton, UK). IR spectra were recorded on an FSM-1201 Fourier transform spectrometer (Infraspek, St. Petersburg, Russia) 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 (ОKB Spectr, St. Petersburg, Russia) (range: 200–1100 nm).

3.2. Synthesis of [[L’(SnEt₂)₂·DMSO]₂]_n (1)

A solution of LH₂ (0.2 g, 0.65 mmol) and Et₂SnCl₂ (0.16 g, 0.65 mmol) in 3 mL of DMSO was treated with 0.3 mL of Et₃N. The resulting reaction mixture was kept at 60 °C for 3 h, during which the solution color turned intense blue. Then, 2 mL of methanol were added, and the reaction mixture was allowed to cool slowly to room temperature. Over 24 h, a dark blue crystalline precipitate with a golden luster formed. The precipitate was separated from the mother liquor by decantation, washed with methanol, and dried under reduced pressure (yield 34% (based on Et₂SnCl₂)). An elemental analysis of [[L’(SnEt₂)₂·DMSO]₂]_n was performed. The calculations (%) for the C₅₂H₆₈Cl₄N₄O₁₀S₂Sn₄ are as follows: C 39.28; H 4.31. The obtained results were as follows (%): C 39.35, H 4.36. IR (Nujol, KBr, cm⁻¹): 1597 (m), 1587 (m), 1551 (m), 1527 (s), 1485 (s), 1436 (m), 1415 (m), 1337 (w), 1327 (m), 1305 (s), 1280 (w), 1267 (w), 1240 (s), 1203 (w), 1173 (s), 1129 (m), 1077 (m), 1015 (m), 965 (s), 906 (w), 892 (w), 884 (w), 876 (w), 852 (m), 819 (m), 808 (m), 772 (w), 684 (m), 657 (m), 638 (m), 610 (w), 580 (s), 555 (s), 544 (s), 494 (m).

3.3. Single-Crystal X-Ray Diffraction Analysis

The single crystal X-ray data for complex 1 was obtained using synchrotron X-ray radiation at the “Belok” beamline [22] of the Kurchatov Synchrotron Radiation Source (National Research Center “Kurchatov Institute”, Moscow, Russia) in the ϕ-scan mode with the Rayonix SX165 CCD detector at 100 K. The experimental X-ray raw data for 1 were indexed, integrated, and scaled with the XDS data reduction program [23]. The structure was solved by direct methods and refined by full-matrix least-squares on F² with anisotropic thermal parameters for all non-hydrogen atoms [24,25]. The sulfur atoms of one DMSO solvate molecule were disordered over two positions (occupancy ratio ca. 0.06 : 0.94) and refined using the SHELX EADP constraint. 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. All calculations were carried out using the SHELXTL program suite [26] and OLEX² X-ray data visualization program package [27].

The crystallographic details are presented in Table S1, while the selected bond lengths and valence angles in complex 1 are listed in Table S2 (see Supplementary Materials). CCDC 2490335 contains the supplementary crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre [28].

Crystal data for 1: C₅₂H₆₈C_l4N₄O₁₀S₂Sn₄, M = 1589.78, P-1, a = 13.1394(17) Å, b = 14.4927(14) Å, c = 18.189(2) Å, V = 2997.9(6) ų, Z = 2, d_calc = 1.761 g/cm³. Grey prism single crystal with dimensions 0.13 × 0.05 × 0.02 mm was selected and intensities of 56638 reflections were collected (μ = 2.266 mm^–1, θ_max = 30.537°). After merging of equivalence reflections and absorption corrections, 14659 independent reflections (R_int = 0.0459) were used for the structure solution and refinement. Final R factors R₁ = 0.0444 [for 11307 reflections with F² > 2σ(F²)], wR₂ = 0.1227 (for all reflections), S = 1.031, and largest diff. peak and hole are 1.466 and −1.466 e/ų, respectively.