N-Cyclohexylcyclohexanaminium {[acetyl(methyl)amino]methyl}bis [4,5-dibromobenzene-1,2-diolato]silicate(IV) 4,5-dibrobenzene-1,2-diol Ethanol Solvate

Abstract

An X-ray diffraction study of a transesterification product of N-methyl-N-(trimethoxysilylmethyl)acetamide by 4,5-dibromobenzene-1,2-diol was carried out. It was established that the coordination polyhedron of the silicon atom in an anionic complex is a distorted octahedron disordered due to the superposition of the Δ and Λ optical isomers. The presence of various hydrogen donor groups in bis(cyclohexyl)ammonium cations, solvent ethanol molecules, and non-coordinated 4,5-dibromobenzene-1,2-diol caused the formation of H-bonded chains, while dibromobenzenediol also took part in stacking interactions.

1. Introduction

Anionic complexes of silicon(IV) with catecholate ligands have demonstrated a number of applications related to catalysis [1] and biochemistry [2,3,4,5,6,7]. These complexes were proved to form supramolecular associates both in crystal and solution via hydrogen bonds and weak intermolecular interactions. Such associates are able to exhibit affinity towards organic cations, solvents, non-coordinated catechol derivatives and other aromatic compounds [8,9,10]. As result, the absorption maxima in UV spectra are shifted [11], thus giving an opportunity to elaborate upon new materials for the application of silicon(IV) complexes with catecholate ligands as sensors. Herein, we describe in detail the molecular structure and intermolecular bonding in crystal of N-cyclohexylcyclohexanaminium {[acetyl(methyl)amino]methyl}bis[4,5-dibromobenzene-1,2-diolato]silicate(IV) 4,5-dibrobenzene-1,2-diol ethanol solvate, (NH₂(cyc)₂)[Si(amam)(dbcat)₂]·H₂dbcat·C₂H₅OH (1) (Scheme 1), which can be described as a supramolecular system based on four different units bonded by several types of interactions.

2. Results and Discussion

The [Si(amam)(dbcat)₂]^– anion contains a hexacoordinated silicon(IV) atom in octahedral geometry that is very similar to previously described anionic chelated complexes with similar substituted catechol ligands [9]. It is noteworthy that the acetyl(methyl)amino]methyl ligand is disordered over two positions in a 2:3 ratio (Figure 1). This fact indicates the superposition of the Δ and Λ optical isomers of the anion in solid 1. Due to this disorder, it is impossible to calculate Si–O and Si–C bond lengths with the acetyl(methyl)amino]methyl ligand with sufficient accuracy. On the other hand, Si1–O1 and Si1–O2 bonds, which are opposite to the corresponding Si–O and Si–C bonds of the [acetyl(methyl)amino]methyl moiety, are equal to 1.766(3) and 1.798(3) Å, which are very close to those in previously described [Si(amam)(Cat^x)₂]^– anions [9]. Thus, the electron-withdrawing effect of the substituted catecholate ligands in the latter anions are comparable to that for 4,5-dibromobenzene-1,2-diolato-anion in the structure described herein.

Due to the presence of NH₂ and OH proton donor groups in conjunction with the negatively charged oxygen atoms of 4,5-dibromobenzene-1,2-diolato anions, all molecular fragments (charged and neutral) take part in complex supramolecular organization (Figure 2) in crystals. A deeper inspection allowed us to distinguish several types of packing motifs. The first one related to the formation of H-bonded macrocycles between all four molecular units (Figure 2a). The conformation of such a macrocycle can be described as a distorted sofa. These H-bonded macrocycles are further assembled into infinite chains parallel to the crystallographic a axis of a unit cell (Figure 2a). Finally, the stacking interaction between two non-coordinated 4,5-dibrobenzene-1,2-diol molecules joins the H-bonded chains into a three-dimensional network. The 4,5-dibrobenzene-1,2-diol molecules in stacking dimers are situated in antiparallel positions with the interplanar and centroid–centroid distances equal to 3.604(6) and 4.3913(14) Å (Figure 2b). Previously, similar co-crystallization was only found for non-substituted catechol- and 4-methyl-benzene-1,2-diol-containing complexes [8,9], while none of the [Si(amam)(Cat^x)₂]^– complexes based on halogen substituted catechol or catechol derivatives with other electron-withdrawing groups co-crystallized with H₂Cat^x. The results of a recently published study by Levernier et al. [11] indicate that motifs described in the present article exist in solution, and hence can be considered as models for H-bonded associates which are able to occur in solution as well.

3. Materials and Methods

The synthesis of the titled compound was carried out according to the route described elsewhere [8,9]. Single crystals of C₃₆H₄₄Br₆N₂O₈Si were obtained from reaction mass under slow evaporation. The intensities of 40834 reflections were collected at the “Belok/XSA” beamline of the Kurchatov Synchrotron Radiation Source [12,13]. Three datasets with different orientations of single crystals relative to the detector were measured using a 1-axis MarDTB goniometer equipped with a Rayonix SX165 CCD 2D positional sensitive CCD detector (λ = 0.745 Å, φ-scanning in 1.0° steps, φ range = 0–180°) in the direct geometry with a detector plane perpendicular to its beam. The datasets thus obtained were integrated, measured, and indexed using the XDS software suite [14]. At 100 K, a crystal system is triclinic; space group P1⁻: a = 10.177(2), b = 11.032(2), c = 20.378(4) Å, α = 86.48(3), β = 80.53(3), γ = 73.38(3) °, V = 2162.2(8) ų, Z = 2, µ = 6.357 mm^–1, D_calc = 1.755 g·cm⁻³, F(000) = 1128.

The structure was solved using the dual-space algorithm [15] and refined with full-matrix least squares against F² using the SHELXL [16] program and Olex2 package [17]. Non-hydrogen atoms were refined in an anisotropic approximation. The analysis of electron density maps and bond lengths in the region of the acetyl(methyl)amino]methyl ligand showed the disorder of the entire ligand. The atomic positions were split into two parts, and their occupancies were refined using free variables, the DFIX and EADP constraints. Hydrogen atoms of aromatic, methylene and amino groups were located on difference Fourier maps and included in the refinement with U_iso(H) = 1.2U_eq(C,N) and fixed bond lengths. In the case of methyl and hydroxyl groups, the ratio between U_iso(H) and oxygen or carbon atoms was equal to 1.5. Refinement converged to R₁ = 0.0526 (for 7523 observed reflections and 499 parameters), wR₂ = 0.1401 and GOF = 1.059 (for 8121 independent reflections, R_int = 0.0392).

In addition, several electron density peaks (up to 3.2e) were found at the regions of 4,5-dibromobenzene-1,2-diolato ligands. Their position allowed one to assume that the composition of single crystals can be more complex than can be concluded on first glance, i.e., a crystal of the title compound mixed with a very small amount of complex similar to {[acetyl(methyl)amino]methyl}bis[3,4,5,6-tetrabromobenzene-1,2-diolato]silicate(IV) [9]. Unfortunately, the model with a disorder of the bromine atom cannot be refined with sufficient accuracy to establish the exact composition of the single-crystal sample. The analysis of spectral data after an attempt to recrystallize the reaction mass from MeCN/EtOH (See Supplementary Materials for details) allowed us to suggest that the sample contained impurities that can be described as catechol or corresponding ligands with at least three atoms of bromine.