Triorganotin carboxylates – synthesis and crystal structure of 2-methyl-1H-imidazol-3-ium catena-O,O′-oxalatotriphenylstannate

Introduction

Organotin (IV) carboxylates are an important class of organotin compounds. They were reported in the literature from a long time and have been widely studied since (Davies, 2004; Davies et al., 2008). Because of its catalytic properties (Meneghetti and Meneghetti, 2015) and biological activity (Gielen et al., 2005; Hadjikakou and Hadjiliadis, 2009), studies on organotin (IV) carboxylates are continuing and still arouse a great interest. Thus, recent studies have been published regarding the catalytic synthesis of aliphatic urethanes (Devendra et al., 2015) as well as the potential anticancer activity of organotin (IV) carboxylates (Amir et al., 2014; Sirajuddin et al., 2014). Lately, the fungicidal activity of triorganotin (IV) 1-methyl-1H-imidazole-4-carboxylates was also reported (Mao et al., 2015). From a structural point of view, numerous papers and review articles have been reported describing the different modes of coordination of carboxylates to organotin (IV) moieties and showing a great diversity of molecular structures (Tiekink, 1991; Chandrasekhar et al., 2002). In this area, several contributions have been reported by the Senegalese group, focusing in particular on the isolation of new oxalates of stannates (IV) (Gueye et al., 2014; Diop et al., 2016). A particular attention was given to the coordination of triorganotin (IV) derivatives (Diop et al., 1997, 2003; Gueye et al., 2011, 2012; Sow et al., 2012). In the field of crystal engineering and supramolecular coordination chemistry, organotin carboxylates can also be viewed as suitable building blocks (Tiekink, 2006). Indeed, in addition to the tin hypervalency, carboxylic acid-based ligands are considered as appropriate connectors to promote the formation of multidirectional architectures (Ivasenko and Perepichka, 2011). Furthermore, the cocrystallization of nitrogen organic compounds significantly increases the potential of linkage and the diversity of topology, via the formation of ammonium cations and the creation of new hydrogen-bonded networks. Imidazolium derivatives are especially well suited for such approach. In 2010, Callear et al. (2010) focused on the crystal packing effects of the methylation of different atoms around the imidazole ring. In this context, we recently started to investigate the contribution of 2-methyl-1H-imidazol-3-ium in the presence of tin oxalates revealing, in particular, an interesting 3,5T1 topological network (Diop et al., 2015). Continuing our investigations, we report herein on the synthesis and characterization of the 2-methyl-1H-imidazol-3-ium catena-O,O′-oxalatotriphenylstannate (1), which is organized in the solid state into the polymeric chains connected by 2-methyl-1H-imidazol-3-ium cations (Scheme 1).

Scheme 1:

Molecular representation of the polymeric structure of 1.

Compound 1 was synthesized by reacting methanolic solutions of bis(triphenyltin) oxalate, [(Ph₃Sn)₂(C₂O₄)], and bis(2-methyl-1H-imidazol-3-ium) oxalate, [C₄H₇N₂]₂[C₂O₄] (Equation 1). Colorless crystals, obtained by a slow evaporation of the solvent at room temperature, were first studied by ATR-FTIR spectroscopy (Figure S1). The strong bands located at 1616 and 1594 cm^-1 and 1424 cm^-1 can be assigned to ν_as(COO^-) and ν_s(COO^-) stretching vibrations of carboxylate groups, respectively. Those observed at 1286 and 1245 cm^-1, both of comparable intensity, are attributed to ν(C-COO^-) vibration bands. Characteristic absorptions of phenyl and imidazole rings are positioned at 732 and 698 cm^-1 and 758 and 662 cm^-1, respectively, corresponding to δ(C_sp2-H) and δ(C=C) elongations. The experimental CHN elemental analysis corroborates the formula of 1 but requires the addition of a half molecule of water to match fully with the theoretical values. Although not detected by XRD, the hygroscopic character of 1 can be explained by its ionic nature.

The molecular structure of 1 was finally elucidated by an X-ray crystallographic analysis performed on suitable crystals. Crystallographic data and refinement details are summarized in Table 1. An Ortep view of the asymmetric unit is shown in Figure 1. The unit cell comprises two distinct entities. Selected bond lengths (Å) and angles (°) are presented in Table 2. The primary structure of 1 can be considered as a coordination polymer chain wherein [Ph₃Sn(C₂O₄)] constitutes the repeating pattern. Oxalates exhibit a bidentate bridging coordination, trans-linking two distinct Ph₃Sn moieties and thus promoting the extension of the chains running along the c axis. The geometry around the tin atoms corresponds to a distorted trigonal bipyramid (TBP). The equatorial positions are occupied by the three phenyl groups [C1-Sn1=2.128(2) Å, C7-Sn1= 2.138(2) Å, and C13-Sn1=2.152(2) Å; C21-Sn2=2.1188(19) Å, C27-Sn2=2.129(2) Å, and C33-Sn2=2.135(2) Å]. The sums of the angles at tin with the ipso-carbon atoms [C1-Sn1-C7=115.02(8)°, C7-Sn1-C13=132.35(7)°, C13-Sn1-C1=112.26(8)°; C21-Sn2-C33=115.09(8)°, C33-Sn2-C27=127.89(7)°, C27-Sn2-C21=117.01(8)°] are 359.63° and 359.99°, respectively, indicating an almost perfect planarity. The variations recorded for the Sn-C bonds and C-Sn-C angles reflect the distortion of the TBP environment. The apical positions are occupied by two oxygen atoms of two distinct oxalate ligands [Sn1-O1 2.2190(14) Å and Sn1-O4 2.2164(13) Å; Sn2-O5 2.2574(13) Å and Sn2-O8 2.2992(13) Å]. These values are in the range of those already observed for polymeric oxalatotriphenylstannates and previously reported in the literature (Ng and Kumar Das, 1993; Ng et al., 1994). The corresponding axial angles O1-Sn1-O4 and O5-Sn2-O8 equal to 173.82(5)° and 174.01(5)°, respectively, indicating a slight deviation from linearity. In the unit cell, oxalate ligands exhibit two distinct conformations: one antiperiplanar [O1-C19-C20-O4=170.6(2)°] and one synclinal [O5-C39-C40-O8=57.8(2)°], leading to a differentiation between the chains. This structural feature is also supported by the infrared spectrum, which exhibits two distinct ν_as(COO^-) absorptions at 1616 and 1594 cm^-1 (Figure S1). The Sn1···O2 and Sn2···O6 distances measure 3.3717(15) and 3.2579(14) Å, respectively, but cannot be considered as significant interactions (Tiekink, 1991). For comparison, in the bis(dicyclohexylammonium) trisoxalatotetrakis(tri-n-butylstannate), which describes a similar tin environment, this distance is 3.043(3) Å (Ng et al., 1990). Two columns are then observed according to the c axis, one composed by the Sn1 antiperiplanar chain and the second with the Sn2 synclinal chain. Furthermore, two separate types of lengths are observed for C-O bonds [C19-O1=1.271(2) Å, C20-O4=1.277(2) Å, C39-O5=1.280(2) Å, C40-O8=1.302(2) Å; C19-O2=1.235(2) Å, C20-O3=1.236(2) Å, C39-O6=1.238(2) Å, C40-O7=1.217(2) Å], thus distinguishing single- and double-bond character, respectively. The negative charge of each [Ph₃Sn(C₂O₄)] moieties is compensated by the presence of a 2-methyl-1H-imidazol-3-ium cation, positioned along the chains but noncoordinated to atoms of the chain. From a supramolecular point of view, each 2-methyl-1H-imidazol-3-ium cation exhibits two N-H···O hydrogen bond interactions with two oxygen atoms of oxalate ligands of two distinct strands [N1-H1···O2= 2.778(2) Å and N2-H2···O8=2.903(2) Å; N3-H3···O3=2.762(2) Å and N4-H4···O6=2.695(2) Å] along the a axis. In the past, similar topologies have been observed in the crystal lattices of the bis(dicyclohexylammonium) trisoxalatotetrakis(tri-n-butylstannate) (Ng et al., 1990) and of diorganoammonium oxalatotrimethylstannates, [R₂NH₂][Me₃Sn(C₂O₄)] (R=i-Bu, cyclohexyl) (Sow et al., 2012). 2-Methyl-1H-imidazol-3-ium cations are positioned perpendicularly to the axis of the tin-based strands and intercalated between the Ph₃Sn groups. The resulting supramolecular arrangement is depicted in Figure 2.

Figure 1:

Molecular structure of 1 (asymmetric unit) showing 50% probability ellipsoids and the crystallographic numbering scheme (Ortep view).

Color code: tin, turquoise; nitrogen, blue; oxygen, red; carbon, gray. Symmetry transformations used to generate equivalent atoms: (i) +x, 0.5-y, 0.5+z; (ii) +x, 0.5-y, -0.5+z.

Figure 2:

Olex2 view showing the infinite chains of 1.

For clarity, only H atoms involved in hydrogen bonds are depicted. The origin of the coordinate system is arbitrary. Red and dotted line: hydrogen bonds; bicolored stipple cone: covalent bonds.

We also sought to characterize 1 in solution by conducting in particular multinuclear NMR measurements. However, whatever the nature of the solvent used (chlorinated organic solvents, aromatics, and alcohols), we have not been able to record its ¹¹⁹Sn NMR fingerprint. Referring to the previous literature on oxalatotriphenylstannates (Ng and Kumar Das, 1993; Ng et al., 1994), we also found that this type of data was not available, leading us to assume that the polymeric nature of these derivatives strongly prevents their solubilization. Finally, the ¹H spectrum was obtained in CD₃OD (solvent used for crystallization). It highlights the characteristic signals of the 2-methyl-1H-imidazol-3-ium cation and phenyl groups and confirms the 1:3 ratio, respectively (Figure S2).

Experimental