Synthesis, spectroscopic study, and crystal structure of a new organotin(IV) selenate derivative

In the past, interactions between oxoanions and organotin(IV) compounds aroused a lot of attention and led to countless synthetic and structural studies. The pioneering work in this area can be attributed to Lambourne, published as early as in the 1920s (Lambourne, 1924). During the 1980s, the work of the Holmes group revived the dynamics on this area (Holmes, 1989), which has since been continued in particular by the Chandrasekhar group (Chandrasekhar et al., 2002, 2007; Metre et al., 2014). This renewed interest has led to the isolation of many new complexes exhibiting a wide variety of polynuclear structures, as follows: ladder, O-capped, cube, prismanes, butterfly drum, football cage, and cyclic trimer. Although structural considerations initially took precedence, in order to discover original molecular edifices, the study of this reactivity also had more applied objectives. Thus, several works have been carried out in particular relating to the use of organotin(IV) dichlorides as selective receptors for the recognition of phosphates and thiocyanates (Hargrove et al., 2011). In a similar setting, colorimetric anion sensors based on trimethyl- and triphenyltin complexes were described (Villamil-Ramos and Yatsimirsky, 2011). Strong interest also exist in materials science, with the aim of preparing heterobimetallic oxides (Schubert, 2016).

Our group in Dakar has been studying interactions between oxoanions and organotin(IV) compounds for a long time, and is particularly interested in the isolation and characterization of new compounds involving Sn-O-Se moieties (Diassé-Sarr et al., 1997; Diop et al., 2001, 2007; Diallo et al., 2007). This topic has recently been the subject of a review article consisting of an inventory of existing crystallographic structures (Plasseraud, 2018). Previously, we reported the preparation and characterization of [n-Bu₂NH₂]₃[Ph₃Sn(SeO₄)₂] by reacting H₂SeO₄ and Ph₃SnOH in the presence of n-Bu₂NH (Diallo et al., 2014). This compound corresponded to the first triorganotin(IV) complex with terminally coordinated selenato ligands. Applying a comparable synthetic method, we report here the synthesis and the spectroscopic and structural characterization of [(Me₃Sn)₃(SeO₄)(OH)]_n (1), which constitutes a new example of an organotin(IV) selenate derivative (Figure 1).

Figure 1:

Molecular representation of 1 (Me=CH₃) showing its zigzag chain arrangement.

From a synthetic point of view, the preparation of 1 required two steps (Figure 2):

Figure 2:

Plausible equations of reactions leading to the formation of 1.

1. (Me₂NH₂)₂SeO₄ salt was first synthesized in aqueous solution by mixing selenic acid (H₂SeO₄) and dimethylamine at room temperature [Eq. (1)]. According to a similar methodology, we recently reported the synthesis of two selenato salts, bis(di-iso-propylammonium) selenate and di-n-butylammoniumhydrogenoselenate, characterized in the solid state by X-ray crystallographic analysis (Diallo et al., 2017).

2. The resulting white powder of (Me₂NH₂)₂SeO₄ was then treated with the tin(IV) precursor, Me₃SnCl, in methanol and in a 1:2 molar ratio [Eq. (2)]. Colorless single crystals were collected from the supernatant solution and were characterized in the solid state as [(Me₃Sn)₃(SeO₄)(OH)]_n (1).

Together with the crystallization of 1, we suggest the co-formation of dimethylammonium chlorotrimethyltin hydrogen selenate, [Me₂NH₂][(Me₃SnCl)(HSeO₄)], which could be related to a sulfate analogue, tetrabutylammonium chlorotrimethyltin hydrogenosulfate [Bu₄N][(Me₃SnCl)(HSO₄)], previously structurally characterized by our group (Diallo et al., 2009). This can also explain the modest yield recorded for 1 (37%). Further work is nevertheless in progress to experimentally verify this hypothesis.

With regard to compound 1, the C, H, and Se contents, experimentally determined by elemental analysis and inductively coupled plasma-optical emission spectrometry (ICP-OES) assay, respectively, corroborate its composition (see details in the Experimental section).

The structure of 1 was definitively solved by an X-ray diffraction study from a suitable single crystal (colorless, prism shaped). Selected crystallographic data and refinement details are reported in the Experimental section. An Ortep view with selected bond lengths and angles (Å, °) is shown in Figure 3. The asymmetric unit of 1 contains three distinct tin(IV) atoms, each bearing three coplanar methyl groups [Sn1-C1=2.118(2), Sn1-C2=2.118(2), Sn1-C3=2.123(2); Sn2-C4=2.121(2), Sn2-C5=2.126(2), Sn2-C6=2.125(2); Sn3-C7=2.116(3), Sn3-C8=2.114(2), Sn3-C9=2.112(2) Å]. All Sn atoms adopt a trigonal bipyramidal geometry (TBP) and are trans-coordinated by two oxygen atoms. The O-Sn-O angles deviate from linearity [O1-Sn1-O3#=174.09(6), O1-Sn2-O2=172.00(6), O4-Sn3-O5=169.41(5)°]. The structural index τ values of the Addison parameter for Sn1, Sn2, and Sn3 (0.85, 0.82, and 0.79, respectively) indicate a marked distorted character for each TBP (Addison et al., 1984). Interestingly, for the Sn1 atom, the second apical position is occupied by an oxygen atom from a selenate group and via a long Se-O···Sn distance interaction [Sn1···O3#=2.8211(16) Å, Sn1-O3-Se=137.17(8)]. Generally, the Sn-O-Se distances observed for organotin(IV) organoselenate complexes are in the range of 2.1–2.3 Å (Plasseraud, 2018). Sn1 and Sn2 atoms are bridged by an OH group [Sn1-O1=2.1016(17), Sn2-O1=2.2044(17) Å] with an Sn1-O1-Sn2 angle of 132.33(9)°. These values are in the range of previous examples including such trimethyltin(IV) hydroxide fragments in hydrogen bonding interaction (Otte et al., 2016). The longest Sn-O distance can be related to a coordinate dative interaction (Pavel et al., 2001). Sn2 and Sn3 are connected by a tetrahedral selenate group acting as tridentate and triply bridging ligand. One oxygen atom of SeO₄ is not coordinating [Se-O3=1.6376(15) Å].

Figure 3:

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

Selected bond lengths and angles [Å, °]: Se-O2 1.6332(15), Se-O3 1.6376(15), Se-O4 1.6500(15), Se-O5ⁱ 1.6449(16), O2-Se-O3 110.39(8), O3-Se-O4 110.03(8), O4-Se-O5ⁱ 109.15(8), O2-Se-O4 107.76(8), O3-Se-O5ⁱ 109.20(8), O2-Se-O5ⁱ 110.28(8), Sn2-O1 2.2044(17), Sn2-O2 2.3580(15), Se-O2-Sn2 136.71(9), O1-Sn2-O2 172.00(6), C4-Sn2-O1 91.86(8), C4-Sn2-O2 81.20 (7), C4-Sn2-C5 118.36(9), C5-Sn2-O1 96.60(8), C5-Sn2-O2 90.24(7), C6-Sn2-O1 93.06(8), C6-Sn2-O2 87.41(8), C5-Sn2-C6 117.68(10), C4-Sn2-C6 122.66(10), Sn1-O1 2.1016(17), Sn1-O1-Sn2 132.33(9), O1-Sn1-C1 99.48(8), O1-Sn1-C2 99.58(8), O1-Sn1-C3 95.64(8), C1-Sn1-C2 112.88(10), C2-Sn1-C3 118.01(10), C3-Sn1-C1 123.12(10), Sn3-O4 2.2787(16), Se-O4-Sn3 127.89(9), Sn3-O5 2.3210(16), O4-Sn3-O5 169.41(5), O4-Sn3-C7 93.30(9), O4-Sn3-C9 94.71(8), O5-Sn3-C7 92.04(8), O5-Sn3-C9 90.93(8), C7-Sn3-C8 120.77(12), C8-Sn3-O4 84.74(8), C8-Sn3-O5 84.67(8), C8-Sn3-C9 121.73(11), C7-Sn3-C9 117.44(11), Seⁱⁱ-O5-Sn3 135.30(9), [symmetry code: (i)=–½+x, ½ – y, 1 – z; (ii)=½+x, ½ – y, 1 – z].

From a supramolecular point of view, the succession of SeO₄ and Me₃Sn(3) moieties leads to the propagation of a polymeric zigzag chain along the a-axis (Figure 4). Pendant (Me₃Sn)₂OH groups are branched via Sn-O-Se bonds to the chain in a syndiotactic sequence. In the past, this type of chain-like organization was already described for organotin(IV) selenite complexes [(Me₃Sn)₂SeO₃·H₂O (Diassé-Sarr et al., 1997), (Ph₃Sn)₂SeO₃ (Diallo et al., 2007)], as well as for selenate derivatives [{Ph₄P[(SeO₄)(Ph₃Sn)(Ph₃SnX)]}_n with X=Br or Cl (Diop et al., 2001)]. In addition, intermolecular OH···O hydrogen bonds [O1-H1···O3=2.799(2) Å] and the long Se-O···Sn distance interactions promote the association of chains together and lead to the propagation of a three-dimensional network (Figure 5).

Figure 4:

Infinite chain of 1 (mercury view) [Sn (gray), Se (yellow), O (red)].

Hydrogen atoms and methyl groups are omitted for clarity.

Figure 5:

Mercury view of the interchain hydrogen bond (blue dotted) network.

Hydrogen atoms and methyl groups are omitted for clarity.

The examination of 1 by Fourier transform-infrared (FT-IR) spectroscopy in attenuated total reflection (ATR) mode corroborates the presence of OH groups and their hydrogen interaction by highlighting a broad absorption centered at 3214 cm⁻¹ (Figure S1). Intense bands are mainly present in the range 950–500 cm⁻¹. Those at 872, 857 and 758 cm⁻¹ can be assigned to νSeO₄ (Baran et al., 1997; Ben Hassen et al., 2014, 2015; Soukrata et al., 2014). In addition, the vibration at 549 and 520 cm⁻¹ are attributed to ν_asSnMe₃ and ν_sSnMe₃, respectively (Diassé-Sarr et al., 1997; Diop et al., 2007).

The characterization of 1 in solution was also investigated. Insoluble in usual halogenated solvents (CH₂Cl₂, CHCl₃), we finally succeeded in dissolving 1 in methanol. A slight heating was nevertheless required. In CD₃OD, the ¹H and ¹³C nuclear magnetic resonance (NMR) spectra display only one set of signals accompanied with ^119,117Sn satellite peaks [²J(¹¹⁹Sn,¹H) and ²J(¹¹⁹Sn,¹³C), respectively] corresponding to methyl groups linked to Sn (there are no differentiation between Sn1, Sn2, and Sn3 atoms) (Figures S2 and S3). The ¹¹⁹Sn{¹H} NMR spectrum of 1 also reveals only one resonance, but broad (width at half height=130 Hz) and located at 49.0 ppm (SnMe₄ as an external reference, Figure S4). In a first approximation, such a chemical shift would support the presence of tetracoordinated tin species (Mridula and Nath, 2016). However, the determination of the C-Sn-C angle calculated from the Lockhart and Manders equation for methyltin(IV) compounds [θ=0.0161|²J(¹¹⁹Sn,¹H)|² – 1.32²J(¹¹⁹Sn,¹H)+133.4] (Lockhart and Manders, 1986) leads to a value of 118°, which argues in favor of pentacoordinated tin species. Due to the non-equivalence of tin atoms in the polymeric structure of 1, a more complex ¹¹⁹Sn{¹H} NMR spectrum was particularly expected. Therefore, the spectrum recorded in CD₃OD probably provides evidence of the fragmentation of 1 into moieties exhibiting an equivalency of all tin atoms and stabilized by solvent molecules. The ⁷⁷Se NMR spectrum recorded for 1 shows a characteristic resonance of selenate derivatives (δ=1040 ppm, Figure S5) (Diallo et al., 2017).

The electrospray mass spectrum (positive mode) of 1, also measured in methanol, corroborates the previous NMR data and provides complementary indices on its stability in solution. Several intense mass clusters displaying the characteristic isotope pattern distributions of species containing Sn-O-Se moieties are observed in the range of 0–1500 Da (Figure S6). The major mass cluster (100%) is detected at m/z=634.8, and can be assigned to [M-OH]⁺. The calculated spectrum, shown in Figure S7, is well in agreement. The second most intense mass cluster at m/z=454.8 (47%) can be assigned to [M-OH-(CH₃)₃Sn-O]⁺. The mass cluster at m/z=699.2 (16%) matches with the solvation of the major mass cluster by two molecules of methanol [M-OH+2CH₃OH+H]⁺. Moreover, the heavier mass groups at m/z=863.1 (36%) and 1104.6 (11%) indicate the presence of slightly longer fragments, attributed to [M-OH+2CH₃OH+(CH₃)₃Sn+H]⁺ and [M-OH+SeO₄+2(CH₃)₃Sn]⁺, respectively. Thus, we can conclude that the original organization of 1 described in the solid state is not preserved in methanolic solution, leading to the cleavage of polymeric chains into monomeric units.

In summary, a new polymeric organotin(IV) selenate has been synthesized and fully characterized. The isolation of [(Me₃Sn)₃(SeO₄)(OH)]_n (1) expands this class of compounds that until now consisted of only five entities, analyzed by single-crystal X-ray diffraction. To the best of our knowledge, 1 constitutes the second example for a trimethyltin(IV) derivative. The first one was a discrete complex characterized as [(Me₃Sn)₂(SeO₄)]·2H₂O (Diop et al., 2007). Further work is under way to extend the structural inventory of such organotin(IV) complexes containing Sn-O-Se moieties.