Molecular structures of pyridinethiolato complexes of Sn(II), Sn(IV), Ge(IV), and Si(IV)

Introduction

The anion of methimazole (2-mercapto-1-methylimidazole, 1-methylimidazoline-2-thione, Hmt) proved to be a suitable buttress to link soft transition metal atoms (from group 10) with hard group 14 element atoms, thus supporting bonds between the group 10 and group 14 elements (e.g., Scheme 1, compounds I and II) (Wagler et al., 2008, 2010a; Brendler et al., 2011; Truflandier et al., 2011; Autschbach et al., 2012). The ambidentate nature of the methimazolide (mt^-) ligand supports the formation of heteronuclear complexes that bear this buttress S-bound to the group 10 metal and N-bound to the group 14 element. In a similar manner, this ligand was shown to buttress heteronuclear complexes of different group 14 elements (Scheme 1, III) (Wagler et al., 2010b), whereas in case of related homo-dinuclear complexes, a so-called (2,2)-paddlewheel arrangement is favored (Scheme 1, IV) (Wagler et al., 2013).

Scheme 1

mt-bridged hetero- and homo-dinuclear group 14 complexes.

Recently, the anion of 2-mercaptopyridine (better called the anion of 1,2-dihydropyridine-2-thione, pyridine-2-thiolate, PyS^-) has been successfully utilized as a related (S,N) ambidentate buttress for the syntheses of heteronuclear complexes of group 10 and group 14 elements (Scheme 2, V and VI) (Martincová et al., 2011, 2012; Berrenguer et al., 2013). This tempted us to explore the chemistry of this buttressing ligand in systems related to those with methimazolyl buttresses. In terms of group 14 element coordination chemistry, to date, the Cambridge Structural Database contains examples of only Sn and Pb compounds of pyridine-2-thiolate; Ge and Si compounds are lacking (ConQuest 1.15, 2012). In tin(IV) chemistry, PyS^- and related ligands have been shown capable of enhancing the tin coordination number to 5, 6, 7, and even 8 (Scheme 3, VII, VIII and IX, X, XI, respectively) (Masaki et al., 1976, 1978; Castaño et al., 1990; Damude et al., 1990; Bouâlam et al., 1992; Schmiedgen et al., 1994; Couce et al., 1996; Huber et al., 1997; de Castro et al., 2001; Sousa-Pedrares et al., 2003; Ismaylova et al., 2012), thus underlining their already intriguing ligand features in mononuclear group 14 element coordination chemistry. In addition, HPyS has been shown to enter the tin coordination sphere as a charge-neutral ligand via S-donor action (Valle et al., 1987; Wu et al., 2000). Therefore, this article will deal with the molecular structures of some analogous Si and Ge compounds and some hitherto not crystallographically characterized Sn compounds of PyS^-.

Scheme 2

PyS-bridged heteronuclear group 14 complexes.

Scheme 3

Different coordination patterns of Sn(PyS)-complexes.

Results and discussion

With focus on the utilization of (PyS) and chlorine-substituted derivatives of Sn, Ge, and Si in further studies (i.e., syntheses of heteronuclear complexes of group 14 and, e.g., group 10 elements), we aimed at the syntheses and characterization of the compounds E(PyS)₄ and Cl₂E(PyS)₂ (E=Sn, Ge, Si). Although the tin compounds Sn(PyS)₄ and Cl₂Sn(PyS)₂ have already been described in the literature (Masaki et al., 1976; Damude et al., 1990), the crystal structure data of pure Sn(PyS)₄ are lacking (only a structure of Sn(PyS)₄·HPyS has been reported), and the crystal structure of Cl₂Sn(PyS)₂ in its triclinic modification (Masaki et al., 1978), which is related to our novel Ge and Si analogs, was of moderate quality only, and therefore, we included the synthesis of Sn(PyS)₄ and structural characterization of Sn(PyS)₄ and Cl₂Sn(PyS)₂ in this article as well. (Note: after submission of this manuscript, the synthesis and characterization of Cl₂Si(PyS)₂ were published by Baus et al., 2013; the experimental data reported therein are in accord with our data, even though the authors used a different synthesis route, i.e., starting from SiCl₄, 2-mercaptopyridine, and triethylamine without a transsilylation step.)

The compounds Cl₂E(PyS)₂ (E=Ge, Si) were prepared along a transsilylation route (Scheme 4). This rather mild substitution approach gave rise to the desired products in good yields without the risk of threefold or fourfold substitution of the Ge- or Si-bound chlorine atoms. We found all compounds of the series Cl₂E(PyS)₂ (E=Sn, Ge, Si) to crystallize in the triclinic system with similar unit cell parameters (Table 1). As a prerequisite, the crystal structures were founded on molecules of similar complex architecture (Figure 1). Selected bond lengths and angles are listed in Table 2 for comparison. The E-N and E-S bond lengths were in accord with the increasing atomic radii of the central atoms of the complexes. Noteworthy, an increase in thiophilicity was observed from the Si toward the Ge compound. That is, whereas the Ge-N bonds (2.09 and 2.10 Å) were ca. 0.15 Å longer than the Si-N bonds (1.95 Å), less bond length increase was found for the Ge-S bonds (2.32 Å), which were ca. 0.05 Å longer than the Si-S bonds (2.27 Å). In the transition from the Ge to the Sn compound, both the E-N and E-S bonds revealed similar trends (increase by ca. 0.16 and 0.15 Å, respectively; i.e., Sn-N 2.26 and 2.27 Å, Sn-S 2.47 and 2.48 Å). In the compounds Cl₂E(PyS)₂ (E=Sn, Ge, Si), the Cl-E-Cl angles were similar and slightly wider than 90° (ranging between 93° and 97°), which can be attributed to the low steric demand of the four-membered chelates. The cis-arranged nitrogen donor atoms furnished an N-E-N angle of close to 90° in all cases (ranging between 85° and 89°), whereas the S-E-S angles exhibited noticeable differences, i.e., pronounced deviation from linearity with an increasing atomic radius of E (163.3°, 159.4°, and 154.6° or 156.8° for Si, Ge, and Sn, respectively). In principle, some differences can be observed between the molecular structures of Cl₂Sn(PyS)₂ in its two modifications, which hinted at some flexibility of those molecules to respond to crystal packing effects. Probably most interesting was the N-C-S angle within each of the four-membered chelates, which was significantly smaller than 120° (ranging between 107° and 113°). Thus, this angle clearly indicated attraction between the central atom E and both donor atoms S and N of each chelate, rather than chelation as an exclusive result of spatial proximity of E and the N or S atom. Furthermore, the decrease in the N-C-S angle along the series E=Sn>Ge>Si was in support of this attractive interaction, as the E-S bond responded to the decreasing atomic radius of the central atom. This observation clearly distinguished the PyS^- ligand from the related mt^-, which furnished capped tetrahedral coordination spheres in its monosilane derivatives with rather long Si···S separations (>3 Å) (Wagler et al., 2010c).

Scheme 4

Syntheses of (PyS)-complexes of silicon, germanium and tin.

Figure 1

Molecular structures of Cl₂E(PyS)₂ (E=Si, Ge, Sn) in their crystal structures.

ORTEP/POV-RAY diagrams (Farrugia, 1997; Persistence of Vision Pty. Ltd. Persistence of Vision Raytracer Version 3.6 [POV-RAY 3.6], 2004) with thermal displacement ellipsoids are shown at the 30% (Si compound) and 50% (Ge and Sn compound) probability levels. Selected atoms are labeled and hydrogen atoms are omitted for clarity.

For the syntheses of compounds E(PyS)₄ (E=Si, Ge, Sn), different synthetic approaches were used (Scheme 4). In case of compound Sn(PyS)₄, an oxidative addition approach was successful (2,2′-dipyridyldisulfide is generally known to undergo oxidative addition with low-valent tin compounds; Masaki et al., 1976; Damude et al., 1990), which, in contrast to the original literature procedure (Damude et al., 1990), started from a tin(II) complex instead of tin powder, to furnish a product of good purity. A substitution route was chosen for the preparation of Si(PyS)₄ because of the pronounced instability of Si(II) compounds. With respect to the related Ge complex, we have not been able to isolate Ge(PyS)₄. So far, both routes (oxidative addition and substitution) have led to mixtures of products. Thus, only the molecular structures of E(PyS)₄ (E=Si, Sn) will be discussed in the following (Tables 1, 3, and 4, Figure 2). Furthermore, from the filtrate of the synthesis of Sn(PyS)₂, some crystals of a new modification of Sn(PyS)₄·HPyS (which actually is a decomposition product) were obtained, and thus, this structure will be included in the comparison (Figure 3).

Figure 2

Molecular structures of E(PyS)₄ (E=Si, Sn) in their crystal structures.

ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled and hydrogen atoms are omitted for clarity. (Note: The labels of S3,C11,N3 and S4,C16,N4 in compound Sn(PyS)₄ do not correspond to the labels in the CIF but have been assigned for consistence with the corresponding bonds and angles in Table 3.)

Figure 3

Molecular structures of Sn(PyS)₄·HPyS in the crystal structure.

ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled and C-bound hydrogen atoms are omitted for clarity.

In general, the pronounced thiophilicity of tin over silicon was again reflected in the molecular structures of E(PyS)₄ (E=Si, Sn), with the E-N bonds (1.94 and 1.95 Å for E=Si and 2.26 and 2.29 Å for E=Sn) being ca. 0.31–0.35 Å longer in the Sn complex, whereas E-S bond length increased by only 0.20–0.22 Å (2.29 and 2.34 Å for E=Si and 2.50 and 2.55 Å for E=Sn) and by 0.23 Å (2.24 and 2.25 Å for E=Si and 2.47 and 2.49 Å for E=Sn) for the chelate and nonchelate S atoms, respectively. Once again, the noticeably small N-C-S angles within the chelates (ca. 109° for E=Si, ca. 113° for E=Sn), which were smaller for the Si compound, reflected the attraction between the central atom E and both donor atoms of the chelates. In case of compounds E(PyS)₄ (E=Si, Sn), the nonchelate (thus S-bound) (PyS) groups exhibited significantly larger N-C-S angles, which ranged between 116° and 120°. Interestingly, in the three different crystal structures of Sn(PyS)₄ and Sn(PyS)₄·HPyS, the molecule of Sn(PyS)₄ exhibited different coordination modes. Even though the fundamental motif was a distorted octahedral coordination sphere about tin, with the monodentate ligands’ S atoms cis and the chelate S atoms trans, the Sn(PyS)₄ molecules in Sn(PyS)₄ and in the herein reported structure of Sn(PyS)₄·HPyS showed [6+1] coordination of the tin atom, furnished by capping of one octahedral face by one of the nonchelate N atoms (N3). The capping modes were different from each other, as in Sn(PyS)₄, a NS₂ face was capped (N1,S2,S3), whereas in the related molecule in the structure of Sn(PyS)₄·HPyS, an S₃ face was capped (S2,S3,S4).

So far, Cl₂Si(PyS)₂, Cl₂Ge(PyS)₂, and Si(PyS)₄ were the first crystallographically characterized representatives of Si(Cl₂N₂S₂), Ge(Cl₂S₂N₂), and Si(N₂S₄) coordination spheres, respectively (ConQuest 1.15, 2012). (The crystal structures of Cl₂Si(PyS)₂ and Cl₂Ge(PyS)₂ have been part of a presentation on the GTL2013 conference; Wächtler et al., 2013.) Whereas Si(PyS)₄ exhibited sufficient solubility in CDCl₃, which allowed for its ²⁹Si NMR characterization in solution (the ²⁹Si shift of -165 ppm clearly indicated that hexacoordination of the Si atom was retained in the solution, although the detection of a sole set of broad signals in the ¹H and ¹³C NMR spectra indicated rapid exchange between the chelating and the nonchelating PyS moieties), the solubility of Cl₂Si(PyS)₂ was too poor for the acquisition of decent ²⁹Si NMR spectroscopic data from a solution. Thus, ²⁹Si solid-state NMR spectroscopy was employed. To our surprise, the ²⁹Si CP/MAS NMR spectrum of Cl₂Si(PyS)₂ (shown in Figure 4) exhibited a broad signal that was patterned with two peaks and a shoulder. As the crystal structure of this complex would give rise to the expectation of a singlet signal, the pattern observed in the spectrum was interpreted as the result of dipolar coupling with the chlorine atoms, since similar effects have already been observed and successfully described for ³¹P solid-state NMR spectra (Thomas et al., 2001). Indeed, simulation of the signal with model ‘THREE’ in CSOLIDS (Eichele, 2013) produced an excellent explanation for this broad and patterned ²⁹Si resonance. The following parameters were used to describe the couplings with the ²⁹Si nucleus: dipolar coupling with ³⁵Cl, which is representative for Cl, as it was shown to be similar to ³⁷Cl (Brendler et al., 2012); D=-231.058 and -224.585 Hz for the two independent crystallographic chlorine sites (values derived from the crystallographically determined Si-Cl bond lengths); ¹J(²⁹Si-³⁵Cl)=30 Hz as an initial guess (Brendler et al., 2012; Wagler et al., 2013); quadrupolar couplings C_q=29 and 34 MHz for the two independent chlorine sites (C_q was varied to achieve good fit of the simulation); Cl-Si-Cl angle 93.5° (derived from the X-ray data); and a line broadening GB=75 Hz.

Figure 4

²⁹Si CP/MAS NMR spectrum of Cl₂Si(PyS)₂.

Signal traces are as follows (from top): simulated signal with smoothing (red), experimental spectrum (blue), and simulated signal as stick approximation without smoothing (green).

As the tin(II) compound Sn(PyS)₂ (Ichikawa and Mukaiyama, 1985) provided easy access to Sn(PyS)₄ via oxidative addition of a disulfide, the synthesis potential of Sn(PyS)₂ was further probed in the reaction with elemental sulfur. As expected, a tin(IV) compound of the gross composition SnS(PyS)₂ formed. Revealed by X-ray crystallography (Figure 5), this compound is a cyclic dimer. Due to its location around a crystallographic center of inversion, the Sn₂S₂ four-membered ring was constrained to perfect planarity. Furthermore, the anisotropic displacement parameters of the atoms of this cycle hinted at only limited displacement out of this perfectly planar arrangement by effects such as thermal motion or dynamic disorder, and the two independent Sn-S bonds within the Sn₂S₂ four-membered ring exhibited similar lengths (2.45 and 2.46 Å). This Sn₂S₂ arrangement with hexacoordinate Sn was similar to the related motif in other compounds (e.g., Scheme 5, XII and XIII), which have previously been described in the literature (Jurkschat et al., 1992; Nayek et al., 2009; Rotar et al., 2009).

Figure 5

Molecular structure of [SnS(PyS)₂]₂ in the crystal structure of [SnS(PyS)₂]₂·THF.

ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled and hydrogen atoms and solvent molecule are omitted for clarity. The asymmetric unit comprises one half of a centrosymmetric cyclo-dimer. Symmetry-related atomic sites are indicated by * (for inversion 1-x, -y, -z). Selected bond lengths (Å) and angles (°): Sn1-S1 2.5483 (7), Sn1-S2 2.5394 (6), Sn1-S3 2.4456 (6), Sn1-S3* 2.4615 (7), Sn1-N1 2.293 (2), Sn1-N2 2.266 (2), Sn1-S3-Sn1* 86.43 (2), S3-Sn-S3* 93.57 (2), S1-Sn1-S2 145.20 (2), N1-Sn1-N2 86.05 (7), N1-C1-S1 113.2 (2), N2-C6-S2 113.0 (2).

Scheme 5

Selected examples of hexacoordinate tin compounds with Sn₂S₂ motif.

The S3-Sn1-S3* angle (93.6°) was similar to the Cl-Sn-Cl angle in Cl₂Sn(PyS)₂ (94.6° and 96.4°), and the same was true for the N-Sn-N angle (86.1° in [SnS(PyS)₂]₂, 88.2° and 85.7° in Cl₂Sn(PyS)₂). Interestingly, the angle S1-Sn1-S2 (145.2°) deviated far more from linearity than in the compound Cl₂Sn(PyS)₂ (154.6° and 156.8°), which can be attributed to the longer Sn1-S1 and Sn1-S2 bonds in (SnS(PyS)₂)₂ (2.55 and 2.54 Å). Apparently, the decrease in Lewis acidity (caused by substitution of the Sn-bound Cl atoms for S atoms) caused predominant lengthening of the Sn-S bonds to the chelating ligands. At first glance, this might be unexpected in the context of the previously mentioned pronounced thiophilicity of Sn (pronounced attraction of S over N donors), but the canonical form of PyS^- as a 1,2-dihydropyridine-2-thione anion offered an explanation with the formally dative (thus more flexible) Sn-S bond within the chelate. This is still unexpected because in compounds such as VIII and X (Scheme 3), the N atoms appeared to establish the formally dative (weaker, and thus more flexible) bond.

Finally, the solid-state structure of the tin(II) compound Sn(PyS)₂, which had been reported in the literature (Ichikawa and Mukaiyama, 1985) and was used for the synthesis of Sn(PyS)₄, has not been reported yet. Thus, we collected the single-crystal X-ray diffraction data set of this compound, and structure solution and refinement revealed an interesting arrangement of the molecules in the solid state (Figure 6). In principle, the crystal structure comprised two independent Sn(PyS)₂ moieties. Interestingly, one of them was part of a centrosymmetric cyclo-dimer, whereas the other moiety was part of a polymeric chain (which was aligned along the monoclinic b axis around a 2₁ screw axis). Both tin atoms were [3+2] coordinated, but the one in the chain comprised a [SN₂+S₂] coordination, whereas in the cyclo-dimer, the tin coordination sphere was [S₂N+SN]. Their same coordination number was clearly reflected by their similar ¹¹⁹Sn NMR shifts in the solid state (-353 and -368 ppm). The ¹¹⁹Sn chemical shift tensors, however, exhibited notable differences (Figure 7). In the CDCl₃ solution, the additional lone pair donation was absent (or at least significantly less pronounced), as reflected by a noticeable downfield shift of the ¹¹⁹Sn resonance (-206 ppm), and same coordination mode (or at least rapid exchange) caused the presence of only one set of ¹H and ¹³C NMR signals for the (PyS) ligands. This structure once more underlined the coordinative flexibility of the pyridine-2-thiolato ligand, which may thus adapt not only to the electronic features of a central atom of a complex but also to crystal packing environments by operation via pronounced S- or N-donor activity.

Figure 6

Molecular structure of Sn(PyS)₂ in the crystal structure.

ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled and hydrogen atoms are omitted for clarity. The asymmetric unit comprises two independent Sn(PyS)₂ moieties. Symmetry-related atomic sites are indicated by * (for the shift along a 2₁ axis 1.5-x, y+0.5, 0.5-z) or ** (for inversion 2-x, 2-y, -z). Selected bond lengths (Å): Sn1-S1 2.6389 (5), Sn1-S2 3.0351 (6), Sn1-S2* 3.2370 (6), Sn1-N1 2.332 (2), Sn1-N2 2.268 (2), Sn2-S3 2.5709 (5), Sn2-S4 3.2182 (5), Sn2-S4** 2.6627 (5), Sn2-N3 2.672 (2), Sn2-N4 2.328 (2).

Figure 7

¹¹⁹Sn MAS NMR spectrum of Sn(PyS)₂.

Signal traces are as follows (from top): simulated signal 1 (purple), simulated signal 2 (green), simulated spectrum with signals 1 and 2 (red), and experimental spectrum (blue). The characteristics of the chemical shift anisotropy tensors are as follows: for signal 1, δ_iso -368.2 ppm, δ₁₁ -57.1 ppm, δ₂₂ -112.2 ppm, δ₃₃ -935.3 ppm, Ω 878.2 ppm, κ 0.87; for signal 2, δ_iso -353.0 ppm, δ₁₁ 110.0 ppm, δ₂₂ -315.1 ppm, δ₃₃ -853.9 ppm, Ω 963.8 ppm, κ 0.12.

Conclusions

In this article, we have presented the first comparison of structurally related Si, Ge, and Sn compounds with respect to the coordination chemistry of pyridine-2-thiolate, which was enabled by the first crystallographically characterized Si and Ge complexes of this interesting ambidentate ligand. Furthermore, crystallographic characterization of some literature known pyridine-2-thiolato tin complexes underlined the coordinative flexibility and versatility of this ligand in group 14 coordination chemistry. The herein reported Si, Ge, and Sn compounds are currently under investigation as starting materials for the syntheses of heteronuclear complexes with group 14 elements and transition metals.

Experimental