Sn4Si{Si(SiMe3)3}4{SiMe3}2]: A Model Compound for the Unexpected First-Order Transition from a Singlet Biradicaloid to a Classical Bonded Molecule

Abstract

Metalloid cluster compounds of the general formula MnRm (n>m ; M=metal or semi-metal, R= ligand) are ideal model compounds for the system size range encompassed by molecules and the solid state, paving the way for further understanding of element formation from oxidized species on an atomic scale. In the case of tin, metalloid cluster compounds were first synthesized by reductive coupling of Sn compounds, such as SnCl2. [2] Recently it was shown that metalloid cluster compounds of tin can also be synthesized by the disproportionation reaction of tin monohalides. The monohalides are thereby obtained by employing a preparative co-condensation technique. Hence, the reaction of SnBr with LiSi(SiMe3)3 leads to the metalloid cluster compound [Sn10{Si(SiMe3)3}6] (1) in moderate yield of approximately 17%. Because only six of the ten tin atoms in 1 bear a Si(SiMe3)3 ligand, the average oxidation state of the tin atoms is 0.6. Thus, the metalloid cluster 1 is a reduction product of the disproportionation reaction on the way to elemental tin. Because the reaction starts with the monohalide SnBr, oxidized species with an average tin atom oxidation state of greater than 1 must also be present in the reaction solution. Early examples of such compounds were anionic stannylene [Sn{Si(SiMe3)3}3] and cyclotristannene [Sn3{Si(SiMe3)3}4] (2), in which the average oxidation states of the tin atoms is + 2 and + 1.3, respectively. The shortest tin–tin double bond of 258 pm was observed in 2, caused by the steric bulk of the ligands forcing the double bond into a planar arrangement. As 2 is only obtained together with the metalloid cluster compound [Sn10{Si(SiMe3)3}6] (1), subsequent investigations on 2 are always hindered by the presence of 1. Crystallization of 2 from the reaction mixture was attempted to circumvent this problem. During these attempts, another type of black diamond shaped crystals were obtained, and single crystal X-ray diffraction analysis of these crystals revealed a yet unknown crystal system. However, solution of the crystal structure showed that the metalloid cluster compound 1 is present in the crystal lattice, this time crystallizing together with the novel polyhedral cluster compound [Sn4Si{Si(SiMe3)3}4(SiMe3)2] (3). The molecular structure of 3 is best described as a butterfly arrangement of four tin atoms bridged by a Si(SiMe3)2 group (Figure 1). Every tin atom is additionally bound to a Si(SiMe3)3 ligand, with slightly different Sn–Si distances of 261 pm (Sn11–Si7A, Sn13–Si7) and 265 pm (Sn14–Si8, Sn12–Si8A). The capping Si(SiMe3)2 group most likely comes from the degradation of the Si(SiMe3)3 ligand, and a plausible mechanism is given in the supporting information. The tin–tin distances (284 pm) inside the Sn4 butterfly unit in 3 are within the range of a normal single bond (283– 285 pm). The capping silicon atom is bound to two tin atoms, with a Si–Sn distance of 263 pm also within the range of a single bond, leading to a nearly tetrahedral arrangement for these two tin atoms (Sn11, Sn13). In spite of this arrangement,