Reduction vs. Metathesis in the Reactions of Bismuth Tribromide with a Bulky Lithium Silanide – An Experimental and Theoretical Study

Abstract

AbstractOn reaction of BiBr3 with Li(thf)3SiPh2tBu (1) in the corresponding ratios redox/metathesis reactions were observed, yielding dibismuthane (tBuPh2Si)4Bi2 (2) and disilylbismuth halide (tBuPh2Si)2BiBr (3). The latter is a reaction intermediate in the formation of the dark‐red 2. The X‐ray crystal structures of 1–3 were determined by low‐temperature X‐ray diffraction. The Si2Bi–BiSi2 core of 2 is in the semi‐eclipsed conformation. No oligomerization of “nonthermochromic” 2 was observed. Compound 3 is a mixed substituted monomer with a pyramidal environment around the bismuth center. On the basis of quantum chemical calculations, the formation of tertiary bismuthane (tBuPh2Si)3Bi is not expected for steric reasons. According to DFT‐optimized geometries of the simplified model systems n[(H3Si)2Bi]2 (n = 1–3), the closed‐shell attraction between intermolecular Bi centers in the chain provides a moderate elongation of the intramolecular Bi–Bi bond in the dibismuthane unit and a shortening of the intermolecular Bi···Bi contacts. According to MP4(SDQ) computations, such oligomerization is carried out by intermolecular interaction of s lone pairs that are bound together and p‐type orbitals of the Bi–Bi bonds in the bismuth chain. An increase in the number of [(H3Si)2Bi]2 molecules per chain results in a decrease in the HOMO–LUMO gap and leads to a bathochromic shift. TD‐PBE0 computations suggest that the lowest energy electron transition in 2 is metal–metal charge transfer. In addition, the attractive contributions in the chain [(H3A)2Bi]2···[Bi(AH3)2]2 with silyl groups (A = Si) outweigh the repulsion of the Bi···Bi centers, whereas for the alkyl‐substituted bismuth chain (A = C) the repulsive van der Waals force dominates. This fact makes the rectangle oligomerization model more preferred for n[(H3A)2Bi]2 (A = C; n = 2), while for A = Si chain formation is favored in the gas phase.