Irreversible rearrangement in hexacoordinate silicon complexes: From neutral bis(N→Si) chelates to mono(N→Si) zwitterionic λ⁶-silicates

The reaction of (chloromethyl)trichlorosilane (7) with O-trimethylsilylated N,N-dimethylhydrazides yields initially under kinetic control, in nearly quantitative yields, a neutral hexacoordinate bis(N→Si) chelate, bis(N-(dimethylamino)acylimidato-N,O)chloro(chloromethyl)silicon(IV) (8a-c). Upon heating, the octahedral complexes 8 are quantitatively and irreversibly converted to the isomeric hexacoordinate chelates zwitterionic λ⁶-silicates 9a-c, (N-(dimethylamino)acylimidato-N,O)(1-((1,1-dimethyl-2-acyl)hydrazonium)methyl-C, O)-dichlorosilicon(IV). This molecular rearrangement is rather unusual, since it involves chelate-ring expansion from the common five-membered ring to a six-membered ring in the thermodynamically favored product 9. 8b and 9b were characterized by X-ray crystal analyses and in solution by their ¹H, ¹³C, and ²⁹Si NMR spectra. The analogous compounds 8a,c and 9a,c were characterized by the respective analogy of their NMR spectra. Both 8b and 9b have slightly distorted octahedral geometries around the central silicon atom. Both series of chelates 8 and 9 undergo ligand-site exchange processes, observed by the coalescence of signals due to diastereotopic N-methyl and/or CH₂ groups. Each compound (of both series) undergoes two consecutive rate processes: in 8 this sequence of exchange processes is analogous to previously reported stereomutations through a bicapped-tetrahedral intermediate or transition state; in 9 the first (lower barrier) process is a nitrogen-silicon dissociation-recombination, and the high-barrier process is an inversion of configuration at the silicon center.