Irreversible rearrangement in hexacoordinate silicon complexes: From neutral bis(N -> Si) chelates to mono(N -> Si) zwitterionic lambda(6)-silicates

The reaction of (chloromethyl)trichlorosilane (7) with O-trimethylsilylated N,N-dimethylhydrazides yields initially under kinetic control, in nearly q uantitative yields, a neutral hexacoordinate bis(N-->Si) chelate, bis(N-(di methylamino)acylimidato-N,O)chloromethyl)silicon(IV) (8a-c). Upon heating, the octahedral complexes 8 are quantitatively and irreversibly converted to the isomeric hexacoordinate chelates zwitterionic lambda(6)-silicates 9a-c , (N-(dimethylamino)acylimidato-N,O)(1-((1,1-dimethyl-2-acyl)hydrazonium)me thyl-C,O)-dichlorosilicon(IV). This molecular rearrangement is rather unusu al, 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 thei r H-1, C-13, and Si-29 NMR spectra. The analogous compounds 8a,c and 9a,c w ere characterized by the respective analogy of their NMR spectra. Both 8b a nd 9b have slightly distorted octahedral geometries around the central sili con atom. Both series of chelates 8 and 9 undergo ligand-site exchange proc esses, observed by the coalescence of signals due to diastereotopic N-methy l and/or CH2 groups. Each compound (of both series) undergoes two consecuti ve rate processes: in 8 this sequence of exchange processes is analogous to previously reported stereomutations through a bicapped-tetrahedral interme diate or transition state; in 9 the first (lower barrier) process is a nitr ogen-silicon dissociation-recombination, and the high-barrier process is an inversion of configuration at the silicon center.