Synthesis, structure, and bonding of stable dialkylsilaketenimines

Stable (N-aryl)- and (N-alkyl)dialkylsilaketenimines R2SiCNR' [R = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, R' = 2,6-diisopropylphenyl (2a) and 1-adamantyl (2b)] were synthesized as blue and red crystals by the reactions of isolable dialkylsilylene 3 with 2,6-diisopropylphenyl isocyanide and 1-adamantyl isocyanide. X-ray single-crystal analysis disclosed that molecular structures of 2a and 2b were close to each other and were characterized to be allenic rather than zwitterionic or a silylene-isocyanide complex. The bonding characteristics of silaketenimines are found to be affected strongly by the substituents on silicon and nitrogen atoms. Remarkable red-shift of the pi(Si=C) --> pi*(C=N) band of 2a [lambdamax/nm (epsilon) 647(156)] compared with that of 2b [465 nm (109)] is ascribed to lowering of the pi*(C=N) orbital level due to significant interaction between pi*(C=N) and pi*(N-aryl) orbitals.