Crystalline (NN)C–M(NN) complexes: synthesis, structure, bonding and lability [M = Si, Ge, Sn or Pb; (NN) = 1,2-(ButCH2N)2C6H4

Abstract

Each of the red-brown or red, isomorphous, crystalline adducts (NN)C–M(NN) [(NN) = 1,2-(ButCH2N)2C6H4 and M = Si, Ge, Sn or Pb] has been obtained from the carbene (NN)C and the appropriate silylene, germylene, stannylene or plumbylene M(NN) in benzene and crystallisation from a hydrocarbon. They are monomeric, with the three-co-ordinate Ccarb and M atoms in an almost planar (C) or pyramidal (M) environment. The C–M distances are more than ca. 10% longer than for a typical MII–C bond in an MR2 molecule. Variable temperature 13C and xM NMR spectra (xM = 29Si, 119Sn or 207Pb) in toluene-d8, as well as for the appropriate M(NN) and C(NN) precursors, have been recorded. The Ccarb and M chemical shift data show that the compounds readily dissociate in solution, their stability decreasing in the sequence Sn > Pb > Si > Ge. From the magnitude of the chemical shifts, their conformation and C–M distances of the adducts, it is concluded that the C–M bond in each adduct is best formulated as being electrostatic in nature, with the carbene moiety as electron donor and the M(NN) fragment as acceptor.