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-(Bu^tCH₂N)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 C_carb 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 M^II–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 C_carb 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.