Crystalline (NN)C-M(NN) complexes: synthesisl, structure, bonding and lability [M = Si, Ge, Sn or Pb; (NN) = 1,2-(Bu~tCH_2N)_2C_6H_4]

摘要

Each of the red-brown or red, isomorphous, crystalline adducts (NN)C-M(NN) [(NN) = 1,2-(Bu~tCH_2N)_2C_6H_4 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 ? or pyramidal (M) environment. The C-M distances are more than ca. 10% logner than for a typical M~(II)-C bond in an MR_2 molecule. Variable temperature ~(13)C and ~xM NMR spectra (~xM = ~(29)Si, ~(119)Sn or ~(207)Pb) in toluene-d_8, 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 dissociated 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.