Synthesis and Structure of N-Heterocyclic Carbene Complexes of Germanium(II)

The synthesis and structural characterization of a series of N-heterocyclic carbene (NHC) complexes of GeR1R2 are reported, where R1 = R2 = Mes (Mes = 2,4,6-trimethylphenyl) (1), R1 = R2 = F (5), R1 = R2 = Br (6), R1 = Cl, R2 = OTf (OTf = O3SCF3) (7), R1 = R2 = OtBu (10), and R1 = R2 = NCS (11). The bond length between the carbenic carbon and the germanium is observed to vary in accordance with the π-donating ability of the substituent on Ge. Efforts to synthesize stable complexes with small alkyl or aryl substituents were not successful; evidence for oligomerization of the GeR2 fragment is presented. The reaction between 1 and 3 (R1 = R2 = Cl) resulted in the formation of an NHC-coordinated germylgermylene (13). The 1H NMR spectra of 3−7, 10, and 11 display broad signals at room temperature. To rationalize the 1H NMR spectra, mechanisms for conformational interchange, as well as intermolecular exchanges, are discussed.