Equilibrium Coordination of NHCs to Si(IV) Species and Donor Exchange in Donor-Acceptor Stabilized Si(II) and Ge(II) Compounds

We report the reversible coordination of the N-heterocyclic carbene (NHC), NHC ^iPr₂Me₂ (NHC ^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiCl₄, MeSiCl₃, Me₂SiCl₂, and Me₃SiCl. Predicted as well as experimentally determined thermodynamic parameters of these equilibria confirm that the complexation constant increases with the Lewis acidity of the silicon halides. In contrast, the more σ-donating N-heterocyclic carbene, NHC^Me₄ (NHC^Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl₄ and Me₂SiCl₂ adducts even at higher temperatures. As a consequence, NHC ^iPr₂Me₂ in donor-acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, NHC ^iPr₂Me₂·GeMe₂·Fe(CO)₄ and NHC ^iPr₂Me₂·SiMe₂·Fe(CO)₄, is readily replaced by NHC^Me₄.