Synthesis of Mixed Silylene-Carbene Chelate Ligands from N-Heterocyclic Silylcarbenes Mediated by Nickel

The Ni-II-mediated tautomerization of the N-heterocyclic hydrosilylcarbene (LSi)-Si-2(H)(CH2)NHC 1, where L-2 = CH(C=CH2)(CMe)(NAr)(2), Ar=2,6-iPr(2)C(6)H(3); NHC=3,4,5-trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocyclic silylene (NHSi)-carbene (NHC) chelate ligand in the dibromo nickel(II) complex [(LSiD)-Si-1(CH2)(NHC)NiBr2] 2 (L-1 = CH(MeC=NAr)(2)). Reduction of 2 with KC8 in the presence of PMe3 as an auxiliary ligand afforded, depending on the reaction time, the N-heterocyclic silyl-NHC bromo Ni-II complex [L2Si(CH2)NHCNiBr(PMe3)] 3 and the unique Ni-0 complex [eta(2)(Si-H){(LSi)-Si-2(H)(CH2)NHC}Ni(PMe3)(2)] 4 featuring an agostic Si-H -> Ni bonding interaction. When 1,2-bis(dimethylphosphino) ethane (DMPE) was employed as an exogenous ligand, the first NHSi-NHC chelate-ligand-stabilized Ni-0 complex [(LSiD)-Si-1(CH2)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni-0 complex 6, [(LSi:-)-Si-1(CH2)NHCNi(CO)(2)], is easily accessible by the reduction of 2 with K(BHEt3) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada-Corriu-type cross-coupling reactions.