N-Heterocyclic Silylene Complexes of Group VI Transition Metal Carbonyls

Abstract

The reactivity and coordination behavior of several N-heterocyclic silylenes R2NHSi (R = tBu, Mes, Dipp) with respect to group VI transition metal acetonitrile carbonyl complexes fac-[M(CO)3(MeCN)3] (M = Cr, Mo) is reported. The first examples for threefold NHSi substituted chromium(0) and molybdenum(0) carbonyl complexes mer-[Cr(CO)3(tBu2NHSi)3] (1), mer-[Mo(CO)3(tBu2NHSi)3] (2), and mer-[Mo(CO)3(Mes2NHSi)3] (3) were isolated from the reactions of fac-[M(CO)3(CH3CN)3] (M = Cr, Mo) with three equivalents of tBu2NHSi or Mes2NHSi. The NHSi ligands adopt meridional arrangements in these octahedral complexes in solution and in the solid state, which contrats related NHC complexes as they adopt facial geometries. The reaction of fac-[Cr(CO)3(MeCN)3] with Mes2NHSi led to substitution of only two acetonitrile ligands to yield mer-[Cr(CO)3(Mes2NHSi)2(MeCN)] 4, as did the reaction of fac-[Mo(CO)3(MeCN)3] with Dipp2NHSi to give mer-[Mo(CO)3(Dipp2NHSi)2(MeCN)] 5. Thus, the steric bulk of the silylenes Mes2NHSi (for Cr) and Dipp2NHSi (for Mo) can be regarded as steric limits for a threefold substitution. The remaining acetonitrile ligand of 4 is prone to further substitution, as was demonstrated with the reaction of 4 with pyridine (= py), dimethylamino-pyridine (= DMAP), tert-Butylisocyanide (= CN-tBu), and 1,3,4,5-tetramethyl-imidazolin-2-ylidene (= IMeMe), which led to formation of the complexes mer-[Cr(CO)3(Mes2NHSi)2(py)] 6, mer-[Cr(CO)3(Mes2NHSi)2(DMAP)] 7, mer-[Cr(CO)3(Mes2NHSi)2(CN-tBu)] 8, and mer-[Cr(CO)3(Mes2NHSi)2(IMeMe)] 9. In these complexes the NHSi ligands are located in trans positions.