Zwitterionic adducts from addition of imines to the α-carbon atom of pentacarbonyl(vinylidene)chromium and -tungsten complexes

Imines, Im, such as MeNC(Ph)H (5), 2-methyl 4,5-dihydrothiazole (8a), 2-methyl 4,5-dihydrooxazole (8b) and MeNC(OMe)Me (13) add to the α-carbon atom of the vinylidene ligand in [(CO)5CrCCMe2] (4) to give isolable zwitterionic adducts, [(CO)5Cr−–C(CMe2)(Im+)]. The reaction of [(CO)5WCCPh2] (12) with 13 also yields an adduct, [(CO)5W−–C(CPh2){NMeC(OMe)Me}+] (15), whereas from the corresponding reaction of 4 with xanthylideneimine, H–NC(C6H4)2O (16), a carbene complex, [(CO)5CrC(i-Pr)–NC(C6H4)2O] (17), is obtained. Complex 17 presumably is formed by initial addition of 16 to 4 and subsequently rapid rearrangement. In solution, the adduct [(CO)5Cr−–C(CMe2)(NMeC(Ph)H)+] (6) slowly cyclizes to form the 2-azetidin-1-ylidene complex [(CO)5CrC–NMe–C(Ph)H–CMe2] (7). In contrast, when solution of those zwitterions are heated that are formed by addition of 4,5-dihydrothiazole or 4,5-dihydrooxazole to 4, no cyclization is observed but rather the formation of 4,5-dihydrothiazole and 4,5-dihydrooxazole complexes, respectively. The structures of two adducts, [(CO)5Cr−–C(CMe2)(Im+)] (Im=MeNC(Ph)H, 2-methyl 4,5-dihydrothiazole) and of the substitution product [(CO)5W(2-methyl 4,5-dihydrothiazole)] have been established by X-ray structural analyses.