Metal complexes of acyclic diaminocarbenes: links between N-heterocyclic carbene (NHC)- and Fischer-carbene complexes

Starting from free bis(diisopropylamino)carbene, only rhodium and iridium complexes could be synthesized with these carbene ligands so far. The formation of further carbene complexes of Group 18 metals failed, because of the high sensitivity of the free diaminocarbenes, as described by Alder. Here we report the synthesis of diaminocarbene complexes of Group 16 metals by reaction of several acyclic and cyclic carbenes—first synthesized by Alder—with hexacarbonyls of these metals. The reaction of bis(diisopropyl)aminocarbene with photolytically produced (ether)Cr(CO)5 leads to a stable η2-carbene complex, with one amino group of the carbene coordinated to the metal. The chemical and spectroscopic characteristics of this complex are described. The reaction of the same carbene with ethersubstituted Mo(CO)6 resp. W(CO)6 leads to unstable merely η1-bonded complexes. In contrast to the bis(diisopropylamino)carbene, the reaction of the tetrahydropyrimidinocarbene, which was already isolated by Alder, with photolytically activated Cr(CO)6 leads to a simply η1-bonded complex; two of his CO ligands can be replaced with isonitrile even at ambient temperature. The analogous reaction of bis(N-piperidyl)carbene with photolytically activated Cr(CO)6 leads to the formation of a tetracarbonylchromium complex with a completely novel chelating ligand. During all reactions of bis(diisopropylamino)carbene and tetrahydropyrimidinocarbene with the metal carbonyls their reduction to carbonyl metallates could be observed as side reaction.