Mono- and Bimetallic Zwitterionic Chromium(0) and Tungsten(0) Allenyls

A series
of stable chiral (racemic), formally neutral, zwitterionic mono- and
bimetallic M­(CO)5[C­(OEt)CCR­(NHC)] (M =
Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic
carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes.
Different classes of NHCs, (e.g., 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene,
1,3-bis­(2,4,6-trimethylphenyl)­imidazolin-2-ylidene, and their
six- and seven-membered analogues and 1,3-bis­(dimethyl)­imidazol-2-ylidene)
were employed as nucleophiles in these C–C bond-forming reactions
yielding the novel complexes in essentially quantitative yields. A
systematic experimental and computational study of the electronic
properties of the Cr- and W-allenyls shows that their UV–vis
spectra are directly influenced by the structure of the heterocyclic
moiety derived from the NHC (ring size, substituents on the N atoms)
and by the nature of the metal fragment (Cr/W). The electron-releasing
nature of these complexes allows them to participate in electron-transfer
reactions in the ground state, leading to a type of charged α,β-unsaturated
Fischer carbenes that incorporate an NHC fragment in their structure.