Carbene-Pnictinidene Adducts.

The syntheses, characterizations, and X-ray crystallographic structure determinations are described for adducts of stable nucleophilic carbenes with pnictinidenes. The adducts between 1,3-dimesitylimidazol-2-ylidene and phenylphosphinidene, phenylarsinidene, (trifluoromethyl)phosphinidene, and (pentafluorophenyl)arsinidene are reported. These carbene-pnictinidene adducts are formed by the direct reaction of a stable nucleophilic carbene with the corresponding pnictinidene cyclic oligomers. The synthesis and structure of the adduct between 1,3-dimesitylimidazolin-2-ylidene and phenylphosphinidene from the reaction of 1,3-dimesitylimidazolin-2-ylidene with phenylphosphorus dichloride are also reported. These carbene-pnictinidene adducts possess strongly polarized pnictinidene-carbene bonds. The C-Pn-C angles are all typically small at 97-102 degrees, and there is only a 4% shortening of the nominal Pn=C double bond compared to the Pn-C single bond to the second substituent on the pnictogen. The (31)P NMR shifts of the phosphorus adducts suggest strongly shielded phosphorus centers in accord with the polarized structures.