From aminocarbene to aminocarbyne complexes: a study of an unusual transformation

Abstract

On heating in solution, certain aminocarbene complexes of chromium - which can be prepared via the addition of anionic nucleophiles to the carbyne carbon of cationic aminocarbyne complexes - spontaneously abstract one CO ligand and rearrange to yield neutral aminocarbyne complexes. This reaction constitutes a possible step in the general synthesis of carbyne complexes from carbene complexes and halides of Group IIIB elements. The rearrangement of (CO) 5Cr[C(X)NR 2] to trans-X(CO) 4Cr[CNR 2] with loss of carbon monoxide follows a first-order rate law. The activation entropy is small. Since the external CO pressure (up to 100 bar) and the addition of radical initiators, radical inhibitors and free X − ions are of no influence on the reaction rate, an intramolecular mechanism may be deduced. The rate of the rearrangement was found to be mainly determined by the bulk of the amino substituent NR 2. The results of MO calculations are in agreement with a symmetry-allowed, simultaneous CO elimination-C/Cr migration of X. In the case of a related tungsten complex with NR 2 = NEt 2 and X = P(Me)Ph, a stable tetracarbonyl complex was isolated containing a three- membered ▪ ring. This type of compound represents a likely intermediate in the carbene-carbyne complex transformation.