Transmetalation of Acyclic Tungsten Carbenes to Coinage Metals: Distinct Behavior of Silver toward Carbene Transfer and Hydrolysis

The transmetalation of acyclic monoamino and alkoxo carbenes from tungsten to group 11 metals has been studied for all three metals by analyzing the new metal carbenes formed and also their decomposition products in solution. The ease of transmetalation follows the trend Au > Cu > Ag, and the gold carbenes can be isolated: a copper carbene could be detected, but we found no experimental evidence of the formation of any silver carbene. The electrophilic character of copper and gold carbenes is supported by the identity of the products formed upon hydrolysis of the carbenes involved. Silver shows a distinct behavior, and the presence of Ag+ ions in an acetonitrile solution of a tungsten aminocarbene leads to the formal protonation of the carbene fragment to give an iminium salt. This outcome, a seemingly nucleophilic behavior, is unexpected for group 11 carbenes, but DFT calculations show that the protonation of a putative cationic silver carbene is energetically accessible.