Metallacarbenes from Diazoalkanes: An Experimental and Computational Study of the Reaction Mechanism

PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ({3-[(di-tert-butyl-phosphanyl)-methyl]-benzyl}-diethyl-amine) based rhodium dinitrogen complexes (1 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN−Rh carbene complexes
(3 and 5, respectively). At low temperature (−70 °C), PCP and PCN phenyl diazomethane complexes (4
and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the
diazo moiety is η1 coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6
at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the
major products of decomposition being the dinitrogen complexes 1 and 2 and stilbene. This and competition
experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate
under the reaction conditions and attack the metal center through the diazo carbon producing a η1-C bound
diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In
the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are
possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr
substituents, the η1-C bound diazo complex appears to be a critical intermediate for carbene complex
formation, in good agreement with the experimental results. Our results and the analysis of reported data
suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can
be anticipated considering steric requirements relevant to η1-C diazo complex formation.