Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R-NHC, X-NHC and X-X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study.

The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M/M catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, M/M catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R] cations. In this study, the relative stabilities of (NHC)M(R)(X)L and (NHC)M(R)(X)L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M and M), while it is thermodynamically and kinetically more facile for the M complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the M complexes were significantly stronger than that for the M complexes. In particular, for the (NHC)M(Ph)(Br) complexes, Ph-NHC coupling was facilitated dramatically from Pt (ΔG = -36.9 kcal mol, ΔG = 37.5 kcal mol) to Pd (ΔG = -61.5 kcal mol, ΔG = 18.3 kcal mol) and Ni (ΔG = -80.2 kcal mol, ΔG = 4.7 kcal mol). For the M oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the M oxidation state of the metal as the (NHC)M(R)(X) complexes were kinetically (4.3-15.9 kcal mol) and thermodynamically (8.0-23.2 kcal mol) significantly less stable than the (NHC)M(R)(X)L (L = pyridine) complexes. For the Ni and Pd complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X] cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising M intermediates.