Theoretical study on interactions of N-heterocyclic carbene with the bare first-row transition metals

Abstract

The electronic and structural features of the free and substituted N-heterocyclic carbenes (NHCs) and their complexes with the bare first-row transition metals (TMs) have been explored theoretically. The present results indicate that the electron-donating behavior of NHC can be tuned by introduction of the substituent, and the NHC–TM σ-bonding interactions strongly depend on the d-electron configuration of TMs. The B3LYP- and CCSD(T)-predicted bond dissociation energies show a double-peak profile, and the maxima appear in [NHC–V] (26.2 kcal/mol) and [NHC–Ni] (29.7 kcal/mol) with the stable half- and full-filled d configurations, respectively, after accepting the σ-lone pair of NHC. Theoretical calculations on the η2-type structure of [NHC–TM] from π interactions between TM and C=C double bond in NHC are also done by B3LYP-D3. For Zn with d 10, no σ-type structure was located, and all geometry optimization attempts converge to only metastable η6-type configuration, arising from the weak aromatic π interactions. For other TMs, the π interactions between the double C=C bond and TM are responsible for their η2-type structures. The σ- and π-type [NHC–TM] complexes can accommodate additional NHC ligand to form stable [NHC–TM–NHC], and the introduction of second NHC can enhance the bonding interactions between TM and NHCs for most of the bare TMs, showing cooperation effect of the multiple NHC ligands on the TM–NHC bonding.