1,2,4-triazol-3-ylidenes with an N-2,4-dinitrophenyl substituent as strongly π-accepting N-heterocyclic carbenes

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4-triazol-3-ylidenes with a N-2,4-dinitrophenyl (N-DNP) substituent are described. IR, NMR, single-crystal X-ray diffraction and computational analyses of the Rh complexes revealed that the N-heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE _bb) to the bond dissociation energies (BDE) of the Rh-C _carbene bond for [RhCl(NHC)(cod)] (cod=1,5-cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2-methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom. π-Acidic carbene complexes: A series of Rh and Au complexes bearing 1,2,4-triazol-3- ylidenes with a N-2,4-dinitrophenyl (DNP) substituent were synthesised. Experimental and theoretical analyses revealed that these N-heterocyclic carbenes (NHCs) behave as strong π acceptors and weak σ donors, and the metal centers exibit enhanced π acidity compared to complexes with traditional σ-donating NHCs (see figure, EWG=electron withdrawing group).