Synthesis, electronic structure and reactivity of rhodium(I) and iridium(I) bis(imino)acenaphthene complexes

Abstract

The bis(arylimino)acenaphthalene (BIAN) group of α-diimine compounds has shown promising electron sink behaviour and redox non-innocent activity as ligands to main group and transition metals. Here we present a series of rhodium(I) and iridium(I) complexes of BIAN derivatives with 2,6-diisopropylphenyl (DippBIAN, DB) and 2,4,6-trimethylphenyl (MesBIAN, MB), featuring various electron withdrawing and donating ancillary ligands. The crystal structures of square planar complexes 1b [RhMB(cod)]PF6, 1d [IrMB(cod)]PF6, 2a [RhDB(CO)2]PF6, 2b [RhMB(CO)2]PF6, 2c [IrDB(CO)2]PF6 and 3a [RhDB(CO)(PEt3)]PF6 are reported, as well as the square pyramidal structure of 4c [IrDB(cod)Cl]. The C–N and C–C bond lengths within the bisimine moiety for the structures of 2a and 4c suggest that the DippBIAN ligands are present in a reduced state, however, all other data from our multi-technique analyses correspond to results for neutral BIAN ligands. These contrasting results are indicative of the DippBIAN’s non-innocent behaviour, accepting additional electron density from the metal centre due to push-pull mechanism between the ancillary and BIAN ligands. The electrochemical study in non-coordinating solvent CH2Cl2 revealed that all complexes featured at least one reversible, ligand-centred reduction event at less negative potentials (above –1.0 V vs Ag/Ag+). In addition, preliminary results from our electrocatalytic CO2 reduction study has shown a promising interaction between CO2 and complex 3a, paving the way for exploring heterogeneous catalysis on these class of compounds. In addition, the statistically-based Peak Characterisation Model (PCM) was developed from Linear Free Energy Relationships (LFERs) between experimentally recorded DPV peak potentials, and various calculated theoretical properties of each reported compound. The PCM aided differentiation between voltammetric peaks associated with the primary electrochemical reaction of the analyte and voltammetry peaks of impurities or secondary reactions in the electrolyte solution.