Rhodium(III) and Iridium(III) Complexes of a NHC-BasedMacrocycle: Persistent Weak Agostic Interactions and Reactions withDihydrogen

摘要

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) 2,2′-biphenyl complexes [M(CNC-12)(biph)][BAr^F4] (M = Rh (>1a), Ir (>1b)), featuring the macrocyclic lutidine- and NHC-based pincer ligand CNC-12 are reported. In the solid state these complexes are notable for the adoption of weak ε-agostic interactions that are characterized by M···H–C contacts of ca. 3.0 Å by X-ray crystallography and ν(CH) bands of reduced wavenumber by ATR IR spectroscopy. Remarkably, these interactions persist on dissolution and were observed at room temperature using NMR spectroscopy (CD2Cl2) and solution-phase IR spectroscopy (CCl4). The associated metrics point toward a stronger M···H–C interaction in the iridium congener, and this conclusion is borne out on interrogation of >1 in silico using DFT-based NBO and QTAIM analyses. Reaction of >1 with dihydrogen resulted in hydrogenolysis of the biaryl and formation of fluxional hydride complexes, whose ground state formulations as [Rh(CNC-12)H2][BAr^F4] (>2a″) and [Ir(CNC-12)H2(H2)][BAr^F4] (>2b‴) are proposed on the basis of inversion recovery and variable-temperature NMR experiments, alongside a computational analysis. Reactions of >1 and >2 with carbon monoxide help support theirrespective structural properties.