Rhodium(III) and Iridium(III) Complexes of a NHC-Based Macrocycle: Persistent Weak Agostic Interactions and Reactions with Dihydrogen

Abstract

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) 2,2'-biphenyl complexes [M(CNC-12)(biph)][BAr^F ₄] (M = Rh (<b>1a</b>), Ir (<b>1b</b>)), 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 <i>M</i>···H-<i>C</i> 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 <b>1</b> in silico using DFT-based NBO and QTAIM analyses. Reaction of <b>1</b> with dihydrogen resulted in hydrogenolysis of the biaryl and formation of fluxional hydride complexes, whose ground state formulations as [Rh(CNC-12)H₂][BAr^F ₄] (<b>2a″</b>) and [Ir(CNC-12)H₂(H₂)][BAr^F ₄] (<b>2b‴</b>) are proposed on the basis of inversion recovery and variable-temperature NMR experiments, alongside a computational analysis. Reactions of <b>1</b> and <b>2</b> with carbon monoxide help support their respective structural properties.