Crystallographic, Electronic Structure, and Computational Studies of PHOX–Ni Aryne Complexes: Origins of Regioselectivity in Metal-Bound Aryne Synthesis and Difunctionalization

Late transition metal aryne complexes are stable, isolable counterparts to free aryne intermediates. However, their utility has largely been limited since the Aryne Distortion Model (ADM) cannot be applied to substituted aryne complex reactivity, leading to nonselective reactions. Our group recently reported the first regioselective synthesis and difunctionalization of a CyPHOX–Ni o-methoxybenzyne complex. However, to increase the utility of these complexes in synthesis, their electronic structure, reactivity, and the impact of aryne substituents on selectivity must be understood. Herein, we report the first comprehensive experimental electronic structure study of aryne complexes, which has been carried out via UV/vis spectroscopy and cyclic voltammetry (CV) with an array of o-substituted arynes. CyPHOX–Ni aryne complexes exhibit a metal-to-ligand charge transfer (MLCT), and this transition as well as their oxidation potentials trend with Hammett parameters for the aryne substituents. To gain further insight into the origins of regioselectivity in CyPHOX–Ni aryne complex formation and difunctionalization, a combination of single-crystal X-ray crystallographic and density functional theory (DFT) structural studies were carried out. Our findings lead us to propose a Metal Aryne Reactivity/Selectivity (MAR/S) Model, which shows that CyPHOX–Ni aryne binding selectivity is governed by a combination of sterics and aryne distortion, whereas selectivity in functionalizations is directed by the phosphine trans influence.