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

TL;DR

This paper explores the electronic structure and reactivity of PHOX–Ni aryne complexes to explain regioselectivity in their synthesis and functionalization.

Contribution

The study introduces a new model combining sterics and distortion to explain selectivity in metal-bound aryne reactions.

Findings

CyPHOX–Ni aryne complexes show metal-to-ligand charge transfer and oxidation potentials linked to Hammett parameters.

A new MAR/S model explains regioselectivity in CyPHOX–Ni aryne complex formation and functionalization.

Steric effects and aryne distortion govern binding selectivity, while phosphine trans influence directs functionalization selectivity.

Abstract

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.

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