Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

Abstract

A series of tetradentate tris(phosphinimine) ligands (R3P3tren) was developed and bound to CuIto form the trigonal pyramidal,C3v-symmetric cuprous complexes [R3P3tren-Cu][BArF4] (1PR3) (PR3= PMe3, PMe2Ph, PMePh2, PPh3, PMe2(NEt2), BArF4= B(C6F5)4). Electrochemical studies on the CuIcomplexes were undertaken, and the permethylated analog,1PMe3, was found to display an unprecedentedly cathodic CuI/CuIIredox potential (−780 mVvs.Fc/Fc+in isobutyronitrile). Elucidation of the electronic structures of1PR3viadensity functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in thexy-plane that destabilize 2e(dxy/dx2−y2) orbital sets and uniquely stabilizeda1(dz2) orbitals. Support is provided that thea1stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties inR3P3tren. This view is corroboratedvia1-dimensional electrostatic potential maps along thez-axes of1PR3and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of1PMe3to the cupric complex [Me3P3tren-Cu][OTf]2(2PMe3), which displays a characteristic Jahn-Teller distortion in the form of a see-saw, pseudo-Cs-symmetric geometry. A systematic anodic shift in the potential of the CuI/CuIIredox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the idealCs-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' propertiesviaa method orthogonal to classic ligand field tuning.