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 Cu^I to form the trigonal pyramidal, <i>C</i> _3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F ₄] (<b>1PR3</b>) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F ₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, <b>1PMe3</b>, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (-780 mV <i>vs.</i> Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of <b>1PR3</b> <i>via</i> density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the <i>xy</i>-plane that destabilize 2<i>e</i> (d _<i>xy</i> /d _{<i>x</i> ²-<i>y</i> ²} ) orbital sets and uniquely stabilized <i>a</i> ₁ (d _{<i>z</i> ²} ) orbitals. Support is provided that the <i>a</i> ₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated <i>via</i> 1-dimensional electrostatic potential maps along the <i>z</i>-axes of <b>1PR3</b> and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of <b>1PMe3</b> to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (<b>2PMe3</b>), which displays a characteristic Jahn-Teller distortion in the form of a see-saw, pseudo-<i>C</i> _s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal <i>C</i> _s-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' properties <i>via</i> a method orthogonal to classic ligand field tuning.