Using internal electrostatic fields to manipulate the valence manifolds of copper complexes. Issue 12 (2nd February 2021) - Weberg, Alexander B.
Abstract : Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials. Abstract : A series of tetradentate tris(phosphinimine) ligands ( R3 P3 tren) was developed and bound to Cu I to form the trigonal pyramidal, C 3v -symmetric cuprous complexes [ R3 P3 tren-Cu][BAr F 4 ] (1 PR3 ) (PR3 = PMe3, PMe2 Ph, PMePh2, PPh3, PMe2 (NEt2 ), BAr F 4 = B(C6 F5 )4 ). Electrochemical studies on the Cu I complexes were undertaken, and the permethylated analog, 1 PMe3, was found to display an unprecedentedly cathodic Cu I /Cu II redox potential (−780 mV vs. Fc/Fc + in isobutyronitrile). Elucidation of the electronic structures of 1 PR3 via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy -plane that destabilize 2 e (d xy /d x 2 − y 2 ) orbital sets and uniquely stabilized a 1 (d z 2 ) orbitals. Support is provided that the a 1 stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in R3 P3 tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z -axes of 1 PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1 PMe3 to the cupric complex [ Me3 P3 tren-Cu][OTf]2 (2 PMe3 ), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo- C 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 C 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 via a method orthogonal to classic ligand field tuning.