Multifrequency EPR Studies of [Cu^1.5Cu^1.5]⁺ for Cu₂(μ-NR₂)₂ and Cu₂(μ-PR₂)₂ Diamond Cores

Abstract

Multifrequency electron paramagnetic resonance (EPR) spectroscopy is used to explore the electronic structures of a series of dicopper complexes of the type {(LXL)Cu}₂⁺. These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two [LXL]⁻ ligands featuring bridging amido or phosphido ligands and associated thioether or phosphine chelate donors. Specific chelating [LXL]⁻ ligands examined in this study include bis(2-<i>tert</i>-butylsulfanylphenyl)amide (SNS), bis(2-di-<i>iso</i>-butylphosphinophenyl)amide (PNP), and bis(2-di-<i>iso</i>-propylphosphinophenyl)phosphide (PPP). To better map the electronic coupling to copper, nitrogen, and phosphorus in these complexes, X-, S-, and Q-band EPR spectra have been obtained for each complex. The resulting EPR parameters implied by computer simulation are unusual for typical dicopper complexes and are largely consistent with previously published X-ray absorption spectroscopy and density functional theory data, where a highly covalent {Cu₂(μ-XR₂)₂}⁺ diamond core has been assigned in which removal of an electron from the neutral {Cu₂(μ-XR₂)₂} can be viewed as ligand-centered to a substantial degree. To our knowledge, this is the first family of dicopper diamond core model complexes for which the compendium of X-, S-, and Q-band EPR spectra have been collected for comparison to Cu_A.