Multifrequency EPR Studies of [Cu^(1.5)Cu^(1.5)]+ for Cu_2(μ-NR_2)_2 and Cu_2(μ-PR_2)_2 Diamond Cores

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-tert-butylsulfanylphenyl)amide (SNS), bis(2-di-iso-butylphosphinophenyl)amide (PNP), and bis(2-di-iso-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 {Cu_2(μ-XR_2)_2}^+ diamond core has been assigned in which removal of an electron from the neutral {Cu_2(μ-XR_2)_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.