Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes

Abstract

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds (^Mesdmx, ^tBudmx: dimethylxanthine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes (^Rdmx)Cu₂(μ²-NAr) (R: Mes, ^tBu; Ar: 4-MeOC₆H₄, 3,5-(F₃C)₂C₆H₃) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of (^Mesdmx)Cu₂(μ²-N(C₆H₄OMe)) with potassium graphite initiates an intramolecular, benzylic C-H amination at room temperature, chemical reduction of (^tBudmx)Cu₂(μ²-NAr) leads to isolable [(^tBudmx)Cu₂(μ²-NAr)]^- product salts. The electronic structures of the thermally robust [(^tBudmx)Cu₂(μ²-NAr)]^0/- complexes were assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L_2,3/K-edge, N K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence complexes of the type [(^Rdmx)Cu₂(μ²-NAr)]^- feature significant NAr-localized spin following reduction from electronic population of the [Cu₂(μ²-NAr)] π* manifold, contrasting previous methods for engendering iminyl character through chemical oxidation. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g., nitrene transfer and H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N-radical character afforded from different aryl substituents.