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 L2,3/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.