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

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.