Redox Cascades and Making of a C–C Bond: 1,2-Benzodiazinyl Radicals and a Copper Complex of a Benzodiazine

Two 1,2-benzodiazinyl radicals, cinnolinyl radicals by name, were successfully isolated by cascade routes using 1,4-naphthoquinone as a precursor. Reaction of 1,4-naphthoquinone with hydrazine hydrate promotes a (5e + 5H⁺) redox cascade affording benzo­[g]­naphtho­[1,2-c]­cinnolinyl-7,12,14-trione (Cn^•) in 69% yields, while the similar reaction with 2-hydrazinopyridine is a (7e + 7H⁺) oxidative cascade and furnishes N-pyridinecinnolinyl radical (^PyCn^•). The cascades are composed of C–N and C–C bond making reactions. The neutral even alternate arenes are always diamagnetic; thus, the isolation of Cn^• and ^PyCn^• is a breakthrough. The Cn^•/Cn^– and ^PyCn^•/^PyCn^– redox couples are reversible, and the reaction of Cn^• with [Cu^I(PPh₃)₃Cl] in the presence of hydrazine hydrate and Et₃N affords a Cn^– complex of copper­(I), [(Cn^–)­Cu^I(PPh₃)₂] (1). Similar to phenalenyl radical, ^PyCn^• exists in three redox states, ^PyCn⁺, ^PyCn^•, and ^PyCn^–, in a smaller potential range (−0.30 V to −0.60 V vs Fc⁺/Fc couple) and can be used as an oxidant as well as a reductant. ^PyCn^• acts as a catalyst for the oxidative cleavages of benzil to benzoic and 2,2′-pyridil to picolinic acids in methanol in the presence of air. The molecular and electronic structures of Cn^•, ^PyCn^•, and 1·1/2MeOH were confirmed by single crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.