The multielectron reduction of small molecules (e.g., CO) is a key aspect of fuel synthesis from renewable electricity. Transition metals have been researched extensively in this role due to their intrinsic redox properties and reactivity, but more recently, strategies that forego transition metal ions for p-block elements have emerged. In this vein, we report an analogue of boranthrene (9,10-diboraanthracene) stabilized by N-heterocyclic carbenes and its one- and two-electron oxidized congeners. This platform exhibits reversible, two-electron redox chemistry at mild potentials and reacts with O, CO, and ethylene via formal [4+2] cycloaddition to the central diborabutadiene core. In an area traditionally dominated by transition metals, these results outline an approach for the redox activation of small molecules at mild potentials based on conjugated, light element scaffolds.

Exposure of [K­(18-crown-6)­(THF)2]­[CPh3] (THF = tetrahydrofuran; Ph = phenyl) to an atmosphere of nitric oxide (NO) cleanly generates [K­(18-crown-6)]­[O2N2CPh3] (1) in excellent yields. A subsequent reaction of [ZnCl2(THF)2] with 3 equiv of 1 affords the C-diazeniumdiolate complex [K­(18-crown-6)]­[Zn­(O2N2CPh3)3] (2). Both 1 and 2 were characterized by 1H and 13C­{1H} NMR spectroscopy, and their structures were confirmed by X-ray crystallography. Photolysis of 2 using 371 nm light resulted in the formation of three trityl-containing products, namely, Ph3CH, 9-phenylfluorene, and Ph3CN­(H)­OCPh3 (3). In addition, we detected nitrous oxide (N2O), as well as small amounts of NO in the reaction mixture. In contrast, oxidation of 2 with 1.2 equiv of [Ag­(MeCN)4]­[PF6] resulted in the formation of O­(CPh3)2 as the major trityl-containing product; N2O was also detected in the reaction mixture, but NO was not apparently formed in this case. The observation of these fragmentation products indicates that the [O2N2CPh3]− ligand is susceptible to both C–N bond and N–N bond cleavage. Moreover, the different product distributions suggest that [O2N2CPh3]− is susceptible to different modes of fragmentation.

Reaction of MBr2 with 3 equiv of [K(18-crown-6)][O2N2CPh3] generates the trityl diazeniumdiolate complexes [K(18-crown-6)][M(O2N2CPh3)3] (M = Co, 2; Fe, 3) in good yields. Irradiation of 2 and 3 using 371 nm light led to NO formation in 10 and 1% yields (calculated assuming a maximum of 6 equiv of NO produced per complex), respectively. N2O was also formed during the photolysis of 2, in 63% yield, whereas photolysis of 3 led to the formation of N2O, as well as Ph3CN(H)OCPh3, in 37 and 5% yields, respectively. These products are indicative of diazeniumdiolate fragmentation via both C–N and N–N bond cleavage pathways. In contrast, oxidation of complexes 2 and 3 with 1.2 equiv of [Ag(MeCN)4][PF6] led to N2O formation but no NO formation, suggesting that diazeniumdiolate fragmentation occurs exclusively via C–N bond cleavage under these conditions. While the photolytic yields of NO are modest, they represent a 10- to 100-fold increase compared to the previously reported Zn congener, suggesting that the presence of a redox-active metal center favors NO formation upon trityl diazeniumdiolate fragmentation.