Reductive Binding of Nitro Substrates at a Masked Dinickel(I) Complex and Proton-Coupled Conversion to Reduced...

The transition-metal-mediated reductive activation of nitro compounds and subsequent proton-coupled N-O bond cleavage reactions are key steps of important processes such as the commercially relevant conversions of nitroaryls to aniline derivatives. Here we report the reactivity of selected nitro substrates RNO₂ (R = Me, Ph, p-C₆H₄CHO) with pyrazolate-based dinickel(II) dihydride complexes [ML(NiH)₂] (M = Na, K); the latter eliminate H₂ upon substrate binding and serve as a masked dinickel(I) platform. The products [MLNi₂(O₂NR)] (R = Me, 3-M; R = Ph, 3-M) host a μ-κO,κO' bridging twice deprotonated dihydroxy amine [RNO₂]^2- within the dinickel pocket, and structural analysis as well as NMR evidence show that the alkali cation (Na⁺ or K⁺) is closely associated with the reduced substrate. In the case of p-nitrobenzaldehyde, chemoselective reduction of the nitro group is observed to give 3-K. The 3-M complexes in solution are unstable and show first order decay to a mixture of complexes [LNi₂(μ-OH)] (4) and [LNi₂(ON═CH₂)] (5), with the latter containing a μ-κO,κN formaldoximato ligand. The decay rate of 3-M strongly depends on the alkali cation (k = 2.38 (±0.03) × 10⁴ s^-1 for 3-K and 4.69 (±0.06) × 10^-6 s^-1 for 3-Na), and a mechanistic scenario is proposed. Protonation of 3-K induces disproportionation of the bound [PhNO₂]^2- to give free PhNO₂, 4, and [LNi₂(ON(H)Ph)] (2-H) featuring an O-deprotonated μ-κO,κN hydroxylamine in the dinickel(II) cleft; abstraction of the cation K⁺ from 3 via addition of cryptand gives the analogous complex [LNi₂(ONPh)][K(crypt)] (2-K[crypt]) with a twice deprotonated hydroxylamine ligand. The results are discussed in light of the intermediates that are proposed to be relevant in the sequence of nitro group reduction and protonation steps, as implicated in the conversion of nitroaryls to anilines.