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

Abstract

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, 3Me-M; R = Ph, 3Ph-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 3Bna-K. The 3Me-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 3Me-M strongly depends on the alkali cation (k = 2.38 (±0.03) × 104 s-1 for 3Me-K and 4.69 (±0.06) × 10-6 s-1 for 3Me-Na), and a mechanistic scenario is proposed. Protonation of 3Ph-K induces disproportionation of the bound [PhNO2]2- to give free PhNO2, 4, and [LNi2(ON(H)Ph)] (2Ph-H) featuring an O-deprotonated μ-κO,κN hydroxylamine in the dinickel(II) cleft; abstraction of the cation K+ from 3Ph-K via addition of cryptand gives the analogous complex [LNi2(ONPh)][K(crypt)] (2Ph-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.