Reductive coupling of nitric oxide (NO(g)) to give nitrous oxide (N2O(g)) has been proposed to occur through the key hyponitrite (N2O22–) intermediate in both biological (i.e., nitric oxide reductase (NOR)) systems and coordination complex models. In our pursuit to understand the factors that dictate the formation, stability and reactivity of this hyponitrite intermediate, we have systematically added and varied the secondary coordination sphere of the (tris-(2-pridylmethyl)amine (TMPA) ligand framework with acidic and non-acidic H-bonding groups and studied their CuI/NO chemistry. Incorporation of an acidic H-bonding moiety (NHCOCMe3) on the TMPA framework reveals detailed insights into the mechanistic cycle of conversion of the trans-hyponitrite (N2O22–) intermediate to further downstream decomposed products; i.e., deprotonation of the H-bonding -NH moiety leads to intramolecular transfer of two protons to N2O22–, thereby effecting the overall 2H+/2e– reductive coupling of NO producing near-quantitative yields of N2O(g) and H2O. Crystallographic evidence for the final, deprotonated LCuII complex is also obtained. As expected, the CF3 analog (cf. CMe3) reacts even faster with NO(g), and the N2O22– intermediate is not at all observed, highlighting the cruciality of the pKa value of the NH-functionality in N–N bond formation and/or N–O bond cleavage. However, employment of non-acidic groups is shown to stabilize the hyponitrite intermediate without further decay, as studied with TMPA derivatives possessing one and two NH2 groups. These hyponitrite species also show substantial reversible binding of carbon-dioxide at room temperature, possibly forming a Lewis adduct which might open up new avenues to perturb the hyponitrite moiety with varied Lewis acids and tune their reactivity. Possible oxene/oxo transfer reactivities of the coordinated hyponitrite are also being studied currently.

Hydrogen bonding vs acidity: Ligand tuning alters CuI/NO reductive coupling product in a Cu-chelate model of NOR

2019 Spring National Meeting

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