Reductive coupling of nitric oxide (NO(g)) to yield nitrous oxide (N2O(g)) has been proposed to occur through the key hyponitrite (N2O22–) intermediate in both biological systems and coordination complex models of nitric oxide reductase (NOR). We have previously employed the tris-(2-pyridylmethyl)amine (TMPA) ligand framework to isolate an O,O’-bound trans-hyponitrito complex at RT in methanol that yields stoichiometric N2O(g) upon acidification. Employing secondary sphere acidic hydrogen bonding groups (NHCOCMe3 and NHCOCF3), we also demonstrated how ligand derived proton transfer facilitates N–O bond cleavage of the putative O,O’-bound trans-hyponitrite intermediate releasing N2O(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)chemical NO(g) coupling systems. However, details pertaining to the stepwise N-N bond formation event between two initial mononuclear Cu(I)-NO units and the subsequent isomerization to the O,O’-bound hyponitrite complex remained elusive. Herein we report the direct vibrational characterization of a new cupric hyponitrito intermediate detected at cryogenic temperatures prior to it’s isomerization to O,O’-bound hyponitrite. Resonance Raman spectroscopy reveals a ν(N=N) stretch at 1393 (Δ15N = -47) cm-1 for this intermediate. The insensivity of the ν(N=N) mode at 1393 cm-1 to 18O-labeling and the lack of 15N-sensitivity of the δ(ONNO) mode at 581 cm-1 is suggestive of a symmetric coordination of the bridging hyponitrite ligand. Additional isotope sensitive modes at 1117 and 1164 cm-1 are assigned to νs(N-O) and νas(N-O) modes. DFT calculations are presently underway to support a plausible and likely structural analysis of N2O22– coordination to the Cu-center/s. The brown-colored hyponitrito intermediate shows unique UV-Vis spectroscopic features at 485 nm, 630 nm and 825 nm in methanol/ethanol, bearing reverse-axial EPR signatures. The effect of intramolecular hydrogen bonding in perturbing the electronic-geometric arrangements of this novel hyponitrito complex are currently being studied by synthetically incorporating varying –NHR moieties onto the TMPA framework.

Characterization of a new cupric hyponitrito complex: Insights into the stepwise reduction pathway of nitric oxide in a Cu-chelate model of NOR

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.