Spectroscopic characterization of a Ni-organic radical intermediate in the aerobic oxidation of methanol catalyzed by a Ni(II)(polyoximate) complex

In the aerobic oxidation of methanol catalyzed by a Ni(II)(TRISOX) complex [H3TRISOX = tris(1-propan-2-onyl oxime)amine], an intermediate is observed spectroscopically. The intensities of both the UV–Vis absorption and electron paramagnetic resonance (EPR) spectra associated with this intermediate maximize during the time period of maximum formaldehyde production, and decrease as the methanol oxidation activity decreases. The UV–Vis spectrum has prominent features at 350, 420, and 535 nm. The EPR spectrum is centered at g = 2.00 and shows splittings of 28 ± 5 G. Both of these spectra are consistent with characterization of the intermediate as including one or more iminoxyl radicals derived from the oximate groups of the TRISOX ligand. Spectroscopic features very similar to those in the air-oxidized intermediate are observed in electrochemically oxidized samples, suggesting that the electrochemically generated complex will be a useful model for the intermediate observed during catalytic turnover. The crystal structure of a Ni(II) complex with an intermediate protonation state of the ligand, [Ni(II)2(H2TRISOX)2(μ2:η1-ONO2)](NO3) · (CH3CN) · 5(H2O), 4, has been structurally characterized. Comparison to the previously reported [Ni(II)(H2TRISOX)(CH3CN)]2(ClO4)2, 3, shows that bis(μ-oximate) dimers can form either with or without an additional bridging ligand. Addition of the nitrato bridge decreases the Ni–Ni distance from 3.5752(13) Å in 3 to 3.2014(4) Å in 4. It is intriguing to note that the reactions catalyzed by the Ni(II)(TRISOX) complex, the net transfer of two hydrogen atoms from an alcohol or amine substrate to O2, are the same reactions catalyzed by several different metalloenzymes that also incorporate both a redox active metal and a redox active organic component in their active sites.

Graphical abstractSpectroscopic characterization of an intermediate in the aerobic oxidation of methanol catalyzed by a Ni(II) complex of a polyoximate ligand demonstrates that the ligand oximates are involved as redox-active components of the reaction, being oxidized to iminoxyl radicals during catalytic turnover.Figure optionsDownload full-size imageDownload as PowerPoint slide