X‑ray Crystallographic, Multifrequency Electron Paramagnetic Resonance, and Density Functional Theory Characterization of the Ni(P^Cy₂N^<i>t</i>Bu₂)₂^<i>n</i>+ Hydrogen Oxidation Catalyst in the Ni(I) Oxidation State

The Ni­(I) hydrogen oxidation catalyst [Ni­(PCy2NtBu2)2]+ (1+; PCy2NtBu2 = 1,5-di­(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane) has been studied using a combination of electron paramagnetic resonance (EPR) techniques (X-, Q-, and D-band, electron–nuclear double resonance, hyperfine sublevel correlation spectroscopy), X-ray crystallography, and density functional theory (DFT) calculations. Crystallographic and DFT studies indicate that the molecular structure of 1+ is highly symmetrical. EPR spectroscopy has allowed determination of the electronic g tensor and the spin density distribution on the ligands, and revealed that the Ni­(I) center does not interact strongly with the potentially coordinating solvents acetonitrile and butyronitrile. The EPR spectra and magnetic parameters of 1+ are found to be distinctly different from those for the related compound [Ni­(PPh2NPh2)2]+ (4+). One significant contributor to these differences is that the molecular structure of 4+ is unsymmetrical, unlike that of 1+. DFT calculations on derivatives in which the R and R′ groups are systematically varied have allowed elucidation of structure/substituent relationships and their corresponding influence on the magnetic resonance parameters.