Terpyridine Molybdenum Dinitrogen Chemistry: Synthesis of Dinitrogen Complexes That Vary by Five Oxidation States

A bimetallic molybdenum complex bridged by an activated dinitrogen ligand and supported by phosphine and terpyridine ligands, [{(<sup>Ph</sup>Tpy)­(PPh<sub>2</sub>Me)<sub>2</sub>Mo}<sub>2</sub>(μ<sub>2</sub>-N<sub>2</sub>)]­[BArF<sup>24</sup>]<sub>2</sub> [<sup>Ph</sup>Tpy = 4′-Ph-2,2′,6′,2″-terpyridine; ArF<sup>24</sup> = (C<sub>6</sub>H<sub>3</sub>-3,5-(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>], was synthesized and structurally characterized, and its electronic structure was determined using a combination of experimental and density functional theory computational methods. Each molybdenum atom is best described as molybdenum­(II) bridged by a modestly activated [N<sub>2</sub>]<sup>2–</sup> ligand. The cyclic voltammogram of [{(<sup>Ph</sup>Tpy)­(PPh<sub>2</sub>Me)<sub>2</sub>Mo}<sub>2</sub>(μ<sub>2</sub>-N<sub>2</sub>)]<sup>2+</sup> displays two reversible reductive and two reversible oxidative features, prompting the preparation and characterization of a series of molybdenum dinitrogen compounds spanning five oxidation states ([{(<sup>Ph</sup>Tpy)­(PPh<sub>2</sub>Me)<sub>2</sub>Mo}<sub>2</sub>(μ<sub>2</sub>-N<sub>2</sub>)]­[BArF<sup>24</sup>]<sub><i>n</i></sub>, where <i>n</i> = 4, 3, 2, 1, 0). Raman and <sup>15</sup>N NMR spectroscopic data establish that the bridging nitrogen ligand remains intact across the redox series. Electron paramagnetic resonance spectroscopy was used to probe the nature of the unpaired electron in the mixed-valent electronic oxidized and reduced products. The singly occupied molecular orbital is principally metal-based in [{(<sup>Ph</sup>Tpy)­(PPh<sub>2</sub>Me)<sub>2</sub>Mo}<sub>2</sub>(μ<sub>2</sub>-N<sub>2</sub>)]<sup>3+</sup> and ligand-localized in [{(<sup>Ph</sup>Tpy)­(PPh<sub>2</sub>Me)<sub>2</sub>Mo}<sub>2</sub>(μ<sub>2</sub>-N<sub>2</sub>)]<sup>+</sup>.