Coordination-Induced N–H Bond Weakening in a Molybdenum Pyrrolidine Complex: Isotopic Labeling Provides Insight into the Pathway for H₂ Evolution Article Swipe

The synthesis and characterization of a cationic molybdenum pyrrolidine complex is described that exhibits significant coordination-induced N–H bond weakening. Here, the N–H bond dissociation free energy (BDFE) of the coordinated pyrrolidine in [(<sup>Ph</sup>Tpy)(PPh<sub>2</sub>Me)<sub>2</sub>Mo(NH(pyrr))][BArF<sup>24</sup>] ([1-NH(pyrr)]<sup>+</sup>; PhTpy = 4'-Ph-2,2',6',2''-terpyridine, NH(pyrr) = pyrrolidine, 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 determined to be between 41–51 kcal mol<sup>-1</sup> by thermochemical analysis and supported by a density functional theory (DFT) computed value of 48 kcal mol<sup>-1</sup>. The complex [1-NH(pyrr)]<sup>+</sup> underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl radical, as well as spontaneous H<sub>2</sub> evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex, [(<sup>Ph</sup>Tpy)(PPh<sub>2</sub>Me)<sub>2</sub>Mo(N(pyrr))][BArF<sup>24</sup>] ([1-N(pyrr)]<sup>+</sup>). Thermolysis of the deuterated isotopolog, [1-ND(pyrr)]<sup>+</sup> still produced H<sub>2</sub> with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N(pyrr-d<sub>n</sub>)]<sup>+</sup> (n = 0–2), consistent with an H<sub>2</sub> evolution pathway involving intramolecular H–H bond formation followed by an intermolecular product-forming PCET step. These observations provide context for understanding H<sub>2</sub> evolution in the nonclassical ammine complex [(<sup>Ph</sup>Tpy)(PPh<sub>2</sub>Me)<sub>2</sub>Mo(NH<sub>3</sub>)][BArF<sup>24</sup>] ([1-NH<sub>3</sub>]<sup>+</sup>) and are supported by DFT-computed reaction thermochemistry. Overall, these studies offer rare insight into the H<sub>2</sub> formation pathway in nonclassical amine complexes with N–H BDFEs below the thermodynamic threshold for H<sub>2</sub> evolution and inform the development of well-defined, thermody-namically potent PCET reagents.