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

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journal contribution
posted on 2020-08-04, 15:27 authored by Máté J. Bezdek, István Pelczer, Paul J. Chirik
The synthesis and characterization of a cationic molybdenum pyrrolidine complex are described that exhibits significant coordination-induced N–H bond weakening. The N–H bond dissociation free energy (BDFE) of the coordinated pyrrolidine in [(PhTpy)­(PPh2Me)2Mo­(NH­(pyrr))]­[BArF24] ([1-NH­(pyrr)]+; PhTpy = 4′-Ph-2,2′,6′,2″-terpyridine, NH­(pyrr) = pyrrolidine, ArF24 = [C6H3-3,5-(CF3)2]4) was determined to be between 41 and 51 kcal mol–1 by thermochemical analysis and supported by a density functional theory (DFT) computed value of 48 kcal mol–1. The complex [1-NH­(pyrr)]+ underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl radical, as well as spontaneous H2 evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex [(PhTpy)­(PPh2Me)2Mo­(N­(pyrr))]­[BArF24] ([1-N­(pyrr)]+). Thermolysis of the deuterated isotopologue [1-ND­(pyrr)]+ still produced H2 with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N­(pyrr-dn)]+ (n = 0–2), consistent with an H2 evolution pathway involving intramolecular H–H bond formation followed by an intermolecular product-forming PCET step. These observations provide the context for understanding H2 evolution in the nonclassical ammine complex [(PhTpy)­(PPh2Me)2Mo­(NH3)]­[BArF24] ([1-NH3]+) and are supported by DFT-computed reaction thermochemistry. Overall, these studies offer rare insight into the H2 formation pathway in nonclassical amine complexes with N–H BDFEs below the thermodynamic threshold for H2 evolution and inform the development of well-defined, thermodynamically potent PCET reagents.

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