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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
journal contribution
posted on 2020-08-04, 15:27 authored by Máté
J. Bezdek, István Pelczer, Paul J. ChirikThe 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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H 2 EvolutionBDFEmolybdenum pyrrolididenonclassical ammineDFT-computed reaction thermochemistryArF 24H 2 formation pathwayH 2 evolution pathwayH 2 evolutionunderstanding H 2 evolutionH 2nonclassical amine complexesbondkcalproton-coupled electron transferthermochemical analysisstudies offerNHNDCFPh Tpypyrrolidide ligandProvides InsightPCET reagentscationic molybdenum pyrrolidineMolybdenum Pyrrolidine Complexproduct-forming PCET step
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