Search for: All records

Coordination of the amine-rich Cp^N3ligand to cobalt is reported. Ligand displacement leads to ligand protonation and evaluation of the metal-free C–H bond thermochemistry, revealing a weak homolytic C–H bond dissociation free energy (52 kcal mol⁻¹). 

Cyclopentadienyl (Cp), a classic ancillary ligand platform, can be chemically noninnocent in electrocatalytic H−H bond formation reactions via protonation of coordinated η5-Cp ligands to form η4-CpH moieties. However, the kinetics of η5-Cp ring protonation, ligand-to-metal (or metal-to-ligand) proton transfer, and the influence of solvent during H2 production electrocatalysis remain poorly understood. We report in-depth kinetic details for electrocatalytic H2 production with Fe complexes containing amine-functionalized CpN3 ligands that are protonated via exogenous acid to generate via η4-CpN3H intermediates (CpN3 = 6-amino-1,4-dimethyl-5,7-diphenyl-2,3,4,6-tetrahydrocyclopenta[b]pyrazin-6-yl). Under reducing conditions, state-of-the-art DFT calculations reveal that a coordinated solvent plays a crucial role in mediating stereo- and regioselective proton transfer to generate (endo-CpN3H)Fe(CO)2(NCMe), with other protonation pathways being kinetically insurmountable. To demonstrate regioselective endo-CpN3H formation, the isoelectronic model complex (endo-CpN3H)Fe(CO)3 is independently prepared, and kinetic studies with the on-cycle hydride intermediate CpN3FeH(CO)2 under CO cleanly furnish the ring-activated complex (endo-CpN3H)Fe(CO)3 via metal-to-ligand proton migration. The on-cycle complex CpN3FeH(CO)2 reacts with acid to release H2 and regenerate [CpN3Fe(CO)2(NCMe)]+, which was found to be the TOF-determining step via DFT. Collectively, these experimental and computational results underscore the emerging importance of Cp ring activation, inner-sphere solvation, and metal−ligand cooperativity to perform proton-coupled electron transfer catalysis for chemical fuel synthesis.