Origin of the Exceptional Activity in Molybdenum-Catalysed Dinitrogen Reduction with Samarium Diiodide and Water
Year: 2026
Type: preprint
Abstract: Molybdenum complexes supported by phosphine–ligand–phosphine (PLP) pincers, combined with samarium diiodide and water, convert dinitrogen to ammonia under mild conditions with nitrogenase-like activities. Using explicitly correlated coupled-cluster calculations, spin–orbit corrections, and realistic concentration and speciation models, we elucidate the cleavage mechanism from N 2 -bound Mo(I) complexes to ammonia release. Dimerisation proceeds through pentacoordinate Mo(I)–N 2 complexes, and N 2 activation is controlled by terminal N 2 dissociation, which converts a [Mo]–N≡N–[Mo] unit into cleavage-ready [Mo]=N=N=[Mo] before spin crossover and bond scission. The resulting nitrido intermediates are reduced by outer-sphere, Sm-mediated concerted proton-coupled electron transfer (cPCET). This pathway avoids the high-energy charge-separated intermediates of stepwise PT/ET and the loss of strong azaphilic Sm(III)–N interactions upon Sm–OH formation in inner-sphere alternatives. Aggregation into ammonia-binding Sm(III) hydroxide oligomers and precipitation stabilise the product states and render reduction effectively irreversible. These features explain the exceptional activity of Mo/SmI 2 /H 2 O systems. The first cPCET is rate-determining under typical conditions, whereas N 2 activation becomes limiting at low catalyst concentrations or with electron-withdrawing ligands. Pincer geometry and π-acidity facilitate cPCET, but efficient catalysis requires ligands that avoid over-stabilising low Mo oxidation states during N 2 activation while accommodating changing coordination numbers across the cycle.
Source: ChemRxiv
Institution: University of Bergen
Language: English
Subfield: Catalysis
Field: Chemical Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7213328440