Origin of the Exceptional Activity in Molybdenum-Catalysed Dinitrogen Reduction with Samarium Diiodide and Water
- 1Department of Chemistry, University of Bergen, Allégaten 41, Bergen, Norway, 5072
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.
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BibTeX
@misc{himmelstrup2026origin,
title = {{Origin of the Exceptional Activity in Molybdenum-Catalysed Dinitrogen Reduction with Samarium Diiodide and Water}},
author = {Jonas Himmelstrup and Leif E. Hertwig and Vidar Remi Jensen},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15008872/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008872/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Origin of the Exceptional Activity in Molybdenum-Catalysed Dinitrogen Reduction with Samarium Diiodide and Water
AU - Jonas Himmelstrup
AU - Leif E. Hertwig
AU - Vidar Remi Jensen
PY - 2026
DA - 2026/09/15
PB - ChemRxiv
DO - 10.26434/chemrxiv.15008872/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008872/v1
AB - 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.
ER -