Reduction of small molecule substrates such as N2 and CO2 to form value-added products hinges on efficient, selective delivery of multiple protons and electrons. Proton-coupled electron transfer (PCET) strategies focused on forming weak X–H bonds have advanced catalyst development in these reactions. Recently it has been demonstrated that coordination of alcohols to the single-electron reductant samarium diiodide (SmI2) weakens their O–H bonds to form strong PCET reagents capable of transferring a hydrogen atom to organic substrates for which electron transfer alone is not feasible (lactones, arenes). Unlike most strong reductive PCET reagents, SmI2(H2O)n(THF)m is kinetically stable to hydrogen evolution despite estimates of &gt;50 kcal mol–1 driving force for H2 release. However, the unknown speciation of SmI2 in THF/alcohol mixtures limits reliable thermodynamic analyses. Moreover, all reported PCET reactions of SmI2 with alcohols are stoichiometric in Sm due to the inertness of the insoluble, multimeric [SmIII–OR] products of PCET. Here, a bulky, strongly chelating macrocyclic ligand maintains solubility and eliminates dimerization pathways in a well-defined functional model for the PCET reactivity of SmII with coordinating proton sources. Electrochemical and thermochemical studies of this system will be presented. From this data, new strategies for electrocatalytic turnover of SmII-based PCET reagents will be discussed.

Development of well-defined Sm(II) complexes for PCET reactivity with proton sources

ACS Spring 2022

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