Sm(II)-Mediated Proton-Coupled Electron Transfer: Quantifying Very Weak N-H and O-H Homolytic Bond Strengths and Factors Controlling Them

Abstract

Coordination of alcohols to the single-electron reductant samarium diiodide (SmI₂) results in substantial O–H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI₂ in tetrahydrofuran (THF)/alcohol mixtures limits reliable thermodynamic analyses of such systems. Rigorous determination of bond dissociation free energy (BDFE) values in such Sm systems, important to evaluating their reactivity profiles, motivates studies of model Sm systems where contributing factors can be teased apart. Here, a bulky and strongly chelating macrocyclic ligand ((^(tBu2)ArOH)₂Me₂cyclam) maintains solubility, eliminates dimerization pathways, and facilitates clean electrochemical behavior in a well-defined functional model for the PCET reactivity of Sm^(ɪɪ) with coordinating proton sources. Direct measurement of thermodynamic parameters enables reliable experimental estimation of the BDFEs in 2-pyrrolidone and MeOH complexes of ((^(tBu2)ArO)₂Me₂cyclam)Sm^(ɪɪ), thereby revealing exceptionally weak N–H and O–H BDFEs of 27.2 and <24.1 kcal mol⁻¹, respectively. Expanded thermochemical cycles reveal that this bond weakening stems from the very strongly reducing Sm^(ɪɪ) center and the formation of strong Sm^(ɪɪɪ)–alkoxide (and –pyrrolidonate) interactions in the PCET products. We provide a detailed analysis comparing these BDFE values with those that have been put forward for SmI₂ in THF in the presence of related proton donors. We suggest that BDFE values for the latter systems may in fact be appreciably higher than the system described herein. Finally, protonation and electrochemical reduction steps necessary for the regeneration of the PCET donors from Sm^(ɪɪɪ)–alkoxides are demonstrated, pointing to future strategies aimed at achieving (electro)catalytic turnover using Sm^(ɪɪ)-based PCET reagents.