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

Boyd, Emily A. and Peters, Jonas C. (2022) Sm(II)-Mediated Proton-Coupled Electron Transfer: Quantifying Very Weak N-H and O-H Homolytic Bond Strengths and Factors Controlling Them. Journal of the American Chemical Society, 144 (46). pp. 21337-21346. ISSN 0002-7863. doi:10.1021/jacs.2c09580.

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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.

Item Type:Article
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URLURL TypeDescription
Boyd, Emily A.0000-0003-0150-5396
Peters, Jonas C.0000-0002-6610-4414
Additional Information:E.A.B. acknowledges the NSF for support via a Graduate Research Fellowship (Grant No. DGE-1745301). The Beckman Institute at Caltech supports the X-ray crystallography facility. This study was supported by funding through the NIH (GM070757).
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1745301
Issue or Number:46
Record Number:CaltechAUTHORS:20221128-494241100.47
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118085
Deposited By: Research Services Depository
Deposited On:21 Dec 2022 17:19
Last Modified:21 Dec 2022 17:19

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