CO-Photolysis-Induced H‑Atom Transfer from Mn^IO–H Bonds

The formation of TEMPOH from a mixture of [Mn­(CO)3(μ3-OH)]4 (1) and (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO) is shown to occur through a light-initiated CO photolysis from 1 (illumination at 300–375 nm). One hypothesis is that the loss of carbon monoxide (CO) causes significant O–H bond weakening to render proton-coupled electron transfer (PCET) to TEMPO favorable. For instance, the ground-state O–H bond dissociation free energy (BDFEO–H) of 1 (computed with density functional theory and estimated using effective BDFE reagents) is too high to transfer an H-atom to TEMPO. We also demonstrate that TEMPO and 1 interact in the dark through a hydrogen-bonded “precomplex” (1···TEMPO). We suggest that the PCET reaction that forms TEMPOH is the result of a H-atom-transfer reaction that occurs immediately after photolysis of a CO ligand(s)