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).