Affiliation:
1. Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812
2. Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, MN 55455
Abstract
The hydroxylation of C–H bonds can be carried out by the high-valent Co
III,IV
2
(µ-O)
2
complex
2a
supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co
III
2
(µ-O)(µ-OH) intermediate (
3a
). Complex
3a
can be independently generated either by H-atom transfer (HAT) in the reaction of
2a
with phenols as the H-atom donor or protonation of its conjugate base, the Co
III
2
(µ-O)
2
complex
1a
. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm
−1
associated with the symmetric Co–O–Co stretching mode of the Co
2
O
2
diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for
1a
and
2a
by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of
3a
allows us to investigate HAT reactions of
2a
with phenols in detail, measure the redox potential and p
K
a
of the system, and calculate the O–H bond strength (
D
O–H
) of
3a
to shed light on the C–H bond activation reactivity of
2a
. Complex
3a
is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal
2a
to be 10
6
-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe
III,IV
2
(µ-O)
2
and Mn
III,IV
2
(µ-O)
2
analogs, an unexpected outcome that raises the prospects for using Co
III,IV
2
(µ-O)
2
species to oxidize alkane C–H bonds.
Funder
HHS | National Institutes of Health
National Science Foundation
Publisher
Proceedings of the National Academy of Sciences