Dalton Transactions
Communication
via a proton coupled electron transfer mechanism based on
the rate dependence of the reaction on the one-electron oxi-
dation potentials of the phenol substrates, as well as deuter-
ium kinetic isotope effects of magnitude less than 2. Thus,
9 J. Ralph, K. Lundquist, G. Brunow, F. Lu, H. Kim,
P. F. Schatz, J. M. Marita, R. D. Hatfield, S. a. Ralph,
J. H. Christensen and W. Boerjan, Phytochem. Rev., 2004, 3,
29–60.
while the oxygen atoms of the CuIII(µ-O)2NiIII core in 2 are 10 R. Vanholme, K. Morreel, J. Ralph and W. Boerjan, Curr.
nucleophilic, they prefer to oxidize phenol by a concerted
Opin. Plant Biol., 2008, 11, 278–285.
PCET mechanism similar to what has been observed before for 11 G. W. Morrow, Anodic oxidation of oxygen-containing com-
the corresponding CuIII(µ-O)2CuIII species involving electro-
philic oxygen atoms. In contrast, for complex 1, which differs
from 2 with respect to the substitution pattern of the ligand
pounds, in Organic Electrochemistry, ed. H. Lund and
O. Hammerich, Marcel Dekker, New York, 4th edn, 2001,
pp. 589–620.
attached to the Cu center, but possesses identical spectro- 12 M. H. V. Huynh and T. J. Meyer, Chem. Rev., 2007, 107,
scopic properties, both HAT and PCET mechanisms may be 5004–5064.
feasible for the oxidation of phenols. Specifically, the oxidation 13 D. E. Lansky and D. P. Goldberg, Inorg. Chem., 2006, 45,
of 2,4-di-tert-butylphenol proceeds by a HAT mechanism, while 5119–5125.
oxidations of 2,6-di-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 14 J. Cho, J. Woo, J. Eun Han, M. Kubo, T. Ogura and W. Nam,
4-phenylphenol and 4-phenoxyphenol proceed by a PCET Chem. Sci., 2011, 2, 2057–2062.
mechanism. The different mechanisms observed for the oxi- 15 T. Osako, K. Ohkubo, M. Taki, Y. Tachi, S. Fukuzumi and
dation of 2,4-di-tert-butylphenol by 1 and 2, therefore, high- S. Itoh, J. Am. Chem. Soc., 2003, 125, 11027–11033.
light the importance of subtle electronic changes in 16 (a) G. Litwinienko and K. U. Ingold, J. Org. Chem., 2003, 68,
modulating the reactivity of biologically relevant metal-
dioxygen intermediates.
3433–3438; (b) G. Litwinienko and K. U. Ingold, J. Org.
Chem., 2004, 69, 5888–5896; (c) M. C. Foti, C. Daquino and
C. Geraci, J. Org. Chem., 2004, 69, 2309–2314.
We gratefully acknowledge financial support of this work
from the Cluster of Excellence “Unifying Concepts in Catalysis” 17 S. Kundu, F. F. Pfaff, E. Miceli, I. Zaharieva, C. Herwig,
(EXC 314/1), Berlin. XAS data were obtained on beamline X3B
of the National Synchrotron Light Source (Brookhaven
National Laboratory, Upton, NY, USA), which is operated by
S. Yao, E. R. Farquhar, U. Kuhlmann, E. Bill,
P. Hildebrandt, H. Dau, M. Driess, C. Limberg and K. Ray,
Angew. Chem., Int. Ed., 2013, 52, 5622–5626.
the Case Western Reserve University Center for Synchrotron 18 M. S. Ram and J. T. Hupp, J. Phys. Chem., 1990, 94, 2378–
Biosciences, supported by NIH grant P30–EB–009998. NSLS is 2380.
supported by the United States Department of Energy, Office 19 S. C. Weatherly, I. V. Yang and H. H. Thorp, J. Am. Chem.
of Science, Office of Basic Energy Sciences, under contract DE– Soc., 2001, 123, 1236–1237.
AC02–98CH10886. We also thank Prof. Dr Peter Hildebrandt 20 S. Yao, E. Bill, C. Milsmann, K. Wieghardt and M. Driess,
and Dr Uwe Kuhlmann for the resonance Raman measure- Angew. Chem., Int. Ed., 2008, 47, 7110–7114.
ment of 2 and Prof. Dr Matthias Driess and Dr Shenglai Yao 21 The formation of the bis(hydroxo)CuIINiII product in
for the supply of the nickel superoxide precursor.
Scheme 2 is suggested on the basis of the analysis of the
reaction mixture of 1 and 2,4-di-tert-butylphenol by ESI-MS
(Fig. S2†) and EPR (Fig. S3†) spectroscopic methods. We
emphasize, however, that the formation of other metal-con-
taining products in addition to the bis(hydroxo) species
cannot be excluded at this point.
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