C O M M U N I C A T I O N S
consistent with our calculations on (TPP)Ga+.15 The gas-phase
proton affinity of H2O2 is 4 kcal mol-1 less than that that of H2O
(161 vs 165 kcal mol-1).16 Very weak binding of H2O2 to CoIII has
previously been suggested on the basis of kinetic studies.6 That
H2O2 is poorer ligand than H2O may be understood by considering
that changing from H2O to H2O2 involves replacing H by the more
electron-withdrawing OH. Weak binding of H2O2 is likely a general
feature of its chemistry, in the absence of a base to form hydro-
peroxo or peroxo complexes.
Scheme 2. Experiments Indicating the Lack of Binding of H2O2 to
(TPP)GaIII Complexes in CD2Cl2
maintaining -4 °C, and the mixture was stored overnight. Only
glassware cleaned with Caro’s acid (H2O2/H2SO4) was used.
Aliquots of such solutions removed with a glass pipet have 1H NMR
spectra that show only a single resonance at δ 7.55 (other than the
residual CHDCl2 peak) and thus have [H2O] < 3% [H2O2]. The
addition of small amounts of H2O results in the appearance of a
second resonance, at δ 1.57 for H2O, without affecting the H2O2
resonance (Figure S1). The addition of Ph3P to the H2O2/CD2Cl2
solutions results in rapid and quantitative formation of Ph3PO and
H2O. While caution must be exercised in any procedure using either
perchlorate salts or 50% H2O2, we have experienced no difficulties
with these solutions, which are stable for months at -4 °C.
Solutions of 1-100 mM H2O2 have been prepared (measured by
1H NMR using an internal standard); typical procedures have used
∼10 mM solutions.
Acknowledgment. We are grateful to the U.S. National Insti-
tutes of Health for support (Grant R01 GM50422). We thank Dr.
W. Kaminsky, Dr. M. Sadilek, and Prof. X. Li for assistance with
X-ray crystallography, GC-MS, and computations, respectively.
Supporting Information Available: Full experimental details and
CIF files. This material is available free of charge via the Internet at
References
(1) (a) ActiVe Oxygen in Chemistry; Foote, C. S.; Valentine, J. S.; Greenberg,
A.; Liebman, J. F.; Eds.; Blackie: London, 1995. (b) Jones, C. W. Appli-
cations of Hydrogen Peroxide and DeriVatiVes; Royal Society: Cambridge,
U.K., 1999. (c) Lancaster, M. Green Chemistry; Royal Society: Cam-
bridge, U.K., 2002. (d) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed
Oxidation of Organic Compounds; Academic Press: New York, 1981.
(2) (a) Cytochrome P450: Structure, Mechanism, and Biochemistry; Ortiz
de Montellano, P. R.; Ed.; Kluwer/Plenum: New York, 2005. (b) Groves,
J. T. J. Inorg. Biochem. 2006, 100, 434-447. (c) Denisov, I. G.; Makris,
T. M.; Sligar, S. G.; Schlichting, I. Chem. ReV. 2005, 105, 2253-2278.
(d) Shaik, S.; Kumar, D.; de Visser, S. P.; Altun, A.; Thiel, W. Chem.
ReV. 2005, 105, 2279-2328.
(3) (a) Chandrasena, R. E. P.; Vatsis, K. P.; Coon, M. J.; Hollenberg, P. F.;
Newcomb, M. J. Am. Chem. Soc. 2004, 126, 115-126. (b) Newcomb,
M.; Toy, P. H. Acc. Chem. Res. 2000, 33, 449-455. (c) Coon, M. J. Annu.
ReV. Pharmacol. Toxicol. 2005, 45, 1-25.
(4) Leading refs: (a) Reference 2. (b) Derat, E.; Kumar, D.; Hirao, H.; Shaik,
S. J. Am. Chem. Soc. 2006, 128, 473-484. (c) Newcomb, M.; Chandra-
sena, R. E. P.; Lansakara-P., D. S. P.; Kim, H.-Y.; Lippard, S. J.; Beauvais,
L. G.; Murray, L. J.; Izzo, V.; Hollenberg, P. F.; Coon, M. J. J. Org.
Chem. 2007, 72, 1121-1127. (d) Davydov, R. M.; Perera, R.; Jin, S.;
Yang, T.-C.; Bryson, T. A.; Sono, M.; Dawson, J. H.; Hoffman, B. M. J.
Am. Chem. Soc. 2005, 127, 1403-1413.
(5) Koppenol, W. H. J. Am. Chem. Soc. 2007, 129, 9686-9690. The author
estimates ∆G (pH 7) J -47 kJ mol-1 for formation of P450FeIIIOOH
(eq 8) and 0-9 kJ mol-1 for its protonation, suggesting stability for the
H2O2 complex.
(6) (a) Mirza, S. A.; Bocquet, B.; Robyr, C.; Thomi, S.; Williams, A. F. Inorg.
Chem. 1996, 35, 1332-1337. (b) Wolak, M.; van Eldik, R. Chem. Eur.
J. 2007, 13, 4873-4883.
(7) OEPH2 ) octaethylporphyrin. For example: (a) Ueno, T.; Nishikawa,
N.; Moriyama, S.; Adachi, S.; Lee, K.; Okamura, T.; Ueyama, N.;
Nakamura, A. Inorg. Chem. 1999, 38, 1199-1210. (b) Okamura, T.;
Nishikawa, N.; Ueyama, N.; Nakamura, A. Chem. Lett. 1998, 199-200.
(c) Vo, E.; Wang, H. C.; Germanas, J. P. J. Am. Chem. Soc. 1997, 119,
1934-1940. (d) Kersting, B.; Telford, J. R.; Meyer, M.; Raymond, K. N.
J. Am. Chem. Soc. 1996, 118, 5712-5721. (e) Kazanis, S.; Pochapsky,
T. C.; Barnhart, T. M.; Penner-Hahn, J. E.; Mizra, U. A.; Chait, B. T. J.
Am. Chem. Soc. 1995, 117, 6625-6626. (f) Maelia, L. E.; Koch, S. A.
Inorg. Chem. 1986, 25, 1896-1904.
(8) (a) Rivera, M.; Caignan, G. A.; Astashkin, A. V.; Raitsimring, A. M.;
Shokhireva, T. Kh.; Walker, F. A. J. Am. Chem. Soc. 2002, 124, 6077-
6089 and refs. therein. (b) Davydov, R.; Satterlee, J. D.; Fujii, H.; Sauer-
Masarwa, A.; Busch, D. H.; Hoffman, B. H. J. Am. Chem. Soc. 2003,
125, 16340-16346. (c) Arasasingham, R. D.; Cornman, C. R.; Balch, A.
L. J. Am. Chem. Soc. 1989, 111, 7800-7805. (d) Tajima, K.; Shigematsu,
M.; Jinno, J.; Ishizu, K.; Ohya-Nishiguchi, H. J. Chem. Soc., Chem.
Commun. 1990, 144-145. (e) Tajima, K.; Oka, S.; Edo, T.; Miyake, S.;
Mano, H.; Mukai, K.; Sakurai, H.; Ishizu, K. J. Chem. Soc., Chem. Comm.
1995, 1507-1508.
(9) Balch, A. L.; Hart, R. L.; Parkin, S. Inorg. Chim. Acta 1993, 205, 137-143.
(10) Kadish, K. M.; Cornillon, J. L.; Coutsolelos, A.; Guilard, R. Inorg. Chem.
1987, 26, 4167-4173.
(11) Full experimental details are given in the Supporting Information.
(12) Kastner, M. E.; Scheidt, M. E.; Mashiko, T.; Reed, C. A. J. Am. Chem.
Soc. 1978, 100, 666-667.
Surprisingly, the gallium triflate complex 1 does not react with
H2O2/CD2Cl2 as determined by 1H NMR. The resonances for 1 and
H2O2 remain unperturbed and no new peaks appear. Adding PPh3
to solutions of 1 and H2O2 causes rapid quantitative formation of
OPPh3 and H2O (1H NMR), showing that H2O2 is still present.
Similarly, successive additions of H2O2/CD2Cl2 to purified samples
1
of the perchlorate complex 2 causes no change in the H NMR
chemical shifts. (Multiply recrystallized 2 is required because trace
AgClO4 appears to disproportionate H2O2.) Some broadening and
then sharpening of the resonances for 2 are observed with increasing
H2O2 (Figure S2), perhaps because of changes in solvation. The
addition of PPh3 to solutions of 2 + H2O2 quantitatively yield OPPh3
and the H2O generated in this reaction forms 5. In sum, H2O2 does
not bind significantly to the gallium triflate or perchlorate complexes.
The aquo complex 5 was generated in situ by dissolving 2 in
1
CD2Cl2 saturated with H2O, yielding a mixture of 2 and 5 (by H
NMR), together with some precipitated bis(aquo) complex. The
addition of H2O2/CD2Cl2, such that there was roughly twice as much
1
H2O2 as H2O, caused only very minor changes in the H NMR
spectra. The H2O resonance of 5 shifted downfield very slightly
(<0.02 ppm) and the concentration of 5 actually increased slightly
(Figure S3), possibly as a result of the greater solubility of the bis-
(aquo) complex in the presence of H2O2. Thus, as summarized in
Scheme 2, H2O2 does not displace the water ligand in 5.
(TPP)Ga(OOtBu), (TPP)GaOH (3), and (TPP)GaCH3 (4) are also
inert to H2O2/CD2Cl2 at 25 °C. Complex 4 does react with HOTf
to form 1 but is unreactive with H2O. Solutions containing (TPP)-
GaClO4 and H2O2 in CD2Cl2 did not show any reactivity with
cyclohexene, norbornene, or trans-stilbene by NMR or GC-MS.11
If a small amount of an H2O2 complex is present under these
conditions, it is not highly reactive.
In sum, H2O2 is a very poor ligand to (TPP)GaIII. An excess of
H2O2 in CD2Cl2 does not displace H2O or ClO4- from the gallium
center. While tetraphenylporphyrin-gallium salts in CD2Cl2 are
distant models for the heme-iron(III) center in P450, these results
do not lend support to the suggestions of a hydrogen peroxide
complex as an important oxidant. To our knowledge there are no
reports of observable M(H2O2) or M(ROOH) complexes. Gas-phase
calculations indicate that η2-binding of H2O2 to Li+ and Na+ is
about 5 kcal mol-1 weaker than binding of H2O,14 which is
(13) (a) Han, H. Shiyou Huagong 1994, 23, 298-300. (b) Titova, K. V.; Kol-
makova, E. I.; Rosolovskii, V. A. Zh. Neorg. Khim. 1987, 32, 2849-2850.
(14) (a) Daza, M. C.; Dobado, J. A.; Molina, J. M.; Salvador, P.; Duran, M.;
Villaveces, J. L. J. Chem. Phys. 1999, 110, 11806-11813. (b) Stefanovich,
E. V.; Truong, T. N. J. Chem. Phys. 1996, 104, 2946-2955.
(15) DiPasquale, A. G. Peroxide Complexes of Non-redox ActiVe Metal Centers:
Models for AlternatiVe Mechanisms in Cytochrome P450 Oxidations?
Ph.D. Thesis. University of Washington, Seattle, WA, 2006, pp 59ff.
(accessed Dec. 16, 2007).
JA077598W
9
J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008 1813