Oxidation of hydrocarbons by [(phen)2Mn(μ-O)2Mn(phen)2]3+ via hydrogen atom abstraction

Full text of DOI:10.1021/ja963180e

1
470
J. Am. Chem. Soc. 1997, 119, 1470-1471
A solution of 1 in acetonitrile (20 mM) reacts with 9,10-
Oxidation of Hydrocarbons by
3+
[(phen)
2
Mn(µ-O)
2
Mn(phen)
2
]
via Hydrogen Atom
dihydroanthracene (DHA, 40 mM) over 11 h at 65 °C, with a
change in color from olive green to light brown (eq 3; all
reactions done in the absence of air). GC analysis of the organic
products revealed anthracene (19.4 mM) and traces of an-
thraquinone (0.4 mM) and anthrone (0.15 mM). Iodometric
Abstraction
Kun Wang and James M. Mayer*
Department of Chemistry, Box 351700
UniVersity of Washington
Seattle, Washington 98195-1700
ReceiVed September 10, 1996
The oxidation of hydrocarbons by homogeneous, heteroge-
neous, and enzymatic reagents is of much fundamental and tech-
nological interest. We report here the first example of C-H
bond oxidation by a (µ-oxo)manganese complex, [L2Mn(µ-O)2-
MnL2](PF6)3 (1; L ) 1,10-phenanthroline).1 Activation of C-H
bonds by µ-oxo compounds has been suggested to occur in the
catalytic cycles of iron and copper enzymes.2 This study
complements a recently characterized diiron system that oxidizes
9
titration of the isolated manganese product gave an average
oxidation state of 2.37(4), roughly consistent with the formation
3+
2+
of the Mn Mn dimer 3. The observed products account for
98% of the oxidizing equivalents of 1 consumed. When only
half an equivalent of DHA is used (10 mM), the manganese
product has an average oxidation state of 2.92(4), consistent
with predominant formation of the Mn Mn dimer 2. The
organic products (8 mM anthracene, 0.9 mM anthraquinone,
and trace anthrone) account for 93% of the manganese oxidizing
equivalents consumed. An isotope effect of 4.2(3) was found
on analyzing the products of the oxidation of a 50/50 mixture
of DHA-h12 and DHA-d12 by 1 at 55 °C. The reaction, as
monitored by UV/vis spectroscopy, shows no induction period,
and there are no isosbestic points. Absorbance vs time traces
indicate a consecutive reaction pattern, suggested to be 1 f 2
f 3. Species 2 and 3 are generated independently from 1 and
2a
2b
2c
3+
3+
cumene and methylcyclohexadiene, as well as dicopper and
3
dicobalt systems that undergo intramolecular C-H activation.
3+
4+
Mixed valent 1 (Mn Mn ) has long been considered a model
for a part of the oxygen-evolving complex in photosystem II,
where oxidation of water has been proposed to occur by H•
abstraction from metal-bound water or hydroxide by an adjacent
4
tyrosine radical. We have proposed that the ability of metal
-
oxo species such as MnO4 and CrO2Cl2 to oxidize hydrocar-
bons by initial hydrogen atom abstraction is directly related to
•
the thermodynamic affinity of these reagents for H , rather than
any radical character in the oxidant.5 With this approach, we
,6
10
respective stoichiometric amounts of hydroquinone.
accurately predicted that 1 and 2 would abstract hydrogen atoms
The reaction of 1 with fluorene in MeCN at 55 °C gives
roughly equal yields of bifluorenyl and 9-fluorenone, accounting
for 76% of the oxidizing equivalents consumed. The formation
of bifluorenyl indicates that fluorenyl radicals are involved, as
was confirmed by trapping with CBrCl3 to give 9-bromofluo-
rene, as well as 9-fluorenone and bifluorenyl. The reaction with
DHA is similarly proposed to involve 9-hydroanthracenyl
•
from weak C-H bonds because of its affinity for H (eqs 1 and
7
,8
2
, L ) phen) based on electrochemical and pKa data.
•
radicals (HA ), by H atom transfer from DHA to 1, forming 2
(
Scheme 1). In order to explain the kinetic traces, it is necessary
•
to propose that 2 can also react with DHA by H abstraction.
HA is then rapidly oxidized by 1 or 2 to give anthracene and
•
(
1) Complex 1 was prepared and fully characterized (with water of
crystallization) following previous reports (see Supporting Information).
a) Manchanda, R.; Brudvig, G. W.; de Gala, S.; Crabtree, R. H. Inorg.
2
or 3. The kinetic traces for the DHA reaction are successfully
simulated with this scheme using the computer programs
KINSIM/FITSIM. At 25 °C, k1 and k3 are well defined as
(
1
1
Chem. 1994, 33, 5157-5160. (b) Stebler, M.; Ludi, A.; B u¨ rgi, H.-B. Inorg.
Chem. 1986, 25, 4743-4750. (c) Cooper, S. R.; Calvin, M. J. Am. Chem.
Soc. 1977, 99, 6623-6630.
-3
-1 -1
-4
-1 -1
1
.56(2) × 10
M
s
and 4.2(7) × 10
M
s , but the fit
is not sensitive to the much faster rate constants for trapping of
(
2) For leading references, see: (a) Dong, Y.; Fuji, H.; Hendrich, M. P.;
•
4
5
-1 -1
Leising, R. A.; Pan, G.; Randall, C. R.; Wilkinson, E. C.; Zang, Y.; Que,
HA (k2, k4 roughly 10 -10 M
s
from the fit). Independent
L., Jr.; Fox, B. G.; Kauffmann, K.; M u¨ nck, E. J. Am. Chem. Soc. 1995,
1
17, 2778-2792. L. Que, Jr., 1996, personal communication. (b) Zang,
(7) Our initial estimates of the O-H bond strengths were based on
aqueous electrochemical data reported (Manchanda, R.; Thorp, H. H.;
Brudvig, G. W.; Crabtree, R. H. Inorg. Chem. 1992, 31, 4040-4041).
However, a reviewer opined that these data are not for 1 but rather for a
phosphate-bridged complex. In our hands, 1 reacts immediately on
dissolution in phosphate buffer, apparently confirming the reviewer’s view.
We thank the reviewer for this information.
Y.; Pan, G.; Que, L., Jr.; Fox, B. G.; M u¨ nck, E. J. Am. Chem. Soc. 1994,
1
16, 3653-3654. (c) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan,
G.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. Ibid. 1995, 117, 8865-6.
Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young, V. G.,
Jr.; Que, L., Jr.; Zuberb u¨ ler, A. D.; Tolman, W. B. Science 1996, 271, 1397-
1
1
8
400. (d) For recent reviews, see: Que, L., Jr.; Dong, Y. Acc. Chem. Res.
996, 29, 190-196. Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-
05. Fox, S.; Karlin, K. D. In ActiVe Oxygen in Biochemistry; Valentine, J.
(8) (a) The O-H bond strengths were calculated using the equation
described by Parker and Tilset8b for measurements in MeCN and Cp2Fe
couple as the reference: BDE ) 23.06E1/2 + 1.37pKa + 59.5 (kcal/mol).
Baldwin and Pecoraro4c used a related approach based on their measurement
of E1/2 for H2 in MeCN vs aqueous SCE, which is apparently equivalent to
using a constant 3 kcal/mol lower (Baldwin, M. J. Personal communication,
+/o
S., Foote, C. S., Greenberg, A., Liebman, J. F., Eds.; Chapman & Hall:
Glasgow, Scotland, 1995; pp 188-231.
(
3) Reinaud, O. M.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 6979-
6
980.
4) (a) Hoganson, C. W.; Lydakis-Simantiris, N.; Tang. X.-S.; Tommos,
+/o
III
IV
III
(
1996). E1/2 values (in MeCN, vs Cp2Fe ): for Mn (O)2Mn f Mn (O)2-
III III III III II
C.; Warncke, K.; Babcock, G. T.; Diner, B. A.; McCracken, J.; Styring, S.
Photosynth. Res. 1995, 46, 177-184. (b) Manchanda, R.; Brudvig, G. W.;
Crabtree, R. H. Coord. Chem. ReV. 1995, 144, 1-38 and references therein.
Mn , -0.01(0.05 V; for Mn (O)(OH)Mn f Mn (O)(OH)Mn ,
-0.03(0.05 V. The pKa values in MeCN were determined electrochemically
following the method of Baldwin and Pecoraro:4c 2, 14.6(0.5; 3, 11.5(0.5.
See Supporting Information for details. (b) Parker, V. D.; Handoo, K. L.;
Roness, F.; Tilset, M. J. Am. Chem. Soc. 1991, 113, 7493-7498.
(9) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C. Vogel’s
Textbook of QuantitatiVe Chemical Analysis 5th ed.; Wiley: New York,
1989; pp 384-391.
(
c) Baldwin, M. J.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 11325-
1
1326.
(
5) (a) Cook, G. K.; Mayer, J. M. J. Am. Chem. Soc. 1995, 117, 7139-
7
1
156. (b) Cook, G. K.; Mayer, J. M. J. Am. Chem. Soc. 1994, 116, 1855-
868. (c) Correction. Ibid. 1994, 116, 8859.
(
6) (a) Gardner, K. A.; Mayer, J. M. Science 1995, 269, 1849-1851.
(10) Following related chemistry of the bipyridine analog of 1: Ghosh,
M. C.; Reed, J. W.; Bose, R. N.; Gould, E. S. Inorg. Chem. 1994, 33, 73-
78. For detailed characterizations of 2 and 3, see Supporting Information.
(
(
b) Gardner, K. A.; Kuehnert, L. L.; Mayer, J. M. Submitted for publication.
c) Gardner, K. A. Ph.D. Thesis, University of Washington, 1996.
S0002-7863(96)03180-0 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 6, 1997 1471
Scheme 1
oxidations of DHA with 2 (prepared from 1 + hydroquinone)
-
3
-1 -1
occur with a rate constant of 4.2 × 10
M
s
at 50 °C, in
-
3
-1
excellent agreement with the simulation result (3.9 × 10
q q
M
-1
s
at 50 °C). Over the temperature range of 25-55 °C, the
activation parameters are ∆H 1 ) 14.5(10) kcal/mol, ∆S 1 )
q
q
-
23(3) eu; ∆H 3 ) 16.0(10) kcal/mol, ∆S 3 ) -21(3) eu. The
Figure 1. Rate constants for hydrogen atom abstraction from dihy-
droanthracene (30 °C) vs the strength of the O-H bond formed by
negative entropies of activation are consistent with bimolecular
rate-limiting steps and are similar to what we have observed in
t
•
sec
•
-
3+
2
BuO , BuOO , MnO , [L Mn(O) MnL ] , and [L Mn(O)(OH)-
4 2 2 2
5
,6
q
other hydrogen atom transfer processes. ∆H 3 is larger than
MnL
2
]3
+ 13
.
The straight line is defined by the values for BuO and
t
•
q
∆
1
H 1 presumably because the O-H bond formed on converting
sec
•
BuOO .
to 2 is stronger than that formed from 2 to 3.
Rates of hydrogen atom abstraction by main group radicals
t
•
t
•
such as BuO and BuOO have long been known to correlate
faster than 2 given the 4 kcal/mol stronger O-H bond formed.
•
with the ∆H° for the H transfer step (the Evans-Polanyi
These deviations from a simple correlation with driving force
1
2
equation). This ∆H° for reaction A-H + B f A + B-H is
the difference in the bond strength between A-H and B-H.
We have previously found that this relation connects the rate
14
may reflect different intrinsic barriers, perhaps due to structural
•
reorganizations on transferring an H to a bridging oxygen.
Structural reorganizations have been implicated as the cause of
•
-
constants for H abstraction by MnO4 and CrO2Cl2 with
abstraction by oxygen radicals.5 Figure 1 shows this correla-
tion, log k vs the strength of the O-H bond formed by the
oxidant, for H abstraction from DHA. The rate constants for
H abstraction by both 1 and 2 roughly correlate with those for
BuO and BuOO (the straight line is defined by the rate
constants for the oxygen radicals). It appears that 1 and 2 react
slightly slower than predicted, while the MnO4 oxidation may
15
slow proton transfer to some µ-oxo groups.
,6
3+
In summary, both [L2Mn(µ-O)2MnL2] (1) and [L2Mn-
3+
(
µ-O)(µ-OH)MnL2] (2; L ) 1,10-phenanthroline) can abstract
•
13
•
H from hydrocarbons. The rate constants for these reactions
can be roughly predicted based on the strengths of the O-H
bonds that are formed. Studies with related substrates and
oxidants are in progress to define the scope of these reactions
and to understand the activation barriers in more detail.
•
t
•
sec
•
-
be slightly faster, although this kind of correlation is rarely
precise.12 It is surprising, however, that 1 reacts only three times
Acknowledgment. We are grateful to the National Institutes of
Health for financial support of this work. We thank Drs. M. Balwin
and V. Pecoraro for valuable comments and an advance copy of ref
(
11) Barshop, B. A.; Wrenn, R. F.; Frieden, C. Anal. Biochem. 1983,
1
30, 134. The KINSIM/FITSIM programs were obtained from Washington
4
c. We thank Jim Roe and Bob Morley for technical support, Tom
University (St. Louis, MO) and run on an SGI computer. We thank Prof.
Carl Frieden for allowing us to use the programs.
Crevier for a sample of dihydroanthracene-d12, Darin DuMez for
assistance, and Dr. K. A. Gardner for valuable comments.
(
12) (a) Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973. See
especially: Ingold, K. U. Ibid. Vol. 1, Chapter 2, pp 67-83. Russell, G.
A. Ibid.; Vol. 1, Chapter 7, pp 275-331. (b) Knox, J. H. In Oxidation of
Organic Compounds; Mayo, R. R., Ed., American Chemical Society:
Washington, DC, 1968; pp 1-22. See refs 5 and 6 for more detailed
discussions.
Supporting Information Available: Full experimental details,
including preparation and characterization of 1-3, kinetics and
a
simulations, electrochemical measurements of E1/2 and pK values, and
t
•
(
13) (a) For the rate constant (21 °C) for BuO see: Mulder, P.; Arends,
a description of the O-H bond strengths estimation (8 pages). See
any current masthead page for ordering and Internet access instructions.
I. W. C. E.; Clark, K. B.; Wayner, D. D. M. J. Phys. Chem. 1995, 99,
8
182-8189. Rates are relatively insensitive to small temperature changes
for this type of fast reactions, therefore the rate was assumed to be the
JA963180E
same at 30 °C. For example, only 6% increase in rate from 21 to 30 °C for
t
•
the reaction of BuO with 1,3-cyclopentadiene (Wong, P. C.; Griller, D.;
(14) Intrinsic barriers for an individual redox couple are widely used in
electron transfer reactions (Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J.
Phys. Chem. 1996, 100, 13148-13168 and references therein), but their
utility in hydrogen atom transfer reactions has yet to be established.
(15) Kramarz, K. W.; Norton, J. R. In Progress in Inorganic Chemistry;
Karlin, K. D., Ed.; Wiley: New York, 1994; Vol. 42, pp 1-65 and
references therein.
Scaiano, J. C. J. Am. Chem. Soc. 1982, 104, 5106). (b) For the rate constant
sec
•
for BuOO (30 °C), see: Howard, J. A.; Ingold, K. U. Can. J. Chem.
sec
1
968, 46, 2661. (c) The O-H bond strength in BuOOH is taken as 89
t
kcal/mol, equal to that in BuOOH (Colussi, A. J. In Chemical Kinetics of
Small Organic Radicals; Alfassi, Z. B., Ed.; CRC Press: Boca Raton, FL,
1
n
988; p 33. (d) Data for Bu4NMnO4 in toluene from ref 6b,c.