Reactions of tetrabromocatecholatomanganese(III) complexes with dioxygen

Article abstract of DOI:10.1021/ic702390q

New five- and six-coordinate complexes containing the [Mn ^IIIBr₄cat)₂]^- core (Br ₄cat^2- = tetrabromo-1,2-catecholate) have been prepared. Homoleptic [Mn^III(Br₄cat)₃]^3- reacts rapidly with O₂ to produce tetrabromo-1,2-benzoquinone (Br ₄bq). The [Mn^III(Br₄cat)₂] ^- fragment is a robust catalytic platform for the aerobic conversion of catechols to quinones. The oxidase activity apparently derives from the coupling of metal- and ligand-centered redox events.

Full text of DOI:10.1021/ic702390q

Inorg. Chem. 2008, 47, 1892-1894
Reactions of Tetrabromocatecholatomanganese(III) Complexes with Dioxygen
Clarence J. Rolle III,† Kenneth I. Hardcastle,‡ and Jake D. Soper*,†
School of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400, and X-ray Crystallography Center, Department of Chemistry,
Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322
Received December 10, 2007
New five- and six-coordinate complexes containing the
[MnIII(Br4cat)2]- core (Br4cat2- ) tetrabromo-1,2-catecholate) have
been prepared. Homoleptic [MnIII(Br4cat)3]3- reacts rapidly with O2
to produce tetrabromo-1,2-benzoquinone (Br4bq). The
[MnIII(Br4cat)2]- fragment is a robust catalytic platform for the
aerobic conversion of catechols to quinones. The oxidase activity
apparently derives from the coupling of metal- and ligand-centered
redox events.
multielectron redox capacity to mononuclear first-row metal
complexes, which typically prefer only single-electron redox
changes.2 Aside from metalloporphyrin complexes, such ligand-
derived multielectron reaction chemistry has been largely
unexplored for redox transformations of small-molecule sub-
strates but is receiving increased interest.2,5
We are developing new multielectron redox cycles based
on the ability of quinoid (o-dioxolene) ligands6a and their
derivatives6b to store and deliver charge in reactions with
small-molecule substrates. In this regard, we were intrigued
by reported reactions of O2 with complexes containing the
anionic bis(catecholato)manganese(III) core,7 particularly the
catalytic production of hydrogen peroxide from O2 by
[MnIII(Cl4cat)2(EtOH)(H2O)]- (Cl4cat2- ) tetrachloro-1,2-
catecholate) with hydroxylamine as a sacrificial reductant.8
It had been postulated that a vacant coordination site at the
manganese center was a prerequisite to reaction with O2.7,8b
However, there was ambiguity regarding the mechanism of
the reaction, particularly the speciation of the O2-sensitive
complexes and the possibility of ligand-based redox events
during catalytic turnover.8a Presented herein are the syntheses
of new five- and six-coordinate complexes based on the
[MnIII(Br4cat)2]- fragment (Br4cat2- ) tetrabromo-1,2-cate-
cholate) and studies of their reaction with O2.
Significant recent attention has been directed toward the
development of new redox catalysts that functionally mimic
enzymes that mediate selective aerobic oxidation reactions.1–3
These may act as oxygenases, by incorporating one or both
oxygen atoms from O2 into organic substrates, or oxidases,
which couple substrate oxidation with the reduction of O2 to
water or hydrogen peroxide.1,3 Catalysts that couple selective
bond activation and functionalization to O2 reduction must
incorporate a multielectron capacity to avoid the loss of
specificity that typically accompanies odd-electron autoxida-
tion.3,4 Traditional inorganic and organometallic catalysts for
such reactions often utilize second- and third-row transition-
metal ions with redox-inert ancillary ligands.1,3 In these systems,
the multielectron redox activity is entirely metal-derived. In
contrast, redox-active “noninnocent” ligands may impart a
Heating acetone solutions of MnII(ClO4)2 · 6H2O with 2
n
equiv of Br4catH2 and BuN4OH to reflux in air generates
* To whom correspondence should be addressed. E-mail:
jake.soper@chemistry.gatech.edu.
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44, 5559–5561. (b) Haneline, M. R.; Heyduk, A. F. J. Am. Chem.
Soc. 2006, 128, 8410–8411. (c) Bouwkamp, M. W.; Bowman, A. C.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13340–
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J. Am. Chem. Soc. 2007, 129, 7212–7213. (e) Edison, S. E.; Hotz,
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†
Georgia Institute of Technology.
Emory University.
‡
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1892 Inorganic Chemistry, Vol. 47, No. 6, 2008
10.1021/ic702390q CCC: $40.75
2008 American Chemical Society
Published on Web 02/19/2008

COMMUNICATION
Accordingly, reaction of Br4bq with isolated 1 in MeCN
under N2 leads to the slow formation of hexabromo-2,3-
oxanthrenequinone and decomposition of 1.
The apparent intermediacy of free Br4bq under the aerobic
conditions employed for the preparation of 1 and the isolated
yields of ∼60% for 1 suggest that 1 equiv of Br4catH2 is
consumed as a sacrificial reductant for each equivalent of 1
produced.15 Consistent with this hypothesis, anaerobic ad-
dition of Br4cat2- (generated in situ from Br4catH2 and
LiOMe) to 1 in MeCN gives immediate conversion to a dark-
green, air-sensitive material (4). Complex 4 is independently
obtained from the reaction of MnIICl2 · 4H2O with 3 equiv
of Br4catH2 and 6 equiv of LiOMe in MeOH at ambient
temperature, followed by air exposure and precipitation with
PPNCl [PPN+ ) bis(triphenylphosphine)iminium]. Recrys-
tallization from THF-pentane under N2 afforded single
crystals suitable for analysis by X-ray crystallography (Figure
1b). The structure reveals a manganese tris(tetrabromo-o-
catecholato) trianion [MnIII(Br4cat)3]3- in 4. The complex
has distorted octahedral geometry with three distinct sets of
Mn-O bond lengths and an axial elongation typical of a
Jahn–Teller distorted, high-spin manganese(III) ion.10 Two
Li(THF)2+ cations are in close contact with the axial oxygen
atoms (Figure S2 in the Supporting Information). This
successful isolation of 4 refutes earlier reports that suggested
that homoleptic tris(catecholato)manganese(III) complexes
are inaccessible because of geometric constraints imposed
by the ligands.8,16,17
Figure 1. Solid-state structures of the anions in (a) (nBu4N)-
[MnIII(Br4cat)2(Me2CdO)2] (1) and (b) (PPN)Li2[MnIII(Br4cat)3]·5THF (4)
shown with 50% probability ellipsoids. Hydrogen atoms, THF solvate
molecules, and countercations are omitted for clarity. Selected bond lengths
(Å) and angles (deg) are as follows. For 1: Mn1-O1 1.905(4), Mn1-O2
1.897(4), Mn1-O3 2.403(4), O1-Mn1-O2 86.17(15), O1-Mn1-O3
93.41(16), O2-Mn1-O3 95.27(17). For 4: Mn1-O1 1.898(3), Mn1-O2
1.917(3), Mn1-O3 2.161(3), Mn1-O4 1.959(3), Mn1-O5 1.970(3),
Mn1-O6 2.154(3), O1-Mn1-O2 85.05(13), O3-Mn1-O4 79.96(11),
O5-Mn1-O6 79.84(11), O1-Mn1-O3 102.28(12), O1-Mn1-O6
96.79(11).
an olive-green solution, which upon cooling to -20 °C
deposits (nBu4N)[MnIII(Br4cat)2(Me2CdO)2] (1) as an ana-
lytically pure, dark-green solid in ca. 60% yield. Recrystal-
lization from acetone gave crystals suitable for X-ray
diffraction (Figure 1a). The crystal structure of 1 contains a
six-coordinate manganese anion located on a crystallographi-
cally defined inversion center and bound to two coplanar
o-dioxolene ligands and two acetone molecules. The quinoid
chelates have C-C and C-O bond lengths indicative of fully
reduced catecholate ligands.9 The axial Mn-Oacetone bond
distance of 2.403(4) Å is elongated relative to the 1.901(6)
Exposure of 4 to O2 in MeCN, acetone, or THF results in
an immediate color change from green to dark blue/violet
5. The UV–vis spectrum of 5 (Figure 2a) is nearly identical
Å averaged Mn-OBr cat bond lengths, consistent with a high-
4
spin 3d4 electron configuration.10 Recrystallization of 1 from
MeOH affords the five-coordinate alcohol adduct
(nBu4N)[MnIII(Br4cat)2(MeOH)] (2) with distorted square-
pyramidal geometry (Figure S1 in the Supporting Informa-
tion). The UV–vis spectra of both 1 and 2 are identical in
either acetone or MeCN, suggesting that the axial ligands
are substitution labile while the [MnIII(Br4cat)2]- core is
preserved in solution. However, contrary to the previous
reports,7b,8 this fragment reacts sluggishly with O2. Exposure
of green 1 or 2 to 1 atm of O2 in acetone or MeCN affords
conversion to a dark-blue species over weeks at ambient
temperature. Likewise, the triphenylphosphine oxide complex
(nBu4N)[MnIII(Br4cat)2(OPPh3)] (3) is indefinitely stable
under air in MeCN.11 In light of these observations, the
reported facile reactions of closely related species with O2
merited reexamination.7b,8,12
7b,8a,12
with the previously reported dianion [MnIV(Cl4cat)3]2-
and is tentatively formulated as [MnIV(Br4cat)3]2-. Consistent
with this assignment, 5 is independently obtained from the
addition of 1.0 equiv of AgPF6 to 4 under N2. The color of
5 in MeCN is temperature-dependent, reversibly converting
from dark blue at -20 °C to green at +65 °C. This color
change may reflect conversion to a [MnIII(Br4cat)2(Br4sq•)]2-
(Br4sq•- ) tetrabromo-1,2-semiquinonate) intravalence iso-
mer. The electronic structure of 5 and related materials is a
(11) (a) For O2 sensitivity studies, 3 was generated in situ from the addition
of excess OPPh3 to 1. The UV-vis spectrum of 3 in MeCN is
unchanged with air exposure over weeks at ambient temperature.
Isolated 3 was prepared by a modification of a literature procedure.11b
An X-ray structure confirmed the five-coordinate monomeric nature
of the anion in 3 with no close-cation contacts. (b) Larsen, S. K.;
Pierpont, C. G.; DeMunno, G.; Dolcetti, G. Inorg. Chem. 1986, 25,
4828–4831.
(12) We have found chemical and physical properties of the Br4cat2-
complexes to be very similar to those of their Cl4cat2- analogues
(Figure S4 in the Supporting Information).
A clue toward the identity of the true air-sensitive species
came from observation of a second red-orange product in
the reaction to prepare 1. Independent synthesis confirmed
that this material is hexabromo-2,3-oxanthrenequinone,13
which arises from coupling of tetrabromo-1,2-benzoquinone
(Br4bq) with Br4cat2-.14 The yield of the coupled product
varies and is inversely proportional to the yield of 1.
(13) Pure hexabromo-2,3-oxanthrenequinone was independently obtained
from the addition of excess Br4bq to 1 in MeCN under air (Figure S3
in the Supporting Information).
(14) (a) Buchanan, R. M.; Fitzgerald, B. J.; Pierpont, C. G. Inorg. Chem.
1979, 18, 3439–3444. (b) Larsen, S. K.; Pierpont, C. G.; DeMunno,
G.; Dolcetti, G. Inorg. Chem. 1986, 25, 4828–4831.
(15) Assuming Br4catH2 is a limiting reagent in the synthesis of 1, the
isolated yields of ∼60% for 1 are close to the theoretical yield of
66%. The discrepancy is likely due to the reaction of Br4bq with 1.
(16) Hartman, J. R.; Foxman, B. M.; Cooper, S. R. Inorg. Chem. 1984,
23, 1381–1387.
(9) Carugo, O.; Castellani, C. B.; Djinoviæ, K.; Rizzi, M. J. Chem. Soc.,
Dalton Trans. 1992, 837–841.
(10) Krzystek, J.; Yeagle, G. J.; Park, J.-H.; Britt, R. D.; Meisel, M. W.;
Brunel, L.-C.; Telser, J. Inorg. Chem. 2003, 42, 4610–4618.
(17) A [Mn. (cat)3]3– ion was inferred from a spectrophotometric titration.
See: Sever, M. J.; Wilker, J. J. Dalton Trans. 2004, 1061–1072.
Inorganic Chemistry, Vol. 47, No. 6, 2008 1893

COMMUNICATION
and gas chromatography-mass spectrometry indicates that
3,5-tBu2bq is the only catechol-derived species formed,
implying 1 is not active for intradiol or extradiol dioxy-
genase-type activity.6a,19 Control experiments confirm that
1 is required for catalytic turnover (Figure 2b). If mixed-
ligand [Mn(Br4cat)2(3,5-tBu2cat)]3-/2- species analogous to
4 and 5 are intermediates during catalytic turnover, the
reaction is >99% selective for internal oxidation of the
tBu2cat2- ligand over Br4cat2-. This redox selectivity reflects
the >500 mV difference in the oxidation potentials of 3,5-
tBu2catH2 versus Br4catH2,20 which also contributes to the
robust nature of the [MnIII(Br4cat)2]- fragment in aerobic
catechol oxidation catalysis.
Figure 2. (a) UV–vis absorption spectra for a reaction of 2.9 × 10-4 M 4
(green line) with 1 atm of O2 in MeCN to generate 5 (blue line) in 100 min
at 25 °C. (b) Selected data for the oxidation of 1.0 × 10-3 M 3,5-tBu2catH2
to 3,5-tBu2bq (λmax ) 397 nm) under 1 atm of O2 in acetone at 25 °C with
2.1 × 10-6 M 1 (red O) and without manganese (gray )).
In summary, whereas the [MnIII(Br4cat)2]- fragment is itself
surprisingly inert to O2, in the presence of catechol substrates
it reacts rapidly to produce quinone in an oxidase-type dehy-
drogenation reaction. Although such catecholase chemistry is
not rare for metal-quinone complexes,6a,21 our preliminary
investigations of the title complex have already uncovered
several salient features of these reactions. In particular, despite
proceeding via a series of 1e- steps, the reaction is selective
for the 2e- dehydrogenation of catechols. This multielectron
capacity apparently derives from both metal- and ligand-
centered redox events during catalytic turnover, a consequence
of the close match in metal and ligand valence orbital energies.
Additionally,therequirementforpreassemblyofacatalyst-substrate
complex prior to reaction with O2 facilitates intramolecular
redox processes over competing intermolecular redox events.
Our ongoing work in this field focuses on the development of
new aerobic multielectron redox cycles, including amine and
alcohol oxidations that derive specificity from the ability of a
coordinatively unsaturated 3d metal ion to bind substrate prior
to reaction with O2. For instance, the affinity of the
[MnIII(Br4cat)2]- fragment for both alcohol and ketone substrates
in solution (as in 1 and 2, respectively) demonstrates the viability
of this approach by modeling two key components in a cycle
for catalytic aerobic alcohol oxidation.
Scheme 1
subject of ongoing studies in our laboratory. Although 5 is
indefinitely stable under N2, it is unstable under O2, affording
1 and Br4bq-derived products over hours at 25 °C.18 The
fate of the reduced oxygen species is under investigation but
is likely H2O2 based on literature precedent.2b,7a,8
The relative stabilities of 1-3 in oxygen suggest that their
slow reaction with O2 proceeds via an unfavorable pre-
equilibrium
comproportionation
reaction
between
[MnIII(Br4cat)2]- anions to generate trace amounts of air-
sensitive 4. However, the addition of 1 equiv of Cp*2Co to
1 in MeCN gives immediate conversion to a light-yellow
product, which rapidly reacts with O2 to generate green 4
and blue 5, as evidenced by UV–vis spectroscopy. An
identical yellow complex is obtained from the combination
of MnIICl2 · 4H2O with 3 equiv of Br4catH2 and 6 equiv of
LiOMe in MeOH under N2. These observations suggest that
[MnII(Br4cat)3]4- is the initial product generated in the aerobic
synthesis of 1 from manganese(II) salts and that Br4cat2-
ligand redistribution on reduced [MnII(Br4cat)2]2- is facile.
The net conversion of Br4catH2 to Br4bq by reaction with
1 and O2 (via 4 and 5; Scheme 1) comprises a complete
catalytic cycle for aerobic catechol oxidation by the
[MnIII(Br4cat)2]- fragment. Indeed, heating 1 to reflux in air-
saturated MeCN with >20 equiv of Br4catH2, and without
base, results in high-yield formation of hexabromo-2,3-
oxanthrenequinone. We postulated that the [MnIII(Br4cat)2]-
core could effect rapid catalytic quinone production from a
more easily oxidized catechol substrate. Accordingly, quan-
titative conversion of 3,5-di-tert-butylcatechol (3,5-tBu2catH2)
to 3,5-di-tert-butylquinone (3,5-tBu2bq) is achieved with 0.2
mol % 1 in ca. 400 min under 1 atm of O2 at 25 °C, without
added base (Figures 2b and S5 in the Supporting Informa-
Acknowledgment. We gratefully acknowledge financial
support from the American Chemical Society Petroleum
Research Fund (45130-G3) and from the Georgia Institute
of Technology. We thank David Bostwick for assistance with
mass spectrometry.
Supporting Information Available: Details of general experi-
mental procedures; synthesis, spectroscopic, and analytical char-
acterization of 1, 2, and 4; a representative procedure for aerobic
catechol oxidation; UV–vis absorption spectra for a catalytic
oxidation of 3,5-tBu2catH2 to tBu2bq; X-ray crystal structures of 2,
4, and hexabromo-2,3-oxanthrenequinone; and X-ray crystal-
lographic information in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC702390Q
(19) Que, L., Jr.; Ho, R. Y. N Chem. ReV. 2006, 96, 2607–2624.
(20) Pascaly, M.; Duda, M.; Schweppe, F.; Zurlinden, K.; Müller, F. K.;
Krebs, B. J. Chem. Soc., Dalton Trans. 2001, 828–837.
1
tion). Analysis of the products by H NMR spectroscopy
(21) (a) For example, see: Triller, M. U.; Pursche, D.; Hsieh, W.-Y.;
Percoraro, V. L.; Rompel, A.; Krebs, B. Inorg. Chem. 2003, 42, 6274–
6283. (b) Hitomi, Y.; Ando, A.; Matsui, H.; Ito, T.; Tanaka, T.; Ogo,
S.; Funabiki, T. Inorg. Chem. 2005, 44, 3473–3478.
(18) Complex 1 was isolated as an insoluble PPN+ salt from the reaction of
4 with O2 in THF. Its identity was confirmed by UV-vis spectroscopy
in MeCN and by electrospray ionization mass spectrometry.
1894 Inorganic Chemistry, Vol. 47, No. 6, 2008