A stable manganese(V)-oxo corrolazine complex

Article abstract of DOI:10.1021/ja028651d

The synthesis and characterization of an oxomanganese(V) corrolazine, (TBP) ₈(Cz)Mn(V)O≡ (2), are reported. This remarkably stable high-valent complex is obtained from the stoichiometric reaction of (TBP) ₈(Cz)Mn(III) (1) with m-CPBA and is easily purified by standard chromatographic methods on silica gel at room temperature. Complex 2 exhibits a diamagnetic ¹H NMR spectrum indicative of a low-spin d ² Mn(V)O≡ species. LDI-TOFMS of 2 shows the predicted isotopic envelope at m/z 1426.8. This envelope shifts to higher mass as expected after the facile exchange of the terminal oxo group with H₂¹⁸O. The resonance Raman spectrum of 2 either in solution or in the solid state shows a strongly enhanced Raman band for the stretching mode of the Mn-oxo bond, which also shifts as expected upon ¹⁸O substitution: 2(¹⁶O), 979 cm^-1; 2(¹⁸O), 938 cm^-1 (in CH₂Cl₂). Initial reactivity studies show that 2 rapidly transfers the terminal oxo ligand to PPh₃, resulting in the quantitative formation of OPPh₃ and concomitant reduction of 2 back to 1. Complex 2 is the first example of an oxomanganese(V)-porphyrinoid complex that can be isolated at room temperature. Copyright

Full text of DOI:10.1021/ja028651d

Published on Web 11/27/2002
A Stable Manganese(V)-Oxo Corrolazine Complex
†
†
†
†
‡
Beaven S. Mandimutsira, Bobby Ramdhanie, Ryan C. Todd, Hailin Wang, Adelajda A. Zareba,
‡
,†
Roman S. Czernuszewicz, and David P. Goldberg*
Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218, and
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003
Received September 23, 2002
Scheme 1 a
High-valent metal-oxo species play important roles in a large
1
number of synthetic and biological transformations. In particular,
2
3
manganese-oxo complexes of both porphyrin and salen catalysts
are implicated in various oxygen atom transfer reactions of
considerable synthetic importance and have also served as models
for intermediates in heme protein-catalyzed oxidations and related
reactions. Moreover, manganese-oxo species are postulated as
2 2
important intermediates in the conversion of H O to O during
4
photosynthesis. Thus, the direct spectroscopic observation and
isolation of Mn(V)tO complexes has been a highly desirable yet
extremely difficult goal to accomplish because of the synthetic
challenges involved in preparing ligands suitable for stabilizing such
species. Spectroscopic characterization of an Mn(V)tO porphyrin
a
Conditions: (a) 10 equiv of Mn(acac)3, pyridine, 115 °C, 1 h; (b) 1.0
equiv of m-CPBA, CH2Cl2, room temperature, 1 min.
obtained by dissolution in CH Cl followed by the slow diffusion
2
2
of MeOH at -20 °C.
As expected for high-spin Mn(III), 1 exhibits a paramagnetic
NMR spectrum. In contrast, the H NMR spectrum of 2 in CD Cl
is diamagnetic, as is apparent from the sharp
appearing in the normal aromatic (δ 7-9 ppm) and alkyl (δ 1-2
ppm) regions (Figure S1). There are a total of four inequivalent
(4-tert-butyl)phenyl substituents in 2, which give rise to the
observed pattern of overlapping AB doublets in the aromatic region
and the four, well-resolved singlets in the alkyl region. A
diamagnetic ground state is expected on theoretical grounds for a
Mn(V) (d ) ion with a triply bonded, terminal oxo ligand. Moreover,
the few examples of spectroscopically characterized (L)Mn(V)tO
compounds uniformly exhibit a diamagnetic NMR spectrum.5,6,8
Results from high-resolution LDI-TOF mass spectroscopy confirm
the characterization of 2 as an Mn complex with a terminal oxo
ligand. The isotopic cluster centered at the molecular ion for 2 (m/z
1426.8) is in excellent agreement with the calculated pattern for
5
was not achieved until very recently, and the three Mn(V)tO
complexes that are isolable at room temperature contain similar
non-porphyrin, tetraanionic ligands with either bis-amido bis-alkoxo
1
2
2
1
H NMR resonances
6
or tetraamido donors. We have recently prepared a new type of
porphyrinoid ligand called corrolazine that is a hybrid of a
7
tetraazaporphyrin and corrole ring. We report herein a remarkably
stable Mn(V)tO species, (TBP) (Cz)Mn(V)tO (2) (TBP ) 4-tert-
8
butylphenyl), which was synthesized using a corrolazine ligand.
This compound is, to our knowledge, the first oxomanganese(V)-
porphyrinoid complex that can be purified by standard chromato-
2
8
graphic methods and isolated at room temperature. The extraor-
dinary stability of 2 has allowed for complete characterization of
the Mn(V)tO moiety, including the first resonance Raman spectra
of a (porphyrinoid)Mn(V)tO complex. Interestingly, (TBP)
Mn(V)tO is still thermodynamically competent to act as an oxygen
atom transfer agent toward PPh
8
(Cz)-
3
.
The synthesis of the Mn(V)tO complex 2 was accomplished
as shown in Scheme 1. Insertion of manganese ion into the metal-
[(TBP) (Cz)Mn(O)] (Figure S2). Stirring a solution of 2 in CH2-
+
8
Cl with H218O (95%) under Ar for 12 h followed by evaporation
2
free (TBP)
8
CzH
3
went smoothly by reaction with 10 equiv of Mn-
(Cz)Mn(III) (1),9
of the solvent results in the facile exchange of the oxo ligand to
give the O-labeled product, as evidenced by a new LDI-TOFMS
pattern consistent with 70% 18O incorporation (Figure S2). Previous
work on manganese porphyrin-mediated oxidations has shown that
18O exchange with water is one of the hallmarks of an oxoman-
ganese(V) porphyrin intermediate.5,10 The NMR and mass spectral
data provide compelling evidence that 2 is a bona fide Mn(V)tO
complex.
(
acac) in refluxing pyridine. The product, (TBP)
3
8
18
was purified by flash chromatography to give 1 as a dark brown
solid in good yield (85%). Treatment of compound 1 with a
stoichiometric amount of m-CPBA in CH Cl resulted in an
2 2
immediate color change from brown to the deep green of a new
species, 2 (λmax ) 418, 634 nm). This new color persisted at room
temperature for seVeral days, and analysis of the reaction mixture
by TLC showed that 2 migrated on silica gel as a well-behaved,
green spot and was the only product in the reaction. Thus, we were
encouraged to attempt to purify 2 by standard chromatographic
methods. Remarkably, compound 2 was easily isolated by flash
chromatography under room-temperature, aerobic conditions,
giving 2 as a stable green solid in good yield (81%) after
evaporation of the solvent. A microcrystalline solid of 2 can be
Finally, direct evidence for the oxomanganese(V) (permanganyl)
species was obtained from resonance Raman (RR) spectroscopy.
Excitation near the absorption maxima of the Soret band (418 nm)
of 2 in CH Cl results in the appearance of a strongly enhanced
2
2
polarized Raman band at 979 cm-1 (Figure 1a). This new Raman
band11 is attributed to the stretching mode of the Mn-oxo bond,
ν(MnO), because it shifts to 938 cm-1 with 18O substitution; the
-
1
expected shift for a Mn-O diatomic oscillator is 43 cm . Solid 2
*
To whom correspondence should be addressed. E-mail: dpg@jhu.edu.
The John Hopkins University.
University of Houston.
-1
†
gives a similar ν(MnO) RR band at 997 cm , which shifts to 954
‡
-1
18
cm when the oxo group is exchanged with O (Figure 1b). A
15170
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10.1021/ja028651d CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
show good isosbestic behavior. The shoulder at 722 nm is indicative
of weak coordination of OPPh to the Mn(III) corrolazine complex,
3
as determined by independent experiments (data not shown).
Reverse titration with m-CPBA quantitatively regenerates 2. The
amount of phosphine oxide produced (83%) was measured by
18
standard GC-FID techniques. The use of O-labeled 2 in the same
18
reaction gave ( O)PPh
evidence for a direct oxygen atom transfer mechanism between 2
and PPh
3
as shown by MALDI-MS, providing strong
3
.
Complex 2 was also reduced by the substrate dimethyl sulfide
DMS), as observed by UV-vis spectroscopy. Oxidation of this
(
thioether substrate is considerably more difficult than PPh
thermodynamic standpoint, and a complete reactivity profile
product analysis, reaction rates) of the Mn(V)tO complex 2
3
from a
(
toward thioethers as well as other substrates will form the basis of
future reports.
-
1
16
Acknowledgment. We thank the NSF (CHE0094095 and
CHE0089168 to D.P.G.) and the Robert A. Welch Foundation (E-
184 to R.S.C.) for financial support. R.C.T. acknowledges support
Figure 1. Resonance Raman spectra (855-1065 cm ) of 2( O) (top)
1
8
and its Mn O isotopomer (bottom) in (a) CH2Cl2 (∼1 mM) and (b) solid
-
1
state, excited at 413.1 nm (100 mW) and 7 cm slit widths.
1
from a Howard Hughes Summer Fellowship through the Johns
Hopkins University. D.P.G. is also grateful for an Alfred P. Sloan
Research Fellowship.
Supporting Information Available: Details of syntheses, mass
spectroscopy, and resonance Raman spectroscopy (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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(
(
(
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Figure 2. UV-vis titration of 2 (0.013 mM) and PPh3 (0-1 equiv) in
CH2Cl2 (5 mL) at 23 °C.
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1
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(
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18
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with a force constant of 7.25 mdyn/Å and predicts an O shift of
-
1
4
4 cm , in excellent agreement with experiment. To our knowl-
edge, the only other Mn(V)tO complexes that have been charac-
(8) A recent report describes the characterization of a (corrole)Mn(V)O which
is stable at room temperature for several hours if kept in dilute solution.
See: Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed.
2000, 39, 4045-4047.
terized by vibrational spectroscopy (RR, IR) are those containing
1
6
-1
a tetraamide macrocycle, for which ν(Mn O) ) 981-970 cm
18
-1 6c,d
(9) The structure of this compound has been determined by X-ray crystal-
lography and shows an axial MeOH coordinated to the Mn ion in the
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and ν(Mn O) ) 942-933 cm
.
Similar vibrational properties
have been determined for the isoelectronic Mn(V)tN and Cr(IV)t
O units in five-coordinate porphyrin complexes having a triply
bonded axial nitrido or oxo ligand.12 In contrast, five-coordinate
oxomanganese(IV) (manganyl) porphyrins give the IR and RR
bands of Mn(IV)dO at the characteristically low frequency of ∼755
(
(
8
11) The corresponding band is absent for the (TBP) (Cz)Mn(III) parent
complex which has no axial oxo ligand (spectra not shown).
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(
-
1 13
cm
Complex 2 is capable of oxidizing PPh
Reaction of 2 with increasing amounts of PPh
.
3
, as shown in Scheme 2.
was monitored by
3
(13) Czernuszewicz, R. S.; Su, Y. O.; Stern, M. K.; Macor, K. A.; Kim, D.;
Groves, J. T.; Spiro, T. G. J. Am. Chem. Soc. 1988, 110, 4158-4165.
UV-vis spectroscopy and revealed the spectral changes indicated
in Figure 2, which are complete within the time of mixing and
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J. AM. CHEM. SOC.
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