An isolable, nonreducible high-valent manganese(V) imido corrolazine complex

Article abstract of DOI:10.1021/ic0609251

The manganese(V) imido complex [(TBP₈Cz)Mn^v(NMes)] (2) was synthesized from the Mn^III complex [(TBP₈Cz) Mn^III] (1) and thermolysis of mesityl azide. An X-ray structure of 2 reveals a short Mn-N distance [1.595(4) A], consistent with the Mn-N triple bond expected for a manganese(V) imido species. This high-valent species is remarkably inert to one- and two-electron reductive processes such as NR group transfer to alkenes or H-atom abstraction from O-H bonds. Electrochemical studies support this lack of reactivity. In contrast, oxidation of 2 is easily accomplished by treatment with [(4-BrC₆H₄) ₃N]^?+SbCl₆, giving a π-radical-cation complex.

Full text of DOI:10.1021/ic0609251

Inorg. Chem. 2006, 45, 8477−8479
An Isolable, Nonreducible High-Valent Manganese(V) Imido Corrolazine
Complex
David E. Lansky, Joseph R. Kosack, Amy A. Narducci Sarjeant, and David P. Goldberg*
Department of Chemistry, Johns Hopkins UniVersity, 3400 North Charles Street,
Baltimore, Maryland 21218
Received May 27, 2006
The manganese(V) imido complex [(TBP8Cz)MnV(NMes)] (2) was
synthesized from the MnIII complex [(TBP8Cz)MnIII] (1) and ther-
molysis of mesityl azide. An X-ray structure of 2 reveals a short
Scheme 1
Mn−N distance [1.595(4) Å], consistent with the Mn−N triple bond
expected for a manganese(V) imido species. This high-valent spe-
cies is remarkably inert to one- and two-electron reductive pro-
cesses such as NR group transfer to alkenes or H-atom abstraction
from O
tivity. In contrast, oxidation of 2 is easily accomplished by treatment
with [(4-BrC6H4)3N]•+SbCl6, giving a
-radical-cation complex.
−H bonds. Electrochemical studies support this lack of reac-
π
species have been implicated as key intermediates in NR
group transfer reactions,5 and thus their direct characterization
is of interest. More generally, the synthesis of mid- to late-
transition-metal imido complexes has been challenging, and
only recently have well-characterized examples appeared.6
Herein we report the synthesis of the MnV terminal imido
complex [(TBP8Cz)MnV(NMes)] [2; TBP8Cz ) octakis(p-
tert-butylphenyl)corrolazinato]. Structural characterization
reveals a short, linear Mn-N triple bond for the imido ligand.
This complex is remarkably resistant to either one- or two-
electron reductive processes, even though it contains a high-
valent MnV center.
High-valent Mn complexes have been postulated as the
active intermediates in many synthetic and biologically
relevant transformations. For example, MnV terminal oxo
species have been invoked as the key oxidizing intermediates
in epoxidations and hydroxylations.1 However, the direct
characterization or isolation of such species has only been
accomplished in a few cases.2 We recently synthesized one
example of a stable MnV terminal oxo complex by using
the corrolazine platform.3 Even more elusive has been the
definitive characterization of an isolectronic manganese(V)
imido complex, of which only one example is known, the
corrole complex [(tpfc)MnV(NMes)] [tpfc ) tris(pentafluo-
rophenyl)corrole].4 Manganese(V) imido and related nitrido
The synthesis of the manganese(V) imido complex was
accomplished by heating mesityl azide with [(TBP8Cz)MnIII]
(1) in toluene (Scheme 1), following the method established
for [(tpfc)MnV(NMes)].4 This method presumably involves
* To whom correspondence should be addressed. E-mail:
dpg@jhu.edu.
(4) (a) Eikey, R. A.; Khan, S. I.; Abu-Omar, M. M. Angew. Chem., Int.
Ed. 2002, 41, 3592-3595. (b) Edwards, N. Y.; Eikey, R. A.; Loring,
M. I.; Abu-Omar, M. M. Inorg. Chem. 2005, 44, 3700-3708.
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Am. Chem. Soc. 1999, 121, 9120-9132. (b) DuBois, J.; Tomooka, C.
S.; Hong, J.; Carreira, E. M. Acc. Chem. Res. 1997, 30, 364-372. (c)
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Am. Chem. Soc. 2003, 125, 13350-13351. (c) Hu, X.; Meyer, K. J.
Am. Chem. Soc. 2004, 126, 16322-16323. (d) Shay, D. T.; Yap, G.
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College Press: London, 2000; pp 91-169. (b) Palucki, M.; Finney,
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Rheingold, A. L.; Goldberg, D. P. Inorg. Chem. 2005, 44, 4485-
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10.1021/ic0609251 CCC: $33.50
Published on Web 09/16/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 21, 2006 8477

COMMUNICATION
Figure 1. 1H NMR spectra of 2 (bottom) and 3 (top) in CD2Cl2.
capture of a nitrene intermediate by the MnIII complex,7
resulting in oxidation of the Mn center to MnV and release
of N2. The reaction was easily monitored by thin-layer chro-
matography and UV-vis, indicating loss of the starting
material and production of the manganese(V) imido complex
(2). After evaporation of the solvent, the resulting dark-brown
solid was purified by column chromatography (silica gel,
CH2Cl2) and then washed with CH3CN to remove mesity-
lamine. Complex 2 was obtained as a dark-green solid in
90% yield.
Figure 2. ORTEP diagram of 2 at the 50% probability level. H atoms are
not shown for clarity.
two independent molecules. Such short distances are con-
sistent with
a
Mn-N triple bond. The cor-
role complexes (tpfc)MnV(NMes) and (tpfc)CrV(NMes)4
exhibit M-N bond lengths of 1.613(4) and 1.635(2) Å, re-
spectively, both of which are slightly longer than those found
for 2. For [(TBP8Cz)MnV(O)] (3), the Mn-O distance from
extended X-ray absorption fine structure data [1.56(2) Å] is
slightly shorter than the Mn-Nimido distance, as is expected
for the less sterically encumbered terminal oxo ligand. The
Mn-Nimido-C angle for 2 [179.7(4)°, 176.9(4)°] is closer
to linearity than those for (tpfc)MnV(NMes) [170.4(4)°] and
(tpfc)CrV(NMes) (169.59(18)°) and, taken together with the
slightly shorter Mn-Nimido distance in 2, suggests that there
may be stronger π overlap between the terminal imido ligand
and the empty metal dxz/dyz orbitals for the corrolazine
complex. The Mn-Npyrrole distances in 2 range from 1.869-
(4) to 1.907(4) Å and are similar to the Mn-Npyrrole distances
[average ) 1.884(7) Å] in the MnIIICz complex [(TBP8Cz)-
MnIII(HOMe)],3b despite the higher oxidation state for 2. The
Mn-Npyrrole distances for the manganese(v) imido corrole
complex [1.891(4)-1.947(4) Å] are longer than those in 2
and likely reflect the larger cavity size of the corrole ligand.
The Mn ion sits 0.55 Å above the mean plane of the four
pyrrole N atoms in both independent molecules of 2, which
is slightly greater than the displacement of the Mn ion in
(tpfc)MnV(NMes) (Mn-Nplane ) 0.513 Å).
1
The aromatic region of the H NMR spectrum for 2 is
shown in Figure 1. The spectrum is clearly diamagnetic,
indicative of a low-spin MnV (d2) species. The phenyl (5.74
ppm) and o-methyl (-0.05 ppm) peaks (not shown) for the
NMes ligand are shifted upfield because of ring current
effects. The eight doublets appearing between 7.2 and 8.4
ppm are assigned to the four types of inequivalent para-
substituted tert-butylphenyl substituents. Three sets of dou-
blets overlap, resulting in an integration ratio of 8:4:8:8:4.
A similar aromatic pattern of doublets in a 8:4:8:8:4 ratio is
found for the isoelectronic manganese(V) oxo complex (3;
Figure 1). The phenyl protons do not display magnetic
inequivalence based upon their orientation with respect to
the axial mesitylimido or oxo ligands, implying that there is
free rotation about the C(pyrrole)-C(phenyl) bonds in both
compounds. Variable-temperature 1H NMR experiments
(198-300 K) for 2 reveal only some line broadening at the
lowest temperature reached.8
In addition to the NMR spectrum, the UV-vis, matrix-
assisted laser desorption ionization mass spectrometry, and
elemental analysis for 2 are all consistent with the proposed
structure. Confirmation of this structure was obtained by
X-ray crystallography, and an ORTEP diagram for 2 is
shown in Figure 2. The crystal structure contains two
independent molecules per asymmetric unit, and only one
molecule is shown in Figure 2. The independent molecules
are oriented in slipped dimer pairs, with the imido axial
ligands pointing away from each other. However, the Mn-
Mn distance is 10.0150(11) Å, indicating that the two Cz
rings are not in a position for π-stacking interactions. The
Mn-Nimido distances are 1.595(4) and 1.611(4) Å for the
Attempted reactions of 2 with alkene substrates did not
lead to aziridine products. For example, prolonged heating
of 2 with excess cyclooctene in toluene afforded only the
starting imido complex. Other alkenes (e.g., cis-stilbene,
styrene) were equally unreactive. The manganese(V) oxo
complex 3 shows a similar lack of reactivity toward alkenes
to give epoxides, a formal two-electron process. However,
we have recently shown that 3 is capable of the one-electron
process of abstracting a H atom from O-H bonds (e.g.,
substituted phenols) and weak C-H bonds (e.g., cyclohexa-
diene). Kinetic studies of these reactions showed that they
proceed through an H-atom transfer (HAT) mechanism.9 By
analogy with 3, we speculated that the imido complex 2
should be capable of H-atom abstraction even though it does
not effect the two-electron oxidation of alkene to aziridine.
(7) Abu-Omar, M. M.; Shields, C. E.; Edwards, N. Y.; Eikey, R. A. Angew.
Chem., Int. Ed. 2005, 44, 6203-6207.
(8) For a discussion of the rotational barriers in â-aryl-substituted
porphyrins, see: Medforth, C. J.; Haddad, R. E.; Muzzi, C. M.; Dooley,
N. R.; Jaquinod, L.; Shyr, D. C.; Nurco, D. J.; Olmstead, M. M.; Smith,
K. M.; Ma, J. G.; Shelnutt, J. A. Inorg. Chem. 2003, 42, 2227-2241.
(9) Lansky, D. E.; Goldberg, D. P. Inorg. Chem. 2006, 45, 5119-5125.
8478 Inorganic Chemistry, Vol. 45, No. 21, 2006

COMMUNICATION
Figure 4. UV-vis spectrum of the reaction between 2 and [(4-
BrC6H4)3N]•+SbCl6.
Figure 3. Cyclic voltammogram of 2 in CH2Cl2 at a scan rate of 50 mV/
s. Inset: First cycle from 0 to -1.4 V.
Although reductive transformations appear to be out of
reach for 2, the cyclic voltammetry data show a reversible
oxidation at 0.71 V, indicating that 2 could be reversibly
oxidized with an appropriate chemical oxidant. In compari-
son, the manganese(V) oxo complex 3 is not oxidized until
1.02 V, and (tpfc)MnV(NMes) exhibits an oxidation at 1.21
V. Reaction of 2 with the one-electron oxidant [(4-
BrC6H4)3N]•+SbCl6 (E1/2 ) 1.16 V)12 results in isosbestic
conversion of the UV-vis spectrum for 2, as shown in Figure
4. The original spectrum is restored upon the addition of 1
equiv of Cp2Co, indicating that the oxidation is fully
reversible. Characterization of the oxidized product by EPR
spectroscopy (Figure S1 in the Supporting Information)
shows a sharp singlet at g ) 2.006 (∆H ) 25 G) and no
hyperfine splitting from Mn. This spectrum is indicative of
a corrolazine π-radical-cation ([(TBP8Cz•+)MnV(NMes)]).
In summary, the first example of a manganese(V) imido
corrolazine complex has been synthesized and structurally
characterized. The X-ray structure and NMR data show
conclusively that 2 is a bona fide high-valent MnV (low-
spin, d2) complex. However, 2 is extremely resistant to
reduction via electron transfer, NR group transfer, or HAT.
It is much more difficult to reduce, and easier to oxidize,
than the isoelectronic terminal oxo complex 3. This discrep-
ancy may arise from significantly better π overlap between
the imido ligand and MnV as compared to an oxo ligand.
Complex 2 is even more difficult to reduce than the corrole
analogue, which is surprising because the electron-withdraw-
ing meso-N atoms of corrolazines are expected to make Cz
complexes easier to reduce than their corrole counterparts.14
In the case of 2, we suggest that the “pull” effect of the
meso-N atoms is more than counterbalanced by a strong
“push” effect from the π donation of the terminal imido
ligand.
Interestingly, 2 was found to be completely unreactive
toward H-atom donors such as C6H5OH, 2,4-di-tert-bu-
tylphenol, and even TEMPO-H. The latter substrate has an
O-H bond dissociation energy (BDE) of 70 kcal/mol,10
which is even lower than that of the phenol substrates (BDE
) 78-83 kcal/mol)11 and indicates that it is a highly reactive
H-atom donor. The absence of reactivity toward TEMPO-H
shows that the manganese(V) imido complex is incapable
of even weak H-atom abstraction.
To shed more light on this lack of reactivity for 2,
electrochemical measurements were carried out as shown in
Figure 3. The cyclic voltammogram reveals two quasi-rever-
sible oxidations centered at 0.71 and 1.31 V and irreversi-
ble reductions at -1.36 and -1.68 V. The peaks at -0.66
and -0.49 V appear only after an initial scan is carried out
past -1.3 V, indicating that these peaks are not from the
original imido complex but rather from one or more species
generated by reduction beyond -1.3 V. A clean single sweep
between 0 and -1.3 V reveals no electrochemical activity
(inset, Figure 3), confirming that complex 2 is not reduced
until at least -1.3 V! In support of these results, we have
found that 2 does not react with an excess of cobaltocene
(E1/2 ) -0.95 V).12,13 Thus, although 2 is formally a high-
valent MnV species, it is remarkably resistant to reduction.
In contrast, the manganese(V) oxo complex 3 exhibits a MnV/
MnIV couple at -0.05 V,3b,13 and the analogous redox couple
for (tpfc)MnV(NMes) appears at -0.36 V.4b The first irrever-
sible reduction observed at -1.36 V for 2 is assigned to a
corrolazine ring reduction, Cz/Cz•-, based on electrochemical
and spectroelectrochemical data for other metallocorrolazines
including manganese(III) and manganese(V) oxo complexes.3
The complete absence of H-atom abstraction reactivity for
2 is understandable in light of the electrochemical data
because the thermodynamic driving force for this reaction,
in part, needs to come from the ease of the MnV/MnIV
reduction.9 Because the MnV/MnIV reduction is not accessible,
the thermodynamic affinity of 2 for an H atom is exceedingly
low.
Acknowledgment. The NSF (Grants CHE0094095 and
CHE0606614) is gratefully acknowledged for support of this
work.
Supporting Information Available: Synthetic details and EPR
(PDF) and X-ray crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(10) Bordwell, F. G.; Liu, W. J. Am. Chem. Soc. 1996, 118, 10819-10823.
(11) Lucarini, M.; Pedrielli, P.; Pedulli, G. F. J. Org. Chem. 1996, 61,
9259-9263.
(12) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(13) All E1/2 values have been converted from their original reference
potential to that of Ag/AgCl using the following: Pavlishchuk, V.
V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97-102.
IC0609251
(14) Kerber, W. D.; Goldberg, D. P. J. Inorg. Biochem. 2006, 100, 838-
857.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8479