Inverse axial-ligand effects in the activation of H2O 2 and ROOH by an MnIII corrolazine

Article abstract of DOI:10.1002/anie.200603139

(Chemical Equation Presented) Upside-down push: Reaction of an Mn ^III corrolazine complex with H₂O₂ or ROOH leads to O-O-bond cleavage to give an Mn^V≡O complex (see scheme). Dramatic and unexpected axial-ligand (or "push") effects are observed, namely, the more electron-poor the axial ligand L, the faster and more heterolytic the cleavage.

Full text of DOI:10.1002/anie.200603139

Communications
Ligand Effects
DOI: 10.1002/anie.200603139
Inverse Axial-Ligand Effects in the Activation of
III
H O and ROOH by an Mn Corrolazine**
2
2
David E. Lansky, Amy A. Narducci Sarjeant, and
David P. Goldberg*
The activation of dioxygen and its two-electron-reduced
analogues H O and ROOH by both heme- and non-heme-
2
2
related metal complexes has been the focus of intense efforts
because of its relevance to biochemical processes, as well as
the potential applications of this chemistry in oxidative
catalysis. Heme enzymes such as cytochrome P450, catalases,
and peroxidases react with dioxygen and H O to give high-
2
2
valent iron–oxido intermediates such as iron(IV)–oxido p-
+
IV
cation radical ((porph ·)Fe =O), which is presumed to be the
[
1]
critical oxidizing intermediate in cytochrome P450. Delin-
eating the mechanism of formation of these high-valent
species, and in particular understanding the OꢀO-bond
cleavage events, remains an important goal. In this regard,
many synthetic porphyrin model compounds have been
examined to determine the mechanism of OꢀO-bond cleav-
age with H O , ROOH, and organic peracids as oxidants. In
2
2
these studies, both heterolytic and homolytic cleavage path-
ways were observed, depending upon the nature of the
oxidant and porphyrin employed. However, much remains
unknown about what factors (e.g. identity of porphyrin,
oxidant, or axial ligand) control the cleavage mechanism.
Herein we describe the reactivity of a manganese(III)
corrolazine complex (1; Scheme 1) toward H O and cumene
2
2
hydroperoxide (CmOOH). Corrolazines, which are ring-
contracted porphyrinoid ligands, have been shown to stabilize
V
Scheme 1. Synthesis of the stable Mn ꢁO complex 2.
[*] D. E. Lansky, Dr. A. A. Narducci Sarjeant, Prof. Dr. D. P. Goldberg
Department of Chemistry
Johns Hopkins University
3400 North Charles Street, Baltimore, MD 21218 (USA)
Fax: (+1)410-516-8420
E-mail: dpg@jhu.edu
[**] This research was supported by the NSF (CHE-0606614).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
8214
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Angewandte
Chemie
[
2]
high oxidation states of transition metals. For example,
treatment of 1 with the oxidants PhIO or meta-chloroperoxy-
V
benzoic acid (mCPBA) gives a stable Mn ꢁO complex (2;
[
3]
Scheme 1), which can be isolated at room temperature. This
system is thus ideally suited to test ideas regarding the
mechanism of oxidation by H O or ROOH, as the expected
2
2
high-valent metal–oxido porphyrinoid product can be directly
identified. In contrast, when working with conventional
porphyrins, one usually has to infer the generation of a
high-valent metal–oxido species from indirect observations
because of their instability. Herein we report that 1 is indeed
capable of activating H O and CmOOH, and, moreover,
2
2
there is a remarkable axial-ligand effect upon these reactions.
+
III
ꢀ
Also, the axially ligated complex [Et N] [(TBP Cz)Mn Cl]
4
8
(3) was structurally characterized, which to our knowledge is
the first example of a structurally characterized anionic
corrole species.
Figure 1. ORTEP diagram of the anion of 3 showing the 50% proba-
bility thermal ellipsoids. Hydrogen atoms have been omitted for clarity.
We speculated that reaction of 1 with H O as the
2
2
oxidizing agent in place of PhIO or mCPBA should give 2.
Surprisingly, this reaction was a failure; when 1 was dissolved
in CH Cl or benzene and stirred with excess H O (30%
of pentane into a solution of 1/Et NCl in CH Cl . The Mn ion
4
2
2
ꢀ
in 3 is five coordinate with a single axial Cl ligand, as
2
2
2
2
+
aqueous solution) at 238C, no 2 was produced, as evidenced
by TLC and UV/Vis spectroscopy. However, the coordination
of axial ligands to 1 caused a dramatic change in reactivity
toward H O . It has been shown previously by UV/Vis
expected. The presence of the Et N counterion confirms that
4
the complex is negatively charged and the Mn ion is formally
in the + 3 oxidation state. A UV/Vis spectrum of crystals of 3
redissolved in CH Cl matched the spectrum obtained from
2
2
2
2
ꢀ
[3c]
spectroscopy that Cl forms a 1:1 complex with 1. Addition
of H O to a solution of 1 and Et NCl in CH Cl led to an
addition of Et NCl to 1 in situ, thus confirming that the
4
complex generated in situ is the same as 3. The MnꢀN and
2
2
4
2
2
immediate color change from the brown of 1 to the deep
MnꢀCl bond lengths in 3 are normal, and the Mn ion sits
V
green of the Mn ꢁO complex (Scheme 2). Analysis of the
significantly further out of the macrocyclic plane
III
(
(
0.6609(7) Š), than in the MeOH adduct [(TBP Cz)Mn -
8
[
3c]
HOMe)] (0.373(2) Š). This large out-of-plane displace-
ment may be due to the steric congestion caused by the large
ꢀ
size of the Cl ion. The tendency of the corrole to stabilize
higher oxidation states appears to make the isolation of
negatively charged species rare, whereas corrolazines allow
for the isolation of both relatively low- and high-valent
complexes.
Scheme 2. Activation of H O by axially ligated manganese(III) corrola-
zine.
2
2
ꢀ
The strong influence of the axial Cl ligand on the H O
2
2
reaction prompted us to examine other potential anionic axial
donors. The addition of a series of para-substituted arylthi-
ꢀ
+
reaction mixture by UV/Vis spectroscopy and TLC showed
that 2 was the major product of this reaction. Isolation after
chromatography resulted in a 63% yield of 2. A likely
mechanism for the generation of 2 involves coordination of
olate donors (p-X-C H S Na ; X = H, OMe, CH , Cl, NO )
6 4 3 2
ꢀ
had the same general influence as Cl on the reactivity of 1
toward H O : a rapid decay of 1 with concomitant formation
2
2
V
of the Mn ꢁO complex. However, a more quantitative
comparison made by measuring the rates of reaction of [1ꢀ
H O2 to the open site on the Mn center, followed by
2
ꢀ
ꢀ
heterolytic cleavage of the OꢀO bond (2e reduction) to
L] (L = axial ligand) with H O revealed significant differ-
2
2
ꢀ
give 2 and OH /H O. Independent experiments involving
ences in the activity of the axial ligands examined. The
2
ꢀ
ꢀ
addition of Cl to 2 show no change in the UV/Vis spectrum,
suggesting that once 2 is formed, the Cl ion is no longer
kinetics were monitored by following the decay of [1ꢀL]
ꢀ
(absorbance loss at 685 nm; Figure 2). The decay curves were
fit to give pseudo-first-order rate constants (kobs), and plots of
kobs versus [H O ] showed good linearity; the slopes of these
bound. The strong donation from the triply bonded terminal
oxido ligand may discourage binding of negatively charged
axial ligands, although we have suggested that a neutral donor
2
2
0
0
plots yielded the second-order rate constants (k ). The
observed rate constants include the step involving coordina-
tion of H O (Scheme 2) and are not a direct measurement of
x
[
3b]
(
PhIO) may coordinate to 2. Some homolytic cleavage may
IV
also occur to give a putative Mn ꢀO(H) complex that likely
2
2
[
4]
rapidly decays. This pathway would account for the less than
the OꢀO-bond-cleavage step. However, it is reasonable to
quantitative yield of 2.
assume that the coordination of H O is fast and OꢀO-bond
2
2
ꢀ
Definitive proof of axial ligation by Cl was obtained by
cleavage is the rate-determining step being monitored.
[
5]
+
ꢀ
X-ray crystallography (Figure 1). Crystallization of [Et N] -
Furthermore, the fact that the reaction requires L to proceed
to any measurable extent suggests that OꢀO-bond cleavage is
4
III
ꢀ
[
(TBP Cz)Mn Cl] (3) was accomplished by vapor diffusion
8
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8215

Communications
[
9]
employed as the oxidant. This latter effect appears to be
operative when either the electronic properties of the
porphyrin ligand itself, or the coordinated axial ligands are
varied, and a rationale for these observations has been
[
9b]
offered.
We have clearly established that the electronic
properties of anionic axial ligands bound to a manganese(III)
corrolazine have a strong influence on the mechanism of Oꢀ
O-bond cleavage for peroxide-type oxidants. Furthermore,
this influence is exactly opposite to the expected trend from a
classical push effect. In future work it would be of interest to
determine if neutral donors (e.g. pyridine, imidazole) show
similar trends, and if the origin of these effects can be
discerned. These observations should help in understanding
the mechanism of OꢀO-bond cleavage in porphyrinoid
Figure 2. a) Time-dependent UV/Vis spectral changes for 1+p-CH -
3
ꢀ
+
C H S Na immediately after addition of H O . b) Hammett plot for
6
4
2
ꢀ
2
ꢀ
the reaction of [1-L] +H O (L=p-X-C H S ).
2
2
6
4
ꢀ
affected by L , because it is unlikely that the coordination of
ꢀ
H O2 (or OOH ) would be strongly enhanced by the
systems of both synthetic and biological origin. Moreover,
the insights gained regarding the activation of oxidants such
as hydrogen peroxide may lead to the design of practical
corrolazine-based catalysts that can utilize the highly desir-
able terminal oxidant H O , or perhaps even O itself.
2
ꢀ
formation of [1-L] , a negatively charged complex.
A Hammett analysis of the data (Figure 2) shows that the
rate of the H O reaction clearly increases with an increase in
2
2
the electron-withdrawing nature of the arylthiolate ligand
1 = 0.63 ꢂ 0.079). Assuming this trend arises from an influ-
ence on the OꢀO-bond-cleavage step, this unusual inverse
electron demand is opposite to that expected for the normal Experimental Section
2
2
2
(
[
6]
“
push” effect invoked for porphyrin/heme systems such as
2: H
O (30% aq, 0.0125 mL, 0.11 mmol) was added to a solution of 1
2 2
(
15 mg, 11 mmol) and Et NCl (2.2 mg, 13 mmol) in CH Cl , and the
acylperoxidoiron(III) porphyrins, in which the more electron-
4
2
2
reaction mixture was stirred for 1 h. During this time the color
changed from the dark brown of 1 to the deep forest green of 2. The
crude product was purified by chromatography (silica gel, CH Cl ) to
donating axial ligand increases the rate of OꢀO-bond
[
6g,7]
cleavage for both heterolytic and homolytic pathways.
2
2
To shed more light on this intriguing axial-ligand effect
give 2 as a green solid. Yield: 9.5 mg, (63%).
3: Under aerobic conditions, 1 (5 mg, 3.55 mmol) and Et NCl
ꢀ
and obtain a direct measure of the influence of L on the
4
mechanism of OꢀO-bond cleavage, the reaction of cumene
hydroperoxide and 1 in the presence of different axial ligands
was examined. CmOOH is a useful probe for distinguishing
between heterolytic and homolytic cleavage mechanisms, as
the former pathway gives 2-phenyl-2-propanol whereas the
(0.7 mg, 4.26 mmol) were dissolved in CH Cl (5 mL) and allowed to
2 2
stand. Vapor diffusion of pentane into this solution over 7 days
yielded green-brown crystals of 3, which were isolated by decantation
and air-dried. UV/Vis spectroscopy (CH Cl ): l = 435, 480, 635 nm.
2
2
max
Elemental analysis (%) calcd for C110H138Cl MnN (3·CH Cl ·pentane
3
8
2
2
ꢀ1
1
5
733.6 gmol ): C 76.21, H, 8.02, N, 6.46; found: C 75.53, H, 8.42, N,
.95.
[
1c,8]
latter pathway gives acetophenone.
The results are
summarized in Scheme 3. It is clear that the ratio of
Received: August 2, 2006
Published online: November 14, 2006
Keywords: ligand effects · manganese · peroxides ·
.
porphyrinoids · reaction mechanisms
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Scheme 3. Cleavage pattern of CmOOH+1 with different axial ligands.
heterolytic versus homolytic cleavage products are strongly
influenced by the nature of the axial ligand. There is a
ꢀ
dramatic increase in heterolytic cleavage with F , which is
ꢀ
ꢀ
significantly more electronegative (F = 4.2) than Cl
EN
[
ꢀ
ꢀ
ꢀ
(
Cl = 2.8) or Br (Br = 2.7). Similarly, heterolytic cleav-
EN EN
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2
2
The axial-ligand effect described herein is in stark contrast
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certain iron porphyrins, in particular, when H O or ROOH is
2
2
8216
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Angew. Chem. Int. Ed. 2006, 45, 8214 –8217

Angewandte
Chemie
[
[
4] D. E. Lansky, D. P. Goldberg,Inorg. Chem. 2006, 45, 5119 – 5125.
5] Data collection for 3·(CH Cl ) : T= 110 K, M = 1788.82, crystal
2
2
2.5
r
¯
3
dimensions: 0.29 ” 0.24 ” 0.20 mm , triclinic (P1), a = 15.2032(13),
b = 18.1447(16), c = 21.632(2) Š, a = 69.001(9), b = 87.647(8), g =
3
ꢀ3
6
0
9.974(8)8, V= 5211.4(8) Š , Z = 2, 1 = 1.140 gcm , (MoKa) =
ꢀ1
.328 mm , MoKa radiation (l = 0.71073 Š), 2qmax = 52.748. X-
ray diffraction intensities were collected on an Xcalibur3
diffractometer; total number of reflections 104404, independent
reflections 21023 [Rint = 0.0277]. Final R factors [I > 2s(I)]: R1 =
0.0712, wR2 = 0.2124, GoF = 1.071. Data were collected, inte-
grated, and corrected for absorption and interframe scaling with
the CrysAlis Pro suite of programs (Oxford Diffraction). The
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of programs (G. Sheldrick, Bruker XRD, Madison, WI, 2000). The
structure was solved by using direct methods and completed by
subsequent difference Fourier syntheses and refined by full
2
matrix least-squares procedures on F . All non-hydrogen atoms
were refined with anisotropic displacement coefficients; several
additional disordered solvent molecules could not be modeled
appropriately. Electronic contributions from these molecules
were removed via SQUEEZE (Platon). H atoms were placed in
calculated positions and refined by using a riding group model.
CCDC-616545 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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[
[
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Angew. Chem. Int. Ed. 2006, 45, 8214 –8217ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8217