Synthesis, structure, and catalase-like activity of dimanganese(III) complexes of 1,5-Bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]- pentan-3-ol (X ) H, Br, OCH3)

Article abstract of DOI:10.1021/ic8019793

New diMn^III complexes of general formula [Mn₂L(μ- OR)(μ-OAc)]BPh₄ (H₃L = 1,5-bis[(2-hydroxy-5-X-benzyl) (2-pyridylmethyl)amino]pentan-3-ol, 1: X = H, R = Me, 2: X = OMe, R = Me, 3: X = Br, R = Me, 4: X = Br, R =

Full text of DOI:10.1021/ic8019793

Inorg. Chem. 2009, 48, 3205-3214
Synthesis, Structure, and Catalase-Like Activity of Dimanganese(III)
Complexes of 1,5-Bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]-
pentan-3-ol (X ) H, Br, OCH3)
Herna´n Biava,† Claudia Palopoli,† Carine Duhayon,‡ Jean-Pierre Tuchagues,‡ and Sandra Signorella*,†
Instituto de Qu´ımica Rosario - CONICET, UniVersidad Nacional de Rosario, Suipacha 531,
S2002LRK Rosario, Argentina, and Laboratoire de Chimie de Coordination, UPR CNRS 8241,
205 Route de Narbonne, 31077 Toulouse Cedex 04, France
Received October 16, 2008
New diMnIII complexes of general formula [Mn2L(µ-OR)(µ-OAc)]BPh4 (H3L ) 1,5-bis[(2-hydroxy-5-X-benzyl)(2-
pyridylmethyl)amino]pentan-3-ol, 1: X ) H, R ) Me, 2: X ) OMe, R ) Me, 3: X ) Br, R ) Me, 4: X ) Br, R
) Et) have been prepared and structurally characterized. The synthesized complexes possess a triply bridged
(µ-alkoxo)2(µ-acetato)Mn23+ core, a short intermetallic distance of 2.95/6 Å modulated by the aliphatic spacers
between the central alcoholato and N-amino donor sites, and the remaining coordination sites of the two MnIII
centers occupied by the six donor atoms of the polydentate ligand. In dimethylformamide, complexes 1-3 are able
to disproportionate more than 1500 equiv of H2O2 without significant decomposition, with first-order dependence on
catalyst and saturation kinetic on [H2O2]. Spectroscopic monitoring of the reaction mixtures revealed that the catalyst
converts into [Mn2III(µ-O)(µ-OAc)L], which is the major active form during cycling. Overall, kinetics and spectroscopic
studies of H2O2 dismutation by these complexes converge at a catalytic cycle between MnIII and MnII2 oxidation
2
levels. Comparison to other alkoxo-bridged complexes suggests that the binding mode of peroxide to the metal
center of the MnIII form of the catalyst is a key factor for tuning the Mn oxidation states involved in the H2O2
2
dismutation mechanism.
Introduction
Because of the potential use as catalytic scavengers of H2O2
for preventing oxidative stress injuries, a large number of
dimanganese complexes involving different types of ligands
and Mn oxidation states have been synthesized and charac-
terized in an attempt to mimic the catalytic activity of the
enzyme.7-15 However, the best functional models are still
Manganese is present in the active site of many enzymes
involved in biological redox processes and plays an important
role in the metabolism of dioxygen and its reduced forms.1
Among the Mn redox enzymes, manganese catalases ef-
fectively catalyze the disproportionation of harmful H2O2 into
H2O and O2 by using a Mn2(µ-O2CR)(µ-O/OH/H2O)2
structural unit as the active site.2,3 The catalytic cycle is
known to involve a redox transformation between MnII2 and
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New York, 1999; Vol. 37, p 535.
MnIII but the mechanism is still poorly understood.4-6
2
(7) Wu, A. J.; Penner-Hahn, J. E.; Pecoraro, V. L. Chem. ReV. 2004, 104,
903–938.
* To whom correspondence should be addressed. E-mail: signorel@
infovia.com.ar.
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Spek, A. L.; Bouwman, E. Eur. J. Inorg. Chem. 2005, 305–313.
(9) Dubois, L.; Xiang, D. F.; Tan, X. S.; Latour, J. M. Eur. J. Inorg.
Chem. 2005, 1565–1571.
†
Instituto de Qu´ımica Rosario.
‡
Laboratoire de Chimie de Coordination du CNRS.
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V. L. , Ed.; VCH Publishers, Inc.: New York, 1992; pp 1-28.
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116.
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Harrison, P. M.; Artymiuk, P. J.; Whittaker, J. W. Structure 2001, 9,
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10.1021/ic8019793 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 7, 2009 3205
Published on Web 03/09/2009

Biava et al.
several orders of magnitude slower than catalase enzymes
in terms of kcat and kcat/KM.16 This limitation stimulates the
interest in establishing structure/activity relationships to
afford mechanistic insight into the catalysis of H2O2
disproportionation.
The highly efficient dismutation of H2O2 by manganese
catalases requires a two-electron redox cycle that involves
MnII /MnIII oxidation levels. Thus, the fine-tuning of Mn
2
2
Experimental Section
redox states is a critical feature when using artificial
compounds to model the enzymatic activity.4 Mechanistic
studies of H2O2 dismutation performed on diMn complexes
including polydentate ligands with a central bridging alco-
holato10,11,16-23 or phenolato9,24-26 have shown that, de-
pending upon the Mn · · ·Mn separation and Mn coordination
Materials. All reagents or AR chemicals were used as purchased.
Solvents were purified by standard methods. The concentration of
H2O2 stock solution was determined by iodometric titration.
Synthesis of Ligands. 1,5-Bis[(2-hydroxybenzyl)(2-pyridylm-
ethyl)amino]pentan-3-ol (H3L1), 1,5-bis[(2-hydroxy-5-methoxyben-
zyl)(2-pyridylmethyl)amino]pentan-3-ol (H3L2), and 1,5-bis[(5-
bromo-2-hydroxybenzyl)(2-pyridylmethyl)amino]pentan-3-ol (H3L3)
were prepared through a synthetic procedure slightly modified from
that published for similar ligands.27 Condensation of 5 mmol of
1,5-diaminopentan-3-ol28 with 2 equiv of the corresponding sali-
cylaldehyde in 20 mL of ethanol yielded the corresponding Schiff-
base as a yellow solid. The Schiff-base was reduced in situ with
NaBH4 (3 equiv), added carefully in small amounts, and then heated
at 60 °C during 30 min. The resulting solution was stirred for 1
day, and then treated with concentrated HCl to reach pH 6, and 4
M NaOH up to pH 10. Precipitated sodium borate was filtered off,
and the filtrate was added to 20 mL of an aqueous solution of
previously neutralized 2-pyridylmethyl choride hydrochloride (2
equiv). The reaction mixture was heated at 70 °C during 5 h. Over
this period, 4 M NaOH was added in small portions so that the pH
never exceeded 10. The reaction mixture was then stirred overnight.
The 5 h preheating of the mixture shortened the reaction time from
5 days (without heating) to 1 day. The red solution was cooled to
room temperature, extracted with chloroform (3 × 20 mL), and
the organic phase was dried over MgSO4. Evaporation of the solvent
yielded the H3L1-3 ligands as red residues. The ligands were
environment, either MnII /MnIII2, MnIIMnIII/MnIIIMnIV, or
2
MnIII2/MnIV2 couples are involved in the catalysis. The redox
couples of these complexes fall within the same range of
potentials, so that there must be other factors that modulate
Mn redox states during catalysis. With the aim of gaining
new insights on the structural features that control the Mn
oxidation states involved in the catalase activity, we report
here the synthesis, structure, properties, and catalase-like
activity of new alkoxo-bridged diMn complexes that combine
short (<3.0 Å) Mn· · · Mn separation and metal coordination
sphere saturated by non-labile donors: [Mn2L1-3(µ-OAc)(µ-
OR)]BPh4 (R ) Me or Et), obtained with the heptadentate
ligand 1,5-bis[(2-hydroxybenzyl)(2-piridylmethyl)amino]pen-
tan-3-ol (H3L1) and two phenyl-ring substituted derivatives
(5-methoxy (H3L2) and 5-bromo (H3L3)), and compare their
catalytic activity with that of other alkoxo-bridged diMn
complexes. The present study adds support to the proposal
that the binding mode of peroxide to the diMnIII center is a
critical feature for determining the Mn oxidation states
involved in the catalytic cycle.
1
characterized by H NMR, IR, and electrospray ionization mass
spectrometry (ESI-MS) and were used without further purification.
H3L1. Yield 22.36%. H NMR (CDCl3) δ: 8.5 (m, 2H, py),
1
7.7-6.6 (m, 14 H, ph and py), 3.9-3.5 (m, 9H, N-CH2-ph,
N-CH2-py, H-C(OH)-), 2.7 (m, 4H, -CH2-N), 1.6 (m, 4H,
(HO)C-CH2-). Significant IR bands (KBr,νcm-1): 3380, 3210
(broad), 3060, 2940, 2840, 1593, 756. ESI-MS (CH3CN): m/z )
513.65 [H4L1]+.
(13) Singh, U. P.; Tyagi, P.; Upreti, S. Polyhedron 2007, 26, 3625–3632.
(14) Shin, B. K.; Kim, Y.; Kim, M.; Han, J. Polyhedron 2007, 26, 4557–
4566.
(15) Dubois, L.; Pe´caut, J.; Charlot, M. F.; Baffert, C.; Collomb, M. N.;
Deronzier, A.; Latour, J. M. Chem.sEur. J. 2008, 14, 3013–3025.
(16) Signorella, S.; Rompel, A.; Buldt-Karentzopoulos, K.; Krebs, B.;
Pecoraro, V. L.; Tuchagues, J.-P. Inorg. Chem. 2007, 46, 10864–
10868.
H3L2. Yield 21.15%. 1H NMR (CDCl3) δ: 8.55 (m, 2H, py
protons), 7.7-6.5 (m, 12 H, ph and py), 4-3.6 (m, 9H, N-CH2-
ph, N-CH2-py, H-C(OH)-), 3.72 (s, 6H, -OCH3), 2.75 (m, 4H,
-CH2-N), 1.6 (m, 4H, (HO)C-CH2-). Significant IR bands (KBr,ν
cm-1): 3250 (broad), 3060, 2933, 2833, 1593, 1250, 810, 759. ESI-
MS (CH3CN): m/z ) 573.70 [H4L2]+.
(17) Palopoli, C.; Chansou, B.; Tuchagues, J.-P.; Signorella, S. Inorg. Chem.
2000, 39, 1458–1462.
(18) Palopoli, C.; Gonza´lez-Sierra, M.; Robles, G.; Dahan, F.; Tuchagues,
J.-P.; Signorella, S. J. Chem. Soc., Dalton Trans. 2002, 3813–3819.
(19) Daier, V.; Biava, H.; Palopoli, C.; Shova, S.; Tuchagues, J.-P.;
Signorella, S. J. Inorg. Biochem. 2004, 98, 1806–1817.
(20) Signorella, S.; Tuchagues, J.-P.; Moreno, D.; Palopoli, C. In Inorganic
Biochemistry Research Progress; Hughes, J. G., Robinson, A. J., Eds.;
Nova Sci. Publ. Inc.: New York, 2008; pp 243-279.
(21) Velasco, A.; Bensiek, S.; Pecoraro, V. L. Inorg. Chem. 1998, 37, 3301–
3309.
(22) Pessiki, P. J.; Dismukes, G. C. J. Am. Chem. Soc. 1994, 116, 898–
903.
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3028.
(24) Okawa, H.; Sakiyama, H. Pure Appl. Chem. 1995, 67, 273–280.
(25) Sakiyama, H.; Okawa, H.; Isobe, R. J. Chem. Soc., Chem. Commun.
1993, 882–884.
(26) Dubois, L.; Caspar, R.; Jacquamet, L.; Petit, P. E.; Charlot, M. F.;
Baffert, C.; Collomb, M. N.; Deronzier, A.; Latour, J. M. Inorg. Chem.
2003, 42, 4817–4827.
H3L3. Yield 21.00%. 1H NMR (CDCl3) δ: 8.5 (m, 2H, py
protons), 7.8-6.6 (m, 12 H, ph and py), 4-3.5 (m, 9H, N-CH2-
ph, N-CH2-py, H-C(OH)-), 2.7 (m, 4H, -CH2-N), 1.55 (m, 4H,
(HO)C-CH2-). Significant IR bands (KBr,νcm-1): 3350 (broad),
3060, 2930, 2835, 1594, 817, 757, 734. ESI-MS (CH3CN): m/z )
671.11 [H4L3]+.
Synthesis of Complexes. [Mn2L1(µ-OMe)(µ-OAc)]BPh4 (1).
Mn(OAc)3.2H2O (0.48 g, 1.80 mmol) was added to a solution of
H3L1 (0.46 g, 0.9 mmol) in methanol (25 mL). The mixture was
(27) Krebs, B.; Schepers, K.; Bremer, B.; Henkel, G.; Althous, E.; Warmuth,
W.; Griesar, K.; Haase, W. Inorg. Chem. 1994, 33, 1907–1914.
(28) Murase, I.; Hatano, M.; Tanaka, M.; Ueno, S.; Okawa, H.; Kida, S.
Bull. Chem. Soc. Jpn. 1982, 55, 2404–2408.
3206 Inorganic Chemistry, Vol. 48, No. 7, 2009

Catalase-Like ActiWity of Dimanganese(III) Complexes
stirred for 20 min, and the resulting brown solution was filtered. A
methanol solution of NaBPh4 (0.31 g, 0.9 mmol) was then added,
and the reaction mixture was stirred for 24 h. A green microcrys-
talline precipitate formed, which was collected by filtration, washed
with cold methanol, and dried under vacuum. Yield: 0.52 g (0.5
mmol, 28%). Anal. Calcd for C58H59BMn2N4O6: C 67.71, H 5.78;
N 5.45; Mn 10.68, B 1.05. Found: C 69.23; H 5.70; N 5.52; Mn
9.09; B 1.16. Molar conductivity ) 104 Ω-l cm2 M-l. ESI-MS:
m/z ) 709 [1 - BPh4]+. Significant IR bands (KBr,νcm-1): νCH
3056, 3033, 3000, 2985, 2924, 2858, νAr,Py 1604, 1574, νCO2- 1544/
1427, νCH Ar/Py out-of-plane 829, 760, 735, 707. UV-vis λ nm (ε M-1
cm-1) in dimethylformamide (DMF): 450 (sh), 645 (570).
[Mn2L2(µ-OMe)(µ-OAc)]BPh4 (2). Following the same proce-
dure as for 1, H3L2 (0.64 g, 1.12 mmol) afforded 0.32 g of 2 (0.30
mmol, 26%). Anal. Calcd for C60H63BMn2N4O8: C 66.17, H 5.79;
N 5.14; Mn 10.11; B 1.01. Found: C 66.89; H 5.79; N 4.82; Mn
10.11; B 0.80. Molar conductivity ) 124 Ω-l cm2 M-l. ESI-MS:
m/z ) 769 [2 - BPh4]+. Significant IR bands (KBr,νcm-1): νCH
3057, 3034, 2930, 2986, 2833, νAr,Py 1604, 1573, νCO2- 1545/1426,
described conditions are used, the ferrocene/ferrocenium redox
couple was observed at E1/2 ) 396 mV in CH3CN and E1/2 ) 481
mV in DMF.
Volumetric Measurements. The stoichiometry of the reaction
was measured by volumetric determination of the evolved O2 from
reaction mixtures in DMF, CH3CN, or methanol as previously
described.17
Kinetic Measurements. Oxygen evolution studies were carried
out polarographically using a Clark-type oxygen electrode with an
YSI oxygen-monitoring system (Model 5300, Yellow Springs
Instruments Co., Inc.). The initial rate method was used to determine
the rate constants (see reference 17 for further details). Each rate
constant reported here represents the mean value of multiple
determinations that fall within (5%. Experiments were carried out
at 10 °C. The solubility of oxygen in air-saturated solution of DMF
was determined by a modified Winkler method.29 The O2 concen-
tration measured in DMF equilibrated with air (20.9% O2) at 1 atm
and 283 K was 1.47 ( 0.01 mM.
X-ray Crystal Structure Determination. Crystallographic data
for compound [MnL3(µ-OEt)(µ-OAc)]BPh4 ·0.75CHCl3 ·1.5H2O
were collected at 180 K on an Oxford-Diffraction XCALIBUR CCD
Diffractometer equipped with a Cryojet cooler device from Oxford
Instruments and a graphite-monochromated Mo-KR radiation
source. The unit cell determination and data integration were carried
out using the CrysAlis package.30 The structure was solved using
SIR 9231 and refined by full-matrix least-squares on Fo with
CRYSTALS.32 Mn and Br atoms were refined with anisotropic
displacement parameters. Owing to the poor quality of collected
data, B, C, N, O, and Cl atoms could be refined only with isotropic
displacement parameters. Moreover, to reduce the number of free
parameters, the BPh4 anions were refined as rigid groups. H atoms
were treated as riding on their parent atom with fixed displacement
parameters (1.2-1.5 times Ueq of the parent atom). Hydrogen atoms
of water molecules could not be located. Absorption corrections
were applied using Multiscan.33 The molecular plot was drawn with
Cameron.34 Crystal data collection and refinement parameters are
summarized in Table 1.
ν
CH Ar/Py out-of-plane 845, 813, 760, 745, 734, 707. UV-vis λmax nm (ε
M-1 cm-1) in DMF: 295 (9700), 390 (sh), 665 (650).
[Mn2L3(µ-OMe)(µ-OAc)]BPh4 ·2H2O (3). Following the same
procedure as for 1, H3L3 (0.66 g, 0.9 mmol) afforded 0.40 g of
3 · 2H2O (0.33 mmol, 36,4%). Anal. Calcd for C58H57BBr2Mn2-
N4O6 ·2H2O: C 56.98, H 5.03; N 4.58; Br 13.07; Mn 8.97; B 0.88.
Found: C 56.43; H 4.32; N 4.36; Br 13.03; Mn 9.13; B 0.90. Molar
conductivity ) 123 Ω-l cm2 M-l. ESI-MS: m/z ) 867 [3 - BPh4]+.
Significant IR bands (KBr,νcm-1): νOH 3436, νCH 3055, 2989, 2925,
2852, νAr,Py 1604, 1577, νCO2- 1542/1418, νCH
820,
Ar/Py out-of-plane
796, 760, 752, 734, 707. UV-vis λ nm (ε M-1 cm-1) in DMF:
290 (sh), 380 (sh), 630 (846). The content of 2 molecules of non-
coordinated water was confirmed by thermogravimetric analysis
of the complex which showed a 2.9% mass loss at 120 °C. Single
crystals of [MnL3(µ-OEt)(µ-OAc)]BPh4 ·0.75CHCl3 ·1.5H2O (4)
suitable for X-ray diffraction were obtained from a solution of 3
in a 1:1 CHCl3/EtOH mixture upon standing in air for several days.
Physical Measurements. Electronic spectra were recorded on
a JASCO V550 spectrophotometer with thermostatted cell compart-
ments. IR spectra were recorded on a Perkin-Elmer Spectrum One
FT-IR spectrophotometer. ESI-mass spectra were recorded on a
Perkin-Elmer SCIEX 365 LCMSMS mass spectrometer, using ∼
10-5 M solutions of the complexes or ligands in CH3CN at a flow
rate of 5 µL min-1. DMF reaction mixtures were diluted with
CH3CN to a ≈ 10-5 M concentration. Electron paramagnetic
resonance (EPR) spectra were obtained on a Bruker ESP 300 E
spectrometer with a microwave frequency generated with a Bruker
ER 04 (9-10 GHz). The microwave frequencies were measured
with a Racal-Dana frequency meter, and the magnetic field was
measured with a Bruker NMR probe Gaussmeter. 1H NMR spectra
were acquired on a Bruker AVANCE II 600 NMR spectrometer at
ambient probe temperature (ca. 26 °C), with nominal operating
frequency of 600.13 MHz, using super WEFT sequence, with
acquisition time of 23 ms. Conductivity measurements were
performed using a Horiba D24 conductivity meter, on 1.0 mM
solutions of the complexes in 1:1 methanol/CH3CN mixtures. The
electrochemical experiments were performed with a computer-
controlled Princeton Applied Research potentiostat, model VER-
SASTAT II, with model 270/250 Research Electrochemistry
Software. Studies were carried out under Ar, in DMF or CH3CN
solutions using 0.1 M Bu4NPF6 as supporting electrolyte and ∼10-3
M of the complex. The working electrode was a Pt wire, and the
reference electrode was Ag/AgCl with Pt as the auxiliary electrode.
All potentials are referred to the Ag/AgCl electrode. When the
Results
Characterization of Complexes 1-4. Crystal Structure
of [MnL3(µ-OEt)(µ-OAc)]BPh4 · 0.75CHCl3 · 1.5H2O (4·
0.75CHCl3 · 1.5H2O). Efforts to obtain single crystals of
complexes 1-3 by recrystallization from methanol, DMF,
or CH3CN were unsuccessful. Several green needles of 4
obtained from a solution of 3 in an 1:1 EtOH/CHCl3 mixture
were used for the X-ray diffraction study, and the best set
of collected data was used to solve the structure. Despite
the poor structural resolution, the main features of the
molecular structure of 4 are clearly established. The asym-
metric unit consists of two crystallographically independent
diMn complex cations, non-coordinated Ph4B- anions, sol-
vate water, and CHCl3 molecules in a molar ratio 1:1:0.75:
1.5. The molecular structure of complex 4 is illustrated in
(29) James, H. J.; Broman, R. F. Anal. Chim. Acta 1969, 48, 411–417.
(30) CrysAlis RED, Version 1.70, Oxford Diffraction Ltd.: Oxford, U.K.,
2002.
(31) Altomare, A.; Cascarano, G.; Giaccovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(32) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin,
D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(33) Blessing, R. H. Acta Crystallogr. 1995, 51, 33.
(34) Watkin, D. J.; Prout, C. K.; Pearce, L. J. CAMERON; Chemical
Crystallography Laboratory, University of Oxford: Oxford, U.K., 1996.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3207

Biava et al.
Table 1. Summary of Crystal Data for 4·0.75CHCl3 ·1.5H2O
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complex
4·0.75CHCl3 ·1.5H2O
empirical formula
M
C59H59Br2BMn2N4O6(CHCl3)0.75(H2O)1.5
1317.17
180
Bond Lengths
temperature, K
wavelength, Å
crystal system,
space group
a, Å
Mn(1)-N(1)
Mn(1)-N(2)
Mn(1)-O(1)
Mn(1)-O(2)
Mn(1)-O(4)
Mn(1)-O(6)
Mn(2)-N(3)
Mn(2)-N(4)
Mn(2)-O(2)
Mn(2)-O(3)
Mn(2)-O(5)
Mn(2)-O(6)
2.17(2)
2.26(2)
1.821(19) Mn(3)-O(7)
1.940(17) Mn(3)-O(8)
2.148(19) Mn(3)-O(10)
1.920(18) Mn(3)-O(12)
Mn(3)-N(5)
Mn(3)-N(6)
2.15(2)
2.24(2)
0.71073
triclinic, P1
j
1.852(17)
1.966(17)
2.133(19)
1.926(19)
2.14(2)
10.971(2)
b, Å
23.169(4)
c, Å
25.272(4)
2.12(2)
2.30(2)
Mn(4)-N(7)
Mn(4)-N(8)
R, deg
75.590(10)
78.68(2)
76.840(10)
5991.6(18)
2.24(3)
ꢀ, deg
1.974(16) Mn(4)-O(8)
1.923(18)
1.855(19)
2.200(18)
1.964(18)
γ, deg
1.857(18) Mn(4)-O(9)
V, Å3
2.11(2)
Mn(4)-O(11)
Z
F
µ
4
1.46
1.91
2678
1.988(18) Mn(4)-O(12)
(calcd.), Mg/m3
Angles
Mo, mm-1
N(1)-Mn(1)-O(6)
N(2)-Mn(1)-O(4)
O(1)-Mn(1)-O(2)
N(3)-Mn(2)-O(6)
N(4)-Mn(2)-O(5)
O(2)-Mn(2)-O(3)
172.0(8)
170.0(8)
171.5(8)
166.9(8)
170.5(8)
170.7(8)
N(5)-Mn(3)-O(12)
170.3(8)
170.0(8)
171.6(8)
171.3(8)
170.2(9)
171.0(8)
98.6(8)
F(000)
N(6)-Mn(3)-O(10)
O(7)-Mn(3)-O(8)
N(7)-Mn(4)-O(12)
N(8)-Mn(4)-O(11)
O(8)-Mn(4)-O(9)
Mn(3)-O(8)-Mn(4)
crystal size, mm
θ range, deg
index ranges
no. of reflections:
measured
0.5 × 0.06 × 0.03
2.720-29.070
-11 e h e 14, -31 e k e 31, -34 e l e 34
31298
3667
518
Mn(1)-O(2)-Mn(2) 98.2(7)
unique (I > 2σ(I))
no. of refined
parameters
Mn(1)-O(6)-Mn(2) 98.4(8)
Mn(3)-O(12)-Mn(4) 98.5(8)
GOF for F2
aR [I > 2σ(I)]
bwR
1.1353
0.0973
0.0956
av. 2.26 Å, Mn-Oac av. 2.15 Å) distinctly longer than the
equatorial Mn-N/O ones (av. 1.97 Å), clear signature of
Jahn-Teller distorted co-ordination sphere of MnIII ions. The
values of the trans-angles in the coordination environment
of each Mn ion are in the range of 167-172° (Table 2)
indicating distortion from a pure octahedral geometry. The
Mn · · ·Mn separation (2.95/6 Å) and the Mn-O-Mn angles
(98.2-98.6°) defining the core of the asymmetric unit are
similar to those found in triply bridged bis(µ-alkoxo)(µ-
carboxylato) diMnIII complexes of the X-salpentOH (sal-
pentOH ) 1,5-bis(salicylideneamino)pentan-3-ol) family,
which also bear the 3-pentanol backbone and possess axial
elongation perpendicular to the bridging plane.10,11,17-19 This
intermetallic distance is shorter than that of MnIII2 complexes
of polydentate ligands derived from 1,3-diaminopropan-2-
ol (≈3.2 Å), either with bis(µ-alkoxo), bis(µ-alkoxo)(µ-
carboxylato), or bis(µ-acetato)(µ-alkoxo) bridging motifs,
which possess a Jahn-Teller elongation axis oriented along
the bridging core.36,38-41 These results suggest that the length
of the spacers between the central O-alkoxo and the N-donor
groups modulates the Mn · · · Mn separation in the resulting
a R ) ∑||Fo| - |Fc||/∑|Fo|. wR ) {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2
.
b
2
2
Figure 1. Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagram of the cation
[Mn2L3(µ-OEt)(µ-OAc)]+ (4). The thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Figure 1, and relevant bond lengths and angles are collected
in Table 2. In each complex cation, L3(3-) acts as a
heptadentate ligand through the N4O3 donor set. The two
Mn atoms are six-coordinated and triply bridged by the
central alcoholato of L3(3-), and exogenous syn,syn-acetato
and ethanolato ligands. The phenolato-O, amino-N and
alcoholato-O atoms of the ligand and the ethanolato-O donor
atom form a meridional geometry around each Mn. One of
the axial positions of each Mn atom is occupied by an
acetato-O atom, and the other one is occupied by the
pyridyl-N atom of L3(3-). The Mn-N/O bond lengths are
typical of the MnIII oxidation state.35-38 The coordination
polyhedra of the Mn centers may be described as elongated
octahedra, with the axial Mn-O/N bonds distances (Mn-Npy
MnIII complexes, independently of the bridging motif.
2
FT-IR Spectroscopy. Comparison of the IR spectra of
1-4 evidences the “fingerprint” pattern of L1-3(3-) coordi-
nated to the Mn ions and confirms their analogous structures.
The IR spectra of complexes 1-3 (Figure 2) show absorption
peaks at 1545-1542 cm-1 and 1427-1418 cm-1 that identify
the antisymmetrical and symmetrical stretching vibrations
of the syn-syn 1,3-bridging acetate.42,43 All three complexes
(39) Zhang, Z.; Brouca-Cabarrecq, C.; Hemmert, C.; Dahan, F.; Tuchagues,
J.-P. J. Chem. Soc., Dalton Trans. 1995, 1453–1460.
(40) Zhang, J. J.; Luo, Q. H.; Duan, C. Y.; Wang, Z. L.; Mei, Y. H. J. Inorg.
Biochem. 2001, 86, 573–579.
(41) Miyasaka, H.; Cle´rac, R.; Wernsdorfer, W.; Lecren, L.; Bonhomme,
C.; Sugiura, K.; Yamashita, M. Angew. Chem., Int. Ed. Engl. 2004,
43, 2801–2805.
(35) Gelasco, A.; Pecoraro, V. L. J. Am. Chem. Soc. 1993, 115, 7928–
7929.
(36) Gelasco, A.; Kirk, M. L.; Kampf, J. W.; Pecoraro, V. L. Inorg. Chem.
1997, 36, 1829–1837.
(37) Bertoncello, K.; Fallon, G.; Murray, K.; Tiekink, E. Inorg. Chem. 1991,
30, 3562–3568.
(38) Mikuriya, M.; Yamato, Y.; Tokii, T. Bull. Chem. Soc. Jpn. 1992, 65,
1466–1468.
(42) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227–250.
(43) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 5th ed.; Wiley-Interscience: New York, 1997;
Part B, p 60.
3208 Inorganic Chemistry, Vol. 48, No. 7, 2009

Catalase-Like ActiWity of Dimanganese(III) Complexes
Figure 3. Electronic spectra of compounds 1-3, in DMF.
Figure 2. FT-IR spectra of complexes 1-3.
exhibit strong phenolato and pyridyl absorptions between
1605 and 1575 cm-1 that are shifted by ≈ 10 cm-1 from
those in the free ligands because of the coordination of the
metal to these groups, and three strong bands at 611, 707,
-
and 734 cm-1 attributable to non-coordinated BPh4 , in
agreement with their molecular structure. In line with the
loss of H2O evidenced by the thermogravimetric analysis at
≈ 120 °C, compound 3 displays a broadband at ≈ 3440 cm-1
assigned to non-coordinated water molecules. Powdered
samples of 3 and crystals of 4 show nearly identical IR
spectra. The L1-3(3-) and µ1,3-carboxylato patterns are retained
in the FT-IR spectra of solutions of complexes 1-4 in
CHCl3, showing that the solid state structure of these
complexes is preserved upon dissolution.
Figure 4. 1H NMR spectra of complexes 1, 2, and 3 in D6-DMSO. [catalyst]
) 10 mM.
also observed for other diMnIII complexes with phenoxo
ligands.10,11,44-47 Absorptions at 645 nm (ε ) 570, 5-H),
665 nm (ε ) 650, 5-OMe), and 630 nm (ε ) 846, 5-Br) can
be assigned to d-d transitions in agreement with reported
values for related diMnIII complexes.44,45,48,49
ESI-Mass Spectrometry (ESI-MS). ESI-mass spectra of
the complexes 1-3 in CH3CN confirmed their chemical
composition as well as retention of their nuclearity in
solution. For complex 1, the parent peak is observed at m/z
) 709.4 (100%) in the positive mode ESI-mass spectra and
originates from the [Mn2L1(OAc)(OMe)]+ monocation. Other
peaks generated during the electrospray experiments cor-
respond to the exchange of OMe by OH ([Mn2L1(OAc)-
(OH)]+, m/z ) 695.4, 16%) and substitution of both
exogenous bridging ligands by an oxo-bridge ([Mn2L1O]+,
m/z ) 635.4, 2%). Complexes 2-3 afford similar mass
spectra with the parent peak at m/z ) 769 ([Mn2L2(OAc)-
(OMe)]+) and m/z ) 867 ([Mn2L3(OAc)(OMe)]+), respec-
tively. The isotopic patterns of these peaks match very well
their simulated spectra. In addition, negative ion ESI-mass
spectra of the three complexes show only one peak at m/z
1H NMR Spectroscopy. The paramagnetic 1H NMR
spectra of complexes 1-3 in D6-DMSO show a common
pattern with signals ranging from +97 to -95 ppm (Figure
4). The spectral pattern for protons of the phenol/pyridyl arms
should result from a mechanism of σ/π-spin delocalization
through the C bonds of the coordinated phenolato and pyridyl
rings that generates large chemical shifts and relaxation
values for the ring protons of the ligand.50-53 Assuming that,
2
like in the crystals of 4, the dz orbital is oriented along the
Npy-Mn-OAc bonds, the phenolato ring is expected to be
predominantly influenced by a π spin-delocalization pathway
-
) 319 corresponding to BPh4 anion, in agreement with
(44) Dubois, L.; Xiang, D. F.; Tan, S. S.; Pecaut, J.; Jones, P.; Baudron,
S.; Le Pape, L.; Latour, J. M.; Baffer, C.; Chardon-Noblat, S.; Collomb,
M. N.; Deronzier, A. Inorg. Chem. 2003, 42, 750–760.
(45) Karsten, P.; Neves, A.; Bortoluzzi, A.; Stra¨hle, J.; Maichle-Mo¨ssmer,
C. Inorg. Chem. Commun. 2002, 5, 434–438.
(46) Neves, A.; Erthal, S. M. D.; Vencato, I.; Ceccato, A. S.; Mascarehas,
Y. P.; Nascimento, O. R.; Ho¨rner, M.; Batista, A. A. Inorg. Chem.
1992, 31, 4749–4755.
(47) Hirotsu, M.; Kojima, M.; Mori, W.; Yoshikawa, Y. Bull. Chem. Soc.
Jpn. 1998, 71, 2873–2884.
(48) Suzuki, M.; Mikuriya, M.; Murata, S.; Uehara, A.; Oshio, H.; Kida,
S.; Saito, K. Bull. Chem. Soc. Jpn. 1987, 60, 4305–4312.
(49) Gultneh, Y.; Tesema, Y. T.; Yisgedu, T. B.; Butcher, R. J.; Wang,
G.; Yee, G. T. Inorg. Chem. 2006, 45, 3023–3033.
conductometric measurements indicating that these com-
plexes are 1:1 electrolytes in solution.
Electronic Spectroscopy. The electronic spectra of the
complexes 1-3 in DMF exhibit similar features (Figure 3).
All three complexes show an intense absorption at 267 nm
which can be attributed to intraligand π-π* transitions.
Absorptions in the 350-500 nm range correspond to LfM
charge-transfer transitions from pπ orbitals of the phenoxo
oxygen to the partially filled dπ orbitals of the MnIII ion, as
Inorganic Chemistry, Vol. 48, No. 7, 2009 3209

Biava et al.
time-scale. A slow exchange process is not surprising because
bridging acetato in these complexes is not easily substituted,
as shown by the formation of 4 which implies replacement
of the µ-OMe bridge by a µ-OEt one with retention of
µ-OAc. These points together with the analogous pattern
observed for the complex treated with 2.0 equiv of Bu4NOH
or sodium benzoate (Figure 5e) suggest that it is indeed the
basic medium that causes the spectral changes, most probably
through conversion of the starting methoxo-bridged complex
into a µ-oxo one. For this µ-oxo complex we may assume a
higher symmetry than that of 3 (in accordance with the
simpler spectrum (Figure 5d,e), that results in the equivalence
of the two pyridyl rings as well as of the phenolato rings
2
with the dz orbital of the Mn atom oriented along the short
Mn-O-oxo bond. The methoxo/oxo exchange in basic
medium was confirmed by mass spectrometry. ESI-mass
spectra obtained after addition of 2 equiv of Bu4NOH over
the catalyst solution, were dominated by the peak at m/z 853
belonging to the [Mn2L3(OAc)(OH)]+ monocation. Electronic
spectroscopy provided another piece of evidence supporting
formation of the µ-oxo substituted complexes. UV-vis
spectra of DMF solutions of complexes 1-3 plus 2 equiv
of Bu4NOH (or AcO-) showed a d-d band at ≈ 500 nm
characteristic of (µ-oxo)(µ-carboxylato)MnIII2 complexes.55
Figure 5. 1H NMR spectra of 3 (10 mM), before (a) and after addition of
(b) 0.5, (c) 1.0; (d) 2.0 equiv of NaOAc, and (e) 2.0 equiv of NaBzO, in
D6-DMSO.
involving molecular dπ orbitals while the pyridyl ring should
be influenced by both σ and π spin delocalization involving
2
the occupied dz and dπ orbitals. Besides, if the two phenolato
and pyridyl rings in these complexes are not in phase (or
1
are not symmetrically related) the H NMR signals for the
two phenolato and pyridyl moieties should be observed at
different positions. Consequently, sixteen (1) and fourteen
(2-3) resonances are expected from two magnetically non-
equivalent terminal pyridyl and phenolato ring protons,
shifted down or upfield depending on the dominating, σ or
π, spin-delocalization. Figure 4 appears to show all of these
peaks (although we have not attempted to assign them) as
well as the broader peaks of methylene protons spanning
the full range of the spectra.50
Kinetic Stability of the Complexes. Complexes 1-3 are
EPR silent in the solid state and in DMF or CH3CN frozen
solution (T ≈ 117 K), a fact consistent with the presence of
MnIII ions. No EPR signal was observed even after 24 h or
when air was bubbled through the complex solutions. The
complexes are EPR silent in the presence of water (up to
10%), meaning that water does not induce disproportionation
of the MnIII2 core. The stability of complexes 1-3 was also
checked by UV-vis spectroscopy. Spectra of the complexes
registered at different time-lengths after preparation of Ar
saturated solutions in anhydrous DMF or CH3CN, even when
water was added (up to 10%), showed identical λmax and
molar absorbance coefficients.
For a number of Mn2III-µ-OAc complexes, the methyl
group of the bridging acetato has been characterized by a
broad resonance with large downfield shift.50,54 With the aim
of identifying the bridging acetato resonance, sodium acetate
was added to the D6-DMSO solution of complex 3. Shown
in Figure 5b-d are the spectra taken after addition of 0.5-2.0
equiv of acetate to the solution used to obtain the spectrum
of Figure 5a. The significant points regarding the spectral
pattern changes upon addition of acetate are as follows: (i)
none of the signals showed the expected upfield shift as a
consequence of acetate addition (if chemical exchange takes
place, an upfield shift for the bridging acetato resonance is
expected to be a linear function of acetate added); (ii) a lower
number of resonances is now observed suggesting formation
of a compound with higher symmetry. Result (i) may imply
that acetate is either bound as a terminal ligand in solution
with a chemical shift in the diamagnetic region or it is
strongly bound and the exchange process is slow on the NMR
Electrochemical Studies. The electrochemical properties
of complexes 1-3 were investigated by cyclic voltammetry
in DMF and CH3CN solutions containing 0.1 M Bu4NPF6.
In DMF, the three complexes exhibit one quasi-reversible
reduction wave at E1/2 -37 mV (1), -115 mV (2), and 32
mV (3) (Figure 6a-c). Linear voltammetry confirmed that
these waves correspond to reduction processes. W1/2 values
of 114 mV for 1, 127 mV for 2, and 115 mV for 3 in the
square-wave voltammetry experiments suggest that this
reduction corresponds to a one-electron process and may be
attributed to the MnIII2/MnIIIMnII redox couple. In the case
of complexes 2 and 3, an additional re-oxidation wave
appeared at 170 mV for 2 and 300 mV for 3, associated to
the reduction wave. This extra peak could correspond to the
oxidation of the chemically transformed MnIIMnIII form of
the complex. In CH3CN, the three complexes showed a one-
electron quasi-reversible reduction wave at E1/2 -46 mV (1),
-120 mV (2), and 38 mV (3).
(50) Wright, D. W.; Mok, H. J.; Dube´, C. E.; Armstrong, W. H. Inorg.
Chem. 1998, 37, 3714–3718.
(51) Bermejo, M. R.; Gonza´lez, A. M.; Fondo, M.; Garc´ıa-Deibe, A.;
Maneiro, M.; Sanmart´ın, J.; Hoyos, O.; Watkinson, M. New J. Chem.
2000, 24, 235–241.
(52) Pyrz, J. W.; Roe, A. L.; Stern, L. J.; Que, L., Jr. J. Am. Chem. Soc.
1985, 107, 614–620.
(53) Aromi, G.; Bhaduri, S.; Artu´s, P.; Huffman, J. C.; Hendrickson, D. N.;
Christou, G. Polyhedron 2002, 21, 1779–1786.
(54) Hage, R.; Gunnewegh, E. A.; Nie¨l, J.; Tjan, F. S. B.; Weyhermu¨ller,
T.; Weighardt, K. Inorg. Chim. Acta 1998, 268, 43–48.
(55) Albela, B.; Chottard, G.; Girerd, J. J. J. Biol. Inorg. Chem. 2001, 6,
430–434.
3210 Inorganic Chemistry, Vol. 48, No. 7, 2009

Catalase-Like ActiWity of Dimanganese(III) Complexes
Figure 7. Effect of the [H2O2] on the initial rate of H2O2 disproportionation
at 10 °C, in DMF.
ring size results in an increased stability of the MnIII ion
toward oxidation. A similar effect was observed for com-
plexes differing only in the length of their chelating arms.49
H2O2 Dismutation Studies. Stoichiometry. The ability
of complexes 1-3 to catalyze H2O2 disproportion was tested
in DMF, CH3CN, and methanol. Addition of H2O2 to a
solution of the catalyst causes an immediate vigorous
evolution of dioxygen coupled to color change: from green
to brown in DMF and CH3CN, and from green to yellow in
methanol. Turnovers as high as 1500 without significant
decomposition were measured for the three complexes in
DMF. In CH3CN, complexes 1-3 were able to dispropor-
tionate up to 300 equiv of H2O2, while rapidly inactivated
in methanol.
Figure 6. Cyclic (black line) and square-wave voltammogram (gray line)
of (a) 1, (b) 2, and (c) 3, in DMF. Conditions: Pt/Pt/Ag-AgCl; conc. ) 1
mM; supporting electrolyte ) Bu4NPF6. Scan rate (CV) ) 100 mV/s. Scan
frequency (SWV) ) 5 Hz. (d) Hammet plot of redox potentials of complexes
1-3 in DMF and CH3CN against substituent parameter, σp.
As expected, the substituent on the phenolato arms
influences the redox potentials. Therefore, the MnIII
/
2
MnIIMnIII redox potential of 1-3 increases with the electron-
withdrawing ability of the substituent, yielding a good linear
correlation when plotted against the Hammett substituent
constants (σp), as shown in Figure 6d.
On the oxidative side, 1-3 exhibit one irreversible wave
at Ep 1.03 V (DMF) and 0.9 V (CH3CN), corresponding to
-
the oxidation of the free BPh4 anion. No metal centered
oxidation is observed in the anodic scan probably because
-
the intense oxidation wave of BPh4 around 1 V hides the
Kinetics. The initial rate of H2O2 disproportionation by
complexes 1-3 in DMF was measured as a function of the
complex and substrate concentrations at 10 °C. At constant
[H2O2]0, the initial rate of H2O2 disproportionation varies
linearly with the [catalyst], meaning that the reaction is first-
order on catalyst. At constant [catalyst], the initial rate of
H2O2 dismutation exhibits saturation kinetics with [H2O2]0
(Figure 7), and the experimental data could be fitted to the
Michaelis-Menten equation from which the catalytic turn-
over number (kcat) and the Michaelis constant (KM) were
determined. At 25 °C, the uncertainty on kcat and KM values
is large because the reaction rate is far from its maximal
value, even for 500 mM H2O2. This was especially evident
for 3 which possess the higher KM value. For this reason,
kinetic parameters obtained at this temperature were disre-
garded. Values of kcat and KM obtained for complexes 1-3
at 10 °C in DMF are listed in Table 3 together with those of
other alkoxo-bridged diMn complexes for which kinetic
studies reported the kcat and KM values.
wave corresponding to the oxidation of complexes 1-3 to
the MnIIIMnIV mixed valence. It is however evident that
reduction and oxidation processes take place at widely
different potentials yielding electrochemical stability of these
complexes over a ≈ 1 V potential range. A similar overall
electrochemical behavior has also been observed for the
related [MnIII2(µ-OAc)2(dbhbpmmpO)]+ complex (dbhbpm-
mpOH ) 2,6-bis{[(3,5-di-tert-butyl-2-hydroxybenzyl)(2-
pyridylmethyl)amino]methyl}-4-methylphenol),56 in which
each Mn possesses an N2O4 coordination sphere and the
MnIII2/MnIIMnIII and MnIII2/MnIIIMnIV redox processes take
place at 0.04 and 0.95 V, respectively. At variance with these
complexes, [Mn2(µ-OAc)(µ-OMe)(hbpmpnO)]+ (hbpmpnOH
) 1,3-bis[(2-hydroxy-benzyl)(2-pyridylmethyl)amino]pro-
pan-2-ol) is stable in a narrower ∆E range (≈ 0.6 V,
CH3CN), and the MnIII2/MnIIIMnIV couple is shifted to
significantly lower potentials: 0.23 V.20 Although the three
types of ligands bear two N2O2 donor sets and the same
chelating arms, they differ in the length of the spacers
between the central O-alcoholato/phenolato and the N-amino
donor groups: one C-atom for hbpmpnOH versus two
C-atoms for H3L1-3 and dbhbpmmpOH. These changes in
the ligand framework modulate the oxidation potentials of
the resulting diMnIII complexes: an increase in the chelate
Spectroscopic Monitoring of the Catalase-Like Reac-
tion. Room temperature UV-visible absorption spectra taken
in DMF during the progress of the reaction of complexes
1-3 with excess H2O2 exhibited a new absorption band at
≈500 nm that grew in and persisted after complete con-
sumption of H2O2 concurrently with the decrease and
disappearance of the absorbance at 630-665 nm (shown in
Figure 8 for 1). The position and ε of the absorption band at
(56) Lomoth, R.; Huang, P.; Zheng, J.; Sun, L.; Hammarstro¨m, L.;
Akermark, B.; Styring, S. Eur. J. Inorg. Chem. 2002, 2965–2974.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3211

Biava et al.
Table 3. Kinetic Parameters for Complexes 1-3 and Comparison with Other Dimanganese Catalysts Including Polydentate Ligands with a Central
Bridging Alcoholatoa
catalyst
kcat (s-1
)
KM (mM)
solvent, T (°C)
ref
1
2
3
l1.69(7)
1.31(5)
2.8(2)
1.1(1) × 102
8.8(9) × 10
1.7(2) × 102
(1.5-6.0) × 102
10-1.2 × 102
6
DMF, 10
DMF, 10
DMF, 10
DMF, 25
CH3CN, 25
MeOH:H2O, 25
DMF, 25
this work
this work
this work
20
21
23
[Mn2(µ-OAc)2(X-hbpmpnO)]+
[Mn(X-salpnO)]2
3.4-23
4.2-21.9
[Mn2(µ-O)(OAc)(OH)(benzimpnO)]+
2.7
[Mn2(µ-OMe)(µ-OAc)(X-salpentO)S2]+
0.75-7.9
16-78
10, 11, 17-19
a benzimpnOH ) N,N,N′,N′-tetrakis(2-methylenebenzimidazolyl)-1,3-diaminopropan-2-ol; hbpmpnOH ) 1,3-bis[(2-hydroxybenzyl)(2-pyridylmethyl)-
amino]propan-2-ol; salpnOH ) 1,3-bis(salicylidenamino)propan-2-ol; salpentOH ) 1,5-bis(salicylidenamino)pentan-3-ol. S ) solvent. X) aromatic
substituents.
obtained upon addition of 2 equiv of base (or AcO- or BzO-)
to the solution of catalyst, indicating that the oxo-diMn
species formed either during catalysis or in basic medium
are essentially the same.
The EPR spectra of complexes 1-3 recorded in frozen
DMF at various times following addition of 150 equiv of
H2O2 were essentially EPR silent, except for a very small
six-line signal at g ≈ 2 (hyperfine splitting of ∼90 G), which
slowly increased for long reaction times. This signal, which
includes weak doublets inserted between the six absorptions
(∆m ) 1 nuclear-spin-forbidden transitions characteristic of
the hyperfine structure of an uncoupled MnII ion59), persisted
and intensified upon successive additions of H2O2, indicating
formation of an irreversible final product, and not an
Figure 8. Electronic and ESI-mass spectra of 1 before (black line) and
after (gray line) addition of 100 equiv of H2O2.
intermediate species. The absence of EPR signal during the
reaction course, except for the inactive uncoupled MnII
species, suggests that the MnIII form of the catalyst is the
2
500 nm are comparable to those assigned to a d-d transition
in (µ-oxo)(µ-carboxylato)MnIII2 complexes, and ε is clearly
lower than that reported for the phenolato-to-MnIII charge
transfer transition.55,57,58 The absorption band at 500 nm
persisted upon successive additions of 100 equiv of H2O2 to
the DMF solution of the catalyst, and the O2 production rate
measured after each new addition was essentially constant,
indicating that the oxo-bridged species is the active form of
the catalyst. Conversion of the starting complex into
[Mn2(OAc)(O)L] during the reaction was confirmed by mass
spectrometry. ESI-mass spectra obtained after addition of
100 equiv H2O2 to the catalyst solution were dominated by
the peak of [Mn2(OAc)(OH)L]+ (shown for 1 (m/z ) 695)
in the inset of Figure 8). These results provided a clear
indication that the starting MnIII2 complex converts into the
[Mn2(OAc)(O)L] one during the catalytic cycle (the proto-
nated form of which is observed in the mass spectra) and
that the active form of the catalyst contains bound acetato.
In CH3CN, ESI-mass spectra taken at different reaction times
showed substantial intensity decrease of the peak corre-
sponding to [Mn2(OAc)(OH)L]+, indicating partial loss of
catalyst in agreement with the lower turnover numbers
dominant species in solution since all other known forms of
the diMn complexes are EPR active. The intensity of the
weak 6-line signal rapidly increased upon addition of
p-toluensulphonic acid to the reaction mixture in DMF,
concomitantly to the loss of catalytic activity of the complex.
Therefore, protonation of the catalyst favors formation of
an inactive uncoupled MnII species, which is only a minor
species when the reaction is performed in the absence of
proton source (the intensity of the EPR signal of a 150:1
H2O2/catalyst mixture in DMF was 2-6% of that for the
same reaction performed in DMF plus p-toluensulphonic
acid). This can explain the loss of activity observed for these
complexes in methanol, a protic solvent.
Discussion
Ligands H3L1-3 yield complexes 1-4 with a triply bridged
bis(µ-alkoxo)(µ1,3-carboxylato) diMnIII core that are structural
mimics of the MnIII form of Mn catalases. X-ray crystal-
2
lographic data of 4, where the methoxo is replaced by an
ethoxo bridge, show that the Mn · · · Mn distance (2.95/6 Å)
in these complexes is only 0.07-0.19 Å shorter than the
Mn · · · Mn separation of 3.03 and 3.14 Å found in the MnIII
1
2
observed for complexes 1-3 in CH3CN. H NMR spectra
of 10:1 H2O2/catalyst reaction mixtures in D6-DMSO ob-
tained at the end of the reaction were identical to those
form of L. plantarum3 and T. thermophilus,4 respectively,
where the two Mn ions are triply bridged through a µ1,3-
carboxylato and two solvent-derived single O-bridges. Solid
state and solution spectroscopic studies indicate that the
nature of the phenolato ring substituent does not affect
(57) Sheats, J. E.; Czernuszzewicz, R. S.; Dismukes, G. C.; Rheingold,
A. L.; Petrouleas, V.; Stubbe, J.; Armstrong, W. H.; Beer, R. H.;
Lippard, S. J. J. Am. Chem. Soc. 1987, 109, 1435–1444.
(58) Wieghardt, K.; Bossek, U.; Nuber, B.; Weiss, J.; Bonvoisin, J.;
Corbella, M.; Vitols, S. E.; Girerd, J. J. J. Am. Chem. Soc. 1988, 110,
7398–7411.
(59) Bleaney, B.; Rubins, R. S. Proc. Phys. Soc. London 1961, 77, 103–
112.
3212 Inorganic Chemistry, Vol. 48, No. 7, 2009

Catalase-Like ActiWity of Dimanganese(III) Complexes
Scheme 1. Proposed Catalytic Cycle for the H2O2 Disproportionation
by Complexes 1-3
the coordination mode of the ligand, or the structure of the
dimetallic core of complexes 1-3, and that their structure
is retained in methanol, CH3CN, or DMF solution, even in
the presence of water.
The three complexes catalyze dismutation of H2O2 with
saturation kinetics on substrate (Figure 7) and first order
dependence on catalyst. The lack of any pattern different
from that of MnIII in the electronic spectra, the retention of
the dinuclearity evidenced in the ESI-mass spectra, and the
absence of EPR signal during the reaction course, except
for the inactive uncoupled MnII species present in very low
proportion, suggest that the major active form of the catalyst
occurs in the MnIII oxidation state during cycling: MnIII
2
2
complexes being EPR silent, no EPR signal is expected for
the catalyst in this oxidation state.
Although the substituent on the phenolato arms has
negligible influence on the structure of the complexes in this
series, it affects their redox potential and their catalase
activity. Figure 6d and Table 3 show that the redox potentials
and the catalytic activity of the complexes increase with
increasing electron-withdrawing ability of the substituent, and
a linear correlation results when log(kcat) are plotted against
the redox potentials of complexes 1-3 (not shown), the last
used as a measure of electronic effects of substituents. As a
result, complex 3, with the Br-substituent, is the best oxidant
(electron-withdrawing effect stabilizes lower oxidation states)
and has the highest kcat among the complexes of this family.
However, this complex possesses higher KM value than the
unsubstituted and 5-OMe derivatives, meaning that the
electron-withdrawing group lowers the affinity of the catalyst
for the substrate. Thus, although the Br derivative (3) is more
effective at oxidizing H2O2 it has difficulty at binding the
substrate, implying that maximal rate can only be achieved
at higher [H2O2] than for the unsubstituted (1) or the 5-OMe
(2) derivatives. The fact that the catalyst which is easier to
reduce reacts faster (electron-withdrawing > H > electron-
donating substituent) indicates that reduction of the catalyst
with simultaneous H2O2 oxidation should be the slow step
in the catalytic cycle. This implies that the oxidized form of
the catalyst is the major catalytic species in solution, which,
from the spectroscopic studies, is a MnIII2 species. Therefore,
kinetic and spectroscopic results converge at a catalytic cycle
that runs between MnIII and MnII oxidation states, which
0.93),60 or upon addition of acid to the reaction in DMF,
protonation of these bridges results in the inactivation of the
catalyst; in DMF, a non protic solvent (RDMF ) 0), these
catalysts disproportionate H2O2 with only minute loss of
activity. In CH3CN, a solvent with intermediate proton donor
ability (RCH3CN ) 0.19), complexes 1-3 show moderate
activity, disproportionating up to 300 equiv of H2O2.
Spectroscopic monitoring of catalyzed H2O2 dispropor-
tionation revealed that the starting complex converts into a
new species that retains bound acetate, while the methoxo
bridge is replaced by an oxo one. This species dismutates
H2O2 at constant rate upon successive additions of H2O2,
suggesting that it is the true catalyst. The first order
dependence of the reaction rate on catalyst and the lack of
time-lag at the onset of the reaction suggest that the starting
µ-methoxo-MnIII complex reacts with H2O2 at a rate
2
comparable to that of the active µ-oxo-MnIII complex
2
involved in the catalytic cycle. A possible mechanism
including an active µ-oxo- MnIII species is presented in
2
Scheme 1. It is proposed that initial reaction of H2O2 with
the starting complex yields O2 and the reduced complex
[MnII (µ-OAc)L1-3]. Proton transfer from terminally bound
2
hydroperoxide to the methoxo ligand may account for the
dissociation of methanol and facilitates the release of O2.
The reduced Mn2II complex may rapidly react with H2O2 to
yield the oxidized catalyst: [MnIII2(µ-O)(µ-OAc)L1-3]. This
MnIII2(µ-O)(µ-OAc) species is the catalytic form that par-
ticipates in the slow oxidative half-reaction (turnover-limiting
step). Observation of substrate saturation kinetics indicates
that a catalyst-peroxide adduct is formed at the turnover-
limiting step, the formation of which requires a ligand-shift
to free some coordination position.61,62 Binding of peroxide
to the oxidized catalyst might take place through an acetato-
shift61 with the endogenous µ-oxo-ligand acting as internal
base to assist deprotonation of the terminally bound peroxide
coupled to its two-electron oxidation. This results in O2
2
2
are the oxidation states of Mn in the MnCAT catalytic cycle.4
Oxidation of the reduced form of the catalyst should occur
in a fast step within the catalytic cycle thus preventing its
spectroscopic observation.
In DMF, catalysts 1-3 disproportionate more than 1500
equiv of H2O2 with only slight decrease of activity. However,
when the reaction is performed in CH3CN, these complexes
lose activity after 300 turnovers, and in methanol (or when
an acid is added to the reaction mixture in DMF) the
[catalyst] rapidly decreases, activity is lost, and Mn2+
accumulates in the reaction mixture. It is likely that the
bridging ligands of the complexes serve as internal bases
facilitating deprotonation of the peroxide coupled to the redox
(60) Shorter, J. Correlation Analysis of Organic ReactiVity; Research
Studies Press: New York, 1983; pp 146-153.
(61) Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759–805.
(62) Rosenzweig, A. C.; Frederick, C. A.; Lippard, S. J.; Nordlund, P.
Nature 1993, 366, 537–543.
reaction. Therefore, in the protic MeOH solvent (RMeOH
)
Inorganic Chemistry, Vol. 48, No. 7, 2009 3213

Biava et al.
release and formation of the reduced Mn2II catalyst: [MnII (µ-
dismutate H2O2.10,11,17-19 Complexes of this family contain
polydentate Schiff-base ligands derived from 1,5-diaminopen-
tan-3-ol that modulate the intermetallic distance to 2.91-2.94
Å, possess a MnIII2(µ-OAc)(µ-OR)23+ core with two labile
coordination sites in cis-position to each other, and dismutate
H2O2 with saturation kinetics (bottom entry in Table 3).
Although the redox potentials of these later compounds fall in
the same range as those of complexes of the other three classes,
the H2O2 disproportionation cycle involves interconversion
2
OAc)(H2O)L1-3]. The trend in KM as a function of the
aromatic substituent supports the ligand-shift in terms of the
strength of the phenoxo-Mn bond: the stronger the donation
from the O-phenolato, the weaker the Mn-acetato interaction.
Thus, the derivative including an electron-donating substitu-
ent can participate in the carboxylate-shift more easily (lower
KM) than the unsubstituted or 5-Br-substituted complexes.
In the fast reductive half-reaction, the reduced MnII2 catalyst
reacts with another H2O2 molecule to yield H2O and restore
[MnIII2(µ-O)(µ-OAc)L1-3], thus closing the cycle. This
mechanistic scheme is fully consistent with the kinetic and
spectroscopic studies of H2O2 disproportionation by com-
plexes 1-3.
between the MnIII and [MnIV)O]2 forms. In this case, both
2
the short Mn · · ·Mn separation and the occurrence of two labile
coordination sites might facilitate the µ-bridging mode of
peroxide leading to O-O cleavage and formation of the
[MnIV)O]2 form of catalyst. This is not possible for complexes
1-3 because, despite the short Mn · · ·Mn distance (2.95/2.96
Å), saturation of the coordination sphere of the Mn ions by
nonlabile donors enforces peroxide to bind Mn as a terminal
ligand (through ligand-shift) resulting in a catalytic cycle that
Three other classes of alkoxo-bridged diMn complexes
display catalase activity by redox cycling between MnIII and
2
MnII oxidation states.20,21,23 These complexes contain poly-
2
dentate ligands derived from 1,3-diaminopropan-2-ol that
modulate the Mn · · ·Mn separation to ≈ 3.2-3.3 Å, and possess
MnIII2(µ-OR)24+, MnIII2(µ-O)(µ-OR)(OH)(OAc)+, or MnIII2(µ-
OAc)2(µ-OR)3+ cores with the remaining coordination sites of
the two Mn ions occupied by donor atoms of the ligand. These
compounds undergo ligand-shift (alkoxide-21 or carboxilate-
shift20), as in the case of 1-3, or possess one labile position,23
thus offering a terminal coordination site to H2O2, which is then
oxidized to O2 concomitantly with reduction of the catalyst to
the MnII2 form. These three classes of complexes disproportion-
ate H2O2 with saturation kinetics (entries 4-6 of Table 3), and
the reported kcat values are similar to those of complexes 1-3
(it should be noted that kinetic studies for complexes 1-3 were
performed at 10 °C while the other complexes in Table 3 were
studied at 25 °C and, in two cases, in different solvents). These
values are significantly higher than kcat obtained for complexes
involves the MnIII2/MnII couple. Thus, terminal coordination
2
of peroxide to the MnIII catalyst should be the key for O2
2
evolution and metal reduction observed for catalysts with none
or one labile position in the MnIII form. This general feature
2
should be considered to delineate new catalase models that
employ the MnIII2/MnII cycle in the future.
2
Acknowledgment. We thank the National University of
Rosario, CONICET, and the National Agency for Sciences
Promotion for financial support, and CONICET and CNRS
for a bilateral agreement (Res.709/2005). H.B. thanks the
France Embassy, Paul Sabatier University (Toulouse III) and
MCyT-Argentina for fellowships. We thank Alejandro J. Vila
for the use of the NMR spectrometer. The Bruker Avance
II 600 MHz NMR spectrometer at Rosario was purchased
with funds from ANPCyT (PME2003-0026) and CONICET.
2+
2+(or 4+)
possessing Mn2(µ-OAc)2 or Mn2(µ-OPh)2
cores
(0.02-0.2 s-1).16 KM values in Table 3 also reflect the relative
affinity of the alkoxo-bridged catalysts for the substrate:
complexes with one labile position > complexes undergoing
alkoxide-shift > complexes undergoing carboxylate-shift. At
variance with these compounds, another family of alkoxo-
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
bridged diMn complexes employs the MnIII2/MnIV couple to
IC8019793
2
3214 Inorganic Chemistry, Vol. 48, No. 7, 2009