Unique example of flexible phenol coordination in mononuclear manganese compounds

Article abstract of DOI:10.1039/b316305a

The synthesis and characterization of six novel mononuclear Mn^II and Mn^III complexes are presented. The tripodal ligands 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol(HL1), 2-[[((6-methylpyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino]methyl] -4-nitrophenol(HL2), (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)(2- hydroxybenzyl)amine (HL3) and 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4- bromophenol were used. All ligands provide an N₃O donor set. The compounds [Mn^III(HL1)Cl₂]·CH₃OH (1), [Mn^III(L1)Cl₂] (2), [Mn^II(HL2)(EtOH)Cl ₂] (3), [Mn^II(HL3)Cl₂]· CH₃OH (4), [Mn^III(HL4)Br₂] (5) and [Mn^III(L1)(tcc)] (6), with tcc = tetrachlorocatecholate dianion, were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR and UV-vis spectroscopy, cyclic voltammetry, and elemental analysis. Compound 1 crystallizes in the triclinic space group P1, compounds 2, 3 and 4 were solved in the monoclinic space group P2₁/c, whereas the structure determination of 5 and 6 succeeded in the orthorhombic space groups Pbca and P2₁2₁2₁, respectively. Notably, the crystal structures of 1 and 3 are the first Mn^II complexes featuring a non-coordinating phenol moiety. Compound 2 oxidizes 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations with a turnover number of k _cat = 173 h^-1. The electronic influence of different substituents in para position of the phenol group is lined out.

Full text of DOI:10.1039/b316305a

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Unique example of flexible phenol coordination in mononuclear
manganese compounds
Nicole Reddig, Daniel Pursche and Annette Rompel*
Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität,
Wilhelm-Klemm-Str. 8, D-48149 Münster, Germany. E-mail: rompela@uni-muenster.de;
Fax: ϩ49-251-83-38366; Tel: ϩ49-251-83-33155
Received 15th December 2003, Accepted 23rd March 2004
First published as an Advance Article on the web 6th April 2004
The synthesis and characterization of six novel mononuclear MnII and MnIII complexes are presented. The tripodal
ligands 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (HL1), 2-[[((6-methylpyridin-2-yl)methyl)(pyridin-2-
ylmethyl)amino]methyl]-4-nitrophenol (HL2), (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)(2-hydroxybenzyl)amine
(HL3) and 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-bromophenol were used. All ligands provide an N3O donor
set. The compounds [MnII(HL1)Cl2]ؒCH3OH (1), [MnIII(L1)Cl2] (2), [MnII(HL2)(EtOH)Cl2] (3), [MnII(HL3)Cl2]ؒ
CH3OH (4), [MnIII(HL4)Br2] (5) and [MnIII(L1)(tcc)] (6), with tcc = tetrachlorocatecholate dianion, were
synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR and
UV-vis spectroscopy, cyclic voltammetry, and elemental analysis. Compound 1 crystallizes in the triclinic space group
¯
P1, compounds 2, 3 and 4 were solved in the monoclinic space group P21/c, whereas the structure determination of
5 and 6 succeeded in the orthorhombic space groups Pbca and P212121, respectively. Notably, the crystal structures
of 1 and 3 are the first MnII complexes featuring a non-coordinating phenol moiety. Compound 2 oxidizes 3,5-di-tert-
butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations with a
turnover number of kcat = 173 hϪ1. The electronic influence of different substituents in para position of the phenol
group is lined out.
Tyr-447) as endogenous protein ligands and one hydroxide
Introduction
ligand. Spectroscopic studies revealed that the axial Tyr-447–
Among Nature’s variety of biocatalysts, redox active metallo-
FeIII bond is weaker than the equatorial Tyr-408–FeIII bond.20
enzymes play a considerable role. Manganese containing
This leads to the dissociation of Tyr-447 as well as the
enzymes are widespread:1 the tetranuclear cluster found in the
labile hydroxide during substrate binding. Both dynamic
oxygen evolving complex (OEC) of photosystem II (PS II),2,3
ligands are expected to act as proton acceptors resulting in
the dinuclear manganese catalase4–8 and the mononuclear Mn
diionic coordination of the substrate.18,19
superoxide dismutase9 are just a few examples of this large
For this purpose we used tripodal ligands as previously
diversity. It is notable that many mono- and dinuclear mangan-
described in the literature.21–23 All ligands provide an N3O
ese containing enzymes exhibit activities similar to iron
donor set consisting of one aliphatic and two pyridine nitrogen
enzymes, with active sites such as superoxide dismutase9 and
donor atoms as well as one oxygen donor from the phenolic
part of the corresponding ligand.
Herein we present a series of novel manganese() com-
pounds that show flexible phenol coordination depending on
some extradiol cleaving catechol dioxygenases10–12 that appear
remarkably alike. Whereas the structural similarity for super-
oxide dismutase is confirmed by X-ray structure analysis,9 there
is no crystal structure available for a manganese dependent
catechol dioxygenase. However, a sequence analysis of the
the ring substituents in the para position of the phenol group.
Ligands (Fig. 1) with a nitro group, a bromine or a hydrogen
enzyme from Arthrobacter globiformis is highly homologous to
atom in the para position to the phenolic oxygen have been used
the sequence of the crystallographically characterized iron
to synthesize the corresponding MnII compounds in order to
dependent enzyme from Pseudomonas cepacia.13 The obtained
investigate the electronic influence of para situated substituents.
data indicate that the first coordination sphere is conserved in
One MnII compound with a nitro group containing ligand was
Mn-containing A. globiformis.13
oxidized to the MnIII complex with 3-chloroperbenzoic acid
Although no manganese containing intradiol cleaving
where in contrast to the MnII compound the coordination of
catechol dioxygenase is known to date, it is a reasonable strat-
the phenolate arm has been achieved.
egy to synthesize and investigate manganese model complexes
for iron containing enzymes due to their frequent similarity.
Therefore, the synthesis and investigation of model complexes
for oxidation reactions are of great promise for the develop-
ment of new and efficient catalysts.
In addition to histidine, tyrosine is an essential ligand in met-
alloenzymes with several functions. In galactose oxidase and PS
II tyrosine is responsible for charge transfer.2,14–16 In catechol
oxidase, a dinuclear copper enzyme, the tyrosine can be
regarded as a dynamic shield for the active site during substrate
binding.17 A structural function of tyrosine is reported for some
intradiol cleaving catechol dioxygenases such as protocate-
Fig. 1 Schematic drawing of the utilized tripodal tetradentate ligands
chuate 3,4-dioxygenase (3,4-PCD), where it functions as a flex-
ible ligand of the first coordination sphere. 3,4-PCD catalyzes
the cleavage of o-diphenol to yield derivatives of cis,cis-
muconic acid.18,19 The active site of 3,4-PCD consists of two
histidines (His-460, His-462) and two tyrosines (Tyr-408,
HL1–HL4.
Most interestingly, this MnIII compound exhibits high
catechol oxidase activity using the substrate 3,5-di-tert-butyl-
catechol (3,5-DTBC). Additionally, the MnII compound with a
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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nitro group containing ligand was reacted with tetrachloro-
catechol (H2tcc) under basic conditions that succeeded in the
formation of the substrate adduct MnIII(tcc) complex.
(0.25 mmol, 50 mg) were reacted in 10 mL dichloromethane.
After 12 h of stirring the resulting pale yellow solid was filtered
off and dissolved in 10 mL ethanol. Vapor diffusion of diethyl
ether yielded rhombic pale yellow crystals suitable for X-ray
diffraction. Yield: 51 mg (0.09 mmol, 38%). Found: C, 49.45; H,
4.77; N, 10.23. Calc. for MnCl2C22H26N4O3: C, 49.27; H, 4.89;
N, 10.45%. IR (KBr pellets, cmϪ1): 3434, 3069, 2971, 2927,
1603, 1523, 1496, 1440, 1394, 1340, 1284, 1232. MS (MALDI):
535 [M Ϫ H]Ϫ.
Experimental
Elemental analyses were performed on a Perkin-Elmer 2400
Series 2 analyzer, an Elementar Vario ELIII analyzer and a
Heraeus CHN–O-Rapid analyzer. IR spectra were recorded on
a Perkin-Elmer Spectrum GX FT-IR spectrometer and a
Bruker IFS 48 spectrometer in the range 4000–400 cmϪ1. Sam-
ples were prepared as KBr pellets. UV-vis spectra were meas-
ured at 25 ЊC in methanol on a Hewlett-Packard 8453 diode
Preparation of [MnII(HL3)Cl2]ؒCH3OH (4)
The complex was prepared similarly to 1. Equimolar amounts of
HL3 (0.25 mmol, 80 mg) and MnCl2ؒ4H2O (0.25 mmol, 50 mg)
were reacted in 10 mL dichloromethane. After the reaction mix-
ture was stirred for 12 h the resulting pale yellow precipitate was
filtered off and dissolved in 10 mL methanol. Vapor diffusion of
diethyl ether led to the formation of rhombic pale yellow crystals
suitable for X-ray diffraction. Yield: 39 mg (0.08 mmol, 33%).
Found: C, 51.44; H, 5.07; N, 8.63. Calc. for MnCl2ON3C20H20ؒ
CH3OH: C, 52.85; H, 5.28; N, 8.80%. IR (KBr pellets, cmϪ1):
3473, 3070, 2901, 1605, 1576, 1502, 1464, 1442, 1394, 1349, 1295,
1250. MS (MALDI): 443 [M Ϫ H]Ϫ.
1
array spectrometer using quartz cuvettes (1 cm). H and 13C
NMR spectra were recorded on a Bruker WH 300 spectro-
meter. MALDI experiments were performed on a Bruker
Daltonics MALDI Reflex IV spectrometer. The compounds
were dissolved in methanol. Cyclic voltammetric experiments
were performed on a BAS CV-27 voltammograph equipped
with a BAS C-1B cell stand and a BAS RXY recorder. The
concentration of the samples for 1 and 2 was 1 × 10Ϫ3 mol LϪ1
in acetonitrile–DMF (1 : 1). In the case of 3, 4 and 5, 1 × 10Ϫ3
mol LϪ1 solutions in acetonitrile were used. The cyclic voltam-
mogram of 6 was measured in dichloromethane (c = 1 × 10Ϫ3
mol LϪ1). Prior to use, the solvents were purified by standard
literature methods.24 Tetrabutylammonium hexafluoro-
phosphate (recrystallized from ethanol) was used as supporting
electrolyte at a concentration of 0.1 mol LϪ1. A three elec-
trode array consisting of a glassy carbon working electrode, a
platinum wire counter electrode and a Ag/AgCl/3 M NaCl
reference electrode was used. For ease of comparison, all poten-
tials were referenced vs. SCE. Using the conditions described
above, the ferrocene/ferrocenium redox couple was observed at
E1/2 = 0.41 V vs. SCE and at E1/2 = 0.53 V vs. Ag/AgCl/3 M
NaCl. All tripodal ligands have been synthesized according to
previously reported methods.21–23 All reagents were purchased
from Fluka and Aldrich and used without further purification.
All solvents were of analytical grade and used without further
purification unless stated otherwise.
Preparation of [MnII(HL4)Br2] (5)
0.25 mmol (54 mg) MnBr2 were suspended in 1.5 mL methanol
and treated with a solution of 0.25 mmol (96 mg) of HL4 in
5 mL dichloromethane. After the reaction mixture was stirred
for 1 hour the resulting solution was layered with n-hexane.
Colorless crystals suitable for X-ray analysis were obtained
after three days. Yield: 36 mg (0.06 mmol, 24%). Found: C,
38.41; H, 3.18; N, 6.78. Calc. for MnBr3C19H18N3O: C, 38.10;
H, 3.03; N, 7.01%. IR (KBr pellets, cmϪ1): 3200, 3084, 3031,
2891, 1603, 1570, 1491, 1480, 1442, 1420, 1390, 1336, 1304,
1291, 1262. MS (MALDI): 598 [M Ϫ H]Ϫ.
Preparation of [MnIII(L1)(tcc)] (6)
0.1 mmol (27 mg) tetrachlorocatechol monohydrate was dis-
solved in 2 mL chloroform–dimethylformamide (1 : 1) and
0.3 mmol (42 µL) triethylamine. This solution was combined
with a solution of compound 1 (0.1 mmol, 51 mg) in 6 mL
chloroform–dimethylformamide (1 : 1). The resulting reddish
brown mixture was stirred for 1 h and filtered off. Red crystals
suitable for X-ray diffraction were obtained by layering with
n-hexane. Yield: 48 mg (0.07 mmol, 73%). Found: C, 45.95; H,
2.56; N, 8.74. Calc. for MnCl4O5N4C25H17: C, 46.18; H, 2.64; N,
8.62%. IR (KBr pellets, cmϪ1): 3061, 2996, 2916, 1603, 1577,
1502, 1479, 1440, 1374, 1333, 1247, 1093. MS (MALDI): 649
[M Ϫ H]Ϫ.
Preparation of [MnII(HL1)Cl2]ؒCH3OH (1)
The complex was prepared by reacting equimolar amounts of
HL1 (0.25 mmol, 88 mg) and MnCl2ؒ4H2O (0.25 mmol, 50 mg)
in 10 mL dichloromethane. The reaction mixture was stirred for
12 h. The resulting pale yellow precipitate was filtered off and
dissolved in 10 mL methanol. Vapor diffusion of diethyl ether
led to the formation of rhombic pale yellow crystals suitable for
X-ray diffraction. Yield: 66 mg (0.13 mmol, 53%). Found: C,
46.45; H, 4.53; N, 12.43. Calc. for MnCl2C19H18N4O3ؒCH3OH:
C, 47.26; H, 4.36; N, 11.02%. IR (KBr pellets, cmϪ1): 3418,
3064, 2942, 2910, 1605, 1590, 1525, 1497, 1442, 1386, 1338,
1289, 1242. MS (MALDI): 474 [M Ϫ H]Ϫ.
Crystallography
Data collection and processing. Data for 1 were collected at
200(2) K on a Bruker SMART APEX CCD diffractometer
using graphite monochromated Mo-Kα radiation (λ = 0.71073
Å). Data for 2, 3, 4 and 6 were collected at 100(2) K (4), 150(2)
K (2) and 200(2) K (3, 6) on a Bruker SMART 6000 CCD
diffractometer using Cu-Kα radiation (λ = 1.54184 Å). Data for
5 were collected at 213(2) K on a STOE-IPDS diffractometer
using graphite monochromated Mo-Kα radiation (λ = 0.71073
Å). All structures were solved by direct methods and refined
using full-matrix least squares (SHELXTL).25 The programs
SADABS26 (1–4, 6) and DECAY27 (5) were applied for absorp-
tion corrections. All hydrogen atoms were placed on calculated
positions and allowed to ride on their corresponding carbon
atoms with isotropic thermal parameters except for H4 in com-
pound 3. This was located on the difference map and refined
isotropically. All non-hydrogen atoms were refined aniso-
tropically. Further crystallographic details are summarized in
Table 1.
Preparation of [MnIII(L1)Cl2] (2)
Compound 1 (0.1 mmol, 51 mg) was dissolved in 10 mL ethanol
and treated with 100 µL nitric acid. After the mixture was
stirred for 5 min a solution of 3-chloroperbenzoic acid
(0.1 mmol, 17 mg) in 2 mL ethanol was added. Brown single
crystals were obtained by vapor diffusion of diethyl ether into
the solution. Yield: 11 mg (0.02 mmol, 23%). Found: C, 47.83;
H, 3.54; N, 11.83. Calc. for MnCl2C19H17N4O3: C, 48.02; H,
3.61; N, 11.79%. IR (KBr pellets, cmϪ1): 3442, 3064, 2950, 2921,
1602, 1574, 1474, 1440, 1386, 1333, 1277. MS (MALDI): 439
[M Ϫ Cl]ϩ.
Preparation of [MnII(HL2)(EtOH)Cl2] (3)
Compound 3 was prepared analogously to 1. Equimolar
amounts of HL2 (0.25 mmol, 91 mg) and MnCl2ؒ4H2O
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
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Table 1 Crystal data and X-ray experimental parameters for complexes 1, 2, 3, 4, 5 and 6.
1
2
3
4
5
6
Formula
M/g molϪ1
Crystal system
Space group
T /K
a/Å
b/Å
c/Å
α/Њ
β/Њ
MnCl2C20H22N4O4 MnCl2C19H17N4O3 MnCl2C22H26N4O4 MnCl2N3O2C21H25 MnBr3C19H18N3O MnCl4N4O5C26H20
508.26
475.21
Monoclinic
P21/c
150(2)
20.623(4)
7.902(2)
12.378(2)
–
98.70(3)
–
1993.9(7)
4
536.31
Monoclinic
P21/c
200(2)
15.715(7)
9.199(4)
17.272(7)
–
99.04(3)
–
2465.8(1)
4
477.29
Monoclinic
P21/c
100(2)
19.358(3)
13.137(2)
17.353(3)
–
96.13(1)
–
4387.6(1)
8
599.03
Orthorhombic
Pbca
213(2)
13.523(3)
17.092(3)
18.230(4)
–
665.20
Orthorhombic
P212121
200(2)
12.487(3)
13.979(4)
14.905(4)
–
Triclinic
¯
P1
200(2)
7.462(3)
9.435(3)
16.115(5)
97.94(2)
92.76(2)
98.72(2)
1108.0(1)
2
–
–
–
–
γ/Њ
V/Å3
4213.6(2)
8
2601.7(1)
4
Z
Dc/g cmϪ3
µ/mmϪ1
R1 [I > 2σ(I )]a
1.523
0.872
1.583
8.105
1.445
6.648
1.444
7.314
1.889
6.330
1.698
8.321
0.0569
0.0583
0.1498
0.998
0.0751
0.1879
0.970
0.0434
0.1141
1.063
0.0672
0.1339
1.059
0.0587
0.1439
0.998
wR2 [I > 2σ(I )]b 0.1480
GOFc
1.009
2
½
½
a R1 = Σ||Fo| Ϫ |Fc||/Σ|Fobs|. b wR2 = {Σ[w(Fo2 Ϫ Fc2)2]/Σ[w(Fo )2]} . c GOF = {Σ[w(Fo2 Ϫ Fc2)2]/(ndata Ϫ nvar} .
Table 2 Selected distances (Å) and angles (Њ) for 1, 2 and 3
CCDC reference numbers 226271 (1), 226272 (2), 226274 (3),
226512 (4), 226513 (5) and 226273 (6).
See http://www.rsc.org/suppdata/dt/b3/b316305a/ for crystal-
lographic data in CIF or other electronic format.
1
2
3
Mn(1) ؒ ؒ ؒ O(1)
Mn(1)–Cl(1)
Mn(1)–Cl(2)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
5.249(7)
2.366(1)
2.351(1)
2.301(3)
2.237(3)
2.236(3)
1.880(2)
2.476(1)
2.286(1)
2.112(3)
2.051(3)
2.280(3)
5.371(6)
2.542(1)
2.487(2)
2.358(4)
2.242(4)
2.262(4)
Kinetic measurements
The kinetics of the oxidation of 3,5-di-tert-butylcatechol (3,5-
DTBC) by dioxygen to 3,5-di-tert-butylquinone (3,5-DTBQ)
were determined by the method of initial rates. 1 mL of a 1 ×
10Ϫ4 M solution of 2 in air-saturated methanol was treated with
1 mL of cumulatively concentrated 3,5-DTBC solutions in
methanol. The increasing band at 400 nm of the product 3,5-di-
tert-butylquinone (3,5-DTBQ) was monitored by UV-vis
spectroscopy as described elsewhere.28–30 Initial rates were
determined by linear regression from the slope of the concen-
tration vs. time plots and averaged over three independent
measurements. The data satisfied Michaelis–Menten kinetics,
originally developed for enzyme kinetics. The turnover number
was determined from the double reciprocal Lineweaver–Burk
plot.31
Cl(1)–Mn(1)–Cl(2)
Cl(1)–Mn(1)–O(1)
Cl(2)–Mn(1)–O(1)
Cl(1)–Mn(1)–N(1)
Cl(1)–Mn(1)–N(2)
Cl(1)–Mn(1)–N(3)
Cl(2)–Mn(1)–N(1)
Cl(2)–Mn(1)–N(2)
Cl(2)–Mn(1)–N(3)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
115.5(4)
–
–
97.4(4)
92.1(8)
94.7(8)
93.5(8)
89.0(8)
170.7(8)
166.9(9)
94.8(9)
91.6(8)
78.1(1)
78.0(1)
92.8(1)
171.6(5)
89.9(1)a
85.7(1)b
88.4(9)
90.5(1)
88.3(1)
97.1(1)
97.1(1)
87.1(1)
73.6(1)
75.0(1)
148.6(2)
143.5(9)
101.9(9)
97.7(9)
101.0(8)
99.3(9)
97.9(9)
73.4(1)
73.6(1)
145.1(1)
a Cl(1)–Mn(1)–O(4). b Cl(2)–Mn(1)–O(4).
Results and discussion
MnII center can be described as distorted square pyramidal.
The nitrogen donor atoms of HL1 occupy three coordination
sites. Two chloride ligands complete the coordination sphere.
The Mn–N bond distances range from 2.236(3) Å for Mn(1)–
N(3) to 2.301(3) for Mn(1)–N(1). As expected the longest bond
lengths can be found for Mn(1)–Cl(2) with 2.351(1) Å and
Mn(1)–Cl(1) with 2.366(1) Å. Although the phenol moiety of
HL1 provides a potential oxygen donor atom no bond between
the metal center and O(1) has been observed. This is proved by
a distance of 5.249(7) Å between Mn(1) and O(1). Additionally,
the phenolic hydrogen atom H(1a) forms a hydrogen bond to an
adjacent methanol molecule with a distance of the donor atom
O(1) to the acceptor atom O(4) of 2.600(4) Å. The distortion is
further manifested in the average N(1)–Mn(1)–N(X) (X = 2, 3)
angle of 73.5Њ which differs significantly from the ideal 90Њ.
The crystal structure of [MnIII(L1)Cl2], 2 is depicted in Fig. 3.
Selected distances and angles are given in Table 2. Complex 2
crystallizes in the monoclinic space group P21/c with four metal
complexes per unit cell.
Description of structures
The crystal structure of the metal complex in [MnII(HL1)Cl2]ؒ
CH3OH, 1 is depicted in Fig. 2. Selected distances and angles
are given in Table 2. Compound 1 crystallizes in the triclinic
¯
space group P1 with two metal complexes and two methanol
solvent molecules per unit cell. The environment around the
The three nitrogen donor atoms and the phenolic oxygen
donor atom of the ligand HL1 and two coordinated chloride
ions provide a distorted octahedral coordination environment.
In contrast to 1 the phenol moiety is deprotonated and
coordinated to the central Mn(1) within a distance of Mn(1)–
O(1) = 1.880(2) Å. Analogous to 1 the longest bond distances
Fig. 2 Ellipsoid plot of [MnII(HL1)Cl2]ؒCH3OH, 1 (50% probability);
hydrogen atoms are omitted for clarity (except H1a).
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
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Fig. 3 Ellipsoid plot of [MnIII(L1)Cl2], 2 (50% probability); hydrogen
atoms are omitted for clarity.
Fig. 5 Ellipsoid plot of [MnII(HL3)Cl2]ؒCH3OH 4 (50% probability);
are observed for Mn(1)–Cl(1) and Mn(1)–Cl(2) with values of
2.476(1) and 2.286(1) Å, respectively. The differences in the
metal–chloride bond lengths and the difference in the bond
lengths for Mn(1)–N(2) and Mn(1)–N(3) with values of
2.051(3) and 2.280(3) Å, respectively, indicate a Jahn–Teller
compression along the axis O(1)–Mn(1)–N(2). This arises from
the d4 configuration of the manganese center. The average
N(1)–Mn(1)–N(X) (X = 2, 3) angle of 78.1Њ differs significantly
from 90Њ and underlines the distortion from ideal octahedral
symmetry.
hydrogen atoms are omitted for clarity (except H1).
The crystal structure of [MnII(HL2)(EtOH)Cl2], 3 is depicted
in Fig. 4. Selected distances and angles are given in Table 2. Like
2, complex 3 crystallizes in the monoclinic space group P21/c
with four metal complexes per unit cell.
Fig. 6 Ellipsoid plot of [MnII(HL4)Br2] 5 (50% probability); hydrogen
atoms are omitted for clarity (except H1).
In the asymmetric unit of 4 two molecules of the MnII com-
plex and the solvent methanol are found. A comparison of both
complex molecules in the asymmetric unit indicates that in one
of them the phenol arm bends towards the unsubstituted pyrid-
ine arm, whereas it is inclined to the methyl substituted pyridine
ring in the other one. To simplify the structural description of
[MnII(HL3)Cl2]ؒCH3OH (4) all discussed bond lengths and
angles originate from the complex molecule with the phenol
moiety bent towards the unsubstituted pyridine.
The coordination environment around the MnII centers in 4
and 5 can be best described as distorted octahedral. Each of the
corresponding tripodal ligands provides one aliphatic and two
pyridine nitrogen donor atoms as well as one oxygen donor
atom from the phenolic moiety. The coordination sphere of 4
and 5 is completed by two chloride or two bromide ligands,
respectively. As expected, the longest bond distance can be
found for Mn-X (X = Cl, Br) in both complexes. As shown in
Table 3 a variation between 2.382(2) and 2.624(2) Å has been
revealed. The Mn–N bonds are in the typical range for MnII–N
bonds and exhibit values between 2.213(7) and 2.337(7) Å.
Manganese oxygen distances of 2.342(2) Å (4) and 2.396(6) Å
(5) have been determined. The distortion of the N3OX2
coordination sphere (X = Cl, Br) is further manifested in the
angles around the metal centers (see Table 3).
Fig. 4 Ellipsoid plot of [MnII(HL2)(EtOH)Cl2], 3 (50% probability);
hydrogen atoms are omitted for clarity (except H1 and H4).
The distorted octahedral coordination sphere of the MnII
core is set up by two chloride ions, one coordinated ethanol
solvent molecule and the ligand HL2. Analogously to 1, the
nitrophenol residue does not coordinate, resulting in a tri-
dentate meridional coordination mode of the tripodal ligand.
The bond lengths for the N3Cl2O donor set vary between
2.186(4) Å for Mn(1)–O(4) and 2.542(1) Å for Mn(1)–Cl(1) and
are in good agreement with other MnII chloride complexes
described in the literature.28,32–34
With respect to the nature of the N donor moieties, the larg-
est metal–nitrogen distance occurs between the manganese cen-
ter Mn(1) and the aliphatic nitrogen atom N(1), with a value of
2.358(4) Å. Analogously to 1, and as expected for a d5 transi-
tion metal compound, 3 exhibits no Jahn–Teller distortion.
Hence no significant differences in the metal–chloride bonds
were observed. The trans situated chloride ions Cl(1) and Cl(2)
exhibit distances of 2.542(1) and 2.487(2) Å, respectively, to
Mn(1). Again, the average angle of cis situated nitrogen
atoms and the central manganese ion of 74.3Њ varies signifi-
cantly from 90Њ. Also the N(2)–Mn(1)–N(3) angle of 148.6(2)Њ
differs notably from the ideal 180Њ for trans situated atoms.
Fig. 5 and Fig. 6 illustrate the crystal structures of
[MnII(HL3)Cl2]ؒCH3OH, 4, and [MnII(HL4)Br2], 5, respect-
ively. Compound 4 crystallizes in the monoclinic space group
P21/c whereas the structure determination for 5 succeeded in
the orthorhombic space group Pbca.
The crystal structure of the complex [MnIII(L1)(tcc)], 6 is
depicted in Fig. 7. Selected distances and angles are given in
Table 4. The structure of 6 was solved in the orthorhombic
space group P212121 with four molecules of the neutral
compound per unit cell.
The manganese center ion is in the oxidation state ϩIII. Its
positive charge is neutralized by the deprotonated phenol oxy-
gen atom provided by the tripodal ligand and the catecholate
dianion. The generated N3O3 donor set results in a distorted
octahedral coordination sphere. Both types of donor atoms are
coordinated meridionally to each other. The distortion of the
coordination environment is caused by the ligand’s geometry,
which allows the formation of five- and six-membered chelate
rings. The values of the cis-angles N(1)–Mn(1)–N(X) are
75.3(2)Њ (X = 2) and 75.8(1)Њ (X = 3). This is notably smaller
compared to the ideal angle of 90Њ. The distortion of the octa-
hedral environment is also caused by the Jahn–Teller-effect
found in 6. A comparison of the distances O(1) ؒ ؒ ؒ O(3),
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
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Table 3 Selected distances (Å) and angles (Њ) for 4 and 5
Table 5 UV-vis data for 1–6 taken in methanol
4
5
Complex
λ/nm
ε/MϪ1 cmϪ1
Mn(1)–X(1)
Mn(1)–X(2)
Mn(1)–O(1)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
2.514(1)
2.382(2)
2.342(2)
2.325(2)
2.291(2)
2.288(2)
2.624(2)b
2.550(2)b
2.396(6)
2.337(7)
2.213(7)
2.219(6)
1
2
3
4
5
6
261, 328, 383
261, 343, 553, 671
267, 328, 383
267
259
229, 256, 356
7469, 5767, 4909
10597, 10392, 1660, 877
8397, 6013, 4443
6513
6952
2276, 1349, 1307
X(1)–Mn(1)–X(2)
X(1)–Mn(1)–O(1)
X(1)–Mn(1)–N(1)
X(1)–Mn(1)–N(2)
X(1)–Mn(1)–N(3)
X(2)–Mn(1)–O(1)
X(2)–Mn(1)–N(1)
X(2)–Mn(1)–N(2)
X(2)–Mn(1)–N(3)
O(1)–Mn(1)–N(1)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
97.4(3)a
172.9(2)a
92.2(2)a
95.4(2)a
91.0(2)a
89.0(2)a
166.5(2)a
96.1(1)a
116.8(2)a
82.1(1)
103.0(6)b
171.8(2)b
92.9(2)b
95.7(2)b
88.5(2)b
84.4(2)b
163.9(2)b
103.0(2)b
107.7(2)b
79.6(2)
Table 6 Redox potentials of compounds 1 and 2 in a 1 : 1 mixture of
acetonitrile and DMF, 3–5 in acetonitrile, and compound 6 in
dichloromethane
E
(∆E)/V vs. SCE
½
Complex
MnII/MnIII
TCSQa/TCC
TSCQ/TCQb
1
2
3
4
5
6
0.50 (0.10)
0.54 (0.12)
0.51 (0.11)
0.72 (0.26)
0.73 (0.12)
–
87.2(1)
83.4(2)
73.4(1)
72.4(1)
85.5(2)
86.0(2)
73.5(2)
73.7(2)
Ϫ0.21 (0.15)
0.20/0.73c
145.4(1)
147.1(3)
a X = Cl. b X = Br.
a TCSQ: tetrachlorosemiquinone. b TCQ: tetrachloroquinone. c Irre-
versible.
lengths of 1.342(6) and 1.352(6) Å are characteristic for the
catecholate form, while for the semiquinonate form, an average
bond length of 1.29 Å would be expected.35
Electronic spectroscopy
Table 5 summarizes the UV-vis data for all six complexes.
Electrochemistry
Redox potentials are reported in Table 6. All values are given
vs. SCE. The cyclic voltammograms of 1, 2, 3, 4 and 5 show a
quasi-reversible transition, which can be attributed to the redox
couple Mn(/).28 The number of electrons involved in this
slow electron transfer can be determined using an equation
published before.36,37 From the calculated data it has been
shown that exactly one electron is involved in this redox process,
confirming the assignment in Table 6.
Fig. 7 Ellipsoid plot of [MnIII(L1)(tcc)] 6 (50% probability); hydrogen
atoms are omitted for clarity.
The cyclic voltammograms of the mononuclear mangan-
ese() compounds 3, 4 and 5 show a second transition. To
further investigate the second oxidation peak of these MnII
compounds, additional electrochemical studies on pure ligand
samples have been performed. In analogy to the data reported
by Blondin and co-workers, oxidation of the phenolic part of
the ligands occurs at higher potentials.36 Therefore, those
irreversible, ill-defined cyclic voltammetric traces overlay any
supposed oxidation from MnIII to MnIV. The CV of the MnIII-
O(2) ؒ ؒ ؒ N(1), and N(2) ؒ ؒ ؒ N(3) via Mn(1) indicates an elon-
gated N(2)–Mn(1)–N(3) axis (see Table 4).
The shortness of the Mn(1)–O(1) bond is favored by the
increased basic character of the oxygen donor atoms compared
to the nitrogen donor atoms and the formation of the geo-
metrically favored six-membered chelate ring.
Examination of the bond lengths within the tetrachloro-
catecholate ligand leads to the conclusion that it is coordinated
as catecholate and not as semiquinonate. The C–O bond
catecholate compound
6
indicates more complex redox
Table 4 Selected distances (Å) and angles (Њ) for 6
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–O(3)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
1.866(3)
1.917(3)
1.887(3)
2.212(4)
2.225(4)
2.210(4)
O(2)–C(25)
O(3)–C(20)
1.342(6)
1.352(6)
O(1) ؒ ؒ ؒ O(3) (via Mn(1))
O(2) ؒ ؒ ؒ N(1) (via Mn(1))
N(2) ؒ ؒ ؒ N(3) (via Mn(1))
3.75(1)
4.13(1)
4.44(1)
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–O(3)
O(2)–Mn(1)–O(3)
O(1)–Mn(1)–N(1)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(3)
O(2)–Mn(1)–N(1)
O(2)–Mn(1)–N(2)
90.0(2)
175.3(2)
85.4(2)
91.7(2)
91.7(2)
91.9(2)
168.5(2)
116.1(2)
O(2)–Mn(1)–N(3)
O(3)–Mn(1)–N(1)
O(3)–Mn(1)–N(2)
O(3)–Mn(1)–N(3)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
92.8(1)
92.9(2)
89.5(2)
89.3(2)
75.3(2)
75.8(1)
150.9(2)
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
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Table 7 Catalytic activity of synthetic catecholase mimics
Compound
kcat/hϪ1
KM/mM
(kcat/KM)/sϪ1 MϪ1
Ref.
MnIII(L1)Cl2], 2
173
230
130
225
4
0.90
1.3
0.8
–
–
0.24
54
49
45
–
–
248
[Mn(bpia)Cl2](ClO4)a
[Mn(bipa)Cl2](ClO4)b
[Mn(diclofenac)2(H2O)]c, d
27
27
40
41
29
c, e
[Mn(tpa)2](ClO4)2
[Cu2(L1)(OH)(EtOH)(H2O)](ClO4)2
214
f
a bpia
= =
bis(picolyl)(N-methylimidazole-2-yl)amine. b bipa Bis(N-methylimidazole-2-yl)(picolyl)amine. c No saturation kinetics reported.
d diclofenac = (2-((2,6-Dichlorophenyl)amino)phenyl)acetate. e tpa = Tris(2-pyridylmethyl)amine. f L1 = 4-Bromo-2,6-bis(4-methylpiperazin-1-
ylmethyl)phenol.
processes. It displays a quasi-reversible peak at E1/2 = Ϫ0.21 V
(0.15 V). Additionally, irreversible redox processes occur at
0.20 V (oxidation)/0.73 V (reduction). In comparison to
redox potentials given in the literature for other transition
metal catecholate complexes28,38–40 the redox transitions can be
assigned as follows. The irreversible peaks correspond to the
semiquinonate–quinone transition of the bound ligand, while
the quasi-reversible peak stems from the transition semi-
quinonate–catecholate.
with tripodal ligands featuring a pure nitrogen donor set.28 The
obtained catalytic data of 2 are comparable to the kinetics
reported by Triller et al. The MnII compounds 1, 3, 4 and 5
exhibit no catalytic activity. This is in agreement with a similar
MnII complex reported by Triller et al., [MnII(Hmimppa)Cl2].28
Conclusions
Six different manganese complexes have been synthesized and
characterized that are structurally related to a manganese sub-
stituted form of an intradiol cleaving catechol dioxygenase.
Exchange of the p-substituent in the phenol moiety of the tri-
podal N3O donor ligand 2-((bis(pyridin-2-ylmethyl)amino)-
methyl)-4-nitrophenol (HL1) by a nitro and bromine group,
respectively, leads to four MnII complexes with different Mn–O
distances.
Kinetic investigations
Air-saturated methanol solutions of 1, 2, 3, 4 and 5 (1 × 10Ϫ4
mol LϪ1) were treated with 10 equivalents of 3,5-di-tert-butyl-
catechol (3,5-DTBC). To suppress an autoxidation of the
substrate, no base was added. The first apparent result, while
monitoring the reaction by UV-vis spectroscopy, is a consider-
able difference in the reactivity of the complexes. While all MnII
compounds (1, 3, 4 and 5) show no catalytic activity, the form-
ation of a band at 400 nm occurs in the case of 2. This is
indicative of an oxidation of the substrate 3,5-DTBC towards
Most recently, Merkel et al. described a functional FeIII
model system for intradiol cleaving catechol dioxygenases con-
sisting of six FeIII compounds sharing the same tripodal ligand
N-[(6-bromopyridin-2-yl)methyl]-N,N-bis(pyridin-2-yl)methyl)-
amine mimicking the dynamic structural behaviour of the
departing tyrosine ligand.41 In Merkel’s system the steric
hindrance caused by the bromide ring substituent pushes the
o-substituted pyridine arm of the ligand away from the co-
ordination site upon the presence of nucleophiles. In contrast to
the results of Merkel et al. the different Mn–O distances
reported here for the MnII compounds 1, 3, 4 and 5 have to
originate from electronic reasons, since no steric hindrance of
the para situated groups occurs. This is in good agreement
3,5-di-tert-butylquinone (3,5-DTBQ).
A decreasing band
around 600 nm underlines that no semiquinone byproduct is
formed. After the formation of 3,5-DTBQ has reached a
maximum, the extinction at 400 nm revealed a concentration of
2.74 × 10Ϫ4 mol LϪ1 oxidized 3,5-DTBC. This is 2.74 times
higher than the starting concentration of the complex indi-
cating a catalytic process. Consequently, the catechol oxidase
activity of 2 was investigated and saturation kinetics at high
substrate concentrations have been revealed (see Fig. 8).
II
with a series of phenolate bridged dinuclear Mn2 com-
pounds reported previously,22 where a coherence between the
Mn ؒ ؒ ؒ Mn distance and the type of ring substituent has been
determined. Especially, the reported influence of the electronic
withdrawing effect of an NO2-group situated in p-position to a
phenolic oxygen is noteworthy. Because of its ϪM-effect the
electron density of the bridging phenolic oxygen is reduced
resulting in larger Mn–O bonds and larger Mn ؒ ؒ ؒ Mn separ-
ations compared to compounds without p-situated nitro
groups.22 Similar behaviour is found in the work presented here.
Although coordination of the phenol moiety would be possible
by the ligands’ geometry, it can be concluded that the non-
coordination of the phenol arm in the manganese() complexes
1 and 3 is also a consequence of the ϪM-effect of the p-situated
nitro groups. This has further been proven by the crystal struc-
ture of compound 5. Here, the phenolic oxygen atom is weakly
bound to the metal() center within a distance of 2.396(6) Å,
although the utilized ligand HL4 contains a bulky bromine
substituent in para position. Moreover, compound 4 with no
substituent in para position to the phenolic oxygen atom also
shows a coordination of the phenol moiety (Mn(1)–O(1) =
2.342(2) Å). Therefore, the electron withdrawing effect of the
nitro group is supposed to be the main reason on the non-
coordination of the phenol moiety in the MnII compounds 1
and 3.
Fig. 8 Initial rate of 3,5-DTBQ formation vs. 3,5-DTBC concen-
tration at a constant concentration of 2 (1 × 10Ϫ4 mol LϪ1).
The turnover number kcat = 173 2 hϪ1 and the Michaelis
constant KM = 0.90
0.01 mM were determined from the
double reciprocal Lineweaver–Burk plot. The kinetic data
obtained for 2 places this compound into the upper range of
catechol oxidizing compounds. Table 7 summarizes the kinetic
data of 2 and selected synthetic catechol oxidase mimics.
It is noteworthy, that similar kinetics were described by
Triller et al. for a number of mononuclear MnIII compounds
Oxidation of 1 by 3-chloroperbenzoic acid under acidic con-
ditions led to the formation of the MnIII compound 2, where in
D a l t o n T r a n s . , 2 0 0 4 , 1 4 7 4 – 1 4 8 0
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contrast to 1 a coordination of the phenol moiety to the MnIII
center has been achieved. In addition, by applying the same
conditions as described for the preparation of 2 we were able to
oxidize compound 3. For the oxidized species of 3 d–d-bands
were detected at 534 nm (ε = 1140 MϪ1 cmϪ1) and 681 nm
(ε = 680 MϪ1 cmϪ1).
Compound 2 exhibits high catalytic activity for the oxidation
of 3,5-DTBC to 3,5-DTBQ in air-saturated methanol. The
catalytic data have been determined to be KM = 0.90 mM and
kcat = 173 hϪ1 placing 2 in the upper range of catechol oxidizing
compounds.
11 A. K. Whiting, Y. R. Boldt, M. P. Hendrich, L. P. Wackett and
L. Que, Jr., Biochemistry, 1996, 35, 160.
12 L. Que, Jr., J. Widom and R. L. Crawford, J. Biol. Chem., 1981, 256,
10941.
13 S. Han, L. D. Eltis, K. N. Timmis, S. W. Muchmore and J. T. Bolin,
Science, 1995, 270, 976.
14 M. M. Whittaker, V. L. DeVito, S. A. Asher and J. W. Whittaker,
J. Biol. Chem., 1989, 264, 7104.
15 J. W. Whittaker, in Bioinorganic Chemistry of Copper, ed. K. D.
Karlin and Z. Tyeklar, Chapman Hall, New York, 1993, p. 447.
16 P. Chaudhuri, M. Hess, J. Müller, K. Hildenbrand, E. Bill,
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To summarize, all presented compounds can be regarded as
structural models for both the reduced and oxidized form of a
manganese substituted intradiol cleaving iron catechol di-
oxygenase. The series of four novel manganese() compounds
show flexible phenol coordination depending on the ring substi-
tuents in para position to the phenol group. Additionally, the
MnIII substrate adduct complex 6 represents an important
structural intermediate in the understanding of catechol
metabolism. The revealed kinetics for the oxidation of 3,5-
DTBC by the MnIII compound 2 represents a further step in
developing synthetic manganese model compounds that can act
as catechol oxygenase mimics.
17 T. Klabunde, C. Eicken, J. C. Sacchettini and B. Krebs, Nat. Struct.
Biol., 1998, 5, 1084.
18 M. W. Vetting, D. A. D’Argenio, L. N. Ornston and D. H.
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19 A. M. Orville, J. D. Lipscomb and D. H. Ohlendorf, Biochemistry,
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20 M. I. Davis, A. M. Orville, F. Neese, J. M. Zaleski, J. D. Lipscomb
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22 N. Reddig, D. Pursche, M. Kloskowski, C. Slinn, S. Baldeau and
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Acknowledgements
Financial support by the DFG (Deutsche Forschungsgemein-
schaft, SPP 1118) and the FCI (Fonds der Chemischen Indus-
trie) is gratefully acknowledged.
29 P. Gentschev, N. Möller and B. Krebs, Inorg. Chim. Acta, 2000, 300,
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