Tuning the coordination mode in mononuclear manganese complexes by changing the steric bulk of the carboxylates

Article abstract of DOI:10.1039/b104975p

Three mononuclear manganese complexes of the tridentate ligand biap [ = N,N-bis(2-ethyl-5-methylimidazol-4-ylmethyl)aminopropane] with various carboxylates as co-ligands have been synthesized and structurally characterised by X-ray diffraction. All three complexes can be described by the general formula [Mn^II(biap)(RCOO)₂(H₂O)_n] (n = 0, 1). The manganese(II) ion in [Mn^II(biap)(C₆H₅COO)₂] (1) is five coordinated by three nitrogen donors from biap and two oxygen donors of monodentate benzoate molecules. This monodentate binding mode is confirmed by IR analysis. In the compound [Mn^II(biap)((C₆H₅)₂CHCOO)₂ (H₂O)] (2), the manganese ion is coordinated in a distorted octahedron with an additional water molecule as the sixth ligand. This aqua ligand forms strong intramolecular hydrogen bonds to both carboxylate groups as is reflected in the IR. Finally the manganese ion in the complex [Mn^II (biap)((C₆H₅)₃CCH₂COO)₂] (3) is again five coordinated with triphenylpropionate as coordinating counter ion. In 3 both carboxylates are bound monodentately, but one shows a semi coordination of the free oxygen to the manganese center (Mn-O = 3.169 A). Both carboxylates are involved in hydrogen bonding albeit to a different degree. This asymmetric binding mode is reflected in the IR spectrum, which shows two pairs of carboxylate vibrations. The structural differences of the three complexes are expressed by the very different X-band EPR spectra of the solid compounds. In solution, however, all three complexes are most likely octahedrally coordinated as indicated by their similar EPR spectra in frozen MeOH-EtOH solution.

Full text of DOI:10.1039/b104975p

View Article Online / Journal Homepage / Table of Contents for this issue
Tuning the coordination mode in mononuclear manganese
complexes by changing the steric bulk of the carboxylates
Sabine T. Warzeska,a Fabrizio Miccichè,a M. Chiara Mimmi,a Elisabeth Bouwman,*a
Huub Kooijman,b Anthony L. Spekb and Jan Reedijka
a Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502,
2300 RA Leiden, The Netherlands
b Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received 6th June 2001, Accepted 3rd October 2001
First published as an Advance Article on the web 8th November 2001
Three mononuclear manganese complexes of the tridentate ligand biap [ = N,N-bis(2-ethyl-5-methylimidazol-
4-ylmethyl)aminopropane] with various carboxylates as co-ligands have been synthesized and structurally characterised
by X-ray diffraction. All three complexes can be described by the general formula [MnII(biap)(RCOO)2(H2O)n] (n = 0,
1). The manganese() ion in [MnII(biap)(C6H5COO)2] (1) is five coordinated by three nitrogen donors from biap and
two oxygen donors of monodentate benzoate molecules. This monodentate binding mode is confirmed by IR
analysis. In the compound [MnII(biap)((C6H5)2CHCOO)2(H2O)] (2), the manganese ion is coordinated in a distorted
octahedron with an additional water molecule as the sixth ligand. This aqua ligand forms strong intramolecular
hydrogen bonds to both carboxylate groups as is reflected in the IR. Finally the manganese ion in the complex
[MnII(biap)((C6H5)3CCH2COO)2] (3) is again five coordinated with triphenylpropionate as coordinating counter
ion. In 3 both carboxylates are bound monodentately, but one shows a semi coordination of the free oxygen to
the manganese center (Mn–O = 3.169 Å). Both carboxylates are involved in hydrogen bonding albeit to a
different degree. This asymmetric binding mode is reflected in the IR spectrum, which shows two pairs of
carboxylate vibrations. The structural differences of the three complexes are expressed by the very different
X-band EPR spectra of the solid compounds. In solution, however, all three complexes are most likely
octahedrally coordinated as indicated by their similar EPR spectra in frozen MeOH–EtOH solution.
Introduction
To date five different mononuclear manganese enzymes are
known:1 manganese superoxide dismutase (MnSOD), man-
ganese peroxidase (MnP), manganese dioxygenase and the
recently discovered manganese lipoxygenase (MnLO)2 and
oxalate oxidase.3,4 In all these enzymes the manganese ion is
believed to shuttle between the Mn() and Mn() oxidation
state, catalysing one-electron redox reactions. Except for man-
ganese peroxidase, a unique enzyme in many respects that will
not be discussed further here, and the oxalate oxidases all the
other enzymes also exist in an iron-containing form. MnSOD
contains a trigonal bipyramidal coordinated manganese ion
in the active site.5 The coordination sphere consists of three
histidines, an aspartate and a water, or hydroxide, molecule.
Germin is a manganese-containing enzyme with oxalate oxidase
and superoxide dismutase activities.3 The manganese() ion
is in an octahedral geometry comprising three histidines, a
glutamate and two water molecules.3 The active-site structure
of the remaining two enzymes is not known as yet. Structure
Fig. 1 Comparison of the active sites of manganese() superoxide
proposals are based on structural similarities between the man-
ganese and iron-containing enzymes (see Fig. 1). For the Mn()
dioxygenase a square-pyramidal arrangement of two histidines,
a glutamate and two water molecules, like in the corresponding
extradiol Fe() dioxygenases might be possible.6 Recently a
manganese lipoxygenase has been discovered2 and also in this
case a structure analogous to the iron-containing soy bean
lipoxygenase (LO) is a topic of discussion.7 The iron ion in the
active site of LO is coordinated to three nitrogens of histidine
residues, one oxygen of a monodentate isoleucine and one
oxygen of a weakly coordinated asparagine.8 A water mole-
cule, stabilised by a hydrogen bond with the non-coordinated
dismutase (a), manganese() oxalate oxidase (b), extradiol iron()
dioxygenase (c) and iron() lipoxygenase-1 (d).
carboxylate oxygen of isoleucine, completes the distorted
octahedral coordination sphere.8 In summary, the manganese
ions in all three enzymes are believed to be coordinated by
imidazoles, carboxylates and water molecules as ligands in
varying coordination geometries.
The ligand N,N-bis(2-ethyl-5-methylimidazol-4-ylmethyl)-
aminopropane (biap) has been designed as a structural mimic
for the nitrogen donors in several enzymes. Despite the steric
crowding at the periphery of the ligand, the coordination
DOI: 10.1039/b104975p
J. Chem. Soc., Dalton Trans., 2001, 3507–3512
This journal is © The Royal Society of Chemistry 2001
3507

View Article Online
compounds of Co(), Ni() and Zn() with biap show a large
variety of coordination modes.9 Recently biap has been used
successfully in copper model complexes for nitrite reductase.10
In the present study, biap is used to model the histidine residues
in manganese containing enzymes. To study the influence of the
carboxylates on the coordination mode of the Mn() complexes
several carboxylates with different steric requirements have
been used.
µeff = 5.87 µB (at 22 ЊC). Anal. calc. for C59H63MnN5O4: C, 73.7;
H, 6.6; N, 7.3, Mn, 5.7. Found: C, 73.9; H, 6.9; N, 7.6; Mn,
5.6%. IR (ν/cmϪ1): 3055 (w), 2967 (w), 1636 (m), 1609 (s), 1559
(s), 1494 (m), 1447 (s), 1410 (s), 1386 (s), 1080 (m), 1045 (m),
762 (m), 744 (m), 698 (s). Recrystallisation from DMF led to
another white manganese() complex (3b) as characterised by
the differences in IR. µeff = 5.75 µB (at 22 ЊC). IR (ν/cmϪ1): 3053
(w), 2920 (w), 2832 (w), 1596 (s), 1557 (s), 1540 (s), 1491 (m),
1468 (m), 1456(w), 1445 (w), 1394 (s), 1084 (m), 1056 (m), 755
(m), 696 (s).
Experimental
Methods and instrumentation
Materials
FTIR spectra were obtained on a Perkin-Elmer Paragon 1000
FTIR spectophotometer equipped with a Golden Gate ATR
device, using the reflectance technique (4000–400 cmϪ1, 4 cmϪ1
resolution). Elemental Analyses were performed on a Perkin-
Elmer 2400 Series II analyzer. X-Band EPR spectra were
recorded on a Jeol RE2x electron spin resonance spectrometer
using DPPH (diphenylpicrylhydrazyl, g = 2.0036) as a stand-
ard. Manganese analysis was carried out complexometrically
with Na2H2EDTA as the complexing agent.
Manganese() perchlorate hexahydrate, sodium benzoate,
diphenylacetic acid, and 3,3,3-triphenylpropionic acid were
obtained from Acros. The ligand biap was synthesised accord-
ing to published methods.9 All reagents were used without
further purification. The complex syntheses were carried out in
an inert atmosphere to prevent oxidation to MnIII.
CAUTION! Organic perchlorate salts are potentially explos-
ive. Although we have experienced no accidents so far, all com-
pounds containing perchlorate should be handled with care and
in small amounts.
X-Ray structure determination
Crystals of suitable size were placed in the cold nitrogen
stream of a Nonius-KappaCCD diffractometer on a rotating
anode. Data in the range 1.6Њ < θ < 25.25Њ were collected at
150 K, using Mo-Kα radiation (graphite monochromator,
λ = 0.71073 Å). No absorption correction was applied. The
structures were solved by direct methods or Patterson
methods, as indicated below. Refinement on F 2 was carried out
by full-matrix least-squares techniques using SHELXL-97.11
All non-hydrogen atoms were refined with anisotropic thermal
parameters with the exception of those in the disordered
solvent of 1. Hydrogen atoms were included in the model at
calculated positions, riding on their carrier atoms unless stated
otherwise. Hydrogen atom displacement was described with a
fixed isotropic parameter related to the equivalent isotropic
parameter of the carrier atom. Pertinent data for each of the
structure determinations are given in Table 1.
Syntheses
[MnII(biap)(C6H5COO)2] (1). A solution of biap (1 mmol,
0.30 g in 1 ml MeOH) was added to a solution of Mn(ClO4)2ؒ
6H2O (1 mmol, 0.36 g) in 2 ml MeOH while stirring, resulting in
a clear pale yellow solution. Sodium benzoate (2 mmol, 0.29 g)
was dissolved in 3 ml of distilled H2O and added to the reagents
mixture, provoking the precipitation of 1ؒH2O [0.46 g, 74%
based on Mn(ClO4)2] as an air-stable white powder. µeff
=
5.70 µB (at 22 ЊC). Anal. calc. for 1ؒH2O: C31H41MnN5O5: C,
60.2; H, 6.7; N, 11.3. Found: C, 60.0; H, 6.2; N, 11.6%. IR
(ν/cmϪ1): 3126 (m), 3062 (m), 2932 (m), 2874 (m), 1597 (s), 1538
(s), 1455 (m), 1385 (vs), 1171 (w), 1080 (w), 1065 (m), 1045 (m),
1023 (m), 802 (m), 713 (vs), 645 (s), 539 (m), 418 (s).
[MnII(biap)((C6H5)2CHCOO)2(H2O)] (2). 0.36 g of Mn-
(ClO4)2ؒ6H2O (1 mmol) was dissolved in 2 ml of methanol to
give a colourless solution. With stirring a solution of biap in
methanol was added (0.30 g, 1 mmol in 2 ml MeOH). After ten
minutes with stirring the methanolic solution of diphenylacetic
acid (0.21 g, 1 mmol in 2 ml MeOH) and several drops of
triethylamine were added to the colourless reaction mixture.
The clear solution was stirred for four hours at room temper-
ature. By slowly adding 20 ml of distilled water a white precip-
itation formed. After half an hour with stirring the precipitate
was collected by filtration in air. A white air-stable powder
resulted. It was washed with some water and a small amount of
acetone and dried in vacuo. The yield of 2 was 70% (0.28 g),
calculated from the amount of diphenylacetic acid used.
µeff = 5.88 µB (at 22 ЊC). Anal. calc. for C45H53MnN5O5: C, 67.7;
H, 6.7; N, 8.8. Found: C, 67.0; H, 6.2; N, 9.0%. IR (ν/cmϪ1):
3027 (s), 2973 (s), 1590 (s), 1386 (s), 1435 (m), 1496 (m), 715 (s),
695 (s).
CCDC reference numbers 153639, 165397 and 165398.
See http://www.rsc.org/suppdata/dt/b1/b104975p/ for crystal-
lographic data in CIF or other electronic format.
[MnII(biap)(C6H5COO)2]ؒ0.4EtOH (1ؒ0.4EtOH). Crystals of
1ؒ0.4EtOH suitable for X-ray structure determination could be
grown from an ethanol–dimethylsulfoxide (10 : 1) solution of
1 by diethyl ether diffusion at room temperature. Data were
collected on a crystal of dimensions 0.1 × 0.2 × 0.3 mm. The
structure was solved by automated direct methods (SHELXS-
86).12 The Mn ion is located on a crystallographic 2-fold
rotation axis which results in a disorder of the propyl and
methylene moieties bound to the tertiary amine nitrogen. The
crystal packing can accommodate both orientations of the
propyl moiety without intermolecular collisions. The structure
contains an ethanol solvent molecule in the lattice. This mole-
cule is also situated on a crystallographic 2-fold axis, and is
therefore disordered.
[MnII(biap)((C6H5)3CCH2COO)2] (3). To a stirred solution
of Mn(ClO4)2ؒ6H2O (0.36 g, 1 mmol in 2 ml of EtOH), the
biap containing solution was added (0.30 g, 1 mmol in 2 ml of
EtOH) and a colourless reaction mixture resulted. After 10
minutes with stirring a solution of 3,3,3-triphenylpropionic
acid (0.30 g, 1 mmol) in 3 ml of EtOH containing some drops
of triethylamine was added. The colourless reaction mixture
was stirred for 14 hours. At the end of this period, about 20 ml
of distilled water was added drop-by-drop resulting in a white
precipitate. After 30 minutes with stirring, the precipitate was
collected by filtration in air. It was washed with some water and
dried in vacuo, yielding 0.24 g (48% based on acid) of 3.
In the crystal packing, the disordered region contains either
two propyl moieties or a disordered ethanol molecule.
[MnII(biap)((C6H5)2CHCOO)2(H2O)] (2). Single crystals of
this compound have been obtained from a solution of MeOH,
acetone and water (2 : 1 : 1) by slow evaporation in air. A block
shaped crystal of dimensions 0.10 × 0.15 × 0.20 mm was used
for data collection. The structure was solved by automated
direct methods (SHELXS-86).12 The measured crystal turned
out to be a non-merohedral twin. The twin operation is a 2-fold
rotation axis parallel to c*, which causes overlap of crystal-
lographically unrelated reflections in the zones h1 = 5n. This
3508
J. Chem. Soc., Dalton Trans., 2001, 3507–3512

View Article Online
Table 1 Crystallographic data for [MnII(biap)(C6H5COO)2]ؒ0.4EtOH (1ؒ0.4EtOH), [MnII(biap)((C6H5)2CHCOO)2(H2O)] (2) and [MnII(biap)-
((C6H5)3CCH2COO)2]ؒH2O (3ؒH2O)
1ؒ0.4 EtOH
2
3ؒH2O
Empirical formula
FW
C31.8H41.4MnN5O4.4
619.07
C45H53MnN5O5
798.86
C59H63MnN5O5
979.10
Crystal system
Space group
a/Å
b/Å
c/Å
a/Њ
β/Њ
Orthorhombic
Pccn (no. 56)
21.073(3)
8.964(1)
16.568(1)
—
—
—
3129.5(6)
4
1.3139(3)
9760
Monoclinic
P21 (no. 4)
9.756(1)
19.356(3)
22.421(4)
—
92.59(1)
—
4230(1)
4
Triclinic
P1 (no. 2)
¯
9.743(1)
15.554(2)
17.412(3)
78.23(1)
87.74(2)
84.03(2)
2568.7(7)
2
γ/Њ
V/Å3
Z
Dc/g cmϪ3
1.2546(3)
55995
1.2659(3)
25581
No. of measured reflections
No. of unique reflections
No. of parameters
µ(Mo-Kα)/mmϪ1
GOF
2820 (Rint = 0.0443)
14934 (Rint = 0.0878)
8943 (Rint = 0.0738)
217
1027
657
0.47
1.068
0.1678
0.0591
0.362
1.054
0.1019
0.0487
0.311
1.047
0.1167
0.0559
wR2 (all data)a
R1[Fo > 4σ(Fo)]a
4
a R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|, wR2 = [Σw(Fo2 Ϫ Fc2)2/ΣwFo ]1/2. GOF = [Σw(Fo2 Ϫ Fc2)2/(n Ϫ p)]1/2, where n is the number of reflections and p the number
of parameters.
Table 2 Selected bond distances (Å) and angles (Њ) at the manganese
centre for [MnII(biap)(C6H5COO)2]ؒ0.4EtOH (1ؒ0.4EtOH), [MnII(biap)-
((C6H5)2CHCOO)2(H2O)] (2) and [MnII(biap)((C6H5)3CCH2COO)2]ؒ
H2O (3ؒH2O)
overlap was taken into account during refinement with
SHELXL-97.11 The crystal was also an inversion twin. The
water hydrogen atoms were located on a difference Fourier map
and included in a rigid group with the oxygen as pivot atom.
1a
2
3
[MnII(biap)((C6H5)3CCH2COO)2]ؒH2O (3ؒH2O). Crystals of
3ؒH2O could be obtained by recrystallisation of the crude
product from ethanol (97%). Data were collected on a crystal of
dimensions 0.05 × 0.15 × 0.35 mm. The structure was solved
using Patterson methods (DIRDIF99).13 The crystal structure
contains a co-crystallised water molecule. The water and N–H
hydrogens were located on a difference Fourier map; their
coordinates were included as parameters in the refinement.
Mn1
Mn2
Mn1–N13
Mn1–N23
Mn1–N30
Mn1–O41
Mn1–O42
Mn1–O61
Mn1–O62
Mn1–O1
2.191(4)
2.191(4)
2.397(3)
2.083(3)
3.121(4)
2.083(3)
3.121(4)
2.228(3)
2.171(3)
2.415(3)
2.150(2)
2.236(3)
2.195(3)
2.418(3)
2.160(2)
2.150(3)
2.140(3)
2.502(3)
2.079(2)
3.169(2)
2.061(2)
3.551(2)
2.125(2)
2.262(3)
2.117(2)
2.277(3)
Results and discussion
O41–Mn1–O61
O41–Mn1–N13
O41–Mn1–N23
O41–Mn1–N30
N13–Mn1–N23
N13–Mn1–N30
N23–Mn1–N30
O61–Mn1–N13
O61–Mn1–N23
O61–Mn1–N30
O1–Mn1–O41
O1–Mn1–O61
O1–Mn1–N13
O1–Mn1–N23
O1–Mn1–N30
105.1(1)
98.9(1)
100.4(1)
127.5(1)
148.0(1)
74.0(1)
74.0(1)
100.4(1)
98.9(1)
94.1(1)
164.1(1)
85.7(1)
94.6(1)
99.2(1)
72.2(1)
75.9(1)
99.6(1)
102.2(1)
170.9(1)
84.0(1)
93.3(1)
87.2(1)
161.9(1)
90.2(1)
92.0(1)
161.2(1)
86.1(1)
92.0(1)
100.0(1)
72.4(1)
76.8(1)
103.4(1)
104.4(1)
175.8(1)
84.2(1)
91.6(1)
84.8(1)
161.5(1)
87.8(1)
92.2(1)
126.1(1)
120.8(1)
90.3(1)
103.6(1)
73.1(1)
74.0(1)
111.0(1)
98.6(1)
Synthesis of the complexes
The complex syntheses have to be carried out in an inert atmos-
phere. The aqueous manganese containing solutions rapidly
turn brown when the reaction is performed in air, indicating
the formation of higher oxidation state complexes. The man-
ganese() complexes, once isolated, seem to be quite resistant to
oxidation, they are stable in air in the solid state for a prolonged
period of time. Furthermore, the complexes seem to be resistant
to electrochemical oxidation (see below), however, this could be
due to the non-aqueous conditions used in cyclic voltammetry.
The syntheses have not been optimised in terms of yields. It
appears that the use of an alkali salt of the carboxylate ligand
has a positive effect on the isolated yield of the complex. For
the complexes 2 and 3, however, a substoichiometric amount of
the corresponding acid was used, primarily because it was
assumed that the bulk of these carboxylates would preclude the
binding of two such moieties to the metal centre. It was
intended that the binding of only one carboxylate would result
in the formation of possible models for those active sites in
enzymes in which only one carboxylate group and two water
molecules are bound to the metal ion. Unexpectedly, it
appeared that two carboxylates bind to the manganese() ion in
complexes 2 and 3.
127.5(1)
172.4(1)
a Due to the location of 1 on the 2-fold axis, N23 = N13a (1/2 Ϫ x,
3/2 Ϫ y, z), O61 = O41a (1/2 Ϫ x, 3/2 Ϫ y, z), O62 = O42a (1/2 Ϫ x,
3/2 Ϫ y, z).
the complex is shown in Fig. 2. The crystal structure consists of
a neutral, mononuclear manganese() complex. The Mn() ion
is five coordinated in an N3O2 donor set. The ligand biap is
coordinated meridionally to the central manganese ion in an
almost planar conformation. Both benzoate anions act as
monodentate co-ligands. Attempts to analyse the coordination
sphere by the method of Addison and Reedijk et al.14 or the
method described by Holmes15 both led to intermediate results.
The τ descriptor14 of 0.34 is closest to a distorted square-
pyramidal coordination sphere. In this description the apex of
the square pyramid would be O41 or, for symmetry reasons,
Description of the structures
[MnII(biap)(C6H5COO)2]ؒ0.4EtOH (1ؒ0.4EtOH). Selected
bond distances and angles are listed in Table 2. The structure of
J. Chem. Soc., Dalton Trans., 2001, 3507–3512
3509

View Article Online
Fig. 3 Projection of 2, only one of the two independent molecules is
shown. All hydrogen atoms except H(1) and H(2) of the water molecule
have been omitted for clarity.
Fig. 2 Projection of 1, only one of the disordered components (related
by crystallographic symmetry; see Experimental section) is shown. All
hydrogen atoms have been omitted for clarity.
amine-N, the other trans to an imidazole-N. A water molecule
occupies the sixth position in the distorted octahedron trans to
imidazole. This aqua ligand forms strong intramolecular
hydrogen bonds to the non-coordinated oxygen of each carb-
oxylate group (O ؒ ؒ ؒ O distance 2.72–2.76 Å). Another hydro-
gen bonding interaction is present between the carboxylates
and the imidazole N–H of neighbouring molecules (O ؒ ؒ ؒ N
distances 2.77–2.80 Å), thus forming a two-dimensional net-
work parallel to the a,b plane. The bond distances around the
metal ion are normal for divalent manganese ions in an octa-
hedral geometry, although the distance from the manganese ion
to the tertiary amine is rather long [Mn1–N30: 2.415(3) Å;
Mn2 ؒ ؒ ؒ N130: 2.418(3) Å]. The octahedral geometry around
the manganese ion is severely distorted. This distortion is dic-
tated by the bite angle of the biap ligand in combination with
the large ionic radius of Mn() resulting in bond angles N13–
Mn1–N30 and N23–Mn1–N30 in both independent molecules
significantly smaller than 90Њ.
O41a. With a dihedral angle δ24 of 36.88Њ on the other hand
the structure would be better described as distorted TBPY
geometry,15 with the imidazole nitrogens occupying the axial
positions. In any case these analyses stress that the coordination
sphere is severely distorted. The small bite angle of the ligand
[N13–Mn1–N13a = 148.0(1)Њ] in the manganese complex in
comparison to the related copper complexes (N13–Cu1–
N23 = 161.6/162.9Њ),10 and the larger ionic radius of the man-
ganese ion forces the metal ion into this distorted coordination
geometry.
The non-coordinated carboxylate O42 accepts an inter-
molecular hydrogen bond from atom N11 of a neighbouring
molecule [O ؒ ؒ ؒ N distance 2.806(4) Å] and a long hydrogen
bond from the disordered ethanol solvent [O ؒ ؒ ؒ O distance
3.16(3) Å]. One might also consider a weak interaction to the
central metal atom [O42 ؒ ؒ ؒ Mn1: 3.121(4) Å, i.e. 0.55 Å
shorter than the sum of the contact radii].
Interestingly, two very similar mononuclear complexes
[Mn(bba)(OAc)2] [bba = bis(benzimidazol-2-ylmethyl)amine]16
and [Mn(terpy)(dmb)2] (terpy = 6Ј,2Љ-terpyridine, dmb = 2,6-
dimethoxybenzoate)17 have been synthesised and characterised
by single crystal X-ray diffraction. Also in these two complexes
the manganese ion is five coordinated with an N3O2 donor set,
the tridentate nitrogen donating ligands are coordinated
meridionally and the coordinated carboxylates are bound in a
monodentate manner. Both complexes also exhibit very irreg-
ular coordination geometries, which are neither TBPY nor
SPY. In [Mn(bba)(OAc)2] an additional interaction of one
of the unbound oxygens to the manganese ion is present, the
Mn–O distance (2.62 Å) is much shorter than in 1.
Octahedral mononuclear manganese() complexes with carb-
oxylate containing ligands in a N3O3 donor set are quite rare in
the literature. One example is the manganese() complex [Mn-
(Hquin)3]Ϫ (H2quin = 2,3-pyridinedicarboxylic acid), in which
the chelating ligand is bound with one pyridine nitrogen atom
and a monodentate carboxylate.18
[MnII(biap)((C6H5)3CCH2COO)2]ؒH2O (3ؒH2O). In the com-
plex 3ؒH2O the manganese ion is five coordinated in an N3O2
donor set as depicted in Fig. 4. The ligand biap is coordinated
facially with an angle between the imidazole planes of 72.2(2)Њ.
The coordination sphere can best be described as distorted
trigonal bipyramidal. This is also confirmed by the para-
meters τ = 0.77 and δ24 = 47.97. In the trigonal bipyramid the
axial positions are occupied by the amine nitrogen and O61,
whereas the imidazole nitrogens and O41 occupy the equatorial
positions. In a first approximation both carboxylates are
coordinated in a monodentate fashion, but one might discuss a
semi-coordination of O42 to Mn1 [Mn1 ؒ ؒ ؒ O42: 3.169(2) Å]
since this distance is significantly (0.5 Å) shorter than the sum
of the van der Waals radii. The other carboxylate is clearly
monodentate [Mn1 ؒ ؒ ؒ O62: 3.551(2) Å]. The two “free” carb-
oxylate oxygens O42 and O62 also show differences in their
participation in hydrogen bonds. O42 accepts a hydrogen bond
from a neighbouring imidazole N–H [O42 ؒ ؒ ؒ N21: 2.794(4) Å]
and from the lattice water molecule [O42 ؒ ؒ ؒ O94: 2.967(4) Å].
On the other hand O62 accepts only one short hydrogen bond
from the water molecule O94 [O62 ؒ ؒ ؒ O94: 2.658(4) Å]. The
[MnII(biap)((C6H5)2CHCOO)2(H2O)] (2). A drawing of the
crystal structure of 2 is shown in Fig. 3, selected interatomic
distances and bond angles are listed in Table 2. The asymmetric
unit contains two independent molecules of [Mn()(biap)-
((C6H5)2CHCOO)2(H2O)]. The only major difference between
these molecules is a 180Њ rotation along one of the C–CPh2
bonds. The remaining parts of the molecules are related by a
non-crystallographic inversion operation. The three nitrogen-
donor atoms from the ligand biap are folded around the man-
ganese ion in a facial fashion with an angle of 61Њ between the
imidazole planes. Both carboxylates are coordinated in a
monodentate coordination mode, one carboxylate trans to the
3510
J. Chem. Soc., Dalton Trans., 2001, 3507–3512

View Article Online
Table 3 Carboxyl stretching frequencies (in cmϪ1) of the complexes 1, 2, 3, 3b and of the alkali salts of the carboxylates
1
Na(PhCOO)
2
K(Ph2CHCOO)
3
3b
K(Ph3CCH2COO)
νa(CO2)
νs(CO2)
∆ν
1549
1386
163
1547
1405
142
1590
1383
207
1595
1360
235
1609
1391
218
1596
1394
202
1560
1408
152
1564
1384
180
Fig. 5 EPR spectra of polycrystalline samples of 1, 2 and 3 at the
X-band frequency.
number of carboxylate containing complexes and arrived at the
following conclusions:
• Monodentate carboxylates show ∆ν values which are much
Fig. 4 Projection of 3. All hydrogen atoms have been omitted for
clarity.
larger than that of the ionic carboxylate (∆ν Ϫ ∆νionic
>
50 cmϪ1);
hydrogen bond network is completed by hydrogen bonds
donated from the imidazole N–H to the oxygen of the lattice
water molecule [O94 ؒ ؒ ؒ N11: 2.754(4) Å]. All these hydrogen
bonds result in a chain parallel to the a-axis.
• Chelating (didentate) carboxylates exhibit ∆ν values which
are significantly less than the ionic values (∆νionic Ϫ ∆ν > 50
cmϪ1);
• Bridging carboxylates exhibit ∆ν values that are close to the
ionic values (∆νionic Ϫ ∆ν ≈ 0 cmϪ1).
Comparison of the structures
The ∆ν values of 1 and 2 are close to those of sodium
benzoate and potassium diphenylacetate, respectively. These
values would thus suggest polynuclear complexes with bridging
carboxylates, although the monodentate binding mode of the
carboxylates has been confirmed by X-ray analysis. However, in
our case this unusual ∆ν value must be due to the strong hydro-
gen bonds in the complexes; such strong bonds have been
described before for other complexes.20,21 These hydrogen bonds
are more predominant in 2 than in 1, where only intermolecular
hydrogen bonds are found. In fact the carboxylates in 2 can be
considered as bridging between the metal centre and the water
molecule. Interestingly, in 3 two different pairs of carboxyl
stretching frequencies are found. This is probably due to the
two different binding modes of the coordinated carboxylates.
Recrystallisation of 3 in a DMF–H2O mixture results in a
different manganese() species 3b. In 3b only one pair of carb-
oxyl stretching frequencies is observed. This might suggest that
in 3b a solvent molecule is bound as a sixth ligand resulting in a
structure similar to 2, or that the carboxylates are bound more
symmetrically as in 1.
The complexes 1, 2 and 3 were all expected to show signals in
the EPR spectra due to the presence of Mn(). Therefore the
X-band EPR spectra of the three complexes were recorded in a
powdered sample (Fig. 5) and in a MeOH–EtOH (9 : 1) glass at
77 K (Fig. 6). The EPR spectrum of 2 shows a broad pattern
around g ≈ 2 typical for manganese() in a distorted octahedral
environment.22 The EPR signals of 1 and 3 are even more com-
plicated due to zero-field splitting effects. A detailed analysis of
the EPR behaviour at various frequencies is beyond the scope
of the present study and will be described elsewhere.
It is interesting to compare the coordination sphere of the
present complexes [Mn(biap)(RCOO)2(H2O)n] (n = 0, 1) with
respect to the growing steric bulk of the carboxylates. The com-
plexes seem to follow two strategies in response to the steric
stress. The first one is a different folding of biap. The mer con-
formation is much more space filling than the fac conformation.
Despite the larger carboxylate group in 2, the fac conformation
of biap even allows the binding of a sixth ligand. The second
strategy is to reduce the coordination number from 6 to 5 again,
with biap remaining in the fac conformation as realized in 3.
It is noteworthy that all non-coordinated carboxylate oxy-
gens in the three structures are involved in extensive hydrogen
bonding, both intra- and inter-molecularly, thereby adding to
the stability of the complexes. This hydrogen bonding clearly
affects the infrared stretching frequencies of the carboxylate
groups and indeed the interpretation of the IR data is not
straightforward.
Spectroscopic properties of the complexes
The antisymmetric and symmetric stretching modes of the
carboxylates dominate the infrared spectra of the manganese
complexes 1–3. The carboxyl stretching frequencies of the
complexes and their corresponding sodium/potassium salts are
listed in Table 3. A comparison between the ∆ν value
[∆ν = νa(COO) Ϫ νs(COO)] of the corresponding sodium or
potassium salts of the carboxylates and the ∆ν value of
the complexes gives interesting insights into the binding mode
of the carboxylate ion. Deacon and Phillips19 analysed a
J. Chem. Soc., Dalton Trans., 2001, 3507–3512
3511

View Article Online
numbers 5 and 6 could be realized using these donors. The same
coordination numbers are believed to play an important role in
the catalytic cycle of the manganese enzymes. The coordination
mode of the carboxylates, and even more their ability to form
hydrogen bridges to assist neighbouring groups, seems to be
crucial for tuning the coordination number of the manganese
centres. Complex 2 in particular can be regarded as a close
structural model of manganese containing oxalate oxidase and
of iron containing lipoxygenase. Both in 2 and in the enzyme
the nitrogen donors are bound facially in the octahedron and
both structures contain a monodentate carboxylate, which
accepts a hydrogen bond from the coordinated water molecule.
The second carboxylate group in 2 simulates the weakly
coordinated amide of asparagine.
Acknowledgements
F. M. (University of Palermo, Italy) and M. C. M. (University
of Pavia, Italy) were involved in the project through the
Socrates student exchange programme. This work was sup-
ported in part by the Netherlands Research Council for
Chemical Sciences (NWO/CW) with financial aid from the
Netherlands Technology Foundation (STW) in the Priority
Programme Materials.
Fig. 6 EPR spectra of a frozen solution of 2 in MeOH–EtOH (9 : 1)
at the X-band frequency.
The spectra of the complexes in frozen solution are all quite
similar. The signals are better resolved and show an additional
55Mn 6-line pattern, visible on a few of the main bands (AMn
=
References
9
1 mT). Only for the octahedral complex 2 do the solid state
and frozen solution spectra resemble one another (Fig. 6).
Given the similarity between the solution spectra of the three
complexes, it is therefore also likely that the five coordinated
complexes 1 and 3 bind a sixth ligand in solution. The solution
spectra of the complexes show a striking resemblance to that of
barley oxalate oxidase.4
The electrochemical properties of the complexes have been
studied in dry and degassed DMF. Although the synthesis of
the complexes has to be carried out in an inert atmosphere, the
complexes appear to be remarkably stable towards oxidation.
Within the potential window of DMF all the complexes were
found to be electrochemically inactive. The discrepancy with
the oxidation sensitivity during synthesis may be attributed
to the use of strictly anhydrous conditions in the cyclic
voltammetry.
1 N. A. Law, M. T. Caudle and V. L. Pecoraro, Adv. Inorg. Chem.,
1999, 46, 305 and references cited therein.
2 C. Su and E. H. Oliw, J. Biol. Chem., 1998, 273, 13072.
3 E. J. Woo, J. M. Dunwell, P. W. Goodenough, A. C. Marvier and
R. W. Pickersgill, Nat. Struct. Biol., 2000, 7, 1036.
4 L. Requena and S. Bornemann, Biochem. J., 1999, 343, 185.
5 G. E. Borgstahl, H. E. Parge, M. J. Hickey, W. F. Beyer, Jr., R. A.
Hallewell and J. A. Trainer, Cell, 1992, 71, 107.
6 S. Han, L. D. Eltis, K. N. Timmis, S. W. Muchmore and J. T. Bolin,
Science, 1995, 270, 976.
7 C. Su, M. Sahlin and E. H. Oliw, J. Biol. Chem., 2000, 275, 18830.
8 W. Minor, J. Steczko, B. Stec, Z. Otwinowski, J. T. Bolin, R. Walter
and B. Axelrod, Biochemistry, 1996, 35, 10687.
9 E. Bouwman, B. Douziech, L. Gutierrez-Soto, M. Beretta, W. L.
Driessen, J. Reedijk and G. Mendoza-Díaz, Inorg. Chim. Acta, 2000,
304, 250.
10 M. Beretta, E. Bouwman, L. Casella, B. Douziech, W. L. Driessen,
L. Gutierrez-Soto, E. Manzani and J. Reedijk, Inorg. Chim. Acta,
2000, 310, 41.
11 G. M. Sheldrick, SHELXL-97, Program for crystal structure
refinement, University of Göttingen, Germany, 1997.
12 G. M. Sheldrick, SHELXS-86, Program for crystal structure
determination, University of Göttingen, Germany, 1986.
13 P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman,
S. García-Granda, R. O. Gould, J. M. M. Smits and C. Smykalla,
DIRDIF99 Program System, Technical Report, Crystallography
Laboratory, University of Nijmegen, The Netherlands, 1999.
14 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C.
Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
15 R. R. Holmes, Prog. Inorg. Chem., 1984, 32, 119.
16 H. Adams, N. A. Bailey, J. D. Crane and D. E. Fenton, J. Chem.
Soc., Dalton Trans., 1990, 1727.
17 L. S. Erre, G. Micera, E. Garribba and A. Cs. Bényei, New J. Chem.,
2000, 24, 725.
18 M. A. S. Goher, A. A. Youssef, Z. Y. Zhou and T. C. W. Mak,
Polyhedron, 1993, 12, 1871.
19 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33,
227.
20 L. S. Erre, G. Micera, F. Cariati, G. Ciani, A. Sironi, H. Kozlowski
and J. Baranowski, J. Chem. Soc., Dalton Trans., 1988, 363.
21 T. Glowiak, H. Kozlowski, L. S. Erre and G. Micera, Inorg. Chim.
Acta, 1995, 236, 149.
Conclusions
The results presented in this paper show that in complexes of
the general formula [Mn(biap)(RCOO)2(H2O)n] (n = 0, 1) a
variety of structures is possible. The differences in the three
structures discussed is mainly caused by the coordination mode
of biap (fac or mer), the absence or presence of a sixth ligand,
the tendency of the free carboxylate oxygen to bind to the metal
ion and the possible formation of inter- and intra-molecular
hydrogen bonds. The three structures can be distinguished by
X-ray crystallography, IR analysis and EPR spectroscopy. In
solution, however, it seems that all the complexes are able to
bind a sixth ligand to form an octahedral complex independent
of the bulk of the coordinated carboxylates.
From the studies described above it is clear that extreme care
has to be taken in assignment of the binding mode of carboxyl-
ate groups based on infrared studies alone. The presence of
strong hydrogen bridges will affect the stretching frequencies of
the carboxylates to such an extent that solid statements on their
origin cannot be made. The coordinating groups used in this
study are similar to those found in mononuclear manganese
enzymes (imidazoles, carboxylates, water). The coordination
22 R. D. Dowsing, B. Nieuwenhuijse and J. Reedijk, Inorg. Chim. Acta,
1971, 5, 301.
3512
J. Chem. Soc., Dalton Trans., 2001, 3507–3512