Molecular structure and spectral properties of bis(2,6-dimethoxybenzoato)(2,2': 6',2-terpyridine)manganese(II): A five-coordinate Mn(II) complex

Article abstract of DOI:10.1039/b002119i

The mixed complex formed by Mn(n), 2,6-dimethoxybenzoate (dmb) and 2,2': 6',2_-terpyridine (terpy) was isolated in the solid state and characterised by single crystal X-ray diffraction, IR and EPR, and thermogravimetric analysis. The complex is fivefold co-ordinated with a geometry shifted toward the limiting form of the trigonal bipyranfid. Two carboxylate groups and the central nitrogen atom of terpy occupy the equatorial plane, while the distal nitrogen atoms are in the axial positions. The normalised bite, the angle formed by distal nitrogen (N0 and N0,) atoms at the metal ion, the manganese-nitrogen distances, and the tilt angles between the pyridine rings were compared with those of other terpyridine complexes. Owing to the large dimensions of the Mn(n) ion, the distortions observed in the Na-Mn-Na, angles and in those between the pyridine rings are the largest so far observed with first-row transition metal ions. The metal-central nitrogen and metal-distal nitrogens distances are longer than with other metal ions.

Full text of DOI:10.1039/b002119i

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Molecular structure and spectral properties of
bis(2,6-dimethoxybenzoato)(2,2º : 6º,2/-terpyridine)manganese(II):
a Ðve-coordinate Mn(II) complex
Liliana Strinna Erre,a Giovanni Micera,*a Eugenio Garribbaa and Attila Cs. Benyeib
a Department of Chemistry, University of Sassari, V ia V ienna 2, I-07100 Sassari, Italy
b Department of Chemistry, L aboratory for X-Ray Di†raction, Kossuth L ajos University,
H-4010 Debrecen, Hungary
Received (in Strasbourg, France) 14th March 2000, Accepted 3rd July 2000
Published on the Web 14th August 2000
The mixed complex formed by Mn(II), 2,6-dimethoxybenzoate (dmb) and 2,2@ : 6@,2A-terpyridine (terpy) was isolated
in the solid state and characterised by single crystal X-ray di†raction, IR and EPR, and thermogravimetric analysis.
The complex is Ðvefold co-ordinated with a geometry shifted toward the limiting form of the trigonal bipyramid.
Two carboxylate groups and the central nitrogen atom of terpy occupy the equatorial plane, while the distal
nitrogen atoms are in the axial positions. The normalised bite, the angle formed by distal nitrogen (N and N )
d
d{
atoms at the metal ion, the manganeseÈnitrogen distances, and the tilt angles between the pyridine rings were
compared with those of other terpyridine complexes. Owing to the large dimensions of the Mn(II) ion, the
distortions observed in the N ÈMnÈN angles and in those between the pyridine rings are the largest so far
d
d{
observed with Ðrst-row transition metal ions. The metal-central nitrogen and metal-distal nitrogens distances are
longer than with other metal ions.
The manganese complexes of carboxylic and nitrogenous
ligands provide models for the active sites of manganese
Experimental
Synthesis
enzymes. In this light, much e†orts has been directed towards
the synthesis of Mn(II), (III) and (IV) complexes with ligands
like 2,2@-bipyridine (bpy), 1,10-phenanthroline (phen) and
2,2@ : 6@,2A-terpyridine (terpy).1h3 A dinuclear complex with
[Mn(dmb) (H O) ] É nH O was prepared as described pre-
2
2
3 n
2
viously.10 2,2@ : 6@,2A-Terpyridine was used as commercially
available. Equimolar amounts (1 mmol) of
[Mn(dmb) (H O) ] É nH O and terpy were dissolved in
2,2@-bipyridine,
[Mn (dmb) (bpy) (H O) ] É bpy(dmb \ 2,6-
2
4
2
2
2
2
2
3 n
2
dimethoxybenzoate) was prepared in our laboratory and char-
20 ] 10~3 dm3 of absolute EtOH under stirring. A yellow
crystal-like precipitate formed by slow evaporation at room
temperature. It was Ðltered o†, washed with EtOH and air-
acterised through single crystal X-ray analysis.4
We have isolated the compound [MnII(dmb) (terpy)], which
2
exhibits a characteristic Ðvefold co-ordination with a distorted
dried at room temperature. [Mn(dmb) (terpy)]: Anal. calcd.
2
trigonal bipyramidal geometry. Although various co-
ordination geometries are accessible to high-spin d5 metal
ions, this stereochemistry is still rare for Mn(II), which prefers
for MnC
H
O N : C, 60.93; H, 4.49; N, 6.46; Mn O ,
33 29
8
3
2
3
12.1%. Found C, 60.70; H, 4.61; N, 6.48; Mn O 11.8%.
2
3
Physical measurements
octahedral hexa-co-ordinated arrangements. Only
examples of a trigonal bipyramidal environment involving a
monodentate ligand are known: [MnCl (2-Meim) ](2-
a few
Elemental analyses (C, H and N) were obtained with a
PerkinÈElmer 240 B elemental analyser. IR spectra were
recorded with a FT Bruker IFS-66 interferometer using KBr
(4000È600 cm~1) or polyethylene (500È100 cm~1) disks. Ther-
mogravimetric data were obtained with a PerkinÈElmer
TGS-2 apparatus in air and under nitrogen Ñow. X-band EPR
spectra (9.15 GHz) were recorded on polycrystalline samples
at 298 K with a Varian E-9 spectrometer and simulated by
use of the Bruker WIN-EPR SimFonia program. Magnetic
susceptibility data were obtained at room temperature with a
Bruker BM-4 system and corrected for diamagnetism using
PascalÏs constants.
2
3
Meim \ 2-methylimidazole),5 [Mn(dmu) Br ](dmu \ N,N@-
3
2
dimethylurea)6
and
[Mn(OBz)(3,5-i-Pr pzH)MHB(3,5-i-
2
Pr pz) N] [3,5-iPr pz \ 3,5-diisopropylpyrazole and HB(3,5-i-
2
3
2
Pr pz) \ hydrotris(3,5-diisopropylpyrazolylborate)].7 This co-
2
ordination geometry arouses great interest because binuclear
or mononuclear divalent manganese complexes with N
ligands are structural analogues for the active site of Mn con-
3
taining proteins. For instance, a trigonal bipyramidal arrange-
ment, with an N O ligand donor set, is adopted by Mn(II) in
3
2
Mn-SOD.8 A Ðve-co-ordinate geometry is stabilised in the
mixed complexes formed by a tridentate ligand, like terpy, and
a unidentate ligand. An analysis of complexes of the type
Crystal structure analysis
Mn(II)LX (L \ tridentate, X \ unidentate) showed that the
A summary of crystal parameters and details of the data col-
lection are given in Table 1; selected bond lengths and angles
are listed in Table 2.
2
geometry is irregular and corresponds neither to a trigonal
bipyramid of C symmetry nor to a regular square pyramid
2v
of
C
symmetry.9 In this study the structure of
Intensity data were taken on an EnrafÈNonius MACH3
computer-controlled di†ractometer.11 Data reduction was
carried out with XCAD4.12 The structure was solved by the
SIR-97 software13 and reÐned on F2 by the program
s
[Mn(dmb) (terpy)] has been determined by single crystal
X-ray di†raction. The co-ordination mode of terpyridine is
2
discussed comparatively with those of other structures.
Further insights derive from the spectral properties (IR and
EPR) and the thermogravimetric behaviour.
SHELXL93.14 The P2 /a space group assigned to the com-
1
pound is consistent with the setting originally determined. Of
DOI: 10.1039/b002119i
New J. Chem., 2000, 24, 725È728
725
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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Table 1 Crystal data experimental details for [Mn(dmb) (terpy)]
dentate ligand spanning basal-apical-basal sites. At b \ 1.73
the stereochemistry reverts to trigonal bipyramid with the tri-
dentate ligand bound in the three equatorial sites.
2
Formula
Formula weight
Crystal system
Space group
a/A
MnC
650.53
Monoclinic
H
O N
33 29
8
3
A
perspective view of the molecular structure of
[Mn(dmb) (terpy)] with atom numbering is presented in Fig.
P2 /a
2
1
1. The metal ion is in the center of a distorted trigonal bipyra-
13.559(2)
b/A
14.745(3)
16.186(1)
111.93(2)
3001.9
4
0.498
293(2)
4612
4235
2070
0.01
0.0588
0.1085
0.1475
mid. The equatorial plane is occupied by the O(1@) and O(1A)
atoms of the carboxylate groups of dmb and the central nitro-
c/A
b(¡)
gen (N ) atom of terpy, N(2). The distal nitrogen atoms (N
)
c
d, d{
U/AŽ 3
of terpy are in the axial positions, N(1) and N(3). The magni-
tude of the normalized bite of the terpy ligand is 1.17. The
angle formed by the MnÈO bonds is 99.9¡. The MnÈO(1@)
bond (2.156 A) is longer than MnÈO(1A) (2.099 A). In addition,
the angle O(1@)ÈMnÈN(2) (149.7¡) is much greater than O(1A)È
MnÈN(2) (108.0¡). The stereochemistry is distorted toward a
square pyramid with the N(1), N(2), N(3) and O(1A) atoms
approaching planarity. The Mn(II) distance from the O(1@)È
O(1A)ÈN(2) plane is 0.18 A; the torsion angle of O(1@)ÈO(1A)È
N(2)ÈMn is 8¡. The angle between N(1)ÈN(2)ÈN(3)ÈMn and
O(1@)ÈO(1A)ÈN(2)ÈMn planes is 78.7¡.
Z
k/mm~1
T /K
Measured reÑections
Independent reÑections
ReÑections with I [ 2p(I)
R
int
R (F)
wR (F2) [I [ 2p(I)]a
wR (F2) (all data)a
a w \ 1/[p2 (F2) ] (0.0371 P)2 ] 6.6740 P] where P \ (F2 ] 2 F2)/3.
0
0
c
The magnitude of the N ÈMnÈN angle is reduced from
d
d{
180¡, as is found in other mono- and bis-terpy complex-
es.18h25 Because of the larger size of the Mn(II) ion, the devi-
ation is greater than for other Ðrst-row transition metal ions
(Table 3). The N ÈMÈN angle follows closely the size of the
course, it could be transformed to the standard P2 /c setting.
1
The best data set involved more than half of the reÑections
with I less than 2p(I). However, this factor is not critical as the
R value, the residual electron density, the data-parameter
ratio, etc. are very good. Atomic scattering factors were taken
from ref. 15. No extinction corrections were applied. The soft-
ware used to prepare material for publication was WINGX.16
All non-hydrogen atoms were anisotropically reÐned. Hydro-
gen atoms were placed in their calculated positions and
reÐned using a riding model.
d, d{
c
metal ion, the angle being smaller the larger is the cation.
Indeed, large metal ions give rise to longer MÈN distances
and to a greater distortion of the terpyridine molecule.
The manganese(II)Ènitrogen bonds are normal, though the
MnÈN distance (2.226 A) is shorter than the average
c
MnÈN
(2.279 A) bond length. This distortion is always
d, d{
observed in terpy complexes and is attributed to a more effi-
CCDC reference number 440/196. See http://www.rsc.org/
suppdata/nj/b0/b002119i/ for crystallographic Ðles in .cif
format.
cient overlap of the metal t orbitals with the p* orbitals of
the central pyridyl moiety in comparison with the distal
2g
pyridyl rings.26 In our complex the di†erence in bond lengths
(0.05 A) is smaller than seen for other Ðrst-row transition
metal ions (Table 3) and comparable with that observed for
high-spin [MnII(terpy) ](I ) (0.06 A).19
Results and discussion
2
3 2
A linear correlation may be found between the magnitude
Structure of [Mn(dmb) (terpy)]
2
of the N ÈN ÈN angle and the average MÈN distance (see
d
c
d{
The Mn(II)LX (L \ tridentate ligand, X \ unidentate ligand)
Fig. 2 in ref. 23). The angle of 109¡ is well correlated to the
average MnÈN distance of our structure (2.261 A). A linear
correlation between the normalised bite and the average MÈN
distance was also established (see Fig. 2 in ref. 23). The dis-
tance of 2.261 A is in fairly good agreement with the ligand
bite of 1.17.
2
stereochemistry has been studied in terms of a repulsion
energy model.9,17 A distinctive parameter is the normalized
bite, deÐned as b \ 2 sin(d/2), where d is the average angle
formed by the axes joining the metal ion to the central and to
the distal donors of the tridentate ligand. For b \ 1.40 the
stereochemistry is close to trigonal bipyramid with the tri-
dentate ligand spanning axial-equatorial-axial sites. With
b \ 1.55 the stereochemistry is square pyramid with the tri-
Table 2 Selected bond lengths (A) and angles (¡)
MnÈO(1A)
2.099(4)
2.156(4)
2.226(5)
2.248(5)
2.310(5)
99.9(2)
C(7@)ÈO(1@)
C(7@)ÈO(2@)
C(7A)ÈO(1A)
C(7A)ÈO(2A)
1.259(7)
1.249(7)
1.255(7)
1.223(7)
MnÈO(1@)
MnÈN(2)
MnÈN(3)
MnÈN(1)
O(1A)ÈMnÈO(1@)
O(1A)ÈMnÈN(1)
O(1@)ÈMnÈN(1)
O(1A)ÈMnÈN(2)
O(1@)ÈMnÈN(2)
O(1A)ÈMnÈN(3)
O(1@)ÈMnÈN(3)
N(1)ÈMnÈN(2)
N(1)ÈMnÈN(3)
N(2)ÈMnÈN(3)
C(1)ÈN(1)ÈMn
C(5)ÈN(1)ÈMn
C(6)ÈN(2)ÈMn
C(10)ÈN(2)ÈMn
C(11)ÈN(3)ÈMn
C(15)ÈN(3)ÈMn
C(1)ÈN(1)ÈC(5)
C(6)ÈN(2)ÈC(10)
C(11)ÈN(3)ÈC(15)
N(1)ÈC(5)ÈC(6)
N(2)ÈC(6)ÈC(5)
N(2)ÈC(10)ÈC(11)
N(3)ÈC(11)ÈC(10)
C(4)ÈC(5)ÈC(6)
123.4(5)
86.1(2)
99.4(2)
119.6(6)
119.5(6)
117.5(5)
116.1(6)
114.7(6)
113.7(6)
115.2(5)
121.9(7)
124.3(7)
123.6(6)
123.8(6)
122.1(7)
125.5(6)
108.0(2)
149.7(2)
117.8(2)
106.2(2)
71.6(2)
140.4(2)
71.1(2)
123.8(5)
116.0(4)
119.3(4)
121.1(5)
118.7(4)
C(5)ÈC(6)ÈC(7)
C(9)ÈC(10)ÈC(11)
C(10)ÈC(11)ÈC(12)
O(1@)ÈC(7@)È O(2@)
O(1A)ÈC(7A)È O(2A)
Fig. 1 Perspective view of [Mn(dmb) (terpy)] (50% probability dis-
2
placement ellipsoids).
726
New J. Chem., 2000, 24, 725È728

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Table 3 Metrical details of terpy complexes (average values)
Ionic radius/A
N
ÈMÈN /¡ N ÈMÈN /¡ MÈN /A MÈN /A *a/A
*
b/¡
Ref.
d,d{
c
d
d{
c
d,d{
ring
[CrIII(terpy) ]ClO
2
0.63
0.80
0.80
0.74
0.72
78.7
72.3
71.3
80.6
78.9
76.7
82.1
77.5
73.0
77.4
157.3
144.5
140.4
161.1
157.9
153.6
164.3
154.8
146.1
154.7
1.975
2.19
2.056
2.25
0.081
0.06
0.053
18
19
4
[MnII(terpy) ](I )
9.7
2
3 2
[MnII(dmb) (terpy)]
2.226
1.891
1.927
2.028
1.858
2.047
2.102
1.988
2.279
1.988
2.094
2.137
1.928
2.159
2.460
2.085
10.4È13.5 This work
2
[FeII(terpy) ](ClO )
0.097 \2
20
21
22
23
24
2
4 2
[CoII(terpy) ]I É 2H O
0.167
0.109
0.07
2.6
1.4È2.7
1.9È4.4
2
2
2
[CoII(terpy) ](ClO ) É 1.3H O 0.72
2
4 2
2
[CoIII(terpy) ]Cl
0.63
0.69
0.96
0.72
2
3
[NiII(terpy)(NCO) (H O)]
0.112
0.358
0.097
2
2
[CuI(PPh ) (terpy)]ClO
27.5È31.3 29
3 2
4
[CuII(terpy) ](NO )
25
2
3 2
a Di†erence between MÈN
and MÈN . b Dihedral angle between the adjacent NC rings in terpy.
d,d{
c
5
The pyridyl rings are planar with a maximum deviation of
0.014 A. The rings are tilted with respect to each other in
order to permit the accommodation of the MnÈN bonds. The
dihedral angles between the adjacent N(1), N(2) and N(2), N(3)
rings are 13.5¡ and 10.4¡, respectively, if the planes are deÐned
as the rms planes with all the atoms: [N(1), C(1), C(2), C(3),
C(4), C(5)], [N(2), C(6), C(7), C(8), C(9), C(10)], and [N(3),
C(11), C(12), C(13), C(14), C(15)]. Smaller deviations from
planarity are observed in terpyridine co-ordinated to metal
ions of smaller size (Table 3). Dihedral angles in the range
0.65È3.29¡ are measured between the pyridyl rings of terpy
complexes of Cu(II).27,28 In contrast, angles of 27.5 and 31.3¡
are measured with the larger Cu(I) ion.29
and those of Mn(II), the inter-pyridine angles decrease from
120¡ to an average value of 114.9¡ for N(1)ÈC(5)ÈC(6), C(5)È
C(6)ÈN(2), N(2)ÈC(10)ÈC(11) and C(10)ÈC(11)ÈN(3). In com-
parison, values of 112.8¡ and 112.7¡ were measured in
[Fe(terpy) ](ClO ) 20 and [Co(terpy) ]Cl ,23 respectively, as
2
4 2
2
3
an e†ect of a shorter metalÈnitrogen distance and a more effi-
cient orbital overlap. As consequence, the inter-ring
a
““externalÏÏ angles C(4)ÈC(5)ÈC(6), C(5)ÈC(6)ÈC(7), C(9)ÈC(10)È
C(11) and C(10)ÈC(11)ÈC(12) increase to 121.9È124.3¡.
The metrical details of the 2,6-dimethoxybenzoato ligand
are normal.4 The MnÈO(1A) and MnÈO(1@) distances are 2.099
and 2.156 A, respectively, consistent with the distortion of the
trigonal bipyramid toward the square pyramid with the longer
MnÈO(1@) bond pointing along the axial direction.
A signiÐcant strain is evident at the junctions between the
adjacent pyridine rings as a decrease of up to 6.3¡ from the
ideal 120¡ value is measured in the inter-ring angles. In con-
trast, the deviations in the intra-ring angles are generally less
than 2.0¡ and, at most, 4.1¡. The distortion of the pyridine
rings is indicated by the magnitude of the angles at the nitro-
gen atoms. The C(6)ÈN(2)ÈC(10) angle in the central ring is
almost regular (119.5¡), whereas the corresponding angles at
N(1) and N(3) are 119.6 and 117.5¡, respectively. It can be seen
that the smallest CÈNÈC angle is C(11)ÈN(3)ÈC(15). Neverthe-
less, N(3) is close to Mn as N(2). Instead, N(1) and N(2) exhibit
similar angles in spite of the very di†erent bond lengths. The
angles MnÈN(2)ÈC(i) (i \ 6, 10) (average value \ 120.2¡) are
more regular than MnÈN(1)ÈC(5) and MnÈN(3)ÈC(11)
(average value \ 117.3¡) and than MnÈN(1)ÈC(1) and MnÈ
N(3)ÈC(15) (average value \ 123.6¡). Similar values are
observed for [Fe(terpy) ](ClO ) .20 The largest distortions
Infrared and thermal properties
The terpyridine vibration modes do not shift appreciably
upon complex formation. Indeed, the absorption bands of
bpy, phen and terpy are not very metal-sensitive, since they
originate from the heterocyclic or aromatic ring of the ligand.
Thus, the main interest may be focused on the l(MÈN) vibra-
tions. For a Mn(II) ion in high-spin conÐguration the stretch
of the metalÈnitrogen bond is expected in the 180È290 cm~1
region. Our compound displays an absorption at 234 cm~1,
comparable with that observed at 224 cm~1 for
[Mn(bpy) ]2`.30 Distinctive IR bands are the stretching
3
vibrations of the carboxylate (l and l : 1593 and 1437 cm~1,
as
s
respectively). The value of *(l [ l ), 156 cm~1, is in agree-
as
s
ment with the monodentate co-ordination of dmb.4,10
2
4 2
Thermogravimetric measurements performed on the nitrog-
involve the decrease of NÈCÈC angles within the chelated
enous
adducts
of
Mn(dmb) ,
like
[Mn(dmb) (2-
rings. To improve the overlap between the orbitals of terpy
2
2
Mepz) (H O) ] and [Mn (dmb) (bpy) (H O) ] É bpy,9 indi-
cated that the release of the water molecules and nitrogenous
bases, yielding a binary Mn(II)-dmb complex, occurs below
170 ¡C, whereas the Ðnal decomposition occurs above
2
2
2
2
4
2
2
2
230 ¡C.31 The behaviour of [Mn(dmb) (terpy)] is di†erent and
2
conÐrms the stable tridentate co-ordination of terpy. Indeed,
the decomposition occurs at about 340 ¡C in a very narrow
temperature range, giving directly a Mn O residue. This indi-
2
3
cates that the strong binding of terpyridine to manganese
enhances the thermal stability of the complex.
Magnetic data
A magnetic moment of 6.02 k was measured at room tem-
B
perature, consistent with the expected spin-only value of an
S \ 5/2 Mn(II) complex. For a high-spin d5 metal ion the fol-
lowing spin Hamiltonian may be considered:
H \ bS Æ g Æ H ] D[S 2 [ 1/3S(S ] 1)] ] E(S 2 [ S 2)
z
x
y
The higher terms are usually negligible and could be omitted.
D and E correspond to the tetragonal and rhombic distortion,
respectively. The j \ E/D ratio ranges from 0, in the case of
Fig. 2 (a) Experimental and (b) simulated EPR spectra recorded on a
polycrystalline sample of [Mn(dmb) (terpy)] at the X-band frequency.
2
New J. Chem., 2000, 24, 725È728
727

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D. L. Kepert, J. Chem. Soc., Dalton T rans., 1974, 612.
10 L. Strinna Erre, G. Micera, F. Cariati, G. Ciani, A. Sironi, H.
Kozlowski and J. Baranowski, J. Chem. Soc., Dalton T rans., 1988,
363.
9
an axial symmetry, to 1/3, in the limit of the maximum
rhombic distortion.
The main features of the polycrystalline EPR spectrum of
our compound, collected at the X-band frequency and shown
11 MACH3/PC Software, Enraf-Nonius, Delft, The Netherlands,
in Fig. 2, are a strong band around g D 2, three bands at
1996.
eff
D0.070, D0.150 and D0.230 T and two resonances above
12 K. Harms and S. Wocadlo, XCAD for CAD-4 Software, Uni-
versity of Marburg, Germany, 1996.
0.400 T, a shoulder at D0.460 T and a band at 0.570 T. These
features indicate 0 \ j \ 0.33, consistent with a structure
intermediate between the trigonal bipyramid and the square
pyramid. Based on the diagrams published by Dowsing and
Gibson,32 the position of the high-Ðeld absorption gives an
indication of the D value: the higher the Ðeld value of the
band, the higher is the value of D. In our case the position of
the high-Ðeld absorption at 0.570 T is characteristic of a D
value approximately in the range 0.05È0.08 cm~1. A reason-
ably good Ðt of the EPR spectrum was obtained with the fol-
lowing parameters: g \ 2.01, g \ 2.01, g \ 2.05, D \ 0.0567
13 A. Altomare, G. Cascarano, G. Giacovazzo and A. Guagliardi, J.
Appl. Cryst., 1993, 26, 343.
14 G. M. Sheldrick, SHEL XL 93, Program for the ReÐnement of
Crystal Structures, University of Gottingen, Gottingen, Germany,
1993.
15 International T ables for X-Ray Crystallography, ed. A. J. C.
Wilson, Kluwer Academic Publishers, Dordrecht, Netherlands,
1992, vol. C.
16 L. J. Farrugia, W INGX, A W indows Program for Crystal Struc-
ture Analysis, University of Glasgow, Glasgow, UK, 1998.
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Acknowledgements
Financial support from TEMPUS JEP Grant No. 9252-95 for
the purchase of the X-ray di†ractometer is gratefully acknow-
ledged.
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