Tetranuclear manganese carboxylate clusters with imidazole-carboxylate chelating ligands. X-ray crystal structure of the 4-imidazoleacetate complex

Article abstract of DOI:10.1016/S0277-5387(00)00516-7

Procedures are described for the synthesis of tetranuclear manganese carboxylate complexes with ligands derived from 4-imidazoleacetic acid (imidacH) and 4-imidazolecarboxylic acid (imidcarbH). Reaction mixtures containing NBu₄/(n)imidac or NHex₄/(n)imidcarb generated in situ and either [Mn₃O(O₂CMe)₆(py)₃](ClO₄) or Mn(O₂CR)₂, RCO₂H (R = Me, Ph) and NR₄/(')MnO₄ (R' = Bu(n), Hex(n)) give red-brown solutions from which the products (NBu₄/(n))[Mn₄O₂(O₂CR)₇(imidac)₂] (R = Me (1); R = Ph (2)) and (NHex₄(n))[Mn₄O₂(O₂CR)₇(imidcarb)₂] (R = Me (3); R = Ph (4)) can be obtained. The X-ray crystal structure of 2 reveals that the anion consists of an [Mn₄([μ₃-O)₂]⁸⁺ core with the four Mn atoms disposed in a 'butterfly' arrangement and the O atoms triply bridging each 'wing'. Peripheral ligation is provided by seven bridging O₂CPh^- ligands and two chelating imidac^- groups. All available characterization data are consistent with the same core for complexes 1, 3 and 4, with the structures varying only in the peripheral ligation. ¹H NMR spectroscopy of all four complexes indicates that the structure is maintained in solution and the majority of the resonances have been assigned on the basis of T₁ measurements and substitution and exchange experiments. Cyclic voltammetry reveals the presence of a single quasi-reversible oxidation processes for complexes 1 and 2. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00516-7

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 2111–2118
Tetranuclear manganese carboxylate clusters with
imidazole-carboxylate chelating ligands. X-ray crystal structure of
the 4-imidazoleacetate complex
Colette Boskovic, Kirsten Folting, George Christou *
Department of Chemistry and Molecular Structure Center, Indiana Uni6ersity, Bloomington, IN 47405-4001, USA
Received 9 May 2000; accepted 17 July 2000
Abstract
Procedures are described for the synthesis of tetranuclear manganese carboxylate complexes with ligands derived from
4-imidazoleacetic acid (imidacH) and 4-imidazolecarboxylic acid (imidcarbH). Reaction mixtures containing NBun4 imidac or
NHexn4 imidcarb generated in situ and either [Mn3O(O2CMe)6(py)3](ClO4) or Mn(O2CR)2, RCO2H (R=Me, Ph) and NR%4MnO4
(R%=Bun, Hexn) give red–brown solutions from which the products (NBun4 )[Mn4O2(O2CR)7(imidac)2] (R=Me (1); R=Ph (2))
and (NHexn4 )[Mn4O2(O2CR)7(imidcarb)2] (R=Me (3); R=Ph (4)) can be obtained. The X-ray crystal structure of 2 reveals that
the anion consists of an [Mn4(m3-O)2]8+ core with the four Mn atoms disposed in a ‘butterfly’ arrangement and the O atoms triply
bridging each ‘wing’. Peripheral ligation is provided by seven bridging O2CPh− ligands and two chelating imidac− groups. All
available characterization data are consistent with the same core for complexes 1, 3 and 4, with the structures varying only in the
1
peripheral ligation. H NMR spectroscopy of all four complexes indicates that the structure is maintained in solution and the
majority of the resonances have been assigned on the basis of T1 measurements and substitution and exchange experiments. Cyclic
voltammetry reveals the presence of a single quasi-reversible oxidation processes for complexes 1 and 2. © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: Polynuclear manganese-carboxylate clusters; Imidazole-carboxylate chelating ligands; Paramagnetic NMR
As part of a continuing exploration of tetranuclear
Mn carboxylate clusters with chelating ligands, we have
1. Introduction
recently turned our attention to the use of imidazole-
containing chelates in order to compare and contrast
the products with those already obtained with pyridine-
based chelates. We describe herein our initial results
comprising the syntheses and characterization of te-
tranuclear clusters possessing imidazole-based N/O
chelating ligands derived from 4-imidazoleacetic acid
(imidacH) and 4-imidazolecarboxylic acid (imidcarbH).
These species allow informative comparisons with those
possessing pyridine-based chelating ligands with which
they are isostructural, as well as providing access to
compounds that possess either five- or six-membered
chelate rings but are otherwise identical. In addition,
imidazole ligation to tetranuclear Mn clusters is of
interest due to the histidine ligation of the tetranuclear
Mn complex responsible for water oxidation in Photo-
system II [7].
The chemistry of Mn carboxylate clusters is an area
of considerable interest, and numerous examples vary-
ing widely in both nuclearity and structure have been
reported [1]. The interest in these species is in part due
to their application to the modeling of Mn-containing
proteins and enzymes, in which up to four Mn nuclear-
ities have been identified [2,3]. We have previously
described procedures that allow the synthesis of te-
tranuclear Mn carboxylate compounds possessing the
[Mn4(m3-O)2] core and of formulation [MnI4IIO2(O2-
CR)7(L)2]z, where L is a pyridine-based N/N or N/O
chelate derived from 2,2%-bipyridine (bpy), picolinic acid
(picH), 8-hydroxyquinoline (hqnH) or 2-(hydrox-
ymethyl)pyridine (hmpH) [4–6].
* Corresponding author. Tel.: +1-812-855-2399; fax: +1-812-855-
2399.
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00516-7

2112
C. Bosko6ic et al. / Polyhedron 19 (2000) 2111–2118
imidacH·HCl (0.11 g, 0.68 mmol) in MeCN (3 cm3) and
1.0 M NBun4 OH in MeOH (1.4 cm3, 1.4 mmol). The
reaction mixture was stirred overnight and a brown
precipitate was removed by filtration. The filtrate was
reduced in volume to 10 cm3 and Et2O (20 cm3) was
added, leading to the formation of a red–brown precip-
itate which could be recrystallized from MeCN–Et2O.
The yield was 0.14 g (47%). The product was spectro-
scopically identical to that obtained by method A.
2. Experimental
2.1. Synthesis
2.1.3. (NBun4 )[Mn4O2(O2CPh)7(imidac)2] (2)
To a stirred solution of Mn(O2CPh)2·2H2O (1.34 g,
4.0 mmol) and PhCO2H (0.86 g, 7.0 mmol) in MeCN
(100 cm3) was slowly added NBu4n MnO4 (0.36 g, 1.0
mmol) in small portions to give a deep brown solution.
To this was added a solution resulting from the combi-
nation of imidacH·HCl (0.41 g, 2.5 mmol) in MeCN (5
cm3) and 1.0 M NBun4 OH in MeOH (5.0 cm3, 5.0
mmol). The resulting solution was stirred overnight,
filtered through celite, reduced in volume to 40 cm3,
and Et2O (100 cm3) added dropwise with vigorous
stirring. This led to the formation of a red–brown
precipitate, which could be recrystallized from MeCN–
Et2O. The yield was 1.45 g (73%). Crystals suitable for
crystallography were grown by layering an MeCN solu-
tion of the complex with Et2O, and they were kept in
contact with the mother liquor to prevent interstitial
solvent loss. Drying in vacuo leads to the unsolvated
form. Elemental analysis. Anal. Found: C, 56.77; H,
5.66; N, 4.34. Calc. for C75H81N5O20Mn4: C, 56.58; H,
5.13; N, 4.40. Selected IR data (cm−1): 1606(s), 1571(s),
1501(w), 1490(m), 1466(w), 1448(m), 1384(s), 1331(s),
1314(m), 1303(m), 1265(m), 1231(w), 1173(m),
1143(w),1089(w), 1068(w), 1026(m), 940(m), 838(m),
720(s), 690(m), 676(m), 650(s), 630(m), 610(m), 468(m),
437(m).
All manipulations were performed under aerobic
conditions, using materials as received. Trinuclear
[Mn3O] complexes were available from previous work
[8]. NBun4 MnO4 and Mn(O2CPh)2·2H2O were prepared
as described [4,9] and NHexn4 OH (Hex=hexyl) was
prepared according to Giovannellio et al. [10]. WARN-
ING: Appropriate care should be taken in the use of
NRn4 MnO4, and readers are referred to the detailed
warning given elsewhere [9].
2.1.1. NHexn4 MnO4
To a vigorously stirred solution of KMnO4 (5.00 g,
31.6 mmol) in H2O (100 cm3) was added a solution of
NHexn4 Br (13.50 g, 31.6 mmol) in H2O (100 cm3),
resulting in the formation of a purple precipitate. This
was filtered, washed copiously with H2O and Et2O and
dried in vacuo at room temperature. The yield was
13.49 g, 92%.
2.1.2. (NBun4 )[Mn4O2(O2CMe)7(imidac)2] (1)
Method A: to a stirred solution of Mn(O2CMe)2·
4H2O (0.74 g, 3.0 mmol) and glacial acetic acid (1 cm3)
in MeCN (70 cm3) was slowly added NBu4n MnO4 (0.27
g, 0.75 mmol) in small portions to give a deep brown
solution. To this was added a solution resulting from
the combination of imidacH·HCl (0.31 g, 1.9 mmol) in
MeCN (5 cm3) and 1.0 M NBun4 OH in MeOH (3.8 cm3,
3.8 mmol). The reaction mixture was stirred overnight
and a brown precipitate removed by filtration. The
filtrate was reduced in volume to 50 cm3 and Et2O (100
cm3) was added, leading to the formation of a red–
brown precipitate which could be recrystallized from
MeCN–Et2O. The yield was 0.60 g (55%). Drying in
vacuo leads to the unsolvated form. Elemental analysis,
Anal. Found: C, 41.09; H, 5.78; N, 6.08. Calc. for
C40H67N5O20Mn4: C, 41.50; H, 5.83; N, 6.05. Selected
IR data (cm−1): 1647(m), 1608(s), 1493(m), 1467(w),
1401(s), 1387(s), 1333(s), 1320(s), 1266(w), 1227(w),
1176(w), 1087(w), 1028(m), 943(m), 935(sh), 882(w),
802(w), 737(sh), 721(m), 678(m), 650(s), 635(sh),
613(m), 436(m).
2.1.4. (NHexn4 )[Mn4O2(O2CMe)7(imidcarb)2] (3)
Method A: to a stirred solution of Mn(O2CMe)2·
4H2O (0.74 g, 3.0 mmol) and glacial acetic acid (1 cm3)
in MeCN (70 cm3) was slowly added NHex4n MnO4
(0.35 g, 0.75 mmol) in small portions to give a deep
brown solution. To this was added a solution resulting
from the combination of imidcarbH (0.21 g, 1.9 mmol)
in MeCN (10 cm3) and 0.2 M NHexn4 OH in MeOH (9.5
cm3, 1.9 mmol). The reaction mixture was stirred
overnight and a brown precipitate removed by filtra-
tion. The filtrate was reduced in volume to 50 cm3 and
Et2O (100 cm3) was added, leading to the formation of
a red–brown precipitate which could be recrystallized
from MeCN–Et2O. The yield was 0.55 g (47%). Drying
in vacuo leads to the unsolvated form. Elemental analy-
sis. Anal. Found: C, 44.34; H, 6.28; N, 5.12. Calc. for
C46H79N5O20Mn4: C, 44.49; H, 6.41; N, 5.64. Selected
IR data (cm−1): 1681(s), 1615(s), 1579(sh), 1504(w),
1486(w), 1465(w), 1397(s), 1332(s), 1313(s), 1263(w),
Method B: to a stirred solution of [Mn3O(O2CMe)6-
(py)3](ClO4) (0.30 g, 0.34 mmol) in MeCN (20 cm3) was
added a solution resulting from the combination of

C. Bosko6ic et al. / Polyhedron 17 (2000) 2111–2118
2113
1188(m), 1154(w), 1099(w), 1076(w), 1043(w), 1013(m),
933(w), 822(m), 800 (w), 776(w), 679(s), 654(s), 614(m),
569(w), 502 (m).
precipitate, which could be recrystallized from MeCN–
Et2O. The yield was 0.12 g (54%). Drying in vacuo
leads to the unsolvated form. Elemental analysis. Anal.
Found: C, 57.50; H, 5.57; N, 4.18. Calc. for
C81H93N5O20Mn4: C, 58.03; H, 5.59; N, 4.18. Selected
IR data (cm−1): 1680(s), 1610(s), 1572(s), 1508(w),
1490(w), 1464(w), 1448(m), 1375(s), 1344(sh), 1305(s),
1188(w), 1174(m), 1156(w), 1140(w), 1068(m), 1025(m),
1012(m), 936(w), 827(m), 780(w), 718(s), 688(m),
676(m), 654(s), 617(m), 563(w), 502(m), 468(m).
Method B: to a stirred solution of [Mn3O(O2CMe)6-
(py)3](ClO4) (0.50 g, 0.57 mmol) in MeCN (60 cm3) was
added a solution resulting from the combination of
imidcarbH (0.097 g, 0.86 mmol) in MeCN (5 cm3) and
0.2 M NHexn4 OH in MeOH (4.3 cm3, 0.86 mmol). The
reaction mixture was stirred overnight and a brown
precipitate was removed by filtration. The filtrate was
reduced in volume to 30 cm3 and Et2O (60 cm3) was
added, leading to the formation of a red–brown precip-
itate which could be recrystallized from MeCN–Et2O.
The yield was 0.27 g (50%). The product was spectro-
scopically identical to that obtained by method A.
2.2. X-ray crystallography
Data for complex 2 were collected on a Bruker
platform goniometer equipped with a SMART 6000
CCD detector at −163°C; details of the diffractome-
try, low temperature facilities and computational proce-
dures employed by the Molecular Structure Center are
available elsewhere [11]. Data collection parameters are
summarized in Table 1.
The initial survey of a limited portion of reciprocal
space located a set of reflections with no symmetry or
systematic absences. Solution and refinement of the
structure confirmed the choice of the centrosymmetric
2.1.5. (NHexn4 )[Mn4O2(O2CPh)7(imidcarb)2] (4)
To a stirred solution of Mn(O2CPh)2·2H2O (0.49 g,
1.5 mmol) and PhCO2H (0.32 g, 2.6 mmol) in MeCN
(80 cm3) was slowly added NHex4n MnO4 (0.17 g, 0.36
mmol) in small portions to give a deep brown solution.
To this was added a solution resulting from the combi-
nation of imidcarbH (0.10 g, 0.92 mmol) in MeCN (5
cm3) and 0.2 M NHexn4 OH in MeOH (4.6 cm3, 0.92
mmol). The resulting solution was stirred overnight,
filtered through celite, reduced in volume to 40 cm3,
and Et2O (100 cm3) added dropwise with vigorous
stirring. This led to the formation of a red–brown
(
space group P1. The structure was solved using a
combination of Patterson and direct methods (DIRDIF
-
96 [12]). Most of the atoms in the cation and anion
were located from DIRDIF-96. The remaining atoms
were located in successive iterations of least-squares
refinement followed by generation of a difference
Fourier map. In addition to the main anion of interest,
the asymmetric unit contained a NBun4+ cation and
five molecules of MeCN. Many of the hydrogen atoms
were visible in a later difference Fourier map. On three
of the solvent molecules, at least one hydrogen atom
was located on the methyl carbon atom. Hydrogen
atoms were then introduced in fixed calculated posi-
tions and assigned a thermal parameter of 1.0 plus the
isotropic equivalent of the parent atom. In the final
cycles of least-squares refinement the non-hydrogen
atoms were refined using anisotropic thermal parame-
ters, while the hydrogen atoms were fixed. The final
Table 1
Crystallographic data for complex 2·5MeCN
a
Formula
C85H96Mn4N10O20
1797.50
Formula weight
Space group
Crystal system
(
P1
triclinic
12.1886(3)
17.8130(5)
22.0268(5)
68.1714(8)
84.7029(8)
83.0696(7)
4401.2
,
a (A)
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
2
Temperature (°C)
−163
difference map was essentially featureless, the largest
Crystal size (mm)
0.60×0.40×0.40
0.71073
1.356
−3
,
,
peak was 1.42 e A
located 1.05 A from H(55) and
b
,
Radiation (A)
z (g cm−3
v (cm−1
,
1.92 A from C(93) (in the cation). The next largest peak
also in the region of the cation,
indicating a possible small disorder.
)
−3
,
)
6.344
was 0.98 e A
Collected data (°)
Total data
4.9252q554.57
35889
Unique data
Rmerge
20163
0.038
2.3. Other measurements
Observed data (F\2|(F))
10514
6.07
6.19
R (%) c
IR spectra (KBr pellet) were recorded on a Nicolet
model 510P spectrometer. 1H NMR spectra were
recorded in CD3CN at ꢀ20°C with a 300 MHz Varian
Gemini 2000 NMR spectrometer. Cyclic voltammetry
employed a BAS CV-50W voltammetric analyzer and a
standard three-electrode assembly (glassy-carbon work-
Rw (%) d
a Including solvate molecules.
b Graphite monochromator.
c R=100 SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
2 1/2
d Rw=100 [Sw(ꢀFoꢀ−ꢀFcꢀ)2/SwꢀFoꢀ ] where w=1/|2(ꢀFoꢀ).

2114
C. Bosko6ic et al. / Polyhedron 19 (2000) 2111–2118
which 1, NBun4 [Mn4O2(O2CPh)7(imidac)2] (2), 3 and
NHexn4 [Mn4O2(O2CPh)7(imidcarb)2] (4) were obtained
with minimal workup in good yield (55, 73, 47 and
54%, respectively). This reaction employed a 4:1
MnII:MnVII ratio, as required for preparation of a MnI4II
product (Eq. (1))
4Mn2+ +Mn7+ “5/4Mn34+
(1)
Compound 2 was structurally characterized, and all
available characterization data for 1 are consistent with
it having the same structure as 2. Crystals of 3 and 4
suitable for X-ray crystallography could not be ob-
tained; however, all of the characterization data are
consistent with structures analogous to those of 1 and
2.
Fig. 1. ORTEP representation for the anion of complex 2 at the 50%
probability level. For clarity, only the ipso carbon atom of each
benzoate ring is included.
3.2. Description of structure
ing, Pt-wire auxiliary, Ag–AgNO3 reference) with 0.1
M NBun4 PF6 as supporting electrolyte. Potentials are
quoted versus the ferrocene–ferrocenium couple under
the same conditions. Elemental Analyses were per-
formed by Atlantic Microlab Inc., Norcross, Georgia
or in-house with a Perkin–Elmer Series II CHNS/O
Analyzer 2000.
An ORTEP representation of the anion of complex 2
is given in Fig. 1. Crystallographic data and selected
bond distances and angles are listed in Tables 1 and 2,
respectively.
(
Complex 2 crystallizes in the triclinic space group P1.
The asymmetric unit contains the complete anion, the
NBun4+ cation and five MeCN solvate molecules; the
latter two will not be further discussed. The anion of 2
possesses a Mn4O2 core with the familiar ‘butterfly’
structure. Mn(1) and Mn(2) occupy the ‘body’ posi-
tions, while Mn(3) and Mn(4) occupy the ‘wingtip’
positions and O(5) and O(6) triply bridge each wing.
The structure can be considered as two triangular
Mn3O units edge-shared through Mn(1) and Mn(2)
with a dihedral angle of 142.2°. Unlike the planar core
of the Mn3O complexes, the m3-O atoms of the triangu-
lar units of the ‘butterfly’ structure lie slightly below the
plane (Fig. 1). Two distinct Mn···Mn distances are
observed, with the ‘body’ Mn(1)···Mn(2) separation of
3. Results and discussion
3.1. Syntheses
In earlier reports we have described two procedures
for the preparation of [MnI4IIO2(O2CR)7(L)2]z com-
plexes, where L is derived from 2,2%-bipyridine (bpy),
picolinic acid (picH), 8-hydroxyquinoline (hqnH) and
2-(hydroxymethyl)pyridine (hmpH) [4–6] The first pro-
cedure involves treatment of trinuclear [Mn3O(O2CR)6-
(py)3]+ complexes with salts of the chelating ligand L,
or with the acid form of the ligand and an equivalent of
base. The second procedure involves a comproportion-
ation reaction between MnII and MnO4− in the presence
of RCO2H and L. In the present study both procedures
have allowed the synthesis of the analogous imidac−
,
ca. 2.8 A significantly shorter than the four ‘body’ to
,
‘wingtip’ separations of 3.3–3.4 A. These separations
are consistent with the presence of two and one bridg-
ing oxygen atoms, respectively [13]. Peripheral ligation
is by seven bridging benzoate ligands and two imidac−
and imidcarb− complexes with R=Me and Ph. NBun+ ligands chelating the ‘wingtip’ Mn atoms. Each Mn
salts of the imidac− species were isolated, while
atom is six-coordinate with axially elongated octahedral
geometry typical of Jahn–Teller distorted high spin d4
MnIII ions. The axially elongated sites are all occupied
by benzoate oxygen atoms, with both of the oxygen
atoms of the unique benzoate ligand (bridging the
‘body’ Mn atoms) occupying JT elongated positions.
The imidac− oxygen atoms are coordinated trans to the
4
NHexn4+ salts of the imidcarb− species were preferred
in order to obtain soluble materials. Thus, treatment of
an MeCN solution of [Mn3O(O2CMe)6(py)3]+ with
NBun4 (imidac) or NHexn4 (imidcarb) generated in situ
afforded a brown solution from which NBun4 [Mn4O2-
(O2CMe)7(imidac)2] (1) or NHexn4 [Mn4O2(O2CMe)7-
(imidcarb)2] (3) were obtained with minimal workup in
good yield (47 and 50%, respectively). Employment of
the second procedure, whereby a reaction mixture con-
,
m3-O atoms at a distance of 1.9 A, which is comparable
to the other (non-JT elongated) Mn-benzoate bonds.
The molecule has idealized C2 point symmetry and,
overall, the structure is extremely similar to those of the
analogous bpy, pic−, hqn− and hmp− complexes, with
the cores of each species essentially superimposable
taining Mn(O2CR)2, NR%MnO4 (R%=Bun or Hexn) and
4
RCO2H was similarly treated with NBun4 (imidac) or
NHexn4 (imidcarb) also resulted in brown solutions from

C. Bosko6ic et al. / Polyhedron 17 (2000) 2111–2118
2115
1
3.3. H NMR spectroscopy
[4–6]. The structural differences between complex 2 and
the previously reported ‘butterfly’ complexes arise in
the vicinity of the chelating ligand. In particular, the
imidac− species possesses a significantly non-planar
six-membered chelate ring in contrast to the planar
five-membered chelate rings of the bpy, pic−, hqn− and
hmp− species.
The 1H NMR spectra of complexes 1 and 2 in
MeCN-d3 are shown in Fig. 2 and the resonance posi-
tions are listed in Table 3 using the labeling scheme
given in Fig. 2. Assignments were made on the basis of:
(i) comparison with the spectrum of the O2CMe-d3
analog of 1 to distinguish between the resonances of the
imidac− protons and those associated with the car-
boxylates; (ii) addition of D2O to the NMR solution to
assign the amine resonance of the imidac− ligand; (iii)
T1 measurements (inversion recovery method) corre-
lated with the relative proton to metal centre distances
to assign the remaining resonances associated with the
imidac− protons; and (iv) comparison with the spectra
of structurally analogous ‘butterfly’ complexes with
other chelating ligands [5,6]. Both complexes display
five resonances due to the protons of the imidac−
ligand in addition to resonances due to the carboxylate
protons.
For complex 1 four resonances due to the methyl
protons of the four inequivalent acetate ligands are
observed with an approximate intensity ratio of 2:2:2:1,
indicating that the peak at 16.9 ppm (C%) can be as-
signed to the unique acetate bridging the ‘body’ Mn
atoms Mn(1) and Mn(2). The resonance positions and
T1 values of these four resonances are similar to those
observed for the pic− and hqn− ‘butterfly’ complexes
and are consistent with idealized C2 point symmetry,
implying retention of the solid state structure in solu-
tion. A more complex spectrum is evident for 2 due to
the presence of four inequivalent benzoate ligands.
Assignment of the resonances as due to ortho, meta or
para protons may be broadly made on the basis of the
relative chemical shifts and T1 values [14], although it is
not possible to ascertain which peaks correspond to the
unique benzoate ligand bridging the ‘body’ Mn atoms.
Nevertheless, the pattern of benzoate resonances is
again similar to that observed for other benzoate ‘but-
terfly’ complexes.
Five resonances of approximately equal intensity are
observed for the protons of the imidac− ligands for
both complexes 1 and 2. For complex 1 the most
downfield imidac− resonance at ca. 26.3 ppm is ob-
scured by a resonance due to the protons of the unique
acetate ligand and is only evident in the spectrum of the
O2CMe-d3 analogue. Addition of a small amount of
D2O to the NMR solutions of 1 and 2 leads to the
immediate disappearance of the resonances at −2.5
and −4.3 ppm, respectively. This is attributed to ex-
change broadening and allows assignment of these reso-
nances to the amine proton of the imidac− ligand, HB.
Assignment of the remaining resonances relies predomi-
nantly on the correlation between the line widths, mea-
sured T1 values and the proton to ‘wingtip’ metal centre
[Mn(3) and Mn(4)] separations derived from the crystal
Table 2
,
Selected interatomic distances (A) and angles (°) for complex 2
Mn(1)···Mn(2)
Mn(1)···Mn(3)
Mn(1)···Mn(4)
Mn(1)ꢀO(5)
Mn(1)ꢀO(6)
Mn(1)ꢀO(7)
Mn(1)ꢀO(16)
Mn(1)ꢀO(25)
Mn(1)ꢀO(52)
Mn(2)ꢀO(5)
Mn(2)ꢀO(6)
Mn(2)ꢀO(9)
Mn(2)ꢀO(34)
Mn(2)ꢀO(43)
Mn(2)ꢀO(61)
2.816(1)
3.446(1)
3.319(1)
1.915(3)
1.872(3)
2.156(3)
1.958(3)
1.981(3)
2.175(3)
1.869(3)
1.898(3)
2.240(3)
1.971(4)
1.942(3)
2.205(3)
Mn(2)···Mn(3)
Mn(2)···Mn(4)
3.293(1)
3.444(1)
Mn(3)ꢀO(5)
Mn(3)ꢀO(18)
Mn(3)ꢀO(36)
Mn(3)ꢀO(63)
Mn(3)ꢀO(77)
Mn(3)ꢀN(70)
Mn(4)ꢀO(6)
Mn(4)ꢀO(27)
Mn(4)ꢀO(45)
Mn(4)ꢀO(54)
Mn(4)ꢀO(86)
Mn(4)ꢀN(79)
1.848(3)
2.244(4)
2.201(4)
1.933(3)
1.929(4)
2.012(4)
1.876(3)
2.166(3)
2.176(3)
1.957(3)
1.903(3)
2.027(4)
Mn(1)ꢀO(5)ꢀMn(2)
Mn(1)ꢀO(6)ꢀMn(2)
Mn(1)ꢀO(5)ꢀMn(3)
O(5)ꢀMn(1)ꢀO(6)
O(5)ꢀMn(1)ꢀO(7)
O(5)ꢀMn(1)ꢀO(16)
O(5)ꢀMn(1)ꢀO(25)
O(5)ꢀMn(1)ꢀO(52)
O(6)ꢀMn(1)ꢀO(7)
O(6)ꢀMn(1)ꢀO(16)
O(6)ꢀMn(1)ꢀO(25)
O(6)ꢀMn(1)ꢀO(52)
O(7)ꢀMn(1)ꢀO(16)
O(7)ꢀMn(1)ꢀO(25)
O(7)ꢀMn(1)ꢀO(52)
O(16)ꢀMn(1)ꢀO(25)
O(16)ꢀMn(1)ꢀO(52)
O(25)ꢀMn(1)ꢀO(52)
O(5)ꢀMn(2)ꢀO(6)
O(5)ꢀMn(2)ꢀO(9)
O(5)ꢀMn(2)ꢀO(34)
O(5)ꢀMn(2)ꢀO(43)
O(5)ꢀMn(2)ꢀO(61)
O(6)ꢀMn(2)ꢀO(9)
O(6)ꢀMn(2)ꢀO(34)
O(6)ꢀMn(2)ꢀO(43)
O(6)ꢀMn(2)ꢀO(61)
O(9)ꢀMn(2)ꢀO(34)
O(9)ꢀMn(2)ꢀO(43)
O(9)ꢀMn(2)ꢀO(61)
O(34)ꢀMn(2)ꢀO(43)
O(34)ꢀMn(2)ꢀO(61)
O(43)ꢀMn(2)ꢀO(61)
96.2(2)
96.7(2)
132.6(2)
82.7(2)
86.0(2)
91.7(2)
172.6(2)
98.8(2)
92.8(2)
172.4(2)
99.7(2)
89.6(2)
92.0(2)
86.8(2)
174.8(2)
86.5(2)
86.1(2)
88.3(2)
83.2(2)
89.8(2)
98.4(2)
174.4(2)
86.8(2)
85.0(2)
171.8(2)
94.8(2)
98.6(2)
87.0(2)
95.3(2)
174.7(2)
84.4(2)
89.5(2)
88.3(2)
Mn(1)ꢀO(6)ꢀMn(4)
Mn(2)ꢀO(5)ꢀMn(3)
Mn(2)ꢀO(6)ꢀMn(4)
O(5)ꢀMn(3)ꢀO(18)
O(5)ꢀMn(3)ꢀO(36)
O(5)ꢀMn(3)ꢀO(63)
O(5)ꢀMn(3)ꢀO(77)
O(5)ꢀMn(3)ꢀN(70)
O(18)ꢀMn(3)ꢀO(36)
O(18)ꢀMn(3)ꢀO(63)
O(18)ꢀMn(3)ꢀO(77)
O(18)ꢀMn(3)ꢀN(70)
O(36)ꢀMn(3)ꢀO(63)
O(36)ꢀMn(3)ꢀO(77)
O(36)ꢀMn(3)ꢀN(70)
O(63)ꢀMn(3)ꢀO(77)
O(63)ꢀMn(3)ꢀN(70)
O(77)ꢀMn(3)ꢀN(70)
O(6)ꢀMn(4)ꢀO(27)
O(6)ꢀMn(4)ꢀO(45)
O(6)ꢀMn(4)ꢀO(54)
O(6)ꢀMn(4)ꢀO(86)
O(6)ꢀMn(4)ꢀN(79)
O(27)ꢀMn(4)ꢀO(45)
O(27)ꢀMn(4)ꢀO(54)
O(27)ꢀMn(4)ꢀO(86)
O(27)ꢀMn(4)ꢀN(79)
O(45)ꢀMn(4)ꢀO(54)
O(45)ꢀMn(4)ꢀO(86)
O(45)ꢀMn(4)ꢀN(79)
O(54)ꢀMn(4)ꢀO(86)
O(54)ꢀMn(4)ꢀN(79)
O(86)ꢀMn(4)ꢀN(79)
124.7(2)
124.8(2)
131.7(2)
92.3(2)
96.0(2)
94.2(2)
176.6(2)
90.9(2)
171.5(2)
87.3(2)
84.8(2)
93.1(2)
90.3(2)
87.0(2)
88.6(2)
87.5(2)
175.0(2)
87.5(2)
92.5(2)
89.7(2)
94.9(2)
176.1(2)
90.2(2)
174.1(2)
91.6(2)
91.1(2)
86.1(2)
93.7(2)
86.6(2)
88.4(2)
86.7(2)
174.5(2)
88.4(2)

2116
C. Bosko6ic et al. / Polyhedron 19 (2000) 2111–2118
Fig. 2. Proton NMR spectra of complexes 1 (top) and 2 (bottom) in MeCN-d3. The peaks are labeled according to the scheme shown as an inset.
The band of resonances in the −1 to 5 ppm region is from the NBun4+ protons, residual protio solvent and adventitious water.
Table 3
Proton NMR data for complexes 1 and 2 in MeCN-d3 at Tꢀ20°C
Complex 1
l (ppm)
Complex 2
l (ppm)
Assignment a
T1 (ms) b
T1 (ms) b
39.2
32.4
26.3 c
26.1
19.3
16.9
2.2
2.4
0.9 c
2.0
1.3
2.2
C (A)
C (A)
HD
C (A)
HE
31.2
12.5
0.8
2.2
C (A%) d
C (m-B)
C (m-B)
C (m-B)
C (o-B)
C (o-B)
C (o-B)
C (p-B)
C (p-B)
HB
11.5
11.3
10.0
8.1
7.2
6.6
5.1
3.9
−4.3
−8.9
−44.2
17.3
16.1
12.2
2.5
9.2
5.2
32.1
31.2
4.0
4.9
0.5
e
−2.5
−7.3
−42.9
3.3
0.5
HC
HA
a Numbering of imidac− protons as in Fig. 2.
b Inversion recovery method.
Fig. 3. Proton NMR spectra of complexes 3 (top) and 4 (bottom) in
MeCN-d3. The peaks are labeled according to the scheme shown as
an inset. The band of resonances in the −1 to 5 ppm region is from
the NHexn4+1 protons, residual protio solvent and adventitious water.
c Determined from (O2CMe-d3) analog of 1.
d Unique acetate bridging Mn(1) and Mn(2).
e Could not be determined.

C. Bosko6ic et al. / Polyhedron 17 (2000) 2111–2118
2117
Table 4
is assigned to the proton closest to the wingtip Mn
atoms, HA. The significant upfield chemical shift of this
resonance is consistent with those observed for the
proton at the position adjacent to the Mn-coordinated
N atoms in the pic− and hqn− ‘butterfly’ compounds.
The sharpest resonance with the largest T1 value is
assigned to the proton most distant from the Mn
centre, HC. Protons HD and HE are diastereotopic with
HD significantly closer to the Mn atom (ca. 3.5 versus
Proton NMR data for complexes 3 and 4 in MeCN-d3 at Tꢀ20°C
Complex 3
l (ppm)
Complex 4
l (ppm)
Assignment a
T1 (ms) b
T1 (ms) b
39.9
31.3
25.6
16.8
2.4
2.4
2.3
2.4
C (A)
C (A)
C (A)
C (A%) c
C (m-B)
C (m-B)
C (m-B)
C (o-B)
C (o-B)
C (o-B)
C (o-B)
C (p-B)
C (p-B)
HC
,
11.3
11.2
10.0
8.2
7.6
6.8
5.7
4.7
3.4
19.0
18.1
4.2 A), and are assigned accordingly. The marked
chemical shift difference between the resonances of
these protons indicates the significant contribution of a
dipolar mechanism to the observed isotropic shifts. The
chemical shifts of resonances HA, HB and HC are
relatively consistent between complexes 1 and 2, with
less than 2 ppm difference observed in each case. How-
ever, slightly larger chemical shift differences of ca. 5
and 7 ppm are observed for HD and HE, respectively.
The reason for this is unclear in the absence of a crystal
structure of complex 1.
13.8
d
8.9
12.8
2.4
36.2
32.5
−5.8
2.6
−6.1
−25.8
−33.9
4.5
d
d
d
HB
HA
−32.0
0.3
0.6
a Numbering of imidcarb− protons as in Fig. 3.
b Inversion recovery method.
The 1H NMR spectra of complexes 3 and 4 in
MeCN-d3 are shown in Fig. 3 and the resonance posi-
tions are listed in Table 4 using the labeling scheme
given in Fig. 3. Assignments were made analogously to
those for 1 and 2. The pattern of resonances due to the
carboxylate protons of 3 and 4 are very similar to those
observed for 1 and 2. Three resonances due to the
imidcarb− protons are observed for 4 while only two
are evident for 3. The one missing is that due to HB,
which is presumably because of exchange with adventi-
tious water. The resonances due to HA and HC display
very similar isotropic shifts to the equivalent resonances
in 1 and 2, while the resonance due to HB is shifted by
ca. 20 ppm, presumably due to exchange effects.
c Unique acetate bridging Mn(1) and Mn(2).
d Could not be determined.
3.4. Electrochemistry
The cyclic voltammograms (CVs) of complexes 1 and
2 (1 mM) have been recorded in MeCN solution (0.1 M
NBun4 PF6) (Fig. 4). Similar results are observed for
both complexes and the CV of 2 is shown in Fig. 3.
Both complexes display a quasi-reversible one-electron
oxidation at 0.58 and 0.59 V versus ferrocene for 1 and
2, respectively. The peak separations at a scan rate of
100 mV s−1 are 187 and 154 mV, respectively, com-
pared with that for the ferrocene–ferrocenium couple
of 116 mV under the same conditions. Plots of ip versus
w1/2 are linear in the scan rate (w) range of 10–500 mV
Fig. 4. Cyclic voltammogram (1 mM, 100 mV s−1) for complex 2 in
MeCN (0.1 M NBun4 PF6). The potentials (V) are quoted versus
ferrocene–ferrocenium.
s
−1, indicating a diffusion-controlled process. Both
complexes also display an irreversible oxidation at peak
potentials of 1.35 and 1.30 V, respectively at a scan rate
of 100 mV s−1. In addition two irreversible reductions
are observed at peak potentials of −0.94 and −1.37
for 1 and −0.91 and −1.37 V for 2.
structure of 2. This is on the basis of previous studies
that have concluded that the major contibution to the
line widths in MnIII complexes is dipolar broadening,
which is proportional to r−6 [15–18]. The broadest
resonance, with the smallest T1 value, at ca. −40 ppm
The quasi-reversible oxidation at 0.59 V for 2 in
MeCN is assigned to oxidation to the MnI3IIMnIV level

2118
C. Bosko6ic et al. / Polyhedron 19 (2000) 2111–2118
as shown in Eq. (2). The oxidation potential can be
compared with the
Data Centre, CCDC No. 141954. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336033; e-mail: deposit@
ccdc.ac.uk or www: http://www.ccdc.cam.ac.uk).
[Mn4O2(O2CPh)7(imidac)2]−0.59 V
X
[Mn4O2(O2CPh)7(imidac)2]0
(2)
value of 0.69 V found for the corresponding couple in
[Mn4O2(O2CPh)7(pic)2]− under the same conditions.
This is consistent with the greater acidity of picH (pKa
of 5.25 [19]) versus imidacH (pKa measured as ca. 7.0),
as the more electron-donating imidac− is better able to
stabilize higher oxidation states of Mn than pic−.
Well-defined cyclic voltammograms of complexes 3
and 4 could not be obtained due to their relative
insolubility in MeCN in the presence of 0.1 M
NBun4 PF6 as support electrolyte. However, poorly re-
solved oxidation processes with peak potentials of ca.
0.60 and 1.2 V and reduction processes at ca. −1.2 and
−1.6 V were evident for 4 under these conditions.
The presence of apparently accessible oxidation pro-
cesses for complexes 1 and 2 suggested the possibility of
oxidation of these complexes. Indeed, oxidation of
other ‘butterfly’ complexes has resulted in the forma-
tion of new species [20]. An investigation of similar
reactivity for 1 and 2 is in progress, with preliminary
studies involving electrochemical oxidation of these spe-
cies resulting in insoluble red–brown solids. Further
efforts are required to fully characterize these materials,
although their IR spectra do not display bands associ-
ated with NBun4+ , implying that they are uncharged.
Acknowledgements
This work was funded by NIH grant GM 39083.
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4. Conclusions
Tetranuclear Mn clusters possessing imidazole-car-
boxylate based chelating ligands have been synthesized
and found to be essentially isostructural with those
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-
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1
possessing pyridine-based chelating ligands. H NMR
spectroscopy indicates that the species retain their
structure in solution and the resonances can be assigned
using the methods described in the text. An investiga-
tion of the electrochemistry of the species reveals a
quasi-reversible oxidation process, similar to those ob-
served for the analogous compounds possessing pyri-
dine-based chelating ligands.
[13] G. Christou, J.B. Vincent, in: L. Que (Ed.), ACS Symposium
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5. Supplementary data
Supplementary structural data for compound 2 have
been deposited with the Cambridge Crystallographic
.