Novel linear transition metal clusters of a heptadentate bis-β-diketone ligand

Article abstract of DOI:10.1021/ic062075v

The synthesis and the structure of the new potentially heptadentate ligand 1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxybenzene (H₅L) is described. The reaction in pyridine or DMF of this ligand with various M(AcO)₂ salts (M = Ni^II, Co^II, Mn^II) leads to very different products depending on the metal. Thus, the dinuclear complexes [M₂(H₃L)₂(py)4] (M = Ni^II, 1; Co^II, 2) or the linear zigzag tetranuclear clusters [Mn ₄(H₂L)₂(AcO)₂(py)₅] (3) and [Mn₄-(H₂L)₂(AcO)₂(dmf) ₄] (4) have been synthesized and characterized crystallographically. Slow oxidation of complex 3 leads to the formation of the novel mixed-valence linear complex [Mn₃(HL)₂(py)₆] (5), displaying an unprecedented asymmetric Mn^IIIMn^IIIMn^II topology. The coordination geometry of complexes 1 to 5 has been analyzed and discussed by means of continuous shape measures. Magnetic measurements of 3 and 5 demonstrate that the metals within these complexes weakly interact magnetically with coupling constants of J₁ = -1.13 cm^-1 and J₂ = -0.43 cm^-1 (S = 0) for complex 3 and J ₁ = -5.4 cm^-1 and J₂ = -0.4 cm^-1 (S = 5/2) for complex 5 (using the H = -Σ2J_ijS_iS _j convention). These results are consistent with X-band EPR measurements on these compounds.

Full text of DOI:10.1021/ic062075v

Inorg. Chem. 2007, 46, 2519−2529
Novel Linear Transition Metal Clusters of a Heptadentate Bis-
Ligand
â-diketone
Guillem Arom´ı,*† Patrick Gamez,‡ J. Krzystek,§ Huub Kooijman,| Anthony L. Spek,|
,#
Elizabeth J. MacLean, Simon J. Teat, and Harriott Nowell∇
Departament de Qu´ımica Inorga`nica, UniVersitat de Barcelona, Diagonal 647, 08028 Barcelona,
Spain, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity, PO Box 9502,
2300 RA Leiden, The Netherlands, National High Magnetic Field Laboratory, Florida State
UniVersity, Tallahassee, Florida 32310, BijVoet Center for Biomolecular Research, Crystal and
Structural Chemistry, Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands,
CCLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, U.K., AdVance Light Source,
Lawrence Berkeley Lab, 1 Cyclotron Rd, MS2-400, Berkeley, California 94720, and Diamond
Light Source, Ltd., Diamond House, Chilton, Didcot, Oxfordshire OX11 0DE, U.K.
Received October 30, 2006
The synthesis and the structure of the new potentially heptadentate ligand 1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-
propionyl)-2-methoxybenzene (H5L) is described. The reaction in pyridine or DMF of this ligand with various M(AcO)2
salts (M
[M2(H3L)2(py)4] (M
)
NiII, CoII, MnII) leads to very different products depending on the metal. Thus, the dinuclear complexes
)
NiII, 1; CoII, 2) or the linear zigzag tetranuclear clusters [Mn4(H2L)2(AcO)2(py)5] (3) and [Mn4-
(H2L)2(AcO)2(dmf)4] (4) have been synthesized and characterized crystallographically. Slow oxidation of complex 3
leads to the formation of the novel mixed-valence linear complex [Mn3(HL)2(py)6] (5), displaying an unprecedented
asymmetric MnIIIMnIIIMnII topology. The coordination geometry of complexes 1 to 5 has been analyzed and discussed
by means of continuous shape measures. Magnetic measurements of 3 and 5 demonstrate that the metals within
1
1
these complexes weakly interact magnetically with coupling constants of J1 ) −1.13 cm- and J2 ) −0.43 cm-
1
1
(S
)
0) for complex 3 and J1 ) −5.4 cm- and J2 ) −0.4 cm- (S
)
5/2) for complex 5 (using the H ) −Σ2JijSiSj
convention). These results are consistent with X-band EPR measurements on these compounds.
Introduction
made in the synthetic arena to increase the spin state of
molecules or control the factors that govern the magne-
toanisotropy of these systems.3,4 Thus, in the past decade
the reports on novel discrete and extended coordination
compounds with relevance to magnetism have proliferated,
boosting the discipline enormously. It is interesting to note
that most clusters prepared with this aim are made by self-
assembly from paramagnetic metal ions and small bridging
ligands, although this approach rarely allows the prediction
of the final products structure.5-9 Some authors, however,
have engaged in the design and synthesis of complicated
multidentate ligands to use in reactions with transition metals,
One of the pillars of molecular magnetism is the synthesis
of novel materials aimed at discovering new properties or
deepening the understanding of known magnetic phenomena.
For example, since the discovery of single-molecule magnets
(SMMs)1 or single-chain magnets2 vast efforts have been
* To whom correspondence should be addressed. E-mail:
guillem.aromi@qi.ub.es.
† Universitat de Barcelona.
‡ Leiden University.
§ Florida State University.
| Utrecht University.
Daresbury Laboratory.
# Lawrence Berkeley Lab.
(3) Arom´ı, G.; Brechin, E. K. Struct. Bonding 2006, 122, 1-67.
(4) Coulon, C.; Misayaka, H.; Cle´rac, R. Struct. Bonding 2006, 122, 163-
206.
(5) Milios, C. J.; Stamatatos, T. C.; Perlepes, S. P. Polyhedron 2006, 25,
134-194.
(6) Winpenny, R. E. P. AdV. Inorg. Chem. 2001, 52, 1.
(7) Winpenny, R. E. P. J. Chem. Soc., Dalton Trans. 2002, 1-10.
(8) Brechin, E. K. Chem. Commun. 2005, 5141-5153.
3 Diamond House.
(1) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S. Y.; Vincent, J. B.;
Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am.
Chem. Soc. 1993, 115, 1804-1816.
(2) Caneschi, A.; Gatteschi, D.; Lalioti, N.; Sangregorio, C.; Sessoli, R.;
Venturi, G.; Vindigni, A.; Rettori, A.; Pini, M. G.; Novak, M. A.
Angew. Chem., Int. Ed. 2001, 40, 1760-1763.
10.1021/ic062075v CCC: $37.00
Published on Web 02/28/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 7, 2007 2519

Arom´ı et al.
Scheme 1
Figure 1. Labeled ORTEP representation of H5L at the 50% probability
level. Hydrogen bonds are shown as dashed lines.
members.18 We report here the synthesis and structure of
the longest ligand made of this family, H5L (1,3-bis-(3-oxo-
3-(2-hydroxyphenyl)-propionyl)-2-methoxybenzene), con-
taining five ionizable protons and a string of seven oxygen
donors in the form of two 1,3-diketones and three phenol
adjacent groups. The reactivity of H5L toward M(AcO)2 (M
) NiII, CoII, MnII) was found to vary dramatically depending
on the metal. Thus, dinuclear complexes were obtained with
nickel(II) and cobalt(II) whereas manganese(II) led to a
tetranuclear cluster comprising a very interesting [Mn4O6]
core in the form of a zigzag chain. On the other hand, the
mixture MnIII/MnII favors the formation of a unique mixed-
valence linear compound exhibiting a rare asymmetric
[MnIII-MnIII-MnII] sequence. The synthesis and structure
of these compounds are described, as well as the magnetic
properties of the manganese aggregates. Some of these results
have been previously communicated.19
with the aim of enforcing certain topological patterns or even
introducing functionality aspects to the resulting coordination
complexes.10-13 For a few years, some of us have devoted
efforts to designing and synthesizing a class of ligands
displaying linear arrays of oxygen donors, based on the
adjacent disposition of phenol and 1,3-diketone groups, with
the aim of favoring the formation of chains of closely spaced
transition metals.14,15 This topology is of relevance in
molecular magnetism, since it might lead to highly aniso-
tropic systems and allow a better understanding of the
phenomenon of magnetic anisotropy. Indeed, one of the
postulated strategies to improve the properties of SMMs is
to aim at species with increased magnetoanisotropy. In this
sense, it is important to understand the factors that enhance
it and to reach the ability to control these within synthetic
schemes. The first members of the family of poly-1,3-
diketone ligands mentioned above exhibited five and six
O-donors in a linear fashion. Different degrees of occupancy
of their coordination pockets have led to several complexes,
which are dinuclear16 or exhibit chains with three17 or four
Results and Discussion
Synthesis. The heptadentate ligand H5L (Scheme 1) was
prepared from the related bis-â-diketone/methoxy-ether
(H4L′) derivative through ether cleavage by BBr3. The latter
polyketone was synthesized through the coupling between
the appropriate diester (dimethyl 2-methoxyisophthalate) and
the corresponding ketone (2-hydroxyacetophenone) by means
of a Claisen condensation (Scheme 1).15 In this reaction, the
product is obtained as the tetra-anionic sodium salt together
with other salts such as NaMeO, and the system is later
acidified in order to obtain the neutral form of the sought
molecule. Invariably, the target compound was obtained
together with approximately 15-20% of the monocoupled
derivative. The former could only be isolated pure by
solubilization of the impurity in acetone. The conversion to
the bis-â-diketone/trisphenol derivative (H5L) necessitated
1.5 equiv of BBr3, albeit each mole of the tribromide can
potentially cleave 3 mol of ether.20 Very likely, the formation
of the orthoboric ester necessary for the process to occur
with maximal efficiency is prevented for steric reasons. Mass
spectrometry produced solid evidence for the formation of
the desired product. 1H NMR spectroscopy, however,
revealed a lower symmetry than that suggested in Scheme
1, presumably as a consequence of the particular keto-enolic
(9) Arom´ı, G.; Aubin, S. M. J.; Bolcar, M. A.; Christou, G.; Eppley, H.
J.; Folting, K.; Hendrickson, D. N.; Huffman, J. C.; Squire, R. C.;
Tsai, H. L.; Wang, S.; Wemple, M. W. Polyhedron 1998, 17, 3005-
3020.
(10) Yoshino, A.; Miyagi, T.; Asato, E.; Mikuriya, M.; Sakata, Y.; Sugiura,
K.; Iwasaki, K.; Hino, S.; Hino, D. Chem. Commun. 2000, 1475-
1476.
(11) Thompson, L. K.; Kelly, T. L.; Dawe, L. N.; Grove, H.; Lemaire, M.
T.; Howard, J. A. K.; Spencer, E. C.; Matthews, C. J.; Onions, S. T.;
Coles, S. J.; Horton, P. N.; Hursthouse, M. B.; Light, M. E. Inorg.
Chem. 2004, 43, 7605-7616.
(12) Peng, S. M.; Wang, C. C.; Jang, Y. L.; Chen, Y. H.; Li, F. Y.; Mou,
C. Y.; Leung, M. K. J. Magn. Magn. Mater. 2000, 209, 80-83.
(13) Ottenwaelder, X.; Cano, J.; Journaux, Y.; Riviere, E.; Brennan, C.;
Nierlich, M.; Ruiz-Garcia, R. Angew. Chem., Int. Ed. 2004, 43, 850-
852.
(17) Arom´ı, G.; Berzal, P. C.; Gamez, P.; Roubeau, O.; Kooijman, H.;
Spek, A. L.; Driessen, W. L.; Reedijk, J. Angew. Chem., Int. Ed. 2001,
40, 3444.
(14) Arom´ı, G.; Boldron, C.; Gamez, P.; Roubeau, O.; Kooijman, H.; Spek,
A. L.; Stoeckli-Evans, H.; Ribas, J.; Reedijk, J. Dalton Trans. 2004,
3586-3592.
(18) Arom´ı, G.; Ribas, J.; Gamez, P.; Roubeau, O.; Kooijman, H.; Spek,
A. L.; Teat, S.; MacLean, E.; Stoeckli-Evans, H.; Reedijk, J. Chem.s
Eur. J. 2004, 10, 6476-6488.
(15) Arom´ı, G.; Gamez, P.; Berzal, P. C.; Driessen, W. L.; Reedijk, J. Synth.
Commun. 2003, 33, 11-18.
(16) Arom´ı, G.; Gamez, P.; Roubeau, O.; Carrero-Berzal, P.; Kooijman,
H.; Spek, A. L.; Driessen, W. L.; Reedijk, J. Eur. J. Inorg. Chem
2002, 1046-1048.
(19) Arom´ı, G.; Gamez, P.; Boldron, C.; Kooijman, H.; Spek, A. L.;
Reedijk, J. Eur. J. Inorg. Chem. 2006, 1940-1944.
(20) Benton, F. L.; Dillon, T. E. J. Am. Chem. Soc. 1942, 64, 1128-1129.
2520 Inorganic Chemistry, Vol. 46, No. 7, 2007

NoWel Linear Transition Metal Clusters
Table 1. Crystallographic Data for Compounds H5L, [Ni2(H3L)2(py)4] (1), [Co2(H3L)2(py)4] (2), [Mn4(H2L)2(AcO)2(py)5] (3),
[Mn4(H2L)2(AcO)2(dmf)4] (4), and [Mn3(HL)2(py)6] (5)
H5L
1
2
3
4
5
formula
C24H18O7
C78H62N5-
Ni2O14
1424.72
triclinic
P1h
8.929(2)
11.205(2)
17.579(6)
103.625(15)
90.170(15)
106.18(2)
1637.1(8)
1
C39H28Co-
N3O7
709.57
monoclinic
P21/c
12.932(2)
9.0376(17)
28.463(5)
90.00
92.615(2)
90.00
3323.1(10)
4
1.418
150(2)
plate
blue
C77H61Mn4-
N5O18
1564.07
triclinic
P1h
10.8441(10)
18.591(2)
20.904(3)
85.172(15)
77.893(15)
87.872(15)
4105.1(9)
2
C64H64.6Mn4-
N4O22.3
1466.00
monoclinic
P21/c
15.8846(11)
15.8321(11)
14.0294(10)
90.00
113.446(2)
90.00
3236.9(4)
2
C
85.5H66Mn3-
N7.5O14.2
fw
418.38
orthorhombic
Pbcn
13.9229(15)
8.5263(12)
32.190(5)
90.00
90.00
90.00
3821.3(9)
8
1.454
150
block
yellow
0.08 ×0.06
× 0.01
3410
1591.28
triclinic
P1h
crystal syst
space group
a/Å
b/Å
c/Å
10.5020(3)
17.7237(5)
19.9453(5)
98.608(2)
94.671(2)
91.647(2)
3655.34(17)
2
1.446
150(2)
rod
brown
R/°
â/°
γ/deg
V/Å3
Z
Fcalcda/g cm-3
T/K
1.445
150
plate
1.265
150
block
1.504
180(2)
plate
yellow
crystal shape
color
dimens/mm
pale green
0.03 × 0.10
× 0.20
pale orange
0.05 × 0.18
× 0.35
0.08 × 0.06
× 0.01
4937
0.14 × 0.10
0.08 × 0.01
× 0.03
× 0.01
unique data
7480
14 832
9794
21 645
unique data with I > 2 σ(I)
2302
0.0468,
0.1254
5679
0.0386,
0.0877
3855
0.0486,
0.1140
11 825
0.0443,
0.0407
7410
0.0384,
0.0901
16 334
0.0554,
0.1460
R1, wR2b
2
2
4
a For 3, excluding disordered solvent contribution. b R1 ) Σ ||Fo| - |Fc||/Σ |Fo|, wR2 ) [(Σw(Fo - Fc )2 /ΣwFo )]1/2
.
form present in solution. One of these forms was trapped in
the solid state, and its structure was determined by X-ray
crystallography (see below). The H NMR spectrum (see
with formula [Mn4(H2L)2(AcO)2(py)5] (3), also during the
course of a clean transformation
1
4Mn(AcO)2 + 2H5L + 5py f
Supporting Information) was consistent with the molecular
symmetry in the solid state and could be assigned with help
of COSY experiments and, for the protons involved in the
keto-enolic processes, assuming the correlation predicted
for resonance-assisted hydrogen-bonded (RAHB) systems,21
between the [O-H‚‚‚O] distance and the chemical shift.
The ability of the heptadentate donor H5L to assemble
transition metals into linear arrays was investigated with
various metallic salts, always using 2 equiv of metallic ion
per ligand. The reaction in pyridine with Ni(AcO)2 produced
crystals of [Ni2(H3L)2(py)4] (1). This complex contains only
two separated metals and results from the sole deprotonation
of the ligand at the 1,3-diketone moieties, the phenol groups
remaining protonated. The latter are certainly stabilized by
internal hydrogen bonds, the acetate group not being a
sufficiently strong base to subtract them. Indeed, the use of
stronger bases led to precipitates with a probable higher metal
content, and attempts to crystallize these compounds are
currently in progress. On the other hand, changing the ligand-
to-metal ratio only does not lead to any identifiable change
in reactivity. Similarly, the reaction with Co(AcO)2 generated
the complex [Co2(H3L)2(py)4] (2), which could be character-
ized by X-ray crystallography, using a synchrotron radiation
source. These simple processes are perfectly described by
eq 1 (with M ) NiII or CoII)
[Mn4(H2L)2(AcO)2(py)5] + 6AcOH (2)
The major differences between this manganese complex
and the previous compounds are the nuclearity, the fact that
3 incorporates the ligand carboxylate, and the higher degree
of deprotonation of the polydentate ligand H5L. The reasons
for such differences in reactivity are difficult to appraise. A
possible explanation is the ability of MnII centers to adapt
to more distorted coordination environments, as compared
with CoII and NiII and as can be seen with the two analogous
complexes [M3(HL1)3] (M ) MnII, CoII)17,22 where H3L1 is
a ligand similar to H5L, without the external OH groups.
Important structural disparities between the product with MnII
vs the products with CoII and NiII were already observed
with H3L1 and explained in terms of subtle differences of
crystal field stabilization energies.16,22 The differences in
reactivity, however, do not seem to be associated to the
ability of Mn to exist in different oxidation states, since the
oxidation of Mn is only observed at latter stages (see below),
after isolation of the MnII complexes. When the reaction to
prepare 3 was performed in DMF, the analogous cluster
[Mn4(H2L)2(AcO)2(dmf)4] (4) was obtained, which could be
characterized by single-crystal X-ray diffraction (synchrotron
source). Unfortunately, no product could be isolated when
performing the reaction in a noncoordinating solvent. All
the above complexes were obtained in crystalline form by
layering the corresponding reaction mixtures with Et2O. A
quite remarkable process was observed when the system
producing crystals of 3 was left unperturbed for a few weeks.
2M(AcO)2 + 2H5L + 4py f [M2(H3L)2(py)4] +
4AcOH (1)
The same reaction was performed with Mn(AcO)2, and
surprisingly, this resulted in a completely distinct product,
(21) Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Chem. Soc., Perkin
Trans. 1997, 945-952.
(22) Arom´ı, G.; Stoeckli-Evans, H.; Teat, S. J.; Cano, J.; Ribas, J. J. Mater.
Chem. 2006, 16, 2635-2644.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2521

Arom´ı et al.
Table 2. Selected Interatomic Distances (Å) for Compound H5L
O(1)-C(22)
O(2)-C(20)
O(3)-C(30)
O(4)-C(40)
O(5)-C(50)
O(6)-C(60)
O(7)-C(62)
C(20)-C(21)
C(20)-C(31)
C(21)-C(22)
C(21)-C(26)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(25)-C(26)
C(30)-C(31)
C(30)-C(42)
C(40)-C(41)
C(40)-C(42)
1.354(2)
1.234(2)
1.237(2)
1.341(2)
1.331(2)
1.277(2)
1.359(2)
1.468(3)
1.521(3)
1.415(3)
1.403(3)
1.388(3)
1.365(3)
1.394(3)
1.372(3)
1.508(3)
1.467(3)
1.408(3)
1.422(2)
C(41)-C(43)
C(41)-C(50)
C(42)-C(44)
C(43)-C(45)
C(44)-C(45)
C(50)-C(51)
C(51)-C(60)
C(60)-C(61)
C(61)-C(62)
C(61)-C(66)
C(62)-C(63)
C(63)-C(64)
C(64)-C(65)
C(65)-C(66)
O(1)-H‚‚‚(2)
O(3)-H‚‚‚O(4)
O(5)-H‚‚‚O(6)
O(6)-H‚‚‚O(7)
1.400(3)
1.479(2)
1.392(3)
1.381(3)
1.372(3)
1.365(3)
1.437(2)
1.470(3)
1.413(2)
1.400(3)
1.387(3)
1.371(3)
1.389(3)
1.378(3)
2.578(2)
2.533(2)
2.488(2)
2.536(2)
Figure 2. ORTEP representation at 50% probability level of [Ni2(H3L)2-
(py)4] (1). Only hydroxyl H atoms are shown. Intramolecular hydrogen
bonds are shown as dashed lines. This representation also describes
accurately the structure of [Co2(H3L)2(py)4] (2), if the metal label is changed
accordingly.
Scheme 2
During that period, these crystals slowly redissolved with
concomitant deposition of dark brown crystals of a new
product. This reaction took place to completion, and thus, a
new complex could be isolated, whose formula, [Mn3(HL)2-
(py)6] (5), was established by synchrotron X-ray diffraction.
Complex 5 comprises one MnII and two MnIII ions, thus
representing an increase of the average oxidation state of
the metals from +2 to +2.7. In addition, the deprotonation
degree of the polydentate ligand has risen from three ionized
protons to four through the transformation of complex 3 to
5. After identification of the new complex 5, the preparation
from its components was accomplished in pyridine through
the comproportionation between MnVII and MnII in the
appropriate ratio to give an [MnIII2MnII] product, as described
in eq 3. This was verified by infrared spectroscopy, elemental
analysis, and magnetic measurements.
cyclic chain, [OdCsCsCdOsH‚‚‚], and intramolecular
hydrogen bonding within such a cycle mutually reinforce
one other in a synergistic manner. This translates into very
short O‚‚‚O distances, d(O‚‚‚O) (Table 2). In the molecule
H5L, this phenomenon occurs four times, three times within
the 2-hydroxybenzoketone moieties and once as a 1,3-
diketone (or ketoenol). The four hydrogen atoms involved
in these interactions have been located crystallographically.
Among all the possibilities, this distribution of internal
hydrogen bonds leads to the most stable combination of
tautomers, and it is retained in CHCl3 solution, as shown by
1H NMR spectroscopy. The correlation found to exist
Bu4NMnO4 + 13/2Mn(AcO)2 + 5H5L + 6py f
15/6[Mn3(HL)2(py)6] + Bu4NAcO + 12AcOH + 4H2O (3)
Description of the Structures. Representations of the
molecular structures of compounds H5L, 1, 2, 3, 4, and 5
are depicted in Figures 1-6. Crystallographic data are
reported in Table 1, whereas selected interatomic distances
and angles are in Tables 2-4 and in the captions of Figures
3 and 5.
H5L. The single-crystal X-ray diffraction of this molecule
reveals that in the solid state it exhibits one of the â-diketone
groups in the enolic form (O5, O6), whereas the other is
present as the biscarbonyl tautomer (O2, O3). This is
reflected by the positions of the relevant hydrogen atoms
and by the various C-O bond distances (Table 2). The
simultaneous presence of both forms within the same
molecule is extremely rare, and we are aware of only one
previous example, corresponding to a cyclic polyketide
lactone, where the strain imposed by the molecular structure
may enforce the mixed tautomeric display.23 Compound H5L
is a very rich example of the manifestation of RAHB,21 by
which the existence of π-delocalization within a heterodienic
1
between d(O‚‚‚O) and the H NMR chemical shift of the
proton involved in the corresponding RAHB was used to
help assign the spectrum of H5L (see Supporting Informa-
tion). All the atoms of the molecule (except the methylenic
H atoms) are essentially contained in either of two planes
that form a dihedral angle of approximately 76° and share
one atom, C30. A complicated network of π-π stacking
interactions maintains the molecules in the solid state.
[Ni2(H3L)2(py)4] (1). Complex 1 (Figure 2) is a dinuclear
complex of NiII ions bridged by two crystallographically
equivalent bis-deprotonated H5L ligands (i.e., in the H3L2-
form; Scheme 2, top). The latter have lost their protons from
the 1,3-diketone moieties, which now function as chelates
at the equatorial positions of the pseudo-octahedral metal
ions, their axial sites being occupied by solvate pyridine
ligands. The phenol protons remain at their original locations,
being part of intramolecular hydrogen bonds as observed in
the free ligand (see above), although H3L2- has varied its
(23) Tatsuta, K.; Chino, M.; Kojima, N.; Shinojima, S.; Nakata, M.;
Morooka, M.; Ohba, S. Tetrahedron Lett. 1993, 34, 4957-4960.
2522 Inorganic Chemistry, Vol. 46, No. 7, 2007

NoWel Linear Transition Metal Clusters
Table 3. Selected Interatomic Distances (Å) and Angles (deg) for
Complexes [M2(H3L)2(py)4] (M ) Ni, 1; Co, 2) in the 1/2 Format
M(1)-O(2)
2.0120(14)/2.040(2)
2.0241(14)/2.045(2)
2.1073(17)/2.167(3)
2.1504(18)/2.202(3)
1.9862(14)/2.013(2)
2.0138(14)/2.057(3)
87.13(5)/85.43(9)
87.64(6)/90.03(10)
90.42(6)/88.72(10)
175.93(6)/177.64(8)
93.40(5)/93.59(9)
91.38(6)/92.65(9)
92.66(6)/88.56(9)
90.64(5)/92.37(9)
178.73(5)/176.12(9)
175.42(6)/178.18(10)
89.02(6)/90.91(10)
89.79(6)/91.11(10)
93.09(6)/90.39(10)
86.19(6)/87.66(10)
88.89(5)/88.56(9)
M(1)-O(3)
M(1)-N(70)
M(1)-N(80)
M(1)-O(5)a
M(1)-O(6)a
O(2)-M(1)-O(3)
O(2)-M(1)-N(70)
O(2)-M(1)-N(80)
O(2)-M(1)-O(5)a
O(2)-M(1)-O(6)a
O(3)-M(1)-N(70)
O(3)-M(1)-N(80)
O(3)-M(1)-O(5)a
O(3)-M(1)-O(6)a
N(70)-M(1)-N(80)
N(70)-M(1)-O(5)a
N(70)-M(1)-O(6)a
N(80)-M(1)-O(5)a
N(80)-M(1)-O(6)a
O(5)a-M(1)-O(6)a
Figure 3. ORTEP representation at 50% probability level of [Mn4(H2L)2-
(AcO)2(py)5] (3). Only hydroxyl H atoms are shown. Intramolecular
hydrogen bonds are shown as dashed lines. Ranges of selected interatomic
distances (Å) and angles (deg): Mn-O(AcO), 2.120(2)-2.152(2); Mn-
O(H2L), 2.066(2)- 2.403(2); Mn-N, 2.266(2)-2.488(2); Mn1‚‚‚Mn2,
3.2876(6); Mn2‚‚‚Mn3, 3.4950(7); Mn3‚‚‚Mn4, 3.3502(6); Mn1‚‚‚Mn3,
5.8984(9); Mn2‚‚‚Mn4, 5.7526(9); Mn1‚‚‚Mn4, 8.8046(13); Mn1-Mn2-
Mn3, 120.80(2); Mn2-Mn3-Mn4, 114.35(2); Mn1-Mn2-Mn3-Mn4,
-176.32(2); O107-H‚‚‚O106, 2.513(2); O207-H‚‚‚O206, 2.500(2); O101-
H‚‚‚O102, 2.501(2); O201-H‚‚‚O202, 2.495(2).
a The [1 - x, 1 - y, -z] symmetry operation was utilized.
conformation upon coordination with respect to solid H5L.
The intramolecular NiII‚‚‚NiII distance is 7.714(3) Å whereas
the shortest intermolecular metal-metal separation is 8.929-
(3) Å. Other selected interatomic distances and angles are
collected in Table 3. The degree of coplanarity of the ligand
H3L2- can be gauged by the angle formed by the idealized
chelate rings. For complex 1, this angle is 20.01(6)°. The
packing in the solid state occurs because of interactions
between the pyridine ligands from different molecules and
as a consequence of mutual interdigitation. In addition to
complex 1, the unit cell contains one molecule of uncoor-
dinated pyridine per dinuclear entity, establishing π-π
interactions with one pyridine ligand.
[Co2(H3L)2(py)4] (2). Complex 2 (Figure 2) is the CoII
analogue of 1. This similarity in reactivity between NiII and
CoII is often encountered,24 and it has been observed with
ligand systems very similar to H5L.16,22 The intermetallic
distances here are 7.807 (intramolecular) and 9.038 Å
(intermolecular). The packing between the molecules occurs
through various weak van der Waals interactions involving
aromatic rings but in a different manner than in complex 1.
In this case, there is no interdigitation between the pyridine
rings of different molecules, and the rings of H3L2- also
participate in these interactions. This may explain the
differences in conformation of the polydentate ligand when
comparing 1 with 2. For complex 2, the ligand is more
removed from planarity; the angle between idealized chelate
rings is 34.0(1)°. The uncoordinated pyridine ligand present
in complex 1, not observed in 2, may also explain these
differences in part.
Figure 4. POV-ray representation of the core of [Mn4(H2L)2(AcO)2(py)5]
(3), showing the spin-coupling scheme used to model its magnetic behavior.
1,3-diketonate moieties for coordination, thereby chelating
and bridging the metals through four adjacent [O-C-C-
C-O] coordination pockets (Scheme 2, middle). The external
phenol moieties of the ligands are not involved in coordina-
tion and retain their protons, which form intramolecular
hydrogen bonds as in complexes 1 and 2. Additional bridging
within the cluster is ensured by two syn,syn-OAc- ligands,
supporting the Mn1-2 and Mn3-4 pairs, respectively. The
central pair is in turn bridged only by two alkoxide-type
oxygen atoms. Terminal ligation is completed by five
pyridine ligands, one on each metal except for Mn3, which
is bound to two such molecules and is therefore heptacoor-
dinated. The remaining manganese ions are pseudo-
octahedral. The bond distances to the metals (see Figure 3
caption) agree well with the oxidation state of +2, which is
consistent with the electroneutrality of the molecule. The
intramolecular metal-metal separations are (in Å) 3.2876-
(6) (Mn1-2), 3.4950(7) (Mn2-3), 3.3502(6) (Mn3-4), 5.7526-
(9) (Mn2-4), 5.8984(9) (Mn3-1), and 8.8046(13) (Mn1-4). The
angles of the “zigzag” can be gauged with the parameters
of Mn1-Mn2-Mn3 (120.80(2)°) and Mn2-Mn3-Mn4
(114.35(2)°). Interestingly, the H2L3- ligand is found here
in a conformation highly reminiscent of its free form in the
solid state. The moderate degree of flexibility of the ligand,
together with its linear array of O-donors, facilitates the
formation of this interesting [Mn4O6] architecture. The
improper torsion angle formed by the four Mn ions within
the molecule is -176.32(2)°, with these atoms thus being
close to lying within the same plane. Tetranuclear manganese/
oxo aggregates have been perceived as potential models for
the oxygen-evolving center of photosystem II.25 Of the many
published examples, relatively few consist of linear arrays
[Mn4(H2L)2(AcO)2(py)5] (3). Complex 3 (Figure 3) is a
tetranuclear cluster of MnII with a [Mn4O6] zigzag linear core
(Figure 4). This arrangement is built up by the action of two
H2L3- ligands located opposite to each other and running
along the chain, each engaging its central phenolate and both
(24) Arom´ı, G.; Batsanov, A. S.; Christian, P.; Helliwell, M.; Roubeau,
O.; Timco, G. A.; Winpenny, R. E. P. Dalton Trans. 2003, 4466-
4471.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2523

Arom´ı et al.
Scheme 3
Figure 6. ORTEP representation at 50% probability level of [Mn3(HL)2-
(py)6] (5). H atoms are not shown.
lographic symmetry). No remarkable features are observed
on the packing diagram of this complex.
[Mn3(HL)2(py)6] (5). The structure of 5 (Figure 6)
comprises a mixed-valence trinuclear [MnIII2MnII] complex
supported by two HL4- ligands, one on each side of the
chain, which balance the metallic charges. The O-donors of
these ligands occupy all the equatorial positions of the
metals as chelates, the axial sites being saturated by pyridine
ligands. The ligand HL4- is found here at almost its highest
possible level of deprotonation, coordinating the metals in
an asymmetric manner, by using three nonadjacent coordina-
tion pockets (Scheme 2, bottom). This comes about through
the involvement of two phenoxide groups and â-diketonate
moieties per ligand. The third phenol moiety of each ligand
retains its proton, and both groups are found in front of each
other at the same end of the molecule. The MnIII centers are
identified within this complex (Mn1 and Mn2) by the
presence of Jahn-Teller elongation axes (corresponding to
the bonds with pyridines). On the other hand, the bond
distances around Mn3 (Table 4) are consistent with the
assignment of the oxidation state +2. The intermetallic
distances in this complex are (in Å) 5.2463 (Mn1-2), 5.1553
(Mn2-3), and 10.3819 (Mn1-3), whereas the angle of Mn1-
Mn2-Mn3 is 172.92°. There are other linear trinuclear
complexes in the literature, featuring mixtures of Mn ions
in the oxidation states of +2 and +3, either as [MnIIIMnII-
MnIII]35,36 or as [MnIIMnIIIMnII],37,38 the former being about
twice as numerous as the latter. All these complexes are,
however, symmetric, thus featuring the unique Mn ion as
the central atom of the array. Complex 5 is the first
asymmetric compound of this category and thus the only
one to exhibit the sequence [MnIIIMnIIIMnII]. This has
interesting implications with regard to the magnetic properties
of this molecule (see below). The molecules of complex 5
are cemented within the crystal by means of van der Waals
interactions as well as a network of π-π bonds established
Figure 5. ORTEP representation at 50% probability level of [Mn4(H2L)2-
(AcO)2(dmf)4] (4). Only hydroxyl H atoms are shown. Intramolecular
hydrogen bonds are shown as dashed lines. Ranges of selected interatomic
distances (Å) and angles (deg): Mn-O(AcO), 2.092(2)-2.119(1); Mn-
O(H2L), 2.082(1) -2.274(1); Mn-O(dmf), 2.185(1)-2.260(2); Mn1‚‚‚Mn2,
3.3182(5); Mn2‚‚‚Mn2a, 3.3877(5); Mn1‚‚‚Mn2a, 5.8618(6); Mn1‚‚‚Mn1a,
8.9031(8); Mn1-Mn2-Mn2a, 121.88(1); Mn1-Mn2-Mn2a-Mn1a, 180;
O1-H‚‚‚O2, 2.605(2); O7-H‚‚‚O6, 2.544(2).
of fused [Mn2(µ-O)2] moieties as in 3.26-30 The latter is the
first MnII example where the oxo-bridged dimers are in
relative trans configuration (zigzag) as opposed to cis
(Scheme 3). The trans configuration is, however, for other
metals, known to coordination chemistry.31-34 The packing
diagram of this compound does not reveal any special
intermolecular interactions other than a plethora of van der
Waals contacts (not shown).
[Mn4(H2L)2(AcO)2(dmf)4] (4). Complex 4 (Figure 5) is
analogous to 3 with dmf terminal ligands instead of pyridine.
Now, each manganese ion is bound to one solvate ligand
and all metal centers are thus octahedral. As a result, the
compound now displays crystallographic inversion sym-
metry. The remaining structural parameters (Figure 5 caption)
are essentially the same as in 3, with the following inter-
metallic distances (in Å) of 3.3182(5) (Mn1-2), 3.3877(5)
(Mn2-2a), 5.8618(6) (Mn1-2a), and 8.9031(8) (Mn1-1a). The
zigzag and torsion angles are 121.88(1)° (Mn1-Mn2-Mn2a)
and 180°, respectively (the latter by virtue of the crystal-
(25) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong, W. H.
Chem. ReV. 2004, 104, 3981-4026.
(26) Dave, B. C.; Czernuszewicz, R. S. New J. Chem. 1994, 18, 149-155.
(27) Philouze, C.; Blondin, G.; Girerd, J. J.; Guilhem, J.; Pascard, C.; Lexa,
D. J. Am. Chem. Soc. 1994, 116, 8557-8565.
(28) Yoo, J.; Yamaguchi, A.; Nakano, M.; Krzystek, J.; Streib, W. E.;
Brunel, L. C.; Ishimoto, H.; Christou, G.; Hendrickson, D. N. Inorg.
Chem. 2001, 40, 4604-4616.
(29) Jeffery, J. C.; Thornton, P.; Ward, M. D. Inorg. Chem. 1994, 33, 3612-
(35) Kitajima, N.; Osawa, M.; Imai, S.; Fujisawa, K.; Morooka, Y.;
Heerwegh, K.; Reed, C. A.; Boyd, P. D. W. Inorg. Chem. 1994, 33,
4613-4614.
(36) Tangoulis, V.; Malamatari, D. A.; Spyroulias, G. A.; Raptopoulou,
C. P.; Terzis, A.; Kessissoglou, D. P. Inorg. Chem. 2000, 39, 2621-
2630.
(37) Liu, J. C.; Xu, Y.; Duan, C. Y.; Wang, S. L.; Liao, F. L.; Zhuang, J.
Z.; You, X. Z. Inorg. Chim. Acta 1999, 295, 229-233.
(38) Tan, X. S.; Sun, J.; Hu, C. H.; Fu, D. G.; Xiang, D. F.; Zheng, P. J.;
Tang, W. X. Inorg. Chim. Acta 1997, 257, 203-210.
3615.
(30) Bardwell, D. A.; Jeffery, J. C.; Ward, M. D. J. Chem. Soc., Dalton.
Trans. 1995, 3071-3080.
(31) Kemmitt, T.; Gainsford, G. J.; Al-Salim, N. I. Acta Crystallogr., Sect.
C: Cryst. Struct. Commun. 2004, 60, M42-M43.
(32) Dey, M.; Pao, C. P.; Saarenketo, P. K.; Rissanen, K. Inorg. Chem.
Commun. 2002, 5, 380-383.
(33) Burger, J.; Klufers, P. Z. Anorg. Allg. Chem. 1997, 623, 1547-1554.
(34) Cotton, F. A.; Elder, R. C. Inorg. Chem. 1965, 4, 1145-1151.
2524 Inorganic Chemistry, Vol. 46, No. 7, 2007

NoWel Linear Transition Metal Clusters
Table 4. Selected Interatomic Distances (Å) and Angles (deg) for
Complex [Mn3(HL)2(py)6] (5)
Table 5. CShMs of Coordination Polyhedra within Complexes
[M2(H3L)2(py)4] (M ) Ni, 1; Co, 2), [Mn4(H2L)2(AcO)2(py)5] (3),
[Mn4(H2L)2(AcO)2(dmf)4] (4), and [Mn3(HL)2(py)6] (5)
Mn1-O1
1.876(2)
1.916(2)
1.886(2)
1.893(1)
2.331(2)
2.294(2)
1.889(1)
1.883(1)
1.894(2)
1.872(1)
2.378(2)
2.352(2)
2.109(2)
2.144(2)
2.112(2)
2.150(2)
2.321(2)
2.266(2)
5.2462(4)
5.1547(4)
10.3811(5)
2.555(3)
2.503(2)
89.72(7)
92.62(7)
176.61(7)
90.78(9)
89.90(8)
177.64(6)
87.00(6)
92.63(7)
90.05(7)
90.67(6)
87.03(8)
90.27(8)
O9-Mn1-N1
O9-Mn1-N2
N1-Mn1-N2
O3-Mn2-O4
O3-Mn2-O10
O3-Mn2-O11
O3-Mn2-N3
O3-Mn2-N4
O4-Mn2-O10
O4-Mn2-O11
O4-Mn2-N3
O4-Mn2-N4
O10-Mn2-O11
O10-Mn2-N3
O10-Mn2-N4
O11-Mn2-N3
O11-Mn2-N4
N3-Mn2-N4
O5-Mn3-O6
O5-Mn3-O12
O5-Mn3-O13
O5-Mn3-N5
O5-Mn3-N6
O6-Mn3-O12
O6-Mn3-O13
O6-Mn3-N5
O6-Mn3-N6
O12-Mn3-O13
O12-Mn3-N5
O12-Mn3-N6
O13-Mn3-N5
O13-Mn3-N6
N5-Mn3-N6
Mn1-Mn2-Mn3
90.20(8)
89.27(7)
177.24(7)
89.53(6)
87.55(6)
174.96(7)
88.67(7)
90.58(7)
177.08(6)
92.87(6)
90.52(7)
86.77(6)
90.04(6)
89.37(7)
93.97(7)
86.88(7)
93.97(7)
177.19(7)
83.82(6)
93.91(6)
173.68(6)
91.14(7)
94.71(7)
176.46(6)
101.84(6)
86.76(7)
88.92(7)
80.56(6)
96.02(7)
88.24(7)
86.43(7)
88.24(7)
172.31(7)
172.92(1)
Mn1-O2
Complexes 1-5
Mn1-O8
ion (complex)
dist to OCa
dist to TRPb
Mn1-O9
Mn1-N1
Ni1 (1)
0.20
0.19
3.08
8.92
3.89
12.15
10.59
6.92
1.01
1.31
0.74
14.85
15.55
7.45
2.49
6.29
2.32
1.87
3.51
Mn1-N2
Co1 (2)
Mn1 (3)
Mn2 (3)
Mn4 (3)
“Mn3” (3)c
Mn1 (4)
Mn2 (4)
Mn1 (5)
Mn2 (5)
Mn3 (5)
Mn2-O3
Mn2-O4
Mn2-O10
Mn2-O11
Mn2-N3
Mn2-N4
Mn3-O5
16.48
16.62
12.59
Mn3-O6
Mn3-O12
Mn3-O13
Mn3-N5
Mn3 of Complex 3 (Heptacoordinated)
Mn3-N6
dist to PBPYd
dist to OCFe
dist to TPRSf
Mn1...Mn2
Mn2...Mn3
Mn1...Mn3
O6-O7
3.98
2.23
1.10
a OC ) octahedron. b TRP ) trigonal prism. c Measures of Mn3 after
removing N41. d PBPY ) pentagonal bipyramid. e OCF ) face-capped
octahedron. f TPRS ) side-face-capped trigonal prism.
O13-O14
O1-Mn1-O2
O1-Mn1-O8
O1-Mn1-O9
O1-Mn1-N1
O1-Mn1-N2
O2-Mn1-O8
O2-Mn1-O9
O2-Mn1-N1
O2-Mn1-N2
O8-Mn1-O9
O8-Mn1-N1
O8-Mn1-N2
regular octahedra (with CShM distances to OC of 0.20 and
0.19, respectively) and are consequently far from being the
ideal trigonal prism (distances to TRP of 14.85 and 15.85;
note that the distance between the perfect polyhedra OC and
TRP is calculated as 16.73). By contrast, the formation of
complexes 3 and 4 requires a severe distortion from the
octahedral geometry (Mn1, Mn4 in 3) or coordination
arrangements close to the trigonal prism (see Table 5). In
fact, Mn2 in 3 and Mn1 in 4 have been calculated to be
very close to ideal TRP (measures, in the TRP/OC format,
of 2.49/8.92 and 1.87/10.59, respectively). In addition, one
MnII ion in 3 (Mn3) displays seven-coordination; CShMs
have shown that the geometry of this center is best described
as a trigonal prism capped on one of its side faces (distance
to TRPS; 1.10). Indeed, if the donor atom capping the prism
(N41) is removed, the remaining shape is an almost perfect
trigonal prism (with TRP/OC distances of 2.32/12.15). The
three manganese atoms of complex 5 display shapes very
near perfect octahedra (Table 5). Interestingly, the MnII
(Mn3) center is closer to the ideal polyhedron than the MnIII
chromophores, which reflects the tetragonal elongation
caused by the Jahn-Teller distortion on the latter.
Magnetic Properties. The magnetic properties of the
tetranuclear and trinuclear complexes of 3 and 5 were
examined by bulk magnetization measurements and by
X-band (∼9.7 GHz) electron paramagnetic resonance (EPR).
Preliminary high frequency (50-400 GHz) EPR experiments
were also performed. The magnetometric behavior of 3 has
been previously communicated41 and will only be briefly
discussed here.
The øMT vs T plot for 3 (See Supporting Information
Figure S2, where øM is the molar paramagnetic susceptibility)
decreases from a value of 18.43 cm3 K mol-1 at room
temperature to 1.22 cm3 K mol-1 near 2 K, indicating
dominance of intramolecular antiferromagnetic interactions.
between the pyridine ligands from different molecules, along
the b-axis (see Suppporting Information Figure S1)
Continuous Shape Measures (CShMs). The dramatic
difference in reactivity toward H5L exhibited by CoII and
NiII with respect to MnII is ascribed to the flexibility of the
latter to adopt various coordination geometries without
significant changes of crystal field energy, in contrast to that
of the other two ions, which are more restricted to certain
geometries. Discussion of this and other aspects dependent
on coordination geometries can be made more precisely in
light of the accurate description of the coordination environ-
ment around the metals delivered by CShMs.39,40 These
measures are normalized “distances” (in a scale that goes
from 0 to 100) with respect to ideal shapes of the polyhedra
formed by a metal and the atoms of its first coordination
sphere. In the case of six-coordination, the distances of the
different chromophores to the regular octahedron (OC) and
the ideal trigonal prism (TPR) will be calculated.39 The only
case of heptacoordination encountered here (within complex
[Mn4(H2L)2(AcO)2(py)5]) will be compared to the penta-
gonal bipyramid (PBPY), the face-capped octahedron
(OCF), and the side-face-capped trigonal prism (TRPS).40
The results of these measures are summarized in Table 5.
These calculations show that the NiII and CoII ions in
complexes 1 and 2, respectively, describe almost perfect
(39) Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M. New J. Chem. 2002,
26, 996-1009.
(40) Casanova, D.; Alemany, P.; Bofill, J. M.; Alvarez, S. Chem.sEur. J.
2003, 9, 1281-1295.
(41) Arom´ı, G.; Gamez, G.; Boldron, C.; Kooijman, H.; Spek, A. L.;
Reedijk, J. Eur. J. Inorg. Chem. 2006, 1940-1944.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2525

Arom´ı et al.
These data were fitted using the program CLUMAG42 by
diagonalization of the spin Hamiltonian H ) -2J1(S1S2 +
S3S4) -2J2(S2S3), where Si ) 5/2, and by using the numbering
scheme for the spins given in Figure 4. A Hamiltonian
involving interactions between nonadjacent ions was not used
to avoid overparametrization of the fit. Two sets of param-
eters gave a good fit of the data, both of which predicted
dominant antiferromagnetic coupling leading to an S ) 0
ground state taking place within the Mn1-2 and Mn3-4 pairs,
while giving either weak ferro- or antiferromagnetic coupling
for the interaction between Mn2 and Mn3. The fit that
produced a better error factor was the second, yielding the
parameters of J1 ) -1.13 cm-1, J2 ) -0.43 cm-1, and g )
2.08.43 The value of g is only slightly larger than expected.
However, this value must be taken with caution, since this
parameter is the most affected by errors in the molecular
weight used for treatment of the magnetic data. These
parameters confirm that the coupling is very weak. This is
due to a large extent to the distortion in coordination
environment of the MnII ions, which reduces the efficiency
of the overlap between magnetic orbitals. The coupling
described by J1 has the same sign as those of previously
reported systems showing the same type of connection
between MnII ions (one acetate and two alkoxide ligands).44,45
The Mn2‚‚‚Mn3 pair is connected only by two alkoxide
oxygen atoms, and thus, few magnetic orbitals participate
in the exchange, which together with the distortion of the
coordination geometry, leads to even weaker coupling. The
systems in the literature exhibiting this coupled moiety have
shown either type of interaction and are always very
weak.29,46-49
X-band EPR spectra of complex 3 collected at several
temperatures are shown in Figure 7. At low temperature (5
K) the spectrum is asymmetric and shows multiple compo-
nents hidden under a broad envelope, in particular a shoulder
appearing on the low-field side of the spectrum. This feature
consistently reappears at high frequencies (not shown) and
is thus not an artifact. Upon experiencing an increase in
temperature, the spectrum becomes progressively symmetric
and the line width becomes smaller, assuming a value of
about 300 mT at room temperature. However, even at that
temperature, it cannot be simulated using a single Gaussian-
or Lorentz-shaped curve. The above EPR characteristics are
in very good agreement with the magnetization measure-
ments. In particular, EPR qualitatively confirms the small
values of the exchange constants Ji, since even at the lowest
conveniently reachable temperature (5 K) there is a signifi-
Figure 7. X-band (9.717 GHz) EPR spectra of [Mn4(H2L)2(AcO)2(py)5]
(3) at varying temperatures. Microwave power, 0.2 W (-30 dB); modulation
amplitude, 10 mT. The narrow line at 178 mT originates from the dielectric
resonator.
Figure 8. øMT vs T plot of [Mn3(HL)2(py)6] (5). The solid line is a fit to
the experimental data. The inset displays isofield reduced magnetization
lines at different fields.
cant population of the thermally accessible paramagnetic (S
> 0) excited states. The small Ji values result in a loss of
resolution and an inability to spectrally separate the particular
states, although the presence of those states can be deduced
from the traces of fine structure, in particular, the low-field
shoulder observable at low temperatures at any frequency.
The temperature behavior of the EPR spectrum closely
follows that observed for the magnetic susceptibility, with
the intensity of the EPR absorption rising from low temper-
ature to about 100-120 K and then gradually falling off due
to the Curie law. Finally, the g value of the room-temperature
line is 2.05, which also confirms the magnetic data. High-
frequency and field electron paramagnetic resonance (HFEPR)
experiments (not shown) have not succeeded in increasing
the spectral resolution; thus, only qualitative observations
can be reported here.
The plot of øMT vs T for complex 5, measured at 0.3 T in
the 2-300 K range, is shown in Figure 8 and indicates again
the predominance of intramolecular antiferromagnetic inter-
actions. The value of øMT at room temperature (9.8 cm3 K
mol-1) is slightly lower than expected for two uncoupled
MnIII ions (S ) 2) and one independent MnII center (S )
5/2) with average g ) 2 (10.4 cm3 K mol-1) and diminishes
upon cooling to 4.2 cm3 K mol-1 at 2 K. The extent of the
antiferromagnetic exchange was assessed by diagonalization
using the program CLUMAG,42 with the exchange model
described by the spin Hamiltonian H ) -2J1S1S2 - 2J2S2S3,
where S1 ) S2 ) 2 and S3 ) 5/2. The best fit (Figure 8, solid
line) was achieved for J1 ) -5.4 cm-1, J2 ) -0.4 cm-1,
(42) Gatteschi, D.; Pardi, L. Gazz. Chim. Ital. 1993, 123, 231.
(43) In the previous communication the coupling constants were mistakenly
reported using the H ) -ΣJS1S2 convention.
(44) Brooker, S.; McKee, V.; Metcalfe, T. Inorg. Chim. Acta 1996, 246,
171-179.
(45) Zhong, Z. J.; You, X. Z. Polyhedron 1994, 13, 2157-2161.
(46) Zhang, Y. Z.; Wei, H. Y.; Pan, T.; Wang, Z. M.; Chen, Z. D.; Gao,
S. Angew. Chem., Int. Ed. 2005, 44, 5841-5846.
(47) Mikuriya, M.; Nakadera, K.; Tokii, T. Inorg. Chim. Acta 1992, 194,
129-131.
(48) Luneau, D.; Savariault, J. M.; Cassoux, P.; Tuchagues, J. P. J. Chem.
Soc., Dalton Trans. 1988, 1225-1235.
(49) Chang, H. R.; Larsen, S. K.; Boyd, P. D. W.; Pierpont, C. G.;
Hendrickson, D. N. J. Am. Chem. Soc. 1988, 110, 4565-4576.
2526 Inorganic Chemistry, Vol. 46, No. 7, 2007

NoWel Linear Transition Metal Clusters
and g ) 2.01. A previously reported dinuclear complex with
two MnIII ions bridged in a manner similar to that found in
3 exhibited a coupling constant of -1.48 cm-1.50 Most likely,
the MnIII‚‚‚MnIII exchange occurs through the π system of
the bridging ligands. The fact that the coupling through the
MnIII‚‚‚MnII pathway (J2) is 1 order of magnitude smaller
than J1 is probably caused by the disruption of the conjuga-
tion occurring along the bridging chain of atoms (because
of the rotation around one of the C-C bonds within this
chain). The combination of exchange constants is conducive
to a total spin ground state of ST ) 5/2, with the first excited
3
state (ST ) /2) located 10 cm-1 greater in energy. This is
consistent with the topology of the cluster; all previous linear
clusters with the same distribution of metal oxidation states
are symmetric with a central MnII; consequently, in these
complexes there is only one type of coupling constant
between adjacent metals, in contrast with complex 3. If this
constant is ferromagnetic,51 the compound has ST ) 13/2,
Figure 9. Representative X-band (9.722 GHz) EPR spectra of
[Mn3(HL)2(py)6] (5) at varying temperatures. Experimental conditions are
as in Figure 7.
3
whereas for antiferromagnetic cases, ST ) /2.52 Exceptions
to this can occur if the interaction between the terminal Mn
ions is not negligible and competes with the interaction
between first neighbors, resulting in a different ground state.53
Complex 5 is the first linear mixed-valence MnII/III trimer
with the asymmetric topology MnIII-MnIII-MnII and is
magnetic susceptibility at the highest frequency used (1340
Hz) down to 2 K.
X-band EPR spectra of complex 5, shown in Figure 9,
are remarkably different from those of complex 3 and display
a rich structure at any temperature (at low temperatures (<20
K) the spectra are distorted by fast-passage effects). With
the help of preliminary HFEPR experiments (not shown),
that structure can be unambiguously ascribed to the zero-
5
therefore the only one leading to an ST ) /2 ground state,
which is expected here if both interactions are antiferromag-
netic. This ground state is in agreement with the results from
reduced magnetization measurements (Figure 8, inset), which
were performed at variable temperature for various fixed
magnetic fields. The curve at the highest field saturates for
a value near 5, which is the expected situation for this ground
state. The various isofield lines are not exactly superimpos-
able, a situation which may be caused by the presence of
zero field splitting (ZFS). This should not be surprising, since
this molecule contains two MnIII ions with mutually parallel
Jahn-Teller axes, a situation known to favor single-molecule
magnet behavior.54 Reduced magnetization measurements at
constant temperature (2 K) and variable field were performed
and the curve was fit (Supporting Information, Figure S3)
through a numerical procedure based on the full diagonal-
ization of the spin Hamiltonian H ) âBgST + D(STz2 - ST-
(ST + °1)/3) for ST ) 5/2 (the remaining terms have the usual
values). This procedure yielded the parameters of g ) 2.03
and D ) +0.4 cm-1. The appropriateness of the model used
for the fitting procedure is supported by the fact that at 2 K
and zero field, 99.9% of the molecules lie at the spin ground
state. As expected for a positive value of ZFS, ac susceptibil-
ity measurements yielded no out-of-phase signal of the
5
field splitting of the ground ST ) /2 state of the complex.
Simulations established the value of the D parameter as
+0.085(5) cm-1, i.e., typical for mononuclear Mn(II) com-
plexes in the conditions of low symmetry.55 This value is,
almost by a factor of 5, smaller than that obtained from
magnetization data (+0.4 cm-1). EPR, being a spectroscopic
technique, is more accurate than bulk magnetization mea-
surements, and it therefore provides more reliable parameters.
The ZFS tensor may be rhombic, although the rhombic
parameter E used in the simulation of the 20 K spectrum
(0.01 cm-1) needs to be re-evaluated at high frequencies.
HFEPR also shows other well-defined thermally accessible
states that are not observable at low frequencies. It appears
that these states are of non-Kramers (integer-spin number)
nature, unlike the spin state responsible for the X-band
spectra shown in Figure 9. Because of the richness and
complexity of HFEPR spectra and phenomena observable
in complex 5 in high-frequency and high-field conditions as
well as the preliminary status of the HFEPR investigation,
we will present those results in a subsequent paper. For the
purpose of the current work we may stress the confirmation
of the magnetometric results by EPR in terms of the nature
of the ground spin state (ST ) 5/2) and the presence of sizable
zero-field splitting in that state. The appearance of non-
Kramers state(s) in the spectra at elevated temperatures can
be tentatively explained by the very small coupling constant
J2 between the Mn(II) ion and the two Mn(III) moieties. In
effect, this means that complex 5 consists, magnetically
(50) Arom´ı, G.; Gamez, P.; Roubeau, O.; Berzal, P. C.; Kooijman, H.;
Spek, A. L.; Driessen, W. L.; Reedijk, J. Inorg. Chem. 2002, 41, 3673-
3683.
(51) Scott, R. T. W.; Parsons, S.; Murugesu, M.; Wernsdorfer, W.; Christou,
G.; Brechin, E. K. Chem. Commun. 2005, 2083-2085.
(52) Tangoulis, V.; Malamatari, D. A.; Soulti, K.; Stergiou, V.; Rapto-
poulou, C. P.; Terzis, A.; Kabanos, T. A.; Kessissoglou, D. P. Inorg.
Chem. 1996, 35, 4974-4983.
(53) Birkelbach, F.; Florke, U.; Haupt, H. J.; Butzlaff, C.; Trautwein, A.
X.; Wieghardt, K.; Chaudhuri, P. Inorg. Chem. 1998, 37, 2000-2008.
(54) Miyakasa, H.; Cle´rac, R.; Wernsdorfer, W.; Lecren, L.; Bonhomme,
C.; K., S.; Yamashita, M. Angew. Chem., Int. Ed. 2004, 43, 2801-
2805.
(55) Krzystek, J.; Ozarowski, A.; Telser, J. Coord. Chem. ReV. 2006, 250,
2308-2324.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2527

Arom´ı et al.
followed by the addition of 1 M HCl until it acquired an
approximate pH ) 1. At this point, the solid was completely
dissolved and the organic yellow phase was decanted, dried over
Na2SO4, and rotaevaporated. The resulting yellow-green solid was
suspended in acetone (40 mL), and the mixture was boiled for a
few minutes. The system was allowed to reach room temperature,
and the remaining solid was collected by filtration and washed with
a small amount of acetone. The yield was 27% (4.3 g). Anal. Calcd
(Found) for H4L′: C, 69.44 (69.06); H, 4.66 (4.16). ESI MS (>0):
m/z 455 [M + Na]+, 433 [MH]+, 415 [M - (OH-)]+. FAB MS
speaking, of two halves, one consisting of the Mn(II) moiety
and the other of the two Mn(III) ions and their coordination
spheres.
Conclusions
In the absence of any other base, the reactivity of the novel
heptadentate bis-â-diketone ligand H5L with various transi-
tion metal M(AcO)2 salts is very dependent on the nature of
the metal. With M ) NiII and CoII, the reaction in pyridine
leads to dinuclear complexes [Ni2(H3L)2(py)4] (1) and [Co2-
(H3L)2(py)4] (2), with the metals within the molecule widely
separated from each other (near 8 Å). By contrast, with Mn-
(AcO)2 the product is a very rare linear [Mn4] cluster with
a zigzag configuration and formula of [Mn4(H2L)2(AcO)2-
(py)5] (3), presumably because MnII is subject to less crystal
field constraints to distort its coordination geometry. The
same reaction in DMF leads to the analogous cluster [Mn4-
(H2L)2(AcO)2(dmf)4] (4), indicating that this arrangement is
a robust topology. Complex 3 was found to slowly oxidize
and recrystallize in the form of the mixed-valent complex
[Mn3(HL)2(py)6] (5), which displays a unique asymmetric
MnIII-MnIII-MnII linear sequence. Compound 5 can also
be prepared directly by comproportionation from the ap-
propriate amounts of MnVII and MnII in the presence of H5L.
The paramagnetic ions within 3 and 5 were found, by means
of bulk magnetic measurements, to weakly interact as
described by small antiferromagnetic coupling constants. The
polydentate ligand H5L thus constitutes a suitable scaffold
for the preparation of novel linear transition metals in a
variety of topologies and nuclearities.
1
(>0): m/z 433 [MH]+, 401 [MH- OMe]+. H NMR (CDCl3): δ
(ppm) ) 3.87 (s, 3H, CH3), 6.95 (t, 2H, CaroH), 7.03 (d, 2H, CaroH),
7.15 (s, 2H, CH), 7.37 (t, 1H, CaroH), 7.50 (t, 2H, CaroH), 7.78 (d,
2H, CaroH), 7.97 (d, 2H, CaroH), 12.04 (s, 2H, Caro-OH), 15.52 (s,
2H, CenolOH).
1,3-Bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxy-
benzene (H5L). A round-bottomed flask containing H4L′ (400 mg,
0.96 mmol) and CH2Cl2 (20 mL) was purged with argon and cooled
with an ice bath. To this mixture was added very carefully a 1 M
solution of BBr3 in MeOH (1.5 mL, 1.5 mmol) with a syringe. An
orange precipitate started to form immediately. The mixture was
stirred vigorously for 3 h while the ice bath melted. The mixture
was further washed with CH2Cl2 and dumped into an ice bath
containing an aqueous solution of NaCl. After the whole system
was stirred for another 4 h, the two phases formed were separated
and the organic part was rotaevaporated to obtain a dark glassy
film. This film was dissolved in the minimum amount of CH2Cl2,
to the solution was added MeOH (20 mL), and the mixture was
left to stand at 4 °C. After 2 days, yellow crystals were collected
by filtration. The yield was 40%. Anal. Calcd (Found) for H5L
(C24H18O7): C, 68.90 (69.06); H, 4.34 (4.16). ESI MS (>0): m/z
1
419 [MH]+, 401 [M - (OH-)]+. H NMR (CDCl3): δ (ppm) )
4.72 (s, 2H, CH2), 6.92 (t, 1H, CaroH), 6.98 (t, 1H, CaroH), 6.99 (d,
1H, CaroH), 7.04 (d, 1H, CaroH), 7.10 (t, 1H, CaroH), 7.46 (t, 1H,
Experimental Section
CaroH), 7.54 (s, 1H, CH), 7.55 (t, 1H, CaroH), 7.79 (d, 2H, CaroH),
Syntheses. All reactions were performed in air and using solvents
7.96 (d, 1H, CaroH), 8.31 (d, 1H, CaroH), 11.77 (s, 1H, CaroOH),
12.03 (s, 1H, CaroOH), 13.32 (s, 1H, CaroOH), 15.56 (s, 1H,
and reagents as received. NBun MnO4 was prepared according to
4
a procedure reported in the literature.56 Warning! Quaternary
ammonium permanganates may detonate during drying at eleVated
temperatures. We therefore recommend using them only in small
amounts and that the heating of them be aVoided.
C
enolOH).
[Ni2(H3L)2(py)4] (1). Solid Ni(AcO)2‚4H2O (32 mg, 0.13 mmol)
and H5L (26 mg, 0.06 mmol) were mixed in pyridine (5 mL) and
stirred for a few minutes. The green solution was layered with ether,
and after a few days, green crystals suitable for X-ray crystal-
lography had settled, which were then collected by filtration, washed
with ether, and dried in air. The yield was 40%. Anal. Calcd (Found)
for 1‚py (C73H57N5O14Ni2): C, 65.16 (64.87); H, 4.27 (4.45); N,
5.20 (5.39).
1,3-Bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxy-
benzene (H4L′). Liquid dimethyl 2-methoxyisophthalate (8.40 g,
37.46 mmol) and liquid 2-hydroxyacetophenone (14 mL, 112.38
mmol) were dissolved under argon in anhydrous dimethoxyethane
(DME, 200 mL). In a different flask, 60% oil dispersion of NaH
(14 g, 360 mmol) was washed twice under argon with hexane and
the solvent extracted by means of a filter cannula. To this solid
was added anhydrous DME (50 mL) under an inert atmosphere,
and the resulting suspension was added carefully with a syringe
onto the above mixture, the color turning bright yellow immediately
(CAUTION!! As a result of hydrogen eVolution, heaVy bubbling
occurs upon addition of the NaH suspension; thus, this addition
needs to be carried out with extreme caution). After the addition
was complete, the mixture was brought to reflux as it gradually
turned orange, and this was maintained overnight. Approximately
after 24 h, the mixture was allowed to cool to room temperature
and to come into contact with the atmosphere. An abundant yellow
precipitate was collected by filtration and washed with DME. This
solid was suspended in a biphasic system of EtAcO and water,
[Co2(H3L)2(py)4] (2). Solid Co(AcO)2‚4H2O (32 mg, 0.13 mmol)
and H5L (26 mg, 0.06 mmol) were mixed in pyridine (5 mL) and
stirred for a few minutes. The orange solution was layered with
ether, and after a few days, orange crystals suitable for X-ray
crystallography had grown, which were then collected by filtration,
washed with ether, and dried in air. The yield was 34%. Anal. Calcd
(Found) for 2 (C68H52N4O14Co2): C, 64.46 (64.20); H, 4.14 (4.15);
N, 4.42 (4.98).
[Mn4(H2L)2(AcO)2(py)5] (3). Solid Mn(AcO)2‚4H2O (32 mg,
0.13 mmol) and H5L (26 mg, 0.06 mmol) were mixed in pyridine
(10 mL) and stirred for a few minutes. The orange solution was
layered with ether, and after a few days, orange crystals suitable
for X-ray crystallography were deposited, which were then collected
by filtration, washed with ether, and dried in air. The yield was
30%. Elemental analysis reveals partial substitution of pyridine by
water upon exposure to air. Anal. Calcd (Found) for 1 (-1.5py +
(56) Vincent, J. B.; Folting, K.; Huffman, J. C.; Christou, G. Inorg. Chem.
1986, 25, 996.
2528 Inorganic Chemistry, Vol. 46, No. 7, 2007

NoWel Linear Transition Metal Clusters
1.5H2O); (C69.5H56.5N3.5O19.5Mn4): C, 56.69 (56.86); H, 3.87 (3.68);
N, 3.33 (3.09).
Tables for Crystallography.61 Validation, geometrical calculations,
and illustrations were performed with PLATON.60
For complexes 2, 4, and 5, data were collected at 150 K (2 and
5) or 180 K (4) using a Bruker APEX II CCD diffractometer on
station 9.8 (4 and 5) or 16.2 smx (2) of the Synchrotron Radiation
Source at CCLRC Daresbury Laboratory, at 0.8464 (2), 0.6711 (4),
and 0.6894 (5) Å, from a Silicon 111 monochromator. The structure
was solved by direct methods and refined using the SHELXTL62
suite of programs. All non-H atoms of complex 4 (except those in
a solvent molecule) are anisotropic. Hydrogens were placed
geometrically where possible; for the CH3 groups, the hydrogens
were found in the rotational difference map. For the OH, the
hydrogen was placed to form the best hydrogen bonds and then
refined using a riding model. The H atoms on the water molecule
can neither be geometrically placed nor found in the difference
map and were therefore omitted. For complex 5, all non-hydrogen
atoms were modeled isotropically except the partial water molecule
and the very disordered pyridine site. The hydrogens were placed
geometrically where possible with the exception of the two hydroxyl
groups where the hydrogens were found in the difference map, and
those of the partial water which could not be placed or found in
the difference map were therefore omitted from the refinement.
All hydrogens were then constrained and refined using a riding
model. Both geometrical and displacement parameter restrains were
used in the modeling of the disordered pyridine. A peak of 2.9 e-
remains in the different map at 1.2 Å from the one on the Mn atoms,
which is in an unreasonable position for sensible modeling.
Inspection of the final refinement does indicate that there is probably
a small degree of twinning present; e.g., K values in analysis of
[Mn4(H2L)2(AcO)2(dmf)4] (4). The same procedure as above
was carried out in DMF instead of pyridine. Orange crystals of 4
were obtained from the DMF/Et2O layers after a few days. The
yield was 26%. Anal. Calcd (Found) for 4 (C64H64N4O22Mn4): C,
52.62 (52.23); H, 4.42 (4.52); N, 3.83 (3.85).
[Mn3(HL)2(py)6] (5). Method 1. The py/Et2O layers leading to
complex 3 were left unperturbed for about a month. During this
time, the orange crystals of 3 slowly redissolved with concomitant
recrystallization of dark brown crystals of 5. At the end of that
period, all orange crystals had disappeared, the only solid present
in the system being the brown crystals. These were suitable for
X-ray crystallography (synchrotron radiation source). This product
was collected by filtration. The yield was 25%. Anal. Calcd (Found)
for 5‚py (C83H63N7O14Mn3): C, 64.43 (64.20); H, 4.09 (4.09); N,
6.34 (6.44).
Method 2. Solid Bu4NMnO4 (9 mg, 0.03 mmol) and Mn(AcO)2‚
4H2O (40 mg, 0.16 mmol) were mixed and dissolved in pyridine
(7 mL) to produce a dark brown solution. To this mixture was added
a solution of H5L (52 mg, 0.13 mmol) in pyridine (13 mL). The
resulting brown solution was stirred for about 30 min and layered
with Et2O. After a week, crystals of 5 had deposited, which were
collected by filtration. The yield was 24%.
Physical Measurements. Field-cooled measurements of the
magnetization of smoothly powdered microcrystalline samples of
3 and 5 were performed in the range of 2-300 K with a Quantum
Design MPMS-5XL SQUID magnetometer with an applied field
of 5 kG. Corrections for diamagnetic contributions of the sample
holder to the measured magnetization and of the sample to the
magnetic susceptibility were performed experimentally and by using
Pascal’s constants, respectively. Measurements under an ac mag-
netic field in the 2-40 K range at 1339 and 107 Hz were also
performed. IR spectra were recorded as KBr pellet samples on a
Nicolet 5700 FT-IR spectrometer. Elemental analyses were per-
formed in-house on a Perkin-Elmer Series II CHNS/O Analyzer
2400, at the Servei de Microana`lisi of CSIC, Barcelona, Spain.
X-ray Crystallography. Pertinent data for the structure deter-
minations are given in Table 1. Data of H5L, 1, and 3 were collected
on a Nonius KappaCCD diffractometer on rotating anode (graphite-
monochromated Mo KR radiation, λ ) 0.71073 Å). Data were
collected at 150 K. The structures were solved with direct methods
using SHELXS8657 (compounds H5L and 1) or SHELXS9758 for
structure 3. Refinement on F2 was performed with SHELXL-97.59
The hydroxyl hydrogen atoms of 1 and 3 and all hydrogen atoms
of H5L were located on a difference-Fourier map, and their
coordinates were included as parameters in the refinement; all other
hydrogen atoms were included on calculated positions riding on
their carrier atoms. Structure 3 showed a large area of disordered
solvent for which no satisfactory atomic model could be obtained.
The contribution of the disordered solvent to the scattered intensity
was taken into account with the SQUEEZE procedure, as incor-
porated in PLATON.60 Neutral atom scattering factors and anoma-
lous dispersion corrections were taken from the International
variance for reflection are higher than 1 for Fc/Fmax and F2 is
obs
larger than F2 for the weak data, where no suitable twin law
calc
could be found. This twinning probably accounts for the 2.9 e-
peak left in the difference map.
Electron Paramagnetic Resonance. The X-band EPR spec-
trometer was a commercial Bruker ElexSys E680X product,
equipped with an Oxford Instruments helium-flow cryostat and a
dielectric FlexLine resonator. The HFEPR instrument was a home-
built device, described elsewhere.63 The latter instrument operates
in transmission mode and employs no resonator.
Acknowledgment. This work was supported by Spanish
Ministerio de Ciencia y Tecnolog´ıa through Grant No. BQU
2003/00539 and a “Ramo´n y Cajal” contract (G.A.) by the
Council for the Chemical Sciences (A.L.S.) of The Nether-
lands Organization for Scientific Research (CW-NWO). We
acknowledge the provision of time at the CCLRC Daresbury
Laboratory via the support by the European Union. EPR
studies were supported by the National High Magnetic Field
Laboratory, which is funded by the National Science
Foundation through Cooperative Agreement DMR 0084173,
the State of Florida, and the Department of Energy.
Supporting Information Available: CIF file; PDF file contain-
ing additional information including Figures S1-S3. This material
is available free of charge via the Internet at http://pubs.acs.org.
(57) Sheldrick, G. M. SHELXS86; University of Go¨ttingen: Go¨ttingen
Germany, 1986.
(58) Sheldrick, G. M. SHELXS97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
IC062075V
(61) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1992; Vol C.
(59) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(60) Spek, A. L. J. Appl. Cryst. 2003, 36, 7-13.
(62) Bruker AXS: Madison, WI, 2005.
(63) Hassan, A. K.; Pardi, L. A.; Krzystek, J.; Sienkiewicz, A.; Goy, P.;
Rohrer, M.; Brunel, L. C. J. Magn. Reson. 2000, 142, 300-312.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2529