A zig-zag [MnII4] cluster from a novel bis(β-diketonate) ligand

Article abstract of DOI:10.1002/ejic.200600105

A new ligand, H₅L3, has been synthesized featuring seven linearly arranged oxygen donors in form of two 1,3-diketone and three phenol groups. The X-ray structure of H₅L3 unveils a rare case where one of the diketones is in the enolic

Full text of DOI:10.1002/ejic.200600105

SHORT COMMUNICATION
DOI: 10.1002/ejic.200600105
A Zig-Zag [MnII ] Cluster from a Novel Bis(β-diketonate) Ligand
4
Guillem Aromí,*[a] Patrick Gamez,[b] Christophe Boldron,[b] Huub Kooijman,[c]
Anthony L. Spek,[c] and Jan Reedijk[b]
Keywords: Ligand design / O ligands / NMR spectroscopy / Magnetism / Mn
A new ligand, H5L3, has been synthesized featuring seven
linearly arranged oxygen donors in form of two 1,3-diketone
and three phenol groups. The X-ray structure of H5L3 unveils
a rare case where one of the diketones is in the enolic form
and the other one in the bis(carbonyl) form. This structure is
shown by 1H NMR to persist in solution. Reaction of H5L3
with Mn(AcO)2 in pyridine leads to the novel tetranuclear
cluster [Mn4(H2L3)2(OAc)2(py)5] (1), which displays an un-
usual core in form of a zig-zag chain. Bulk magnetic mea-
surements revealed the existence of weak antiferromagnetic
coupling within the molecule. Numerical fits to a model de-
scribed by the Hamiltonian H = –2J1(S1S2 + S3S4) – 2J2(S2S3)
yield coupling constants of J1 = –2.23 cm–1, J2 = –0.85 cm–1
and g = 2.08.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
The preparation of molecular cluster complexes of inter- Scheme 1),[15,16] aimed at forming molecular strings of
acting transition-metal ions is useful to areas as important metal ions in close proximity. Maximum occupancy of their
and diverse as molecular magnetism,[1,2] homogeneous ca- coordination pockets would result in tetranuclear clusters
talysis[3] or bioinorganic chemistry.[4,5] A very successful ap- in form of [M4] chains[17] for H3L1 and aligned [M2]2 di-
proach for accessing such species has been the assembly of mers of dimers for H4L2. Reactions of the latter with
metal ions by means of small bridging ligands, such as car- M(OAc)2 salts have resulted invariably in the formation of
boxylates, where growth into infinite arrays is often pre- complexes of the type [M2(H2L2)2(S)n] (n = 2 or 4; S =
vented by additional terminal ligands.[6,7] Such a method solvent; M = Mn, Ni, Co, Cu).[16] Similar dinuclear com-
has been sometimes termed “serendipitous approach” since pounds have been obtained with H3L1.[18] However, in the
it does not allow prediction of the structure of the final absence of a coordinating solvent, molecules exhibiting
species.[8] An alternative strategy has been the design and higher occupancy, [Co3(HL1)3] and [Mn3(HL1)3], were
preparation of complicated multidentate ligands favoring characterized, which represented a new asymmetric top-
the assembly of metal ions into aggregates with topologies ology within the context of coordination helicates.[19] In-
that may be anticipated from the structure of these ligands. spired by these early results, we have now designed and syn-
This second course of action has led to the production of a thesized a new ligand displaying as many as six adjacent
large amount of polynuclear edifices displaying sophisti- coordination pockets (H5L3, Scheme 1). Reaction of H5L3
cated shapes and architectures, such as helicates,[9] grids,[10] with MnII produces a very rare magnetically exchanged tet-
cylinders,[11] catenates,[12] and many more. Of these molecu- ranuclear complex featuring an [Mn4O6] core in form of a
lar species, only a minority have the metal ions disposed in zig-zag chain.
close proximity so as to show cooperative effects resulting
from strong magnetic-exchange interactions[13] or metal–
metal bonding.[14] We have been interested in preparing new
ligands incorporating various β-diketone units and other
donor groups in a linear fashion (H3L1 and H4L2 in
The new ligand H5L3 {2-hydroxy-1,3-bis[3-(2-hydroxy-
phenyl)-3-oxopropionyl]benzene} was prepared from the
methoxide derivative H4L4 (Scheme 1).[20] The latter is
also a new molecule not described in the literature and con-
stitutes a bis(β-diketone) with potentially interesting coor-
dination chemistry. The NMR spectra (1H and 13C) of
[a] Departament de Química Inorgànica, Universitat de Barcelona, H5L3 (Figure 1) do not reflect the symmetry presumed for
Diagonal 647, 08028 Barcelona, Spain
this molecule in Scheme 1. The crystal structure of the com-
Fax: +34-93-490-7725
pound (Figure 1),[21] obtained following its recrystallization
E-mail: guillem.aromi@qi.ub.es
[b] Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden
University,
P. O. Box 9502, 2300 RA Leiden, The Netherlands
[c] Bijvoet Center for Biomolecular Research, Crystal and Struc-
tural Chemistry, Utrecht University,
from MeOH, threw light into the solution structure respon-
1
sible of its H NMR pattern, allowing its assignment, with
help of a COSY spectrum (see Supporting Information). In
fact, the molecule of H5L3 crystallizes with one of its 1,3-
diketone groups in its enolic form and the other one in the
bis(carbonyl) form, constituting a very rare example where
Padualaan 8, 3584 CH Utrecht, The Netherlands
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
1940
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 1940–1944

A Zig-Zag [MnII4] Cluster from a Novel Bis(β-diketonate) Ligand
SHORT COMMUNICATION
Scheme 1.
both forms are present simultaneously in one molecule. The
only such previous example featured by the Cambridge data
base is a cyclic polyketide lactone where the strain of the
macrocycle is forcing the formation of the mixed tauto-
mer.[22] In H5L3, the simultaneous occurrence of both
forms may be favored by the formation of five intramolecu-
lar [O–H···O] hydrogen bonds, which are all part of π-delo-
calized systems. The structural parameters within these de-
localized systems follow indeed the criteria of resonance-
assisted hydrogen bonding (RAHB).[23] According to this
theory, intramolecular hydrogen bonds within β-diketone
enols are reinforced by the existence of π-delocalization
within the heterodienic O=C–C–C=O–H chain, translating
into abnormally short O···O distances. The same phenome-
non occurs, although less pronounced, within 2-hydroxy-
benzoketones, such as these present in H5L3, and this trend
is clearly observed on the crystal structure of the molecule.
RAHB predicts a good correlation between d(O···O) and
the 1H NMR chemical shift of the proton concerned by the
H-bond, and this criterion was used to assign this class of
protons in H5L3 (Figure 1).
Figure 1. Top: ORTEP representation at the 50% probability level
of the ligand H5L3. Only the oxygen atoms are labeled. Intramolec-
ular hydrogen bonds are shown as dashed lines. H-bond [O–H···O]
distances [Å]: 2.578(2) (O1,O2), 2.533(2) (O3,O4), 2.488(2)
The reaction of H5L3 with 2 equiv. of Mn(OAc)2·H2O in
pyridine affords a dark orange solution that produces
orange crystals upon layering with Et2O. This product
was identified by single-crystal X-ray diffraction to be
[Mn4(H2L3)2(OAc)2(py)5] (1).[24] The structure of 1 (Fig-
ures 2 and 3)[21] reveals a linear tetranuclear array of MnII,
assembled by the action of two triply deprotonated µ4-
(H2L3)3– ligands located face-to-face, through four adjacent
1
(O5,O6), 2.536(2) (O6,O7). Bottom: 300 MHz H NMR spectrum
of H5L3 in CDCl3 reflecting the symmetry of the solid state. The
assignment of the non-aromatic protons is shown (see text for de-
tails). Full assignments are given in the Supporting Information.
chelating units per side constituted by the five central oxy- accommodate the four metal ions in a linear array with the
gen donors of each ligand. All chelating units form six- chelating units disposed equatorially (i.e. trans). The result
membered rings. The peripheral ones, formed by the 1,3- is an [Mn(µ-O)2Mn(µ-O)2Mn(µ-O)2Mn] chain with a very
diketonate units, deviate significantly from planarity unusual “zig-zag” shape. In complex 1, each peripheral pair
[Cremer and Pople puckering amplitude in the range of metal ions is further bridged by one µ-OAc– ligand, while
0.0756(13)–0.2321(15) Å],[25] although not as much as the pyridine ligands complete the six- (Mn1, Mn2 and Mn4)
central ones, involving the phenoxide moiety [puckering or seven-donor (Mn3) coordination spheres present in this
amplitudes in the range 0.3804(15)–0.8351(16) Å]. The lat- system. Thus, the external Mn2 pairs are bridged by one
ter can be described as in a distorted boat, twist-boat or syn,syn-OAc and two alkoxide-type µ-O donors, while the
screw-boat conformation. The ionizable protons remaining central Mn2 is linked by just two phenoxide-type bridges.
on each (H2L3)3– moiety are those of the peripheral neutral The bond lengths around the metal ions range from
phenol groups and are involved in internal hydrogen bonds, 2.066(1) to 2.488(2) Å, within the limits expected for MnII
as seen with the free ligand. Interestingly, each chelating ions (ranges for each type of bond are listed in the caption
pocket binds a metal ion disposed in cis configuration with of Figure 2). The average nearest and second-nearest
respect to its counterpart from the other ligand. This is pre- neighbor Mn···Mn separations are 3.378 and 5.826 Å,
sumably because the ionic radius of MnII is too large to respectively.
Eur. J. Inorg. Chem. 2006, 1940–1944
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1941

G. Aromí, P. Gamez, C. Boldron, H. Kooijman, A. L. Spek, J. Reedijk
SHORT COMMUNICATION
1 is the first example of the trans form as forced presumably
by the structure of the (H2L3)3– ligand.
Scheme 2.
The magnetic exchange within complex 1 was examined
by means of variable-temperature (2–300 K) bulk-magne-
tization measurements under a constant magnetic field of
0.3 T. The results are given in Figure 4 in form of a χMT vs.
T plot. The product χMT at 300 K is 18.43 cm3·K·mol–1
(quite near to the value of 17.5 cm3·K·mol–1 for four inde-
pendent high-spin MnII centers with g = 2), and it decreases
slowly and then more rapidly near 70 K to reach a value
of 1.22 cm3·K·mol–1 at 2 K. This indicates the presence of
dominant antiferromagnetic interactions within the cluster.
A fit of the experimental data was obtained through a ma-
trix diagonalization procedure using the program CLU-
MAG,[33] by considering the Heisenberg spin Hamiltonian
H = –2J1(S1S2 + S3S4) – 2J2(S2S3), where Si is the spin
angular momentum operator of Mni (Si = 5/2) and the in-
teraction between the metal ions of both external Mn2 pairs
was assumed to be equal (see inset in Figure 4). Good fits
were obtained for two sets of solutions. One was reached
for values J1 = –1.37 cm–1, J2 = +0.77 cm–1 and g = 2.06,
which leads to a total spin ground state of ST = 0. Since J1
Ͼ J2, a very similar behavior is expected if J2 were instead
negative but with comparable absolute value. This turns out
to be the case if the fit is performed by restricting both
coupling constants to be antiferromagnetic, yielding the pa-
rameters J1 = –2.23 cm–1, J2 = –0.85 cm–1 and g = 2.08. The
latter fit is preferred since it yields a smaller error factor R
(R = Σ[χMTexp – χMTcalc]2/Σ[χMTexp]2) than the first fit (R =
7.73·10–5 vs. 1.62·10–4). The value of g is similar in both
cases; however, it 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. The bridg-
ing arrangement found within the Mn1–2 and Mn3–4 pairs
(see above) is very uncommon for MnII ions; however, the
few reports showing preliminary magnetic studies on sys-
tems containing this moiety suggest weak antiferromagnetic
coupling as found in 1.[34,35] The central [Mn2O2] unit on
the other hand, is bridged by two phenoxide-type ligands.
Such a type of link between divalent Mn centers has been
Figure 2. Labeled Pov-ray representation of [Mn4(H2L3)2(AcO)2-
(py)5] (1). Only hydroxy H-atoms are shown. Intramolecular hydro-
gen bonds are shown as dashed lines. Ranges of selected in-
teratomic distances [Å] and angles [°]: Mn–O 2.0661(14) to
2.4031(14), Mn–N 2.2660(18) to 2.488(2), Mn1···Mn2 3.2876(6),
Mn2···Mn3 3.4950(7), Mn3···Mn4 3.3502(6), Mn2···Mn4 5.7526(9),
Mn3···Mn1 5.8984(9), Mn1···Mn4 8.8046, Mn1–Mn2–Mn3
120.80(2), Mn2–Mn3–Mn4 114.35(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).
Figure 3. Platon representation of the core of [Mn4(H2L3)2-
(AcO)2(py)5] (1) emphasizing two different views. Code for atoms:
large, Mn; small hatched, O; shaded, N.
Tetranuclear manganese clusters are interesting from the reported to lead to both, ferro-[31,36–39] and antiferromag-
magnetic point of view,[26] or as synthetic models of the netic interactions.[40–43] The actual magnetic behaviour
dioxygen-evolving complex of photosystem II.[27] Of the depends strongly on the efficiency of the overlap between
dozens of examples existing in the literature only three dx2–y2 orbitals of the metal ions through the phenoxide O-
types consist of linear arrays of fused [Mn2(µ-O)2] moieties donors, which is considered the main pathway to antiferro-
III
as in 1; the [Mn4IVO6(bpy)6]4+ cation,[28,29] the [Mn2
-
magnetism. In 1, the latter is expected to be much reduced,
Mn2II(Me-hmp)6Cl4] cluster [Me-hmpH = 2-(hydroxy- given the weakening of antiferromagnetic pathways that
methyl)-6-methylpyridine],[30] and the cations [Mn4IIL6]2+ might be caused by the severe distortion in the coordination
[HL = a (2-hydroxyphenyl)bipyridine or -phenanthroline li- geometry shown by the metal ions. The coupling appears
gand].[31,32] All of these display a cis conformation with re- thus antiferromagnetic and very weak in any case, as for all
spect to the central [Mn2O2] unit (Scheme 2), while complex previous analogous cases.
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Eur. J. Inorg. Chem. 2006, 1940–1944

A Zig-Zag [MnII4] Cluster from a Novel Bis(β-diketonate) Ligand
SHORT COMMUNICATION
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Figure 4. Plot of χMT vs. T per mol of [Mn4(H2L3)2(AcO)2(py)5]
(1). The solid line is a fit to the experimental data (see text for
details). The inset shows the model and spin coupling scheme used
for the fitting procedure.
[20]
[21]
See Supporting Information for experimental details on or-
ganic synthesis and characterization of H5L3 and H4L4.
Crystals of H5L3 and 1 were measured with a Nonius Kap-
paCCD diffractometer on a rotating anode (θmax = 25.3°, T =
150 K, Mo-Kα radiation, λ = 0.71073 Å, no absorption correc-
tion). The structures were solved with direct methods and re-
fined on F2 (SHELXL-97). Pertinent data for H5L3: C24H18O7,
yellow block-shaped crystal (0.10×0.25×0.35 mm), ortho-
rhombic, space group Pbcn (no. 60) with a = 13.9229(15), b =
8.5263(12), c = 32.190(5) Å, V = 3821.3(9) Å3, Z = 8, 18194
The preparation of H5L3 and its association with MnII
ions have demonstrated that rigid ligands with linearly dis-
posed O-donors do provide access to metallic coordination
chains with interesting magnetic properties, difficult to ob-
tain otherwise. The preparation of 1 offers a very promising
prospect in this direction. We are currently exploring the
coordination chemistry of this new ligand with other metals
and in the presence of other small co-ligands.
reflections measured, 3410 independent, Rint = 0.1106, Rσ
=
Supporting Information (see footnote on the first page of this arti-
0.0664. The structure was solved with SHELXS-86. All hydro-
gen atoms could be located on a difference Fourier map and
their coordinates were refined. Refinement of 334 parameters
converged at a final wR2 value of 0.1254, R1 = 0.0468 [for 2302
reflections with I Ͼ 2σ(I)], S = 1.044, –0.22 Ͻ ∆ρ Ͻ 0.20 e·Å–3.
Pertinent data for 1: C77H61Mn4N5O18+solvent (vide infra),
pale orange blockshaped crystal (0.05×0.18×0.35 mm), tri-
cle): This material includes the description of the preparation of
1
ligands H5L3 and H4L4, full H NMR characterization of the li-
gands and the COSY diagram of H5L3.
Acknowledgments
¯
clinic, space group P1 (no. 2) with a = 10.8441(10), b =
18.591(2), c = 20.904(3) Å, α = 85.172(15), β = 77.893(15), γ =
87.872(15)°, V = 4105.1(9) Å3, Z = 2, 83947 reflections mea-
sured, 14832 independent, Rint = 0.0443, Rσ = 0.0330. The
structure was solved with SHELXL-97. Electron density in a
cavity with a volume of 896 Å3, filled with a disordered mixture
of Et2O and pyridine, was taken into account in the refinement
with PLATON/SQUEEZE. A total number of 321 electrons
per unit cell were found. Where relevant, data cited above are
given without disordered solvent contribution. The coordi-
nated pyridine molecule containing N11 is disordered and is
described with a two-site disorder model. The hydroxy hydro-
gen atoms could be located on a difference Fourier map and
their coordinates were refined. All other hydrogen atoms were
included with calculated positions and riding on their carrier
atoms. Refinement of 967 parameters converged at a final wR2
value of 0.1203, R1 = 0.0407 [for 11825 reflections with I Ͼ
2σ(I)], S = 1.074, –0.44 Ͻ ∆ρ Ͻ 0.59 e·Å–3. CCDC-293439
and -293440 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Elemental analysis reveals partial substitution of pyridine by
water upon exposure to air. Calcd. (found) for 1 (– 1.5py +
1·5H2O): C 56.69 (56.86), H 3.87 (3.68), N 3.33 (3.09).
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This work was supported by grants from the Spanish Ministerio de
Ciencia y Tecnología, the Agència de Gestió d’Ajuts Univesitaris i
de Recerca (Generalitat de Catalunya), the Dutch WFMO (Werk-
groep Fundamenteel-Materialen Onderzoek) and the CW-NWO
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Published Online: April 3, 2006
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