Synthesis and studies of a tetradecanuclear manganese(II)/(III) cage

Article abstract of DOI:10.1039/b205157p

A novel Mn₁₄ cluster is reported; this is a new nuclearity for manganese cages and highly unusual in that the ligands are not exclusively oxygen donors.

Full text of DOI:10.1039/b205157p

Synthesis and studies of a tetradecanuclear manganese(II)/(III) cage
Guillem Aromí,a Aidan Bell,a Simon J. Teat,c A. Gavin Whittakerb and Richard E. P. Winpenny*a
a Department of Chemistry, The University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: richard.winpenny@man.ac.uk
b Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ
c
CCLRC Daresbury Laboratory, Warrington, Cheshire, UK WA4 4AD
Received (in Cambridge, UK) 28th May 2002, Accepted 15th July 2002
First published as an Advance Article on the web 26th July 2002
A novel Mn14 cluster is reported; this is a new nuclearity for
manganese cages and highly unusual in that the ligands are
not exclusively oxygen donors.
further purification steps are necessary.† Deprotonation of
Hppo (320 mg, 2 mmol) with NBu4OH (2 mL of a 1 M
methanolic solution, 2 mmol) in the presence of hydrated
Mn(NO3)2 (500 mg, 2 mmol) in MeCN (40 mL) led to a brown
solution that produced crystals of the title compound in ca. 25%
yield upon diffusion of Et2O into the reaction mixture. These
crystals were suited for X-ray diffraction experiments using a
synchrotron source.
Much of the current interest in the magnetism of polymetallic
cage complexes is based on observations originally made on
manganese cages. The first ‘single molecule magnet’1 (SMM)
discovered was a {Mn12} cage, and this metal features in the
majority of SMMs known to this date.2 Given the excitement of
this chemistry the range of ligands reported to stabilise Mn
cages is surprisingly small, and dominated by oxygen donor
ligands such as carboxylates or b-diketonates.3,4 A rare
exception is a {Mn8} cage featuring amino-pyrimidine ligands.5
Here we report studies of a new manganese cage which has an
unusual nuclearity and structure, and which features a new
bridging ligand.
The ligand used is the anion of 3-phenyl-3-pyrazolin-5-one
(Hppo, see Scheme 1). Hppo is itself straightforwardly made
from reaction of ethyl benzoylacetate (22 g, 0.114 mmol) with
62% hydrazine hydrate solution (9 mL, 0.114 mmol) in
refluxing ethanol (100 mL). The yield of Hppo is 67%, and no
The unit cell contains two independent {Mn14} cages, with
almost identical structures.‡ Each is centro-symmetric, with the
formula [Mn14O2(OH)4(ppo)18(Hppo)4(NO3)4(MeCN)4]
(Fig. 1). The molecule is mixed-valent, containing two Mn(III
1
)
and twelve Mn(II) centres. A possible description of the core
(Fig. 2) is based on {Mn7} fragments, two of which are found in
the asymmetric unit. Each fragment contains a {Mn3O4}
distorted incomplete cubane, lacking one Mn site (Fig. 2). Two
of the O-centres within this unit are hydroxide [O(2), O(3)], and
each of these is bound to one further Mn(II) site [Mn(7) and
Mn(5), respectively]. A single O-atom of the {Mn3O4} is part of
a ppo ligand [O(31)], and it also bridges to one further Mn(II
[Mn(4)]. The last O-atom [O(1)] is an oxide, and this is capped
by two Mn(III) sites [Mn(1) and Mn(1a)]. These two Mn(III
)
)
sites form the shared edge of a bitetrahedron and serve to link
the two {Mn3O4} distorted cubanes. Hexanuclear Mn bitetrahe-
dra have been reported previously.6
The coordination geometries of the Mn sites vary. Mn(1) is
clearly a Mn(III) site, with a Jahn–Teller elongated octahedral
geometry and an O5N coordination sphere. Five of the six
Scheme 1 Hppo (left) and ppo in its 4.31 coordination mode (right).
Fig. 1 SCHAKAL representation of 1 showing the intramolecular hydrogen bondings. Only the ipso carbon atoms of the phenyl rings are shown. Colours:
C, grey; N, blue; O, red; Mn, green.
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This journal is © The Royal Society of Chemistry 2002

Notes and references
† Anal. Calcd (Found) for C9H8N2O: C, 67.47 (67.17); H, 5.04 (5.01); N,
17.50 (17.57)%. 1H NMR (CD3OD): d (ppm) = 7.32 (t, 1H, CaromH), 7.40
(t, 2H, CaromH), 7.61 (d, 2H, CaromH). CI, m/z = 161 [M + H]+.
‡ Crystal data for C206H174N52Mn14O40·Et2O·3.5MeCN, 1·Et2O·3.5MeCN:
¯
triclinic, P1, a = 20.9499(18), b = 22.6189(19), c = 27.567(2) Å, a =
74.856(2), b = 77.870(2), g = 69.345(2)°, V = 11697.7(17) Å3, M =
5004.9, Z = 2, T = 123(2) K, R1 = 0.0625. Data collection, structure
solution and refinement used programs SMART,7 SAINT7 and SHELXL.8
Figures were produced using SCHAKAL.9 Full details have been deposited
and will be published later. CCDC reference number 186867. See http://
www.rsc.org/suppdata/cc/b2/b205157p/ for crystallographic data in CIF or
other electronic format.
Fig. 2 SCHAKAL representation of the core of 1, showing MnIII in purple
and MnII in green. The dashed line is a guide to the eye.
§ Anal. Calcd (Found) for 1a·H2O: C, 50.46 (50.43); H, 3.68 (3.53); N,
14.27 (14.27)%.
¶ Variable temperature magnetic measurements on 1 in the region 1.8–325
K were made using a SQUID magnetometer (Quantum Design) with
samples sealed in gelatine capsules in a 100 G field. The data have been
adjusted for the diamagnetism of the sample using Pascal’s constants.
Mn(II) sites are six coordinate, with O5N, O4N2, and O3N3
donor sets. The final site is five coordinate, bound to three O-
and two N-donors. The bond lengths and geometries are
consistent with the oxidation states required for charge
balance.
In each of the fragments there are nine ppo and two Hppo
ligands. The former, which have been deprotonated at the
hydroxy group, show three distinct bridging modes which differ
in the number of Mn-centres bound to the oxygen (Scheme 1).
One ppo ligand shows the 4.31 mode (Harris notation);10 five
show the 3.21 and three the 2.11 mode. The two Hppo ligands
are bound via an N-atom, and form a H-bond through their
protonated O-atom to an O-atom of a deprotonated ppo ligand.
What differentiates the two independent molecules in the lattice
is that in one (Fig. 1), both N-atoms of the Hppo groups are
hydrogen bonded to the same oxygen atom, whereas in the other
they H-bond to oxygen atoms from different ligands.
There are two terminal nitrate ions per asymmetric unit. Each
shows an unusual orientation as the Mn–O bond is approx-
imately 34° to the plane of the nitrate, rather than in plane as is
conventional. This is caused by the H-bonds between the
uncoordinated O-atoms of the nitrate and the protonated N-
atoms of the two ppo ligands bound to the same metal as the
NO32. The two MeCN ligands per asymmetric unit are attached
to the Mn centres to which the nitrate ions are also attached.
Repeated microanalysis determinations show that these ligands
are substituted completely by H2O molecules upon air exposure
to form the corresponding tetrahydrate [Mn14O2(OH)4-
(ppo)18(Hppo)4(NO3)4(H2O)4] 1a.§
1 R. Sessoli, H.-L. Tsai, A. R. Schake, S. Wang, J. B. Vincent, K. Folting,
D. Gatteschi, G. Christou and D. N. Hendrickson, J. Am. Chem. Soc.,
1993, 115, 1804.
2 D. N. Hendrickson, G. Christou, H. Ishimoto, J. Yoo, E. K. Brechin, A.
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therein.
5 C. S. Aluarez, A. D. Bond, E. A. Harron, R. A. Layfield, J. A.
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6 e. g. A. R. E. Baikie, A. J. Howes, M. B. Hursthouse, A. Quick and P.
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7 Bruker AXS Systems, 2001.
The presence of two {Mn14} cages within the unit cell may be
due to the efficient packing of the cages. The phenyl-rings of the
ppo ligands provide a hydrophobic periphery for each cage.
However it is noticeable that in some cases Ph groups from one
ring inter-digitate with Ph groups from the second cage. The
shortest contacts between phenyl ring centroids are ca. 4.0 Å.
Bulk, variable temperature magnetic susceptibility data were
collected from a polycrystalline sample of 1a under a constant
magnetic field of 100 G.¶ The value of cmT (cm3 K mol21) per
[Mn14] decreases from 52.2 at 300 K to 4.6 at 1.8 K. The value
at room temperature is slightly smaller than the expected for an
uncoupled [MnIII2MnII12] system with g = 2 (58.5) and the drop
with decreasing temperature reflects the presence of strong
antiferromagnetic interactions within the cluster, presumably
leading to a spin ground state of ST = 0.
No previous coordination chemistry has been reported for
ppo, and little for related ligands. For 3-methyl-3-pyrazolin-
5-one we have reported a {Ni24} cage.11 Tetradecanuclear cages
are rare — not only for Mn but for any 3d-metal. Several
polyoxovanadate cages are known,12 and copper sulfide13 and
phosphide clusters.14 1 is therefore only the second {M14} cage
containing N- or O-donor ligands, after the beautiful {Fe14}
cage reported by Klüfers.15 This family of ligands therefore
look immensely promising candidates for the generation of new
cages.
8 G. M. Sheldrick, SHELXL-93, program for crystal structure refinement,
University of Göttingen, 1993.
9 E. Keller, SCHAKAL 99, program for representation of crystallo-
graphic models, University of Freiburg, 1999.
10 Harris notation describes the binding mode as [X.Y1Y2Y3…Yn], where
X is the overall number of metals bound by the whole ligand, and each
value of Y refers to the number of metal atoms attached to the different
donor atoms. Therefore for ppo, there will be two values for Y. The
ordering of Y is listed by the Cahn–Ingold–Prelog priority rules, hence
O before N. See: R. A. Coxall, S. G. Harris, D. K. Henderson, S.
Parsons, P. A. Tasker and R. E. P. Winpenny, Dalton Trans., 2000,
2349–2356.
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Commun., 1997, 851; A. Muller, K. Hovemeier, E. Krickemeyer and H.
Bogge, Angew. Chem., Int. Ed. Engl., 1995, 34, 779.
13 P. J. M. W. L. Birker and H. C. Freeman, J. Amer. Chem. Soc., 1977, 99,
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Felthouse, M. S. Haddad, D. N. Hendrickson, W. Furey Jr. and R. A.
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14 A. Eichhofer, D. Fenske and W. Holstein, Angew. Chem., Int. Ed. Engl.,
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15 J. Burger and P. Klüfers, Angew. Chem., Int. Ed. Engl., 1997, 36,
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This work was supported by the EPSRC (UK).
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