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Cite this: DOI: 10.1039/c0xx00000x
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Molecular Coolers: A Carbonate-Panelled [Gd6Cu3] Tridiminished
Icosahedron
Thomas N. Hoopera, Mateusz B. Pitak,b Simon J. Coles,b Marco Evangelisti,*c and Euan K. Brechin*a
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
A
CO32--panelled [GdIII6CuII3] cage conforming to a
tridiminished icosahedron is an excellent candidate for low
temperature magnetic refrigeration.
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The number of molecular nanomagnets and molecule-based
materials proposed as potential cryogenic magnetic refrigerants
has increased exponentially in the last 2-3 years.1 Refrigeration
proceeds adiabatically via the magnetocaloric effect (MCE),
which describes the changes of magnetic entropy (ΔSm) and
adiabatic temperature (ΔTad), following a change in the applied
magnetic field (ΔB),2 and can be greatly enhanced in molecular or
molecule-based materials with particular physical characteristics.3
One of the key ingredients in the recipe4 required for constructing
high performance molecular coolers is a high magnetic density;
i.e. the supporting ligand framework should be as lightweight as
possible in order to limit the presence of passive, non-magnetic
elements. This has recently resulted in the use of, for example,
actetate and formate, as key bridging ligands.5 An alternative, and
much underused, option is the carbonate anion, CO32-. While
there a number of molecular cage complexes containing this ion
as a bridging ligand,6 its appearance is more often than not
serendipitous in nature, resulting from the unplanned fixation of
atmospheric CO2 during aerobic reactions. However, as pointed
out by Murray and Batten, this appears to be a growing
phenomenon in 4f cluster chemistry.6a,b In order to investigate
the potential application of the CO32- anion for the construction of
molecular nanomagnets, and in particular Gd(III)-containing
magnetic refrigerants, we have begun a systematic investigation
into the use of CO2 as a reaction ingredient, and herein report the
synthesis, structure and magnetic properties of the complex
[Gd6Cu3O(pdm)3(O2CtBu)9(CO3)4(MeOH)3]·7MeOH (1·7MeOH;
Figure 1).
Cu(NO3)2·2.5H2O, Gd(NO3)3·6H2O, H2pdm (pyridine-2,6dimethanol)7 and NaO2CtBu were dissolved in methanol and
upon slow addition of triethylamine a dark blue colour was
observed.‡ The sample was filtered and CO2 gas was then
bubbled through the filtrate for 1 minute. Slow evaporation of the
resulting solution resulted in X-ray quality hexagonal blue
crystals of 1·7MeOH, after 3 days. Complex 1 crystallises8 in the
trigonal space group P3c1 with a three fold rotational axis. The
metallic skeleton describes a distorted tridiminished icosahedron
(Figure 1D), one of the Johnson solids.8 The Gd…Gd distances
within the upper [Gd3] triangle (as drawn in Figure 1; Gd2 and
symmetry equivalent) are ~4 Å in length, those between Gd1 and
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symmetry equivalent are ~6 Å, and those between Cu1 and
symmetry equivalent are ~5 Å. The distances between the Gd2Gd1, Gd1-Cu1 mean planes are ~4 Å and 1.4 Å, respectively.
Fig.1 The molecular structure of 1 viewed (A) parallel and (B)
perpendicular to the three fold rotation axis. (C) The core of the complex
highlighting the bridging modes of the CO32- ions and the sole OH- ion.
(D) The metallic skeleton of the complex. Colour code: Gd = yellow; Cu
= green; O = red; N = blue; C = black. H-atoms and some C atoms have
been omitted for clarity.
The CO32- ions are of two types: three are µ5-bridging,
panelling the pentagonal [Gd4Cu] faces of the prism, with each
O-atom (O4, O9, O10 and symmetry equivalents) bridging two
metal centres. The remaining CO32- ion is µ6-bridging, sitting in
the lower [Cu3] triangular face (as drawn in Figure 1) and further
bridging to three Gd ions in the [Gd3] triangle sitting above it.
Each O-atom (O3 and syymetry equivalent) is therefore bonded
to one Gd ion and one Cu ion. The sole hydroxide ion (O13) caps
the upper [Gd3] triangular face (O13…Gd2, ~2.4 A), sitting ~0.84
Å above the [Gd3] plane, with each edge of this [Gd3] triangle
bridged by a µ-pivalate. The six remaining carboxylates are of
two types: three span the Gd2…Gd1 edges of the [Gd4Cu]
pentagons in a η1,η2,µ-fashion, while three simply chelate to Gd1.
The three µ3-pdm2- ligands each bond to one Cu ion through the
N- and both O-atoms, with the latter further bridging to Gd1 ions.
The Cu(II) ions are all 5-coordinate and in distorted squarepyramidal geometries, with [O4N] donor sets. The Jahn Teller
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axis of each lies along the apex of the square-based pyramid
towards O4, one of the carbonate O-atoms (Cu1…O4, 2.341 Å).
The Gd2 ions are 8-coordinate ans in square-antiprismatic
geomteries, whilst the Gd1 ions are 9-coordinate and in capped
trigonal antiprismatic geometries.
In the crystal each molecule has three close contacts to three
neighbouring cluster units in the ab plane. These are mediated via
π-π stacking of the pdm2- rings, C-H(pdm2-)…O(carboxylate) and
C-H(pdm2-)…O(carbonate) contacts, all in the ~3.3 – 3.5 Å
range. Down the c-axis the clusters are stacked on top of one
another, with the closest intermolecular contacts being between
the Me-groups of the pivalates and the rings of the pdm2- ligands
at a distance of approximately 3.7 Å. The result is an aesthetically
pleasing honeycomb-like framework when viewed down the caxis (Figure 2)
There are no other [LnIII6MII3] complexes in the literature, with
the only other reported trimidiminished icosahedron being the
complex [Fe9O4(O3PPh)3(O2CtBu)13].8c
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Fig.2 The crystal packing of 1 viewed down the c-axis. Colour code: Gd
= yellow; Cu = green; O = red; N = blue; C = grey.
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Magnetics
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In conclusion, 1 is a rare example of a Ln-based cluster to contain
bridging CO32- ions, built employing a reaction scheme which
included CO2 as a deliberately added ingredient. Given its small
relative molecular mass and its ability to flexibly coordinate a
number of metal ions - particulary the oxophilic Gd(III) - the
CO32- ion appears to be an excellent candidate for the constrution
of molecular cryocoolers and molecular magnets. The magnetic
properties of 1 reveal it to be…..
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Notes and references
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a
School of Chemistry, The University of Edinburgh, West Mains Road,
Edinburgh, Scotland, EH9 3JJ. Fax: +44-131-650-6453; Tel: +44-1316507545; E-mail: ebrechin@staffmail.ed.ac.uk
b
UK National Crystallography Service, School of Chemistry,
University of Southampton, Highfield Campus, Southampton SO17 1BJ,
UK
c
Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de
Zaragoza, Departamento de Física de la Materia Condensada, 50009
Zaragoza, Spain. E-mail: evange@unizar.es
Web: http://molchip.unizar.es
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†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Cu(NO3)2·2.5H2O (0.058 g, 0.25 mmol), Gd(NO3)3·6H2O (0.226 g, 0.50
mmol), pyridine-2,6-dimethanol (0.035 g, 0.25 mmol) and NaO2CtBu
(0.124 g, 1.0 mmol) were dissolved in methanol (20 cm3). NEt3 (0.14 cm3,
1.0 mmol) was added and the mixture stirred for 5 minutes. The sample
was filtered and CO2 gas was then bubbled through the filtrate for 1
minute. Slow evaporation of the solution resulted in X-ray quality blue
crystals after 3 days. 1: Yield = 0.098 g, 39%. Calculated (found) for 1: C,
31.43 (31.43); H, 4.53 (4.56); N, 1.48 (1.52). FT-IR: 3301 (w, br), 2962
(w), 1574 (s), 1557 (s), 1482 (s), 1423 (s), 1374 (s), 1361 (s), 1226 (m),
1094 (w), 1071 (m), 1028 (m), 896 (m) 840 (m), 805 (m), 794 (m) 755
(w), 655 (m) cm-1 (s = strong, m = medium, w = weak, br = broad).
Crystallographic data for 1. C73H111Cu3Gd6N3O40, Mr = 2804.77,
Trigonal, P-3c1, a = 22.2578(15) Å, b = 22.2578(15) Å, c = 26.9738(18)
Å,  =  = 90°,  = 120°, Z = 4, Dc = 1.610 Mg/m3, μ = 3.996 mm-1,
T=100K. Data were collected on Rigaku AFC12 goniometer equipped
with an enhanced sensitivity (HG) Saturn724+ detector mounted at the
window of an FR-E+ SuperBright molybdenum rotating anode generator
with VHF Varimax optics (70µm focus).9 A total of 74053 collected
reflections, of which 8821were unique (Rint = 0.0302). The final R1 (I
>2(I)) = 0.0329, wR2 (I >2(I))= 0.0901. The goodness of fit on F2 =
1.053.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
CCDC depository number = xxxxxxx.
1 For examples published in 2012 see: (a) G. Lorusso, M. A. Palacios, G.
S. Nichol, E. K. Brechin, O. Roubeau and M. Evangelisti, Chem.
Commun., 2012, 48, 7592; (b) M.-J. Martínez-Pérez, O. Montero, M.
Evangelisti, F. Luis, J. Sesé, S. Cardona-Serra and E. Coronado, Adv.
Mater., 2012, 24, 4301; (c) F.-S. Guo, Y.-C. Chen, J.-L. Liu, J.-D.
Leng, Z.-S. Meng, P. Vrábel, M. Orendáč and M.-L. Tong, Chem.
Commun., 2012, 48, 12219; (d) R. Sibille, T. Mazet, B. Malaman and
M. François, Chem. Eur. J., 2012, 18, 12970; (e) T. N. Hooper, J.
Schnack, S. Piligkos, M. Evangelisti and E. K. Brechin, Angew. Chem.
Int. Ed., 2012, 51, 4633; (f) Y.- Z. Zheng, M. Evangelisti, F. Tuna and
R. E. P. Winpenny, J. Am. Chem. Soc., 2012, 134, 1057; (g) T. Birk, K.
S. Pedersen, C. A. Thuesen, T. Weyhermuller, M. Schau-Magnussen, S.
Piligkos, H. Weihe, S. Mossin, M. Evangelisti and J. Bendix, Inorg.
Chem., 2012, 51, 5435.
2 W. F. Giauque and D. P. MacDougall, Phys. Rev. B, 1933, 43, 768.
3 M. Evangelisti, F. Luis, L. J. de Jongh and M. Affronte, J. Mater.
Chem., 2006, 16, 2534.
4 M. Evangelisti and E. K. Brechin, Dalton Trans., 2010, 39, 4672.
5 (a) M. Evangelisti, O. Roubeau, E. Palacios, A. Camón, T. N. Hooper,
E. K. Brechin and J. J. Alonso, Angew. Chem. Int. Ed., 2011, 50, 6606;
(b) G. Lorusso, M. A. Palacios, G. S. Nichol, E. K. Brechin, O.
Roubeau and M. Evangelisti, Chem. Commun., 2012, 48, 7592.
6 A search of the CCDC database reveals 122 hits for metal complexes
containing bridging CO32- ions. Of these 97 are with 1st row transition
metals and only seven contain Ln ions. For examples of Ln-based
clusters, see: (a) I. A. Gass, B. Moubaraki, S. K. Langley, S. R. Batten,
K. S. Murray, Chem.Commun., 2012, 48, 2089; (b) A. S. R. Chesman,
D. R. Turner, B.Moubaraki, K. S. Murray, G. B. Deacon and S. R.
Batten, Chem. Eur. J., 2009, 15, 5203; (c) L. Natrajan, J. Pécaut and M.
Mazzanti, Dalton Trans., 2006, 1002; (d) P. C. Andrews, T. Beck, C.
M. Forsyth, B. H. Fraser, P. C. Junk, M. Massi and P. W. Roesky,
Dalton Trans., 2007, 5651; (e) H. Ke, L. Zhao, G.-F. Xu, Y.-N. Guo, J.
Tang, X.-Y. Zhang and H.-J. Zhang, Dalton Trans., 2009, 10609.
7 The pro-ligand H2pdm has been extensively employed by Christou and
co-workers. See for example: (a) M. Murugesu, A. Mishra, W.
Wernsdorfer, K. A. Abboud and G. Christou, Polyhedron, 2006, 25,
613; (b) J. Yoo E. K. Brechin, A. Yamaguchi, M. Nakano, J. C.
Huffman, A. L. Maniero, L. C. Brunel, K. Awaga, H. Ishimoto, G.
Christou and D. N. Hendrickson, Inorg Chem., 2000, 39, 3615.
8 (a) N. W. Johnson, Canad. J. Math., 1966, 18, 169; (b) A. RuizMartínez, D. Casanova and S. Alvarez, Dalton Trans., 2008, 2583; (c)
E. I. Tolis, L. P. Engelhardt, P. V. Mason, G. Rajaraman, K. Kindo, M.
Luban, A. Matsuo, H. Nojiri, J. Raftery, C. Schröder, G. A. Timco, F.
This journal is © The Royal Society of Chemistry [year]
Tuna, W. Wernsdorfer and R. E. P. Winpenny, Chem. Eur. J., 2006, 12,
8961.
9 S. J. Coles and P. A. Gale, Chem. Sci., 2012, 3, 683.
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