Molecular amino-phosphonate cobalt-lanthanide clusters

Article abstract of DOI:10.1039/c3cc40907d

The use of 1-amino-1-cyclohexyl phosphonic acid, a functionalised phosphonate, leads to the synthesis of two new structural types for 3d-4f phosphonate cages with unusual structural cores and which show high magnetocaloric effects.

Full text of DOI:10.1039/c3cc40907d

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Molecular amino-phosphonate cobalt–lanthanide
clusters†
Cite this: Chem. Commun., 2013,
49, 3522
Eufemio Moreno Pineda, Floriana Tuna, Robin G. Pritchard, Andrew C. Regan,
Richard E. P. Winpenny* and Eric J. L. McInnes*
Received 2nd February 2013,
Accepted 8th March 2013
DOI: 10.1039/c3cc40907d
www.rsc.org/chemcomm
The use of 1-amino-1-cyclohexyl phosphonic acid, a functionalised
phosphonate, leads to the synthesis of two new structural types
for 3d–4f phosphonate cages with unusual structural cores and
which show high magnetocaloric effects.
2ꢀ
Phosphonates of general formula RPO3 are now well estab-
lished as binding groups for molecular d-block transition metal
clusters through use of large R groups or co-ligands that inhibit
formation of polymeric materials.1 Such compounds are being
intensively studied for their magnetic properties. However, with
some exceptions,2 the range of R groups used has tended to be
limited to aliphatic or aromatic organic groups. Here we report the
use of phosphonates where R contains an additional binding
group, and the synthesis of two families of 3d–4f molecular clusters
using an amino-functionalised phosphonate, which exploits the
differences in donor atom preferences of the 3d and 4f ions. There
is very little literature on phosphonate-bound molecular 3d–4f
Fig. 1 Crystal structure of the {Co4Dy10P12} cluster. Scheme: Dy, purple; Co, blue;
P, green; O, orange; C, grey; N, cyan; (H omitted for clarity).
Reaction of (1-NH2-1-C6H10)PO3H2 with [Co2(m-OH2)(O2CtBu)4-
clusters,3 although recent results have shown that 3d–4f clusters (HO2CtBu)4]6 and [Ln2(O2CtBu)6(HO2CtBu)6],7 (Ln = Gd or Dy)
in general can display impressive magnetocaloric effects.3,4 There with Et3N as base, in MeCN at 150 1C under solvothermal
are a few examples of the use of imine- and amino-phosphonates in conditions, followed by slow cooling to room temperature gives
3d and metalloid clusters2a,b and coordination polymers,2c,d and a pink crystals of [Co4Ln10(O2CtBu)12(O3PC6H10NH2)8(PO4)2(O2CMe)2-
3d–4f 2D materials using R = CH2N(CH2CO2)2.2e,f 1-Amino-1-cyclo- (O3PC6H10NH3)2] (Ln = Gd, 1; Dy, 2) in 10% yield (Fig. 1).
hexylphosphonic acid, (1-NH2-1-C6H10)PO3H2, was prepared by
Equivalent reactions with other LnIII ions failed to give crystal-
a modification of a literature method5 (Scheme 1; see ESI† for line products. Compounds 1 and 2 both crystallise in the triclinic
%
synthetic details).
space group P1 and have the same molecular structures (Fig. 1).‡
The {Dy10P12} core of 2 can be described as based on two centered
12-vertex polyhedra sharing a face (Fig. 2). The two atoms at the
centre of each polyhedron (P11 and P12) are from m6-bridging
phosphates (a 6.222 binding mode in Harris notation),8 with the
centres in the shared face being P5, P6, Dy5 and Dy6. The 12-vertex
polyhedra are distorted, but can be derived from an icosahedron,
with the distortion increasing dramatically towards the shared
face, i.e. in an icosahedron there should be triangular faces, not a
diamond face as in P5-Dy5-P6-Dy6. One icosahedron consists of
Dy1 to Dy6 and P1 to P6; if P5 and P6 moved towards the centre of
the polyhedron (Fig. 2a) the distortion would be much less,
however as these sites are shared between the icosahedra then
this might be the reason for the distortion. We have previously
noted the tendency of phosphorus centres to act as additional
Scheme 1 Synthesis of 1-amino-1-cyclohexylphosphonic acid.
School of Chemistry and Photon Science Institute, The University of Manchester,
Oxford Road, Manchester, M13 9PL, UK. E-mail: eric.mcinnes@manchester.ac.uk;
Fax: +44 (0)161-275-4606
† Electronic supplementary information (ESI) available: Synthetic method,
crystallographic details, crystallographic figures, magnetic information. Experi-
mental details, magnetism and Diamond figures. CCDC 922115 (2), 922116 (1),
922117 (3). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc40907d
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Fig. 3 (a) Crystal structure of {Co6Gd4P6} cluster (pivalate groups were omitted for
Fig. 2 (a) 12-Vertex polyhedral sharing a face {Dy10P12} core; (b) phosphonate
simplicity); (b) phosphonate coordination modes in {Co6Gd4P6} (colour as Fig. 1).
coordination modes in {Co4Dy10P10} (colours as Fig. 1).
Two CoII ions (Co2 and symmetry equivalent, s.e.) with octa-
hedral {O5N} geometry sit at the centre of the cluster, bridged by
two m3-hydroxides which further bridge to a LnIII ion (Gd1 and s.e.).
Three amino-phosphonates bridge this butterfly-like core, on
either side, to a further two cobalts, a lanthanide and a sodium
ion. Co2 is N,O-chelated by one amino-phosphonate (P2), as above.
Co1 and s.e. have a distorted {N2O3} coordination sphere, com-
prising one N,O-chelating amino-phosphonate (P1) and one
N-bound (P3). The remaining cobalt sites (Co3 and s.e.) have
tetrahedral {O4} geometry from two phosphonates and two
carboxylates. The amino-phosphonates adopt three different
binding modes (Fig. 3b): that containing P1 is 5.2221 bound
(Gd2Co2Na; including an N,O-chelated Co1); that containing P2
is 4.2111 bound (Gd2Co2; N,O-chelating Co2); that containing P3 is
5.2211 bound (Gd2Co3). In each case the amino group is terminally
bound to Co. As in 1 and 2, all the Ln ions are exclusively O-bound.
The magnetic behaviour of 1 to 5 was studied on polycrystalline
samples using a SQUID magnetometer (Fig. 4 and Fig. S3–S5, ESI†).
The room temperature values of wT (w = molar magnetic suscepti-
bility) for 1 is 91 emu K molꢀ1, decreasing slowly with decreasing
temperature down to about 10 K, before decreasing more rapidly. For
the dysprosium analogue 2, wT is 154 emu K molꢀ1 and decreases
more steadily with decreasing temperature. This more rapid decrease
reflects the nature of the orbitally degenerate DyIII ions in 2 cf.
the spin-only GdIII ions of 1. Compounds 3, 4 and 5 have room
vertices of larger polyhedra in phosphonate clusters, including
{M9P3} icosahedra (M = FeIII, VIII).9 The {Ln10P12} core structures of
1 and 2 are then an extension of this. Bi-icosahedral structures are
known, including an {Au13Ag12} cluster reported by Teo, which
features a single shared vertex.10
The four CoII ions cap the {Ln10P12} core, sitting above triangular
faces P2-P4-Dy4, P1-P3-Dy3, P8-P10-Dy9 and P7-P9-Dy7 in 2. This is
a direct consequence of the second functional group in the
aminophosphonates. Eight of the aminophosphonates (P1–4 and
P7–10) each bind three DyIII ions, with each O-arm binding a single
Dy, with the amino-group binding to a cobalt (giving a 4.2111
coordination mode; Fig. 2b).8 Hence, each Co is N,O-chelated by
two aminophosphonates, with a final coordination site being taken
by a carboxylate giving a distorted cis-N2O3 geometry at Co. All Dy
ions are exclusively O-bound, with either capped octahedral or
dodecahedral geometry. Hence, the preferential binding of the
amino-phosphonate O- and N-donors for LnIII and CoII, respec-
tively, forces the Co centres to the periphery of the molecule. The
remaining two aminophosphonates (P5, 6), which form part of the
shared face of the {Ln10P12} core, only bind to three Dy centres
(3.1111 mode; Fig. 2b), with the amino group uncoordinated.
Neither the acetates nor the phosphates in the structure of 1 and 2
were present in the reaction starting materials; hence presumably
they result from hydrolysis of acetonitrile and phosphonate under the
solvothermal reaction conditions. We attempted to improve the yields
of 1 and 2 by introducing Na3PO4 as a phosphate source. However,
under otherwise identical reaction conditions, we instead obtained
[Na2Co6Ln4(m3-OH)2(O2CtBu)12(O3PC6H10NH2)6(MeCN)2] (Fig. 3; Ln =
Gd, 3; Dy, 5) in low yields. The syntheses can be modified to give
improved yields of 15–20%. This reaction also works for Ln = Tb (4).
Curiously, these products do not contain phosphate, but do incorpo-
rate Na ions and a much less regular structure results.
Compounds 3–5 crystallize in the space group P21/n.‡ These
inversion-symmetric {Ln4Co6P6Na2} clusters contain proportio-
nately much more cobalt than 1–2, and with three different
geometric environments (tetrahedral, distorted 5-coordinate,
and octahedral). Three different amino-phosphonate binding
modes are found. There is also a higher proportion of carboxylate
in the products.
Fig. 4 Magnetization as a function of applied field at 2 K for 1–5.
Chem. Commun., 2013, 49, 3522--3524 3523
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b = 27.2388(6), c = 19.440(5) Å, b = 111.837(3)1, V = 7797.9(3) Å3. Crystal
data for 5 (unit cell) [C100H186Co6Dy4N8Na2O44P6]: monoclinic, space
group P21/n, T = 100.15 K, a = 15.810(3), b = 27.065(6), c = 19.386(4) Å,
b = 111.888(5), V = 7697.12 Å3.
temperature wT values of 52, 65 and 81 emu K molꢀ1, respectively
which are close to the values calculated for non-interacting ions
52 emu molꢀ1 K (for six CoII, g = 2.70,3f,11 S = 3/2 and four GdIII, g =
2.00, S = 7/2), 68 emu molꢀ1 K (for six CoII, g = 2.70, S = 3/2 and four
TbIII, gJ = 3/2, J = 6) and 77 emu molꢀ1 K (for six CoII, g = 2.70, S = 3/2
and four DyIII, gJ = 4/3, J = 15/2). For all three complexes there is a
minimum in wT at a few K; this may be a signature of weak intra-
molecular magnetic interactions. Given the typically weak nature of
interactions between phosphonate-bridged metal ions, and also
that it is observed in all three compounds, this may be due to the
central {Co2O2} unit. The molar magnetisations (M) as a function
applied magnetic field (H) of 1–5 at 2 K rise to values of 64, 58, 41,
32 and 35 Nb, respectively, at 7 T without achieving saturation
(Fig. 4). We have not attempted any quantitative analysis given the
complexity of both the molecular structures and the orbitally
degenerate nature of most of the metal ions.
˜
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Given the recent interest in 3d–4f clusters as potential
magnetic refrigerants, particularly those with the isotropic and
high spin GdIII ion, we have investigated the magnetocaloric
properties of the Gd-containing compounds 1 and 3. The magnetic
entropy changes of 1 and 3 for changing applied field were
calculated indirectly from the magnetization behaviour as a func-
tion of applied field and temperatures (Fig. 4, Fig. S4 and S5, ESI†)
R
using the standard relationship DS ¼ ½@MðT; MÞ=@TꢁHdH.
This gives magnetic entropy changes for 1 and 3 of 32.6 and
19.7 J kgꢀ1 Kꢀ1 for DH = 0–7 T at 2 K.
The magnetic entropy value observed for 1 is among the highest
observed to date for a 3d–4f mixed metal clusters,3,4 surpassed only
by {Co10Gd42} 41.3 J kgꢀ1
{Ni12Gd36} 36.3 J kgꢀ1
K ,
ꢀ1, {Ni10Gd42} 38.2 J kgꢀ1 Kꢀ1
K
ꢀ1, {Mo4Gd12} 35.3 J kgꢀ1 Kꢀ1 and
{Mn4Gd6P6} 33.7 J kgꢀ1 Kꢀ1. 1 has the second highest magnetic
entropy change for 3d–4f phosphonate compounds, just
exceeded by {Mn4Gd6P6}. Gd-cage complexes with higher
entropy values have been reported.12
In conclusion, functionalization of phosphonate ligands is a
very promising route to 3d–4f clusters, with the potential to exploit
the differing coordination preferences of d-block and f-block ions.
This work was supported by the EPSRC (UK), the University of
Manchester and the Panamanian government agency SENACYT-
IFARHU. REPW thanks the Royal Society for a Wolfson Merit Award.
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Notes and references
‡ Crystal data for
5226.25, triclinic, space group P1, T = 100.15 K, a = 18.2190(2), b =
22.4563(3), 27.5752(4) Å, 70.1022(12), 84.5267(11),
g = 79.3776(12)1, V = 10420.1(2) Å3, Z = 2, r = 1.666 g cmꢀ3, total data
84 001, independent reflections 42 000 (Rint = 0.083), m = 3.61 mmꢀ1
1
[C124H214Co4Gd10N10O66P12ꢂ3(C2H3N)]: Mr
=
%
c
=
a
=
b =
,
9 (a) E. I. Tolis, L. P. Engelhardt, P. V. Mason, G. Rajaraman, K. Kindo,
¨
1975 parameters, R1 = 0.095 for I Z 2s(I) and wR2 = 0.218. Crystal data
for 2 [C124H228Co4Dy10N10O66P12ꢂC4N2H6]: Mr = 5229.68, triclinic, space
M. Luban, A. Matsuo, H. Nojiri, J. Raftery, C. Schroder, G. A. Timco,
F. Tuna, W. Wernsdorfer and R. E. P. Winpenny, Chem.–Eur. J., 2006,
12, 8961; (b) S. Khanra, M. Kloth, H. Mansaray, C. A. Muryn, F. Tuna,
E. C. Sanudo, M. Helliwell, E. J. L. McInnes and R. E. P. Winpenny,
Angew. Chem., Int. Ed., 2007, 46, 5568; (c) S. Konar, N. Bhuvanesh
and A. Clearfield, J. Am. Chem. Soc., 2006, 128, 9604.
%
group P1, T = 100.15 K, a = 17.0917(3), b = 20.9589(3), c = 30.4298(4) Å,
a = 108.0775(13), b = 101.8103(13), g = 92.1947(12)1, V = 10082.3(3) Å3,
Z = 2, r = 1.725 g cmꢀ3, total data 79 235, independent reflections 40 171
(Rint = 0.062), m = 4.14 mmꢀ1, 1877 parameters, R1 = 0.087 for I Z 2s(I) and
wR2 = 0.194. Crystal data for 3 [C100H186Co6Gd4N8Na2O44P6ꢂ2(C2H3N)]: 10 B. K. Teo, X. Shi and H. Zhang, J. Am. Chem. Soc., 1991, 113, 4329.
´
Mr = 3501.06, triclinic, space group P21/n, T = 100.15 K, a = 15.8683(5), 11 (a) F. Lloret, M. Julve, J. Cano, R. Ruiz-Garcıa and E. Pardo, Inorg.
b = 27.0973(8), c = 19.4683(6) Å, b = 112.274(3)1, V = 7746.5(4) Å3, Z = 2,
r = 1.501 g cmꢀ3, total data 42 633, independent reflections 15 760
(Rint = 0.060), m = 2.45 mmꢀ1, 771 parameters, R1 = 0.055 for I Z 2s(I) and
Chim. Acta, 2008, 361, 3432; (b) A. Mondal, S. Durdevic, L.-M.
Chamoreau, Y. Journaux, M. Julve, L. Lisnard and R. Lescouezec,
¨
Chem. Commun., 2013, 49, 1181.
wR2 = 0.139. Crystal data for 4 (unit cell) [C100H186Co6Tb4N8Na2O44P6]: 12 M. Evangelisti, O. Roubeau, E. Palacios, A. Camon, T. N. Hooper,
monoclinic, space group P21/n,
T
=
100.15 K,
a
=
15.8615(4),
E. K. Brechin and J. J. Alonso, Angew. Chem., Int. Ed., 2011, 50, 6606.
c
3524 Chem. Commun., 2013, 49, 3522--3524
This journal is The Royal Society of Chemistry 2013