Synthesis and structural and magnetic characterisation of copper(II) complexes of mixed phosphonate-antimonate ligands

Article abstract of DOI:10.1039/b913467k

A polynucleating oxygen donor ligand based on condensation of p-chlorophenylstibonic acid (ArSbO₃H₂) and t-butylphosphonic acid is reported. [(SbAr)₂O(HO₃P ^tBu) ₆] contains two antimony centres, bridged by an oxide and two hydrogen phosphonates. Reaction with copper acetate under solvothermal conditions produces four new polymetallic copper complexes. With pyridine used as a base the major product is a tetracopper cage, [Cu₄O₂ (SbAr)₂(O₃P^tBu)₂(O ₂CMe)₂(OMe)₆], with the four copper centres arranged in a rhombus; a minor product is a tricopper complex, [Cu ₃O₄(SbAr)₂ (O₃P^tBu) ₄ (py)₃]. With LiOMe as base the major product is again the tetranuclear cage, but the minor product is a heterometallic cage, [Cu ₅Li₄O₆(SbAr) ₄(O₃P ^tBu)₆(O₂CMe)₂(OMe)₄ (MeOH)₄]. With 2,6-lutidine as base an octametallic complex is formed, [Cu₈O₄(SbAr) ₂(O₃ P ^tBu)₆(O₂CMe)₄(lutidine) ₂]. Magnetic studies show both anti-ferromagnetic and weak ferromagnetic exchange between the copper centres in these new complexes. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b913467k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and structural and magnetic characterisation of copper(II)
complexes of mixed phosphonate-antimonate ligands†
Shoaib Ali, Christopher A. Muryn, Floriana Tuna and Richard E. P. Winpenny*
Received 7th July 2009, Accepted 24th October 2009
First published as an Advance Article on the web 20th November 2009
DOI: 10.1039/b913467k
A polynucleating oxygen donor ligand based on condensation of p-chlorophenylstibonic acid
(ArSbO3H2) and t-butylphosphonic acid is reported. [(SbAr)2O(HO3PtBu)6] contains two antimony
centres, bridged by an oxide and two hydrogen phosphonates. Reaction with copper acetate under
solvothermal conditions produces four new polymetallic copper complexes. With pyridine used
as a base the major product is a tetracopper cage, [Cu4O2(SbAr)2(O3PtBu)2(O2CMe)2(OMe)6],
with the four copper centres arranged in a rhombus; a minor product is a tricopper complex,
[Cu3O4(SbAr)2(O3PtBu)4(py)3]. With LiOMe as base the major product is again the tetranuclear cage,
but the minor product is a heterometallic cage, [Cu5Li4O6(SbAr)4(O3PtBu)6(O2CMe)2(OMe)4
(MeOH)4]. With 2,6-lutidine as base an octametallic complex is formed, [Cu8O4(SbAr)2(O3PtBu)6-
(O2CMe)4(lutidine)2]. Magnetic studies show both anti-ferromagnetic and weak ferromagnetic
exchange between the copper centres in these new complexes.
an investigation of the coordination chemistry of copper(II) with
one of these phosphonate-antimonate ligands.
Introduction
A number of groups have been examining the use of phosphonate
ligands to make polymetallic cage complexes. Our work in this area
was originally inspired by a paper by Kingsley and Chandrasekhar,
who showed that phosphonates could be used as bridging ligands
to make a large copper(II) cage, with pyrazoles as co-ligands.1
Using this approach we have made a number of cobalt,2 nickel3 and
vanadium cages.4 We also examined the reaction of phosphonates
with pre-formed carboxylate cage compounds, producing new
cages with iron5 and manganese.6 This reaction, where carboxy-
lates are displaced by phosphonates causing growth of the cage
complex due to the greater denticity of the phosphonate ligand,
has been pursued with great success by Konar and Clearfield,7
and a number of other groups.8 Both tactics are used to avoid
the insolubility of metal phosphonates which had previously
restricted their uses in making cage complexes, with a much greater
prevalence of phosphonates being used in making 2D-materials.9
To extend this work we examined antimonates as possible
ligands, and found polycondensation products which suggest
antimonates could be used as inorganic cryptands.10 As no
condensation is observed for phosphonates, and polycondensation
is found for antimonates we reasoned that mixing phosphonic
acids and stibonic acids might lead to partial condensation and
polydentate oxygen donor ligands. We reported a preliminary
communication on this subject recently, and the resulting cobalt(II)
complexes.11 Main group phosphonates have been studied for
a number of years by Roesky12 and others,13 but not, to our
knowledge, used as ligands for transition metals. Here we report
Experimental
Preparation of compounds
All reagents, metal salts and ligands were used as obtained from
Aldrich. Analytical data were obtained by the microanalytical
service of the University of Manchester and are given in Table 1.
[(SbAr)2O(HO3PtBu)6] 1
p-Chlorophenylstibonic acid (ArSbO3H2) (0.4 g, 1.4 mmol) and
t-butylphosphonic acid 0.43 g (2.8 mmol) were added to MeCN
(20 ml) and the mixture was stirred for 24 h. It was then filtered and
filtrate was allowed to crystallise by slow evaporation. Crystals of
1 formed after one week. Yield: 60%.
[Cu4O2(SbAr)2(O3PtBu)2(O2CMe)2(OMe)6] 2
Compound 1 (0.13 g, 0.10 mmol) and copper acetate (0.08 g,
0.40 mmol) were added to methanol (6 mL) and pyridine (0.1 mL)
in a Parr digestion bomb. The bomb was sealed and heated at
100 ◦C for 12 h and then cooled slowly. Blue crystals of 2 were
obtained directly from the reaction. Yield: 40%
[Cu3O4(SbAr)2(O3PtBu)4(py)3] 3
The same procedure was followed as for 2. The crystals of 2
were removed, and the blue solution remaining was filtered and
allowed to evaporate slowly. Blue crystals of 3 were formed after
several days. Yield: 30%
School of Chemistry, The University of Manchester, Oxford Road,
Manchester, UK M13 9PL. E-mail: richard.winpenny@manchester.ac.uk;
Fax: +44-161-275-4616
† CCDC reference numbers 739330–739333. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b913467k
[Cu5Li4O6(SbAr)4(O3PtBu)6(O2CMe)2(OMe)4 (MeOH)4] 4
Compound 1 (0.13 g, 0.10 mmol) and copper acetate (0.08 g,
0.40 mmol) were added to methanol (6 mL) and LiOMe (0.023 g,
124 | Dalton Trans., 2010, 39, 124–131
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Table 1 Analytical data for compounds 1–5
Analytical dataa
C
Compound
Formula
H
Cu
N
P
Sb
1
2
C36H72Cl2O19P6Sb2
33.72 (33.03)
26.80 (27.10)
33.05 (34.36)
25.97 (28.43)
29.96 (30.20)
4.89 (5.54)
3.72 (3.79)
4.19 (3.95)
3.66 (4.31)
3.90 (4.02)
—
—
—
13.40 (14.19)
3.81 (4.66)
7.70 (8.24)
6.84 (7.10)
7.32 (8.05)
18.06 (18.60)
19.20 (18.31)
14.90 (16.20)
17.95 (18.48)
10.66 (10.56)
C30H50Cl2Cu4O18P2Sb2
C43H59Cl2Cu3N3O16P4Sb2
C60H104Cl4Cu5Li4O36P6Sb4
C58H92Cl2Cu8N2O30P6Sb2
17.77 (19.12)
11.41 (12.68)
11.45 (12.02)
21.88 (22.04)
3
2.62 (2.79)
4b
5
1.31 (1.21)
a Found (calc). b Also measured: Li 0.97(1.07).
Table 2 Crystal data for compounds 1-5
Compound
Formula
1
2
3
4
5
C36H62Cl2O19P6Sb2 C30H50Cl2Cu4O18P2Sb2 C45H67Cl2Cu3N3O19P4Sb2 C61H104Cl4Cu5Li4O37P6Sb4. C58H92Cl2Cu8N2O30P6Sb2
0.26H1.08O0.71
C
Mr
1299.08
Triclinic
P1
10.4506(19)
11.6444(18)
22.684(3)
94.982(13)
99.542(14)
90.679(13)
2711.0 (8)
1329.20
Monoclinic
P21/n
13.2383(4)
11.1442(3)
15.5501(4)
90.00
99.411(3)
90.00
2263.23(11)
0.30 ¥ 0.20 ¥ 0.20
1.951
1582.92
Triclinic
P1
2605.19
Triclinic
P1
2305.88
Monoclinic
P21/n
4.3110(3)
21.4852(4)
14.3972(3)
90.00
103.040(2)
90.00
4312.62(15)
0.30 ¥ 0.20 ¥ 0.15
1.776
Crystal system
Space group
a/A
¯
¯
¯
˚
˚
11.6323(2)
13.2236(4)
21.2764(6)
91.158(2)
102.161(2)
104.612(2)
3086.51(14)
0.30 ¥ 0.20 ¥ 0.20
1.703
14.1112(3)
15.8788(3)
23.7777(5)
92.306(2)
92.089(2)
110.555(2)
4977.32(18)
0.35 ¥ 0.30 ¥ 0.20
1.743
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Crystal size/mm
rc/g cm-3
1.591
Crystal shape and colour colourless block
blue block
15153
green block
26149
green block
47395
blue block
31096
Total reflns
29942
ind.reflns/R int
reflns with I > 2s(I)
Parameters
9551/0.0604
5296
604
0
4603/0.0179
4066
269
12539/0.0270
9374
719
20218
14707
1229
656
8773
7067
500
0
restraints
0
0
R1, wR2
0.0547, 0.1259
0.95
2.45
0.0209, 0.0524
1.094
0.66
0.0285, 0.0666
0.993
0.87
0.0444, 0.0968
1.021
2.62
0.0296, 0.0724
1.054
1.13
goodness of fit
-3
˚
largest residuals/e A
˚
Common features: Z = 2; T = 100(2) K; radiation = Mo-Ka, l = 0.71073 A
0.6 mmol) in a Parr digestion bomb. The bomb was sealed and
heated at 100◦ C for 12 h and then cooled slowly. The blue solution
was filtered and coloured crystals of 4 were obtained by slow
evaporation of the filtrate. Yield: 5%.
SADABS.12 Structure solution and refinement was performed
with the SHELX-package.12 The structures were solved by direct
methods and completed by iterative cycles of DF syntheses and
full-matrix least-squares refinement against F2. Crystal data are
given in Table 2.
[Cu8O4(SbAr)2(O3PtBu)6(O2CMe)4(lutidine)2] 5
Magnetic measurements
Compound 1 (0.13 g, 0.10 mmol) and copper acetate (0.08 g,
0.40 mmol) were added to MeCN (6 mL) and 2,6-lutidine (0.1 mL)
in a Parr digestion bomb. The bomb was sealed and heated at 100◦
C for 12 h and then cooled slowly. Blue crystals of 5 were obtained
directly from the reaction. Yield: 35%.
The magnetic properties of polycrystalline samples were investi-
gated in the temperature range 2–300 K, by using a Quantum
Design MPMS XL SQUID magnetometer equipped with a
7 T magnet. Magnetic susceptibility data were corrected for
the diamagnetism of the compounds by using Pascal constants
and for the diamagnetic contribution of the sample holder by
measurement.
Structure determinations
Data were collected on a Oxford Xcalibur CCD diffractometer
˚
(Mo-Ka, l = 0.71069 A). In all cases the selected crystals were
Results
mounted on the tip of a glass pin using Paratone-N oil and placed
in the cold flow (100 K) produced with an Oxford Cryocooling
device.11 Complete hemispheres of data were collected using w-
scans (0.3◦, 30 s/frame). Integrated intensities were obtained
with SAINT+12 and they were corrected for absorption using
Compound 1 is made straightforwardly from mixing p-chloro-
phenylstibonic acid (ArSbO3H2) and t-butylphosphonic acid
together in MeCN with a 1 : 2 mole ratio. The crystal structure
(Fig. 1) shows two Sb centres bridged by a m2-oxide and two
This journal is
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) in compound 2
Cu1-O5
Cu1-O1
Cu1-O2
Cu2-O8
1.8971(17)
1.9534(17)
2.3813(16)
1.9177(17)
2.0111(17)
168.95(7)
93.25(7)
94.99(7)
78.90(7)
92.63(7)
172.04(7)
94.05(6)
Cu1-O9
Cu1-O7
Cu2-O5
Cu2-O2
1.9197(17)
2.0155(17)
1.8977(17)
1.9571(16)
2.3563(16)
169.26(7)
93.78(7)
93.62(7)
79.11(7)
93.06(7)
172.39(7)
94.61(6)
Cu2-O4
Cu2-O1
O5-Cu1-O9
O5-Cu1-O1
O9-Cu1-O1
O5-Cu1-O7
O9-Cu1-O7
O1-Cu1-O7
O5-Cu1-O2
O9-Cu1-O2
O1-Cu1-O2
O7-Cu1-O2
Cu1-O1-Cu2
Cu1-O5-Cu2
O5-Cu2-O8
O5-Cu2-O2
O8-Cu2-O2
O5-Cu2-O4
O8-Cu2-O4
O2-Cu2-O4
O5-Cu2-O1
O8-Cu2-O1
O2-Cu2-O1
O4-Cu2-O1
Cu1-O2-Cu2
93.73(7)
86.92(6)
94.87(6)
87.98(6)
93.46(7)
87.54(6)
95.68(6)
87.19(6)
134.77(9)
Fig. 1 The structure of 1 in the crystal showing as a polyhedral view.
Colour code: Sb, orange; P, gray; O, red; C, black; Cl, green. A line drawing
of the structure is shown below.
hydrogen phosphonate ligands. Four further hydrogen phospho-
nates are attached to the Sb centres. The Sb–O bond lengths
˚
involving the m2-oxide are 1.931(5) and 1.944(5) A, while the Sb–
O bonds involving the phosphonates are in the range 1.982(5)–
˚
2.034(5) A.
Compound 2 was obtained by the reaction of 1 with copper
acetate in MeOH under solvothermal conditions, using pyridine
as base. The crystals of 2 appeared directly upon slow cooling
of the MeOH solution. The structure is centro-symmetric and
contains four Cu centres arranged in a rhombus (Fig. 2). Selected
bond distances and bond angles for 2 are given in Table 3. The
Cu1 ◊ ◊ ◊ Cu2 edge of the rhombus is bridged by a m3-oxide (O5)
which is also attached to the Sb1 site. This Cu ◊ ◊ ◊ Cu contact on
˚
this edge is 3.50 A. Cu1 is bridged to Cu2a centre by two oxygens
Fig. 2 The structure of 2 in the crystal shown (a) as a ball-and-stick
view (aryl group on Sb excluded for clarity), and (b) as a polyhedral view.
Colour scheme as Fig. 1, plus Cu, blue polyhedra.
(O1a and O2) from phosphonates and by an acetate group; this
leads to a Cu ◊ ◊ ◊ Cu distance of 3.01 A. There are also four m2-
˚
methoxides in the structure, bridging Cu ◊ ◊ ◊ Sb contacts.
˚
Both copper sites have square pyramidal geometry; for each O5,
an oxygen from an acetate, and oxygen from a phosphonate and
an O-atom from the m2-methoxide occupy equatorial positions.
The axial position is occupied by an oxygen from a phosphonate;
O2 for Cu1 and O1 for Cu2. All the equatorial bond distances
2.3813(16) A. All the bond angles are as expected for a square
pyramid.
The two symmetry-equivalent Sb centres, are six co-ordinate
and attached to one terminal methoxide (O6), two m-methoxides
(O4 and O7), one m3-oxide (O5) and one oxygen (O3) from a
phosphonate. The Sb–O bond distances again depend on the type
˚
are around 2.0 A, while the axial Cu(1)-O(2) bond distance is
126 | Dalton Trans., 2010, 39, 124–131
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˚
of oxygen involved; the shortest bond is to O5 at 1.950(2) A; bonds
˚
to methoxide fall in the range 1.962–2.045(2) A, while those to
˚
phosphonate are longest at 2.084(2) A. The bonding mode for both
of the phosphonates is 5.221, with the mononucleating O-donor
bound to antimony. The same product, with almost the same yield
and purity was obtained by using LiOMe, NaOMe, KOMe or
lutidine as a base instead of pyridine. Therefore changing base in
this reaction does not change the major product obtained, however
the by-products are different, with 3 obtained if pyridine is used
and 4 if LiOMe is used.
Compound 3 was obtained by slow evaporation of the mother
liquor after crystals of 2 were collected. The molecular structure
of compound 3 is shown in Fig. 3. The structure contains two Sb
and three Cu centres occupying the corners of a square pyramid.
The two Sb centres are connected through a m2-oxide (O4) and a
˚
m3-oxide (O6), with Sb–O bond distance of around 2.0 A. The
Cu1 and Cu3 sites have square pyramidal geometry with two
oxygen donors from phosphonates (O14 and O15 for Cu1, O10
and O12 for Cu3), a m2-oxide (O1 or O5) and an N-donor from
pyridine in equatorial positions, while a m2-oxide (O19) occupies
the axial position of each copper site. All the equatorial bond
distances around these copper centres are in the range of 1.91-
˚
˚
2.00 A, while the axial Cu1-O19 bond distance is 2.527(2) A. The
bond angles are as expected for a square pyramid. The Cu2 site
has a trigonal bipyramidal coordination geometry in which one
pyridyl ligand and a m3-oxide (O6) occupy the axial positions while
three oxygen donors from phosphonates, (O7, O11, and O16),
occupy the equatorial positions of the trigonal bipyramid. All the
˚
equatorial bond distances around Cu(2) centre are 1.9-2.1 A, while
˚
the axial bond distances are around 1.9 A. There is no trend in
the Sb–O bond lengths which fall in the range 1.971(2)–2.045(2)
˚
A. The phosphonates show two bonding modes; three, involving
P1, P2 and P3, show the 3.111 mode while the fourth bridges Cu1
and Cu3 using the 2.110 mode. The inter atomic distance between
˚
Cu(1) and Cu(3) centres is 4.18 A. The selected bond distances
and bond angles for compound 3 are shown in Table 4.
Compound 4 was obtained by the reaction of 1 with copper
acetate in MeOH under solvothermal conditions, using LiOMe
as base. Crystals of 2 are the major product of the reaction, and
form on slow cooling of the solution within the bomb. Crystals
of 4 appeared by slow evaporation of the MeOH solution after
Fig. 3 The structure of 3 in the crystal shown (a) as a ball-and-stick
view (aryl group on Sb excluded for clarity), and (b) as a polyhedral view.
Colour scheme as Fig. 2, plus N, blue spheres.
crystals of 2 were removed. The low yield of 4 prevented magnetic
characterisation.
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) in compound 3
The structure of 4 (Fig. 4) contains five Cu and four Sb
centres connected through six phosphonates, two acetates and four
methoxides. The copper centres make a bow-tie consisting of two
triangles (Cu1, Cu2 and Cu3, and Cu3, Cu4 and Cu5 respectively)
connected at Cu3. There is one Sb centre above and one below
the plane of each triangle. Thus Sb1 and Sb2 lie above and below
the first triangle while Sb3 and Sb4 are associated with the second
triangle.
Four of the copper sites, Cu1, Cu2, Cu4 and Cu5, have square
pyramidal geometries, while Cu3 is square planar. The Cu1
centre has one m3-oxide (O16), a m2-methoxide (O17), an oxygen
from an acetate (O10) and an oxygen (O9) from a phosphonate
at equatorial positions of the square pyramid, while a further
phosphonate oxygen (O31) occupies the equatorial position. All
Cu1-O14
Cu1-O15
Cu1-O1
Cu1-N1
Cu2-N2
Cu2-O6
Cu2-O7
O14-Cu1-O15
O14-Cu1-O1
O15-Cu1-O1
O14-Cu1-N1
O15-Cu1-N1
O1-Cu1-N1
N2-Cu2-O6
N2-Cu2-O7
O6-Cu2-O7
N2-Cu2-O11
O6-Cu2-O11
1.916(2)
1.919(2)
2.005(2)
2.028(3)
1.979(3)
1.981(2)
1.997(2)
174.70(9)
89.86(9)
91.77(9)
89.01(10)
88.37(10)
168.35(9)
176.24(10)
93.16(10)
89.92(8)
88.69(10)
91.07(8)
Cu3-O12
Cu3-O10
Cu3-O5
Cu3-N3
Cu2-O11
Cu2-O16
1.917(2)
1.933(2)
2.018(2)
2.026(3)
2.050(2)
2.110(2)
O12-Cu3-O10
O12-Cu3-O5
O10-Cu3-O5
O12-Cu3-N3
O10-Cu3-N3
O5-Cu3-N3
O7-Cu2-O11
N2-Cu2-O16
O6-Cu2-O16
O7-Cu2-O16
O11-Cu2-O16
165.67(9)
91.00(9)
90.49(9)
89.13(10)
87.42(10)
171.93(9)
128.77(9)
89.05(9)
87.31(8)
130.73(9)
100.47(8)
˚
the equatorial bond distances are around 1.95 A, while the axial
˚
Cu1-O31 bond distance is 2.390(4) A. The O10-Cu1-O31 bond
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˚
Table 5 Selected bond lengths (A) and angles ( ) in compound 4
Cu1-O10
Cu1-O16
1.950(4)
1.953(3)
Cu4-O32
Cu4-O7
1.935(4)
1.955(3)
Cu1-O17
Cu1-O9
1.970(4)
1.981(4)
Cu4-O6
Cu4-O24
1.984(4)
1.991(4)
Cu1-O31
Cu2-O14
2.390(4)
1.907(3)
Cu4-O5
Cu5-O4
2.385(4)
1.912(3)
Cu2-O16
Cu2-O19
1.948(4)
1.964(3)
Cu5-O34
Cu5-O7
1.950(3)
1.951(3)
Cu2-O18
Cu2-O29
2.003(4)
2.401(3)
Cu5-O28
Cu5-O21
2.013(3)
2.348(4)
Cu3-O19
Cu3-O34
1.921(3)
1.939(3)
Cu3-O26
Cu3-O8
1.969(4)
1.979(3)
Li1-O33
Li1-O8
Li1-O36
Li1-O26
Li2-O29
Li2-O21
Li2-O19
Li2-O34
1.884(13)
1.922(12)
1.933(12)
1.990(12)
1.849(9)
1.868(9)
1.967(9)
Li3-O35
Li3-O5
Li3-O37
Li3-O24
Li4-O38
Li4-O31
Li4-O11
Li4-O9
1.886(15)
1.919(11)
1.953(13)
1.978(11)
1.879(12)
1.915(12)
1.957(12)
1.969(11)
169.70(17)
92.10(18)
78.93(14)
92.32(17)
96.32(14)
174.24(15)
95.74(17)
91.19(14)
102.47(15)
80.76(14)
174.78(15)
94.48(15)
88.51(14)
96.84(14)
78.78(14)
154.91(15)
99.33(15)
84.39(13)
99.00(13)
101.15(14)
93.52(15)
174.93(14)
87.95(15)
111.67(16)
110.29(15)
1.970(9)
O10-Cu1-O16
O10-Cu1-O17
O16-Cu1-O17
O10-Cu1-O9
O16-Cu1-O9
O17-Cu1-O9
O10-Cu1-O31
O16-Cu1-O31
O17-Cu1-O31
O9-Cu1-O31
O14-Cu2-O16
O14-Cu2-O19
O16-Cu2-O19
O14-Cu2-O18
O16-Cu2-O18
O19-Cu2-O18
O14-Cu2-O29
O16-Cu2-O29
O19-Cu2-O29
O18-Cu2-O29
O19-Cu3-O34
O19-Cu3-O26
O34-Cu3-O26
Cu5-O7-Cu4
Cu2-O16-Cu1
163.97(16)
92.53(17)
79.29(15)
92.62(16)
95.48(15)
174.77(15)
104.77(16)
90.03(14)
97.24(15)
82.34(15)
94.18(15)
175.15(15)
88.23(14)
98.68(15)
158.64(15)
77.80(14)
100.67(14)
96.00(13)
83.23(13)
98.28(14)
84.73(14)
175.35(15)
94.17(15)
116.96(17)
116.02(18)
O32-Cu4-O7
O32-Cu4-O6
O7-Cu4-O6
O32-Cu4-O24
O7-Cu4-O24
O6-Cu4-O24
O32-Cu4-O5
O7-Cu4-O5
O6-Cu4-O5
O24-Cu4-O5
O4-Cu5-O34
O4-Cu5-O7
O34-Cu5-O7
O4-Cu5-O28
O34-Cu5-O28
O7-Cu5-O28
O4-Cu5-O21
O34-Cu5-O21
O7-Cu5-O21
O28-Cu5-O21
O19-Cu3-O8
O34-Cu3-O8
O26-Cu3-O8
Cu3-O19-Cu2
Cu3-O34-Cu5
Fig. 4 The structure of 4 in the crystal shown (a) as a ball-and-stick
view (aryl group on Sb excluded for clarity), and (b) as a polyhedral view.
Colour scheme as Fig. 3, plus Li, light blue spheres.
angle is 104.8(2)◦, causing a distortion in the square pyramidal
geometry. The coordination environment around Cu4 involves a
similar set of donor atoms.
The Cu2 centre has a m3-oxide (O16), a m4-oxide (O19), a m2-
methoxide (O18) and an oxygen (O14) from a phosphonate at the
equatorial positions of the square pyramid, while a further oxygen
donor (O29) from another phosphonate at the axial site. All the
There are four Li centres, each is four coordinate and two of
these coordination sites are occupied by oxygen donors from
phosphonates, while occupation of the other sites vary. In case
of Li1 and Li4, the other two sites are occupied by an oxygen
from methanol and a O-atom from acetate. For Li2 centre the two
additional sites are occupied by two m4-oxides, while for Li3 two
methanol molecules occupy these sites.
˚
equatorial bond distances are in the range of 1.9-2.0 A, while
The phosphorous centres occupy the triangular faces made by
Sb and Cu centres, and then bind further to the external Li atoms.
Four of the phosphonates (those involving P1, P2, P5 and P6) have
a bonding mode of 4.211, while final two phosphonates (involving
P3 and P4) have a 5.221 mode. Each phosphonate, with a bonding
mode 4.211, contain one oxygen donor bridging between a Li and
a Cu centre while the other two oxygen donors coordinate to either
a Sb or Cu centre. Similarly each phosphonate with bonding mode
5.221, has two oxygen donors bridging between Li and Cu centres
and one oxygen donor coordinating to an antimony.
˚
axial Cu2-O29 bond distance is 2.401(3) A. The O14-Cu2-O18
bond angle is 98.7(2)◦ which is the chief distortion in the square
pyramidal geometry. The coordination environment around Cu5
involves a similar set of donor atoms to Cu2.
The square planar Cu3 centre is connected to two m4-oxides (O19
and O34) and two phosphonate oxygens (O8 and O26). The Cu3-
O bond distances are in the range of 1.92-1.97 A and all the bond
angles are regular. There is a clear trend in the Sb–O bond lengths,
with bonds involving oxides falling in the range 1.923–1.961(4) A,
˚
˚
while where phosphonates are involved the Sb–O bonds fall in the
Compound 5 was obtained by the reaction of 1 with copper
acetate in MeCN under solvothermal conditions, using 2,6-
lutidine as base. The crystals of 5 appeared by slow cooling of the
˚
range 2.006–2.123(4) A. Selected bond distances and bond angles
for compound 4 are shown in Table 5.
128 | Dalton Trans., 2010, 39, 124–131
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◦
˚
MeCN solution. The structure is centro-symmetric and contains
two Sb and eight Cu centres connected through six phosphonates
(Fig. 5). The core of the structure is an edge-sharing oxo-centred
tetrahedron Cu4Sb2(m4-O)2; Cu2 and Cu2a are in the shared edge,
with Cu3, Cu3a, Sb1 and Sb1a on the exterior vertices. The m4-
oxides are O15 and O15a. The three crystallographically unique
phosphonates all adopt the 3.111 mode, and bridge edges of this
core polyhedron and then on to the additional copper sites. The
phosphonate containing P1 bridges from Sb1 to Cu3a (Fig. 5),
and then on to Cu1. The phosphonate containing P2 bridges from
Sb1 to Cu2, and then to Cu1, while the P3-phosphonate bridges
Sb1 to Cu2a and then on to Cu4. There is a m3-oxide between Sb1,
Cu3 and Cu4 and there is also a 2.11 bridging acetate between Cu4
and Cu3. The coordination site of Cu1 is completed by a chelating
acetate while that of Cu4 has a terminal lutidine attached.
Table 6 Selected bond lengths (A) and angles ( ) in compound 5
Cu1-O6
1.898(2)
1.908(2)
1.990(2)
2.007(3)
1.915(2)
1.939(2)
1.958(2)
2.008(2)
98.49(10)
157.45(11)
99.36(10)
93.58(10)
159.64(10)
65.57(11)
95.19(10)
160.71(8)
99.18(9)
87.95(9)
161.71(9)
82.44(9)
107.15(10)
114.91(10)
Cu3-O14
Cu3-O10
Cu3-O2
Cu3-O15
Cu4-O14
Cu4-O9
Cu4-O11
Cu4-N1
O14-Cu3-O10
O14-Cu3-O2
O10-Cu3-O2
O14-Cu3-O15
O10-Cu3-O15
O2-Cu3-O15
O14-Cu4-O9
O14-Cu4-O11
O9-Cu4-O11
O14-Cu4-N1
O9-Cu4-N1
O11-Cu4-N1
Cu2-O15-Cu2a
Cu3-O15-Cu2
1.928(2)
1.929(2)
1.932(2)
2.000(2)
1.912(2)
1.918(3)
1.941(2)
1.973(2)
93.22(9)
171.17(9)
95.17(9)
80.94(8)
172.36(9)
90.93(9)
99.40(9)
89.33(10)
161.04(11)
159.86(10)
87.28(11)
90.20(11)
97.56(9)
95.91(9)
Cu1-O3
Cu1-O12
Cu1-O13
Cu2-O5
Cu2-O7
Cu2-O15
Cu2-O15
O6-Cu1-O3
O6-Cu1-O12
O3-Cu1-O12
O6-Cu1-O13
O3-Cu1-O13
O12-Cu1-O13
O5-Cu2-O7
O5-Cu2-O15
O7-Cu2-O15
O5-Cu2-O15
O7-Cu2-O15
O15-Cu2-O15
Cu4-O14-Cu3
Cu2-O15-Cu3a
square planar geometry is due to the small O12-Cu1-O13 angle
(65◦) due to the bite angle of acetate. The Cu2 site is coordinated
to two m4-oxides (O15 and O15a) and two phosphonate oxygens
(O5 and O7a). The Cu3 centre is coordinated to a m4-oxide (O15),
a m3-oxide (O14), one oxygen donor (O2) from a phosphonate and
one oxygen donor (O10) from acetate. Finally, Cu4 is coordinated
to a m3-oxide (O14), one oxygen donor (O9) from a phosphonate,
one oxygen donor (O11) from acetate and a lutidine. The Cu–O
bond distances are not exceptional. The distances between the Cu
centres connected through a m4-oxide (O15) are in the range of
˚
2.97-3.34 A. The Sb–O bond involving the m3-oxide is 1.962(2)
˚
A, while the other Sb–O bonds in the structure fall in the range
˚
1.998–2.022(2) A. Selected bond distances and bond angles for
compound 5 are shown in Table 6.
In each case it is necessary to make compound 1 prior to
reacting with the copper salt. If the components that make
1 are added separately to a copper salt, then insoluble and
uncharacterisable materials form, which we believe are polymeric
copper phosphonates. The condensation reaction to give 1 restricts
the formation of these polymeric materials.
Magnetic studies
DC magnetic studies were carried out on polycrystalline samples
of 2, 3 and 5, in the temperature range 2–300 K. The temperature
dependence of the magnetic susceptibility of 2 is shown as a cmT vs.
T plot (cm = molar magnetic susceptibility) in Fig. 6. The product
cmT is 1.42 cm3 K mol-1 at room temperature and decreases rapidly
upon cooling to approach 0.012 cm3 K mol-1 at 1.8 K. In addition,
the magnetic susceptibility curve shows a maximum at ca. 145 K,
which is the signature of dominant antiferromagnetic exchange
between metal centers leading to a S = 0 ground state. The non-
zero value of the cmT at low temperatures is probably due to
both temperature independent paramagnetism (TIP) and a small
amount of paramagnetic impurity (approximately 1.5%). Similar
behaviour has been reported for other Cu(II) tetramers.14
Fig. 5 The structure of 5 in the crystal shown (a) as a ball-and-stick
view (aryl group on Sb excluded for clarity), and (b) as a polyhedral view.
Colour scheme as Fig. 3.
All the Cu centres are approximately square planar. The Cu1
centre is coordinated to two O-donors from phosphonates (O3 and
O6) and the chelating acetate (O12, O13). The chief distortion from
In compound 2 the four copper centres are arranged in a
rhombus with the super-exchange paths along alternate edges of
the rhombus very different. For two edges the exchange is via
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with g = 2.166, J = -165.4 cm-1, q = -1.93 K, with the paramagnetic
impurity fraction r = 0.015, TIP = 240 ¥ 10-6 cm3 mol-1 and R =
ꢀ
1.0 ¥ 105 (R is the reliability factor defined as i[(cmT)obs(i) -
ꢀ
(cmT)calc(i)]2/ i[(cmT)obs(i)]2).
The fit is excellent (solid line, Fig. 6a), but this fit does not
define the edge of the rhombus that corresponds to the stronger
antiferromagnetic exchange. Taking into account the structure
of this compound, it is reasonably to assume that the stronger
magnetic exchange occurs via the m3 oxide bridge as the bridging
angle at this oxygen is most obtuse (Cu1-O5-Cu2 = 135◦). The
other edge is bridged by an acetate ligand, and two m2-oxygen
atoms from phosphonate. The position of these two phosphonate
oxygens is such that they are in an axial position on one Cu
and an equatorial position on the second; such bridging makes
the magnetic orbitals orthogonal and would lead to a weak, and
possibly weakly ferromagnetic, exchange interaction.15
For compound 3 the room temperature value of cmT is
1.32 cm3 K mol-1, above the calculated value of 1.24 cm3
K
mol-1 for three S = 1/2 centers with g = 2.1 (Fig. 6b). The
value declines slowly on cooling to reach 1.29 cm3 K mol-1 at
50 K, before increasing to 1.35 cm3 K mol-1 at 5 K. Below 5 K,
cmT decreases again and reaches 1.32 cm3 K mol-1 at 2 K. Close
inspection of the structure of 3 shows that the Cu2 center is well
isolated from the other two coppers, Cu1 and Cu3, so that we can
model the magnetic behaviour of 3 as the sum of contributions
of a Cu(II) dimer and a Cu(II) monomer, using eqn (2). A Weiss
constant q was incorporated into analysis to take account of weak
intermolecular interactions that occur at low temperatures. Any
interaction between the mono-copper centre and the dimer is also
contributing to this number. The best fit of cm, as well as of cmT,
using eqn (2), gave g = 2.12, J = 5.15 cm-1, q = -0.3 K, with R =
6.58 ¥ 10-6 and TIP = 180 ¥ 10-6 cm3 K mol-1.
Fig. 6 a) Plots of cmT vs. T for compounds 2 and 5, with best fit of the
data for 2 shown as a solid line; b) Plots of cm and cmT vs. T for compound
3 with the best fit of the data shown as a solid line.
2N b2g2
1
N b2g2
cmCu
=
+
+TIP
3
(2)
two O-atoms from phosphonates and a 2.11-bridging carboxylate,
while on the other two edges the bridging is via a m3-oxide, which
also binds to a Sb centre.
k B (T −q) 3+exp(−J /k BT ) 4k B (T −q)
The results show that the magnetic interaction between Cu1
and Cu3 is ferromagnetic (J = 5.15 cm-1), and this can be
rationalized as arising from the orthogonality of the magnetic
orbitals of square-planar copper(II) centers (dx2-y2 orbitals) and
bridging oxygen atoms (p orbitals) along the Cu1-O19-Cu3
connection. The second bridging ligand that connect Cu1 and
Cu3 is a phosphonate group disposed in equatorial-equatorial
position with respect to the adjacent copper ions. This ligand
may promote antiferromagnetic exchange, but exchange coupling
through phosphonate ligands is normally found to be small.6
The magnetic susceptibility data of 5 are plotted as cmT against
T in Fig. 6a. The room temperature cmT value of 3.89 cm3 K
mol-1 indicates that any interaction between the metal centres is
weak. Below 100 K, cmT falls steadily which suggests that the
predominant interactions in the cage are anti-ferromagnetic. The
value of cmT at 2 K is 1.07 cm3 K mol-1; cmT is still falling it
seems likely that the ground state of the cage is S = 0. Given the
low symmetry of the cage with many possible exchange paths, no
attempt to fit this data was made.
Attempts to simulate the magnetic data of 2 on the base of
the general spin Hamiltonian H = -Ja(S1S2 + S3S4) - Jb(S2S3 +
S1S4) - Jc(S1S3 + S2S4) with S1 = S2 = S3 = S4 = 1/2 failed,
because the three exchange couplings are correlated and could not
be determined unambiguously. Setting any one of the exchange
coupling constants to zero also failed to fit the data. However,
with Jb and Jc fixed to zero, and taking into account a small
percentage of paramagnetic impurity (ca. 1.2%) - assumed to be
a copper(II) monomer with a similar g factor - the data could
be fitted reasonably well giving Ja = -164.2 cm-1 and g = 2.17;
this g value is very close to the value of 2.173 determined by
EPR spectroscopy. This suggests that the magnetic exchange along
one edge of the rhombus is much larger than any other exchange
interaction present, so we assumed that the magnetic behaviour
of 2 could also be modeled as two weakly-interacting copper(II)
dimers, and used eqn (1) to fit the data.
4N b2g2
1
N b2g2
cmCu
=
(1− r) + r
+TIP
4
k B (T −q) 3+exp(−J /k BT )
k BT
Conclusion
(1)
A Weiss constant, q, was included to take into account possible
exchange interaction between dimers. The best fit was obtained
Compound 1 is clearly a pro-ligand that can be combined
with copper(II) to produce new polymetallic complexes, with
130 | Dalton Trans., 2010, 39, 124–131
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the nuclearity of the complex dependent on the base used in
the reaction. Initial magnetic studies show anti-ferromagnetic or
weak ferromagnetic exchange between the spin centres in these
compounds.
6 (a) M. Maheswaran, G. Chastanet, S. J. Teat, T. Mallah, R. Sessoli, W.
Wernsdorfer and R. E. P. Winpenny, Angew. Chem., Int. Ed., 2005, 44,
5044–5048; (b) M. Shanmugam, G. Chastanet, T. Mallah, R. Sessoli,
S. J. Teat, G. A. Timco and R. E. P. Winpenny, Chem.–Eur. J., 2006,
12, 8777–8785; (c) M. Shanmugam, M. Shanmugam, G. Chastanet, R.
Sessoli, T. Mallah, W. Wernsdorfer and R. E. P. Winpenny, J. Mater.
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7 S. Konar, N. Bhuvanesh and A. Clearfield, J. Am. Chem. Soc., 2006,
128, 9604–9605(a) S. Konar and A. Clearfield, Inorg. Chem., 2008, 47,
3492–3494; (b) S. Konar and A. Clearfield, Inorg. Chem., 2008, 47,
3489–3491; (c) S. Konar and A. Clearfield, Inorg. Chem., 2008, 47,
5573–5579 .
Acknowledgements
SA thanks the Higher Education Commission of Pakistan for a
studentship, and REPW thanks the Royal Society for a Wolfson
Merit Award. This work was supported by the EPSRC(UK).
8 (a) Y.-S. Ma, Y. Song, Y.-Z. Li and L.-M. Zheng, Inorg. Chem., 2007,
46, 5459–5461; (b) R. Murugavel, N. Gogoi, K. G. Suresh, S. Lavek
and H. C. Verma, Chem.–Asian J., 2009, 4, 923–935.
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