Rational assembly of soluble copper(II) phosphonates: Synthesis, structure and magnetism of molecular tetranuclear copper(II) phosphonates

Article abstract of DOI:10.1021/ic101982c

The reactions of the dinuclear copper complexes [Cu₂(L)(OAc)] [H₃L = N,N′-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) or [Cu₂(L′)(OAc)] (H₃L′ = N,N′-(2- hydroxypropane-1,3-diyl)bis(4,5-dimethylsalicylaldimin

Full text of DOI:10.1021/ic101982c

1420 Inorg. Chem. 2011, 50, 1420–1428
DOI: 10.1021/ic101982c
Rational Assembly of Soluble Copper(II) Phosphonates: Synthesis, Structure
and Magnetism of Molecular Tetranuclear Copper(II) Phosphonates#
,†
†
†
,‡
~
Vadapalli Chandrasekhar,* Tapas Senapati, Atanu Dey, and E. Carolina Sanudo*
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India, and ‡Departament de
´
ꢀ
Quımica Inorganica and Institute for Nanoscience and Nanotechnology, Universitat de Barcelona, Diagonal 647,
08028 Barcelona, Spain
Received September 28, 2010
The reactions of the dinuclear copper complexes [Cu2(L)(OAc)] [H3L = N,N0-(2-hydroxypropane-1,3-diyl)bis-
(salicylaldimine) or [Cu2(L0)(OAc)] (H3L0 = N,N0-(2-hydroxypropane-1,3-diyl)bis(4,5-dimethylsalicylaldimine)] with
various phosphonic acids, RPO3H2 (R= t-Bu, Ph, c-C5H9, c-C6H11 or 2,4,6-i-Pr3-C6H2), leads to the replacement of the
acetate bridge affording tetranuclear copper(II) phosphonates, [Cu4(L)2(t-BuPO3)](CH3OH)2(C6H6) (1), [Cu4(L)2-
(PhPO3)(H2O)2(NMe2CHO)](H2O)2 (2), [Cu4(L0)2(C5H9PO3)](CH3OH)2 (3), [Cu4(L0)2(C6H11PO3](MeOH)4(H2O)2
(4) and [Cu4(L0)2(C30H46P2O5)](PhCH3) (5). The molecular structures of 1-4 reveal that a [RPO3]2- ligand is
involved in holding the four copper atoms together by a 4.211 coordination mode. In 5, an in situ formed [(RPO2)2O]4-
ligand bridges two pairs of the dinuclear subunits. Magnetic studies on these complexes reveal that the phosphonate
ligand is an effective conduit for magnetic interaction among the four copper centers present; a predominantly
antiferromagnetic interaction is observed at low temperatures.
Introduction
compositional diversity but also because of their potential
applications in various fields such as cation exchange,3
sorption,4 catalysis,5 catalyst supports,5 and sensors6 as well
as nonlinear optics.7
In contrast to metal phosphonates that possess extended
structures, molecular metal phosphonates (zero-dimen-
sional) are of recent origin and studies on these compounds
Organophosphonates [RPO3]2- and the related family of
ligands have been attracting attention in recent years.1 Many
research groups, notably those of Clearfield and Zubieta,
have pioneered the use of phosphonate ligands to assemble a
large number of metal phosphonates that possess mainly
extended structures (1D-coordination polymers, 2D-layered
structures and 3D-pillared structures).2 These compounds
are of interest not only because of their structural and
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2751.
# Dedicated to Prof. Y. D. Vankar on his 60th birthday.
*To whom correspondence should be addressed. E-mail: vc@
iitk.ac.in (V.C.).
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r
pubs.acs.org/IC
Published on Web 01/20/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1421
have been limited.8 One of the main reasons for this is that the
multisite coordination capability of the phosphonate ligands
generally leads to the formation of insoluble compounds.2
Synthetically, this difficulty has been addressed by different
strategies. We and others have used appropriate ancillary
ligands which occupy and block a certain number of coordi-
nation sites on the transition metal ion.8 As a result, the
number of coordination sites that are accessible for the phos-
phonate ligand are limited and soluble products can be
isolated. By using this methodology we had initially prepared
a lipophilic dodecanuclear Cu(II) phosphonate cage;8a sub-
sequently different types of molecular metal phosphonates
with varying nuclearity have been assembled.8 A second
strategy is to use a sterically hindered lipophilic phosphonic
acid with or without the use of an ancillary ligand.9 Many
main-group element phosphonates and transition metal
phosphonates have been prepared by this method.9,10 The
combination of a sterically hindered phosphonic acid with an
appropriate ancillary ligand is also an excellent method to
limit the size of the metal phosphonate aggregate.11 Most of
these synthetic strategies, however, do not lead to predictable
assemblies. While the above synthetic methodology, also
known as the serendipity approach12 (because the final out-
come of the product cannot be predicted), has a demon-
strated utility, it is also important to come up with rational
strategies that can lead to a predictable outcome. One
promising approach that can lead to a rational outcome is
the use of preformed metal aggregates as precursors. Reac-
tions of trinuclear metal carboxylates [M3O(O2CR)6(H2O)3]-
X (M=Mn, Fe, V, Cr; R=H, CH3, Ph, t-Bu; X=NO3-, Cl-)
with phosphonate ligands have been shown to generate larger
metal aggregates although the size and structure of the latter
still could not be predicted with certainty.13 In view of our
interest in molecular copper(II) phosphonates8a-g,j,k we were
interested in exploring the use of precursors that would lead
to a single predictable product. We reasoned that if the
starting precursor had only one labile ligand, such as a
carboxylate (RCO2)-, replacement of this with a phospho-
nate (RPO3)2- can lead to a controlled expansion of the size
of the aggregate since the phosphonate ligand contains an
additional donor oxygen to bind. We tested our hypothesis
by the reaction of various phosphonic acids with two di-
nuclear copper(II) complexes {[Cu2(L)(OAc)] (H3L = 1,3-
bis(salicylideneamino)propan-2-ol) and [Cu2(L0)(OAc)]
(H3L0 =1,3-bis(4,5-dimethylsalicylideneamino)propan-2-ol)};
the lability of the acetate ligand in these complexes has been
demonstrated before.14 Accordingly, in this manuscript we
describe the synthesis, structural characterization and mag-
netic studies of the tetranuclear copper(II) phosphonates
[Cu4(L)2(t-BuPO3)](CH3OH)2(C6H6) (1), [Cu4(L)2(PhPO3)-
(H2O)2(NMe2CHO)](H2O)2 (2), Cu4(L0)2(C5H9PO3)](CH3-
OH)2 (3), [Cu4(L0)2(C6H11PO3](MeOH)4(H2O)2 (4) and [Cu4-
(L0)2(C30H46P2O5)](PhCH3) (5).
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Experimental Section
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Reagents and General Procedures. All the reagents and the
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lideneamino)propan-2-ol (H3L0)15 (Scheme 1), t-BuPO3-
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1422 Inorganic Chemistry, Vol. 50, No. 4, 2011
Scheme 1. Synthesis of 1-4a
Chandrasekhar et al.
a 1: R = t-Bu,.2: R = C6H5. 3: R = C5H9. 4: R = C6H11
.
acid,16 2,4,6-tri-isopropylphenylphosphonic acid9c,17 and the dinuc-
lear copper(II) complexes [Cu2(L)(OAc)] and [Cu2(L0)(OAc)]15
were prepared by using procedures that were reported in the
literature. Solvents were purified by conventional methods.18
The following chemicals were used as received: t-BuOH (s. d.
(H3L = N,N0-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine)
or [Cu2(L0)(OAc)] (H3L0
=
N,N0-(2-hydroxypropane-1,3-
diyl)bis(4,5-dimethylsalicylaldimine) were reacted with appro-
priate phosphonic acids, RPO3H2 (R = t-Bu, Ph, c-C5H9, c-
C6H11 or 2,4,6-i-Pr3-C6H2) in the presence of triethylamine in
methanol (30 mL) for a period of 12 h at room temperature. A
clear deep green-colored solution was obtained. This was com-
pletely evaporated in vacuum. The resultant green solid was
redissolved [dichloromethane/benzene (10:1) for 1, methanol/
dimethylformamide (15:1) for 2, methanol/acetonitrile (10:1)
for 3, methanol/chloroform (5:1) for 4, methanol/toluene (10:1)
for 5], filtered, and left undisturbed to allow crystallization.
After 2-3 weeks block-shaped blue-colored crystals were
obtained.
Fine Chemicals, India), Cu(OAc)2 2H2O (Fluka, Switzerland),
3
AlCl3 (s. d. Fine Chemicals, India), PCl3 (s. d. Fine Chemicals,
India), 1,3-diaminopropan-2-ol (Fluka, Switzerland), salicylalde-
hyde (Fluka, Switzerland), 4,5-dimethylsalicylaldehyde (Fluka,
Switzerland) and phenylphosphonic acid (Fluka, Switzerland).
Instrumentation. Melting points were measured using a
JSGW melting point apparatus and are uncorrected. Electronic
spectra were recorded on a Perkin-Elmer-Lambda 20 UV-vis
spectrometer and on a Shimadzu UV-160 spectrometer using
methanol as the solvent. IR spectra were recorded as KBr pellets
on a Bruker Vector 22 FT IR spectrophotometer operating
between 400 and 4000 cm-1. Elemental analyses of the com-
pounds were obtained using a Thermoquest CE instrument
CHNS-O, EA/110 model. ESI-MS analyses were performed
on a Waters Micromass Quattro Micro triple quadrupole mass
spectrometer. The ionization mechanism used was electrospray
in positive ion full scan mode using methanol as the solvent;
nitrogen gas was used for desolvation. Capillary voltage was
maintained at 3 kV, and cone voltage was kept at 30 V. The
temperature maintained for ion source was 100 °C and for
desolvation 250 °C. 1H and 31P{1H} NMR spectra were re-
corded in CD3OD solutions on a JEOL JNM LAMBDA 400
model spectrometer operating at 400.0 and 161.7 MHz respec-
tively. Chemical shifts are reported in ppm and are referenced
with respect to internal tetramethylsilane (1H) and external 85%
H3PO4 (31P).
[Cu4(L)2(t-BuPO3)](CH3OH)2(C6H6) (1). Quantities: [Cu2-
(L)(OAc)] (0.200 g, 0.420 mmol), t-BuPO3H2 (0.029 g, 0.210
mmol) and triethylamine (0.021 g, 0.420 mmol). Yield: 0.153 g,
67% (based on metal). Mp: 246 °C (dec). UV-vis (CH3OH)
λ
max/nm (ε/L mol-1 cm-1): 648 (376). FT-IR ν/cm-1: 3396 (b),
2927 (b), 1637 (s), 1600 (m), 1537 (s), 1470 (m), 1450 (s), 1398
(m), 1312 (m), 1150 (s), 986 (s), 892 (s), 886 (m), 794 (s), 756 (s),
599 (m), 463 (m). ESI-MS m/z, ion: 980.96 [{Cu4(L)2(t-BuPO3)}
þ H]þ. Anal. Calcd for C43H52Cu4N4O12P: C, 46.86; H, 4.76; N,
5.08. Found: C, 46.88; H, 4.87; N, 5.04.
[Cu4(L)2(PhPO3)(H2O)2(NMe2CHO)](H2O)2 (2). Quanti-
ties: [Cu2(L)(OAc)] (0.200 g, 0.420 mmol), PhPO3H2 (0.033 g,
0.210 mmol) and triethylamine (0.022 g, 0.42 mmol). Yield:
0.165 g, 70% (based on metal). Mp: 256 °C (dec). UV-vis
(CH3OH) λmax/nm (ε/L mol-1 cm-1): 652 (385). FT-IR ν/cm-1
:
3466 (b), 3042 (m), 2918 (m), 1635 (s), 1599 (m), 1536 (m), 1450
(s), 1338 (m), 1253 (s), 1131 (s), 1061 (m), 996 (m), 890 (m), 791
(s), 717 (m), 684 (m), 576 (m). ESI-MS m/z, ion: 1000.94
[{Cu4(L)2(PhPO3)} þ H]þ. Anal. Calcd for C43H35Cu4N5O14P:
C, 45.67; H, 3.12; N, 6.19. Found: C, 45.88; H, 3.16; N, 6.14.
[Cu4(L0)2(c-C5H9PO3)](CH3OH)2 (3). Quantities: [Cu2(L0)-
(OAc)] (0.144 g, 0.268 mmol), c-C5H9PO3H2 (0.020 g, 0.134
mmol) and triethylamine (0.027 g, 0. 268 mmol). Yield: 0.098 g,
62% (based on metal). Mp: 260 °C (dec). UV-vis (CH3OH)
Magnetic Measurements. Magnetic data were collected at the
ꢀ
Unitat de Mesures Magnetiques at the Universitat de Barcelona
using crushed crystals of the sample on a Quantum Design
MPMS-XL SQUID magnetometer equipped with a 5 T magnet.
The data were corrected for TIP, and the diamagnetic correc-
tions were calculated using Pascal’s constants; an experimental
correction for the sample holder was applied.
Synthesis. General Synthetic Procedure Used for the Pre-
paration of 1-5. Dinuclear copper complexes [Cu2(L)(OAc)]
λ
max/nm (ε/L mol-1 cm-1): 650 (703). FT-IR ν/cm-1: 3423 (b),
2917 (m), 2860 (m), 1630 (s), 1515 (s), 1475 (m), 1453 (s), 1392 (s),
1279 (s), 1226 (m), 1180 (m), 1147 (m), 1118 (m), 1062 (s), 964 (s),
881 (m), 861 (m), 779 (s), 678 (s), 589 (m), 518 (m), 468 (þm). ESI-
MS m/z, ion: 1105.098 [{Cu4(L0)2(C5H9PO3)} þ H] . Anal.
(18) Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman:
London, 1989.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1423
Table 1. Crystal Data and Structure Refinement Parameters of 1-5
parameters
empirical formula
formula weight
temp (K)
˚
wavelength (A)
1
2
3
4
5
C43H52Cu4N4O12
1102.02
100(2)
0.71073
orthorhombic, Pbca
P
C43H35Cu4N5O14
1130.89
173(2)
0.71073
monoclinic, P21/c
P
C49H63Cu4N4O11
1169.16
100(2)
0.71069
triclinic, P1
P
C52H69Cu4N4O15
1275.29
100(2)
0.71069
triclinic, P1
P
C79H99Cu4N4O11P2
1596.77
100(2)
0.71069
triclinic, P1
cryst syst, space group
unit cell dimens
˚
a (A)
20.533(10)
18.225(9)
24.150(12)
90
90
13.841(2)
33.287(5)
10.577(16)
90
107.414(3)
90
11.927(5)
14.305(5)
16.632(5)
74.943(5)
75.092(5)
68.735(5)
2511.4(16)
2, 1.546
13.168(5)
14.028(5)
16.709(5)
75.594(5)
76.835(5)
67.154(5)
15.806(5)
16.854(5)
17.237(5)
68.379(5)
86.815(5)
64.470(5)
3823(2)
2, 1.387
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
90
9037.3(8)
3
˚
vol (A )
4649.9(12)
4, 1.615
2725.0(16)
2, 1.554
Z, calcd density (mg/m3)
abs coeff
8, 1.620
1.959 mm-1
4520
1.910 mm-1
2284
1.765 mm-1
1208
1.639 mm-1
1320
1.201 mm-1
1670
F(000)
cryst size (mm3)
θ range for data collection
limiting indices
0.30 ꢀ 0.20 ꢀ 0.10
1.98 to 28.32°
-27 e h e 27,
-24 e k e 24,
-16 e l e 32
57925/11242
[Rint = 0.0768]
99.8%
0.16 ꢀ 0.10 ꢀ 0.05
1.97 to 28.32°
-15 e h e 18,
-43 e k e 44,
-13 e l e 13
30571/11428
[Rint = 0.0619]
98.6%
0.21 ꢀ 0.19 ꢀ 0.15
2.03 to 26.00°
-14 e h e12,
-15 e k e17,
-18 e l e 20
14038/9639
[Rint = 0.0493]
97.5%
9639/0/637
0.6 ꢀ 0.5 ꢀ 0.3
2.22 to 25.50°
-15 e h e14,
-16 e k e13,
-20 e l e20
14319/9886
[Rint = 0.0329]
97.6%
9886/0/701
1.041
R1 = 0.0839,
wR2 = 0.2275
R1 = 0.1228,
wR2 = 0.2660
0.30 ꢀ 0.20 ꢀ 0.10
2.04 to 25.50°
-17 e h e19,
-16 e k e20,
-20 e l e20
20508/13909
[Rint = 0.0332]
97.7%
reflns collected/unique
vompleteness to θ
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
11242/1 /571
1.035
11428/0/593
1.038
13909/0/921
1.026
1.025
R1 = 0.0564,
wR2 = 0.1306
R1 = 0.0819,
wR2 = 0.1432
1.693 and -0.707
R1 = 0.0630,
wR2 = 0.1523
R1 = 0.1197,
wR2 = 0.1997
1.138 and -0.694
R1 = 0.0915,
wR2 = 0.2194
R1 = 0.1562,
wR2 = 0.2674
1.177 and -0.726
R1 = 0.0567,
wR2 = 0.1343
R1 = 0.0807,
wR2 = 0.1532
0.969 and -0.621
R indices (all data)
-3
˚
largest diff. peak and hole (e A
)
1.711 and -1.139
3
Calcd for C49H63Cu4N4O11P: C, 50.34; H, 5.43; N, 4.79. Found:
C, 50.21; H, 5.12; N, 4.20.
slow evaporation of dichloromethane/benzene (1), methanol/
DMF (2) methanol/acetonitrile (10:1) (3), methanol/chloroform
(5:1) (4) or methanol/toluene (10:1) (5) solutions. The crystal data
for compounds 1-5 have been collected on a Bruker SMART
CCD diffractometer using a Mo KR sealed tube. The program
SMART19a was used for collecting frames of data, indexing
reflections, and determining lattice parameters, SAINT19a for
integrationof the intensity of reflections and scaling, SADABS19b
for absorption correction, and SHELXTL19c,d for space group
and structure determination and least-squares refinements on
F2. All the structures were solved by direct methods using the
program SHELXS-9713e and refined by full-matrix least-squares
methods against F2 with SHELXL-97.19e Hydrogen atoms were
fixed at calculated positions, and their positions were refined by a
riding model. All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. The disordered water molecules
were refined isotropically. The crystallographic figures have been
generated using Diamond 3.1e software.19f
[Cu4(L0)2(c-C6H11PO3)](MeOH)4(H2O)2 (4). Quantities:
[Cu2(L0)(OAc)] (0.120 g, 0.22 mmol), c-C6H11PO3H2 (0.019 g,
0.115 mmol) and triethylamine (0.022 g, 0.22 mmol). Yield:
0.078 g, 55% (based on metal). Mp: 264 °C (dec). UV-vis (CH3-
OH) λmax/nm (ε/L mol-1 cm-1): 650 (816). FT-IR ν/cm-1: 3416
(b), 2917 (m), 2852 (m), 1629 (s), 1516 (s), 1475 (m), 1453 (s),
1392 (s), 1343 (m), 1278 (s), 1227 (m), 1181 (m), 1138 (s), 1118 (s),
1061 (s), 960 (s), 858 (m), 779 (s), 682 (s), 589 (m), 519 (þm). ESI-
MS m/z, ion: 1119.11 [{Cu4(L0)2(C6H11PO3)} þ H] . Anal.
Calcd for C52H77Cu4N4O15P: C, 48.67; H, 6.05; N, 4.37. Found:
C, 48.28; H, 6.06; N, 4.14.
[Cu4(L0)2(2,4,6-i-Pr3-C6H2PO2)2O)](PhCH3) (5). Quantities:
[Cu2(L0)(OAc)] (0.142 g, 0.264 mmol), 2,4,6-i-Pr3-C6H2PO3H2
(0.038 g, 0.132 mmol) and triethylamine (0.027 g, 0.264 mmol).
Yield: 0.123 g, 58% (based on metal). Mp: 274 °C (dec). UV-vis
(CH3OH) λmax/nm (ε/L mol-1 cm-1): 635 (642). FT-IR ν/cm-1
:
3430 (b), 2957 (m), 2919 (m), 2862 (m), 1630 (s), 1569 (m), 1512
(s), 1456 (m), 1394 (m), 1275 (m), 1250 (m), 1181 (s), 1062 (m),
999 (m), 867 (s), 778 (s), 732 (m), 666 (m), 583 (m), 517 (s), 469
(m), 431 (m). ESI-MS m/z, ion: 1505.35 [{Cu4(L0)2(2,4,6-i-Pr3-
C6H2PO2)2O)} þ H]þ. Anal. Calcd for C79H100Cu4N4O11P2: C,
59.38; H, 6.31; N, 3.51. Found: C, 59.18; H, 6.16; N, 3.34.
X-ray Crystallography. The crystal data and the cell param-
eters for compounds 1-5 are given in Table 1. Single crystals
suitable for X-ray crystallographic analyses were obtained by
Results and Discussion
Synthetic Aspects. Two dinuclear copper complexes
[Cu2(L)(OAc)] and [Cu2(L0)(OAc)] were prepared by the
reaction of H3L and H3L0 with Cu(OAc)2 2H2O respec-
tively (Schemes 1 and 2). The unique structural feature of
3
the deprotonated trianionic ligand platform (L3- or L0
)
3-
allows the encapsulation of two copper centers. The two
copper atoms are bridged by a μ-O as well as a μ-acetate.
An examination of these complexes reveals that the di-
nuclear copper framework is quite robust; on the other
hand the acetate bridge is the weak link and can be
replaced by other ligands.14 We reasoned that replacing
the bidentate acetate ligand in these dinuclear complexes
with the potentially multidentate phosphonate ligand,
[RPO3]2- would lead to an automatic cluster expansion;
the latter should retain the robust dinuclear Cu2 motifs of
(19) (a) SMART & SAINT Software Reference manuals, Version 6.45;
Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2003. (b) Sheldrick,
G. M. SADABS a software for empirical absorption correction, Ver. 2.05;
€
€
University of Gottingen: Gottingen, Germany, 2002. (c) SHELXTL Reference
Manual, Ver. 6.1; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2000.
(d) Sheldrick, G. M. SHELXTL, Ver. 6.12; Bruker AXS Inc.: Madison, WI, 2001.
(e) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refinement;
€
€
University of Gottingen: Gottingen, Germany, 1997. (f) Bradenburg, K. Diamond,
Ver. 3.1eM; Crystal Impact GbR: Bonn, Germany, 2005.

1424 Inorganic Chemistry, Vol. 50, No. 4, 2011
Scheme 2. Synthesis of 5
Chandrasekhar et al.
the starting precursors. Accordingly a two component
reaction involving [Cu2(L)(OAc)] or [Cu2(L0)(OAc)] with
RPO3H2 (R = t-Bu, Ph, c-C5H9-, c-C6H11-, 2,4,6-i-Pr3-
C6H2-) was carried out in the presence of triethylamine
affording tetranuclear Cu(II) compounds [Cu4(L)2(t-
BuPO3)](CH3OH)2(C6H6) (1), [Cu4(L)2(PhPO3)(H2O)2-
(NMe2CHO)](H2O)2 (2), [Cu4(L0)2(C5H9PO3)](CH3-
OH)2 (3), [Cu4(L0)2(C6H11PO3](MeOH)4(H2O)2 (4) and
[Cu4(L0)2(2,4,6-i-Pr3-C6H2PO2)2O)](PhCH3) (5) in good
yields (Schemes 1 and 2). Interestingly while in 1-4 the
phosphonate ligand, [RPO3]2-, is involved in holding the
tetranuclear compound together, in 5, the tetraanionic
anhydride [(RPO2)2O]4-, which is generated in situ, glues
the two dinuclear units.
Electronic spectra of these tetranuclear compounds
revealed a broad d-d transition in the region of 650 ( 2
nm (1-4) and 635 nm (5) (see Experimental Section). ESI-
MS spectra of the tetranuclear Cu(II) complexes 1-5
reveal that they retain their structures in solution as
indicated by the presence of peaks at m/z = 980.9617
corresponding to [Cu4(L)2(t-BuPO3)]þ for 1, m/z = 1000.93
corresponding to [Cu4(L)2(PhPO3)]þ for 2, m/z =
1105.098 for 3 corresponding to [{Cu4(L0)2(C5H9-
PO3)}]þ, m/z = 1119.11 for 4 corresponding to [{Cu4-
(L0)2(C6H11PO3)}]þ, and m/z=1505.35 for 5 correspond-
ing to [{Cu4(L0)2(C30H46P2O5)}]þ.
Molecular Structures of 1-5. The solid-state structures
of 1-5 were confirmed by their single crystal X-ray
diffraction studies. Crystallographic parameters of these
compounds are given in Table 1. All the bond parameters
of these compounds are given in Tables S1-S5 in the
Supporting Information. Although all the compounds
are tetranuclear, there are variations in the structural and
compositional details. In compounds 1-4 one [RPO3]2-
is involved in holding the tetranuclear aggregate together.
On the other hand in 5 the tetraanionic [(RPO2)2O]4- is
involved. Even in 1-4 there are subtle differences; com-
pound 1 belongs to one type while 2-4 belong to another.
The molecular structure of 1 is shown in Figure 1. An
examination of the molecular structure of 1 reveals that
the dinuclear motif of the starting material is completely
Figure 1. Molecular structure of 1 shown with 50% ellipsoidal prob-
ability. Hydrogen atoms are omitted for clarity.
conserved. The two pairs of copper atoms in each di-
nuclear unit (Cu4 and Cu2; Cu1 and Cu3) (which were
bridged by acetate bridges in the starting precursor) are
held together by a single [RPO3]2- ligand in a [4.211]20
coordination mode (Chart 1). Thus, O2 acts as a single
atom bridge between Cu2 and Cu1, while O1 and O3 are
coordinated to Cu4 and Cu3. As a result of this coordina-
tion, two six-membered rings are simultaneously gener-
ated [P1-O1-Cu4-O8-Cu2-O2; P1-O3-Cu3-O9-
Cu1-O2]. The P-O distances for the phosphonate li-
˚
gands are nonequivalent, P(1)-O(1), 1.514(3) A, P-
˚
˚
(1)-O(2), 1.553(3) A and P(1)-O(3), 1.507(3) A. It can
be noted that the longest P-O distance involves O2 which
(20) Coxall, R. A.; Harris, S. G.; Henderson, S.; Parsons, S.; Taskar,
R. A.; Winpenny, R. E. P. J. Chem. Soc., Dalton Trans. 2000, 14, 2349.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1425
Chart 1. Bridging Mode of Phosphonic Acids:20 (a) Complexes 1-4
and (b) Complex 5
is found to bridge two copper atoms. The phosphonate
coordination in 1 forces the phenolate oxygen atoms O7
and O6 to utilize their residual basicity and coordinate to
Cu1 and Cu2 respectively. This causes the formation of
three puckered four-membered rings [Cu1-O2-Cu2-
O6; Cu1-O2-Cu2-O7; Cu1-O6-Cu2-O7]. Among
the four copper atoms, Cu1 and Cu2 are five-coordinate
(4O, 1N; distorted square pyramidal; apical position
occupied by the phenolate oxygen atom O7 or O6) while
Cu3 and Cu4 are tetracoodinate (3O, 1N; square planar).
The Cu-O bond distances observed in 1 are reasonably
similar to each other although the apical Cu-O distances
Figure 2. Molecular structure of 2 shown with 50% ellipsoidal prob-
ability. Hydrogen atoms are omitted for clarity.
˚
[Cu1-O7 (2.487(2) and Cu2-O6 (2.716 (3) A] are clearly
longer than the others. Among the other Cu-O bond
distances, those involving η1 phosphonate oxygen atoms
˚
are shorter [Cu3-O3, 1.915 (3) A, and Cu4-O1, 1.935 (3)
A] in comparison to the one involved in the bridging mode
˚
˚
[Cu1-O2 and Cu2-O2, 1.962(3) (A)].
Since the molecular structures of 2-4 are quite similar,
the structure of 2 is shown as a representative example in
Figure 2. The main difference between the structures of 2
and 1 is that in the former only one of the phenolate
oxygen atoms exercises its residual basicity and coordi-
nates to the copper atom of the other dinuclear subunit.
Thus, Cu3 and Cu1 are bridged by O1, while O6 does not
participate in such a coordination action. This is presum-
ably due to the fact that the fifth coordination site on Cu3
is occupied by the water molecule (O14). As a result the
central part of the tetranuclear ensemble contains two six-
membered rings and only one four-membered ring. All
the four copper atoms in 2 are five-coordinate in a
distorted square pyramidal geometry. It is to be noticed
that the fifth coordination site around Cu1 and Cu4 also
is occupied by a solvent molecule (water or dimethyl-
formamide). The coordination environment of the var-
ious copper atoms is summarized in Table S6 in the
Supporting Information. The two dinuclear segments
present in 1 and 2 are nearly orthogonal to each other
as illustrated in Figure 3. The molecular structures of 3
and 4 are similar to that of 2 except that in these
compounds three of the copper atoms are four-coordi-
nate and square planar while one copper is five-coordi-
nate and distorted square pyramidal (see Figures S1 and
S2 in the Supporting Information).
Figure 3. Representation of the two dinuclear segments of complexes 1
and 2.The angle between two Cu2P planes is 79.804(78)o (1) and
84.925(58)o (2).
situ under the reaction conditions. Precedents for the
formation of such species exist in the literature.21 Each
of the two pairs of the PO2 units of the phosphonic acid
anhydride ligand is involved in bridging two copper
atoms of a dinuclear subunit. This coordination mode
of the PO2 unit is analogous to that of an acetate ligand.
The overall coordination mode of the [(RPO2)2O]4-
ligand is [4.1111] (Chart 1). The P-O bonds involved in
˚
the P-O-P unit [O(9)-P(1) and O(9)-P(2): 1.610(3) (A)
˚
and 1.612(3) (A)] are longer than the remaining P-O
˚
bonds [O(7)-P(1), 1.493(3); O(8)-P(1), 1.492(3) (A);
˚
O(10)-P(2), 1.491(3) (A); and O(11)-P(2), 1.499(3)
(A)]. The flexible P-O-P unit of the [(RPO2)2O]4- ligand
˚
facilitates the two dinuclear copper units to fold over and
to lie nearly on top of each other (Figure 6). Furthermore
all the copper atoms in 5 are tetracoordinate and square
planar (Table S6 in the Supporting Information). It may
be mentioned that the Cu-O bond distances found in the
present instance are comparable to that found in the
starting dinuclear compound [Cu2(L)(OAc)].15
The molecular structure of 5 is different from the others
discussed thus far. The two dinuclear segments found in
this compound are stitched to each other by the
[(RPO2)2O]4- ligand (Figure 4). The former is formed in
(21) Kingsley, S.; Vij, A.; Chandrasekhar, V. Inorg. Chem. 2001, 40, 6057.

1426 Inorganic Chemistry, Vol. 50, No. 4, 2011
Chandrasekhar et al.
Figure 6. Exchange coupling model for complexes 1-5 with the atom
numbering used in the spin Hamiltonian.
vs T plots are shown in Figure 5. The χT value at 300 K for
1 was 0.76 cm3 K mol-1, nearly 1 cm3 K mol-1 below the
expected value for four non-interacting Cu(II) ions. This
indicates strong antiferromagnetic coupling, and at room
temperature, the S = 0 spin ground state is still partly
populated. As temperature decreases, so does the χT
product, until a plateau is reached below 50 K. The signal
becomes very weak at low temperatures, and only the
data above 10 K were used to obtain the exchange
parameters. The Magpack software22 which performs
the full-matrix diagonalization of the spin matrix, calcu-
Figure 4. Molecular structure of 5 shown with 50% ellipsoidal prob-
ability. Hydrogen atoms are omitted for clarity.
^
^ ^
lated using the spin Hamiltonian H = -J1[S2S3] -
^ ^
^ ^
^ ^
^ ^
^ ^
2J2[S1S2 þ S3S4] - J3[S1S3 þ S1S4 þ S2S4], was used
to model the magnetic data following the numbering
shown in Figure 6. There are three main exchange path-
ways, labeled J1, J2 and J3 in Figure 6. The exchange
pathways in J1 and J2 are mediated by bridging oxygen
atoms of the alkoxide groups of the polydentate ligand
(C21H23N2O3)3-
, with Cu-O-Cu angles between
118.05° and 137.15° for J2 and Cu-O-Cu angles of
95.04° and 105.30° for J1. For Cu-alkoxide compounds
with Cu-O-Cu angles larger than 95° strong antiferro-
magnetic coupling is expected.23 However, for J1 one
must take into account the fact that the Cu-O-Cu angle
of 95° corresponds to an axial-equatorial dz2 - dx2 - y2
square interaction, which is expected to afford a weak
ferromagnetic coupling,24 thus the value of J1 will be less
antiferromagnetic than expected. Finally, J3 should be
antiferromagnetic, as expected for coupling mediated by
phosphonate ligands.9a,b,13h-13l,8d-8k To avoid overpar-
ametrization of the system only one average exchange
constant value was used, thus J1 = J2 = J3. A para-
magnetic impurity, probably a Cu(II) salt, was included
in the model to fit the experimental data. The best fittings
are shown in Figure 5 as a solid line. The fitting param-
eters for complex 1 were J1 = -74.13 cm-1, and 0.2% of
paramagnetic Cu(II) impurity; the g was fixed at 2.05, and
the agreement between experimental and calculated data
was F(obj) = 0.001. The χT value at 300 K for 2 was 1.86
cm3 K mol-1, above the expected 1.5 cm3 K mol-1 for
four non-interacting Cu(II) ions with S = 1/2 and g =
2.0. This is due to the presence of a paramagnetic impurity
as well as the thermal population of excited states of S
values larger than zero. As temperature decreases, the χT
product decreases, indicating strong antiferromagnetic
Figure 5. χT vsT plotfor complexes 1 (circles), 2 (dots), 3 (triangles) and
5 (squares). The solid line is the best fit; see text for the fitting parameters.
The measurement wascarriedoutatanappliedfield of1.0T inthe2 to300
K temperature range.
Among the tetranuclear copper phosphonates known
thus far, several structural types are known as shown for
[Cu4(μ3-MePO3)2(μ-Cl)2(Cl)2(3,5-Me2PzH)4],8b [Cu4(μ3-
C6H11PO3)2(μ-Cl)2(bpy)4][NO3]2, [Cu4(μ3-C6H11PO3)2(μ-
CH3COO)2(bpy)4][CH3COO]2, and [Cu4(μ3-C5H9PO3)2(μ-
Cl)2(bpa)4][Cl2] 2MeOH (type I),8e,j [Cu4(μ3-C6H11-
3
PO3)2(μ-OH)(bpy)4(H2O)2][NO3]3, and Cu4(μ3-C6H11-
PO3)2(μ-OH)(phen)4(H2O)2][NO3]3 (type II),8e,j [Cu4(μ3-
OH)2{ArPO2(OH)}2(CH3COO)2(3,5-Me2PzH)4][CH3-
COO]2 CH2Cl2 (type III),9c [Cu4(3,5-t-Bu2PzH)4(μ3-
3
t-BuPO3)4] (type IV)8d and [Cu4(OH)(Ph3CPO3)3-
(Ph3PO2OH)Py4 H2O CH3CN] (type V)9a (Chart 2).
3
3
The compounds discussed in this study represent new
structural types not known hitherto.
Magnetic Studies. Magnetic susceptibility data were
collected for crushed crystalline samples of 1-3 and 5
(due to structural similarity of complexes 3 and 4, we
carried out the magnetic study only for 3) at an applied dc
field of 1.0 T in the 2 to 300 K temperature range. Addi-
tional data were collected at an applied dc field of 500 G
below 30 K, where no field dependence was observed. χT
ꢁ
(22) Borras-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.; Tsukerblat,
B. S. J. Comp. Chem. 2001, 2, 2985 (Magpack Software).
(23) Merz, L.; Haase, W. J. Chem. Soc., Dalton Trans. 1980, 875.
(24) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993; p
26.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1427
Chart 2. Different Tetranuclear Copper Phosphonate Cores Known in the Literaturea
a Carbon, hydrogen and solvent molecules have been deleted for clarity.
coupling within the Cu(II) centers of 2. As temperature
decreases, so does the χT product, until a plateau is
reached below 50 K (Figures 5 and S8 in the Supporting
Information). The same Hamiltonian used for 1 was used
to fit the experimental data with the software package
Magpack. The best fitting is shown in Figure 5 as a solid
line, and the fitting parameters were J1 = -44.22 cm-1
and 0.7% of paramagnetic Cu(II) impurity with a fixed g
value of 2.05 and F(obj) = 0.0305.
The χT product for 3 at 300 K has a value of 0.94 cm3 K
mol-1 per tetranuclear complex, well below the expected
1.5 cm3 K mol-1 for four non-interacting Cu(II) ions with
g = 2.0 and S = 1/2, indicating strong antiferromagnetic
coupling. As temperature decreases, so does the χT
product, following a sinusoidal shape that reaches a value
near zero below 100 K due to the Boltzmann population
of the well-isolated S = 0 spin ground state of 3. The best
simulation of the experimental data was obtained for

1428 Inorganic Chemistry, Vol. 50, No. 4, 2011
Chandrasekhar et al.
g = 2.2, and an average J value, J1 = -86 cm-1. A 3% of
paramagnetic mononuclear Cu(II) impurity was included
in order to model the χT value observed below 50 K. For
complex 5 at 300 K the χT product has a value of 1.47 cm3
K mol-1, in agreement with the expected 1.5 cm3 K mol-1
value for four non-interacting Cu(II) ions with S = 1/2.
As temperature decreases, the χT product decreases until
it reaches a value of nearly 0 cm3 K mol-1 below 50 K, as
shown in Figure 5 due to the Boltzmann population as
temperature decreases of the ground state S = 0 for 5.
Again, there is strong antiferromagnetic coupling be-
tween the Cu(II) ions in 5 that leads to an S = 0 spin
ground state. The χT product does not reach exactly 0 cm3
K mol-1 due to the presence of small amount of para-
magnetic impurity, probably a Cu(II) compound. Due to
the structure of 5 a magnetic model different from that
used for 1-3 must be employed. Now only one exchange
constant is considered: the main exchange pathway is
through the μ-oxygen from an alkoxide function of the
ligand: O2 bridges Cu1-Cu2 and O5 links Cu3-Cu4 (as
labeled in the crystallographic data) through their dx2 -
y2 magnetic orbitals at angles of 136.12° and 133.96°. The
oxygen atoms are located in the axial positions of Cu3 and
Cu2, leading to an axial-equatorial interaction that
should be very weak24 but the distance between the two
dinuclear units formed by Cu1-Cu2 and Cu3-Cu4 is
to strong antiferromagnetic coupling.25 Thus, the mag-
netic data has been modeled as two Cu2 units, using the
^
spin Hamiltonian H = -2J[S1 S2] and writing an analy-
3
tical Van Vleck equation using the equivalent operator
approach. Since the data below 50 K has a value of χT =
0.05 cm3 K mol-1, a paramagnetic Cu(II) impurity has
been included in the fitting. The best agreement is shown
in Figure 5 as a solid line, and the best fitting parameters
were g = 2.31 ( 0.02, J = -85.7 ( 0.2 cm-1 and 3% of
Cu(II) impurity.
Conclusion
We have described an approach that can lead to a con-
trolled expansion of a metal aggregate. In this method a
dicopper precursor was chosen that contained two distinct
types of ligands, a stable Schiff-base ligand platform that held
the two copper atoms together and an acetate bridging ligand
that could be replaced by other ligands. Accordingly repla-
cing the latter with a phosphonate ligand that possessed an
additional oxygen donor site ensured automatic increase in
the nuclearity of resulting metal ensemble. The possibility of
successfully extending this synthetic principle to other sys-
tems depends on the design of the precursor complexes.
Currently we are exploring this area.
˚
around 2.8 A, too long to consider this interaction in the
Acknowledgment. We thank the Department of Science
and Technology, India, and Council of Scientific and
Industrial Research, India, for financial support. V.C. is
thankful for the Department of Science and Technology,
for a J.C. Bose fellowship. T.S. thanks Council of Scien-
tific and Industrial Research, India, for Senior Research
Fellowship. A.D. thanks Council of Scientific and In-
dustrial Research, India, for Senior Research Fellowship.
model. Thus, complex 5 is taken as two antiferromagne-
tically coupled Cu2 units. The diphosphonate ligands
have been proven to provide efficient pathways for
magnetic exchange, however, here the magnitude of the
exchange through the monatomic oxo bridge dominates
the magnetic behavior.9a,8g,8j Even though most of the
structural correlations relating the Cu-O-Cu angle with
the exchange constant are done on Cu2O2 units with
angles smaller than 110°, the results suggest that
Cu-O-Cu angles such as those found in 5 would lead
Supporting Information Available: One cif file. Figures S1 and
S2 (Diamond pictures of compounds 3 and 4), Figures S3-S7
(ESI-MS spectra of 1-5), Figure S8 (χ vs T plots for 1-3 and 5),
Tables S1-S5 (bond parameters of 1-5), Table S6
(coordination environment of 1-5). This material is available
free of charge via the Internet at http://pubs.acs.org.
(25) (a) Merz, L.; Haase, W. J. Chem. Soc. Dalton 1980, 875.
(b) Thompson, K. L.; Mandal, K. S.; Tandon, S. S.; Bridson, N. J.; Park, K. M.
Inorg. Chem. 1996, 35, 3117.