Synthesis, structure, magnetism and nuclease activity of tetranuclear copper(ii) phosphonates containing ancillary 2,2′-bipyridine or 1,10-phenanthroline ligands

Article abstract of DOI:10.1039/b712876b

The reaction of cyclohexylphosphonic acid (C₆H ₁₁PO₃H₂), anhydrous CuCl₂ and 2,2′-bipyridine (bpy) in the presence of triethylamine followed by a metathesis reaction with KNO₃ afforded [Cu<

Full text of DOI:10.1039/b712876b

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www.rsc.org/dalton | Dalton Transactions
Synthesis, structure, magnetism and nuclease activity of tetranuclear
copper(II) phosphonates containing ancillary 2,2ꢀ-bipyridine or
1,10-phenanthroline ligands†‡
Vadapalli Chandrasekhar,*a Ramachandran Azhakar,a Tapas Senapati,a Pakkirisamy Thilagar,a
Surajit Ghosh,a Sandeep Verma,a Ramamoorthy Boomishankar,b Alexander Steinerb and Paul Ko¨gerlerc
Received 21st August 2007, Accepted 16th November 2007
First published as an Advance Article on the web 7th January 2008
DOI: 10.1039/b712876b
The reaction of cyclohexylphosphonic acid (C6H11PO3H2), anhydrous CuCl2 and 2,2ꢀ-bipyridine (bpy)
in the presence of triethylamine followed by a metathesis reaction with KNO3 afforded
[Cu4(l-Cl)2(l3-C6H11PO3)2(bpy)4](NO3)2 (1). In an analogous reaction involving Cu(OAc)2·H2O, the
complex [Cu4(l-CH3COO)2(l3-C6H11PO3)2(2,2ꢀ-bpy)4](CH3COO)2 (2) has been isolated. The
three-component reaction involving Cu(NO3)2·3H2O, cyclohexylphosphonic acid and 2,2ꢀ-bipyridine in
the presence of triethylamine afforded the tetranuclear assembly [Cu4(l-OH)(l3-C6H11PO3)2(2,2ꢀ-bpy)4
(H2O)2](NO3)3 (3). Replacing 2,2ꢀ-bipyridine with 1,10-phenanthroline (phen) in the above reaction
resulted in [Cu4(l-OH)(l3-C6H11PO3)2(phen)4(H2O)2](NO3)3 (4). In all the copper(II) phosphonates
(1–4) the two phosphonate ions bridge the four copper(II) ions in a capping coordination action. Each
3
phosphonate ion bridges four copper(II) ions in a l4, g coordination mode or 4.211 of the Harris
notation. Variable-temperature magnetic studies on 1–4 reveal that all four complexes exhibit
moderately strong intramolecular antiferromagnetic coupling. The DNA cleavage activity of complexes
1–4 is also described. Compounds 1 and 3 were able to completely convert the supercoiled pBR322
DNA form I to nick form II without any co-oxidant. In contrast, 50% conversion occurred with 2 and
40% with 4. In the presence of magnesium monoperoxyphthalate all four compounds achieved rapid
conversion of form I to form II.
acids in conjunction with ancillary ligands is another strategy
that has gained importance for assembling discrete molecular
Introduction
assemblies of transition metal phosphonates.12–15 We have been
interested, for some time, in developing appropriate synthetic
routes for molecular main group and transition metal phos-
phonates.16–18 Our initial discovery of a lipophilic dodecanuclear
copper phosphonate, [Cu12(l4-Cl)4(l3-Cl)2(g -3,5-Me2Pz)6(g -3,5-
Me2Pz)4(l3-t-BuPO3)4(l2-t-BuPO3)2(l2-t-BuPO2OH)2 has spur-
red us to investigate the assembly and structures of molecular
copper phosphonates. Previously we have shown that phosphonic
acids (such as t-BuP(O)(OH)2 or 2,4,6-iPr3–C6H2–PO3H2)
in conjunction with ancillary ligands such as pyrazoles are
extremely effective for the preparation of soluble molecular
multi-metal phosphonates.19–21 In addition to the aforementioned
dodecanuclear Cu(II) cage, we have also been able to isolate tetra-
Transition metal phosphonates have been attracting considerable
interest in recent years.1 One of the reasons for this is the structural
and compositional diversity that is present among these materials.
In addition, they have also been attracting interest in view of
their wide-ranging potential applications. These include their use
as cation exchangers with potential utility in the processing of
radioactive waste streams.2 Other possible applications of these
materials include sorption,3 catalysis,4 catalyst supports,4 sensors5
and nonlinear optics.6 Many transition metal phosphonates
possess extended structures and are generally prepared by
solvothermal and hydrothermal procedures. In recent years, there
have also been efforts to prepare molecular phosphonates that are
soluble in common organic solvents. One of the main challenges in
this endeavour has been to devise new synthetic strategies. Use of
sterically hindered and lipophilic phosphonic acids has been found
to be conducive to promoting solubility.7–11 The use of phosphonic
1
2
nuclear
[Cu4(l3-OH)2{ArPO2(OH)}2(CH3CO2)2(DMPZH)4]-
[CH3COO]2·CH2Cl2 (Ar = 2,4,6-iPr3–C6H2)20 and decanuclear
Cu(II)
cages
[Cu5(l3-OH)2(t-BuPO3)3(2-PyPz)2(MeOH)]2·
10MeOH·2H2O.21 In our quest for new lipophilic phosphonic acids
we were intrigued by the possibility of using cyclohexylphosphonic
acid as it satisfies the steric and lipophilic requirements. Although
cyclohexylphosphonic acid has been known in the literature for
quite some time, it has not been widely used for the preparation
of molecular metal phosphonates. A cobalt(II) phosphonate,
[Co(O3PCy)·H2O]n and a cobalt complex [{Co(H2O)4(DMP)2}-
{CyPO3H}2]n (DMP = 3,5-dimethylpyrazole; Cy = cyclohexyl)
have been previously reported.22 However, the molecular structure
of the former is unknown while the X-ray crystal structure
aDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur,
UP, 208016, India. E-mail: vc@iitk.ac.in; Fax: +91 (0)512 259 7259
bDepartment of Chemistry, University of Liverpool, Liverpool, U.K. , L69
7ZD
cAmes Laboratory and Department of Physics and Astronomy, Iowa State
University, Ames, Iowa, 50011, USA
† CCDC reference numbers 641412–641415. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b712876b
‡ Electronic supplementary information (ESI) available: MS spectra and
selected bond angles and lengths for complexes 1–4, with details of their
supramolecular structures. See DOI: 10.1039/b712876b
1150 | Dalton Trans., 2008, 1150–1160
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of the latter showed that cyclohexylphosphonic acid is not
coordinated to the metal ion but is present only as a counter
anion. Herein we report the first (structurally characterized)
examples of molecular transition metal phosphonates prepared
by using cyclohexylphosphonic acid. Accordingly we describe in
this paper the synthesis and molecular structures of tetranuclear
copper(II) phosphonates [Cu4(l3-C6H11PO3)2(l-Cl)2(bpy)4](NO3)2
(1), [Cu4(l-CH3COO)2(l3-C6H11PO3)2(bpy)4](CH3COO)2 (2),
[Cu4(l-OH) (l3-C6H11PO3)2(bpy)4(H2O)2](NO3)3 (3) and [Cu4(l-
OH)(l3-C6H11PO3)2(phen)4(H2O)2](NO3)3 (4). We also report
the magnetic properties and DNA cleavage activity of these
compounds.
tesla. Experimental DC susceptibility data have been corrected for
diamagnetic contributions. Singlet ground states (and the absence
of significant amounts of impurities such as isolated Cu(II) centres)
were detected by field-dependent measurements. Least-squares
fitting of the Heisenberg model Hamiltonian to the experimental
data employed a modified version of MAGPACK.24
pBR322 cleavage assay
Plasmid cleavage reactions were performed in sodium cacodylate
◦
buffer (10 mM, pH 7.5, 32 C), containing pBR322 (8 ng lL−1,
Bangalore Genei), solution of the complexes 1–4 (1 mM) in
distilled methanol and activating agents magnesium monoperox-
yphthalate (MMPP, 100 lM). For each cleavage reaction, 16–
18 lL of pBR322 supercoiled DNA, and 2 lL of complex 1–4
were used and they were initiated by adding 2 lL of magnesium
monoperoxyphthalate in an Eppendorf tube. For scavenger exper-
iments, concentrations used were 100 mM. All cleavage reactions
were quenched with 5 lL of loading buffer containing 100 mM
of EDTA, 50% glycerol in Tris–HCl, (pH 8.0) and the samples
were loaded onto 0.7% agarose gel (Biozym) containing ethidium
bromide (1 lg ml−1). Electrophoresis was done for 1 h at constant
current (80 mA) in 0.5 X TBE buffer. Gels were imaged with a
PC-interfaced Bio-Rad Gel Documentation System 2000.
Experimental
Reagents and general procedures
Solvents were purified by conventional methods.23 The follow-
ing chemicals were used as received: C6H11Cl (Aldrich, USA),
Cu(OAc)2·H2O (Fluka, Switzerland), CuCl2 (Lancaster, U.K.),
2,2ꢀ-bipyridine (Aldrich, U.S.A), 1,10-phenanthroline (Aldrich,
U.S.A), AlCl3 (s.d. Fine Chemicals, India), PCl3 (s.d. Fine Chem-
icals, India) and Cu(NO3)2·3H2O (s.d. Fine Chemicals, India).
Supercoiled plasmid DNA (pBR322) was purchased from Banga-
lore Genei. Ethidium bromide was purchased from Sigma Aldrich
Ltd. Magnesium monoperoxyphthalate hexahydrate (MMPP) was
procured from Lancaster and was used as supplied, as was the
sodium cacodylate buffer (SRL, Mumbai). Ethylenediaminete-
traacetic acid (EDTA), DMSO, tert-butanol and D-mannitol
were purchased from S.D. Fine Chemicals, Mumbai. All buffer
solutions were prepared using Millipore water.
Plasmid cleavage under anaerobic conditions
Oxygen-free nitrogen was bubbled through cacodylate buffer,
which was then subjected to four freeze–thaw cycles. All reagents
were transferred in an argon-filled glove bag and Eppendorf
tubes were tightly sealed with parafilm in the argon atmosphere.
Reactions were quenched with loading buffer and efforts were
made to ensure strict anaerobic conditions during irradiation and
quenching.
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 from 400–4000 cm−1. ESI-
MS analyses were performed on a Waters Micromass Quat-
tro Micro triple quadrupole mass spectrometer. The ionization
mechanism used was electrospray in positive ion full scan mode
using methanol as solvent and nitrogen gas for desolvation.
Capillary voltage was maintained at 3 kV and cone voltage was
Synthesis
C6H11P(O)(OH)2. To a cooled mixture of AlCl3 (10.0 g,
75.5 mmol) and PCl3 (10.4 g, 75.7 mmol) at 0 ◦C, cyclohexylchlo-
ride (12.0 g, 101.2 mmol) was added slowly with stirring. After
the addition, the reaction mixture became viscous. After standing
overnight, 50 mL of CH◦Cl3 was added to this reaction mixture
slowly with stirring at 0 C. This was added to 150 g of ice and
30 mL of CHCl3. The organic portion was separated and dried
over calcium chloride. It was filtered and the solvent evaporated
from the filtrate to afford cyclohexylphosphonyl dichloride. To
this, 30 mL of water was added and was stirred overnight. The
solution was evaporated to dryness and the residue obtained was
identified as impure cyclohexylphosphonic acid. This product was
recrystallized from a 10 : 1 mixture of toluene and THF.
kept at 30 kV. The temperature maintained for the ion source
◦
was 100 ◦C and for desolvation was 250 C. H and 31P{ H}
NMR spectra were recorded in CD3OD solutions on a JEOL
JNM LAMBDA 400 model spectrometer operating at 400.0 and
161.7 MHz respectively. Chemical shifts are reported in ppm and
referenced with respect to internal tetramethylsilane (1H) and
external 85% H3PO4 (31P). Elemental analyses were carried out
using a Thermoquest CE Instruments CHNS-O, EA/110 model
elemental analyzer.
1
1
Yield: 9.94 g, 80.0% (based on phosphorus). Mp: 165 ◦C. FT-IR
m/cm−1: 2934 (b), 2325 (b), 1451 (s), 1234 (m), 942 (m), 801 (m),
495 (m). 1H NMR (CD3OD): 1.15–1.86 (m). 31P NMR (CD3OD):
+
31.2 (s). ESI-MS analysis: m/z, ion: 165, {M+1} ; anal. calc. for
C6H13PO3: C, 43.90; H, 7.98. Found: C, 43.76; H, 7.78.
Magnetic measurements
[Cu4(l-Cl)2(l3-C6H11PO3)2(bpy)4](NO3)2 (1). A mixture of an-
hydrous CuCl2 (0.20 g, 1.5 mmol), 2,2ꢀ-bipyridine (0.24 g,
1.5 mmol), cyclohexylphosphonic acid (1) (0.13 g, 0.80 mmol) and
triethylamine (0.36 g, 3.60 mmol) was taken in methanol (60 mL)
Magnetic susceptibility measurements were performed using a
Quantum Design MPMS-5 SQUID magnetometer between a
temperature range of 2–290 K and a field range of 0.1 to 5.0
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and stirred for 6 h. The blue coloured reaction mixture was filtered
and treated with solid KNO3 (0.08 g, 9.00 mmol). This mixture
was stirred for further 3 h. The reaction mixture was filtered and
the filtrate was stripped off the solvent in vacuo to afford a blue
solid which was recrystallized from the methanol–CH2Cl2 solvent
mixture to obtain the pure crystalline product.
PO3)2(NO3)(C12H8N2)3]+. Other peaks are seen at 406, 516, 528,
679, 710, 811, 937. Anal. calc. for C60H59N11P2O18Cu4: C, 46.85;
H, 3.87; N, 10.02. Found: C, 46.65; H, 3.78; N, 9.96.
X-Ray crystallography†
Yield: 0.46 g, 88.5% (based on metal). Mp: 218 ◦C (d).
UV-Vis (CH3OH) kmax/nm (e/L mol−1 cm−1): 695 (186). FT-IR
m/cm−1: 3430 (b), 2928 (s), 2675 (s), 2675 (m), 2494 (s), 1604
(m), 1382 (m), 1070 (m), 776 (m), 575 (m). ESI-MS: m/z, ion:
637, [Cu2(Cl)(C6H11PO3)(bpy)2]+. Other peaks are observed at
150, 254, 382, 410, 546, 673, 763, 801 and 927. Anal. calc. for
C52H54N10P2O12Cu4Cl2: C, 44.67; H, 3.89; N, 10.02. Found: C,
44.37; H, 3.76; N, 10.10.
The crystal data and the parameters for the compounds
(1–4) are given in Table 1. Single crystals of 1, suitable for X-
ray crystallographic analysis, were obtained by slow evaporation
of a dichloromethane and methanol (1 : 1) solution at room
temperature. Crystals of 2–4 were obtained by slow evaporation
of their methanolic solutions. The crystal data for 1, 2 and 4 were
collected on a Stoe-IPDS machine while those for 3 were obtained
on a Bruker SMART APEX CCD Diffractometer. All structures
were solved by direct methods using the programs SHELXS-
97 and refined by full-matrix least-squares methods against F2
with SHELXL-97.25 All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were in-
cluded at idealized positions and refined isotropically. The crystals
of 3 are poor and weakly diffracting at 2h above 50◦ and hence
these reflections are omitted. The structure contains large solvated
nitrate and water molecules. Although the electron densities in the
solvated regions are overlapping each other, we were able to locate
all the nitrate positions. Out of the six nitrate molecules four of
them are present in two different positions (N50, N60, N70 and
N80 sets) and one of them (N40 set) is in an inversion centre and
is refined with half occupancies. The other high electron density
peaks are carefully examined and are assigned as full or disordered
water molecules. Restraining some of these water positions as
nitrates did not refine well and hence assigning them as waters
(with the aid of possible meaningful H-bonding patterns) gives
the best model for refinement. The amount of disorder present
in these solvated regions did not allow us to locate the hydrogen
atoms on the water molecules. Some of the carbons and nitrogens
of the 2,2ꢀ-bipyridine are disordered as observed from their slightly
large thermal ellipsoids, but refining them anisotropically gives a
more stable refinement than isotropically refining them in two
different positions. The crystals of the compound 2 diffracted
very poorly, displaying broad and weak reflections. Measured
intensities therefore have a high r(I), As a consequence, Rint for
this is relatively high.
[Cu4(l-CH3COO)2(l3-C6H11PO3)2(bpy)4](CH3COO)2 (2).
A
mixture of copper(II) acetate monohydrate (0.20 g, 1.00 mmol),
2,2ꢀ-bipyridine (0.16 g, 1.00 mmol), cyclohexylphosphonic acid
(0.08 g, 0.50 mmol) and triethylamine (0.22 g, 2.17 mmol) were
taken in methanol (60 mL) and stirred for 6 h. The blue coloured
reaction mixture was filtered and the filtrate was evaporated to
afford a blue coloured solid. This was recrystallized from a mixture
of methanol–ethanol (1 : 1) to get a pure crystalline product.
Yield: 0.28 g, 79.8% (based on metal). Mp: 120 ◦C (d).
UV-Vis (CH3OH) kmax/nm (e/L mol−1 cm−1): 668 (180). FT-
IR m/cm−1: 3387 (b), 1613 (m), 1446 (m), 1260 (s), 1165 (s),
1080 (m), 779 (s), 672 (s), 585 (s). ESI-MS: m/z, ion: 659,
[Cu2(CH3COO)(C6H11PO3)(bpy)2]+. Other peaks are observed at
150, 278, 375, 431, 492, 565, 763 and 886. Anal. calc. for
C60H66N8O14P2Cu4: C, 50.07; H, 4.62; N, 7.79. Found: C, 49.60;
H, 4.32; N, 7.56.
[Cu4(l-OH)(l3-C6H11PO3)2(bpy)4(H2O)2](NO3)3 (3). A mix-
ture of copper(II) nitrate trihydrate (0.18 g, 0.75 mmol), 2,2ꢀ-
bipyridine (0.11 g, 0.70 mmol), cyclohexylphosphonic acid (1)
(0.06 g, 0.35 mmol) and triethylamine (0.15 g, 1.48 mmol) were
taken in methanol (60 mL) and stirred for 6 h. The blue coloured
reaction mixture was filtered and the filtrate was evaporated to
afford a blue coloured solid. This was recrystallized from a mixture
of methanol–ethanol to get a pure crystalline product.
Yield: 0.18 g, 67.0% (based on metal). Mp: 236 ◦C (d). UV-Vis
(CH3OH) kmax/nm (e/L mol−1 cm−1): 677 (226). FT-IR m/cm−1:
3431 (b), 2927 (m), 1605 (m), 1359 (m), 1104 (m), 774 (s), 732
(s), 696 (s), 582 (m). ESI-MS: m/z, ion: 889, [Cu3(C6H11PO3)-
(OH)3(H2O)(bpy)3]+. Other peaks are observed at 492, 637, 662,
765, 793 and 1045. Anal. calc. for C52H59N11P2O18Cu4: C, 43.31;
H, 4.12; N, 10.68. Found: C, 43.10; H, 3.98; N, 10.36.
Results and discussion
Synthesis
[Cu4(l-OH)(l3-C6H11PO3)2(phen)4(H2O)2](NO3)3 (4). A mix-
ture of copper(II) nitrate trihydrate (0.18 g, 0.70 mmol), 1,10-
phenanthroline (0.13 g, 0.70 mmol), cyclohexylphosphonic acid
(0.06 g, 0.35 mmol) and triethylamine (0.15 g, 1.48 mmol) were
taken in methanol (60 mL) and stirred for 6 h. The blue coloured
reaction mixture was filtered and the filtrate was evaporated to
afford a blue coloured solid. This was recrystallized from a mixture
of methanol–ethanol to get a pure crystalline product.
The synthesis of the tetranuclear copper(II) complexes (1–4) using
cyclohexylphosphonic acid and chelating nitrogenous ligands (bpy
and phen) is shown in Scheme 1. In spite of the fact that these
are three-component reactions involving a metal ion and two
independent ligands, in each case, the isolated yields are quite good
and range from 59.3% (4) to 88.5% (1) (vide supra, Experimental).
Complexes 1–2 contain a tetranuclear dicationic part with nitrate
or acetate counter anions, while complexes 3–4 are tricationic with
nitrate counter anions (Chart 1). In each case, the synthesis was
carried out such that every copper is chelated with one bidentate
bpy or phen ligand. All four complexes are held together by the
tridentate coordination mode of the two phosphonate ligands
Yield: 0.17 g, 59.3% (based on metal). Mp: 238 ◦C (d). UV-Vis
(CH3OH) kmax/nm (e/L mol−1 cm−1): 683 (225). FT-IR m/cm−1:
3424 (b), 2924 (m), 1623 (m), 1372 (m), 1214 (s), 1133 (m), 854 (s),
725 (s), 647 (m), 578 (m). ESI-MS: m/z, ion: 1117, [Cu3(C6H11-
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Table 1 Crystal and structure refinement parameters for compounds 1–4
Parameters
1
2
3
4
Empirical formula
Formula weight
Temperature/K
C56 H62 Cl10 Cu4 N10 O12 P2
1737.76
213(2)
C62 H74 Cu4 N8 O18 P2
1535.39
213(2)
C52 H71 Cu4 N11 O24 P2
1514.27
100(2)
C60 H65 Cu4 N11 O21 P2
1592.33
213(2)
˚
Wavelength/A
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Triclinic
P-1
Monoclinic
P2(1)/c
Triclinic
P-1
Monoclinic
C2/c
Unit cell dimensions (lengths in
a = 10.762(3)
b = 12.649(4)
c = 14.253(4)
a = 67.88(3)
b = 72.84(2)
c = 84.47
a = 11.747(3)
b = 17.734(4)
c = 18.981(5)
a = 90
a = 13.3151(10)
b = 15.0690(11)
c = 32.232(2)
a = 81.051(10)
b = 87.441(10)
c = 78.351(10)
6256.4(8), 4
1.608
a = 24.125(5)
b = 15.624(2)
c = 18.860(4)
a = 90
◦
˚
A, angles in
)
b = 104. 22(3)
c = 90
b = 106.76(2)
c = 90
3
˚
Volume/A , Z
1717.2(8), 1
1.680
3832.9(15), 2
1.330
6807(2), 4
Density (calculated)/mg m−3
Absorption coefficient/mm−1
F (000)
1.554
1.361
3264
0.1 × 0.1 × 0.1
4.10 to 22.49
−25 ≤ h ≤ 25,
−16 ≤ k ≤ 16,
−20 ≤ l ≤ 20
15756
1.723
880
1.202
1584
1.478
3112
Crystal size/mm
0.1 × 0.2 × 0.1
1.89 to 24.27
−11 ≤ h ≤ 12,
−14 ≤ k ≤ 14,
−16 ≤ l ≤ 16
10919
0.2 × 0.3 × 0.2
4.10 to 22.49
−12 ≤ h ≤ 12,
−19 ≤ k ≤ 19,
−20 ≤ l ≤ 20
19870
0.2 × 0.2 × 0.2
4.09 to 25.03
−15 ≤ h ≤ 15,
−17 ≤ k ≤ 17,
−38 ≤ l ≤38
46469
h range for data collection/◦
Limiting indices
Reflections collected
Independent reflections
Completeness to h (%)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
5117 (Rint = 0.0747)
91.7 (h = 24.27)
5117/71/421
0.833
R1 = 0.0516,
wR2 = 0.1207
R1 = 0.0955,
wR2 = 0.1346
0.741 and −0.694
4978 (Rint = 0.1674)
99.4 (h = 22.49)
4978/146/490
0.860
R1 = 0.0782,
wR2 = 0.1611
R1 = 0.1686,
wR2 = 0.1910
0.466 and −0.548
21846 (Rint = 0.0245)
98.9 (h = 25.03)
21846/1866/1652
1.047
R1 = 0.0646,
wR2 = 0.1749
R1 = 0.0763,
wR2 = 0.1834
1.293 and −0.936
4429 (Rint = 0.0702)
99.4 (h = 22.49)
4429/190/507
0.921
R1 = 0.0471,
wR2 = 0.1243
R1 = 0.0695,
wR2 = 0.1325
0.656 and −0.421
R indices (all data)
−3
˚
Largest diff. peak and hole/e A
Scheme 1 Synthesis of tetranuclear copper phosphonates 1–4.
(Chart 2a). However, there are important structural differences,
which are discussed, below.
at 889 nm [Cu3(C6H11PO3)(OH)3(H2O)(bpy)3]+ and 1117 nm
[Cu3(C6H11PO3)2(NO3)(C12H8N2)3]+ respectively (see ESI‡).
The electronic spectra of 1–4 reveals a broad d–d transition
in the region of 680
15 nm. ESI-MS spectra of 1–4 were
Molecular structures of 1–4
recorded in the positive ion mode to study the stability of the
tetranuclear core structures in solution. Complexes 1 and 2, which
contain a pair of doubly-bridged copper(II) dimers, show major
peaks at 637 nm and 659 nm corresponding to [Cu2(l-X)2(l3-
C6H11PO3)(bpy)4]+ [X = Cl (1) and CH3COO (2)]. The ESI-
MS spectra of 3 and 4 also show peaks with high nuclearity
The molecular structures (cationic portion only) of 1–4 are given in
Fig. 1–4. Bond parameters for these compounds are summarized
in the ESI.‡ The detailed supramolecular organization of these
compounds is also given in the ESI.‡ The asymmetric unit of 3
contains two independent molecules (Fig. 3). The cationic part
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Chart 1 Cationic part of the complexes 1–4.
3
described as the l4, g coordination mode or 4·211 coordination
mode according to the Harris notation26a (Chart 2b).
The assembly of the tetranuclear complexes of 1 and 2 is
conceptually different from that for 3 and 4. Thus, the former
are built from the fusion of two structurally similar symmetry-
related di-copper sub-units. Both the copper atoms involved in
each sub-unit are doubly-bridged: in the case of 1 by chloride and
oxygen (phosphonate) atoms and in the case of 2 by oxygen atoms
(acetate and phosphonate). As a result of this, symmetry-related
four-membered puckered rings [2Cu,Cl,O (1) and 2Cu,2O (2)] are
formed in these compounds (Chart 1, Fig. 1 and 2).
Furthermore, in both of these compounds, as a result of the
cumulative effects of coordination, two symmetry-related six-
membered rings [2Cu,P,2O,Cl (1) and 2Cu,P,3O (2)] are generated
within the molecular structures. The bridging chloride atoms in 1
and the acetate oxygen atoms in 2 are considerably displaced from
the mean plane of the four copper atoms. Their arrangement with
respect to each other can be described as trans (Fig. 1b and 2b).
In 3 and 4 while a pair of copper atoms is bridged by l-OH the
other two have terminal ligands (H2O) (Chart 1, Fig. 3 and 4). The
presence of the single l-OH leads to a trapezoidal arrangement of
the four copper atoms in 3 and 4 while at the same time causing
the assembly to be non-planar.
Chart 2 Tetranuclear copper unit bridged by two phosphonate groups.
(b) The crystallographically established coordination modes of the cyclo-
hexylphosphonates in the crystal structure of 1–4, and the Harris notation
that describes these modes.
of the complexes 1–4 consists of a tetranuclear copper assembly.
In 1 and 2 all four copper atoms are arranged in a rectangular
plane while for 3 and 4 this may be described as trapezoidal.
Furthermore, in 1 and 2 all four copper atoms are in the same plane
(Fig. 1b and 2b). However, in 3 and 4 the tetra-copper assembly
is non-planar (Fig. 3b, 3c and 4b). Mean-plane information for
the tetra-copper array is given in Table S3 (see ESI‡). In all four
complexes two [C6H11PO3]2− ligands are involved in holding the
tetranuclear assembly together. This is accomplished by a capping
tridentate coordination action by each of the phosphonate ligands
(Chart 2, Fig. 2b). Two oxygen atoms of the [C6H11PO3]2− ligand
coordinate to two independent Cu(II) ions in a monodentate
manner while the third oxygen atom is involved in bridging the
remaining two Cu(II) ions. This mode of coordination can be
In view of the structural similarity of 1 and 2, the bond
parameters and coordination details for these two compounds are
discussed together. As mentioned (vide supra) the only difference
between 1 and 2 is the nature of the bridging ligand. The two
copper atoms present in the dicopper sub-units of 1 and 2 are
five-coordinate (2N,2O,Cl for 1 and 2N,3O for 2). In the case of
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Fig. 1 (a) Cationic part of complex 1. Cyclohexyl groups and hydrogen
atoms have been removed for the sake of clarity. (b) View of the tetranuclear
core of 1. The bridging chloride atoms are displaced from the mean plane
Fig. 2 (a) Cationic part of 2. Cyclohexyl groups and hydrogen atoms have
been removed for the sake of clarity. (b) View of the tetranuclear copper
core. The bridging oxygen atoms of the acetate ligand are displaced by
˚
of the four copper atoms by 0.76 A.
˚
0.55 A from the mean plane of the copper atoms.
1, Cu1 is in a square pyramidal geometry (s = 0.035; basal plane
is made up of 2N and 2O, Cl is present in the apical position)
while Cu2 is in a distorted trigonal bipyramidal geometry (s =
0.707).26b The axial bond angle in the latter (Cl1–Cu2–N4) is
150.23◦. In a slight variation, in 2, both Cu1 and Cu2 are present
in a near square pyramidal geometry [s = 0.068 (Cu1); s = 0.105
(Cu2)]. The bond distances within the four-membered sub-unit
Compounds 3 and 4 do not contain doubly-bridged copper atoms
as found in 1 and 2. However, two copper centres are bridged
symmetrically by a l–OH. The Cu–O distances involved are:
for 3, Cu2–O9 (1.925(4) A) and Cu4–O9 (1.886(4) A); for 4,
Cu1–O4 1.906 (3) A. The phosphonate coordination to the four
copper centres is tripodal. One of the oxygen atoms is involved
in a bridging coordination while the other two are involved in
monodentate coordination. The Cu–O bond distances observed
for 3 and 4 with the phosphonate ligand indicate the highly
unsymmetrical nature of the bridging coordination (see ESI‡).
For example, in 4 the Cu1–O1 bond distance is 2.428(4) A while
the Cu2–O1 distance is 1.961(4) A. One possible reason for this
is the trapezoidal arrangement of the four copper centres in these
assemblies. Accordingly, in 3 and 4, four different inter-copper
distances are observed (see ESI‡).
˚
˚
˚
˚
are not equal. Thus in 1, the Cu1–Cl1 distance is 2.707(2) A while
˚
˚
Cu2–Cl1 is 2.3520(19) A. Similarly Cu1–O3* is 1.969(4) A and
˚
Cu2–O3* is 2.396(4) A. A similar situation is also found in 2
(see ESI‡). The Cu–O distances involving a l-O (phosphonate)
are longer in comparison to the bond distances observed where
a monodentate oxygen atom is involved in coordination (see
ESI‡). The rectangular arrangement of the copper atoms in
these assemblies is gauged by the near perfect planarity of the
tetra-nuclear array (see ESI‡) and the intramolecular copper
distances. Thus, the shortest Cu–Cu edge distances in 1 and 2
˚
˚
˚
˚
are 3.338 A (Cu1–Cu2) and 3.255 A (Cu1–Cu2) respectively. The
Comparison of the structural features of 1–4 with related systems
corresponding longer edge distances (Cu1–Cu2*) are 4.047 and
˚
4.138 A. The coordination environments of the copper atoms in 3
Tetranuclear copper clusters containing phosphate ligands
are more predominant in comparison to those involving
phosphonates.27 In this discussion we restrict ourselves only to
tetranuclear copper phosphonates. The tetranuclear copper cores
found in 1–4 are shown in Chart 3. As discussed (vide supra)
two types of cores are found in these compounds. Compounds
1 and 2 possess a closed type of core where two dimeric [Cu2(l-
X)(l-O)] units are interlinked with each other (Chart 3, Type A).
The phosphonate ligand is involved in connecting the two dimers
and 4 are similar except that the nitrogenous chelating ligand in the
former is 2,2ꢀ-bipyridine while in the latter it is 1,10-phenanthroline
(Chart 1, Fig. 3 and 4). In 4 all the copper atoms are five-
coordinate and in a square pyramidal geometry with a 2N,3O
coordination environment. The asymmetric unit of 3 contains two
molecules. In both of these, three of the four copper atoms are
five-coordinate and square pyramidal; the fourth copper atom is
six-coordinate with an additional water or nitrate ligand (Fig. 3).
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Fig. 4 (a) Cationic part of 4. Cyclohexyl groups and hydrogen atoms have
been removed for clarity. (b) View of the non-planar tetra-copper core of 4.
Chart 3 Three representative framework structures of tetranuclear cop-
per(II) phosphonate assemblies formed by using ancillary nitrogenous
ligands.
of 3 and 4 have a different structural arrangement. These contain
more open cores. Thus, two copper atoms (a and b) are linked
to each other by a l-OH. The other two copper atoms (c and
d) are not bridged to each other but instead have a terminal
water ligand each (Chart 3, Type B). The connectivity of the four
copper atoms is established again by two tripodal phosphonates,
which act slightly differently. However, one of the oxygen atoms
of the phosphonate ligand binds Cua and Cuc in a l bridging
mode while the other two coordinate in a unidentate manner to
bind Cub and Cud. The coordination mode of the phosphonate
ligand is twisted by 90◦ for 3 and 4 in comparison to 1 and 2.
There are only two other tetranuclear copper(II) phosphonates
containing ancillary ligands, to the best of our knowledge. Both
of these have been reported from our laboratory. One of these is
[Cu2(l-Cl)(l3-MePO3)(Cl)(3,5-Me2PzH)3]2 which is obtained in an
indirect desulfurization/hydrolysis reaction involving MeP(S)(3,5-
Me2Pz)2 and CuCl2.
Fig. 3 (a) Cationic part of the two independent molecules present in the
asymmetric unit of 3. Cyclohexyl groups and hydrogen atoms have been
removed for clarity. (b) View of the non-planar tetra-copper core (Cu1,
Cu2, Cu3 and Cu4) of upper molecule shown in Fig. 3(a). (c) View of the
non-planar tetra-copper core of lower molecule shown in Fig. 3(a) (Cu5,
Cu6, Cu7 and Cu8).
together. While one oxygen atom of a phosphonate forms the l-O
linking Cua and Cub the other two oxygen atoms are monodentate
and bind to Cuc and Cud. As noted above, the tetranuclear cores
The tetranuclear core of this compound corresponds to Type
A found in the present instance, although within a dimeric
pair the disposition of the ancillary ligands is not symmetric as
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is the situation in the present instance. The other tetranuclear
copper phosphonate known in literature, [Cu4(l-OH)2{(ArP(O)2-
(OH)}2(CH3CO2)2(DMPZH)4][CH3COO]2 has been obtained in
the reaction involving Cu(CH3COO)2·H2O with ArP(O)(OH)2
(Ar = 2,4,6-iPr–C6H3). In this compound, the tetranuclear copper
assembly is an asymmetric cage containing three continuous
four-membered rings, [Cu2(l-O)(l-OH)], [Cu(l-OH)]2 and [Cu2(l-
O)(l-OH)]. Bridging ligands in the form of [ArP(O)2(OH)]−
and CH3COO− hold the cage together (Type C, Chart 3). The
important difference between Type C cores vis-a`-vis the Type
A and B cores lies in the fact that one of the oxygen atoms
of the [ArP(O)2(OH)]− ligand in the former is not involved in
coordination. Because of this, the phosphinate ligand holds three
copper atoms together in contrast to the situation in Type A and
Type B structures where four copper atoms are bound by a capping
phosphonate ligand (Chart 3). The metric parameter variations
in these three structural types are not very significant. For
contributions to the overall exchange energies that depend on such
bridges. Whereas the types of exchange pathways are identical
in nature for compounds 3 and 4, the difference in the terminal
(bridging N-donor ligands (bpy for 3, phen for 4) slightly affects
the geometry of the exchange pathways in the Cu4(PO3)2(l-OH)
cores.
Correspondingly, the experimental susceptibility curves for 3
and 4 differ only slightly (below approx. 100 K). Furthermore,
the presence of two crystallographically distinct core geometries
in compound 3 (with planar and non-planar Cu4 arrangements,
see Fig. 3) complicates the magnetic analysis since the exchange
energies cannot be unambiguously assigned to the two species
involved, effectively limiting the interpretation of bulk suscep-
tibility data for these mixed species. Since the coordination
environments of all Cu(II) centres in compounds 1 to 4 are
approximately square pyramidal (with minor deviations from the
square cis-N2O2/N2OCl coordination plane (see ESI‡) with an
elongated axial bond, i.e. Jahn–Teller-distorted as is typical for
these d9 systems, the g tensors can in a first approximation be
characterized by the axial and perpendicular components gz and
gx/gy, respectively. Using uniform typical values of gz = 2.30 and
gx = gy = 2.05, least-squares fits to the experimental susceptibility
data sets (Fig. 6 and 7) were performed based on the simplified
isotropic model. Heisenberg spin operators as defined in Fig. 5.
These models yielded the following effective, all-antiferromagnetic
exchange energies: 1: J1/kB = −6.2 K, J2/kB = −5.8 K; 2: J1/kB =
−6.8 K, J2/kB = −6.3 K; 4: J1/kB = −18.4 K, J2/kB = −9.2 K,
J3/kB = −6.4 K. As stated above, the mixture of core geometries
in 3 prevented such fitting procedure; a qualitative comparison
between the susceptibility vs. temperature curves of 3 and 4 (Fig. 7)
indicates that the overall antiferromagnetic coupling observed in
3 should be slightly weaker.
1
example the average Cu–O(g ) and Cu–O(l3) distances in [Cu2(l-
˚
Cl)(l3-MePO3)(Cl)(3,5-Me2PzH)3]2 are 2.003(5) and 1.990(4) A
respectively, which are comparable to the situation found in
compounds 1–4. Also, as found in 1 and 2, the four copper atoms
in [Cu2(l-Cl)(l3-MePO3)(Cl)(3,5-Me2PzH)3]2 lie in a single plane.
Magnetic studies
Magnetic measurements indicate that all the presented compounds
(1–4) exhibit moderately strong antiferromagnetic intramolecular
coupling between the spin-1/2 Cu(II) centres resulting in singlet
ground states.
The local symmetry of the central Cu4(PO3)2(l-X)n core (X = O,
Cl; n = 1, 2) suggests the following superexchange connectivities
for a Heisenberg exchange model (Fig. 5): two types of exchange
pathways for compound 1 (where J1 involves a phosphonate l3-
oxo centre, an O–P–O bridge and a bridging chloride, J2 involves
two O–P–O pathways) and 2 (where J1 involves a phosphonate
l3-oxo centre, an O–P–O bridge and a bridging acetate oxo centre,
J2 as in compound 1) and four exchange pathways for compounds
3 and 4 (J1: two O–P–O bridges and a l-hydroxo group, J2: an
O–P–O bridge and a phosphonate l3-oxo centre, J3: two O–P–O
bridges) (see Fig. 5).
Fig. 5 Exchange coupling scheme for compounds 1 (left), 2 (left), 3 (right)
and 4 (right), using the same perspectives as Chart 1. The effective exchange
pathways are represented by bold bonds: J1 black, J2 blue, J3 orange.
Corresponding isotropic Heisenberg spin Hamiltonians: H = −J1(S1S2 +
S3S4) − J2(S1S3 + S1S4 + S2S3 + S2S4) (for 1 and 2); H = −J1(S1S2) −
J2(S1S4 + S2S3) − J3(S1S3 + S2S4 + S3S4) (for 3 and 4).
Fig. 6 Experimental temperature dependence of vT for compounds 1
(0.1 tesla, left, grey circles) and 2 (0.1 tesla, left, black squares).
Overall these exchange constants are slightly smaller than for
the comparable phosphonate-bridged polynuclear Cu(II) species,
probably due to the relatively large Cu–Cu distances obtained in
the presented cluster compounds.28
Note that these models are simplified in that small variations
in the exact local geometries of the Cu–O–P–O–Cu fragments
are assumed to have no significant impact on the resulting
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Fig. 9 pBR322 DNA cleavage by 1–4 in the presence of MMPP (complex
1 corresponds to gel A, 2 to gel B, 3 to gel C and 4 to gel D) at different
time intervals. Lane 1: DNA alone; lanes 2–3: DNA + complex + MMPP.
Fig. 7 Experimental temperature dependence of vT for compounds 3
(0.5 tesla, right, grey circles), and 4 (0.5 tesla, right, black squares) and
representative least-squares fit to the proposed Heisenberg model for 4
(red graph). Note the different scale to Fig. 6.
mechanism of 1–4. In the presence of EDTA, the cleavage reaction
is completely inhibited demonstrating the crucial role of copper
in plasmid modification. Hydroxyl radical scavengers, dimethyl
sulfoxide (DMSO), D-mannitol or tert-butylalcohol do not inhibit
cleavage reactions, demonstrating that radicals are not involved in
the cleavage process. On the other hand, NaN3 is a well-known
quencher of singlet oxygen.
This suggests that in situ generation and possible involvement
of singlet oxygen is involved in the plasmid cleavage (Fig. 10).
However, the hydrolytic pathway for plasmid modification also
appears potent in the current instance as demonstrated by the fact
that DNA cleavage does not decrease under anaerobic conditions
(Fig. 11). Thus, it appears that both multiple reaction pathways
involving singlet oxygen30 as well as hydrolysis are involved in the
plasmid cleavage activity mediated by 1–4.
Cleavage of plasmid DNA
We have systematically examined the DNA cleavage ability29 of
complexes 1–4 (Fig. 8). Time-course experiments reveal that 1 and
3 were able to mediate complete conversion of the supercoiled
pBR322 DNA form I to nick form II in 120 min (Fig. 5). In
contrast, only partial conversion occurred in the presence of
2 (50%) and 4 (40%). To foster the reaction we have added
magnesium monoperoxyphthalate (MMPP). In all cases, we
observed rapid conversion of the super coiled pBR322 DNA form
I to form II within 2 min (Fig. 9).
DNA cleavage mechanism
Copper-based artificial nucleases function through oxidative
and/or hydrolytic pathways. In view of this, we probed the cleavage
Fig. 10 pBR322 cleavage experiments involving 1–4 in the presence of free
radical scavengers and singlet oxygen quencher assisted by the complexes
(complex 1 corresponds to gel A, 2 to gel B, 3 to gel C and 4 to gel D)
in a 2 min reaction. Lane 1: DNA alone; lane 2: pBR322 + complex +
MMPP; lane 3: pBR322 + complex + MMPP + DMSO; lane 4: pBR322 +
complex + MMPP + D-mannitol; lane 5: pBR322 + complex + MMPP +
t-BuOH; lane 6: pBR322 + complex + MMPP + EDTA; lane 7: pBR322 +
complex + MMPP + NaN3.
Fig. 8 Time-course experiment for the conversion of supercoiled pBR322
DNA form I to nick form II in the presence of 1–4 at different time intervals
(complex 1 corresponds to gel A, 2 to gel B, 3 to gel C and 4 to gel D).
Lane 1: DNA alone; lanes 2–5: DNA + complexes (30, 60, 90 and 120 min
respectively).
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7, p. 151; (c) J. D. Wang, A. Clearfield and G.-Z. Peng, Mater. Chem.
Phys., 1993, 35, 208; (d) B. Zang and A. Clearfield, J. Am. Chem. Soc.,
1997, 119, 2751.
3 (a) F. Fredoueil, D. Massiot, P. Janvier, F. Gingl, M. Bujoli-Doeuff,
M. Evian, A. Clearfield and B. Bujoli, Inorg. Chem., 1999, 38, 1831;
(b) K. Maeda, Y. Kiyozumi and F. Mizukami, J. Phys. Chem. B, 1997,
101, 4402; (c) Q. Yue, J. Yang, G.-H. Li, G.-D. Li and J.-S. Chen, Inorg.
Chem., 2006, 45, 4431; (d) L. A. Gallagher, S. A. Serron, X. Wen, B. J.
Hornstein, D. M. Dattelbaum, J. R. Schoonover and T. J. Meyer, Inorg.
Chem., 2005, 44, 2089; (e) K. Maeda, Microporous Mesoporous Mater.,
2004, 73, 47.
4 (a) B.-Z. Wan, R. G. Anthony, G.-Z. Peng and A. Clearfield, J. Catal.,
1994, 19, 101; (b) D. Deniaud, B. Schollorn, J. Mansury, J. Rouxel,
P. Battion and B. Bujoli, Chem. Mater., 1995, 7, 995; (c) A. Hu, H.
Ngo and W. Lin, J. Am. Chem. Soc., 2003, 125, 11490; (d) A. Hu,
G. T. Yee and W. Lin, J. Am. Chem. Soc., 2005, 127, 12486; (e) G.
Nonglaton, I. O. Benitez, I. Guisle, M. Pipeler, J. Leger, D. Dubreuil,
C. Tellier, D. R. Talham and B. Bujoli, J. Am. Chem. Soc., 2004, 126,
1497.
5 (a) S. B. Ungashe, W. L. Wilson, H. E. Katz, G. R. Scheller and T. M.
Putvinski, J. Am. Chem. Soc., 1992, 114, 8717; (b) M. J. Sanchez-
Moreno, A. Fernandez-Botello, R. B. Gomez-Coca, R. Griesser, J.
Ochocki, A. Kotynski, J. Niclos-Gutierrez, V. Moreno and H. Sigel,
Inorg. Chem., 2004, 43, 1311; (c) Z.-Y. Du, H.-B. Xu and J.-G. Mao,
Inorg. Chem., 2006, 45, 6424.
6 (a) A. Cabeza, M. A. G. Aranda, S. Bruque, D. M. Poojary, A. Clearfield
and J. Sanz, Inorg. Chem., 1998, 37, 4168; (b) B. Zhang, D. M. Poojary
and A. Clearfield, Inorg. Chem., 1998, 37, 249; (c) P. Ayyappan, O. R.
Evans, Y. Cui, K. A. Wheeler and W. Lin, Inorg. Chem., 2002, 41,
4978.
7 Yu. Yang, M. G. Walawalkar, J. Pinkas, H. W. Roesky and H.-G.
Schmidt, Angew. Chem., Int. Ed., 1998, 37, 96.
8 V. Baskar, M. Shanmugam, E. C. San˜udo, M. Shanmugam, D. Collison,
E. J. L. McInnes, Q. Wei and R. E. P. Winpenny, Chem. Commun., 2007,
37.
Fig. 11 pBR322 DNA cleavage in anaerobic conditions by 1–4 (complex
1 corresponds to gel A, 2 to gel B, 3 to gel C and 4 to gel D) at different
time intervals. Lane 1: DNA alone; lanes 2–3: DNA + complex + MMPP.
Conclusion
We have synthesized four tetranuclear copper(II) phosphonate
cages (1–4) in three-component reactions involving a Cu(II) salt, an
ancillary ligand such as 2,2ꢀ-bipyridine and cyclohexylphosphonic
acid. In all the cases two phosphonate ligands act as bicapping
ligands and hold the four copper atoms together in a tridentate
coordination action. The role of the chelating ligand is also crucial.
It blocks two coordination positions around copper and reduces
the chances of other side-products. The magnetic studies on these
complexes reveal an overall anti-ferromagnetic behaviour. The
metal phosphonates 1–4 also show impressive plasmid cleavage
activity.
9 G. Anantharaman, M. G. Walawalkar, R. Murugavel, B. Ga´bor, H.-I.
Regine, M. Baldus, B. Angerstein and H. W. Roesky, Angew. Chem.,
Int. Ed., 2003, 42, 4482.
10 G. Anantharaman, V. Chandrasekhar, M. G. Walawalkar, H. W.
Roesky, D. Vidovic, A. J. Magull and M. Noltemeyer, Dalton Trans.,
2004, 1271.
11 Y. Yang, H.-G. Schmidt, M. Noltemeyer, J. Pinkas and H. W. Roesky,
J. Chem. Soc., Dalton Trans., 1996, 3609.
12 H.-C. Yao, Y.-Z. Li, Y. Song, Y.-S. Ma, L.-M. Zheng and X.-Q. Xin,
Inorg. Chem., 2006, 45, 59.
13 H.-C. Yao, Y.-Z. Li, L.-M. Zheng and X.-Q. Xin, Inorg. Chim. Acta,
2005, 358, 2523.
14 P. Yin, Y. Peng, L.-M. Zheng, S. Gao and X.-Q. Xin, Eur. J. Inorg.
Chem., 2003, 726.
15 R. Murugavel and S. Shanmugan, Chem. Commun., 2007, 1257.
16 V. Chandrasekhar, V. Baskar, A. Steiner and S. Zacchini,
Organometallics, 2002, 21, 4528.
17 V. Chandrasekhar and S. Kingsley, Angew. Chem., Int. Ed., 2000, 39,
2320.
Acknowledgements
We thank the Council of Scientific and Industrial Research
(CSIR), New Delhi, India and the Department of Science and
Technology, India for financial support. VC is thankful to IIT
Kanpur for the Lalit Kapoor Chair Professorship. TS, PT and
RA thank IIT Kanpur and CSIR for fellowships. AS thanks the
EPSRC (Engineering and Physical Sciences Research Council),
U. K. for financial support.
18 V. Chandrasekhar, S. Kingsley, A. Vij, K. C. Lam and A. L. Rheingold,
Inorg. Chem., 2000, 39, 3238.
19 V. Chandrasekhar, S. Kingsley, B. Hatigan, M. K. Lam and A. L.
Rheingold, Inorg. Chem., 2002, 41, 1030.
20 V. Chandrasekhar, P. Sasikumar, R. Boomishankar and G. Ananthara-
man, Inorg. Chem., 2006, 45, 3344.
21 V. Chandrasekhar, L. Nagarajan, K. Gopal, V. Baskar and P. Ko¨gerler,
Dalton Trans., 2005, 3143.
22 R. Murugavel, P. Davis and M. G. Walawalkar, Z. Anorg. Allg. Chem.,
2005, 631, 2806.
23 B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell,
Vogel’s Textbook of Practical Organic Chemistry, 5th edn, Longman,
London, 1989.
24 J. J. Borra´s-Allmenar, J. J. Clemente, E. Coronado and B. S. Tsukerblat,
Comput. Chem., 2001, 22, 985.
25 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
26 (a) R. A. Coxall, S. G. Harris, S. Henderson, S. Parsons, R. A. Taskar
and R. E. P. Winpenny, J. Chem. Soc., Dalton Trans., 2000, 14, 2349;
(b) A. W. Addison, T. N. Rao, J. Reedijk, J. V. Rijn and G. C. Verschoor,
J. Chem. Soc., Dalton Trans., 1984, 1349.
Notes and references
1 (a) A. Clearfield, Progress in Inorganic Chemistry: Metal–Phosphonate
Chemistry, ed. K. D. Karlin, John Wiley & Sons, New York, 1998, vol.
47, p. 371, and references therein; (b) G. Cao, H.-G. Hong and T. E.
Mallouk, Acc. Chem. Res., 1992, 25, 420; (c) A. Clearfield, Curr. Opin.
Solid State Mater. Sci., 2002, 6, 495; (d) A. Clearfield, Curr. Opin. Solid
State Mater. Sci., 1996, 1, 268; (e) M. E. Thompson, Chem. Mater.,
1994, 6, 1168–1175; (f) A. Subbiah, D. Pyle, A. Rowland, J. Huang,
A. R. Narayanan, P. Thiyagarajan, J. Zon´ and A. Clearfield, J. Am.
Chem. Soc., 2005, 127, 10826; (g) S.-Y. Song, J.-F. Ma, J. Yang, M.-H.
Cao and K.-C. Li, Inorg. Chem., 2005, 44, 2140; (h) W. Ouellette, M. H.
Yu, C. J. O’Connor and J. Zubieta, Inorg. Chem., 2006, 45, 7628; (i) W.
Ouellette, M. H. Yu, C. J. O’Connor and J. Zubieta, Inorg. Chem., 2006,
45, 3224; (j) E. Burkholder, V. Golub, C. J. O’Connor and J. Zubieta,
Inorg. Chem., 2004, 43, 7014; (k) B.-P. Yang and J.-G. Mao, Inorg.
Chem., 2005, 44, 566.
2 (a) A. Clearfield, Inorganic Ion Exchange Materials, CRC Press, Boca
Raton, Florida, 1982; (b) G. Alberti, in Comprehensive Supramolecular
Chemistry, ed. G. Alberti and T. Bein, Pergamon, New York, 1996, vol.
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©

View Article Online
27 (a) R. P. Doyle, P. E. Kruger, B. Moubaraki, K. S. Murray and M.
Nieuwenhuyzen, Dalton Trans., 2003, 4230; (b) M. Kato and T. Tanase,
Inorg. Chem., 2005, 44, 8; (c) S. Youngme, P. Phuengphai, N. Chaichit,
G. A. van Albada, O. Roubeau and J. Reedijk, Inorg. Chim. Acta,
2005, 358, 849; (d) S. Youngme, P. Phuengphai, N. Chaichit, G. A. van
Albada, O. Roubeau and J. Reedijk, Inorg. Chim. Acta, 2005, 358, 2262.
28 V. Baskar, M. Shanmugam, E. C. San˜udo, M. Shanmugam, D. Collison,
E. J. L. McInnes, Q. Wei and R. E. P. Winpenny, Chem. Commun., 2007,
37.
29 (a) S. G. Srivatsan, M. Parvez and S. Verma, J. Inorg. Biochem., 2003,
97, 340; (b) V. Chandrasekhar, P. Deria, V. Krishnan, A. Athimoolam,
S. Singh, C. Madhavaiah, S. G. Srivatsan and S. Verma, Bioorg. Med.
Chem. Lett., 2004, 14, 1559; (c) V. Chandrasekhar, S. Nagendran, R.
Azhakar, R. M. Kumar, A. Srinivasan, K. Ray, T. K. Chandrashekar,
C. Madhavaiah, S. Verma, U. D. Priyakumar and N. G. Sastry, J. Am.
Chem. Soc., 2005, 127, 2410; (d) V. Chandrasekhar, A. Athimoolam, V.
Krishnan, R. Azhakar, C. Madhavaiah and S. Verma, Eur. J. Inorg.
Chem., 2005, 8, 1482; (e) C. Madhavaiah and S. Verma, Chem.
Commun., 2003, 6, 800; (f) S. Das, C. Madhavaiah, S. Verma and P. K.
Bharadwaj, Inorg. Chim. Acta, 2005, 358, 3236; (g) L. M. Pope, K. A.
Reich, D. R. Graham and D. S. Sigman, J. Biol. Chem., 1982, 257,
12121; (h) J. Sankar, H. Rath, V. Prabhuraja, S. Gokulnath, T. K.
Chandrashedkar, C. S. Purohit and S. Verma, Chem.–Eur. J., 2006, 13,
105.
30 (a) A. Fortner, S. Wang, G. K. Darbha, A. Ray, H. Yu, P. C. Ray, R. R.
Kalluru, C. K. Kim, V. Rai and J. P. Singh, Chem. Phys. Lett., 2007, 434,
127; (b) B. Macias, M. V. Villa, F. Sanz, J. Borras, M. Gonzalez-Alvarez
and G. Alzuet, J. Inorg. Biochem., 2005, 99, 1441; (c) A. K. Patra, S.
Dhar, M. Nethaji and A. R. Chakravarty, Chem. Commun., 2003, 13,
1562; (d) Y. Li and M. A. Trush, Carcinogenesis, 1993, 14, 1303.
1160 | Dalton Trans., 2008, 1150–1160
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