Dinuclear metal phosphonates and -phosphates

Article abstract of DOI:10.1016/j.ica.2010.03.038

The reaction of Cu(II) or Cd(II) salts with 2,4,6-iPr₃C ₆H₂PO₃H₂, 2,4,6-iPr ₃C₆H₂CH₂PO₃H₂ or 2,6-iPr₂C₆H₃

Full text of DOI:10.1016/j.ica.2010.03.038

Inorganica Chimica Acta 363 (2010) 2920–2928
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Inorganica Chimica Acta
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Dinuclear metal phosphonates and -phosphates
*
Vadapalli Chandrasekhar , Palani Sasikumar, Tapas Senapati, Atanu Dey
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of Cu(II) or Cd(II) salts with 2,4,6-iPr3C6H2PO3H2, 2,4,6-iPr3C6H2CH2PO3H2 or 2,6-iPr2C6H3O-
PO3H2 in the presence of strong chelating nitrogen ligands such as 2,20-bipyridine (bpy), 1,10-phenan-
throline (phen), 2-pyridylpyrazole (pypz) or 3,5-dimethyl pyrazole (dmpz) as the ancillary ligands
afforded dinuclear copper or cadmium complexes [Cu2(2,4,6-iPr3C6H2PO3H)4(bpy)2] (4), [Cu2(2,
6-iPr2C6H3OPO3H)2(bpy)2(OAc)2(CH3OH)2]ꢀ(CH3OH) (5), [Cd2(2,6-iPr2C6H3OPO3H)4 (bpy)2(CH3OH)2]ꢀ
2(CH3OH) (6), [Cd2(2,6-iPr2C6H3OPO3H)4(phen)2] (7), [Cu2(2,6-iPr2C6H3OPO3H)2(PyPz)2(CH3OH)2] (8)
and [Cu2(2,4,6-iPr3C6H2CH2PO3H)2(DMPz)2Cl2]ꢀ(CH3OH) (9) The molecular structures of 4–7 are grossly
similar. The common structural features in these complexes are that the two metal centers are bridged
by two bidentate [RPO2(OH)]ꢁ ligands generating a central eight-membered ring. Each of the metal cen-
ters also contains a chelating nitrogen ligand and a monodentate phosphonate or a phosphate ligand. In 5
and 6 other terminal ancillary ligands are also present. In compound 8, each of the two copper centers
contains a monodentate [RPO2(OH)]ꢁ ligand along with a molecule of methanol. The two coppers are
bridged by two monoanionic pyridylpyrazole ligands. The molecular structure of 9 is similar to that of
4–7. However, in 9 each of the two copper centers contain only terminal monodentate ligands in the form
of two chlorides and a pyrazole. Magnetic studies on all of these copper complexes reveal an anti-ferro-
magnetic behavior at low temperatures. In addition, these complexes were found to be artificial nucleas-
es and can convert supercoiled pBR322 DNA form I into nick form II in 1 min in the presence of an
external oxidant through a hydrolytic and/or an oxidative pathway.
Received 30 December 2009
Received in revised form 9 March 2010
Accepted 16 March 2010
Available online 19 March 2010
Dedicated to Prof. Animesh Chakravorty on
his 75th birthday
Keywords:
Phosphonates
Phosphates
Dinuclear complexes
Plasmid cleavage
Anti-ferromagnetism
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
behavior as well as their utility as artificial nucleases. We also re-
port the synthesis and structure of a new organophosphonic acid,
Transition metal phosphates, possessing extended structures
are well-known [1–4]. The reaction of transition metal salts with
sterically hindered phosphonic acids or organodihydrogen phos-
phates in conjunction with ancillary nitrogen ligands has been
known to afford molecular complexes whose nuclearity can range
from 2 to 16 [5–9]. Recently we were able to demonstrate that the
use of chelating nitrogen ligands allows greater control over the
resultant assembly. In this paper this theme is elaborated further
and we demonstrate the efficacy of our three-components strategy
(metal salt, phosphorus-based acid, and a chelating nitogen ligand)
by a designed assembly of several dinuclear copper (II) and cad-
mium (II) complexes, [Cu2(2,4,6-iPr3C6H2PO3H)4(bpy)2] (4),
[Cu2(2,6-iPr2C6H3OPO3H)2(bpy)2 (OAc)2(CH3OH)2]ꢀ(CH3OH) (5),
[Cd2(2,6-iPr2C6H3OPO3H)4(bpy)2(CH3OH)2]ꢀ2(CH3OH) (6), [Cd2-
2,4,6-iPr3C6H2CH2PO3H2 (2).
2. Experimental
2.1. Materials and methods
Cu(OAc)2ꢀH2O (Fluka, USA), CuCl2, 2,20-bipyridine, 1,10-phenan-
throline (Aldrich, USA), and Cd(OAc)2ꢀ2H2O (SD fine, India) were
used as received. 2,4,6-tri-isopropylphenylphosphonic acid (1)
and 2,6-di-isopropylphenyldihydrogenphosphate (3) were synthe-
sized according to literature procedures [10]. Solvents were puri-
fied by using standard procedures [11]. Melting points were
measured using a JSGW apparatus and are uncorrected. Elemental
analyses were carried out by using a Thermo quest CE instrument
model EA/110 CHNS-O elemental analyzer. IR spectra were re-
corded as KBr pellets on a Bruker Vector 22 FT IR spectrophotom-
eter operating from 400 to 4000 cmꢁ1. 1H and 31P{1H} NMR spectra
were recorded on a JEOL-JNM LAMBDA 400 model NMR spectrom-
eter in CDCl3 and CD3OD solutions and the chemical shifts are ref-
erenced with respect to tetramethylsilane (1H) and 85% H3PO4
(2,6-iPr2C6H3OPO3H)4(phen)2]
(7),
[Cu2(2,6-iPr2C6H3OPO3H)2
(PyPz)2(CH3OH)2] (8) and [Cu2(2,4,6-iPr3C6H2CH2PO3H)2(DMPz)2-
Cl2]ꢀ(CH3OH) (9). In addition to determining the molecular struc-
tures of these complexes we have also studied their magnetic
* Corresponding author.
E-mail address: vc@iitk.ac.in (V. Chandrasekhar).
(
31P), respectively. ESI-MS analyses were performed on a Waters
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.03.038

V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
2921
Micromass Quattro Micro triple quadrupole mass spectrometer.
2.3. General synthetic procedure used for the preparation of
compounds 4–9
Electro spray ionization (positive ion, full scan mode) was carried
out using 100% methanol as the solvent and nitrogen gas for
desolvation. Capillary voltage was maintained at 3 kV while the
cone voltage was kept at 30 kV.
Stoichiometric amount of the metal (II) salt and the ancillary
ligand was taken in distilled methanol and the resultant mixture
was stirred for 5 min. At this stage a stoichiometric amount of
the phosphorus acid was added to this reaction mixture all at once.
This was allowed to stir at room temperature for 4 h. The volume of
the solvent was reduced to 5 mL under vacuo, filtered and kept for
crystallization at room temperature by slow evaporation. After
about a week block-like crystals suitable for X-ray analysis were
isolated.
2.2. Synthesis
2.2.1. Synthesis of 2,4,6-tri-isopropylbenzylphosphonic acid, 2,4,6-
iPr3C6H2CH2P(O)(OH)2 (2)
2,4,6-tri-isopropylbenzyl chloride (9.86 g, 39,0 mmol and tri-
ethyl phosphite (13.0 g, 78.0 mmol) were heated under constant
stirring in a nitrogen atmosphere at 140 °C for 12 h. Excess of tri-
ethyl phosphite and other volatiles were removed by heating at
100 °C in vacuo. The residue, mainly the diethylphosphonate ester
was hydrolyzed by treating with Conc. HCl (60 mL) and heating the
mixture under reflux for 12 h. Work up of this reaction mixture in-
volved removing the excess of acid in vacuo, treatment of the res-
idue with water (25 mL) followed again by the removal of the
solvent in vacuo and finally treatment of the residue with CH3CN
followed by a final removal of the solvent. This process afforded
the phosphonic acid which was recrystallized from a solvent mix-
ture of dimethylether and CH2Cl2 mixture (1:3) at room tempera-
ture. Yield: 6.9 g (59.2%) Mp: 180–182 °C. Anal. Calc. for
2.3.1. [Cu2(2,4,6-iPr3C6H2PO3H)4(bpy)2] (4)
Quantities: Cu(CH3COO)2ꢀH2O (0.11 g, 0.55 mmol); 2,20-bipyri-
dine (0.09 g, 0.55 mmol); 2,4,6-iPr3C6H2PO3H2 (0.16 g, 0.55 mmol).
Yield: 0.13 g (58.7% based on 2,4,6-iPr3C6H2PO3H2); Mp. 250 °C (d).
Anal. Calc. for C80H112Cu2N4O12P4 (1572.70): C, 61.09; H, 7.18; N,
3.56. Found: C, 61.13; H, 7.21; N, 3.55%. IR (KBr, cmꢁ1): 1105 [s, (ma-
PO3)], 1046 [s, (msym PO3)]. ESI-MS (CH3OH) m/z: 1574.16
sym
[Cu2(2,4,6-iPr3C6H2PO3H)4(bpy)2+H]+, 1638.19 [Cu2(2,4,6-iPr3C6H2-
PO3H)4(bpy)2(CH3OH)2+H]+. TGA, temperature ranges °C (weight
loss %): 30–175 (10); 176–280 (12.7); 281–340 (41.1); 448–550
(6.5).
C
18H33O4P (344.41 amu): C, 62.77; H, 9.66. Found: C, 62.82; H,
9.73%. 1H NMR (400 MHz, CDCl3, d ppm) d 7.03 (s, 2H), 4.98 (s,
2H), 3.33 (septet, 2H), 3.1 (septet, 1H), 1.28 (d, 18H). 31P{1H}
NMR (161.7 MHz, CDCl3, d ppm): 25.4 (s). ESI-MS (m/z): 299.2
2.3.2. [Cu2(2,6-iPr2C6H3OPO3H)2(bpy)2(AcO)2(CH3OH)2]ꢀCH3OH (5)
Quantities: Cu(CH3COO)2ꢀH2O (0.12 g, 0.60 mmol); 2,20-bipyri-
dine (0.09 g, 0.60 mmol); 2,6-iPr2C6H3OPO3H2 (0.16 g, 0.60 mmol).
Yield: 0.32 g (89.6% based on 2,6-iPr2C6H3OPO3H2); Mp. 248–
249 °C. Anal. Calc. for C51H70Cu2N4O15P2 (1168.16): C, 52.44; H,
6.04; N, 4.80. Found: C, 52.56; H, 6.08; N, 4.78%. IR (KBr, cmꢁ1):
1089 [s, (masym PO3)], 1024 [s, (msym PO3)]. ESI-MS (CH3OH) m/z:
1074.01 [Mꢁ2MeOH+H]+. TGA, temperature ranges °C (weight loss
%): 30–140 (8.2); 181–380 (45); 471–545 (6.7); 546–765 (10.4).
(M+H, 100%). IR (CH2Cl2,
3535 (w), 2105 (m), 2055 (m), 1765 (w), 1712 (w), 1602 (s),
1571 (s), 1469 (s), 1301 (s), 1262 (vs), 963 (s). IR (KBr,
cmꢁ1):
m
cmꢁ1): 3820 (s), 3722 (s), 3601 (w),
m
3625–2780 (br), 2963 (vs), 2867 (vs), 2287 (m), 1609 (m), 1577
(m), 1463 (m), 1382 (m), 1361 (m), 1187 (s), 1004 (s), 930 (m),
877 (s), 789 (m), 668 (s), 582 (s).
Table 1
Crystal data and structure refinement parameters of 2–6.
Identification code
Empirical formula
Formula weight
Temperature (K)
Crystal system, space group
Unit cell dimensions
a (Å)
2
C
4
5
6
18H33O4P
C80H112Cu2N4O12P4
1572.7
100(2)
C51H66Cu2N4O15P2
1164.1
100(2)
C74H96Cd2N4O22P4
1742.23
100(2)
344.41
100(2)
triclinic, P-1
monoclinic, P2(1)/c
monoclinic, C2/c
monoclinic, P2(1)/n
8.043(5)
8.546(5)
10.6968(6)
14.1108(8)
26.4412(15)
90.000(5)
94.243(10)
90.000(5)
3980.1(4)
2, 1.312
0.676
1668
21.452(5)
13.284(5)
21.568(5)
90.000(5)
115.103(5)
90.000(5)
5566(3)
4, 1.389
0.889
2432
17.664(5)
11.198(5)
21.756(5)
90.000(5)
106.980(5)
90.000(5)
4116(2)
2, 1.406
0.666
1800
b (Å)
c (Å)
15.274(5)
99.075(5)
96.017(5)
97.380(5)
1019.7(9)
2, 1.122
a
(°)
b (°)
c
(°)
V (Å3)
Z, calculated density (mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
)
0.151
376
Crystal size (mm)
h Range for data collection (°)
Index ranges
0.15 ꢂ 0.1 ꢂ 0.1
2.44–25.99
ꢁ9 6 h 6 9,
ꢁ9 6 k 6 10,
ꢁ18 6 l 6 12
5800
0.15 ꢂ 0.10 ꢂ 0.06
2.11–28.33
ꢁ14 6 h 6 13,
ꢁ18 6 k 6 18,
ꢁ35 6 l 6 28
26 425
0.10 ꢂ 0.08 ꢂ 0.07
2.24–26.00
ꢁ26 6 h 6 26,
ꢁ16 6 k 6 13,
ꢁ26 6 l 6 19
15 213
0.12 ꢂ 0.09 ꢂ 0.05
1.96–26.00
ꢁ21 6 h 6 20,
ꢁ13 6 k 6 12,
ꢁ21 6 l 6 26
22 508
Reflections collected
Independent reflections
3924
9871
5439
8051
[Rint = 0.0229]
97.80
[Rint = 0.0595]
99.3
[Rint = 0.0413]
98.9
[Rint = 0.0371]
99.5
Completeness to h (%)
Data/restraints/parameters
3924/0/208
1.025
9871/0/480
1.068
5439/0/340
1.036
8051/1/530
1.056
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0618,
wR2 = 0.1545
R1 = 0.0769,
wR2 = 0.1656
0.665 and ꢁ0.525
R1 = 0.0607,
wR2 = 0.1251
R1 = 0.0826,
wR2 = 0.1346
0.721 and ꢁ0.460
R1 = 0.0563,
wR2 = 0.1525
R1 = 0.0715,
wR2 = 0.1690
1.680 and ꢁ0.604
R1 = 0.0499,
wR2 = 0.1240
R1 = 0.0614,
wR2 = 0.1324
1.057 and ꢁ0.813
Largest difference in peak and hole (e Åꢁ3
)

2922
V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
Table 2
Crystal data and structure refinement parameters of 7–9.
Identification code
Empirical formula
Formula weight
Temperature (K)
Crystal system, space group
Unit cell dimensions
a (Å)
7
C
8
9
72H88Cd2N4O16P4
C42H56Cu2N6O10P2
993.95
100(2)
C42H68Cl2Cu2N4O6P2
1055.86
100(2)
1614.14
100(2)
orthorhombic, Pbca
ꢀ
ꢀ
triclinic, P1
triclinic, P1
12.5356(8)
20.4442(13)
28.4290(19)
90
90
90
7285.8(8)
4, 1.472
0.740
8.361(5)
11.328(5)
12.669(5)
102.502(5)
100.661(5)
90.956(5)
1149.2(10)
1, 1.436
8.334(5)
9.593(5)
b (Å)
c (Å)
15.733(5)
87.237(5)
85.411(5)
79.543(5)
1232.2(11)
2, 1.414
a
(°)
b (°)
c
(°)
V (Å3)
Z, calculated density (mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
)
1.056
518
1.090
554
3328
Crystal size (mm)
h Range for data collection
Index ranges
0.11 ꢂ 0.06 ꢂ 0.04
2.04 to 26.00 °
ꢁ15 6 h 6 14,
ꢁ23 6 k 6 25,
ꢁ35 6 l 6 33
39 285
0.10 ꢂ 0.05 ꢂ 0.03
2.20 to 26.00 °
ꢁ10 6 h 6 10,
ꢁ9 6 k 6 13,
ꢁ13 6 l 6 15
6478
0.15 ꢂ 0.10 ꢂ 0.10
2.48 to 26.00 °
ꢁ10 6 h 6 10,
ꢁ11 6 k 6 11,
ꢁ14 6 l 6 19
7018
Reflections collected
Independent reflections
7146
4410
4717
[Rint = 0.0930]
99.8
[Rint = 0.0381]
97.7
[Rint = 0.0163]
97.4
Completeness to h (%)
Data/restraints/parameters
7146/2/458
1.046
4410/2/288
1.014
4717/0/294
1.054
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0520,
wR2 = 0.1150
R1 = 0.0801,
wR2 = 0.1320
1.888 and ꢁ0.640 e.Åꢁ3
R1 = 0.0604,
wR2 = 0.1378
R1 = 0.0853,
wR2 = 0.1638
0.993 and ꢁ0.483 e.Åꢁ3
R1 = 0.0353,
wR2 = 0.0867
R1 = 0.0396,
wR2 = 0.0893
0.607 and ꢁ0.392 e.Åꢁ3
Largest diff. peak and hole
2.3.3. [Cd2(2,6-iPr2C6H3OPO3H)4(bpy)2(CH3OH)2]ꢀ2CH3OH (6)
Quantities: Cd(CH3COO)2ꢀ2H2O (0.13 g, 0.48 mmol); 2,20-bipyri-
dine (0.08 g, 0.48 mmol); 2,6-iPr2C6H3OPO3H2 (0.12 g, 0.48 mmol).
Yield: 0.18 g (88.1% based on 2,6-iPr2C6H3OPO3H2); Mp. >260 °C
(d). 31P{1H} NMR (CD3OD, d) 6.7 (s) and 5.6 (s). Anal. Calc. for
2.3.6. [Cu2(2,4,6-iPr3C6H2CH2PO3H)2(DMPz)2Cl2]ꢀCH3OH (9)
Quantities: CuCl2 (0.05 g, 0.37 mmol); 3,5-dimethylpyrazole
(0.04 g, 0.37 mmol); 2,4,6-iPr3C6H2CH2PO3H2 (0.11 g, 0.37 mmol);
0.5 mL 1 M NaOH in water. Yield: 0.09 g (44.6% based on 2,4,6-
iPr3C6H2CH2PO3H2); Mp. >250 °C (d). Anal. Calc. for C42H68Cl2Cu2-
N4O6P2 (1055.86): C, 47.78; H, 6.49; N, 5.31. Found: C, 47.76; H,
6.48; N, 5.30%. IR (KBr, cmꢁ1): 1119 [s, (masym PO3)], 1008 [s, (msym
PO3)]. ESI-MS (CH3OH) m/z: 1052.49 [MꢁCl+MeOH]+. TGA, temper-
ature ranges °C (weight loss %): 30–75 (3.3); 76.115 (6.1); 176.350
(29.6); 351–470 (21.7); 501–605 (13.4); 606–750 (7.2).
C
74H96Cd2N4O22P4 (1760.47): C, 50.55; H, 6.42; N, 3.19. Found: C,
51.57; H, 6.49; N, 3.15. IR (KBr, cmꢁ1): 1082 [s, (masym PO3)], 1020
[s, (msym PO3)]. ESI-MS (CH3OH) m/z: 1567.32 [Mꢁ2MeOH+H]+.
TGA, temperature ranges °C (weight loss %): 30–205 (6); 206–
375 (54.6.0); 435–500 (5.1); 501–750 (2.5).
2.4. X-ray crystallography
2.3.4. [Cd2(2,6-iPr2C6H3OPO3H)4(phen)2] (7)
The crystal data for 2 and 4–9 are given in Tables 1 and 2. The
data were collected on a Bruker Smart Diffractometer. Data were
Quantities: Cd(CH3COO)2ꢀ2H2O (0.11 g, 0.40 mmol); 1,10-phe-
nanthroline (0.07 g, 0.40 mmol); 2,6-iPr2C6H3OPO3H2 (0.10 g,
0.40 mmol). Yield: 0.14 g (89.6% based on 2,6-iPr2C6H3OPO3H2);
Mp. > 250 °C (d). 31P{1H} NMR (CD3OD, d) 6.9 (s) and 5.6 (s). Anal.
Calc. for C72H88Cd2N4O16P4 (1614.20): C, 53.57; H, 5.49; N, 3.47.
Found: C, 53.55; H, 5.49; N, 3.45. IR (KBr, cmꢁ1): 1092 [s, (masym
collected using
a graphite-monochromated Mo Ka radiation
(k = 0.71073 Å). All the structures were solved by direct methods
using SHELXS-97 [12a] and refined by full-matrix least squares on
F2 using SHELXL-97. All hydrogen atoms were included in idealized
positions, and a riding model was used. Non-hydrogen atoms were
refined with anisotropic displacement parameters. The crystallo-
graphic figures were generated by using DIAMOND 3.1f programme
[12b].
PO3)], 1018 [s, (
m
symPO3)]. ESI-MS (CH3OH) m/z: 1615.32 [MꢁH]+.
TGA, temperature ranges °C (weight loss %): 30–220 (2.1); 221–
330 (39.1); 331–485 (9.9); 486–800 (6.7).
2.3.5. [Cu2(2,6-iPr2C6H3OPO3H)2(PyPz)2(CH3OH)2] (8)
3. Results and discussion
Quantities: Cu(CH3COO)2ꢀH2O (0.11 g, 0.53 mmol); 2-pyridylpy-
razole
(0.08 g,
0.53 mmol);
2,6-iPr2C6H3OPO3H2
(0.14 g,
3.1. Synthesis
0.53 mmol). Yield: 0.25 g (92.8% based on 2,6-iPr2C6H3OPO3H2);
Mp. > 250 °C (d). Anal. Calc. for C42H56Cu2N6O10P2 (993.97): C,
50.75; H, 5.68; N, 8.46. Found: C, 50.73; H, 5.69; N, 8.45%. IR
(KBr, cmꢁ1): 1082 [s, (masym PO3)], 1028 [s, (msym PO3)]. ESI-MS
(CH3OH) m/z: 929.16 [Mꢁ2MeOH+H]+. TGA, temperature ranges
°C (weight loss %): 30–190 (2); 191–260 (21.8); 261–400 (24.3);
401–565 (7.4); 566–800 (14).
The new phosphonic acid, 2,4,6-iPr3C6H2CH2P(O)(OH)2 (2), was
prepared according to Scheme 1. The first step involved the Arbu-
zov reaction between 2,4,6-tri-isopropylbenzyl chloride and tri-
ethyl phosphite to afford the corresponding diethylphosphonate
ester. The latter was not isolated, but hydrolyzed further affording
2 in good yield (see Section 2). The 31P{1H} NMR spectrum of 2

V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
2923
OH
OH
OEt
O
O
OEt
P
P
Cl
6% HCl
100 ºC
CH2O, dryHCl
CCl4, AcOH
P(OEt)3
140 ºC
Scheme 1. Synthesis of 2,4,6-tri-isopropylbenzylphosphonic acid.
showed a singlet at 25.4 ppm. ESI-MS spectra of 2 showed a parent
ion peak (M+H, 100%) at m/z 299.2. The solid-state structure of 2
revealed it to be a dimer as a result of intermolecular hydrogen
bonding (see vide infra).
The dinuclear (Cu2 or Cd2) compounds 4–7 are formed in three-
component reactions involving Cu(OAc)2ꢀH2O or Cd(OAc)2ꢀ2H2O/
phosphonic acid or phosphate/2,20-bipyridyl or 1,10-phenanthro-
line (Scheme 2). The strongly chelating ancillary ligand blocks
two of the coordination sites on the metal ion and this together
with the steric bulk of the phosphonic acid or the phosphate does
not allow the cluster to grow. Using pyridylpyrazole as the ligand
also leads to a dinuclear compound 8 albeit with a slight difference
R
P
L1
O
O
O
O
L
N
OH
MeOH
N
M
M(OAc)2 XH2O + RPO3H2
+
M
N
N
N
L1
N
HO
L
P
R
4; M = Cu, R = 2,4,6-iPr3C6H2, N-N = bpy, L = 2,4,6-iPr3C6H2PO3H-, L1
= nothing
-
5; M = Cu, R = 2,6-iPr3C6H3O, N-N = bpy, L = CH3CO2 ,
L1 = CH3OH
6; M = Cd, R = 2,6-iPr3C6H3O, N-N = bpy, L = 2,6-iPr3C6H3OPO3H-, L1 = CH3OH
7; M = Cd, R = 2,6-iPr3C6H3O, N-N = phen, L = 2,6-iPr3C6H3OPO3H-, L1
Scheme 2. Synthesis of 4–7.
=
nothing
R
OH
P
N
O
N
N
L
O
MeOH
Stirr, 4h
Cu
Cu
Cu(OAc)2 H2O + RPO3H2
+
N
N
N
O
L
N
N
H
N
O
P
R = 2,6-iPrC6H3O
R
L = CH3OH
HO
(8)
Scheme 3. Synthesis of 8.
R
P
HN
Cu
O
O
O
O
N
Cl
OH
Cl
HN N NaOH, MeOH
Stirr, 4h
Cu
Cl
CuCl2 + RPO3H2
+
Cl
N
HO
NH
P
R
R = 2,4,6-iPr3C6H2CH2
(9)
Scheme 4. Synthesis of 9.

2924
V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
in the structure in that these multifunctional nitrogen ligands do
not act as chelating ligands, instead they are involved in bridging
the two copper centers (Scheme 3).
The influence of the steric bulk of the phosphonic acid on the
course of the final product is clearly seen in the reaction of CuCl2
with 2,4,6-iPr3C6H2CH2PO3H2 in the presence of 3,5-dimethylpyra-
zole. In this instance also a dinuclear compound (9) is formed
(Scheme 4). ESI-MS studies on all the dinuclear complexes revealed
the presence of prominent molecular ion peaks (see Supporting
Information, Fig. S1). The diamagnetic compounds 6 and 7 show
a pair of singlets (6.7 and 5.6 ppm for 6; 6.9 and 5.6 ppm for 7) con-
sistent with their molecular structures (vide infra).
3.2. Molecular structures
3.2.1. Molecular structure of 2
The asymmetric unit of 2 (Fig. 1) contains one molecule of the
phosphonic acid and one molecule of dimethyl ether. Selected
bond parameters of 2 including those found in hydrogen bonding
are given in Table S1. Two molecules of 2 form an O–HꢀꢀꢀO hydro-
gen-bonded dimer, similar to that seen in many carboxylic acids
[13], leading to the formation of an eight-membered non-planar
ring (2P, 4O 2H). One of the P–OH units acts as a donor while the
P@O group acts as an acceptor. The remaining OH group is not in-
volved in hydrogen-bonding. The OꢀꢀꢀO distance involved in this
intermolecular hydrogen bonding interaction is 2.591 Å and the
O–HꢀꢀꢀO bond angle is 148.17° (see Supporting Information,
Fig. S2, Table S1).
Fig. 2. Molecular structure of 4. Hydrogen atoms from aromatic ring and isopropyl
group from phosphonic acid are omitted for clarity.
3.2.2. Molecular structures of the dinuclear complexes 4–9
The molecular structures of the dinuclear metal compounds 4–
7 are grossly similar and contain a central chair-shaped eight-
membered M2P2O4 [M = Cu (4, 5) or Cd (6, 7)] inorganic ring [Figs.
2–5].The two metal atoms and the two phosphorus atoms lie in
one plane and with respect to this a pair of oxygen atoms lie above
while another pair is located below. In all these cases the bridging
coordination of the singly deprotonated ligand [RP(O)2(OH)]ꢁ can
be considered as anisobidentate and the mode of coordination
may be termed as 2.101 according to the Harris notation (Chart
1) [14].
Fig. 3. Molecular structure of 5. Hydrogen atoms from aromatic ring, methyl group,
and isopropyl group from phosphate are omitted for clarity.
Each metal atom in 4–7 is bound by a chelating ligand (bpy in
the case of 4, 5 and 6; phen in the case of 7). The other coordination
environment around the metal atoms is slightly varied in each
compound. In 4 each copper atom is bound by a monodentate
[2,4,6-iPr3C6H2PO2OH]ꢁ ligand leading to an overall coordination
number of five and a coordination environment of 2N, 3O around
each copper atom. The coordination geometry around the metal
atoms in 4 may be described as distorted square pyramidal where
the base of the pyramid consists of two oxygen and two nitrogen
atoms, while the apical position is occupied by a third oxygen
atom.
In the case of 5 the two terminal copper atoms are six-coordi-
nate (Fig. 3) due to the presence of terminal acetate and methanol
ligands. The copper atoms in 6 on the other hand contain one
monodentate [2,6-iPr2C6H3OPO3H]ꢁ and a methanol molecule as
terminal coordinating groups. In this case also the copper atoms
are six-coordinate (2N, 4O). Finally, in 7 the two copper atoms
Fig. 1. Molecular structure of 2. Hydrogen atoms from methyl group and aromatic
ring are omitted for clarity.

V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
2925
Fig. 4. Molecular structure of 6. Hydrogen atoms from aromatic ring, methyl group
and isopropyl group from phosphate are omitted for clarity.
Fig. 6. Molecular structure of 8. Hydrogen atoms from methyl group, aromatic ring,
pyrazole ring and isopropyl group from phosphate are omitted for clarity.
Fig. 7. Molecular structure of 9. Hydrogen atoms from aromatic ring, methyl group
and isopropyl group from phosphate are omitted for clarity.
Fig. 5. Molecular structure of 7. Hydrogen atoms from aromatic ring and isopropyl
group from phosphate are omitted for clarity.
are 5-coordinate (2N, 3O) and contain only one terminal monoden-
tate [2,6-iPr2C6H3OPO3H]ꢁ ligand each.
The molecular structure of the dinuclear copper phosphate 8 is
slightly different from those discussed above and consists of a six-
membered Cu2N4 inorganic ring. In this instance the 2-pyridylpy-
R
P
R
P
O
O
O
M
N
O
N
N
HO
M
HO
M
M
M
2.101
(a)
1.001
(b)
2.111
(c)
R = 2,4,6-iPr3C6H2, 2,6-iPr3C6H3O
Fig. 8. Complexes 4, 5, 8 and 9 (complex 4 corresponds to gel A, 5 gel B, 8 gel C and
9 gel D) mediated DNA cleavage experiment at different time intervals. Lane 1: DNA
alone; lanes 2–5: DNA (pBR322) + complexes 4, 5, 8 and 9 (30, 60, 90 and 120 min,
respectively).
M = Cu, Cd
Chart 1. Coordination modes of phosphonate and 2-pyridyl pyrazole ligands found
in 2–9.

2926
V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
The molecular structure of 9 consists of an eight-membered
chair-shaped Cu2P2O4 inorganic ring similar to that found in 4–7
(Fig. 7). All the metal and phosphorus atoms lie in one plane with
*
respect to which the two oxygen atoms (O1 and O3 ) are above the
*
plane while the other two (O3 and O1 ) are below the plane.
The eight-membered Cu2P2O4 ring is formed as a result of an
anisobidentate bridging coordination mode of the two [2,4,6-
iPr3C6H2CH2PO2OH]ꢁ ligands. Each metal atom is further coordi-
nated by terminal pyrazole and chloride ligands. The square pyra-
midal coordination environment around each copper atom consists
of a basal plane comprising of two oxygen atoms, one nitrogen
atom of the pyrazole ligand and one chloride ligand. The remaining
chloride atom occupies the axial position. The Cu–Claxial bond is
slightly longer (Table S7, Supporting Information).
Magnetic susceptibility measurements for two representative
compounds viz., 5 and 8 were carried out between 300 and 5 K.
These studies (see Supporting Information) reveal that the room
Fig. 9. pBR322 DNA cleavage by 4, 5, 8 and 9 in presence of MMPP (complex 4
corresponds to gel A, 5 to gel B, 8 to gel C and 9 to gel D) at different time intervals.
Lane 1, DNA alone; lanes 2–5, DNA + complex + MMPP (at 1, 2, 3 and 4 min).
temperature (300 K) magnetic moments for 5 (1.46
lB) and 8
(1.66 B) decrease to 1.28 (5) and 0.41 (8) lB at 5 K indicating an
l
overall anti-ferromagnetic interaction between the two metal cen-
ters. We have not attempted to model the magnetic behavior.
3.3. Cleavage of plasmid DNA
In view of the wide-spread interest in the use copper complexes
in general and dinuclear copper complexes in particular as DNA
cleavage reagents [15], we have systematically examined the
DNA cleavage ability of the complexes 4, 5, 8 and 9 (Fig. 8). Time
course experiments revealed that4, 5, 8 and 9 were able to mediate
partial conversion (20–50%) of the supercoiled pBR322 DNA form I
to nick form II in 2 h. To foster the reaction we have added magne-
sium monoperoxyphthalate (MMPP) and we observed a rapid con-
version of the super coiled pBR322 DNA form I to form II within
1 min (Fig. 9). In the case of complexes 6 and 7 there is no nuclease
activity, even in the presence of the external oxidant (see Support-
ing Information, Fig. S5).
Fig. 10. pBR322 Cleavage experiments involving 4, 5, 8 and 9 in presence of free
radical scavengers and singlet oxygen quencher assisted by complex (complex 4
corresponds to gel A, 5 gel B, 8 gel C and 9 gel D) in a 2 min reaction. Lane 1, DNA
3.3.1. DNA cleavage mechanism
Copper-based artificial nucleases function through oxidative
and/or hydrolytic pathways. In view of this, we probed the cleav-
age mechanism of 4, 5, 8 and 9. In the presence of EDTA, the cleav-
age reaction is completely inhibited demonstrating a crucial role of
copper for plasmid modification. Hydroxyl radical scavengers, di-
methyl sulphoxide (DMSO), D-mannitol or tert-butylalcohol do
not inhibit cleavage reactions demonstrating that radicals are not
involved in the cleavage process. NaN3, a well-known quencher
of singlet oxygen [16], inhibits the cleavage reaction in the case
of 4, 5 and 9, which indicates that singlet oxygen may be involved
in the cleavage process (Fig. 10). In contrast, in the case of complex
8, NaN3 does not inhibit the cleavage reaction, which demonstrates
that singlet oxygen [16] is not involved in the cleavage process (see
Fig. 10).
alone;
lane
2,
pBR322 + complex + MMPP;
lane
3,
pBR322 + com-
plex + MMPP + DMSO; lane 4, pBR322 + complex + MMPP + D-mannitol; lane 5,
pBR322 + complex + MMPP + t-BuOH; lane 6, pBR322 + complex + MMPP + NaN3;
lane 7, pBR322 + complex + MMPP + EDTA.
razole ligand bridges the two copper atoms through the pyrazole
nitrogen atoms (N2 and N3, Fig. 6). In addition to the bridging coor-
dination mode each pyridylpyrazole ligand also functions as a che-
lating ligand through one pyridine and one pyrazole nitrogen
atoms to each of the copper atoms (Fig. 6). Additionally a mono an-
ionic phosphate [2,6-iPr2C6H3OPO2OH]ꢁ is present as a terminal li-
gand on each copper atom.
A solvent methanol molecule
completes the coordination environment around each copper (dis-
torted square pyramidal, 3N, 2O). The coordination modes of the
various ligands involved are summarized in Chart 1. The bond dis-
tance/angle data around copper are summarized in Table S6.
However, hydrolytic pathway for plasmid modification also ap-
pears potent in the current instance as demonstrated by the fact
that DNA cleavage does not decrease under anaerobic conditions
Fig. 11. pBR322 DNA cleavage in anaerobic conditions by 4, 5, 8 and 9 at 2 min time interval. Lane 1, DNA (pBR322) alone; lanes 2 and 3 for complex 4 + pBR322 + MMPP;
lanes 4 and 5 for complex 5 + pBR322 + MMPP; lanes 6 and 7 for complex 8 + pBR322 + MMPP; lanes 8 and 9 for complex 9 + pBR322 + MMPP (lanes 2, 4, 6, 8 aerobic and 3, 5,
7, 9 for anaerobic condition).

V. Chandrasekhar et al. / Inorganica Chimica Acta 363 (2010) 2920–2928
2927
(e) B.-Z. Wan, R.G. Anthony, G.-Z. Peng, A. Clearfield, J. Catal. 19 (1994) 101;
(Fig. 11). This cumulative circumstantial evidence points to the
possibility of multiple reaction pathways in the plasmid cleavage
activity mediated by 4, 5 and 9 where as exclusive hydrolytic path-
way is indicated for complex 8.
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Inorg. Chem. 37 (1998) 4168;
(m) B. Zhang, D.M. Poojary, A. Clearfield, Inorg. Chem. 37 (1998) 249;
(n) P. Ayyappan, O.R. Evans, Y. Cui, K.A. Wheeler, W. Lin, Inorg. Chem. 41 (2002)
4978;
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34 (2001) 80;
In conclusion we have prepared a series of dinuclear copper(II)
and cadmium(II) phosphonates and -phosphates using a multi-
component synthetic strategy involving the metal salt, a nitrogen
ancillary ligand and a phosphorus acid. Most of these compounds
have similar structures containing a central M2P2O4 ring. In the
case of 8, however, a central six-membered ring is formed as a re-
sult of the bridging mode of interaction of the pyridylpyrazole li-
gands. While the copper complexes are active nucleases, the
cadmium complexes are not.
(p) D.-K. Cao, Y.-Z. Li, L.-M. Zeng, J. Solid State Chem. 179 (2006) 573.
[5] V. Baskar, M. Shanmugam, E.C. Sañudo, M. Shanmugam, D. Collison, E.J.L.
McInnes, Q. Wei, R.E.P. Winpenny, Chem. Commun. (2007) 37.
[6] (a) V. Chandrasekhar, S. Kingsley, B. Rhatigan, M.K. Lam, A.L. Rheingold, Inorg.
Chem. 41 (2002) 1030;
Acknowledgment
(b) S. Kingsley, Ph.D. Thesis, Indian Institute of Technology, Kanpur, India,
2001.;
(c) V. Chandrasekhar, L. Nagarajan, R. Clérac, S. Ghosh, S. Verma, Inorg. Chem.
47 (2008) 1067;
(d) V. Chandrasekhar, R. Azhakar, T. Senapati, P. Thilagar, S. Ghosh, S. Verma, R.
Boomishankar, A. Steiner, P. Kögerler, Dalton Trans. (2008) 1150;
(e) V. Chandrasekhar, L. Nagarajan, K. Gopal, V. Baskar, P. Kögerler, Dalton
Trans. (2005) 3143;
(f) V. Chandrasekhar, S. Kingsley, Angew. Chem., Int. Ed. 39 (2000) 2320;
(g) V. Chandrasekhar, S. Kingsley, A. Vij, K.C. Lam, A.L. Rheingold, Inorg. Chem.
39 (2000) 3238;
We thank the Department of Science and Technology, India and
Council of Scientific and Industrial Research, India for financial
support. V.C. is a Lalit Kapoor Chair Professor of Chemistry. V.C.
is thankful for the Department of Science and Technology, for a
J.C. Bose fellowship. P.S. and T.S. thanks Council of Scientific and
Industrial Research, India for Senior Research Fellowship. A.D.
thanks Council of Scientific and Industrial Research, India for Junior
Research Fellowship.
(h) V. Chandrasekhar, P. Sasikumar, R. Boomishankar, Dalton Trans. (2008)
5189;
~
(i) V. Chandrasekhar, T. Senapati, E.C. Sanudo, Inorg. Chem. 47 (2008) 9553;
~
(j) V. Chandrasekhar, T. Senapati, R. Clerac, E.C. Sanudo, Inorg. Chem. 48 (2009)
Appendix A. Supplementary material
6192.
[7] (a) H.-C. Yao, Y.-Z. Li, Y. Song, Y.-S. Ma, L.-M. Zheng, X.-Q. Xin, Inorg. Chem. 45
(2006) 59;
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.03.038.
(b) H.-C. Yao, Y.-Z. Li, L.-M. Zheng, X.-Q. Xin, Inorg. Chim. Acta 358 (2005)
2523;
(c) P. Yin, Y. Peng, L.-M. Zheng, S. Gao, X.-Q. Xin, Eur. J. Inorg. Chem. (2003) 726;
(d) R. Murugavel, S. Shanmugan, Chem. Commun. (2007) 1257;
(e) S. Langley, M. Helliwell, J. Raftery, E.I. Tolis, R.E.P. Winpenny, Chem.
Commun. (2004) 142;
(f) R. Clarke, K. Latham, C. Rix, M. Hobday, J. White, CrystEngComm 7 (2005)
28;
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