Assembly of tetra, di and mononuclear molecular cadmium phosphonates using 2,4,6-triisopropylphenylphosponic acid and ancillary ligands

Article abstract of DOI:10.1039/b803256d

The reaction of ArPO₃H₂ (Ar = 2,4,6-iPr ₃-C₆H₂) with Cd(CH₃COO) ₂2H₂O using various co-ligands such as methanol, dimethylformamide (DMF) and 3,5-dimethylpyrazole (DMPZH) resulted in the formation of tetranuclear assemblies [Cd₄(ArPO₃) ₂(ArPO₃H)₄(CH₃OH) ₄]3(CH₃OH) (1), [Cd₄(ArPO ₃)₂(ArPO₃H)₄(DMF) ₄]3(DMF) (2) and [Cd₄(ArPO₃) ₂(ArPO₃H)₄(DMF)₂(DMPZH) ₂]2(DMF)2(H₂O) (3). In all of these compounds the tetranuclear cadmium array, containing two five-coordinate and two six-coordinate cadmium atoms, is held together by two μ₄ capping [ArPO₃]^2- and four anisobidentate μ₂ [ArPO₂(OH)]^- ligands. Each cadmium atom is bound to an additional ancillary ligand. The reaction of ArPO₃H₂ with Cd(CH₃COO)₂2H₂O in the presence of the chelating ligand 2,2′-bipyridine (bipy) leads to the exclusive formation of the dinuclear assembly [Cd₂(ArPO₃H)₄(bipy) ₂](CH₃OH)(H₂O) (4). The latter contains an eight-membered Cd₂P₂O₄ inorganic ring formed as a result of the bridging coordination action of two anisobidentate μ₂ [ArPO₂(OH)]^- ligands. Each cadmium atom is bound by one chelating bipy and one monodentate [ArPO₂(OH)] ^- ligands. Use of four equivalents of 3,5-dimethylpyrazole leads to the formation of the mononuclear derivative [Cd(ArPO₃H) ₂(DMPZH)₄] (5). The molecular structure of the latter comprises of a central cadmium atom surrounded by six monodentate ligands. Four of these are neutral pyrazole ligands that occupy the equatorial plane; the remaining two are anionic phosphinate ligands which are present trans to each other. The thermal analysis of 1 and 4 reveals that the char residue obtained at 600 °C consists predominantly of Cd₂P₂O₇. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b803256d

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www.rsc.org/dalton | Dalton Transactions
Assembly of tetra, di and mononuclear molecular cadmium phosphonates
using 2,4,6-triisopropylphenylphosponic acid and ancillary ligands†
Vadapalli Chandrasekhar,* Palani Sasikumar and Ramamoorthy Boomishankar
Received 26th February 2008, Accepted 4th June 2008
First published as an Advance Article on the web 11th August 2008
DOI: 10.1039/b803256d
The reaction of ArPO3H2 (Ar = 2,4,6-iPr3-C6H2) with Cd(CH3COO)2·2H2O using various co-ligands
such as methanol, dimethylformamide (DMF) and 3,5-dimethylpyrazole (DMPZH) resulted in the
formation of tetranuclear assemblies [Cd4(ArPO3)2(ArPO3H)4(CH3OH)4]·3(CH3OH) (1),
[Cd4(ArPO3)2(ArPO3H)4(DMF)4]·3(DMF) (2) and [Cd4(ArPO3)2(ArPO3H)4(DMF)2-
(DMPZH)2]·2(DMF)·2(H2O) (3). In all of these compounds the tetranuclear cadmium array,
containing two five-coordinate and two six-coordinate cadmium atoms, is held together by two m4
capping [ArPO3]2- and four anisobidentate m2 [ArPO2(OH)]- ligands. Each cadmium atom is bound to
an additional ancillary ligand. The reaction of ArPO3H2 with Cd(CH3COO)2·2H2O in the presence of
the chelating ligand 2,2¢-bipyridine (bipy) leads to the exclusive formation of the dinuclear assembly
[Cd2(ArPO3H)4(bipy)2]·(CH3OH)(H2O) (4). The latter contains an eight-membered Cd2P2O4 inorganic
ring formed as a result of the bridging coordination action of two anisobidentate m2 [ArPO2(OH)]-
ligands. Each cadmium atom is bound by one chelating bipy and one monodentate [ArPO2(OH)]-
ligands. Use of four equivalents of 3,5-dimethylpyrazole leads to the formation of the mononuclear
derivative [Cd(ArPO3H)2(DMPZH)4] (5). The molecular structure of the latter comprises of a central
cadmium atom surrounded by six monodentate ligands. Four of these are neutral pyrazole ligands that
occupy the equatorial plane; the remaining two are anionic phosphinate ligands which are present trans
to each other. The thermal analysis of 1 and 4 reveals that the char residue obtained at 600 ◦C consists
predominantly of Cd2P2O7.
In contrast to these molecular metal phosphonates those
Introduction
involving Cd(II) are relatively few.4 From earlier studies it is
Molecular metal phosphonates,1–4 unlike those possessing
extended structures,5,6 have been much less studied. One of
the principal challenges in this area is to find appropriate
known that the reactions of Cd(II) salts with MePO3H2 or
PhPO3H2 afford cadmium phosphonates [Cd(MePO3)]·H2O
and [Cd(PhPO3)]·H2O. However these compounds are not
molecular but possess extended structures.6a In a significant
result Roesky and co-workers have isolated a Cd20 molecular
aggregate [(MeCd)10{(thf)Cd}4Cd6(m4-O)2(m3-OH)2(tBuPO3)12]
in a reaction involving Me2Cd and tBuPO3H2.4a Subsequently a
few other molecular cadmium phosphonates have been isolated
and structurally characterized.4b–f We were interested in assessing
if the three-component synthetic methodology (metal salt,
phosphonic acid, co-ligand) that was successfully implemented
in preparing transition metal phosphonates could be applied to
the cadmium system also. Accordingly we chose to react Cd(II)
salts with the sterically hindered phosphonic acid ArPO3H2
(Ar = 2,4,6-iPr3-C6H2) along with co-ligands such as DMPZH
(3,5-dimethylpyrazole) and bipy. In some instances we also used
methanol or dimethylformamide (DMF) as the co-ligand. We
present herein, the synthesis and structural characterization
of [Cd4(ArPO3)2(ArPO3H)4(CH3OH)4]·3(CH3OH) (1), [Cd4-
(ArPO3)2(ArPO3H)4(DMF)4]·3(DMF) (2), [Cd4(ArPO3)2(Ar-
PO3H)4(DMF)2(DMPZH)2]·2(DMF)·2(H2O) (3), [Cd2(ArPO3-
H)4(bipy)2]·(CH3OH)·(H2O) (4) and [Cd(ArPO3H)2(DMPZH)4]
(5). We also have investigated the thermal stability of these
molecular phosphonates and have found that the char residue of
1 and 4 contains the cadmium pyrophosphate Cd2P2O7.
synthetic strategies that will allow the preparation of soluble
metal phosphonates whose nuclearity can be varied. While the
use of sterically hindered and liphophilic phosphonic acids has
led to the isolation of soluble metal phosphonates,1 another
strategy that has also been successful is the use of an ancillary
nitrogen ligand in conjunction with a phosphonic acid.2,3 The
latter strategy has yielded rich dividends for us and allowed
the isolation of trinuclear [Co(II) and Zn(II)],2a,b tetranuclear
[Cu(II) and Zn(II)],1a,2c,d hexanuclear [Zn(II)],2a decanuclear
[Cu(II)]2e and dodecanuclear [Cu(II)]2f metal phosphonates.
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur,
India. E-mail: vc@iitk.ac.in; Fax: +91-512-2590007/2597436; Tel: +91-
512-2597259
† Electronic supplementary information (ESI) available: Fig. S1(a)–(d),
S2(a)–(d), S3(a)–(d), S4(a), (b) and S5: Diamond view of compounds 1–5;
Fig. S6–S10: infrared spectra of compounds 1–5; Fig. S11(a), (b): infrared
spectrum of the char residue of 1 and infrared spectrum of Cd2P2O7; Fig.
S12(a), (b) and S13(a)–(c): ESI-MS of compounds 1 and 5; Fig. S14(a),
(b): EDX analysis of char residue obtained from 1 and 4; Fig. S15: SEM
picture of the char residue obtained in the TGA of 1. Tables S1(a), (b): bond
length and bond angle data for 1 and 2; Table S2: comparison of infrared
spectral data of char residue obtained from 1 with that of Cd2P2O7. CCDC
reference numbers 679265–679269. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b803256d
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[Cd(ArPO3H) (H2O)4]. TGA, temperature ranges ◦C (weight loss
%): 30–173 (10.11); 180–240 (12.9); 180–395 (36.7); 395–560 (4.9);
560–760 (2.9). The EDX of the char residue from TGA shows that
the Cd : P atomic ratio is 1 : 1.
Experimental section
General methods
Solvents and other general reagents used in this work were
purified according to standard procedures.7 2,4,6-Triisopro-
pylphenylphosphonic acid1a and 3,5-dimethylpyrazole7 were pre-
pared according to literature procedures. Cadmium(II) acetate
dihydrate (sd Fine chemicals, Mumbai, India) and 2,2¢-bipyridine
(Aldrich, USA) were used as supplied without any further
purification.
Preparation of [Cd4(ArPO3)2(ArPO3H)4(DMF)4]·3(DMF) (2)
The same procedure as above was followed for the synthesis of
2. The quantities involved are: ArP(O)(OH)2 (0.10 g, 0.35 mmol)
and Cd(CH3COO)2·2H2O (0.09 g, 0.35 mmol). At the end of the
reaction solvent was completely removed in vacuo and the residue
was recrystallized from DMF at room temperature. Yield: 0.11 g
crystals (70.6% based on ArPO3H2); mp 186–188 ◦C. 31P{ H}
1
Instrumentation
NMR (ppm) d 24.7. Anal. calcd for C102H170Cd4N4O22P6 (2439.94
g): C, 50.21; H, 7.02; N, 2.30. Found: C, 50.23; H, 7.01; N, 2.28%.
IR (KBr, cm-1): 1091 [s, (nasymPO3)], 1047 [s, (nsymPO3)]. TGA,
temperature ranges ◦C (weight loss %): 30–130 (3.5); 130–235
(27.7); 235–335 (41.7); 335–500 (5.1); 500–600 (1.4).
Melting points were measured using a JSGW melting point ap-
paratus and are uncorrected. Elemental analyses were carried out
using a Thermoquest CE instruments model EA/110 CHNS-O el-
emental analyzer. CHN analyses are reported for compounds after
removal of lattice solvent. 31P{ H} NMR spectra were recorded
1
on a JEOL-JNM LAMBDA 400 model spectrometer operating at
161.7MHz. These spectra were recorded in CDCl3 solution (unless
otherwise specified) and the chemical shifts were referenced with
respect to 85% H3PO4. IR spectra were recorded as KBr pellets
on a Bruker Vector 22FT IR spectrophotometer operating from
400 to 4000 cm-1. ESI-MS analyses were performed on a Waters
Micromass Quattro Micro triple quadrupole mass spectrometer.
Electrospray ionization (positive ion, full scan mode) was used
keeping methanol as solvent and nitrogen gas for desolvation.
Capillary voltage was maintained at 3 to 9 kV and cone voltage
was kept at 30 kV. The temperature maintained for the ion source
was 100 ◦C and for desolvation 250 ◦C. TGA curves were obtained
on a Perkin Elmer-Pyris6 Thermo Gravimetric Analyzer under a
dry nitrogen atmosphere operating at a heating rate of 10 ◦C min-1.
SEM images were obtained at room temperature on a JEOL JEM-
1011 machine, operated at 80 kV, equipped with a MegaView III
camera. For these measurements, the samples were prepared as
follows. The final char residue of the TGA experiment was placed
on a copper disc (1 cm ¥ 1 cm) surface containing a carbon tape.
The samples were then dried under vacuum and examined by
SEM. The composition of the material was determined by an
EDX analyzer attached to the SEM instrument.
Preparation of [Cd4(ArPO3)2(ArPO3H)4(DMF)2(DMPZH)2]·
2(DMF)·2(H2O) (3)
To a suspension of Cd(CH3COO)2·2H2O (0.14 g, 0.53 mmol) in
methanol (50 mL), 3,5-dimethylpyrazole (0.034 g, 0.35 mmol)
was added and the resultant clear reaction mixture was stirred
for 5 min. At this stage, ArP(O)(OH)2 (0.10 g, 0.35 mmol) was
added to this reaction mixture all at once. This was allowed to
stir at room temperature for 4 h. Removal of methanol in vacuo
afforded a white solid. This was recrystallized in DMF solution
at room temperature. Yield: 0.13 g (83.1% based on ArPO3H2);
mp 186–188 ◦C. 31P{ H} NMR (ppm) d 25.1. Anal. calcd for
1
C106H172Cd4N6O20P6 (2486.01 g): C, 51.21; H, 6.97; N, 3.4. Found:
C, 51.18; H, 6.99; N, 3.3%. IR (KBr, cm-1): 1118 [m, (nasymPO3)],
1014 [m, (nsymPO3)]. TGA, temperature ranges ◦C (weight loss
%): 30–100 (1.6); 100–220 (31.4); 220–260 (5.8); 260–320 (13.8);
260–400 (17.9); 400–550 (2.4).
Preparation of [Cd2(ArPO3H)4(bipy)2]·(CH3OH)·(H2O) (4)
To a suspension of Cd(CH3COO)2·2H2O (0.09 g, 0.35 mmol) in
methanol (50 mL), 2,2¢-bipyridine (0.06 g, 0.35 mmol) was added
and the resultant clear reaction mixture was stirred for 5 min.
At this stage, ArP(O)(OH)2 (0.10 g, 0.35 mmol) 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 vacuum and the clear solution was filtered and kept for crys-
tallization at room temperature by slow evaporation. After about
a week colorless crystals suitable for X-ray analysis were isolated.
A TGA analysis of the crystals was carried out. The char residue
was subjected to EDX analysis which showed that its Cd : P ratio
Preparation of [Cd4(ArPO3)2(ArPO3H)4(CH3OH)4]·3(CH3OH)
(1)
A
mixture of ArP(O)(OH)2 (0.50 g, 1.76 mmol) and
Cd(CH3COO)2·2H2O (0.47 g, 1.76 mmol) were taken in 60 mL
of distilled methanol. The reaction mixture was stirred for 4 h
at room temperature. The volume of the solvent was reduced
under vacuum to about 5 mL, this was filtered and kept for
crystallization at room temperature by slow evaporation. After
about a week colorless crystals suitable for X-ray analysis were ob-
tained. Yield: 0.64 g (92.1% based on ArPO3H2); mp 172–174 ◦C.
◦
is 1 : . Yield: 0.14 g (92.5% based on ArPO3H2); mp 240–244 C.
1
31P{ H} NMR (ppm) d 21.0. Anal. calcd for C80H112Cd2N4O12P4
(1670.48): C, 57.52; H, 6.76; N, 3.35. Found: C, 57.50; H, 6.78; N,
1
31P{ H} NMR (ppm) d 24.7. Anal. calcd for C94H158Cd4O22P6
3.34%. IR (KBr, cm-1): 1093 [s, (nasymPO3)], 1014 [s, (nsymPO3)].
(2275.73 g): C, 49.61; H, 6.99. Found: C, 49.58; H, 6.71%.
IR (KBr, cm-1): 1086 [m, (nasymPO3)], 1018 [m, (nsymPO3)]. ESI-
MS (CH3OH) m/z: 1927.75 [Cd2(ArPO3H)4(ArPO3H2)2 - H]+,
1643.59 [Cd2(ArPO3H)3(ArPO3H2)2]+, 1249.53 [Cd2(ArPO3H)3-
(CH3OH)2(H2O)6]+, 754.27 [Cd(ArPO3H)2(H2O)4 + H], 470.10
ESI-MS (CH3OH) m/z: 1669.36 [Cd2(ArPO3H)4(bipy)2 -
H]+, 1735.17 [Cd2(ArPO3H)4(bipy)2(CH3OH)2 + H]+, 1552.36
[Cd2(ArPO3H)4(bipy)(H2O)2 + H]+. TGA, temperature ranges ◦C
(weight loss %): 30–80 (10); 80–210 (17.0); 210–260 (9.7); 260–350
(32.0); 350–500 (2.0).
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Preparation of [Cd(ArPO3H)2(DMPZH)4] (5)
To a suspension of Cd(CH3COO)2·2H2O (0.09 g, 0.35 mmol)
in methanol (50 mL), 3,5-dimethylpyrazole (0.14 g, 1.41 mmol)
was added and the resultant clear reaction mixture was stirred
for 5 min. At this stage, ArP(O)(OH)2 (0.10 g, 0.35 mmol) 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 vacuum and the clear solution
was filtered and kept for crystallization at room temperature
by slow evaporation. Yield: 0.17 g (90.9% based on ArPO3H2);
mp 172–174 ◦C. 31P{ H} NMR (ppm) d 23.7. Anal. calcd for
1
C50H80CdN8O6P2 (1063.57): C, 56.36; H, 7.57; N, 10.52. Found: C,
56.28; H, 7.59; N, 10.47%. IR (KBr, cm-1): 1119 [m, (nasymPO3)].
ESI-MS (CH3OH) m/z: 1061.43 [Cd(ArPO3H)2(DMPZH)4
-
H]+, 874.35 [Cd(ArPO3H)2(DMPZH)(CH3OH)3 - H]+, 590.19
[Cd(ArPO3H)(DMPZH)(CH3OH)3]+. TGA, temperature ranges
◦C (weight loss %): 30–100 (1.0); 100–200 (13.8); 200–250 (22.3);
250–450 (36.2); 450–600 (3.5); 600–700(0.6).
X-Ray crystallography
The crystal data for the compounds were collected on a Bruker
SMART APEX CCD Diffractometer. The program SMART
(version 6.45) was used for integration of the intensity of reflections
and scaling. The program SADABS was used for absorption
correction. The crystal structures were solved and refined by
full matrix least-squares methods against F2 by using the pro-
gram SHELXL-97.8a All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen positions were
fixed at calculated positions and refined isotropically. Crystal data
for the compounds 1–5 are given in Table 1. The figures are
generated by using Diamond 3.1e programme.8b In compound
2 there are three regions of the asymmetric unit that appear to
be occupied by molecules of dimethylformamide. None of the
solvent molecules are well behaved. All appear to be disordered.
As a result, their contribution to the scattering was removed by
use of the utility, SQUEEZE.8c,d The electron count (125) from
the “squeeze” model converged in good agreement with the three
dimethylformamide solvate molecules.
Results and discussion
Synthetic aspects
The reaction of ArPO3H2 with Cd(CH3COO)2·2H2O in
methanol afforded the tetranuclear compound 1 in about
90% yield (Scheme 1). Although the ratio of ArPO3H2 to
Cd(CH3COO)2·2H2O was kept at 1 : 1 in the reaction, in the
product the P : Cd ratio was always 6 : 4. Compounds 2 and 3 are
analogous to 1 except for a modification in the terminal ligands.
In 2 these are dimethylformamide ligands. In 3 two DMF and two
DMPZH molecules serve as the terminal ligands.
The dinuclear compound 4 is formed in a three component
1 : 1 : 1 reaction involving ArPO3H2, Cd(CH3COO)2·2H2O and
bipy (Scheme 1). Here also the P : Cd ratio used in the reaction
(1 : 1) is not retained in the product (2 : 1). However, the Cd : bipy
ratio remains 1 : 1 in the product. Use of four equivalents of
DMPZH alters the course of the reaction and only a mononuclear
product 5 is formed (Scheme 1). Thus, it appears that the nuclearity
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Scheme 1 Synthesis of 1–5.
of cadmium phosphonates can be varied by changing the nature
and amount of the ancillary ligand.
atoms of such [ArPO3]2- ligands are bidentate and function as
bridging [m2] ligands, the third oxygen atom is monodentate and is
attached only to one cadmium atom. The [Cd4(ArPO3)2] unit,
thus generated, is stitched together by the bidentate bridging
coordination action of four phosphinate, [ArPO2(OH)]- ligands.
Each [ArPO2(OH)]- ligands holds two cadmium atoms together
through two monodentate oxygen atoms. The coordination modes
of [ArPO3]2- and [ArPO2(OH)]- in 1–3 are of the type 5·122 and
2·011 according to the Harris notation [Chart 1(a) and Chart
1(b)].9 The overall result of the phosphonate/phosphinate coordi-
nation is the generation of 8 six-membered Cd2PO3 inorganic rings
which have a puckered conformation. The coordination around
the cadmium atoms is completed by the monodentate binding of
a neutral ancillary ligand. In 1, all the four cadmium atoms are
bound to methanol molecules (O coordination). In 2, all the four
cadmium atoms are bonded to DMF molecules (O coordination).
In 3, two DMF (O coordination) and two 3,5-dimethylpyrazole
molecules (N coordination) are bound to a pair of antipodal
cadmium atoms. Thus, in all the three compounds (1–3) two types
of cadmium atoms are present. One of these has a coordination
number of five [5O (1, 2 and 3)] while the other has a coordination
number of six [6O (1 and 2), 5O, 1 N (3)]. The coordination
geometry of the five-coordinate cadmium atom is approximately
trigonal bipyramidal, while that of the six-coordinate cadmium
atom is distorted octahedral (see ESI†). The t parameters around
the five-coordinate cadmium atoms in 1–3 are 0.77, 0.63 and 0.84
respectively indicating that these are present in a distorted trigonal
bipyramidal geometry (see ESI†).10
In order to examine the solution behavior of 1–5 we examined
their ESI-MS in methanol solution under the positive ion mode.
The tetranuclear aggregates do not retain their structures under
the mass spectroscopic conditions. Although peaks at 1927.75
and 1249.53 were observed these correspond to dinuclear or
1
mononuclear species (see ESI†). The 31P{ H} NMR spectra of
1–3 also show singlets at 24.7, 24.7 and 25.1 ppm (cf . ArPO3H2
1
which shows a singlet at 28.3 ppm in its 31P{ H} NMR spectrum)1a
respectively corroborating the solution mass spectral behavior.
Although the tetranuclear structures do not retain their struc-
tures in solution, the dinuclear compound 4 does so. Thus, the
ESI-MS spectrum of 4 [Fig. 1(a)] shows a peak at 1669.36 which
corresponds to the parent ion peak. Another prominent high
molecular weight peak at 1735.17 is seen [Fig. 1(b)] which is
assigned to a solvate containing two molecules of methanol.
Analogous to 4 the mononuclear compound 5 also shows the
retention of the structure in solution. In its ESI-MS spectrum a
parent ion peak at 1061.43 is observed (see ESI†).
Molecular structures of 1–3
The molecular structures of 1–5 were confirmed by their single
crystal X-ray analysis. All the tetranuclear derivatives (1–3) possess
similar structures. The molecular structure of 3 is shown in
Fig. 2. Those of 1 and 2 are given in the ESI.† The molecular
structures of 1–3 possess a planar tetra cadmium array which is
held together as a result of a concerted coordination action of
two types of phosphonic acid-derived ligands. One of these is
a phosphonate, [ArPO3]2-, ligand. A pair of such ligands binds
to the four cadmium atoms from the top and bottom faces in
a m4 capping coordination mode (see ESI†). While two oxygen
The bond length and bond angle data of 3 are summarized in
Table 2, those of 1 and 2 are given in the ESI.† The Cd–O bond
distances around the five-coordinate cadmium (Cd1) atom vary
˚
from 2.13 to 2.35 A, the longer distances being associated with the
apical Cd–O bonds. The O7–Cd–O10 bond angle of 175.34(12)◦ is
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Fig. 2 Molecular structure of 3. All the carbon and hydrogen atoms have
been omitted. For the dimethylpyrazole ligands, only the coordinating
nitrogen atom is shown. * Symmetry transformations used to generate
equivalent atoms: -x + 2, -y + 2, -z + 1.
Chart 1 Coordination modes of [ArPO3]2- and [ArPO2OH]- observed in
1–5.
Fig. 1 (a) ESI MS spectrum of 4 showing the parent ion peak [M - H]+ at
1669.36. (b) ESI MS spectrum of 4 showing the [M·(MeOH)2 - H]+ peak
at 1735.17.
close to the linear angle anticipated in perfect trigonal bipyramidal
geometries. The Cd–O distances found around the six-coordinate
˚
cadmium (Cd2) range from 2.24 to 2.48 A. The bond angles
around this metal center reveal that the geometry is significantly
distorted from an ideal octahedral situation. Among the P–O bond
distances those involved in the phosphonate ligand, [ArPO3]2-, are
˚
very similar with an average value of 1.528(3) A. In contrast among
the phosphinate ligands [ArP(O)2OH]- the P–O bond distances are
dissimilar; for example the short distances found around P1 have
˚
an average value of 1.505(4) A, while the long distance involving
˚
the P1–O2 bond is 1.588(4) A. A similar trend is seen around P2
also (Table 2).
Fig.
3 Molecular structure of 4. The alkyl substituents on the
[ArPO2OH]- ligand as well as the hydrogen atoms on the Ar groups and
bipyridyl ligands have been omitted. * Symmetry transformations used to
generate equivalent atoms: -x + 1, -y + 2, -z.
Molecular structures of 4 and 5
The molecular structures of 4 and 5 are shown in Fig. 3 and 4
respectively. The bond length/angle data for these compounds are
given in Tables 3 and 4 respectively.
The molecular structure of the dinuclear compound 4 com-
prises of an eight-membered chair-shaped Cd2P2O4 inorganic ring
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result of an anisobidentate bridging coordination mode of two
[ArPO2OH]- ligands. This coordination mode of the phosphinate
is designated as 1·001.9 Each cadmium atom is further coordi-
nated by a chelating 2,2¢-bipyridine ligand and two monodentate
[ArPO2OH]- ligands. The two bipyridine ligands in 4 are present in
a trans orientation with respect to each other. The same description
is also valid for the two monodentate [ArPO2OH]- ligands. As
a result of this multiple coordination from three different types
of ligands the coordination environment around each cadmium
consists of three oxygen and two nitrogen atoms (see ESI†).
The coordination geometry around the cadmium atom 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. The Cd–
O distances in 4 are very similar and are slightly shorter than
the Cd–N distances. All the three P–O distances of the bridging
phosphinate ligand are dissimilar owing to its anisobidentate
coordination mode. Among the monodentate phosphinate ligands
˚
P2–O5 is significantly longer (1.568(4) A) in comparison to the
other two bonds P2–O4 and P2–O6 (Table 3).
The molecular structure of the mononuclear compound 5
(Fig. 4) comprises of a central cadmium atom surrounded by six
monodentate ligands. Four of these are neutral pyrazole ligands
that occupy the equatorial plane, the remaining two are anionic
phosphinate ligands which are present trans to each other (see
ESI†). The coordination geometry around the cadmium atom is
near octahedral as reflected in the bond angles found around it
[O1–Cd1–O1*, N3–Cd1–N3*, N1–Cd1–N1* are all 180◦]. The
Fig. 4 Molecular structure of 5. Alkyl groups of the [ArPO2OH]- ligands
have been omitted. Among the hydrogen atoms only the N–H and O–H
hydrogen atoms are shown, the rest have been omitted. * Symmetry
transformations used to generate equivalent atoms: -x + 1, -y + 1, -z.
(see ESI†). All the cadmium and phosphorus atoms lie in one
plane with respect to which two oxygen atoms (O1 and O3*) are
above the plane while the other two (O3 and O1*) are below
the plane. The eight-membered Cd2P2O4 ring is formed as a
˚
Cd–O bond distance (Cd1–O1, 2.282(2) A) is shorter than the Cd–
˚
N distance (average value: 2.364(3) A). Among the P–O distances
˚
P1–O2 is much longer (1.570(2) A) than the other two distances
Table 2 Selected bond distance/angle data of 3
Geometry around Cd/coordination
˚
mode of phosphorus ligand
Bond distances/A
Bond angles/◦
Cd(1)–O(7)
Cd(1)–O(10)
Cd(1)–O(1)
Cd(1)–O(4)
Cd(1)–O(8)
2.300(3)
2.345(4)
2.136(3)
2.217(3)
2.184(3)
O(7)–Cd(1)–O(10)
O(1)–Cd(1)–O(8)
O(8)–Cd(1)–O(4)
O(1)–Cd(1)–O(4)
175.34(12)
107.29(14)
127.23(14)
125.03(15)
Cd(2)–N(1)
Cd(2)–O(9)
Cd(2)–O(3)*
Cd(2)–O(6)
Cd(2)–O(7)*
Cd(2)–O(8)
2.282(4)
2.275(3)
2.244(3)
2.252(3)
2.355(3)
2.482(3)
O(9)–Cd(2)–N(1)
O(6)–Cd(2)–O(8)
O(3)*–Cd(2)–O(6)
O(3)*–Cd(2)–O(7)*
O(7)*–Cd(2)–O(8)
163.48(13)
79.87(13)
128.03(15)
92.05(12)
60.73(11)
P(1)–O(1)
P(1)–O(2)
P(1)–O(3)
1.503(4)
1.588(4)
1.505(4)
P(1)–O(1)–Cd(1)
P(1)–O(3)–Cd(2)
139.8(2)
133.8(2)
P(2)–O(4)
P(2)–O(5)
P(2)–O(6)
1.508(4)
1.592(4)
1.499(4)
P(2)–O(4)–Cd(1)
P(2)–O(6)–Cd(2)
109.7(2)
132.6(2)
P(3)–O(7)
P(3)–O(8)*
P(3)–O(9)
1.526(3)
1.533(3)
1.529(3)
P(3)–O(7)–Cd(1)
P(3)–O(7)–Cd(2)
P(3)–O(8)–Cd(2)
P(3)–O(8)–Cd(1)
P(3)–O(9)–Cd(2)
128.7(2)
98.86(16)
93.55(16)
133.85(19)
124.85(17)
* Symmetry transformations used to generate equivalent atoms: -x + 2, -y + 2, -z + 1.
5194 | Dalton Trans., 2008, 5189–5196
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Table 3 Selected bond distance/angle data of 4
Geometry around Cd/coordination
mode of phosphorus ligand
Bond angles/◦
˚
Bond distances/A
Cd(1)–O(1)
2.187(4)
O(3)–Cd(1)–N(1)
101.84(18)
Cd(1)–O(3)
Cd(1)–N(2)
Cd(1)–N(1)
2.202(3)
2.324(5)
2.308(5)
O(4)–Cd(1)–N(2)
O(1)–Cd(1)–O(4)
O(3)–Cd(1)–O(1)
O(1)–Cd(1)–N(1)
O(1)–Cd(1)–N(2)
87.57(16)
95.99(16)
105.42(15)
85.34(17)
147.93(17)
P(1)–O(1)
P(1)–O(2)
P(1)–O(3)
1.506(4)
1.534(4)
1.547(4)
P(1)–O(1)–Cd(1)
P(1)–O(3)–Cd(1)*
129.6(2)
117.8(2)
P(2)–O(4)
P(2)–O(5)
P(2)–O(6)
1.505(4)
1.568(4)
1.512(4)
P(2)–O(4)–Cd(1)
133.2(2)
* Symmetry transformations used to generate equivalent atoms: -x + 1, -y + 2, -z.
Table 4 Selected bond distance/angle data of 5
Geometry around Cd/coordination
˚
mode of phosphorus ligand
Bond distances/A
Bond angles/◦
Cd(1)–O(1)
Cd(1)–N(3)
Cd(1)–N(1)
2.282(2)
2.337(3)
2.391(3)
O(1)–Cd(1)–O(1)*
N(1)*–Cd(1)–N(1)
N(3)*–Cd(1)—N(3)
O(1)–Cd(1)–N(1)
O(1)–Cd(1)–N(1)*
O(1)–Cd(1)–N(1)*
O(1)–Cd(1)–N(3)*
N(3)*–Cd(1)–N(1)*
180.00(9)
180.00(1)
180.000(1)
98.56(8)
81.44(8)
81.44(8)
81.31(9)
87.47(9)
P(1)–O(1)
P(1)–O(2)
P(1)–O(3)
1.505(2)
1.570(2)
1.510(2)
P(1)–O(1)–Cd(1)
158.65(14)
* Symmetry transformations used to generate equivalent atoms: -x + 1, -y + 1, -z.
(Table 4). The coordination mode of [ArPO2OH]- in 5 may be
described as 1·001 [(Chart 1(c)].9
Thermal degradation of 1–5
All the cadmium complexes prepared in the present study were
examined for their thermal stability by thermogravimetric analysis.
The TGA traces for these compounds are shown in Fig. 5.
Among the tetrameric derivatives, 1, has the highest char residue
(~ 35%) at 600 ◦C. The decrease in weight beyond this temperature
is not very significant. The weight loss pattern for the dinuclear
derivative 4 reveals a final char yield of about 29% that of the
mononuclear compound; 5 is 22% at 700 ◦C.
The nature of the final char obtained in 1 and 4 was investigated
and was found to be identical. SEM pictures showed that this
material has a sheet-like microstructure (see ESI†). An EDX
analysis (see ESI†) revealed that the Cd : P atomic ratio in the char
residue was 1 : 1 suggesting that this residue has the composition
Cd2P2O7. The IR spectrum of the char was investigated and was
found to match very well with that of an authentic sample reported
earlier (see ESI†).11
Fig. 5 TGA traces of 1–5.
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Conclusion
In conclusion, we have shown that the sterically hindered and
lipophilic phosphonic acid ArP(O)(OH)2 is able to stabilize
cadmium aggregates of varying nuclearity ranging from 4 to 1.
The versatility of the phosphonic acid-derived ligand systems
stems from its ability to function both as a phosphonate [ArPO3]2-
and a phosphinate [ArPO2(OH)]-. While the former functions
as a m4 capping ligand the latter acts both as an anisobidentate
bridging ligand (m2) as well as a monodentate bridging ligand.
The role of the ancillary ligand in the stabilization of the discrete
molecular entities is also quite crucial as demonstrated in the
change of nuclearity from four to one with an increase in the
3,5-dimethylpyrazole content. Finally the thermal analysis of 1
and 4 reveals the formation of the pyrophosphate Cd2P2O7. It
is to be anticipated that the use of lipophilic phosphates such
as ROP(O)(OH)2 would considerably enhance the utility of the
cadmium precursors for cadmium phosphate materials. We are
investigating this possibility.
Acknowledgements
6 (a) G. Cao, V. M. Lynch and L. N. Yacullo, Chem. Mater., 1993, 5,
1000; (b) C. Xu, B. Liu, S.-S. Bao, Y. Chen and L.-M. Zheng, Solid
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7 B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell, Vogel’s
Text book of Practical Organic Chemistry, 5th edn, ELBS, Longman,
London, 1989.
8 (a) G. M. Sheldrick, SHELXL-97, A Program for Crystal Struc-
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Germany, 1997; (b) K. Bradenburg, Diamond, version 3.1e, Crystal
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We thank Council of Scientific and Industrial Research (CSIR),
New Delhi, India and Department of Science and Technology,
India for financial support. VC is a Lalit Kapoor Chair Professor
at IIT Kanpur. VC is thankful to DST for a J. C. Bose National
Fellowship. PS is thankful to CSIR for a Senior Research
Fellowship.
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5196 | Dalton Trans., 2008, 5189–5196
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