Pyrazolylcyclotriphosphazene containing pendant polymers: Synthesis, characterization, and phosphate ester hydrolysis using a Cu(II)-metalated cross-linked polymeric catalyst

Article abstract of DOI:10.1021/ic011159v

A multi-pyrazolyl cyclotriphosphazene containing polymerizable group N₃P₃(3,5-Me₂Pz)₅ (O-C₆H₄-p-C₆H₄-p- CH=CH₂) (2) has been prepared from the corresponding chloro derivative N₃P₃Cl₅ (O-C₆H₄-p- C₆H₄-p-CH=CH₂) (1). The X-ray structures of 1 and 2 have been determined. Compound 2 undergoes ready metalation with CuCl₂ to afford N₃P₃ (3,5-Me₂Pz)₅ (O-C₆H₄-p- C₆H₄-p-CH=CH₂) ·CuCl₂ (3). Model compound N₃P₃ (3,5-Me₂Pz)₅ (O-C₆H₄-p-CHO)· CuCl₂ (6) has been prepared and characterized by spectroscopy and X-ray crystallography. In this compound, the coordination around copper is distorted trigonal bipyramidal, and the cyclotriphosphazene coordinates in a non-gem N₃ mode. Compound 2 has been copolymerized with divinylbenzene to afford cross-linked multisite coordinating polymer CPPL which is readily metalated with CuCl₂ to afford copper-containing polymer CPPL-Cu. The coordination environment around copper in CPPL-Cu has been evaluated by obtaining its EPR, optical, and IR spectra and comparing them with those of model compounds 3 and 6. The utility of CPPL-Cu as a heterogeneous catalyst has been demonstrated in the phosphate ester hydrolysis involving three model phosphate esters: p-nitrophenyl phosphate (pNPP), bis(p-nitrophenyl) phosphate (bNPP), and 2-(hydroxypropyl)-p-nitrophenyl phosphate (hNPP). In all of these reactions, a significant rate enhancement of ester hydrolysis is observed. Detailed kinetic analyses to evaluate Michaelis-Menten parameters have also been carried out along with experiments to elucidate the effect of pH, solvent, and temperature on the rate of hydrolysis. Recycling experiments on the hydrolysis of pNPP with CPPL-Cu shows that it can be recycled several times over without affecting the rates.

Full text of DOI:10.1021/ic011159v

Inorg. Chem. 2002, 41, 5162−5173
Pyrazolylcyclotriphosphazene Containing Pendant Polymers: Synthesis,
Characterization, and Phosphate Ester Hydrolysis Using a
Cu(II)-Metalated Cross-Linked Polymeric Catalyst
,†
Vadapalli Chandrasekhar,* Arunachalampillai Athimoolam,† S. G. Srivatsan,†
P. Shanmuga Sundaram,† Sandeep Verma,† Alexander Steiner,‡ Stefano Zacchini,‡ and
Raymond Butcher§
Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India, Department of
Chemistry, UniVersity of LiVerpool, LiVerpool-L69 7ZD, U.K., and Department of Chemistry,
Howard UniVersity, Washington, D.C. 20059
Received November 13, 2001
A multi-pyrazolyl cyclotriphosphazene containing polymerizable group N P (3,5-Me2Pz)5(O−C H -p-C H -p-CHdCH )
3
3
6
4
6
4
2
(2) has been prepared from the corresponding chloro derivative N P Cl5(OsC H -p-C H -p-CHdCH ) (1). The
3
3
6
4
6
4
2
X-ray structures of 1 and 2 have been determined. Compound 2 undergoes ready metalation with CuCl2 to afford
N P (3,5-Me2Pz)5(OsC H -p-C H -p-CHdCH )‚CuCl2 (3). Model compound N P (3,5-Me2Pz)5(OsC H -p-CHO)‚
3
3
6
4
6
4
2
3
3
6 4
CuCl2 (6) has been prepared and characterized by spectroscopy and X-ray crystallography. In this compound, the
coordination around copper is distorted trigonal bipyramidal, and the cyclotriphosphazene coordinates in a non-
gem N mode. Compound 2 has been copolymerized with divinylbenzene to afford cross-linked multisite coordinating
3
polymer CPPL which is readily metalated with CuCl2 to afford copper-containing polymer CPPL-Cu. The coordination
environment around copper in CPPL-Cu has been evaluated by obtaining its EPR, optical, and IR spectra and
comparing them with those of model compounds 3 and 6. The utility of CPPL-Cu as a heterogeneous catalyst has
been demonstrated in the phosphate ester hydrolysis involving three model phosphate esters: p-nitrophenyl phosphate
(pNPP), bis(p-nitrophenyl) phosphate (bNPP), and 2-(hydroxypropyl)-p-nitrophenyl phosphate (hNPP). In all of these
reactions, a significant rate enhancement of ester hydrolysis is observed. Detailed kinetic analyses to evaluate
Michaelis−Menten parameters have also been carried out along with experiments to elucidate the effect of pH,
solvent, and temperature on the rate of hydrolysis. Recycling experiments on the hydrolysis of pNPP with CPPL-
Cu shows that it can be recycled several times over without affecting the rates.
Introduction
opening polymerization3 of N3P3Cl6 or by a condensation
polymerization involving Cl3PdNsP(O)Cl2 or Cl3PdN-
(SiMe)3.5 Other methods of preparing poly(organophos-
phazenes) include fluoride-assisted6 or thermal polymeriza-
4
Polyphosphazenes, [NdPR2]n, constitute the largest family
of inorganic polymers with over 800 different examples.1 A
large majority of these polymers are assembled by a macro-
molecular nucleophilic substitution on the polydichloro-
phosphazene [NdPCl2]n.1a,2 The latter is obtained by a ring
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* To whom correspondence should be addressed. E-mail: vc@iitk.ac.in.
† Indian Institute of Technology.
‡ University of Liverpool.
§ Howard University.
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5162 Inorganic Chemistry, Vol. 41, No. 20, 2002
10.1021/ic011159v CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/04/2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
tionofphosphoranimines.7,1c Theversatilityofpolyphosphazenes
arises from the fact that the polymer properties can be readily
modulated by varying the substituents on phosphorus.
Another class of related polymers contains an intact cyclo-
phosphazene ring as a pendant group attached at regular
intervals to the backbone of an organic polymer.8 The number
of examples of such pendant polymers is limited in com-
parison to that of the more prolific linear polyphosphazenes.
However, in principle, this family also has the same potential
in terms of tunability of polymer function and property. This
can be accomplished by polymerizing a cyclotriphosphazene
monomer N3P3R5P (P ) a vinyl containing substituent; R
) other substituent).
Polymers containing cyclotriphosphazenes as pendant
groups are particularly attractive from the point of view of
preparing polymeric ligands. Accordingly, we have prepared
a cyclotriphosphazene monomer N3P3(3,5-Me2Pz)5(OsC6H4-
p-C6H4-p-CHdCH2) (2) that contains five pyrazolyl ligands.9
This compound is a potential multisite coordinating ligand
that possesses pyrazolyl pyridinic nitrogens and cyclotriphos-
phazene ring nitrogen atoms for coordination to metal ions.
This has been used for the preparation of new coordinating
cross-linked polymer CPPL and heterogeneous Cu(II)-
containing catalyst CPPL-Cu. To test the potential of CPPL-
Cu as a heterogeneous catalyst, we have chosen to use it in
the hydrolysis of phosphate esters. Phosphate ester hydrolysis
plays a very important role in energy metabolism and in
various cellular signal transduction pathways in biological
systems.10 A number of phosphoesterases require two or more
metal ions for their catalytic activity in these reactions. In
recent years, there has been considerable interest in the design
of synthetic models that can function as catalysts for this
biologically important reaction.11 Many of these catalysts are
homogeneous in nature, and the use of heterogeneous
catalysts has been less frequent.12 In view of this, we have
evaluated the utility of CPPL-Cu as a catalyst in the
hydrolysis reaction of three substrates: a phosphate mono-
ester (p-nitrophenyl phosphate, pNPP), a phosphodiester [bis-
(p-nitrophenyl) phosphate, bNPP], and an RNA model
phosphodiester compound [2-(hydroxypropyl)-p-nitrophenyl
phosphate, hNPP]. To the best of our knowledge, this is the
first time that such studies are reported with this family of
polymers. These results are presented in this paper. The X-ray
crystal structures of N3P3Cl5(OsC6H4-p-C6H4-p-CHdCH2)
(1), N3P3(3,5-Me2Pz)5(OsC6H4-p-C6H4-p-CHdCH2) (2), and
a copper(II) complex of a model compound N3P3(3,5-Me2-
Pz)5(O-C6H4-p-CHO)‚CuCl2 (6) have also been determined
and are discussed in this paper.
Experimental Section
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Inorganic Chemistry, Vol. 41, No. 20, 2002 5163

Chandrasekhar et al.
reported procedures. p-Nitrophenyl phosphate (disodium hexahy-
drate salt), 4-hydroxy biphenyl (SD Fine Chemicals, India), and
anhydrous cupric chloride (Fluka, Switzerland) were used as
received. Bis(p-nitrophenyl) phosphate (Fluka) was converted into
its sodium salt by neutralization with 1 M sodium hydroxide
followed by lyophilization. Hexachlorocyclotriphosphazene (Nippon
Soda, Japan) and p-hydroxy benzaldehyde (SD Fine Chemicals,
India) were recrystallized from n-hexane before use. Divinylbenzene
(Fluka, Switzerland) was treated with 20% sodium hydroxide to
remove tert-butyl catechol, washed with water, and dried over
sodium sulfate prior to use. N-Ethyl morpholine and ethylene glycol
(Spectrochem, India) were distilled before use.
Instrumentation. 1H and 31P NMR spectra were recorded on a
JEOL-JNM LAMBDA 400 model spectrometer operating at 400.0
and 161.7 MHz, respectively. The chemical shifts are reported with
respect to internal tetramethylsilane (1H) and external 85% H3PO4
(31P). Mass spectra were recorded on a JEOL D-300 (EI/CI)
spectrometer in the EI mode or in the FAB mass mode on a JEOL
SX 102/DA 6000 mass spectrometer using xenon (6 kV, 10 mA)
as the FAB gas. Elemental analyses were carried out on a Carlo-
Erba CHNSO 1108 elemental analyzer. The amount of copper
present in the polymeric catalyst was determined by atomic
absorption spectrometry (AAS) on an Integra XL AAS spectrom-
eter. EPR spectra were recorded on a Varian 109E Line Century
Series, X-band spectrometer, at liquid nitrogen temperature. IR
spectra were recorded as KBr pellets on a Bruker Vector 22 FTIR
spectrophotometer operating from 400 to 4000 cm-1. Optical spectra
were recorded on a Shimadzu UV-160 spectrophotometer. TGA
was recorded on a Perkin-Elmer, Pyris 6 thermogravimetric
analyzer.
in vacuo to afford a white solid, which was identified as title
compound 2. This was recrystallized by dissolving in hot n-hexane
and allowing cooling at 5 °C. Yield: 4.2 g, 88%. Mp: 179-181
°C. IR (KBr) cm-1: 3431(s), 3059(s), 3018(s), 2990(s), 2966(s),
2927(s), 1632(w), 1603(w), 1571(s), 1523(w), 1494(s), 1463(m),
1434(m), 1409(s), 1371(m), 1300(s), 1250(vs), 1220(vs), 1169(vs),
1046(s), 1089(s), 1022(m), 964(vs), 913(m), 891(m), 843(m),
825(m), 794(w), 765(w), 752(m), 735(w), 669(w), 638(w), 601(s),
534(s), 519(s), 486(m). 1H NMR (CDCl3): δ 1.97, 2.05, 2.06, 2.07,
2.11 and 2.19 (s, 30 H, CH3), 5.19 (d, 1H, J ) 10.7 Hz, see structure
A for assignment), 5.70 (d, 1H, J ) 17.5 Hz, see structure B for
assignment), 5.71 and 5.75 (s, 5H, pyrazolyl CH), 6.67 (dd, 1H, J
) 17.6 and 10.9 Hz, see structure C for assignment), 7.17-7.38
(m, 8H, aromatic). 31P{1H} NMR (CDCl3): δ 3.0 (t, P(OR)-
(3,5-Me2Pz)), 0.2 (d, P(3,5-Me2Pz)2), 2J(P-N-P) ) 65.5 Hz.
MS(FAB): 806 (M+). Anal. Calcd for C39H46N13OP3 (805.79): C,
58.13; H, 5.75; N, 22.60. Found: C, 57.81; H, 5.63; N, 22.32.
Preparation of N3P3(3,5-Me2Pz)5(OsC6H4-p-C6H4-p-CHd
CH2)‚CuCl2 (3). To a solution of 2 (1.0 g, 10.0 mmol) in
dichloromethane (80 mL) was added anhydrous cupric chloride (0.2
g, 10.0 mmol) at once, and the mixture was stirred for 24 h at
room temperature. The solution was filtered, and the solvent was
removed from the filtrate in vacuo. A green solid identified as 3
was obtained, and it was purified by reprecipitation by dissolving
it in dichloromethane (2 mL) and adding n-hexane (20 mL).
Yield: 0.80 g, 66.4%. Mp: 186 °C. IR (KBr) cm-1: 3098(w),
2963(w), 2925(m), 1492(s), 1464(s), 1440(m), 1411(s), 1376(w),
1297(s), 1243(vs), 1191(vs), 1085(m), 1049(m), 1022(m), 966(s),
902(m), 830(m), 779(w), 755(w), 735(w), 678(w), 630(m), 594(vs),
540(s), 458(m). EPR (CH2Cl2/toluene 1:1 77 K): g| ) 2.24;
g ) 2.06; A| ) 122.0 × 10-4 cm-1. UV-vis {CHCl3, [λmax/nm
(ꢀmax/M-1 cm-1)]}: 897 (269), 350 (2690), 283 (4470), 232 (4470).
MS(FAB): 904 (M+ - Cl, 25%), 869 (base peak, M+ - 2Cl),
807 (M+ - CuCl2). Anal. Calcd for C39H46N13Cl2CuOP3 (940.24):
C, 49.82; H, 4.93; N, 19.37. Found: C, 49.18; H, 4.63; N, 19.15.
Preparation of N3P3Cl5(O-C6H4-p-CHO) (4). N3P3Cl6 (3.48
g, 10.0 mmol) was dissolved in THF (30 mL), and to it was added
a mixture of p-hydroxy benzaldehyde (0.9 g, 10.0 mmol) and
triethylamine (1.0 g, 10.0 mmol) taken together in THF (30 mL)
dropwise over a period of 30 min. The reaction mixture was stirred
for 12 h at room temperature. Triethylamine hydrochloride was
filtered off, and the solvent was removed from the filtrate in vacuo
to afford viscous oil. This was subjected to column chromatography
over a silica gel column. Title compound 4 was obtained in a pure
form by elution with a mixture of ethyl acetate and n-hexane (5:
Preparation of N3P3Cl5(OsC6H4-p-C6H4-p-CHdCH2) (1). The
preparation of 1 was carried out by a slightly modified procedure
compared to the one reported in the literature.8l To a solution of
N3P3Cl6 (7.0 g, 20.0 mmol) in benzene (60 mL) was added a
mixture of 4-hydroxy-4′-vinyl biphenyl (3.9 g, 20.0 mmol) and
triethylamine (2.0 g, 20.0 mmol) in benzene (60 mL) dropwise for
30 min. The reaction mixture was stirred for 6 h at room
temperature, and the resultant triethylamine hydrochloride was
filtered off. Removal of benzene from the filtrate in vacuo afforded
an oily liquid. This was chromatographed over a silica gel column
using n-hexane as the eluent. The first fraction contained unreacted
N3P3Cl6. The second fraction contained title compound 1. Yield:
5.8 g, 56% (lit. yield: 40.0%). Mp: 112 °C. 1H NMR (CDCl3): δ
5.3 (d, 1H, J ) 10.8 Hz, see structure A for assignment), 5.7 (d,
1H, J ) 17.8 Hz, see structure B for assignment), 6.8 (dd, 1H, J )
17.5 and 10.9 Hz, see structure C for assignment), 7.4 (m, 8H,
biphenyl). 31P{1H} NMR (CDCl3): δ 22.5 (d, PCl2), 12.3 (t, P(OR)-
1
95). Yield: 2.5 g, 57%. Mp: 52 °C. H NMR (CDCl3): 7.86 (d,
2H, J ) 8.3 Hz), 7.50 (d, 2H, J ) 9.5 Hz, aromatic), 10.02 (s, 1H,
2
Cl), J(P-N-P) ) 58.2 Hz. MS(EI): 507 (M+).
CHO). 31P{1H} NMR (CDCl3): δ 22.6 (d, PCl2), 11.8 (t, P(OR)-
2
Cl), J(P-N-P) ) 61.5 Hz. MS(EI): 433 (M+). Anal. Calcd for
C7H5N3Cl5O2P3 (433.32): C, 19.40; H, 1.16; N, 9.70. Found: C,
19.16; H, 1.40; N, 9.40.
Preparation of N3P3(3,5-Me2Pz)5(O-C6H4-p-CHO) (5). To a
mixture of 3,5-dimethyl pyrazole (4.8 g, 50.0 mmol) and triethyl-
amine (5.0 g, 50.0 mmol) in THF (50 mL) was added a solution of
3 (4.3 g, 10.0 mmol) in THF (50 mL) dropwise for 15 min at room
temperature. The reaction mixture was stirred at room temperature
for 1 h and was subsequently heated under reflux for 8 h. The
reaction mixture was allowed to come to room temperature and
filtered and the filtrate was stripped off the solvent in vacuo to
afford a white solid identified as 5. This was recrystallized from a
mixture of dichloromethane and n-hexane (1:2) at 0 °C. Yield: 3.7
g, 50%. Mp: 147-148 °C. IR (KBr) cm-1: 3096(m), 2968(m),
2927(s), 1574(vs), 1503(w), 1463(s), 1431(s), 1411(s), 1371(s),
Preparation of N3P3(3,5-Me2Pz)5(OsC6H4-p-C6H4-p-CHd
CH2) (2). To a mixture of 3,5-dimethylpyrazole (2.9 g, 30.0 mmol)
and triethylamine (3.0 g, 30.0 mmol) in THF (40 mL) was added
a solution of 1 (3.0 g, 6.0 mmol) in THF (30 mL) at room
temperature over a period of 45 min. This was further stirred at
the same temperature for 1 h and was subsequently heated under
reflux for 6 h. The reaction mixture was allowed to come to room
temperature and filtered and the solvent removed from the filtrate
(14) Tanigaki, T.; Shirai, M.; Inoue, K. Polym. J. 1987, 19, 881.
(15) Brown, D. M.; Usher, D. A. J. Chem. Soc. 1965, 6558.
5164 Inorganic Chemistry, Vol. 41, No. 20, 2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
Table 1. Crystallographic Data for Compounds 1, 2, and 6
1319(s), 1295(vs), 1217(vs), 1184(vs), 1164(vs), 1144(s), 1085(vs),
1022(m), 967(s), 901(m), 828(s), 801(s), 756(s), 659(w), 635(s),
609(vs), 577(m), 512(s), 475(vs). 1H NMR (CDCl3): δ 1.93, 2.04,
2.05, 2.06, 2.13, 2.16 (s, 30H, CH3), 5.72 and 5.77 (s, 5H, pyrazolyl
CH), 7.19-7.79 (m, 4H, aromatic), 9.61 (s, 1H, CHO). 31P{1H}
NMR (CDCl3): δ 2.3 (t, P(OR)(3,5-Me2Pz)), 0.1 (d, P(3,5-Me2-
Pz)2), 2J(P-N-P) ) 65.5 Hz. MS (FAB): 732 (M+). Anal. Calcd
for C32H40N13O2P3 (731.67): C, 52.53; H, 5.51; N, 24.89. Found:
C, 52.31; H, 5.63; N, 24.72.
1
2
6a
C14H11Cl5N3OP3 C39H46N13OP3 C34H44N13O2P3CuCl6
empirical
formula
fw
space group
cryst color
cryst syst
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
507.42
P21/c
805.80
P21/c
1035.97
P1h
colorless
monoclinic
23.442(4)
7.7938(17)
11.619(3)
90
99.264(15)
90
2095.2 (8)
1.609
colorless
monoclinic
18.882(5)
11.813(5)
19.491(5)
90
99.352(5)
90
4290(2)
1.248
green
triclinic
12.387(3)
13.374(3)
14.833(4)
99.707(4)
101.508(4)
98.858(4)
2328.3(10)
1.478
Preparation of N3P3(3,5-Me2Pz)5(O-C6H4-p-CHO).CuCl2 (6).
To a solution of 5 (1.0 g, 0.1 mmol) in dichloromethane (50 mL)
was added anhydrous cupric chloride (0.2 g, 0.1 mmol), and the
mixture was stirred for 24 h at room temperature. This was filtered,
and the solvent was removed from the filtrate in vacuo. Copper
complex 6 was obtained as a green solid, and it was recrystallized
by layering n-hexane to a solution of 6 in dichloromethane and
keeping the mixture at 0 °C. Yield: 1.0 g, 83%. Mp: 161 °C (d).
IR (KBr) cm-1: 3095(vs), 2973(vs), 2926(s), 1702(s), 1597(m),
1574(vs), 1503(w), 1463(m), 1440(w), 1410(m), 1377(m), 1298(vs),
1249(vs), 1189(vs), 1154(vs), 1087(w), 1049(w), 965(vs), 887(w),
589(vs), 563(w), 544(s), 463(w). EPR (CH2Cl2/toluene, 1:1, 77
K): g| ) 2.28; g ) 2.05; A| ) 130.5 × 10-4 cm-1. UV-vis
{CHCl3, [λmax/nm (ꢀmax/M-1 cm-1)]}: 905(198), 372(889), 351(488),
262(2398). Anal. Calcd for C32H40O2N13P3CuCl2 (866.12): C,
44.38; H, 4.65; N, 21.02. Found: C, 43.78; H, 4.81; N, 21.23.
Preparation of CPPL. To a solution of 2 (1.0 g, 1.2 mmol) in
1,2-dichloroethane (15 mL) was added 1,4-divinylbenzene (80%,
mixture of isomers, 0.4 g, 2.5 mmol) and AIBN (30 mg, 0.2 mmol).
The solution was purged with argon for 45 min and was
subsequently heated at 80 °C for 36 h. The cross-linked polymer
obtained was filtered, washed with toluene (3 × 20 mL), dichloro-
methane (3 × 20 mL), methanol (3 × 20 mL), and acetone (3 ×
20 mL), and dried thoroughly under vacuum at 40 °C. Yield: 1.04
g. IR (KBr) cm-1: 3397(s), 2923(s), 1604(s), 1573(s), 1493(s),
1443(s), 1412(s), 1372(m), 1301(s), 1218(vs), 1089(m), 1023(m),
967(m), 902(m), 824(m), 796(m), 711(w), 597(m), 525(m). Anal.
Found: C, 64.98; H, 6.2; N, 9.32. From the nitrogen analysis, it is
concluded that every gram of the polymer contains 0.54 g of the
cyclophosphazene monomer corresponding to 6.7 × 10-4 mol.
Preparation of CPPL-Cu. To a solution of anhydrous cupric
chloride (0.09 g, 0.7 mmol) in ethanol was added CPPL (1.0 g,
0.67 × 10-3 mol), and the mixture was stirred for 24 h. The
resulting metalated polymer was washed thoroughly with methanol
(5 × 20 mL) to remove any unreacted cupric chloride and dried
under vacuum at 40 °C. Yield: 1.2 g. IR (KBr) cm-1: 3420(m),
3020(m), 2923(s), 2853(m), 1649(w), 1603(w), 1570(w), 1492(m),
1454(m), 1418(m), 1374(w), 1240(vs), 1187(vs), 1047(m), 964(w),
903(m), 824(m), 795(m), 709(w), 588(w), 540(w). Anal. Found:
C, 55.40; H, 5.21; N, 6.39. From the nitrogen analysis, it is
concluded that every gram of the polymer contains 0.33 g of the
cyclophosphazene monomer corresponding to 4.09 × 10-4 mol.
From the AAS analysis, the amount of copper present per gram of
CPPL-Cu was found to be 21.7 mg. EPR (solid, 77 K): g| ) 2.27;
g ) 2.10; A| ) 130.5 × 10-4cm-1. Diffuse reflectance UV-vis
[λmax/nm (absorbance)]: λmax ) 845 (0.355), 351 (0.294), 318
(0.891), 304 (0.878), 251 (0.920).
F
(calcd) (Mg m-3
Z
)
4
4
2
µ (mm-1
λ (Å)
)
0.931
0.186
0.963
0.71073
293(2)
1.024
0.71069
293(2)
0.813
0.71073
150(2)
1.025
temp (K)
GOF
total reflns
indep reflns
Rint
3898
3690
0.0272
9585
4806
0.1509
0.0784
0.1546
11765
8059
0.0183
0.0503
0.1374
R1b [I > 2σ(I)] 0.0490
wR2b [I > 2σ(I)] 0.1078
a Crystallizes with two molecules of dichloromethane. b R ) ∑||Fo| -
2
2
2
|Fc||/∑|Fo|, Rw ) {[∑w(|Fo| - |Fc| )2]/[∑w(|Fo| )2]}1/2
.
collected on Siemens P4S and SMART APEX CCD (Bruker-AXS)
diffractometers, respectively. The structures were solved and refined
using the SHELXTL program.16 Disordered carbon atoms of phenyl
and vinyl groups in 1 were found with occupancies of 50% each
for C2, C3, C5, C6, C8, C9, C11, C12, C14 and C2a, C3a, C5a,
C6a, C8a, C9a, C11a, C12a, C14a. X-ray diffraction data for 2
were collected on an Enraf Nonius FR590 CAD-4 diffractometer.
The structure of 2 was solved by using WINGX version 1.63, a
crystallographic collective package (L. J. Farrugia, WINGX ver
1.63, An integrated Systems of Windows Programs for the solution,
refinement and analysis of single-crystal X-ray diffraction data,
Department of Chemistry, University of Glasgow). The structure
was solved initially with SIR97 and refined with the SHELX-9717
package incorporated in WINGX. The structure was refined against
F2 with a full-matrix least-squares algorithm. The X-ray data
pertaining to data collection, crystal systems, and structure solution
for compounds 1, 2, and 6 are summarized in Table 1.
Kinetic Studies. General. Hydrolytic reactions were performed
in duplicate in centrifuge tubes containing 3 mL of solution of the
substrate prepared in 0.01 M N-ethylmorpholine (pH 8.0) in 50%
aqueous methanol. Methanol was used in the reaction solvent to
ensure optimum wetting of the hydrophobic catalyst. The amount
of polymer was 1 mg/mL of the buffer, corresponding to 0.34 mM
of copper, if the polymer were to be completely soluble in the
buffer. The concentration calculated in this manner was used to
estimate kcat. This is quite reasonable as only coordinated copper
ions promote phosphate ester hydrolysis and the rest of the polymer
matrix merely acts as a scaffold for coordination. In short, this
derivation results in apparent kinetic parameters. The rate of
hydrolysis was followed by monitoring the appearance of p-
nitrophenolate anion (ꢀ400 ) 1.65 × 104 M-1 cm-1). To correct for
autohydrolysis of the substrates, the rate of each reaction was
measured against a reference cell, which was identical in composi-
X-ray Crystallography. Colorless crystals of 1 and 2 were
grown by dissolving these compounds in hot n-hexane and allowing
the solutions to cool. Crystals of 6 were grown from a mixture of
dichloromethane and n-hexane (1:1) at room temperature. All
hydrogen atoms were included in idealized positions, and a riding
model was used. Non-hydrogen atoms were refined with anisotropic
displacement parameters. X-ray data for crystals 1 and 6 were
(16) Sheldrick, G. M. SHELXTL, An Integrated System for SolVing, Refining
and Displaying Crystal Structures from Diffraction Data; University
of Gottingen: Gottingen, Germany, 1991.
(17) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Refine-
ment; University of Gottingen: Gottingen, Germany, 1997.
Inorganic Chemistry, Vol. 41, No. 20, 2002 5165

Chandrasekhar et al.
Scheme 1
Table 2. Concentrations Used for the Calculation of Michaelis
Constants and kobs Values in the Hydrolysis of pNPP, bNPP, and hNPP
by CPPL-Cu
Results and Discussion
Synthetic and Spectroscopic Aspects of Cyclophos-
phazene Derivatives 1-6. Hybrid inorganic-organic poly-
mers containing an organic backbone and a cyclophos-
phazene moiety as the pendant group can be assembled from
an appropriate cyclophosphazene monomer, which contains
a polymerizable vinyl functional group. Allen and others have
shown that the most suitable monomers are those where a
spacer separates the vinyl group from the cyclophosphazene.8d
The spacer minimizes the σ-electron withdrawing effect of
the cyclophosphazene ring on the vinyl group as well as
enables a reduction of the steric encumbrance about it.
Keeping this in view, we have prepared N3P3Cl5(OsC6H4-
p-C6H4-p-CHdCH2) (1) which has previously been shown
by Inoue8j-l and us8f to be an excellent monomer in terms
of its ease of preparation, hydrolytic stability, and ready
polymerizability. An additional advantage of this monomer
is the presence of five reactive PsCl bonds, which can be
readily replaced by a variety of nucleophiles. Using this
feature, we prepared pyrazolyl derivative N3P3(3,5-Me2Pz)5-
(OsC6H4-p-C6H4-p-CHdCH2) (2) in excellent yield by the
reaction of 1 with 3,5-dimethylpyrazole in the presence of
triethylamine as the hydrogen chloride scavenger (Scheme
1). Previously, we and others have shown that pyrazolyl
cyclophosphazenes such as N3P3(3,5-Me2Pz)6 or gem-N3P3-
Ph2(3,5-Me2Pz)4 are extremely versatile multisite coordina-
tion ligands toward several types of transition metal ions as
well as metal carbonyls such as Mo(CO)6.18 In view of this
proven coordination behavior of these ligands, we have
concentration range
of the substratea
concentration
substrate
of the substrateb
pNPP
bNPP
hNPP
0.10-0.60
0.50-2.00
0.10-0.80
2.00
2.00
5.00
a Used for the calculation of Vmax, Km, and kcat, [S] mM. b Used for the
calculation of kobs, [S] mM.
tion except lacking the catalyst. The hydrolyses of pNPP and bNPP
were performed at 30 °C while that of hNPP was performed at 35
°C. The concentrations used for the hydrolysis of the phosphate
esters are summarized in Table 2. The initial velocities (Vi’s) were
determined from concentration versus time plots, and pseudo-first-
order rate constants were determined from ln A∞/A∞ - At versus
time plots. Michaelis-Menten kinetic parameters such as Km, kcat,
and Vmax have also been determined from the corresponding
Lineweaver-Burk plot (1/Vi vs 1/[S]). Reactions were monitored
to less than 10% conversion of substrate to product. In addition to
these general kinetic investigations, the hydrolysis of bNPP was
carried out under various other conditions. The assay used in these
studies is the same as that described vide supra except that the
concentration of bNPP was 2.0 mM.
pH and Solvent Dependence on Hydrolysis. Solutions with
varying pHs ranging from 7.2 to 8.9 were prepared by using
N-ethylmorpholine. Pseudo-first-order constants for the hydrolysis
of bNPP at these pHs were determined, and a pH versus kobs plot
was generated.
Hydrolysis of bNPP by CPPL-Cu was carried out in water,
ethylene glycol, and 50% aqueous ethylene glycol at pH 8.0. Initial
rates were calculated for each solvent system.
Monitoring of Hydrolysis Products of hNPP by 31P NMR.
The hydrolysis of 3.0 mM solutions of hNPP in N-ethylmorpholine
buffer at pH 8.0 with 0.34 mM of the catalyst was investigated by
31P{1H} NMR at 30 °C. The reaction was followed by monitoring
the appearance of the signal at 18.5 ppm corresponding to the cyclic
phosphate formed after the release of p-nitrophenolate anion (this
chemical shift value is with respect to 85% H3PO4 in 50% aqueous
methanol buffer as the external standard).
(18) (a) Chandrasekhar, V.; Nagendran, S. Chem. Soc. ReV. 2001, 30, 193.
(b) Thomas, K. R. J.; Chandrasekhar, V.; Vivekanandan, K.; Senthil
Andavan, G. T.; Nagendran, S.; Kingsley, S.; Teikink, E. R. T. Inorg.
Chim. Acta. 1999, 286, 127. (c) Koo, B. H.; Byun, Y.; Hong, E.; Kim,
Y.; Do, Y. Chem. Commun. 1998, 1227. (d) Thomas, K. R. J.;
Tharmaraj, P.; Chandrasekhar, V.; Bryan, C. D.; Cordes, A. W. Inorg.
Chem. 1994, 33, 5332. (e) Thomas, K. R. J.; Chandrasekhar, V.; Pal,
P.; Scott, S. R.; Hallford, R.; Cordes, A. W. Inorg. Chem. 1993, 32,
606.
5166 Inorganic Chemistry, Vol. 41, No. 20, 2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
Scheme 2
Copper complexes 3 and 6 are nonionic in solution, and
magnetic moment data are unexceptional. The UV-vis
spectra of 3 and 6 show intense high-energy absorptions and
weak low-energy absorptions. The latter, which occur at 897
nm for 3 and 905 nm for 6, are attributed to the d-d
transition. These values are comparable to that found for
N3P3(3,5-Me2Pz)6‚CuCl2.18e The high-energy absorptions are
due to intra-pyrazole π-π* as well as π1(pyrazole) f Cu(II)
and π2(pyrazole) f Cu(II) charge-transfer bands. In these
compounds, the Cl f Cu(II) charge-transfer band is buried
in the absorption at 262 and 283 nm for 3 and 6, respectively.
These assignments are based on the pioneering work of
Schugar and co-workers and our results on several pyrazolyl
cyclophosphazene-metal complexes.18,19 The infrared spectra
of ligands 2 and 5 show strong PdN stretching frequencies
at 1220 and 1218 cm-1, respectively. In copper complexes
3 and 6, the parent PdN stretching frequency is split, and
two sets of peaks are seen for each compound: 1243 and
1191 cm-1 for 3, 1249 and 1189 cm-1 for 6, respectively.
Metalation or protonation of the ring nitrogen atom of the
cyclophosphazene rings is known to cause such a splitting
of the PdN stretching frequency.20
The EPR spectra of copper complexes 3 and 6 have been
recorded in solution at room and liquid nitrogen temperatures.
The spectra at room temperature are not informative.
However, at 77 K, spectra of axial symmetry are obtained
with g| > g . Thus, the g| values for 3 and 6 are 2.28 and
2.24 while the corresponding g values are 2.05 and 2.06,
respectively. The A| values for these compounds are 130.5
× 10-4 cm-1 and 122.0 × 10-4 cm-1, respectively. These
EPR parameters are very similar to that observed for
N3P3(3,5-Me2Pz)6‚CuCl2.18e This, along with the similarity
of optical spectral features, suggests that in all of these
compounds the coordination environment and geometry
around Cu(II) are very similar. This estimate has been
confirmed by the X-ray structure of 6.
X-ray Structures of 1 and 2. The X-ray structural
determination of compounds 1 and 2 confirms the assignment
of their structures from spectroscopic data. The ORTEP
diagrams of these compounds along with the important metric
parameters are given in Figures 1 and 2. The cyclophos-
phazene ring in compounds 1 and 2 is nearly planar. The
P-N bond lengths in 1 are similar and range from 1.565 to
1.579 Å. The P-N-P angles are about 121° while the angles
at phosphorus are slightly smaller. These metric parameters
are comparable to those observed in other related monosub-
stituted cyclophosphazenes such as N3P3Cl5(OC6H3Cl2-2,6).21
The endocyclic P-N bond lengths in the pyrazolyl derivative
2 are slightly longer than those found for 1 and range from
1.556 to 1.583 Å. The shorter bond length is associated with
P-N bonds adjacent to P(3,5-Me2Pz)(OR) while the longer
chosen to incorporate the pyrazolyl moiety in monomer 2.
Ligand 2 readily reacts with copper(II) chloride to afford
the 1:1 complex N3P3(3,5-Me2Pz)5(OsC6H4-p-C6H4-p-CHd
CH2)‚CuCl2 (3) (Scheme 2). However, we were unable to
obtain suitable crystals of this compound. To prove the
structure of 3 beyond any reasonable doubt, we prepared
structurally analogous compound N3P3(3,5-Me2Pz)5(Os
C6H4-p-CHO)‚CuCl2 (6) in a three-step synthesis starting
from N3P3Cl6 (Schemes 1 and 2) and determined its X-ray
structure (vide infra). Compounds 1, 2, 4, and 5 show
prominent parent ion peaks in their FAB or EI mass spectra.
However, compound 3 shows the M+ - Cl peak as the
highest mass peak.
The 31P{1H} NMR spectra of N3P3Cl5(OsC6H4-p-C6H4-
p-CHdCH2) (1), N3P3(3,5-Me2Pz)5(OsC6H4-p-C6H4-p-CHd
CH2) (2), N3P3Cl5(OsC6H4-p-CHO) (4), and N3P3(3,5-
Me2Pz)5(OsC6H4-p-CHO) (5) are of the AX2 type consistent
with their structures. The structures of 1 and 2 have been
further confirmed by X-ray crystallography (vide infra). An
interesting feature of the 31P{1H} NMR spectra is that both
the A and the X region are considerably upfield shifted in
the pyrazolyl derivatives in comparison to the chloro
precursors. Thus, in compounds 1 and 4, a doublet is seen
corresponding to δ PCl2 at 22.5 and 22.6 ppm, respectively,
while a triplet corresponding to δ PCl(OR) is seen at 12.3
and 11.8 ppm. These chemical shifts move upfield in
compounds 2 and 5: 0.2 and 0.1 ppm (doublet), 3.0 and 2.3
ppm (triplet). Such chemical shift changes have been
previously noted in the conversion of N3P3Cl6 (singlet, 19.3
ppm) to N3P3(3,5-Me2Pz)6 (singlet, -3.4 ppm).18e
(19) Bernarducci, E.; Schwindinger, W. F.; Hughey, J. L., IV; Krogh-
Jespersen, J.; Schugar, H. J. J. Am. Chem. Soc. 1981, 103, 1686.
(20) (a) Krishnamurthy, S. S.; Sau, A. C.; Woods, M. AdV. Inorg. Chem.
Radiochem. 1978, 21, 41. (b) Chandrasekhar, V.; Thomas, K. R. J.
Struct. Bonding (Berlin) 1993, 81, 41. (c) Chandrasekhar, V.; Krishnan,
V. AdV. Inorg. Chem., in press.
(21) Allcock, H. R.; Ngo, D. C.; Parvez, M.; Visscher, K. J. Chem. Soc.,
Dalton Trans. 1992, 1687.
Inorganic Chemistry, Vol. 41, No. 20, 2002 5167

Chandrasekhar et al.
Figure 1. ORTEP diagram of compound 1. Hydrogen atoms have been
omitted for clarity. The important distances (Å) and bond angles (deg) are
the following: P(1)-N(1) 1.575(4), P(1)-N(3) 1.574(4), P(2)-N(1)
1.577(4), P(2)-N(2) 1.579(4), P(3)-N(2) 1.565(4), P(3)-N(3) 1.577(4),
P(1)-Cl(1) 1.997(2), P(2)-Cl(2) 1.9825(9), P(1)-O(1) 1.571(3), N(1)-
P(1)-N(3) 118.4(2), N(1)-P(2)-N(2) 118.6(2), N(2)-P(3)-N(3) 118.6(2),
P(1)-N(1)-P(2) 121.4(2), P(2)-N(2)-P(3) 121.1(3), P(1)-N(3)-P(3)
121.0(3), O(1)-P(1)-Cl(1) 103.09(15), Cl(2)-P(2)-Cl(3) 101.43 (8).
Figure 3. ORTEP diagram of compound 6. All hydrogen atoms and the
3,5-dimethyl pyrazolyl groups, which are not involved in the coordination,
are removed for clarity. The important bond lengths (Å) and bond angles
(deg) are the following: Cu(1)-N(9) 1.993(3), Cu(1)-N(5) 2.010(3),
Cu(1)-N(1) 2.295(3), Cu(1)-Cl(1) 2.2501(11), Cu(1)-Cl(2) 2.2764 (10),
P(1)-N(1) 1.594(3), P(1)-N(2) 1.572(3), P(2)-N(2) 1.580(3), P(2)-N(3)
1.590(3), P(3)-N(1) 1.598(3), P(3)-N(3) 1.569(3), P(2)-O(1) 1.586(2),
P(1)-N(4) 1.693(3), P(3)-N(8) 1.685(3), N(9)-Cu(1)-N(5) 161.08(12),
N(9)-Cu(1)-Cl(1) 97.40(9), N(5)-Cu(1)-Cl(1) 93.13(8), N(9)-Cu(1)-
Cl(2) 91.53(8), N(5)-Cu(1)-Cl(2) 88.64(8), Cl(1)-Cu(1)-Cl(2) 145.34(5),
N(9)-Cu(1)-N(1) 81.03(11), N(5)-Cu(1)-N(1) 80.93(11), Cl(1)-Cu(1)-
N(1) 106.49(7), Cl(2)-Cu(1)-N(1) 107.96(8), P(1)-N(1)-Cu(1) 110.60(14),
P(3)-N(1)-Cu(1) 110.93(14), N(2)-P(1)-N(1) 118.59(14), N(2)-P(2)-
N(3) 117.77(15), N(3)-P(3)-N(1) 118.15(15), P(1)-N(1)-P(3) 117.80(17),
P(1)-N(2)-P(2) 120.72(15), P(3)-N(3)-P(2) 122.04(18), O(1)-P(2)-
N(3) 104.59(14), N(3)-P(2)-N(12) 111.62(15).
2.010(3) and 1.993(3) Å, respectively. The cyclophosphazene
ring in 6 is nonplanar with the atoms N(1) and P(1) being
displaced from the mean plane of the cyclophosphazene ring
by +0.15 and -0.13 Å, respectively. The P-N bond lengths
within the cyclophosphazene ring are not equal: thus, the
distances flanking the site of coordination P(1)-N(1) and
P(3)-N(1) are 1.594(3) and 1.598(3) Å while P(1)-N(2)
and P(3)-N(3) are 1.572(3) and 1.569(3) Å. The N-P-N
and P-N-P bond angles within the cyclophosphazene ring
range from 117° to 122°. The coordination behavior observed
in 6 closely resembles that found in other Cu(II) complexes
of pyrazolyl cyclophosphazenes including N3P3(3,5-Me2Pz)6‚
CuCl2.18a,e
A closer inspection of the X-ray structure of 6 reveals
interesting intermolecular C-H‚‚‚Cl secondary interactions
(Figure 4). The metric parameters involved in these interac-
tions are within the norms for C-H‚‚‚Cl hydrogen bonds.
An analysis of such interactions has been performed recently,
and it has been concluded that C-H‚‚‚Cl-M interactions
behave as hydrogen bonds.22 Thus, the chlorine atom [Cl(1)]
is involved in hydrogen bonding to two different hydrogen
atoms of two neighboring molecules, viz., to H(27) of the
aromatic substituent of a neighboring molecule and H(12)
of the pyrazolyl substituent of another neighboring molecule.
These interactions lead to the formation of a sheet type
structural arrangement in the solid-state. This array comprises
two alternating macrocylic rings, a 24-membered ring A and
a 12-membered ring B.
Figure 2. ORTEP diagram of compound 2. Hydrogen atoms are omitted
for clarity. The relevant bond distances (Å) and bond angles (deg) are the
following: P(1)-N(1) 1.583(10), P(1)-N(3) 1.588(4), P(2)-N(1) 1.562(4),
P(2)-N(2) 1.558(4), P(3)-N(2) 1.582(4), P(3)-N(3) 1.556(11), P(1)-O(1)
1.577(10), P(1)-N(4) 1.667(4), P(2)-N(8) 1.680(12), N(1)-P(1)-N(3)
116.8(6), N(1)-P(2)-N(2) 117.8(6), N(3)-P(3)-N(2) 117.2(7), P(1)-
N(1)-P(2) 122.2(2), P(2)-N(2)-P(3) 122.7(8), P(1)-N(3)-P(3) 122.7(8),
N(6)-P(2)-N(8) 100.7(6), P(1)-N(4)-N(5) 113.7(11), O(1)-P(1)-N(4)
103.4(6).
bond lengths are associated with P-N bonds adjacent to
P(3,5-Me2Pz)2. The endocyclic bond angles at nitrogen and
phosphorus in 2 are unexceptional.
X-ray Structure of 6. The ORTEP diagram of 6, along
with the important metric parameters, is given in Figure 3.
The cyclophosphazene ligand functions in the non-gem N3
mode.18d,e The coordination to Cu(II) occurs through two
pyridinic atoms N(5) and N(9) of the nongeminal 3,5-
dimethylpyrazolyl substituents on P(1) and P(3) of the
cyclophosphazene ring and the ring nitrogen atom N(1). The
chlorides take up the other two coordination sites around
the metal atom. The geometry around the copper atom is
distorted trigonal bipyramidal; the two chlorides and the
cyclophosphazene ring nitrogen atom (N(1)) occupy the
equatorial positions while the pyrazolyl pyridinic nitrogen
atoms (N(5) and N(9)) take up the axial positions. The
distortion of geometry around the copper is a result of the
steric constraints of the ligand in this coordination mode.
Thus, the axial N(9)-Cu(1)-N(5) angle is 161.08(12)° while
the equatorial angles N(1)-Cu(1)-Cl(2), N(1)-Cu(1)-
Cl(1), and Cl(1)-Cu(1)-Cl(2) are 107.96(8)°, 106.49(7)°,
and 145.34(5)°. The Cu(1)-N(1) bond distance is 2.295(3)
Å while the Cu(1)-N(5) and Cu(1)-N(9) distances are
Polymeric Ligand CPPL and Metalated Polymer CPPL-
Cu. Monomer 2 has been copolymerized in the presence of
divinylbenzene to afford cross-linked polymer CPPL (Scheme
(22) Thallapally, P. K.; Nangia, A. CrystEngComm 2001, 27.
5168 Inorganic Chemistry, Vol. 41, No. 20, 2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
Figure 4. DIAMOND view of hydrogen bonding present in compound 6. 3,5-Dimethyl pyrazolyl groups, which are not involved in the coordination, have
been omitted. Methyl substituents on the coordinating pyrazole groups have also been omitted. Also, all hydrogen atoms except those that are involved in
C-H‚‚‚Cl hydrogen bonding have been omitted for clarity. The important bond lengths (Å) and bond angles (deg) are the following: (a) for C(12)-H(12)‚
‚‚Cl(1), C-H 0.950(5), H‚‚‚Cl 2.842(7), C-Cl 3.525(9), C-H‚‚‚Cl 129.66(25), and the symmetry is 2 - x, 1 - y, 1 - z; (b) for C(27)-H(27)‚‚‚Cl(1), C-H
0.950(5), H‚‚‚Cl 2.836, C-Cl 3.638(9), C-H‚‚‚Cl 142.74(24), and the symmetry is -1 + x, y, z.
changed. Metalation of CPPL with copper(II) chloride occurs
readily to afford metalated polymer CPPL-Cu (Scheme 3).
To ensure that the material does not have any unreacted
copper(II) chloride, CPPL-Cu was washed several times with
methanol and dried until reproducible and consistent copper
analysis could be obtained from atomic absorption spectra.
The diffuse reflectance spectrum of CPPL-Cu shows peaks
at 845, 351, 318, 304, and 251 nm. Similar to the data for
compounds 3 and 6, the low-energy peak is assigned to the
d-d transition while the high-energy peaks correspond to
intra-pyrazole π-π* and other LMCT transitions. The
infrared spectrum of CPPL-Cu shows that the PdN stretching
frequency is split and two peaks at 1240 and 1186 cm-1 are
seen. This is very similar to the changes observed upon
metalation of cyclophosphazene ligands 2 and 5 (vide supra).
The EPR spectrum of CPPL-Cu at 77 K is of the axial type,
and the parameters obtained, viz., g| ) 2.27, g ) 2.10, and
A| ) 130.5 × 10-4 cm-1, are very similar to those found for
Figure 5. TGA curve for CPPL (- -) and CPPL-Cu (-).
copper(II) complexes 3 and 6. In view of the close similarity
of the IR, optical, and EPR spectra of CPPL-Cu with those
of 3 and 6, we believe that the same type of coordination
environment and geometry is present for the copper atoms
in all of these systems. The TGA curve of CPPL-Cu shows
a final char yield of 17% at 950 °C (Figure 5). The polymer
undergoes a three-step degradation. An initial weight loss
occurs from 281 to 312 °C followed by a slow and
continuous degradation from 437 to 545 °C. The final
degradation occurs from 673 to 824 °C. The nature of the
degraded products has not been investigated.
3). From the nitrogen analysis of this polymer, it is concluded
that every gram of the polymer contains 0.54 g of the
cyclophosphazene moiety. The IR spectrum of CPPL shows
the characteristic PdN stretching frequency at 1216 cm-1.
This value is relatively unchanged with respect to monomer
2 suggesting that the cyclophosphazene unit remains intact
in the cross-linked polymer. The TGA trace of this polymer
shows that it gives a char yield of nearly 31% even at 1000
°C (Figure 5). The polymer undergoes a slow degradation
from 170 °C up to 474.8 °C. At this point, the char yield is
69.3%. A relatively sharp degradation occurs from 475 °C
to 523 °C. After this, the polymer virtually remains un-
Hydrolysis Studies of pNPP, bNPP, and hNPP with
CPPL-Cu. To screen the utility of CPPL-Cu on phospho-
Inorganic Chemistry, Vol. 41, No. 20, 2002 5169

Chandrasekhar et al.
Scheme 3
Scheme 4
Table 3. Pseudo-First-Order Rate Constants and krel Values for the
Hydrolysis of pNPP, bNPP, and hNPPa
substrate
k
obs (min-1
)
krel (kobs/kuncat)
pNPP
bNPP
hNPP
4.42 × 10-4
2.19 × 10-4
5.31 × 10-3
8.98 × 102
2.81 × 105
2.68 × 103
a Hydrolytic reactions were performed in duplicate containing 3 mL of
solution of the substrate prepared in 0.01 M N-ethylmorpholine (pH 8.0)
in 50% aqueous methanol. The amount of catalyst used was 1 mg/mL of
the buffer, corresponding to 0.34 mM of copper, if the polymeric catalyst
were to be completely soluble in the buffer. The hydrolyses of pNPP and
bNPP were performed at 30 °C while that of hNPP was performed at 35
°C. The concentrations of the phosphate esters used for this study are
summarized in Table 2.
tions. The hydrolytic instability of simple cyclophosphazene
metal complexes has been documented.18e,23
Rate constants for the hydrolysis of all these substrates
by CPPL-Cu were determined under pseudo-first-order
conditions. The hydrolysis proceeds with considerable rate
enhancement relative to that of the uncatalyzed24 reaction
for all three substrates. We have carried out control experi-
ments with unmetalated cross-linked polymer CPPL under
the same conditions as those described, and no phosphate
ester hydrolysis was observed. Further, enhancement in the
hydrolytic reaction rates was not observed when the sub-
strates were incubated in the buffer alone in the absence of
a catalyst or even in the presence of simple copper(II)
chloride.
For the hydrolysis reaction of all the three substrates by
CPPL-Cu, pseudo-first-order rate constants (kobs) were
determined from ln A∞/A∞ - At versus time plots (Table 3).
The kobs values obtained are 4.42 × 10-4, 2.19 × 10-4, and
5.31 × 10-3 min-1 for pNPP, bNPP, and hNPP, respectively.
The krel values, which represent the actual enhancement of
the rate of hydrolysis relative to the uncatalyzed reactions
for these substrates, are 8.98 × 102, 2.81 × 105, and 2.68 ×
103, respectively. Thus, there is significant rate enhancement
for all three substrates with the highest rate enhancement
being seen for bNPP hydrolysis. Although for homoge-
neously catalyzed reactions higher rate enhancements have
been observed previously, the data obtained in the present
study are comparable with other heterogeneous polymeric
systems studied recently.12 Thus, Verma and co-workers12a
esterase activity, the hydrolysis reactions of three model
substrates, p-nitrophenyl phosphate (pNPP), bis(p-nitro-
phenyl)phosphate (bNPP), and 2-hydroxypropyl-p-nitro-
phenyl phosphate (hNPP), (Scheme 4) were studied. While
pNPP is a model for phosphomonoester, bNPP acts as a
phosphodiester substrate. Both of these are examples where
the hydrolytic reaction is triggered in an intermolecular
manner by an external nucleophile. hNPP is a phosphodiester
and is also a model for RNA in that the 2′-OH position of
the latter is structurally mimicked. Our results on this
substrate show an intramolecular hydrolytic reaction, through
the activation of an internal hydroxyl group (vide infra). The
hydrolyses of all the three substrates pNPP, bNPP, and hNPP
in the presence of CPPL-Cu were followed in 50% aqueous
methanol/0.01 M N-ethylmorpholine buffer at pH 8.0 by the
time-dependent release of p-nitrophenolate anion at 400 nm.
Attempts to use copper complex 3 or 6 as homogeneous
catalysts for the previous reactions did not succeed as these
complexes decomposed during the hydrolysis reaction condi-
(23) Chandrasekaran, A.; Krishnamurthy, S. S.; Nethaji, M. Inorg. Chem.
1994, 33, 3085.
(24) (a) Vance, D. H.; Czarnik, A. W. J. Am Chem. Soc. 1993, 115, 12165.
(b) Breslow, R.; Huang, D. L. Proc. Natl. Acad. Sci. U.S.A. 1991, 88,
4080.
5170 Inorganic Chemistry, Vol. 41, No. 20, 2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
Figure 6. Lineweaver-Burk plots for pNPP (9) and bNPP (b) hydrolysis
by CPPL-Cu. These experiments were carried out at pH 8.0 and at 30 °C.
The other experimental conditions relating to the data plotted in this figure
are summarized in Table 4.
Figure 7. Lineweaver-Burk plot for hNPP hydrolysis by CPPL-Cu. These
experiments were carried out at pH 8.0 and at 35 °C. The other experimental
conditions relating to the data plotted in this figure are summarized in Table
4.
have shown that an adenine-containing metalated polymeric
resin exhibits a rate enhancement (krel) of 4.8 × 105 for bNPP
hydrolysis.
Table 4. Michaelis Constants, Maximal Velocities, and kcat Values for
the Hydrolysis of pNPP, bNPP, and hNPPa
substrate
K
m (mM)
Vmax (mM min-1
)
kcat (min-1
)
To demonstrate that the catalysis is due to the copper
contained in the cross-linked polymer and not because of
leached out metal ion species, the following tests were carried
out. First, catalyst CPPL-Cu was filtered after 6 h of reaction,
and the release of p-nitrophenolate anion for the hydrolysis
of bNPP in the filtrate was monitored for a further period of
36 h. The reaction was completely arrested, and no further
formation of the hydrolytic product was observed (Supporting
Information). Second, AAS analysis of used and fresh CPPL-
Cu confirmed that the metal does not leach out during the
course of the catalytic reaction as indicated by the invariance
of the copper content in both the samples. Third, used CPPL-
Cu catalyst could be separated from the reaction mixture,
washed, dried, and recycled without affecting the rates of
hydrolysis (vide infra). Thus, cross-linked polymer CPPL
not only provides a multisite coordinating medium for the
preparation of a metal complex but also imparts hydrolytic
stability to the complex so formed. Although it is speculative,
the hydrolytic stability of CPPL-Cu probably arises because
of the cross-linked nature of the polymer which shields the
P-N (pyrazolyl) bonds effectively.
pNPP
bNPP
hNPP
0.18
0.74
3.11
3.44 × 10-5
6.05 × 10-5
2.48 × 10-3
1.02 × 10-4
1.76 × 10-4
6.80 × 10-3
a Hydrolysis was performed in duplicate containing 3 mL of solution of
the substrate prepared in 0.01 M N-ethylmorpholine (pH 8.0) in 50%
aqueous methanol. The amount of the polymeric catalyst was 1 mg/mL of
the buffer. This corresponds to 0.34 mM of copper, if the polymeric catalyst
were to be completely soluble in the buffer. The hydrolyses of pNPP and
bNPP were performed at 30 °C, while that of hNPP was performed at 35
°C. Ranges of concentrations of the substrates were used for the hydrolysis,
and these are summarized in Table 2.
Table 5. Temperature Dependence on the Hydrolysis of bNPP by
CPPL-Cua
relative increase
b
T (K)
kobs × 10-4 min-1
in kobs
303
313
323
333
2.19
7.85
12.26
18.75
3.59
1.56
1.53
a Hydrolytic reactions were performed in duplicate containing 3 mL of
solution of bNPP prepared in 0.01 M N-ethylmorpholine (pH 8.0) in 50%
aqueous methanol. The amount of the polymeric catalyst was 1 mg/mL of
the buffer, and the substrate concentration was 2.0 mM. b For a 10 °C
increase in reaction temperature.
Detailed kinetic studies were carried out for the purpose
of evaluating Michaelis-Menten kinetic parameters Km, Vmax
,
activation was found to be 13.71 kcal mol-1, while the
frequency factor A was 2.18 × 106 min-1. These parameters
are comparable to those obtained from a recently studied
heterogeneous Cu(II) catalyst in the hydrolysis of phos-
phodiester substrates.12a
pH Dependence and Solvent Effect Studies. The effect
of pH on the hydrolytic activity of CPPL-Cu was investigated
using bNPP and hNPP as the substrates in the pH range 7.0-
9.0. The pH-rate profile for bNPP is slightly complicated
and shows that there is an increase in the rate constant of
hydrolysis with an increase in pH up to pH 8.0. The rate
constant slightly drops thereafter and becomes nearly con-
stant in the pH range 8.4-9.0 (Figure 9). This kind of a pH
profile has also been observed in other instances involving
phosphate ester hydrolysis where an increase in the rate of
hydrolysis with pH was attributed to the ease of deprotona-
and kcat for all the substrates from the corresponding
Lineweaver-Burk plots, (1/Vi) versus 1/[S] (Figures 6 and
7). The Km values obtained are 0.18 (pNPP), 0.74 (bNPP),
and 3.11 mM (hNPP), respectively. The corresponding kcat
values are 1.02 × 10-4, 1.76 × 10-4, and 6.80 × 10-3 min-1,
respectively. The Vmax values found for the hydrolysis of these
substrates are 3.44 × 10-5, 6.05 × 10-5, and 2.48 × 10-3
mM min-1. All of these parameters are summarized in Table
4.
The effect of temperature on the rate of hydrolysis of bNPP
was studied. Pseudo-first-order rate constants were obtained
at four different temperatures, viz., 30, 40, 50, and 60 °C.
The steady increase in kobs even at higher temperatures is
indicative of the stability of CPPL-Cu under these conditions
(Table 5). Arrhenius parameters Ea and A were obtained from
the plot of ln kobs versus 1/T (Figure 8). The energy of
Inorganic Chemistry, Vol. 41, No. 20, 2002 5171

Chandrasekhar et al.
Table 6. Solvent Dependence on bNPP Hydrolysis by CPPL-Cua
Vi
b
solvent
(mM min-1 × 10-5
)
krel
ethylene glycol (100%)
water/ethylene glycol (1:1)
water (100%)
0.36
1.93
2.92
1.0
5.4
8.1
a Hydrolysis reactions were performed in duplicate containing 3 mL of
solution of the substrate prepared in 0.01 M N-ethylmorpholine (pH 8.0)
in the corresponding solvent systems at 30 °C. The amount of polymeric
catalyst was 1 mg/mL of the buffer, and the substrate concentration was
2.0 mM. b Relative rates are reported with respect to kobs obtained for pure
ethylene glycol.
Figure 8. Plot of -ln kobs versus 1/T for the hydrolysis of bNPP by CPPL-
Cu. These experiments were carried out at pH 8.0. The other experimental
conditions relating to the data plotted in this figure are summarized in Table
5.
To probe the preferential attacking nucleophile on bNPP
hydrolysis by CPPL-Cu, viz., Cu-OH or Cu-OR, we have
performed solvolytic reactions in water, ethylene glycol, and
50% aqueous ethylene glycol. Ethylene glycol was used
instead of methanol because of the insufficient solubility of
bNPP (sodium salt) in pure methanol. Initial rates (Vi’s) were
determined for the three systems (Table 6). Relative to the
rate observed for pure ethylene glycol (Vi ) 0.36 × 10-5
mM min-1), a 5.4-fold increase in rate for 50% aqueous
ethylene glycol (Vi ) 1.93 × 10-5 mM min-1) and an 8.1-
fold increase in rate for pure water (Vi ) 2.92 × 10-5 mM
min-1) were observed. From these results, it appears that
the deprotonation of coordinated water is preferred over
alcohol.
31P NMR Study on the Hydrolysis of hNPP. The
hydrolysis of hNPP can proceed either by an intermolecular
pathway or an intramolecular pathway. The latter would lead
to the formation of a cyclic phosphate while the former would
lead to the formation of an acyclic phosphate. The hydrolysis
of hNPP was monitored by a time-dependent 31P NMR study.
The 31P NMR spectra obtained at various time intervals are
given in Figure 11. The chemical shift of hNPP is at -4.65
ppm. As the hydrolysis proceeds, the formation of the cyclic
phosphate is observed at +18.53 ppm.11k No evidence for
the formation of any acyclic phosphates was observed. These
results clearly suggest that an intramolecular pathway is
preferred. It is possible to envisage two possible mecha-
nisms: (a) a direct activation of the hydroxyl group of hNPP
by the metal hydroxide acting as a base or (b) activation of
water molecules by the metal hydroxide leading to the
eventual deprotonation of the hydroxyl group on hNPP.
These possibilities are summarized in Scheme 5.
Figure 9. pH versus kobs plot for the hydrolysis of bNPP by CPPL-Cu.
These experiments were carried out at 30 °C. The concentration of the
substrate used was 2.00 mM. Other experimental conditions are as described
in Table 4.
Recycling Experiments. To evaluate the facile recovery
and reusability of the catalyst, we have carried out recycling
experiments using pNPP as the substrate. Typically, CPPL-
Cu was filtered after the reaction, washed with aqueous
methanol (6 × 5 mL), methanol (3 × 5 mL), and acetone (3
× 5 mL), dried in a vacuum at 40 °C for 6 h, and reused.
The initial rates (Vi’s) obtained show that CPPL-Cu can be
recycled several times at different substrate concentrations
without affecting the rates of hydrolysis (Table 7).
Figure 10. pH versus kobs plot for the hydrolysis of hNPP by CPPL-Cu.
These experiments were carried out at 35 °C. The concentration of the
substrate was 1.00 mM. Other experimental conditions are as described in
Table 4.
tion of water molecules that are bound to the metal ion
leading to the formation of metal hydroxide.11a,f,j The reasons
for the slight drop in the rate constant after pH 8.0 appear
to be complex and elude an unequivocal explanation at this
stage. However, the pH-rate profile for hNPP hydrolysis is
relatively more straightforward where a linear dependence
of the rate constant on pH was observed (Figure 10). Such
a linear profile has been documented in the literature.11f,j,k,n
Conclusion
We have shown for the first time that a polymer containing
cyclophosphazene as pendant groups can be designed as a
multisite coordinating polymeric ligand for metal ions. Apart
5172 Inorganic Chemistry, Vol. 41, No. 20, 2002

Pyrazolylcyclotriphosphazene with Pendant Polymers
Scheme 5a
a Pathway 1 represents a direct activation of the hydroxyl group of hNPP by metal hydroxide. Pathway (2) represents the activation of water molecules
by metal hydroxide followed by the activation of the hydroxyl group on hNPP.
Table 7. Recycling Experimentsa
pNPP concentration,
[S] (mM)
Vi
no. cycles
(mM min-1
)
2
3
4
0.2b
0.4
0.6
1.6 × 10-5
1.84 × 10-5
2.03 × 10-5
a Hydrolytic reactions were performed in duplicate containing 3 mL
solution of pNPP prepared in 0.01 M N-ethylmorpholine (pH 8.0) in 50%
aqueous methanol at 30 °C. The amount of polymeric catalyst used for
each cycle was 1 mg/mL of the buffer, and the substrate concentration was
2.0 mM. b For comparison, initial velocity, Vi, for 0.2 mM substrate
concentration at 30 °C using a fresh catalyst was 1.85 × 10-5 mM min-1
.
biological reaction, viz., phosphate ester hydrolysis. This has
been accomplished by studying the hydrolysis of three
activated substrates, viz., pNPP, bNPP, and RNA model
substrate hNPP. The hydrolysis of all of these substrates in
the presence of CPPL-Cu proceeds with considerable rate
enhancement relative to the uncatalyzed reactions. Further,
we have shown that CPPL-Cu is very robust and can be
recycled several times. This augurs well for the development
of new polymer-bound catalysts and reagents based on
polymers containing cyclophosphazenes as pendants.
Figure 11. 31P NMR spectra of the product of hNPP hydrolysis by CPPL-
Cu at different time intervals (the peak at -4.65 ppm is due to hNPP, and
the one at +18.53 ppm is due to cyclic phosphate).
Acknowledgment. We thank one of the referees for
valuable suggestions. We are thankful to the Department of
Science and Technology, New Delhi, India, for financial
support. One of us (A.A.) is thankful to the Council of
Scientific and Industrial Research, New Delhi, India, for the
award of a Senior Research Fellowship. A.S. and S.Z. thank
Engineering and Physical Sciences Research Council, U.K.,
for financial support.
from generating an insoluble polymer, cross-linking of
cyclophosphazene monomer 2 with divinyl benzene also
provides hydrolytic stability to the metal complex (CPPL-
Cu) prepared thereafter. This methodology is quite general
and offers a powerful and alternative synthetic route to the
traditional approaches based mainly on modification of
Merrifield and Wang resins,25 for the preparation of a variety
of polymeric ligands and polymer-bound transition and
lanthanide metal complexes. The heterogeneous catalyst
CPPL-Cu has been used for the catalysis of an important
Supporting Information Available: Additional figure. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(25) Merrifield, R. B. Angew. Chem., Int. Ed. Engl. 1985, 24, 799.
IC011159V
Inorganic Chemistry, Vol. 41, No. 20, 2002 5173