Hydrolysis of Bis(p-nitrophenyl)phosphate by tetravalent metal complexes with kl?ui's oxygen tripodal ligand

Article abstract of DOI:10.1021/ic902018u

The treatment of HfCl₄ with NaL_OEt (L _OEt ^- = [(η-C₅H₅)Co(P(O)(OEt) ₂}₃]^-) in nitric acid afforded L _OEtHf(NO₃)₃ (1). Hydro

Full text of DOI:10.1021/ic902018u

2
232 Inorg. Chem. 2010, 49, 2232–2238
DOI: 10.1021/ic902018u
Hydrolysis of Bis(p-nitrophenyl)phosphate by Tetravalent Metal Complexes with
€
Klaui’s Oxygen Tripodal Ligand
Xiao-Yi Yi, Tony C. H. Lam, Ian D. Williams, and Wa-Hung Leung*
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, People’s Republic of China
Received October 15, 2009
The treatment of HfCl with NaL (LOEt- = [(η -C H )Co{P(O)(OEt) } ] ) in nitric acid afforded L Hf(NO ) (1).
5
-
4
OEt
5
5
2 3
OEt
3 3
Hydrolysis of 1 in acetone/water (4:1, v/v) yielded the hydroxy-bridged dimer [(LOEt)2Hf (H O) (μ-OH) ][NO ] (2). The
2
2
4
2
3 4
treatment of (NH ) [Ce(NO ) ] with 2 equiv of NaL in water afforded (LOEt)2Ce(NO ) (3), whereas that with 1 equiv of
NaLOEt in CH Cl gave L Ce(NO ) (4). While4 is stable in organic solvents such as acetone, it was converted completely to
4
2
3 6
OEt
3 2
2
2
OEt
3 3
1
3
in acetone/water. The treatment of 3 with bis(p-nitrophenyl)phosphate (BNPP) afforded (LOEt)2Ce[η -OPO(OR) ] (5; R =
2 2
p-NO C H ), whereas the reaction of 4 with NaPO (OR) yielded dinuclear [L Ce(NO ) {μ-O P(OR) }] [R = p-NO C H
2
6
4
2
2
OEt
3 2
2
2
2
2 6 4
(
6), Ph (7)]. The addition of 3 in acetone to an aqueous solution of NaH PO yielded the cerium(IV) dihydrogen phosphate
2 4
complex (L ) Ce(PO H ) (8). Complexes 1-5 and 7 have been characterized by X-ray crystallography. The hydrolysis of
OEt 2
4 2 2
1
BNPP with LOEtM(NO ) (M = Zr, Hf, Ce) in acetone/water has been studied by H NMR spectroscopy. At 25 °C, with [Ce] =
0[BNPP], in an acetone-d /N-(2-hydroxyethyl)piperazine-N -2-ethanesulfonic acid (4:1, v/v) buffer solution (50 mM) (4:1, v/v),
the hydrolysis of BNPP with 4 was found to exhibit first-order kinetics with a rate constant of (1.1 ( 0.1) ꢀ 10 s .
3 3
0
2
6
-
3 -1
4þ
Introduction
The Ce -based hydrolysis suffers from the drawback of ease
of formation of an insoluble hydroxide gel in aqueous
solutions at around pH 7. In the presence of stabilizing
ligands such as ethylenediaminetetraacetic acid (EDTA),
1
The aqueous chemistry of metal ions is of significance
because of its relevance to the mechanisms of metal-catalyzed
organic reactions in water and the important roles of metal
ions in biological systems. We are particularly interested in
the hydrolytic chemistry of tetravalent metal ions such as
4þ
aqueous solutions containing Ce are homogeneous and
5
-7
can cleave DNA selectively under neutral conditions.
IV
4
þ
4þ
4þ
4þ
Although the exact mechanism of the Ce -mediated
cleavage of phosphodiesters is not fully understood, it is
believed that the coordination and activation of phosphate
diesters by Ce and nucleophilic attack by Ce-bound hydro-
xide play important roles. The superior hydrolytic activity
of Ce compared with other lanthanide ions has been
attributed to (a) the high electron-withdrawing ability of
Ce and (b) the formation of hybrid orbitals between the
Zr , Hf , Ce , and Th , which are highly active in the
hydrolysis of the phosphodiester linkage. Of note are the
Ce ion and its complexes, which have been used as artificial
nucleases for the cleavage of phosphate esters and DNA.
2
4
þ
IV
3-5
7,8
4þ
*
(
To whom correspondence should be addressed. E-mail: chleung@ust.hk.
1) (a) Bates, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New
York, 1976. (b) Richens, D. T. The Chemistry of Aqua Ions; Wiley: Chichester,
IV
2b,9
U.K., 1997.
(
Ce 4f and O 2p orbitals.
Recently, Komiyama and co-
2) For reviews of the metal-mediated hydrolysis of phosphate diesters,
workers reported the isolation of cerium(IV) dipicolinate
see for examples:(a) Williams, N. H.; Takasaki, B; Wall, M.; Chin, J. Acc.
Chem. Res. 1999, 32, 485. (b) Komiyama, M.; Takeda, N.; Shigekawa, H. Chem.
Commun. 1999, 1443. (c) Ott, R.; Kr €a mer, R. Appl. Mircobiol. Biotechnol. 1999,
10
complexes, which are active in DNA hydrolysis. X-ray
5
2, 761. (d) Franklin, S. J. Curr. Opin. Chem. Biol. 2001, 5, 201. (e) Sreedhara,
A.; Cowan, J. A. J. Biol. Inorg. Chem. 2001, 6, 337. (f) Liu, C.; Wang, M.; Zhang,
T.; Sun, H. Coord. Chem. Rev. 2004, 248, 147.
(6) (a) Branum, M. E.; Que, L. J. Biol. Inorg. Chem. 1999, 4, 593. (b)
Branum, M. E.; Tipton, A. K.; Zhu, S. R.; Que, L. J. Am. Chem. Soc. 2001, 123,
(
3) (a) Bracken, K.; Moss, R. A.; Ragunathan, K. G. J. Am. Chem. Soc.
1
898.
1997, 119, 9323. (b) Moss, R. A.; Zhang, J.; Bracken, K. Chem. Commun. 1997,
1639. (c) Moss, R. A.; Zhang, J.; Ragunathan, K. G. Tetrahedron Lett. 1998, 39,
1529. (d) Moss, R. A.; Morales-Rojas, H.; Vijayaraghavan, S.; Tian, J. Z. J. Am.
(7) (a) Maldonado, A. L.; Yatsimirsky, A. K. Org. Biomol. Chem. 2005, 3,
2859. (b) Sirish, M.; Franklin, S. J. J. Inorg. Biochem. 2002, 91, 253.
(8) Sumaoka, J.; Furuki, K.; Kojima, Y.; Shibata, M.; Hirao, K.; Takeda,
N.; Komiyama, M. Nucleosides Nucleotides Nucleic Acids 2006, 25, 523.
(9) (a) Shigekawa, H.; Ikawa, H.; Yoshizaki, R.; Iijima, Y.; Sumaoka, J.;
Komiyama, M. Appl. Phys. Lett. 1996, 68, 1433. (b) Shigekawa, H.; Ishida, M.;
Miyake, K.; Shioda, R.; Iijima, Y.; Imai, T.; Takahashi, H.; Sumaoka, J.;
Komiyama, M. Appl. Phys. Lett. 1999, 74, 460.
Soc. Chem. 2004, 126, 10923.
(4) Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1994, 116, 1121.
(5) (a) Komiyama, M.; Shiiba, T.; Kodama, N.; Takeda, N.; Smaoka, J.;
Yahsiro, M. Chem. Lett. 1994, 1025. (b) Komiyama, M.; Takeda, N.; Takahashi,
Y.; Uchida, H.; Shiiba, T.; Kodama, T.; Yashiro, M. J. Chem. Soc., Perkin Trans.
1
995, 2, 269. (c) Kitamura, Y.; Sumaoka, J.; Komiyama, M. Tetrahedron 2003,
(10) Katada, H.; Seino, H.; Mizobe, Y.; Sumaoka, J.; Komiyama, M.
J. Biol. Inorg. Chem. 2008, 13, 249.
5
9, 10403.
pubs.acs.org/IC
Published on Web 02/04/2010
r 2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2233
1
Chart 1
240 mg (56%). H NMR (CDCl
CH ), 4.12 (m, 12H, OCH ), 5.15 (s, 5H, C
CDCl ): δ 121.8 (s). IR (KBr, cm ): 1292, 1385, 1542, 1565
3
): δ 1.29 (t, J = 7.0 Hz, 18H,
31 1
3
2
5 5
H ). P{ H} NMR
-
1
(
3
þ
[
ν(NO )]. MS (FAB): m/z 839 (M - NO þ 1). Anal. Calcd for
3
3
17
C H
3 3
35CoHfO18N P : C, 22.69; H, 3.92; N, 4.67. Found: C, 22.58;
H, 3.89; N, 5.00.
Preparation of [(LOEt
tion of 1 (90 mg, 0.10 mmol) in acetone/H
)
2
Hf
2
2
(H O)
4
(μ-OH)
2 3 4
][NO ] (2). A solu-
2
O (4:1, v/v) was left to
stand in air overnight. Slow evaporation of the solvent afforded
yellow crystals that were suitable for X-ray diffraction. Yield: 81
1
crystallography revealed that the Ce atoms in these com-
plexes contain a number of labile coordinated water mole-
cules that are presumably essential for their hydrolytic
mg (91%). H NMR (CDCl ): δ 1.27 (t, J = 7.0 Hz, 36H, CH ),
3
3
1.90 (s, 8H, H
O), 4.13 (m, 24H, CH ), 5.09 (s, 10H, C
2
H
), 9.01
2
5
5
3
1
1
s, 2H, OH). P{ H} NMR (CDCl
(
for C H Co Hf O P N : C, 22.92; H, 4.53; N, 3.15. Found: C,
3
): δ 121.1 (s). Anal. Calcd
IV
3
4
80
2
2
36
6
4
activity. To our knowledge, Ce complexes containing
2
3.04; H, 4.30; N, 3.36.
Preparation of (LOEt)2Ce(NO ) (3). To a solution of NaL
(198 mg, 0.35 mmol) in water (10 mL) was added
NH [Ce(NO ] (97.2 mg, 0.18 mmol) in water (5 mL) drop-
wise over 5 min. The orange precipitate was collected, washed
with water, and dried in vacuo. X-ray-quality crystals were
obtained by the slow evaporation of a saturated CH Cl /hexane
phosphate diester ligands have not been isolated, although
III
3 2
OEt
the structure of a dinuclear Ce complex with a bridged
2a
dimethylphosphate ligand has been reported.
We have a long-standing interest in metal complexes
(
4
)
2
3 6
)
5
-
with Kl €a ui’s tripodal ligand [(η -C H )Co{P(O)(OEt) } ]
5
5
2 3
-
(
Chart 1, denoted as LOEt hereafter), which can serve as
2
2
11
1
models of metal aqua ions and metal oxides. In a previous
paper, we reported that the interaction of zirconyl nitrate
with NaLOEt innitric acid led to formation of the tetranuclear
3
solution. Yield: 220 mg (93%). H NMR (CDCl ): δ 1.30 (t, J =
7.0 Hz, 36H, CH
3
), 4.08-4.12 (m, 24H, CH
). P{ H} NMR (CDCl ): δ 121.1 (s). IR (KBr, cm ):
292 [ν(NO )]. MS (FAB): m/z 1272 (M - NO
2NO ). Anal. Calcd for C H CeCo N O P : C, 30.60; H,
2
), 5.12 (s, 10H,
3
1
1
-1
C
H
5
5
3
þ
þ
4þ
1
3
3
), 1210 (M -
cluster [(L ) Zr (μ -O) (μ-OH) (H O) ] . In acetone/water,
OEt 4
4
3
2
4
2
2
IV
Zr L
complexes such as LOEtZr(NO ) can hydrolyze bis-
3
34 70
2
2
24 6
OEt
3 3
5
.29; N, 2.10. Found: C, 30.21; H, 5.42; N, 2.18.
Preparation of LOEtCe(NO ) (4). A mixture of NaL
mg, 0.42 mmol) and (NH ) [Ce(NO ) ] (220 mg, 0.41 mmol) in
4
(
p-nitrophenyl)phosphate (BNPP) to give p-nitrophenol and a
12
OEt
(232
3 3
zirconium(IV) phosphate cluster, [LOEtZr(μ -PO )] . As an
3
4 4
2
3 6
extension of this study, we examine the hydrolytic chemistry of
2 2
CH Cl (45 mL) was stirred under nitrogen overnight. The
IV
IV
analogous Hf and Ce complexes. In this paper, we describe
solvent was removed in vacuo, and the residue was washed with
hexanes (3 ꢀ 4 mL). Recrystallization from THF/hexanes
IV
IV
the synthesis and solid-state structures of Hf - and Ce L
OEt
complexes and their activity in BNPP hydrolysis. In addition,
afforded dark-brown crystals that were suitable for X-ray
IV
1
the syntheses and structures of Ce LOEt complexes containing
crystallography. Yield: 252 mg (70%). H NMR (CDCl ): δ
3
phosphate diester and phosphate ligands that may serve as
models for intermediate(s) for the Ce -catalyzed hydrolysis of
phosphate diesters will be reported.
1.31 (t, J = 7.0 Hz, 18H, CH
(
cm ): 1259, 1385, 1521, 1542 [ν(NO
3
), 4.10-4.17 (m, 12H, CH
2
), 5.17
IV
31
1
s, 5H, C H ). P{ H} NMR (CDCl ): δ 123.4 (s). IR (KBr,
5
5
3
-
1
3
)]. Anal. Calcd for
C
17
H
35CeCoO18 : C, 23.70; H, 4.10; N, 4.88. Found: C,
3 3
N P
2
3.65; H, 4.12; N, 4.64.
Experimental Section
1
2 2 2 2
Preparation of (LOEt) Ce[η -OP(O)(OR) ] (5; R = p-NO -
6 4
General Considerations. NMR spectra were recorded on a
Bruker AV 400 spectrometer operating at 400 and 162.0 MHz
C H ). To a solution of 3 (40 mg, 0.030 mmol) in water (10 mL)
was added 2 equiv of BNPP (0.06 mmol) in water (5 mL), and
the mixture was stirred at room temperature for 10 min. The
1
31
for H and P, respectively. Chemical shifts (δ, ppm) were
1
( H) and H
31
( P). IR
reported with reference to SiMe
4
3
PO
4
reddish-brown precipitate was collected, washed with H O (2 ꢀ
2
spectra were recorded on a Perkin-Elmer 16 PC Fourier trans-
form infrared spectrophotometer. Electrospray ionization mass
spectrometry was recorded on a Finningan MAT TSQ-7000
spectrometer. Elemental analyses were performed by Medac
Ltd., Surrey, U.K.
2
mL), redissolved in CH Cl , and dried over anhydrous
2 2
2 4 2 2
Na SO . Recrystallization from CH Cl /hexanes afforded red
crystals that were suitable for X-ray diffraction. Yield: 52 mg
(
1
91%). H NMR (CDCl ): δ 1.17 (t, J = 7.0 Hz, 36H, CH ), 4.18
3
3
(
phenyl protons), 8.05 (d, J = 4.5 Hz, 4H, phenyl protons).
m, 24H, CH
2
), 5.03 (s, 10H, C
5
H
5
), 7.44 (d, J = 4.5 Hz, 4H,
The complexesLOEtZr(NO ) and[(L ) Zr (μ -O) (μ-OH) -
3 3
OEt 4
4
3
2
2
1
2
3
1
1
(
The ligand NaLOEt was synthesized according to a literature
H
2
O)
4 3 4
][NO ] were prepared as described elsewhere.
P{ H} NMR (CDCl ): δ -20.1 (s, BNPP), 119.3 (s, LOEt).
3
Despite two attempts, we have not been able to obtain good
analytical data for the compound. The carbon content of the
sample was found to be lower than the calculated value. How-
ever, this compound has been well characterized spectroscopi-
cally and by X-ray diffraction.
13
2 2 2 6 4
method. NaO P(OR) (R = p-NO C H , Ph) complexes were
prepared by the treatment of (RO) P(O)(OH) with 1 equiv of
2
sodium hydride in tetrahydrofuran (THF). The resulting solid
was washed with Et
Preparation of LOEtHf(NO
2
O and dried in vacuo.
(1). To a solution of HfCl
3
)
3
4
Preparation of [LOEtCe(NO
C H (6), C H (7)]. A mixture of 4 (86 mg, 0.10 mmol) and
NaPO (OR) (0.10 mmol) in CH Cl (30 mL) was stirred over-
2
night and filtered. The solvent was removed in vacuo, and the
residue was washed with hexanes (3 ꢀ 4 mL). Recrystallization
fromCH Cl /hexane (for 6) andEt O (for 7) gavebrown crystals.
3 2 2 2 2 2
) {μ-O P(OR) }] [R = p-NO -
(
(
195 mg, 0.61 mmol) in 70% nitric acid (4 mL) was added NaLOEt
6
4
6
5
280 mg, 0.48 mmol) in acetone (1 mL) dropwise, and the reaction
2
2
2
mixture was stirred at room temperature for 30 min. The yellow
precipitate was collected, washed with water (3 ꢀ 1 mL), redissolved
in CH Cl , and dried over anhydrous MgSO . Evaporation of the
2
2
4
2
2
2
solvent afforded an analytically pure yellow crystalline solid. Yield:
1
For 6. Yield: 96 mg (85%). H NMR (CDCl ): δ 1.18 (m, 36H,
3
CH
phenyl protons), 8.14 (d, 8H, phenyl protons). P{ H} NMR
3 2 5 5
), 3.99 (m, 24H, CH ), 5.06 (s, 10H, C H ), 7.42 (d, 8H,
3
1
1
(
11) (a) Kl €a ui, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 627. (b) Leung,
W.-H.; Zhang, Q.-F.; Yi, X.-Y. Coord. Chem. Rev. 2007, 251, 2266.
12) Zhang, Q.-F.; Lam, T. C. H.; Chan, E. Y. Y.; Lo, S. M. F.; Williams,
I. D.; Leung, W.-H. Angew. Chem., Int. Ed. 2004, 43, 1715.
13) Kl €a ui, W. Z. Naturforsch. B 1979, 34, 1403.
-
-1
(
1
CDCl
3
): δ 122.1 (s, LOEt ), -29.4 (s, BNPP). IR (KBr, cm ):
222, 1347, 1494, 1593, and 1614 [ν(NO )]. Despite two at-
(
3
tempts, we have not been able to obtain good analytical data for
the compound. The carbon content was found to be lower than
(

2
234 Inorganic Chemistry, Vol. 49, No. 5, 2010
Yi et al.
Table 1. Crystallgraphic Data and Experimental Details for Complexes 1-5 and 7
1
2
3
4
5
7
formula
C
17
H
35CoHfN
3
-
C
34
H
Hf
84Co
2
-
C
34
H
70CeCo
2
-
C
21
H
43CeCoN
3
-
C
58
H
86CeCo
2
-
C
58
H
90Ce
2 2
Co -
O
18
P
3
2
N
4
O
38
P
6
N
2
24
O P
6
O
19
P
3
N
4
O
35
P
8
N O P
4 38 8
fw
T, K
899.81
173(2)
1817.71
100(2)
1334.72
173(2)
933.54
100(2)
1905.05
253(2)
2097.20
173(2)
cryst syst
space group
monoclinic
P2 /c
17.44590(10)
10.83490(10)
16.73140(10)
90
93.0850(10)
90
3158.06(4)
4
1.893
12.233
1784
43 454
5659
0.0474
1.028
0.0205, 0.0492
0.0231, 0.0499
monoclinic
P2 /n
10.8073(11)
19.274(2)
15.8459(16)
90
99.2430(10)
90
3257.9(6)
2
1.853
3.919
1816
24 485
6359
0.0655
1.020
0.0360, 0.0631
0.0534, 0.0677
monoclinic
C2/c
20.351(4)
10.8702(18)
26.204(4)
90
110.673(2)
90
5423.4(16)
4
1.635
1.684
2728
26 583
5210
0.027
1.037
0.0272, 0.0678
0.0307, 0.0693
orthorhombic
P2
13.4996(8)
13.5632(8)
19.4555(12)
90
90
90
3562.3(4)
4
1.741
1.941
1888
19 610
6184
0.0413
0.965
0.0246, 0.0464
0.0293, 0.0475
monoclinic
P2 /n
12.5077(5)
29.5374(12)
21.9626(9)
90
95.0510(10)
90
8082.5(6)
4
1.566
1.204
3896
43 902
15 827
0.0540
1.075
monoclinic
P2 /n
13.8865(2)
24.9546(3)
24.3263(4)
90
99.742(2)
90
8308.3(2)
4
1.677
13.613
4240
28 516
13 856
0.1216
1.015
1
1
1
2
1
2
1
1
1
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
-
3
F
calcd, g cm
μ, mm
-
1
F(000)
no. of reflns
no. of indep reflns
R
GOF
int
a
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0566, 0.1226
0.0927, 0.1324
0.0747, 0.1533
0.1378, 0.1807
P
a
2
1/2
GOF = [( w|F
o
| - |F
c
|) /(Nobs - Nparam)]
.
the calculated value. However, this compound has been well
characterized spectroscopically. In addition, the structure of the
diphenylphosphate analogue 7 has been established by X-ray
δ 7.01 for at least 3 half-lives. The rate constant (kobs) was
obtained by fitting the first 3 half-lives of the reaction according
3
-
to a first-order kinetics equation. The formation of PO
4
in the
reaction was confirmed by ion chromatography (after treatment
diffraction.
For 7. Yield: 68 mg (65%). H NMR (CDCl
1
IV
of the final mixture with EDTA to remove Ce ).
3
). δ 1.10 (36H,
CH ), 4.00 (24H, OCH
3
protons), 7.20 (16H, phenyl protons) (all broad resonances). P-
2
), 4.99 (10H, C
5
H
5
), 7.04 (4H, phenyl
X-ray Crystallography. Crystallographic data and experi-
mental details for complexes 1-5 and 7 are summarized in
Table 1. Intensity data were collected on a Bruker SMART
APEX 1000 CCD diffractometer using graphite-monochro-
31
1
-
{
(
H} NMR (CDCl ): δ 120.9 (L
), -28.9 [P(O) Ph ]. IR
3
OEt
2
2
-1
KBr, cm ): 1235, 1290, 1494, 1521 [ν(NO
45CeCoO19 : C, 33.22; H, 4.33; N, 2.67. Found: C, 33.15;
H, 4.29; N, 2.56.
Preparation of (LOEt)2Ce(PO H ) (8). To a solution of
3
)]. Anal. Calcd for
C
29
H
N P
2 4
mated Mo KR radiation (λ = 0.710 73 A). The collected frames
˚
1
4
were processed with the software SAINT. Structures were
solved by direct methods and refined by full-matrix least squares
4
2 2
2
15
NaH PO4 2H O (18 mg, 0.12 mmol) in water (1 mL) was added
2
on F using the SHELXTL software package. The atomic
positions of non-hydrogen atoms were refined with anisotropic
parameters. In complex 3, the carbon atoms C18 and C22 of an
ethoxy group were found to be disordered. Each of these carbon
atoms were split into two sites with occupancies of 0.6 and 0.4. In
2
3
3
(40 mg, 0.030 mmol) in acetone (0.20 mL). An orange
precipitate formed immediately, and the suspension mixture
was stirred for 10 min and filtered. The orange solid was
dissolved in CH
Na SO . Evaporation of the filtrate afforded an orange-yellow
solid, which was recrystallized from CH Cl /hexane to give
orange crystals. Yield: 35 mg (85%). H NMR (CDCl ): δ
2 2
Cl /hexane (6:1, v/v) and dried over anhydrous
-
2
4
complex 5, the ethoxy groups of ligand LOEt were found to be
seriously disordered.
2
2
1
3
1
.26 (t, J = 7.0 Hz, 36H, CH
3
), 4.13 (m, 24H, OCH ), 5.04 (s,
2
Results and Discussion
IV
31
1
10H, C
s, LOEt). Anal. Calcd for C H CeCo O P : C, 29.07; H, 5.31.
5
H
5
). P{ H} NMR (CDCl ): δ -5.3 (s, PO H ), 117.2
3
4 2
IV
Hf Complexes. The syntheses of Hf L complexes
OEt
(
Found: C, 28.67; H, 5.18.
34
74
2
26 8
are summarized in Scheme 1. The treatment of HfCl in
4
70% nitric acid with 1 equiv of NaLOEt in acetone
Hydrolysis of BNPP with LOEtM(NO ) (M = Zr, Hf). To a
3 3
solution of LOEtM(NO
(0.48 mL) was added BNPP (0.3 μmol, 0.12 mL of a 2.5 mM
solution in water). The progress of the reaction was followed by
3
)
3
(M = Zr, H; 6 μmol) in acetone-d
6
afforded the tri(nitrate) complex 1. The use of concen-
trated nitric acid is essential for the isolation of 1 in pure
form. The reaction of HfCl with NaL
4
in less concen-
OEt
monitoring the integration of the p-nitrophenol resonance at δ
trated nitric acid (<1 M) gave a mixture of 1 and
uncharacterized species. The IR spectrum of 1 showed
1
.01 (d, JHH = 4.6 Hz) in the H NMR spectrum.
7
Kinetics of the Hydrolysis of BNPP with 4. A buffered BNPP
solution was prepared by adding BNPP (8.5 mg, 0.025 mmol),
-1
the N-O bands at 1542 and 1565 cm , which are typical
for κ -nitrate ligands. Complex 1 exhibited one set of H
NMR signals for the ethyl groups of the LOEt ligands
2
1
NaClO4 (306 mg, 2.5 mmol), and 2.5 mL of a N-(2-hydro-
0
-
xyethyl)piperazine-N -2-ethanesulfonic acid (HEPES) buffer
1 M, pH 7.0) to 7.5 mL of water. To a solution of 4 (6 μmol)
3
1
1
and a sharp singlet in the P{ H} NMR spectrum,
indicative of the high symmetry of the molecule.
(
in acetone-d (0.48 mL) was added 0.12 mL of the above
6
buffered BNPP solution (containing ca. 0.3 μmol of BNPP).
The concentration of HEPES in the resulting the mixture is
approximately 50 mM. The mixture was shaken to ensure
homogeneity and quickly transferred to a 5-mm NMR tube.
The H NMR spectrum of the reaction mixture was recorded at
3.5-min intervals. The rate of hydrolysis was determined by
monitoring the integration of the p-nitrophenol resonance at
Although 1 is stable in CH
hydrolyzes easily in aqueous media. Dissolution of 1 in
Cl and CHCl solutions, it
2 2 3
(
14) Bruker SMART and SAINTþ, version 6.02a; Siemens Analytical X-ray
1
Instruments Inc.: Madison, WI, 1998.
(15) Sheldrick, G. M. SHELXTL-Plus V5.1 Software Reference Manual;
Bruker AXS Inc.: Madison, WI, 1997.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2235
Scheme 1
Figure 1. Molecular structure of 1. Hydrogen atoms are omitted for
clarity. The thermal ellipsoids are drawn at a 30% probability level.
˚
Selected bond length (A) and angle (deg): Hf1-O(LOEt) 2.0809-
Scheme 2
(
15)-2.0913(16), Hf1-O11 2.3641(18), Hf1-O12 2.2637(17), Hf1-O14
2.3620(18), Hf1-O15 2.2615(17), Hf1-O17 2.2759(16), Hf1-O18
2.3599(17); O(LOEt)-Hf1-O(LOEt) 83.16(6)-84.11(6), O11-Hf1-O12
55.11(6), O14-Hf1-O15 54.88(6), O17-Hf1-O18 55.29(6).
in acetone/water (4:1, v/v), followed by the slow evapora-
tion in air, led to the isolation of 3 almost quantitatively
-
IV
(>98% with respect to LOEt ). The remaining Ce
byproduct, presumably a LOEt-free species, has not been
characterized. The facile conversion of 4 to 3 in aqueous
media is presumably due to the high affinity of electron-
IV
deficient Ce for the π-donating oxygen ligand LOEt-
.
Cesium(IV) Phosphate Diester and Phosphate Com-
plexes. Efforts have been made to synthesize Ce com-
IV
plexes with phosphate diester and phosphate ligands
that may serve as models of intermediates of the Ce -
IV
mediated hydrolysis of phosphate diesters. The treatment
of 3 with BNPP in water resulted in precipitation of the
3
1
1
bis(phosphate diester) complex 5. The P{ H} NMR
spectrum of 5 displayed a singlet at δ -20.1 due to the
1
deprotonated η -phosphate diester ligand, which is more
acetone/water (4:1, v/v), followed by the slow evapora-
tion in air, afforded yellow crystals identified as the
hydroxy-bridged dimer 2. It may be noted that the
upfield than that of free BNPP (δ -13.4 in D O). The
2
treatment of 4 with Na[PO (OR) ] (R = p-NO C H ) in
CH Cl afforded dinuclear 6 containing bridged phos-
2
2
2
6
4
2
2
hydrolysis of the Zr analogue LOEtZr(NO ) in water
3 3
phate diester ligands. An analogous complex with diphe-
nylphosphate, 7, has been prepared from 4 and
NaP(O) (OPh) . The P NMR resonance for the diphe-
2 2
nylphosphate ligands in 7 was found at δ -28.9, which is
more upfield than that for the free diphenylphosphate
afforded tetranuclear [(LOEt)4Zr (H O) (μ -O) (μ-OH) ]-
4
2
2
3
2
4
3
1
[
LOEtZr(OTf) hydrolyzes to dinuclear [(LOEt)2Zr (H O) -
NO ] , while depending upon experimental conditions,
3
4
3
2
2
4
(
(
μ-OH) ][OTf] or trinuclear [(L ) Zr (H O) (μ -O)-
2
4
OEt 3
3
2
3
3
1
6
μ-OH) ][OTf] .
(δ -9.0 in D O).
3
4
2
Cerium(IV) Nitrate Complexes. The syntheses of
complexes are summarized in Scheme 2. As
reported previously, the treatment of (NH ) [Ce(NO ) ]
The addition of 3 in acetone to an aqueous solution of
NaH PO led to formation of the dihydrogen phosphate
IV
Ce L
OEt
2
4
4
2
3 6
complex 8. A preliminary X-ray diffraction study showed
that 8 is an eight-coordinated Ce complex containing two
with NaLOEt in water led to isolation of the bis(tripod)
compound 3. Mono(tripod) compounds were not formed
even when excess (NH ) [Ce(NO ) ] (2 equiv) was em-
1
-
cisoid η -H PO ligands (see the Supporting Information).
2
4
31
4
2
3 6
The P NMR resonance for the dihydrogen phosphate
ligand in 8 was observed at δ -5.3. The mono(tripod)
compound 4 was also found to react with NaH PO , as
evidenced by P NMR spectroscopy. Unfortunately, we
were not able to isolate any half-sandwich LOEt cesium(IV)
phosphate complex(es) from the reaction mixture.
ployed. In this work, we found that the reaction of
NH ) [Ce(NO ) ] with 1 equiv of NaL in CH Cl
(
4
2
3 6
OEt
2
2
2
4
31
afforded half-sandwich 4 as the sole isolated product.
Although complex 4 is stable in a CH Cl solution, it
converted rapidly to 3 in aqueous media. Dissolution of 4
2
2
Crystal Structures. Figures 1 and 2 show the molecular
structures of 1 and 4, respectively, which are isostructural
with LOEtZr(NO ) . Both 1 and 4 are nine-coordinated,
(
16) Yi, X. Y.; Zhang, Q.-F.; Lam, T. C. H.; Chan, E. Y. Y.; Willams, I.
D.; Leung, W.-H. Inorg. Chem. 2006, 45, 328.
3 3

2
236 Inorganic Chemistry, Vol. 49, No. 5, 2010
Yi et al.
Figure 4. Molecular structure of 3. Hydrogen atoms are omitted for
clarity. The thermal ellipsoids are drawn at a 30% probability level.
Selected bond length (A) and angle (deg): Ce1-O(LOEt) 2.326(2)-2.350-
(2), Ce1-O10 2.496(2), Ce1-O11 2.748(2); O(LOEt)-Ce1-O(LOEt
)
Figure 2. Molecular structure of 4. Hydrogen atoms are omitted for
clarity. The thermal ellipsoids are drawn at a 30% probability level.
˚
˚
Selected bond length (A) and angle (deg): Ce1-O(LOEt) 2.220(2)-
2.232(2), Ce1-O11 2.461(3), Ce1-O13 2.468(3), Ce1-O14 2.462(2),
72.77(6)-75.92(6), O10-Ce1-O11 47.72(6).
Ce1-O16 2.473(3), Ce1-O17 2.476(3), Ce1-O19 2.471(2); O(LOEt)-
Ce1-O(LOEt) 79.26(9)-80.52(8), O11-Ce1-O13 51.84(8), O14-Ce1-
O16 51.68(8), O19-Ce1-O17 51.36(8).
˚
those in 1. The Hf-OH distance [2.104(3) A] is shorter
˚
than the Hf-OH distances [2.153(3) and 2.189(3) A] and
2
3
compares well with those in [HfCl (μ-OH)(κ -bdmpza)]
2
2
[
2
bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetae; 2.084(3)-
17
˚
.106(3) A].
Figure 4 shows the molecular structure of 3. The
complex is formally 10-coordinated, with the Ce atom
located at the inversion center. The Ce-O(LOEt)
˚
[
2
2.326(2)-2.350(2) A] and Ce-O(nitrate) [2.496(2) and
˚
.748(2) A] distances are longer than those in 4 presum-
ably because of the steric effect of the oxygen tripodal
ligands. Unlike 4, the bidentate nitrate ligands in 3 bind to
the Ce center in a more unsymmetrical fashion with
˚
Ce-ONO distances of 2.496(2) and 2.748(2) A.
2
Figures 5 and 6 show the structures of 5 and 7, respec-
tively. To our knowledge, 5 and 7 are the first cerium(IV)
phosphate diester complexes characterized by X-ray crystal-
lography. In 5, the two phosphate diester ligands bind to the
Ce atom in a unidentate fashion. Unlike 1, the two phos-
phate diesters in 5 are adjacent to each other. The Ce-
Figure 3. Molecular structure of 2. Hydrogen atoms of the LOEt-
ligands are omitted for clarity. The thermal ellipsoids are drawn at a
˚
˚
O(phosphate diester) distances [2.370(3) and 2.373(3) A] are
50% probability level. Selected bond length (A) and angle (deg): Hf1-
slightly longer than the Ce-O(LOEt) distances [2.289(3)-
O(LOEt) 2.056(3)-2.137(3), Hf1-O30 2.092(3), Hf1-O30A 2.104(3),
Hf1-O10 2.189(3), Hf1-O20 2.153(3), Hf1-Hf1A 3.492(1); O(LOEt)-
˚
2.358(3) A]. The terminal P-O distances [1.444(4) and
Hf1-O(LOEt
O20-Hf1-O10, 78.73(12). Symmetry operator: A = -x þ 1, -y þ 1,
z þ 1.
)
79.94(11)-83.97(12), Hf1-O30-Hf1A 112.63(14),
1
˚
.439(4) A] of the phosphate diester ligands are shorter
than the P-O(Ce) distances [1.487(3) and 1.489(3) A],
consistent with the formulation of the PdO double bond.
˚
-
þ
7
consists of two {L Ce(κ -NO ) } fragments that are
OEt
2
3 2
with the nitrate ligands binding to the metal center in a
0
0
bridged by two μ-O,O -diphenylphosphate ligands. Dinuc-
lear phosphate diester complexes of d-block metals, e.g.,
bidentate-O,O fashion. The Hf-O(LOEt) [2.0809(15)-
˚
18
19
2
.0913(16) A] and Hf-O(nitrate) [2.2615(17)-2.3641(18)
Cu and Ni, are well documented. The Ce-O(phosphate
˚
A] distances in 1 are comparable to those in L
Zr(NO ) [2.088(2)-2.095(2) and 2.275(2)-2.365(2) A, res-
pectively]. As expected, the corresponding bond distances
in 4 [2.220(2)-2.232(2) and 2.461(3)-2.476(3), respectively]
are longer than those in 1 because of the bigger ionic radius
of Ce compared with Hf .
Figure 3 shows the structure of the cation [(L ) Hf -
H O) (μ-OH) ] in 2, which is isostructural with that in
2 4 2
16
(L ) Zr (H O) (μ-OH) ][OTf] . The complex cation
consists of two symmetry-related {LOEtHf(H O) (μ-OH)}
-
˚
OEt
diester) distances [2.261(7)-2.346(7) A] in7are shorter than
those in 5.
˚
3
3
12
IV IV
Hydrolysis of BNPP with Zr and Hf Complexes. As
reported previously, reaction of LOEtZr(NO3)3 with
4þ
4þ
(17) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-
OEt 2
2
Sanchez, A.; Sanchez-Barba, L.; Fernandez-Lopez, M.; Lopez-Solera, I.
Inorg. Chem. 2004, 43, 1350.
(18) Kato, M.; Tanase, T.; Mikuriya, M. Inorg. Chem. 2006, 45, 2925.
(19) (a) Goldberg, D. P.; Watton, S. P.; Masschelein, A.; Wimmer, L.;
Lippard, S. J. J. Am. Chem. Soc. 1993, 115, 5346. (b) Pothiraja, R.; Shanmugan,
S.; Walawalkar, M. G.; Nethaji, M.; Butcher, R. J.; Murugavel, R. Eur. J. Inorg.
Chem. 2008, 1834.
4þ
(
[
OEt 2
2
2
4
2
4
2þ
2
2
moieties related by an inversion center. The Hf-O(LOEt
)
˚
distances of 2.056(3)-2.137(3) A are slightly longer than

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2237
appeared. The hydrolysis was found to be complete in ca.
2
h, when the resonances at δ 7.33 and 8.33 almost dis-
appeared and broad signals at δ123 and -24 due to [LOEtZr-
31
1
(
Apart from the resonances for L Zr(NO ) and [LOEt-
μ -PO )] were observed in the P{ H} NMR spectrum.
3 4 4
OEt
3 3
31
Zr(μ -PO )] , no other P NMR signals were found during
3
4 4
the reaction, indicating that either no intermediate(s) was
involved or the concentration of the intermediate(s) was too
low to be detected by NMR spectroscopy.
The hydrolysis of BNPP with a 20-fold excess of LOEt-
Zr(NO ) in acetone-d /water (4:1, v/v) has been followed
3
3
6
by monitoring the integration of the p-nitrophenol signal at
δ 7.01 [I(7.01)]. The plot of I(7.01) versus time was found to
exhibit sigmoidal behavior. A similar plot was found for
4þ
[
(L ) Zr (μ -O) (μ-OH) (H O) ] under the same con-
OEt 4 4 3 2 4 2 2
ditions (see Figure S3 in the Supporting Information).
Previously, we have demonstrated that LOEtZr(NO ) can
be converted to [(L ) Zr (μ -O) (μ-OH) (H O) ] read-
OEt 4 4 3 2 4 2 2
1
Figure 5. Molecular structure of 5. Hydrogen atoms and cocrystallized
water molecules are omitted for clarity. The thermal ellipsoids are drawn at a
˚
3 3
þ
4
3
0% probability level. Selected bond length (A) and angle (deg): Ce1-O-
ily in aqueous media. Therefore, it seems reasonable to
assume that the LOEtZr-mediated hydrolysis of BNPP in
acetone/water involves a tetrameric or other oligomeric Zr
active species. The hydrolysis of BNPP with 1 was also
found to exhibit sigmoidal behavior with a longer induction
period (Figure S3 in the Supporting Information). The
hydrolytic activity of the hydroxyl-bridged Hf dimer 2 was
not studied because of its low solubility in acetone/water.
(
1
1
L
OEt) 2.289(3)-2.358(3), Ce1-O23 2.370(3), Ce1-O27 2.373(3), P7-O23
.489(3), P7-O24 1.439(4), P7-O25 1.622(4), P7-O26 1.610(4), P8-O27
.487(3), P8-O28 1.444(4), P8-O29 1.612(4), P8-O30 1.618(3); O(LOEt)-
Ce1-O(LOEt) 73.34(12)-75.75(12), O23-Ce1-O27 79.38(12), O23-P7-
O24 121.8(2), O25-P7-O26 101.0(2), O27-P8-O28 122.4(2), O29-
P8-O30 100.9(2).
1
7
Neither LOEtTi(OTf) nor (L ) Ti (μ-O) (μ-SO ) is cap-
3
OEt 2
2
2
4
able of hydrolysis of BNPP in acetone/water.
IV
Hydrolysis of BNPP by Ce complexes. As expected,
the bis(tripod) compound 3 cannot hydrolyze BNPP in
aqueous media. The reaction of 3 with BNPP in water led
to precipitation of the bis(phosphate diester) complex (see
the section above), which does not undergo hydrolysis
with bases such as Et N, NaOH, and n-Bu NOH. The
3
4
treatment of 4 with BNPP in acetone/water (4:1, v/v) led
to the formation of 3, as evidenced by NMR spectro-
scopy. When the reaction mixture was left to stand in air
for a long period of time (3-4 days), pale-yellow fine
crystals identified as Ce(PO )(PO H) (H O) formed,
4
4
0.5
2
2
indicating that in an aqueous acetone solution 4 could
hydrolyze BNPP, albeit rather slowly. A preliminary
X-ray diffraction study (see the Supporting Information)
revealed that Ce(PO )(PO H) (H O) has a three-
4
4
0.5
2
2
dimensional framework structure similar to that of
reported Ce(H O)(PO ) (H O) (H O)0.5.
2
0
2
4 1.5
3
0.5
2
Figure 6. Molecular structure of 7. Hydrogen atoms are omitted for
clarity. The thermal ellipsoids are drawn at a 30% probability level. Selected
bond length (A) and angle (deg): Ce1-O(LOEt) 2.264(7)-2.345(8),
IV
Previous studies demonstrated that the Ce -mediated
hydrolysis of phosphate diesters is pH-dependent. Moss
and co-workers reported that the hydrolysis of BNPP
˚
Ce2-O(LOEt) 2.282(8)-2.351(7), Ce1-O21 2.364(7), Ce1-O26 2.261(7),
Ce2-O22 2.332(7), Ce2-O25 2.285(7), P7-O21 1.470(7), P7-O22
IV
with Ce ions in aqueous micellar solutions is very
sensitive to the pH and the highest rate enhancement
1
.493(7), P7-O23 1.576(7), P7-O24 1.592(8), P8-O25 1.475(7),
P8-O26 1.494(8), P8-O27 1.578(8), P8-O28 1.587(8); O(LOEt)-Ce1-O-
OEt) 73.5(3)-78.8(3), O21-P7-O22 118.0(4), O25-P8-O26 117.2(4).
3
a
(L
was found at pH 7. Thus, the hydrolysis of BNPP with 4
in an acetone/HEPES buffer (pH 7) solution was studied.
The treatment of BNPP with excess 4 (20-fold) in an
acetone/HEPES buffer (4:1, v/v) resulted in the immedi-
BNPP in aqueous media such as acetone/water yielded
p-nitrophenol and the cubane cluster [LOEtZr(PO )] .
1
2
4
4
1
(
L
) M(NO ) (M = Zr, Hf) cannot hydrolyze BNPP
OEt 2 3 2
ate formation of p-nitrophenol, as evidenced by H NMR
presumably because the metal center in these complexes is
more sterically crowded and less electrophilic. The hydro-
lysis of BNPP with L Zr(NO ) has been investigated
spectroscopy. The formation of phosphate in the reaction
was confirmed by ion chromatography (after treatment
of the final reaction mixture with EDTA to remove Ce ).
4
þ
OEt
3 3
1
by H NMR spectroscopy. Upon the addition of excess
LOEtZr(NO ) (10 mM) to BNPP (0.5 mM) in acetone-d /
3 3
6
(
20) (a) Nazaraly, M.; Wallez, G.; Chan ꢀe ac, C.; Tronc, E.; Ribot, F.;
water (4:1, v/v), the doublets at δ 7.33 and 8.33 due to
BNPP dropped in intensity and concomitantly new sig-
nals at δ 7.01 and 8.15 attributable to p-nitrophenol
Quarton, M.; Jolivet, J.-P. Angew. Chem., Int. Ed. 2005, 44, 5691. (b)
Nazaraly, M.; Quarton, M.; Wallez, G.; Chan ꢀe ac, C.; Ribot, F.; Jolivet, J.-P. Solid
State Sci. 2007, 9, 672.

2
238 Inorganic Chemistry, Vol. 49, No. 5, 2010
Yi et al.
the Ce complex and the Zr and Hf analogues can be
IV
IV
IV
At 25 °C in an acetone-d /HEPES buffer (4:1, v/v) with [Ce]
=
follow pseudo-first-order kinetics with a rate constant of
6
4þ
20[BNPP], the hydrolysis of BNPP by 4 was found to
attributed to the larger size of Ce and higher affinity of
4þ
electron-deficient Ce for the oxygen tripod ligand. The
-
3 -1
(
1.1 ( 0.1) ꢀ 10 s . It is interesting to note that, unlike
hydrolysis of BNPP by LOEtM(NO ) in acetone/water has
3 3
the Ce analogue, the hydrolysis of BNPP by LOEtZr(NO3)3
was found to be decelerated rather than accelerated by the
addition of a HEPES buffer. In the absence of BNPP, 4in an
acetone/HEPES buffer was converted rapidly to 3 along
been investigated. Unlike the Zr and Hf analogues, 4 can only
hydrolyze BNPP in the presence of a HEPES buffer. We have
IV
isolated and structurally characterized the first Ce complexes
with terminal and bridged phosphate diester and hydrogen
phosphate ligands. The LOEt cesium(IV) phosphate diester
complexes 5-7 are stable with respect to hydrolysis in aqueous
IV
with a byproduct, presumably a LOEt-free Ce species.
Because 3 alone cannot hydrolyze BNPP in an acetone/
HEPES buffer, it is reasonable to assume that the active
species for the Ce -mediated hydrolysis is a LOEt-free Ce
complex. Support for this suggestion also comes from the
formation of LOEt-free Ce(PO )(PO H) (H O) from the
reaction of 4 with BNPP in acetone/water.
IV
media, suggesting that they are not involved in the Ce -
IV
IV
mediated hydrolysis of phosphate diesters. It seems likely that
the active species for the Ce-based hydrolysis of BNPP is a
IV
LOEt-free Ce species derived from the ligand-exchange reac-
4
4
0.5
2
2
tion of 4 in acetone/water.
Concluding Remarks
Acknowledgment. Support from the Hong Kong
Research Grants Council is gratefully acknowledged
(Project 601307). We thank Dr. Herman H. Y. Sung for
solving the crystal structures.
In an attempt to model tetravalent Zr, Hf, and Ce ions in
aqueous solutions, LOEtM(NO ) (M = Zr, Hf, Ce) containing
3 3
Kl €a ui’s oxygen tripodal ligand were synthesized and character-
ized. These complexes exhibit different reactivities in aqueous
media. LOEtZr(NO ) is converted to tetranuclear [(L ) Zr -
Supporting Information Available: X-ray crystallographic
data in CIF format for complexes 1-5 and 7, crystal data and
structures for 8 and Ce(PO )(PO H) (H O) , and plots of
3 3
OEt 4
4
4þ
(
μ-O) (μ-OH) (H O) ] or other oligomeric species depend-
2 4 2 2
12,16
4
4
0.5
2
2
ing upon the experimental conditions,
whereas 1 hydro-
I(7.01) versus time for the hydrolysis of BNPP with LOEtM-
(NO ) (M = Zr, Hf, Ce). This material is available free of
charge via the Internet at http://pubs.acs.org.
lyzes to a hydroxyl-bridged dimer. 4undergoes ligand exchange
in acetone/water to give 3. The difference in reactivity between
3
3