Transesterification vs hydrolysis: The reactivity of metal-bound hydroxo moiety in [TPANi(II)(μ-OH)2Ni(II)TPA](ClO4)2 (TPA = tris(2-pyridylmethyl)amine)

Article abstract of DOI:10.1246/cl.2000.372

In the reaction of triphenyl phosphate with ethanol in the presence of complex 1, the transesterification to give diethyl phenyl phosphate was observed, in which system direct nucleophilic attack of ligated hydroxo moiety in 1 toward triphenyl phosphate a

Full text of DOI:10.1246/cl.2000.372

372
Chemistry Letters 2000
Transesterification vs Hydrolysis: the Reactivity of Metal-Bound Hydroxo Moiety in
[TPANi(II)(µ-OH)2Ni(II)TPA](ClO4)2 (TPA = tris(2-pyridylmethyl)amine)
Masami Ito,* Hiroyuki Kawano, Takashi Takeuchi, and Yu-saku Takita
Research and Development Center, Oita University,700 Dannoharu, Oita, 870-1192.
(Received December 20, 1999; CL-991079)
In the reaction of triphenyl phosphate with ethanol in the
presence of complex 1, the transesterification to give diethyl
phenyl phosphate was observed, in which system direct nucle-
ophilic attack of ligated hydroxo moiety in 1 toward triphenyl
phosphate also proceeded to give µ-phosphate complex, 2.
proposed in the reaction of complex 1 with carboxylic acid ester
under the presence of exogenous ethanol.9,12 However the possi-
biliy of the presence of other reactive species such as
(alkoxo)Ni(II) complex can not be excluded at the present stage.
To elucidate how the conformation of the nickel(II) com-
plex was changed after the reaction, the formed complex was
isolated and characterized from the reaction mixture. The sol-
vent of the reaction mixture was removed in vacuo. The
remained solid was washed by Et2O to remove excess phosphate
ester and dissolved in MeCN again. The blue purple single crys-
tals suitable for the X-ray measurement could be obtained by
Et2O slow diffusion to this MeCN solution. The X-ray crystal-
lography indicates the dinuclear nickel(II) bis(µ-phosphato) com-
plex, [TPANi(II)(OP(OPh)2O)2Ni(II)TPA](ClO4)2 (2) (isolated
yield ca. 85%).13,14 The structure shows center of imposed
symmetry (Figure 1). Each nickel(II) ion adopts a slightly dis-
Hydrolytic enzymes such as phosphatases or nucleases are
known to catalyze the hydrolysis of phosphate ester.1 Many of
these enzymes contain metals such as zinc at the active sites.
The catalytic role of zinc is ascribed to the binding and activa-
tion of substrates, whereas deprotonation of coordinated water
to produce a nucleophilic zinc hydroxide is also proposed as an
essential catalytic hydrolysis function. On the other hand, one
of these zinc containing enzymes such as alkaline phosphatase
can perform transesterification in the presence of exogenous
alcohol.2-4 The reaction mechanism of transesterification cat-
alyzed by this enzyme was not elucidated well at the present
stage. While some model studies of intra-molecular transesteri-
fication by metal complexes have been reported,5-7 the model
study by use of exogenous alcohol is scarcely known.8,9
Recently, we reported the transesterification of carboxylic
acid ester by (hydroxo)dinickel(II) complex, [TPANi(II)(µ-
OH)2Ni(II)TPA](ClO4)2 (TPA = Tris(2-pyridylmethyl)amine)
(1)10 in the presence of exogenous alcohol.9 In this system,
hydrogen-bonded alcohol to the metal-bound hydroxo moiety
was the most plausible nucleophile for the transesterification
toward carboxylic acid ester.
Here we will present the reaction of complex 1 with phos-
phate ester such as triphenyl phosphate under the presence of
external alcohol to investigate how the transesterification of
phosphate ester proceeds in this system.
Triphenyl phosphate (1 mmol) was treated with excess
ethanol (5 mmol) in the presence of 1 (0.05 mmol) in MeCN (5
ml) at room temperature. After stirring for 12 h, the color
change from blue to purple was noted, indicating the shift of d-
d transition band of nickel(II) center, hence conformational
change of complex 1. The GC and GC-MS analysis of this
reaction mixture turned out the formation of diphenyl ethyl
phosphate (yield 150% comparison to the complex 1). When
two equiv of triphenyl phosphate (0.1 mol) was added to the
reaction system, the formation of diphenyl ethyl phosphate
(yield 75%) and further reacted product, diethyl phenyl phos-
phate (yield 30%) were observed under the same reaction con-
dition.11 These results indicate that this system can perform not
only the transesterification of triphenyl phosphate but also the
transesterification of diphenyl ethyl phosphate. While reaction
mechanism of this system is not completely clear at the present
stage, we supposed that hydrogen-bonded alcohol to the metal-
ligated hydroxo moiety can be the good candidate for nucle-
ophilic attack to transesterificate phosphate ester as previously
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
373
torted octahedral coordination environment with diphosphate
anions bridging in syn-anti mode. The intra-ring O(1')-P(1)-
O(2) angle was expanded by ca. 10 ° from the ideal value to
122.2(2)°, whereas the other angles around the phosphorus
atom only deviate slightly from tetrahedral (O(3)-P(1')-O(4'),
104.7(2)°). The distortion around the phosphoryl center
appears to derive from the bridging bidentate coordination
mode to the two nickel atoms. A similar expansion has been
already found in other dimetallic complexes containing µ-η2
phosphate ester ligands.15,16 To our knowledge, 2 is the first
nickel(II) complex which possesses two phosphate esters as (µ-
η2)2 binding mode with no additional ligand between the two
metals,15 while similar bridging mode of phosphate esters has
been already reported in dinuclear zinc complex.16 The phos-
phate diesters in 2 are quite stable, being ineffective for further
hydrolytic reaction by 1 and transesterification by 1 in the pres-
ence of ethanol.17
purified by alumina column chromatography with methyl-
ene chloride elutant, respectively. The transesterificated
products were identified by comparison of their GC reten-
tion times and MS spectra and quantified by GC-FID.
12 The dissociation of dinuclear complex 1 to afford
monomeric (hydroxo)Ni(II) complex was suggested by the
kinetic experiment of hydrolysis reaction of p-nitrophenyl
acetate by complex 1 in acetonitrile, see ref. 9. However
we have not had concrete evidence for that dissociation
reaction yet.
13 Anal. Found C, 51.40; H, 4.06; N, 11.34%. Calcd for 2
Ni2C60N8H28O8P2Cl2: C. 51.66; H. 4.01; N. 11.03%. IR
(KBr, cm-1), ν(C=C) 1603, ν(P-O) 1281, ν(ClO4), 1130.
UV-vis (nm; ε/M-1cm-1) 542 (80), 772 (45), 890 (95).
14 Crystal data for 2, Mr = 697.7; 2 crystallized in the mono-
clinic space system with P21/c, a = 11.193(8), b =
13.070(3), c = 21.069(4) Å, β = 101.90(4)°, V = 3016(2)
Å3, Z = 2, Dc = 1.536 g cm-3, µ(Mo-K) = 0.843 mm-1. The
R(Rw) value is 5.9(5.6)% for 3894 reflections (3°< θ <55°,
Fo>3σ(Fo)). The X-ray data collection was carried out at
room temperature. The structure was solved by direct
methods (MITHRIL) and refined by the full matrix least
squares techniques with TEXSAN. All non-hydrogen
atoms were refined anisotropically, and they were refined
isotropically. Hydrogen atoms were calculated and fixed
in final refinement cycles.
The reaction pathway to afford 2 seems to limit the catalyt-
ic activity for the transesterification. To inhibit the direct
nucleophilic attack toward the phosphate ester by the metal-lig-
ated hydroxo moiety and improve the activity of this transester-
ification system, a synthesis of hydroxo complex with newly-
designed sterically hindered ligand is now in progress.
We are grateful to Prof. M. Akita and Prof. Y. Moro-oka
(Tokyo Institute of Technology) for the X-ray measurement.
15 a) J. W. Yun, T. Tanase, and S. J. Lippard, Inorg. Chem.,
35, 7596 (1996). b) T. Tanase, S. P. Watton, and S. J.
Lippard, J. Am. Chem. Soc., 116, 9401 (1994). c) T.
Tanase, J. W. Yun, and S. J. Lippard, Inorg. Chem., 35,
3585 (1996). d) W. H. Armstrong, and S. J. Lippard, J.
Am. Chem. Soc., 107, 3730 (1985). e) P. N. Turowski, W.
H. Armstrong, M. E. Roth, and S. J. Lippard, J. Am. Chem.
Soc., 33, 636 (1994). f) B. Krebs, K. Schepers, B. Bremer,
G. Henkel, E. Althaus, W. Muller-Warmuth, K. Griesar,
and W. Haase, Inorg. Chem., 33, 1907 (1994). g) H. G.
Jang, M. P. Hendrich, and L. Que, Jr., Inorg. Chem., 32,
911 (1993). h) M. Mahroof-Tahir, K. D. Karlin, and J.
Zubieta, Inorg. Chim. Acta, 207, 135 (1993). i) S. Hikichi,
M. Tanaka, Y. Moro-oka, and N. Kitajima, J. Chem. Soc.,
Chem. Commun., 1992, 814. j) S. Drueke, K. Wieghardt,
B. Nuber, J. Weiss, H.-P. Fleishhauser, S. Gehring, and W.
Hasse, J. Am. Chem. Soc., 111, 8622 (1989). k) R. E.
Norman, S. Yan, L. Que, Jr.,G. Backs, J. Ling, J. Sanders-
Loehr, J. H. Zhang, and C. J. O'Conner, J. Am. Chem. Soc.,
112, 1554 (1990). l) K. Schepers, B. Bremer, B. Krebs, G.
Henkel, E. Althaus, B. Mosel, and W. Muller-Warmuth,
Angew. Chem., Int. Ed. Engl., 29, 531 (1990).
References and Notes
1
N. W. Lipscomb and N. Strãter, Chem. Rev., 96, 2375
(1996), and references therein.
2
3
D. E. Wilcox, Chem. Rev., 96, 2435 (1996).
I. B. Wilson, J. Dayan, and J. K. Cyr, J. Biol. Chem., 239,
4182 (1964).
4
5
M. L. Applebury and J. E. Coleman, J. Biol. Chem., 244,
308 (1969).
a) P. Molenveld, S. Kapsabeils, J. F. J. Engberson, and D.
N. Reinhoudt, J. Am. Chem. Soc., 119, 2949 (1997). b) P.
Molenveld, J. F. J. Engberson, H. Kooijman, A. L. Spek,
and D. N. Reinhoudt, J. Am. Chem. Soc., 120, 6726 (1998).
c) N. H. Williams and J. Chin, J. Chem. Soc. Chem.
Commun., 1996, 131.
6
a) T. Koike, S. Kajitani, I. Nakamura, E. Kimura, and M.
Shiro, J. Am. Chem. Soc., 117, 1210 (1995). b) E. Kimura,
Y. Kodama, T. Koike, and M. Shiro, J. Am. Chem. Soc.,
117, 8304 (1995).
7
8
9
D. S. Sigman and C. T. Jorgensen, J. Am. Chem. Soc., 94,
1724 (1972).
D. Herschlag and W. P. Jencks, J. Am. Chem. Soc., 109,
4665 (1987).
M. Ito, K. Sakai, T. Tsubomura, and Y. Takita, Bull. Chem.
Soc. Jpn., 72, 239 (1999).
16 C. Bazzicalupi, A. Bencini, A. Bianchi, V. Fusi, C. Giorgi,
P. Paoletti, B. Valtancoli, and D. Zanchi, Inorg. Chem., 36,
2784 (1997).
10 M. Ito and Y. Takita, Chem. Lett., 1996, 929.
17 It is supposed that the added alcohol itself doesn't have any
effect to promote the formation of 2 by the treatment of 1
with triphenyl phosphate, since complex 2 can be obtained
quantitatively by the reaction of 1 with triphenyl phosphate
in MeCN even in the absence of alcohol.
11 The authentic samples of diphenyl ethyl phosphate and
diethyl phenyl phosphate were readily prepared by the
reaction of chlorophosphoric acid diphenyl ester and
dichlorophosphoric acid phenyl ester with ethanol and