its acid dissociation constant (pKa = 7.15) using the Henderson–
Hasselbalch equation.24 For comparison, the rate acceleration
of HPNP hydrolysis by “free” Zn(OAc)2 was also studied. The
results are shown in Fig. 3. The observed rate constants for
HPNP transesterification by the complexes in this study are on
the same order of magnitude as some previous systems studied
under similar conditions.17,24–26 The complex with the asymmetric
ligand has a considerably higher observed rate constant than the
complex with the symmetric ligand. The asymmetric complex also
yields higher rates than the reference reaction with Zn(OAc)2
at equal total concentrations of zinc ions. On the other hand,
the symmetric complex shows less activity than Zn(OAc)2 below
pH 8.5 but higher above, due to precipitation of zinc hydroxide
from Zn(OAc)2 solutions at high pH. The asymmetric complex
shows activity at low pH (< 8) in contrast to the other catalysts
and the uncatalyzed reaction. The transesterification of the HPNP
substrate may be initiated via deprotonation of the substrate by
a free or metal-bound hydroxyl moiety,27,28 and it is likely that
the reason for the higher initial rates observed for the asymmetric
complex is due to the available coordination site in this complex,
which facilitates the binding and deprotonation of water, although
enhancement of the catalysis by the presence of the (coordinatively
unsaturated) tetranuclear complex can not be ruled out (vide
supra). Current studies are directed toward further elucidation of
the observed difference in catalytic activity and the possible effect
that the different electronic influences of the ligand environment
on the reactivities for the two types of complexes.
1H, 3J = 4.8 Hz), 8.25 (t, 1H, 3J = 7.9 Hz), 7.78 (t, 1H, 3J = 7.8 Hz), 7.73
(t, 1H, 3J = 6.8 Hz), 7.61 (d, 1H, 3J = 8.0 Hz), 7.34 (dd, 1H, 3J = 5.3 Hz,
3J = 7.1 Hz), 7.25 (d, 1H, 3J = 7.8 Hz), 6.93 (s, 2H), 4.35 (s, 2H, br), 4.18
3
(s, 2H, br), 4.09 (s, 2H), 3.83 (s, 2H), 3.69 (s, 2H), 3.68 (sept, 1H, J =
6.6 Hz), 2.14 (s, 3H), 1.44 (d, 6H, 3J = 6.6 Hz).
Na3BCPMP·4NaOH Elem. Anal. Calc. C25H29N4Na7O9: C, 43.49; H, 4.23;
N, 8.11; Found: C, 44.029; H, 3.814; N, 8.133; IR (KBr) cm−1: 1586(s, –
−
−
CO2 asym.); 1471(m, aromatic C–C); 1442(m); 1412(m, –CO2 sym.);
1
1331(m); 866(w, aromatic C–H bend); 767(m, aromatic C–H bend); H-
NMR (500 MHz) CD3OD d 2.15 (s, 3 H), 3.07 (s, 4 H), 3.56 (m, 4H), 3.65
(m, 4 H), 6.77 (s, 2 H), 7.19 (t, 3J = 6.3 Hz, 2 H), 7.26 (d, 3J = 8 Hz, 2 H),
7.64 (dt, 3J = 7.5 Hz, 2 H), 8.47 (d, 3J = 5 Hz, 2 H).
†† 1: Na3[Zn2(BCPMP)(OAc)2]2PF6·6H2O Elem. Anal. Calc. C58H74F6-
N8Na3O24PZn4: C, 39.97; H, 4.28; N, 6.43; Found C, 40.2; H, 4.4; N, 6.5;
IR (KBr)/cm−1: 2920(w); 1601(s, asym. –CO2); 1590(s); 1477(m); 1418(m,
sym. –CO2); 1397(m, sym. –CO2); 838(s); UV/Vis (acetonitrile)/nm:
234(sh); 260; 298; FAB+ MS: m/z (64Zn) 753 ([Zn2(BCPMP)(OAc)2]−
+
2Na+); 671 ([Zn2(BCPMP)(OAc)] + Na+); 589 ([Zn2(BCPMP)]+;
2: [{Zn2(IPCPMP)(OAc)}2](PF6)2 Elem. Anal. Calc. C56H66F12N8O10P2-
Zn4: C, 43.04; H, 4.26; N, 7.17; Found C, 42.8; H 4.4; N, 7.0 IR
(KBr)/cm−1: 2964(w); 2921(w); 1609(s, asym. –CO2); 1598(s, asym. –CO2);
1562(m); 1486(m); 1435(m, sym. –CO2); 1414(m, sym. –CO2); 844(s);
559(m); UV/vis (acetonitrile)/nm: 234(sh); 260; 298; FAB+ MS: m/z
(64Zn) 829 ([Zn2(IPCPMP)(OAc)2] + NB+, NB+ = 3-nitrobenzyl cation,
matrix); 633 ([Zn2(IPCPMP)(OAc)]+); 619 ([Zn2(IPCPMP)(O2CH)]+); 591
([Zn2(IPCPMP)(OH)]+); 1411 ([{Zn2(IPCPMP)(O2CCH3)}2]2+.PF6−);
3: [{Zn2(IPCPMP)(O2CC(CH3)3}2](PF6)2·2H2O·2C2H5OH Elem. Anal.
Calc. C64H86F12N8O12P2Zn4: C, 44.93; H, 5.07; N, 6.55; Found C,
44.5; H, 5.1; N, 6.4; IR (KBr)/cm−1: 2970(w); 2921(w); 2871(w);
1609(s, asym. –CO2); 1562(m); 1482(m); 1442(w); 1418(m); 844(s);
553(m); UV/Vis (acetonitrile)/nm: 233(sh); 261; 298; FAB+ MS: m/z
(64Zn) 675 ([Zn2(IPCPMP)(O2CC(CH3)3]+); 1495 ([{Zn2(IPCPMP)(O2CC-
−
(CH3)3)}2]2+PF6
)
‡‡ Diffraction data was collected at 120 K on a Bruker AXS BV
CCD diffractometer. The structures were solved using direct methods
(SHELXS) and refined by full-matrix least squares against F2. 1:
Na3[Zn2(BCPMP)(OAc)2]2PF6 ·6H2O; The Na2, O13, and O14 atoms are
disordered over two sites with equal occupancies. All hydrogens connected
to the Na bound oxygens have been omitted. Other hydrogen atoms were
positioned geometrically and were also constrained to ride on their parent
atoms. C58H66F6N8Na3O24PZn4, M = 1734.61, monoclinic, P21/c, a =
◦
˚
˚
˚
19.5230(5) A, b = 10.1399(2) A, c = 18.1745(5) A, b = 90.181(3) ,
3
˚
V = 3597.83(15) A , Z = 2, T = 120(2)K, data/restraints/parameters
7067/12/487, R(int) = 0.0367, R1 = 0.0653, wR2 = 0.1736 (observed
data).
3: [Zn2(IPCPMP)(O2CC(CH3)3]PF6H2O·C2H5OH The idealized positions
of the H2O hydrogens were estimated with HYDROGEN (Nardelli,
1999) program and constrained to ride on their parent atom. These
hydrogen atoms were disordered over two sites with equal occupancies.
The OH hydrogen atom was located from the difference Fourier map
but constrained to ride on its parent atom. Other hydrogen atoms were
positioned geometrically and also constrained to ride on their parent
atoms. Hydrogen atoms were positioned geometrically and refined using
the riding model. C64H86F12N8O12P2Zn4, M = 1710.83, monoclinic C2/◦c,
Fig. 3 A plot of kobs vs. pH for the hydrolysis/transesterification of HPNP
by complexes 1 and 2 as well as Zn(OAc)2 in buffered H2O–MeCN (1 :
1, v/v) solution at 25 ◦C. -·- ꢀ-·- H4IPCPMP(PF6)2 + 2Zn(OAc)2; ---᭺---
Na3BCPMP + 2Zn(OAc)2; –ꢁ– Zn(OAc)2; · · ·ꢀ· · · uncatalyzed.
˚
˚
˚
a = 24.5080(5) A, b = 14.4476(3) A, c = 21.2277(3) A, b = 93.3150(10) ,
3
˚
V = 7503.8(2) A , Z = 4, T = 120(2)K, data/restraints/parameters =
8289/29/479, R(int) = 0.375, R1 = 0.0453, wR2 = 0.1145 (observed
data). See ESI§ for crystallographic details for 2. CCDC reference numbers
659750, 659751 and 667079. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b713664a
We would like to thank the Swedish Research Council (VR)
for financial support and Dr Einar Nilsson for help with the
measurement of mass spectra. This research has been carried
out within the framework of the International Research Training
Group Metal Sites in Biomolecules: Structures, Regulation and
Mechanisms (www.biometals.eu).
§§ Ionic strength and pH were kept constant by using total concentrations
of 0.1 M of NaClO4 and 0.01 M buffer (pH 7.0–7.5 MOPS, pH 8.0–8.5
TRIS, pH 9.0–9.5, pH 10.0 CAPS).
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45, 4546–4550.
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Notes and references
* Analytical data: H4IPCPMP(PF6)2·H2O Elem. Anal. Calc. C26H36F12-
N4O4P2: C, 41.17; H, 4.78; F, 30.06; N, 7.39; Found: C, 41.2; H, 4.8; F,
29.9; N, 7.2; IR (KBr)/cm−1: 1710(s, C O of –CO2H); 1615(m), 1540(w),
=
=
1490(m, aromatic C C); 1241(m, C–OH of –CO2H); 1227(m); 844(s)
(aromatic C–H bending); 558(s); UV/vis (acetonitrile)/nm: 220(sh); 260;
1
3
287; H-NMR (500 MHz) CD3CN d 8.48 (d, 1H, J = 5.8 Hz), 8.46 (d,
This journal is The Royal Society of Chemistry 2008
Dalton Trans., 2008, 993–996 | 995
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