Enhanced hydrolytic activity of Cu(II) and Zn(II) complexes in highly cross-linked polymers

Article abstract of DOI:10.1039/b605676h

The chelate ligand tris[(1-vinylimidazol-2-yl)methyl]amine (5) was synthesized in five steps from commercially available starting materials. Upon reaction with ZnCl₂ or CuCl₂ in the presence of NH ₄PF₆, the complexes [Zn(5)Cl]PF₆ (6) and [Cu(5)Cl]PF₆ (7) were obtained. The structure of both complexes was determined by single-crystal X-ray crystallography. Immobilization of 6 and 7 was achieved by co-polymerization with ethylene glycol dimethacrylate. The supported complexes P6-Zn and P7-Cu were found to be efficient catalysts for the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) at 50 °C. At pH 9.5, the heterogeneous catalyst P7-Cu was 56 times more active than the homogeneous catalyst 7. Partitioning effects, which increase the local concentration of BNPP in the polymer, are shown to contribute to the enhanced activity of the immobilized catalyst. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605676h

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www.rsc.org/dalton | Dalton Transactions
Enhanced hydrolytic activity of Cu(II) and Zn(II) complexes in highly
cross-linked polymers
Alexander Schiller, Rosario Scopelliti and Kay Severin*
Received 20th April 2006, Accepted 26th June 2006
First published as an Advance Article on the web 10th July 2006
DOI: 10.1039/b605676h
The chelate ligand tris[(1-vinylimidazol-2-yl)methyl]amine (5) was synthesized in five steps from
commercially available starting materials. Upon reaction with ZnCl2 or CuCl2 in the presence of
NH4PF6, the complexes [Zn(5)Cl]PF6 (6) and [Cu(5)Cl]PF6 (7) were obtained. The structure of both
complexes was determined by single-crystal X-ray crystallography. Immobilization of 6 and 7 was
achieved by co-polymerization with ethylene glycol dimethacrylate. The supported complexes P6-Zn
and P7-Cu were found to be efficient catalysts for the hydrolysis of bis(p-nitrophenyl)phosphate
(BNPP) at 50 ◦C. At pH 9.5, the heterogeneous catalyst P7-Cu was 56 times more active than the
homogeneous catalyst 7. Partitioning effects, which increase the local concentration of BNPP in the
polymer, are shown to contribute to the enhanced activity of the immobilized catalyst.
a 1 : 1 ratio;7 (b) its nitrogen donors are of the imidazolyl type
Introduction
mimicking the histidine side chains found in the binding sites
Transition-metal complexes with multidentate N-donor chelate
of many hydrolytic metalloenzymes; (c) it contains three vinyl
ligands have been studied extensively as mimics for hydrolytic
metalloenzymes.1 Most efforts have focused on homogeneous,
groups and co-polymerization is expected to proceed with a high
incorporation efficiency; (d) ligand 5 can be prepared easily in a
low molecular weight catalysts but immobilized metal complexes
few steps from commercially available starting materials, which
are increasingly being investigated as potential alternatives.2,3
Advantages of supported catalysts are the ease of separation and
allows to generate larger quantities.
The procedure for the synthesis of ligand 5 is outlined in
the possibility to recycle the catalyst as well as reduced problems
Scheme 1. 1-Methylimidazole-2-carbaldehyde was known to be an
excellent synthon for the preparation of polyimidazole ligands.8a,b,d
Consequently, we synthesized the corresponding polymerizable
building block 1-vinylimidazole-2-carbaldehyde (1), which was
then transformed to 1-vinylimidazole-2-carbaldehyde oxime (2)
and reduced with zinc under alkaline conditions9 to obtain 1-
vinylimidazole-2-methylamine (3). The Schiff-base intermediate
of 1 and 3 was reduced in situ with NaBH4 affording bis[(1-
vinylimidazol-2-yl)methyl]amine (4). A reductive amination re-
action with 1 and 4 gave the crude product 5. Purification of
with catalyst solubility and aggregation. The latter point is par-
ticularly evident for hydrolytically active Cu(II) complexes, which
are known to form catalytically inactive hydroxy-bridged dimers
[LnCu(l-OH)2CuLn]2+ (Ln = multidentate N-donor ligand).4,5 The
supporting polymeric matrix may also be used to achieve some
structural control over the first and/or second coordination
sphere of the metal binding site using the technique of molecular
imprinting.3,6
In the following, we describe the synthesis and the character-
ization of immobilized Cu(II) and Zn(II) complexes. They were
obtained by co-polymerization with ethylene glycol dimethacry-
late (EGDMA) using a vinyl-substituted, tetradendate N-chelate
ligand. In hydrolysis reactions of the activated phosphodiester
bis(p-nitrophenyl)phosphate (BNPP), the polymeric catalysts were
found to be up to 56 times more active than the corresponding
homogeneous catalysts. Partitioning effects, which increase the
local concentration of the substrate in the polymer, are shown to
contribute to the enhanced activity of the immobilized catalyst.
Results and discussion
Synthesis of the polymerizable chelate ligand 5
For our investigations, we first synthesized the chelate ligand 5.
This ligand was chosen due to several reasons: (a) tetradendate
polyamine ligands are known to form stable metal complexes in
Scheme 1 Synthesis of the polymerizable chelate ligand 5. Reagents and
conditions: (i) n-BuLi, DMF, Et2O, 75%; (ii) NH2OH, EtOH–H2O, 87%;
(iii) Zn, NH3 aq, NH4OAc, EtOH, 88%; (iv) NaBH4, MeOH, 79%; (v) 1,
NaBH3CN, MeOH, 46%.
´
Institut des Sciences et Inge´nieries Chimiques, Ecole Polytechnique Fe´de´rale
de Lausanne (EPFL), 1015, Lausanne, Switzerland. E-mail: kay.severin@
epfl.ch; Fax: +41 21 693 93 05; Tel: +41 21 693 93 02
3858 | Dalton Trans., 2006, 3858–3867
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ligand 5 was achieved by synthesizing the zinc- or copper-complex
[M(5)Cl]PF6 (M = Zn (6) or Cu (7)) (see below) followed by
cleavage of the metal with HClconc. After adjusting the pH to ∼14,
the free base ligand was extracted with CHCl3 to obtain the pure
product in moderate yield.
were characterized by mass spectrometry, UV-Vis-near-IR spec-
troscopy, IR spectroscopy, elemental analyses and single-crystal
X-ray crystallography (Fig. 2 and 3). In addition, complex 6 could
be investigated in solution by NMR spectroscopy. The 1H and 13
C
NMR spectra of 6 in CD3CN exhibited only one set of signals
in accordance with a C3 symmetrical structure of the cation. It is
1
The H and 13C NMR spectra of 5 were in accordance with a
C3 symmetrical structure and showed similar chemical shifts as
interesting to note that the pronounced upfield shift, which was
1
tris[(1-methylimidazol-2-yl)methyl]amine.10 A pronounced differ-
found for the H NMR signal of the CH CH2 protons of ligand
=
ence in chemical shift compared to the precursor 4 was found
5 (d 6.08 ppm), was no longer observed for the Zn complex 6
1
=
for the H NMR signal of the CH CH2 vinyl protons of 5, for
which an upfield shift of 1.0 ppm was observed. This could be
explained by ring current effects of the adjacent imidazole groups.
Single crystals of ligand 5 were analyzed by X-ray diffraction. In
the solid state, the ligand shows a concave, pseudo-C3 symmetrical
=
structure (Fig. 1). As indicated by the NMR spectra, the CH CH2
protons are in close proximity to the imidazole rings. The C–N(1)
˚
distances (1.492(3)–1.495(3) A) and C–N(1)–C angles (111.8(2)–
111.9(2)◦) are comparable to what has been observed for other
trisimidazole ligands.11
Fig. 2 Graphic representation of the molecular structure of the cation
of complex 6 in the crystal. The counter ani◦on is omitted for clarity.
˚
Selected bond lengths, distances (A) and angles ( ): Zn(1)–N(2) 2.0214(18),
Zn(1)–N(4) 2.0227(18), Zn(1)–N(6) 2.0269(16), Zn(1)–Cl(1) 2.2625(5),
Zn(1) · · · N(1) 2.5676(15); N(2)–Zn(1)–N(4) 110.68(7), N(2)–Zn(1)–N(6)
117.16(7), N(4)–Zn(1)–N(6) 106.85(7), N(2)–Zn(1)–Cl(1) 103.21(5),
N(4)–Zn(1)–Cl(1) 111.56(5), N(6)–Zn(1)–Cl(1) 107.36(5).
Fig. 1 Graphical representation of the molecular structure of ligand 5 in
the crystal.
Synthesis of the metal complexes 6 and 7
The transition-metal complexes 6 and 7 were prepared by mixing
stoichiometric quantities of ligand 5 with the corresponding metal
chloride in methanol (Scheme 2). The products precipitated upon
addition of NH4PF6 to the reaction mixtures. Both complexes
Fig.
3
Graphic representation of the molecular structure of the
cation of complex 7 in the crystal. The counter an◦ion is omitted
˚
for clarity. Selected bond lengths (A) and angles ( ): Cu(1)–N(1)
2.176(3), Cu(1)–N(2) 2.001(3), Cu(1)–N(4) 2.118(3), Cu(1)–N(6) 2.002(3),
Cu(1)–Cl(1) 2.2367(11); N(2)–Cu(1)–N(6) 130.76(13), N(2)–Cu(1)–N(4)
110.90(12), N(6)–Cu(1)–N(4) 107.79(12), N(2)–Cu(1)–N(1) 79.00(12),
N(6)–Cu(1)–N(1) 80.20(12), N(4)–Cu(1)–N(1) 78.35(11), N(2)–Cu(1)–
Cl(1) 98.03(10), N(6)–Cu(1)–Cl(1) 100.29(9), N(4)–Cu(1)–Cl(1) 104.91(9),
N(1)–Cu(1)–Cl(1) 176.28(8).
Scheme 2 Synthesis of the transition-metal complexes 6 and 7.
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(d 6.91 ppm). The coordination of 5 to Zn(II) prevents the orien-
=
tation of the CH CH2 protons towards the aromatic imidazole
groups.
UV-Vis-near-IR spectra of 6 and 7 have been recorded in
acetonitrile. As expected, complex 6 showed no bands in the
region from 250–1100 nm. Complex 7 displayed an intense
band at 290 nm (e = 5.8 × 103 dm3 mol−1 cm−1) which can
be assigned to intraligand and metal-to-ligand charge transfer
(LMCT) transitions.12 Furthermore, broad bands of low intensity
were found at 816 nm (e = 87 dm3 mol−1 cm−1) and at ∼1100 nm
(e ∼100 dm3 mol−1 cm−1) which is typical for a trigonal bipyramidal
copper(II) complex.8,13
The structures of the isomorphous complexes 6 and 7 in the
crystal are depicted in Fig. 2 and 3. Both complexes contain
mononuclear cations of the formula [M(5)Cl]+. Interesting differ-
ences, however, are observed for the distances of the metal atoms to
the bridgehead sp3-nitrogen atom N(1) of the ligand. The value for
Fig. 4 Graphic representation of the molecular structure of the cation
of complex 8 in the crystal. The hydrogen atoms, the counter anion and
the solvent molecules are omitted for clarity. Only one orientation of the
disordered p-nitrophenolate moieties is shown.
˚
the Zn complex 6 of Zn(1)–N(1) 2.5676(15) A points to a very weak
interaction and the overall geometry is best described as distorted
tetrahedral (Fig. 2). This is in agreement with what has been
observed for zinc complexes with tris(imidazolylmethyl)amine
ligands.14 For the Cu complex 7, however, a value of Cu(1)–N(1)
Immobilization of the metal complexes 6 and 7
˚
2.176(3) A is observed. The geometry of the latter is therefore
Radical initiated (AIBN) co-polymerization of 6 and 7 (each
2.5 mol%) with the crosslinker EGDMA (97.5 mol%) in ace-
tonitrile at 65 ◦C gave the insoluble polymers P6-Zn (colorless)
and P7-Cu (pale green) in high yield (94 and 90%) (Scheme 4).
The polymers were ground, washed with CH3CN, and dried. The
metal content was found to be 5.06 mg Zn/g polymer for P6-Zn
and 3.22 mg Cu/g polymer for P7-Cu as determined by ICP-
OES. These values corresponded to an incorporation rate of 65%
in the case of P6-Zn and 42% for P7-Cu. The moderate incorpora-
tion rate suggests that some of the vinyl groups of the ligand were
not polymerized. It was therefore expected that metal complexes
with one, two and three connections to the polymer backbone can
be found in the polymer. The attachment to the polymer backbone
via one vinyl side chain as shown in Scheme 4 represents just one
possible—but statistically very likely—situation. The complexes
6 and 7 showed a very different behavior in the polymerization
process. While complex 6 co-polymerized with EGDMA in the
typical fashion,16 the polymerization of 7 was slow and resulted in a
glassy green co-polymer. An explanation for the different behavior
could be that the Cu complex participates in atom transfer radical
best described as distorted trigonal bipyramidal (Fig. 3).8,13 The
distances observed for the bonds between the metal ions and the
imidazole nitrogen atoms N(2), N(4) and N(6) (M–N 2.001(3)–
˚
˚
2.118(3) A) or the chloro atoms (Zn(1)–Cl(1) 2.2625(5) A; Cu(1)–
7
˚
Cl(1) 2.2367(11) A) were all within the expected range.
In anticipation of the hydrolysis studies, we have investigated
the coordination chemistry of the Cu complex 7 with the substrate
BNPP. Upon reaction of complex 7 with the silver salt of BNPP in
acetonitrile followed by removal of AgCl, we were able to obtain
complex 8 (Scheme 3).
Scheme 3 Synthesis of complex 8.
Complex 8 was comprehensively characterized including a
single-crystal X-ray analysis (Fig. 4). The quality of the latter was
unfortunately not very high due to the disordered p-nitrophenolate
moieties and a discussion of bond lengths and angles is therefore
not presented. From the data it is clear, however, that the BNPP
anion acts as a monodentate ligand and that the geometry of
the Cu complex is similar to what has been found for the chloro
complex 7 (Fig. 3). It should be noted that 8 represents one of the
few Cu(II)–BNPP complexes known so far.15
Scheme 4 Synthesis of the zinc(II)- and copper(II)-containing polymers
P6-Zn and P7-Cu. Complexes with a two- or threefold attachment to the
polymer backbone are not shown.
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polymerization (ATRP) reactions. In fact, structurally related
tripodal copper catalysts have been used for atom transfer radical
polymerization reactions (ATRP).17
Whereas the initial rate of hydrolysis at pH 9.5 in the presence
of the homogeneous Zn complex 6 was found to be 7.6 ×
10−6 mM min−1, a rate of 1.6 × 10−4 mM min−1 was observed
for reactions with the polymer P6-Zn. This corresponds to a
rate enhancement of a factor of 21. An even more pronounced
difference was found for the Cu complexes. The initial rates
for reactions with 7 and P7-Cu were 2.7 × 10−5 and 1.5 ×
10−3 mM min−1, respectively. The immobilized Cu complex P7-
Cu was thus 56 times more active than the homogeneous complex
7. Hydrolysis of the reaction product p-nitrophenyl phosphate
(NPP) in the presence of P7-Cu was approximately two orders
of magnitude slower than that of BNPP. For the experiments
described above, consecutive hydrolysis reaction can therefore be
neglected.
The BET surface area of P6-Zn was determined to be 429 m2 g−1
and that of P7-Cu 416 m2 g−1. These values are comparable to
those observed for other highly cross-linked EGDMA polymers
prepared with the porogen CH3CN.18 Despite the similar surface
area, the BET isotherms of the polymers P6-Zn and P7-Cu showed
pronounced differences. While P7-Cu displayed a hysteresis indi-
˚
cating mesopores with an average pore diameter of 57 A, P6-Zn
showed no hysteresis at all indicating the absence of detectable
mesopores.
Hydrolysis reactions
The hydrolysis of BNPP promoted by 7 and P7-Cu was
investigated in more detail. The effect of pH on the hydrolytic
activity was studied in the pH range 7.0–11.0. Stable complex
formation for both compounds was confirmed by the absence of
precipitation of metal hydroxide at high pH. The pH vs. initial
rates profiles are shown in Fig. 6. For P7-Cu, a strong increase
of the initial rates of nearly one order of magnitude was observed
in the pH range 7.0–9.5. This pH dependency can be explained
by the assumption that the deprotonation of a coordinated water
molecule is necessary to generate the catalytically active species.21
The sigmoidal curve of P7-Cu was fitted with the Boltzmann model
to give a kinetically determined pKa value of 8.6 0.1. A plot of
the rate enhancements as a function of pH shows that the largest
difference between homogeneous and heterogeneous catalysts are
found at a pH of approximately 9.5 (Fig. 7).
To evaluate the catalytic activity of the complexes 6 and 7
and the corresponding polymers P6-Zn and P7-Cu, we inves-
tigated the hydrolysis of the activated phosphodiester bis(p-
nitrophenyl)phosphate (BNPP). This substrate is frequently used
as a model for biologically relevant phosphodiesters such as
DNA and RNA. Advantages and drawbacks of activated model
substrates have been discussed by Breslow and Singh19 and Menger
and Ladika.20
The reactions were performed in buffered aqueous solution
(H2O–DMSO 95 : 5) at 50 1 ◦C. In the case of the heterogeneous
hydrolysis reactions, the amount of polymer that was employed
corresponds to a metal ion concentration of 1.0 mM if the
polymers would be completely soluble in the buffer. After an
incubation time of 30 min, the reactions were initiated by addition
of a freshly prepared stock solution of the substrate BNPP (final
conc.: 5.0 mM). After a given time, the reaction tubes were
centrifuged and the concentration of the product p-nitrophenolate
in the clear supernatant was determined by UV/Vis spectroscopy
at k = 400 nm. The initial rate was determined by a plot of
p-nitrophenolate concentration vs. time. The initial rate of the
spontaneous hydrolysis was substracted.
The complexes 6 and 7 and their immobilized versions P6-Zn
and P7-Cu displayed very different hydrolytic activities (Fig. 5).
Fig. 6 Plot of the initial rates of BNPP hydrolysis reactions mediated by
7 (᭺) or P7-Cu (᭹) as a function of pH. Reaction conditions: [Cu2+] =
1.0 mM, [BNPP] = 5.0 mM, H2O–DMSO 95 : 5, 64 mM buffer, 50
1
◦C. The following buffers were employed: HEPES (pH 7.0–8.0), CHES
(pH 8.5–10.0), and CAPS (pH 10.5–11.0). The data points represent
averaged values from two independent experiments; the errors are less
than 4%. Dashed line represents a non-linear curve fit by the Boltzmann
model with R >0.993.
Fig. 5 Plot of the p-nitrophenolate concentration vs. time for the P6-Zn
(ꢀ), P7-Cu (᭹), 6 (ꢁ) or 7 (᭺) mediated hydrolysis of BNPP. Reaction
conditions: [M2+] = 1.0 mM, [BNPP] = 5.0 mM, H2O–DMSO 95 : 5, 64 mM
CHES buffer, pH 9.5, 50 1 ◦C. The data points represent averaged values
from two independent experiments; the errors are less than 4%. Dashed
lines represent least-squares linear regressions with R >0.999.
The effect of temperature on the rate of the hydrolysis of BNPP
was studied. Pseudo-first order rate constants were determined
at four different temperatures in the range between 30 and
60 ◦C. Arrhenius parameters Ea and A were obtained from the
plot of ln kobs vs. 1/T (Fig. 8). The activation energy was found to
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Fig. 9 Plot of p-nitrophenolate concentration vs. time for the P7-Cu medi-
ated hydrolysis of BNPP. Reaction conditions: [Cu2+] = 1.0 mM, [BNPP] =
5.0 mM, H2O–DMSO 95 : 5, 64 mM CHES buffer, pH 9.5, 50 1 ◦C. The
data points represent averaged values from two independent experiments;
the errors are less than 1%. Dashed lines represent least-squares linear
regressions with R >0.999.
Fig. 7 Ratio of the initial rates of BNPP hydrolysis reactions mediated by
7 and P7-Cu as a function of pH. For the reaction conditions see Fig. 6.
down by only 13% to 1.3 × 10−3 mM min−1. A variation of the
stirring speed had a negligible influence on the initial hydrolysis
rates as long as the polymer particles were homogeneously
suspended in the solution. Furthermore, we investigated the BNPP
hydrolysis using polymers with a different average particle size
(>100 and 25–50 lm). The initial rates were similar within a 10%
error. These experiments suggested that the observed reaction rates
were not affected by diffusional processes to a significant extent.
As described above, the polymers P6-Zn and P7-Cu showed
a similar surface area but a different pore structure. We were
interested whether this difference in polymer morphology had an
effect on the hydrolysis reactions. Therefore, we exchanged Zn(II)
in P6-Zn for Cu(II). This was accomplished by treating P6-Zn
with HCl (∼20%), followed by neutralization with NaOH (1 M)
and re-metalation with CuCl2 in methanol to give P6-Cu. The
metal content of the polymers was determined by ICP-OES. In
the exchange process, 96% of the zinc ions were cleaved off and
97% of the free binding sites were re-occupied by Cu(II) indicating
a facile access to the binding sites. Polymer P6-Cu was then tested
for its ability to promote the hydrolysis of BNPP. An initial rate
of 1.6 × 10−3 mM min−1 was determined for P6-Cu, which is very
similar to what was found for P7-Cu (1.5 × 10−3 mM min−1). This
data showed that the different pore structure of P6-Zn and P7-
Cu was not responsible for the differences in catalytic activity but
rather the nature of the metal ion.
Fig. 8 Plot of ln kobs vs. 1/T for the P7-Cu mediated hydrolysis of BNPP.
Reaction conditions: [Cu2+] = 1.0 mM, [BNPP] = 5.0 mM, H2O–DMSO
95 : 5, 64 mM CHES buffer, pH 9.5, 30–60 ◦C. The data points represent
averaged values from two independent experiments; the errors are less than
4%. Dashed line represents a linear regression with R >0.998.
be Ea = 65 2 kJ mol−1, while the frequency factor A was 8.4
0.4 × 106 min−1. These values are similar to those obtained for a
polymeric, EGDMA-based Cu(II) catalysts studied by the group
of Verma.2i
P7-Cu was able to act as a true catalyst, as evidenced by the
fact that p-nitrophenolate was observed in 53% yield after 96 h
using a substrate to catalyst ratio of 5 : 1. For this reaction, it is
likely that some hydrolysis of the reaction product NPP occurred
concomitantly to the hydrolysis of BNPP.
Partitioning studies
To verify that immobilized copper complexes were responsible
for catalysis and not a small amount of leached Cu2+, we have
performed “stop-experiments” in which the polymer was sepa-
rated after 50 min by centrifugation and the isolated supernatant
was monitored spectrophotometrically. The hydrolysis reaction of
BNPP was slowed significantly by a factor of 290 (Fig. 9). The
metal-free polymer P7 showed no acceleration of the hydrolysis of
BNPP at all.
The majority of the reactive metal sites of P7-Cu are in the
interior of the polymeric particles. To verify that diffusional
processes were not rate limiting, we have carried out a reaction
without stirring. The initial rate for the BNPP hydrolysis slowed
Recent studies have shown that highly cross-linked EGDMA
polymers can act as potent sorbents for organic compounds
dissolved in fluorous solvents.22 The consequence of such a
partitioning is a high local concentration within the pores of
the polymer. This leads to the possibility that the activity of
a catalyst embedded in such a support may be enhanced by a
favorable substrate concentration gradient. Investigations with
immobilized Rh catalysts have confirmed that rate enhancements
due to partitioning can indeed occur.23 Chang and co-workers have
demonstrated that similar effects can be observed for reactions
performed in aqueous solutions.2c,e
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The partitioning of BNPP into the EGDMA polymers used in
this study could strongly affect the hydrolysis kinetics. We therefore
investigated the adsorption of the substrate BNPP and the
hydrolysis product p-nitrophenolate using a metal-free polymer
P8, which was obtained by homo-polymerization of EGDMA
with CH3CN as the porogen. The amount of P8 was varied in
the range that was used for hydrolysis reactions (0–215 mg). After
an equilibration time of 30 min24 in a buffered aqueous solution
of BNPP, the polymer was separated by centrifugation and the
UV-Vis spectrum of the supernatant was recorded (Fig. 10). The
difference in BNPP concentration before and after addition of the
polymer P8 was calculated from the absorption at k = 288 nm (e =
17.8 × 103 dm3 mol−1 cm−1).
Fig. 11 BNPP-uptake by the polymer P8 as a function of the amount of
polymer added. The content of BNPP in the polymer was calculated from
the UV-Vis data as described in the text. For conditions see Fig. 10. The
data points represent averaged values from two independent experiments;
the errors are less than 1%.
prepared in the presence of a porogen, highly cross-linked
polymers typically show a high surface area between 50 and 500
m2 g−1.18 This ensures efficient access to the reactive sites within
the polymer. Catalytic transformations are thus not restricted to
sites on the outer surface of the polymer particle. The permanent
pore structure of highly cross-linked polymers allows the use
of polar and nonpolar solvents for catalysis. This is in contrast
to lightly cross-linked supports such as Merrifield resins, for
which swelling in an appropriate solvent is necessary for access
to the interior volume.26 A further advantage of highly cross-
linked supports is the possibility to create site-isolated metal
sites, which can be of importance if the catalysts suffer from self-
deactivation.27 Finally, these polymers may act as potent sorbents
for organic molecules, which may lead to favorable concentration
gradients during catalysis. All these factors are of relevance for
the immobilized catalysts P6-Zn and P7-Cu described in this
study. The polymers show surface areas of >400 m2 g−1 and the
accessibility of the active sites is indeed very good as demonstrated
by the nearly quantitative metal exchange reaction which was used
to make P6-Cu. The partitioning studies show that the substrate
BNPP is strongly adsorbed to the EGDMA polymer. This leads to
a high local concentration of BNPP within the polymeric matrix,
which facilitates its hydrolysis. It should be noted that so far
there are only a few studies, which have addressed partitioning
effects as a means to increase the catalytic efficiency of poly-
EGDMA-based catalysts.2c,e,23 The data presented above clearly
highlights the importance and the potential of such effects. A
high local concentration of the substrate BNPP is unlikely to
be the only reason for the superior catalytic performance of
the immobilized catalysts. Otherwise, similar rate enhancements
compared to their homogeneous counterparts would have been
observed for P6-Zn and P7-Cu. In particular the heterogeneous
catalyst P7-Cu is expected to benefit from site isolation effects
since homogeneous Cu(II) catalysts have repeatedly been reported
to partially deactivate via [LnCu(l-OH)2CuLn]2+ dimer formation.4
Several mechanistic studies suggest that for the phosphodiester
hydrolysis with mononuclear Cu(II) or Zn(II) complexes, an
Fig. 10 UV-Vis spectra of BNPP solutions after addition of different
amounts of P8 (0, 24.0, 59.2, 98.6, 146 and 215 mg). The polymer was
removed by centrifugation and an aliquot of the supernatant was diluted
75 times prior to the measurements. Conditions: Vtotal = 3.0 mL, [BNPP] =
5.0 mM, H2O–DMSO 95 : 5, 64 mM CHES buffer, pH 9.5, 50 1 ◦C.
The BNPP concentration in solution decreased significantly
upon addition of P8. This demonstrates that BNPP is strongly
adsorbed to the EGDMA polymer. In a typical hydrolysis
reaction, 59.2 mg of P7-Cu were employed (V = 3.0 mL). The
experiments with P8 show that approximately 30% of the total
BNPP partitions into the polymeric matrix under those conditions
(Fig. 11). As a consequence, a very high local concentration of
BNPP is found within the pores of the polymer. The increased
local concentration is likely to be an important factor for the
superior hydrolytic activity of the polymers P6-Zn and P7-Cu as
compared to the homogeneous catalysts 6 and 7.
The hydrolysis product p-nitrophenolate was also adsorbed
to the polymer. The uptake of p-nitrophenolate by the polymer
P8 was determined for solutions with an initial p-nitrophenolate
concentration of 0.1 mM. This corresponds to a reaction progress
of 2%. 16% of the p-nitrophenolate was absorbed by 59.2 mg of
P8. These results show that the partitioning of p-nitrophenolate is
relatively low under the conditions employed in the kinetic studies.
Conclusion
Highly cross-linked organic polymers are increasingly being used
as supports for immobilized transition-metal catalysts.25 These
materials display a number of favorable characteristics. When
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intramolecular attack a metal-bound hydroxyide to a coordinated
substrate is the crucial step.1 A similar mechanism is like to account
for the hydrolytic activity of P6-Zn and P7-Cu and the isolation
of the BNPP complex 8 supports this assumption.
solution was finally evaporated in vacuum at room temperature.
The resulting brown residue was sublimed twice (50 ◦C, 60 mbar,
air cooling). A colorless crystalline solid was obtained (yield:
9.15 g, 75%); mmax/cm−1 1673vs (CO) and 1640vs (CH CH2); dH
=
3
3
=
(400 MHz; CDCl3) 5.10 (1 H, d, J 8, CH CH2), 5.40 (1 H, d, J
=
16, CH CH2), 7.32 (1 H, s, HIm), 7.52 (1 H, s, HIm), 7.93 (1 H, dd,
Experimental
3JE 16, JZ 8, CH CH2) and 9.85 (1 H, s, CHO); dC (101 MHz;
3
=
=
CDCl3) 105.38 (CH CH2), 120.85, 129.85, 132.29, 142.44 and
General
182.32 (CHO); m/z (MALDI-TOF) 123.06 (100%, M + H+).
All reagents and solvents used were commercially available
and were used as received. Sodium bis(p-nitrophenyl)phosphate
(BNPP) was purchased from Sigma-Aldrich. [AgBNPP] was
obtained as a pale yellow precipitate by reaction of BNPP
with AgNO3 in H2O–MeOH. Polymerizations were performed
in a glovebox under an atmosphere of dinitrogen containing
less than 1 ppm of oxygen and water. Prior to utilization, 2,2ꢀ-
azobisisobutyronitrile (AIBN) was recrystallized from MeOH and
EGDMA was washed with NaOH (1 M) and saturated NaClaq,
dried over Na2SO4 and distilled.
1-Vinylimidazole-2-carbaldehyde oxime (2). Hydroxylamine
hydrochloride (3.66 g, 52.7 mmol) and sodium carbonate (2.79 g,
26.3 mmol) were dissolved in water (50 mL). The solution was
placed in an ice-bath and 1-vinylimidazole-2-carbaldehyde (6.43 g,
52.7 mmol) dissolved in ethanol (25 mL) was added dropwise. The
resulting white suspension was stirred for 90 min at 0 ◦C. The
product was filtered off, washed with 75 mL of cold EtOH–H2O
(1 : 2) and dried in vacuum. A white crystalline powder was
obtained (yield: 6.27 g, 87%); mmax/cm−1 2738, 2678, 2614s
=
(CHNOH) and 1640vs (CH CH2); dH (400 MHz; MeOD) 4.99
3
3
=
=
(1 H, d, J 8, CH CH2), 5.47 (1 H, d, J 16, CH CH2), 7.10
Instrumentation
3
3
(1 H, s, HIm), 7.62 (1 H, s, HIm), 7.80 (1 H, dd, JE 16, JZ 8,
The 1H and 13C NMR spectra were recorded on a Bruker Advance
DPX 400 using the residual protonated solvents as internal
standards. All spectra were recorded at room temperature. The
mass spectra were measured with the following instruments: Shi-
madzu Axima CRFplus spectrometer (MALDI-TOF; a-cyano-4-
hydroxycinnamic acid matrix), MicroMass Q-Tof Ultima spec-
trometer (ESI), and Varian 1200L spectrometer (EI). The IR
spectra were recorded on a Perkin-Elmer Spectrum One FT-IR
spectrometer. The UV-Vis-near-IR spectra and hydrolysis kinetics
were recorded on a Perkin-Elmer Lambda 35 spectrometer. The
elemental analyses (Zn and Cu) were obtained by inductively
coupled plasma atomic emission spectroscopy (ICP-OES) on a
Perkin-Elmer ICP-OES 2000 DV instrument. The pH measure-
ments were carried out with a Metrohm 692 pH/ion meter. The
BET measurements were carried out by Quantachrome GmbH,
Odelzhausen. Dinitrogen adsorption/desorption measurements
were performed on a Quantchrome Autosorb-3 instrument at
77.4 K. Prior to the measurements, the samples were dried under
vacuum at 100 ◦C for 2 h. All fitting procedures were effected by the
Levenberg–Marquardt algorithm with Origin 7.0 from OriginLab
Corporation.
=
CH CH2) and 8.09 (1 H, s, CHNOH); dC (101 MHz; MeOD)
=
104.12 (CH CH2), 119.66, 130.43, 131.79, 141.49 and 141.59
(CHNOH); m/z (MALDI-TOF) 138.07 (100%, M + H+).
1-Vinylimidazole-2-methylamine
(3). 1-Vinylimidazole-2-
carbaldehyde oxime (6.86 g, 50.0 mmol), ammonium acetate
(8.34 g, 108 mmol), concentrated aqueous ammonia (150 mL,
2.58 mol) and ethanol (200 mL) were heated at 85 ◦C under
reflux. After 5 min, zinc powder (13.8 g, 211 mmol) was added
in portions over a period of 20 min. Heating was continued
for 3 h. The excess of zinc was removed by filtration and the
ethanol was evaporated. A solution of sodium hydroxide (42.8 g,
1.07 mol) in water (200 mL) was added, initially producing
a white precipitate which redissolved nearly as the remainder
of the sodium hydroxide solution was added. The colorless
solution was extracted with CH2Cl2 (5 × 50 mL). The organic
phase was dried over mgSO4·2H2O and concentrated to yield a
light yellow oil (yield: 5.36 g, 87%). Contamination of bis[(1-
vinylimidazol-2-yl)methyl]amine (∼6%) was evidenced by 1H and
13C NMR spectroscopy; mmax/cm−1 3358, 3275w (NH2) and 1645vs
=
(CH CH2); dH (400 MHz; CDCl3) 3.92 (2 H, s, CH2), 4.86 (1 H,
3
3
=
=
d, J 8, CH CH2), 5.18 (1 H, d, J 16, CH CH2), 6.93 (1 H, s,
3
3
=
HIm), 6.98 (1 H, dd, JE 16, JZ 8, CH CH2) and 7.05 (1 H, s,
HIm); dC (101 MHz; CDCl3) 38.68 (CH2), 102.18 (CH CH2),
Synthesis
=
1-Vinylimidazole-2-carbaldehyde (1). n-Butyllithium in hex-
ane (1.6 M, 68.0 mL, 108 mmol) was added slowly to a solution of
1-vinylimidazole (9.06 mL, 100 mmol) in diethyl ether (200 mL)
at −30 ◦C. The deep red solution turned to yellow after complete
addition and was allowed to stir at −30 ◦C for 90 min. Dimethylfor-
mamide (7.76 mL, 100 mmol) was quickly added and the resulting
beige suspension was stirred overnight at room temperature. The
reaction was quenched by adding cautiously 50 mL of water
followed by the careful addition of 70 mL of HClaq 12%. The
layers were separated and the organic phase was washed with HClaq
12% (3 × 50 mL). The combined aqueous phases were saturated
with potassium carbonate and extracted with chloroform (5 ×
50 mL). The organic phase was dried over mgSO4·2H2O, filtered
and concentrated under reduced pressure. The resulting brown
115.83, 128.26, 128.37 and 148.12; m/z (MALDI-TOF) 124.10
(100%, M + H+).
Bis[(1-vinylimidazol-2-yl)methyl]amine (4). 1-Vinylimidazole-
2-methylamine (4.09 g, 33.2 mmol) and 1-vinylimidazole-2-
carbaldehyde (4.05 g, 33.2 mmol) were dissolved in methanol
(150 mL) and stirred overnight at room temperature. Sodium
borohydride (1.26 g, 33.2 mmol) was carefully added in portions
at 0 ◦C and the yellow solution was further stirred for 3 h at room
temperature. After evaporating the solvent, water (150 mL) was
added. The solution was extracted with CHCl3 (3 × 40 mL) and
dried over mgSO4·2H2O. The solvent was removed and a yellow
oil was obtained (yield: 6.01 g, 79%); mmax/cm−1 2891, 2820w (NH)
=
and 1645vs (CH CH2); dH (400 MHz; CDCl3) 3.93 (4 H, s, CH2),
3864 | Dalton Trans., 2006, 3858–3867
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©

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3
3
=
=
4.86 (2 H, d, J 8, CH CH2), 5.21 (2 H, d, J 16, CH CH2), 6.98
17.08. C18H21ClCuF6N7P requires C, 37.32; H, 3.65; N, 16.92%);
(2 H, s, HIm), 7.08 (2 H, dd, JE 16, JZ 8, CH CH2) and 7.20 (2
H, s, HIm); dC (101 MHz; CDCl3) 44.91 (CH2), 101.98 (CH CH2),
mmax/cm−1 1652vs (CH CH2) and 832vs (PF6−); kmax(CH3CN)/nm
3
3
=
=
290 (e/dm3 mol−1 cm−1 5.79 × 103), 816 (87) and ∼1100 (∼100);
m/z (ESI-TOF) 398.32 (100%, M − Cl − PF6−) and 433.06
(47, M − PF6−).
=
116.00, 128.43, 128.97 and 145.44; m/z (EI) 108 (80%), 122 (50)
and 230 (100, M + H+).
[Cu(5)BNPP]PF6 (8). [AgBNPP] (45.1 mg, 101 lmol) in 3 mL
of CH3CN was added to a solution of complex 7 (58.5 mg,
101 lmol) in CH3CN (3 mL) in the dark. After 15 min, the
AgCl precipitate was removed by centrifugation. Pale green
crystals were obtained by slow diffusion of diethyl ether into
a CH3CN solution of complex 8 (Found: C, 40.63; H, 3.56; N,
14.33. C30H29CuF6N9O8P2 requires C, 40.80; H, 3.31; N, 14.27%);
Tris[(1-vinylimidazol-2-yl)methyl]amine (5).
A solution of
bis[(1-vinylimidazol-2-yl)methyl]amine (4.92 g, 21.5 mmol), 1-
vinylimidazole-2-carbaldehyde (3.93 g, 32.2 mmol) and glacial
acetic acid (6.3 mL, 110 mmol) in methanol (450 mL) was stirred
overnight at room temperature. NaBH3CN (3.78 g, 60.2 mmol)
was added in portions and the reaction mixture was stirred for
further 7 h. The reaction was quenched by adjusting the pH to
2.0 by careful addition of concentrated HCl. A white precipitate
was formed which was filtered off. After reduction to a volume
of 10 mL, water (100 mL) was added. The aqueous solution was
washed with diethyl ether (3 × 40 mL). Then the pH was adjusted
to about 8 by addition of Na2CO3. The resulting yellow precipitate
was extracted with CHCl3 (3 × 50 mL) and the organic phase
was dried over mgSO4·2H2O. The solvent was evaporated and
the residue was dried in a vacuum. The crude product (4.96 g)
could be purified by first synthesizing the corresponding zinc- or
copper-complex 6 or 7 followed by removal of the metal with HCl
37%. After adjusting the pH to ∼14, the white precipitate was
extracted with CHCl3 and dried over mgSO4·2H2O. The solvent
was evaporated and a white solid was obtained (yield: 3.28 g, 46%).
The product was recrystallized from toluene (Found: C, 64.29; H,
6.54; N, 29.65. C18H21N7 requires C, 64.46; H, 6.31; N, 29.23%);
mmax/cm−1 1650vs (CH CH2), 1089vs (P O) and 830vs (PF6−);
kmax(CH3CN)/nm 285 (e/dm3 mol−1 cm−1 2.31 × 104) and 1023
(114); m/z (ESI-TOF) 738.01 (100%, M − PF6−).
=
=
Polymer P6-Zn. Complex 6 (244 mg, 419 lmol), EGDMA
(3.24 g, 16.4 mmol), and AIBN (70.0 mg, 426 lmol) were dissolved
in CH3CN (3.36 mL) and heated for 24 h at 65 ◦C in a closed screw
cap vial. The colorless, insoluble polymer was isolated, ground in
a mortar, and washed with CH3CN (5 × 80 mL). After drying
in vacuum, a white powder was obtained (yield: 3.28 g, 94%);
mmax/cm−1 1722vs (EGDMA) and 844m (PF6−); ICP-OES: 5.06 mg
of Zn/g of polymer; BET surface area: 429 m2 g−1; average pore
diameter: not determined.
Polymer P7-Cu. Complex 7 (243 mg, 419 lmol), EGDMA
(3.24 g, 16.4 mmol), and AIBN (70.0 mg, 426 lmol) were dissolved
in CH3CN (3.36 mL) and heated for 24 h at 65 ◦C in a closed screw
cap vial. The pale green, glassy, insoluble polymer was isolated,
ground in a mortar, and washed with CH3CN (5 × 80 mL). After
drying in vacuum, a pale green powder was obtained (yield: 3.14 g,
90%); mmax/cm−1 1721vs (EGDMA) and 844m (PF6−); ICP-OES:
3.22 mg of Cu/g of polymer; BET surface area: 416 m2 g−1; average
mmax/cm−1 1643vs (CH CH2); dH (400 MHz; CDCl3) 3.81 (6 H, s,
=
3
3
=
=
CH2), 4.56 (3 H, d, J 8, CH CH2), 5.04 (3 H, d, J 16, CH CH2),
3
3
=
6.08 (3 H, dd, JE 16, JZ 8, CH CH2), 7.01 (3 H, s, HIm) and 7.19 (3
H, s, HIm); dC (101 MHz; CDCl3) 48.88 (CH2), 101.81 (CH CH2),
=
115.96, 128.42, 128.71 and 144.50; m/z (MALDI-TOF): 336.30
(100%, M + H+).
˚
pore diameter: 57 A.
Polymer P6-Cu. The polymer P6-Zn (600 mg) was suspended
in HCl (20 mL, 20%) for 30 min at room temperature. Subse-
quently, it was washed with water (3 × 20 mL), with NaOHaq (2 ×
40 mL, 1 M), and with MeOH (4 × 40 mL). Metalation of P6
(580 mg) was achieved by mixing a solution of CuCl2 in MeOH
(40 mL, 0.5 M) with the polymer for 20 min. The resulting green
polymer P6-Cu was washed with MeOH until the reaction with
dithiooxamide was negative (5 × 40 mL) and then it was dried in a
vacuum (yield: 523 mg); mmax/cm−1 1722vs (EGDMA); ICP-OES:
0.19 mg of Zn/g of polymer, 5.21 mg of Cu/g of polymer.
[Zn(5)Cl]PF6 (6). Anhydrous ZnCl2 (89.7 mg, 659 lmol) was
added to a solution of tris[(1-vinylimidazol-2-yl)methyl]amine
(221 mg, 659 lmol) in methanol (12 mL). After 15 min, NH4PF6
(107 mg, 659 lmol) was added and a white precipitate was
formed immediately. The product was isolated, washed with
methanol, and dried in a vacuum (yield: 324 mg, 85%). Crys-
tals were obtained by slow diffusion of diethyl ether into a
CH3CN solution of complex 6 (Found: C, 37.09; H, 3.91; N,
17.08. C18H21ClF6N7PZn requires C, 37.20; H, 3.64; N, 16.87%);
mmax/cm−1 1651vs (CH CH2) and 833vs (PF6−); dH (400 MHz;
=
3
=
CD3CN) 4.25 (6 H, s, CH2), 5.20 (3 H, d, J 8, CH CH2), 5.50 (3
Polymer P8. EGDMA (3.24 g, 16.4 mmol), and AIBN
(70.0 mg, 426 lmol) were dissolved in CH3CN (3.5 mL) and
heated for 24 h at 65 ◦C in a closed screw cap vial. The white,
insoluble polymer was isolated, ground in a mortar, and washed
with CH3CN (5 × 80 mL). After drying in vacuum, a white powder
was obtained (yield: 3.03 g, 94%); mmax/cm−1 1722vs (EGDMA).
3
3
3
=
=
H, d, J 16, CH CH2), 6.91 (3 H, dd, JE 16, JZ 8, CH CH2), 7.19
(3 H, s, HIm) and 7.50 (3 H, s, HIm); dC (101 MHz; CD3CN) 53.08
=
(CH2), 107.96 (CH CH2), 120.07, 126.81, 128.36 and 147.63; m/z
(ESI-TOF) 434.07 (100%, M − PF6−).
[Cu(5)Cl]PF6 (7). Anhydrous CuCl2 (88.6 mg, 659 lmol) was
added to a solution of tris[(1-vinylimidazol-2-yl)methyl]amine
(221 mg, 659 lmol) in methanol (12 mL). After 15 min, NH4PF6
(107 mg, 659 lmol) was added and a pale green precipitate
was formed immediately. The product was isolated, washed
with methanol, and dried in a vacuum (yield: 286 mg, 75%).
Crystals were obtained by slow diffusion of diethyl ether into
a CH3CN solution of complex 7 (Found: C, 37.14; H, 3.92; N
Kinetic investigations
The hydrolysis of BNPP was carried out in 64 mM buffered
aqueous solution (HEPES, CHES, and CAPS) at 50 1 C. In
the case of heterogeneous hydrolysis the amount of polymer
corresponds to a metal ion concentration of 1.0 mM if the
polymers would be completely soluble in the buffer. The crystalline
◦
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complexes 6 and 7 were dissolved in DMSO and diluted (×20) with
aqueous buffer solution. After an incubation time of 30 min, the
reactions were initiated by addition of a freshly prepared stock
solution of the substrate (final concentration: 5.0 mM). After a
given time, the reaction tubes were centrifuged for 1 min and
the absorbance A of the clear supernatant was determined by
UV/Vis at k = 400 nm. For the determination of initial rates,
the concentration of the hydrolysis product, p-nitrophenolate
anion, was calculated from the absorbance A and the extinction
coefficient (e = 18.7 × 103 dm3 mol−1 cm−1) by correction for the
degree of ionization of p-nitrophenol (pKa 6.87) at 50 1 ◦C. The
pKa = 7.15 at 25 ◦C was corrected by van’t Hoff reaction isobar.
Initial rates were obtained by linear least-squares regression (R
>0.998) from a plot of concentration of hydrolysis product against
time. Reactions were monitored to less than 2% conversion of
substrate to product. The buffers were HEPES (pH 7.0–8.0),
CHES (pH 8.5–10.0), and CAPS (pH 10.5–11.0) and the pH
remained constant within the error range after the experiment.
Correction of spontaneous hydrolysis was effected across the pH
range studied (7.0–11.0).
squares on F2 with all non-H atoms anisotropically defined. The
hydrogen atoms were placed in calculated positions using the
“riding model”. Structure refinement and geometrical calculations
were carried out on all structures with SHELXTL 5.1.30
Crystal data for 5. C18H21N7·2H2O, M = 371.45, triclinic,
¯
˚
space group P1, a = 8.7133(17), b = 10.2885(18), c = 12.011(2) A,
a = 94.105(15), b = 94.136(17), c = 103.709(16)◦, U =
3
−3
˚
1038.9(3) A , T = 140(2) K, Z = 2, Dc = 1.187 g cm , l(Mo-
Ka) = 0.082 mm−1, 6178 reflections measured, 3200 unique (Rint
=
0.0576) which were used in all calculations. The final wR(F2) was
0.1434 (all data).
Crystal data for 6. C18H21ClF6N7PZn, M = 581.21, mono-
clinic, space group I2/a, a = 13.8886(4), b = 24.7471(11), c =
◦
3
˚
˚
14.5306(8) A, b = 108.775(4) , U = 4728.5(4) A , T = 140(2)
K, Z = 8, Dc = 1.633 g cm−3, l(Mo-Ka) = 1.288 mm−1, 13843
reflections measured, 3773 unique (Rint = 0.0243) which were used
in all calculations. The final wR(F2) was 0.0749 (all data).
Crystal data for 7. C18H21ClCuF6N7P, M = 579.38, mono-
clinic, space group I2/a, a = 13.5389(9), b = 24.865(2), c =
The Arrhenius parameters for P7-Cu were obtained by mea-
suring the initial rates at 30, 40, 50 and 60 ◦C. Pseudo-first-order
rate constants were calculated by using the following formula:
initial rate = kobs[BNPP]. These data were plotted as ln kobs against
1/T and fitted to ln kobs = ln A + Ea/(RT). Linear least-squares
regression gave the Arrhenius parameters Ea and A.
◦
3
˚
˚
14.5342(11) A, b = 108.009(6) , U = 4653.1(6) A , T = 140(2)
K, Z = 8, Dc = 1.654 g cm−3, l(Mo-Ka) = 1.192 mm−1, 13536
reflections measured, 3895 unique (Rint = 0.0697) which were used
in all calculations. The final wR(F2) was 0.1135 (all data).
Crystal data for 8. C30H29CuF6N9O8P2·0.5C4H10O, M =
Partitioning studies
¯
920.16, triclinic, space group P1, a = 8.5886(16), b = 13.3393(19),
◦
˚
The uptake of the substrate BNPP and the hydrolysis product p-
nitrophenolate was determined by the difference in concentration
before and after addition of the polymer P8 after an equilibration
time of 30 min in buffer solution at 50 1 ◦C: ([S]ini − [S]final)/[S]ini.
The substrate concentrations were calculated from the absorbance
A and the extinction coefficients (BNPP at kmax = 288 nm: e =
17.8 × 103 dm3 mol−1 cm−1, p-nitrophenolate at kmax = 400 nm:
e = 18.7 × 103 dm3 mol−1 cm−1). Conditions: Vtotal = 3.0 mL,
[BNPP]ini = 5.0 mM, [p-nitrophenolate]ini = 0.1 mM, H2O–DMSO
95 : 5, 64 mM CHES buffer, pH 9.5, 50 1 ◦C.
c = 17.721(3) A, a = 101.074(12), b = 96.831(14), c = 92.850(13) ,
3
−
3
˚
U = 1972.8(5) A , T = 140(2) K, Z = 2, Dc = 1.549 g cm , l(Mo-
Ka) = 0.725mm−1, 11660 reflections measured, 6067 unique(Rint
=
0.1171) which were used in all calculations. The final wR(F2) was
0.3275 (all data).
CCDC reference numbers 604874–604877.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b605676h
Acknowledgements
Elemental analysis of polymers (ICP-OES)
This work was supported by the Swiss National Science Founda-
tion, the EPFL and the DFG.
A suspension of the respective polymer (∼80 mg) was heated in
concentrated sulfuric acid (3 mL) at 140 ◦C for 2 h in a volumetric
flask (25.0 mL). Hydrogen peroxide (30%, 1.0–1.5 mL) was added,
and the colorless solution was heated for 12 h at 140 ◦C. The
resulting clear solution was diluted to 25.0 mL with nitric acid
(2%, aqueous) and analyzed by ICP-OES. Every analysis including
sample preparation was done in duplicates and the error was less
than 1.5%.
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3866 | Dalton Trans., 2006, 3858–3867
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©

View Article Online
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