Hydrolysis of glycosides with microgel catalysts

Article abstract of DOI:10.1021/ic200837z

A dormant macromolecular catalyst was prepared by polymerization of an aqueous styrene-butyl acrylate miniemulsion in the presence of a new polymerizable pentadentate ligand. The catalyst was activated by binding Cu(II) ions to the ligand site and then explored for its ability to hydrolyze glycosidic bonds in alkaline solution. The performance was correlated to the catalytic activity shown by low molecular weight analogs. A turnover rate of up to 43 ×10^-4 min^-1 was previously observed for cleavage of the glycosidic bond in selected p-nitrophenylglycosides with a binuclear, low molecular weight catalyst; by contrast, the same reaction is more than 1 order of magnitude faster and has a turnover rate of up to 380 ×10^-4 min^-1 when using the prepared macromolecular catalyst. The catalyzed hydrolysis is about 10⁵-fold accelerated over the uncatalyzed background reaction under the provided conditions, while a significant discrimination of the α- and β-glycosidic bond or of the galacto- and gluco-configuration in the sugar moiety in the glycoside substrates is not observed.

Full text of DOI:10.1021/ic200837z

ARTICLE
pubs.acs.org/IC
Hydrolysis of Glycosides with Microgel Catalysts
Susanne Striegler,*,†,‡ Michael Dittel,‡ Rami Kanso,† Natasha A. Alonso,† and Evert C. Duin†
†Department of Chemistry and Biochemistry, 179 Chemistry Building, Auburn University, Auburn, Alabama 36849, United States
‡Division of Inorganic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany
S Supporting Information
b
ABSTRACT: A dormant macromolecular catalyst was prepared by polymer-
ization of an aqueous styreneꢀbutyl acrylate miniemulsion in the presence of
a new polymerizable pentadentate ligand. The catalyst was activated by
binding Cu(II) ions to the ligand site and then explored for its ability to
hydrolyze glycosidic bonds in alkaline solution. The performance was
correlated to the catalytic activity shown by low molecular weight analogs.
A turnover rate of up to 43 ꢁ 10ꢀ4 minꢀ1 was previously observed for
cleavage of the glycosidic bond in selected p-nitrophenylglycosides with a
binuclear, low molecular weight catalyst; by contrast, the same reaction is
more than 1 order of magnitude faster and has a turnover rate of up to 380 ꢁ
10ꢀ4 minꢀ1 when using the prepared macromolecular catalyst. The catalyzed
hydrolysis is about 105-fold accelerated over the uncatalyzed background reaction under the provided conditions, while a significant
discrimination of the R- and β-glycosidic bond or of the galacto- and gluco-configuration in the sugar moiety in the glycoside
substrates is not observed.
’ INTRODUCTION
Conventional multistep syntheses of oligosaccharides involve
numerous protection and deprotection steps and obligatory
activation of the glycosyl donors. While many elegant approaches
for the syntheses of oligosaccharides have been developed,1ꢀ14
they remain time consuming and most of them still require expert
knowledge that limits automation. Selective catalytic transforma-
tions on carbohydrates remain particularly cumbersome, espe-
cially when underivatized sugar entities need to be transformed
in aqueous solution, but corresponding enzymes are not known,
are only available in limited quantity, are of insufficient purity, or
possess a short shelf live. Catalytic entities promoting biomimetic
reactions in the natural solvent of carbohydrates, i.e., water,
would shorten time-consuming syntheses of oligosaccharides
and open new directions beyond organic syntheses and medic-
inal chemistry by providing fast and convenient access to defined
Figure 1. Structures of complexes 1a,b and 2aꢀc.
glycosides.
Our long-term goal in this context is preparation of macro-
molecular catalytic entities that allow selective in situ activation
of underivatized glycosyl donors, thereby avoiding any lengthy
activation or protection/deprotection steps of carbohydrates.
This goal might be achieved by combining the catalytic transfor-
mation ability of transition metal complexes with the selectivity
of a soluble templated polymer matrix. Previous results along
these lines revealed that a macromolecular surrounding of a
binuclear Cu(II) catalyst embedded in a water-soluble microgel
matrix enhances its catalytic performance during oxidation
reactions by about 1 order of magnitude compared to its low
molecular weight analog.15,16 Other preliminary data demon-
strated the carbohydrate-discriminating ability of the binuclear
Cu(II) complex N,N0-{bis(2-pyridylmethyl)-1,3-diaminopropan-2-
ol}ato dicopper(II) (μ-acetato) diperchlorate, Cu2(bpdpo)
(1a),16ꢀ18 in alkaline solution (Figure 1); D-mannose is about
30-fold more tightly bound and recognized than D-glucose
under comparable conditions.18
As a first step toward the long-term goal described above, we
studied a variety of mono- and binuclear Cu(II) complexes as
catalysts for hydrolysis of p-nitrophenylglycosides in aqueous
solution.19 Cleavage of the glycosidic bond was typically
Received: April 21, 2011
Published: August 15, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1. Synthesis of Pentadentate Backbone Ligand VB-
(bsdpo) (3)
To overcome this limitation, a different synthetic strategy was
developed by treating 2-(hydroxymethyl)pyridine-3-ol hydro-
chloride (10)23 with 5 and base to afford (3-(4-vinylbenzyloxy)-
pyridin-2-yl)methanol (11).24 Swern oxidation conditions did
not promote any apparent oxidation of 11 to 3-(4-vinylbenzy-
loxy)picolinaldehyde (12). Using commercially available manga-
nese(IV) oxide in dimethoxyethane was futile as well, while
freshly synthesized manganese(IV) oxide25 in dimethoxyethane
provided 12 in more than 91% yield after 8 h under reflux.24
Likewise, selenium oxide in dry dioxane promotes oxidation of
11 into 12 nearly quantitatively in about 3 h.26
Isolation of aldehyde 12 was found to be cumbersome, leading
to complete product loss due to polymerization upon standing in
neat or diluted solutions at ambient temperature or in the cold
under argon atmosphere or in the dark within 24 h. Conse-
quently, only an analytical sample of aldehyde 12 was purified by
flash column chromatography on silica gel using gradient elution
with cyclohexane and ethyl acetate to allow full compound char-
acterization. Attempts to purify 12 over basic aluminum oxide
resulted in complete compound loss. As analysis of the raw
material by 1H and 13C NMR spectroscopy indicated only minor
amounts of impurities, aldehyde 12 was typically not further
purified and used for immediate preparation of ligand 8, as
obtained from the reaction mixture after filtration and evapora-
tion of the solvent.
promoted more efficiently by bi- than by mononuclear Cu(II)
complexes.19 Complex 1a was identified as the most proficient
catalyst among those investigated as it accelerated hydrolysis of
selected p-nitrophenylglycopyranosides up to 11 000-fold over
the background reaction in CAPS buffer at pH 10.5 and 30 °C.19
To some lesser extent, the reaction is also promoted by a
binuclear copper(II) complex Cu2(TEGbsdpo) (2a), which is
derived in situ from N,N0-1,3-bis[{2-hydroxy-4-[2-(2-methoxy-
ethoxy)ethoxy]ethoxy}benzylideneamino]propan-2-ol and cop-
per(II) acetate (Figure 1).19
To enhance the catalytic turnover further, we and others
hypothesized that a reagent immobilized in a polymer matrix
might support selective interactions between a catalytic site and a
substrate when constrained by the rigidity of a three-dimensional
macromolecular matrix as observed in enzymes.16 Correspond-
ing synthetic entities might be assembled by arranging building
blocks that enable hydrophobic interactions, πꢀπ stacking, and/
or hydrogen bonding in addition to the coordinative bonds at a
metal complex-containing active site in a cross-linked polymer
matrix, such as a microgel. Immobilization of catalyst 1a and 2a in
a polymeric environment lends itself as a promising strategy
toward this goal. Along these lines, we herein report the synthesis
and evaluation of macromolecular catalysts derived from poly-
merizable p-vinylbenzyl analogs of 1a and 2a (Figure 1).
Successful condensation of 12 with 1,3-diaminopropanol (7) yield-
ing 1,3-bis((E)-(3-(4-vinylbenzyloxy)pyridin-2-yl)methyleneamino)-
propan-2-ol (13) is indicated by ESI mass spectrometry (see
Supporting Information). Subsequent reduction of the Schiff-
base ligand 13 with sodium borohydride afforded 8 as a yellow-
brownish oil after purification by size exclusion chromatography
on Sephadex LH-20 with methanol as the eluent. Efforts to purify
8 by column chromatography over silica gel and neutral alumi-
num oxide resulted in compound decomposition; an analytical
sample was purified by preparative-scale HPLC over RP-18
eluting with acetonitrileꢀwater. Purified ligand 8 was stored
neat in the dark at ꢀ18 °C prior to preparation of microgels or
used immediately for preparation of binuclear Cu(II) complexes.
Preparation and Characterization of Polymerizable Binuclear
Cu(II) Complexes: Synthesis of Cu2(VBbsdpo). The binuclear
Cu(II) complex N,N0-{1,3-bis[(2-hydroxy-4-(p-vinylbenzyloxy))-
benzylideneamino]-propan-2-ol}ato dicopper (μ-acetate), Cu2-
(VBbsdpo) (2b), was derived from 3 after treatment with Cu(II)
acetate in methanol as an amorphous powder; recrystallization of 3in
the same solvent and slow solvent evaporation yielded single crystals
of 3 suitable for X-ray analysis.16 The composition of 2b in the solid
state was confirmed by elemental analysis and mass spectrometry;16
the coordination of the Cu(II) ions to the backbone ligand 3 was
characterized by IR spectroscopy.16 Single-crystal analysis of complex
2b by X-ray diffraction revealed a square planar coordination
environment for both Cu(II) ions, which are bridged by an
endogenous μ-alkoxo oxygen atom of the backbone ligand 3 and
an exogenous μ-1,3-coordinating acetate group providing N1O3
donor sets for both Cu(II) ions (Figure 2). Very similar bond
lengths and angles are reported for binuclear Cu(II) com-
plexes derived from Schiff-base ligands related to 2b, which
are obtained from other salicylaldehyde derivatives and 7.19,28
Discussion of the structural features of 2b is consequently
shortened herein. Selected bond lengths, bond angles, and
torsion angles are listed in Table 1. The structure of Cu2-
(VBbsdpo) (2b) is confirmed as Cu2(3ꢀ3H)(OAc) in the solid
’ RESULTS AND DISCUSSION
Synthesis of Pentadentate Backbone Ligands. As Cu(II)
ions are paramagnetic limiting or even inhibiting radical polym-
erization reactions, the microgel catalysts are typically obtained
by embedding the backbone ligands in a macromolecular matrix
followed by coordination of Cu(II) ions to activate the dormant
catalysts. Along these lines, a polymerizable ligand N,N0-1,3-
bis[(2-hydroxy-4-vinylbenzyloxy)benzylideneamino]propan-2-ol,
VBbsdpo (3), was prepared as reported recently (Scheme 1).16,20,21
In short, commercially available 4-hydroxysalicylaldehyde (4)
was treated with p-vinylbenzyl chloride (5) in the presence of a
base and potassium iodide in a Williamson ether synthesis
yielding (p-vinylbenzyloxy)salicylaldehyde (6).16 Consecutive
condensation of 6 with 1,3-diaminopropanol (7) afforded 3 in
good yields and high purity after recrystallization from ethyl
acetate. The yellow solid was stored without further precaution at
ambient temperature prior to use for synthesis of the microgel
catalysts.
The same synthetic strategy toward the preparation of 1,3-
bis((3-(4-vinylbenzyloxy)pyridin-2-yl)methylamino)propan-2-ol,
VB(bpdpo) (8), the backbone ligand of 1b, failed, as attempts to
derivatize 3-hydroxypyridinecarbaldehyde (9) in a Williamson
ether synthesis gave no reaction product (Scheme 2).22
state with an intermetallic Cu
Cu distance of 3.514(5) Å.29
3 3 3
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Inorganic Chemistry
ARTICLE
Scheme 2. Synthesis of Pentadentate Backbone Ligand VB(bpdpo) (8)
Figure 2. ORTEP27ꢀPOV ray representation for the structure of 2b, Cu2(VBbsdpo), depicted in ellipsoids with 70% probability; carbon atoms in gray,
copper atoms in green, oxygen atoms in red, nitrogen atoms in blue, and chlorine atoms in yellow; all hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths [Å], Bond Angles [deg], and Dihedral Angles [deg] for Complex 2b, Cu2(VBbsdpo)
bond lengths
bond angles
dihedral angles τ
Cu(1)ꢀO(2)
Cu(1)ꢀO(3)
Cu(1)ꢀO(4)
Cu(1)ꢀN(1)
Cu(2)ꢀO(4)
Cu(2)ꢀO(5)
Cu(2)ꢀO(6)
Cu(2)ꢀN(2)
1.898(2)
1.955(2)
1.916(2)
1.919(2)
1.910(2)
1.947(2)
1.893(2)
1.914(2)
3.514(5)
O(2)ꢀCu(1)ꢀO(4)
175.4(1)
94.2(1)
84.5(1)
87.4(1)
94.0(1)
178.5(1)
176.3(1)
93.7(1)
84.7(1)
87.4(1)
94.2(1)
178.2(1)
133.4(1)
O(2)ꢀCu(1)ꢀO(4)ꢀCu(2)
103.4
176.7
111.1
176.6
O(2)ꢀCu(1)ꢀN(1)
O(4)ꢀCu(1)ꢀN(1)
O(2)ꢀCu(1)ꢀO(3)
O(4)ꢀCu(1)ꢀO(3)
N(1)ꢀCu(1)ꢀO(3)
O(6)ꢀCu(2)ꢀO(4)
O(6)ꢀCu(2)ꢀN(2)
O(4)ꢀCu(2)ꢀN(2)
O(6)ꢀCu(2)ꢀO(5)
O(4)ꢀCu(2)ꢀO(5)
N(2)ꢀCu(2)ꢀO(5)
Cu(1)ꢀO(4)ꢀCu(2)
N(1)ꢀCu(1)ꢀO(4)ꢀCu(2)
O(6)ꢀCu(2)ꢀO(4)ꢀCu(1)
N(2)ꢀCu(2)ꢀO(4)ꢀCu(1)
Cu(1) Cu(2)
3 3 3
Similar intermetallic distances were previously noted for the
benzylideneamino]propan-2-ol}ato (μ-acetato) dicopper com-
plex Cu2(EGbsdpo), 2c (3.506(5) Å, Figure 1).19 Thus,
two Cu(II) ions in {N,N0-1,3-bis[{2-hydroxy-4-(2-methoxyethoxy)}-
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Inorganic Chemistry
ARTICLE
derivatization of the ligand backbone structure of salen-type
pentadentate ligands at their periphery does not influence the
intermetallic distance in binuclear Cu(II) complexes derived
thereof.
Chart 1. Structures of the Polymerizable Ligands 3 and 8 and
Their Water-Soluble Analogs 3a and 8a
Preparation and Characterization of Polymerizable Bi-
nuclear Cu(II) complexes: Synthesis of Cu2(VBbpdpo). The
binuclear copper(II) complex N,N0-{bis((3-(4-vinylbenzyloxy)-
2-pyridylmethyl)-1,3-diaminopropan-2-ol}ato dicopper(II)
(μ-acetato) diperchlorate, Cu2(VBbpdpo) (1b) was obtained
from 8 by treatment with copper(II) acetate in the presence of
sodium perchlorate in ethanol. The residue obtained after eva-
poration of the solvent was recrystallized from dichloromethaneꢀ
methanol, yielding 1b as a blue amorphous powder. Caution:
perchlorate salts are potentially explosive. However, we did not
experience any difficulty in handling or drying the compound below
40 °C.
The composition of 1b in the solid state was confirmed by
elemental analysis. The coordination of the copper(II) ions to
the backbone ligand 8 was indicated by IR spectroscopy. The
NꢀH stretch vibrational band of the secondary amino groups in
ligand 8 is shifted from 1631 to 1620 cmꢀ1 after complexation
with copper(II) ions in 1b; likewise, the CdN stretch vibrational
band of the pyridine groups changed from 1571 to 1577 cmꢀ1
upon metal coordination (see Supporting Information). The
aliphatic CꢀO stretching band found at 1116 cmꢀ1 in 8 cannot
be unambiguously assigned to a vibration in 1b due to overlap
with very strong perchlorate vibrational bands at 1092 cmꢀ1. Never-
theless, comparable assignments of the observed infrared vibra-
tional bands were made previously for related ligands and their
Cu(II) complexes.19,30 Single crystals of 1b suitable for X-ray
diffraction were not obtained yet despite all efforts.
Figure 3. Complex formation upon titration of a solution of (a)
0.02 mM TEGbsdpo ligand (3a) and (b) 0.02 mM bpdpo ligand (8a)
with 10 μL aliquots of a 0.30 mM aqueous Cu(II) acetate solution in
50 mM CAPS buffer at pH 10.5.
Synthesis and Purification of the Microgel Catalysts.
Microgels derived from styrene (14), butyl acrylate (15), and
Schiff-base ligand 3 were prepared by a protocol developed and
described previously.20 However, applying the elaborated pro-
cedure to the preparation of microgels from pentadentate ligand
8 led to visible phase separation in the polymerization mixture
after about 1 h. Consequently, the amount of 8 was lowered to a
nominal concentration of 1 mM for preparation of pol8 and the
amount of 3 for the preparation of pol3 adjusted accordingly. In
short, polymerization of the miniemulsion prepared from 14, 15,
decane, and L (L = 3 or 8) in 5 mM CAPS buffer at pH 10.5
containing sodium dodecyl sulfate was initiated by addition of
potassium peroxysulfate solution to the preheated emulsion at
72 °C. Incorporation of pentadentate ligand 3 in pol3 was
previously shown to be nearly quantitative.20 The same recipe
was applied to prepare a control polymer in the absence of ligand,
while the amount of DMSO solvent was kept constant.
All polymers pol3, pol8, and polblank were subsequently
purified by dialysis in nanopure water and CAPS buffer for 24
h; the microgels pol3 and pol8 were then explored for their
metal-ion binding ability prior to their use as catalysts. A detailed
discussion of the physical characterization of microgel pol3 is
given elsewhere;20 the morphology of pol8 is a topic of ongoing
investigation.
Figure 4. Titration of a 20 mM Cu(OAc)2 solution into 0.1 mM pol8
solution in 5 mM CAPS buffer at pH 10.5 and 30.0 ( 0.1 °C; K1 = 5.8 ꢁ
106, ΔH1 = ꢀ7100 ( 1200 kcal/mol, ΔS1 = 7.6 cal molꢀ1 Kꢀ1; K2 = 1.5 ꢁ
107, ΔH2 = 1800 ( 1200 kcal/mol, ΔS2 = 39.0 cal molꢀ1 Kꢀ1
.
between TEGbsdpo (3a) and Cu(II) acetate dependent upon
the pH of the solution.31 A mononuclear [CuL]2+ complex was
determined as the major species at pH 5.5, and this explained
the observed insufficient metal-ion binding ability of pol3 under
+
these conditions; binuclear species [Cu2L
]
ꢀ2H
2+ and [Cu2L
]
ꢀ3H
(L = TEGbsdpo) may, however, form in alkaline solution.20
Previous studies with low molecular weight complexes further-
more revealed that catalytic hydrolysis of the glycosidic bonds in
p-nitrophenylglycosides as model substrates requires a pH higher
than 9.19
These previous results built the foundation for the herein
described investigation of catalyst activation at pH 10.5 and
subsequent study of their catalytic activity during cleavage of
glycosidic bonds. Characterization of complex formation be-
tween ligands 3a and 8a and Cu(II) ions revealed the presence of
mono- and binuclear copper(II) complexes, depending on the
amount of metal ion, and ensured the absence of free metal ions.
Activation of the Dormant Catalysts by Metal-Ion Load-
ing. The previously observed insufficient binding of Cu(II) ions
to pol3 in nanopure water at pH 5.5 prompted a study of the
metal binding ability of water-soluble TEGbsdpo (3a) as an analog
of the polymerizable, water-insoluble ligand 3 (Chart 1).20
The spectrophotometric titration method developed by
Zuberb€uhler et al. was used to determine complex formation
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Inorganic Chemistry
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Scheme 3. Putative Composition of 2a, Cu2(TEGbsdpo), at pH 10.5
When titrating a solution of the TEGbsdpo ligand (3a) with
aqueous Cu(II) acetate at pH 10.5, a mononuclear Cu(II)
species [CuLꢀ2H] (L = 3a) forms initially and is successively
replaced by a binuclear complex [Cu2Lꢀ3H]+. The ligand is
transformed into a binuclear Cu(II) species at a molar ratio of 2:
1 between Cu(II) ions and 3a. Hypothetical formation of a
trinuclear species was included in the computation of the
spectroscopic titration data but is not verified by the experi-
mental data, even if the molar ratio between Cu(II) ions and the
ligand exceeds 2:1 (Figure 3a). Comparable speciation data are
obtained for the bpdpo ligand (8a) under identical conditions
(Figure 3b).
To further characterize complex formation not only for small
molecular weight analogs but more importantly for the polymers
under investigation, we followed the titration of aqueous Cu(II)
acetate solution into pol8 by isothermal titration calorimetry
(ITC) in CAPS buffer at pH 10.5 (Figure 4). The buffer
concentration was lowered from 50 to 5 mM to reduce the large
background noise; data are corrected for unspecific binding of
Cu(II) ions to a control polymer that does not contain a ligand.
The best fit of the ITC data was provided by a sequential binding
model indicating a molar ratio of 2:1 for coordination of the
metal ions to the ligand binding site in pol8. The titration data
indicate coordination of two metal ions, supporting the results
obtained from the speciation data (vide infra). For the
following kinetic and electron spin resonance (EPR) experi-
ments, appropriate amounts of aqueous Cu(II) acetate solu-
tions were added to the dialyzed microgels pol3 and pol8 to
activate the catalysts polCu23 and polCu28 in CAPS buffer at
pH 10.5; the nominal catalyst concentration is 0.1 mM.
Characterization of the Complex Composition at pH 10.5.
To probe for possible structural changes around the Cu(II) ions
caused by immobilization of the backbone ligand, the low
molecular weight complexes and their microgel analogs were
evaluated by EPR spectroscopy. The spectra observed for the
activated microgels polCu28 and polCu23 are correlated to those
of their corresponding small molecular weight entities 1a and 2a. All
spectra were recorded at 77 K and corrected for background signals
by subtraction of the buffer spectrum. The composition of the low
molecular weight entities at pH 10.5 is known from spectrophoto-
metric titration experiments conducted previously.18,19,32
exist as a [Cu2Lꢀ3H(OH)] species (Scheme 3). By contrast, a
[Cu2LꢀH(OH)]2+ species is predominant (L = 8a) when
dissolving complex 1a at pH 10.5, while about 11% of the complex
is forming a [Cu2LꢀH(OH)2]+ species (Scheme 4).18 Previous
studies revealed that the acetate anion coordinated to the metal
core in 1a in the solid state is preserved in methanol32 but not in
aqueous organic solution.33
The Cu signal in the EPR spectra of all samples is insignificant
with an insufficient signal to noise ratio when imitating the
conditions used during catalysis due to the low nominal complex
concentration in the microgels (0.01 mM, see below). However,
concentration-dependent studies with complex 1a indicate that
sufficient Cu signal is observable for concentrations between 0.05
and 1 mM (data not shown). Due to practical limitations caused
by dialysis of the microgels and subsequent catalyst activation, a
nominal metal complex concentration of 0.05 mM was used for
all EPR experiments thereafter. On average, 80% of the Cu signal
is detectable as determined by spin quantization under nonsatur-
ating conditions.
The electronic environment around the Cu ions in the low
molecular weight complexes 1a and 2a is different due to the
H-bond-donating and H-bond-accepting nature of the backbone
ligands (Chart 1). Consequently, different EPR spectra for
complexes 1a and 2a (Figure 5) are observed. However, the
EPR spectra of complex 1a and its corresponding microgel
polCu28 are alike, indicating that the complex does not undergo
large structural changes due to immobilization in a microgel. Slight
line broadening reflecting restricted orientation of the immobilized
catalyst is noted for the microgel polCu28 in comparison to 1a.
Complex 2a and microgel polCu23 behave likewise.
The results of the EPR experiments suggest that the catalyti-
cally active species in the microgels is very similar to those of the
low molecular weight complexes and does not provide a rational
for the dramatically increased catalytic activity observed by the
microgels (see below). Further experiments in this regard and
modifications of the microgels to increase the catalyst loading are
topics of ongoing investigations.
Evaluation of polCu23 and polCu28 as Catalysts for Clea-
vage of Glycosidic Bonds. In order to evaluate the ability of the
macromolecular entities to hydrolyze glycosidic bonds in com-
parison to their small molecular weight analogs, catalytic hydro-
lysis of p-nitrophenyl-R-D-galactopyranoside (16) into galactose
and p-nitrophenolate was followed using UVꢀvis spectroscopy
(Scheme 5).19 The reaction was previously shown to depend
In brief, the in situ prepared complex 2a exists in about
93% as binuclear [Cu2Lꢀ3H]+ (L = 3a) species coordinating
water molecules to the metal-ion core and less than 7%
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Inorganic Chemistry
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Scheme 4. Putative Composition of 1a, Cu2(bpdpo), at pH 10.5
The low molecular weight catalysts 1a and 2a were previously
found to cleave the glycosidic bond in substrate 16 with a
catalytic turnover of up to 43 ꢁ 10ꢀ4 minꢀ1, a moderate
efficiency (kcat/KM), and an acceleration (kcat/knon) of the
hydrolysis of up to 11 000-fold over the background reaction
(Table 2, entries 1 and 2).19 By contrast, the macromolecular
catalyst polCu23 increases the catalytic efficiency for hydrolysis
of the glycosidic bond in 16 by about an order of magnitude in
comparison to 2a (Table 2, entries 2 and 3); use of macro-
molecular polCu28 instead of small molecular weight complex 1a
increases the catalytic efficiency even further (Table 2, entries 1
and 4). Acceleration of the reaction and the catalytic efficiency of
the hydrolysis of substrates 17ꢀ19 by polCu28 is also compar-
able to hydrolysis of 16, accounting for a high catalytic activity of
the macromolecular catalyst toward cleavage of glycosidic bonds
(Table 2, entries 5ꢀ7); however, catalyst polCu28 is not capable
of differentiating R- or β-glycosidic linkages or the gluco or galacto
configuration of the sugar substrates. Nevertheless, the results are
remarkable as the catalytic proficiency (kcat/(Kmknon)) of polymer
polCu28 toward the investigated substrates is as high as 5.6 ꢁ 107 in
alkaline solution and thus more than 3 orders of magnitude higher
than the proficiency determined for its small molecular weight
analog 1a. Stimulated by these encouraging results, current studies
are directed toward templating the catalyst polCu28 to enable a
selective cleavage of glycosidic bonds with macromolecular catalysts
in the near future.
Figure 5. EPR spectra of complexes Cu2(bpdpo) (1a, black line) and
Cu2(bsdpo) (2a, green line) and of the corresponding activated micro-
gels polCu28 (red line) and polCu23 (blue line) at 77 K.
’ SUMMARY OF RESULTS
linearly on the catalyst concentration, to require a pH above 9,
and to proceed with higher efficiency for the binuclear complex
1a in comparison to 2a or mononuclear analogs thereof
(Figure 1).19 Attempts to increase the substrate concentration
to reach saturation of the catalytic sites are limited by the
solubility of the selected substrates 16, p-nitrophenyl-β-D-galac-
topyranoside (17), p-nitrophenyl-R-D-glucopyranoside (18), and
p-nitrophenyl-β-D-glucopyranoside (19).19 Nevertheless, pseudo-
first-order conditions were provided with excess of substrate in
concentrations between 5 and 35 mM (Table 2).
The turnover rate for hydrolysis of 16 in the presence of
metal-free polymers pol3 and pol8 is on the same order of
magnitude as its free hydrolysis in solution (knon = 3.8 ꢁ 10ꢀ7
minꢀ1; knon,pol3 = 5.2 ꢁ 10ꢀ7 minꢀ1; knon,pol8 = 3.4 ꢁ 10ꢀ7
minꢀ1). Hydrolysis of the substrates is not promoted by a Cu(II)
acetate solution, as precipitation of Cu(II) hydroxide is observed
at pH 10.5 instead.
Macromolecular entities were prepared and explored as
catalysts to enhance the hydrolysis of glycosidic bonds in com-
parison to small molecular weight analogs. Along these lines,
pentadentate ligand 3 was synthesized as previously disclosed,
while a feasible synthesis for the new polymerizable pentadentate
ligand VB(bpdpo) (8) was developed and described herein.
Binuclear copper(II) complexes Cu2(VBbpdpo), 1b, and Cu2-
(VBbsdpo), 2b, were subsequently derived from VB(bpdpo), 8,
and VB(bsdpo), 3, respectively, and characterized in the solid
state. Analysis of single crystals of 2b suitable for X-ray diffrac-
tion revealed that derivatization of the backbone ligand at its
periphery does not alter the geometry of the donor atoms around
the metal ion core nor the metalꢀmetal ion distance in the
binuclear complex.
Polymers prepared from aqueous miniemulsions of styrene or
butyl acrylate in the presence of 3 or 8, respectively, lead to
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dormant catalysts. The nominal ligand concentration should not
exceed 1 mM to avoid phase separation during polymerization.
The dormant catalysts can be activated by coordination of
Cu(II) ions; sufficient metal-ion loading ability between Cu(II)
ions and the backbone ligand in a 2:1 molar ratio was observed
and followed by isothermal titration calorimetry. The results
are in agreement with the Cu(II)-ion binding ability deter-
mined for small molecular weight analogs by spectrophoto-
metric titration.
Glycosidic hydrolysis of p-nitrophenylglycoside model sub-
strates in alkaline aqueous solution was previously observed with
a turnover rate of up to 43 ꢁ 10ꢀ4 minꢀ1 for 1a; the same reaction
is more than 1 order of magnitude faster and has a turnover rate
of up to 380 ꢁ 10ꢀ4 minꢀ1 when using the macromolecular
catalyst polCu28 under otherwise identical conditions. The
reaction is about 105-fold accelerated over the uncatalyzed
background reaction. A similar enhancement was previously
observed for acceleration of catalytic oxidation of derivatized
catechols into quinones by macromolecular catalysts.16 How-
ever, the polCu28 catalyst does not discriminate between R- or
β-glycosidic linkages or the gluco and galacto configurations of
the carbohydrate moieties of the selected substrates. Current
efforts are therefore directed toward templating the matrix
during material preparation to provide microgel catalysts with
sufficient selectivity during hydrolysis of glycosidic bonds in
the near future.
values are reported in ppm relative to the residual signals of these
solvents (CDCl3 δ = 7.29 for 1H and δ = 77.0 for 13C; DMSO δ = 2.59
1
for H and δ = 39.5 for 13C). MS analysis of samples was performed
using an Ultra Performance LC Systems (ACQUITY, Waters Corp.,
Milford, MA) coupled with a quadrupole time-of-flight mass spectro-
meter (Q-TOF Premier, Waters) with electrospray ionization (ESI) in
both ESI-MS and ESI-MS/MS modes operated by the Masslynx soft-
ware (V4.1). X-ray analysis was performed on a Bruker SMART APEX
CCD X-ray diffractometer. IR spectra of oils were recorded as thin films
on KBr discs; solids were measured as KBr pellets with a resolution of
0.5 cmꢀ1 using a Shimadzu IR Prestige-21 FT-Infrared Spectrometer; ν
in cmꢀ1. UVꢀvis spectra were recorded at 30.0 ( 0.1 °C over a range of
200ꢀ800 nm on a Varian Cary 50 with WinUV Analysis Suite software,
version 3.0, using disposable Brandtech macrocells (220ꢀ900 nm) of
1 cm thickness and 4.5 mL volume for determination of the distribution
of species. Disposable 1.5 mL semimicro Brandtech UV cuvettes
(220ꢀ900 nm) of 10 mm light path with caps were used for the
hydrolysis studies. Isothermal titration calorimetry (ITC) was per-
formed on a VP isothermal titration calorimeter (Microcal Inc., North-
ampton, MA) at 300 K; the reference cell was loaded with nanopure
water. EPR spectra were obtained with a Bruker EMX spectrometer at
X-band (9383 MHz) and fitted with the ER-4119-HS perpendicular-
mode cavity. Sonication was performed with a Digital Sonifier from
Branson equipped with a disruptor horn and a tapered 1/8 in. microtip at
a 70% amplitude for 2 min. Samples for elemental analysis were sent to
Atlantic Microlab Inc., Atlanta, GA, or obtained on the CHN-O-RAPID
instrumentation (Heraeus), Ulm University, Germany. The pH values
were measured using a Beckman Φ 250 pH meter equipped with a
refillable long Futura pH electrode of 0.7 mm thickness. The pH meter
was calibrated before each set of readings (3-point calibration). All
melting points were recorded on a Mel-Temp melting point apparatus,
and the values are uncorrected. Thin layer chromatography (TLC) was
performed using silica gel TLC plates from SORBENT Technologies,
200 μm, 4 ꢁ 8 cm, aluminum backed, with fluorescence indicator F254
and detection by UV light or charring with an aqueous vanillinꢀsulfuric
acid reagent and subsequent heating of the TLC plate. Column chromato-
graphy was carried out using silica gel 60 from Silicycle (40ꢀ63 μm,
230ꢀ240 mesh) or basic aluminum oxide (pH range 9.7 ( 0.4, activity I),
32ꢀ63 μm, surface area 150 m2 gꢀ1 from Sorbent Technologies as
stationary phase. Sephadex LH-20ꢀ100 was obtained from Fluka and
equilibrated in methanol for 24 h prior to use. Nanopure water was ob-
tained from an EASYpure II water system from Barnstead (18.2 MΩ/ cm).
The dialysis membrane Spectra/Por Biotech (16 ꢁ 10 mm; 0.79 mL/cm;
volume/length) had a molecular cutoff of 15 000 and was obtained from
Spectrumlabs.
’ EXPERIMENTAL SECTION
1
Instrumentation. H and 13C NMR spectra were recorded on a
Bruker AV400 (400.2 MHz for 1H and 100.6 MHz for 13C). Chemical
shifts (δ) in 1H NMR spectra are expressed in ppm and coupling
constants (J) in Hertz. Signal multiplicities are denoted as s (singlet), d
(doublet), t (triplet), q (quartet), and m (multiplet). Deuterated
chloroform and DMSO-d6 were used as solvents, and chemical shift
Scheme 5. Catalytic Hydrolysis of p-Nitrophenyl-R-D-galac-
topyranoside (16)
Table 2. Summary of Kinetic Parameters for the Catalytic Hydrolysis of p-Nitrophenyl-R/β-D-glycopyranosides in 50 mM CAPS
buffer at pH 10.5 and 30 ( 0.1 °Ca
entry
catalyst
S [M]
kcat ( ꢁ 10ꢀ4) [minꢀ1
]
KM [mM]
kcat/KM [Mꢀ1 minꢀ1
]
kcat/knon [M]
kcat/(Kmknon) ( ꢁ 103)
metal complexesb
210 ( 57
1b
2b
1a
2a
16
16
43 ( 10
13 ( 3
0.02
11 000
3300
54
22
150 ( 43
0.009
activated macromolecular catalysts
3
4
5
6
7
polCu23
polCu28
16
16
17
18
19
20 ( 1
20 ( 4
0.1
3800
192
350 ( 60
360 ( 44
380 ( 20
375 ( 10
2.7 ( 0.1
1.9 ( 0.6
4.6 ( 0.2
3.0 ( 1.4
13
19
8
104 000
106 000
112 000
110 000
39 000
56 000
24 000
37 000
13
a knon = 3.8 ꢁ 10ꢀ7 minꢀ1; knon,pol3 = 5.2 ꢁ 10ꢀ7 minꢀ1; knon,pol8 = 3.4 ꢁ 10ꢀ7 minꢀ1; εsol (p-nitrophenolate, 400 nm) = 16 190 Mꢀ1 cmꢀ1
;
εpol (p-nitrophenolate, 400 nm) = 24 400 Mꢀ1 cmꢀ1. b Data taken from ref 19.
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Chemicals. The polymerizable ligand N,N0-1,3-bis[(2-hydroxy-4-
vinylbenzyloxy)benzylideneamino]propan-2-ol, VBbsdpo (3),16 its
water-soluble analog N,N0-1,3-bis[{2-hydroxy-4-[2-(2-methoxyethoxy)-
ethoxy]ethoxy}benzylideneamino]-propan-2-ol, TEGbsdpo (3a),33
the {N,N0-1,3-bis[2-hydroxy-4-(vinylbenzyloxy)benzylideneamino]-
propan-2-ol}ato (μ-acetato) dicopper complex, Cu2(VBbsdpo) (2b),16
{N,N0-1,3-bis[{2-hydroxy-4-[2-(2-methoxyethoxy)ethoxy]ethoxy}ben-
zylideneamino]-propan-2-ol}ato (μ-acetato) dicopper complex, Cu2-
(TEGbsdpo) (2a), binuclear Cu(II) complex N,N0-{bis(2-pyridylmethyl)-
1,3-diaminopropan-2-ol}ato dicopper(II) (μ-acetato) diperchlorate,
Cu2(bpdpo) (1a),16ꢀ18 and 2-(hydroxymethyl)pyridine-3-ol hydro-
chloride 1023 were synthesized as described. Styrene (14) and butyl
acrylate (15) were distilled under vacuum and stored under argon in the
dark at 10 °C prior to use. All other chemicals were used as received from
commercial suppliers.
Miniemulsion Polymerization: Typical Procedure. The poly-
(acrylateꢀstyrene)ꢀligand microgels, polL (L = 3 or 8), were prepared by
mixing 0.30 g (2.3 mmol) of butyl acrylate, 0.30 g (2.9 mmol) of styrene,
and 50 mg of decane with 4.8 mL of water containing 14 mg (0.05 mmol) of
sodium dodecyl sulfate (SDS). The ligand (L = 3 or 8, 5 μmol) was
dissolved in 50 μL of DMSO and added to the monomer solution. The
resulting mixture was cooled in ice, sonicated at 70% amplitude for 2 min,
and heated to 72 °C under continued stirring. Polymerization was initiated
after 10 min by addition of 0.4 mL of aqueous potassium peroxysulfate
(20 mg, 0.07 mmol) solution and allowed to proceed for 90 min.
Polymer Purification by Dialysis. Dialysis membranes with a
molecular weight cutoff of 15 kDa and a nominal diameter of 10 mm
were soaked in Nanopure water for 30 min prior to immediate use and
filled with 1 mL sample aliquots of the microgel stock solution. The
tubing was subsequently closed and dialyzed against nanopure water for
2 h and four times for 6 h each against 5 mM CAPS buffer at pH 10.5
subsequently. The samples of the purified polymers were then diluted to
10 mL with the same buffer. The nominal concentration of the ligand in
the resulting polymers was 0.1 mM. The control polymer polblank was
treated identically.
Speciation Experiments. In a typical experiment, 2 mL of a
50 mM CAPS buffer solution at pH 10.5 that is 0.02 mM regarding the
ligand (3a or 8a) and 1.0 mM regarding the NaClO4 concentration were
titrated with 10 μL aliquots of an aqueous 0.30 mM Cu(II) acetate
solution at 30.0 ( 0.1 °C. UVꢀvis spectra were taken after each addition
of an aliquot of the Cu(II) solution, and the resulting data were computed
by the global fitting model provided by the program SpecFit.31
EPR Experiments: Typical Procedure. Complex 1a (1.31 mg,
0.002 mmol) was dissolved in 2.0 mL of 50 mM CAPS buffer at pH 10.5
yielding a 1.0 mM stock solution; aliquots of this solution were further
diluted to obtain samples with concentrations in the range of 0.01ꢀ1 mM
and an overall sample volume of 300 μL. Alternatively, a 0.05 mM sample
was prepared by dissolving 1.64 mg (0.003 mmol) of complex 1a into
50.0 mL of 50 mM CAPS buffer at pH 10.5.
Complex 2a was prepared in situ as 0.05 mM solution from 20 μL of a
solution containing ligand 8a (2.34 mg, 0.004 mmol) in 5.0 mL of
50 mM CAPS buffer at pH 10.5 and 20 μL of a solution of copper acetate
monohydrate (1.50 mg, 0.008 mmol) in 5.0 mL of nanopure water; the
total volume of the sample was increased to about 300 μL by addition of
260 μL of buffer, yielding the indicated concentration.
The microgels polCu28 and polCu23 were prepared and dialyzed as
described, yielding solutions with a nominal concentration of 0.1 mM
with respect to the metal complex in 5 mM CAPS buffer. Samples for
EPR measurements were prepared by diluting 150 μL of the dialyzed
microgel solutions with 150 μL of 50 mM CAPS buffer.
aqueous copper perchlorate solution as the standard (10 mM CuSO4;
2 mM NaClO4; 10 mM HCl(aq)).
Isothermal Titration Experiments: Typical Procedure. An
aliquot of the dialyzed polymer pol3, pol8, or polblank with a nominal
ligand concentration of 0.1 mM in 5 mM CAPS buffer at pH 10.5 was
placed in the sample cell. The reference cell was loaded with nanopure
water. The polymers were subsequently titrated with 10 μL aliquots of a
20 mM aqueous Cu(II) acetate solution at 30.0 ( 0.1 °C. The obtained
data were fitted by the sequential binding site model implemented in the
data collecting software Origin 7.0.
Catalytic Hydrolysis of Glycosides: Typical Procedure. In a
typical experiment, the concentration of p-nitrophenylglycosides was
5ꢀ30 mM with a catalyst concentration of 0.01 mM in a solution with a
total volume of 1.0 mL. All solutions were prepared or diluted into
50 mM CAPS buffer at pH 10.5. The substrate aliquots were diluted with
buffer to a total volume of 900 μL and equilibrated at 30 °C for 30 min.
The reaction was initiated by addition of a 100 μL aliquot of the
equilibrated 0.1 mM catalyst solution. The proceedings of the reaction
were monitored over time by recording the formation of the p-
nitrophenolate using UVꢀvis spectroscopy at 400 nm. The background
reaction was observed in a similar manner in the absence of a polymer
catalyst or, alternatively, in the presence of the control polymer polblank
The extinction coefficient for product formation in the presence (εsol
.
)
and absence (εsol) of the microgel catalyst was determined by a
calibration curve and used to convert the absorbance of the reaction
product into molar amounts (p-nitrophenolate: εsol = 16 190 Mꢀ1 cmꢀ1
and εpol = 24 400 Mꢀ1 cmꢀ1). The value for the extinction coefficients
varies due to the turbidity in the solution caused by addition of the
microgel catalysts.
(3-(4-Vinylbenzyloxy)pyridin-2-yl)methanol (11). Synthesis
of 11 was achieved by modifying a procedure for preparation of
2-hydroxymethyl-3-benzyloxypyridine disclosed by Sheehan et al.34 In
short, an aqueous solution (50 mL) of potassium hydroxide (8.1 g, 0.14
mol) and potassium iodide (0.16 g, 1.0 mmol) was added to a solution of
10 in 50 mL of water. p-Vinylbenzylchloride (12 mL, 75 mmol) in
200 mL of methanol was then added dropwise while cooling the solution
in an ice bath. Subsequently, methanol was added until the solution
turned clear (∼150 mL), and stirring was continued for 72 h. The
solution was concentrated after filtration in vacuum below 30 °C until
phase separation occurred. The flask was kept at 8 °C for 24 h, during
which time the darker orange layer solidified. A light yellow, micro-
crystalline product was separated by filtration and washed with ice-
chilled methanol yielding 4.0 g (54%) of 11; mp 95ꢀ96 °C; δH (CDCl3)
8.19 (m, 1H, ꢀC5H3N), 7.46 (dd, 2H, 7.3, 1.0, ꢀC6H4ꢀ), 7.38, (dd,
2H, 7.3, 1.0, ꢀC6H4ꢀ), 7.19 (d, 2H, 3.0, ꢀC5H3N), 6.75 (dd, 1H, 17.7,
10.7, CH2dCH), 5.80 (dd, 1H, 17.7, 0.8, CH2dCH), 5.31 (dd, 1H,
10.5, 0.4, CH2dCH), 5.13 (s, 2H, CH2ꢀC6H4ꢀ), 4.84 (s, 2H,
ꢀCH2ꢀC5H3N); 3.38 (bs, 2H, NH); δC 151.3, 148.7, 139.7; 137.6,
136.2, 135.4, 127.3, 126.5, 122.6, 117.9, 114.4, 69.7, 60.1. ν (KBr): 3433
(NH), 3075, 3023 (arom. CH), 2929, 2868 (aliph. CH), 1573 (CꢀN),
1445, 1408 (arom. CꢀC), 1276 (OH), 1054 (CꢀO) cmꢀ1. Anal. Calcd
for C15H15NO2: C, 74.67; H, 6.27; N, 5.81. Found: C, 74.63; H, 6.24; N,
5.71; HRMS, ESI-TOF (+), calcd for C15H16NO2 (M + H)+: 242.1881.
Found: 242.1886.
3-(4-Vinylbenzyloxy)picolinaldehyde (12). Selenium dioxide
(460 mg, 4.15 mmol) was added to a solution of 11 (1.0 g, 4.2 mmol) in
10 mL of dry 1,4-dioxane under argon at room temperature. The
resulting suspension was stirred under reflux for 3 h and filtered through
glass wool after cooling. The filtrate was concentrated under reduced
pressure at 32 °C to yield 12 as an orange oil that was typically not
further purified. For a full characterization of 12, however, a sample
aliquot of the residue was purified by flash chromatography over silica
gel, eluting with a gradient of cyclohexane/EtOAc (2/1 to 1/1, v/v),
yielding 12 as a colorless solid (TLC, SiO2, cyclohexane/EtOAc, 1/1,
All spectra were recorded with a field modulation frequency of
100 kHz, the microwave power was 2 mW, and cooling of the samples
was performed with a liquid-nitrogen finger Dewar (77 K). Spin quan-
tization was carried out under nonsaturating conditions using an
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ARTICLE
v/v, Rf = 0.6). A large loss of product was observed during purification
due to putative polymerization to a gummy solid particularly during
concentration of any solution containing 12 prior to and after gel
chromatography. Use of aluminum oxide instead of silica gel resulted in
complete product loss and was for that reason not pursued further; mp
62ꢀ63 °C; δH (DMSO-d6) 10.31 (s, 1H, CHO), 8.46 (d, 1H, 4.2,
ꢀCH), 7.89 (d, 1H, 8.7, ꢀCH), 7.72 (dd, 1H, 4.3, 8.5, ꢀCH), 7.59 (s,
4H, ꢀCH), 6.82 (dd, 1H, 17.7, 10.93, ꢀCH), 5.94 (d, 1H, 17.7, ꢀCH2),
5.40 (s, 2H, ꢀOCH2), 5.36 (d, 1H, 10.93, ꢀCH2); δC (DMSO-d6)
190.3, 156.5, 141.9, 140.7, 136.8, 136.2, 135.6, 129.23, 127.6, 126.2,
122.6, 114.5, 69.5. ν (KBr pellet): 3071, 2794, 2707, 1706, 1557, 1455,
1290, 1153, 973 cmꢀ1. HRMS, ESI-TOF (+), calcd for C15H14NO2
(M + H)+: 240.1025. Found: 240.1031.
C35H38Cl2Cu2N4O13 CH2Cl2: C, 43.00; H, 4.01; N, 5.57. Found: C,
3
42.86; H, 4.23; N, 5.59. νmax (KBr)/cmꢀ1 3416, 3265, 3092, 2926, 1622,
1577, 1453, 1409, 1352, 1283, 1227, 1092.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR, IR, and ESI
S
b
mass spectra of all new compounds; cif file for 2b; stability
constants for species derived from 3a, 8a, and Cu(II) acetate
monohydrate. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
1,3-Bis((E)-(3-(4-vinylbenzyloxy)pyridin-2-yl)methylene-
amino)propan-2-ol (13). Synthesis followed a procedure developed
by Mazurek et al. for preparation of related backbone ligands derived
from pyridine-2-carbaldehyde.18,35 A solution of compound 12 (2.18 g,
9.12 mmol) in 25 mL of methanol was added dropwise to a solution of
1,3-diaminopropan-2-ol (7) (0.41 g, 4.6 mmol) in 25 mL of methanol.
The resulting solution was stirred overnight and used without further
purification for preparation of 8. A sample aliquot was subjected to
analysis by ESI mass spectrometry to verify formation of the title
compound. HRMS, ESI-TOF (+), calcd for C33H33N4O3 (M + H)+:
533.2552. Found: 533.2549.
1,3-Bis((3-(4-vinylbenzyloxy)pyridin-2-yl)methylamino)-
propan-2-ol, VB(bpdpo) (8). The volume of the solution of 13 was
increased to about 150 mL, and sodium borohydride (0.35 g, 10 mmol)
was added in small quantities. After termination of gas evolution, the
solution was heated to reflux for 45 min, cooled, and concentrated to
dryness. The residue was dissolved in 100 mL of chloroform and
extracted once with 15 mL of ice-cold water. The organic layer was
separated, dried over anhydrous sodium sulfate, and filtered. The filtrate
was concentrated in vacuum, yielding 8 as a yellow-brown oil (2.0 g,
82%). The compound was dissolved in ethanol to produce a 1 M stock
solution for subsequent preparation of the Cu(II) complex 1b.
A sample aliquot of the raw product of 8 was purified on Sephadex
LH-20ꢀ100 with methanol as eluent; Rf (SiO2; EtOAc/MeOH, 4/1) =
0.1, yielding 8 as a yellowish oil (65%). This fraction was used for the
polymerization experiments. A sample aliquot of this yellowish oil
was further purified by preparative-scale HPLC on RP-18 eluting with
acetonitrileꢀwater to allow full compound characterization; δH (CDCl3)
8.17 (dd, 2H, 4.61, 1.45), 7.42 (dd, 8H, 7.96, 26.53), 7.17 (dd, 2H, 8.34,
1.52), 7.13 (dd, 2H, 8.21, 4.55), 6.74 (dd, 2H, 17.62, 10.93), 5.78 (dd,
2H, 17.56, 0.56), 5.78 (dd, 2H, 17.56, 0.76), 5.29 (dd, 2H, 10.86, 0.76),
5.10 (s, 4H), 4.31 (br s, 1H), 4.05 (s, 4H), 3.83 (s, 1H), 2.81 (dd, 2H,
12.19, 3.73) 2.70 (dd, 2H, 12.19, 8.15); δC (DMSO-d6) 152.0, 149.2,
140.2, 136.8, 136.3, 136.2, 127.8, 126.3, 123.3, 122.6, 119.2, 118.9, 114.5,
79.2, 69.0, 59.9, 53.8, 49.7. IR (thin film on KBr): 3404, 3058, 2918,
1629, 1571, 1443, 1275, 1177, 1116, 992, 910, 829 cmꢀ1. HRMS,
ESI-TOF (+), calcd for C33H37N4O3 (M + H)+: 537.2866. Found:
537.2876.
Corresponding Author
*Phone: ++1-334-844-6954. Fax: ++1-334-844-6959. E-mail:
Susanne.Striegler@auburn.edu.
’ ACKNOWLEDGMENT
A CAREER award from the National Science Foundation
(CHE-0746635) to S.S. is gratefully acknowledged. The authors
are also indebted to Thomas E. Albrecht-Schmidt and his research
group for analysis of 2b by X-ray diffraction and to Thomas
Webb for insightful comments during the preparation of this
manuscript.
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N,N0-{Bis((3-(4-vinylbenzyloxy)2-pyridinmethyl)-1,3-dia-
monopropan-2-ol}ato (μ-acetato) Dicopper Diperchlorate,
Cu2(VBbpdpo) (1b). A sample aliquot (3.7 mL, 3.7 mmol) of the 1 M
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monohydrate (1.6 g, 0.01 mol) in 150 mL of ethanol at ambient
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methanolꢀdichloromethane yielded complex 1b as a dark blue amor-
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needle, crystal dimensions 0.300 ꢁ 0.070 ꢁ 0.066 mm, monoclinic,
P21/n, Z = 4, a = 8.6070(3) Å, b = 20.0239(8) Å, c = 20.9300(8) Å,
β = 101.4446(8)°, V = 3535.5(2) Å3 (T = 193 K), μ = 14.08 cmꢀ1
,
R1 = 0.0440, wR2 = 0.1081. Bruker SMART APEX CCD X-ray
diffractometer: θmax = 56.58°, Mo KR radiation, λ = 0.71073 Å, 0.3°
ω scans, 35 986 reflections measured, 8769 independent reflections all of
which were included in the refinement. The data were corrected for
Lorentz-polarization effects and absorption (analytical and SADABS);
solutions were solved by direct methods; anisotropic refinement of F2 by
full-matrix least-squares, 460 parameters. Further details of the crystal
structure investigations may be obtained from the CIF files.
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