Supramolecular catalytic systems based on bolaform pyrimidinic surfactants: the counterion effect

Article abstract of DOI:10.1016/j.mencom.2010.03.018

The catalytic effect of new amphiphilic pyrimidinic compounds with two ammonium head groups and different kinds of counterions, inorganic bromide anions and hydrophobic tosylate anions, was studied and compared with that of conventional cetyltrimethylammo

Full text of DOI:10.1016/j.mencom.2010.03.018

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 116–118
Supramolecular catalytic systems based on bolaform
pyrimidinic surfactants: the counterion effect
a,b
a
a,b
Lucia Ya. Zakharova,* Victor V. Syakaev, Mikhail A. Voronin,
a
a
a
a
Vyacheslav E. Semenov, Farida G. Valeeva, Rashit Kh. Giniyatullin,
a
a
Shamil K. Latypov, Vladimir S. Reznik and Alexander I. Konovalov
a
b
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy
of Sciences, 420088 Kazan, Russian Federation. Fax: +7 8432 73 2253; e-mail: lucia@iopc.knc.ru
Kazan State Technological University, 420015 Kazan, Russian Federation
DOI: 10.1016/j.mencom.2010.03.018
The catalytic effect of new amphiphilic pyrimidinic compounds with two ammonium head groups and different kinds of
counterions, inorganic bromide anions and hydrophobic tosylate anions, was studied and compared with that of conventional
cetyltrimethylammonium surfactants with bromide and tosylate counterions.
Self-assembling systems based on amphiphilic compounds are
of interest from both fundamental and practical viewpoints.
They are typical biomimetic systems capable of interacting via
chains and their acyclic analogues with two hydrophilic groups
1
6,17
related to gemini or bolaform amphiphiles were studied.
In
this work, new AP compounds with bromide (APB) and tosylate
(APT) counterions are synthesized and their catalytic effect
towards hydrolysis of O-p-nitrophenyl-O-alkylchloromethylphos-
phonates [alkyl = ethyl (1), hexyl (2)] is studied (Scheme 1).
The initial motivation of such investigations is the development
of effective nanoreactors for important chemical reactions. How-
ever, as the information on the catalytic properties of amphiphilic
systems becomes accumulated and systematized it provides
insight into the aggregative behaviour of new amphiphilic com-
pounds through the examination of their catalytic behaviour. In
1–3
the guest–host mechanism. The compartmentalization of guest
molecules within nanosized aggregates results in a sharp increase
in their local concentrations (the concentration factor) and
changes of their microenvironment (the microenvironment factor),
which then affect their thermodynamic stability and correspond-
ing quantitative characteristics. From the practical viewpoint
the capacity of amphiphiles to aggregate and solubilize guest
molecules can be used for designing nanocontainers, nano-
reactors, etc.4 Our sphere is the formation of nanoreactors
for the hydrolysis of phosphorus acid esters by noncovalent
–9
particular, certain regularities are revealed for the hydrolysis of
phosphorus acid esters in direct micellar solutions.1
8–20
One of
strategy in single amphiphile solutions and mixed amphiphile–
polymer systems.1
0–12
The transfer of a phosphoryl group is one
the well established empirical findings is that the sign of the
catalytic effect (catalysis/inhibition) on ion-molecular reactions
is determined by the charge of aggregates. Cationic micelles
accelerate basic hydrolysis by concentrating the reagents in
micelles. Organic substrates are effectively solubilized in the
nonpolar micellar interior, while hydroxide ions are attracted to
the positively charged micellar surface due to electrostatic forces.
On the contrary, the inhibition of hydrolysis in anionic micelles
is due to the electrostatic repulsion of OH ions from the similarly
charged head groups, resulting in the separation of reagents.
Knowledge of this and others tendencies makes it possible to
analyze the reasons of deviations from them, including those
related to the structural factor (i.e., changes in sizes and shapes of
aggregates, in the surface potential, the counterion binding, etc.).
In this work, the kinetics of basic hydrolysis of phosphonates
1 and 2 in aqueous solutions of APB and APT is measured by
spectrophotometry (Figures 1, 2). The aggregation behaviour
of the APB and APT systems is covered elsewhere.21 The mecha-
nism of basic hydrolysis of organophosphorus compounds is
well established.15 Hydrolysis of p-nitrophenyl esters of phos-
porus acids proceeds through P–OAr bond fission with quanti-
tative liberation of the p-nitrophenoxide ion. These substrates
of the most fundamental chemical and biochemical reactions.
Besides, the motivation of such investigations is the search of
effective catalytic compositions for the destruction of phos-
1
3–15
phorus acid esters related to ecological toxicants.
A direc-
tion developed in our recent works concerns the design of
catalytic systems based on amphiphilic pyrimidinic (AP) com-
pounds, in particular, uracil derivatives. The self-assembly of
the macrocyclic pyrimidinic surfactants with two 5(6)-alkyl-
substituted uracil moieties bridged together by polymethylene
ClH C
O
2
P
RO
O
NO2
1
2
R = Et
R = hexyl
X–
N
X–
R
N
R
R
R
R'
R'
O
N
N
O
are widely explored in aqueous solutions and organized systems
of different morphologies.2
,3,15
Therefore, they are very suitable
C16H33
for comparing the catalytic activities of different systems. To test
whether new dimeric AP surfactants obey the typical behaviour,
their catalytic effect is compared with the influence of conven-
tional cetyltrimethylammonium (CTA) surfactants with bromide
APB: X = Br, R = R' = Et
APT: X = OTs, R = Et, R' = Me
Scheme 1 The chemical structures of substrates and bolas.
©
2010 Mendeleev Communications. All rights reserved.
– 116 –

Mendeleev Commun., 2010, 20, 116–118
cmc value of APB (0.4 mM) compared to CTAB (0.85 mM) as
evident from the surface tension data. These systems demonstrate
1
20
2
21
1
20
1
a quite different catalytic effect on the hydrolysis of 1, namely,
about 25-fold and 3-fold accelerations occur for CTAB and
APB, respectively. If the micellar systems are considered as
nanoreactors for the destruction of toxic organophosphorus com-
pounds, the following conclusions can be made: (i) the efficiency
of APB is a little less than that of CTAB; (ii) at the same
time, APB needs the lower concentrations to reach the maxi-
mum catalytic activity as compared to CTAB; (iii) bola APB
demonstrates marked substrate specificity; the kobs values for 1
and 2 differ almost tenfold near the cmc in the APB solution
against 1.2-fold in the CTAB solution.
Kinetic data for the APT system are given in Figure 2. The
micellar rate effect is observed at the concentrations lower than
the tensiometric cmc 0.00013 M,21 which is probably due to the
pre-micellar aggregation and/or the promotion of micellization
by substrates. The kobs vs. CAPT dependences for the hydrolyses
of 1 and 2 in the APT micellar system differ markedly from those
of conventional surfactants CTAB and CTAT and from those of
the dimeric surfactant APB. An inversion in the catalytic effect
8
4
0
9
6
3
0
0
0
0
0
.000 0.003 0.006 0.009
C
CTAB/mol dm–3
2
0
0
1
0
.000 0.001
0.002
0.003
0.004
C
APB/mol dm–3
Figure 1 Observed rate constants of hydrolyses of (1) 1 and (2) 2 as
functions of the APB concentration; 0.001 M NaOH; 25 °C. Inset: observed
rate constants of hydrolyses of (1) 1 and (2) 2 as functions of the CTAB
concentration; 0.001 M NaOH; 25 °C.
(
CTAB) and tosylate (CTAT) counterions. Substantial differences
are observed between the reactivities of phosphonates 1 and 2
in the single-head cationic surfactants. Hydrolysis of more
hydrophobic substrate 2 is accelerated by 47 and 35 times in
the CTAB and CTAT micelles, respectively (Figures 1, 2). Less
hydrophobic phosphonate 1 hydrolyzes in different ways in the
(
catalysis/inhibition) is observed depending on the substrate
hydrophobicity. A low four-fold acceleration of hydrolysis of 2
and nontypical inhibition of basic hydrolysis of 1 for cationic
micelles should be noted. The reasons of such deviation from
the typical behavior for cationic surfactants are revealed when
the pH vs. CAPT dependence for APT solutions is examined.
Unlike the three other surfactants studied, a sharp decrease in
pH occurs in the APT system on reaching the cmc (Figure 3),
which is confirmed by both, the pH-meter measurements and
the indicator probes. In the APB solution like in the systems
based on the conventional CTA surfactants the pH remains near
the neutral value throughout the whole concentration range
measured. A probable reason for this was assumed to be the
geometry of the AP compounds (Scheme 1), in particular, a
steric hindrance around the head groups preventing the counter-
ion binding. As distinct from CTAB and CTAT, the head groups
of bolas APB and APT bear ethyl groups instead of smaller
methyl ones. Besides, the presence of two heads linked by
flexible spacer allowed their approaches to be an additional source
for the steric barrier especially for the bulky p-toluene sulfonate
anions. On the other hand, a rigid uracil fragment in the spacer
prevents the too close approach of heads. This favours the dis-
sociation of the head groups, which is supported by the NMR
micellar solutions of these surfactants. A 25-fold increase in kobs
occurs in the CTAB micelles, while almost no effect is observed
in the CTAT solution. A slight minimum in the kobs vs. [surfactant]
plot near the critical micelle concentration (cmc) is probably
caused by the primary salt effect in the presence of monomer
surfactant. A rate enhancement for the reaction of anionic nucleo-
philes is typical of cationic micelles. The higher acceleration
found for the more hydrophobic substrate 2 is due to its higher
affinity towards the nonpolar micellar microenvironment resulting
in stronger binding with micelles as compared to substrate 1.
The higher catalytic effect of CTAB as compared to CTAT is
probably due to the higher surface potential of the CTAB micelles,
which provides the effective electrostatic attraction between
OH ions and the micelles. This assumption is in line with the
lower degree of the counterion binding for bromide counterions
2
1–24
in comparison with organic ions.
The acceleration of hydrolyses of 1 and 2 in the presence of
APB occurs, which correlates with the cationic character of the
APB head groups (Figure 1). The observed rate constants of
hydrolysis of 2 in the APB and CTAB solutions are commen-
surable (maximum values differ by 1.3 times), with the maximum
in the kobs–CAPB dependence shifted to the lower concentra-
tions, as compared to CTAB solution. This is due to the lower
21
self-diffusion data. Therefore, the strong polarization up to
the ionization of water molecules in solvate shells of head groups
can occur due to cooperative interactions of the dipoles of water
with a high uncompensated micellar charge. Thus, hydroxide ions
and conjugated hydroxonium ions are generated (Scheme 2).
Unlike bulky organic ions, smaller hydroxide ions can bind with
head groups, while residuary free protons acidify the system.
Thus, the inhibition of hydrolysis of 1 and low acceleration of
hydrolysis of 2 are due to the decrease in pH with surfactant
concentration. We tested if other inorganic ions (e.g., bromide
4
3
2
1
0
0
0
0
2
1
2
9
6
1
0
1
2
3
CTAT/mmol dm–3
C
2
7
6
5
4
3
2
1
3
0
1
0
.0
0.2
0.4
0.6
0.8
C
APT/mmol dm–3
0
.00001 0.0001 0.001
CAPT/mol dm–3
0.01
Figure 2 Observed rate constants of hydrolyses of (1) 1 and (2) 2 as
functions of the APT concentration; 0.001 M NaOH; 25 °C. Inset: observed
rate constants of hydrolyses of (1) 1 and (2) 2 as functions of the CTAT
concentration; 0.001 M NaOH; 25 °C.
Figure 3 The dependence of solution pH on the surfactant concentration.
–
117 –

Mendeleev Commun., 2010, 20, 116–118
1
and 2. Surfactants with hydrophobic anions demonstrate a
O
O
different behaviour, namely, hydrolysis of phosphonate 2 is
accelerated by CTAT and APT micelles, while that of less hydro-
phobic substrate 1 is retarded by the APT micelles unless an
alkali excess is used. This deviation from the typical behaviour
of cationic surfactants is due to the decrease in the solution pH
with the APT concentration above the cmc.
H
S
OH
N
O
N
d
H
O
N
HOd
N
N
O
N
O
N
O
O
O
This work was supported by the Russian Foundation for
Basic Research (grant no. 07-03-00392-a).
S
N
O
N
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2
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4
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–
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.000 0.002 0.004 0.006
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Figure 4 Observed rate constants of hydrolyses of (1, 3) 1 and (2) 2 as
functions of the APT concentration; (1, 2) [APT]:[NaOH] = 1:2; (3) 1:3;
2
5 °C. Inset: observed rate constant of hydrolysis of 1 as function of the
–
3
APT concentration; [NaOH] = 0.01 mol dm ; 25 °C.
Received: 13th August 2009; Com. 09/3377
–
118 –