Thermodynamic, Spectroscopic, and Structural Studies of Complexation of Phenol- and
Pyridine-Armed Macrocyclic Ligands with Univalent Metal Ions
Xian Xin Zhang, Andrei V. Bordunov,
†
Xiaolan Kou, N. Kent Dalley, Reed M. Izatt,*
John H. Mangum, Du Li, Jerald S. Bradshaw,* and Paul C. Hellier
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602
ReceiVed October 16, 1996
X
Log K, ΔH, and ΔS values for interactions of a series of pyridinoazacrown ethers each bearing a phenol arm
(2-6) and two macrocycles each bearing a pyridine arm (7, 8) with Na
+
,K
+
, Tl
+
, and Ag
+
have been determined
in absolute methanol at 25 °C by calorimetric titration. In each case, the complex stability has the sequence Na
+
< K
+
< Tl
+
, Ag
+
. The phenol-armed macrocycles exhibit selectivity of more than 4 orders of magnitude for
Ag
+
over Na
+
,K
+
, and Tl
+
. Attachment of a pendant phenol arm having various substituents to parent macrocycle
1 increases the binding abilities of the resulting ligands. Substituents on the para position of the phenol arm have
an appreciable effect on cation-binding constants. Good Hammett correlations are found by plotting log K values
vs σ
p
for interactions of five phenol-armed macrocyclic ligands (2-6) with Na
+
,K
+
, and Tl
+
. The complexation
has been characterized by means of
1
H NMR and UV-visible spectroscopic, and X-ray crystallographic methods.
The crystal data for Na
+
-3: formula, [Na(C
23
H
28.5
N
3
O
5
)](ClO
4
)
0.5
; space group, P1 h; a ) 9.400(9) Å, b )
11.467(10) Å, c ) 12.281(11) Å, R) 77.22(7)°, ) 87.73(7)°, γ ) 86.39(7)°, V ) 1288(2) Å
3
, and Z ) 2. The
study indicates that the phenol OH group of 2-6 is capable of forming an intramolecular hydrogen bond with the
macroring nitrogen atom and that the complexation in absolute methanol generally does not deprotonate these
phenols. In the crystal structure of the Na
+
-3 complex, the Na
+
is coordinated to all seven of the donor atoms
of the ligand and two Na
+
-3 complexes join together to form a dimer. The dimer contains an intermolecular
hydrogen bond formed between the phenol hydrogen atom of one ligand and the phenolate group of a
centrosymmetrically related ligand and two π-π stacking interactions between the electron-deficient pyridine
ring of one molecule and the electron-rich phenol ring of the other.
Introduction
Pyridine-containing macrocycles form stable complexes with
metal ions,
1-10
various ammonium cations,
10-14
and water
molecules.
15
A macrocyclic ligand having a pyridine ring
incorporated into its backbone usually has notably different
complexation properties from its fully saturated analogues.
Sherry and co-workers have recently observed that incorporation
of one or two pyridine groups into tetraaza macrocycles results
in a significant change in both complexation kinetics and
thermodynamic properties.
5
The complexation rates of the
pyridine-containing tetraaza ligands with Gd
3+
are fast enough
for potentiometric titrations to be made while the CYCLEN
derivatives form complexes with Gd
3+
too slowly for normal
potentiometric titrations. Jackels and co-workers have found
that incorporation of two pyridine moieties into an [18]aneN
6
-
type macrocycle results in an increase in binding constants for
its interaction with Ca
2+
, Zn
2+
, and La
3+
by up to 3 orders of
magnitude over those of the parent macrocycle.
6
Thermo-
dynamic quantities determined by Herman and co-workers
indicate that the stability of the Cu
2+
complex with a hexaaza
macrocycle containing two pyridine rings is 1.5 log K units
larger than that of the Cu
2+
complex with the fully saturated
[18]aneN
6
.
7
The higher log K value is entirely due to an entropy
effect. Because of the presence of the two pyridine groups,
the macrocycle is preorganized and shows a small conforma-
tional entropic loss during the complexation.
Among seven pyridinocrown ethers of different macroring
sizes (from 15-crown-5 to 33-crown-11), the 18-membered
ligand was shown to have the maximum binding constants for
all alkali metal cations.
4
Several pyridino macrocycles have
been shown to be more effective membrane carriers for Ag
+
than the parent crown ethers.
10
Kumar, Singh, and co-workers
recently synthesized a series of pyridine-containing macrocycles
and studied their metal ion complexation properties.
8
They
†
Present address: Division of Chemistry and Chemical Engineering 210-
41, California Institute of Technology, Pasadena, CA 91125.
X
Abstract published in AdVance ACS Abstracts, May 15, 1997.
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2586 Inorg. Chem. 1997, 36, 2586-2593
S0020-1669(96)01263-3 CCC: $14.00 © 1997 American Chemical Society