Synthesis and characterization of a novel 32-membered unsymmetrical dinucleating [N12] macrocycle: Preparation of bimetallic complexes M2LX2(ClO4)2 (M = Zn, Cd, or Hg; X = Cl, NCS, or NO3)

Article abstract of DOI:10.1081/SIM-120037515

The condensation reaction of N-acetylaniline with 1,3-diaminopropane in 2:1 mole ratio under reflux condition generates an intermediate species having C=N bonds. The in situ reaction of the intermediate with HCHO and 1,3-diaminopropane in the presence of perchloric acid results in an off-white solid product. Analytical, FAB mass, IR, ¹H NMR and electronic spectral data of this solid are consistent with the formation of the dihydroperchlorate salt of an unsymmetrical 32-membered [N₁₂] macrocycle [L·2HClO ₄] via a capping mechanism of the intermediate species. Reactions of [L·2HClO₄] with Group 12 metal salts, MX₂, and their precursors, [M(Ph₃P)₂Cl₂], produce bimetallic complexes with the stoichiometries M₂LX ₂(ClO₄)₂ (M = Zn, Cd or Hg; X = Cl, NO ₃, or NCS) and M₂LCl₂(ClO₄) ₂, respectively, with liberation of 2 mol equiv. of the ancillary ligand, Ph₃P, from the precursors. Physico-chemical and spectroscopic data of the complexes confirm encapsulation of two metal ions in the macrocyclic cavity via coordination through nitrogen atoms of the unsymmetrical aza groups and the counter anions occupy apical positions in the octahedral geometry around the metal ions. The thermodynamic first ionic association constant and related parameters have been determined from conductometric studies using the Fuoss and Edelson method and are discussed.

Full text of DOI:10.1081/SIM-120037515

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 5, pp. 897–917, 2004
Synthesis and Characterization of a Novel
32-Membered Unsymmetrical Dinucleating
[N12] Macrocycle: Preparation of Bimetallic
Complexes M2LX2(ClO4)2 (M 5 Zn, Cd, or
Hg; X 5 Cl, NCS, or NO3)
*
Zafar Ahmad Siddiqi and Mohammad Mansoob Khan
Department of Chemistry, Division of Inorganic Chemistry, Aligarh
Muslim University, Aligarh, India
ABSTRACT
The condensation reaction of N-acetylaniline with 1,3-diaminopropane in
2 : 1 mole ratio under reflux condition generates an intermediate species
having C55N bonds. The in situ reaction of the intermediate with
HCHO and 1,3-diaminopropane in the presence of perchloric acid results
1
in an off-white solid product. Analytical, FAB mass, IR, H NMR and
electronic spectral data of this solid are consistent with the formation
of the dihydroperchlorate salt of an unsymmetrical 32-membered [N12]
.
macrocycle [L 2HClO4] via a capping mechanism of the intermediate
*
Correspondence: Zafar Ahmad Siddiqi, Department of Chemistry, Division of Inor-
ganic Chemistry, Aligarh Muslim University, Aligarh 202 002, India; E-mail:
nabilah@sancharnet.in.
897
DOI: 10.1081/SIM-120037515
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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.
species. Reactions of [L 2HClO4] with Group 12 metal salts, MX2, and
their precursors, [M(Ph3P)2Cl2], produce bimetallic complexes with the
stoichiometries M2LX2(ClO4)2 (M ¼ Zn, Cd or Hg; X ¼ Cl, NO3, or
NCS) and M2LCl2(ClO4)2, respectively, with liberation of 2 mol equiv.
of the ancillary ligand, Ph3P, from the precursors. Physico-chemical
and spectroscopic data of the complexes confirm encapsulation of two
metal ions in the macrocyclic cavity via coordination through nitrogen
atoms of the unsymmetrical aza groups and the counter anions occupy
apical positions in the octahedral geometry around the metal ions. The
thermodynamic first ionic association constant and related parameters
have been determined from conductometric studies using the Fuoss and
Edelson method and are discussed.
Key Words: Unsymmetrical macrocycle; Electronic spectral data;
Bimetallic complex; Macrocyclic ligand.
INTRODUCTION
Macrocyclic ligands that can incorporate two or more metal ions giving
discrete homo- or heterodinuclear complexes have been a subject of consider-
able interest during recent years. Some of these heterodinuclear complexes are
interesting as they mimic the active sites[1] of metalloenzymes, such as cyto-
chrome-c oxidase and bovine erythrocyte superoxide dismutase. Such systems
are also helpful in investigating mutual influences[2] of the two metal centers
on electronic, magnetic, and electrochemical properties. The polyazamacro-
cycle of the type (3k)-ane[Nk] has been shown to produce bimetallic com-
plexes[3] when k ꢀ 6. However, for the macrocycle (24)-ane[N6], e.g.,
[1,5,9,13,17,21]hexaazacyclotetracosane, encapsulation of only one metal
ion has been reported.[4] The metal-template condensation of a diamine
with 2,6-diformyl-4-methylphenol was the first one-pot reaction to yield a
dinuclear bimetallic macrocyclic complex.[5] Large-size macrocycles contain-
ing different lateral chains are obtained[1] through stepwise cyclization using
two dissimilar alkyldiamines by a template procedure. However, most of the
earlier reports[6–9] describe isolation of symmetrical macrocycles because the
preparations of such macrocycles, in general, involves simple synthetic routes.
Symmetrical polyaza macrocycles are those, which provide identical binding
sites to encapsulate metal ions in their cavities. Macrocycles that contain two
or more different types of coordinating sites to bind metal ions have been
classified[10–12] as unsymmetrical macrocycles. The large unsymmetrical
macrocycles reported[10,13–15] in the literature are those, which, usually,
involve heteroatomic binding centers like [N4O2], [N4OS], [N7O], [N4O10],

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899
and [N4O8S2] etc. A small unsymmetrical hexaaza [N6] macrocycle binding
metal ions in two different environments of aza groups recently has been
reported.[12] This [N6] macrocycle is obtained through a multistep procedure
using N-acetyl-1,3-diaminopropane as a reagent to generate an intermediate
that undergoes cyclization with 1,3-diaminopropane in the presence of
metal ions as templating agent. However, to our knowledge large unsym-
metrical polyaza macrocycles [Nk] with k . 8 to encapsulate more than one
metal ion are not reported. It was our interest to investigate condensation
reactions of diamines with suitable reagents, which may lead to cyclization
in the absence of metal ions as templating tool to produce metal ion-free
unsymmetrical polyaza macrocycles. Herein, we are the first to report that
N-acetylaniline undergoes condensation with 1,3-diaminopropane under
refluxed conditions generating an intermediate species with C55N bonds,
which is followed by a capping reaction with 1,3-diaminopropane and
HCHO in the presence of HClO4 yielding an off-white dihydroper-
chlorate salt of a 32-membered unsymmetrical [N12] macrocycle, 8,14,
24,30-tetramethyl-7,15,23,31-tetraphenyl[1,5,7,9,13,15,17,21,23,25,29,31]
dodecaazacyclodotriaconta-8,14,24,30-tetraene dihydroperchlorate, i.e.,
.
[L 2HClO4]. Its reactivity towards MX2 (X ¼ Cl, NCS, or NO3) and a few
derivative complexes [M(Ph3P)2Cl2] (M ¼ Zn, Cd, or Hg) has also been
investigated.
EXPERIMENTAL
Reagents
All the chemicals viz. aniline (E. Merck, India), 1,3-diaminopropane (E.
Merck, Germany), ethyl acetate (BDH, India), formaldehyde (37–41% aqu-
eous, E. Merck, India), HClO4 (70% aqueous, BDH, India), triphenylphos-
phine (SISCO, India), and metal salts viz. ZnCl2 (E. Merck, India), CdCl2
(E. Merck, India), Hg(NO3)2 (E. Merck, India), Hg(SCN)2 (LOBA chemicals,
India), HgCl2 (E. Merck, India), were commercially pure samples used as
received, while the solvents were purified by reported methods.[16] The pre-
cursors [M(Ph3P)2Cl2] (M ¼ Zn, Cd, or Hg) were prepared and purified by
reported methods.[17] N-Acetylaniline was obtained as a syrupy liquid in
60% yield according to the procedure reported elsewhere[18] for acylation
of diamines. The analytical and spectroscopic data of the oily mass are as
follows: Calcd. for C8H9NO (FW 135): C, 71.11; H, 6.66; N, 10.37. Found:
C, 71.00; H, 6.50; N, 10.30%. IR: 1700 s, 3320 b cm21; UV-visible: lmax
320 nm (characteristic[19,20] of –CO–NH– group) are consistent with N-acetyl-
aniline.

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Siddiqi and Khan
Preparation of the Macrocyclic Ligand, 8,14,24,30-
Tetramethyl-7,15,23,31-tetraphenyl-
[1,5,7,9,13,15,17,21,23,25,29,31]dodecaazacyclodotriaconta-
.
8,14,24,30-tetraene Dihydroperchlorate [L 2HClO4]
1,3-Diaminopropane (5.1 mL, 0.06 mol) and HClO4 (6.3 mL, 0.06 mol) in
150 mL ethanol was placed in a three-necked round-bottomed flask fitted with
a condenser to which N-acetylaniline (18.36 g, 0.12 mol) was added drop-
wise with continuous stirring for ꢀ1 hr and then it was refluxed for
ꢀ12 hr. The reaction has been monitored by recording UV-visible spectra
of the reaction mixture at intervals. The intensity of the 320 nm band of N-
acetylaniline started diminishing with the concomitant appearance of a new
band at ꢀ385 nm characteristic of the presence of a C55N bond[21] suggesting
the progress of the condensation process. Refluxing was stopped to bring the
temperature down to room temperature. Formaldehyde (15 mL, ꢀ0.2 mol)
and 1,3-diaminopropane (5.1 mL, 0.06 mol) dissolved separately in 50 mL
ethanol were added, simultaneously, through two dropping funnels to the
above reaction mixture with stirring. Stirring was continued for about
2 days at room temperature giving an off-white precipitate which was
filtered, washed with methanol, and dried in vacuo, m.p. 178–180 8C;
yield, 18 g (70%).
Preparation of Complexes
Preparation of M2LCl2(ClO4)2 [M ¼ Zn (1), (2); M ¼ Cd (4), (5)]
The ligand (0.64 g, 0.63 mmol) dissolved in 20 mL THF was added
drop-wise to the solution of ZnCl2 (0.17 g, 1.26 mmol) or CdCl2 (0.22 g,
1.26 mmol) dissolved in 30 mL methanol with stirring, which was contin-
ued for about 48 hr at room temperature. The precipitate formed was
filtered and washed with methanol followed by 10 mL portions of
THF till washings were colorless and then dried in vacuo; (1) m.p. 270–
272 8C (dec.); yield, 0.18 g (ꢀ36%); (4) m.p. 200–203 8C; yield, 0.21 g
(ꢀ35%).
The mother liquor was concentrated to 1/3 of its original volume under
reduced pressure and kept in a refrigerator for a fortnight which afforded a
second crop; (2) m.p. 258–260 8C; yield, 0.16 g (ꢀ32%); (5) m.p. 268–
270 8C (dec.); yield, 0.28 g (ꢀ36%). Total yield: (1) þ (2): 0.34 g (ꢀ34%);
(4) þ (5): 0.49 g (ꢀ36%).

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Preparation of Hg2LX2(ClO4)2 [X ¼ NO3 (8) or NCS (9)]
.
Hg(NO3)2 H2O (0.43 g, 1.262 mmol) or Hg(SCN)2 (0.468 g,
1.479 mmol) dissolved in 10 mL methanol was added drop-wise to the ligand
(0.64 g, 0.631 mmol) solution in 30 mL THF with stirring, which was
continued for about 48 hr at room temperature. The precipitate formed was
filtered and washed with methanol followed by THF till washings were
colorless and then dried in vacuo; (8) m.p. 228–230 8C (dec.); yield, 0.28 g
(ꢀ58%); (9) m.p. 318–320 8C (dec.); yield, 0.21 g (ꢀ43%). The mother liquor
in both cases, however, did not produce second crops.
.
Reactions of L 2HClO4 with the Precursors [M(Ph3P)2Cl2] (M ¼ Zn,
Cd, or Hg); Preparation of Zn2LCl2(ClO4)2 (3); Cd2LCl2(ClO4)2 (6), (7),
and Hg2LCl2(ClO4)2 (10)
[M(Ph3P)2Cl2] (M ¼ Zn 0.977 g, Cd 1.046 g, Hg 1.177 g, 1.479 mmol)
dissolved/suspended in 20 mL CH2Cl2 was reacted with the ligand (0.75 g,
0.739 mmol) in 25 mL THF at room temperature with stirring, which was con-
tinued for ꢀ48 hr. The solid products formed were filtered, washed with THF
and CH2Cl2 and then dried in vacuo; (3) m.p. 258–260 8C; yield, 0.39 g
(ꢀ56%); (6) m.p. 200–202 8C; yield, 0.32 g (ꢀ37%); (10) m.p. 218–
220 8C; yield, 0.38 g (ꢀ54%). The filtrate and washings were combined and
concentrated to 1/4 of the total volume, kept in a refrigerator for 5 days to
give colorless crystals. The crystals (m.p. 79 8C) have been identified as
free Ph3P by microanalytical, 1H and 31P NMR spectroscopic data. The
amount of free Ph3P collected corresponded approximately to 2 mol equiv.
of the precursors used in the reaction, suggesting a total release of Ph3P
from the precursor during the course of this reaction. The mother liquor was
again concentrated under vacuum, which produced a second crop (7) for the
reaction of the Cd(II)-precursor only, (7) m.p. 278–280 8C (dec.); yield,
0.29 g (ꢀ31%).
Warning: The compounds contain perchlorate groups, care should
be taken as these are liable to explode at the decomposing temperature on a
sudden strong heating.
Physical Measurements
IR spectra were recorded as KBr discs on a Perkin Elmer model spectrum
GX, 1H and 31P NMR spectra were recorded in DMSO-d6 on a Bruker DRX-
300 spectrometer using SiMe4 as internal reference or 85% H3PO4 as external
reference, respectively. The FAB mass spectra were recorded on a Jeol

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Siddiqi and Khan
102/DA-6000 mass spectrometer using argon (6 kV, 10 mA) as the FAB gas.
The accelerating voltage was 10 kV and the spectra were recorded in a
m-nitrobenzyl alcohol (NBA) matrix. The matrix peaks appeared at
m/z ¼ 136, 137, 154, 289, and 307. Microanalyses were obtained from the
Micro-Analytical Laboratory of the Central Drug Research Institute, Luck-
now, India. Conductivity measurements were carried out at a frequency of
1 kHz on a Systronics conductivity bridge using distilled DMSO as solvent.
The cell content was thermostated at 25 + 0.5 8C and the cell constant of
the conductivity cell (1.00 cm21) was determined by using 0.01 mol L21 of
KCl as standard solution. The concentration range of the complex solution
studied was 1023–1024 mol L21
.
RESULTS AND DISCUSSION
The reaction of N-acetylaniline with 1,3-diaminopropane in 2 : 1 mole
ratio in the presence of HClO4 in ethanol under reflux condition has been moni-
tored by recording the UV-visible spectrum of the reaction mixture at inter-
vals. The reagent N-acetylaniline shows a band at 320 nm characteristic[20]
of the n ! pꢁ transition of the –CO–NH– function. However, the reaction
mixture exhibited an additional new band at ꢀ385 nm, which gradually inten-
sified with a concomitant decrease in the intensity of the band at ꢀ320 nm as
the reaction progressed. The new band at ꢀ385 nm is assignable to the
p ! pꢁ excitation[21] of electrons from low-lying HOMO (i.e., p orbital)
to LUMO (i.e., pꢁ orbital) of the C55N bonds. The C55N bond is formed
due to the condensation process between 1,3-diaminopropane and the reagent
N-acetylaniline, similar to that reported[12] for the reaction of a diamine with
N-acetyl-1,3-diaminopropane, giving an intermediate species in the reaction
mixture. The capping reaction (in situ) of this intermediate with HCHO and
1,3-diaminopropane has resulted in the formation of an off-white amorphous
solid. The mechanism of the reaction is presented by the scheme shown
in Fig. 1. The first step (1st step) of the scheme shows formation of the
intermediate species while the 2nd step indicates the capping mechanism
involving a cyclization process resulting in the macrocyclic moiety. The
presence of mineral acids like HCl or HClO4 is known[22] to facilitate the
cyclization process in non-template procedures. Its presence, usually, mini-
mizes the extent of various unwanted side reactions, which may reduce
the yield of the desired macrocycles. Moreover, in non-template procedures
the presence of mineral acids also helps in the isolation of stable salts of the
macrocycles. Here, too, the presence of HClO4 has afforded a good yield of
the final product in the form of a solid dihydroperchlorate salt of the macro-
.
cycle, i.e., [L 2HClO4]. The analytical data of the final product (Table 1)

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903
.
Figure 1. Scheme showing the mechanism for the formation of L 2HClO4.
are consistent with the molecular formula C48H70Cl2N12O8. Its FAB mass
spectrum recorded in NBA matrix failed to show the molecular ion peak,
which may be due to its low thermal stability. It readily eliminates a Ph2þ
ion, evidenced by the appearance of a high-abundance peak (100%) at
m/z ¼ 153 (Table 2) and a peak at m/z ¼ 658 (10% abundance) consistent
with the L-2Ph molecular ion peak. In addition, the spectrum contained
peaks (Table 2) assignable to various fragments arising from the thermal clea-
vage of the macrocycle and, therefore, supports its molecular formula (Fig. 1).
The IR spectrum exhibited various important bands (Table 3) characteristic of

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Siddiqi and Khan
p
p
p
p
p
p
p
p
p
p
l
l

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Bimetallic Macrocyclic Complexes
905
p
p
p
p
p
p
p
p
p
p

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Siddiqi and Khan
Table 2. Abundance (%) of fragments with their assignments observed in FAB mass
.
spectrum of L 2HClO4.
Abundance
%
Abundance
%
m/z
Assignments
[L–2C6H5]
m/z
Assignments
658
391
314
239
371
10
30
6
449
603
16
10
[L0 –C6H5 þ Bz1]
[L0 –C6H5 þ Bz2]
[L0 –CH3 þ Bz1]
[L0 –2C6H5 þ Bz2]
[(C6H5)2]þ
0
;;
1/2[L–2Me] [L ]
[L0 –C6H5]
512
391
30
30
25
16
[L0 –2C6H5]
[L0 –CH3-4H]
153, 154
100
Note: Bz (m/z ¼ 136, 137) and Bz (m/z ¼ 154) are fragments of matrix m-nitroben-
1
2
zyl alcohol.
n(N–H), n(C55N), and n(C–N) stretching vibrations along with those arising
from n(C55C), n(C–C), and n(C–H) fundamental vibrations.[19] The absence
of a strong broad band at ꢀ1700 cm21 due to the –CO–NH– group of N-acetyl-
analine and appearance of a new band at 1610 cm21 characteristic of the
n(C55N) stretching vibration[12] are in accordance with the capping (2nd
step of Fig. 1) reaction. The IR spectrum contained additional bands
(Table 3) characteristic of the fundamental frequencies[23] of the ClO4 ion.
The observed splitting of the n2 stretching vibration (Table 3) of the ClO4
group in the region 1000–1200 cm21 is consistent with the lowering of its
symmetry relative to the perfect Td symmetry of the free anion. It is well
known[23] that the perfect Td symmetry of the ClO4 anion is lowered to C3v
or C2v as a result of its involvement in a weak to medium strength attachment
or bonding to a cation. This indicates that the ClO4 counter ion is complexed or
encapsulated in the cavity of the macrocyclic cation [L 2H]2þ. The protonated
.
form of a few polyaza macrocycles, e.g., [1,4,7,10,13,16]hexaazacycloocta-
decane [1,5,9,13,17,21]hexaazacyclooctatetracosane and [1,5,9,13,17,21,25,29]-
octaaza-cyclodotriacontane are reported[4] to form stable complexes with
various anions, such as Cl, NO3, succinate, and metal complex anions. The
magnitude of the molar conductance (Lm) measured in DMSO (Table 1) in
an ionizing high-dielectric constant solvent (1 ꢀ 46), indicates its non-electro-
lytic[24] nature and suggests that the salt [L 2HClO4] does not dissociate or
.
ionize to produce ClO4 ion even in solution. This is due to a strong attachment
1
of the counter ion with the macrocyclic cation. The H NMR spectrum of
[12]
.
L 2HClO4 recorded in DMSO-d6 (Table 4) exhibited signals assignable
to the protons from C6H5 (d 6.5), CH3C55N (d 7.0), –CH2–N55C (d 5.9),
N–CH2–N (d 4.0), NH (d 4.9), and –CH2– (d 2.5). The electronic spectrum
(Table 1) exhibited a band at 384 nm (1 ¼ 1.507 ꢂ 103 L mol21 cm21) arising

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Siddiqi and Khan
Table 4. Resonance peaks (ppm) observed in 1H NMR of the ligand and complexes.
N55C–
CH3
C55N–
CH2–
N–CH2–
NH2
C–CH2–
C
Compound
C6H5
NH
.
L 2HClO4
7.0 s
(12H)
6.8 s
7.0 s
7.2 s
7.0 s
7.0 s
6.5 m
(20H)
6.6 m
6.5 m
6.5 m
6.6 m
6.5 m
5.9 br
(16H)
3.6 br
5.8 br
4.0 br
4.4 br
3.6 br
4.0 s
(8H)
4.0 s
4.0 s
4.0 s
3.7 s
4.0 s
2.5 s
(8H)
2.5 s
2.5 s
2.5 s
2.5 s
2.5 s
4.9 br
(4H)
(2), (3)
(5), (6)
(8)
4.2 br
4.8 br
5.2 br
5.3 br
5.0 w,br
(9)
(10)
Note: w, weak; br, broad; m, multiplet; s, singlet.
from the excitation[21] of p electrons (HOMO) of the C55N bond to an anti-
bonding pꢁ orbital (LUMO), i.e., p ! pꢁ transition. The above physico-
chemical and spectroscopic data agree with the presence of unsymmetrical
aza groups[12] in the molecular formula of the macrocyclic ligand 8,14,24,30-
tetramethyl-7,15,23,31-tetraphenyl[1,5,7,9,13,15,17,21,23,25,29,31]dodeca-
.
azacyclodotriaconta-8,14,24,30-tetraene dihydroperchlorate, i.e., L 2HClO4,
which is formed through a 2 : 2 cyclization as shown in the scheme of Fig. 1.
Metal Encapsulated Derivatives
The macrocyclic ligand exhibited considerable reactivity towards metal
salts, MCl2, producing two crops [M ¼ Zn (1), (2); M ¼ Cd (4), (5)] in
good yields. The decomposition temperatures of the two crops are different,
however, their analytical data (Table 1) are very close and are consistent
with the stoichiometry [M2LCl2(ClO4)2]. Reactions with HgX2 produced a
single crop with the stoichiometry [Hg2LX2(ClO4)2] [X ¼ NO3 (8);
X ¼ NCS (9)]. Reactions with the precursors [M(Ph3P)2Cl2] (M ¼ Zn, Cd,
or Hg) are accompanied by the liberation of 2 mol equiv. of ancillary Ph3P
as free ligand in the mother liquor (see Experimental). A complete displace-
ment of Ph3P from the precursors in the complexation reaction affords
products consistent with the stoichiometries [M2LCl2(ClO4)2] [M ¼ Zn (3),
M ¼ Hg (10), and M ¼ Cd (6), (7)]. The analytical (Table 1) and spectro-
scopic data (vide infra, Tables 3 and 4) for (3), (6), and (7) have been found
identical to that of the corresponding products (2), (4), and (5) obtained
from reactions using MCl2.

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909
Conductance and Spectroscopic Studies
The molar conductivities (Lm) of the complexes have been measured in
DMSO except for (1) as it does not dissolve in any solvents. The magnitudes of
Lm (Table 1) indicate a non-electrolytic nature for (2), (3), (4), (6), and (10). How-
ever, a full-fledged 1 : 2 electrolytic nature for (8) and (9) was indicated in DMSO
solution. The magnitudes of Lm (49 cm22 ohm21 mol21) for the complexes (5), (7)
lie at the borderline of a 1 : 1 electrolyte[24] (50–75cm22 ohm21 mol21) supporting
an appreciable degree of dissociation of the semi-coordinated counter ClO4 ion in
these complexes in solution giving an equilibrium as shown below by Eq. (1)
½Cd2LCl2ðClO4Þ2ꢃ
½Cd2LCl2ClO4ꢃþ þ ClO4ꢄ
ð1Þ
A similar behavior for metal complexes containing BF4 counter ions has also
been reported.[25]
IR spectra (Table 3) of complexes contained characteristic frequencies of
the macrocyclic ligand, as well as those of ClO4, NCS, or NO3 as counter ion.
A small shift in the positions of the n(N–H) and n(C55N) bond frequencies rela-
tive to the free ligand has been observed after complexation. The observed split-
ting of the fundamental n2 stretching vibration of the ClO4 group in (1), (2), (3),
(4), (5), (6), (7), (8), and (10) suggest that the counter ion is coordinated to the
metal ions, resulting in a decrease in the symmetry of the free ClO4 ion from per-
fect Td to a C3v or C2v symmetry upon coordination in a monodentate (Fig. 2) or a
bidentate (Fig. 3) manner.[23] The strong broad band at 1382cm21 and a band of
medium intensity at 817 cm21 observed in (8) are characteristic of an uncoordi-
nated[23] NO3 counter ion. It has been reported that the free NO3 ion has D3h
symmetry exhibiting a strong broad band at ꢀ1400cm21 which, however, splits
if the symmetry is reduced to C2v or C3v due to its involvement in coordination to
metal ions. It, therefore, indicates that in complex (8), NO3 is not involved in
coordination compatible with its 1 : 2 electrolytic nature in solution.
The thiocyanate group is a well-known ambidentate ligand providing both
N as well as S atoms as potential sites for coordination[26] depending on the
soft or hard nature of the metal ions. In general, soft metal ions tend to bind
through S centers while hard counter parts favor N coordination. There may
be some exceptions depending on the nature of the main supporting ligands
already bound or chelated to the metal ions. IR spectroscopic studies are useful
in identifying the nature and mode of coordination of this ambidentate group.
;;
It has been concluded that the n(C N) stretching vibration, invariably, lies in
the region 2100–2150 cm21 whether it is N- or S-bonded. However, the
position of the n(C–S) stretching vibration is a more reliable guide[26] to
ascertain the coordination mode. Thiocyanate bridged bimetallic complexes,
21
;;
M–N–C–S–M, exhibit n(C N) at ꢀ2090 cm and a n(C–S) band at

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Siddiqi and Khan
Figure 2. Molecular structure of metal encapsulated macrocyclic complexes (a)
M ¼ Zn (1), (2), and (3); M ¼ Cd (4) and (6); M ¼ Hg (10). (b) M ¼ Cd (5) and (7).
ꢀ790 cm21. The appearance of medium-intensity bands at 2100 and
790 cm21 in (9) favors a thiocyanate-bridged structure. The spectrum exhib-
ited an additional unsplit strong band at 1070 cm21 assignable to the n2
stretching frequency characteristic of the presence of uncoordinated[23]
ClO4 anions in the complex. This has been substantiated by the observed
1 : 2 electrolytic nature of (9) in solution (vide supra). The encapsulation
of metal ions in the macrocyclic cavity is indicated from the observed
n(M–N) stretching frequency in the 400–450 cm21 regions.[19] The observed
splitting of the n(M–N) bond frequency is probably, due to coordination of
unsymmetrical aza groups of the macrocyclic moiety.
The UV-visible spectra of (1)–(10) contained bands arising from the p ! pꢁ
excitation characteristic of the C55N bond in the ligand moiety as well as
ligand ! metal charge transfer (CT) transition.[27] The position of the p ! pꢁ

ORDER
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Bimetallic Macrocyclic Complexes
911
Figure 3. Molecular structure of metal encapsulated macrocyclic complex
X ¼ ClO4, Y ¼ NO3 (8); X ¼ NCS, Y ¼ ClO4 (9).
transition in the complexes is shifted to shorter wavelength (blue-shift) or higher
energy compared to that observed in the free uncomplexed ligand. This shift is
due either to a destabilization of LUMO (pꢁ energy level) or a stabilization of
HOMO (p energy level) after chelation from the aza group of the (C55N) function.
The latter seems more plausible in view of the macrocyclic effect.
The 1H NMR spectra of (2), (3), (5), (6), (8), (9), and (10) were nearly identical
and exhibited resonance peaks arising from ligand protons (Table 4). A small
change in the position of some of the resonance peaks observed in the complexes
relative to the free macrocycle may be the consequence of metal ion encapsulation.
The physico-chemical and spectroscopic data presented here agree
with the cyclization mechanism to produce the salt of the macrocyclic
ligand 8,14,24,30-tetramethyl-7,15,23,31-tetraphenyl[1,5,7,9,13,15,17,21,23,
25,29,31]dodecaazacyclodotriaconta-8,14,24,30-tetraene dihydroperchlorate, i.e.,
.
L 2HClO4. The macrocycle possesses unsymmetrical aza groups, i.e., –C55N–
as well as C–NH–C in the molecular unit and the ClO4 counter anion is encapsu-
lated in the cavities as shown in Fig. 1. The macrocycle encapsulates two metal ions
resulting in bimetallic complexes where each metal ion attains hexa-coordination,
the counter ions occupy, preferably, the apical positions of the octahedral structure
as illustrated in Figs. 2(a and b) and 3.
Evaluation of Thermodynamic First Ionic Association
Constant (K1) and Free Energy Change (DG)
The thermodynamic ionic association constant K1 as well as the corre-
sponding free energy change (DG) of the complexes (5), (7), (8), and (9) in

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Siddiqi and Khan
DMSO have also been determined by carrying out a detailed conductometric
study at room temperature. Conductometric data have been analysed using the
modified Onsagar limiting equation according to the Fuoss and Edelson
method.[28,29] The Onsagar limiting equation at the low concentration range
of the electrolyte is given by the relation:
L ¼ L8 ꢄ S Co1=2
ð2Þ
Here, L is the equivalent conductance at each concentration Co, Lo are the
limiting equivalent conductances and S represents slope of the plot of L vs.
C1o/2. The consecutive association equilibria of the complexes in solution
may be represented by Eqs. (3) and (4) as follows:
A
2þ þ Xꢄ
AXþ
ð3Þ
and
AXþ þ Xꢄ
AX2
ð4Þ
where A2þ indicates the complex cations and X2 is the counter anion. The
corresponding ionic association constants, K1 and K2, are given by the
expressions:
½AXþꢃ fþ
K1 ¼
K2 ¼
ð5Þ
ð6Þ
½A2þꢃ½Xꢄꢃ f2þfꢄ
½AX2ꢃ f
½AXþꢃ½Xꢄꢃ fþfꢄ
The quantities in brackets indicate molar concentrations of the corresponding
species while f2þ, f , f and f2 are the activity coefficients of A2þ, AXþ, AX2,
þ
and X2, respectively. According to the Fuoss and Edelson method, an ade-
quate limiting description of the ionic complexes could be obtained assum-
ing[29] K2 = 0 and K1 s K2. With this assumption, there is a negligible
association to give an electrically neutral AX2 structure and a certain amount
of pair-wise association of complex cations A2þ with the counter anions X to
give singly charged AXþ ions. This method works very well to determine the
first ionic association constant (K1) using Eq. (5), which expresses the
ionic equilibrium for the formation of ionic species in solution. The Onsagar
limiting law becomes:
DK1
LF ¼ L8 ꢄ
ð7Þ
L8

ORDER
REPRINTS
Bimetallic Macrocyclic Complexes
913
where
ꢀ
ꢁ
L8
2
D ¼ Co f2þLF LF ꢄ
ð1 ꢄ dCo1=2Þꢄ1 þ ðL8 ꢄ loꢄÞ=2L
F ¼
1 þ ðLo ꢄ loꢄÞ2L8
Here, F is a function which corrects the conductance ratio (L/L8) for the
effect of interionic forces on the mobility, d is the ratio of the slope (S) and
limiting equivalent conductance (L8) of the complex cations and lo2 is the
limiting equivalent conductance of counter anions X. The ionic activity coef-
ficient ( f2þ) of A2þ has been determined[28] using the following Debye–
Hu¨ckel equation:
rffiffiffiffiffiffiffiffi
3Co
13=2
ꢄ log f2þ ¼ 250:3ZB2
ð8Þ
where ZB ¼ 2 (effective charge on the complex cation) and 1, dielectric
constant of the solvent.
The experimental equivalent conductivity data for the complexes (5), (7),
(8), and (9) measured in DMSO at 25 8C and the determined several relevant
parameters, like limiting equivalent conductance (L8), the activity coefficient
( f2þ), and D are summarized in Table 5. These parameters and K1 (Table 6)
have been evaluated using expressions given by Eqs. (2), (5), (7), and (8).
The magnitude of the limiting equivalent ionic conductance (loþ) of the
complex cation (Table 6) has been determined according to Kohlrausch
law[29] given by Eq. (9):
L8 ¼ loþ þ loꢄ
ð9Þ
The free energy changes (Table 6) due to first ionic association reaction have
been obtained using the thermodynamic relationship given by Eq. (10):
DG ¼ ꢄRT ln K1
ð10Þ
It can be concluded from Table 6 that the ionic association constants (K1)
for the complexes is in the order (7) . (9) . (5) . (8). The plot of L vs.
C1o/2 for the complexes also provides[30] information regarding the limiting
concentration (i.e., concentration at which the curve deviates from linearity)
where ion-pair formation in solution becomes more significant. The

ORDER
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914
Siddiqi and Khan
Table 5. Magnitudes of several parameters calculated for the metal complexes from
conductivity data.
Compounds
Concentration
L
L8
f2þ
F
D
(5)
0.8657 ꢂ 1024
1.7315 ꢂ 1024
3.4631 ꢂ 1024
5.1947 ꢂ 1024
6.9263 ꢂ 1024
0.8657 ꢂ 1023
92.28
75.00
57.67
53.83
51.93
49.60
120
0.8900 1.2955
0.8481 1.4496
0.7921 1.8720
0.7517 2.2476
0.7192 2.7118
0.6918 3.3509
0.5435
0.7778
1.4468
2.9088
5.6909
10.7927
(7)
(8)
(9)
0.8657 ꢂ 1024
1.7315 ꢂ 1024
3.4631 ꢂ 1024
5.1947 ꢂ 1024
6.9263 ꢂ 1024
0.8657 ꢂ 1023
85.00
74.97
60.55
51.90
49.02
48.44
102
0.8900 1.1712
0.8481 1.3368
0.7921 1.5672
0.7517 1.3569
0.7192 1.5201
0.6918 1.7001
0.8686
0.7254
1.1695
0.4919
0.8715
1.4959
0.8853 ꢂ 1024
1.7706 ꢂ 1024
3.5412 ꢂ 1024
5.3118 ꢂ 1024
7.0825 ꢂ 1024
0.8853 ꢂ 1024
62.02
62.02
56.38
54.50
52.15
50.00
67.5 0.8888 1.0926
0.8465 1.1376
0.7900 1.2041
0.7493 1.2624
0.7166 1.3156
0.6889 1.3738
0.1814
0.3892
0.6487
0.9599
1.2142
2.4837
0.8787 ꢂ 1024
1.7574 ꢂ 1024
3.5149 ꢂ 1024
5.2723 ꢂ 1024
95.91
70.52
56.42
52.66
105
0.8892 1.4697
0.8470 1.8965
0.7907 3.0684
0.7502 5.9970
0.9744
1.6180
5.8036
32.8869
magnitudes of this limiting concentration determined for the complexes (5),
,
(7), (8), and (9) are 5 ꢂ 1024, 5 ꢂ 1024, 7 ꢂ 1024, and 4 ꢂ 1024 equiv. L21
respectively. The magnitude is in the order (8) . (5), (7) . (9) suggesting
that there is the least tendency to give ion-pair in solution for (8), while it is
highest for (9). The present conductivity data, therefore, indicate that ion-
Table 6. Magnitudes of ionic association constant (K1), free energy (DG) and
limiting equivalent conductance of the complex cations (loþ) for the complexes.
Compounds
K1 (L mol21
)
2DG (kJ mol21
)
loþ(ohm21 cm2 mol21
)
(5)
(7)
(8)
(9)
1,164
2,652
162
17.4907
19.5300
12.6040
17.5660
49.1
31.1
24.3
34.1
1,200

ORDER
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Bimetallic Macrocyclic Complexes
915
pair association in DMSO solution for the complex [Hg2L(ClO4)2](NO3)2 (8)
is very low when compared to that of [Hg2L(NCS)2](ClO4)2 (9).
ACKNOWLEDGMENT
The authors are thankful to the Chairman, Department of Chemistry, Ali-
garh Muslim University, Aligarh, India, for providing the necessary research
facilities. Financial support from the University Grants Commission, New
Delhi, India, in the form of a Major Research Project is also gratefully
acknowledged.
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Received March 27, 2003
Accepted February 14, 2004
Referee I: K. Moedritzer
Referee II: R. D. Pike

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