Synthesis and characterization of new trans N,N′-disubstituted macrocyclic "tet a" ligands and their copper(II) and nickel(II) complexes: Structural, electrochemical, magnetic, and catalytic studies

Article abstract of DOI:10.1246/bcsj.77.1153

New trans-disubstituted macrocyclic ligands (L¹), trans-1,8-[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-1,4,8, 11-tetraaza-5,5,7,12,12,14-hexamethylcyclotetradecane, and (L²), 1,8-[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-4

Full text of DOI:10.1246/bcsj.77.1153

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1153–1159 (2004) 1153
Synthesis and Characterization of New trans N,N0-Disubstituted
Macrocyclic ‘‘tet a’’ Ligands and Their Copper(II) and Nickel(II)
Complexes: Structural, Electrochemical, Magnetic, and Catalytic Studies
Nallathambi Sengottuvelan, Duraisamy Saravanakumar, Vengidusamy Narayanan,
ꢀ
Muthusamy Kandaswamy, Kandaswamy Chinnakali,1 and Gopal Senthilkumar1
Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, India
1Department of Physics, Anna University, Chennai-600 025, India
Received October 30, 2003; E-mail: mkands@yahoo.com
New trans-disubstituted macrocyclic ligands (L1), trans-1,8-[N,N0-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-
1,4,8,11-tetraaza-5,5,7,12,12,14-hexamethylcyclotetradecane, and (L2), 1,8-[N,N0-bis{(3-formyl-2-hydroxy-5-methyl)-
benzyl}]-4,11-dimethyl-1,4,8,11-tetraaza-5,5,7,12,12,14-hexamethylcyclotetradecane, were synthesized. The ligands
were characterized by elemental analysis, IR, 1H and 13C NMR spectral analyses. The ligand L1 crystallized as monoclinic
in the space group P21=n, and the final R-value was found to be 0.062. Copper(II) and nickel(II) complexes of these li-
gands were prepared and characterized by elemental analysis, IR, UV–visible, FAB-mass spectral studies. Electrochem-
ical studies of the nickel(II) complexes show one-electron quasireversible reduction around ꢁ1:00 to ꢁ1:13 V and one-
electron quasireversible oxidation around 1.00 to 1.14 V. The copper(II) complexes show one-electron quasireversible
reduction around ꢁ0:93 to ꢁ1:10 V. ESR spectral studies of the copper(II) complexes show hyperfine splitting with a
g value of 2.31, a g value of around 2.02 and an A value of 160 G. The magnetic moment values (ꢀeff) of the Cu(II)
k
?
complexes fall in the range of 1.71 to 1.72 B.M. at room temperature (298 K). The rate-constant values of the catalytic
hydrolysis of p-nitrophenyl phosphate for the copper(II) complexes were in the range 6:3 ꢂ 10ꢁ3 to 12:2 ꢂ 10ꢁ3 minꢁ1
and for the nickel(II) complexes in the range 10 ꢂ 10ꢁ3 to 14 ꢂ 10ꢁ3 minꢁ1. The rate-constant values of the catalytic
oxidation of catechol to o-quinone were in the range 9:4 ꢂ 10ꢁ3 to 13:4 ꢂ 10ꢁ3 minꢁ1 for the copper(II) complexes.
The copper(II) and nickel(II) complexes of ligand L2 have higher catalytic activity than that of ligand L1.
The chemistry of macrocyclic ligands with coordinating side
arms1–7 has been the subject of growing interest during the past
few years. Synthetic macrocycles have been, and still are, a rap-
idly growing class of compounds having different molecular
topologies,8,9 a different set of donor atoms and thus a great va-
riety of ligational properties towards protons,10–12 metal cations
and anions.13 This variegated chemistry, which arose from the
endless fantasy of synthetic chemists, has led to macrocycles
and their complexes being used as catalysts, selective metal
ion binders, NMR contrast agents, molecular recognition,14
photochemical devices, selective extractants, analytical re-
agents, etc.15,16 The copper-catalysed oxidation of phenols to
catechols and subsequent oxidation of diphenols to quinones
(tyrosinase) are important chemical processes in biological sys-
tems.17,18 Hydrolysis is found to be the key process in the active
sites of a number of enzymes, namely urease and carbon mon-
oxide dehydrogenase mediated by nickel ions.19 Hence, here
we report on the synthesis of new tetraaza macrocyclic ligands
(L1) and (L2) with coordinating side arms. Both of the ligands
contain two side arms with phenolic oxygen as an additional
coordinating site and formyl groups for further extension. This
may be used as a spacer in long-chain molecules or dendrons to
prepare dendrimers.20 The structure of ligand L1 has been
solved by the X-ray crystallographic method. The ligands can
be used to prepare mono-, di-, and tri-nuclear metal com-
plexes.21,22 The cavity size and the shape of the host molecule
can be easily tuned in order to study the coordination properties
of the ligands. Synthesis and electrochemical studies of ferro-
cene attached macrocycle are also in progress. Here, we report
on the synthesis of mononuclear copper(II) and nickel(II) com-
plexes of these ligands. The spectral, electrochemical, magnet-
ic, and catalytic studies of these complexes are also discussed.
Experimental
An elemental analysis was carried out on a Carlo Erba Model
1106 elemental analyzer. The Nuclear magnetic resonance spectra
were measured at 400.13 (1H) and 100.40 (13C) on a Jeol GSX400
spectrometer. Electronic spectral studies were carried out on a
Hitachi 320 spectrophotometer in the range of 200–850 nm. IR
spectra were recorded on a Hitachi 270-50 spectrophotometer on
KBr disks in the range of 4000–400 cmꢁ1. FAB mass spectra of
the complexes were obtained on a JEOL SX 102/DA-6000; m-ni-
trobenzyl alcohol (NBA) was used as the matrix. The molar con-
ductivity of a freshly prepared solution of the complex in DMF
was measured using an Elico Model SX80 conductivity bridge. Cy-
clic Voltammograms were obtained on a CHI600A electrochemi-
cal analyzer. The measurements were carried out under an oxy-
gen-free condition using a three-electrode cell in which a glassy
carbon electrode was a working electrode, a saturated Ag/AgCl
electrode was a reference electrode and a platinum wire was used
as an auxiliary electrode. The concentration of the complexes was
10ꢁ3 M. Tetrabutylammonium perchlorate (10ꢁ1 M) was used as a
Published on the web June 9, 2004; DOI 10.1246/bcsj.77.1153

1154 Bull. Chem. Soc. Jpn., 77, No. 6 (2004)
Cu(II) and Ni(II) Complexes of ‘‘tet a’’ Ligands
supporting electrolyte. A ferrocene/ferrocenium (1+) couple was
used as an internal standard and E1=2 of the ferrocene/ferrocenium
(Fc/Fcþ) couple under the experimental condition was 470 mV in
DMF, and ꢀEp for Fc/Fcþ was 70 mV. The room-temperature
magnetic moment was measured on a PAR vibrating sample mag-
netometer (Model-155). X-band ESR spectra of Cu(II) complexes
were recorded using DMF as a solvent at liquid-nitrogen tempera-
ture on a Varian EPR-E 112 spectrometer with diphenylpicrylhy-
drazine (DPPH) as a reference. The catalytic oxidation of catechol
to o-quinone and the hydrolysis of 4-nitrophenyl phosphate by the
complexes (10ꢁ3 M, DMF solution) were studied. The catechol ox-
idation reaction was followed spectrophotometrically by choosing
the strongest absorbance at 390 nm (o-quinone), and the phosphate
hydrolysis reaction was followed at 420 nm (nitrophenolate anion).
The increase in the absorbance at these wavelengths was monitored
ture. Ten equivalents of NaBH4 (1.3 g, 16 mmol) were then added,
and the mixture was refluxed for 3 h. After cooling to room temper-
ature, 10 mL of HCl (3 M in water) was added. The mixture was
concentrated to dryness on a rotary evaporator, and the residue
was then dissolved in 100 mL of water and KOH (1 M) was added
until the solution attained a pH of 12. After extraction with CHCl3
(5 ꢂ 30 mL), the organic phase was dried with anhydrous MgSO4
and concentrated to give a yellow compound.26–28 Light-yellow
microcrystals of the ligand 1,8-[N,N0-bis{(3-formyl-2-hydroxy-5-
methyl}benzyl)]-4,11-dimethyl-1,4,8,11-tetraaza-5,5,7,12,12,14-
hexamethylcyclotetradecane (L2) were obtained upon recrystalli-
zation from chloroform.
Yield: 73%. mp: 290 ꢃC (dec). Analytical data for C36H56O4N4:
Calcd: C, 71.02; H, 9.27; N, 9.20%. Found: C, 71.32; H, 9.53; N,
9.54%. Selected IR (KBr): 3442 (br), 1658 (s), 1345 (s) cmꢁ1
.
as a function of time. A plot of logðA =A1 ꢁ AtÞ vs time was
1H NMR Spectra: ꢁ (ppm in CDCl3) 1.01 to 1.10 (s, 18H, C–
CH3), 1.64 (s, 4H, CH2), 2.21 (s, 6H, Ar–CH3), 2.27 (s, 6H, N–
CH3), 2.68 (m, 2H, CH), 2.80 (m, 4H, ꢂ-CH2), 3.27 (m, 4H, ꢃ-
CH2), 3.96 (d, 4H, N–CH2–Ar, J ¼ 1:04 Hz), 7.26 (d, 4H, Ar–H,
J ¼ 1:78 Hz), 10.50 (s, 2H, Ar–CHO), 12.94 (br. s, 2H, Ar–OH).
13C NMR Spectra: ꢁ (ppm in CDCl3) 18.50, 20.43, 27.30, 36.05,
37.45, 47.17, 51.80, 55.20, 117.82, 124.04, 126.3, 132.4, 138.6,
156.2, 193.7.
1
made for each complex and the initial rate constant was calculated.
Materials. 5-methylsalicylaldehyde,23 3-chloromethyl-5-meth-
ylsalicylaldehyde,24 and 1,4,8,11-tetraazatricyclo[9.3.1.14;8]hexa-
decane25 were prepared by following literature methods. All of oth-
er chemicals were of analytical grade and were used as received.
Synthesis of Precursor Compound (P.C.-1). The compound
1,4,8,11-tetraazatricyclo[9.3.1.14;8]hexadecane (1 g, 0.03 mol) was
dissolved in acetonitrile (30 mL) and two equivalents of 3-chloro-
methyl-5-methylsalicylaldehyde (1.2 g, 0.066 mol) in acetonitrile
(30 mL) wereꢃrapidly added. This solution was stirred at room tem-
perature (25 C) for three days and the white precipitate formed
was filtered, washed with a small quantity of CH3CN, and dried un-
der a vacuum. This crude compound, 1,8-[N,N0-bis{(3-formyl-2-
hydroxy-5-methyl)benzyl]-4,11-diazaniatricyclo[9.3.1.14;8]-5,5,7,
12,12,14-hexamethylhexadecane dichloride, was recrystallized
from water to give white crystals.
Synthesis of Macrocyclic Mononuclear Complexes. [CuL1]
(PF6)2: A methanolic solution of copper(II) perchlorate hexahy-
drate (0.63 g, 1.7 mmol) was added to a hot solution of L1 (1.0 g,
1.7 mmol) in methanol. The solution was refluxed on a water bath
for 24 h, and filtered while hot. Then, KPF6 (0.3 g, 1.7 mmol) in 10
mL of methanol was added to the solution. The resulting solution
was stirred overnight at 25 ꢃC. The crude product was precipitated
ꢃ
by slow evaporation of the solvent at room temperature (25 C).
The complex was obtained as a green powder upon recrystalliza-
tion of the crude product from CH3CN.
Yield: 89%. mp: 275 ꢃC (dec). Analytical data for C36H54O4N4:
Calcd: C, 71.25; H, 8.97; N, 9.23%. Found: C, 71.36; H, 9.11; N,
.
9.41%. Selected IR (KBr): 3450 (br), 1668 (s) cmꢁ1 1H NMR
Spectra: ꢁ (ppm in D2O) 0.72 to 1.30 (s, 18H, C–CH3), 1.64 (s,
4H, CH2), 2.36 (s, 6H, Ar–CH3), 2.48 (m, 2H, CH), 2.94 (m,
4H, ꢂ-CH2), 3.27 (m, 4H, ꢃ-CH2), 3.96 (d, 4H, N–CH2–Ar),
5.06 (d, 4H, N–CH2–N), 7.26 (d, 4H, Ar–H), 10.50 (s, 2H, Ar–
CHO), 12.94 (br. s, 2H, Ar–OH).
Yield: 81%. Analytical data for C34H52O4N4Cu(PF6)2: Calcd:
C, 43.71; H, 5.61; N, 6.00; Cu, 6.80%. Found: C, 43.68; H, 5.23;
N, 6.38; Cu, 6.55%. Selected IR (KBr): 3428 (br), 3241 (m),
1659 (s) cmꢁ1. Conductance (ꢁm/S cm2 molꢁ1) in CH3CN: 125.
ꢄ
max/nm ("/Mꢁ1 cmꢁ1) in CH3CN: 547 (171), 388 (14800), 300
(38600). g ¼ 2:31, g ¼ 2:02; ꢀeff: 1.72 B.M.
k
?
[CuL2](PF6)2: Complex [CuL2](PF6)2 was synthesized by fol-
lowing the above procedure using ligand L2 instead of ligand L1.
The complex was obtained as a dark-green powder upon recrystal-
lization of the crude product from CH3CN. Yield: 76%. Analytical
data for C36H56O4N4Cu(PF6)2: Calcd: C, 44.93; H, 5.87; N, 5.82;
Cu, 6.60%. Found: C, 44.68; H, 5.62; N, 6.02; Cu, 6.43%. Selected
IR (KBr): 3436 (br), 1672 (s), 1342 (s) cmꢁ1. Conductance (ꢁm/
S cm2 molꢁ1) in CH3CN: 143. ꢄmax/nm ("/Mꢁ1 cmꢁ1) in CH3CN:
Synthesis of Ligand (L1). The compound P.C.-1 (0.5 g, 0.08
mmol) was dissolved in 100 mL of an aqueous NaOH solution (3
M) with stirring. After stirring for 4 h, the solution was extracted
with CHCl3 (5 ꢂ 30 mL). The combined CHCl3 extracts were
dried with anhydrous MgSO4, and concentrated under a vacuum
to give a yellow compound. Light-yellow crystals of the ligand 1,8-
[N,N0-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-1,4,8,11-tetra-
aza-5,5,7,12,12,14-hexamethylcyclotetradecane (L1) were ob-
tained upon recrystallization from chloroform.
562 (176), 395 (11300), 304 (47400). g ¼ 2:32, g ¼ 2:05; ꢀ
:
k
?
eff
1.71 B.M.
Yield: 67%. mp: 318 ꢃC (dec). Analytical data for C34H52O4N4:
Calcd: C, 70.31; H, 9.02; N, 9.65%. Found: C, 70.02; H, 9.55; N,
[NiL1](PF6)2: The complex was prepared by the procedure
used for copper(II) complex by using Ni(ClO4)2 6H2O (1 g, 1.7
mmol) instead of Cu(ClO4)2 6H2O. The complex was obtained
ꢄ
9.92%. Selected IR (KBr): 3445 (br), 3289 (s), 1666 (s) cmꢁ1
.
ꢄ
1H NMR Spectra: (L1) ꢁ (ppm in CDCl3) 0.63 to 1.10 (s, 18H,
C–CH3), 1.64 (s, 4H, CH2), 2.19 (s, 6H, Ar–CH3), 2.48 (m, 2H,
CH), 2.65 (br. s, 2H, NH), 2.94 (m, 4H, ꢂ-CH2), 3.27 (m, 4H,
ꢃ-CH2), 3.97 (d, 4H, N–CH2–Ar, J ¼ 1:05 Hz), 7.26 (d, 4H,
Ar–H, J ¼ 1:78 Hz), 10.40 (s, 2H, Ar–CHO), 12.94 (br. s, 2H,
Ar–OH). 13C NMR Spectra: (L1) ꢁ (ppm in CDCl3) 19.3, 20.3,
31.2, 38.3, 46.2, 49.17, 51.2, 53.4, 55.2, 119.8, 124.5, 127.4,
133.4, 138.8, 158.2, 195.6.
as a dark-green powder upon recrystallization of the crude product
from CH3CN. Yield: 77%. Analytical data for C34H52O4N4Ni-
(PF6)2: Calcd: C, 43.94; H, 5.64; N, 6.03; Ni, 6.32%. Found: C,
43.79, H, 5.78; N, 6.34; Ni, 6.12%. MS (EI) m=z ¼ 926 (mþ). Se-
lected IR (KBr): 3452 (br), 3254 (m), 1667 (s), 1345 (s) cmꢁ1
.
Conductance (ꢁm/S cm2 molꢁ1) in CH3CN: 137. ꢄmax/nm ("/
M
ꢁ1 cmꢁ1) in CH3CN: 480 (181), 384 (15800), 302 (31600).
1H NMR Spectra: ꢁ (ppm in CDCl3) 0.55 to 1.51 (s, 18H, C–
CH3), 1.64 to 1.9 (s, 4H, CH2), 2.24 (s, 6H, Ar–CH3), 2.55 (br s,
2H, NH), 2.68 (m, 2H, CH), 2.80 (m, 4H, ꢂ-CH2), 3.27 (m, 4H,
Synthesis of Tetrasubstituted Ligand (L2). The compound
P.C.-1 (1 g, 1.6 mmol) was dissolved in an EtOH/H2O (95:5) mix-

N. Sengottuvelan et al.
Bull. Chem. Soc. Jpn., 77, No. 6 (2004) 1155
X-ray Structural Study of the Ligand (L1). The ligand L1
was crystallized from a chloroform–methanol solution in the
space group P21=n with two molecules in a unit cell. The crys-
tal structure of the ligand along with the atomic labeling is giv-
en in Fig. 1. The crystal data and structure refinement for the
ligand (L1) is given in Table 1. The crystal structure29 reveals
that the ligand has a center of inversion due to the 3-chloro-
methyl-5-methyl salicylaldehyde group attached to the trans ni-
trogen atoms. The cyclam unit is more buckled compared to the
unsubstituted cyclam due to the attachment of a bulkier group
in the ring. The overall C–C distance varies between 1.363(4)
ꢃ-CH2), 3.61 (d, 4H, N–CH2–Ar), 7.64 (d, 4H, Ar–H), 9.65 (s, 2H,
Ar–CHO). 13C NMR Spectra: ꢁ (ppm in CDCl3) 19.05, 20.17, 34.5,
41.95, 44.93, 48.42, 49.84, 51.89, 66.06, 118.2, 126.21, 128.9,
130.9, 152.17, 184.37.
[NiL2](PF6)2:
Complex [NiL2](PF6)2 was synthesized by
following the above procedure using ligand L2 instead of ligand
L1. The complex was obtained as a green powder upon recrystalli-
zation of the crude product from CH3CN. Yield: 72%. Analytical
data for C36H56O4N4Ni(PF6)2: Calcd: C, 45.15; H, 5.90; N, 5.85;
Ni, 6.13%. Found: C, 45.45; H, 5.62; N, 5.51; Ni, 6.42%. MS
(EI) m=z ¼ 956 (mþ). Selected IR (KBr): 3433 (br), 1664 (s)
cmꢁ1. Conductance (ꢁm/S cm2 molꢁ1) in CH3CN: 136. ꢄmax
/
ꢂ
and 1.538(4) A. The overall C–N distance varies between
ꢂ
1.448(4) and 1.515(3) A. No anomalous bond distances or bond
nm ("/Mꢁ1 cmꢁ1) in CH3CN: 492 (193), 392 (13300), 305
(58400). 1H NMR Spectra: ꢁ (ppm in CDCl3) 0.52 to 1.58 (s,
18H, C–CH3), 1.65 to 1.80 (s, 4H, CH2), 2.47 (s, 6H, Ar–CH3),
2.38 (s, 6H, N–CH3), 2.60 (m, 4H, ꢂ,ꢃ-CH2), 3.05 (m, 2H, CH),
4.04 (d, 4H, N–CH2–Ar), 7.64 (d, 4H, Ar–H, J ¼ 2:42 Hz), 9.45
(s, 2H, Ar–CHO). 13C NMR Spectra: ꢁ (ppm in CDCl3) 18.15,
21.17, 33.59, 40.97, 45.53, 47.32, 48.64, 50.89, 67.16, 115.26,
124.20, 127.5, 130.6, 151.15, 182.57.
angles were found in the structure, and all of the bond distances
and bond angles agree well with the reported literature.30
Spectral Studies. The ligand L1 shows ꢅ(C=O), ꢅ(N–H),
and ꢅ(OH) peaks at 1666, 3289, and 3445 cmꢁ1, respectively.
Ligand L2 shows ꢅ(C=O) and ꢅ(OH) peaks at 1658 and 3442
cmꢁ1, respectively. The infrared spectra of all the complexes
show a sharp band in the range 1650–1674 cmꢁ1, correspond-
ing to ꢅ(C=O) stretching. Complexes of ligand L1 exhibit a
peak due to ꢅ(N–H) stretching at 3256 cmꢁ1. The FAB mass
spectrum of the complex [NiL1](PF6)2 shows a molecular ion
peak at 926. The electronic spectra of all the complexes exhibit
three main peaks. One peak below 300 nm (" ꢅ 31400
Results and Discussion
The precursor compound-1 (P.C.-1) was prepared from a re-
action of hexamethylated 1,4,8,11-tetraazatricyclo[9.3.1.14;8]-
hexadecane with two equivalents of 3-chloromethyl-5-methyl-
salicylaldehyde in acetonitrile. Compound P.C.-1 hydrolyzed
with aqueous NaOH to yield the macrocycle ligand L1. The li-
gand L1 crystallized as a yellow crystal, which was suitable for
single crystal X-ray analysis. The ligand L2 was prepared by the
reduction of P.C.-1 with sodium borohydride in ethanol. Mon-
onuclear copper(II) and nickel(II) complexes were prepared by
mixing an equimolar amount of the ligand and metal(II) per-
chlorate salts in a methanol medium. The synthetic routes for
the preparation of ligands and their complexes are shown in
Schemes 1 and 2, respectively.
ꢀ
ꢁ1 cmꢁ1) is assigned to the intraligand transitions (ꢆ–ꢆ );
M
a moderately intense peak in the range 380–400 nm
(" ꢅ 12300 Mꢁ1 cmꢁ1) is due to ligand-to-metal charge trans-
fer transitions, and a broad peak in the region 480–562 nm is
due to d–d transitions. The copper(II) complexes show ꢄmax
value in the region 545–562 nm (" ꢅ 170 Mꢁ1 cmꢁ1), which
are consistent with reported tetraaza macrocyclic Cu2þ com-
plexes.31 The nickel(II) complexes show a single band in the re-
gion 480–495 nm (" ꢅ 181 Mꢁ1 cmꢁ1), which indicates a
square-plannar geometry of the complexes32 with N4 coordina-
Scheme 1.

1156 Bull. Chem. Soc. Jpn., 77, No. 6 (2004)
Cu(II) and Ni(II) Complexes of ‘‘tet a’’ Ligands
Scheme 2.
Table 1. Crystal Data and Structure Refinement for (L1)
Compound
Ligand-L1
C34H52N4O4
580.78
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
293(2) K
ꢂ
0.71073 A
Monoclinic
P21=n
ꢂ
a
10.1862(6) A
ꢂ
12.2663(8) A
b
ꢂ
13.4698(8) A
c
ꢂ
ꢃ
90.00ꢃ
107.13(1)ꢃ
90.00ꢃ
1608.4(2) A
Fig. 1. ORTEP diagram of ligand (L1) with atom labels
and numbering scheme. (Hydrogen atoms are omitted for
clarity.)
ꢇ
Volume
ꢂ 3
Z
2
Density (calculated)
Absorption coefficient
Fð000Þ
1.183 Mg mꢁ3
0.075 mmꢁ1
620
tion.
Electrochemical Properties of the Complexes. The molar
conductance values of the complexes in DMF are in the range
125 to 143 ꢁm/S cm2 molꢁ1, which indicates that the com-
plexes are 1:2 electrolyte.33 The electrochemical properties of
the copper(II) and nickel(II) complexes reported in the present
work were studied by cyclic voltammetry in dimethylforma-
mide containing 10ꢁ1 M tetra(n-butyl)ammonium perchlorate
(TBAP) (Caution! TBAP is potentially an explosive; hence,
care should be taken in handling the compound); the electro-
chemical data are summarized in Table 2. Cyclic voltammo-
grams of copper(II) complexes are shown in Fig. 2 and for
nickel(II) complexes are shown in Fig. 3.
Reduction Process at Negative Potential. The cyclic vol-
tammograms of the copper(II) and nickel(II) complexes were
recorded in the potential range 0 to ꢁ1:4 V. Each voltammo-
gram shows one quasireversible reduction wave at a negative
potential in the range ꢁ0:92 to ꢁ1:13 V. The reduction process
for the complexes is quasireversible in nature, as is evident
from the following criteria: ꢀE increases with increasing scan
Crystal size
Index ranges
0:34 ꢂ 0:20 ꢂ 0:20 mm
ꢁ11 ꢆ h ꢆ 11,
ꢁ13 ꢆ k ꢆ 9,
ꢁ14 ꢆ l ꢆ 14
7400
2298 [R(int) = 0.1164]
99.3%
Full-matrix least-squares on F2
2298/0/195
0.93
Reflections collected
Independent reflections
Completeness to 2ꢈ
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2ꢉðIÞ]
R indices (all data)
R1 ¼ 0:062, wR2 ¼ 0:146
R1 ¼ 0:097, wR2 ¼ 0:172
ꢂ ꢁ3
Largest diff. peak and holes
0.33 and ꢁ0:32 e A
rate, and is always greater than 60 mV. The peak currents, Ipc
and Ipa, were not equal. That a controlled potential electrolysis
carried out at 100 mV more negative than the reduction wave
conveys the consumption of one electron per molecule

N. Sengottuvelan et al.
Table 2. Electrochemical Data of Copper(II) and Nickel(II) Complexes
Bull. Chem. Soc. Jpn., 77, No. 6 (2004) 1157
Oxidation [M(II) ꢀ M(III)]
Complexes
Reduction [M(II) ꢀ M(I)]
Epc
/V
Epa
/V
E1=2
/V
ꢀE
/mV
Epa
/V
Epc
/V
E1=2
/V
ꢀE
/mV
½NiL1](PF6)2
½NiL2](PF6)2
½CuL1](PF6)2
½CuL2](PF6)2
ꢁ1:13
ꢁ1:06
ꢁ1:04
ꢁ0:92
ꢁ0:99
ꢁ0:94
ꢁ0:76
ꢁ0:72
ꢁ1:06
ꢁ1:00
ꢁ0:90
ꢁ0:82
140
120
280
200
1.05
1.14
—
0.98
0.90
—
1.02
1.02
—
70
240
—
—
—
—
—
Measured by cyclic voltammetry at 50 mV/s in DMF. E vs Ag/AgCl conditions, GC working and Ag/AgCl
reference electrodes, supporting electrolyte TBAP, concentration of complex 1 ꢂ 10ꢁ3 M, concentration of TBAP
1 ꢂ 10ꢁ1 M.
Fig. 2. Cyclic voltammograms of the copper(II) complexes.
(a) [CuL1](PF6)2; (b) [CuL2](PF6)2.
(n ¼ 0:92).
The reduction potential of the copper(II) complexes shifts to-
wards anodically from ligand L1 (E1 ¼ ꢁ1:04) to ligand L2
pc
(E1 ¼ ꢁ0:92). This may be due to steric effects of the methyl
pc
group present in two trans-nitrogen atoms of the macrocycle,
which causes a distortion of the geometry of the Cu(II) com-
plexes (due to macrocyclic ring distortion); also, the number
of tertiary nitrogen atoms increases in L2, which stabilize the
low valent metal Cu(I).34 The same trend was also observed
for the nickel(II) complexes. Thus, the N-methylation of the
macrocyclic frame was shown to stabilize the monovalent met-
al ions by increasing the cavity size of the macrocyclic ligand
and by decreasing the ligand field strength imparted by the ni-
trogen donor atoms.35
Fig. 3. A: Cyclic voltammograms of the nickel(II) com-
plexes in the cathodic region. (a) [NiL1](PF6)2; (b)
[NiL2](PF6)2. B: Cyclic voltammograms of the nickel(II)
complexes in the anodic region. (a) [NiL1](PF6)2; (b)
[NiL2](PF6)2.
Oxidation Process at Positive Potential. The cyclic vol-
tammograms for the nickel(II) complexes at positive potential
were recorded in the potential range 0 to 1.3 V. Cyclic voltam-
mograms of nickel(II) complexes at a positive potential show a
single redox wave in the potential range þ1:05 to þ1:14 V. The
oxidation process for the complexes is quasireversible in na-
ture. Controlled potential electrolysis indicates that the waves
correspond to a one-electron transfer process. The oxidation
potential of nickel(II) complexes shifts towards a positive val-
ue36 (Epa ¼ 1:05 to 1.14 V) as the N-methylation of ligand L1 to
L2. Since the ligand L2 stabilize the lower oxidation state, as
mentioned above, the oxidation of NiL2 complex is more diffi-
cult than the NiL1 complex.
plex, the observed g value (avg) is 2.31, the g value is 2.02,
k
?
and the A value is 160 G; for the [CuL2] complex, the g value
(avg) is 2.32, the g value is 2.05, and the A value is 165 G.
k
k
?
k
The relation g > g is typical of Cu(II) having one unpaired
k
?
electron in a dx2ꢁy2 orbital.37 Figure 4 shows the ESR spectrum
of the mononuclear copper(II) complex of L1. Room-tempera-
ture (at 298 K) magnetic moment studies of the copper(II) com-
plexes show a ꢀeff value of 1.72 B.M. for [CuL1], and 1.71
B.M. for [CuL2], which is nearer to the spin-only value of
the copper(II) ion.38 The nickel(II) complexes are diamagnetic.
Magnetic Properties of the Complexes. The ESR spec-
trum of copper(II) complexes shows four lines due to hyperfine
splitting with nuclear hyperfine spin 3/2. For the [CuL1] com-
Kinetic Studies.
copper(II) and nickel(II) complexes towards the hydrolysis of
4-nitrophenyl phosphate is given in Table 3, and a plot of
The catalytic reaction rate of the

1158 Bull. Chem. Soc. Jpn., 77, No. 6 (2004)
Cu(II) and Ni(II) Complexes of ‘‘tet a’’ Ligands
Fig. 4. ESR spectrum of [CuL1](PF6)2.
Fig. 5. Hydrolysis of 4-nitrophenyl phosphate by copper(II)
and nickel(II) complexes. (Inset: Absorbance maximum
at 420 nm.) (a) [CuL1](PF6)2; (b) [NiL1](PF6)2; (c)
[CuL2](PF6)2; (d) [NiL2](PF6)2.
Table 3. (a) Hydrolysis of 4-Nitrophenyl Phosphate by
Copper(II) and Nickel(II) Complexes, (b) Catecholase
Activity of Copper(II) Complexes
Complexes
(a) Rate constant (k)
k ꢂ 103 minꢁ1
(b) Rate constant (k)
k ꢂ 103 minꢁ1
½NiL1](PF6)2
½NiL2](PF6)2
½CuL1](PF6)2
½CuL2](PF6)2
10.0
14.0
6.3
—
—
9.4
13.4
12.2
Measured spectrophotometrically in DMF medium. Concen-
tration of the complex 1 ꢂ 10ꢁ3 M, concentration of p-nitro-
phenyl phosphate 1 ꢂ 10ꢁ1 M, and concentration of pyrocate-
chol 1 ꢂ 10ꢁ1 M.
logðA =A1 ꢁ AtÞ vs time for the complexes is shown in Fig. 5.
1
Hydrolysis of Phosphate Ester. Copper(II) and nickel(II)
complexes were subjected to hydrolysis of the phosphate ester.
The rate of hydrolysis of 4-nitrophenyl phosphate for [NiL2]
(14 ꢂ 10ꢁ3 minꢁ1) was slightly higher than [NiL1] (10 ꢂ
10ꢁ3 minꢁ1). The catalytic activities of the nickel(II) com-
plexes were found to increase with an increase in N-alkylation,
which causes a distortion of the geometry of the metal ion in the
complexes and favors axial substrate coordination. A similar
trend was also observed for copper(II) complexes. From the re-
sults, it is found that the Ni(II) complexes have a rate-constant
value higher than that of the Cu(II) complexes for phosphate
hydrolysis.
Fig. 6. Catecholase activity of the copper(II) complexes.
(Inset: Absorbance maximum at 390 nm.) (a) [CuL1]-
(PF6)2; (b) [CuL2](PF6)2.
per(II) complexes has a rate-constant value of 2:9 ꢂ 10ꢁ3
minꢁ1. The catecholase activity of the copper(II) complex re-
ported in the present study is 9:4 ꢂ 10ꢁ3 minꢁ1, which is higher
than that of the non-substituted macrocyclic complexes. This
higher catalytic activity may be due to a greater distortion in
the geometry of the substituted macrocyclic complexes than
the non-substituted macrocyclic complexes. This distortion in
geometry is inferred from the wavelength of the absorption of
the d–d transition of the complexes. The side-arm substituted
copper(II) complex (CuL1) has a d–d band at 547 nm, and
the non-substituted macrocyclic copper(II) has a d–d band at
517 nm. The side-arm substituted nickel(II) complex (NiL1)
has a d–d band at 480 nm and the non-substituted macrocyclic
nickel(II) complex has a d–d band at 471 nm.41 The red shift in
the wavelength indicates that the coordination geometry in the
side-arm substituted complexes is more distorted than that of
the non-substituted complexes. It is evidenced from the litera-
ture,42 that the rate constant for the more-distorted complexes is
higher than that of the less-distorted complexes.
Oxidation of Catechol (Catecholase Activity). A plot of
logðA =A1 ꢁ AtÞ vs time for the oxidation of catechol to qui-
1
none by the copper(II) complexes is shown in Fig. 6. The rate-
constant values are also reported in Table 3. The complex
[CuL2](PF6)2 has higher activities (13:4 ꢂ 10ꢁ3 minꢁ1) than
complex [CuL1](PF6)2 (9:4 ꢂ 10ꢁ3 minꢁ1). The oxidation of
catechol to o-quinone by the complexes was found to increase
as the macrocyclic N-methylation increases.39,40 N-alkylation
may cause a greater distortion of the geometry of the com-
plexes. This geometry may favour substrate coordination in
the axial position, which in turn increases the reaction rate. It
is expected that the catalytic activity of complexes with side
arms is suppose to be higher than that of complexes with no
side arms due to the flexibility resulting from the distorted ge-
ometry.
From these studies, it is well understood that substitution in
the ring nitrogen atom deviates the planarity of the ring, which
The catecholase activity of non-substituted macrocyclic cop-

N. Sengottuvelan et al.
Bull. Chem. Soc. Jpn., 77, No. 6 (2004) 1159
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Financial support received by authors N. S. and D. S.
from ISRO major project sanctioned to M. K. is gratefully
acknowledged.
Supplementary Material.
Full crystal data and structure
refinement details of the atomic coordinates, equivalent isotropic
displacement parameters, full interatomic distances and angles an-
isotropic displacement parameters, hydrogen coordinates and iso-
tropic displacement parameters for ligand L1 CCDC No. 196207
are available from the Cambridge Crystallographic Data Centre.
Copies of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44-1223-336033, e-mail: deposit@ccdc.cam.ac.uk [or]
www.http//www.ccdc.cam.ac.uk.)
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