Indian Journal of Chemistry
Vol.
~ 9A
April2000, pp. 400-406
Template synthesis of 1-(8 '-amino-a-naphthyl)-4-(8 '-amino-a-naphthylanline)1-azabuta-1 ,3-diene, a tetraaza half-cyclised ligand, and
characterisation of its copper(II) complexes
Nita A Lewis
Department of Chemistry, University of Miami , Coral Gables, Florida 33 124, USA
and
Derek A Toc her
Department of Chemistry, University College London, 20 Gordon Street, London WCI H OAJ, UK
and
Goutam K Patra, Jnan P Naskar & Dipankar Datta*
Department of Inorganic Chemistry, Indian Association for the Culti:vation of Science, Calcutta 700 032, India
Received 20 July 1999
Reaction of I, I ,3,3-tetramethoxypropane with I ,8-diaminon aphthalene in presence of a nickel(ll ) templa te in aqueous
medium generates the free acycli c ligand 1-(8 '-amino-a-naphthyl )-4-(8 '-amino-a-naphthylamine)-1-azabuta-1 ,3-diene (LH ;
H: dissociable proton) as a 1.5 hydrate. The ligand undergoes acid dissociation in non-aqueous solvents which is reflected in
its nc NMR spectrum in deuteriated dimet hyl su lphcxide (DMSO). Its pK, value determined in DMSO from co nductivity
data is 5. 18. With copper( II ) perchlorate, LH forms air sensitive complexes of type CuL(Cl0 4).3 H 20 and CuL(Cl04).DMF
(DMF: dimeth ylformam ide). In both th e co mplexes, the perchlorate anion is weakly bound to th e metal as revealed by their
conductivity in DM F suggesting a CuN 40 2+ chro moph ore. Their EPR spectra in DMF-toluene gl a~s at 77 K are almost
identical and have rhombic features (<g 3>, 2.40; <g 2>, 2.08; <g 1>, 2.01; <A 3>, l37xl0-4 cm- 1; <A 2>, 18x l0-4 cm- 1)
indicating th at the coo rdin ation sphere around the copper ion is of di storted square-pyramid al type. ZINDO calculations
show that the CuL+ fragment has a butterfly like shape wit h the nap hthalene fragments spanning like two wings and t'le
metal in the N4 plane. In cyclic voltamrnetri c experiments in DMF at a platinum electrode, the co mplexes show a quasi~
reversible two-electron ligand oxidation with an E 112 {)f 0. 12 Y vs saturated calomel electrode (SCE) and a irreversible
metal oxidation process at a low potential (< 0.5 Y vs SCE) .
Jager's macrocycle, reported in 1964, is a 2 + 2
condensate of I ,2-phenylenediamine and acety lacetone1. Later many derivat ives of it have been
prepared. They have an extensive conj ugati on, like
that in naturally occurri ng porphyrins. This class of
tetraaza macrocycles (1) can be said to be true
sy nthetic analog of the porphyrins . They have given
rise to a very rich and versatile coordination
chemistry. For some selected aspects, see refs 2-7 .
Similar interesting chemistry is likely to emerge from
thei r 1,8-diaminonaphthalene ( 1,8-DAN) counterpart
(2) which is not yet known. As such, no tetraaza
macrocycle containing fused naphthalene ring(s) has
been reported so far. Recently we have initiated a
study in this direction. The results obtained so far are
reported here.
lf1
OC
N
NH
HNY ' )
N~
V'
Materials and Methods
I ,8-DAN (97% ), I, I ,3,3-tetramethoxypropane (99+
%) and NaCI0 4 .H20 were purchased from Lancaster,
England, and deuteriated dimethyl sulphoxide
(DMSO-d6 ; 99.9 atom % D) was purchased from
Aldrich, USA . Other chemicals used were of AR
grade. Fresh analytical grade DMF, procured from S.
D. Fine-Chem Ltd., India, was used directly for
electrochemistry without further purification. C, H
and N analyses were performed by using a Perkin-
LEWIS eta!.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND
Elmer 24001! analyzer. Copper was estimated
gravimetrically as CuSCN. The melting point of LH
was determined by a melting point apparatus
procured from CBC Power Corporati<''l (Calcutta,
India) and is uncorrected. IR spectra (KBr disc;
4000-400 cm- 1) were recorded on a Perkin-Elmer
783 spectrophotometer, UV -vis spectra on a
Shimadzu UV-l60A spectrophotometer, 1H and ''c
NMR on a Brucker DPX300 spectr9meter, X-band
EPR spectra on a JEOL RE IX spectrometer and
mass spectra on a VG-ZAB-SE instrument. Solution
conductivity was measured by a Systronics (India)
t(irect reading conductivity meter (model 304).
Magnetic susceptibility was determined at room
temperature by a PAR 155 vibrating sample
magnetometer. The magnetometer was calibrated
with HgCo(SCN) 4 and the susceptibility data were
corrected for diamagnetism using Pascal's constants.
Cyclic voltammetry and coulometry were performed
using EG&G PARC electrochemical analysis system
(Model 250/5/0) under dry nitrogen atmosphere in
conventional three electrode configurations with
tetraethylammonium perchlorate as the supporting
electrolyte. A planar EG&G PARC G0228 platinum
milli electrode was used as the working electrode in
cyclic voltammetry. The potentials are reported here
with respect to SCE (saturated calomel electrode) and
are uncorrected for liquid junction potentials. Under
the experimental conditions employed here, the
ferrocene-ferrocenium couple appears at 0.406 V vs
SCE. Constant-potential coulometry was performed
using a platinum wire gauge working electrode and a
PAR 377 A cell system.
Syntheses
1-(8 '-amino-a-naphthyl)-4- (8 '-amino-a-naphthylamine)-1-azabuta-1,3-diene 1.5 hydrate (LH.l.5H20)
-1.582 g (10 mmol) of pulverised 1,8-DAN and 1.19
g (5 mmol) of NiCh.6H 20 were taken in 500 cm3 of
water. To this suspension was added 1.6 cm3 ( 10
mmo1) of 1, l ,3,3-tetramethoxypropane and the
reaction mixture was stirred for 2 h. The resulting
bluish green mixture was then refluxed for 9 h to
obtain a light green solution which was decanted in
hot condition and kept in the refrigerator overnight. A
shinning fluffy compound precipitated (the colour of
the compound varied from greenish to beige). It was
filtered , washed thoroughly with water and dried in
vacuo over fused CaCh It was . stored under N2
atmosphere; yield, 0.73 g (83%); m.p. 186-190°C. It
401
analysed as LH.l.5H 20 [Found: C, 72.92; H, 6.01; N,
14.83. Calc. for C23H23N401.5 : C, 72.79; H, 6.11; N,
14.77%]. FAB mass spectrum: m/z 389 (LH + 2H 20 +
W). EI mass spectrum: m/z 387 (L+ + 2H20), 370
(LW + H20), 352 (LW). Calcd for LH (C23H2oN4)
352.16. IR data (em-'): 3440 (vb), 3190 (w), 3110
(w), 3040 (s), 2860 (w), 2735 (w), 2710 (w), 2370
(w), 1905 (w), 1630 (s), 1610 (s), 1590 (vs), 1480 (s),
1440 (s), 1415 (s), 1375 (vs), 1335 (s), 1295 (s), 1205
(vs), 1170 (m), 1155 (m), 1110 (m), 1080 (s), 1030 (s,
split), 960 (w), 875 (s), 820 (vs), 790 (w), 755 (vs),
660 (s), 640 (m, split), 590 (m), 525 (m), 490 (m),
450 (m) .
CuLCl0 4.3H2 0 (3a)--0.44 g (1.16 mmol) of LH
and 0.1 g ( 1.22 mmol) of sodium acetate were
dissolved in 20 cm3 of methanol and filtered . To the
filtrate, 5 cm3 methanolic solution of 0.4 g ( 1.08
mmol) of Cu(Cl04)2.6H 20 was added dropwise with
stirring. Immediately a green flocculent precipitate
appeared. It was stirred for another 2 min and was left
in the air for 15 min . The green complex was filtered
and washed with 2 cm3 of cold methanol. It was dried
by keeping in the air overnight and then stored under
nitrogen; yield, 0.39 g (68%). It analysed as a
trihydrate of CuLCl04 [Found: C, 48.72; H, 4.31; N,
9.76; Cu, 11.10. Calc. for C 23 H25N4CuCl07 : C, 48.57 ;
H, 4.43; N, 9.85; Cu, 11.18%]. IR data (cm- 1): 3440
(vb), 3050 (w), 2460 (w), 1630 (s), 1610 (s), 1550
(w), 1480 (s), 1420 (w), 1375 (m), 1340 (w), 1310
(w), 1280 (w), 1210 (w) 1 1150 (vs), 1120 (vs), 1080
(vs), 950 (w), 835 (s), 810 .(w), 775 (m), 680 (w), 630
(s, split), 500 (b).
CuLCl04.DMF (3bf-To a 20 cm 3 DMF solution of
0.3 g (0.53 mmol) of CuLC104.3H 20 was added 10
cm3 aqueous solution of 0.2 g NaC10 4.H 20 dropwise
with stirring. Immediately after the addition was
complete, a green compound started precipitating.
The reaction mixture was left in the air for 1 h, after
which the green complex was filtered, washed
thoroughly with diethylether and dried in vacuo over
fused CaCh. It was stored under nitrogen; yield, 0.22
g (70%). It analysed as CuLCI04.DMF [Found: C,
53.00; H, 4.42 ; N, 11.98; Cu , 10.73 . Calc. for
C26H26NsCuCIOs : C, 53.12; H, 4.46 ; N, 11.92; Cu ,
10.82%]. IR data (cm- 1): 3445 (vb), 1650 (s), 1625
(s), 1600 (s), 1550 (s), 1485 (s), 1360 (b), 1305 (w),
1210 (w), 1175 (w), 1145 (s), 1120 (vs, split), 1090
(s), 850 (w), 830 (m) , 770 (b), 700 (s, split), 650 (s,
split).
402
INDIAN J CHEM, SEC. A, APRIL 2000
Caution-Though we have not met with any
incident during our studies, care should be taken in
handling these compounds as perchlorate salts are
potentially explosive. These should not be prepared
and stored in larger amounts.
Results and Discussion
Derivatives of 1 can be prepared by various
·
· to synt hes1se
· Its
·
method s. 189
· · A convement
way IS
8
nickel(m complex lb by Scheme 1 and subsequent
stripping of the metal (by anhydrous HCl). In Scheme
1, the half-cyclised intermediate la can be isolated in
the solid state. Here we have followed Scheme 1 for
our purpose. Use of l ,8-diaminonaphthalene (I ,8DAN) and NiCb.6H 20 in Scheme I leads to the
isolation.of the free ligand LH (H: dissociable proton)
straight away as a I.5 hydrate; the tetraaza
macrocycle 2 or its nickel(ll) complex is not formed .
The reaction is carried out in aqueous medium.
LH
LH.l.5H 20 is moderately air-stable in solid state as
well as in solution. But prolonged exposure to air
should be avoided. It undergoes acid dissociation in
non-aqueous solvents [reaction (I)]. We have tried to
estimate its pKa value in dimethylsulphoxide (DMSO)
from its conductivity in DMSO (Table 1). The molar
conductance of LH.1.5H 20 increases with dilution
(Table I). The range of molar conductance for I: I
10
electrolytes in DMSO as specified by Geary is
1
1
2
50-70
cm mor • Assuming an average molar
1
conductance of 60
cm2 mor 1 for a I : l electrolyte
in DMSO, our estimation of pKa of LH . l.5Hz0 in
DMSO yields a value of 5.18 (cf. pKa of acetic acid in
water, 4.74). This acid dissociation of LH.I.5Hz0 is
reflected in its NMR spectra. Not much can be
inferred from the 1H-NMR spectrum of LH.l .5H 20 in
n-
n-
1b
Scheme 1
DMSO-d6 as it consists of three sharp signals in the
region 6.95-7.29 ppm, an ill resolved and very broad
signal (half-width : 0.58 ppm) around 6.38 ppm and a
relatively less broad signal (half-width: 0.25 ppm) at
10.56 ppm. However, the 13C NMR spectrum is quite
revealing (Fig. 1); the naphthalene C atoms resonate
in the region 1I5.89-I47.24 ppm (thi s assignment is
done by comparing the 13C NMR spectra of I ,8-DAN
in DMSO-d6 ), three distinct signals due to the carbon
atoms marked as I, 2 and 3 in LH in reaction (1)
appear at 101.82, 104.82 and 105 .26 ppm, and two
distinct signals due to the carbon atoms marked as 1,
2 and I ' (I and 1' are magnetically equ ivalent) in Lin reaction (I) appear at 53.52 and 61 .63 ppm. From
our pKa value, the population of L- in the solution of
13
LH. l.5H 20 used for C NMR is estimated as - I %.
Reaction of Cu(CI04)z.6H20 with LH.I .5Hz0 in
equimolar
proportion
in
methanol
gives
CuLCIOdH 20 (Ja) in poor yield(- 10%) . But when
the reaction is carried out in presence of sodium
acetate, 3a is obtained in 70% yield (see
Experimental Section). Attempt to recrystallise 3a
from
dimethylformamide
(DMF)
yields
CuLCI04 .DMF (3b). The presence of DMF in 3b is
confirmed by the appearance of a strong peak at I650
cm- 1 in its IR spectra assignable to the carbonyl
func tion of DMF (this peak is absent in the IR spectra
of 3a). The copper(ll) complexes 3a and 3b are air
sensitive in solid state as well as in solution. Freshly
prepared samples of 3a and 3b are completely soluble
in DMF but sparingly soluble in other polar solvents
like methanol, acetonitrile etc. Their solubility in
DMF decreases gradually if kept in air. However,
their solubility properties are retained if stored under
N 2. The freshly prepared solutions of 3a and 3b are
light greenish brown in colour; however, after several
LEWIS eta!.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND
403
Table I - Conductivity data for LH.l .5H 20 in DMSO at various concentrationsh
Solute
concentration
Solution
conductancec
Molar
conductanced
[LHr
[LT
[W]e
pK/
11.920
16
1.34
11.653
0 .267
0 .267
5 .21
5.960
12
2.01
5.670
0.200
0.200
5. 16
2.980
8
2.68
2.847
0 . 133
0.133
5.21
1.490
6
4.03
1.390
0 . 100
0 . 100
5. 14
0 .745
4
5.37
0 .678
0.067
0.067
5. 18
h Various concentrations are given in M.
c In mho.
2
1
d In mho cm mol- .
e Equilibrium concentration.
r Avearge value is 5.1 8.
150
130
Fig. I - 300 MHz
110
13
6 (ppm)
90
C NMR spectra ofLH.I .5H 20 in OM SO-d~.
hours of standing in air these become intense bluegreen. Both 3a and 3b show molar conductance (in
degassed DMF; Table 2) less than that stipulated for a
10
I: 1 electrolyte indicating that the perchlorate anion
is weakly bound to the copper atom. This is al so
reflected in the IR spectra of 3a and 3b where the
perchlorate anion shows very well resolved three to
four vibrations in the region I 080-1150 em_,
11
characteristic of a C 3v perturbation of the anion •
Macrocycles of type 1 give rise to saddle shaped
complexes. T his property has been utilised very
recently in a very ingenious manner to trap globular
molecules like
So far, we have not been able to
grow single crystals of 3a or 3b. In order to find what
sort of geometry is produced by our ligand LH, we
have performed ZINDO (Zemer-derived Intermediate
Neglect of Differential Overlap) calculations 12 , with
the CAChe suite of programs available from Oxford
Molecular Group Inc . 13 , on the CuL+ moiety, The
minimum energy structure obtained is displayed in
Fig. 2. The CuL+ moiety is predicted to have a very
beautiful butterfly like conformation ; the two
c61.t
404
INDIAN J CHEM, SEC. A, APRIL 2000
naphthalene fragments span like two wings. The
copper atom is predicted to lie in the N4 plane. Since
all the three water molecules of 3a are replaced by a
single DMF molecule upon recrystallisation of 3a
from DMF to yield 3b, it is evident that no solvent
molecule is coordinated to the copper atom in 3a or
3b. Considering the results of our conductivity
measurements on 3a and 3b together with Fig. 2, we
can say that the copper atom in 3a and 3b possibly
has a square-pyramidal N40 coordination with the
oxygen atom of the CI04 anion occupying the apex.
The ligand LH.l.5H 20 displays only one band at
334 nm (E = 30,500 dm 3 mor 1 cm- 1) in its electronic
spectra in DMF. The electronic spectra of 3a and 3b
in degassed DMF are essentially similar with rrunor
variations in the intensities (Table 2). The spectra
comprise two closely spaced humps - one at :- 623
nm and another at 550 nm, and a very intense band
around 340 nm. All the electronic transitions are of
charge transfer origin. The high energy band seems to
be the intra-ligand -charge transfer modulated by the
metal. The electronic spectra of 3a and 3b in nujol
mull (Table 2) indicate,· though the high intensity
band could not be observed clearly, that 3a and 3b
have more or less same strucmre in the solid and
solution states.
The room temperature magnetic moments of 3a
and 3b correspond to one unpaired electron (Table 2)
as expected. Their X-band EPR spectra in solid state
at room temperature as well as at 77 K are identical -
Fig. 2- Minimum energy stru cture of CuV fr ag ment obtai ned by ZINDO calcu lations.
(Colour code: red , Cu; violet , N; grey, C; white, H. )
Table 2 - Some physical properties of CuLCI04 .3H 20 (3a) and CuLC104 .DMF (3b)
Property
3a
3b
Conductivity"
44
29
Magnetic momenth
1.71
1.79
Electronic spectrac
nujol
625,540
615 ,530
624 (600), 550 (870),
622 ( 1,080), 550 ( I ,350) ,
DMF
336 (22,000)
EPR spectra
solid stated
DMF-toluene glassc
344 ( 18,770)
g.l
= 2.01
g.l =
gJ,
2.39 ; g2, 2.08 ; gJ . 2.01;:
gJ,
A3 ,137.9x l0-4; A2 ,17.4x l0-4
"In DMF; in mho cm 2 mol- 1
hAt room temperature; in flB ·
cAmaxlnm (Eidm 3 mol- 1 cm- 1).
dAt room temperature and 77 K.
cAt 77 K. The A values are given in cm- 1•
2.01
2.41; g2, 2.07 ; gl> 2.01
A3 , 134.9x i0-4; A 2 , 19.3xl0-4
405
LEWIS et al.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND
O.lt
4- O.lt
3
-1.2
-H
- 2,01...-----J----L----L----'
0.6
1.0
0.2
-0.2
-0.6
E IV l vs S CE
{b)
ItO
Fig. 3 - X-band EPR spectra of CuLCI04 .3H 20 (3a). Upper
trace, in solid state at 7'7 K; lower trace, in DMF-toluene glass at
77K.
<
0
3
-40
-.so
60
1.0
<
20
-0.2
0.2
E :vl vs SCE
-0·6
Fig. 5 - Cyclic voltammograms of CuLCI0 4 .DMF (3b) in DMF
at a platinum electrode under N2 atmosphere; solute
concentration, 0.507 mmol dm- 3; supporting electrolyte, 0.1 mol
dm- 3 tetraethylammonium perchlorate. (a) scan rate v = 0.050 Y
s- 1; (b) v =IV s- 1.
~
-20
-6QL---~----~-----L----~
l.O
0.6
0.6
0.2
-0.2
-0.6
E (V) vs SCE
Fig. 4 - Cyclic voltammogram of LH.l .5H 20 in DMF at a
platinum electrode under N2 atmosphere; solute concentration,
0.812 mmol dm- 3 ; supporting electrolyte, 0.1 mol dm- 3
tetraethylammonium perchlorate; scan rate v = I V s- 1•
sort of axial type with the gJ. component appearing at
2.01 (Table 2; Fig. 3). These resolve into rhombic
spectra in DMF-toluene glass at 77 K showing clear
nuclear hyperfine splitting for the two higher g
factors (Table 2; Fig. 3). The ratio (g2 - g1)/(g3 - g2),
which can be considered as a measure of
rhombiciti 4, is quite small for the two complexes; it
is 0.187 for 3a and 0.176 for 3b. This ratio indicates
that the pentacoordinate copper(II) ion in these two
complexes has a ground state which is quite close to a
dx2- / type rather than a dz 2 type . The fact that A3 > A2
r:-. state. For a dz2 ground
also supports a dx 2 _ / ground
state [resulting from a trigonal bipyramidal geometry
for a 5-coordinate copper(II)], largest A value is
14
associated with the lowest g factor . Thus, our EPR
spectra suggest that the N40 coordination sphere
around the copper(II)-ion in 3a and 3b is of somewhat
distorted square-pyramidal type.
We have examined the electrochemical behaviour
of the ligand LH.l.5H 20 and its copper(II) complexes
by cyclic voltamrnetry and coulometry in DMF using
platinum working electrodes under N2 atmosphere.
The ligand shows quasi-reversible voltamrnograms
with half-wave potential £ 112 = 0.10 V vs SCE
(saturated calomel electrode). Faster scan rates yield
voltamrnograms of relatively better quality (Fig. 4) .
Our coulometry at 0.4 V vs SCE establishes that the
electrode process is oxidative and involves two
electrons. The oxidation probably arise from the
amino ends as I ,8-DAN itself is known to undergo an
15
irreversible oxidation around 0.4 V vs SCE • The
complexes 3a and 3b . give identical voltammograms
in DMF. At slow scan rates, these show two waves on
the anodic side but only one wave on the cathodic
side; at faster scan rates the two anodic waves
coalesce (Fig. 5). At the scan rate v = 10 mV s- 1, the
two anodic peaks appear at 0. 15 and 0.35 V vs SCE
and the cathodic peak appears at 0.08 V vs SCE
406
INDIAN J CHEM, SEC. A, APRIL 2000
[Fig. 5(a)]. Our coulometry at 0 .6 V vs SCE shows
that the observed electrochemical process is oxidative
and involves three electron. Our conclusion is that in
Fig. 5(a) the peaks at 0.15 and 0.08 V vs SCE
cmTespond to the quasi-reversible two-electron ligand
oxidation (£ 112 = 0 . 12 V vs SCE) and the peak at
0 .35 V vs SCE with no cathodic counterpart
corresponds to irreversible oxidation of Cu(II) to
Cu(III) . Since the metal oxidation process is
irreversible, it is evident that the ligand framework
cannot stabilise the copper(III) state. Neverthe less,
the potential for the oxidation of Cu(II) to Cu(III) is
qu ite low and deserves some comments. Wh ile to the
best of our knowledge there is no data on the redox
potential for the Cu(II)/Cu(III) couple in an N40
coordination sphere, thi s couple in N4 and N 20 2
coordination spheres is known to occur at very low
potentials ( < 0.5 V vs SCE) in some copper
complexes of ligands containing anionic amide
17
16
N-donors and phenolate 0-donors .
Concluding remarks
Here we have demonstrated that like I ,2phenylenediamine, I ,8-diamjno-naphthalene does not
yield a tetraaza macrocycle in its reaction with
I , I ,3,3-tetramethoxypropane in presence of nickel(II)
chloride. The reaction product is a free acyclic
tetraaza ligand, LH . Thi s we call half-cyclisation . The
ligand birrds copper(II) in its monoan ionic form
giving rise to butterfly shaped air sensitive copper(II)
complexes, Their sensitivity towards air can be
attributed to their low oxidation potentials . The
ligand framework is incapable of stabilising
copper(III). This is possibly because the ligand itself
gets oxidised before the metal oxidation takes place.
In the perchlorate salts of our synthesised copper(II)
complexes of LH, the anion is found to be weakly
bound to the metal giving rise to penta coordinate
complexes. This is in line with the general
characteristics of the sadd le shaped complexes which
show a marked tendency to yield penta coordinate
. 9
spec1es .
So far, all our attempts. to synthes ise Ni (II)
complexes of LH have failed. Since LH does not form
any complex with Ni(II), the reacti on of I ,8-DAN
with I , I ,3,3-tetramethoxypropane in presence of a
nickei(II) template does not proceed a ll the way (see
Scheme I) to yield our desired macrocycle 2. It is
noted that use of a copper(II) template instead of a
nickel(II) one does not bring about even the halfcyclisation.
Acknowledgement
DO wishes to thank the Depa1tment of Science and
Technology, New Delhi for financial support.
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