Angle isomerism, as exemplified in a five-coordinate, dimeric copper(ii) Schiff base complex. Observation of Ostwald ripening

Article abstract of DOI:10.1039/c5nj02765a

The 1:1 condensate of benzil and 2-hydrazinopyridine is the ligand (LH; H: a dissociable proton) here. Its reaction with CuCl₂·2H₂O in methanol at room temperature in equimolar proportion affords a mixture of two types of dark green (with metallic luster) single crystals - hexagonal (1a) and rectangular (1b). They are separated mechanically. The yield of 1a is higher. X-ray crystallography shows that 1a and 1b are penta-coordinate, dichloro-bridged dimers of the type Cu₂L₂Cl₂ with very similar centrosymmetric structures. All the bonding parameters except for two mutually dependent bond angles in the N₂OCl₂ coordination sphere of Cu(ii) are the same. Correspondingly, two different minima are located in DFT calculations on 1a and 1b. Energetically 1b is more stable than 1a in the gas phase by 3-4 kcal mol^-1. Their X-band EPR spectra in the solid state at 77 K, which are axial, reveal that (d_x2-y2)¹ is the ground state in 1a (g_∥ > g_⊥) and (d_z2)¹ in 1b (g_∥ < g_⊥). In keeping with Ostwald ripening, the energetically less stable isomer 1a crystallizes first. As the crystallization time is allowed to be longer, more of 1b is formed. The transformation of 1a to 1b in methanol solution is found to follow the kinetics of a zero order reaction. The reverse transformation is not possible.

Full text of DOI:10.1039/c5nj02765a

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Angle isomerism, as exemplified in a
five-coordinate, dimeric copper(II) Schiff base
Cite this:DOI: 10.1039/c5nj02765a
complex. Observation of Ostwald ripening†
Shanti G. Patra,a Nirmal K. Shee,a Michael G. B. Drewb and Dipankar Datta*a
The 1 : 1 condensate of benzil and 2-hydrazinopyridine is the ligand (LH; H: a dissociable proton) here.
Its reaction with CuCl2Á2H2O in methanol at room temperature in equimolar proportion affords a
mixture of two types of dark green (with metallic luster) single crystals—hexagonal (1a) and rectangular
(1b). They are separated mechanically. The yield of 1a is higher. X-ray crystallography shows that 1a and
1b are penta-coordinate, dichloro-bridged dimers of the type Cu2L2Cl2 with very similar centrosymmetric
structures. All the bonding parameters except for two mutually dependent bond angles in the N2OCl2
coordination sphere of Cu(II) are the same. Correspondingly, two different minima are located in DFT
calculations on 1a and 1b. Energetically 1b is more stable than 1a in the gas phase by 3–4 kcal molÀ1
Their X-band EPR spectra in the solid state at 77 K, which are axial, reveal that (dx Ày
)1 is the ground state
in 1b (gJ o g>). In keeping with Ostwald ripening, the energetically less stable
.
Received (in Montpellier, France)
8th October 2015,
Accepted 2nd February 2016
2
2
1
in 1a (gJ 4 g>) and (dz )
2
isomer 1a crystallizes first. As the crystallization time is allowed to be longer, more of 1b is formed. The
transformation of 1a to 1b in methanol solution is found to follow the kinetics of a zero order reaction.
The reverse transformation is not possible.
DOI: 10.1039/c5nj02765a
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who first recognized this type of structure for coordination
compounds.4,5 Prior to Werner, it was assumed that there can
1. Introduction
The concept of ‘‘isomer’’ was introduced in chemistry by Berzelius be only associative bonds. For example, he showed that in
in 1831.1 In Greek, it means ‘‘equal parts’’. It all started with the CoCl3Á6NH3, there is an octahedral Co atom with six ammonia
observations that despite having the same elemental composition, molecules bound at the apices and the three chloride ions are
silver cyanate (AgOCN) and silver fulminate (AgCNO) possess free from any type of bonding. Since six ammonia molecules
different physicochemical properties.1 Berzelius argued that the are attached to the central Co3+, the coordination number of
connectivities of the atoms in these two compounds are different. cobalt in CoCl3Á6NH3 is six. Ammonia is a ‘‘ligand’’ here and
This remarkable idea paved the way for the concept of structure in the chloride anion is not. In CoCl3Á4NH3, two of the three
chemical compounds. It is noteworthy that present day’s ball and chloride anions behave as ligands giving rise to ‘‘cis’’ and
stick model of a molecule emerges only from Bader’s theory of ‘‘trans’’ isomers (geometric), maintaining six coordination for
atoms-in-molecules (AIM),2 since from X-ray crystallography one the metal. The number of isomers of an inorganic complex
can know3 only about the shape of a molecule. Isomers, as now depends on the coordination number of the central atom and
understood, are molecules with the same chemical formula but the nature of the ligands. Coordination chemistry now is rife
different structures.
with varieties of isomerism.6 Herein, we describe a hitherto
Inorganic complexes are different from organic molecules undiscovered type of isomers in some five-coordinate copper(II)
in many ways. They usually contain a central atom which is complexes. In five-coordinate complexes, the most charac-
surrounded by other neutral molecules or ions. It was Werner teristic isomerism is the adoption of square pyramidal (SP)
and trigonal bipyramidal (TBP) geometries. For example,
3À
a Department of Inorganic Chemistry, Indian Association for the Cultivation of
Science, Calcutta 700 032, India. E-mail: icdd@iacs.res.in
b Department of Chemistry, University of Reading, Whiteknights,
Reading RG6 6AD, UK
the CuCl5
ion in (piperazinium)2CuC16ÁCH3OH is square
pyramidal7 but in [Cr(NH3)6][CuCl5] it is trigonal bipyramidal.8
In our pair of five-coordinate complexes, all the bonding para-
meters except for two angles are the same in their X-ray crystal
structures, a feature which we call ‘‘angle isomerism’’. Additionally,
we point out the relevance of Ostwald ripening in the formation of
our isomers.
† Electronic supplementary information (ESI) available: The Cartesian coordi-
nates of the DFT optimized structures of the complexes. CCDC 1063592 and
1063593. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5nj02765a
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2. Results and discussion
The ligand involved here is the 1 : 1 Schiff base of benzil and
2-hydrazinopyridine (LH where H is a dissociable proton).
It has been first reported by Chiswell et al. in 1964.9 It can exist
in two forms—keto and enol. In its X-ray crystal structure,
reported very recently by Hu et al.,10 the keto form is observed.
This is commensurate with our DFT calculations. At the BP86/
LanL2DZ level, the keto form is found to be more stable than the
enol form by 27.96 kcal molÀ1 and at the B3LYP/6-311++G(2d,p)
level, by 21.39 kcal molÀ1. Crucial bond distances in the tauto-
mers of LH are given in Table 1. The bond distances calculated
for a particular tautomer at two different levels of DFT are com-
parable. As expected, the N–N bond is much shorter in the enol
form than in the keto form as it assumes the character of a
double bond in the enol form. Similarly, the C–O bond is much
longer in the enol form than in the keto form as its bond order
is two in the keto form but one in the enol form. The copper(II)
complex CuLCl has been reported by Chiswell et al.9 And the
use of LH as a very sensitive colorimetric detector for the Cu2+
ion has been described by Hu et al.10 where the related copper(II)
complex in solution is CuL2. It is argued qualitatively that LH
prefers to bind to a metal in the anionic enolate form as a
tridentate N,N,O donor ligand.9,10 But to date, there has been no
X-ray crystal structure available for any of its metal complexes
(Chart 1).
Fig. 1 Morphology of the single crystals of 1a (a) and 1b (b).
We have found that LH reacts with CuCl2Á2H2O in methanol
at room temperature in equimolar proportion to yield a mixture
of two types of single crystals (Fig. 1) – hexagonal (1a) and
rectangular (1b) which are dark green with metallic sheen. On
crushing, they become red powder. This, we believe, is an optical
phenomenon. The colour of their solutions is deep red. The two
types of crystals were separated mechanically. They both were
analysed as CuLCl. Their FTIR spectra in KBr, devoid of an N–H
peak, are essentially similar. It is clear from their electronic
spectra that these two are different species. The ligand is light
yellow in colour in methanol showing strong absorptions at
Fig. 2 Electronic spectra of LH (black), 1a (blue) and 1b (red) in methanol.
The intensity e for 1a and 1b shown is per copper.
333 and 234 nm together with a shoulder at 257 nm (Fig. 2). The
band at 333 nm (e = 34,300 dm3 molÀ1 cmÀ1) has been assigned
to the n - p* transition of the carbonyl group by Bahgat11 from
DFT calculations. This band upon complexation shifts to a
longer wavelength at 506 nm in 1a and 1b giving rise to their
intense red colour in solution (Fig. 2). The e of this band in 1b is
25 300 dm3 molÀ1 cmÀ1. It is 90% of that in 1a. Interestingly, the
other strong band observed at 294 nm in the copper(II) complexes
has an e of 15 600 dm3 molÀ1 cmÀ1 in 1b which is also 90% of
that in 1a.
Table 1 Selected bond distances (in Å) in the two tautomers of LHa
The crystal structure of 1a is shown in Fig. 3. The X-ray
structure of 1b is similar.
Keto form
X-rayb
Enol form
BP86
Bond
BP86
B3LYP
B3LYP
Both are centrosymmetric dimers, Cu2L2Cl2, with equivalent
connectivities but have slightly different geometries. Selected
crystallographic data are given in Table 2 and the dimensions
are compared in Table 3. LH binds the copper atoms in the
anionic mode and behaves as a tridentate (N,N,O) ligand. Bond
lengths from the metal to the three donor atoms in the ligand
are very similar in 1a and 1b. A slight difference is observed in
the metal–chlorine bonds with Cu(1)–Cl(1) being 2.266(1) and
2.242(1) Å in the equatorial plane. The metal atom is 0.168(1) Å
from the plane in 1a and 0.295(1) Å in 1b. The CuÁ Á ÁCu
distances in 1a and 1b are 3.269(1) and 3.344(1) Å, respectively.
In the two complexes, the bonds of the ligand fragment listed
in Table 1 are found to be closer to the enol form of LH.
For example, experimental N–N bond lengths in 1a and 1b are
C–O
1.239
1.496
1.301
1.328
1.277
1.503
1.338
1.362
1.221
1.495
1.295
1.328
1.403
1.401
1.413
1.320
1.359
1.366
1.391
1.260
C(Ph)–C(Ph)
C(Ph)–N(NH)
N–N
a
The basis set used in conjunction with the BP86 functional is
b
LanL2DZ and that with the B3LYP functional is 6-311++G(2d,p). From
ref. 10.
Chart 1 Tautomers of the ligand LH.
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Table 3 Experimental (X-ray) and calculated (by DFT using the BP86
functional and the LanL2DZ basis set) dimensions (Å, 1) in the metal
coordination spheres of 1a and 1ba
1a
1b
X-ray
DFT
2.379
X-ray
DFT
2.397
Cu(1)–Cl(1)
Cu(1)–Cl(1)$1
Cu(1)–N(18)
Cu(1)–N(11)
2.266(1)
2.242(1)
2.575(1)
1.938(2)
1.996(3)
1.996(2)
92.86(3)
99.71(8)
155.94(9)
97.20(6)
96.41(9)
111.19(9)
94.90(8)
79.07(10)
159.11(9)
80.48(9)
87.14(3)
2.619
2.011
2.014
2.038
93.89
98.25
164.06
99.69
97.05
102.04
96.27
79.28
156.81
79.43
86.11
2.614(1)
1.955(2)
1.983(2)
1.988(1)
95.75(2)
100.84(5)
171.57(5)
97.70(4)
95.96(5)
92.65(5)
94.05(5)
78.97(7)
157.90(6)
80.93(6)
84.26(2)
2.567
2.011
2.005
2.043
94.47
98.11
153.86
98.33
97.77
111.67
94.77
79.46
158.43
79.61
85.53
Cu(1)–O(21)
Cl(1)–Cu(1)–Cl(1)$1
Cl(1)–Cu(1)–N(11)
Cl(1)–Cu(1)–N(18)
Cl(1)–Cu(1)–O(21)
Cl(1)$1–Cu(1)–N(11)
Cl(1)$1–Cu(1)–N(18)
Cl(1)$1–Cu(1)–O(21)
N(11)–Cu(1)–N(18)
N(11)–Cu(1)–O(21)
N(18)–Cu(1)–O(21)
Cu–Cl(1)–Cu
Fig. 3 The centrosymmetric structure of 1a with ellipsoids at 50%
probability.
Table 2 Some crystallographic data for complexes 1a and 1b
a
Symmetry operation $1: 1 À x, 1 À y, 1 À z in 1a and 2 À x, 1 À y, 1 À z
in 1b. For the atom labeling scheme, see Fig. 3. Note that the optimized
structures from DFT showed very small deviations in the last digit
between dimensions in the two halves of the molecule, so only dimen-
sions in one half of the molecule are given.
1a
1b
Colour
Dark green with
metallic luster
C38H28N6Cu2Cl2O2
798.64
Same as 1a
Formula
M
Space group
C38H28N6Cu2Cl2O2
798.64
Triclinic, P1
%
Monoclinic, C2/c
possibly aspirin.13,14 Aspirin crystallizes in the space group P21/c
which gives a wide choice of spatial arrangements in the solid.15
The polymorphs can have differences in their energies as low
as 0.50 kcal molÀ1. But isomers can have much larger energy
differences between them, as found here.
A further feature should be mentioned here, namely the
interaction in the dimer between an ortho proton of the pyridine
ring of the ligand and O(21)$1. The dimensions for HÁ Á ÁA, DÁ Á ÁA,
D–HÁ Á ÁA (D = donor, A = acceptor O(21)$1) are 2.79, 3.518 Å,
1361 in 1a and 3.16, 3.604 Å, 1111 in 1b, showing a much larger
interaction in the former. While this difference is significant,
it seems likely to be a consequence of the angle isomerism rather
than the driving force behind it, particularly as the interactions
can be considered as weak.
Cell dimensions (Å, 1)
a
b
c
21.798(3)
9.7924(9)
17.180(3)
111.803(18)
3404.9(8)
4, 1.558
1.451
1624
4801
3459
7.1801(6)
10.2796(7)
12.3788(8)
95.428(6)
846.66(11)
1, 1.566
1.459
406
4734
4115
b
U (Å3)
Z, dcalc (g cmÀ3
)
m (mmÀ1
F(000)
)
Unique reflections
Observed reflections
[I 4 2s(I)]
Parameters
226
226
R1, wR2 [I 4 2s(I)]
R1, wR2 (all data)
Largest peak/hole (e ÅÀ3
0.0619, 0.1515
0.0788, 0.1635
1.460/À1.556
0.0356, 0.0858
0.0432, 0.0899
0.400/À0.669
)
1.304 Å and 1.298 Å, respectively. On the other hand, this bond
In order to show that they are two distinct species, we have
in the enol form of LH is calculated to be 1.260 Å at the B3LYP/ examined the two structures via DFT calculations at BP86/
6-311++G(2d,p) level and found to be 1.328 Å in the X-ray crystal LanL2DZ and BP86/6-31G(2d,p) levels in quest of two different
structure of LH which, as pointed out above, is essentially the minima. Spin multiplicity is taken as 3. Single point energies
keto form of LH.
show 1b to have energy lower than 1a by 6.37 kcal molÀ1 using
The angles around the metal are very similar in the two struc- the LanL2DZ basis set and 3.78 kcal molÀ1 using the larger
tures, within 31 as shown in Table 3, apart from Cl(1)–Cu(1)–N(18) basis set. Upon geometry optimisation, this difference changes
being 155.94(9) and 171.57(5)1 and Cl(1)$1–Cu(1)–N(18) being to 3.21 kcal molÀ1 at the BP86/LanL2DZ level but the structures
111.19(9) and 92.65(5)1 in 1a and 1b, respectively. Interestingly, do not converge to the same one. The difference comes out to be
the sum of these two angles remains the same (155.941 + 111.191 = 4.49 kcal molÀ1 at the BP86/6-31G(2d,p)//BP86/LanL2DZ level.
267.131 and 171.571 + 92.651 = 264.221). This indicates that The fact that the two optimized structures correspond to true
the angle Cl(1)–Cu(1)–Cl(1)$1 remains the same in 1a and 1b minima has been established by frequency calculations. No
(within 31; 92.86(3)1 and 95.75(2)1). Since all the bonding para- imaginary frequency is encountered. Theoretically determined
meters except for two mutually dependent angles are the same, we bond parameters are close to the experimental values (Table 3).
call the relation between 1a and 1b as angle isomerism. In a much In particular, the two Cl–Cu–N(18) angles retain the differences,
broader sense, they are manifestations of polymorphism which is as observed in the X-ray crystal structures.
‘‘the ability of a compound to crystallize in more than one crystal
The X-band EPR spectra of 1a and 1b are shown in Fig. 4.
structure’’.12 The most enigmatic example of polymorphism is They are axial. They reveal that (dx2Ày2)1 is the ground state for
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Fig. 5 Variation in the concentration of a 3 Â 10À5 mol dmÀ3 methanolic
solution of 1a with time t as observed by monitoring the absorbance of the
solution at 506 nm. Coefficient of determination r2 = 0.952.
Fig. 4 Solid state X-band EPR spectra of 1a (black) and 1b (blue) at 77 K.
1a (gJ 4 g>) and (dz2)1 for 1b (gJ o g>). It is remarkable to find
that such a small difference in the structures of the metal
coordination spheres in 1a and 1b affects the electronic nature
of the ground state so dramatically. Very few examples of such
effects are known.16 The AJ value in 1a is unusually small. While
the normal mononuclear copper(II) complexes show it around
16 mT, it is only 6 mT in 1a. Such small AJ values occur in blue
copper proteins.17 The magnetic moments of 1a and 1b corre-
spond to one unpaired electron per copper at room temperature.
In keeping with this, no EPR signals at g = 4 are observed for
them indicating insignificant coupling between the two unpaired
electrons on the metals.18 Such a situation is indeed expected19,20
on the basis of the near 901 values of the Cu–Cl–Cu bridge angles
(Table 3). This justifies our consideration of S = 1 (i.e. triplet
ground state) in our DFT calculations on 1a and 1b.
on crystallization of a melt or a solution is the least stable poly-
morph.21 This can be justified by intricate thermodynamics.22–24
It should be mentioned that Ostwald’s law is concerned only
with the order of appearance of polymorphs in a single experi-
ment; the appearance of more stable forms later is a result of
transition. We have followed the transformation of 1a to 1b by
monitoring the absorbance at 506 nm with time t, spectro-
photometrically. Since the absorption maxima of both the iso-
mers occur at the same wavelength with only a 10% difference
in the extinction coefficient e in methanol, we have studied the
decrease in the absorbance of a methanolic solution of 1a in
the initial few hours of dissolution to avoid complications. The
transformation is found to follow the kinetics of a zero order
reaction (Fig. 5).
The relative yield of the two isomers 1a and 1b constitutes
an interesting phenomenon; it depends on the time allowed for
crystallization. When the allowed crystallization time is less, the
yield of 1a is found to be more than 1b. When the crystallization
3. Concluding remarks
is carried out for a longer time, the relative yield of 1a and 1b is Here, we have demonstrated a new type of isomerism where all the
reversed. The crystallization time is controlled by varying the bonding parameters except for two mutually dependent bond angles
volume of the solvent (methanol) used initially to carry out the are the same. It is reminiscent of the case of previously reported
reaction of LH with CuCl2Á2H2O. Typically, fixed amounts of bond stretch isomerism where all the bonding parameters except for
the two reagents (maintaining the 1 : 1 molar proportion) are one bond length were presumed to be the same.25,26 Anyway, it
mixed in methanol which almost instantaneously turns deep seems to have been mistaken as no two separate minima could be
red indicating the completion of the reaction. Then, the reac- located clearly on the related potential energy surface. But in 1a and
tion mixture is left for aerial evaporation. Crystals are deposited 1b, we obtain two distinct minima in our DFT calculations.
when the volume of the reaction mixture reduces to B5 ml. Thus,
We do not understand the reason for the occurrence of angle
the more the amount of methanol taken initially, the longer the isomerism in 1a and 1b. But a related observation of ours is
two species remain in solution. When the synthesis is carried out that replacement of the four phenyl rings in 1a and 1b by H
by taking the amounts of the reagents LH and CuCl2Á2H2O atoms leads to the convergence of the two structures to the same
specified in the Experimental section in 10 ml of methanol, structure in DFT calculations where the two relevant Cl–Cu–N(18)
hexagonal crystals (1a) are obtained after 24 h as almost the sole angles become 159.2 and 106.71 with the sum of these two angles
product. In our DFT calculations, we have seen that energetically (265.91) almost the same as those obtained in the crystal
1a is less stable than 1b. The time dependent pattern of relative structures of 1a and 1b. Thus, the skewness of benzil compared
yield of 1a and 1b indicates that the less stable isomer forms to glyoxal27 may have a role here.
first. This observation is in line with the concept of Ostwald
A remarkable result of our present work is the observation
ripening. In 1897, Ostwald observed that the solid first formed of Ostwald ripening in the formation of isomers 1a and 1b.
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The transformation of 1a to 1b follows the kinetics of a zero 1141s, 1093m, 1070w, 1012m, 931m, 844w, 777m, 689m, 599w,
order reaction. It should be noted that the reverse transforma- 511w. LM/mho cm2 molÀ1 (MeOH): hexagonal (1a), 27.4; rectangular
tion is not possible. Ostwald’s law finds practical application in (1b), 24.5 (non-electrolyte). UV-Vis lmax/nm (emax/dm3 molÀ1cmÀ1):
the area of materials processing.28–31 Its present identification rectangular, 506 (25 300), 294 (15 600), 228 (sh); hexagonal, 506
is possibly the first in coordination chemistry.
(28 400), 292 (17 700), 230 (sh). meff/mB per copper (at 298 K):
hexagonal (1a), 1.84; rectangular (1b), 1.89.
4.4 Computation
4. Experimental
DFT calculations were performed using GAUSSIAN 09 suite of
programs.32 Inputs were given from the X-ray crystal structures.
4.1 Materials and physical measurements
Microanalyses were performed using a Perkin-Elmer 2400II CHNS
analyser. Molar conductance was measured using a Syntronics
(India) conductivity meter (model 306) in methanol. FTIR spectra
(KBr) were recorded on a Shimadzu FTIR-8400S spectrometer
and UV-Vis spectra (in CH3OH) on a Perkin Elmer Lambda 950
spectrophotometer. The 500 MHz NMR spectrum of LH was
recorded on a Bruker Avance III 500 spectrometer in CDCl3.
X-band EPR spectra of the copper(II) complexes were recorded on a
JEOL JES-FA200 spectrometer and ESI mass spectra (in CH3CN)
on a Waters Qtof Micro YA263 spectrometer. Room temperature
magnetic moments were measured using a magnetic susceptibility
balance procured from Sherwood Scientific, UK. The diamagnetic
correction was evaluated using Pascal’s constants.
4.5 X-ray crystallography
Data for 1a and 1b were collected with MoKa at 150 K using
the Oxford Diffraction X-Calibur CCD system. The crystals were
positioned at 50 mm from the CCD and 321 frames were
measured with counting times of 10 s. Data analyses were carried
out using the CrysAlis program.33 Both structures were solved by
direct methods using the Shelxs97 program.34 The non-hydrogen
atoms were refined using anisotropic thermal parameters. The
hydrogen atoms bonded to carbon were included in geometric
positions and given thermal parameters equivalent to 1.2 times
those of the atom to which they were attached. Absorption
corrections were carried out using the ABSPACK program.35
The two structures were refined using Shelxl9734 on F2.
4.2 Synthesis of LH
LH was prepared by modifying the procedure reported by
Chiswell et al.9 in the following manner. 2-Hydrazinopyridine
(1.09 g, 10 mmol) was added to a solution of benzil (2.10 g,
10 mmol) in ethanol (40 ml). The resulting orange yellow solution
was refluxed for 2 h and then it was left in the air. After 16 h, the
precipitated light yellow microcrystalline compound was filtered
and washed successively with cold ethanol (20 ml) and diethyl
ether (25 ml). The compound was dried in air. Yield: 1.8 g (60%).
m.p. 138–141 1C (lit. 140 1C).9 Anal. calc. for C19H15N3O: C, 75.73;
Acknowledgements
M. G. B. D. thanks EPSRC and the University of Reading for
funds for the Oxford Instruments X-Calibur system. S. G. P.
thanks the Council of Scientific and Industrial Research,
Government of India, New Delhi, for a fellowship.
References
1
H, 5.02; N, 13.94 found: C, 75.59; H, 4.99; N, 13.98%. H NMR
d/ppm: 7.73–7.8 (m, 2H), 7.31–7.39 (m, 8H), 7.19–7.25 (m, 8H),
7.02 (t, 1H). FTIR n/cmÀ1: 3315m, 3049w, 1631s, 1595s, 1554s,
1492s, 1440s, 1338s, 1292s, 1265s, 1247s, 1176m, 1081m,
1026w, 929w, 896w, 779m, 698m, 673w, 532m, 445w, 408w.
UV-Vis lmax/nm (emax/dm3 molÀ1 cmÀ1): 234 (29 600), 257
(23 000), 333 (34 500). ESI-MS m/z: 302.20 (LH + H+, 100%).
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4.3 Syntheses of 1a and 1b
A solution of CuCl2Á2H2O (0.034 g, 0.2 mmol) in methanol (10 ml)
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1556w, 1515m, 1456m, 1440m, 1359s, 1330s, 1215s, 1176s,
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