Synthesis and characterization of nickel(II) and copper(II) complexes with tetradentate Schiff base ligands

Article abstract of DOI:10.1016/j.ica.2010.10.010

The 1:1 condensation of 1,2-diaminopropane and 1-phenylbutane-1,3-dione at high dilution gives a mixture of two positional isomers of terdentate mono-condensed Schiff bases 6-amino-3-methyl-1-phenyl-4-aza-2-hepten-1-one (HAMPAH) and 6-amino-3,5-dimethyl-1

Full text of DOI:10.1016/j.ica.2010.10.010

Inorganica Chimica Acta 366 (2011) 62–67
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Inorganica Chimica Acta
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Synthesis and characterization of nickel(II) and copper(II) complexes
with tetradentate Schiff base ligands
Prasanta Bhowmik a, Michael G.B. Drew b, Shouvik Chattopadhyay a,
⇑
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700032, India
b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The 1:1 condensation of 1,2-diaminopropane and 1-phenylbutane-1,3-dione at high dilution gives a mix-
ture of two positional isomers of terdentate mono-condensed Schiff bases 6-amino-3-methyl-1-phenyl-
4-aza-2-hepten-1-one (HAMPAH) and 6-amino-3,5-dimethyl-1-phenyl-4-aza-2-hexen-1-one (HADPAH).
The mixture of the terdentate ligands has been used for further condensation with pyridine-2-carboxal-
dehyde or 2-acetylpyridine to obtain the unsymmetrical tetradentate Schiff base ligands. The tetraden-
tate Schiff bases are then allowed to react with the methanol solution of copper(II) and nickel(II)
perchlorate separately. The X-ray diffraction confirms the structures of two of the complexes and shows
that the condensation site of the diamine with 1-phenylbutane-1,3-dione is the same.
Ó 2010 Elsevier B.V. All rights reserved.
Received 14 September 2010
Received in revised form 5 October 2010
Accepted 8 October 2010
Available online 14 October 2010
Keywords:
Crystal structure
Nickel(II)
Copper(II)
Schiff base
Cyclic voltametry
1. Introduction
structure analysis showed that the condensation site of the dia-
mine with 2,4-pentanedione is different in copper(II) and nickel(II)
The condensation of an amine with an aldehyde or ketone,
forming what is called a Schiff base, is one of the oldest reactions
in chemistry [1]. Schiff base ligands coordinate to a metal through
the imine nitrogen and another group, usually oxygen, situated on
the original carbonyl compound. When a diamine is combined
with one equivalent of a b-diketone, a terdentate Schiff base is
formed and when this terdentate Schiff bases are made to undergo
condensation with pyridine-2-carboxaldehyde or 2-acetylpyridine,
tetradentate unsymmetrical Schiff base ligands come into being
[2–24]. The ligands have four donor sites. This makes the ligands
ideal for the equatorial coordination of transition metals, leaving
the two axial sites open for auxiliary ligands. They are very much
like porphyrins in this regard, but unlike porphyrins, these ligands
are easy to prepare and inexpensive [25].
The 1:1 condensation of 2,4-pentanedione and 1,2-diaminopro-
pane under the condition of high dilution produces isomeric
mixtures of mono-condensed terdentate Schiff bases 7-amino-4-
methyl-5-aza-3-octene-2-one (HAMAO) or 7-amino-4,6-di-
methyl-5-aza-3-heptene-2-one (HADAH) [26]. This mixture could
be made to undergo condensation reaction with 2-acetylpyridine
and pyridine-2-carboxaldehyde to form isomeric mixtures of tetra-
dentate unsymmetrical Schiff bases which could be used to form
complexes with transition metal ions. Interestingly, the crystal
complexes i.e. the methyl group of the diamine is located in the
vicinity of 2,4-pentanedione in the nickel(II) complex whereas it
is in the remote position in the copper(II) one [26]. On putting
the nickel(II) complex under reflux and taking the 1H NMR spectra
before, during and after the reflux, it was concluded [26] that the
tetradentate ligands reacted readily with nickel(II) to result imme-
diately in a mixture of the two isomeric complexes that finally con-
verted into single products where methyl group of the diamine is
located in the vicinity of 2,4-pentanedione. Under similar condi-
tions, copper(II) yielded a complex having a remote methyl group.
From this observation, it was concluded that rearrangement of the
ligand and the stability of its isomeric forms depend upon the nat-
ure of the metal ions forming the complexes.
To examine whether the conclusion was general one or applica-
ble to that particular case only, we planned to prepare similar
complexes using 1-phenylbutane-1,3-dione instead of 2,4-pen-
tanedione and synthesised nickel(II) and copper(II) complexes with
similar tetradentate ligands. The result is quite interesting, because
here we get both the nickel(II) and copper(II) complexes with
methyl group remote from 2,4-pentanedione moiety.
2. Experimental
⇑
All chemicals were of reagent grade and used without further
purification.
Corresponding author. Tel.: +91 9903756480.
E-mail address: shouvik.chem@gmail.com (S. Chattopadhyay).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.10.010

P. Bhowmik et al. / Inorganica Chimica Acta 366 (2011) 62–67
63
2.1. Preparations
2.1.5. Synthesis of [Ni(L2)]ClO4 (4)
It was prepared in the same manner except the fact that here
we use 2-acetylpyridine (10 mmol, 1.21 g) instead of pyridine-2-
carboxaldehyde. It was recrystallised from acetonitrile to obtain
dark-red single crystals suitable for X-ray diffraction. Yield:
4.21 g (88%) Anal. Calc. for C20H22O5NiN3Cl (478.6): C, 50.20; H,
4.63; N, 8.78, Ni 12.26. Found: C, 50.2; H, 4.6; N, 8.8, Ni 12.2%.
UV–Vis, kmax (nm), (emax (dm3 molÀ1 cmÀ1)) (acetonitrile), 331
(444), 434 (362) Magnetic moment diamagnetic.
2.1.1. Synthesis of the terdentate ligand
A
solution (25 ml) of 1-phenylbutane-1,3-dione (10 mmol,
1.62 g) in chloroform was added drop wise to a solution of 1,2-
diaminopropane (10 mmol, 0.76 cm3) in chloroform. After comple-
tion of addition, the solution was stirred for an additional 3 h and
chloroform was evaporated under reduced pressure, yielding a
mixture of neutral monodentate ligand, 6-amino-3-methyl-1-phe-
nyl-4-aza-2-hepten-1-one (HAMPAH) and 6-amino-3,5-dimethyl-
1-phenyl-4-aza-2-hexen-1-one (HADPAH), as
(Scheme 1).
a viscous liquid
2.2. Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was per-
formed using a Perkin–Elmer 240C elemental analyzer. IR spectra
in KBr (4500–500 cmÀ1) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer. Electronic spectra in acetonitrile
(1200–350 nm) were recorded in a Hitachi U-3501 spectropho-
tometer. Copper content in the complex was estimated spectro-
photometrically and the nickel content in the complexes was
estimated gravimetrically using DMG. The magnetic susceptibility
measurements were done with an EG & PAR vibrating sample mag-
netometer, model 155 at room temperature and diamagnetic cor-
rections were made using Pascal’s constants. Electrochemical
measurements were performed under a dry nitrogen atmosphere
in a three-electrode configuration using Pt disk working electrode,
Pt auxiliary electrode and SCE (reference electrode) on a Sycopel
model AEW2 1820F/S instrument and were uncorrected for junc-
tion contribution. The value for the ferrocenium–ferrocene couple
under our conditions is 0.40 V.
2.1.2. Synthesis of [Cu (L1)]ClO4 (1)
A mixture of HAMPAH and HADPAH (10 mmol) was dissolved in
methanol (10 cm3). Pyridine-2-carboxaldehyde (10 mmol, 1.07 g)
was added to it. The mixture was warmed at 50 °C for 1 h and
cooled to room temperature.
A
solution of Cu(ClO4)2Á6H2O
(10 mmol, 3.7 g) in methanol (20 cm3) were added to the resulting
solution of the tetradentate ligand with continuous stirring to pre-
cipitate dark-red solid. It was put under reflux for ca. 6 h and the
resulting solid was recrystallised from acetonitrile solution to ob-
tain dark-red single crystal for complex 1 suitable for X-ray
diffraction.
Yield: 4.10 g (85%). Anal. Calc. for C19H20O5CuN3Cl (469.4): C,
48.62; H, 4.29; N, 8.95, Cu 13.54. Found: C, 48.6; H, 4.3; N, 8.9,
Cu 13.5%. UV–Vis, kmax (nm), (emax (dm3 molÀ1 cmÀ1)) (acetoni-
trile), 545 (145), 363 (1159), Magnetic moment
l = 1.73 BM.
2.1.3. Synthesis of [Cu(L2)]ClO4 (2)
2.3. X-ray crystallography
It was prepared in the same manner except the fact that here
we use 2-acetylpyridine instead of pyridine-2-carboxaldehyde. It
was recrystallised from acetonitrile, but no suitable single crystals
were grown for complex 2.
Yield: 3.80 g (81%). Anal. Calc. for C20H22O5CuN3Cl (483.4): C,
49.69; H, 4.59; N, 8.69, Cu 13.15. Found: C, 48.7; H, 4.5; N, 8.7,
Cu 13.2%. UV–Vis, kmax (nm), (emax (dm3 molÀ1 cmÀ1)) (acetoni-
Crystal data for the two crystals are given in Table 1. 5410 inde-
pendent data of complex 1 were collected with Mo K
a radiation
using the Oxford Diffraction X-Calibur CCD System. The crystal
was positioned at 50 mm from the CCD. 321 frames were mea-
sured with a counting time of 10 s. Data analysis was carried out
with the CRYSALIS program [27]. The structure was solved using
trile), 533 (130), 373 (1170), Magnetic moment
l = 1.73 BM.
2.1.4. Synthesis of [Ni(L1)]ClO4 (3)
Table 1
A mixture of HAMPAH and HADPAH (10 mmol) was dissolved in
methanol (10 cm3). Pyridine-2-carboxaldehyde (10 mmol, 1.07 g)
was added to it. The mixture was warmed at 50 °C for 1 h and
Crystal data for compounds 1 and 4.
1
4
Formula
C
19H20O5CuN3Cl
C20H22O5NiN3Cl
478.57
298(2)
monoclinic
P21/n
8.617(11)
21.06(2)
11.974(14)
(90)
cooled to room temperature.
A
solution of Ni(ClO4)2Á6H2O
Formula weight
Temperature (K)
System
Space group
a (Å)
469.37
150(2)
monoclinic
P21/n
(10 mmol, 3.65 g) in methanol (20 cm3) were added to the result-
ing solution of the tetradentate ligand with continuous stirring to
precipitate dark-red solid. It was put under reflux for ca. 6 h and
then was recrystallised from acetonitrile, but no suitable single
crystals were grown for complex 3.
7.3101(3)
11.9007(5)
21.7059(10)
(90)
b (Å)
c (Å)
Yield: 3.84 g (83%). Anal. Calc. for C19H20O5NiN3Cl (464.6): C,
49.13; H, 4.34; N, 9.05, Ni 12.64. Found: C, 49.2; H, 4.3; N, 9.1, Ni
12.6%. UV–Vis, kmax (nm), (emax (dm3 molÀ1 cmÀ1)) (acetonitrile),
338 (450), 443 (394) Magnetic moment diamagnetic.
a
(°)
b (°)
97.144(4)o
(90)
99.78(1)
(90)
2141.2
c
(°)
V (Å3)
1873.65(8)
4
1.664
Z
4
Dcalc (mg mÀ3
)
1.485
1.067
l
(mmÀ1
)
1.347
F(0 0 0)
964
992
Crystal size (mm)
Total reflection
Independent reflections
Rint
Collected reflections [I > 2
R1, wR2 (all data)
0.25 Â 0.04 Â 0.03
0.30 Â 0.25 Â 0.25
12657
8359
O
O
5410
0.0688
2263
R1 = 0.1153
wR2 = 0.1404
R1 = 0.0485,
wR2 = 0.1031
3114
0.0709
1628
R1 = 0.1645
wR2 = 0.1913
R1 = 0.0851
wR2 = 0.1747
NH2
NH2
H
N
H
N
r(I)]
HAMPAH
HADPAH
R1, wR2 [I > 2r(I)]
Scheme 1. Schematic representation of the terdentate ligands.

64
P. Bhowmik et al. / Inorganica Chimica Acta 366 (2011) 62–67
direct methods with the SHELX97 program [28]. The non-hydrogen
atoms were refined with 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. An absorption cor-
rection was carried out using the ABSPACK program [29]. The struc-
ture was refined on F2 using SHELX97 [30] to R1 0.0485, wR2
using SHELX97 [28] to R1 0.0851, wR2 0.1747 for 1637 reflections
with I > 2 (I).
r
3. Results and discussion
3.1. Syntheses
0.1031 for 2263 reflections with I > 2
Intensity data of complex 4 was measured at 298 K using the
Marresearch Image Plate diffractometer with Mo K radiation.
r(I).
The formation of the complexes is shown in Scheme 2. Conden-
sation of the free amino group of the singly condensed isomeric
ligands, HAMPAH and HADPAH, with pyridine-2-carboxaldehyde
(and 2-acetylpyridine) results in the formation of the isomers of
unsymmetrical tetradentate Schiff base ligands HL1 (and HL2)
and their positional isomers. Both the isomeric forms of the tetra-
dentate ligand react with copper(II) and nickel(II) perchlorate sep-
arately to produce immediately a mixture of isomeric complexes. It
is known that Schiff bases are subject to rearrangement in pres-
ence of H+ and/or metal ion, [33]. So we put the compounds under
reflux for about 6 h to convert the mixture into a single product.
We cannot grow suitable single crystals for complexes 2 and 3,
a
The crystal was positioned at 70 mm from the Image Plate. 100
frames were measured with a counting time of 5 m. Data analysis
was carried out with the XDS program [31] to give 3114 indepen-
dent reflections. The structure was solved using direct methods
with the SHELX97 program [28]. The non-hydrogen atoms were re-
fined with 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. An Absorption correction was carried
out using the DIFABS program [32]. The structure was refined on F2
O
O
+
NH
NH2
NH
NH2
H3C
H3C
H3C
CH3
HADPAH
HAMPAH
R
N
O
N
OH
N
OH
N
+
( R = H, CH3 )
N
N
R
R
N
H3C
H3C
CH3
H3C
Ni(ClO4)2
Reflux in methanol
Cu(ClO4)2
Reflux in methanol
N
O
N
O
Cu
Ni
ClO4
ClO4
R
R
N
N
N
N
H3C
H3C
CH3
CH3
Complex 1 (R = H)
Complex 2 (R = CH3)
Complex 3 (R = H)
Complex 4 (R = CH3)
Scheme 2. Synthetic scheme of the complexes.

P. Bhowmik et al. / Inorganica Chimica Acta 366 (2011) 62–67
65
Table 2
but complexes 1 and 4 forms diffraction quality single crystalsand
Selected bond lengths (Å) and bond angles (°) for compounds 1 and 4.
X ray structure determination showed that the methyl group re-
sides in the remote position from 1-phenylbutane-1,3-dione
moiety.
1 (M = Ni)
4 (M = Cu)
Bond lengths
M(1)–N(11)
M(1)–N(18)
M(1)–N(21)
M(1)–O(31)
1.908(8)
1.888(7)
1.845(9)
1.867(6)
1.987(4)
1.913(3)
1.912(4)
1.872(3)
3.2. Description of the structure of complexes 1 and 4
The ORTEP views of the asymmetric units, consisting of discrete
[Cu(L1)]+ and [Ni(L2)]+ of the complexes 1 and 4 respectively, to-
gether with (common) atom numbering schemes are shown in
Figs. 1 and 2. In both the complexes, the deprotonated ligands L1
and L2 are quadridentate forming one six membered and two five
membered chelate rings (6–5–5). The metal ions have square pla-
ner co-ordinations, being bonded to the four donor atoms of the
tetradentate ligand, with a slight tetrahedral distortion in both
the complexes. The sum of the different angles around the metal
Bond angles
N(11)–M(1)–N(18)
N(18)–M(1)–N(21)
N(21)–M(1)–O(31)
O(31)–M(1)–N(11)
N(11)–M(1)–N(21)
N(18)–M(1)–O(31)
82.8(3)
86.7(4)
97.2(3)
93.4(3)
169.5(4)
176.1(3)
81.4(1)
85.0(1)
97.4(1)
96.5(1)
165.5(1)
175.9(1)
is 341.4° in complex 1 whereas it is 345.6° in complex 4 indicating
slightly distorted square-planer geometry around the metal ions.
The distortion may conveniently be measured by the trans angles
that are ideally 180° for a square-planar complex and 109.5° in a
tetrahedral complex. The two trans angles [N(11)–M(1)–N(21)
and O(31)–M(1)–N(18)] are 165.5(1)° and 175.9(1)° in complex 1
and 169.5(4)° and 176.1(3)° in complex 4, respectively.
The deviations of the coordinating atoms N(11), N(18), N(21)
and O(31) from the least square plane through them are
À0.068(3), 0.076(3), À0.068(3), 0.060(2) Å in complex 1 and
0.002(8), À0.002(8), 0.002(8) and À0.026(5) Å in complex 4,
respectively. Deviation of the metal atom from the same plane is
0.011(4) and 0.011(1) Å in complexes 1 and 4, respectively. The
six-membered benzoylacetone rings around the metal ions are
essentially planar with no atom deviating from the least square
plane by more than 0.041 and 0.019 Å in complexes 1 and 4,
respectively. The five-membered ring M(1)–N(18)–C(19)–C(20)–
N(21) in complex 1 (M = Cu) is twisted on C(19)–C(20) with puck-
ering parameters Q(2) = 0.253(4),
ring in complex 4 (M = Ni) has an envelope configuration on
C(20) with puckering parameters Q(2) = 0.131(13) and = 100(4)
u = 85.8(7) whereas the same
u
[34]. The phenyl ring in complex 1 intersects the equatorial plane
at an angle of 5.9(1)°. Thus the molecule is approximately planar
which facilitates the stacking of two molecules related by a centre
of symmetry. The perchlorate anion has no close contacts with the
metal ions. The phenyl ring in complex 4 is twisted by 8.6(4)° from
this equatorial plane.
Fig. 1. The structure of the [CuL1]+ cation in 1 with ellipsoids at 50% probability.
The dimensions of the metal coordination sphere are given in
Table 2. In both the complexes the bond lengths and angles are
comparable with those for the related copper(II) and nickel(II)
complexes of the non-symmetric Schiff base ligands and are close
to the expected values [12,13,35,36].
3.3. IR and electronic spectra and magnetic moments
The bands corresponding to azomethine group (C@N) are dis-
tinct in all the four complexes and occur within 1587–
1520 cmÀ1. The lowering of the positions of the bands indicates
their coordination to the metal centers. The sharp, strong, single
Table 3
Electrochemical data for complexes 1–2.
Complex
E1/2/V (D
Ep/mV)a
CuII/I couple
1
2
0.53 (77)
0.52 (72)
a
Fig. 2. The structure of the [NiL2]+ cation in 4 together with the atomic numbering
E1/2 is calculated as the average of anodic (Epa)
and cathodic (Epc) peak potentials;
Ep = (Epa À Epc).
D
scheme, ellipsoids at 50% probability.

66
P. Bhowmik et al. / Inorganica Chimica Acta 366 (2011) 62–67
peak at 1088, 1092, 1089 and 1090 cmÀ1 for complexes 1, 2, 3 and
4, respectively gives evidence for the presence of ionic perchlorate
in all of them [37].
The appearance of two bands at 545 and 363 nm in the absorp-
tion spectrum of complex 1 and two bands at 533 and 373 nm in
that of complex 2 is consistent with the square planar geometry
of copper(II) [26,38–40]. The electronic spectra show that absorp-
tion bands of the nickel(II) complexes (3 and 4) occur below
600 nm. The lack of any electronic transition at longer wavelengths
indicates a large crystal-field splitting and is consistent with the
square planar geometry of nickel(II) complexes [26].
pattern in electron transfer behavior of 3 and 4 at about same po-
tential establishes the fact that they have closely similar structures.
All the redox signals remain virtually invariant under different
scan rates (0.01–1.0 V sÀ1) in the temperature range 300–280 K.
Solvent dependent shift and change in electrochemical reversibil-
ity of redox couples are not significant.
3.5. Luminescence studies
Complexes 1–4 show fluorescence. These are assigned as intra-
1
ligand fluorescence
(p ? p*). The spectroscopic data are given in
Room temperature magnetic susceptibility measurements
show that complexes 1 and 2 have magnetic moments close to
1.73 BM as expected for discrete magnetically non-coupled spin-
only value for copper(II) ion, as was observed in similar systems
[33,41,42]. Compounds 3 and 4 are diamagnetic as expected for
the square planar nickel(II) [39].
Table 5 (without solvent correction).
4. Summary
A mixture of two (positional) isomeric terdentate mono-condensed
Schiff bases 6-amino-3-methyl-1-phenyl-4-aza-2-hepten-1-one
(HAMPAH) and 6-amino-3,5-dimethyl-1-phenyl-4-aza-2-hexen-
1-one (HADPAH), produced by the 1:1 condensation of 1,2-diami-
nopropane and 1-phenylbutane-1,3-dione at high dilution, has
been used for further condensation with pyridine-2-carboxalde-
hyde or 2-acetylpyridine to obtain the unsymmetrical tetradentate
Schiff base ligands. This tetradentate Schiff bases are allowed to re-
act with the methanol solution of copper(II) and nickel(II) perchlo-
rate separately. Crystal structure analysis showed that the
condensation site of the diamine with 1-phenylbutane-1,3-dione
is the same.
3.4. Electrochemistry
The redox behavior of all the complexes were studied by using
cyclic voltammetry (CV) in acetonitrile (0.1 M NEt4ClO4) in the po-
tential range 2 V by using platinum auxiliary electrode and Pt disc
working electrode at ambient temperature (300 K) with no trace of
decomposition as reflected in smooth curve. The reported poten-
tials (Tables 3 and 4) are referenced to the saturated calomel elec-
trode (SCE).
Cyclic voltammetric studies of the complexes 1 and 2 in aceto-
nitrile solution under nitrogen atmosphere are quasi-reversible
Acknowledgments
with
D
Ep ꢀ 75 mV. The redox reaction at E1/2 ꢀ +0.5 V corresponds
This work was supported by the CSIR, India (Fellowship for
Prasanta Bhowmik, Sanction No. 09/096(0607)/2010-EMR-I) and
the University Grants Commission, CAS-UGC, New Delhi. We also
thank EPSRC and the University of Reading for funds for the
X-Calibur system.
to the electron abstraction from the complex and is consistent with
[Cu(L1)]2+/[Cu(L1)]+ couple (for complex 1) or [Cu(L2)]2+/[Cu(L2)]+
couple (for complex 2). The similar pattern of cyclic voltammo-
gram of 1 and 2 indicates the similarity of their structures. The re-
sults of cyclic voltammetry also closely resemble that of the similar
reported compounds, which serve as further evidences for similar
structural and electronic properties [43].
Appendix A. Supplementary material
Complexes 3 and 4 display two one electron cyclic voltammet-
ric responses at about 0.95 and 1.70 V. The irreversible redox signal
encountered near 1.70 V is attributable to the Ni4+/3+ response,
CCDC 757588 and 757589 contain the supplementary crystallo-
graphic data for complex 1 and complex 4, respectively. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.10.010.
whereas quasi reversible (
D
Ep ꢀ 115 mV) one electron oxidative
response at about 0.95 V is attributable to the Ni3+/2+ couple. The
absence of any satellite response is consistent with the exclusive
occurrence of single isomer in the solution. No voltammetric re-
sponse consistent with Ni2+/+ reduction is observed. The similar
References
Table 4
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Ep/mV)a
NiIII/II couple
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3
4
0.97 (110)
0.95 (118)
1.69
1.71
a
E1/2 is calculated as the average of anodic (Epa) and cathodic
(Epc) peak potentials;
Ep = (Epa À Epc).
D
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Table 5
Photophysical data for the complexes 1–4.
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Complex
Absorption (nm)
Emission (nm)
1
2
3
4
280
281
282
280
458
435
449
455
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