Synthesis, spectra, structure, redox properties and DFT computation of copper(I)-triphenylphosphine-pyridyl Schiff bases

Article abstract of DOI:10.1016/j.ica.2012.01.030

The reaction of 2-(2′-pyridylmethyleneamino)pyridine (L ¹)/3-(2′-pyridylmethyleneamino)pyridine (L²)/4- (2′-pyridylmethyleneamino)pyridine (L³) with CuX (X = Cl, Br, I) and triphenylphosphine (PPh₃) affords comp

Full text of DOI:10.1016/j.ica.2012.01.030

Inorganica Chimica Acta 387 (2012) 240–247
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, spectra, structure, redox properties and DFT computation
of copper(I)–triphenylphosphine–pyridyl Schiff bases
1
2
⇑
Gunomoni Saha , Kamal Krishna Sarkar , Tapan Kumar Mondal, Chittaranjan Sinha
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
The reaction of 2-(20-pyridylmethyleneamino)pyridine (L1)/3-(20-pyridylmethyleneamino)pyridine (L2)/
4-(20-pyridylmethyleneamino)pyridine (L3) with CuX (X = Cl, Br, I) and triphenylphosphine (PPh3) affords
complexes of composition [Cu(L)(PPh3)X]. These are characterized by microanalytical data and spectro-
scopic studies (IR, UV–Vis, 1H NMR). The single crystal X-ray structure determines the distorted tetrahe-
dral geometry about Cu(I) in case of [Cu(L2)(PPh3)I]. Cyclic voltammogram shows Cu(II)/Cu(I) couple at
0.5–0.6 V, in addition [Cu(L)(PPh3)I] shows iodide oxidation peak at 0.3 V. Redox couple negative to
SCE may be assigned to the reduction of chelated diimine function. Electronic structure, spectra, emission
properties and redox activities are explained by DFT calculations of optimized geometry of
[Cu(L2)(PPh3)X] and have been used to determine the composition and energy of the molecular levels.
Ó 2012 Elsevier B.V. All rights reserved.
Article history:
Received 6 August 2011
Received in revised form 12 January 2012
Accepted 13 January 2012
Available online 28 January 2012
Keywords:
Copper(I)–pyridyl-Schiff base
X-ray structure
Electrochemistry
DFT computation
1. Introduction
2. Experimental
Metal complexes of Schiff bases are known since 1860 [1].
Significant development has been started on and from 1950 [2,3].
Ligands containing polyimine function are of potential interest
in the development of coordination chemistry of lower valent
metal redox states. Polypyridine, a class of diimine function
(AN@CAC@NA), is useful in the development of transition and
non-transition metal complexes [4–8]. The condensation of pyri-
dinecarboxaldehyde with aromatic amine has synthesized diimine
function [9–15]. The condensation of aminopyridine(s) and pyri-
dine-2-carboxaldehyde produces useful Schiff base which can
stabilize low valent metal ion like copper(I) [14–21].
Herein we describe the synthesis, spectral characterization of
copper(I) halide complexes of 2-/3-/4-(20-pyridylmethyleneami-
no)pyridine incorporating phosphine coordination. In one case
the structural confirmation is achieved by single crystal X-ray
structure analysis. The DFT and TD-DFT computations have been
carried out to explain the spectroscopic and redox properties of
the complexes.
2.1. Materials
2-Aminopyridine, 3-aminopyridine, 4-aminopyridine and pyri-
dine-2-carboxaldehyde were purchased from Sigma–Aldrich. CuX
(X = Cl, Br, I) and triphenylphosphine were purchased from
E. Merck, India. All other chemicals and solvents were of reagent
grade and used as received.
2.2. Physical measurements
Microanalytical data (C, H, N) were collected from Perkin–Elmer
2400 CHNS/O elemental analyzer. Spectroscopic data were ob-
tained using the following instruments: UV–Vis spectra, Perkin–El-
mer; model Lambda 25, IR spectra (KBr disk, 4000–400 cmꢀ1),
Perkin–Elmer; model spectrum RX-1, 1H NMR spectra, Bruker
(AC) 300 MHz FTNMR spectrometer. Electrochemical measure-
ments were performed using computer-controlled CH Instruments
Electrochemical Workstations with Pt-disk electrodes. All mea-
surements were carried out under nitrogen environment at 298 K
at scan rate 50 mV Sꢀ1 with reference to SCE in acetonitrile using
[nBu4N][ClO4] as supporting electrolyte. The reported potentials
are uncorrected for junction potential.
⇑
Corresponding author. Tel.: +91 9433309907; fax: +91 033 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present address: Department of Chemistry, Barrackpore Rastraguru Surendranath
2.3. Synthesis of ligands
1
College, Barrackpore, Kolkata 700 120, India.
Present address: Department of Chemistry, Mahadevananda Mahavidyalaya,
Barrackpore, Monirampore, Kolkata 700 120, India.
2
The ligands were synthesized following a common procedure
[22]. Detail of synthesis for L1 is given below.
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.030

G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
241
2-Aminopyridine (1.213 g, 12.9 mmol) was dissolved in 30 ml
methanol, pyridine-2-carboxyaldehyde (1.38 g, 12.9 mmol) was
added to it and the mixture was refluxed for 6 h in the presence
of molecular sieves, it was then filtered and filtrate was collected
in a beaker and solvent was allowed to evaporate slowly. The crys-
talline product at the bottom of the beaker was collected, washed
with methanol (2 ꢁ 5 ml) and dried over CaCl2 in a desiccator.
Yield was 64% and m.p. 196 2 °C.
in MeCN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 405 (0.845), 284
(10.59). [Cu(L2)(PPh3)Br] (2b) Anal. Calc. for C29H24CuBrN3P:
59.15; H, 4.11; N, 7.14. Found: 59.03; H, 4.04; N, 7.ꢀ119%. FT-IR
(KBr disc, cmꢀ1),
m(PPh3), 597, 708; m(C@N), 1612 cm . UV–Vis
spectral data in MeCN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1cmꢀ1))):
401 (0.568), 288 (11.71). [Cu(L3)(PPh3)Br] (2c) Anal. Calc. for
C
29H24CuBrN3P: 59.15; H, 4.11; N, 7.14. Found: 59.19; H, 4.17; N,
7.08%. FT-IR (KBr disc, cmꢀ1),
m
(PPh3), 607, 719; (C@N),
m
1599 cmꢀ1. UV–Vis spectral data in MeCN (kmax(nm) (10ꢀ3
2
(dm3 molꢀ1 cmꢀ1))): 404 (0.946), 285 (9.86). [Cu(L1)(PPh3)I] (3a)
Anal. Calc. for C29H24CuIN3P: C, 54.77; H, 3.80; N, 6.61. Found: C,
2.4. Synthesis of [Cu(L2)(PPh3)(I)] (3b) and analytical data of the
complexes
54.71; H, 3.88; N, 6.53%. FT-IR (KBr disc, cmꢀ1),
m(PPh3), 610,
719; m
(C@N), 1597 cmꢀ1. UV–Vis spectral data in MeCN (kmax(nm)
(10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 410 (0.464), 288 (10.10). [Cu(L2)(P-
Ph3)I] (3b) Anal. Calc. for C29H24CuIN3P: C, 54.77; H, 3.80; N,
6.61. Found: C, 54.84; H, 3.87; N, 6.68%. FT-IR (KBr disc, cmꢀ1),
To a stirred solution of CuI (0.11 g, 0.58 mmol) in MeCN (10 ml),
PPh3 (0.162 g, 0.62 mmol) was added in small portion. To the resul-
tant suspension, L2, 3-(20-pyridylmethyleneamino)pyridine
(0.114 g, 0.62 mmol) in methanol (10 ml) was added in drops
and refluxed for 3 h. The solution was filtered through G4 crucible
and was allowed to evaporate slowly in air. Block shaped dark red
microcrystal deposited on the wall of the beaker. These were col-
lected by filtration and dried over CaCl2 in desiccator. Yield:
0.297 g (77%).
m
(PPh3), 622, 706; m
(C@N), 1620 cmꢀ1. UV–Vis spectral data in
MeCN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 409 (1.72), 285
(19.79). [Cu(L3)(PPh3)I] (3c) Anal. Calc. for C29H24CuIN3P: C,
54.77; H, 3.80; N, 6.61. Found: C, 54.73; H, 3.72; N, 6.ꢀ519%. FT-IR
(KBr disc, cmꢀ1),
m(PPh3), 609, 703; m(C@N), 1598 cm . UV–Vis
spectral data in MeCN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))):
All other complexes were prepared by identical procedure.
Yield: 70–80%. Microanalytical data of the complexes are as fol-
lows: [Cu(L1)(PPh3)Cl] (1a) Anal. Calc. for C29H24CuClN3P: C,
63.97; H, 4.44; N, 7.72. Found: C, 63.91; H, 4.49; N, 7.68%. FT-IR
401 (1.02), 287 (12.32).
2.5. X-ray crystal structure analyses
(KBr disc, cmꢀ1), (C@N), 1597 cmꢀ1. UV–Vis
m(PPh3), 611, 725; m
Single crystals suitable for X-ray diffraction study of [Cu(L2)(P-
Ph3)I] (3b) (0.30 ꢁ 0.30 ꢁ 0.35 mm) were obtained on slow evapo-
ration of acetonitrile–methanol (1:2 v/v) solution of the complex.
Diffraction data were collected with the CrysAlis CCD, Oxford Dif-
fraction Ltd. area-detector diffractometer using fine focused 0sealed
spectral data in MeCN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))):
406 (0.430), 289 (9.42). [Cu(L2)(PPh3)Cl] (1b) Anal. Calc. for
C
33H29CuClN4P: C, 63.97; H, 4.44; N, 7.72. Found: C, 64.03; H,
4.42; N, 7.70%. FT-IR (KBr disc, cmꢀ1),
m
(PPh3), 605, 707; (C@N),
m
1616 cmꢀ1. UV–Vis spectral data in MeCN (kmax(nm) (10ꢀ3
2
tube graphite-monochromatized MoKa radiation (0.71073 ÅA). Cell
(dm3 molꢀ1 cmꢀ1))): 401 (0.802), 286 (9.24). [Cu(L3)(PPh3)Cl] (1c)
Anal. Calc. for C29H24CuClN3P: C, 63.97; H, 4.44; N, 7.72. Found:
refinement and data reduction were carried out with CrysAlis RED,
Oxford Diffraction Ltd. Unit cell parameters were determined from
least-squares method. A summary of the crystallographic data and
structure refinement parameters are given in Table 1. Data were
corrected for Lorentz and polarization effects. The structure was
solved by direct method using SHELXS-97 [23] and successive differ-
ence Fourier syntheses. All non-hydrogen atoms were refined
C, 63.88; H, 4.39; N, 7.69%. FT-IR (KBr disc, cmꢀ1),
m
(PPh3), 604,
701;
m
(C@N), 1590 cmꢀ1. UV–Vis spectral data in MeCN (kmax(nm)
2
(10ꢀ3
(dm3 molꢀ1 cmꢀ1))): 403 (1.027), 282 (9.93).
[Cu(L1)(PPh3)Br (2a) Anal. Calc. for C29H24CuBrN3P: C, 59.15; H,
4.11; N, 7.14. Found: C, 59.11; H, 4.07; N, 7.21%. FT-IR (KBr disc,
cmꢀ1), (C@N), 1592 cmꢀ1. UV–Vis spectral data
m(PPh3), 608, 702; m
2
anisotropically. Full matrix least squares refinements on F0 were
carried out using SHELXL-97 [24] with anisotropic displacement
parameters for all non-hydrogen atoms. Hydrogen atoms were
constrained to ride on the respective carbon or nitrogen atoms
with anisotropic displacement parameters equal to 1.2 times the
equivalent isotropic displacement of their parent atom in all cases.
All calculations were carried out using SHELXS 97 [23], PLATON 99 [25],
ORTEP-3 [26] programs.
Table 1
Summarized crystallographic data summarized crystallographic data for [Cu(L2)(P-
Ph3)I] (3b).
Empirical formula
Formula weight
T (K)
C29H24CuIN3P
635.92
293(2)
Crystal system
Space group
Crystal size (mm)
Unit cell dimensions
a (Å)
monoclinic
C2/c
0.30 ꢁ 0.30 ꢁ 0.35
2.6. Computational methods
All computations were performed using the GAUSSIAN03 (G03)
program [27]. Full geometry optimizations of the complexes 1b,
2b and 3b were carried out using the DFT method at the B3LYP le-
vel of theory [28,29]. The 6-31G(d) basis set was used for C, H and
N, while LanL2TZ(f) basis set with effective core potential was em-
ployed for the Cu atom [30] and the MIDI! basis functions for P, Cl,
Br and I atoms [31]. The LanL2TZ(f) and MIDI! basis set for the
respective elements were downloaded from EMSL basis set library.
The vibrational frequency calculations were performed to ensure
that the optimized geometries represent the local minima of po-
tential energy surface and there are only positive eigen values.
The lowest 30 singlet–singlet vertical electronic excitations based
on B3LYP optimized geometries were computed using the time-
dependent density functional theory (TD-DFT) formalism [32–34]
in methanol using conductor-like polarizable continuum model
(CPCM) [35–37] using the same B3LYP level and basis sets.
33.739(4)
9.5890(10)
19.633(2)
117.113(11)
5653.8(11)
8
1.942
2.94–25.0
b (Å)
c (Å)
b (°)
V (Å)3
Z
l
(MoK
h range
hkl range
Dcalc (mg mꢀ3
a
) (mmꢀ1
)
ꢀ40 6 h 6 39; ꢀ11 6 k 6 11; ꢀ23 6 l 6 23
)
1.494
316
25613
4951
0.0650
0.1605
1.223
Refine parameters
Total reflections
Unique reflections
a
R1 [I > 2
r
(I)]
b
wR2
Goodness of fit (GOF)
a
R =
wR2 = [
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
2
R
w(Fo ꢀ Fc2)2/
w(Fo2)2]1 2, w = 1/[ 2(F02) + (0.0490P)2 + 63.9973P].
0
R r

242
G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
N
N
N
Dry MeOH
Reflux, 5 h
445
+
N
H
N
NH2
447
N'
CHO
N
5/
5/
4/
4/
3/
6/
6/
5 /
4/
6/
3/
N
N
3 /
N
7
N
7
N
H
H
7
H
N
2
2
3
6
6
N
6
N
5
5
N
4
5
3
4
L1
L2
L3
Scheme 1. Ligands (L1, L2 and L3) used this work.
Fig. 1. ORTEP plot of [Cu(L2)(PPh3)I] (3b) (35% ellipsoidal probability).
GaussSum [38] was used to calculate the fractional contributions
of various groups to each molecular orbital.
unit consists of CuN2PI fragment. The atomic arrangements Cu,
N(1), C(5), C(6), N(2), constitute the chelate plane (deviation
<0.02 Å). Cu(I) is positioned at distorted tetrahedron centre. The
pendant pyridyl ring makes a dihedral angle 10.9(5)° with chelated
diimine function. The acute bite angle 78.4(3)° is extended by L2 on
coordination to Cu(I). The Cu–N(1)(pyridyl) (2.161(7) Å) is longer
than Cu(I)–N(2) (2.084 (7) Å) distance which reflects stronger
interaction of Cu(I) with N(2) compared to N(1).
3. Results and discussion
3.1. Synthesis and formulation
The ligands used in this work are 2-(20-pyridylmethyleneami-
no)pyridine (L1), 3-(20-pyridylmethyleneamino)pyridine (L2) and
4-(20-pyridylmethyleneamino)pyridine (L3). They are synthesized
by the condensation of pyridine-2-carboxaldehyde and aminopyri-
dine in dry methanol (Scheme 1) in presence of molecular sieves as
dehydrating agent. The ligands (L) act as N,N0 bidentate chelating
agent. The reaction between CuX and PPh3 forms the tetramer
[Ph3PCuX]4 [39] which reacts with L in MeOH–MeCN (2:1, v/v) mix-
ture to afford the coordination complexes of composition
[Cu(L)(PPh3)X] (X = Cl (1); Br (2), I (3); L1 = (a), L2 = (b), L3 = (c))
(Scheme 2). The compounds are non-conducting and their composi-
tion has been supported by microanalytical data. The structure has
been established for 3b by single-crystal X-ray diffraction study.
. . .
. . .
. . .
and CAH
The presence of I H,
erated a supramolecular sheet (Fig. 2). The I H(3)AC(3) bond
parameters are 3.073 Å and I H(3)AC(3), 146.48° (symmetry: x,
p
p
p
interactions has gen-
•
•
•
. . .
•
. . .
•
. . .
ꢀ1 + y, z). The heterochelate
p p interaction observed between
•
Cg(3) (C(7)–C(8)–C(9)–C(10)–C(11)–N(3); symmetry: ꢀx, ꢀy, ꢀz)
and Cg(2) (C(1)–C(2)–C(3)–C(4)–C(5)–N(1); symmetry: ꢀx, ꢀy,
. . .
p
•
ꢀz) of 3-pyridyl ring (3.814 Å) constitutes a
p-sheet. The CAH
is observed between the phenyl rings of PPh3 and pyridyl ring of
neighboring molecular unit enhances the strength of supramolec-
ular union.
The molecular geometry of [Cu(L2)(PPh3)X] (X = Cl (1b), Br (2b)
and I (3b)) in the gas phase singlet state have been optimized and
the selected bond parameters are given in Table 2. The experimen-
tal data obtained by single crystal X-ray diffraction study is closer
to the calculated metric parameters for 3b; the bond lengths are
increased by 0.01–0.07 Å and the bond angles are changing within
3.2. Molecular structure of [Cu(L2)(PPh3)I] (3b)
The molecular structure of [Cu(L2)(PPh3)I] is shown in Fig. 1.
The bond parameters are listed in Table 2. Each discrete molecular
5/
5/
5/
4/
6/
4/
3/
4/
3/
6/
6/
3/
X
X
N
N
N
X
Cu
Cu
7
N
7
N
Cu
7
N
PPh3
H
H
PPh3
PPh3
H
2
2
3
6
6
6
N
N
5
5
N
5
3
4
4
[Cu(L1)(PPh3)X] (1)
[Cu(L2)(PPh3)X] (2)
[Cu(L3)(PPh3)X] (3)
[Cu(L1)(PPh3)Cl] (1a), [Cu(L2)(PPh3)Cl] (1b), [Cu(L3)(PPh3)Cl] (1c); [Cu(L1)(PPh3)Br] (2a),
[Cu(L2)(PPh3)Br] (2b), [Cu(L3)(PPh3)Br] (2c); [Cu(L1)(PPh3)I] (3a), [Cu(L2)(PPh3)I] (3b),
[Cu(L3)(PPh3)I] (3c)
Scheme 2. The complexes, [Cu(L)(PPh3)X] (where, L = L1 (1), L2 (2) and L3 (3) and X = Cl (a), Br (b) and I (c)).

G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
243
3.5
0.25
0.20
0.15
0.10
0.05
0.0
2.8
2.1
1.4
0.7
0.0
300
400
500
600
700
800
Wavelength(nm)
. . .
Fig. 2. Supramolecular sheet formation through I H (
. . .
p
•
),
p
(
) and
•
Fig. 3. Solution (in MeOH) (—) and theoretically (- - -) calculated spectra (oscillator
strength vs. wavelength (nm)) of [Cu(L2)(PPh3)I] (3b).
interactions of 3b. # The distances are as follows:
. . .
CAH
p
(
)
•
. . .
C(23)AH(23) Cg(1), 2.912 Å (Cg(1): C(1), N(1),C(5),C(6), N(2); symmetry: ꢀx, ꢀy,
•
. . .
ꢀz); C(9)AH(9) Cg(5), 2825 Å (Cg(5): C(18) to C(23) of PPh3, symmetry: ꢀx, y, ꢀ1/
•
. . .
2 ꢀ z); C(20)AH(20) Cg(6), 3.160 Å (Cg(6): C(24) to C(29) of PPh3; symmetry: 1/
•
. . .
2 ꢀ x, ꢀ1/2 ꢀ y, z); C(26)AH(26) Cg(4), 3.382 Å (Cg(4): C(12) to C(17) of PPh3;
•
6
2
symmetry: x, y, z).
4
0.30
0.25
0.20
0.15
0.10
0.05
0.00
70
60
50
40
6
8
8
3
7
1–3° limiting value. Thus close resemblance of experimental struc-
ture and theoretical data may calculate accurate frontier molecular
functions to account the electronic and redox properties. This con-
jecture has been extended to two other complexes 1b and 2b
whose experimental structure have not been determined. How-
ever, the spectroscopic characterization of the complexes supports
the proposed structure.
3
5
9
10
7
1
0.3
0.4
f(D,n2)
0.5
0.6
4
2
9
10
3.3. Spectral studies
The ligands (L) show sharp stretch at 1600–1630 cmꢀ1 due to
the presence of
er frequency at 1590–1620 cmꢀ1 that suggests the coordination of
the function to Cu(I). The (PPh3) appears at 600–612, 700–
m(C@N). In the complexes m(C@N) is shifted to low-
400
500
600
700
800
900
m
Wavelength(nm)
725 cmꢀ1. The solution electronic spectra of the complexes show
an intense UV band (2 ꢂ 104 molꢀ1 dm3 cmꢀ1) at 282–289 nm that
is referred to the ligand centered charge transitions. A weak band is
observed at 400–410 nm (2 ꢂ 103 molꢀ1 dm3 cmꢀ1) which is char-
acteristic to MLCT transition of tetrahedral [Cu(diimine)2]+ [11]
(Fig. 3). DFT and TD-DFT calculation using optimized geometry of
representative complexes has assigned the transition (400–
Fig. 4. Solvatochromic effect on visible band of [Cu(L2)(PPh3)I] (3b) in different
solvents. Inset figure is the plot of EMLCT vs. f(D,
2) values. kmax(solvent, dielectric
g
constant): (1) 404 (dimethylsulfoxide (DMSO), 47.2); (2) 407 (N,N-dimethylform-
amide (DMF), 38); (3) 409 (acetonitrile (MeCN), 36.6), (4) 447 (methanol (MeOH),
32.6); (5) 452 (ethanol (EtOH), 24.3); (6) 456 (acetone, 20.7), (7) 466 (ethylene-
glycolmonomethyl ether (EGME), 17); (8) 467 (dichloromethane (CH2Cl2), 9.1), (9)
645 (tetrahydrofuran (THF), 7.6), (10) 708 nm (chloroform (CHCl3), 4.9).
410 nm) as admixture of MLCT (d
(I(
) ? p⁄(diimine)) and the UV band (282–289 nm) is ligand
centered,
p⁄, transitions. The effect of solvent polarity on the
p
(Cu) ? p⁄(diimine)) and XLCT
p
non-hydroxylic and hydroxylic solvents, shifts from the lower
p
?
energy (higher k) to the higher energy (shorter k) with increase
visible band is significant. The kmax, when recorded in a series of
Table 2
Selected bond distances (Å) and angles (°) for [Cu(L2)(PPh3)X] (for X = I (3b), both X-ray structure and DFT calculated data) (X = Cl (1b) and Br (2b) only DFT calculated data).
Bonds (Å)
X-ray structure (X = I), 3b
DFT optimized (X = I), 3b
DFT optimized (X = Br), 2b
DFT optimized (X = Cl), 1b
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–X(1)
Cu(1)–P(1)
2.161(7)
2.084(7)
2.6016(11)
2.2011(19)
2.174
2.151
2.577
2.276
2.191
2.150
2.452
2.249
2.195
2.140
2.312
2.250
Angles (°)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–X(1)
N(1)–Cu(1)–P(1)
N(2)–Cu(1)–X(1)
N(2)–Cu(1)–P(1)
X(1)–Cu(1)–P(1)
78.4(3)
77.19
105.6
117.6
108.5
124.1
116.4
77.01
105.3
117.0
108.5
125.9
115.7
77.02
107.5
116.7
108.2
127.4
113.6
107.16(18)
114.58(19)
111.76(18)
124.27(19)
114.34(6)

244
G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
E = -2.37 eV; L, 96%
E = -5.46 eV; Cu, 52%;
E = -2.48 eV; L, 94%
[Cu(L2)(PPh3)Cl] (1b), gas phase
E = -5.00 eV; Cu, 41%;
Cl, 35%; PPh3, 21%
Cl, 19%; PPh3, 23%
[Cu(L2)(PPh3)Cl] (1b), methanol
E = -2.48 eV; L, 95%
E = -4.94 eV; Cu, 27%;
Br, 56%; PPh3, 14%
E = -5.49 eV; Cu, 47%;
Br, 30%; PPh3, 20%
E = -2.41 eV; L, 93%
[Cu(L2)(PPh3)Br] (2b), methanol phase
[Cu(L2)(PPh3)Br] (2b), gas phase
E = -2.35 eV; L, 97%
LUMO
E = -5.47 eV; Cu, 42%;
I, 39%; PPh3, 17%
E = -4.80 eV; Cu, 15%;
I, 76%; PPh3, 07%
HOMO
E = -2.51 eV; L, 94%
LUMO
HOMO
[Cu(L2)(PPh3)I] (3b), gas phase
[Cu(L2)(PPh3)I] (3b), methanol phase
Fig. 5. Contour plots of [Cu(L2)(PPh3)X] in gas and methanol phase.
in polarity of the solvents. This implies that the polar solvents sta-
bilize the ground state of the molecules relative to the excited state
[40]. Fig. 4 shows solvatochromic effect in the visible region while
the transition remains unshifted at UV region with the change of
the solvent polarity. The superior solvation of the dipolar ground
state (HOMO) and inferior solvation of the less dipolar excited state
by the polar solvent may cause the shift of kmax. This observation is
also supported by the plot of EMLCT (calculated from spectral data of
(a)
800
600
400
200
0
(d)
(b)
(c)
different solvents) versus f(D,
and refractive index ( ) of solvent (Fig. 4, inset) obtained from lit-
g
2) (function of dipole moment (D)
g
erature [40,41]). The energy shifting between Frank–Condon
ground and excited states is related to the dielectric constant (D)
and the refractive index (g) of the solvent. This is related to the dif-
ference in dipole moment of the solute in the ground and excited
energy states. If the excited state dipole is greater than the ground
state then the solvent effect is described as positive solvatochro-
mism and the reverse is negative solvatochromism. Fig. 5 shows
some of the molecular orbitals and their energy and composition.
The HOMO–LUMO energy difference increases on moving from
300
350
400
Wavelength (nm)
450
500
gas phase (
anol phase), 3.12 eV).
DE (gas phase), 2.29 eV) to methanol phase (DE (meth-
Fig. 6. Emission spectra of (a) [Cu(L1)(PPh3)Cl] (1a), (b) [Cu(L1)(PPh3)Br] (2a), (c)
[Cu(L1)(PPh3)I] (3a), and (d) L1 in MeCN.

G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
245
Table 3
1H NMR spectral data of L and [Cu(L)(PPh3)(X)] (X = Cl (1), Br (2), I (3)) complexes in CDCl3 at 300 K.
Compd.
D (ppm) (J (Hz))
2-H
3-H
4-H
5-H
6-Hd
7-Hs
30-Hd
40-Hm
50-Hm
60-Hd
PPh3
L1 (Ref. [18])
–
8.75d (7.0)
–
7.81m
8.71 (6.0)
7.81m
8.20 (6.5)
8.52 (6.0)
8.26 (7.0)
8.31 (6.0)
7.89 (7.0)
7.89 (7.0)
8.29 (7.0)
7.86 (7.0)
7.93 (6.0)
8.28 (7.0)
7.84 (7.0)
7.91 (6.0)
9.19
9.29
9.98
10.01
10.10
10.04
9.99
10.08
10.01
9.98
7.36 (7.0)
7.38 (7.0)
7.36 (7.0)
7.42 (7.0)
7.45 (7.0)
7.46 (6.5)
7.41 (6.5)
7.45 (7.0)
7.40 (7.0)
7.40 (7.0)
7.42 (6.5)
7.42 (6.5)
7.77
7.55
7.60
7.93
7.83
7.93
7.90
7.89
7.91
7.87
7.88
7.87
7.46
7.42
7.46
7.45
7.41
7.43
7.47
7.40
7.37
7.40
7.40
7.39
8.52 (7.0)
8.20 (7.0)
8.05 (7.0)
8.77 (7.0)
8.80 (7.0)
8.76 (7.0)
8.74 (7.0)
8.77 (7.0)
8.73 (6.5)
8.71 (7.0)
8.73 (7.0)
8.10 (6.5)
d
L2
8.59s
8.26d (7.0)
–
8.48m
L3
8.81d (7.0)
8.84d (7.0)
–
–
8.81d (7.0)
[Cu(L1)(PPh3)Cl] (1a)
[Cu(L2)(PPh3)Cl] (1b)
[Cu(L3)(PPh3)Cl] (1c)
[Cu(L1)(PPh3)Br] (2a)
[Cu(L2)(PPh3)Br] (2b)
[Cu(L3)(PPh3)Br] (2c)
[Cu(L1)(PPh3)I] (3a)
[Cu(L2)(PPh3)I] (3b)
[Cu(L3)(PPh3)I] (3c)
7.91m
7.91m
7.28–7.39
7.29–7.43
7.25–7.41
7.26–7.41
7.27–7.42
7.24–7.40
7.22–7.39
7.27–7.42
7.25–7.41
8.52s
7.89
–
8.49
7.93
–
8.50
7.91
8.76 (7.0)
7.53
d
m
8.41d (7.0)
8.80d (6.5)
–
–
8.41d (7.0)
d
7.91m
7.91
7.50
m
s
d
m
8.74 (7.0)
8.38d (7.0)
8.84d (7.0)
–
–
7.89
8.72 (7.0)
8.38 (7.0)
d
d
m
m
7.89
7.48
s
d
m
10.08
10.00
8.35d (7.0)
–
8.35 (7.0)
d
d
d = doublet, m = multiplet, s = singlet.
at 358, 367 and 362 nm at 298 K. The complexes [Cu(L)(PPh3)X]
(1–3) show emission at ꢂ355 nm when excited at pp⁄ state (com-
plex (kex and kem): 1a (289, 359 nm); 1b (286, 357); 1c (282, 354);
2a (284, 360); 2b (288, 364); 2c (285, 362) 3a (288, 356); 3b (283,
353); 3c (287, 355 nm). The complexes do not show significant
emission when they are excited at MLCT band maxima
(>400 nm). Thus, the emission may belong to intraligand charge
transfer (ILCT), ligand-to-ligand charge transfer (LLCT) transitions
or mixture of them. The mechanism of fluorescence enhancement
for the complexes is believed to work through photo-induced elec-
tron transfer (PET) and rigidity to chelated L in the complexes may
suppress vibrational relaxation. In PET, the complex fails to fluo-
resce or very weakly fluoresce because the excited state is
quenched by electron transfer, unless the relative energies of the
fluorophore are perturbed. Heteroatom containing fluorophores
develop partial charges due to internal charge transfer (ICT) and
interaction with charged groups can affect its energy. It has been
reported that the metal ions can enhance or quench the fluores-
cence emission of pyridine containing compounds [42]. In the com-
plexes, the PET process is effectively reduced due to the presence of
metal ion and
p-acidic PPh3 which also helps to populate the ex-
cited states [43]. Intersystem crossing due to spin–orbit coupling
introduced by the metal centre may also influence the emission
intensity. This enhances fluorescence intensity upon coordination
to the metal ion.
Fig. 7. Cyclic voltamogram of [Cu(L2)(PPh3)I] (3b).
The 1H NMR spectral data are given in Table 3. The NMR char-
acterization of L1 is known in literature [20] while L2 and L3 are
new and the protons have been assigned with reference to spin–
spin interaction and on comparing with L1. There are two pyridyl
groups about CH@N function. Important observation is the down-
field shifting of 30-H–60-H protons of [Cu(L)(PPh3)X] from those of
Photoluminescence study of the ligands (L) and the complexes
are carried out at room temperature in CH3CN (Fig. 6). The com-
pounds upon excitation at 280–290 nm exhibit fluorescence at
350–370 nm. Free ligands are excited at pp⁄ state at 291, 290,
292 nm for L1, L2 and L3, respectively and emission are observed
Table 4
Cyclic voltammetric data of [Cu(L)(PPh3)X].a
Complex
Metal redox couple E, V (
D
Ep, mV)
Ligand redox couple E, V (DEp, mV)
I/Iꢀb
CuII/CuI
[Cu(L1)(PPh3)Cl] (1a)
[Cu(L2)(PPh3)Cl] (1b)
[Cu(L3)(PPh3)Cl] (1c)
[Cu(L1)(PPh3)Br] (2a)
[Cu(L2)(PPh3)Br] (2b)
[Cu(L3)(PPh3)Br] (2c)
[Cu(L1)(PPh3)I] (3a)
[Cu(L2)(PPh3)I] (3b)
[Cu(L3)(PPh3)I] (3c)
0.51 (140)
0.53 (150)
0.58 (160)
0.46 (160)
0.50 (150)
0.53 (150)
0.52 (160)
0.58 (160)
0.54 (150)
ꢀ0.68 (140)
ꢀ0.70 (150)
ꢀ0.78 (150)
ꢀ0.69 (160)
ꢀ0.67 (150)
ꢀ0.74 (160)
ꢀ0.75 (160)
ꢀ0.74 (150)
ꢀ0.79 (140)
0.32
0.35
0.38
a
Solvent: MeCN, Pt-disk working electrode, supporting electrolyte, [nBu4](ClO4) (0.01 M); reference, SCE; solute concentration, 10ꢀ3 M; scan rate, 0.05 V sꢀ1
; DEp = |E-
pa ꢀ Epc|, mV; Epa = anodic peak potential, Epc = cathodic peak potential, V; E1/2 = 0.5(Epa + Epc).
b
Epa
.

246
G. Saha et al. / Inorganica Chimica Acta 387 (2012) 240–247
Table 5
Calculated electronic transitions and their assignment to [Cu(L2)(PPh3)I] (3b) by TD DFT data in methanol using optimized geometry.
Excitation energy (eV)
Wavelength (nm)
Osc. strength (f)
Key transitions
Character
2.2905
2.9696
3.6179
541.3
417.5
342.7
0.0215
0.0351
0.0215
(83%) HOMO ? LUMO
Cu(d
Cu(d
Cu(d
Cu(d
p
p
p
p
)/I(p
)/I(p
p
p
) ? L(p⁄
) ? L(p⁄
)
)
(87%) HOMOꢀ3 ? LUMO
(51%) HOMOꢀ5 ? LUMO
(22%) HOMOꢀ7 ? LUMO
(73%) HOMOꢀ10 ? LUMO
(81%) HOMOꢀ11 ? LUMO
)/L(
p
) ? L(p⁄
)
)/PPh3(p )
) ? L(p⁄
4.0799
4.2243
303.9
293.5
0.2175
0.1782
L(p
p
) ? L(p⁄
) ? L(p⁄
)
)
L(
free ligand (L) data. Protons of pendant pyridyl ring 2-H to 6-H are
influenced by the position of pyridyl-N at 2-/3- or 4-Py-N.
293.5 nm (f, 0.1782) those are assigned to the intra-ligand
transitions of electrons localized on the azomethine group of the
Schiff base, L(
) ? L(p⁄). Three weak bands calculated at 541.3 (f,
0.0215), 417.5 (f, 0.0351) and 342.7 nm (0.0215) are assigned to
Cu/I(
) ? L(p⁄) transitions and because of weak intensity these
p ?
p⁄
p
3.4. Electrochemistry
p
transitions may not appreciably visible. The emission spectra
Redox properties of the complexes are examined by cyclic vol-
tammetric experiment and the electrochemical behavior of the
molecules are alike. Fig. 7 shows the cyclic voltammogram of
[Cu(L2)(PPh3)I] (3b) in MeCN at a Pt-disk milli electrode in the po-
tential range +1.5 to ꢀ1.5 V versus SCE reference electrode (Table 4)
at 50 mV Sꢀ1 scan rate. [Cu((L)(PPh3)Cl] (1) and [Cu((L)(PPh3)Br]
(Fig. 6) are also originated from pp⁄ transition.
There are two oxidative responses in [Cu(L2)(PPh3)I] those may
be assigned to Cu(I) ? Cu(II) and Iꢀ ? 1/2 I2. This is confirmed
from the composition of HOMO and HOMOꢀ1. The LUMO is basi-
cally made of ligand orbitals (>90%) and the reduction is explained
as electron accommodation into the LUMO.
(2) show
a quasireversible oxidative response at 0.5–0.6 V
(D
Ep = 140–160 mV). This is corresponding to Cu(II)/Cu(I) couple
[44,45]. [Cu((L)(PPh3)I] (3) shows an anodic peak (Epa) at 0.3 V
which may be assigned to ½ I2/Iꢀ couple [45] and Cu(II)/Cu(I) is ob-
served at 0.52–0.58 V and the quasireversibility is supported by
4. Conclusion
Pyridyl Schiff bases of diimine family are used to synthesize
copper(I)–halide complexes. The structure has been confirmed by
spectroscopic data and by single crystal X-ray structure determina-
tion in one representative case. Both ligands and their complexes
are emissive upon excitation at pp⁄ state. The cyclic voltammetric
experiment shows Cu(II)/Cu(I) redox couple and reduction of dii-
mine function. The electronic structure calculation has proved
the redox activity and spectral transitions of the compounds.
large peak-to-peak potential separation (DEp = 140–160 mV). The
complexes, [Cu(L(PPh3)X], show Cu(II)/Cu(I) couple at more posi-
tive value than oxidative response of [Cu(diimine)2]+ [11] which
may be due to the influence of coordination of strongly
p-acidic
PPh3. On scanning to ꢀve direction 0.0 to ꢀ1.5 V we observe an
irreversible response, Epc at ꢀ0.65 to ꢀ0.80 V that may be assigned
to reduction of imine group [(AC@NA)/(AC@NA)ꢀ] of the chelated
ligands. Free ligand does not show any oxidation but irreversible
reductive responses appear at <ꢀ1.0 V.
Acknowledgments
3.5. DFT calculation: Structure, electronic structure and redox
properties
Financial supports from the Council of Scientific and Industrial
Research and University Grants Commission (under CAS pro-
gramme), New Delhi are thankfully acknowledged. One of us, GS,
thanks the University Grants Commission, New Delhi for sanction-
ing a project under minor scheme.
The electronic structure of [Cu(L2)(PPh3)X] (X = Cl (1b), Br (2b), I
(3b)) has been calculated by DFT computation technique from
optimized structure. The structural agreement has been observed
from the comparison of bond distances and angles between calcu-
lated and X-ray determined structure of 3b (Table 2). The orbital
energies along with contributions from the ligands and metal both
in gas and methanol phase are given in Supplementary table (Table
S1) and surface plots of some selected MOs are given in Fig. 5 and
detail are available from Supplementary material (Fig. S1). The
HOMO–LUMO energy gap in gas phase is [Cu(L2)(PPh3)Cl] (1b),
2.52 eV; [Cu(L2)(PPh3)Br] (2b), 2.46 eV and [Cu(L2)(PPh3)I] (3b),
2.29 eV and is lower than methanol phase (1b, 3.09 eV; 2b,
3.08 eV; 3b, 3.12 eV)). This implies the blue shifting of charge
transfer transition in polar medium and better stabilization of
Appendix A. Supplementary material
Supplementary material CCDC 778766 contains the supplemen-
tary crystallographic data for the structure of [Cu(L2)(PPh3)I] (3b).
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/da-
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2012.01.030.
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D
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[45] T.K. Misra, D. Das, C. Sinha, Polyhedron 16 (1997) 4163.