Synthesis, spectroscopic properties, structural characterization and electrochemistry of mixed ligand complexes of copper(I) halide with PPh 3and naphthylazoimidazole

Article abstract of DOI:10.1016/j.poly.2010.04.007

The reaction of CuX (X = Cl, Br, I) with a mixture of PPh₃ and 1-alkyl-2-(naphthyl-α/β-azo)imidazole has synthesized mixed ligand complexes of the composition, [Cu(α/β-NaiR)(PPh₃)X]. The spectroscopic characterization (IR, UV-Vis, ¹H NMR) supports this formulation. The single crystal X-ray diffraction study of [Cu((α-NaiMe) (PPh₃)I] (7a) (α-NaiMe = 1-methyl-2-(naphthyl-α-azo) imidazole) shows a distorted tetrahedral geometry about Cu(I). Cyclic voltammograms of the complexes show a high potential Cu(II)/Cu(I) couple and azo reductions. The [Cu(α/β-NaiR)(PPh₃)I] complexes show an additional oxidative response at 0.4 V that is assigned to I/I^- A sharp anodic peak at ～-0.2 V is assigned to the oxidation of metallic Cu, deposited on electrode surface upon scanning to the negative side of the SCE. DFT and TD-DFT computations of [Cu((α-NaiMe)(PPh₃)I] (7a), [Cu((α-NaiMe)(PPh₃)I]⁺ (7a⁺) and [Cu((α-NaiMe)(PPh₃)I]^- (7a^-) were carried out to examine the electronic configuration and to explain the spectral and redox properties of the complexes.

Full text of DOI:10.1016/j.poly.2010.04.007

Polyhedron 29 (2010) 2098–2104
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, spectroscopic properties, structural characterization
and electrochemistry of mixed ligand complexes of copper(I) halide
with PPh3 and naphthylazoimidazole
G. Saha a,1, K.K. Sarkar a,2, P. Datta a, P. Raghavaiah b, C. Sinha a,
*
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b School of Chemistry, National Single Crystal X-ray Diffractometer Facility, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of CuX (X = Cl, Br, I) with a mixture of PPh3 and 1-alkyl-2-(naphthyl-a/b-azo)imidazole has
Received 3 December 2009
Accepted 5 April 2010
Available online 14 April 2010
synthesized mixed ligand complexes of the composition, [Cu(
acterization (IR, UV–Vis, 1H NMR) supports this formulation. The single crystal X-ray diffraction study of
[Cu(( -NaiMe)(PPh3)I] (7a) ( -NaiMe = 1-methyl-2-(naphthyl- -azo)imidazole) shows a distorted tetra-
hedral geometry about Cu(I). Cyclic voltammograms of the complexes show a high potential Cu(II)/Cu(I)
a/b-NaiR)(PPh3)X]. The spectroscopic char-
a
a
a
Keywords:
Copper(I)–naphthylazoimidazole-halide
X-ray structure
Electrochemistry
DFT computation
couple and azo reductions. The [Cu(a/b-NaiR)(PPh3)I] complexes show an additional oxidative response
at 0.4 V that is assigned to I/Iꢀ A sharp anodic peak at ꢁꢀ0.2 V is assigned to the oxidation of metallic Cu,
deposited on electrode surface upon scanning to the negative side of the SCE. DFT and TD-DFT computa-
tions of [Cu((a-NaiMe)(PPh3)I] (7a), [Cu((a a
-NaiMe)(PPh3)I]+ (7a+) and [Cu(( -NaiMe)(PPh3)I]ꢀ (7aꢀ) were
carried out to examine the electronic configuration and to explain the spectral and redox properties of the
complexes.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
methanol and the copper(II) complexes thus obtained are reduced
by ascorbic acid to the copper(I) derivatives. A theoretical calcula-
Imidazole is ubiquitous in chemistry and biology [1]. The syn-
thesis of polydentate ligands using imidazole and their metal com-
plexes are of interest [2–4]. We have been engaged in the synthesis
of azo functionalized imidazoles such as 1-alkyl-2-(arylazo)imid-
azole for several years [5–12]. The arylazoheterocycle stabilizes
metals in their lower oxidation state [8–12], such as Cu(I), Ag(I),
Fe(II), Co(II), Ru(II), Os(II). The azoimine function, –N@N–C@N– is
the chelating group contributing to the chemical, spectral and elec-
trochemical properties of the molecules. It has been proposed that
tion using the DFT computation technique is used to define the
electronic configuration of the molecules and to give an explana-
tion of their electronic properties. We have attempted this proce-
dure to correlate the electronic and redox properties of the
complexes. The calculation was carried out using the neutral, oxi-
dized and reduced compound.
2. Experimental
the stability, visible spectra and Cu(II)/Cu(I) redox potential of
Cu(I)-arylazoheterocycles are due to dp(Cu(I)) ? p (ligand orbital)
2.1. Materials
*
transitions [10–12]. The presence of p-acidic co-ligands, like PPh3
Imidazole, naphthyl amines, CuX (X = Cl, Br, I) and triphenyl
phosphine were purchased from E. Merck India. All other chemi-
cals and solvents were of reagent grade and were used as received.
The 1-alkyl-2-(naphthyl-azo)imidazoles were prepared by the
reported procedure [10].
and CO, may assist the azoimine function to stabilize the lower oxi-
dation state more efficiently [8,9] compared to the azoimine func-
tion alone. In this work, we report mixed ligand complexes of
copper(I) halide with PPh3 and 1-alkyl-2-(naphthyl-a/b-azo)imida-
zoles ( /b-NaiR). The complexes are oxidized by chlorine in
a
2.2. Physical measurements
* Corresponding author.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present Address: Department of Chemistry, Mahadevananda Mahavidyalaya,
Barrackpore, Monirampore, Kolkata 700 120, India.
Present Address: Department of Chemistry, Barrackpore Rastraguru Surendranath
College, Barrackpore, Kolkata 700 120, India.
Microanalytical (C,H,N) data were obtained from a Perkin–
Elmer 2400 CHNS/O elemental analyzer. Spectroscopic data were
obtained using the following instruments: UV–Vis, Perkin–Elmer
Lambda-25; IR (KBr disc, 4000–200 cmꢀ1), Perkin–Elmer RX-1
1
2
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.04.007

G. Saha et al. / Polyhedron 29 (2010) 2098–2104
2099
spectrophotometer, and 1H NMR, Bruker 300 MHz FT NMR spec-
trometer. Electrochemical measurements were performed using
computer-controlled PAR model 250 VersaStat electrochemical
instruments with Pt-disc electrodes. All measurements were car-
ried out under a nitrogen environment at 298 K with reference to
SCE in acetonitrile using [nBu4N][ClO4] as a supporting electrolyte.
The reported potentials are not corrected further for junction
potentials. Room temperature (298 K) magnetic susceptibility
was measured using Sherwood Scientific Cambridge, UK at 298 K.
ESR spectra were recorded in MeCN solution at room temperature
(298 K) and at liquid nitrogen temperature (77 K) using a Bruker
ESR spectrometer model EMX 10/12, X-band ER 4119 HS cylindri-
cal resonator.
NaiMe)(PPh3)I] (7a): Anal. Calc. for C32H27CuClN4P: C, 55.78; H,
3.92; N, 8.13. Found: C, 55.72; H, 3.90; N, 8.14%. FT-IR (KBr disc,
cmꢀ1),
m
(N@N), 1345;
CH3CN (kmax (nm) (
, 10ꢀ4 dm3 molꢀ1 cmꢀ1)): 376 (4.99), 394
(4.11), 585 (0.85). [Cu( -NaiEt)(PPh3)I] (7b): Anal. Calc. for
C33H29CuClN4P: C, 56.38; H, 4.13; N, 7.97. Found: C, 56.32; H,
4.17; N, 7.96%. FT-IR (KBr disc, cmꢀ1),
(N@N), 1342; (C@N),
m(C@N), 1557. UV–Vis spectral data in
e
a
m
m
1552. UV–Vis spectral data in CH3CN (kmax(nm)
(e,
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 379 (5.47), 398 (4.20), 570 (0.89). [Cu(b-
NaiMe)(PPh3)I] (8a): Anal. Calc. for C32H27CuClN4P: C, 55.78; H,
3.92; N, 8.13. Found: C, 55.72; H, 4.00; N, 8.10%. FT-IR (KBr disc,
cmꢀ1),
CH3CN (kmax(nm)
m
(N@N), 1335;
m
(C@N), 1555. UV–Vis spectral data in
(e,
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 375 (5.61), 393
(4.49), 595 (0.77). [Cu(b-NaiEt)(PPh3)I] (8b): Anal. Calc. for
2.3. Synthesis [Cu(
a
-NaiMe)(PPh3)I] (7a) and analytical data
C33H29CuClN4P: C, 56.38; H, 4.13; N, 7.97. Found: C, 56.33; H,
4.10; N, 7.92%. FT-IR (KBr disc, cmꢀ1),
m(N@N), 1340; m(C@N),
To a methanol solution (25 ml) of 1-methyl-2-(naphthyl-
a
-
1554. UV–Vis spectral data in CH3CN (kmax(nm)
(e,
azo)imidazole -NaiMe) (0.09 g, 0.38 mmol), CuI (0.07 g,
(a
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 375 (5.29), 396 (4.19), 582 (0.90).
0.37 mmol) in acetonitrile (25 ml) and PPh3 (0.1 g, 0.38 mmol)
were added, stirred and refluxed for 3 h. The solution was filtered
under hot conditions through a G4 crucible and was allowed to
evaporate slowly in air. Microcrystals, deposited on the wall of
beaker, were collected by filtration. These crystals were ground
and a DMF solution was prepared. Methanol (2 volumes) was
added to this solution. After a week X-ray quality crystals sepa-
rated out at the bottom of the beaker. These were collected and
dried over CaCl2 in a desiccator. The yield was 0.15 g (71%).
All the other complexes were prepared by the same procedure.
In all cases, crystalline products were obtained. The yield varied
from 70–80%. Microanalytical data of the complexes are as follows:
2.4. X-ray crystal structure analyses 7a
Crystals suitable for an X-ray diffraction study of [Cu(a-Nai-
Me)(PPh3)I] (7a) (0.38 ꢂ 0.28 ꢂ 0.06 mm) were prepared by slow
evaporation of a N,N-dimethylformamide–methanol (1:2 v/v) solu-
tion of the compound under ambient conditions. Diffraction data
were collected with a Bruker SMART 1 K CCD area-detector diffrac-
tometer using fine focused sealed tube graphite-monochromatized
Mo Ka radiation (k = 0.71073 Å). Unit cell parameters were deter-
mined by the least-squares method with the data in the range
ꢀ11 6 h 6 11, ꢀ23 6 k 6 22, ꢀ18 6 l 6 22 and angles 3.30° < 2h <
52.10° for 7a. The crystallographic data and structural refinement
parameters are given in Table 1. Data were corrected for Lorentz
and polarization effects. Data reduction was carried out by using
the SAINT program [13]. The structure was solved by direct methods
using SHELXS-97 [14] and successive difference Fourier syntheses.
All non-hydrogen atoms were refined anisotropically. Full matrix
least-squares refinements on F2o were carried out using SHELXL-97
[15] with anisotropic displacement parameters for all non-hydro-
gen atoms. Hydrogen atoms were constrained to ride on their
respective carbon or nitrogen atoms with anisotropic displacement
parameters equal to 1.2 times the equivalent isotropic displace-
[Cu(
64.32; H, 4.50; N, 9.30. Found: C, 64.35; H, 4.54; N, 9.37%. FT-IR
(KBr disc, cmꢀ1):
(N@N), 1365; (C@N), 1580. UV–VIS spec-
tral data in CH3CN (kmax (nm) (
, 10ꢀ4 dm3 molꢀ1 cmꢀ1)): 375
(5.08), 393 (4.01), 580 (0.81). [Cu( -NaiEt)(PPh3)Cl] (3b): Anal.
Calc. for C33H29CuClN4P: C, 64.81; H, 4.78; N, 9.16. Found: C,
64.79; H, 4.73; N, 9.10%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1350;
a-NaiMe)(PPh3)Cl] (3a): Anal. Calc. for C32H27CuClN4P: C,
m
m
e
a
m
m
(C@N), 1577. UV–Vis spectral data in CH3CN (kmax (nm) (e,
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 371 (6.01), 394 (4.38), 575 (0.89).
[Cu(b-NaiMe)(PPh3)Cl] (4a): Anal. Calc. for C32H27CuClN4P: C,
64.32; H, 4.55; N, 9.38. Found: C, 64.35; H, 4.50; N, 9.31%. FT-IR
(KBr disc, cmꢀ1):
data in CH3CN (kmax(nm) (
m
(N@N), 1355;
m(C@N), 1571. UV–Vis spectral
e
, 10ꢀ4 dm3 molꢀ1 cmꢀ1)): 376 (5.06),
395 (4.86), 590 (0.55). [Cu(b-NaiEt)(PPh3)Cl] (4b): Anal. Calc. for
Table 1
Summarized crystallographic data for [Cu(
a
-NaiMe)(PPh3)I] (7a).
[Cu( -NaiMe)(PPh3)I] (7a)
C33H29CuClN4P: C, 64.81; H, 4.78; N, 9.16. Found: C, 64.73; H,
4.72; N, 9.13%. FT-IR (KBr disc, cmꢀ1),
m(N@N), 1345; m(C@N),
a
1537. UV–Vis spectral data in CH3CN (kmax(nm)
(
a
e
-
,
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C32H27N4PICu
688.99
298(2)
monoclinic
P21/c
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 379 (5.85), 396 (4.38), 590 (0.76). [Cu(
NaiMe)(PPh3)Br] (5a): Anal. Calc. for C32H27CuBrN4P: C, 59.87; H,
4.21; N, 8.73. Found: C, 59.82; H, 4.26; N, 8.71%. FT-IR (KBr disc,
cmꢀ1):
m
(N@N), 1340;
CH3CN (kmax(nm)
(4.81), 570 (0.95). [Cu(
C33H29CuBrN4P: C, 60.42; H, 4.46; N, 8.54. Found: C, 60.43; H,
4.41; N, 8.50%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1325; (C@N),
m
(C@N), 1540. UV–Vis spectral data in
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 378 (5.26), 391
-NaiEt)(PPh3)Br] (5b): Anal. Calc. for
(e,
9.395(2)
19.268(4)
18.435(3)
119.445(8)
2906.1(10)
4
0.71073
1.897
1.575
354
5438
0.0665
0.1198
1.283
a
b (Å)
c (Å)
b (°)
m
m
V (Å)3
1550. UV–Vis spectral data in CH3CN (kmax (nm)
(e,
Z
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 379 (5.31), 394 (4.31), 580 (0.80). [Cu(b-
NaiMe)(PPh3)Br] (6a): Anal. Calc. for C32H27CuBrN4P: C, 59.87; H,
4.21; N, 8.73. Found: C, 59.83; H, 4.24; N, 8.70%. FT-IR (KBr disc,
k (Å)
l(Mo K
a )
) (mmꢀ1
Dcalc (Mg mꢀ3
)
cmꢀ1),
CH3CN (kmax (nm) (
m
(N@N), 1331;
m(C@N), 1555. UV–Vis spectral data in
Refine parameters
Total reflection
e
, 10ꢀ4 dm3 molꢀ1 cmꢀ1): 380 (5.19), 395
a
R1 [I > 2
r(I)]
b
(4.37), 590 (0.87). [Cu(b-NaiEt)(PPh3)Br] (6b): Anal. Calc. for
wR2
Goodness-of-fit (GOF)
C33H29CuBrN4P: C, 60.42; H, 4.46; N, 8.54. Found: C, 60.40; H,
4.40; N, 8.50%. FT-IR (KBr disc, cmꢀ1),
m(N@N), 1335; m(C@N),
P
P
a
R = |Fo ꢀ Fc|/ Fo.
P
P
wR = [ w(F2o ꢀ F2c )/ wFo4]1/2
where
w = 1/[r
2(F2) + (0.0212P)2 + 6.3126P]
b
1550. UV–Vis spectral data in CH3CN (kmax(nm)
(e
,
-
10ꢀ4 dm3 molꢀ1 cmꢀ1)): 374 (5.43), 396 (4.21), 588 (0.75). [Cu(
a
where P = (F2o þ 2Fc2)/3.

2100
G. Saha et al. / Polyhedron 29 (2010) 2098–2104
5
4
4
5
N
N
N
N
N
N
R
R
N
N
N
N
N
N
9
6
8
13
10
7
12
11
9
13
8
10
12
11
R
α-NaiR (1a, 1b)
NaiR (2a, 2b)
179
β -
5
4
4
5
N
PPh3
X
N
PPh3
N
N
Cu
R
Cu
N
6
N
X
N
8
N
9
13
7
9
12
11
13
12
8
10
10
11
[Cu((α-NaiR)(PPh3)X] (3, 5, 7)
[Cu((β-NaiR)(PPh3)X] (4, 6, 8)
[Cu(α-NaiMe)(PPh3)Cl] (3a), [Cu(α-NaiEt)(PPh3)Cl](3b), [Cu(β-NaiMe)(PPh3)Cl] (4a)
[Cu(β-NaiEt)(PPh3)Cl] (4b), [Cu(α-NaiMe)(PPh3)Br] (5a), [Cu(α-NaiEt)(PPh3)Br] (5b),
[Cu(β-NaiMe)(PPh3)Br] (6a), [Cu(β-NaiEt)(PPh3)Br] (6b), [Cu(α-NaiMe)(PPh3)I] (7a),
[Cu(α-NaiEt)(PPh3)I] (7b), [Cu(β-NaiMe)(PPh3)I] (8a), [Cu(β-NaiEt)(PPh3)I] (8b)
Scheme 1.
ment of their parent atom in all cases. All calculations were carried
out using SHELXS 97 [14], PLATON 99 [16] and ORTEP-3 [17] programs.
NaiR in a MeOH–MeCN (1:1, v/v) mixture to afford complexes of
the composition [Cu( /b-NaiR)(PPh3)X] (X = Cl (3, 4); Br (5, 6), I
(7, 8) (Scheme 1)). [Ph3PCuX]4 has also been isolated from the reac-
tion mixture and then reacted with -NaiMe, to separate the iden-
tical complexes. To save time we opted for the in situ reaction
a
a
2.5. Theoretical calculations
Full geometry optimizations were carried out using the density
functional theory method at the B3LYP level for 7a, 7a+ and 7aꢀ
using the GAUSSIAN 03 program package [18,19]. The SDD basis set
with the corresponding effective core potentials was employed
for Cu and I atoms [20,21]. For C, H and N, we have used the 6-
31G(d) basis set. The vibrational frequency calculations were per-
formed to ensure that the optimized geometries represent the local
minima and there are only positive eigen values. Vertical electronic
excitations based on the B3LYP optimized geometry were com-
puted using a time-dependent density functional theory (TD-
DFT) formalism [22–23] in acetonitrile using a conductor-like
polarizable continuum model (CPCM) [24].
3. Results and discussion
3.1. Synthesis and formulation
1-Alkyl-2-(naphthyl-a/b-azo)imidazoles (a-NaiR, 1 and b-NaiR,
2 where R = –CH3 (a), –CH2–CH3 (b)) are used in this work to pre-
pare the copper(I) complexes. The reaction between CuX and PPh3
Fig. 1. ORTEP structure of [Cu(
a-NaiMe)(PPh3)I] (7a) with 30% ellipsoidal probability
forms the tetramer [Ph3PCuX]4 [25] which reacts in situ with a/b-
(H atoms omitted for clarity).

G. Saha et al. / Polyhedron 29 (2010) 2098–2104
2101
Table 2
Selected bond distances (Å) and angles (°) for [Cu(a-NaiMe)(PPh3)I] (7a).
Bond distances(Å)
Experimental
Bond angles(°)
Theoretical
Experimental
Theoretical
Cu(1)–I(1)
Cu(1)–P(1)
Cu(1)–N(1)
Cu(1)–N(3)
N(1)–N(2)
N(1)–C(1)
N(2)–C(11)
2.5955(9)
2.2138(15)
2.171(4)
2.051(5)
1.268(6)
1.417(7)
1.359(7)
2.634
2.291
2.206
2.084
1.280
1.413
1.363
I(1)–Cu(1)–P(1)
I(1)–Cu(1)–N(1)
I(1)–Cu(1)–N(3)
P(1)–Cu(1)–N(1)
P(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
111.15(4)
119.65(11)
102.75(13)
117.08(12)
125.51(13)
76.88(18)
111.3
119.1
103.3
118.4
123.8
76.59
strategy in the preparation of the complexes. The compounds are
non-conducting and their composition has been supported by
microanalytical data. The spectroscopic studies (IR, UV–Vis and
1H NMR) determine the structure of the complexes. The single
0.02 Å compared to the free ligand value (1.250(2) Å) [30,31],
*
which indirectly supports dp(Cu) ? p (azo) charge transfer.
3.3. Spectral characterization
crystal X-ray structure of [Cu(a-NaiMe)(PPh3)I] (7a) has also been
The IR stretching at 1530–1580 and 1340–1365 cmꢀ1 for
[Cu(( /b-NaiR)(PPh3)X] (3–8) corresponds to (C@N) and (N@N),
respectively. The
determined.
a
m
m
m
(N@N) stretch is shifted to lower frequency by
3.2. Molecular structure of [Cu(a-NaiMe)(PPh3)I] (7a)
25–30 cmꢀ1 compared to the free ligand data. This supports the
coordination of the azo-N to Cu(I). The UV–Vis spectra of the com-
plexes are recorded in MeCN solution.
The molecular structure of [Cu(
in Fig. 1, and the bond parameters are listed in Table 2.
a
-NaiMe)(PPh3)I] (7a) is shown
-NaiMe
a
The solution spectra of the complexes exhibit two close par-
acts as an N(imidazole), N(azo) chelating ligand. The atomic
arrangements Cu, N(1), N(2), C(11), N(3) constitute a chelate plane
with a maximum deviation <0.063 Å. The pendant naphthyl ring
makes a dihedral angle of 49.5(2)° with the chelating azoimidazole
ring and shows distortion from planarity. The acute Cu(N(imidaz-
tially overlapping intense bands
(
e
ꢁ 104 molꢀ1 dm3 cmꢀ1
) at
370–380 and 390–400 nm, and are referred to intraligand charge
transfer transitions (n–p* *) along with a weak band at 570–
, p–p
595 nm (
e
ꢁ 103 molꢀ1 dm3 cmꢀ1) assigned to an admixture of
MLCT and XLCT transitions. The spectral identity of the complexes
ole), N(azo)) chelate angle, 76.88(18)°, extended by
a-NaiMe on
has suggested a similar electronic configuration and hence struc-
ture of the complexes. One of the complexes, [Cu(a-NaiMe)(PPh3)I]
(7a) has been structurally confirmed by single crystal X-ray diffrac-
tion analysis (Fig. 1) and supports the structural similarity of the
other complexes.
coordination to Cu(I) is comparable with reported data [26–29]
(76.5° for a ruthenium(II) complex [28] and 77.8° for a osmium(II)
compound [29]). The Cu–N(imidazole) distance (2.051(5) Å) is
shorter than the Cu(I)–N(azo) distance (2.171(4) Å), which reflects
a stronger interaction of Cu(I) with N(imidazole) compared to
N(azo). The N@N distance is 1.268(6) Å, which is elongated by
Table 3
Cyclic voltammetric data.a
Complex
Metal redox couple E, V (
D
Ep, mV)
Ligand redox couple E, Vb
c
I/Iꢀ
CuII/CuI
0.85
(160)
0.83
(150)
0.78
(170)
0.85
(160)
0.80
(170)
0.88
(170)
0.85
(180)
0.86
(170)
0.90
(160)
0.89
(175)
0.90
(180)
0.88
CuI/Cu0
azo/azoꢀ
ꢀ0.90
(140)
ꢀ0.89
(150)
ꢀ0.88
(150)
ꢀ0.89
(160)
ꢀ0.83
(170)
ꢀ0.84
(160)
ꢀ0.85
(160)
ꢀ0.84
(150)
ꢀ0.79
(140)
ꢀ0.81
(160)
ꢀ0.80
(170)
ꢀ0.80
(170)
azo=/azoꢀ
ꢀ1.25
(160)
ꢀ1.26
(180)
ꢀ1.25
(170)
ꢀ1.26
(170)
ꢀ1.23
(180)
ꢀ1.23
(170)
ꢀ1.24
(190)
ꢀ1.24
(170)
ꢀ1.18
(180)
ꢀ1.15
(180)
ꢀ1.16
(180)
ꢀ1.22
(190)
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
ꢀ0.22b
ꢀ0.21b
ꢀ0.21b
ꢀ0.22b
ꢀ0.24
ꢀ0.26
ꢀ0.28b
ꢀ0.24
ꢀ.24
Fig. 2. Cyclic voltamogram of [Cu(a-NaiMe)(PPh3)I] (7a).
Table 4
Selected electronic transitions at the TD-DFT/B3LYP level for [Cu(
a
-NaiMe)(PPh3)I]
Character
0.41
0.43
0.40
0.40
(7a).
ꢀ.23
Excited
state
k (nm)
Oscillator
strength
Key transitions
ꢀ.24
1
2
623.1
583.6
0.0182
0.0405
(60%) HOMOꢀ1 ? LUMO
(55%) HOMO ? LUMO
(27%) HOMOꢀ2 ? LUMO
(50%) HOMOꢀ2 ? LUMO
(24%) HOMO ? LUMO
(54%) HOMOꢀ6 ? LUMO
(23%) HOMOꢀ7 ? LUMO
(84%) HOMOꢀ8 ? LUMO
MLCT, XLCT
MLCT, XLCT
ꢀ0.25
(180)
3
7
9
548.9
396.3
368.8
0.0943
0.2166
0.1176
MLCT, XLCT
a
Solvent: MeCN, Pt-disc working electrode, supporting electrolyte, TBAP
(0.01 M); reference, SCE; solute concentration, 10ꢀ3 M; scan rate, 0.05 V sꢀ1
;
ILCT
YLCT
ILCT, YLCT
D
Ep = |Epa ꢀ Epc|, mV; Epa = anodic peak potential, Epc = cathodic peak potential, V;
E1/2 = 0.5(Epa + Epc).
b
Epc.
Epa
c
.
M = metal, X = iodide, Y = PPh3, L = a-NaiMe.

2102
G. Saha et al. / Polyhedron 29 (2010) 2098–2104
The 1H NMR spectra (Supplementary Table S1) were assigned
Other naphthyl protons (7-H to 13-H) appear at 7.5–8.2 ppm. The
NMR spectral similarity accounts for the structural equivalence
of the complexes.
on comparison with the free ligand values [11]. An important
observation is the downfield shift of the imidazole protons, 4-H
and 5-H, by 0.6–0.8 ppm. The 4-H is shifted by 0.7–0.8 ppm where
as 5-H is shifted by 0.5–0.6 ppm. This supports the coordination
of the imidazole-N (N(3)) to the metal centre. The spectra of
4. Electrochemistry
[Cu(a-NaiR)(PPh3)I] (7) show a significantly higher chemical shift
difference (
D
d) between 4-H and 5-H ( d = 0.5 ppm). The naphthyl
D
The cyclic voltammograms of the complexes, [Cu((NaiR)-
(PPh3)Cl] (3, 4) and [Cu((NaiR)(PPh3)Br] (5, 6) (Supplementary Fig-
ures) show one quasireversible oxidative response at 0.8 V, while
the iodo complexes, [Cu(NaiR)(PPh3)I] (7, 8), exhibit two quasire-
versible redox responses at ꢁ0.4 and ꢁ0.8 V (Fig. 2, Table 3 and
also Supplementary Figures). The quasireversible redox couple at
0.4 V for [Cu(NaiR)(PPh3)I] may be referred to the
protons (6-H to 13-H) experience small perturbations and the
chemical shifts are comparable with the free ligand data. This im-
plies chelation by the naphthylazo-N to Cu(I). The 1-alkyl group
shows the usual splitting pattern: 1-Me is
4.1 ppm); 1-CH2–CH3 shows a quartet to –CH2– (4.5 to 4.6 ppm,
a singlet (4.0–
J = 10.0 Hz) and a triplet to –CH3 group (1.5–1.6 ppm, J = 8.0 Hz).
E= -5.45 eV; Cu, 29 %; E= -5.36 eV; Cu, 38
E= -3.99 eV; Cu, 04 %;
α-NaiMe, 92 %
E= -2.40 eV; α-NaiMe,
97 %
%; I, 41 %; -NaiMe,
α
I, 53 %; PPh3, 14 %
18 %
[Cu(α-NaiMe)(PPh3)I] (7a)
E = -8.75 eV
E = -6.08 eV
E = -5.95 eV
E = -3.94 eV
Cu, 15%; I, 11%; L,
42%; PPh3, 32%
Cu, 41%; I, 32%; L,
24%; PPh3, 03%
Cu, 02%; L, 97%
Cu, 02%; L, 97%
[Cu(α-NaiMe)(PPh3)I]+ (7a+)
SOMO
E = -1.80 eV
E = 0.23 eV
E = 1.49 eV
E = 1.55 eV
Cu, 28%; I, 55%; L,
13%; PPh3, 04%
Cu, 02%; L, 97%
(N=N, 56%)
PPh3, 100%
PPh3, 100%
[Cu(α-NaiMe)(PPh3)I]- (7a-)
HOMO-1
HOMO
LUMO
LUMO+1
Fig. 3. Contour plots of some selected molecular orbitals with energy and composition for [Cu(
(7aꢀ).
a-NaiMe)(PPh3)I] (7a), [Cu(a a
-NaiMe)(PPh3)I]+ (7a+) and [Cu( -NaiMe)(PPh3)I]ꢀ

G. Saha et al. / Polyhedron 29 (2010) 2098–2104
2103
I/Iꢀ couple. Copper(I)–azoimine complexes show the Cu(II)/Cu(I)
couple at 0.5–0.6 V [10–12] and in the present series of complexes
it is shifted to a more positive value which may be due to addi-
The redox responses in the cyclic voltammogram are satisfacto-
rily explained by the DFT calculation. The voltammogram
of [Cu((a-NaiMe)(PPh3)X] (X = Cl (3, 4), Br (5, 6)) differ from
tional
p-acidity of the coordinated PPh3. On scanning to the nega-
[Cu(( -NaiMe)(PPh3)I] (7, 8). An additional oxidative quasirevers-
a
tive direction (0.0 V to ꢀ2.0 V), we observed a sharp anodic signal
ible response is observed at 0.4 V in 7 and 8. The DFT calculation
of 7a shows that >40% contribution of I is observed to HOMO
(ꢀ5.36 eV) and HOMOꢀ1 (ꢀ5.45 eV). It is certain that the electron
extraction would take place from the iodide (Cu–I) and the redox
response may be referred to I/Iꢀ. It is difficult to extract a second
at ꢁꢀ0.2 V, an irreversible response Epc at ꢁꢀ0.9 V and a quasire-
versible couple at ꢀ1.2 V (DEP > 150 mV) (Fig. 2). The sharp anodic
signal at ꢁꢀ0.2 V, may be the Cu(I)/Cu(0) couple [10,32]. The re-
duced Cu(0) is absorbed on the electrode surface as evidenced from
the narrow width of the anodic response with a large peak current.
To account for the Cu(I)/Cu(0) redox response, we did a few more
experiments with varying scan rates (20 mV/s to 500 mV/s at
20 mV intervals), starting potential other than 0.0 V and end po-
tential to back scan. A well defined sharp cathodic current is ob-
served at 20–150 mV/s, while broadening of the cathodic peak is
observed at higher scan rates. This observation suggests that the
heterogeneous electron transfer rate is influenced by the applied
potential [33]. A clear Cu(I)/Cu(0) couple was not observed when
scanning started in the potential range 0.3–0.7 V. An interesting
observation is recorded on fixing the potential to back scan; no
cathodic signal is observed at potential above ꢀ0.4 V. With a
decreasing fixed potential (ꢀ0.4 to ꢀ1.3 V) to back scan, the inten-
sity of the sharp cathodic peak at ꢀ0.2 V increases. It is quite obvi-
ous that the rate of reduction, Cu(I) ? Cu(0), increases with the
decreasing fixed potential to back scan and amount of deposited
Cu(0) is increased. It is reoxidised on scan reversal and shows a
sharp peak. We did not examine the voltammetric behaviour of
the Cu(I)/Cu(0) response beyond 0.7 V. To get a true picture, the
sweep of the CV was started from 0.0 V in the negative direction
up to ꢀ2.0 V. The reductive responses at <ꢀ0.7 V may be assigned
to the accommodation of electrons at the azo group [azo/azoꢀ;
azoꢀ/azo=] of the chelated ligands. The free ligand does not show
any oxidation, but irreversible reductive responses appear at
<ꢀ1.0 V.
electron from iodide. The calculation of [Cu((a
-NaiMe)(PPh3)I]+
(7a+) shows the contribution of Cu to the HOMO is 41% (Fig. 3),
thus the second oxidative response may be identified to Cu(I) ?
Cu(II). The irreversible oxidation at 0.4 V is thus assigned to iodide
oxidation and the couple at 0.8 V refers to the Cu(II)/Cu(I) couple.
The composition of the MOs of the reduced species 7aꢀ shows that
the HOMO is composed of mainly
a-NaiMe (97%) out of which –
N@N– contributes 56%. Thus the electron is localized on the azo
function in the reduced species. HOMOꢀ1 contains 28% Cu charac-
ter and 55% iodide character. It implies that the possibility of fur-
ther electron accommodation to the copper centre and the
reduction of Cu(I) to Cu(0). Accommodation of an electron to the
I
ꢀ centre may be excluded due to its anionic charge. Hence the con-
clusion of electron accommodation at the azo dominated orbital of
the ligand is justified along with Cu(I) reduction.
5. Conclusion
We have synthesized and structurally characterized complexes
of halide-copper(I)–naphthylazoimidazoles using PPh3 as a sup-
porting p-acidic ligand. The single crystal X-ray structural charac-
terization shows a tetrahedral arrangement around the Cu(I)
centre. Electrochemistry shows a high potential Cu(II)/Cu(I) couple.
The spectral and redox assignments are supported by DFT and TD-
DFT calculations.
Acknowledgments
4.1. Electronic structure and correlation with spectral and redox
properties
Financial support from the Department of Science Technology
(PURSE), New Delhi and CAS-UGC (New Delhi) sanctioned to the
Department of Chemistry, Jadavpur University are gratefully
acknowledged. We thank Dr. T.K. Mondal, Department of Chemis-
try, Jadavpur University for some calculations and corrections. Our
sincere thanks to the Reviewers for suggestions and we designed
some experiments (Cyclic Voltammetry) and calculations with an
improved basis set.
To understand the electronic structure and spectral properties
of the complexes, DFT and TD-DFT calculations have been carried
out on the representative complex [Cu((a-NaiMe)(PPh3)I] (7a),
and the calculated bond parameters support the experimental re-
sults (Table 4). The electronic structure explains the spectral and
electrochemical properties of the compound. To verify the origin
of two oxidative responses at positive to SCE, we have also carried
out calculations using the theoretically defined optimized struc-
ture of [Cu((a
-NaiMe)(PPh3)I]+ (7a+). The DFT calculation on the
Appendix A. Supplementary data
anionic form 7aꢀ was also carried out to find the site of the
reduction.
CCDC 767451 contains the supplementary crystallographic data
for [Cu((a-NaiMe)(PPh3)I] (7a). These data can be obtained free of
The orbital energies and the composition are given as Supple-
mentary material (Table S2), and Fig. 3 depicts selected occupied
and unoccupied frontier orbitals. The highest occupied molecular
orbitals (HOMO to HOMOꢀ2) are concentrated on the iodide and
Cu centres (Fig. 3). The LUMO and LUMO+1 are constituted of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. 1H NMR spectral data (Table S1), DFT calcu-
lated orbital energies, orbital compositions (Tables S2a and 2b) and
cyclic voltammetric figures of different complexes are given as
Supplementary data.
> 90%
a
-NaiMe functions, while the other unoccupied molecular
-NaiMe
orbitals (LUMO+2, LUMO+3 etc.) are an admixture of
a
and PPh3 functions. The calculated electronic transitions are
grouped in Table 4. The intensity of these transitions has been
assessed from the oscillator strength (f). The calculated moderately
intense transitions at 623, 584 and 549 nm correspond to HOMO/
HOMOꢀ1/HOMOꢀ2 ? LUMO transitions and are assigned to the
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2010.04.007.
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