Structure, electrochemistry and photochromism of [Cu(RaaiR′)(PPh 3)X] (RaaiR′ = 1-alkyl-2-(arylazo)imidazole; X = Cl, Br, I) and correlation with theoretical calculations

Article abstract of DOI:10.1016/j.poly.2012.07.039

The reaction between CuX (X = Cl, Br, I), PPh₃ and 1-alkyl-2-(arylazo)imidazole (RaaiR′) has synthesized [Cu(RaaiR′) (PPh₃)X]. The composition has been established by spectroscopic (UV-Vis, IR, ¹H NMR) data and the single crystal X-ray diffraction study of [Cu(MeaaiH)(PPh₃)Cl] and [Cu(MeaaiH)(PPh₃)Br] (MeaaiH = 2-(p-tolylazo)imidazole) have confirmed the structures. These complexes show trans-to-cis (E-to-Z) photoisomerisation upon UV light irradiation. Quantum yields (φ_E→Z) of [Cu(RaaiR′) (PPh₃)X] are lower than the free ligand values. The rate of isomerisation follows the sequence [Cu(RaaiR′)(PPh₃)Cl] < [Cu(RaaiR′)(PPh₃)Br] < [Cu(RaaiR′)(PPh₃)I]. The cis-to-trans (Z-to-E) isomerisation is very slow upon light irradiation and has been achieved by a thermal route. The activation energy (E_a) of the Z-to-E isomerisation has been calculated by a controlled temperature reaction. DFT calculations of the optimized geometry of representative complexes have been used to determine the composition and energy of the molecular levels.

Full text of DOI:10.1016/j.poly.2012.07.039

Polyhedron 45 (2012) 158–169
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Polyhedron
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Structure, electrochemistry and photochromism of [Cu(RaaiR0)(PPh3)X]
(RaaiR0 = 1-alkyl-2-(arylazo)imidazole; X = Cl, Br, I) and correlation with
theoretical calculations
Gunomoni Saha a,1, Kamal Krishna Sarkar a,2, Dibakar Sardar a,3, Jack Cheng b, Tian-Huey Lu b,
Chittaranjan Sinha a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
The reaction between CuX (X = Cl, Br, I), PPh3 and 1-alkyl-2-(arylazo)imidazole (RaaiR0) has synthesized
[Cu(RaaiR0)(PPh3)X]. The composition has been established by spectroscopic (UV–Vis, IR, 1H NMR) data
and the single crystal X-ray diffraction study of [Cu(MeaaiH)(PPh3)Cl] and [Cu(MeaaiH)(PPh3)Br]
(MeaaiH = 2-(p-tolylazo)imidazole) have confirmed the structures. These complexes show trans-to-cis
(E-to-Z) photoisomerisation upon UV light irradiation. Quantum yields (/E?Z) of [Cu(RaaiR0)(PPh3)X]
are lower than the free ligand values. The rate of isomerisation follows the sequence [Cu(R-
aaiR0)(PPh3)Cl] < [Cu(RaaiR0)(PPh3)Br] < [Cu(RaaiR0)(PPh3)I]. The cis-to-trans (Z-to-E) isomerisation is very
slow upon light irradiation and has been achieved by a thermal route. The activation energy (Ea) of the Z-
to-E isomerisation has been calculated by a controlled temperature reaction. DFT calculations of the opti-
mized geometry of representative complexes have been used to determine the composition and energy of
the molecular levels.
Article history:
Received 6 March 2012
Accepted 1 July 2012
Available online 20 July 2012
Keywords:
Copper(I)-azoimidazole
Photochromism
X-ray structure
Redox interconversion
DFT
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
light irradiation [19]. 1-Alkyl-2-(arylazo)imidazole (RaaiR0) exhibits
photochromism [20,21], a reversible photo-induced transformation
Azo dyes, an important class of chemicals in pigment industries,
have been used in coordination chemistry as a target to synthesize
photostable and weather stable pigments [1]. Recently, some metal
complexes of dyes have found applications in the high-technology
realm, for example as photoelectronic devices, optical recording
media [2], light emitting diodes [3], field effect transistors [4], pho-
tovoltaic cells (PVCs) [5] etc. Studies have been made on different
organic dyes for efficient dye sensitized solar cells [6–14]. Azo dyes
are interesting materials for solar batteries and non-linear optical
appliances [15]. The chelating property of azo dyes has been uti-
lized in a number of analytical methods [16], including sensors
[17,18] to determine metal ions.
between cis and trans geometries, whose absorption spectra differ
[22–27] significantly. This molecule has been used as a ligand
for the synthesis of some transition and non-transition metal
complexes [28–38]. The photochromic behavior of 1-alkyl-2-(ary-
lazo)imidazole (RaaiR0) [20,21] and some complexes like Hg(II)-
[39], Cd(II)- [40] and Pd(II)-azoimidazoles [41] has inspired us to
examine the photochromic property of copper(I) halide complexes.
Halide compounds of metals with a d10 configuration show var-
ious coordination numbers and configurations [42,43]. The chem-
istry of group 11 metals (Cu, Ag, Au) has been given considerable
attention [44] because of their interesting electronic, optical, struc-
tural, biological and catalytic properties [45–47]. Azoimidazole
Metal complexes containing azo groups have aroused great inter-
est due to the possibility of conformational modification under UV
carries a
p-acidic azoimine (–N@N–C@N–) chelating group and
stabilizes low valent metal redox states. The presence of other
p-
acidic molecules, like PPh3, CO and NO, cooperates with the azoim-
ine function to stabilize low redox states like Cr(0), Mo(0), W(0),
Ru(0) [48–50], Cu(I) [51,52] etc. Herein we report Cu(I)–halide–
PPh3 complexes of RaaiR0. The complexes are oxidized by chlorine
in methanol and the copper(II) complexes so obtained are reduced
by ascorbic acid to precursor copper(I) derivatives. This work
describes the structures, redox and photochromism of
Cu(I)–PPh3–RaaiR0. Theoretical calculations using the DFT
⇑
Corresponding author.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present address: Department of Chemistry, Barrackpore Rasthraguru Surendr-
1
anath College, Barrackpore, Kolkata 700 120, India.
2
Present address: Department of Chemistry, Mahadevananda Mahavidyalaya,
Barrackpore, Monirampore, Kolkata 700 120, India.
3
Present address: Department of Chemistry, Dinabandhu Andrews College, Garia,
Kolkata 700 084, India.
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.039

G. Saha et al. / Polyhedron 45 (2012) 158–169
159
computation technique for the optimized geometries of the com-
pounds has been used to define the electronic configuration and
to explain the spectroscopic and redox properties of the molecules.
aiH)(PPh3)Cl] (4b) Anal. Calc. for C28H25CuClN4P: C, 61.42; H,
4.60; N, 10.23. Found: C,61.44; H, 4.63; N, 10.20%. FT-IR (KBr disc,
cmꢀ1):
m (N@N) 1435; m (C@N) 1598; m (PPh3) 516, 470, 416. UV–
Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 262(7.4),
360(16.30), 425(4.62), 564(0.52). [Cu(HaaiMe)(PPh3)Cl] (5a) Anal.
Calc. for C28H25CuClN4P: C, 61.42; H, 4.60; N, 10.23. Found: C,
2. Experimental
61.43; H, 4.62; N, 10.24%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1427;
(PPh3) 525, 476, 416. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 264(14.52), 370(17.6), 434(4.49),
2.1. Materials
m
(C@N) 1600; m
Imidazole, different aromatic amines, CuX (X = Cl, Br, I) and tri-
phenyl phosphine were purchased from E. Merck India. All other
chemicals and solvents were of reagent grade and used as received.
The 1-alkyl-2-(arylazo)imidazoles (RaaiR0) were prepared by a re-
ported procedure [51,52].
580(0.46).
[Cu(MeaaiMe)(PPh3)Cl]
(5b)
Anal.
Calc
for
C
29H27CuClN4P: C, 62.03; H, 4.85; N, 9.98. Found: C, 62.05; H,
4.81; N, 10.01%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1429;
m
(C@N)
1597;
m (PPh3) 507, 474, 425. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 275(6.95), 373(14.35), 436(3.88),
482(0.62). [Cu(HaaiEt)(PPh3)Cl] (6a) Anal. Calc. for C29H27CuClN4P:
C, 62.03; H, 4.85; N, 9.98. Found: C, 62.06; H, 4.83; N, 9.95%. FT-IR
2.2. Physical measurements
(KBr disc, cmꢀ1):
m (N@N) 1435; m (C@N) 1599; m (PPh3) 522, 473,
Microanalytical (C,H,N) data were obtained from a Perkin–El-
mer 2400 CHNS/O elemental analyzer. Spectroscopic data were ob-
tained using the following instruments: UV–Vis, Perkin–Elmer
Lambda-25; IR (KBr disk, 4000–200 cmꢀ1), Perkin–Elmer RX-1
spectrophotometer; 1H NMR, Bruker 300 MHz FT-NMR spectrome-
ter. Electrochemical measurements were performed using com-
puter-controlled PAR model 250 VersaStat electrochemical
instruments with Pt-disk electrodes. All measurements were car-
ried out under a nitrogen environment at 298 K with reference to
SCE in acetonitrile using [nBu4N][ClO4] as the supporting electro-
lyte. The reported potentials are uncorrected for junction potential.
Room temperature (298 K) magnetic susceptibility was measured
using Sherwood Scientific Cambridge, UK at 298 K. EPR spectra
were recorded in MeCN solution at room temperature (298 K)
and liquid nitrogen temperature (77 K) using a Bruker ESR spec-
trometer model EMX 10/12, X-band ER 4119 HS cylindrical
resonator.
420. UV–Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)):
279(7.86), 372(11.45), 420(6.32), 574(0.45). [Cu(MeaaiEt)(PPh3)Cl]
(6b) Anal. Calc. for C30H29CuClN4P: C, 62.60; H, 5.07; N, 9.73.
Found: C, 62.63; H, 5.03; N, 9.77%. FT-IR (KBr disc, cmꢀ1):
m
(N@N): 1434;
m (C@N): 1598; m (PPh3): 522, 475, 424. UV–Vis
(CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 263(3.92),
379(10.13), 441(3.43), 580(0.60). [Cu(HaaiH)(PPh3)Br] (7a) Anal.
Calc. for C27H23CuBrN4P: C, 56.11; H, 4.01; N, 9.69. Found: C,
56.14; H, 3.98; N, 9.71%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1423; m
(C@N) 1597;
m (PPh3) 524, 470, 420. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 271(7.25), 364(13.39), 443(5.49),
566(0.52). [Cu(MeaaiH)(PPh3)Br] (7b) Anal. Calc. for C28H25CuB-
rN4P: C, 56.81; H, 4.25; N, 9.46. Found: C, 56.84; H, 4.24; N,
9.43%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1436; m (C@N) 1596; m
(PPh3) 521, 470, 422. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 265(5.38), 358(16.97), 440(5.31),
550(0.48). [Cu(HaaiMe)(PPh3)Br] (8a) Anal. Calc. for C28H25CuB-
rN4P: C, 56.81; H, 4.25; N, 9.46. Found: C, 56.84; H, 4.22; N,
9.51%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1432; m (C@N) 1594; m
2.3. Synthesis
(PPh3) 516, 466, 424. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1): 278(8.47), 376(13.57), 423(6.23),
578(0.91). [Cu(MeaaiMe)(PPh3)Br] (8b) Anal. Calc. for C29H27CuB-
rN4P: C, 57.47; H, 4.49; N, 9.24. Found: C,57.51; H, 4.63; N,
2.3.1. [Cu(PPh3)(MeaaiH)Cl] (4b)
To a methanol solution (25 ml) of 2-(p-tolyl-azo)imidazole
(MeaaiH) (65 mg, 0.35 mmol), an acetonitrile solution of CuCl
(45 mg, 0.35 mmol) and PPh3 (92 mg, 0.35 mmol) were added, stir-
red and refluxed for 3 h. The solution was filtered through a G4
crucible and allowed to evaporate slowly in air. Block shaped dark
micro crystals deposited on the wall of the beaker. These were col-
lected by filtration and washed with cold methanol. X-ray quality
block shaped crystals were obtained by recrystallising from a
DMF–MeOH solution (1:2 v/v). Yield: 0.15 g (71%).
9.20%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1431; m (C@N) 1598; m
(PPh3) 534, 478, 426. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1): 275(6.64), 379(17.3), 437(5.94),
575(0.67). [Cu(HaaiEt)(PPh3)Br] (9a) Anal. Calc. for C29H27CuBrN4P:
C, 57.47; H, 4.49; N, 9.24. Found: C, 57.49; H, 4.52; N, 9.24%. FT-IR
(KBr disc, cmꢀ1):
m (N@N) 1434; m (C@N) 1599; m (PPh3) 536, 470,
428. UV–Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)):
282(6.89), 375(12.80), 423(5.37), 578(0.80). [Cu(MeaaiEt)(PPh3)Br]
(9b) Anal. Calc for C30H29CuBrN4P: 58.12; H, 4.71; N, 9.03. Found: C,
2.3.2. [Cu(PPh3)(MeaaiH)I] (10b)
To a methanol solution (25 ml) of 2-(p-tolyl-azo)imidazole
(MeaaiH) (65 mg, 0.35 mmol), an acetonitrile solution of CuI
(70 mg, 0.35 mmol) and PPh3 (92 mg, 0.35 mmol) were added, stir-
red and refluxed for 3 h. The solution was filtered through a G4
crucible and allowed to evaporate slowly in air. Block shaped black
micro crystals deposited on the wall of the beaker and were
washed with cold methanol. These were collected by filtration.
X-ray quality block shaped crystals were obtained by recrystallis-
ing from a DMF–MeOH solution (1:2 v/v). Yield: 0.15 g (71%).
All other complexes were prepared by the same procedure. In
all cases, crystalline products were obtained. The yields varied
from 70% to 80% and microanalytical data of the complexes are
as follows: [Cu(HaaiH)(PPh3)Cl] (4a) Anal. Calc. for C27H23CuClN4P:
C, 60.79; H, 4.60; N, 10.50. Found: C, 60.81; H, 4.59; N, 10.53%. FT-
58.13; H, 4.76; N, 9.00%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1431; m
(C@N) 1598;
m (PPh3) 516, 470, 416. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 278(6.34), 375(15.36), 433(5.93),
580(0.66). [Cu(HaaiH)(PPh3)I] (10a) Anal. Calc. for C27H23CuIN4P:
C, 51.89; H, 3.71; N, 8.97. Found: C, 51.90; H, 3.70; N, 8.98%. FT-
IR (KBr disc, cmꢀ1):
m (N@N) 1423; m (C@N) 1595; m (PPh3) 530,
470, 420. UV–Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)):
261(5.93), 367(11.77), 441(4.71), 470(0.54). [Cu(MeaaiH)(PPh3)I]
(10b) Anal. Calc. for C28H25CuIN4P: C, 52.63; H, 3.94; N, 8.77.
Found: C, 52.60; H, 3.93; N, 8.79%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1433;
m (C@N) 1595; m (PPh3) 525, 470, 425. UV–Vis
(CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 245(6.46),
362(10.72), 433(5.32), 545(0.65). [Cu(HaaiMe)(PPh3)I] (11a) Anal.
Calc. for C28H25CuIN4P: C, 52.63; H, 3.94; N, 8.77. Found: C,
IR (KBr disc, cmꢀ1):
420. UV–Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)):
267(13.34), 364(15.21), 432(3.97), 560(0.45). [Cu(Mea-
m
(N@N) 1424;
m
(C@N) 1599;
m
(PPh3) 520, 474,
52.61; H, 3.95; N, 8.79%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1422;
(PPh3) 524, 474, 416. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 282(8.77), 370(15.42), 426(5.04),
m
(C@N) 1600;
m

160
G. Saha et al. / Polyhedron 45 (2012) 158–169
576(0.60).
[Cu(MeaaiMe)(PPh3)I]
(11b)
Anal.
Calc.
for
trate was left for slow evaporation at room temperature in a bea-
ker. A dark crystalline compound was deposited on the wall of
container after a week. The product was then washed with water
and dried in a CaCl2 desiccator. Recrystallisation was carried out
by diffusion of a CH2Cl2 solution of the compound into hexane.
Yield 31 mg (66%).
C
29H27CuIN4P: C, 53.34; H, 4.17; N, 8.58. Found: C, 53.31; H,
4.16; N, 8.60%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1432;
m
(C@N,
1599;
m (PPh3) 525, 472, 420. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 281(5.38), 381(11.20), 440(5.14),
581(0.75). [Cu(HaaiEt)(PPh3)I] (12a) Anal. Calc. for C29H27CuIN4P:
C, 53.34; H, 4.17; N, 8.58. C, 52.63; H, 3.94; N, 8.77. Found: C,
53.32; H, 4.18; N, 8.61%. FT-IR (KBr disc, cmꢀ1):
m (N@N) 1436; m
2.5. X-ray diffraction study
(C@N) 1597;
m (PPh3) 527, 467, 420. UV–Vis (CH3CN) kmax (nm)
(10ꢀ3 e (dm3 molꢀ1 cmꢀ1)): 282(8.52), 374(17.18), 426(6.43),
578(0.75). [Cu(MeaaiEt)(PPh3)I] (12b) Anal. Calc. for C30H29CuIN4P:
C, 54.02; H, 4.38; N, 8.39. Found: C, 54.04; H, 4.36; N, 8.40%. FT-IR
The crystallographic data are shown in Table 5. Suitable single
crystals of complexes 4b (0.10 ꢂ 0.15 ꢂ 0.28 mm) and 7b
(0.14 ꢂ 0.20 ꢂ 0.23 mm) were mounted on a Siemens CCD diffrac-
(KBr disc, cmꢀ1):
m (N@N) 1432; m (C@N) 1596; m (PPh3) 536, 476,
tometer equipped with graphite monochromated Mo
Ka
430. UV–Vis (CH3CN) kmax (nm) (10ꢀ3 e (dm3 molꢀ1 cmꢀ1)):
(k = 0.71073 Å) radiation. The unit cell parameters and crystal-ori-
entation matrices were determined for the two complexes by least
squares refinements of all reflections. The intensity data were cor-
rected for Lorentz and polarization effects, and an empirical absorp-
tion correction was also employed using the SAINT program [53]. Data
273(13.32), 378(16.37), 435(5.93), 584(0.67).
2.4. Cu(II)–Cu(I) interconversion
Cl2solution
were collected applying the condition I > 2r(I). All these structures
½CuðMeaaiHÞðPPh ÞClꢁð4bÞ
½CuðMeaaiHÞðPPh ÞCl ꢁð13bÞ
ƒƒƒƒƒƒ!
3
3
2
were solved by direct methods and followed by successive Fourier
and difference Fourier syntheses. Full matrix least squares refine-
ments on F2 were carried out using SHELXL-97 with anisotropic dis-
placement parameters for all non-hydrogen atoms. Hydrogen
atoms were constrained to ride on their respective carbon or nitro-
gen atoms with isotropic displacement parameters equal to 1.2
times the equivalent isotropic displacement of their parent atom
in all cases. Complex neutral atom scattering factors were used
throughout in all cases. All calculations were carried out using SHELXS
97 [54], SHELXL97 [55], PLATON 99 [56] and ORTEP-3 [57] programs.
[Cu(MeaaiH)(PPh3)Cl] (4b) (120 mg, 0.220 mmol), was dis-
solved in CH3CN and chlorine saturated methanol was added in
drops. The resultant brown solution was stirred for 6 h at room
temperature with time-to-time addition of chlorine saturated
methanol. The solution color turned permanently to deep brown.
A methanol solution of LiCl (2 ml, 0.2 mmol) was added and stirred
for a few minutes. It was then filtered and allowed to evaporate in
air. The solid mass so obtained was washed with a profuse amount
of water, recrystallised from methanol and then dried in a CaCl2
desiccator. Yield 74 mg (58%).
2.6. Photometric measurements
½CuðMeaaiHÞðPPh3ÞCl2ꢁð13bÞ ! ½CuðMeaaiHÞðPPh3ÞClꢁð4bÞ
To a stirred dry methanol solution of [Cu(MeaaiH)(PPh3)Cl2]
(13b) (50 mg, 0.09 mmol), ascorbic acid (20 mg, 0.11 mmol) was
added pinchwise in excess. The resultant dark brown–red solution
was stirred for 3 h at room temperature and then filtered. The fil-
Absorption spectra were taken with a PerkinElmer Lambda 25
UV–Vis spectrophotometer in a 1 ꢂ 1 cm quartz optical cell main-
tained at 25 °C with a Peltier thermostat. The light source of a Perk-
inElmer LS 55 spectrofluorimeter was used as the excitation light,
Scheme 1.

G. Saha et al. / Polyhedron 45 (2012) 158–169
161
Fig. 1. Molecular structures of (a) [Cu(MeaaiH)(PPh3)Cl] (4b) and (b) [Cu(MeaaiH)(PPh3)Br] (7b).
Table 1
Selected bond distances (Å) and angles (°) for [Cu(PPh3)(MeaaiH)Cl] (4b) and [Cu(PPh3)(MeaaiH)Br] (7b), and the structures generated from DFT calculations for 4b, 7b and
[Cu(PPh3)(MeaaiH)I] (10b).
Bond distances (Å)
Bond angles (°)
X-ray
DFT/B3LYP
X-ray
DFT/B3LYP
[Cu(PPh3)(MeaaiH)Cl] (4b)
Cu–Cl
Cu–P
Cu–N(1)
Cu–N(4)
N(3)–N(4)
N(3)–C(3)
N(4)–C(4)
2.3127(6)
2.2001(6)
2.0997(1)
2.1352(16)
1.269(2)
2.322
2.268
2.123
2.140
1.277
1.373
1.408
Cl(1)–Cu–P
117.34(2)
105.32(5)
101.23(5)
119.61(5)
128.45(5)
76.95(6)
118.7
108.1
102.6
116.9
126.5
75.68
Cl(1)–Cu–N(1)
Cl(1)–Cu–N(4)
P–Cu–N(1)
P–Cu–N(4)
N(1)–Cu–N(4)
1.381(3)
1.430(3)
[Cu(PPh3)(MeaaiH)Br] (7b)
Cu–Br
Cu–P
Cu–N(1)
Cu–N(4)
N(3)–N(4)
N(3)–C(3)
N(4)–C(4)
2.436(6)
2.202(3)
2.1037(17)
2.1320(16)
1.266(2)
1.345(3)
1.432(3)
2.449
2.254
2.131
2.151
1.277
1.374
1.415
Br(1)–Cu–P
117.573(17)
105.31(5)
101.95(4)
119.43(5)
127.69(5)
76.90(7)
117.6
105.6
102.0
118.9
127.8
76.77
Br(1)–Cu–N(1)
Br(1)–Cu–N(4)
P–Cu–N(1)
P–Cu–N(4)
N(1)–Cu–N(4)
[Cu(PPh3)(MeaaiH)I] (10b)
Cu–I
Cu–P
Cu–N(1)
Cu–N(4)
N(3)–N(4)
N(3)–C(3)
N(4)–C(4)
–
–
–
–
–
–
–
2.654
2.271
2.140
2.160
1.277
1.371
1.410
I–Cu–P
–
–
–
–
–
–
121.2
105.0
102.9
116.6
125.3
75.9
I–Cu–N(1)
I–Cu–N(4)
P–Cu–N(1)
P–Cu–N(4)
N(1)–Cu–N(4)
with a slit width of 10 nm. An optical filter was used to cut off
overtones when necessary. The absorption spectra of the cis iso-
mers were obtained by extrapolation of the absorption spectra of
a cis-rich mixture for which the composition was known from 1H
NMR integration. Quantum yields () were obtained by measuring
The thermal cis-to-trans isomerisation rates were obtained by
monitoring absorption changes intermittently for a cis-rich solu-
tion kept in the dark at a constant temperature in the range from
298 to 313 K. The activation energy (Ea) and the frequency factor
(A) were obtained from the Arrhenius plot,
the initial trans-to-cis isomerization rates (
tion within the above instrument using the equation,
m) in a well-stirred solu-
lnk ¼ lnA ꢀ Ea=RT
where k is the measured rate constant, R is the gas constant, and T is
m
¼ ð/I0=VÞð1 ꢀ 10ꢀAbs
Þ
the absolute temperature. The values of activation free energy (D
G⁄)
and activation entropy
relationships,
(D
S⁄) were obtained through the
where I0 is the photon flux at the front of the cell, V is the volume of
the solution, and Abs is the initial absorbance for the azobenzene
(/ = 0.11 for
conditions.
p –
p⁄ excitation [58]) under the same irradiation
D
Gꢃ ¼ Ea ꢀ RT ꢀ T
D
Sꢃ and
D
Sꢃ ¼ ½ln A ꢀ 1 ꢀ lnðkBT=hÞ=R

162
G. Saha et al. / Polyhedron 45 (2012) 158–169
N
N
N
N
N
N
R
N
N
N
N
N
N
R
Cu
Cu
N
R
N
R
N
N
N
Cu
N
R
N
N
Free ligand
Coordinated ligand
E- or trans-
Z- or cis-
Scheme 2. Photochromism of free and coordinated RaaiR0.
where kB and h are Boltzmann’s and Plank’s constants, respectively.
(HaaiEt, 3a) and 1-ethyl-2-(p-tolylazo)imidazole (MeaaiEt, 3b), are
used in this work (Scheme 1) to prepare copper(I) complexes. The
reaction of CuX and PPh3 forms the tetramer [Ph3PCuX]4 [66],
which upon reaction in situ with RaaiR0 in MeCN has yielded com-
plexes of the composition [Cu(RaaiR0)(PPh3)X] (X = Cl (4, 5, 6), Br
(7, 8, 9), I (10, 11, 12)). We have also isolated [Ph3PCuX]4 and have
reacted it with RaaiR0 under the same reaction conditions, and this
gives complexes of the same composition. However, the reactions
have been carried out in situ as this is more economic in terms of
time. The compounds are non-conducting and their composition
has been supported by microanalytical data. The structures have
been established in representative cases by a single-crystal X-ray
diffraction study.
2.7. DFT calculations
The density functional theory (DFT) calculations on the crystal
structures of 4b and 7b were carried out using the GAUSSIAN 03w
program package [59] with the aid of the GAUSSVIEW visualization
program [60] The hybrid functional by Becke, B3LYP [61], was
used, which included a mixture of Hartree–Fock exchange with
DFT exchange–correlation. For C, H, N and I, we used MIDI! basis
functions [62] as this basis set is applicable to all these elements
and includes polarization functions. For Cu, we used the LanL2DZ
basis set, including Los Alamos Effective Core Potentials [63–65].
MeCN—MeOH
ƒƒƒƒƒƒƒ!
reflux
MeCN—MeOH
ƒƒƒƒƒƒƒ!
reflux;RaaiR
CuXþPPh3
1=4½Ph PCuXꢁ
½CuðRaaiRÞðPPh ÞXꢁ
3
3
4
3. Results and discussion
3.1. Synthesis of complexes
3.2. Molecular structures
1-Alkyl-2-(arylazo)imidazoles (RaaiR0), like 2-(phenylazo)imid-
azole (HaaiH) (1a), 2-(p-tolylazo)imidazole (MeaaiH) (1b),
1-methyl-2-(phenylazo)imidazole (HaaiMe, 2a), 1-methyl-2-(p-
tolylazo)imidazole (MeaaiMe, 2b), 1-ethyl-2-(phenylazo)imidazole
The molecular structures of [Cu(MeaaiH)(PPh3)Cl] (4b) and
[Cu(MeaaiH)(PPh3)Br] (7b) are shown in Fig. 1a and b, respectively.
Bond parameters are listed in Table 1. Each discrete molecular unit
Fig. 2. C–Hꢄ ꢄ ꢄp (a) and p–p (b) interactions in [Cu(MeaaiH)(PPh3)Cl] (4b).

G. Saha et al. / Polyhedron 45 (2012) 158–169
163
HOMO-1
HOMO
LUMO
LUMO+1
E = -5.18 eV; Cu,
47 %; MeaaiH, 13
%; Cl, 38 %, PPh3,
2%
E = -2.70 eV; Cu, 7 E = -0.77 eV; Cu,
E = -5.00 eV; Cu,
40 %; MeaaiH, 8%,
Cl, 31
%; MeaaiH, 90 %;
Cl,
1%; MeaaiH, 1%;
PPh3, 98 %.
[Cu(PPh3)(MeaaiH)Cl] (4b)
E = -0.84 eV; PPh3,
99 %
E = -2.64 eV; Cu, 6
%; MeaiH, 90 %;
Br, 3%.
E = -5.01 eV; Cu,
27 %; Br, 54%;
MeaaiH, 8%;
E = -4.89 eV; Cu,
27 %; Br, 54%;
MeaaiH, 4%; PPh3,
15 %
[Cu(PPh3)(MeaaiH)Br] (7b)
E = -4.84 eV; Cu,
E = -3.05 eV; Cu,
5%; MeaaiH, 90%;
I, 3%; PPh3, 2%
E = -4.80 eV; Cu,
E = -0.86 eV; PPh3,
99%
12 %; I, 83%;
MeaaiH, 3%;
PPH3,2%.
11%; I, 82%;
MeaaiH, 2%; PPh3,
4%.
[Cu(PPh3)(MeaaiH)I] (10b)
Fig. 3. Surface plots of some selected MOs of [Cu(MeaaiH)(PPh3)X] (X = Cl (4b), Br (7b), I (10b)).
consists of the coordination unit CuN2PX. The chelating ligand,
MeaaiH acts as a N,N0-donor (N refers to N(imidazole) and N0 refers
to N(azo)). The atomic arrangements Cu, N(4), N(3), C(3) and N(1),
constitute a chelate plane with a deviation of <0.04 Å in 4b and
0.035 Å in 7b. The pendant aryl ring makes a dihedral angle of
9.82(10)° for 4b and 8.76(10)° for 7b with the respective chelated
azoimidazole ring. The acute bite angle, Cu(N,N0), 76.95(6)° (4b)
and 76.90(7)° (7b), is extended by MeaaiH on coordination to
Cu(I) and is comparable with reported results in the series of che-
lated arylazoimidazole complexes of d10 metal complexes
[51,52,67,68]. The small chelate angle may be one of the reasons
for geometrical distortion. The ligand (MeaaiH) is planar (maxi-
mum deviation ꢅ0.08 Å). The Cu–N(imidazole) bond length
(2.0997(1) Å, 4b 2.1037(17)) Å, 7b) is shorter than that of Cu(I)–
N(azo) (2.1352(16) Å, 4b and 2.1320(16) Å, 7b), which reflects
the stronger interaction of Cu(I) with N(imidazole) compared to
N(azo). Because of the long Cu(I)–N(azo) distance, the molecule
may exhibit photoactivation via cleavage of this bond followed
by rotation to introduce photoisomerisation (Scheme 2, vide infra).
The N@N distance is 1.269(2) Å in 4b and 1.266(2) Å in 7b. These
are elongated by a small amount compared to that of the free li-
p
-network, while the slippage between both imidazole rings is
too large (ca. 4.32(5) Å) to establish effective interactions (Fig. 2).
These two interactions ( ) have endowed a 1D
p–p
and C–Hꢄ ꢄ ꢄ
p
chain to the solid state geometry of this molecule.
DFT calculations have been performed for three complexes to
establish the structural relationship between the crystal structures
of 4b and 7b (Fig. 1) and the theoretically generated optimized
geometry. The structure of 10b was generated by using DFT calcu-
lations only. The structural agreement has been observed from a
gand value (1.250(1) Å) [20], which supports d(Cu) ?
p (azoimine)
charge transfer.
A
p–p interaction is observed between the chelate plane of
Cu(N,N0) and the imidazole ring of two adjacent molecules (Cg(1)
(chelated Cu(N,N0) plane)–Cg(1)(imidazole), 3.79(1) Å) in 4b and
3.86(4) Å (Cg(1) (Cu(N,N0) plane)–Cg(2) (imidazole)) in 7b. The
phenyl rings of PPh3 interact with the partners of a neighboring
molecule and the C–Hꢄ ꢄ ꢄ
p distance is 2.89(3) Å (symmetry, 1+X,
Y, Z) in 4b and 2.86(4) Å (symmetry, 1+X, Y, Z) in 7b. These two
Fig. 4. Correlation amongst calculated energy levels of [Cu(MeaaiH)(PPh3)X] (X = Cl
(4b), Br (7b), I (10b)).
interactions increase the intermolecular attraction to generate a

164
G. Saha et al. / Polyhedron 45 (2012) 158–169
comparison of bond distances and angles between the calculated
and X-ray determined structures of 4b and 7b (Table 1). The theo-
retical bond distances are longer by 0.01–0.07 Å compared to those
obtained from the X-ray determined structures (Fig. 1).
state, nature of the ligand, presence of other coordinating ligand(s)
etc. play an important role in the photoisomerisation of the coordi-
nated ligand in the complexes [23,70–72]. Upon UV light irradia-
tion the trans structure of RaaiR0 changes to the cis structure
about the azo (–N@N–) function and the cis molar ratio reaches
>95% (Scheme 2).
3.3. Spectroscopic studies
The complexes are resistant to photo degradation upon re-
peated irradiation at least up to 15 cycles in each case.
IR spectra of the complexes show moderately intense stretching
bands at 1595–1600 and 1435–1445 cmꢀ1 which are assigned to
m
UV-light irradiation of a solution of [Cu(RaaiR0)(PPh3)X] may
cleave the Cu–N(azo) bond and make the ligand free to rotate
about the azo-aryl group (–N@N–Ar), which isomerizes it from
the trans(E) to the cis (Z) form. This is apprehended on considering
the longer Cu(I)–N(azo) bond length than Cu–N(imidazole) dis-
tance in the complexes (Table 1). The rates of photoisomerisation
and also the quantum yields (trans-to-cis (E–Z), t?c) increase with
the decreasing electronegativity of X in the complexes; the rate fol-
lows [Cu(RaaiR0)(PPh3)Cl] (5, 6) < [Cu(RaaiR0)(PPh3)Br] (8,
9) < [Cu(RaaiR0)(PPh3)I] (11, 12) (Table 2). It is observed that the
(C@N) and
m (N@N), respectively. Other vibrations are shifted to a
lower energy in the copper complexes (4–12) compared to the free
ligand values [69].
The solution UV–Vis spectra of the complexes 4–12 are re-
corded in MeCN in the range 200–900 nm. The bands in the UV re-
gion are intense (
e
ꢅ104 molꢀ1 dm3 cmꢀ1) at 240–255 and 365–
380 nm, and are referred to intramolecular charge transfer transi-
tions (n–p⁄
,
p
–
p⁄
)
[69].
A
band at 550–600 nm
(
e
ꢅ103
molꢀ1 dm3 cmꢀ1) is characteristic of a MLCT transition of tetrahe-
dral [Cu(azoimidazole)2]+. The electronic configuration of the three
representative complexes 4b, 7b and 10b are used to explain the
origin of the transitions. The selected MOs of the complexes are
shown in Fig. 3. The HOMOs of these three complexes are com-
posed of copper (25%), PPh3 (10%) and halogen (30% of Cl in 4b,
65% of Br in 7b and 83% of I in 10b) (Supplementary Material, Table
S1). Other occupied MOs (HOMO-1, HOMO-2 etc.) are composed of
Cu and X (halogen). The LUMO is a p⁄ orbital delocalized over the
entire arylazoimidazole moiety (90%), while LUMO+1, LUMO+2 etc.
are dominated by PPh3. As a consequence, a significant mixing of
orbitals of MeaaiH and PPh3 may be considered. The energies of
the MOs are correlated in Fig. 4. The calculated transitions are gi-
ven as Supplementary Material (Table S2). The intensity of these
transitions has been assessed from oscillator strengths (f). The cal-
culated bands are of MLCT, XLCT, YLCT and ILCT types (MLCT, me-
tal-to-ligand charge transfer; XLCT, halide-to-ligand charge
transfer, YLCT, phosphine-to-ligand charge transfer; ILCT, intra-li-
gand charge transfer where the ligand refers to MeaaiH). The tran-
sitions in UV region (<400 nm) are a mixture of ILCT and YLCT
(Y = PPh3). The observed transitions appear closer to the calculated
wavelength in the complexes. The transitions appearing in the re-
gion >600 nm are described as an admixture of MLCT and XLCT.
The 1H NMR spectra of the complexes are recorded in CDCl3 and
the signals are assigned (Supplementary Material, Table S1) unam-
biguously by spin–spin interactions, the effect of substitution
therein and on comparing with previously reported compounds
[67–69]. The atom numbering pattern is shown in the structure
(Scheme 1). Data reveal that the signals in the spectra of the
complexes in general are shifted downfield compared to the spec-
tra of the free ligands [69]. The signals of the imidazole protons, 4
HOMO–LUMO energy difference (DE) decreases with decreasing
electronegativity of X: 2.3 eV ([Cu(MeaaiH)(PPh3)Cl], 4b), 2.25 eV
([Cu(MeaaiH)(PPh3)Br], 7b) and 1.75 eV ([Cu(MeaaiH)(PPh3)I],
10b) (Fig. 4, correlation diagram). The composition of the MOs
0.7
44
84
(i)
0.7
0.6
trans
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
0.4
0.3
0.2
0.1
0.0
cis
300
350
400
450
500
550
600
460 nm
331nm
300
350
400
450
500
550
600
wavelength (nm)
0.6
0.5
0.4
0.3
0.2
trans
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(ii)
and 5H, shift significantly (D, 0.2–0.5 ppm), whereas shifting for
the aryl protons, 7 and 11H, is in the range 0.06–0.15 ppm. Protons
8–10H remain almost unshifted. This supports chelation by
N(imidazole) and N(azo) in which the imidazole-N binds more
effectively than the azo-N to Cu(I) (vide supra, X-ray structures).
The electron donating effect of the (Ar-)Me group in the Cu(I) com-
plexes of MeaaiR0 shifts aryl protons (7–11H) to the upfield side
compared to Cu(I)–HaaiR0.
cis
0.1
0.0
300
350
400
450
500
550
600
332 nm
463 nm
3.4. Photochromism of [Cu(RaaiR0)(PPh3)X]
The chelated ligands RaaiR0 in the complexes [Cu(R-
aaiR0)(PPh3)X] undergo photo-induced trans(E)-to-cis(Z) isomerisa-
tion. The effect of light irradiation on [Cu(RaaiR0)(PPh3)X] (5, 6, 8, 9,
11 and 12) in acetonitrile solution was investigated by UV–Vis
spectroscopy (Fig. 5). The complexes, [Cu(RaaiH)(PPh3)X] (4, 7
and 10) show very slow spectral change upon UV light irradiation
and have not been used further. The metal ion and its oxidation
300
350
400
450
500
550
600
wavelength (nm)
Fig. 5. Spectroscopic changes of (i) [Cu(MeaaiMe)(PPh3)Br] (8b) and (ii) [Cu(Haai-
Me)(PPh3)I] (11a) in MeCN upon repeated irradiation at 380 nm at 3 min intervals
at 25 °C. Inset figure shows spectra of the cis and trans isomers of the complex.

G. Saha et al. / Polyhedron 45 (2012) 158–169
165
Table 2
Excitation wavelength (kp, p⁄), rate of trans (t) ? cis (c) conversion and quantum yield (t?c) in MeCN.
Compounds
k
p, p⁄ (nm)
Isosbestic point (nm)
Rate of t ? c conversion x 108 (sꢀ1
)
t?c
[Cu(HaaiMe)(PPh3)Cl] (5a)
[Cu(MeaaiMe)(PPh3)Cl] (5b)
[Cu(HaaiEt)(PPh3)Cl] (6a)
[Cu(MeaaiEt)(PPh3)Cl] (6b)
[Cu(HaaiMe)(PPh3)Br] (8a)
[Cu(MeaaiMe)(PPh3)Br] (8b)
[Cu(HaaiEt)(PPh3)Br] (9a)
[Cu(MeaaiEt)(PPh3)Br] (9b)
[Cu(HaaiMe)(PPh3)I] (11a)
[Cu(MeaaiMe)(PPh3)I] (11b)
[Cu(HaaiEt)(PPh3)I] (12a)
[Cu(MeaaiEt)(PPh3)I] (12b)
377
382
376
381
375
380
377
381
374
381
375
380
335, 466
334, 462
331, 464
330, 474
336, 468
331, 460
335, 474
334, 473
332, 463
333, 463
332, 462
331, 472
2.64
2.61
2.64
2.56
2.71
2.67
2.69
2.62
2.82
2.71
2.74
2.65
0.1731
0.1639
0.1678
0.1581
0.1863
0.1754
0.1782
0.1629
0.1972
0.1816
0.1865
0.1769
Table 3
Rate and activation parameters for cis (c) ? trans (t) thermal isomerisation.
Compound
Temp (K)
Rate of thermal c ? t conversion ꢂ 104 (s–1
)
Ea
D
H⁄
D
S⁄
D
G⁄c
kJ molꢀ1
kJ molꢀ1
J molꢀ1Kꢀ1
kJ molꢀ1
[Cu(HaaiMe)(PPh3)Cl] (5a)
298
303
308
313
1.3215
2.1958
2.9726
3.6825
52.52
49.98
51.27
46.0107
59.29
57.028
57.68
53.099
65.35
64.19
63.91
58.20
49.98
ꢀ150.83
94.93
94.39
95.14
94.47
95.81
94.54
96.28
94.65
95.53
96.05
94.93
96.13
[Cu(MeaaiMe)(PPh3)Cl] (5b)
[Cu(HaaiEt)(PPh3)Cl] (6a)
[Cu(MeaaiEt)(PPh3)Cl] (6b)
[Cu(HaaiMe)(PPh3)Br] (8a)
[Cu(MeaaiMe)(PPh3)Br] (8b)
[Cu(HaaiEt)(PPh3)Br] (9a)
[Cu(MeaaiEt)(PPh3)Br] (9b)
[Cu(HaaiMe)(PPh3)I] (11a)
[Cu(MeaaiMe)(PPh3)I] (11b)
[Cu(HaaiEt)(PPh3)I] (12a)
[Cu(MeaaiEt)(PPh3)I] (12b)
298
303
308
313
1.703
47.44
48.74
43.47
56.75
54.49
55.14
50.56
62.81
61.6484
61.37
55.66
ꢀ157.56
ꢀ155.73
ꢀ171.13
ꢀ131.05
ꢀ134.40
ꢀ138.04
ꢀ147.94
ꢀ109.79
ꢀ115.42559
ꢀ112.60
ꢀ135.79
2.5955
3.4167
4.5429
298
303
308
313
1.2196
1.9864
2.672
3.3164
298
303
308
313
1.654
2.3965
3.281
3.9973
298
303
308
313
0.9524
1.8479
2.6884
3.7925
298
303
308
313
1.5462
2.6486
3.7173
4.6894
298
303
308
313
0.7973
1.2529
1.8136
2.4310
298
303
308
313
1.5023
2.4351
3.3351
4.2243
298
303
308
313
1.0573
1.8479
2.6884
3.7925
298
303
308
313
0.8945
1.41
2.154
3.076
298
303
308
313
1.2928
2.4373
3.5326
4.4931
298
303
308
313
0.8729
1.2629
1.9974
2.6149

166
G. Saha et al. / Polyhedron 45 (2012) 158–169
calculate the activation energies (Table 3, Fig. 6). In the complexes
the Eas are severely reduced, which means faster cis-to-trans (Z-to-
E) thermal isomerisation of the complexes. The entropies of activa-
-13.2
-13.5
-13.8
-14.1
-14.4
-14.7
-15.0
(11b)
tion (
D
S⁄) are highly negative in the complexes compared to the
free ligand. This is also in support of an increase in rotor volume
in the complexes.
(8b)
(5b)
3.5. Electrochemistry and redox interconversion, CuII M CuI
The cyclic voltammograms of the copper(I) complexes [Cu((R-
aaiR0)(PPh3)X] are recorded in MeCN at a Pt-disk milli electrode
in the potential range +1.5 to ꢀ2.0 V versus the SCE reference elec-
trode (Fig. 7; Table 4). [Cu((RaaiR0)(PPh3)Cl] (4–6) and [Cu((R-
aaiR0)(PPh3)Br] (7–9), show a quasireversible oxidative response
of ꢅ1 V while the complexes [Cu((RaaiR0)(PPh3)I] (10–12) show
two quasireversible redox responses at ꢅ0.4 and ꢅ1 V. A signifi-
cant observation is the appearance of a quasireversible redox cou-
ple at 0.4 V for [Cu((RaaiR0)(PPh3)I] (10–12) which may be assigned
to the Iꢀ/I response. [Cu(RaaiR0)2](ClO4) shows a Cu(II)/Cu(I) couple
at 0.5–0.6 V [51,52,69], and in [Cu((RaaiR0)(PPh3)X] this redox cou-
ple shifts to a more positive redox value which may be due to the
0.00320
0.00324
0.00328
1/T
0.00332
0.00336
Fig. 6. Eyring plots of Z-to-E thermal isomerisation of (a) [Cu(MeaaiMe)(PPh3)Cl]
(5b, N); slope, ꢀ7415.01126; intercept, 9.87672; R2, 0.998. (b) [Cu(Mea-
aiMe)(PPh3)Br] (8b, d) slope, ꢀ6553.86192; intercept, 7.595; R2, 0.968. (c)
[Cu(MeaaiMe)(PPh3)I] (11b, j); slope, ꢀ6441.33079; intercept, 7.3841; R2, 0.969
at different temperatures.
p
-acidity of the coordinated PPh3. On scanning to the ꢀve direction
up to ꢀ2.0 V, we observed a sharp anodic signal at ꢀ0.2, an irre-
versible response, Epc at ꢀ0.9 V and a quasireversible couple at
ꢀ1.2 V ( EP > 150 mV) (Fig. 7). The sharp anodic signal at ꢀ0.2 V
D
is possibly the Cu(I)/Cu(0) couple. The reduced Cu(0) is absorbed
on the electrode surface as is evident from the narrow width of
the anodic response with a large peak current. To account for the
Cu(I)/Cu(0) redox response we carried out a few more experiments
with varying scan rates (20–500 mV/s at 20 mV interval) and end
potential 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 a higher scan rate. This observation suggests that the
heterogeneous electron transfer rate is influenced by the applied
potential [74,75]. An interesting observation is recorded on chang-
ing the back scan potential; no cathodic signal is observed at a po-
tential above ꢀ0.4 V. On decreasing the back scan potential (ꢀ0.4
to ꢀ1.3 V), it is recorded that the intensity of the cathodic peak
at ꢀ0.2 V increases. It is quite obvious that the rate of reduction,
Cu(I) ? Cu(0), increases with decreasing back scan potential and
the amount of deposited Cu(0) increases. It is reoxidised on scan
reversal and shows a sharp peak. The reductive responses at ꢀ0.9
and ꢀ1.2 V may be assigned to reduction of 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.
Fig. 7. Cyclic voltammogram of [Cu(MeaaiEt)(PPh3)I] in MeCN using a Pt-disk
working electrode, Pt-wire auxiliary and SCE reference electrode in the presence of
[n-Bu4N](ClO4) supporting electrolyte at 50 mV Sꢀ1 scan rate, at 300 K.
may be used to explain this effect. The HOMO is composed of 31%
Cl in 4b; 65% Br in 7b and 83% I in 10b and the LUMO is mainly
dominated by MeaaiH (90%). The bonded X may serve as an anten-
na to receive a photon and to transfer energy to the participating
The DFT calculations of three representative complexes, 4b, 7b
and 10b, suggest that the HOMO is composed of Cu and X (vide su-
pra). Because of the higher iodide contribution to the HOMO (83%),
the redox response may be assigned to the I/Iꢀ couple at 0.4 V, fol-
lowed by the metal oxidation Cu(II)/Cu(I). The unoccupied MOs are
significantly dominated by the azoimine function of the chelated
ligand MeaaiH; thus the reduction is rightly referred to electron
accommodation at the azo dominated orbital of the ligand.
The [Cu(RaaiR0)(PPh3)Cl] complexes are oxidized to the Cu(II)
complexes by chlorine and are characterized by spectroscopic
and magnetic measurements as [Cu(RaaiR0)(PPh3)Cl2] (13, 14).
Again, the Cu(II) complexes are reduced by ascorbic acid to the
Cu(I) complexes. The Cu(II)–Cu(I) inter-conversion has been car-
ried out with [Cu(MeaaiMe)(PPh3)Cl] (5b) and [Cu(Meaai-
Et)(PPh3)Cl] (6b). Magnetic moment (room temperature) data
(13b, 1.83 BM; 14b, 1.88 BM) indicates a Cu(II) (d9) redox state,
which supports the composition [Cu(HaaiMe)(PPh3)Cl2] (13b)
and Cu(MeaaiMe)(PPh3)Cl2] (14b) at 300 K, and the complexes are
EPR active. The EPR spectra were recorded at liquid N2 temperature
groups. Thus, a function of higher X contribution and of lower
DE
may act as an efficient energy acceptor and communicator
[71,72]. Once the energy is transferred to the LUMO, the long
Cu–N(azo) bond may cleave (possibly the rate controlling factor)
to initiate photoisomerisation. This indeed happens. The t?c values
of the complexes are lower than that of the free ligand
(t?c = 0.25 0.03) [20,21]. The presence of the coordinated
{Cu(PPh3)X} motif may increase the molar mass and rotor volume
of the complexes and severely interfere with the motion of the –
N@N–Ar moiety. The rotor volume has a significant influence on
the isomerisation rate and quantum yields [23,73,74]. The com-
plexation increases the rotor volume, leading to less efficient
isomerisation.
The cis-to-trans (Z-to-E) isomerisation of the complexes has
been carried out by UV–Vis spectroscopy at various temperatures,
298–313 K. The Eyring plots in the range 298–313 K are used to

G. Saha et al. / Polyhedron 45 (2012) 158–169
167
Table 4
Cyclic voltammetric data.a
Complex
Metal redox couple E, V (
D
Ep, mV)
Ligand redox couple E, V (
D
Ep, mV)c
azo=/azoꢀ
X/Xꢀb
CuII/CuI
CuI/Cu0b
azo/azoꢀ
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
10a
1.04
1.03
1.06
1.05
1.06
1.05
1.00
1.06
1.07
1.05
1.07
1.02
1.00
(150)
1.06
(150)
1.10
(150)
1.07
(160)
1.09
(130)
1.07
(140)
ꢀ0.22
ꢀ0.21
ꢀ0.21
ꢀ0.22
ꢀ0.21
ꢀ0.22
ꢀ0.22
ꢀ0.21
ꢀ0.21
ꢀ0.22
ꢀ0.24
ꢀ0.26
ꢀ0.28
ꢀ0.90
ꢀ0.89
ꢀ0.88
ꢀ0.89
ꢀ0.88
ꢀ0.89
ꢀ0.90b
ꢀ0.89
ꢀ0.88
ꢀ0.89
ꢀ0.83
ꢀ0.84
ꢀ0.85
ꢀ1.25
ꢀ1.26
ꢀ1.25
ꢀ1.26
ꢀ1.25
ꢀ1.26
ꢀ1.25
ꢀ1.26
ꢀ1.25
ꢀ1.26
ꢀ1.23
ꢀ1.23
ꢀ1.24
0.39
0.40
0.41
0.33
0.30
0.32
10b
11a
11b
12a
12b
ꢀ0.24
ꢀ0.24
ꢀ0.23
ꢀ0.24
ꢀ0.28
ꢀ0.84
ꢀ0.79
ꢀ0.79
ꢀ0.79
ꢀ0.79
ꢀ1.24
ꢀ1.18
ꢀ1.15
ꢀ1.16
ꢀ1.18
a
Solvent: MeCN, Pt-disk working electrode, supporting electrolyte, TBAP (0.01 M); reference, SCE; solute concentration, 10ꢀ3 M; scan rate, 0.05 V sꢀ1
;
D
Ep = Epa ꢀ Epc, mV;
Epa = anodic peak potential, Epc = cathodic peak potential, V; E1/2 = 0.5(Epa + Epc), V.
b
Epa
.
c
Epc.
Fig. 9. Uv–Vis spectrum of [Cu(MeaaiMe)(PPh3)Cl2] (13b) in MeCN.
and 620–630 nm, which are referred to as d–d transitions of Cu(II)
complexes, and the higher energy bands correspond to n–p⁄ (380–
390 nm) and
445–450 nm is assigned to a MLCT (d
p–
p⁄ (280–300 nm) transitions. An intense band at
Fig. 8. EPR spectrum of [Cu(MeaaiMe)(PPh3)Cl2] (13b) in MeOH at 77 K (the
complex is obtained by the reaction of [Cu(MeaaiMe)(PPh3)Cl] with Cl2 in MeOH).
p
(Cu) ? p⁄ (azoimine))
transition.
The copper(II) complexes [Cu(HaaiMe)(PPh3)Cl2] (13b) and
Cu(MeaaiMe)(PPh3)Cl2] (14b) are reduced by ascorbic acid and
are identified by matching the spectroscopic and cyclic voltammet-
ric results. The compounds are diamagnetic. The structures of the
coordinated Cu(I) compounds have been established by their 1H
NMR spectra and a comparison with the spectra of the directly syn-
thesized complexes. Microanalytical data (C, H, N) also confirm the
composition of the compounds.
(77 K). The four line (63Cu, I = 3/2) EPR spectra are anisotropic at
higher magnetic field (Fig. 8). Three peaks are of low intensity in
the weaker field and are considered to originate from the g||
(2.240) component and g\ is 2.032. The relation g|| > g\ > 2.0023
2
agrees with the ground state configuration of dx 2. The solution
–y
spectrum (Fig. 9) shows two additional weak bands at 730–740

168
G. Saha et al. / Polyhedron 45 (2012) 158–169
Acknowledgements
Financial support from the Department of Science & Technology
and the University Grants Commission (under CAS programme),
New Delhi are thankfully acknowledged. One of us, G.S., thanks
the University Grants Commission, New Delhi for sanctioning a
project under the minor scheme. We sincerely thank Mr. Rajat
Saha, Department of Physics, Jadavpur University for his help.
Table 5
Summarized crystallographic data for [Cu(MeaaiH)(PPh3)Cl] (4b) and [Cu(MeaaiH)
(PPh3)Br] (7b).
Appendix A. Supplementary data
[Cu(MeaaiH)(PPh3)Cl]
[Cu(MeaaiH) (PPh3)Br]
(4b)
(7b)
CCDC 654254 and 654255 contain the supplementary crystallo-
graphic data for [Cu(MeaaiH)(PPh3)Cl] (4b) and [Cu(Mea-
aiH)(PPh3)Br] (7b). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.07.039.
Empirical formula
Formula weight
T (K)
C
28H25ClCuN4P
C28H25BrCuN4P
591.94
273(2)
547.48
290(2)
k (Å)
0.71073
triclinic
0.71073
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
ꢀ
P1
P1
9.0911(2)
9.6045(2)
14.9520(4)
98.6060(12)
93.5880(11)
92.5750(10)
1286.36(5)
2
9.0953(2)
9.6363(2)
15.2467(3)
98.9030(12)
93.8130(11)
93.9290(10)
1313.06(5)
2
a
(°)
b (°)
References
c
(°)
V (Å3)
[1] M. Adachi, T. Bredow, K. Jug, Dyes Pigm. 63 (2004) 225.
[2] P.F. Gordon, P. Gregory, Organic Chemistry in Color, Springer-Verlag, Berlin,
Heidelberg, New York, 1983.
Z
l
(Mo K
a
) (mmꢀ1
)
1.039
1.413
2.438
1.497
Dcalc (Mgmꢀ3
)
[3] S.R. Forrest, Org. Electron. 4 (2003) 45.
Crystal size (mm3)
0.10 ꢂ 0.15 ꢂ 0.28
ꢀ13 ꢆ h ꢆ 13,
ꢀ13 ꢆ k ꢆ 13,
ꢀ21 ꢆ l ꢆ 21
1.38–31.16
316
0.14 ꢂ 0.20 ꢂ 0.23
ꢀ14 ꢆ h ꢆ 14,
ꢀ15 ꢆ k ꢆ 15,
ꢀ23 ꢆ l ꢆ 23
2.15–34.01
317
[4] J. Veres, S. Ogier, G. Lloyd, D. de Leeuw, Chem. Mater. 16 (2004) 4543.
[5] C. Winder, N.S. Sariciftci, J. Mater. Chem. 14 (2004) 1077.
[6] N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki, K. Hara, J. Am. Chem.
Soc. 128 (2006) 14256.
[7] T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N.A. Anderson, X. Ai, T. Lian, S.
Yanagida, Chem. Mater. 16 (2004) 1806.
hkl range
h range (°)
Refine parameters
Total reflection
Unique data
8052
4525
10705
5492
[8] P. Qin, X. Yang, R. Chen, L. Sun, T. Marinado, T. Edvinsson, G. Boschloo, A.
Hagfeldt, J. Phys. Chem. C 111 (2007) 1853.
[9] M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen, Z. Li, J. Phys. Chem. C 111
(2007) 4465.
[10] Y. Ooyama, Y. Shimada, Y. Kagawa, Y. Yamada, I. Imae, K. Komaguchi, Y.
Harima, Tetrahedron Lett. 48 (2007) 9167.
[11] S. Tan, J. Zhai, H. Fang, T. Jiu, J. Ge, Y. Li, L. Jiang, D. Zhu, Chem. Eur. J. 11 (2005)
6272.
[12] X. Zhang, C. Li, W.-B. Wang, X.-X. Cheng, X.-S. Wang, B.-W. Zhang, J. Mater.
Chem. 17 (2007) 642.
[13] H. Otaka, M. Kira, K. Yano, S. Ito, H. Mitekura, T. Kawata, F. Matsui, J.
Photochem. Photobiol., A: Chem. 164 (2004) 67.
[14] S. Hwang, J.H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K.
Kim, N.-G. Park, C. Kim, Chem. Commun. (2007) 4887.
[15] J. Griffiths, Mol. Electron. Bioelectron. 3 (1992) 182.
[16] F. Szurdoki, D. Ren, D.R. Walt, Anal. Chem. 72 (2000) 5250.
[17] K.M. Wang, K. Seiler, B. Rusterholz, W. Simon, Analyst 117 (1992) 57.
[18] N. Malcik, O. Oktar, M.E. Ozser, P. Caglar, L. Bushby, A. Vaughan, B. Kuswandi,
R. Narayanaswamy, Sens. Actuators, B 53 (1998) 211.
[19] I. Nor, R. Ene, V. Hurduc, M. Bercea, J. Optoelectron. Adv. Mater. 10 (2008) 683.
[20] J. Otsuki, K. Suwa, K. Narutaki, C. Sinha, I. Yoshikawa, K. Araki, J. Phys. Chem. A
109 (2005) 8064.
[21] J. Otsuki, K. Suwa, K.K. Sarker, C. Sinha, J. Phys. Chem. A 111 (2007) 1403.
[22] H. Rau, in: H. Dürr, H. Bounas-Laurent (Eds.), Photochromism, Molecules and
Systems, Elsevier, Amsterdam, 1990, p. 165.
[23] H. Nishihara, Bull. Chem. Soc. Jpn. 77 (2004) 407.
[24] N. Tamai, H. Miyasaka, Chem. Rev. 100 (2000) 1875.
[25] S. Yagai, V. Karatsu, A. Kitamura, Chem. Eur. J. 11 (2005) 4054.
[26] M. Ire, Chem. Rev. 100 (2000) 1683.
[I > 2
r
r
(I)]
(I)]
a
R1 [I > 2
0.0361
0.0762
0.965
0.460
ꢀ0.693
0.0354
0.0954
0.925
0.611
ꢀ0.770
b
wR2
Goodness of fit
Dmax (eÅꢀ3
Dmin (eÅꢀ3
)
)
a
R =
wR = [
R
F0 – Fc/
R
R
F0.
w(F02 – Fc2)/
R
wF0
]
are general but
w
are different, w = 1/
b
4 1/2
[2(F2) + (0.0400P)2
P = (F02 + 2Fc2)/3.
]
for (4b); w = 1/[2(F2) + (0.0600P)2] for (7b) where
4. Conclusion
The mixed ligand complexes [Cu((RaaiR0)(PPh3)X] (RaaiR0 = 1-
alkyl-2-(arylylazo)imidazole) are described. Some of the com-
plexes have been structurally confirmed by single crystal X-ray
diffraction measurements. The complexes show a trans-to-cis
(E-to-Z) isomerisation of the coordinated azoimidazole upon light
irradiation in the UV range. Quantum yields (t?c) of photoisomer-
isation were calculated. The cis-to-trans (Z–E) isomerisation is a
thermally induced process. The activation energy (Ea) of isomerisa-
tion was calculated by a controlled temperature experiment. An in-
crease in electronegativity of X decreases the rate of isomerisation.
Upon treatment with a chlorine saturated alcohol solution, the
Cu(I) complexes are oxidized to the respective Cu(II) complexes,
which then can be further reduced by ascorbic acid back to the pre-
cursor Cu(I) complexes. DFT and TD-DFT calculations have been
used to determine the electronic structures, and these support
the spectra and cyclic voltammetric properties of the complexes.
[27] T. Ikeda, O. Tsutsumi, Science 268 (1995) 1873.
[28] B. Chand, U. Ray, G. Mostafa, T.-H. Lu, C. Sinha, J. Coord. Chem. 57 (2004) 627.
[29] U. Ray, D. Banerjee, G. Mostafa, T.-H. Lu, C. Sinha, New J. Chem. 28 (2004) 1432.
[30] J. Dinda, S. Jasimuddin, G. Mostafa, C.-H. Hung, C. Sinha, Polyhedron 23 (2004)
793.
[31] S. Jasimuddin, C. Sinha, Transition Met. Chem. 29 (2004) 566.
[32] B. Chand, U. Ray, G. Mostafa, J. Cheng, T.-H. Lu, C. Sinha, Inorg. Chim. Acta 358
(2005) 1927.
[33] J. Dinda, S. Senapoti, T. Mondal, A.D. Jana, M. Chiang, T.-H. Lu, C. Sinha,
Polyhedron 25 (2006) 1125.
[34] T. Mathur, U.S. Ray, B.N. Baruri, C. Sinha, J. Coord. Chem. 58 (2005) 399.
[35] S.S.S. Raj, H.-K. Fun, X.-F. Chen, X.-H. Zhu, X.-Z. You, Acta Crystallogr. C55
(1999) 1644.

G. Saha et al. / Polyhedron 45 (2012) 158–169
169
[36] M.N. Ackermann, M.P. Robinson, I.A. Maher, E.B. LeBlanc, R.V. Raz, J.
Organomet. Chem. 682 (2003) 248.
[37] M. Endo, K. Nakayama, Y. Kaida, T. Majima, Tetrahedron Lett. 44 (2003) 6903.
[38] N. Fukuda, J.Y. Kim, T. Fukuda, H. Ushijima, K. Tamada, Jpn. J. Appl. Phys. 45
(2006) 460.
[39] K.K. Sarker, B.G. Chand, K. Suwa, J. Cheng, T.-H. Lu, J. Otsuki, C. Sinha, Inorg.
Chem. 46 (2007) 670.
[40] K.K. Sarker, B.G. Chand, K. Suwa, J. Cheng, T.-H. Lu, J. Otsuki, C. Sinha, Inorg.
Chem. 46 (2007) 670.
[41] P. Pratihar, T.K. Mondal, A.K. Patra, C. Sinha, Inorg. Chem. 48 (2009) 2760.
[42] G. Boche, F. Basold, M. Marsch, K. Harms, Angew. Chem., Int. Ed. 37 (1998)
1684.
[43] H.-H. Li, Z.-R. Chen, J.-Q. Li, C.-C. Huang, Y.-F. Zhang, G.-X. Jia, Cryst. Growth
Des. 7 (2006) 1813.
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C.Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J.
Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas,
D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.
Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,
C. Gonzalez, J.A. Pople, Gaussian Inc., Wallingford CT, 2004.
[60] GaussView, Version 3.09, R. Dennington II, T. Keith, J. Millam, K Eppinnett, W.L.
Ken, R. Gilliland, Semichem Inc., Shawnee Mission, KS, 2003.
[61] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[62] R.E. Easton, D.J. Giesen, A. Welch, C.J. Cramer, D.G. Truhlar, Theor. Chim. Acta
93 (1996) 281.
[44] J.L. Zhou, Y.Z. Li, H.G. Zheng, X.Q. Xin, T. Yin, Y.X. Wang, Y.L. Song, Transition
Met. Chem. 29 (2004) 185.
[63] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270.
[45] S. Shi, W. Ji, S.H. Tang, J.P. Lang, X.Q. Xin, J. Am. Chem. Soc. 116 (1994) 3615.
[46] J.B. Howard, D.J. Low, Chem. Rev. 96 (1996) 2965.
[64] W.R. Wadt, P.J. Hay, J. Chem. Phys. 82 (1985) 284.
[65] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.
[47] R.H. Holm, Adv. Inorg. Chem. 38 (1992) 1.
[66] F.A. Cotton, Geoffrey Wilkinson, Carlos A. Murillo, Manfred Bochmann,
Advanced Inorganic Chemistry, sixth ed., Wiley, New York, 1999.
[67] K.K. Sarker, B.G. Chand, A.D. Jana, G. Mostafa, C. Sinha, Inorg. Chim Acta. 359
(695) (2005) 700.
[68] B.G. Chand, U.S. Ray, G. Mostafa, T.-H. Lu, L.R. Falvello, T. Soler, M. Tomas, C.
Sinha, Polyhedron 22 (2003) 3161.
[69] T. K. Misra, Ph. D. Thesis, Transition Metal Chemistry of 2-Arylazoimidazoles:
Synthesis, Characterisation and Electrochemical Studies, Burdwan University,
Burdwan, India, 1999.
[70] D.J. Fitzmaurice, M. Esche, H. Frei, J. Moser, J. Phys. Chem. 97 (1993) 3806.
[71] A.T. Hutton, H.M.N.H. Irving, J. Chem. Soc., Dalton Trans. (1982) 2299.
[72] G. Zentai, L. Partain, R. Pavlyuchkova, C. Proano, G. Virshup, L. Melekshov, A.
Zuck, B.N. Breen, O. Dagan, A. Vilensky, M. Schieber, H. Gilboa, P. Bennet, K.
Shah, Y. Dimitriev, J. Thomas, M. Yaffe, D. Hunter, Proc. SPIE MI (2003) 5030.
[73] T. Yutaka, M. Kurihara, H. Nishihara, Mol. Cryst. Liq. Cryst. 343 (2000) 193.
[74] T. Yutaka, L. Mori, M. Kurihara, J. Mizutani, K. Kubo, S. Furusho, K. Matsumura,
N. Tamai, H. Nishihara, Inorg. Chem. 40 (2001) 4986.
[48] P. Datta, P. Gayen, C. Sinha, Polyhedron 25 (2006) 3435.
[49] P. Datta, A.K. Patra, C. Sinha, Polyhedron 28 (2009) 525.
[50] T. Mathur, J. Dinda, P. Datta, J.-C. Liou, T.-H. Lu, C. Sinha, Polyhedron 25 (2006)
2503.
[51] T.K. Misra, D. Das, C. Sinha, Polyhedron 16 (1997) 4163.
[52] J. Dinda, U.S. Ray, G. Mostafa, T.-H. Lu, A. Usman, I. Abdul Razak, S.
Chantrapromma, H.-K. Fun, C. Sinha, Polyhedron 22 (2003) 247.
[53] Bruker, SMART and SAINT, Bruker AXS Inc., Madison, WI, USA, 1998.
[54] G.M. Sheldrick, SHELXS-97, Program for the Solution of Crystal Structure,
University of Gottingen, Germany, 1997.
[55] G.M. Sheldrick, SHELXL 97, Program for the Refinement of Crystal Structure,
University of Gottingen, Germany, 1997.
[56] A.L. Spek, PLATON, Molecular Geometry Program, University of Utrecht, The
Netherlands, 1999.
[57] L.J. Farrugia, ORTEP-3 for windows, J. Appl. Cryst. 30 (1997) 565.
[58] G. Zimmerman, L. Chow, U. Paik, J. Am. Chem. Soc. 80 (1958) 3528.
[59] Gaussian 03, Revision C.02, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E.
Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N.
Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
[75] E.A. Ambundo, M.-V. Deydier, A.J. Grall, N. Aguera-Vega, L.T. Dressel, T.H.
Cooper, M.J. Heeg, L.A. Ochrymowycz, D.B. Rorabacher, Inorg. Chem. 38 (1999)
4233.