Cu(II) complexes with square pyramidal (N2S)CuCl2 chromophore: Jahn-Teller distortion and subsequent effect on spectral and structural properties

Article abstract of DOI:10.1016/j.ica.2011.01.068

One monomeric neutral Cu(II) complex [(pmtpm)CuCl₂] (1) is reported by Lindoy and Livingstone [8]. Two new complexes namely, μ-Cl bridged binuclear Cu(II) complex [{(pmtpm)Cu(Cl)}₂ μ-Cl](ClO ₄) (2) and a bis μ-Cl bridged b

Full text of DOI:10.1016/j.ica.2011.01.068

Inorganica Chimica Acta 370 (2011) 247–253
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Cu(II) complexes with square pyramidal (N2S)CuCl2 chromophore: Jahn–Teller
distortion and subsequent effect on spectral and structural properties
Suprakash Roy a, Partha Mitra b, Apurba K. Patra a,
⇑
a Department of Chemistry, National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, India
b Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A and 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2010
Received in revised form 19 January 2011
Accepted 20 January 2011
Available online 2 March 2011
One monomeric neutral Cu(II) complex [(pmtpm)CuCl2] (1) is reported by Lindoy and Livingstone [8].
Two new complexes namely,
l
-Cl bridged binuclear Cu(II) complex [{(pmtpm)Cu(Cl)}2
l-Cl](ClO4) (2)
and a bis -Cl bridged binuclear Cu(II) complex [{(pmtpm)Cu}2(
l
l
-Cl)2](ClO4)2 (3) derived from a triden-
tate Schiff base ligand, 2-pyridyl-N-(20-methylthiophenyl)methyleneimine (pmtpm) were synthesized and
characterized by various spectroscopic methods and by X-ray crystallography. (N2S)CuCl2 chromo-
phore(s) of distorted square pyramidal coordination geometries around Cu(II) ion(s) have been observed
for all the complexes 1–3. The equatorial sites of the square plane comprise two N and a thioether S donor
atoms of the pmtpm ligand as well as one Clꢀ ion (terminal in 1 and 2, and bridging in 3) while the
remaining axial site is occupied by a terminal Clꢀ ion (for 1) or a bridging Clꢀ ion (for 2 and 3). The equa-
torial Cu–Cl distances are much shorter [1: 2.2511(4) Å, 2: 2.2307(12) Å, 3: 2.2513(12) Å] than the axial
Cu–Cl distances [1: 2.4394(4) Å, 2: 2.5597(9) Å, 3: 2.7037(12) Å]. The correlation of an axial Cu–Cl bond
elongation with a lower g|| value in the solid state EPR spectrum and a blue shifted ligand field transition
in the solid and solution phase absorption spectrum has been observed.
Keywords:
Mono-l-chloro and di-l-chloro bridged
binuclear copper(II) complexes
Tridentate Schiff’s base ligand
N,S donor ligand
Jahn–Teller distortion
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
molecular structures have been studied by Reinen and Atanasov
[4b]. Examples of penta-coordinated Cu(II) complexes are known
The Jahn–Teller effect is commonly encountered in octahedral
complexes of the transition metal ions with a d9, d7(low-spin)
and d4 (high-spin) electronic configuration [1]. As the Cu(II) ion
has a d9 electronic configuration (t26ge3g ), the Jahn–Teller effect is
found to be very often and prominent in case of octahedral Cu(II)
complexes [2]. A long list of such Cu(II) complexes with a variety
of coordination environment (CuO6, CuN6, CuS6, CuN4O2, CuO4N2,
and other) have been reported by Malcolm A. Halcrow [3] (see
the Supporting information of Ref. [3]). The existence of pseudo
or a second order Jahn–Teller effect is also known for the penta-
coordinated Cu(II) complexes where the two limiting geometries
such as square pyramidal (SP) or trigonal bipyramidal (TBP) is pos-
sible [4] depending on the nature of the ligand and its ligator
atoms. In these two limiting geometries the non-degenerate
ground electronic state is associated with a low lying degenerate
excited state and the distortion of symmetry will result in mixing
of the ground and excited states to lower down as a whole the en-
ergy of the ground state. The trigonal bipyramidal geometry is less
stable than the square pyramidal one due to pseudo Jahn–Teller
coupling; and the pseudorotation between these two limiting
with (CuN5) [5a and b], (CuN4O) [5c], (CuN3O2) [6] coordination
environment where axial elongation is structurally evident. In fact,
quite a large number of such Cu(II) complexes (penta or hexa-coor-
dinated) are reported as a single complex only where the Jahn–
Teller effect has been invoked to explain the elongation of axial
bond(s) in an octahedral, square pyramidal or trigonal bipyramidal
geometry. But with a single compound it is not possible to observe
the changes of spectral properties due to this Jahn–Teller effect,
unless otherwise a proper series of complexes with same coordina-
tion environment containing the same ligands are known. Such
series of complexes will enable one to correlate unambiguously
the spectral data with the extent of Jahn–Teller distortion
observed.
Literature survey of the chloro ligated Cu(II) complexes reveal at
least two such series of complexes [7]. D. Reinen and coworkers
have reported a series of square pyramidal Cu(II) complexes of
the general formula Cu(terpy)X2ꢁnH2O (where X = Clꢀ, Brꢀ, Iꢀ,
NO2ꢀ, NO3ꢀ, Fꢀ; and terpy = terpyridine) where the second order
Jahn–Teller effect has been well established for the two chloro li-
gated compounds [7a] such as Cu(terpy)Cl2 and Cu(terpy)Cl2ꢁH2O.
Here a water molecule as a solvent of crystallization, affects the
crystal structure, with the consequence of a more pronounced
elongation of the Cu–Cl bond relative to that of the other complex.
The UV–Vis and EPR spectral properties of these two complexes are
⇑
Corresponding author. Tel.: +91 0343 275 5475; fax: +91 0343 2547375.
E-mail addresses: apurba.patra@ch.nitdgp.ac.in, apurba_69@yahoo.com (A.K.
Patra).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.068

248
S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
methanolic solution (30 mL) of anh CuCl2 (0.59 g, 4.38 mmol). A
dark green color develops to which solid NaClO4 (1.07 g,
8.73 mmol) was added. The reaction mixture was stirred for 2 h
then kept undisturbed for slow evaporation. Within a day dark
green needle shaped crystals obtained was filtered off and washed
with cold MeOH, ether and dried under vacuum (yield: 1.3 g, ca
75%). Anal. Calc. for C26H24N4S2Cl4O4Cu2: C, 39.55; H, 3.06; N,
7.10. Found: C, 39.52; H, 3.01; N, 7.02%. IR (KBr, cmꢀ1, selected
peaks): 1596s imine C@N), 1100vs and 623 ClO4), 646 C–S). Molar
N
N
S
Me
pmtpm
Scheme 1. Chem draw structure of the tridentate Schiff base ligand
conductance, KM (MeCN, 298 K) = 130
X
ꢀ1 cm2 molꢀ1. Absorption
spectrum (in MeCN) [kmax, nm (
e
dm3 molꢀ1 cmꢀ1]: 752(260),
also reported. Inspecting the other series of Cu(II) complexes with
differing tridentate nitrogen donor ligand, bis(2-pyridyl-
methyl)amine, having N3Cl2 coordination zone [7b] around the
Cu(II) ion, only structural data have been published. In this paper,
we report the synthesis, structure and spectral properties (solid
and solution phase UV–Vis, EPR) including redox chemistry of
333(28 000), 275sh (19 860), 227(32 080). ESI-MS (positive ion
mode, MeCN): m/z 688.64 ([{(pmtpm)CuCl}2Cl]+, 25%), 325.81
([{(pmtpm)CuCl}2]2+, 100).
a
2.4. Synthesis of[{(pmtpm)Cu}2(l-Cl)2](ClO4)2 (3)
two new binuclear Cu(II) complexes [{(pmtpm)Cu(Cl)}2
l-Cl](ClO4)
To methanolic suspension/solution (10 mL) of
a
2 (0.1 g,
(2) and [{(pmtpm)Cu}2( -Cl)2](ClO4)2 (3) derived from a tridentate
l
0.126 mmol) was added drop wise a solution of (AgClO4)2ꢁC6H6
(0.062 g, 0.252 mmol) in 5 mL MeOH. The reaction mixture was re-
fluxed for 2 h. White precipitate of AgCl was formed. After cooling
the reaction mixture to room temperature AgCl was filtered off
through celite pad using a G-4 frit and filtrate was kept for slow
evaporation that affords green needle shaped crystals of 3 (yield:
0.11 g, ca 65%). Anal. Calc. for C26H24N4S2Cl4O4Cu2: C, 36.59; H,
2.83; N, 6.56. Found: C, 36.53; H, 2.81; N, 6.51%. IR (KBr, cmꢀ1, se-
lected peaks): 1594vs imine C@N); 1145vs, 1080vs and 624w
Schiff base ligand, 2-pyridyl-N-(20-methylthiophenyl)methylenei-
mine, pmtpm (Scheme 1). These complexes and a reported mono-
nuclear Cu(II) complex [(pmtpm)CuCl2] (1) with the same
N2SCuCl2 chromophore [8c] comprises a series, where the correla-
tion between the spectral properties and the extent of Jahn–Teller
distortion could be studied.
2. Experimental
ClO4),
298 K) = 245
nm (
dm3 molꢀ1 cmꢀ1: 657(190), 333(24 500), 275sh (23 900),
646w
C–S).
Molar
conductance, KM (MeCN,
cm2 molꢀ1. Absorption spectrum in MeCN [kmax
,
2.1. Materials
X
e
Pyridine-2-carboxaldehyde, 2-(methylthio)aniline, sodium per-
chlorate (NaClO4), Ag2O (Aldrich) were purchased and used with-
out further purification. Anhydrous CuCl2 was prepared from
CuCl2ꢁ2H2O (Aldrich) following the reported procedure [9]. (Ag-
ClO4)2ꢁC6H6 has been prepared by reacting Ag2O with HClO4 then
recrystallizing from benzene. Solvents used for spectroscopic stud-
ies and for synthesis were purified and dried by standard proce-
dures before used [10].
229(37 800). ESI-MS (positive ion mode, MeCN): m/z 325.83
([{(pmtpm)CuCl}2]2+, 100%).
Caution: Perchlorate salts are potentially explosive and should
be handled with small quantities and care. We did not encounter
any difficulties during the work.
2.5. Physical measurements
The Fourier transform infrared spectra of the ligand and the
complexes were recorded on a Thermo Nicolet iS10 spectropho-
tometer using KBr pellet in the range 4000–400 cmꢀ1. The solid
state diffuse reflectance spectra and the solution phase electronic
spectra were recorded on a Shimadzu UV-2401 PC and on an Agi-
lent 8453 diode array spectrophotometer, respectively. Elemental
analyses were carried out on a Perkin–Elmer 2400 series-II CHNS
Analyzer. Electron paramagnetic resonance (EPR) spectrum of 2
and 3 were recorded on the polycrystalline samples as well as in
MeCN solution with an X-band JEOL-JES-200 spectrometer at
298 K and 77 K. Mass spectra of 2 and 3 were recorded on Micro-
2.2. Synthesis of 2-pyridyl-N-(20-methylthiophenyl)-methyleneimine
(pmtpm)
The ligand has been synthesized by modifying the reported pro-
cedures [11,8c]. To a solution of pyridine-2-carboxaldehyde (1.0 g,
9.33 mmol) in 15 ml of freshly distilled methanol was added a
methanolic solution (15 mL) of 2-(methylthioaniline) (1.299 g,
9.33 mmol) and refluxed under N2 for 6 h. Solvent removal affor-
ded a thick yellow oil that was dissolved in 70 mL chloroform
and washed successively with distilled water, brine solution and fi-
nally with distilled water. The organic layer separated was then
dried over anh Na2SO4 for 1 h, filtered and solvent was removed
completely. The yellow oil kept at 4 °C for overnight to get a yellow
solid. The solid was then recrystallized from diethyl ether that
afforded yellow block/needle shaped crystals of the ligand pmtpm
after a day (yield: 1.80 g, ca. 84%).
mass Q-Tof microTM 1H NMR spectrum of pmtpm has been re-
.
corded on Bruker DPX-300. Solution conductivity and redox
potentials were measured using Eutech CON-510 and CHI 1120A
potentiometer, respectively.
2.6. Data collection and structure refinement
IR (KBr, m
max/cmꢀ1, selected peaks): 1622vs (C@N), 693 m (C–S).
1H NMR (300 MHz, CDCl3): d 8.70 (1H, d, pyridine proton adjacent
to N), 8.57 (1H, s, HC@N), 8.33 (1H, d, pyridine proton), 7.81 (1H, t,
pyridine proton), 7.37 (1H, t, pyridine proton), 7.28–7.16 (3H, m,
phenyl ring proton), 7.08 (1H, d, phenyl ring proton), 2.47 (3H, t,
methyl group of S–Me).
Diffraction intensities for the two complexes 2 and 3 were col-
lected at 150(2) K using a Bruker Smart APEX II CCD area detector
with Mo Ka radiation (k = 0.71073 Å). The cell refinement, indexing
and scaling of the data set were carried out using APEX2 v2.1-0 pro-
gramme [12]. Both structures were solved by direct methods with
SHELXS [13a], and refined by full-matrix least square based on F2
with SHELXL [13b] whereas the other calculations were performed
using the WINGX programme, Ver 1.80.05 [14]. All of the non-hydro-
gen atoms were refined anisotropically. The positions of the hydro-
gen atoms were calculated assuming ideal geometries of the atom
2.3. Synthesis of [{(pmtpm)Cu(Cl)}2l-Cl](ClO4) (2)
To a yellow solution of the ligand pmtpm (1 g, 4.38 mmol) in
150 mL of a mixed solvent CH2Cl2/MeOH (2:1 v/v) was added a

S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
249
Table 1
Data collection and structure refinement parameters for the complexes [{(pmtpm)
Cu(Cl)}2 -Cl](ClO4) (2) and [{(pmtpm)Cu}2( -Cl)2](ClO4)2 (3).
Table 2
Selected bond lengths (Å) and angles (°) of 1, 2 and 3.
l
l
Compound 1c
Compound 2
Compound 3
Complex
2
3
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–S(1)
Cu(1)–Cl(1)a
Cu(1)–Cl(2)b
Cu(1)–Cl(1)i
Cu(1)–Cu(1)i
C(6)–N(2)
2.0218(11)
2.0084(11)
2.3380(4)
2.2511(4)
2.4394(4)
–
2.0196(19)
1.9943(19)
2.3253(11)
2.2307(12)
2.5597(9)
–
2.020(3)
1.975(3)
2.3271(11)
2.2513(12)
–
2.7037(12)
3.416
1.280(3)
Chemical formula
Formula weight
Crystal system
Color
Space group
a (Å)
C
26H24N4S2Cl4O4Cu2 C26H24N4S2Cl4O8Cu2
789.5
triclinic
green
853.49
triclinic
green
ꢀ
ꢀ
P1
P1
–
5.119
1.286(4)
7.698(5)
8.269(5)
12.777(5)
83.349(5)
83.078(5)
66.989(5)
741.0(7)
1
1.769
1.978
0.714/0.717
398
8592/212
2606
8.284(4)
9.421(4)
11.442(5)
74.082(9)
70.352(10)
72.987(10)
788.7(6)
1
1.797
1.875
0.713/0.717
430
6836/209
2767
1.2889(17)
b (Å)
c (Å)
(o)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–S(1)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–S(1)
N(2)–Cu(1)–Cl(1)
S(1)–Cu(1)–Cl(1)
Cl(2)–Cu(1)–N(1)
Cl(2)–Cu(1)–N(2)
Cl(2)–Cu(1)–S(1)
Cl(2)–Cu(1)–Cl(1)
80.44(5)
162.50(4)
98.15(4)
80.89(8)
160.64(5)
97.47(7)
84.14(6)
159.93(5)
92.39(4)
101.06(5)
98.94(5)
93.31(4)
101.00(3)
81.62(11)
162.31(8)
97.80(8)
85.76(8)
173.30(8)
93.29(4)
106.74(8)
93.21(8)
86.22(4)
93.35(3)
a
b (o)
c
(o)
V (Å3)
84.06(3)
158.03(4)
93.178(14)
98.12(4)
96.11(4)
91.540(14)
105.756(16)
Z
Dcalc (g cmꢀ3
)
Absorption coefficient (mmꢀ1
)
)
Transmission factor (Tmin/Tmax
F(0 0 0)
Reflections/parameters
Unique reflections
a
b
c
Equatorial Cl(1) coordination.
Goodness-of-fit (GOF) on F2
1.052
1.127
Axial Cl(2) coordination (in case of compound 3, Cl(1)i is axial).
a
R1 [I > 2
r
]
0.0245
0.0658
0.0257
0.0666
0.47
0.0375
0.0976
0.0399
0.0990
0.66
Data from Ref. [8c].
b
wR2 [I > 2
r
]
R1 (all data)
wR2 (all data)
Highest peak
Deepest hole
time, the conversion of 1 to the cationic 2 with a Clꢀ counter anion
occurs that in presence ofꢀNaClO4 produce 2 with an exchanged,
preferred bigger size ClO4 counter ion. The halide displacement
from 2 by reacting with AgClO4 afforded 3 in high yield. Although
2 equivalent of AgClO4 is used in boiling MeOH the displacement of
one out of two terminal chloride ions from 2 has been observed.
This fact indirectly proved that the labile chloride dissociation of
3 in polar solvent like MeOH will be less than 2.
ꢀ0.48
ꢀ0.85
P
P
a
R = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
wR2 = { [w(F2o ꢀ F2c )2]/ w[(F2o )2]}1/2
.
b
concerned and their positions and thermal parameters were not re-
fined. The perchlorate anion of 2 was found to be disordered and
was solved considering two positions for each of the oxygen atom
with site occupancy factors of 0.5 so that the total multiplicity for
each oxygen atom is unity. The perchlorate Cl atom (Cl3) resides on
a center of inversion. The H atom on C6 is acidic and a very weak
hydrogen bonding to the two oxygen atoms of the perchlorate an-
ion is observed (average C6–O1, C6–O4 distance is 3.291 and C6–
O2, C6–O3 distance is 3.506 Å, respectively). On the other hand
no such disorder of the perchlorate anion is encountered for 3.
For 3 the inversion center is in the middle of the Cu2Cl2 plane
and symmetry operation will provide the full molecule view. Fur-
ther crystallographic analysis and ORTEP generation were carried
out using ORTEP programmes [15]. The crystallographic data such
as data collection parameters and other crystallographic informa-
tion for the complexes 2 and 3 are summarized in Table 1. Selected
bond lengths and angles are given in Table 2.
The IR spectrum of pmtpm display an intense stretch at
1622 cmꢀ1 and 693 cmꢀ1 correspond to mC=N [16] of imine and
m
C-S, respectively. The mC=N of pmtpm for 1 (1592 cmꢀ1) [8c], 2
(1594 cmꢀ1) and 3 (1596 cmꢀ1) are red shifted than the mC=N of
the free ligand (1622 cmꢀ1), indicating imine N coordination. Sim-
ilar red shift of mC–S from 693 cmꢀ1 (free pmtpm) to ꢂ645 cmꢀ1 in
complexes 1, 2 and 3 indicates the thioether S coordination to
Cu(II) ion. The intense and broad stretches for the perchlorate
counter anion have been found at ꢂ1100 cmꢀ1 and 623 cmꢀ1 for
both 2 and 3. The molar conductivity of 2 (KM = 130 Mꢀ1 cmꢀ1
)
and 3 (KM = 245 Mꢀ1 cmꢀ1) in MeCN are fairly consistent with a
1:1 and a 2:1 electrolyte, respectively [17]. The ESI positive mass
spectra of the complexes 2 and 3 were recorded in MeCN solution
that display the 100% molecular ion peaks at m/z, 325.81 and
325.83, respectively that corresponds to [{(pmtpm)Cu}2(l
-Cl)2]2+
for both the complexes. In case of 2 an additional less intense
3. Results and discussion
(ꢂ25%) peak at m/z 688.64 corresponds to [{(pmtpm)Cu(Cl)}2
l
-Cl]+ was observed (supporting information, Figs. S1 and S2 for
3.1. Synthesis and characterization of complexes
2 and 3, respectively) which is absent in the spectrum of 3. Based
on the microanalytical data in addition to the above facts, we
preliminarily proposed the formulation of these two new com-
Complex 1 was first reported by Lindoy and Livingstone [8a] in
1968 and later it was structurally characterized by A. Mangia and
coworkers [8b] in the year 1971. The better structure of 1 is re-
ported (same cell parameters and space group but with much low-
er R value) again in 2006 by Palaniandavar and coworkers [8c] with
it’s detailed spectroscopic studies. Structural and spectral data of 1
has been considered from Ref. [8c] for comparing to that of the
complexes 2 and 3 when necessary. Complex 2 has been prepared
by reacting the ligand and anh CuCl2 in a solvent mixture CH2Cl2/
MeOH (2:1 v/v) following the addition of NaClO4 and kept for slow
evaporation. Complex 2 can be synthesized separately from the
solution of 1 and NaClO4 in CH2Cl2/MeOH (2:1 v/v). Complex 1 is
insoluble in MeOH and moderately soluble in CH2Cl2. Conse-
quently, when a solution of 1 in CH2Cl2/MeOH store for prolong
plexes as [{(pmtpm)Cu(Cl)}2
-Cl)2](ClO4)2 (3), which were finally confirmed from their X-ray
crystal structure determination.
l-Cl](ClO4) (2) and [{(pmtpm)Cu}2
(l
3.2. Description of the structures of [{(pmtpm)Cu(Cl)}2
and [{(pmtpm)Cu}2( -Cl)2](ClO4)2 (3)
l-Cl](ClO4) (2)
l
Perspective views of the cationic part of the complexes 2 and 3
have been shown in Figs. 1 and 2, respectively along with atom
labeling scheme. Selected bond distances and angles are tabulated
in Table 2. For both the complexes the coordination geometry
around the Cu(II) ions are distorted square pyramidal as are evi-
denced from the values of trigonality index,
s (s for 1, 2 and 3

250
S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
much shorter [for 2: 2.2307(12) Å and for 3: 2.2513(12) Å] than
the axial Cu–Cl distances [for 2: 2.5597(9) Å and for 3:
2.7037(12) Å]. The Cu(II) ions are not perfectly situated in the
square plane and it is 0.322 Å and 0.184 Å out of plane towards
the axial Clꢀ ion in 2 and 3, respectively. X-ray structure of 2 and
3 clearly reveals that two square pyramids are fused together at
the apex in 2 and at the base-to-apex edge in 3 (Scheme 2).
The equatorial and axial Cu–Cl distances of 2 are comparable to
those of the similar type reported complexes [7,19], where square
pyramidal coordination environment around Cu(II) has been ob-
served with a tridentate supporting ligand along with terminal/
bridging chloride ions. However, Cu–Cl–Cu angle of 2 (180°) differs
noticeably than the reported complex [Cu2(bzpy)2Cl3]ClO4ꢁ0.5H2O
[19] where it is 136.9(1)°. For other two reported oligomeric com-
plexes [7b] this angle ranges from 144.57(6)° to 144.69(6)°. Other
one penta-coordinated mono-chloro bridged dicopper(II) complex,
[Cu2(py2)2Cl3](PF6) (where py2 is bis(2-(2-pyridyl)ethylamine)
[20] with the Cu(II) ions at a distorted trigonal bipyramidal geom-
etry is also known where the long Cu–Cl distance of 2.549 Å and a
Cu–Cl–Cu bridging angle of 178.0(1)° is observed.
Fig. 1. ORTEP view of the cationic part of 2. The thermal ellipsoids are at the 50%
probability scale and the hydrogen atoms are omitted for clarity.
The Cu–Cl distances of 2.2513(12) Å and 2.7037(12) and the
Cu–Cl–Cu angle of 93.35(3)° in 3 are fairly consistent with the cor-
responding Cu–Cl distances and the Cu–Cl–Cu angles of the similar
bis-l-chloro-bridged dicopper(II) complexes with a supporting
bidentate [21] or tridentate ligand [22].
are 0.07, 0.01 and 0.18, respectively) defined as
where h and are the two largest coordination angles. The ex-
pected values of are 0 and 1 for perfect square-pyramidal (SP)
s
= [|h ꢀ
u|/60],
u
The Cu–Npy, Cu–Nimine [8c,21,22] and Cu–Sthioether [8c,23] dis-
tances for both 2 and 3 are within the ranges found for other re-
ported complexes. The Cu–Nimine distances trans to the equatorial
Cu–Cl bond follow a trend of shorter distances like 2 > 3 [for 2:
1.9943(19) Å and for 3: 1.975(3) Å]. Due to this trans effect the
equatorial Cu–Cl distance of 3 (2.2513(12) Å) is 0.021 Å longer than
that of 2 (2.2307(12) Å) is observed.
It is necessary to mention, that 1 and 3 are the two extreme
structures so far the axial Cu–Cl elongation is concerned. The equa-
torial Cu–Cl distances for 1 [2.2511(4) Å] and 3 [2.2513(12) Å] are
almost equal while all the donor atoms of pmtpm to Cu(II) dis-
tances are shorter in 3 than 1 (see Table 2). This situation gives
s
and ideal trigonal-bipyramidal (TBP) coordination geometry,
respectively [18]. The pmtpm ligand coordinate meridionally to
the central Cu(II) ion like other reported complexes of this ligand
[8c]. Three corners of the square plane are occupied by the pyridine
nitrogen N(1), imine nitrogen N(2) and the thioether sulfur S(1)
atoms. The remaining one corner of the square plane is occupied
by a chloride ion, namely Cl(1) which is terminally coordinated
in case of 2 while it is a bridging chloride ion in case of 3. The
bridging chloride ion Cl(2) in 2 and the symmetry generated
Cl(1)i in 3 are situated at the apex of the square pyramidal struc-
tures. The equatorial Cu–Cl distances in both the complexes are
Fig. 2. ORTEP view of the cationic part of 3. The thermal ellipsoids are at the 50% probability scale and the hydrogen atoms are omitted for clarity.

S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
251
N2i
S1i
N1i
Cu1i
Cl1i
S1i
Cu1i
N1i
Cl1i
N2i
Cl2
N1
Cu1
Cl1
Cl2
N2
Cl1
S1
Cu1
N1
S1
N2
Cl1
3
S1
Cu1
N2
N1
1
2
Scheme 2. Arrangement of polyhedra (square pyramids) for 1, 2 and 3: for 2 apex [Cl2] sharing and for 3 apex-to-base edge [Cl1–Cl1i] sharing, atom labeling is according to
the X-ray structures.
6x104
distance) is 95 nm and 67 nm blue shifted than that of the com-
plexes 2 (752 nm) and 1 (724 nm) [8c], respectively. Similar trend
in the solid state diffuse reflectance spectrum of 2 and 3 has been
found that display the broad d-d band centered at 738 nm and
678 nm, respectively (shown in the inset of Fig. 3; for larger view
see supporting information: Fig. S3). These broad bands are the en-
500
0.40
0.35
2
0.30
5x104
0.25
400
0.20
0.15
3
4x104
0.10
0.05
300
0.00
3
450 500 550 600 650 700 750 800
velop of the transitions from B1 (d
) ground state to the excited
x2ꢀy2
3x104
2x104
λ / nm
A1 (d2), B2 (dxy) and E (dxz, dyz) states in C4v symmetry. A weaken-
z
2
200
100
0
ing of the axial bond along z-axis is expected to shift the absorp-
tions involving electronic z-components to higher energies. Thus
2
3
1x104
0
200
400
600
/ nm
800
1000
λ
Fig. 3. Electronic absorption spectra of 2 and 3 in MeCN (the inset shows the solid
state reflectance spectra of 2 and 3).
Table 3
Absorption spectral data for the complexes 1–3.
a
Complex kmax (nm) (
e
, Mꢀ1 cmꢀ1
)
in MeCN solution
kmax (nm) in
solid state
1b
2
238(8385), 277(6255), 332(11340), 724(60)
227(32080), 275 (sh, 19860), 333(28000),
752(260)
746
738
Fig. 4. X-band EPR spectra of (a) powder sample and (b) MeCN solution of 3 at 77 K.
3
229(37800), 275 (sh, 23900), 333(24500),
657(190)
678
a
e
(Mꢀ1 cmꢀ1) values for 2 and 3 are per dimer.
b
Data from Ref. [8c].
-20
-10
us an opportunity to conclude that pmtpm acts as a stronger donor
ligand in 3 than in 1. The greater is the in plane ligand donor
strength the longer will be the axial Cu–Cl bond. In fact, with no
exception here, 0.264 Å longer Cu–Claxial bond is found in 3
(2.7037(12) Å) than in 1 (2.4394(4) Å).
0
A
10
3.3. Electronic spectra
C
B
The electronic spectra of 2 and 3 in MeCN solution and their so-
lid state diffuse reflectance spectra were shown in Fig. 3. The spec-
tral data have been tabulated in Table 3. Complexes 2 and 3 display
a lower energy band at 752 nm and 657 nm, respectively with low
extinction coefficient values that correspond to the d–d transition.
This d–d transition band at 657 nm for 3 (longest Cu–Claxial bond
D
-0.4
20
0.0
0.4
0.8
1.2
E (V)
Fig. 5. Cyclic voltammograms (scan rate 50 mV/s) of 3 in MeCN solution of 0.1 M
TBAP, using platinum working electrode. Potentials are vs. non-aquous Ag/Ag+.

252
S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
S
S
CuII
N
N
N
CuI
Cl
N
N
Cl
+ e-
- e-
CuII
S
CuII
S
N
Cl
MeCN
B
+ e-
Cl
N
N
N
S
N
A
N
=
N
S
S
N
N
Me
N
N
CuI
Cl
+ e-
- e-
Cl
CuI
CuI
N
N
Cu0 +
+
Cl
N
N
Association
Cl
MeCN
C
S
S
S
D
Scheme 3. Structures of the complexes involved in the most plausible pathway of consecutive electron transfer reaction for 3 in MeCN.
trend will be followed [7,25]. The g values for these complexes
are comparable to other reported binuclear bis
l-chloro bridged
Table 4
Electrochemical dataa for the complexes 2 and 3.
Cu(II) complexes [21,22]. The g|| and g? values reported for the
complex 1 measured in the polycrystalline form at 77 K are 2.202
and 2.041, respectively thus the solid state g|| values decreases in
the order 1(2.202) > 2(2.183) > 3(2.166). Thus the larger axial Cu–
Cl distance, smaller g|| value and the blue shifted ligand field tran-
sition have been observed for the complexes 1–3.
Complex
Epc (V)
Epa (V)
D
Ep (mV)
E1/2 (V)
ipa/ipc
1b
2
0.132
0.192
ꢀ0.290
ꢀ0.591
0.183
0.218
0.295
–
ꢀ0.230
0.299
–
86
103
–
361
116
–
0.175
0.243
–
ꢀ0.410
0.241
–
1.1
0.95
–
3
1.1
ꢀ0.296
3.5. Redox chemistry
ꢀ0.493
ꢀ0.215
278
ꢀ0.354
Potentials are vs. non-aqueous Ag/Ag+ reference electrode. Fc/Fcþ couple in
a
The cyclic voltammograms (CV) of the complexes 2 and 3 were
recorded in MeCN solvent at room temperature. Three electrode
cell set up such as platinum, Ag/Ag+ (non-aquous) and a platinum
wire as a working, reference and auxiliary electrode, respectively
have been used for measurements. The cyclic voltammograms of
one representative complex 3 have been shown in Fig. 5 and the
electrochemical data have been tabulated in Table 4. The com-
plexes 2 and 3 exhibit a quasireversible reductive response at
E1/2 value ꢄ +0.24 V versus Ag/Ag+ (non-aquous) correspond to
Cu(II) to Cu(I) reduction. The appearance of same potential
tempted us to examine whether it originates from the ligand oxi-
dation/reduction or not. The CV of a mixture of ZnCl2 and ligand
(1:1 ratio) in MeCN does not display any response1 at 0.24 V indi-
cating that this E1/2 ꢄ 0.24 V response is not due to the ligand cen-
tered oxidation or reduction. Small differences in the DEp values
(86 mV, 103 mV and 116 mV for 1, 2 and 3, respectively) have been
observed that increases in the order 1 < 2 < 3. The more DEp value
indicates more structural reorganization upon reduction to Cu(I).
Thus the lowest DEp value for 1 is reasonable as it can adopt a tet-
rahedral structure (with a dangling donor atom like shown in
Scheme 3) for Cu(I) state more easily with two terminal chloride
ions in it’s coordination zone (less restrain) unlike the cases ob-
served for 2 and 3 which are mono and doubly chloro bridged,
respectively thus more and more restrain imposed to attain the
tetrahedral geometry for reduced Cu(I) state.
MeCN, E1/2 = 0.436 V, scan rate 50 mV/s, supporting electrolyte: tetra-N-butylam-
monium perchlorate (0.1 M).
b
Data from Ref. [8c].
for the solid state reflectance spectra at high temperature (298 K),
the energy-averaged d–d transitions at 746, 738 and 678 nm for 1,
2 and 3 were observed which is the expected sequence. The elec-
tronic spectra of a Cu(II) complex namely, [Cu(L5)2(H2O)].H2O
(where HL5 is N-(2-chloro-6-methylphenyl)pyridine-2-carboxamide
and H is the dissociable amide proton) in two different geometries
such as trigonal bipyramidal (solid state) and square pyramidal
(solution phase) has been reported by Mukherjee and coworkers
[24].
The higher energy band at ꢂ330 nm for all three complexes
with high extinction coefficient values are due to the coordinated
thioether, S(r) ? Cu(II) charge transfer (LMCT) transition. The
other higher energy intense transitions at 275 nm and 227 nm
are due to the intraligand
sitions (ILCT).
p ?
p⁄ and n ? p⁄ charge transfer tran-
3.4. EPR spectra
The EPR spectra of the complexes 2 and 3 in polycrystalline so-
lid state and in MeCN solution at 77 K and 293 K have been mea-
sured. The solid sample of 2 display two peaks at g values 2.183
and 2.053 corresponds to g|| and g?, respectively, while in MeCN
solution it display an isotropic signal at g = 2.087. Complex 3 dis-
play similar EPR spectra (77 K, solid state: g|| = 2.166, g? = 2.057;
77 K in MeCN: g = 2.079 isotropic signal) that have been shown
Other two irreversible reductive responses at Epc values
ꢀ0.296 V and ꢀ0.493 V for 3 have been observed correspond to
B ? C and C ? D transformation according to the most possible
Scheme 3 (the possibility of dangling pyridine N instead of thioe-
ther S might also be possible as both will prefer Cu(I) state almost
equally) shown below. The reverse scan from ꢀ0.6 V towards
+1.1 V shows the anodic stripping potential at ꢀ0.215 V for 3
which is due to metallic Cu(0) to Cu(I) re-oxidation.
in Fig. 4. The forbidden singlet-triplet transition (
detected for a dimeric copper(II) sample is only observable with
high instrumental gain. This spectral behavior of the complexes
D
Ms = ꢃ2), often
are attributed to an axial symmetry with dx2
ground state [25]
ꢀy2
1
No increase of current has been observed in the range +0.8 V to –0.8 V. The Cl-/Cl2
having elongated octahedral, square pyramidal or square planar
coordination geometry around the Cu(II) ion. For a trigonal-bipyra-
midal coordination geometry around Cu(II) ion the g? > g||ꢄg0
oxidation at 1.1 V has been observed indicates the Clꢀ liberation from the ZnCl2 and
ligand reaction in MeCN medium. This Clꢀ to Cl2 oxidation has been confirmed from
the CV measurement of LiCl in MeCN that display this Clꢀ/Cl2 potential at 1.1 V.

S. Roy et al. / Inorganica Chimica Acta 370 (2011) 247–253
253
According to the scheme the reduction potential from A ? B
References
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can be adopted to make it same through the Clꢀ bridge that com-
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ꢀ0.591 V) than 3 (Epc value is ꢀ0.491 V). Cu(ClO4)2 in organic sol-
vent display similar cyclic voltammograms [26]. The electrochem-
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Financial support from the Department of Science and Technol-
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for their helps with EPR spectral measurement and X-ray structure
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CCDC 776513 and 776514 contain the supplementary crystallo-
graphic data for complexes 2 and 3, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
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