Synthesis, structure, spectra and redox of Cu(II) complexes of chelating bis(benzimidazole) - Thioether ligands as models for electron transfer blue copper proteins

Article abstract of DOI:10.1039/b103511h

The tridentate ligand 1,5-bis(benzimidazol-2-yl)-3-thiapentane (L¹) with N₂S donor set forms the complex [Cu(L¹)-(H₂O)Cl]Cl 1a and the linear quadridentate ligand 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (L²) with N₂S₂ donor set forms the complexes [Cu(L²)](ClO₄)₂·2H₂O 2a and [Cu(L²)(NO3)]NO₃ 2b. The linear pentadentate ligand 1,11-bis(pyrid-2-yl)-3,6,9-trithiaundecane (L³) with N₂S₃ donor set forms the complex [Cu(L³)](ClO₄)₂ 3. The perchlorate complex [Cu(L⁴)](ClO₄)₂·2CH₃CN 4 of the pentadentate ligand,N,N-bis(benzimidazol-2-ylmethylthioethyl)methyl-amine (L⁴) with N₃S₂ donor set has also been isolated. In 1a Cu(II) is coordinated to the two benzimidazole nitrogens and thioether sulfur of the ligand L¹, a chloride ion and a water molecule. The coordination geometry around copper is intermediate between trigonal bipyramidal and square pyramidal geometries and is better described as trigonal bipyramidal distorted square based pyramidal (TBDSBP) with the sulfur and nitrogen atoms and the chloride ion in the equatorial positions and the oxygen of water in the apical position. The coordination geometry around copper(II) in 2b is best described as trigonal bipyramidal, with both the thioether sulfur atoms [Cu-S(1), 2.529(5) and Cu-S(2), 2.438(6) A] and one of the oxygen atoms of the nitrate ion [Cu-O(1), 2.066(13) A] constituting the trigonal plane and both the benzimidazole nitrogens [Cu-N, 1.985(14) and 1.953(13) A] occupying the axial positions. The bulky benzimidazole moieties of the ligand prevent the other nitrate ion from coordinating and favours trigonal bipyramidal geometry in spite of the presence of two six-membered chelate rings. In 4 the coordination plane of Cu(II) is comprised of two benzimidazole nitrogens, one thioether sulfur and N-methyl substituted amine nitrogen atom with the other thioether sulfur atom coordinated axially. The coordination geometry is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). The ligand field and EPR spectra of 1a, 2a and 2b are consistent with trigonal bipyramidal geometry in the solid state, whereas two ligand field bands in solution and an axial EPR spectrum in frozen solution were observed suggesting a change in coordination geometry to a square-based one on dissolution. The complexes 3 and 4 exhibit only one ligand field band in the solid state and axial EPR spectrum consistent with a square based geometry. All the complexes exhibit an intense S(σ)→Cu(II) CT band in the range 330-380 nm and a high positive Cu^II/Cu^I redox potential.

Full text of DOI:10.1039/b103511h

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Synthesis, structure, spectra and redox of Cu(II) complexes of
chelating bis(benzimidazole)—thioether ligands as models for
electron transfer blue copper proteins†
Mathrubootham Vaidyanathan, Ramalingam Balamurugan, Usha Sivagnanam and
Mallayan Palaniandavar*
Department of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India.
E-mail: palani@bdu.ernet.in
Received 19th April 2001, Accepted 5th September 2001
First published as an Advance Article on the web 15th November 2001
The tridentate ligand 1,5-bis(benzimidazol-2-yl)-3-thiapentane (L1) with N2S donor set forms the complex [Cu(L1)-
(H2O)Cl]Cl 1a and the linear quadridentate ligand 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (L2) with N2S2 donor
set forms the complexes [Cu(L2)](ClO4)2ؒ2H2O 2a and [Cu(L2)(NO3)]NO3 2b. The linear pentadentate ligand 1,11-
bis(pyrid-2-yl)-3,6,9-trithiaundecane (L3) with N2S3 donor set forms the complex [Cu(L3)](ClO4)2 3. The perchlorate
complex [Cu(L4)](ClO4)2ؒ2CH3CN 4 of the pentadentate ligand, N,N-bis(benzimidazol-2-ylmethylthioethyl)methyl-
amine (L4) with N3S2 donor set has also been isolated. In 1a Cu() is coordinated to the two benzimidazole nitrogens
and thioether sulfur of the ligand L1, a chloride ion and a water molecule. The coordination geometry around copper
is intermediate between trigonal bipyramidal and square pyramidal geometries and is better described as trigonal
bipyramidal distorted square based pyramidal (TBDSBP) with the sulfur and nitrogen atoms and the chloride ion in
the equatorial positions and the oxygen of water in the apical position. The coordination geometry around copper()
in 2b is best described as trigonal bipyramidal, with both the thioether sulfur atoms [Cu–S(1), 2.529(5) and Cu–S(2),
2.438(6) Å] and one of the oxygen atoms of the nitrate ion [Cu–O(1), 2.066(13) Å] constituting the trigonal plane
and both the benzimidazole nitrogens [Cu–N, 1.985(14) and 1.953(13) Å] occupying the axial positions. The
bulky benzimidazole moieties of the ligand prevent the other nitrate ion from coordinating and favours trigonal
bipyramidal geometry in spite of the presence of two six-membered chelate rings. In 4 the coordination plane of
Cu() is comprised of two benzimidazole nitrogens, one thioether sulfur and N-methyl substituted amine nitrogen
atom with the other thioether sulfur atom coordinated axially. The coordination geometry is best described as
trigonal bipyramidal distorted square based pyramidal (TBDSBP). The ligand field and EPR spectra of 1a, 2a and
2b are consistent with trigonal bipyramidal geometry in the solid state, whereas two ligand field bands in solution
and an axial EPR spectrum in frozen solution were observed suggesting a change in coordination geometry to a
square-based one on dissolution. The complexes 3 and 4 exhibit only one ligand field band in the solid state and
axial EPR spectrum consistent with a square based geometry. All the complexes exhibit an intense S(σ)
Cu() CT
band in the range 330–380 nm and a high positive CuII/CuI redox potential.
dentate bis(benzimidazol-2-yl)dithioether ligands,6–10 as they
offer a CuN2S2 chromophore as in blue proteins. A remarkable
feature of this class of complexes is that many of them possess
a trigonal bipyramidal component while the corresponding
4-imidazolyl ligand complexes form a surprising number of
‘compressed’ octahedral structures10,11 illustrating the import-
ance of the bulky benzimidazole (bzim) moieties. Thus in the
CuN2S2 complex [Cu(bbdh)Cl]Cl [bbdh = 1,6-bis(benzimidazol-
2-yl)-2,5-dithiahexane]6 with a 555 chelate ring system a
trigonal bipyramidal distorted square based pyramidal
(TBDSBP)12 geometry is observed. On adding a methylene
group between the thioether sulfurs in this complex to obtain
[Cu(bmdhp)](ClO4)2 [bmdhp, 1,7-bis(N-methylbenzimidazol-2-
yl)-2,6-dithiahexane]8 the geometry becomes more trigonal
bipyramidal distorted square based pyramidal (TBDSBP)12
and significant changes in spectral and redox properties13 are
observed. So, it is of great interest to probe the consequences of
enlarging the ligand by varying the chelate ring sizes on struc-
ture and spectral and redox properties of CuN2S2 complexes.
In the present report the consequences of incorporating one
more methylene group between sulfur and benzimidazole
donors in bbdh to generate the ligand 1,8-bis(benzimidazol-2-
yl)-3,6-dithiaoctane (bbdo, L2, Scheme 1) and removing one
thioether sulfur donor from L2 to obtain the ligand 1,5-
Introduction
The active sites in blue copper proteins contain two histidine
imidazoles and a cysteine thiolate coordinated to Cu() and a
methionine thioether present at a long but bonding distance
with a geometry closer to trigonal (bi)pyramidal.1–3 They are in
fact variable with subtle changes in their copper coordination
geometry, which determine the spectral and redox properties
of the proteins. Even though there is a possibility that the
apical thioether ligand may control the rather high CuII/CuI
redox potential of the proteins, its definite role still remains
ambiguous.4 So the synthesis and study of structure, spectra
and redox properties of Cu() complexes as models for the
proteins are essential to further address the structure/redox
potential relationship.5
There is a growing interest in the Cu() complexes of tetra-
† Dedicated to Professor P. Natarajan on the occasion of his sixtieth
anniversary.
Electronic supplementary information (ESI) available: Fig. S1 show-
ing the preliminary X-ray structure of the [Cu(L3)]ϩ cation, Figs. S2, S4
and S5 showing the cyclic and differential pulse voltammograms of
complexes 1a, 3 and 4 respectively and Fig. S3 showing the cyclic vol-
tammograms of complexes 2a and 2b. See http://www.rsc.org/suppdata/
dt/b1/b103511h/
3498
J. Chem. Soc., Dalton Trans., 2001, 3498–3506
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103511h

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recrystallised from aqueous ethanol and tested for the absence
of bromide. Methanol for electrochemistry was distilled over
magnesium turnings and acetonitrile over P4O10.
Syntheses of ligands
The ligands 1,5-bis(benzimidazol-2-yl)-3-thiapentane (L1)21
and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (L2)13 were
synthesised as reported previously.
1,11-Bis(pyrid-2-yl)-3,6,9-trithiaundecane (L3, pttu). The
ligand pttu was prepared by adding 2-vinylpyridine (2.1 g,
20 mmol) slowly with stirring to bis(2-mercaptoethyl)sulfide
(1.5 g, 10 mmol) and stirring continued for two hours. To
the resulting solution was added water with stirring and the
precipitate obtained was collected, recrystallised from aqueous
1
ethanol and dried over P4O10. Yield, 67%, mp 63–65 ЊC. H
NMR (DMSO-d6): δ 2.45 (s, 2 –SCH2CH2S–), 3.40 (t, –SCH2–),
3.48 (t, –CH2py), 7.54–7.69 (m, py-3,5 protons), 8.02 (m, py-4
protons) and 8.95 (d, py-6 protons). Found: C, 59.5; H, 6.73;
N, 7.53. Calc. for C18H24N2S3: C, 59.3; H, 6.63; N, 7.68%.
Scheme 1
N,N-Bis(benzimidazol-2-ylmethylthioethyl)methylamine (L4).
N-Methyldiethanolamine (5.96 g, 0.05 mol) was suspended
in dichloromethane (50 ml) and an excess of thionyl chloride
(23.8 g, 2.0 mol) was added dropwise with constant stirring
at 0 ЊC. The reaction mixture was refluxed for an hour and
then cooled. The excess thionyl chloride was removed by adding
a small amount of methanol, the bis(chloroethyl)-N-methyl-
amine hydrochloride formed was collected and dried over
CaCl2 in vacuum. Sodium hydroxide (0.80 g, 20 mmol)
and 2-mercaptomethylbenzimidazole16 (4.02 g, 20 mmol)
were combined in ethanol (50 ml) and the mixture stirred for
30 minutes under nitrogen. Then an ethanol solution (30 ml)
of bis(chloroethyl)-N-methylamine hydrochloride (1.93 g,
10 mmol) was added to this and the resulting solution mixture
was stirred for an hour and then refluxed for an hour. The
pH of the resulting mixture was adjusted to 9.0 with sodium
hydroxide solution. The volume of ethanol was reduced after
treatment with charcoal, by rotary evaporation and 100 ml of
water was added. The oily product obtained was used as such
for the complex preparation without further purification.
bis(benzimidazol-2-yl)-3-thiapentane (bbes, L1, Scheme 1) on
the structure and spectral and electrochemical behaviour of
their Cu() complexes are discussed. The present work also
provides an opportunity to gain insight into the relative
importance of the contributions from the coordination of
thioethers14–16 and the benzimidazole,13,16,17 both of which are
known to contribute to the positive CuII/CuI redox potential.
Addison et al. have studied Cu() complexes of linear N2S3
ligands containing pyridyl,18 quinolyl18 and benzimidazolyl13
termini. Recently Liu et al. and Adhikary and Lucas have
described the structure and properties of Cu() complexes
of trithioether ligands containing substituted pyrazolyl19
[1,9-bis(3,5-dimethyl-1-pyrazolyl)-2,5,8-trithianonane, pzttn],
benzimidazolyl19
[1,9-bis(3,5-benzimidazol-2-yl)-2,5,8-tri-
thianonane, bzttn] and pyridyl19,20 [(1,9-bis(pyrid-2-yl)-2,5,8-
trithianonane, pttn] moieties. Recently we have described17 the
effect of incorporating increasing numbers of sulfur donors
and varying chelate ring size on the structure and spectra of
copper() complexes of bis(benzimidazol-2-yl)tetra- and penta-
thioether ligands. The present work deals with our attempt to
understand the effect of ligand enlargement on the spectral
and redox properties of the Cu() complex of the new ligand
1,11-bis(pyrid-2-yl)-3,6,9-trithiaundecane (pttu, L3, Scheme 1)
generated by addition of one more methylene group between
pyridyl (py) and sulfur in pttn, which has also been isolated
and investigated. Also we have enlarged the bbdh ligand by
incorporating an amino nitrogen donor in between the two
thioether sulfurs to obtain the ligand L4 (Scheme 1) and its
copper() complex and such a study would also enable us
to understand the effect of the ligand enlargement and
incorporating the third thioether sulfur on the spectral and
electrochemical properties.
Preparation of complexes
The complexes [Cu(L1)(H2O)Cl]Cl 1a, [Cu(L2)](ClO4)2ؒ2H2O
2a,13 [Cu(L2)(NO3)]NO3 2b and [Cu(L3)](ClO4)2 3 and [Cu(L4)]-
(ClO4)2ؒ2CH3CN 4 were obtained by adding the corresponding
Cu() salt in methanol to a methanolic solution of the ligands
in 1 : 1 mole ratio and then cooling the solutions. The products
formed were filtered, washed with a small amount of cold
methanol and the complex then dried over P4O10. Found:
C, 43.42; H, 4.32; N, 11.78. Calc. for C18H20N4OSCl2Cu (1a) C,
43.53; H, 4.24; N, 11.80%. Found: C, 37.5; H, 3.72; N, 8.75.
Calc. for C20H22S2N4O8Cl2Cu (2a) C, 37.2; H, 3.44; N, 8.69%.
Found: C, 41.8; H, 3.96; N, 14.4. Calc. for C20H22S2N6O6Cu (2b)
C, 42.1; H, 3.89; N, 14.7%. Found: C, 34.3; H, 3.91; N, 4.28.
Calc. for C18H24S3N2O8Cl2Cu (3) C, 34.5; H, 3.86; N, 4.47%.
Found: C, 39.82; H, 4.21; N, 12.89. Calc. for C25H31S2N7O8-
Cl2Cu (4) C, 39.71; H, 4.13; N, 12.97%. Single crystals of
[Cu(L1)(H2O)Cl]Cl 1a suitable for X-ray diffraction were
obtained on slow evaporation of the 1 : 1 water : acetonitrile
solution of the complex. Single crystals of [Cu(L2)(NO3)]NO3
2b suitable for X-ray structure analysis were obtained on slow
evaporation of the methanolic solution obtained by mixing
Cu() nitrate and the ligand in 1 : 1 ratio. Single crystals of
[Cu(L4)](ClO4)2ؒ2CH3CN 4 suitable for X-ray diffraction were
obtained on slow evaporation of the 1 : 1 acetonitrile : toluene
solution of the complex.
Experimental
Materials
CuCl2ؒ2H2O, Cu(NO3)2ؒ3H2O (BDH, India), bis(2-mercapto-
ethyl)sulfide, 1,2-ethanedithiol (Aldrich), Cu(ClO4)2ؒ6H2O, 3-
chloropropionic acid, sodium borohydride and 2-vinylpyridine
(Fluka), thionyl chloride (Ranbaxy, India) and 1,2-diamino-
benzene, 3,3-thiodipropionic acid, thioglycolic acid and N-
methyldiethanolamine (Sisco, India) were used as supplied.
Tetra-n-hexylammonium perchlorate (THAP, G. F. Smith,
USA) was recrystallised twice from aqueous ethanol. Tetra-n-
butylammonium perchlorate was prepared by the addition of
sodium perchlorate to a hot ethanol solution of tetra-n-butyl-
ammonium bromide (G. F. Smith, USA). The product was
Caution. During handling of the perchlorate salts of organic
ligands care should be taken because of the possibility of
J. Chem. Soc., Dalton Trans., 2001, 3498–3506
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Table 1 Crystallographic data for the complexes [Cu(L1)(H2O)Cl]Cl 1a, [Cu(L2)(NO3)]NO3 2b and [Cu(L4)](ClO4)2ؒ2CH3CN 4
1a
2b
4
Chemical formula
M
C18H20N4CuOSCl2
474.88
C20H22N6O6S2Cu
570.10
C23H28N6O8S2Cl2Cu
715.07
Space group
Crystal system
a/Å
b/Å
c/Å
α/Њ
β/Њ
P1 (no. 2)
Triclinic
9.466(3)
9.480(3)
11.519(2)
75.880(18)
81.362(18)
80.03(2)
981.1(4)
2
P21/a (no. 14)
Monoclinic
15.990(7)
13.706(4)
10.972(4)
P21/n (no. 14)
Monoclinic
10.9367(5)
15.6280(6)
17.6216(8)
93.90(3)
100.9692(15)
γ/Њ
V/Å3
2399(2)
4
2956.8(2)
4
Z
T /ЊC
λ(Mo-Kα)/Å
ρcalc/g cmϪ3
µ/mmϪ1
R[I > 2σ(I )]
wR2
20
0.71069
1.608
1.508
0.0487
20
20
0.71069
1.578
1.134
0.0695
0.1336
0.71069
1.606
1.117
0.0680
0.1688
0.1474
explosion. [Cu(L3)](ClO4)2 led to an explosion during sample
transfer in our laboratory.
Fourier maps but their positions were not refined and their iso-
tropic thermal parameter (U) values were fixed at 0.05 Å. The
other hydrogen atoms were included in calculated positions
and refined using a riding model. In the case of 2b and 4, all
the hydrogen atoms were included in their calculated positions
and refined using a riding model. All structure solutions22 and
refinement calculations were carried out using SHELXL-9723
(1a, 4) and SHELXL-9324 (2b) crystallographic software pack-
ages. The molecular structures were drawn using ZORTEP.25
Selected bond lengths and angles are given in Table 2.
CCDC reference numbers 156236, 156237 and 171722.
See http://www.rsc.org/suppdata/dt/b1/b103511h/ for crystal-
lographic data for 1a (156236) and 4 (156237) in CIF or other
electronic format.
Physical measurements
Elemental analyses were performed at City University, London.
The solid state reflectance and the solution spectra in DMF,
MeCN and MeOH were measured on a HITACHI U-3410
double beam UV-Vis-NIR spectrophotometer. EPR spectra
were obtained on a JES-FE2XG X-band spectrometer, the
field being measured using a NMR field meter. The values of
go/gav were measured at ambient temperature and g|| and A|| at
77 K. Cyclic voltammetry and differential pulse voltammetry
were performed at 25.0
electrochemical cell was maintained by
0.2 ЊC. The temperature of the
cryocirculator
a
(HAAKE D8 G). A three-electrode cell configuration was used,
the reference electrode being Ag(s)/AgNO3 (0.01 M), tetra-n-
hexylammonium perchlorate (THAP) (0.1 M) in acetonitrile.
The E1/2 and ∆Ep values observed under identical conditions
for Fc/Fcϩ couple in methanol were 0.094 V and 96 mV respec-
tively. The experimental solutions were deoxygenated by
bubbling research grade nitrogen. The potentials were not
corrected for the liquid junction. The instrument utilized
included an EG&G PAR 273 potentiostat/galvanostat and
an IBM PS-2 computer. EG&G M270 software (Princeton
Applied Research, USA) was employed to carry out the
experiments and to acquire the data.
Results and discussion
The present ligands were synthesised according to known
procedures which involve the Phillip’s condensation of 1,2-
diaminobenzene with the dicarboxylic acids dithiopropionic
acid, mercaptoacetic acid and 4,7-dithiaoctane dicarboxylic
acid to obtain respectively 1,5-bis(benzimidazol-2-yl)-3-thia-
pentane (L1, bbes), mercaptoethylbenzimidazole and 1,8-
bis(benzimidazol-2-yl)-3,6-dithiaoctane (L2, bbdo). The tri-
dentate ligand N,N-bis(benzimidazol-2-ylmethylthioethyl)-
methylamine (L4) and the acid 4,7-dithiaoctane dicarboxylic
acid were synthesised by the nucleophilic substitution of the
chlorides, viz. bis(2-chloroethyl)methylamine and chloropro-
pionic acid by respectively mercaptomethylbenzimidazole
and 1,2-ethanedithiol in the presence of alkali. Copper() com-
plexes of these ligands were obtained by their reaction with
equimolar amounts of the appropriate copper salt in methanol.
All the complexes are soluble in common solvents like meth-
anol, acetonitrile and dimethylformamide.
The electronic and EPR spectra of the complexes in solution
are consistent with structural changes occurring on dissolution
and reveal anion–copper interaction. The conductivity
measurements of the complexes 1a and 2b in MeOH solution
suggest that they exist as 1 : 1 electrolytes and those of 2a and
4 in MeOH solution and 3 in CH3CN solution suggest that
they exist as 1 : 2 electrolytes. The benzimidazole and imidazole
moieties have pKa(HBϩ) values26 of 5.5 and 6.0 respectively
suggesting that Cu–benzimidazole interaction in the complexes
would duplicate the properties of Cu–imidazole interaction in
proteins. The extra rigidity and bulk of benzimidazole relative
to imidazole may be advantageous in attempts to mimic the
constrained biological coordination environments in copper
proteins. Thus the N2S2 ligand L2 represents a good ligand
Crystallographic data collection and structure analysis
Crystals of 1a, 2b and 4 showing sharp extinction under
a polarising microscope were used for data collection with
Enraf-Nonius-CAD4, Rigaku AFC-5 and Siemens three-circle
diffractometers, respectively, with a CCD area detector using
Mo-Kα radiation. The ω–2θ scan technique was used to record
the intensity data for the crystals 1a and 2b and the ω scan
technique was used for 4. The data were corrected for Lorentz
and polarisation but not for absorption effects. The details of
data collection and structure analysis are given in Table 1. The
structures were solved by direct methods.22 The non-hydrogen
atoms were located from the difference Fourier maps and fur-
ther refined by the least-squares technique. All non-hydrogen
atoms were refined anisotropically but in the case of crystal
2b, due to the relatively low number of unique and observed
reflections [I > 2σ(I )] and to maintain a reasonable data-
to-parameter ratio, the non-hydrogen atoms other than ring
carbons atoms were refined anisotropically, while the ring
carbons (C1–C6 and C15–C20) were refined isotropically. In 1a,
the hydrogen atoms of water were located from the difference
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J. Chem. Soc., Dalton Trans., 2001, 3498–3506

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Table 2 Selected bond distances (Å) and angles (Њ)
[Cu(L1)(H2O)Cl]Cl 1a
Cu(1)–S(1)
Cu(1)–N(3)
Cu(1)–Cl(1)
2.4201(12)
1.969(3)
2.3017(12)
Cu(1)–O(1)
Cu(1)–N(1)
2.217(3)
1.988(3)
N(3)–Cu(1)–N(1)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–Cl(1)
N(3)–Cu(1)–S(1)
O(1)–Cu(1)–S(1)
176.64(13)
92.28(14)
91.33(10)
91.09(10)
94.32(10)
N(3)–Cu(1)–O(1)
N(3)–Cu(1)–Cl(1)
O(1)–Cu(1)–Cl(1)
N(1)–Cu(1)–S(1)
Cl(1)–Cu(1)–S(1)
87.87(13)
91.56(10)
118.11(11)
85.56(10)
147.53(5)
[Cu(L2)NO3]NO3 2b
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–S(1)
1.953(13)
2.066(13)
2.529(5)
Cu(1)–N(3)
Cu(1)–S(2)
1.985(14)
2.438(6)
N(2)–Cu(1)–N(3)
N(3)–Cu(1)–O(1)
N(3)–Cu(1)–S(2)
N(2)–Cu(1)–S(1)
O(1)–Cu(1)–S(1)
177.6(6)
90.0(5)
92.6(4)
92.4(4)
122.4(5)
N(2)–Cu(1)–O(1)
N(2)–Cu(1)–S(2)
O(1)–Cu(1)–S(2)
N(3)–Cu(1)–S(1)
S(2)–Cu(1)–S(1)
89.8(5)
86.4(4)
148.4(4)
89.7(4)
89.1(2)
[Cu(L4)](ClO4)2ؒ2CH3CN 4
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–N(5)
1.982(6)
1.993(6)
2.118(6)
Cu(1)–S(2)
Cu(1)–S(1)
2.335(3)
2.559(2)
N(1)–Cu(1)–N(3)
N(3)–Cu(1)–N(5)
N(3)–Cu(1)–S(2)
N(1)–Cu(1)–S(1)
N(5)–Cu(1)–S(1)
96.6(3)
153.2(3)
82.8(2)
82.71(19)
85.70(19)
N(1)–Cu(1)–N(5)
N(1)–Cu(1)–S(2)
N(5)–Cu(1)–S(2)
N(3)–Cu(1)–S(1)
S(2)–Cu(1)–S(1)
92.9(3)
175.8(2)
86.0(2)
120.31(19)
101.19(9)
Fig. 1 ZORTEP drawing of [Cu(L1)(H2O)Cl]Cl 1a showing the atom
numbering scheme and thermal motion ellipsoids (50% probability
level) for the non-hydrogen atoms. The hydrogen atoms are omitted for
clarity.
model with donors similar to those in blue copper proteins,
with one of the weak thioether donors simulating the Cu–
cysteinate interaction in the latter.
Description of the crystal structures of [Cu(L1)(H2O)Cl]Cl 1a,
[Cu(L2)(NO3)](NO3) 2b and [Cu(L4)](ClO4)2ؒ2CH3CN 4 and
comparison with related structures
Fig. 2 ZORTEP drawing of [Cu(L2)(NO3)]NO3 2b showing the atom
numbering scheme and thermal motion ellipsoids (50% probability
level) for the non-hydrogen atoms. The hydrogen atoms are omitted for
clarity.
The ZORTEP views of 1a, 2b and 4 are illustrated in Figs. 1, 2
and 3 respectively, with atom numbering schemes shown.
Selected bond lengths and angles are given in Table 2. In the
complex cation of [Cu(L1)(H2O)Cl]Cl 1a, Cu() is located in an
approximately trigonal bipyramidal geometry in which the
equatorial positions are occupied by the sulfur atom of L1,
a chloride ion and the oxygen atom of water and the axial
positions by the two benzimidazole nitrogen atoms [Cu–N(1),
1.988(3), Cu–N(3), 1.969(3) Å] with the N(1)–Cu(1)–N(3) angle
of 176.64(13)Њ being close to the ‘ideal’ value of 180Њ which is
usual for such systems.27 However, as seen from the trigonality
index9 τ of 0.49 [τ = (β Ϫ α)/60 where β = N(1)–Cu(1)–N(3) =
176.64(13)Њ and α = Cl(1)–Cu(1)–S(1) = 147.53(5)Њ; for perfect
square pyramidal and trigonal bipyramidal geometries the
values of τ are zero and unity respectively], the coordination
geometry around copper is intermediate between trigonal
bipyramidal and square pyramidal geometries and is better
described as trigonal bipyramidal distorted square based
pyramidal (TBDSBP)12 with copper displaced 0.587 Å above
the N2SCl coordination plane and towards the elongated apical
oxygen atom. In the present complex only one of the counter-
ions is coordinated, possibly due to the steric hindrance from
J. Chem. Soc., Dalton Trans., 2001, 3498–3506
3501

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benzene]8 (τ, 0.64).8 This illustrates that both the replacement
of five-membered chelate ring(s) in [Cu(bbdh)Cl]ϩ by six-
membered one(s) and the fusion of a benzene ring between the
thioether donors as in [Cu(bbtb)]2ϩ lead to increased constraint
at copper and enhanced distortion towards trigonality. Further,
interestingly, the addition of a CH2 group in between the two
sulfur atoms lead to an equatorial disposition of both the sulfur
atoms as in [Cu(bmdhp)(H2O)](ClO4)2. A comparison of the
present structure with that of its diamino analogue30 viz. [Cu-
(bbeda)]2ϩ [bbeda = N,N Ј-bis(benzimidazolyl-2-ylethyl)ethane-
1,2-diamine], which possesses a square planar CuN4 chromo-
phore, suggests that the presence of Cu–S bond(s) is essential
for distortion towards trigonal bipyramidal geometry. This is
in conformity with the earlier observation31 that π-bonding
atoms confer on Cu() with ability to distort its coordination
sphere toward a tetrahedral geometry even in the absence of
steric forces. On the other hand, the pyridine analogue32 of the
present complex viz. [Cu(pdto)(ClO4)]ϩ [pdto = 1,8-bis(pyrid-2-
yl)-3,6-dithiaoctane] possesses a square pyramidal structure
(τ, 0.12). So it is evident that, in addition to coordinated thio-
ethers, sterically hindered benzimidazole groups are required
to confer on Cu() a geometry with considerable trigonal
bipyramidal distortion.
Fig. 3 ZORTEP drawing of [Cu(L4)](ClO4)2ؒ2CH3CN 4 showing
the atom numbering scheme and thermal motion ellipsoids (50%
probability level) for the non-hydrogen atoms. The hydrogen atoms and
solvent molecules are omitted for clarity.
the bulky benzimidazole groups. Interestingly, the Cu–O and
Cu–S distances [Cu–O(H2O), 2.217(3), Cu–S, 2.4201(12) Å] in
1a are longer than in [Cu(L1)(H2O)(CH3OH)]ClO4 1b28 [Cu–S,
2.297, Cu–O(H2O), 1.995 Å] obviously because of the increase
in formal charge on 1a by coordination of chloride. Both the
hydrogens of the coordinated water molecule are hydrogen
bonded to the uncoordinated chloride anion and both the
benzimidazole hydrogens (–NH) are hydrogen bonded to co-
ordinated chloride of the next molecule and so on.
The copper atom in [Cu(L4)](ClO4)2ؒ2CH3CN 4 is coordin-
ated by two thioether sulfur atoms, two benzimidazole nitrogen
atoms and the tertiary amine nitrogen atom. The value of the
geometric parameter τ (0.38) indicates that the coordination
geometry around copper() is best described as (TBDSBP)12
with the corners of the square plane being occupied by one
thioether sulfur atom (S2), two benzimidazole nitrogens and
the tertiary amine nitrogen and the apical position by the other
sulfur atom (S1). As in other bis(benzimidazolyl)dithioether
systems, the sterically demanding benzimidazole groups pre-
vent the sulfur and benzimidazole nitrogen atoms from lying
in the same plane but still introduce trigonality in the co-
ordination geometry around copper(). The Cu–Namine bond
[2.118(6) Å] is longer than the Cu–Nbzim bond [1.982(6) and
1.993(6) Å],6,8–10,27 as expected of the sp3 and sp2 hybridizations
respectively of amine and benzimidazole nitrogen atoms. The
basal Cu–S(2) distance [2.335(3) Å] is significantly shorter
than the Cu–S(1) bond distance [2.559(2) Å]. The significant
deviation of the S(1)–Cu–N(3) bond angle [120.31(19)Њ] from
the ideal value of 90Њ illustrates the tendency of the ligand,
because of all-five-membered chelate rings, to become folded,
as also found in the [Cu(pttn)]2ϩ and [Cu(pzttn)]2ϩ com-
plexes,19,20 and also that of the N–Me group to become flexible
like a thioether donor. The benzimidazole –NH is hydrogen-
bonded to the uncoordinated perchlorate oxygen.
The coordination geometry around copper() in [Cu(L2)-
(NO3)](NO3) 2b is best described as trigonal bipyramidal,
with both the thioether sulfur atoms and one of the oxygen
atoms of the nitrate ion in the trigonal plane and both the
benzimidazole nitrogens occupying the axial positions. The
bulky benzimidazole groups on the ligand apparently prevent
the other nitrate ion from coordinating. The thioether sulfur S2
is coordinated at a short distance [Cu–S(2), 2.438(6) Å] with the
other at a rather long distance [Cu–S(1), 2.529(5) Å], revealing
that the very symmetric ligand is coordinated in an asymmetric
fashion, which is allowed by the flexibility of the Cu()
coordination geometry. The Cu–S distances are similar to that
in 1a and those6 in [Cu(bbdh)Cl]ϩ; however, they are longer
than those9 in [Cu(bmdhp)(H2O)]2ϩ on account of a decrease in
formal charge on anion coordination to copper() in 2b, as
illustrated above for 1a. It is also possible that the addition of
a methylene group between the thioethers as in the bmdhp
complex, rather than that between thioether and benzimidazole
groups as in the present complex, leads to stronger coordin-
ation of thioethers. The Cu–Nbzim distances [1.953(13),
1.985(14) Å] are in good agreement with those in 1a and similar
compounds.6,8–10,27 The N(2)–Cu–N(3) angle of 177.6(6)Њ is
close to the ‘ideal’ angle of 180Њ, whereas the S(1)–Cu–S(2)
bond angle of 89.1(2)Њ is much smaller than the ‘ideal’ angle
of 120Њ. The latter is a consequence of the limited bite of the
ethylene bridge between the two sulfur atoms, as also found
in [Cu(bbdh)Cl]ϩ. Based on the geometrical criteria already
established,29 the nitrate ion is considered to engage in a
monodentate mode of coordination as evident from a ∆d
value of 0.6649 Å (>0.6 Å) and ∆angle of 30.55Њ (>28Њ). The
benzimidazole –NH is intermolecularly hydrogen-bonded to
one of the oxygen atoms of the uncoordinated nitrate anion
another oxygen atom of which is further hydrogen-bonded to
–NH of benzimidazole of the next molecule and so on.
Spectral properties
An intense band observed in the range 330–380 nm for all the
complexes is assigned to a S(σ) Cu() charge transfer (CT)
transition.27,33 The solid state reflectance spectra of 1a–4 show
two poorly resolved ligand field (LF) bands; however, in
methanol or acetonitrile solutions two well separated (>3000
cmϪ1) bands of almost equal intensity are observed suggesting
that on dissolution34,35 the complexes undergo a change from a
geometry with a trigonal bipyramidal component,36 as revealed
in the X-ray crystal structures of 1a, 2b and 4, to a square-
derived geometry with considerable axial interaction. Further,
the spectral features of 3 observed in the solid state and in
solution are similar to those of [Cu(pttn)]2ϩ, which possesses
a square based pyramidal distorted trigonal bipyramidal
(SBPDTB)12 geometry in its X-ray crystal structure,19,20 and are
consistent with a distorted square-based geometry. The chloro
complex 1a displays more intense ligand field bands at energies
slightly higher than its perchlorate analogue28 1b; however, the
S(σ) Cu() CT band energy is lower which is in line with the
elongation of the Cu–S bond on chloride coordination. Ligand
The trigonality index9 τ for 2b (0.49) illustrates that the
coordination geometry is best described also as TBDSBP.12 The
geometry is distorted towards trigonal bipyramidal (with more
constraint at copper) more than [Cu(bbdh)Cl]Cl (τ, 0.40),
[Cu(bmdhp)(H2O)](ClO4)2 (τ, 0.48) but less than [Cu(bbtb)-
(H2O)](ClO4)2 [bbtb = 1,2-bis(benzimidazol-2-ylmethylthio)-
3502
J. Chem. Soc., Dalton Trans., 2001, 3498–3506

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field and CT spectral titrations of 1b with chloride ions (LiCl)
reveal the coordination of two chloride ions to copper. These
observations illustrate that the location of the CT band at very
low energies in blue copper proteins may be partly due to strong
copper()–cysteinate interaction.
The polycrystalline EPR spectra of 1a, 2a and 2b are rhom-
bic with three g values [lowest g <2.03, Table 3], which is con-
sistent with the TBDSBP12 coordination geometry observed
in the X-ray crystal structures of 1a and 2b. The low values for
g1 with R > 1 [R = (g3 Ϫ g2)/(g2 Ϫ g1)] are characteristic
of copper() complexes containing one or two thioether bonds
and are consistent with the trigonal bipyramidal distorted
2
y2
geometry with a predominantly dx
ground state.34,37,38 The
Ϫ
rhombic spectra of 2a and 2b are in contrast to the polycrystal-
line axial spectra of the analogous CuN2S2 complexes [Cu-
(bbdh)Cl]ϩ and [Cu(bmdhp)(H2O)]ϩ, both with lesser trigonal
bipyramidal components in their geometries. Thus the spectral
properties are in support of the description of the geometry of
Table 3 Spectral and electrochemical data for Cu() complexes
Redoxd
Solvent
supporting
electrolyte
Electronic spectra λmax/nm
EPR spectraa
(εmax/MϪ1 cmϪ1
)
Frozen
∆Ep/
mV
106D/
Complex
LF
CT
Solid
solution
E1/2/V
cm2 sϪ1
MeOH
695 (160)
912 (128)
694 (266)
884 (352)
Solid
710
870
MeOH
680 (206)
870 (150)
H2O
MeOH
g|| 2.234
g⊥ 2.06 A|| 151
[Cu(L1)(H2O)(CH3OH)](ClO4)2 1b
ϩLiCl (excess)
327 (1740)
353 (1720)
312 (2420)
352 (2070)
MeOH
g|| 2.234
g⊥ 2.014b
A|| 153
MeOH/THAP
0.110
92
4.5
[Cu(L1)(H2O)Cl]Cl 1a
305
380
g1 2.018
g2 2.107
g3 2.194
305 (2206)
365 (2260)
670 (155)
900 (110)
Solid
625
900
MeOH
600 (220)
890 (260)
MeCN
590 (220)
860 (210)
H2O
330 (1421)
MeOH
g|| 2.227
g⊥ 2.071
A|| 146
MeOH/THAP
H2O/NaCIO4
0.299
0.483
82
2.5
1.2
[Cu(L2)](ClO4)2 2a
g1 2.009
g2 2.214
g3 2.207
127
345 (2840)
600 (170)
880 (190)
Solid
640
870
MeOH
605 (220)
885 (220)
MeCN
615 (160)
890 (110)
340 (3760)
405 (1420)
MeOH
g|| 2.210
2.241c
MeOH/THAP
MeOH/NaNO3
0.263
0.227
90
90
2.5
2.8
[Cu(L2)(NO3)]NO3 2b
g1 2.029
g2 2.134
g3 2.185
A|| 145
g⊥ 2.026
345 (2300)
340 (3020)
157c
DMF
g|| 2.210
2.215c
g⊥ 2.071
A|| 145
131c
CH3CN/THAP
0.247
98
2.6
Solid
600
900
H2O
H2Ob
MeOH/THAP
H2O/NaClO4
0.364
0.470
84
80
3.7
4.9
[Cu(L3)](ClO4)2 3
g|| 2.174
g⊥ 2.031
g|| 2.172
A|| 170
600 (300)
900 (120)
MeOH
610 (430)
900 (110)
MeCN
615 (490)
900 (100)
Solid
870
615
MeCN
620 (265)
836 (120) sh
380 (1910)
380 (2540)
380 (3280)
DMSO
g|| 2.193
g⊥ 2.077
A|| 172
CH3CN/THAP
Ϫ0.039
94
1.4
[Cu(L4)](ClO4)2ؒ2CH3CN 4
337
416 (sh)
g|| 2.175
g⊥ 2.053
332 (5520)
a A|| values are in units of 10Ϫ4 cmϪ1 b At least five N-shf lines are located on the perpendicular feature. c Minor species. d Measured vs. non-aqueous
.
Ag/Agϩ reference electrode for non-aqueous solvents (Add 544 mV to convert into NHE) and vs. SCE for aqueous solutions. Glassy carbon working
electrode for complexes 1a and 4 and Pt sphere for 2a, 2b and 3. Scan rate 50 mV/s; concentration of the complexes 0.001 M.
J. Chem. Soc., Dalton Trans., 2001, 3498–3506
3503

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2b as trigonal bipyramidal distorted square based pyramidal
(TBDSBP).12 The frozen solution spectrum of 1a is axial
supporting changes in structure on dissolution, inferred from
electronic spectral studies. The g|| value of 1a is higher than the
perchlorate28 1b, consistent with the higher constraint (higher
τ) at copper in 1a. The frozen solution spectrum of 2b shows
clearly resolved peaks in the g|| region, consistent with the
superposition of spectra of three species. As the solution is
diluted, the signals corresponding to solvated cupric ion grew in
intensity relative to those of other species. On the addition of
excess NaNO3, the signals corresponding to the less abundant
species with high g|| value decreased in intensity and a well-
defined axial spectrum with a weak rhombic component for the
other species tend to appear; however, the signals for solvated
cupric ion still remain. A similar dissociative equilibrium
involving ligand and solvent has been observed previously
even in frozen methanol solution for other copper–thioether
complexes,7,19 the g|| values being in the same range as those
observed for copper() thioether complexes.9,18,27,33 The less
abundant species, as it has a high g|| value, is likely to have
at least one of the weakly coordinated thioethers replaced by
solvent. In contrast to 2b, the frozen solution spectrum of 2a
is axial and no dissociative equilibrium could be discerned. The
g|| and A|| values (Table 3) are close to those observed for the
more abundant species of 2b.
The polycrystalline EPR spectra of 3 and 4 are clearly axial
34,38
2
2
suggesting that the ground state is predominantly dx
.
Ϫ
y
The frozen solution spectra of 3 and 4 are also clearly axial
(Fig. 4). The observed g|| and A|| values of 3 are similar to
32
[Cu(pdto)]2ϩ (g||, 2.176; A||, 174 × 10Ϫ4 cmϪ1
)
with the N2S2
equatorial plane supporting the presence of a similar square-
based coordination environment, suggested by electronic spec-
tral studies. The g|| and A|| values of 4 are less than those for
complexes with a CuN4 chromophore (g||, 2.200; A||, 190–200 ×
10Ϫ4 cmϪ1). Further, the A|| rather than g|| value is very sensitive
to the inclusion of thioether sulfur in the equatorial plane.
Fig. 4 EPR spectrum of 3 in aqueous solution (a) and 4 in DMSO
solution (b) at 77 K.
those observed in blue copper proteins on electron transfer,
which involve very small changes in geometry (bond lengths,
<0.15 Å). The pentadentate ligand complexes [Cu(L3)]2ϩ 3 and
[Cu(L4)]2ϩ 4, both with square pyramidal structures as dis-
cussed above, are expected to undergo relatively less reversible
electron transfer with the rupture of weak Cu–S or, most
possibly, the terminal Cu–Npy bonds as part of the structural
Electrochemistry
All the present complexes exhibit reversible to irreversible one-
electron transfer involving CuII/CuI couple9,39,40 (diffusion co-
efficients D, 1.2–4.9 × 10Ϫ6 cm2 sϪ1, Table 3). The value of ∆Ep
in excess of 59 mV (for reversible one-electron transfer ∆Ep is
reorganization to achieve
a four coordinated pseudo-
59 mV; for the Fc/Fcϩ couple of ferrocene in MeOH: E1/2
,
tetrahedral structure suitable for the copper() ion. In fact, as
revealed in its preliminary X-ray crystal structure42 [Cu(L3)]ϩ
possesses a tetrahedral structure with a terminal Cu–Npy bond
dissociated. Thus the reversibility of the CuII/CuI couple
appears to depend on bond-breaking of not only chelating
ligands but also of coordinated anions and solvent.
0
0
0.094 V; ∆Ep , 80 mV under identical conditions; ∆Ep is
∆Ep extrapolated to zero scan rate) is considered to reflect the
involvement of, apart from other factors, stereochemical
reorganization accompanying electron transfer, which can be
qualitatively understood from a comparison of the structures
of the copper() and copper() forms of the present complexes.
The CuII/CuI redox potentials of 1a and 2a are higher
than those of their amine analogues,30,43 [Cu(bba)]2ϩ and
[Cu(bbedo)]2ϩ (Ϫ0.294 V, Ϫ0.508 V vs. Ag/Agϩ respectively),
the latter with the same chelate ring sizes as 2a. This suggests
that the sulfur donor atom, by virtue of its greater π-acceptor
ability, stabilizes Cu() with respect to the Cu() form more than
the amino nitrogen. Thus the incorporation of a sulfur donor
into L1 to form the complex 2a, and into L2 to form copper()
complexes of 1,9-bis(benzimidazol-2-yl)-2,5,8-trithianonane
(bttn)and1,11-bis(N-methylbenzimidazol-2-yl)-3,6,9-trithiaun-
decane (bmbtu), both with N2S3 donor sets, raises the redox
potential (ϩ0.23, ϩ0.35 V vs. Ag/Agϩ)44 whereas the addition
of one amine nitrogen into 2b to give 4 decreases the redox
potential, as expected. Further, on incorporation of a third
sulfur donor into [Cu(pdto)]2ϩ to obtain 3 a rise in E1/2 by 95 mV
is expected,45 which is in fair agreement with the observed rise
0
Thus the slightly higher ∆Ep and ∆Ep values of 2a and 2b
suggest that the differences expected in the structures of their
Cu() and Cu() forms would be small and hence would imply
lower structural reorganization energy involved during the
heterogeneous electron transfer41 which follows dissociation of
weakly coordinated solvent/anion.
In contrast, upon one-electron transfer, the strongly bound
chloride ion and a solvent molecule in 1a with trigonal bipy-
ramidal based geometry are displaced from the copper center to
give possibly a T-shaped structure for the d10 Cu() ion, similar
to that observed for its homologue27 [Cu(i-Pr-bbes)]ϩ. Further,
assuming that N-i-Pr substitution in the homologue is only
peripheral and does not influence the bond length, it may be
noted that the decrease in coordination number from five to
three is accompanied by enormous changes in bond angles,
elongation of the Cu–S bond [2.469(9) Å] and a significant
decrease in the Cu–Nbzim bond distance [1.912(5) and 1.910(5)
Å]. Such intricate structural changes, though difficult to verify
in solution, account for the enhanced departure from electro-
of 108 mV; however, a similar rise in the E1/2 of [Cu(pttn)]2ϩ
,
a homologue of 3 is only 25 mV. This clearly suggests that an
increase in number of coordinated sulfur atoms from one to
two to three increases the redox potential, while incorporation
of a five-membered chelate ring decreases the redox potential.
0
chemical reversibility (∆Ep , 88 mV) for 1a. They are unlike
3504
J. Chem. Soc., Dalton Trans., 2001, 3498–3506

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The redox potentials of 1a and 2b are lower than their per-
chlorate complexes (1b, E1/2, 0.110 V) and 2a respectively; this is
consistent with the stabilization of the Cu() form by chloride
and nitrate ion coordination as evident from the above elec-
tronic and EPR spectral behaviour. In fact, addition of excess
chloride (LiCl) to 1b and use of NaNO3 as supporting electro-
lyte for 2b decreases the E1/2 values significantly. This implies
that the covalent copper()–cysteinate interaction in blue
copper proteins would also lead to a decrease in CuII/CuI redox
potential; however, the unusual distorted tetrahedral geometry
for copper appears to offset this effect and raises the redox
potential of the protein. Further, the E1/2 values of both 2a
bipyramidal component in the coordination geometry. On the
other hand, increasing the number of coordinated thioethers,
though enhancing the redox potential, does not seem to exercise
a preference for a geometry with enhanced trigonal distortion
because of other influential factors like type of donor atoms
and chelate ring size. Though we were successful in designing
models with high redox potentials and decreased hyperfine
splitting constants (A||), the latter is only remotely comparable
with that of blue copper proteins and the square-based struc-
tures and spectral properties are comparable only to the normal
copper() species. Valid models for blue copper proteins may
be constructed by incorporating bulky ligand moieties,
six-membered chelate rings and a minimum of two sulfur
donors i.e. N2S2 ligands.46 Thus very recently success has been
attained50 in reproducing type I coordination geometry by
characterising a Cu() complex with N2S(thiolate)S(thioether)
ligation; however, though relatively low A|| values and a low
energy S Cu() LMCT band have been observed, the positive
redox potential of the proteins is not reproduced. Moreover, a
study on interaction of ClϪ, a stronger coordinating anion, with
CuN2S complexes with trigonal bipyramidal geometry may
simulate the effect of strong cysteinyl sulfur coordination in
increasing the Cu–thioether bond length and shifting the charge
transfer band to lower energy. This result demands a further
study on the interaction of this complex with heavily protected
thiolates with an aim to isolate four-coordinate complexes with
a geometry approaching that in blue copper proteins.
and 2b are higher than their pyridine analogue [Cu(pdto)]2ϩ
,
obviously due to the bulky benzimidazole groups which confer
distorted geometries as illustrated above. In fact, the E1/2 value
calculated for 2a using Addison’s ∆EL parameters45 (∆Eim,
52 mV) is lower than the adjusted value [Eadj = Eexptl Ϫ 0.215 V
(in methanol)].45 This implies that the contribution by bulky
benzimidazole to E1/2 is much higher than that by pyridine or
imidazole and thus a better agreement with the observed values
is seen when a ∆Ebzim value17 of ϩ165 mV is used for calculating
an E1/2 of [Cu(bbdh)]2ϩ. The value of E1/2 calculated for 2a,
however, is still lower (60 mV) than the observed one, illus-
trating that its copper() structure in solution may still possess
significant trigonal bipyramidal distortion.40 Among the bis-
(benzimidazolyl)dithioether complexes the E1/2 value varies as
[Cu(bbdh)]2ϩ 13 < [Cu(bmdhp)]2ϩ 13 < [Cu(L2)]2ϩ 2a in methanol,
suggesting that six-membered chelate rings46,47 in the presence
of π-donors increase the redox potential and that larger chelate
rings can accommodate the pseudo-tetrahedral structure
suitable for the Cu() state as expected.20,41,48,49 Thus the incor-
poration of thioether donors or benzimidazole moieties or six-
membered chelate rings and enlargement of the ligand back-
bone elevates the CuII/CuI redox potential by readily facilitating
access to a pseudo-tetrahedral coordination geometry suitable
for the Cu() oxidation state.
Acknowledgements
We thank the Department of Science and Technology, India
(Grant No. SP/S1/F02/96) for the financial support of this
work and the University Grants Commission, India for fellow-
ships (M. V. and U. S.). We thank Professor N. Kitajima
(deceased) for providing X-ray diffraction data for the crystal
2b, Professor C. N. R. Rao, FRS and Dr G. U. Kulkarni,
JNCASR, Bangalore for providing X-ray diffraction facilities
for the crystal 4 and the Regional Sophisticated Instru-
mentation Centre, IIT, Chennai for collecting X-ray data
for the crystal 1a. Professor M. Chikira, Chuo University and
Professor J. Subramanian, Pondicherry University are thanked
for EPR facilities for a few of the complexes.
Conclusions
The CuN2S2 complexes exhibit a square-based geometry with
enhanced trigonality, in spite of the presence of two ethylene
bridges in between thioether and benzimidazole donors; this
is interesting because the bis(pyridyl)dithioether and bis(benz-
imidazolyl)diamino analogues, which contain such bridges
between –S– and pyridine and –NH– and benzimidazole
respectively, possess square-based geometries with lesser tri-
gonality. This is obviously because of geometric distortion
induced by steric effects of the bulky benzimidazole groups,
facilitated by coordinated thioethers. Further, the present study
leads to the significant conclusion that in the solid state Cu()
complexes of all tetradentate bis(benzimidazol-2-yl)dithioether
ligands possess coordination geometries with the trigonality
increasing with increase in number of six-membered chelate
rings and with incorporation of fused benzene rings.6,8 Similar
to other members of its class, the present CuN2S2 complex has a
solid state geometry which, because of its sensitivity to inter-
actions with solvents and anions, tends to become square-based
in solution. A water molecule or a halogenide or a relatively
strongly coordinating anion like nitrate can be bound to
copper() as the fifth ligand. The above variations in structures
are reflected in their spectral and electrochemical behaviour and
thus an enhanced redox potential expected of ligand enlarge-
ment is observed.
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