Copper(II) complexes of a new tetradentate bis-benzimidazolyl diamide ligand with disulfanediyl linker: Synthesis, characterization, and oxidation of some pyridyl, napthyl, and benzyl alcohols

Article abstract of DOI:10.1080/00958972.2013.835403

The bis-benzimidazolyl diamide ligand 3,3’-disulfanediyl-bis-(N-((1H-benzo[d]imidazol-2-yl)methyl) propanamide) (L) having N, S, and O donors has been synthesized and utilized to prepare copper(II) complexes of general composition [Cu(L)X]·X·nH₂O, where X is Cl^-or (Formula Found). Ligands and complexes were characterized by elemental analysis, UV-visible, IR spectroscopy, 1H NMR, mass spectrometry, conductance, electron paramagnetic resonance, cyclic voltammetry, fluorescence, single-crystal, and PXRD. [Cu(L)(NO₃)]NO₃·H₂O crystallizes in the monoclinic C2/c space group with one molecule in the asymmetric unit. Copper adopts a distorted octahedral geometry with four coordination sites occupied by the ligand and two sites by a bidentate nitrate. [Cu(L)(NO₃)] NO₃·H₂O has been utilized as a catalyst to oxidize alcohols with tert-butyl hydroperoxide as an alternative source of oxygen. The oxidized products have been characterized by GCMS.

Full text of DOI:10.1080/00958972.2013.835403

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Copper(II) complexes of a new tetradentate bis-
benzimidazolyl diamide ligand with disulfanediyl
linker: Synthesis, characterization and oxidation of
some pyridyl, napthyl and benzyl alcohols
NIDHI TYAGI a , RAVINDER KUMAR a , KULDEEP MAHIYA a & PAVAN MATHUR a
a Department of Chemistry , University of Delhi , Delhi , India
Accepted author version posted online: 16 Aug 2013.
To cite this article: Journal of Coordination Chemistry (2013): Copper(II) complexes of a new tetradentate bis-benzimidazolyl
diamide ligand with disulfanediyl linker: Synthesis, characterization and oxidation of some pyridyl, napthyl and benzyl
alcohols, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2013.835403
To link to this article: http://dx.doi.org/10.1080/00958972.2013.835403
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Copper(II) complexes of a new tetradentate bis-benzimidazolyl diamide ligand
with disulfanediyl linker: Synthesis, characterization and oxidation of some
pyridyl, napthyl and benzyl alcohols
NIDHI TYAGI, RAVINDER KUMAR, KULDEEP MAHIYA and PAVAN MATHUR*
Department of Chemistry, University of Delhi, Delhi, India
The bis-benzimidazolyl diamide ligand 3,3'-disulfanediyl-bis-(N-((1H-benzo[d]imidazol-2-
yl)methyl)propanamide) (L) having N, S and O donors has been synthesized and utilized to
-
prepare copper(II) complexes of general composition [Cu(L)X]·X·nH2O, where X is Cl- or NO3 .
Ligands and complexes were characterized by elemental analysis, UV-visible, IR spectroscopy,
1H NMR, mass spectrometry, conductance, electron paramagnetic resonance, cyclic
voltammetry, fluorescence, single crystal and PXRD. [Cu(L)(NO3)]NO3·H2O crystallizes in the
monoclinic C2/c space group with one molecule in the asymmetric unit. Copper adopts a
distorted octahedral geometry with four coordination sites occupied by the ligand and two sites
by a bidentate nitrate. [Cu(L)(NO3)]NO3·H2O has been utilized as a catalyst to oxidize alcohols
with tert-butyl hydroperoxide (TBHP) as an alternative source of oxygen. The oxidized products
have been characterized by GCMS.
Keywords: Copper(II) complexes; N, S, O donor benzimidazolyl diamide ligands; Crystal
structure; Galactose oxidase
1. Introduction
Transition metal complex-catalyzed oxidation of organic compounds has been extensively
investigated. Introduction of efficient catalysts for the selective insertion of one oxygen from
oxygen donors (like O2, H2O2, t-BuOOH, or NaOCl) into various organic molecules, under mild
conditions, has been a challenge in chemical catalysis [1]. The search for efficient oxidation
catalysts is increasingly being guided by considering the active site structures of
metalloenzymes. An example is the fungal enzyme galactose oxidase (GOase) catalyzing
selectively aerobic oxidation of a wide range of primary alcohols to the corresponding aldehydes
[2]. Stack and co-worker described the use of a functional mononuclear Cu(II) model compound
*Corresponding author. Email: pavanmat@yahoo.co.in
1

as a catalyst of aerobic oxidation of activated benzyl and allylic alcohols with high turnover
numbers but in the presence of an excess of the base (alkoxide) as co-catalyst [3], while
Wieghardt and co-workers reported a mononuclear 2,2'-thio-bis-(phenolato) triethylamine
copper(II) compound that oxidized both primary and secondary alcohols to the corresponding
aldehydes and ketones, respectively, and/or to 1,2-glycol derivatives [4, 5]. Oxidation of benzyl
alcohol have been widely discussed by Valodkar et al. using polymer-Cu(II) metal complexes as
catalysts and TBHP as oxidant [6].
Both homogenous and heterogeneous catalysts based on various metals including Mn, V,
Fe, Ni, Cu, Ag and Au are effective in oxidation of many alcohols [7]. Ag and Au [8, 9] catalysts
are reported in heterogeneous media while copper is reported to be efficient under both
homogeneous and heterogeneous conditions [10a,b].
In the present research, a N2O2S2 donor benzimidazolyl-based ligand and its copper(II)
complexes have been synthesized and characterized. One Cu(II) complex has been used as
homogeneous catalyst in the oxidation of alcohols to carbonyl compounds and carboxylic acids
using t-butylhydroperoxide as source of oxygen.
2. Experimental
2.1. Materials
All solvents used were of HPLC grade. 3,3'-Thiodipropionic acid, 3-pyridyl carbinol, 1,2,3,4-
tetrahydronaphthol, 4-methoxybenzyl alcohol, Geraniol and 4-nitrobenzyl alcohol were obtained
from Sigma Aldrich. All other chemicals were obtained from commercial sources and used as
received. 2-Aminomethyl benzimidazolyl dihydrochloride was prepared following the procedure
reported by Cescon and Day [11].
2.2. Physical measurements
Carbon, hydrogen, nitrogen and sulfur were estimated using an Elemental Analyzer at USIC,
University of Delhi, Delhi. Electronic spectra were recorded in HPLC grade DMF on a
Shimadzu 1601 spectrophotometer from 200-1100 nm. IR spectra were recorded in the solid
state as KBr pellets on a Perkin-Elmer FTIR-2000 spectrometer. 1H NMR (400 MHz) spectra of
ligands were recorded in DMSO-d6 on a JEOL ECX-400P NMR spectrometer at the Department
of Chemistry, University of Delhi, Delhi. Conductance values were measured on a Digital
2

Conductivity Bridge from Popular Traders, India (Model: PT-825) at the Department of
Chemistry, University of Delhi. Electrochemistry was studied on a BAS CV 50W
electrochemical analysis system at the Department of Chemistry, University of Delhi, Delhi.
Tetrabutyl ammonium perchlorate (TBAP) (0.1 M) was used as supporting electrolyte. A three-
electrode configuration composed of Pt disc working electrode (3.1 mm2 area), a Pt-wire counter
electrode, and a Ag/AgNO3 reference electrode was used for the measurements. E1/2 of ferrocene
was +56 mV. ESI-MS were obtained from CDRI, Lucknow, India. X-band EPR spectra were
recorded on a Bruker Spectrospin with a variable temperature liquid nitrogen cryostat at 120 K at
IIT Bombay, India. GCMS spectra were recorded at Advanced Instrumentation & Research
Facility, Jwaharlal Nehru University, New Delhi, on a GCMS-QP 2010(plus) Shimadzu
instrument. Fluorescence spectra were recorded in methanol on a Varian CARY Eclipse
fluorescence spectrophotometer. Powder X-ray diffraction patterns were recorded using a high
resolution D8 Discover Bruker X-ray diffractometer equipped with a point detector (scintillation
counter), employing monochromated Cu Kα1 radiation obtained through a globel mirror with a
scan rate of 1.0 s/step and step size of 0.02° at 298 K from 2θ=10-40. Single crystal diffraction
data were collected on an X’Calibur single crystal X-ray diffractometer having CCD camera at
the University of Delhi, Delhi, India.
2.3. Synthesis of 3,3'-disulfanediyl-bis (N-((1H-benzo [d] imidazol-2-yl) methyl) propanamide) (L)
L was prepared following the procedure reported by Mathur et al. [12]. A solution of
2-aminomethyl benzimidazolyl dihydrochloride (5.0 g, 22.7 mol) and 3,3'-dithiodipropionic acid
(2.38 g, 11.4 mol) in 20 ml pyridine was stirred for 5 min until a white precipitate was obtained
(during the reaction pyridine forms pyridine hydrochloride). Then, triphenyl phosphate (TPP)
(6.38 ml, 22.7 mol) was added to the reaction mixture at a temperature of 50 C and the
temperature was slowly raised to 80-85 C. Within half an hour, the precipitate redissolved and
the clear solution was stirred for 30 h. The resulting orange brown solution was allowed to cool
and washed with saturated NaHCO3 solution in a separating funnel until no effervescence could
be seen. Then, it was washed with water 2-3 times. While washing, the oil solidified into a light
yellow solid (scheme 1). Upon further washing with acetone, the solid turned white. This was
dried and recrystallized by hot methanol:H2O (1:1). The recrystallized white product was
filtered, washed with cold water and dried in air. Yield: 55%, Melting point: 185-186 C. Anal.
3

found for C22H24N6O2S2·CH3OH: C, 54.5; H, 6.1; N, 17.4; S, 12.0 %. Calcd: C, 55.2; H, 5.6; N,
16.8; S, 12.8%. UV/Vis spectrum (DMF) max, nm (log , M-1 cm-1) = 283 (4.35), 276 (4.38). IR
(KBr pellets, cm-1):  = 3310 (O-H), 3235 (N-H amide), 3062 (N-H benzimidazole), 1646 (C=O
amide I), 1540 (C-N amide II), 1431 (C=N-C=C (benzimidazole)), 740 (C=C benzene), 677
(C-S). 1H-NMR (DMSO-d6, ppm) : 12.2 (s, 2H), 8.70-8.67 (t, 2H), 7.53-7.45 (d, 4H), 7.14-7.13
(q, 4H), 4.50-4.47 (d, 4H), 2.97-2.93 (t, 4H), 2.63-2.61 (t, 4H). ESI/MS+ (m/e): 468.
O
H
N
S
OH
NH2. 2HCl
S
HO
N
O
80-850C
30 hrs
Pyridine, TPP
O
N
H
N
H
S
N
S
N
N
H
H
N
O
Scheme 1. Synthesis of 3,3'-disulfanediyl-bis(N-((1H-benzo[d]imidazol-2-
yl)methyl)propanamide) (L).
2.4. Synthesis of copper(II) complexes
2.4.1. [Cu(L)Cl]Cl. To a solution of L (100 mg, 0.21 mmol) in 10 mL CH3OH, a methanolic
solution (5 mL) of CuCl2·2H2O (36.4 mg, 0.21 mmol) was added. The reaction mixture was
stirred for 4 h at room temperature. The reaction mixture was then concentrated on a rotary
evaporator and kept in a refrigerator for a few hours, when a green product separated. The
product was filtered and washed with cold MeOH. Yield: 50%. Anal. found for
C22H24N6O2S2Cl2Cu·2H2O (638.5): C, 40.7; H, 4.2; N, 12.7; S, 10.0 %. Calcd.: C, 41.3; H, 4.3;
N, 13.1; S, (10.0) %. UV/Vis spectrum (DMF) max, nm (log , M-1 cm-1): 276 (4.10), 282 (4.06),
760 (1.88). IR (KBr pellets, cm-1):  = 3421 (O-H), 3250 (N-H amide), 3065 (N-H
(benzimidazole)), 1654 (C=O amide I), 1540 (C-N amide II), 1455 (C=N-C=C (benzimidazole)),
745 (C=C benzene),  = 663 (C-S).
4

2.4.2. [Cu(L)NO3]NO3. The procedure was similar to [Cu(L)Cl]Cl except that Cu(NO3)2·3H2O
(51.6 mg, 0.21 mmol) was used instead of CuCl2·2H2O for complexation. The reaction mixture
was then kept for crystallization. After a few days green crystals were obtained. Yield: 65%.
Anal. found for C22H24N8O8S2Cu·H2O (673.5): C, 38.8; H, 4.1; N, 16.5; S, 9.3 %. Calcd.: C,
39.1; H, 3.9; N, 16.6; S, 9.5 %. UV/Vis (DMF) max, nm (log  M-1 cm-1): 275 (4.03), 282 (3.98),
720 (1.79). IR (KBr pellets, cm-1):  = 3450 (O-H), 3257 (N-H amide), 3058 (N-H
(benzimidazole)), 1625 (C=O amide I), 1543 (C-N amide II), 1453 (C=N-C=C (benzimidazole)),
1384 (N-O), 745 (C=C benzene), 663 (C-S). IR (KBr pellets, cm-1):  = 3460 (O-H), 3290 (N-H
amide), 1661 (C=O amide I), 1538 (C-N amide II), 1432 (C=N-C=C benzimidazole), 747 (C=C
benzene), 670 (C-S).
2.5. X-Ray crystallography
Single crystals of [Cu(L)NO3]NO3·H2O suitable for X-ray diffraction studies were grown by
slow evaporation in methanol at room temperature. The intensity data were collected at 298 K on
an X’calibur CCD diffractometer with graphite monochromated Mo/K radiation. A total of
40794 reflections were measured of which 5543 were unique and 4113 were considered
observed (I > 2 (I)]. The data were corrected for Lorentz and polarization effects and multi-scan
absorption correction was applied. The structure was solved by direct methods using
SHELXS-97 [14a] and refined by full-matrix least-squares refinement on F2 using SHELXL-97
[14a]. All calculations were done with the help of WinGX package of crystallographic programs
[14b]. All non-hydrogen atoms except the disordered lattice water were refined anisotropically.
All hydrogens were fixed geometrically with Uiso values of 1.2 times the Uiso values of their
respective carrier. Hydrogens on lattice water were not located. The final residual indices are
R 0.0779, Rw 0.2236 for the observed and R 0.1055, Rw 0.2431 for all reflections using 378
parameters and 2 restraints. Thermal ellipsoid plot (ORTEP) for [Cu(L)NO3]NO3 is shown in
figure 1. Crystal and structure refinement data and selected geometrical parameters are
summarized in tables 1 and 2, respectively.
5

3. Results and discussion
3.1. Crystal structure description
The ORTEP diagram of [Cu(L)NO3]NO3·H2O with atomic numbering scheme is shown in
figure 1. The complex crystallizes in the monoclinic C2/c space group with one molecule in the
asymmetric unit. The copper in [Cu(L)NO3]NO3·H2O adopts a distorted octahedral geometry.
The ligand provides four coordination sites through two benzimidazole imine nitrogens, N2 and
N6 (Cu(1)–N(6) = 1.950(5) and Cu(1)–N(2) = 1.974(5) Å), and two amide carbonyl oxygens, O1
and O2 (Cu(1)–O(1) = 2.061(4) and Cu(1)–O(2) = 2.157(5) Å). The remaining two coordination
sites O3 and O4 are provided by bidentate nitrate (Cu(1)–O(3) = 2.176(8) Å and Cu(1)–O(4) =
2.433(7) Å). The third uncoordinated oxygen atom of the bound nitrate was disordered over two
positions O5B and O5A (disordered ratio 67:33, respectively). A comparative study of bond
lengths and angles is given in table 2. One nitrate is present in the crystal lattice to balance the
charge. A highly disordered water is also present in the lattice for which the disorder could not
be modeled and it was refined only isotropically.
3.2. PXRD Studies
As suitable single crystals for [Cu(L)Cl]Cl could not be grown, the powder XRD pattern of the
compound was matched with [Cu(L)NO3]NO3 with lattice constant a = 18.4094, b = 15.6079, c =
23.7095, α = 90.00, β = 112.336, γ = 90.00; Monoclinic, C 2/c.
The Le-Bail fit of the powder X-ray diffraction pattern of [Cu(L)Cl]Cl (figure 2(a-b)
yielded the lattice constant a = 18.2124, b = 15.6691, c = 33.0812, α = 90.00, β = 112.1068, γ =
90.00; Monoclinic, C 2/c.
On the basis of a close fit of the a, b, c and α, β, γ parameters with that for
[Cu(L)NO3]NO3 it is proposed that [Cu(L)Cl]Cl also has a similar lattice structure [15].
3.3. Characterizations
1
1
3.3.1. H NMR Spectra. The H NMR spectra of the ligands were measured in d6-DMSO. The
benzimidazolyl N-H protons appear as a singlet at 12.2 ppm. The protons due to amide N-H give
a triplet at 8.70-8.67 ppm. The benzene protons which are adjacent to benzimidazole N-H appear
as a quartet at 7.14-7.13 ppm and the remaining benzene protons show a doublet at
7.53-7.45 ppm. The protons of -CH2- attached to the amide appear as a doublet at 4.50-4.47 and
6

protons of –CH2-CH2- appear as triplets at 2.97-2.93 and 2.63-2.61 ppm, respectively
(Supplementary Material).
3.3.2. Mass spectra analysis. A main peak was found in the ESI/MS+ at 468 that corresponds to
the molecular ion peak for L.
3.3.3. Electronic spectroscopy. The electronic spectra of all the Cu(II) complexes show two
peaks at 276-287 nm. These peaks are assigned to –* transition of benzimidazole in the
ligands [16]. Absorption at this position is high as indicated by their extinction coefficient. The
Cu(II) complexes exhibit a single symmetrical but broad d–d band in the region 700-770 nm,
characteristic of copper(II) in a tetragonal geometry [17]. These bands are assigned to the
transition dxz, dyz  d
and dxy  d 2. The difference in absorption band position may be
x -y
2 2
x -y
2
due to the ligand field splitting of the coordinated ligand. The decrease in the energy of the
absorption maxima on going from [Cu(L)NO3]NO3 to [Cu(L)Cl]Cl in these complexes can be
attributed to a decrease in the plane ligand field splitting which reflects different degrees of
distortions of copper(II) bound to the ligands.
3.3.4. IR Spectral studies. IR stretching bands for the ligand and complexes are listed in table 3.
A broad band at 3300-3500 cm-1 in ligand and complexes indicates the presence of associated
H2O molecules. Small shifts are observed for the C–S stretch, while shifts in amide I and amide
II stretches due to the –CO–NH– group indicates coordination of the ligand through the carbonyl
oxygen present in the ligands [15, 18]. The nitrate complex shows bands at 1372 and 830 cm-1
due to O-N-O symmetric and O-N-O anti-symmetric stretching of the coordinated nitrate [19].
3.3.5. Molar conductance. The molar conductivity values were measured in DMF. The values
are 37.2 Ohm-1 cm2 mol-1 and 48.0 Ohm-1 cm2 mol-1 for [Cu(L)Cl]Cl and [Cu(L)NO3]NO3,
respectively. This indicates that at least one anion (chloride/nitrate) is dissociated in solution
[23b].
3.3.6. Cyclic voltammetry. Cyclic voltammograms of the copper(II) complexes were recorded
in DMSO. The Cu(II) complexes show a quasi reversible redox wave (Supplementary Material)
7

due to the Cu(II)/Cu(I) couple (table 4). Anodic shifts in E1/2 values indicate retention of the

anion in the coordination sphere of Cu(II). E1/2 values become cathodic in the order Cl < NO3
for the ligand L. Earlier reported E1/2 values of the benzimidazole-based Cu(II) complexes
[17, 20, 21] are less positive than for the present complexes. Copper(II) complexes of bis-
benzimidazolyl thioether with five-membered chelate rings show that the potential increases with
increase in the number of thioether donors [22, 23].
The present E1/2 values are also more cathodic than previously reported for
benzimidazolyl amide based complexes [12, 13b]. Thus it may be concluded that there could be
a role of the sulfur in the ligating system for stabilizing the Cu(II) complexes.
3.3.7. EPR Data. X-band EPR spectra of the copper(II) complexes were recorded in DMF
(Supplementary Material and table 5). Solution spectra of both copper complexes typically
indicate a d
ground state (g > g > 2.0023). EPR spectra of some Cu(II) complexes show
2 2
x -y
less than four lines and broadening of g line, indicating a severe distortion of tetragonal
geometry [24]. The values of hyperfine coupling constant A are also low in comparison to the
normal range found for other copper(II) complexes. Further, no nitrogen super hyperfine splitting
could be observed, implying non-planarity of the complexes. The values of g/A follow the
order [Cu(L)NO3]NO3 ~ [Cu(L)Cl]Cl, implying a similar solution structure for both the
complexes.
3.3.8. Fluorescence. Figure 3 shows the solvent dependence of fluorescence (λexc 277 nm) for L
in MeOH, DMF and MeCN. The intensity of the emission at ~297 nm is attributed to
benzimidazolyl in the ligand that is a known fluorophore [25]. The λmax of the emission band
slightly shifts in various solvent systems in the range ~295-297 nm (figure 3a). These small
shifts are indicative of varying degree of solvation of the excited state and imply that there is
minimal change in the dipole moment of the fluorophore in the excited state, with varying
solvent system [26]. The emission intensity for L decreases in the order MeOH > MeCN > DMF,
showing a loss of almost 27% from MeOH to DMF (figure 3a). The fluorescence quenching is
strongly affected by the nature of the solvent. Quenching is higher in DMF than MeOH and
CH3CN. As PET mediated quenching occurs relatively more rapidly in a polar solvent, the
quenching effect can be attributed to a PET process [27]. Fluorescence spectra also reveal the
8

presence of an emission band at ~487 nm (figure 3b); small shifts are observed with change in
solvent system. Quenching of this band is again largest in the more polar DMF in comparison to
MeOH.
The fluorescence emission spectra of the respective complexes [Cu(L)Cl]Cl and
[Cu(L)NO3]NO3 were recorded in MeOH, showing a nearly complete quenching of the emission
band at ~297 nm observed in the free ligand (figure 3a), and a quenching of nearly 40-60% is
found in the emission band at 487 nm (figure 3b). Such a quenching is attributed to the CHEQ
effect, however since the geometry of the two complexes is the same, the role of the anion in the
quenching process (chloride vs nitrate) cannot be ruled out.
4. Catalytic studies of [Cu(L)NO3]NO3
4.1. Oxidation of alcohol
A number of catalytic systems have been reported that oxidize aryl and alkyl based alcohols to
the corresponding carbonyl compounds. These include Ru(II) carbonyl [28] and phenanthroline
complexes [29] that mainly use N-methyl morpholine oxide and NaIO4 as respective oxidants
with product yields from 74-98%. Cu(II) azo [30] and a Ni Schiff base complex [31] have also
been utilized for oxidation of benzyl, piperonyl, cinnamyl and a variety of alkyl and benzyl
alcohols. Oxidants used were H2O2 and periodic acid, respectively, and product yields vary
between 60-90%. Cr(VI) has been utilized for oxidation of sugars, ethane and propane diols in
miceller media [32]. Selective oxidation of benzylic alcohol has been reported using a hybrid
catalyst, substituted polyoxometalates using oxidants H2O2, periodic acid and tert-butyl-
hydroperoxide. Yields of 100% were reported with H2O2 while they were only 48% for tert-
butyl-hydroperoxide [33].
We have utilized the present copper(II) complex [Cu(L)NO3]NO3 to catalyze the
oxidation of pyridyl and naphthyl alcohols using tert-butyl-hydroperoxide (TBHP) as alternative
source of active oxygen for which few reports are available [29, 34].
4.2. General procedure
The complex (10 mg) was dissolved in MeOH (5 ml) and methanolic solution (5 ml) of
substrates (various alcohols) and 0.02 ml of TBHP was added. The ratio catalyst:substrate:TBHP
was 1:20:20. The reaction mixture was stirred at 45 C on a water bath for 12 h. The reaction
9

mixture was analyzed by TLC. The reaction mixture was evaporated to near dryness on a rotary
evaporator and extracted with dichloromethane. The products in this extracted reaction mixture
were analyzed using GCMS.
When the same reaction was performed without complex, no product could be obtained
during the same period, implying that [Cu(L)NO3]NO3 catalyzes the oxidation of alcohols to the
corresponding aldehydes. The general reaction may be represented as follows:
4.3. Oxidation of alcohols using [Cu(L)NO3]NO3
4.3.1. Oxidation of 3-pyridyl carbinol. 3-Pyridyl carbinol oxidizes to form nicotinaldehyde
(82.0%, mol. weight 107) and nicotinic acid (18.0%, mol. weight 123). These two products were
found at retention times 10.4 and 14.8 using GCMS technique (table 6).
4.3.2. Oxidation of 1,2,3,4-tetrahydronaphthol. 1,2,3,4-Tetrahydronaphthol (mol. weight 148,
retention time 16.0) oxidizes to form 3,4-tetrahydronaphthalenal-(2H)-one (56.9%). This product
was found at retention times 16.4 and 28.1 (two isomeric forms, mol. weight 146) using the
GCMS technique (table 6).
4.3.3. Oxidation of 4-methoxybenzyl alcohol. 4-Methoxybenzyl alcohol (mol. weight 138,
retention time 14.3) oxidizes to form 4-methoxybenzaldehyde (mol. weight 136) (15.4%) and
4-methoxybenzoic acid (mol. weight 152) (8.22%). These two products were found at retention
times 13.8 and 15.4 using the GCMS technique (table 6).
4.3.4. Oxidation of 4-nitrobenzyl alcohol. 4-Nitrobenzyl alcohol (mol. weight 153, retention
time 21.9) oxidizes to form 4-nitrobenzaldehyde (mol. weight 151) (4.9%) and 4-nitrobenzoic
acid (mol. weight 166) (11.0%). These two products were found at retention times 18.0 and 21.5
using the GCMS technique (table 6).
10

5. Conclusion
The present work describes the synthesis and characterization of a new N2O2S2 donor
benzimidazolyl-based ligand (L) and its copper(II) complexes. A main peak in the ESI/MS+ at
468 corresponds to the molecular ion peak for L. SCXRD on [Cu(L)NO3]NO3·H2O reveals that
copper has a distorted octahedral geometry. The ligand provides four coordination sites while the
remaining two coordination sites are provided by a bidentate nitrate. One nitrate group is
uncoordinated, supported by molar conductance data which indicate that at least one anion is
dissociated in solution. The presence of nitrate is supported by bands in the IR spectra. The a, b,
c and α, β, γ parameters obtained from the powder XRD of [Cu(L)Cl]Cl were matched with that
of [Cu(L)NO3]NO3 and a close fit suggests that both possess a similar lattice structure.
The Cu(II) complexes exhibit a broad d–d band at 700-790 nm, characteristic of
copper(II) in a tetragonal geometry. Cyclic voltammograms show that the E1/2 values are more
cathodic than previously reported for benzimidazolyl amide based complexes. Anodic shifts in
E1/2 indicate the retention of an anion in the coordination sphere of Cu(II). EPR spectra of both
copper complexes indicate a dx2-y ground state (g > g > 2.0023). The values of the hyperfine
2
coupling constant A are low, implying non-planarity of the complexes. The fluorescence
emission intensity for L decreases in the order MeOH > MeCN > DMF, while the emission
spectra of the respective complexes show a nearly complete quenching of the emission band at
~297 nm in the free ligand. Such quenching is attributed to the CHEQ effect. [Cu(L)NO3]NO3 is
used to catalyze the oxidation of pyridyl, naphthyl, and benzyl alcohols, with highest yields and
high selectivity for pyridyl carbonyl oxidation, in comparison to naphthyl and benzyl alcohols.
Supporting information
Crystal data in CIF format for [Cu(L)(NO3)]NO3·H2O. CCDC-921436 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgement
We gratefully acknowledge financial support from the University of Delhi for a special grant.
Thanks to AIRF (JNU) for the GCMS data.
11

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13

Table 1. Crystal data and structure refinement for [Cu(L)(NO3)]NO3·H2O.
Empirical formula
Formula weight
Temperature
C22H24CuN8O9S2
672.15
298(2) K
Wavelength
0.71073 Å
Monoclinic
C 2/c
Crystal system
Space group
Unit cell dimensions
a = 18.4094(8) Å, α = 90°
b = 15.6079(8) Å, β = 112.336(4)°
c = 23.7095(13) Å, γ = 90°
Volume
6301.4(5) Å3
Z
8
Density (calculated)
Absorption coefficient
F(000)
1.417 mg/m3
0.884 mm-1
2760
Crystal size
0.12 × 0.08 × 0.06 mm3
Theta range for data collection
Index ranges
2.89 to 25.00°
-21≤h≤21, -18≤k≤18, -28≤l≤28
40794
Reflections collected
Independent reflections
Completeness to θ = 25.00°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
5543 [R(int) = 0.0720]
99.8%
Semi-empirical from equivalents
0.9489 and 0.9014
Full-matrix least-squares on F2
5543 / 2 / 378
1.075
R1 = 0.0779, wR2 = 0.2236
R1 = 0.1055, wR2 = 0.2431
0.951 and -0.509 e.Å-3
14

Table 2. Selected bond lengths (Å) and angles (°) for [Cu(L)(NO3)]NO3.
Cu(1)-N(6)
Cu(1)-N(2)
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-O(3)
Cu(1)-O(4)
S(1)-S(2)
1.950(5)
1.974(5)
2.061(4)
2.157(5)
2.176(8)
2.433(7)
2.025(3)
N(6)-Cu(1)-N(2)
N(6)-Cu(1)-O(1)
N(2)-Cu(1)-O(1)
N(6)-Cu(1)-O(2)
N(2)-Cu(1)-O(2)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-O(3)
O(2)-Cu(1)-O(3)
O(1)-Cu(1)-O(4)
173.9(2)
94.06(19)
85.91(19)
90.35(19)
95.73(19)
96.60(19)
165.8(2)
97.0(2)
113.2(2)
15

Table 3. IR Spectral data of ligands and Cu(II) complexes.
Assignments (cm-1)
OH
NH
NH
C=O
C=O
-C=C-N=C
group
freq
benz.
C-S

stretch
amide
benzimi
amide I
amide II
special
for anions
stretch
Complexes
Ligand [L]
3310
3421
3450
3235
3250
3257
3062
3065
3058
1646
1654
1625
1540
1540
1543
1426
1455
1453
-
-
740
745
745
677
663
663
[Cu(L)Cl2]
[Cu(L)(NO3)2]
1384
16

Table 4. Electrochemical data of the Cu(II) complexes.
Complexes
Ec (mV)
Ea (mV)
E1/2 (mV)
[Cu(L)Cl2]
-138
-254
-41
-48.5
-69
[Cu(L)(NO3)2]
+116
*values with respect to Fc/Fc+ of +56 mV
17

Table 5. X-Band EPR data of the Cu(II) complexes.
2
g/A × 104
Complexes
A
g
g
[Cu(L)Cl2]
143
147
2.28
2.33
2.07
2.07
0.558
0.702
160
159
[Cu(L) (NO3)2]
18

Table 6. Oxidation products of alcohols using [Cu(L)(NO3)]·NO3.
Products Retention Time (min)
Substrates
Yield
O
3-Pyridyl carbinol
Nicotinaldehyde
10.4
82.0 (%)
N
N
HO
O
OH
Nicotinic acid
14.8
18.0 (%)
56.9 (%)
N
O
1,2,3,4-Tetrahydronaphthol
3,4-Dihydronaphthalen-1(2H)-one
16.4
OH
O
4-Methoxybenzyl alcohol
4-Methoxybenzaldehyde
4-Methoxybenzoic acid
13.8
15.4
15.4 (%)
8.2 (%)
HO
O
O
O
O
HO
-O
O
4-Nitrobenzyl alcohol
4-Nitrobenzaldehyde
4-Nitrobenzoic acid
18.0
21.5
4.9 (%)
-O
OH
N+
N+
O
O
O
O
11.0 (%)
N+
HO
O-
19

Figure 1. ORTEP diagram of [Cu(L)(NO3)]NO3·H2O drawn in 20% thermal probability
ellipsoids showing the atom numbering scheme. Hydrogen and lattice water have been removed
for clarity. Only one site of disordered oxygen (O5A) of bound nitrate is shown.
20

Figure 2(a). PXRD pattern of [Cu(L)Cl] and Le-Bail fit of the powder X-ray diffraction pattern.
Red (observed), black (calc.), blue (Obs-Calc), vertical line (Bragg reflection).
S
A
5
10
15
20
25
30
35

Figure 2(b). (A) Powder XRD pattern of Cu(L)NO3. (S) Simulated pattern obtained from single
crystal data.
21

A
200
150
100
50
A: 5uM Ligand in MeOH
B: 5uM Ligand in CH3CN
C: 5uM Ligand in DMF
D: 5uM Cu(L)Cl in MeOH
E: 5uM Cu(L)NO3 in MeOH
B
C
E
D
0
300
350
400
Wavelength(nm)
Figure 3(a). Fluorescence spectrum of L (Curves A-C) in different solvents. Curves D and E
represent the fluorescence of respective complexes in methanol.
600
A: Ligand in MeOH
B: Lignad in CH3CN
A
500
400
300
200
100
0
C: Ligand in DMF
D: Cu(L)Cl in MeOH
E: Cu(L)NO3 in MeOH
B
C
D
E
400
500
600
700
Wavelength(nm)
Figure 3(b). Fluorescence spectrum (480 nm) of L (Curves A-C) in different solvents. Curves
D and E represent the fluorescence of respective complexes in methanol.
22