Solvent dependent disproportionation of Cu(II) complexes of N2O2-type ligands: Direct evidence of formation of phenoxyl radical: An experimental and computational study

Article abstract of DOI:10.14233/ajchem.2015.19193

Four Cu(II) complexes (1, 2, 3 and 4) with N₂O₂-type ligand, H₂L₁, H₂L₂, H₂L₃ and H₂L₄, respectively have been synthesized as the functional model for galactose oxidase. In presence of acetonitrile the Cu(II) centres in the complexes, undergo reduction with simultaneous oxidation of the ligands. The ligand oxidized products are isolated and characterized. Spectroscopic studies indicate that this disproportionation goes through the formation of a Cu(II)-phenoxyl intermediate. The complexes also undergoes the same reaction with pyridine, which indicates the involvement of the exergonic N-donor ligand for the formation of Cu(II)-phenoxyl complex. The Cu(II)-phenoxyl complexes are found to be stable in methanol in presence of a strong base. The paramagnetic centers in the Cu(II)-phenoxyl complexes were found to be weakly ferromagnetically coupled. The complexes, in acetonitrile solvent, have been found to oxidize primary alcohols to corresponding aldehydes. In absence of single crystal structures of the complexes, we optimized the structures using density functional theory (DFT). The UV-visible peaks of complexes as found from time dependent density functional theory (TDDFT) calculations match well with the observed experimental results.

Full text of DOI:10.14233/ajchem.2015.19193

Asian Journal of Chemistry; Vol. 27, No. 12 (2015), 4490-4500
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.19193
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
O -Type Ligands: Direct
2
Evidence of Formation of Phenoxyl Radical: An Experimental and Computational Study
1
2
3
RAJIB KUMAR DEBNATH , APURBA KALITA , SOURAB SINHA ,
3
2
1,*
PRADIP KR. BHATTACHARYYA , BIPLAB MONDAL and JATINDRA NATH GANGULI
1
2
3
Department of Chemistry, Gauhati University, Guwahati-781 014, India
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781 039, India
Department of Chemistry, Arya Vidyapeeth College, Guwahati-781 016, India
*
Corresponding author: E-mail: jatin_ganguli_gu@yahoo.co.in
Received: 24 March 2015; Accepted: 20 May 2015;
Published online: 29 August 2015;
AJC-17496
Four Cu(II) complexes (1, 2, 3 and 4) with N
2
O
2
-type ligand, H
2
L
1
, H
2
L
2
, H
2
L
3
and H
2
L , respectively have been synthesized as the
4
functional model for galactose oxidase. In presence of acetonitrile the Cu(II) centres in the complexes, undergo reduction with simultaneous
oxidation of the ligands. The ligand oxidized products are isolated and characterized. Spectroscopic studies indicate that this
disproportionation goes through the formation of a Cu(II)-phenoxyl intermediate. The complexes also undergoes the same reaction with
pyridine, which indicates the involvement of the exergonic N-donor ligand for the formation of Cu(II)-phenoxyl complex. The Cu(II)-
phenoxyl complexes are found to be stable in methanol in presence of a strong base. The paramagnetic centers in the Cu(II)-phenoxyl
complexes were found to be weakly ferromagnetically coupled. The complexes, in acetonitrile solvent, have been found to oxidize
primary alcohols to corresponding aldehydes. In absence of single crystal structures of the complexes, we optimized the structures using
density functional theory (DFT). The UV-visible peaks of complexes as found from time dependent density functional theory (TDDFT)
calculations match well with the observed experimental results.
Keywords: Galactose oxidase, N
2
2
O type ligands, Phenoxyl radical, DFT study.
Cu(II)-phenolate form has not been detected in the catalytic
INTRODUCTION
9,15,16
process
.
Galactose oxidase (GO), an extracellular copper-containing
A number of model studies for the active site of galactose
10,17-28
enzyme, is known to catalyze the two-electron oxidation of
primary alcohols to the corresponding aldehydes with a
oxidase have been reported in last few years
. Here we
report our observation of the solvent dependent formation of
mononuclear Cu(II)-phenoxyl radical complexes of ortho- and
1
-6
simultaneous reduction of dioxygen to hydrogen peroxide .
para-tert-butylated or methylated N
2
O ligands (Fig. 1) which
2
Galactoseoxidase
RCH OH + O   →RCHO+ H O
2
2
2
2
appeared to be valuable functional models for galactose
oxidase.At the same time, we observed that the reduction of the
Cu(II) to Cu(I) in Cu(II)-phenoxyl complexes are accompanied
In contrast to other copper proteins which catalyze the
multi-electron redox reactions, in galactose oxidase, an isolated
monocopper center effects the two-electron redox process
7
-10
29
.
by simultaneous decomposition of the ligands .
This paradox has been explained by the involvement of an
additional redox center, a coordinated tyrosyl radical from the
Density functional theory (DFT) has persistently substan-
tiated its significance in solving problems based on molecular
structure, providing platform to the researchers to explain the
subject under investigation which are a bit challenging using
11-14
protein chain . Structural studies revealed that there are two
distinct tyrosine residues in the active site of the inactive form
of galactose oxidase; one of them is a cysteine modified phenol
of tyrosine 272 which is coordinated to the copper center from
equatorial position whereas the other phenol group is from
30
experimental techniques . Time dependent density functional
31
theory (TDDFT) is one of the fascinating tools provided by
DFT to calculate the properties of molecules in its excited states.
As we are unable to get good quality crystals for structure deter-
mination, we have used DFT and TDDFT to get information of
structure and excited state chemistry of the complexes.
9
tyrosine 495 that binds the metal from an apical position . The
proposed mechanistic cycle of the oxidation process involves
Cu(I)-phenol and Cu(II)-phenoxyl radical states; however,

Vol. 27, No. 12 (2015)
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
2
O -Type Ligands 4491
3
3
OH
and XPREP software .Multi-scan empirical absorption correc-
34
N
tions were applied to the data using the program SADABS .
Structures were solved by direct methods using SHELXS-97
and refined with full-matrix least squares on F using SHELXL-
OH
N
N
N
2
35
9
7 .All non-hydrogen atoms were refined anisotropically. The
HO
hydrogen atoms were located from the difference Fourier maps
and refined. Structural illustrations have been drawn with
ORTEP-3 for Windows.
HO
2 2
H L
2 1
H L
Synthesis of ligand H
L
2 1
(Fig. 1) (C34
H
56
2
N O
2
): To a
OH
OH
solution of 2,4-di-tert-butylphenol (4.80 g, 0.023 mol) in EtOH
N
N
(15 mL) and water (3 mL) was added aqueous formaldehyde
37 %, 2.6 g, 0.032 mol), N,N-dimethyl-ethylenediamine (0.88
N
N
(
g, 0.010 mol) and triethylamine (0.500 mL, 0.004 mol) as a
catalyst. The resulting solution was kept in an oil bath (at 50
°C) for 48 h. The white precipitate formed was filtered, washed
HO
HO
with cold MeOH and dried under vacuo over P
g (75 %). Elemental analyses: Calcd. for C34
H, 10.11; N, 5.87; O, 6.66. Found (%): C, 77.81; H, 10.76; N,
2
O
5
.Yield: 4.586
H
2
L
4
H
56
N
2 2
O : C, 77.21;
2 3
H L
1
5
.34; O, 6.10. H NMR (400 MHz, CDCl
3
), δ ppm 1.25(18H,
2 2
Fig. 1. Structure of N O type ligands
s), 1.40(18H, s), 2.35(6H, s), 2.63(4H, t), 3.61(4H, s), 6.87(2H,
13
s), 7.20(2H, s); C NMR (100 MHz, CDCl
3
), δ ppm, 29.75,
1.91, 34.27, 35.22, 45.05, 49.25, 56.12, 56.77, 121.837,
23.53, 125.01, 136.28, 140.37, 153.47.
Synthesis of ligand H (Fig. 1) (C37
tion of 2,4-di-tert-butylphenol (4.80 g, 0.023 mol) in EtOH
COMPUTATIONAL METHODS
3
1
The geometrical minima of the species were optimized
with 6-31G(d,p) basis set with Becke three parameter exchange
and Lee, Yang and Parr correlation functional, B3LYP in gas
phase. Real frequencies of the systems confirmed that they are
at minima. Time dependent density functional theory calcula-
tions were performed using polarizable continuum model
2 2
L
H
54
N
2
O ):To a solu-
2
(
(
15 mL) and water (3 mL) was added aqueous formaldehyde
37 %, 2.6 g, 0.032 mol), 2-(2-aminoethyl)pyridine (1.22 g,
0
.010 mol) and triethylamine (0.500 mL, 0.004 mol) as a catalyst.
The resulting solution was kept in an oil bath (at 50 °C) for
8 h. The white precipitate formed was filtered, washed with
3
32
(
PCM)8 .All calculations were performed using Gaussian 09 .
4
EXPERIMENTAL
cold MeOH and dried under vacuo over P O . Yield: 4.680 g
2
5
(
72 %). Elemental analyses: Calcd. for C37
H
54
N
2
O : C, 79.56;
2
All reagents and solvents were purchased from commercial
H, 9.67; N, 5.02; O, 5.73. Found (%): C, 79.52; H, 9.74; N,
5
s), 1.27 (18H, s), 2.75 (2H, t), 3.04 (2H, t), 3.66 (4H, s), 6.8
(
8
3
1
sources and were of reagent grade. Acetonitrile was distilled
from calcium hydride. Deoxygenation of the solvent and solutions
were affected by repeated vacuum/purge cycles or bubbling
with nitrogen for 0.5 h. UV-visible spectra were recorded on a
Perkin Elmer Lambda-25 spectrophotometer. FT-IR spectra
were taken on a Perkin Elmer spectrophotometer with samples
prepared as KBr pellets. Solution electrical conductivity was
1
.01; O, 5.73. H NMR (400 MHz, CDCl
3
), δ ppm, 1.25 (18H,
2H, s), 6.97 (1H, d), 7.06 (2H, s), 7.17 (1H, t), 7.46 (1H, t),
13
.60 (1H, d). C NMR (100 MHz, CDCl ), δ ppm, 29.83,
3
1.90, 34.08, 35.13, 54.17, 57.11, 121.80, 122.09, 123.43,
23.72, 125.5, 136.31, 137.66, 140.60, 149.07, 153.19, 159.18.
1
Synthesis of ligand H
L
2 3
(Fig. 1) (C26
H
40
N
2
O ): (Schiff
2
checked using a Systronic 305 conductivity bridge. H NMR
base reaction) One equivalent of N,N-dimethylethane-1,2-
diamine (1.76 g, 20 mmol) and equivalent amount of salicyl-
aldehyde (2.44 g, 20 mmol) in methanol was stirred for 3 h. A
yellow coloured imine was formed. The imine was reduced
with 2.5 equivalents of sodium borohydride (1.90 g, 50 mmol).
After complete reduction a colourless solution was obtained.
The solution was stirred for 1 h. Then it was neutralized by
acetic acid to pH 7 and was extracted with chloroform to afford
spectra were obtained with a 400 MHzVarian FT spectrometer.
Chemical shifts (ppm) were referenced either with an internal
standard (Me Si) for organic compounds or to the residual
4
solvent peaks for copper complexes. The X-band electron
paramagnetic resonance (EPR) spectrum of complex 1 and of
the reaction mixture was recorded on a JES-FA200 ESR spectro-
meter, at room temperature as well as variable temperature.
Elemental analyses were obtained from a Perkin Elmer Series
II Analyzer. The magnetic moment of complex 1 is measured
on a Cambridge Magnetic Balance.
Crystal structure analysis: Single crystals were grown
by slow diffusion followed by slow evaporation technique.
The intensity data were collected using a Bruker SMART
APEX-II CCD diffractometer, equipped with a fine focus 1.75
/
/
L
3
(yield: 2.30 g, about 60 %). The L (0.97 g, 5 mmol) was
3
dissolved in methanol-water mixture (1:4), one equivalent of
formaldehyde (0.15 g, 5 mmol), one equivalent of triethyl-
amine (0.50 g, 5 mmol) and one equivalent of 2,4-di-tert-
butylphenol (1.03 g, 5 mmol) was added into reaction mixture
and refluxed for 3 days at 60 °C. A white precipitate was
observed, it was filtered, washed with water and dried (yield:
kW sealed tube MoK
with increasing w (width of 0.3° per frame) at a scan speed of
s/frame. The SMART software was used for data acquisition.
Data integration and reduction were undertaken with SAINT
α
radiation (λ = 0.71073 Å) at 273(3) K,
1
g, about 50 %) (Scheme-I). The NMR signals of H
2
L
3
are as
), δ ppm 1.25 (18H, s),
.40 (18H, s), 2.35 (6H, s), 2.63 (4H, t), 3.61 (4H, s), 6.87
1
follows: H NMR (400 MHz, CDCl
1
3
3

4
492 Debnath et al.
Asian J. Chem.
OH
OH
MeOH
+
N
Stirring for 3 h
N
H N
2
N
CHO
MeOH
NaBH4
N
OH
N
1
. MeOH/H O (4:1)
2
OH
2. HCHO (1 eq.)
H
N
3
4
5
. NEt3
N
. 2,4-ditert-butylphenol (1 eq.)
. Reffluxed for 60 h
HO
/
(
L )
3
H L
2
3
Scheme-I
1
(
2
1
2H, s), 7.20 (2H, s); H NMR (100 MHz, CDCl
3
), δ ppm,
triethylamine (0.50 g, 5 mmol) and one equivalent of 2,4-
dimethylphenol (0.61 g, 5 mmol) was added into reaction
mixture and refluxed for 3 days at 60 °C. A white precipitate
appears, it was filtered, washed with water and dried (yield:
9.75, 31.91, 34.27, 35.22, 45.05, 49.25, 56.12, 56.77,
21.837, 123.53, 125.01, 136.28, 140.37, 153.47.
Synthesis of 1igand H
2
L
4
(Fig. 1) (C20
H
28
N
2
O ): The Schiff
2
base reaction was carried out using one equivalent of N,N-
dimethyl-ethane-1,2-diamine (1.76 g, 20 mmol) and equivalent
amount of salicylaldehyde (2.44 g, 20 mmol) in methanol
medium.A yellow coloured imine was formed. The imine was
reduced with 2.5 equivalents of sodium borohydride (1.90 g,
0.82 g, about 50 %) (Scheme-II). The NMR signals of H
2
L
4
1
are as follows: H NMR (400 MHz, CDCl ), δ ppm, 1.25 (18H,
3
s), 1.27 (18H, s), 2.75 (2H, t), 3.04 (2H, t), 3.66 (4H, s), 6.8
(2H, s), 6.97 (1H, d), 7.06 (2H, s), 7.17 (1H, t), 7.46 (1H, t),
13
8.60 (1H, d). C NMR (100 MHz, CDCl ), δ ppm, 29.83,
3
5
0 mmol). After complete reduction a colourless solution was
31.90, 34.08, 35.13, 54.17, 57.11, 121.80, 122.09, 123.43,
123.72, 125.5, 136.31, 137.66, 140.60, 149.07, 153.19, 159.18.
obtained. The solution was stirred for 1 h. Then it was neutra-
lized by acetic acid to pH 7 and was extracted with chloroform
Synthesis of complex 1 [CuL
1
(CH
was dissolved in 10 mL distilled
methanol. To this solution, 0.524 g (1 mmol) of the ligand
3
OH)]: 0.370 g (1.0
/
4
/
4
II
to afford L
(yield: 2.30 g, about 60 %). The L
(0.97 g, 5 mmol)
mmol) of [Cu (H
2
O)
6 4 2
](ClO )
was dissolved in methanol-water mixture (1:4), one equivalent
of formaldehyde (0.15 g, 5 mmol), one equivalent of
2
H L
1
was added slowly with constant stirring. The colour of
OH
OH
MeOH
Stirring for 3 h
+
N
N
2
H N
CHO
N
MeOH
NaBH
4
N
OH
N
1
. MeOH/H
2
O (4:1)
OH
2. HCHO (1 eq.)
H
N
3
4
5
. NEt3
. 2,4-dimethyl phenol
. Reffluxed for 60 h
N
HO
(
L4
/
)
H L
2
4
Scheme-II

Vol. 27, No. 12 (2015)
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
2
O -Type Ligands 4493
the solution turned into violet from light blue. The stirring
was continued for 1 h at 298 K. The volume of the solution
then reduced to about 2 mL and kept it overnight at 273 K.
This resulted into violet colour microcrystalline precipitate of
complex 1. Yield: 0.588 g (63.36 %). Elemental analyses:
CDCl
(1H, s), 9.86 (1H, s), 11.66 (1H, s); C NMR (100 MHz,
CDCl ), δ ppm, 29.46, 31.52, 34.50, 35.21, 120.18, 128.05,
132.09, 95 137.67, 141.81, 159.29, 197.55. For L
(400 MHz, CDCl ), δ ppm; 1.28 (9H, s),1.40 (9H, s), 2.39
(6H, s), 2.69(4H, t), 3.66 (2H, s), 6.90 (1H, s), 7.20(1H, s);
3
), δ ppm, 1.33(9H, s), 1.43 (9H, s), 7.35 (1H, s), 7.60
13
3
/
1
1
, H NMR
3
Calcd. for CuC34
%): C, 69.41; H, 9.59; N, 4.76. ESI-mass (m/z) = 588.30,
magnetic susceptibility = χ , 1.20 B.M.
Synthesis of complex 2 [CuL (CH
was dissolved in 10 mL distilled
methanol solvent. To this solution, 0.558 g (1.0 mmol) of the
ligand H was added slowly with constant stirring. The colour
H
56
2 2
N O : C, 69.39; H, 9.52; N, 4.76. Found
13
(
C NMR (100 MHz, CDCl ), δ ppm, 29.88, 31.80, 34.45,
3
M
35.06, 45.87, 49.37, 51.56, 82.09, 119.18, 122.09, 136.78,
2
3
OH)]: 0.370 g (1.0
142.22, 150.83.
II
/
mmol) of [Cu (H
2
O)
6 4
](ClO )
2
Isolation of L
2
2 6 4 2
: 185 mg (0.5 mmol) of [Cu(H O) ](ClO )
was dissolved in 15 mL acetonitrile in a round bottom flask.
280 mg (0.5 mmol) of the ligand H was added to it and
L
2 2
2
L
2
of the solution turned into violet from light blue. The stirring
was continued for 1 h at 298 K. The volume of the solution
then reduced to about 2 mL and kept it overnight at 273 K. The
violet colour complex 2 was obtained as microcrystalline
precipitate.Yield: 0.622 g (67 %). Elemental analyses: Calcd.
stirred for 1 h. The transient green solution became yellow
and finally converted into the course of reaction. The volume
of the solution is then reduced using rotavapour and was kept
at 273 K for overnight. [Cu(CH
3
CN)
4
](ClO ) was crystallized
4
out. The solution was then dried and subjected to column
for CuC37
7
H
54
N
2
O
2
: C, 71.38; H, 8.68; N, 4.50. Found (%): C,
0.40; H, 8.75; N, 4.50. ESI-mass (m/z) = 621.35 and molar
susceptibility = χ , 1.28 B.M.
Synthesis of complex 3 [CuL
chromatographic separation. The 3,5-di-tert-butyl-2-
hydroxybenzaldehyde was eluted first by hexane and the amine
/
M
product, L
2
, was eluted with 10 % methanol in chloroform
/
1
3
(CH
was dissolved in 10 mL distilled
MeOH. To this solution, 0.412 g (1 mmol) of the ligand H
3
OH)]: 0.370 g (1
mixture. For L
2
, H NMR (400 MHz, CDCl ), δ ppm, 1.25 (9H,
3
II
mmol) of [Cu (H
2
O)
6 4
](ClO )
2
s), 1.27 (9H, s), 115, 2.70 (2H, t), 3.08 (2H, t), 3.68 (2H, s), 6.8
(2H, s), 6.97 (1H, d), 7.09 (1H, s), 7.15 (1H, t), 7.49 (1H, t), 8.62
2
L
3
13
was added slowly with constant stirring. The colour of the
solution turned into green from light blue. The stirring was
continued for 1 h at 298 K. The volume of the solution then
reduced to about 2 mL. To this, 10 mL of benzene was added to
make a layer on it and kept it overnight at 273 K. This resulted
into green colour crystal of complex 3. Yield: 0.594 g (80.45
(1H, d). C NMR (100 MHz, CDCl ), δppm, 29.75, 31.75, 34.11,
3
34.96, 54.03, 56.92, 121.68, 121.94, 123.25, 123.60, 125.30,
136.13, 137.63, 140.41, 148.86, 153.02, 159.05.
/
/
Isolation of L
was dissolved in 15 mL acetonitrile in a round bottom flask.
206 mg (0.5 mmol) of the ligand H or 164 mg (0.5 mmol)
of the ligand H was added to it and stirred for 1 h. The
3
/L
4
: 185 mg (0.5 mmol) of [Cu(H
2
O)
6
](ClO
4
)
2
2
L
3
%
). Elemental analyses: Calcd. for C27
H41CuN
2 3
O : C, 64.19;
2
L
4
H, 8.18; N, 5.55; Found (%): C, 64.56; H, 8.12; N, 5.43. Molar
transient green solution became yellow and finally converted
into the course of reaction. The volume of the solution is then
reduced using rotavapour and was kept at 273 K for overnight.
-1
2
-1
conductivity: [Λ
magnetic susceptibility = χ
Synthesis of complex 4 [CuL
M
= 23 W cm mol ]. ESI-mass (m/z) = 505.61,
, 1.28 B.M.
(CH
was dissolved in 10 mL distilled
MeOH. To this solution, 0.328 g (1 mmol) of the ligand H
M
4
3
OH)]: 0.370 g (1
[Cu(CH
then dried and subjected to column chromatographic separa-
tion. The 3,5-di-tert-butyl-2-hydroxybenzaldehyde (for H
and 3,5-di-methyl-2-hydroxybenzaldehyde (for H ) was
3
CN)
4
](ClO
4
) was crystallized out. The solution was
II
mmol) of [Cu (H
2
O)
6 4 2
](ClO )
2
L
4
2
L )
3
was added slowly with constant stirring. The colour of the
solution turned into green from light blue. The stirring was
continued for 1 h at 298 K. The volume of the solution then
reduced to about 2 mL. To this, 10 mL of benzene was added
to make a layer on it and kept it overnight on freezer. This
resulted into green colour crystal of complex 4.Yield: 0.512 g
2
L
4
/
/
eluted first by hexane and the amine product, L
3
or L was
4
/
eluted with 10 % methanol in chloroform mixture. For L
3
or
), δ ppm, 2.05 (6H, s), 2.22
(2H, t), 2.31 (2H, t), 2.95 (1H, s), 3.90 (2H, s), 7.02 (1H, t),
/
1
L
4
H NMR (400 MHz, CDCl
3
13
7.13 (1H, t), 7.29 (1H, d), 8.22 (1H, d), C NMR (100 MHz,
CDCl ), δ ppm, 44.25, 45.38, 54.51, 59.96, 77.03, 122.04,
126.08, 136.13, 149.25, 160.01.
(
5
5
82.23 %). Elemental analyses: Calcd. for C21
H
29CuN
2
O
3
: C,
9.91; H, 6.94; Cu, 15.09; N, 6.65; O, 11.40; Found (%): C,
9.77; H, 7.05; N, 6.73; O, 11.64; Cu, 14.81. Molar conductivity:
3
-1
2
-1
RESULTS AND DISCUSSION
The ligands, H , H ,H and H have been synthe-
sized using previously reported method of one-pot Mannich
[
Λ
M
=37 W cm mol ]. ESI-mass (m/z) = 421.14, magnetic
susceptibility = χ , 1.33 B.M.
M
2
L
1
2
L
2
2
L
3
2
L
4
/
Isolation of L
was dissolved in 15 mL acetonitrile in a round bottom flask.
62 mg (0.5 mmol) of the ligand H was added to it and
1
: 185 mg (0.5 mmol) of [Cu(H
2
O)
6
](ClO
)
4 2
3
6-38
synthesis . The formation of both the ligands have been
1
2
2
L
1
confirmed by various spectroscopic techniques like FT-IR, H
13
stirred for 1 h. The transient greenish solution became orange
in course of reaction. The volume of the solution was reduced
using rotavapour and was kept at 273 K for overnight.
NMR and C NMR, elemental analysis and single crystal
structures. The X-ray quality single crystals of the ligands,
2
H L
1 2
, H L
2 2
, H L
3
and H
2
L were grown by the slow diffusion
4
[
Cu(CH
was then dried and subjected to column chromatography. The
,5-di-tert-butyl-2-hydroxybenzaldehyde was separated by
3
CN)
4
](ClO
4
) was crystallized out. The remaining part
of dichloromethane solutions of the ligand into hexane
followed by slow evaporation at 298 K. The ORTEP diagrams
3
of the neutral H
in Fig. 2. The crystallographic data, bond angle and bond length
for neutral H , H ,H and H molecules are shown in
Tables 1-3, respectively.
2
L
1
, H
2
L
2 2
,H L
3
and H
2
4
L molecules are shown
/
hexane, yield (75 %). The amine product, L , was eluted with
methanol:chloroform (v/v, 1:10) mixture. Yield (70 %). 3,5-
di-tert-butyl-2-hydroxybenzaldehyde: H NMR (400 MHz,
1
2
L
1
2
L
2
2
L
3
2
L
4
1

4
494 Debnath et al.
Asian J. Chem.
(a)
(b)
(c)
(d)
Fig. 2. ORTEP diagram of the ligands: (a) H
2
L
1
(b) H
2
L
2
(c) H
2
L
3
and (d) H
2
L
4
(50 % thermal ellipsoid)
(a)
(b)
Fig. 3. Optimized structure of (a) complex 1 and (b) complex 2 at 6-31G (d,p) level of theory (for better visualization, H-atoms from the
structure has been omitted)

Vol. 27, No. 12 (2015)
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
2
O -Type Ligands 4495
TABLE-1
CRYSTALLOGRAPHIC DATA OF LIGANDS H L , H L , H L AND H L
2
1
2
2
2
3
2 4
Parameters
Formulae
m.w. (g)
Crystal system
Space group
Temperature (K)
Wavelength (Å)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
H L1
H L2
H L3
H L4
2
2
2
2
C H N O
C H N O
C H N O
C H N O
20 28 2 2
328.44
Monoclinic
P21/c
296(2)
0.71073
9.9314(5)
13.9518(7)
14.4574(7)
90.004(3)
108.314(3)
90.016(3)
1901.76(16)
4
1.147
0.128
712
24158
3376
28.30
--12<= h <=12
-16<= k <=16
-19 <= l <=17
81.60
3
4
56
2
2
37 54
2
2
26 40
2
2
524.81
558.82
Tetragonal
I4(1)/a
412.59
Monoclinic
P21/c
296(2)
0.71073
12.314(4)
13.416(4)
15.629(4)
90.00
96.895(6)
90.00
2563.3(13)
4
1.069
0.067
904.0
17936
4586
28.40
-16<= h <=15
-17<= k <=17
-20<= l <=20
98.10
Monoclinic
P2(1)/c
296(2)
0.71073
15.3374(9)
100.036(4)
21.7414(13)
90.00
100.047(2)
90.00
3399.2(4)
4
1.025
0.062
1160.0
23171
3554
296(2)
0.71073
27.9655(10)
27.9655(10)
17.7352(7)
90.00
90.00
90.00
13870.2(9)
16
1.070
0.065
4896.0
41097
2403
25.00
--28<= h <=22
-30<= k <=32
-19 <= l <=21
93.50
γ (°)
3
V (Å )
Z
-
3
Density (Mg m )
-
1
Abs. coefficient (mm )
F(000)
Total no. of reflections
Reflections, I > 2σ(I)
Max. 2θ (°)
28.36
-
20<= h <=20
-11<= k <=13
28<= l <=28
97.00
Ranges (h, k, l)
-
Complete to 2θ (%)
Full-matrix least-squares Full-matrix least-squares Full-matrix least-squares Full-matrix least-squares
Refinement method
2
2
2
2
on F
on F
on F
on F
Data/Restraints/Parameters
8249 / 0 / 360
0.1620
0.944
5731/0/384
0.0795
1.818
6321 / 0 / 281
0.1603
0.774
4630/0/223
0.1312
0.862
WR (all data)
2
2
Goof (F )
R indices [I > 2σ(I)]
R indices (all data)
0.0535
0.1473
0.0650
0.1841
0.0581
0.1617
0.0485
0.1221
TABLE2
SELECTED BOND LENGTH (Å) OF
LIGANDS H L , H L , H L AND H L
2
1
2
2
2
3
2
4
Bond
length (Å)
H L1
2
H L
H L
H L4
2
2
2
2
3
(a)
N(1)-C(1)
N(1)-C(5)
N(1)-C(8)
N(1)-C(20)
N(1)-C(23)
N(2)-C(2)
N(2)-C(3)
N(2)-C(4)
N(2)-C(7)
O(1)-C(11)
O(1)-C(26)
O(1)-C(10)
O(2)-C(29)
1.464(2)
1.472(2)
1.468(3)
1.464(2)
1.472(2)
1.468(3)
-
-
-
1.482(2)
-
1.482(2)
1.469(2)
-
1.469(2)
-
-
1.468(3)
-
1.468(3)
1.458(3)
1.459(3)
1.459(3)
-
1.369(2)
1.370(2)
-
-
1.458(3)
1.459(3)
1.459(3)
-
1.369(2)
1.370(2)
-
-
1.332(3)
1.332(3)
-
-
1.336(4)
-
1.336(4)
-
-
-
1.372(3)
1.374(3)
1.372(3)
1.374(3)
-
-
(b)
Since we could not grow good quality crystals for single
crystal X-ray analysis, optimized structures of the complexes
were obtained using DFT at B3LYP/6-31G (d,p) level of theory
32
using Gaussian 09 . The optimized structure of complexes
and 2 are shown in Fig. 3 and the shapes of HOMO are
1
presented in Fig. 4. The calculated values of bond lengths and
bond angles are shown in Tables 4 and 5, respectively. The
density functional studies showed that the complexes adopt
distorted square pyramidal geometry at the copper center. Some
of the key parameters of the complexes such as the bond length
Fig. 4. HOMO of (a) complex 1 and (b) complex 2, respectively

4
496 Debnath et al.
Asian J. Chem.
TABLE-3
SELECTED BOND ANGLES (°) FOR LIGANDS H L , H L , H L AND H L
2
1
2
2
2
3
2
4
Bond angle (°)
C(1)-N(1)-C(5)
C(1)-N(1)-C(8)
C(1)-N(1)-C(20)
C(20)-N(1)-C(5)
C(23)-N(1)-C(1)
C(23)-N(1)-C(8)
C(2)-N(2)-C(4)
C(2)-N(2)-C(3)
C(3)-N(2)-C(7)
C(4)-N(2)-C(3)
N(1)-C(1)-C(2)
N(1)-C(5)-C(6)
N(1)-C(20)-C(21)
N(1)-C(8)-C(9)
N(1)-C(23)-C(24)
N(2)-C(3)-C(2)
N(2)-C(3)-C(4)
N(2)-C(7)-C(6)
N(2)-C(2)-C(1)
O(1)-C(11)-C(6)
O(1)-C(11)-C(10)
O(1)-C(10)-C(9)
O(1)-C(10)-C(11)
O(2)-C(26)-C(21)
O(2)-C(26)-C(25)
O(2)-C(29)-C(24)
O(2)-C(29)-C(28)
H L
112.41(15)
-
110.97(14)
111.10(14)
H L2
-
109.60(2)
H L3
112.41(15)
H L4
-
109.60(2)
2
1
2
2
2
-
-
-
110.97(14)
111.10(14)
-
-
-
-
109.70(2)
111.10(2)
-
-
109.70(2)
111.10(2)
111.18(18)
110.50(2)
-
-
111.18(18)
110.50(2)
-
-
-
117.20(3)
-
117.20(3)
110.04(19)
113.61(16)
111.78(15)
112.33(15)
-
110.04(19)
113.61(16)
111.78(15)
112.33(15)
-
114.80(2)
114.80(2)
-
-
-
-
-
-
113.20(2)
113.20(2)
-
117.40(4)
122.60(4)
124.30(4)
-
-
-
113.20(2)
113.20(2)
-
117.40(4)
122.60(4)
124.30(4)
-
112.63(17)
112.63(17)
-
-
-
-
-
-
121.55(17)
117.79(17)
121.55(17)
117.79(17)
-
-
-
-
121.00(3)
117.70(3)
-
-
-
121.00(3)
117.70(3)
-
119.33(16)
119.29(16)
119.33(16)
119.29(16)
-
-
-
-
119.00(3)
119.10(3)
-
-
119.00(3)
119.10(3)
TABLE-4
IMPORTANT BOND LENGTHS OF COMPLEXES 1, 2, 3 AND 4 (OBTAINED FROM DFT STUDY)
Complex 1
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-O2
Cu1-O3
O1-C1
O2-C6
N1-C18
N2-C3
Bond length (Å)
Complex 2
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-O2
Cu1-O3
O1-C25
O2-C1
Bond length (Å)
Complex 3
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-O2
Cu1-O3
O1-C1
O2-C6
N1-C18
N2-C3
Bond length (Å)
Complex 4
Cu1-N1
Cu1-N2
Cu1-O1
Cu1-O2
Cu1-O3
O1-C25
O2-C1
Bond length (Å)
2.36
2.01
1.90
1.94
2.10
1.32
1.33
1.47
1.50
1.51
2.35
1.98
1.90
1.94
2.13
1.33
1.33
1.49
1.34
1.40
2.36
2.01
1.90
1.94
2.10
1.32
1.33
1.47
1.50
1.51
2.35
1.98
1.90
1.94
2.13
1.33
1.33
1.49
1.34
1.40
N1-C8
N2-C1
C1-C2
N1-C8
N2-C1
C1-C2
C2-C3
C2-C3
TABLE-5
IMPORTANT BOND ANGLES OF COMPLEXES 1, 2, 3 AND 4 (OBTAINED FROM DFT STUDY)
Complex 1
N1-Cu1-O1
N2-Cu1-O1
N1-Cu1-O2
N2-Cu1-O2
O1-Cu1-O2
O2-Cu1-O3
Cu1-O1-C1
Cu1-O2-C6
N2-C2-C3
Bond angle (°)
Complex 2
N1-Cu1-O1
N2-Cu1-O1
N1-Cu1-O2
N2-Cu1-O2
O1-Cu1-O2
O2-Cu1-O3
Cu1-O1-C25
Cu1-O2-C14
O1-Cu1-O3
Cu1-N1-C7
Bond angle (°)
Complex 3
N1-Cu1-O1
N2-Cu1-O1
N1-Cu1-O2
N2-Cu1-O2
O1-Cu1-O2
N1-Cu1-O3
Cu1-O1-C1
Cu1-O2-C6
N2-C3-C2
Bond angle (°)
Complex 4
N1-Cu1-O1
N2-Cu1-O1
N1-Cu1-O2
N2-Cu1-O2
O1-Cu1-O2
N1-Cu1-O3
Cu1-O1-C1
Cu1-O2-C6
N2-C3-C2
Bond angle (°)
102
97
114
96
142
77
127
119
115
105
98
108
94
108
140
75
124
128
93
95
97
116
97
146
92
126
118
114
105
96
94
111
96
150
99
127
113
115
106
Cu1-N2-C3
112
Cu1-N2-C3
Cu1-N2-C3
of Cu1-O1 (1.90), Cu1-O2 (1.94) for complex 1, Cu1-O1
The conductivity study shows that the complexes are non-
ionic in nature and the IR study shows that the ligand to metal
binding is phenolate in nature. The ESI-mass study shows that
the absence of perchlorate in the complexes.
(
(
1.94), Cu1-O2 (1.90) for complex 2, Cu1-O1 (1.89), Cu1-O2
1.92) for complex 3 and Cu1-O1 (1.89), Cu1-O2 (1.93) for
complex 4 obtained after optimization are, are comparable to
bond length data obtained from X-ray crystal structure of
UV-visible spectral studies: The complexes 1, 2, 3 and 4
in methanol solution, display absorptions in UV-visible region
6
,13
related galactose oxidase model complexes
.

Vol. 27, No. 12 (2015)
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
2
O -Type Ligands 4497
0
.40
.35
characteristic to the typical square-pyramidal phenolato-Cu(II)
charge transfer. Fig. 5 shows the representative spectrum of
0
II
complex 2 in methanol solvent. The Ophenolato → Cu charge
-1
-1
0.30
0.25
transfer bands appear at 490 (ε = 1430 M cm ) and 497 nm
-
1
-1
-1
-1
(
1
ε = 1270 M cm ), 490 (ε = 987 M cm ) and 442 nm (ε =
-1
-1
39,40
146 M cm ) in case of 1, 2, 3 and 4, respectively
.
0.20
0.15
0.10
0.05
0
1.2
3
15
415
515
615
715
815
915
Wavelength (nm)
Fig. 7. UV-visible spectrum of Cu(II)-phenoxyl radical complex generated
from complex 1 in methanol in presence of NaOEt
0.2
0
2
70
370
470
570
670
770
870
970
Wavelength (nm)
Fig. 5. UV-visible spectrum of complex 2 in methanol at room temperature
However, the phenolate Cu(II) transition, in dry and distilled
acetonitrile has been found to decrease in intensity very rapidly
which is attributed to the reduction of Cu(II) to Cu(I) (Fig. 6)
presumably through the very unstable Cu(II)-phenoxyl radical
29
complex .
-0.0085
350
450
550
650
750
Wavelength (nm)
0
.4
.2
II
2
Fig. 8. UV-visible spectral monitoring of the reduction of Cu(II) in [L Cu ]
in presence of pyridine in methanol solvent. The intensity of the
phenolate → Cu charge transfer band at about 500 nm decreases
II
II
with time in presence of pyridine {[L
2
Cu ]:Pyridine = 1:5]
0
In order to assign the UV-visible spectra, we performed
TDDFT calculations on the optimized structures of the comp-
lexes at B3LYP/6-31G(d,p) level of theory in methanol using
the PCM model . The calculated wavelength (λcal), major
molecular orbitals involved in the transitions, oscillator strengths
4
1
0
3
80
480
580
680
780
880
980
(
f) and coefficients of the wave function corresponding to a
) along with the experimental values
Wavelength (nm)
particular transition (Φ
c
Fig. 6. UV-visible monitoring of the formation of Cu(II)-phenoxyl radical
complex by the addition of acetonitrile in methanolic solution of
complex 1
of the wavelengths (λexp) are given in Table-6.
Time dependent density functional theory calculations
confirmed that, in complex 1 there is one intense band at 481.22
nm with a high oscillator strength (f = 0.0126). This peak is
close to the experimentally observed peak at 497 nm. There is
also a weak band at 523.85 nm (with f = 0.0094), the 481.22
nm band originates mainly from the HOMO-2 → HOMO
transition with a large coefficient (0.5544) for this complex.
However for complexes 2 and 4, we observed intense bands at
465 nm and 487 nm (with f = 0.0032 and 0.0416 respectively)
which are in well accord with the experimentally observed
peaks. There are also some weak bands arising at 536 and 418
nm (with f = 0.0261 and 0.0032). The 465 and 487 nm bands
(in complexes 2 and 4, respectively) originate mainly from
the molecular orbitals HOMO-2 → HOMO and HOMO-1 →
HOMO respectively and have the largest coefficients (0.6639
and 0.7534, respectively). Interestingly, in complex 3, one
It has been found that these phenoxyl radical species are
thermally unstable. However, they are found to be stable in
presence of base (NaOEt) (Fig. 7) .
It has been found that addition of pyridine instead of
acetonitrile to the methanolic solution of complexes 1, 2, 3
and 4 at room temperature also results into the gradual decrease
in intensity of the Ophenolato → Cu charge transfer bands. This
fact can be attributed to the reduction of Cu(II) center to Cu(I)
via the formation of intermediate Cu(II)-phenoxyl complex
Fig. 8). However, we are not able to trap the Cu(II)-phenoxyl
29
II
(
radical complex in this case because of the rapid rate of reaction
at room temperature. These results indicate that the exergonic
nitrogen donor ligands/solvents play an important role to
initiate the Cu(II)-phenoxyl radical complex formation.

4
498 Debnath et al.
Asian J. Chem.
TABLE-6
CALCULATED WAVELENGTHS (nm), MAJOR ORBITALS INVOLVED, OSCILLATOR STRENGTH (f) AND
COEFFICIENTS OF THE WAVE FUNCTION CORRESPONDING TO A PARTICULAR TRANSITION (Φ )
c
OBTAINED FROM THE TD-DFT CALCULATIONS IN ADDITION WITH THE EXPERIMENTAL RESULTS
Complexes
1
λcal (nm)
481
Orbitals involved
HOMO-2 → HOMO
HOMO-2 → HOMO
HOMO-1 → HOMO
HOMO-1 → HOMO
HOMO-2 → HOMO
HOMO-1 → HOMO
HOMO-2 → HOMO
f
Φc
λexp (nm)
497
0.0126
0.0032
0.0261
0.0417
0.0032
0.0416
0.0032
0.55444
0.66393
0.71122
0.75535
0.65192
0.75340
0.65299
465
490
2
3
4
5
36*
490
490
442
4
4
20*
87*
418
** No experimental peaks observed.
350
intense peak at 490 nm is in concurrence with the experi-
mental data which arises from the transition from HOMO-1
(a)
→
LUMO, having oscillation strength (f = 0.0032) and
coefficient (0.7553).
EPR studies: The X-band EPR spectra in methanol
solution of 1, 2, 3 and 4 at 298 K exhibit axial four line spectra
29
0
characteristic to S = ½ signals with Cu-hyperfine splitting .
II
The spectral features are typical to the square-pyramidal Cu -
complexes. The calculated parameters are as follows. Complex
-
4
-1
1
2
g
2
, g|| = 2.285; g
.310; g = 2.065;A = 110 × 10 cm , Complex 3, g|| = 2.297;
⊥
= 2.031; A = 111 × 10 cm ; complex 4, g|| = 2.335; g =
⊥
= 2.022; A = 109 × 10 cm ; complex 2, g|| =
-4 -1
⊥
-4
-1
⊥
-4
-1
-350
.073;A = 113 × 10 cm . These values match well with those
100
200
300
400
reported for similar model compounds of the inactive form of
Filed (mT)
1
0,18,19,21,42-44
350
galactose oxidase
.
It is interesting to note that in acetonitrile solvent, the
complexes display two distinct signals in X-band EPR at room
temperature which is characteristic of spin triplet state with
(b)
7,11,29
very weak ferromagnetic coupling
. One signal appears at
g = 2.06 as a broad one for Cu(II) center and other one is very
sharp isotropic signal for organic radical at g = 1.99 charac-
teristic of phenoxyl radical (Fig. 5).
0
The broad signal for Cu(II) and the radical signal have
been found to be disappeared with time and with equal
proportion indicating an immediate reaction between the Cu(II)
center and phenoxyl radical (Fig. 9). This further supports the
formation of only one phenoxyl radical though there are two
-350
320
330
340
350
Filed (mT)
phenolate groups. Shimazaki et al. have reported that N
type tripodal ligands with stoichiometric amounts of Cu(II)
perchlorate results into a Cu(II) phenoxyl radical and Cu(I) in
2
O
2
1
500
000
(c)
1
2
9,45-48
disproportionation reaction
. On the other hand, Stack et
49
al. have reported Cu(III) complexes by disproportionation
of Cu(II). Our present observations clearly indicate that the
disproportionation take place through Cu(II) phenoxyl radical
complex formation. Shimazaki et al. have also suggested that
a dimeric intermediate plays a key role in the copper(II) dispro-
portionation.
0
46
-1000
On the other hand, in absence of base, the green solution
which has been observed after addition of acetonitrile to the
methanolic solution of 1, 2, 3 and 4 readily turned colourless
in air at room temperature and yielded the colourless crystals
-2000
2500
-
100
200
300
Filed (mT)
400
500
Fig. 9. (a) X-band EPR spectrum of the Cu(II)-phenoxyl radical complex
generated from complex 1 in acetonitrile at room temperature. The
signals were found to decrease in intensity rapidly with time. (b)
the sharp radical signal of (a). (c) X-band EPR spectrum of complex
1 in methanol
of [Cu(CH
3
CN)
4
]ClO . This reaction has been found to take
4
place even under dinitrogen condition; which indicates that
the radical formation is not due to the oxidation by oxygen. These
reductions are found to be associated with the concomitant

Vol. 27, No. 12 (2015)
Solvent Dependent Disproportionation of Cu(II) Complexes of N
2
2
O -Type Ligands 4499
decomposition of the ligands H
2
L
1
, H
2
L
2
, H
2
L
3
and H
2
L
4
to
Conclusion
The present study demonstrates a new set of Cu(II)
complexes with N -type ligand as the functional model for
/
/
/
/
L
2
1
, L
-hydroxybenzaldehyde in case of H
dimethyl-2-hydroxybenzaldehyde in case of H
2
, L
3
and L
4
, respectively along with 3,5-di-tert-butyl-
, H , H and 3,5-
(Scheme-
2
L
1
2
L
2
2
L
3
2
O
2
2
L
4
galactose oxidase active site. It has been found that the Cu(II)
centers in the complexes, in presence of acetonitrile, undergo
reduction with a concomitant oxidation of the ligands. The
ligand oxidized products are isolated and characterized.
Spectroscopic studies indicate that this disproportionation goes
through the formation of a Cu(II)-phenoxyl intermediate.
Instead of acetonitrile, pyridine can also results into the same
reaction which indicates the involvement of the exergonic
N-donor ligand for the formation of Cu(II)-phenoxyl complex.
The Cu(II)-phenoxyl complex is found to be stable in methanol
in presence of base. The complexes show galactose oxidase
activity to oxidize primary alcohols to corresponding aldehydes
in actonitrile solvent. The present study demonstrates examples
of the formation of Cu(II)-phenoxyl complex where the para-
magnetic centers are very weakly coupled. A distorted square
pyramidal geometry of the complexes is confirmed by DFT
29
III) .
Galactose oxidase reactivity: The radical complexes
generated from complexes 1, 2, 3 and 4 are observed to oxidize
primary alcohols to corresponding aldehydes successfully. A
number of substituted benzyl alcohols have been found to result
into corresponding aldehydes when stirred with complex 1 in
38
acetonitrile solvent (Table-7) . The same reaction has not been
observed in methanol solvent even after stirring for 24 h in
presence of air at room temperature. However, addition of few
drops of acetonitrile to the methanolic solution leads to the
oxidation. This can be attributed to the fact that in presence of
acetonitrile the Cu(II)-phenoxyl radical forms, which actively
takes part in the oxidation of the primary alcohols. The
complexes 2, 3 and 4 have also been observed to induce the
same reaction with a variable turn over numbers.
N
MeCN
O
N
N
Cu
MeHO
O
O
MeOH
Cu
O
HO
N
+
[Cu(H
2
O)
6
](ClO
4
)
2
3
CH CN
N
N
OH
H L
[1]
2
1
[1']
(
i) [Cu(H O) ](ClO )
2 6 4 2
(
ii) CH CN
3
OH
OH
N
CHO
N
H
+
3 4
[Cu(CH CN)
]+
+
L '
1
Scheme-III
TABLE-7
SELECTED PRIMARY ALCOHOLS AND THEIR OXIDATION PRODUCTS
AFTER REACTION WITH COMPLEXES 1-4 IN ACETONITRILE
S. No.
1
Reactant alcohol
OH
Product aldehyde*
TON for 1
TON for 2
TON for 3
TON for 4
CHO
210
210
175
165
CHO
OH
2
3
230
170
230
170
185
155
170
145
CHO
OH
O N
2
O N
2
*All the products have been quantified by GC.

4
500 Debnath et al.
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2
2
2
2
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matched with the experimental value.
ACKNOWLEDGEMENTS
The authors acknowledge to CSIR for financial support
to R.K. Debnath. Thanks are also due to DST-FIST for single
crystal X-ray, CIF, IIT Guwahati for NMR & Mass spectro-
photometer, Arya Vidyapeeth College for conducting DFT
study and Department of Chemistry for providing the necessary
research facilities.
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