Copper(II) complexes of a new N-picolylated bis benzimidazolyl diamide ligand: Synthesis, crystal structure and catechol oxidase studies

Article abstract of DOI:10.1016/j.ica.2011.06.013

Copper(II) complexes of a new bis benzimidazole diamide ligand N-picolyl-N,N′-bis(2-methylbenzimidazolyl)hexanediamide [Pic-GBHA = L ₂] have been synthesized and characterized. One of the compound [Cu(L₂)(NO₃)₂] has been structurally characterized. The copper atom is bound to two benzimidazolyl nitrogen atoms, two amide carbonyl oxygen atoms and a bidentate nitrate ion, resulting in a distorted octahedral geometry. EPR spectra obtained at low temperature indicate a tetragonal geometry in the solution state. Complexes display a quasi-reversible redox wave due to the Cu(II)/Cu(I) reduction process having fairly cathodic E_1/2. These Cu(II) complexes were utilized to carry out oxidation of ditertbutylcatechol (DTBC) in methanol using molecular oxygen as the oxidant in. Low temperature EPR study of the oxidation reaction implicates the formation of an active copper species with fairly low A value. The presence of picolyl groups on the ligand also serve as a proton sponge giving 2-3 times higher rates of reaction in comparison to the non-picolylated ligand, implying a role of free basic groups in the pH control of enzymatic oxidation of catechols by catechol oxidase and tyrosinase.

Full text of DOI:10.1016/j.ica.2011.06.013

Inorganica Chimica Acta 376 (2011) 175–188
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Copper(II) complexes of a new N-picolylated bis benzimidazolyl diamide
ligand: Synthesis, crystal structure and catechol oxidase studies
Ruchi Bakshi a, Miriam Rossi b, Francesco Caruso c, Pavan Mathur a,
⇑
a Department of Chemistry, University of Delhi, Delhi, India
b Vassar College, Department of Chemistry, Poughkeepsie, NY 12604, USA
c Istituto di Chimica Biomolecolare, CNR c/o University of Rome ‘‘La Sapienza’’, Istituto Chimico, Piazzale Aldo Moro 5, 00185, Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Copper(II) complexes of a new bis benzimidazole diamide ligand N-picolyl-N,N0-bis(2-methylbenzimi-
dazolyl)hexanediamide [Pic-GBHA = L2] have been synthesized and characterized. One of the compound
[Cu(L2)(NO3)2] has been structurally characterized. The copper atom is bound to two benzimidazolyl
nitrogen atoms, two amide carbonyl oxygen atoms and a bidentate nitrate ion, resulting in a distorted
octahedral geometry. EPR spectra obtained at low temperature indicate a tetragonal geometry in the
solution state. Complexes display a quasi-reversible redox wave due to the Cu(II)/Cu(I) reduction process
having fairly cathodic E1/2. These Cu(II) complexes were utilized to carry out oxidation of ditertbutylcate-
chol (DTBC) in methanol using molecular oxygen as the oxidant in. Low temperature EPR study of the oxi-
Article history:
Received 17 March 2011
Received in revised form 3 June 2011
Accepted 7 June 2011
Available online 7 July 2011
Keywords:
Bis benzimidazole diamide
Copper(II)
Crystal structure
Catechol oxidation
dation reaction implicates the formation of an active copper species with fairly low A value. The presence
k
of picolyl groups on the ligand also serve as a proton sponge giving 2–3 times higher rates of reaction in
comparison to the non-picolylated ligand, implying a role of free basic groups in the pH control of enzy-
matic oxidation of catechols by catechol oxidase and tyrosinase.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
proposed that ligand flexibility is an essential condition for the
reactivity of copper complexes and geometric factors are impor-
Catechol oxidases are enzymes that possess a dinuclear copper
center in their active site, and the copper center is known to be
coordinated by three N-atoms from three histidines [1–4]. This en-
zyme catalyzes the substrate specific oxidation of o-diphenols to o-
diquinones in plants [5,6]. The structure of catechol oxidase from
sweet potatoes shows that the native state has both the coppers
in +2 oxidation state. This is called the oxidized met-form. The
reduction of the met form gives rise to the deoxy state where both
the coppers are in +1 oxidation state. The oxy-form results when
molecular oxygen oxidizes the deoxy-form of the enzyme [7].
The mechanistic cycle begins with the native met-form and a stoi-
chiometric oxidation of diphenol to the quinone takes place imme-
diately, thus this excludes the oxy-form in the beginning of the
cycle [8,9]. The activity of this enzyme is highest at pH ꢀ 8.0. The
loss of activity is observed at pH <5.0 and >10.0 [8–10]. A number
of coordination compounds have been synthesized in the recent
past so as to gain insight into the factors that control the oxidation
cycle, binding of the catechol to the active center and reactive
intermediates involved [11–15]. The above studies have suggested
that there is no direct relation between rates of reaction and redox
potentials of the biomimic coordination complexes. It has been
tant in understanding the difference in the reactivity of copper(II)
compounds [16]. Further it is necessary to have free coordination
site on the copper(II) coordination compound for weak binding of
the catechol. Therefore, ligands having small number of donor
atoms are important in generating the functional mimics of cate-
chol oxidase. In this context we have earlier reported new tetra
dentate N2O2 type bis benzimidazole diamide ligands and their
copper(II) complexes [17–20]. The present study reports the syn-
thesis of a new N-picolyl tetradentate bis benzimidazolyl diamide
ligand and its copper(II) compounds. It studies the effect of biolog-
ically innocent anions like acetate, phosphate, presence of an
intrinsic uncoordinated basic group in the ligand, impact of pH
changes and E1/2 on the rates of oxidation reaction.
2. Experimental
2.1. Materials required
Glycine benzimidazole dihydrochloride was prepared following
the procedure reported by Cescon and Day [21]. Freshly distilled
solvents were employed for all the synthesis. Spectroscopic grade
solvents were employed for spectral experiments. All other chem-
icals were of AR grade.
⇑
Corresponding author.
E-mail address: pavanmat@yahoo.co.in (P. Mathur).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.06.013

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R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
2.2. Physical measurements
program psi-scan was applied. The structure was solved by direct
methods (SHELXXS) and refined by full matrix least squares against
Elemental analyses of copper(II) complexes were obtained from
USIC, University of Delhi, Delhi and CDRI, Lucknow, India. Elec-
tronic spectra were recorded on a Shimadzu 1601 Spectrometer
at the Department of Chemistry, University of Delhi, Delhi. IR spec-
tra were recorded in the solid state as KBr pellets on a Perkin-Elmer
FTIR-2000 Spectrometer in the region of 400–4000 cmꢁ1. Cyclic
voltammetric measurements were carried out on a BAS-CV 50W
electrochemical analysis system. Cyclic voltammograms of all the
complexes were recorded in DMSO or DMSO/acetonitrile (2:3)
solutions, with 0.1 M TBAP as a supporting electrolyte. A three
electrode configuration composed of a Pt-disk working electrode,
a Pt wire counter electrode and Ag/AgNO3 reference electrode
was used for measurements. The reversible one-electron Fc/Fc+
couple has an E1/2 of 0.1 V versus an Ag/AgNO3 electrode. Solution
state EPR spectra were recorded on an X-Band Bruker-Spectrospin
with a variable temperature liquid Nitrogen cryostat at 120 K at
IIT-Kanpur, Kanpur, India. X-ray diffraction data for [Cu(L)(NO3)]
(NO3) were collected on a Bruker SMART APEX2 CCD single-crystal
diffractometer equipped with an Oxford systems liquid nitrogen
cryostream for collection at 125 K at Department of Chemistry,
Vassar College, New York. It was difficult to obtain a suitable crys-
tal since there was significant visible solvent loss from the sample
crystals (and backed up by the unusual anisotropic displacement
parameters for the atoms in the methanol solvent molecules that
indicate atomic motion). Data were collected from a crystal of
0.33 ꢂ 0.16 ꢂ 0.05 mm3 mounted on a cryo-loop with Paratone
oil. Relevant crystallographic data and structure refinement details
are listed in Tables 1 and Supplementary Tables 1.1–1.4. Mono-
F2 using SHELXL-97 software [22].
2.3. Synthesis of ligands and complexes
2.3.1. N,N0-Bis(2-methylbenzimidazolyl)hexanediamide [GBHA] [L1]
The ligand was prepared by following the procedure reported
earlier [17].
The crystalline product so obtained was analyzed for composi-
tion C22N6O2H24ꢃ2H2O.
1H NMR (d6-DMSO) d ppm: 1.5 (s, 4H), 2.2 (s, 4H), 4.4 (d, 4H,
J = 5.5 Hz), 7.1–7.2 (m, 4H), 7.4–7.5 (m, 4H), 8.4 (t, 2Hamide NH
J = 5.1 Hz), 12.1 (s, 2H)
,
H2C
C
H2
C
O
NH
NH
C
O
CH2
H2C
N
NH
HN
N
2.3.2. N-Picolyl-N,N0-bis(2-methylbenzimidazolyl)hexanediamide [Pic-
GBHA] [L2]
A solution of L1 (500 mg, 1.185 mmol) was suspended in 20 ml
dry DMF taken in a RB flask (100 ml) and stirred for 4–5 h with dry
K2CO3 (700 mg, 4.739 mmol) at 70–75 °C. When turbidity appeared,
2-picolyl chloride (800 mg, 4.739 mmol) was added and the solution
was left for stirring for the next 72 h at 70–75 °C. Subsequently, the
solution was centrifuged and the clear solution so obtained was
stripped off on a rotary evaporator. The residue was extracted with
chloroform. Upon adding hexane to the chloroform extract, an off
white precipitate was deposited. This was centrifuged and washed
with hexane. The dried compound was recrystallized using (1:1)
methanol:H2O solution. It analyzed for following composition:
chromated Mo Ka radiation (k = 0.71073 Å) was used for the mea-
surements. Appropriate empirical absorption correction using the
Table 1
Crystal data and structure refinement for Cu(L2)(NO3)2.
Empirical formula
Formula weight
T (K)
k (Å)
Crystal system
Space group
C34H34CuN9O5ꢃNO3ꢃ2CH3OHꢃH2O
846.27
125(2)
0.71073
triclinic
ꢀ
P1
2.3.2.1. C34N8O2H34ꢃH2O
. Anal. Calc.: C, 67.5; N, 18.5; H, 5.9. Found:
Unit cell dimensions
C, 67.0; N, 18.9; H, 6.0%. M.p. 206 °C; Yield: 347 mg (52%); 1H NMR
(d6-DMSO): 1.3 (s, 4H), 2.0 (s, 4H), 4.6 (d, 4H, J = 5.0 Hz), 5.6 (s, 4H),
7.1 (m, 6H), 7.2 (m, 2H), 7.3–7.4 (m, 2H), 7.5–7.6 (m, 2H), 7.7 (t, 2H,
a (Å)
b (Å)
c (Å)
9.573(5)
11.732(6)
17.830(10)
82.052(7)
84.214(7)
72.382(6)
1886.5(17)
2
1.490
0.649
846
0.33 ꢂ 0.16 ꢂ 0.05
1.83–23.35
ꢁ10 6 h 6 10, ꢁ13 6 k 6 13,
ꢁ19 6 l 6 19
16 201
a
(°)
J = 7.4 Hz), 8.4 (m, 4H) (Fig. 1); IR (KBr pellets): 3415 (
NH amide), 3067 ( NH benzim), 1643 ( C@O amide I), 1542 ( C–N amide II),
), 743 ( C@C benz); kmax (nm) (log ): 268(4.1),
mOH), 3316
b (°)
(m
m
m
m
c
(°)
V (Å3)
1446 (
m
m
e
benzim
275(4.0)C,@2N–8C@2C–(4.0)
Z
Dcalc (mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
H2C
C
)
H2
C
O
NH
N
NH
N
C
O
CH2
H2C
Crystal size (mm)
h (°)
Index ranges
H2
C
H2
C
N
N
N
N
Reflections collected
Independent reflections (Rint
Completeness to h = 23.35°
Absorption correction
Maximum and minimum
transmission
)
5424 (0.1536)
99.0%
empirical
0.9683 and 0.8143
Refinement method
full-matrix least-squares on F2
5424/483/503
0.994
2.3.3. [Cu(L2)X2] where L2 = Pic-GBHA; X = Clꢁ, NO3
ꢁ
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Solution (5 ml) of ligand L2 (0.5 mmol) in methanol was added
to a solution (5 ml) of CuX2ꢃnH2O (0.5 mmol) in methanol. The
resulting solution was stirred for 2 h. A green colored product
formed slowly. This was filtered, washed with small amount of
methanol and dried under vacuum over anhydrous CaCl2 and ana-
lyzed for following composition:
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.1066, wR2 = 0.2664
R1 = 0.2430, wR2 = 0.3600
0.008(3)
0.984 and ꢁ0.840
Extinction coefficient
Largest difference in peak and hole
(e Åꢁ3
)

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
177
b
O
O
a
C
C
a
b
e
e
e
e
N
N
NH
HN
N
N
f
f
j
j
f
f
CcH2
H C
c2
CdH2
g
H2C
d
g
h
h
N
N
i
i
j
j
* = DMSO
** = moisture
Fig. 1. 1H NMR of ligand L2 in d6-DMSO.
2.3.3.1. CuC34N8O2H34ꢃCl2ꢃ0.5H2O. Anal. Calc.: C, 55.9; H, 4.8; N, 15.4.
catechol was added. The final molarities are 2.01 mM (com-
plex) and 42.0 mM (DTBC). Addition of catechol to the
solution of the Cu(II) complex is accompanied by the appear-
ance of a new absorption band in the range 390–400 nm
(Fig. 2a and 2b). The growth of the new band with time
was studied spectrophometrically. This new band at
390 nm is assigned to the o-benzoquinone being formed as
a result of catechol oxidation [17,18].
Found: C, 55.2; H, 4.8; N, 15.3%. kmax (nm) (log e) (MeOH): 285(4.0),
277(4.0), 268(4.1), 765(1.8), E1/2 (mV) = ꢁ76, IR (cmꢁ1, KBr): 3410
(
(
m
m
OH), 3120 (mNH amide), 3044 (mNH benzim), 1657 (mC@O amide I), 1531
C–N amide II), 1454 (
m
C@N–C@C– benzim), 752 (
m
C@C benz). Yield (%): 85.
2.3.3.2. CuC34N9O5H34ꢃ(NO3)ꢃCH3OHꢃ0.5H2O1,2
.
Anal. Calc.: C, 51.6;
H, 4.8; N, 17.2. Found: C, 51.5; H, 4.4; N, 17.6%. kmax (nm) (log e)
(MeOH): 284(3.9), 276(3.9), 268(3.9), 786(1.6), E1/2 (mV) = ꢁ201,
IR (cmꢁ1, KBr): 3415 (
OH), 3101 ( NH amide), 2931 ( NH benzim),
1618 ( C@O amide I), 1545 ( C–N amide II), 1478 ( C@N–C@C– benzim), 748
C@C benz), 1383( O–N–O (symm.)), 879( O–N–O(asymm.)). Yield (%): 80.
(b) Effect of anion: The reaction in (a) was also carried out in the
presence of sodium acetate (0.03 mmol). The growth of the
new band at 390 nm was found to be faster with respect
to the reaction carried out in (a). Effect of changing sodium
acetate with sodium phosphate (0.03 mmol) was also
studied.
(c) The same set of experiment (a and b) were also carried
out with the complex [Cu(L2)Cl2] (7 ꢂ 10ꢁ3 mmol) and
DTBC (0.135 mmol) the final ratios being 1:20 {complex:
DTBC}.
m
m
m
m
m
m
(m
m
m
2.4. Oxidation of 3,5-ditertbutylcatechol by [Cu(L2)(NO3)2] and
[Cu(L2)Cl2] using molecular oxygen
In the present study, two complexes [Cu(L2)(NO3)2] and
[Cu(L2)Cl2] were utilized for a detailed study of the oxidation of
3,5-ditertbutylcatechol (DTBC). These experiments were followed
spectrophotometrically, and by EPR studies.
3. Results and discussions
2.5. Experimental procedure
3.1. Description of the crystal structure
(a) A methanolic solution of the complex [Cu(L2)(NO3)2] (3 ml,
6 ꢂ 10ꢁ3 mmol) saturated with O2 displays a d–d band at
ꢀ780 nm. To this solution 0.126 mmol of 3,5-ditertbutyl-
The crystal structure of the complex [Cu(L2)(NO3)2] is shown in
Fig. 3a–3d. The coordination environment around the copper atom
is 6-coordinate consisting of two benzimidazolyl nitrogen atoms
and two amide carbonyl oxygen atoms and two oxygen atoms of
the nitrate ion, the nitrato chelate is asymmetric, i.e., one of its O
atoms shows a longer Cu–O bond length creating a distorted octa-
hedral system (Fig. 3a). This asymmetric type of bonding by nitrate
1
Solvent molecules are found to be different from crystal stoichiometry as
compound tends to loose solvent molecules at room temperature.
2
Presence of water of crystallization was ascertained by IR and TGA studies.

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R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
Fig. 2a. UV–Vis spectra of DTBC + Cu(L2)(NO3)2 at neutral pH; curve 1 – for [Cu(L2)(NO3)2] complex of nitrate system (6 ꢂ 10ꢁ3 mmol, O2 saturated methanol), curve 2 – on
adding DTBC (0.126 mmol) to the complex.
anion is seen in many structures [19,20]. The crystal structure has
centers of inversion in between the C54 and its equivalent C540,
and C52 and C520 ligand atoms (Fig. 3b); this allows the ligand to
extend its arms to bind with a symmetry related copper atom so
that the complex forms a one-dimensional polymer. Each copper
atom is coordinated through one benzimidazole N (N1) and one
amide carbonyl (O2) of one arm of the ligand, while the other
arm is coordinated to the second inversion center related copper
atom. The one-dimensional polymer can be described with the
bonding sequence being: –Cu–ligand–Cu–ligand– such that the
two different benzimidazole units are coordinated to a given Cu(II)
ion are from different ends of the ligand (Fig. 3c). Complexes with
similar ID chain structures are available in literature [18,23]. The
equatorial Cu–N bond distances of 1.975(9) Å (Cu–N1) and
1.976(9) Å (Cu–N3) are in the range found for similar benzimid-
azole and imidazole ligated compounds. These Cu–N bonds are less
than 2 Å, indicating a strong coordination of benzimidazoles [24].
The Cu–O (amide) bond distances of 2.013(8) Å (Cu–O2), 2.19(1) Å
(Cu–O1) are in the range of those reported in the Cambridge Struc-
tural Database for amide carbonyl compounds. The nitrate ion
binds asymmetrically to Cu(II) and there is a long Cu–O (nitrate)
(O76) bond, 2.587(10) Å, compared with the strongly bound O70
of 2.060(9) Å. The 2nd nitrate anion is not coordinated to the metal.
The apical Cu–O bond is not exactly perpendicular to the square
plane, the angles N1–Cu–O1, N3–Cu–O1 and O2–Cu–O1 being near
95°. The equatorial bond angles deviate slightly from the expected
value of 90°, the maximum deviation being 5.1°. Two trans O atoms
are at angle of 175.5(5)° and trans nitrogen atoms are not com-
pletely linear N1–Cu–N3, bond angle being 170.3(4)°. The slight
bending of N1–Cu–N3 axis indicates that two benzimidazole rings
are not lying in the same plane. The sum of equatorial bond angles
is 360° indicating that Cu atom resides in the square plane. Two
methanol and one water molecule are hydrogen bonded to each
other and to the complex. Fig. 3d shows this arrangement. The
hydrogen bonding linkage is N43 (picolyl)–O1S (methanol 1)–O2S
(methanol 2)–O1W (water)–O1W (water)–O2S–O1S–N43. O1W
also has two hydrogen bonds with two oxygen atoms of the bound
nitrate ligand (O1W–O76 and O1W–O75). The solvent molecules
fill a hole in the crystal packing.
3.2. 1H NMR, IR spectra and electronic spectroscopy
3.2.1. 1H NMR
The 1H NMR spectrum of the free ligand L2 shows signals for ali-
phatic and aromatic protons with theoretically predicted splitting.
A signal observed at 8.4 ppm is assigned to amide NH protons (cou-
pled with adjacent CH2 protons). The CH2 protons give rise to a
doublet at 4.4–4.6 ppm due to coupling with adjacent amide NH
proton [17,18]. A singlet at 5.6 is observed which is assigned to
the CH2 group attached to pyridine ring. A multiplet is observed

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
179
Fig. 2b. UV–Vis spectra of DTBC + Cu(L2)(NO3)2 at neutral pH; curve 1 – on adding DTBC (0.126 mmol) to the complex at 15 min and curve 2 – at 285 min on 100-fold
dilution.
in the range of 7.1–7.8 ppm and is assigned to the protons of the
benzimidazole ring.
mC@N–C@C– stretching frequencies, respectively. On complexation,
a decrease in amide I and an increase in amide II stretching
frequencies have been observed which is in accordance with the
coordination of the ligand through amide carbonyl oxygen [17–
20]. The nitrate complex shows bands at 1383 and 879 cmꢁ1 due
to mO–N–O (symm.) and mO–N–O (asymm.) stretching, confirming a
coordinated nitrate group [25].
3.2.2. IR spectral properties
The free ligand L2 has characteristic IR bands at 1638, 1553 and
1433 cmꢁ1. These are assigned to amide I (mainly mC@O amide I
stretch), amide II (mainly mC@N amide II stretch) and benzimidazole

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R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
Fig. 3a. Coordination scheme of [Cu(L2)(NO3)2].
Fig. 3b. The asymmetric unit of the complex (without solvent molecules) showing the coordination scheme and atom labeling; atoms C52 and C54 ligand atoms are where
the structure extends itself through inversion centers to create a one-dimensional polymer.
3.2.3. Electronic spectroscopy
The electronic spectra of copper(II) complexes were recorded in
HPLC grade MeOH. Two peaks in the range 270–285 nm are
transition of benzimidazole moiety in the ligand. The bands show
small shifts towards higher energy upon complexation with
change in their extinction coefficients when compared with the
ligand. The complexes exhibit a symmetrical but broad d–d band
observed for the complexes, and are assigned to intraligand
p–
p⁄

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
181
10
8
6
4
2
0
-2
-4
-6
-8
400
200
0
-200
-400
-600
Potential, mV
Fig. 4a. Cyclic voltammogram of [Cu(L2)Cl2] in DMSO at scan rate 50 mV/s.
3.3. Cyclic voltammetry and EPR spectroscopy
Fig. 3c. This shows the one-dimensional polymer. Atoms C52 and C54 are shown as
balls; the inversion centers lie between the C54–C54 and the C52–C52 bonds.
3.3.1. Cyclic voltammetry
The complexes display a quasi-reversible redox wave in their
cyclic voltammograms due to the Cu(II)/Cu(I) reduction process
(Fig. 4a and 4b). Cathodic shifts in E1/2 values indicate the retention
of anion in the coordination sphere of Cu(II). E1/2 values are found
to be cathodic in the order Clꢁ < NO3ꢁ. E1/2 values are quite nega-
tive contrary to that observed earlier [17–20].
in the region of 750–800 nm. This is characteristic of a tetragonal
geometry for bound Cu(II). A shoulder is observed in the region
of 300–350 nm in both the cases and is assigned to a Xꢁ ? Cu(II)
LMCT charge transfer band [13].
ꢁ
Fig. 3d. Showing the H-bonded interactions between solvent molecules and NO3 ion.

182
R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
3.3.2. EPR spectroscopy
10
8
X-band EPR spectra of the copper(II) complexes were recorded
in MeOH or DMSO:MeOH (1:6) as a frozen sample at liquid nitro-
gen temperature (Figs. 5a, 6a and Table 2). The spectra typically
6
indicate a dx2
ground state (g > g\ > 2.0024) with low hyperfine
ꢁy2
k
coupling constant.
4
2
3.4. Oxidase studies
0
3.4.1. EPR studies and d–d band changes during the oxidation reaction
(a) The EPR spectrum of the parent complex [Cu(L2)(NO3)2] in
MeOH solution (saturated with molecular O2) at 120 K shows a line
-2
-4
-6
-8
with g\ = 2.07, and four hyperfine lines with g = 2.31 and A of
k
k
133 G (Fig. 5a).
Major changes are observed in its EPR spectrum on addition of
DTBC (Fig. 5 and Table 3). The blow up of the spectrum shows, that
the four hyperfine lines are shifted to lower fields with g = 2.41
k
400
200
0
-200
-400
and A = 114 G (Fig. 5d). An increase in g with a corresponding de-
k
k
Potential, mV
crease in A value is indicative of formation of a relatively less pla-
k
nar Cu(II) species in solution. This is also indicated by the
Fig. 4b. Cyclic voltammogram of [Cu(L2)(NO3)2] in DMSO at scan rate 50 mV.
parameter g /A which changes from 173 to 211 [26].
k
k
20
(a)
(b)
20
0
-20
-40
-60
0
2000
3000
[G]
4000
2600 2700 2800 2900 3000 3100 3200
[G]
(d)
5
(c)
10
4
3
2
1
0
0
-10
-20
-30
-1
2600
2800
3000
3200
2000
3000
[G]
4000
[G]
Fig. 5. X-band EPR spectra of DTBC + Cu(L2)(NO3)2 at neutral pH (a) blank [Cu(L2)(NO3)2] complex (b) blow up of the blank complex (c) at 0 min after adding DTBC in the
Cu(L2)(NO3)2 complex solution (d) blow up at 0 min.

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
183
Fig. 6. X-band EPR spectra of DTBC + Cu(L2)Cl2 at neutral pH (a) blank [Cu(L2)Cl2] complex (b) blow up of {a} (c) at 0 min after adding DTBC in the Cu(L2)Cl2 complex solution
(d) blow up of {c} [N = new species; O = old species].
Table 2
Table 3
X-band EPR data of copper(II) complexes with ligand Pic-GBHA (L2) at 120 K.
EPR data observed with DTBC and [Cu(L2)(NO3)2] as catalyst at pH 7.
Complexes
Solvent
A
g
g\
(g /A ) ꢂ 104
A
g
g\
(g /A ) ꢂ 104
k
k
k
k
k
k
k
k
[Cu(L2)Cl2]
[Cu(L2)(NO3)2]
MeOH:DMSO (6:1)
MeOH
130
133
2.36
2.31
2.07
2.07
182
173
Complex
+DTBC (0 min)
30 min
133
114
123
2.31
2.41
2.41
2.07
2.08
2.08
173
211
195
This species continues to exist for the next 60 min over which
no further changes in EPR parameters are observed (Fig. 5). Thus,
it is this low planarity Cu(II) species existing as an intermediate
which carries out the catalytic oxidation of DTBC.
(b) The EPR spectrum of a solution of [Cu(L2)Cl2] in MeOH:DM-
SO (6:1) solution (saturated with molecular oxygen) at 120 K
also accompanied with a nearly 15% reduction in d–d band inten-
sity (Supplementary Fig. 1.1(b)). This decrease in band intensity
implies the partial reduction of parent Cu(II) complex to a Cu(I)
species.
Similar results were obtained in case of oxidation reaction with
the [Cu(L2)Cl2], where slight broadening of the d–d band at 765 nm
is accompanied with a nearly 20% reduction in d–d band intensity
on adding DTBC.
(d) The changes observed in d–d band and EPR spectral param-
eters during oxidation reaction, taken together suggests that the
shows
a line with g\ = 2.07, and four hyperfine lines with
g = 2.36 and A = 130 G (Fig. 6a). Addition of DTBC to the above
k
k
solution causes the number of hyperfine lines to increase as shown
in Fig. 6 and Table 4. A careful examination of seven lines shows
that four lines correspond to the field position as found for the par-
ent complex, while the remaining three lines are due to a new spe-
cies that forms because of interaction of the DTBC with [Cu(L2)Cl2]
complex. The new species has a g = 2.40 and A = 110 G. This spe-
k
k
Table 4
cies then continues to exist for the next 60 min as shown by EPR
spectra (Fig. 6d). This new relatively non-planar species has EPR
parameters similar to the species generated in the case of
[Cu(L2)(NO3)2].
(c) Addition of 3,5-ditertbutylcatechol (DTBC) to the [Cu(L2)
(NO3)2] complex causes a slight broadening of the d–d band and
the band shifts to lower energies, (kmax = 800 nm). This change is
EPR data observed with DTBC and [Cu(L2)Cl2] as catalyst at pH 7.
A
g
g\
(g /A ) ꢂ 104
k
k
k
k
Complex
+DTBC (0 min)
30 min
130
110
110
110
2.36
2.40
2.40
2.40
2.07
2.07
2.07
2.07
182
215
215
215
1 h

184
R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
Table 5
addition of 3,5-ditertbutylcatechol causes a more distorted Cu(II)
species to form in solution with low A value. This species then ex-
EPR data observed for [Cu(L2)(NO3)2] and sodium phosphate with subsequent
addition of DTBC.
k
ists during the oxidation cycle. Copper(II) species with such low A
k
A
g
g\
(g /A ) ꢂ 104
167
160
156
values have been reported for highly distorted five coordinated
complexes [26]. It is proposed that this low symmetry, non-planar
Cu(II) species could form by the loss of a donor group after the re-
dox reaction between DTBC and the Cu(II) center.
k
k
k
k
Complex + phosphate
+DTBC (0 min)
30 min
140
146
146
2.28
2.30
2.29
2.03
2.06
2.05
3.4.1.1. Effect of anion. Earlier work has shown binding of exoge-
nous anions to the half met-form of tyrosinase [27]. Keeping this
in mind, we have utilized PO43ꢁ and OAcꢁ anion to probe their role
in terms of binding to copper(II) as ligand or in the generation of
intermediate species during the catalytic oxidation cycle.
the low symmetry species generated by the addition of DTBC to
the parent complex in its absence.
(b) A similar experiment was conducted with acetate ions,
added in the form of solution of sodium acetate. The EPR spectrum
shows a Cu(II) species with g\ = 2.06 and g of 2.31 and A of 130 G.
k
k
(a) Thus the EPR spectrum of the complex [Cu(L2)(NO3)2] in
methanolic solution (saturated with molecular O2) was taken in
the presence of phosphate ions, added as a solution of sodium
phosphate. The EPR parameters are found to be g\ = 2.03 and
Thus the addition of acetate ions to the parent complex does not
lead to changes in the parent Cu(II) species. However, subsequent
addition of DTBC generates a Cu(II) species with g of 2.30 and A of
k
k
150 G. This species then remains unchanged for the next 60 min
g = 2.28 and A = 140 G (Fig. 7). Addition of 3,5-diterybutylcate-
during the catalytic cycle (Fig. 8 and Table 6).
k
k
chol to this gives an EPR spectrum with g of 2.30 and A of
146 G (Fig. 7c and Table 5). This Cu(II) species remains unaltered
for the next 60 min during the oxidation cycle.
It is important to note that the presence of phosphate ions
causes the intermediate species to be relatively more planar than
Further a free radical is observed at g ꢀ 2.0 immediately
(Fig. 8b). This radical signal is found to exist even after 30 min of
oxidation cycle. Such a signal at g ꢀ 2.0 has been earlier reported
to be due to benzosemiquinone anion radical formed from diter-
tbutylcatechol [28].
k
k
20
(a)
(b)
10
10
0
-10
-20
-30
(a)
(b)
0
-40
2600
2700 2800 2900 3000 3100 3200
2000
3000
[G]
4000
[G]
5
(d)
(c)
5
0
5-
10-
15-
0
(d)
(c)
2000
3000
[G]
4000
2600
2800
3000
3200
[G]
Fig. 7. X-band EPR spectra of (a) Cu(L2)(NO3)2 + phosphate ion (b) blow up of {a} (c) at 0 min after adding DTBC to the complex solution (c) blow up of {c}.

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
185
Table 6
(c) Addition of phosphate/acetate ions in the case of [Cu(L2)Cl2]
complex causes broadening and generation of weak shoulders on
EPR data observed for [Cu(L2)(NO3)2] and sodium acetate with subsequent addition of
DTBC.
the four hyperfine lines in the g region (Supplementary Figs. 1.4,
k
A
g
g\
(g /A ) ꢂ 104
1.5 and Tables 7, 8). It appears that two different types of species
exist. The signature of this multiple species does not change on
the addition of DTBC to the reaction mixture. However in the case
of acetate ions, a free radical signal is also observed at g ꢀ 1.98.
(d) When sodium phosphate ions are added to a methanolic
solution of the parent complex [Cu(L2)(NO3)2] and ditertbutylcate-
chol, it results in a slight broadening and a small shift of the d–d
band maxima to higher energies (Supplementary Fig. 1.3). Thus,
in this case there appears to be no large change in the equatorial
ligand field of the parent complex. Similarly when sodium phos-
phate is added to the mixture of [Cu(L2)Cl2] complex and ditertbu-
tylcatechol no substantial change in the Visible region is found.
However the addition of acetate ions leads to a very large increase
in band intensity in the visible region and a band with kmax at
715 nm is observed (Supplementary Fig. 1.2). It has been reported
earlier that a 3,5-ditertbuty-1,2-semiquinone anion radical has a
band at 730 nm [28]. It is proposed that the shift in kmax and an in-
crease in the band intensity (in the visible part of the spectrum) are
due to the formation of such a radical anion in the presence of so-
dium acetate.
k
k
k
k
Complex + acetate
+DTBC (0 min)
30 min
133
150
150
2.31
2.30
2.30
2.06
2.06
2.06
173
157
157
Table 7
EPR data observed [Cu(L2)Cl2] and sodium acetate with subsequent addition of DTBC.
A
g
g\
(g /A ) ꢂ 104
k
k
k
k
Complex + acetate
+DTBC (0 min)
30 min
120
140
135
140
2.41
2.40
2.39
2.40
2.03
2.07
2.06
2.04
209
171
177
171
1 h
Table 8
EPR data observed [Cu(L2)Cl2] and sodium phosphate with subsequent addition of
DTBC.
A
g
g\
(g /A ) ꢂ 104
k
k
k
k
A similar effect is observed when acetate ions are added to the
mixture [Cu(L2)Cl2] complex and ditertbutylcatechol, a change in
the kmax towards 700 nm occurs with an increased intensity in
the region of 700–750 nm. No substantial change in the visible
region is found when sodium phosphate is added to the mixture
Complex + phosphate
+DTBC (0 min)
30 min
120
123
123
123
2.33
2.32
2.32
2.32
2.06
2.08
2.06
2.06
194
188
188
188
1 h
10
(a)
(b)
5
0
1
0
-1
-2
-3
-5
-10
-15
-20
-25
2000
3000
[G]
4000
2400 2600 2800 3000 3200 3400 3600 3800 4000
[G]
2
(c)
1
0
-1
-2
-3
3150
3200
3250
3300
[G]
3350
3400
3450
Fig. 8. X-band EPR spectra of (a) Cu(L2)(NO3)2 + acetate ion (b) at 0 min after adding DTBC to the complex solution ion (d) at 0 min (blow up).

186
R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
Table 9
Rate of reaction observed with DTBC and [Cu(L2)(NO3)2] as catalyst.
Condition of complex
kmax (nm) of product o-benzoquinone
Rate of reaction (ꢂ10ꢁ5 M lꢁ1 minꢁ1
)
% conversion after 24 ha
Turnover no.
E1/2 (mV)
pH 7
Acetate ion
Phosphate ion
393
391
393
1.7(0.6)
Very fast(2.2)
3.3(1.2)
27(285)
97(1020)
38(406)
11
41
16
ꢁ201.5
ꢁ213.5
ꢁ208.0
a
Based on initial conc. of the substrate (catalyst).
Fig. 9. Plot of conc. of o-benzoquinone formed (mmol/l) vs. time (min) for oxidation of DTBC using [Cu(L2)(NO3)] as catalyst at (a) pH 7, (b) effect of phosphate ion, (c) effect of
acetate ion.
of [Cu(L2)Cl2] complex and ditertbutylcatechol. Thus, the visible
spectral data is in conformity with the EPR work.
sion to the quinone was found after 24 h with a turnover of 11-fold
(Scheme 1).
3.4.2. Kinetic studies
During the oxidation of 3,5-ditertbutylcatechol by the [Cu(L2)
(NO3)2], the concentration of corresponding quinone produced
3.4.2.2. Oxidation with [Cu(L2)(NO3)2] in presence of [PO43ꢁ] anion. -
When the oxidation reaction is performed in the presence of phos-
phate ions added as Na3PO4, the rate of reaction found for this is
3.3 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1 for the initial fast phase followed by a
slower rate of reaction (1.2 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1). When com-
pared to the reaction carried out in the absence of phosphate ion.
was calculated by using
e
= 1585 molꢁ1 cmꢁ1 [23,24]. A plot of
concentration of quinone produced versus time shows a stoichi-
ometric fast phase followed by slower catalytic reaction, for all
the cases. Initial rates have been calculated by drawing a tangent
to the curve at t = 0 and the rates have been given in Table 9 and
Fig. 9.
(a) The rate of reactions in presence of phosphate ion is found to
be twice as large. This is also supported by 39% conversion
found after 24 h with a turnover of 16-fold.
3.4.2.1. Oxidation with [Cu(L2)(NO3)2]. Rate of reaction was found to
be 1.7 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1 for the fast phase followed by a slower
average rate found to be 0.6 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1. A 27% conver-
(b) Based on EPR studies it is evident that presence of phosphate
ion generates an active Cu(II) species which is more planar
than the parent complex. It is proposed that the access of

R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
187
Mechanism of the Oxidation of 3, 5-Ditertbutylcatechol
DTBC
2CuIIL
DTBQ + 2H+
2-
O2
+
2CuIL'
2H+
H2O + 1/2O2
O2
2CuIIL'
Scheme 1. Mechanistic pathway for the oxidation of DTBC to o-benzoquinone in absence and presence of phosphate ions.
2.2 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1 with a turnover of 41 folds in 24 h. The
large formation of quinone and the presence of free 3,5-ditertbutyl
benzosemiquinone anion radical (as found by EPR and optical stud-
ies), suggests that the mechanism of oxidation must be quite dif-
ferent from the above two cases. It is proposed that molecular
oxygen rapidly oxidizes the benzosemiquinone radical formed as
the intermediate to the quinone. The superoxide radical generated
then oxidizes Cu(I) to Cu(II). This facilitates electron transfer and
enhances the rate of reaction in comparison to the above two cases.
This is depicted in Scheme 2.
2 DTBC2- + 2 H2O
DTBC2-
2 DTBC + 2OH-
CuIIL
2-
O2
.-
CuIL'.DTBC
3.4.2.4. Oxidation with [Cu(L2)Cl2] as catalyst. When oxidation reac-
tions were done using [Cu(L2)Cl2] as the catalyst, the rates of oxi-
dation reactions were found to be relatively faster than the
analogous [Cu(L2)(NO3)2] under similar conditions (Table 10 and
Supplementary Fig. 1.6).
O2
.
CuIL'O2
3.4.3. Role of uncoordinated pyridyl groups in the complex [Cu(L2)
(NO3)2] in the oxidation reaction
DTBQ
An experiment as described in procedure (a) was carried out
using the copper(II) complex [Cu(L1)(NO3)2] and DTBC, at pH ꢀ 5
(using 5 mM HCl). A plot of quinone formed versus time is shown
in Supplementary Fig. 1.7. Decomposition of the quinone formed
occurs after 20 min and the rate of the reaction is found to be
0.04 ꢂ 10ꢁ5 Mꢁ1 lꢁ1 minꢁ1. This is 10 times slower than found with
the complex [Cu(L2)(NO3)2]. The two complexes differ by the pres-
ence of two picolyl groups in the ligand L2 in comparison to L1. The
two extra picolyl groups having pyridyl nitrogen’s, are likely to act
as a proton sponge. Thus, they appear to be capable of compensat-
ing the lowering in pH to a large extent than the ligand L1, and thus
enhancing the rate of the reaction.
Scheme 2. Mechanistic pathway for the oxidation of DTBC to o-benzoquinone in
presence of acetate ions.
catechol to the Cu(II) center is probably easier in a relatively
more planar active species leading to relatively facile elec-
tron transfer.
(c) Further a tri negative ion would facilitate the forward reac-
tion as it would be capable of picking up the H+ ions gener-
ated during the deprotonation of DTBC and its subsequent
oxidation.
3.4.2.3. Oxidation with [Cu(L2 )(NO3)2] in presence of [CH3COOꢁ]
anion. In the presence of CH3COONa (0.03 mmol), the initial rate of
the oxidation of catechol is very fast and one observes a sudden
jump in the concentration of quinone. A near stoichiometric con-
version to the quinone is observed in the fast phase (Fig. 9c). Sub-
sequently a relatively lower rate of oxidation reaction is found,
3.4.4. Electrode potentials
The parent complex [Cu(L2)(NO3)2] in DMSO solution shows an
E1/2 of ꢁ201 mV (Supplementary Fig. 1.8(a)). Similarly in the pres-
ence of phosphate ion, E1/2 of the parent complex is found to shift
to ꢁ208 mV (Supplementary Fig. 1.8(c)), while in the presence of
acetate ions, it is found to shift at ꢁ213 mV (Supplementary
Table 10
Rate of reaction observed with DTBC and [Cu (L2) Cl2] as catalyst.
Condition of complex
kmax (nm) of product o-benzoquinone
Rate of reaction (ꢂ10ꢁ5 M lꢁ1 minꢁ1
)
% conversion after 24 ha
Turnover no.
E1/2 (mV)
pH 7
Acetate ion
Phosphate ion
398
396
398
2.0(1.4)
Very fast(5.6)
2.0
34(231)
90(810)
65(584)
8
28
20
ꢁ75.75
ꢁ79.00
ꢁ76.00
a
Based on initial conc. of the substrate.

188
R. Bakshi et al. / Inorganica Chimica Acta 376 (2011) 175–188
Fig. 1.8(b)). However, the rates of reaction of oxidation of catechol
in presence of phosphate and acetate ions are much larger as
compared to parent complex. The above observations therefore,
suggest that E1/2 has a minimal role in controlling the rate of reac-
tion during the oxidation cycle.
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2011.06.013.
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3ꢁ
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Acknowledgements
We gratefully acknowledge financial support from the Univer-
sity Grants Commission (UGC) and University of Delhi, Delhi, India
for special grant. X-ray diffraction facilities were provided by the
US National Science Foundation, Grant No. 0521237.
Appendix A. Supplementary material
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CCDC 768148 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/