Catecholase Activities of Copper(II) Complexes With N4 Ligands

Article abstract of DOI:10.1002/bkcs.12303

Two new N₄ ligands, 1,2-bis(2-((pyridine-2-yl)methyleneamino) phenyl)ethane (L¹) and tris((2-benzylideneamino) ethyl)amine (L²), derived from 2,2′-ethylenedianiline and tren, respectively, were developed and their copper(I

Full text of DOI:10.1002/bkcs.12303

Article
DOI: 10.1002/bkcs.12303
BULLETIN OF THE
S. Sarkar et al.
KOREAN CHEMICAL SOCIETY
Catecholase Activities of Copper(II) Complexes With N Ligands
4
†,‡
†
†,
*
Shuranjan Sarkar, Minyoung Kim, and Hong-In Lee
†Department of Chemistry and Green-Nano Research Center, Kyungpook National University, Daegu
4
1566, Republic of Korea. *E-mail: leehi@knu.ac.kr
‡
Chemistry Division, Bangladesh Jute Research Institute, Dhaka 1207, Bangladesh
Received March 16, 2021, Accepted May 2, 2021
1
Two new N4 ligands, 1,2-bis(2-((pyridine-2-yl)methyleneamino) phenyl)ethane (L ) and tris((2-ben-
2
0
zylideneamino) ethyl)amine (L ), derived from 2,2 -ethylenedianiline and tren, respectively, were devel-
1
2
oped and their copper(II) complexes [Cu(L )(ClO )](ClO )CH OHꢀH O and [Cu(L ) (CH COO)](ClO )
4
4
3
2
3
4
were synthesized. Crystal structures of the complexes were determined to show that both the complexes
have distorted square pyramidal coordinations with very similar geometrical parameters. Catecholase
activities of the complexes were investigated for the oxidation of 3,5-di-tert-butylcatechol (3,5-H dtbc) to
2
3
,5-di-tert-butyl-o-quinone (3,5-dtbq) in the presence of O . Kinetic behaviors and spectroscopic measure-
2
ments on the catalytic reactions were supportive to the proposal in which the mechanical pathway of the
catecholase activities by the complexes were initiated by the anaerobic oxidation of 3,5-H dtbc to
2
3
,5-dtbq, followed by the catalytic cycles with Cu(I) active forms.
Keywords: Cu(II) complex, N ligand, Crystal structure, Catecholase activity
4
Introduction
novel geometries and electronic properties of such coordina-
22,24,48–51
tion compounds.
We have been interested in devel-
Catecholase (catechol oxidase), a type-3 copper enzyme,
catalyzes the oxidation reactions of catechols to the
oping N ligands, which can form monomeric coordination
compounds, derived from 2,2 -ethylenedianiline to diversify
the ligand candidates for biomimetic complexes and to com-
4
0
1
,2
corresponding o-quinones. Many biomimetic compounds
with multicopper centers modeling the active site of
catecholase have been reported to show catecholase activi-
pare the coordination abilities of N ligands derived from tren
4
0
and 2,2 -ethylenedianiline.
3–23
ties.
In addition, model complexes with monocopper
In this study, we have developed two new ligands, 1,2-bis
centers have been studied to show that the complexes effec-
1
(
2-((pyridine-2-yl)methyleneamino)phenyl)ethane (L ) and
2
4–31
tively catalyze the oxidation of catecholes.
Mechanistic
2
tris((2-benzyl-ideneamino)ethyl)amine (L ), derived from
,2 -ethylenedianiline and tren, respectively, to compare the
investigations on the activity using the oxidation of 3,5-di-
0
2
tert-butylcatechol (3,5-H dtbc) to 3,5-di-tert-butyl-o-
2
spectroscopic and structural properties of their copper(II)
complexes. And the catecholase activities of the complexes
quinone (3,5-dtbq) by the monocopper complexes in the
presence of O have proposed three major catalytic path-
2
(Scheme 1) were investigated for searching the relationship
ways depending on the complexes: (i) formation of Cu(I)-
between the complex structure and mechanistic pathway of
catecholase activity.
ꢁ
(
semiquinonate) intermediate, followed by the generation
30,31
ꢁ
of 3,5-dtbq
; (ii) formation of Cu(I)-(semiquinonate)
intermediate, followed by the generation of free semi-
quinonate anion radical, subsequently forming 3,5-dtbq by
Experimental
2
4,29
disproportionation of the anion radical
; and (iii) anaero-
0
Materials. Tris(2-aminoethyl)amine, 2,2 -ethylenedianiline,
-pyridinecarboxaldehyde, benzaldehyde, copper(II)perchlo-
bically initial two-electron oxidation of 3,5-H dtbc to 3,5-
2
2
dtbq, followed by O activation for reoxidizing the
2
26,32,33
rate hexahydrate, copper(II)acetate hydrate, and sodium per-
chlorate were purchased from Sigma-Aldrich Chemicals and
were used without further purification. Acetonitrile and meth-
anol were purchased from J. T. Baker Chemicals. The sol-
vents were purified before use according to the standard
procedures.
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. Only small quantities of
material should be prepared, and the samples should be han-
dled with care.
copper(I) centers.
Tripodal N4 ligands derived from tren (tren = tris(2-
aminoethyl)amine) have been intensively developed for
34–40
investigating coordination properties to metal ions.
These
ligands have abilities of generating structurally diversified
metal complexes, which can act as attractive model com-
4
1–47
plexes mimicking enzymatic activities.
On the other
0
hands, N ligands derived from 2,2 -ethylenedianiline showed
4
a characteristics of forming polymeric or macrocyclic metal
coordination compounds, which are suitable for investigating
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Article
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KOREAN CHEMICAL SOCIETY
1
21.53 (ArC), 117.40 (ArC), 32.87 (N(PhCH CH Ph)N).
2
2
ꢂ
Melting point > 150 C.
Preparation of tris((2-benzylideneamino)ethyl)amine
2
2
(
L ). The ligand L was prepared by using the literature
52
method with slight modification. The 10 mL of a dried
methanol solution of benzaldehyde (1.53 mL, 15.0 mmol)
was added to a magnetically stirred 10 mL of a freshly
dried methanol solution of tren (tren = tris(2-aminoethyl)
Scheme 1. Catecholase activity by Cu(II) complex.
amine) (0.77 g, 5.0 mmol). The solution was refluxed for
ꢂ
2
h at 70 C. A yellow solid, obtained from vacuum evapo-
ration, was washed several times with acetonitrile and dried
under vacuum at ambient temperature to produce 1.85 g of
Preparation
of
1,2-bis(2-((pyridine-2-yl)
1
methyleneamino)phenyl)ethane (L ). The 10 mL metha-
nol solution of 2-pyridinecarboxaldehyde (0.97 mL,
2
the product (Scheme 2(b)). Yield: 90%. The ligand L was
1
13
identified by elemental analysis (EA), and H and
NMR (Supporting Information Figure S2) recorded in
CDCl . Elemental analysis calculated (%) for C H N : C,
C
10.0 mmol) was added into 10 mL solution prepared by
0
dissolving solid 2,2 -ethylenedianiline (1.08 g, 5.0 mmol)
in freshly dried methanol. The mixed solution was refluxed
for 2 h at 70 C. Yellow powder was obtained from vacuum
evaporation. The powder was washed several times with
methanol and dried under vacuum at ambient temperature
3
27 30 4
ꢂ
7
1
9.03; H, 7.31; N, 13.65. Found: C, 78.90; H, 7.32; N,
1
13
3.64. H and C NMR (Supporting Information Figure S2)
52
showed identical features to the previously reported data.
Melting point >200 C.
ꢂ
to give the final product of 1.85 g (Scheme 2(a)). Yield:
1
1
9
5%. The ligand L was identified by elemental analysis
Synthesis of [Cu(II)(L )(CH OH)](ClO ) ꢀCH OH (1).
3
4 2
3
1
13
(EA), and H and C NMR (Supporting Information -
Copper(II)perchlorate hexahydrate (0.76 g, 2.0 mmol) dis-
solved in 10 mL dry methanol was added drop by drop to
ꢂ
Figure S1) recorded in CDCl at 25 C. Elemental analysis
calculated (%) for C H N : C, 79.97; H, 5.68; N, 14.35.
3
1
2
6 22 4
10 mL methanolic solution of L (0.78 g, 2.0 mmol) and
1
Found: C, 79.90; H, 5.62; N, 14.30. H NMR data
400 MHz, CDCl , 298 K): δ (ppm) = 8.66 (d, 2H PyH),
stirred. After 1 h, a green crystalline powder was collected
by filtration and dried in vacuum under pressure to give
1.32 g of compound 1 (Scheme 2(a)). Yield: 90%. Elemen-
tal analysis calculated (%) for C H Cl CuN O
4 11
(
3
8
7
.31 (s, 4H PyH), 8.15 (d, 2H, NCH), 7.72 (t, 2H PyH)
.33–6.89 (m, 6H, ArH), 3.15 (s, 4H, N(PhCH CH Ph)N).
C NMR data (CDCl , 298 K): δ (ppm) = 159.37 (NCH),
2
2
28 32
2
1
3
3
(C H N O Cl Cu + H O): C, 45.75; H, 4.39; N, 7.62.
28 30 4 10 2 2
1
1
54.90 (PyC), 149.41 (ArC), 136.46 (PyC), 129.88 (ArC),
27.05 (PyC), 126.48 (PyC), 124.81 (PyC), 127.64 (ArC),
Found: C, 44.80; H, 3.72; N, 7.89. FAB MS analysis: m/z,
ion, base; 453.2, {[Cu(L )] H } , 100%; 552.1, {[Cu
1
+ +
Scheme 2. Syntheses of (a) compound 1 and (b) compound 2.
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Catecholase Activities of Copper(II) Complexes
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1
+
(
L )](ClO )} , 57%. (Supporting Information Figure S3).
Table 1. Crystal data and structure refinement for 1 and 2.
4
The powder was again dissolved in dry methanol (1 mM
solution) and taken in a test tube with diethyl ether (1:1,
v/v). Suitable crystals for X-ray analysis were obtained
from the solution after 1 day.
1
2
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
C H N O Cl Cu
717.02
293
0.71073
Triclinic
P1
C H N O ClCu
632.61
293
2
8
30
4
10
2
29 33 4 6
2
Synthesis of [Cu(L )(CH COO)](ClO ) (2). Copper(II)
3
4
0.71073
Triclinic
P1
10.6037(4)
10.8030(5)
14.6785(6)
79.966(1)
74.995(1)
72.890(1)
1531.78(11)
2
7302
(Rint = 0.0267)
1.072
acetate hydrate (0.37 g, 2.0 mmol) dissolved in 20 mL dry
methanol was added drop by drop to 10 mL methanol solu-
2
tion of L (0.82 g, 2.0 mmol) and stirred. After a blue
8.234(2)
solution appeared, 5 mL methanol solution of sodium per-
chlorate (0.25 g 2.0 mmol) was added. After 1 h, a blue
crystalline powder was collected by filtration and dried in
vacuum under pressure to give 1.14 g of compound
b, Å
11.476(3)
18.335(5)
78.169(4)
89.494(4)
75.156(4)
1637.3(7)
2
6029
(Rint = 0.0518)
1.047
c, Å
o
α,
β,
γ,
o
2
(Scheme 2(b)). Yield: 90%. Elemental analysis calculated
o
(
%) for C H ClCuN O : C, 55.06, H, 5.26, N, 8.86.
3
29
33
4
6
Volume, Å
Found: C, 54.92, H, 5.35, N, 8.80. FAB MS analysis: m/z,
Z
2
+ +
2
ion, base; 473.4, {[Cu(L )] H } , 100%, 532.4, {[Cu(L )
Independent
reflections
+
(
CH COO)]} , 84% (Supporting Information Figure S4).
3
The powder was again dissolved in dry methanol (1 mM
solution) and taken in a test tube with diethyl ether (1:1,
v/v). Suitable crystals for X-ray analysis were obtained
from the solution after 1 day.
Goodness-of-fit
2
on F
Final R indices
R1 = 0.0758,
R1 = 0.0458,
a,b
[I > 2σ(I)]
wR = 0.1544
wR = 0.1159
2
2
R indices (all data)
R
1
= 0.1941,
wR = 0.2090
R
1
= 0.0692,
wR = 0.1303
0.838 and ꢁ0.696
X-ray crystallography. The crystals of 1 and 2 were
mounted on a glass fiber, respectively. X-ray data were col-
lected on a Bruker SMART APEXII diffractometer
equipped with graphite monochromated MoKα radiation
2
2
Largest diff. peak
1.509 and ꢁ1.866
and hole, eꢀÅꢁ
3
a
R
1
= ΣjjF
0
j-jF
c
jj/ΣjF j.
0
2 2
(
λ = 0.71073 Å) at room temperature. (Korea Basic Sci-
b
2
4 1/2
wR = [Σw(F
2
0
-F
c
0
) /ΣwF ] .
ence Institute, Seoul Western Center) The SMART and
SAINT software package were used for data collection
53
and integration. Absorption corrections were carried out
54
spectral simulations were performed using the program
using SADABS. The crystal structure was solved by the
58
EasySpin.
direct method and refined by full-matrix least-squares cal-
5
5
56,57
culation with SHELXTL
and Olex2.
All non-
hydrogen atoms were refined anisotropically. The hydrogen
atoms were assigned with isotropic displacement coeffi-
cients U(H) = 1.2 U (C) for C(H) and C(H,H) groups and
U(H) = 1.5 U (C) for C(H,H,H) and O(H) groups. Their
coordinates were allowed to ride on their respective atoms.
A summary of the crystals and some crystallography data
are given in Table 1. CCDC-1992122 for 1 and CCDC-
Results and Discussion
Syntheses and Structures. solution of 2,2 -
0
A
ethylenedianiline and 2-pyridinecarboxaldehyde was refluxed
in dry methanol to yield the crystalline solid Schiff-base
product, 1 1,2-bis(2-((pyridine-2-yl)methyleneamino)phenyl)
1
13
ethane (L ). The purified ligand was identified by H and
C
1992123 for 2 contain the supplementary crystallographic
NMR in CDCl . The reaction of copper(II)perchlorate hexa-
3
data for this article. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
1
hydrate with the ligand L in dry methanol produced a green
1
powder of [Cu(II)(L )(CH OH)](ClO ) ꢀCH OHꢀH O, com-
3
4 2
3
2
2
pound 1. For L (= tris((2-benzylideneamino)ethyl)amine), a
solution of tris(2-aminoethyl)amine and benzaldehyde were
refluxed and purified. The ligand showed identical features of
Physical Measurements.
Fourier transform infrared
(
6
4
FTIR) spectra of samples were recorded on a Bruker IFS
ꢁ
1
1
13
52
6v FTIR spectrophotometer in the region of 400 cm to
H and H NMR to the previously reported data. The reac-
ꢁ
1
000 cm using KBr pellets. UV/Vis electronic spectra
tion of copper(II)acetate hydrate in the presence of sodium
2
were recorded with Scinco UV S-2100 spectrophotometer.
perchlorate with L in dry methanol solution formed a blue
1
13
2
H and C NMR is taken in CDCl solution at 298 K
solid compound of [Cu(II)(L ) (CH COO)](ClO ).
3
3
4
using Bruker Advance Digital 400 NMR spectrometer. The
elemental analysis was done using a Perkin-Elmer 2400
CHN analyzer. X-Band (9 GHz) electron paramagnetic res-
onance (EPR) spectra were recorded on a Bruker EMXplus
EPR spectrometer using a 100 kHz field modulation. EPR
(Scheme 2) It is noteworthy that for the preparation of com-
pound 2, one molar ratio of sodium perchlorate was added to
the reaction mixture. Both 1 and 2 were soluble in CH OH,
3
CH CH OH, CH CN, DMF, and DMSO. The crystals were
3
2
3
obtained by using diffusion method in diethyl ether.
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Catecholase Activities of Copper(II) Complexes
KOREAN CHEMICAL SOCIETY
Compounds 1 and 2 were structurally characterized by
Table 2. Selected bond lengths and angles for compounds
1 and 2.
single crystal X-ray crystallography. Both crystallized in a
triclinic unit cell system. Table 1 lists the crystal data and
structure refinements. Figure 1 shows the molecular struc-
1
2
1
Bond lengths (Å)
Cu1 N1
Cu1 N2
Cu1 N3
Cu1 N4
tures of the first coordination spheres, [Cu(II)(L )
2+
2
+
2.020 (5)
2.016 (6)
2.371 (6)
2.007 (6)
1.985 (4)
Cu1 N1
Cu1 N2
Cu1 N3
Cu1 N4
Cu1 O1
Cu1 O2
2.349 (2)
2.008 (2)
2.063 (2)
2.036 (2)
1.9504 (18)
2.703 (2)
(
CH OH)]
and [Cu(II)(L )(CH COO)] , of 1 and 2,
3
3
respectively, with the labeling scheme used. Selected bond
lengths and angles are summarized in Table 2. The coordi-
nation polyhedrons around the copper centers of both the
complexes are best described as distorted square pyramids
Cu1 O1
with CuN O chromophores. A simple metric to assess
4
ꢂ
Bond angles ( )
molecular shape is exemplified by the angular structural
parameter τ (τ = │α-β│/60) for five-coordinate transition
metal complexes with the two largest bond angles, α and β,
N1 Cu1 N2
N1 Cu1 N3
N1 Cu1 N4
N2 Cu1 N3
N2 Cu1 N4
N3 Cu1 N4
N1 Cu1 O1
N2 Cu1 O1
N3 Cu1 O1
N4 Cu1 O1
81.1 (3)
94.8 (2)
96.4 (3)
110.9 (2)
171.3 (2)
77.6 (3)
163.3 (2)
89.5 (2)
N1 Cu1 N2
N1 Cu1 N3
N1 Cu1 N4
N2 Cu1 N3
N2 Cu1 N4
N3 Cu1 N4
N1 Cu1 O1
N2 Cu1 O1
N3 Cu1 O1
N4 Cu1 O1
97.47 (9)
81.24 (9)
99.25 (9)
83.97 (9)
156.87 (9)
82.91 (9)
114.08 (8)
92.69 (8)
164.71 (8)
94.97 (8)
5
9
introduced by Addison et al. In the scheme, a trigonal
bipyramidal structure with D3h symmetry has a τ = 1,
while a square pyramidal structure with C symmetry has
4
v
1
2+
a τ = 0. τ values for both [Cu(II)(L )(CH OH)] and [Cu
3
2
+
(
II)(L )(CH COO)] are calculated to be 0.13, indicating
3
that both complexes have distorted square pyramidal coor-
dination geometries.
101.50 (18)
90.8 (2)
1
2+
In [Cu(II)(L )(CH OH)]
of 1, the equatorial plane
consists of methanol oxygen (O1), two pyridine nitrogens
N1, N4), and imine nitrogen (N2) with the bond lengths of
.985 (4) Å (Cu O) and 2.007 (6) – 2.020 (5) Å (Cu N).
3
(
1
Another imine nitrogen (N3) is at the apical position with
longer Cu N bond length of 2.371 (6) Å than the equato-
2
rial nitrogens’ due to Jahn–Teller distortion. In [Cu(II)(L )
+
(
CH COO)] of 2, carboxylate oxygen (O1), two imine
3
nitrogens (N2, N4), and tertiary nitrogen (N3) are on the
basal plane with the bond lengths of 1.9504 (18) Å
(Cu O) and 2.008 (2) – 2.063 (2) Å (Cu N). Jahn–Teller
effect is also observed along the polar direction with the
Cu N1 bond length of 2.349 (2) Å. The other carboxylate
oxygen (O2) is at the opposite side of N1 with the Cu1 O2
distance of 2.703 (2) Å and the angle O2 Cu1 N1 of
ꢂ
1
67.0 . The Cu1 O2 distance is longer than the usual bond
lengths found at Cu(II) Oaxial in Cu(II) complexes, indicat-
ing that a pseudo coordination presents between Cu1 and
O2. Considering the pseudo coordination of Cu1 O2, the
2
+
coordination geometry of [Cu(II)(L )(CH COO)] can be
3
viewed as a distorted octahedron.
Spectroscopic and electrochemical characterizations.
Compounds 1 and 2 were spectroscopically characterized
by using UV/Vis absorption, IR, and electron paramagnetic
resonance (EPR) spectrophotometries. UV/Vis electronic
absorption spectra of the compounds were taken from
ꢁ
5
5
ꢃ 10
M
solutions in acetonitrile (Supporting
Information Figure S5). Compound 1 displays very intense
4
1
2+
absorption at 290 nm (ε = 3.3 ꢃ 10 M/cm), which can be
3
Figure 1. ORTEP diagrams of (a) [Cu(II)(L )(CH OH)] portion
2
+
due an intraligand transition. The same kind of absorption
3
of compound 1 and (B) [Cu(II)(L )(CH COO)] portion of com-
4
pound 2 with 30% probability ellipsoids and atom numbering
scheme. Hydrogen atoms are omitted for clarity.
peak is observed at 250 nm (ε = 2.6 ꢃ 10 M/cm) for com-
pound 2. A strong shoulder absorption band at 360 nm
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Catecholase Activities of Copper(II) Complexes
KOREAN CHEMICAL SOCIETY
4
(
ε = 1.3 ꢃ 10 M/cm) of 1 can be attributed to the ligand
The electrochemical properties of the complexes were
investigated. Figure 3 displays the cyclic voltamograms of
compounds 1 and 2, respectively, measured at a platinum
electrode in an acetonitrile solution containing 0.1 M tris(p-
bromophenyl)amine electrolyte with AgCl/Ag reference
electrode. The reduction potentials of Cu(II)/Cu(I) are
6
0
to metal charge transfer (LMCT) and a weak absorption
2
band at 630 nm (ε = 7.4 ꢃ 10 M/cm) is assigned to the d-
d transition of Cu(II) ion. The LMCT and d-d transition
bands of compound 2 are observed very weakly at around
3
90 nm and 640 nm, respectively.
Figure S6 displays the FTIR spectra of the ligands, L
and L , and their corresponding complexes. The stretching
vibrations of imine C N bonds are observed at 1630 and
645 cm for L and L , respectively. These are almost
1
observed at 0.28 V and ꢁ 0.26 V for 1 and 2, respectively.
2
ꢂ
Though E1
cannot be determined due nonreversible
/2
redox behaviors of the complexes on the electrode, 1 is
ꢁ
1
1
2
ꢂ
1
expected to have higher E1/2 than 2. Strong cathodic peaks
ꢁ
invariant in the complexes. ClO , counter ion, shows
strong bands near 1100 cm . For compound 2, the coordi-
at ꢁ1.14 V for 1 and ꢁ1.04 V for 2 are probably associated
4
ꢁ
1
with imine group of the ligands.
nated acetate anion gives rise to a strong peak for COO
In general, two major factors are involved to tune the
redox potentials of copper complexes, the geometry of
ꢁ
1
asymmetric stretching, νasym(COO), at 1610 cm
and a
67–69
weak band for COO symmetric stretching, νsym(COO), at
the complexes and the types of donors.
Although com-
ꢁ
1
1455 cm
.
pounds 1 and 2 have a similar angular structural parameter
τ of 0.13, which can be viewed as distorted square pyrami-
dal coordination geometries, a pseudo coordination between
Cu(II) and the second O atom of the carboxylate group is
present to complete the coordination sphere in 2. This may
more stabilize Cu(II) state of 2 than 1 to lower the reduc-
tion potential of 2. The donor ligand effects, alcoholic O of
1 vs. carboxylic O of 2, should be also considered.
π-donating ability of acetate ligand reduces ligand field to
stabilize Cu(II), as previously studied five-coordinated
The X-band EPR spectra of the compounds were
recorded in frozen acetonitrile solution at 110 K, as shown
9
in Figure 2. The spectra display typical Cu(II) (d , S = 1/
2) EPR features with the g-tensors of [2.069 2.069 2.270]
for 1 and [2.060 2.060 2.239] for 2, respectively. g(par)
regions are further split by the copper (I = 3/2) nuclear
hyperfine couplings of 160 G for 1 and 163 G for 2. Since
the g(par) > g
> ge (=2.0023), the unpaired electron
(per)
2
2
resides on d(x -y ) orbital at the ground state as expected
69,70
for the complexes in a square pyramidal geometry with a
Cu(II) complexes with different donor anions.
6
1–66
Jahn–Teller effect or a slight equatorial distortion.
Catecholase activity. The catecholase activities of com-
pounds 1 and 2 were investigated in acetonitrile solutions
by treating the compounds with 3,5-di-tert-butylcatechol
(
3
3,5-H dtbc) in the presence of air. Since the product,
,5-di-tert-butyl-o-quinone (3,5-dtbq), of the catecholase
2
(
(
a)
b)
activity has a strong absorption at λmax = 400 nm
(
can be determined by using UV/Vis spectroscopy.
Figure 4 shows the changes of UV/Vis spectra upon adding
ε = 1900 M/cm), the reaction rate of the catalytic activity
17,22,24
ꢁ
3
ꢁ6
3
,5-H dtbc (1.25 ꢃ 10 M, 3.75 ꢃ 10 mol) into the
2
3
Figure 2. EPR spectra obtained from the frozen CH CN solutions
of (a) compound 1 and (b) compound 2. Their corresponding
numerical simulations are represented in blue lines. Experimental
conditions: Microwave frequency, 9.219 GHz; microwave power,
1
1
.2 mW; modulation amplitude, 5 G; modulation frequency,
00 kHz; time constant, 0.3 s; scan speed, 400 G/min; tempera-
ꢁ
3
ture, 110 K; [compound 1] = 1.0 ꢃ 10
M; [compound
ꢁ
5
ꢁ3
2] = 5.0 ꢃ 10 M. Simulation parameters: (a) g = [2.069 2.069
2.270]; ACu = [0 0160] G, (b) g = [2.060 2.060 2.239]; ACu = [0
0163] G, and Gaussian linewidths = 55 G.
Figure 3. Cyclic voltammograms of 1 ꢃ 0
(a) 1 and (b) 2, respectively, in CH
at a scan rate of 100 mV/s with AgCl/Ag reference electrode.
M compounds
3
CN containing 0.1 M TBAP
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2
1
1
0
0
.0
.5
.0
.5
.0
(a)
(a)
300
400
500
600
700
Wavelength (nm)
2
1
1
0
0
.0
.5
.0
.5
.0
(b)
(b)
300
400
500
600
700
Wavelength (nm)
Figure 5. The concentration changes of the reaction product,
Figure 4. UV/Vis absorption spectral changes of (a) compound 1
3.5-H dtbc and (b) compound 2 + 3.5-H dtbc in acetonitrile.
Each reaction was initiated by adding 3,5-H dtbc solution
1.25 ꢃ 10 M, 3.75 ꢃ 10 mol) into the compound solution
3,5-dtbq vs. the reaction time for the oxidation of 3,5-H dtbc by
2
+
2
2
(a) compound 1 and (b) compound 2. [compound 1] = [compound
0
ꢁ
5
2
2] = 5.0 ꢃ 10 M.
0
ꢁ
3
ꢁ6
ꢁ7
(
(
ꢁ
5
5.0 ꢃ 10 M, 1.5 ꢃ 10 mol). Each spectrum was obtained
ꢁ
6
compound 1 and V
ꢃ 10 M, and kcat = 3.0 ꢃ 10
= 1.5 ꢃ 10
M/s, KM = 4.8
every 120 s up to 2400 s for (a) and every 60 s up to 1320 s for
b) after mixing.
max
ꢁ
4
ꢁ2 ꢁ1
(
s
for compound 2, respec-
tively, were derived. The reaction rates were more than one
order of magnitude faster than those of the blank activity tests
1
2
ꢁ
5
with the free ligands, L and L , or copper salts.
solutions of compounds 1 and 2 (5.0 ꢃ 10
M,
ꢁ
1
ꢁ
7
Wide range of kcat values of 1–32400 h has been reported
1
.5 ꢃ 10 mol), respectively. As observed in the figure, the
from the previous catecholase activity studies of copper model
quinone bands at 400 nm increased after mixing 3,5-H dtbc
2
2
3
complexes using 3.5-H dtbc. The observed k value of
3
2
cat
ꢁ2 ꢁ1
and the compounds until their intensities were saturated. The
amounts of 3,5-dtbq generated by the reactions were deter-
ꢁ
1
ꢁ2 ꢁ1
ꢁ1
6 h (=1.0 ꢃ 10 s ) and 108 h (=3.0 ꢃ 10 s ) for
ꢁ
ꢁ4
6
ꢁ4
the mononuclear complexes 1 and 2, respectively, is compara-
mined to be 2.1 ꢃ 10
mol (6.9 ꢃ 0
M) and
ꢁ
6
ble to those with moderate catecholase activities.
2
.6 ꢃ 10 mol (8.7 ꢃ 10 M) for 1 and 2, respectively,
which were more than the amount of the copper complexes
used, indicating the catalytic activity of both the compounds.
Mechanistic implication for the catecholase activity.
Above electrochemical, spectroscopic, and kinetic studies
give some insights on the mechanistic pathway for the
The kinetic study of the oxidation of 3,5-H dtbc by the com-
2
pounds was carried out by using the initial rate method.
Figure 5 displays the concentration changes of the reaction
product, 3,5-dtbq vs. the reaction time. Figure 6 is the
corresponding Michaelis–Menten and Lineweaver–Burk plots.
catalytic conversion of 3,5-H dtbc to 3,5-dtbq by com-
2
pounds 1 and 2. Mechanical pathways, proposed by sev-
eral studies of catecholase model complexes containing
monocopper(II) active sites on the oxidation of
ꢁ
7
From the plots, the kinetic parameters of V
= 5.0 ꢃ 10
3,5-H dtbc, can be classified into three categories, as
max
2
M/s, KM = 7.8 ꢃ 10ꢁ M, and kcat = 1.0 ꢃ 10
4
ꢁ2 ꢁ1
s
for
23,24,26,29–33
depicted in Figure 7 and Table 3.
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(a)
(b)
Figure 6. (a) Michaelis–Menten and (b) Lineweaver–Burk plots
corresponding to the data of Figure 5. Data and fittings are
depicted in blue for compound 1 and in red for compound 2,
respectively.
The catalytic cycles of both pathways A and B are initiated by
●ꢁ
forming Cu(I)-(3,5-dtsq ) intermediate. (Figure 7(a) and (b)).
Here, 3,5-dtsq● is 3,5-di-tert-butyl-1,2-benzosemiquinonate
anion radical. Two pathways are differentiated in the final oxida-
ꢁ
●
ꢁ
tion stage of 3,5-dtsq . In pathway A, O binds to Cu(I)-(3,-
2
●ꢁ
30,31
●ꢁ
5
-dtsq ) to produce 3,5-dtbq,
from the intermediate in pathway B. In B, the oxidation product,
,5-dtbq, is generated via two paths, either disproportionation or
while 3,5-dtsq
is freed
3
•
- 24,29
air oxidation of 3,5-dtsq .
The first step of pathway C is an
anaerobic oxidation of 3,5-H dtbc to 3,5-dtbq, consuming two
2
Cu(II) ions for the production of one 3,5-dtbq. This leaves
Cu(I) ion that is the active form of the catalytic cycle. 3,5-H dtbc
2
6,32,33
2
binds to Cu(I) and aerobically oxidizes to 3,5-dtbq.
A
noticeable difference between paths A, B, and C is the catalyti-
cally active oxidation state of copper ion; Cu(II) in A and B, and
Cu(I) in C.
EPR measurement on a reaction mixture is useful to distin-
guish the mechanistic pathways. If a Cu(II) complex shows the
catecholase activity through either the pathway A or B, the
Figure 7. Previously proposed stoichiometrically represented
2
mechanistic pathways of the oxidation of 3,5-H dtbc by
●ꢁ
monocopper(II) model complexes. (a) Cu(I)-(3,5-dtsq ) interme-
●
ꢁ
diate pathway, (b) free 3,5-dtsq
(
intermediate pathway, and
2
3,24,29–33
c) anaerobic oxidation and catalytic Cu(I) pathway.
Cu(I)/Cu(II) represents Cu(I)/Cu(II) complexes.
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a
Table 3. Stoichiometrically represented mechanistic pathways of the oxidation of 3,5-H
2
dtbc by monocopper(II) model complexes.
Steps
Pathway A
Pathway B
Pathway C
b
Conversion
2 Cu(II) + 3,5-H dtbc ! 2 Cu(I) + 3,5-
2
+
dtbq +2H
Initiation
Activation
Oxidation
Cu(II) + 3,5-H dtbc ! Cu(II)-(3,5-
Cu(II) + 3,5-H dtbc ! Cu(II)-(3,5-
2 Cu(I) + 3,5-H dtbc ! 2 Cu(I)-(3,5-
2
2
2
ꢁ
+
ꢁ
+
ꢁ +
Hdtbc ) + H
Hdtbc ) + H
Hdtbc ) + 2H
ꢁ
ꢁ
ꢁ
! 2 O2●ꢁ
Cu(II)-(3,5-Hdtbc ) ! Cu(I)-(3,5-
Cu(II)-(3,5-Hdtbc ) ! Cu(I) + 3,5-
2 Cu(I)-(3,5-Hdtbc ) + 2 O
2
-
●
ꢁ
+
●ꢁ
+
ꢁ
dtsq ) + H
dtsq + H
Cu(II)-(3,5-Hdtbc )
●ꢁ
●ꢁ
+
●ꢁ
ꢁ
+
Cu(I)-(3,5-dtsq
)
3,5-dtsq + H ! 1/2 3,5-H dtbc
2 O2 -Cu(II)-(3,5-Hdtbc ) + 2H ! 2
2
+
+
O
2
+ 2H ! Cu(II) + 3,5-
+ 1/2 3,5-dtbq
2 2
Cu(I) + 2 3,5-dtbq + 2 H O
Cu(I) + 1 O + H ! Cu(II) + H O
+
dtbq + H
2 2
O
2
2 2
2
c
[Cu(II)] + 1 3,5-H dtbc +
1
2
Overall
[Cu(II)] + 3,5-H dtbc + O ! [Cu
[2 Cu(I)] + 2 3,5-H dtbc + 2 O ! [2
2
2
2
2
2
2
(
II)] + 3,5-dtbq + H O
1
2
Cu(I)] + 2 3,5-dtbq + 2 H O
2
2
O ! [Cu(II)] + 3,5-
2
2
2
2 2
dtbq + H O
a
Cu(I)/Cu(II) represents Cu(I)/Cu(II) complexes.
Conversion implies the conversion of Cu(II) to Cu(I) in pathway C.
Overall means the sum of conversion, initiation, activation, and oxidation steps. Brackets denote catalytically active forms.
b
c
9
EPR feature of Cu(II) (d , S = 1/2) is readily observed. Further,
the pathway B can be verified by observing the EPR signal of
compare the spectroscopic, structural, and catalytic proper-
ties of their copper(II) complexes. Though 2,2 -
0
a free 3,5-dtsq● radical.
ꢁ
24,29,71
However, the pathway C,
where Cu(II) is consumed for the initial anaerobic oxidation of
ethylenedianiline and tren have different electronical and1
coordination properties, the copper(II) complexes of L
2
3
,5-H dtbc and the catalytic cycle maintains a Cu(I) state,
and L showed very similar structural characteristics with
2
would make the EPR intensity of Cu(II) disappear. When the
the angular structural parameter τ of 0.13, which can be
views as a distorted square pyramidal coordination.
Catecholase activities of the complexes were investigated
reactions were initiated by adding 3,5-H dtbc into the solution
2
of compounds 1 and 2, respectively, EPR signals of the
Cu(II) complexes were diminished, implying that the observed
catecholase activities of both compounds 1 and 2 were likely
arising from the pathway C. Although it is less evident, the
curve shape of the 3,5-dtbq concentration change on reaction
time is supportive of pathway C proposal. The shape is not a
simply asymptotic increase as seen in Figure 5, rather there are
likely two steps in the production of 3,5-dtbq when compounds
on the oxidation of 3,5-di-tert-butylcatechol (3,5-H dtbc) to
2
3,5-di-tert-butyl-o-quinone (3,5-dtbq). Results of kinetic
behaviors and EPR measurements are supportive to suggest
that the mechanistic pathway of the catecholase activitiy by
the complexes is the pathway C in which the first step is an
anaerobic oxidation of 3,5-H dtbc to 3,5- dtbq, followed by
2
a catalytic cycle with Cu(I) active form. To propose more
conclusive mechanical pathway, we are planning a
chatecholase activity test in anaerobic condition.
1
and 2 were used, respectively.
As discussed earlier, the oxidation of 3,5-H dtbc by com-
2
pounds 1 and 2 likely occurs via pathway C. Electrochemical
behaviors of compounds 1 and 2 indicate that the reduction of
Cu(II) in compound 1 is thermodynamically more favorable
that in 2. (Figure 3) If a linear free-energy relationship can be
applied, conversion of Cu(II) to Cu(I) in compound 1 would be
kinetically more labile in the first step than in compound 2.
However, the kinetic investigations on the catecholase activities
revealed that the compound 2 is kinetically more efficient for
Acknowledgments. This work was supported by the
National Research Foundation of Korea Grant funded by
the Korean Government (NRF-2018R1D1A1B07051018).
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
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