Air oxygenation chemistry of 4-TBC catalyzed by chloro bridged dinuclear copper(ii) complexes of pyrazole based tridentate ligands: Synthesis, structure, magnetic and computational studies

Article abstract of DOI:10.1039/c2dt30983a

Four dinuclear bis(μ-Cl) bridged copper(ii) complexes, [Cu ₂(μ-Cl)₂(L^X)₂](ClO ₄)₂ (L^X = N,N-bis[(3,5-dimethylpyrazole-1-yl)- methyl]benzylamine with X = H(1), OMe(2), Me(3) and C

Full text of DOI:10.1039/c2dt30983a

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Air oxygenation chemistry of 4-TBC catalyzed by chloro
bridged dinuclear copper(II) complexes of pyrazole
based tridentate ligands: synthesis, structure, magnetic
and computational studies†
Cite this: Dalton Trans., 2013, 42, 1879
Ishita Banerjee,a Pabitra Narayan Samanta,a Kalyan Kumar Das,a Rodica Ababei,b,c,d,e
Marguerite Kalisz,b,c Adrien Girard,b,c Corine Mathonière,d,e M. Nethaji,f
Rodolphe Clérac*b,c and Mahammad Ali*a
Four dinuclear bis(μ-Cl) bridged copper(II) complexes, [Cu2(μ-Cl)2(LX)2](ClO4)2 (LX = N,N-bis[(3,5-dimethyl-
pyrazole-1-yl)-methyl]benzylamine with X = H(1), OMe(2), Me(3) and Cl(4)), have been synthesized and
characterized by the single crystal X-ray diffraction method. In these complexes, each copper(II) center is
penta-coordinated with square-pyramidal geometry. In addition to the tridentate LX ligand, a chloride
ion occupies the last position of the square plane. This chloride ion is also bonded to the neighboring
Cu(II) site in its axial position forming an SP-I dinuclear Cu(II) unit that exhibits small intramolecular ferro-
magnetic interactions and supported by DFT calculations. The complexes 1–3 exhibit methylmono-
oxygenase (pMMO) behaviour and oxidise 4-tert-butylcatechol (4-TBCH2) with molecular oxygen in
MeOH or MeCN to 4-tert-butyl-benzoquinone (4-TBQ), 5-methoxy-4-tert-butyl-benzoquinone (5-MeO-4-
TBQ) as the major products along with 6,6’-But-biphenyl-3,4,3’,4’-tetraol and others as minor products.
These are further confirmed by ESI- and FAB-mass analyses. A tentative catalytic cycle has been framed
based on the mass spectral analysis of the products and DFT calculations on individual intermediates that
are energetically feasible.
Received 5th May 2012,
Accepted 18th October 2012
DOI: 10.1039/c2dt30983a
www.rsc.org/dalton
peptidylglycine α-amidating monooxygenase (PHM),23,24
methane monooxygenase (p-MMO)25–27 etc. In these oxidation
Introduction
Dioxygen activation and transition-metal-catalyzed oxidation processes, the copper(II) metal ions in mono- or dinuclear
processes play a crucial role in many biological systems,1–12 active cores are reduced into copper(I) by an external reductant
environmental chemistry,13 medicinal chemistry14 and indus- prior to binding and activation of molecular oxygen. Monooxy-
trial applications.15–17 Copper monooxygenases and copper genases play a crucial role in the activation by inserting singlet
dioxygenases are the best known copper proteins that bind oxygen into relatively strong C–H bonds. The products from
and/or activate molecular oxygen in many biological these reactions are vital for the organisms in which these pro-
processes.18–20 The copper monooxygenases are mainly teins are formed. On the other hand, in copper dioxygenase
involved in hydrocarbon oxidations such as tyrosinase (Tyr),20 systems, such as quercetin 2,3-dioxygenase (2,3-QD), the reac-
catechol oxidase (CO),21 dopamine β-monooxygenase, DβM,22 tion is believed to occur through substrate activation by an
inner-sphere coordination of the deprotonated polyhydroxylated
organic substrate (RO−). The electron transfer to copper(II)
aDepartment of Chemistry, Jadavpur University, Kolkata 700 032, India.
initially occurs to generate copper(I) and an organic radical
intermediate (RO˙) of the substrate.28 The generated RO˙/
E-mail: m_ali2062@yahoo.com
bCNRS, CRPP, UPR 8641, F-33600 Pessac, France
cUniv. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France.
E-mail: clerac@crpp-bordeaux.cnrs.fr
dCNRS, ICMCB, UPR 9048, F-33600 Pessac, France
eUniv. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France
fDepartment of Inorganic and Physical Chemistry, Indian Institute of Science,
copper(I) pair readily reacts with O2 to promote the substrate
oxygenation reaction.
Different possible active intermediates in mononuclear
copper monooxygenases are CuII–O–O (end-on superoxo),29
CuII–O–O–H (hydroperoxo)30 and CuIIIvO (higher valent oxo)
species (Fig. 1).29–32
Bangalore 560012, India
†Electronic supplementary information (ESI) available. CCDC 716786 (1),
893810 (2), 716784 (3), 716785 (4). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt30983a
Therefore, independent synthesis of small molecules con-
taining these motifs and examination of their reactivities
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Experimental section
Materials and reagents
The starting materials for the synthesis of the ligands like
acetylacetone (Merck, India), hydrazine hydrate (Rankem), for-
malin (Merck, India), benzyl amine derivatives (Aldrich),
Na2SO4 (Merck, India) are of reagent grade and used as received.
Copper(II) perchlorate hexahydrate and copper(II) chloride dihy-
drate (Loba Chemie, India) are used for the preparation of
complexes. Tetrabutylammonium perchlorate was purchased
from Aldrich. Solvents like methanol, ethanol, chloroform,
diethyl ether, acetonitrile (Merck, India) were of reagent grade
and dried before use.
Fig. 1 Schematic presentation of the proposed binding sites of type 2 Cu2+ in
pMMO from ref. 36 and representative [Cu(LH)Cl]+ complex in solution of the
present study.
Synthesis of the bis(pyrazoline) derivative of tripodal ligands
(LX): 3,5-Dimethylpyrazole and 3,5-dimethyl-1-hydroxymethyl-
pyrazole were prepared by following the literature procedure.37
Preparation of ligands (LX, X = Cl, H, Me and OMe): All ligands
(LX) were synthesized by following the literature procedure.37
A
Scheme 1
solution of 12 mmol of the corresponding amine and 24 mmol
of 3,5-dimethyl-1-hydroxymethyl pyrazole in 50 ml acetonitrile
were stirred in a stoppered vessel at room temperature for
24 hours. The water produced during the reaction was removed
by drying over anhydrous Na2SO4. After filtration and evapo-
ration under reduced pressure, an oily substance was obtained.
A white crystalline product was isolated after dissolution of the
oily material in diethyl ether and cooling in the refrigerator.
The white crystals were harvested by filtration and dried in air.
LH: Yield 60%. Ana. Cal. value for molecular formula
C19H25N5: C, 66.86; H, 7.92; N, 19.62; Found C, 65.26; H, 7.26;
N, 20.5%.1H NMR(CDCl3, δ, ppm): 1.99 (6H,s); 2.17 (6H,s); 3.69
(2H,s); 4.93 (4H,s); 5.42 (2H,s); 7.1–7.3 (5H,m). 13C NMR(CDCl3, δ,
ppm): 149.4 (C3); 140.9 (C5); 105.4 (C4); 127.3 (C13); 135.6 (C10);
128.5 (C11, 12, 14, 15); 61.4 (C8); 55.7 (C6); 18.0 (C7); 11.2 (C6).
LOMe: Yield 65%. Ana. Cal. value for molecular formula
C20H27N5O: C, 67.90; H, 7.64; N, 19.80; Found C, 68.16; H, 7.23;
N, 20.06%.1H NMR(CDCl3, δ, ppm): 1.98 (6H,s); 2.19 (6H,s); 3.62
(2H,s); 3.76 (2H,s); 4.88 (4H,s); 5.77 (2H,s); 6.78–7.09 (4H,d). 13C
NMR(CDCl3, δ, ppm): 147.52 (C3); 139.91 (C5); 160.25 (C13); 129.9
(C11, 15); 127.9 (C10); 113.69 (C12, 14); 105.64 (C4); 64.74 (C8);
55.24 (C16); 51.98 (C9); 13.50 (C7); 10.67 (C6); 55.24 (C16, OMe).
LMe: Yield 60%. Ana. Cal. value for molecular formula
C20H27N5: C, 71.21; H, 8.01; N, 20.70; Found C, 70.15; H, 7.49;
N, 20.80. 1H NMR(CDCl3, δ, ppm): LMe (X = Me) 1.99 (6H,s);
2.18 (3H,s); 3.63 (2H,s); 4.90 (4H,s); 5.77 (2H,s); 7.06 (4H,q).
13C NMR(CDCl3, δ, ppm): 147.52 (C3); 139.91 (C5); 136.78
(C13); 134.96 (C10); 128.98 (C11, 12, 14, 15); 105.63 (C4); 64.74
(C8); 52.22 (C9); 13.50 (C7); 10.67 (C6); 21.08 (C16, Me).
should provide some insights into how these monooxygenases
work and should help to develop bio-inspired oxidation cata-
lysts. This type of investigation has been less developed in
model systems involving mononuclear copper(II) complexes in
comparison to the comprehensive copper(I) dioxygen studies.
In dihalogen-bridged copper complexes, metal centers
usually adopt a tetra- or penta-coordinated geometry with
different types of terminal/capping ligands leading to a wide
variety of ferro- or antiferro-magnetic interactions. For five
coordinated metal centers, three different geometries of di-
halogen-bridged complexes have been experimentally observed
(Scheme 1): (i) SP-I, for which the two shared halogens occupy
at the same time the axial position of the first Cu(II) site and
one of the four equatorial positions of the second Cu(II) center;
this configuration usually leads to small ferromagnetic or anti-
ferromagnetic coupling; (ii) SP-II, for which the two halogens
form a sharing edge of both Cu(II) basal planes leading to
quasi-coplanar pyramidal bases showing different magnetic
behaviors that depend strongly on the nature of co-ligands;
and (iii) SP-III, for which the first halogen is shared between
the two Cu(II) sites in their axial positions while the second
one occupies simultaneously equatorial positions for both
metal ions. This arrangement, that is relatively rare, usually
leads to almost perpendicular Cu(II) basal planes and ferro-
magnetic coupling.33–35
In this work, we report four dinuclear [Cu2(μ-Cl)2(LX)2]2+
complexes of the SP-I type that exhibit weak intramolecular
ferromagnetic interactions and also act as a copper monooxy-
genases model. The observed EPR spectrum for the type 2
center in the native pMMO consisted of g = 2.24 with A (Cu) =
k
k
185 G and g⊥ = 2.0436 which are comparable to our system
(vide infra). The combined CW and ESEEM data for pMMO
indicate that the coordination environment comprises four
nitrogen atoms coordinated to the Cu center and that three or
four of these nitrogens are histidine imidazoles. In this respect
the mononuclear species of our systems closely resembles
structurally and spectroscopically the active site of pMMO.
1880 | Dalton Trans., 2013, 42, 1879–1892
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LCl: Yield 55%. Ana. Cal. value for molecular formula recorded from KBr pellets on a Nickolet Magna IR 750 series-II
C19H24N5Cl: C, 63.77; H, 6.71; N, 19.58; Found C, 64.01; H, 6.89; FTIR spectro-photometer. 1H and 13C NMR were recorded in
N, 19.40%. 1H NMR(CDCl3, δ, ppm): 2.32 (6H,s); 3.62 (6H,s); 4.80 CDCl3 on a Bruker 300 MHz NMR Spectrophotometer using
(4H,s); 5.82 (2H,s), 7.78 (4H,s). 13C NMR(CDCl3, δ, ppm): 148.32 tetramethylsilane (δ = 0) as an internal standard. Electronic
(C3); 139.92 (C5); 132.8 (C13); 129.9 (C11, 15); 128.02 (C12, 14); spectra were recorded on an Agilent-8453 diode array UV-vis
106.04 (C4); 64.84 (C8); 55.24 (C6); 13.50 (C7); 11.2 (C6).
spectrophotometer. Electrochemical measurements were
Preparation of the [Cu(LX)Cl]2(ClO4)2 complexes: 0.25 mmol carried out using a computer-controlled AUTOLAB (model
of Cu(ClO4)2·6H2O was solubilized in MeOH (15 ml) and 263A VERSASTAT) electrochemical instrument with a platinum
stirred for a few minutes before the addition of 0.25 mmol of tip as the working electrode.
CuCl2·2H2O. The corresponding LX ligand was added to the
Crystallography
light green copper solution that turned dark green. After six
hours of stirring, the solution was filtered and kept in a
refrigerator. After two to three days, green crystals formed that
were isolated by filtration and air dried. [{Cu(LH)Cl}]2(ClO4)2
(1): Yield 60%, Anal. Calcd for C38H50Cl4Cu2N10O8: C, 43.69;
H, 4.79; N, 13.41. Found: C, 43.60; H, 4.78; N, 13.63. [{Cu
(LOMe)-Cl}]2(ClO4)2 (2): Yield 70%, Anal. Calcd for C40H54Cl4-
Cu2N10O10: C, 43.52; H, 4.89; N, 12.69. Found: C, 43.17;
H, 4.75; N, 12.33. [{Cu(LMe)Cl}]2(ClO4)2 (3): Yield 64%, Anal.
Calcd for C40H54Cl4Cu2N10O8: C, 44.82; H, 5.08; N, 13.07.
Found: C, 44.69; H, 4.99; N, 12.57. [{Cu(LCl)Cl}]2(ClO4)2 (4):
Yield 54%, Anal. Calcd for C38H46Cl6Cu2N10O8: C, 41.09; H,
4.31; N, 12.61. Found: C, 40.77; H, 4.18; N, 12.27.
Single crystal X-ray data of 1–4 were collected at room temp-
erature on a Bruker SMART APEX-II CCD diffractometer using
graphite monochromated MoKα radiation (λ = 0.71073 Å). Data
integration and reductions were processed with SAINT+ soft-
ware.38 Structures were solved by the direct method and then
refined on F2 by the full matrix least squares technique with
SHELX-97 software.39 During the refinement of these struc-
tures some restraints were imposed, for example, Cl–O bond
distances were fixed in space and in most cases oxygen atoms
of perchlorate ions were refined considering the ISOR model.
The crystallographic data for 1–4 are given in Table 1.
Magnetic measurements
The magnetic susceptibility measurements were carried out on
an MPMS-XL Quantum Design SQUID magnetometer between
1.8 and 400 K for dc applied fields ranging from −7 to 7 T on
Physical measurements
Elemental analyses were carried out using a Perkin-Elmer 240 polycrystalline samples of 1–4. ac susceptibilities were
elemental analyzer. Infrared spectra (400–4000 cm−1) were measured with an oscillating ac field of 3 Oe with frequencies
Table 1 Crystal data and structural refinement parameters
CCDC 716786 (1)
38H50Cl4Cu2N10O8
1043.76
293(2)
CCDC 893810 (2)
CCDC 716784 (3)
CCDC 716785 (4)
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
C
C42H62Cl4Cu2N10O12
1167.92
293(2)
C40H54Cl4Cu2N10O8
1071.83
293(2)
C38H48Cl6Cu2N10O8
1112.64
293(2)
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions (Å, °)
Monoclinic
P21/c (No. 14)
a = 8.427(15)
b = 21.205(4)
c = 12.694(2)
α = 90.00(3)
β = 96.680(3)
Triclinic
P1 (No. 2)
Triclinic
P1 (No. 2)
Triclinic
P1 (No. 2)
ˉ
ˉ
ˉ
a = 9.4248(14)
b = 11.5255(17)
c = 12.6536(19)
α = 85.280(8)
β = 86.063(9)
γ = 78.893(8)
1342.2(3)
a = 9.187(6)
b = 10.264(6)
c = 12.987(8)
α = 76.354(9)
β = 74.315(9)
γ = 89.607(9)
1143.6(12)
1
1.556
1.226
554
0.39 × 0.42 × 0.46
1.68 to 27.25
−11 ≤ h ≤ 12,
−13 ≤ k ≤ 13,
−16 ≤ l ≤ 16
12 698
a = 9.1275(12)
b = 10.1836(14)
c = 13.1067(18)
α = 75.730(2)
β = 73.331(2)
γ = 89.528(2)
1128.5(3)
Volume (Å3)
2252.949(7)
2
1.539
1.242
1076
0.40 × 0.43 × 0.47
1.88 to 25.05
−10 ≤ h ≤ 10,
−25 ≤ k ≤ 25,
−16 ≤ l ≤ 14
16 028
Z
1
1
Density calcd (g cm−3
)
1.445
1.056
606
0.20 × 0.15 × 0.12
2.21 to 27.01
−12 ≤ h ≤ 12,
−14 ≤ k ≤ 14,
−16 ≤ l ≤ 16
17 891
1.637
1.360
570
0.39 × 0.42 × 0.45
1.68 to 27.50
−11 ≤ h ≤ 12,
−13 ≤ k ≤ 13,
−17 ≤ l ≤ 16
13 118
Absorption coefficient (mm−1
F(000)
)
Crystal size (mm3)
θ range (°)
Limiting indices
Reflections collected
Independent reflections
Data [I > 2σ(I)]/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
3987 [R(int) = 0.035]
3986/302
1.041
5858 [R(int) = 0.1090]
3874/326
1.032
5190 [R(int) = 0.040]
3883/294
0.909
5217 [R(int) = 0.013]
4701/293
0.695
R1 = 0.0601, wR2 = 0.1668
R1 = 0.0936, wR2 = 0.2496
R1 = 0.0623, wR2 = 0.1715
R1 = 0.0325, wR2 = 0.1014
w = 1/[σ2 (Fo2) + (0.1588P)2 + 10.3950P], where P = (Fo2 + 2Fc2).
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between 1 and 1500 Hz. It is worth noting that no out-of-phase concentration of the complex. The formation of 3-methoxy-4-
ac susceptibility signal has been detected above 1.8 K. The TBQ was monitored with time at a wavelength of 386 nm. The
magnetic data were corrected for the sample holder (plastic observed rate constants (ki) were extracted by
a
linear
bag) and the diamagnetic contribution.
regression fit of the kinetic traces (initial rate method) for
more than 3-half-lives. A weighted average of triplicate runs
of each experiment indicates that the error limit falls well
within 5%.
DFT calculations
Geometries of the complexes [Cu(LX)Cl]2 (X = H(1), OMe(2),
Me(3), Cl(4)) were fully optimized starting with the geometries
obtained from single-crystal X-ray diffraction studies by DFT
computations using the Gaussian 03 program40 with the Becke
three-parameter exchange and Lee–Yang–Parr correlation func-
tional (B3LYP).41 The LanL2DZ42 basis functions and effective
core potentials (ECP) were used for all atoms including copper.
These basis functions were augmented with additional polari-
zation functions of exponents: Cu (ζf = 0.8), C (ζd = 0.8), N (ζd =
0.8), O (ζd = 0.8), Cl (ζd = 0.75). The nature of all stationary
points was confirmed by harmonic vibrational frequency
analysis. For all the four complexes, the ground state belongs
to a triplet multiplicity. The exchange coupling constant (J)
between the two magnetic Cu-centers of the chloro bridged
Cu(II) dimers is calculated from the energy difference between
the triplet (T) ground state and singlet (S) excited state ener-
gies as ET − ES = −2J of the complex using the Heisenberg–
Dirac–van Vleck spin Hamiltonian. The broken-symmetry (BS)
formalism proposed by Noodleman43 within the framework of
DFT has been found to be efficient in calculating the exchange
coupling constant, J. The BS state is not the eigenstate of the
spin Hamiltonian but an equal mixture of a singlet and triplet
state which is obtained when magnetic orbitals are allowed to
overlap in a self-consistent field (SCF) manner. The energy of
the BS state is basically a specific weighted average of energies
of the pure spin multiplets. The exchange coupling constant, J,
for a binuclear system can be determined from the energies of
the BS and the triplet states following the formula of Yamaguchi
and coworkers:44
2+
Results and discussion
Synthesis, characterizations and structural description of
complexes 1–4
The four complexes were synthesized in a similar synthetic
approach. A 1 : 1 mixture of perchlorate and chloride salts of
Cu(II) in methanol was allowed to react with a stoichiometric
amount of functionalized N,N-bis[(3,5-dimethylpyrazole-1-yl)-
methyl]benzylamine ligands at room temperature for about six
hours. Green crystals suitable for single-crystal X-ray diffrac-
tion studies were easily obtained in a couple of days by the
slow evaporation technique at about 4 °C. The dicationic parts
of the complexes 1–4 are shown in Fig. 2 and selected bond
distances and angles are listed in Table 2. Complex 1 crystal-
lizes in the monoclinic space group P21/c while the three other
ˉ
complexes adopt a triclinic space group P1. In all these com-
pounds, the Cu(II) metal ions are the dichloro-bridged dinu-
clear units which are five-coordinated and adopt a distorted
square-based pyramidal geometry. The LX ligands coordinate
the Cu(II) centers by three nitrogen atoms (two from two pyr-
azole rings and one from the functionalized benzyl amine)
that occupy three of the four Cu(II) equatorial positions. One
chlorine atom completes the basal plane and is shared with
the neighboring Cu(II) metal center in its single apical posi-
tion. In the four compounds, the equatorial planes of the two
copper sites in the dinuclear centrosymmetrical cation moiety
are almost coplanar with the dihedral angle ∼3.69°. The
copper center lies perfectly in its [N3Cl] basal plane and is not
displaced toward the axial chlorine atom as is often observed
in square-pyramidal geometry. The distortion from ideal
DFT
ð
E
ꢀ
DFTETÞ
BS
J ¼
ð1Þ
kS2lT ꢀ kS2lBS
where EBS and ET are the energies of the broken symmetry and
triplet state, respectively. 〈S2〉T and 〈S2〉BS denote the square of
the total spin for the triplet and BS state, respectively.
Kinetic experiments
All kinetic experiments were performed with an Agilent diode-
array UV-VIS spectrophotometer under pseudo-first-order con-
ditions, with the copper complexes as the minor component.
The kinetic procedures involve the preparation of stock solu-
tions of the complexes and the substrate 4-TBCH2 at higher
concentrations in pure MeCN. From these stock solutions, a
set of 11–12 solutions of [4-TBCH2] = 1.0–50 mM were pre-
pared. A 2 ml portion of each solution was pipetted out into a
quartz cell and thermostated for 15 min at 25 °C by inserting
into the shell holder which is attached to a peltier temperature
controller system. 20 μl of a stock solution of the complex was
Fig. 2 Ortep-type view of the four [Cu2(μ-Cl)2(LX)2](ClO4)2 (with X = H (1),
OMe (2), Me(3) and Cl (4)) complexes. Thermal ellipsoids are at the 30% prob-
added to the 4-TBCH2 solution to achieve a 0.50 mM ability level.
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Table 2 Selected bond distances and bond angles of complexes 1–4. Values in square brackets are from DFT calculations
Complex 1
Complex 2
2.228 (15) [2.315] Cu1–Cl1
Complex 3
2.2329(14) [2.316] Cu1–Cl1
Complex 4
Cu1–Cl1
Cu1–N1
Cu1–N11
Cu1–N9
2.2397(14) [2.315] Cu1–Cl1
2.2340(6) [2.317]
2.0822(18) [2.166]
1.9704(19) [2.018]
1.982(2) [2.044]
2.8301(7) [2.843]
3.522 [3.706]
178.86(5) [178.9]
99.02(6) [99.9]
98.53(6) [100.3]
1.972(4) [2.166]
1.977(4) [2.015]
2.084(4) [2.034]
2.896(1) [2.858]
3.542 [3.723]
99.20(13) [92.3]
98.21(13) [91.8]
Cu1–N1
Cu1–N3
Cu1–N5
Cu1–Cl1_a
Cu⋯Cu
1.987(4) [2.040] Cu1–N1
2.083(4) [2.016] Cu1–N3
1.980(4) [2.160] Cu1–N5
2.831(1) [2.878] Cu1–Cl1_a
1.965(3) [2.011]
Cu1–N1
Cu1–N3
Cu1–N4
Cu1–Cl1_a
Cu⋯Cu
Cl1–Cu1–N1
Cl1–Cu1–N3
1.986(3) [2.031]
2.088(3) [2.166]
2.844(1) [2.878]
3.526 [3.742]
98.97(9) [100.7]
98.47(9) [99.2]
Cu1–Cl1_a
Cu1⋯Cu1_a
N1–Cu1–Cl1
N11–Cu1–Cl1
N9–Cu1–Cl1
Cl1–Cu1–Cl1_a
N1–Cu1–N11
N1–Cu1–N9
Cl1_–Cu1–N1
N11–Cu1–N9
Cl1_a–Cu1–N11
Cl1_a–Cu1–N5
3.449 [3.731]
98.29(15) [100]
176.52(10) [99.9] Cl1–Cu1–N3
98.51(14) [179.4] Cl1–Cu1–N5
Cu⋯Cu
Cl1–Cu1–N1
Cl1–Cu1–N3
178.64(13) [178.3] Cl1–Cu1–N5
Cl1–Cu1–N1
178.95(8) [178.5] Cl1–Cu1–N4
Cl1–Cu1–Cl1_a 92.70(2) [88.8]
162.34(11) [159.5] N1–Cu1–N3
93.79(4) [88.6]
162.57(17) [159.3] N1–Cu1–N3
Cl1–Cu1–Cl1_a 94.92(5) [88.8]
Cl1–Cu1–Cl1_a 92.89(3) [88.5]
81.93(17) [159.6] N1–Cu1–N3
162.04(2) [79.4] N1–Cu1–N5
81.88(8) [80.5]
80.53(8) [79.3]
80.88(17) [79.4]
90.58(8) [93.6]
81.69(10) [80.6]
89.24(8) [92.3]
8758 (8) [91.8]
N1–Cu1–N5
81.90(12) [79.0]
N1–Cu1–N4
Cl1_a–Cu1–N1 87.97(5) [92.2]
N3–Cu1–N4 162.08(8) [159.2]
Cl1_a–Cu1–N1 91.05(1) [92.6]
N3–Cu1–N5
Cl1_a–Cu1–N3 88.54(1) [91.8]
Cl1_a–Cu1–N5 90.45(1) [91.0]
Cu1–Cl1–Cu1_a 85.08(5) [91.2]
Cl1_a–Cu1–N1 91.81(9) [92.4]
81.12(17) [80.6] N3–Cu1–N5
80.63(12) [79.3]
Cl1_a–Cu1–N3 90.00(8) [93.0]
Cl1_a–Cu1–N5 87.65(8) [91.6]
Cu1–Cl1–Cu1_a 87.11(3) [91.5]
Cl1_a–Cu1–N3 92.48(6) [93.1]
Cl1_a–Cu1–N4 90.26(5) [92.7]
Cu1–Cl1–Cu1_a 87.30(2) [91.2]
Cu1–Cl1_a–Cu1_a 86.21(4) [91.4]
square pyramidal geometry estimated from Reedijk’s τ factor45
(τ = 0 for a square pyramid, and τ = 1 for a trigonal bipyramid)
for all four complexes was found to lie between 0.27 and 0.28.
In the equatorial plane, the Cu–Namine (Namine = tertiary amine
nitrogen) distance range 2.034–2.086 Å is slightly longer than
the two other Cu–NPZ (NPZ = coordinated nitrogen atom of the
pyrazole ring) distances that range between 1.967 and 1.998 Å.
The axial Cu–Cl distance in the four complexes ranges as
2.830–2.896 Å that is considerably longer than the basal Cu–Cl
distance 2.228–2.239 Å. However, the two Cu–Cl distances in
these compounds are comparable to those found in the related
[Cu2N6Cl2] moiety in which the Cu(II) ion adopts a similar geome-
try.46 It is worth noting that all the bridging Cu–Cl–Cu angles are
smaller than 90°, between 85.03 and 87.30°. Consequently, the
Cu⋯Cu distances are remarkably shorter (3.449–3.541 Å) than
the previously reported dichloro bridged Cu(II) complexes.47,48
Fig. 3 Cyclic voltammograms of 1, 3 and 4 (1 mmol L−1) in MeCN under a
pure N2 atmosphere with a sweep rate of 50 mV s−1 at 25 °C with 0.1 M tetra-
butylammonium perchlorate as the supporting electrolyte. The CV for 2 is sup-
plied as ESI (Fig. S1†).
ELECTROCHEMICAL
STUDIES. Cyclic
voltammograms
were
recorded for 1, 2, 3 and 4 at 25 °C vs. the Ag/AgCl electrode in
MeCN under a pure N2 atmosphere with 0.1 M tetrabutyl-
ammonium perchlorate as the supporting electrolyte. For the
three compounds, the cyclic voltammograms show three metal
centered quasi-reversible waves (Fig. 3). As the dinuclear com-
plexes convert into mononuclear [Cu(LX)Cl]+ species in solu-
tion by the cleavage of the Cu–Cl bond (vide infra), the
sequence of redox processes can be represented as
agreement with the presence of two Cu(II) S = 1/2 spin carriers
considering g values around 2.0 and 2.2. When the temp-
erature is lowered, the χT products at 1000 Oe remain roughly
constant up to 25 K and then increase to reach a value at 1.8 K
of 0.93 cm3 K mol−1 for 1, 0.97 cm3 K mol−1 for 2, 0.95 cm3 K
mol−1 for 3 and 0.86 cm3 K mol−1 for 4. This thermal behavior
clearly indicates the presence of intra-molecular ferromagnetic
interactions between S = 1/2 CuII spins.
A1
A2
A3
Based on the dinuclear copper(II) core topology, this system
can be viewed as an S = 1/2 spin dimer with the following
Heisenberg spin Hamiltonian:
0
I
II
III
ꢀ*
ꢀ*
ꢀ*
Cu )ꢀ Cu )ꢀ Cu )ꢀ Cu
ð2Þ
C1
C2
C3
Details of peak potentials and E1/2 values are shown in
Table 3. The (E1/2)3 for CuII → CuIII conversion is well within
the range of the reported values.49
MAGNETIC PROPERTIES. The magnetic properties of the
[Cu2(μ-Cl)2(LX)2](ClO4)2 complexes have been studied by dc
susceptibility measurements on polycrystalline samples at
1000 Oe between 1.8 and 300 K. The experimental data for the
four compounds are shown in Fig. 4 as a χT vs. T plot. As
shown in Fig. 4, the χT product at room temperature for 1, 2, 3
and 4 ranges between 0.8 and 0.9 cm3 K mol−1 in good
H ¼ ꢀ2JðSCu1 ꢁ SCu2
Þ
ð3Þ
where J is the CuII⋯CuII magnetic interaction and Si is the spin
operator for the two CuII metal ions. The theoretical expression
of the magnetic susceptibility can be estimated by applying the
van Vleck equation50,51 in the weak field approximation:
ꢀ
ꢁ
g2Nμ2B
kB
2
χ0T ¼
ð4Þ
3 þ eꢀ2J=k T
B
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Table 3 Cyclic voltammogram data for 1, 2, 3 and 4
1
2
3
Complexes
Epa1
Epc1
(E1/2
)
Epa2
Epc2
(E1/2
)
Epa3
Epc3
(E1/2)
1
2
3
4
−0.229
−0.188
−0.263
−0.224
−0.570
−0.587
−0.544
−0.545
−0.399
0.006
0.006
0.018
0.033
−0.220
−0.288
−0.300
−0.270
−0.107
−0.282
−0.141
−0.119
0.570
0.630
0.616
0.546
0.280
0.260
0.180
0.324
0.425
0.445
0.398
0.435
−0.3876
−0.4035
−0.3845
Fig. 4 Magneto-structural correlation, J vs. Φ/R plot, in [Cu2(μ-Cl)2]2+ dinuclear
complexes.
As shown in Fig. 4, very good fits of the experimental data
have been achieved with J/kB = +0.37(2) K and g = 2.17(5) for 1,
J/kB = +0.44(2) K and g = 2.16(5) for 2, J/kB = +0.47(6) K and
g = 2.12(5) for 3 and J/kB = +0.27(6) K and g = 2.05(5) for 4. The
sign of J is consistent with ferromagnetic coupling and implies
that these complexes possess an ST = 1 triplet ground state.
MAGNETO-STRUCTURAL CORRELATION. For penta-coordinated Cu(II)
involved in CuII(μ-Cl)CuII dinuclear complexes, the average
bridging Cu–Cl–Cu angle (Φ) and the axial Cu⋯Claxial bond
distance (R) are the most important geometrical parameters
that govern the sign and amplitude of the exchange-coupling
constant (J). According to the empirical magneto-structural
Fig. 5 Temperature dependence of the χT product for 1, 2, 3 and 4 at 1000 Oe
(with χ defined as molar magnetic susceptibility and equal to M/H per dinuclear
complex). The solid lines are the best fit of the experimental data using the
model described in the text.
correlation established by Hatfield and co-workers,52
J
depends on Φ/R. In Fig. 5, J is plotted as a function of Φ/R and
a parabolic function has been used to fit the experimental data
according to ref. 48. The data extracted from the literature48
are plotted in red while the present data are given in blue. We
have also plotted J values obtained from DFT calculations,
[single point (brown) and fully optimized (green)] vs. Φ/R
(Fig. 5), and found to be well within the range.
Fig. 6 X-band EPR spectra of 1 at 77 K: (1, green) in solid state; (2, blue) in
MeOH. Inset: EPR signal in the magnetic field range 100–200 mT in solid state.
EPR STUDIES. The X-band EPR spectra of the copper(II)
complex [Cu(LH)(Cl)]2(ClO4)2 (1), as a representative one, were
performed in solid state as well as in MeOH solution at 77 K.
The EPR spectrum in the solid state at 77 K exhibits two
The EPR spectrum of 1 in frozen methanol is displayed in signals, one with g|| = 2.18, g⊥ = 1.89 and the other is a weak
signal at g = 4.14 with ΔM ∼ 1.9655 thus indicating that these
Fig. 6. This is typical of a monomeric tetragonal Cu(II) complex
complexes are dimers in solid state while in solution they are
with a dx2−y2 ground-state doublet. This experiment suggests
that the bis(μ-Cl)-bridged species, [(LX)Cu(μ-Cl)2Cu(LX)]2+, monomers. In solution at 77 K the observation that (g||
=
2.28 > g⊥ = 2.025) with A (Cu) = 170 G is consistent with a
undergo dissociation in solution presumably through the
k
tetragonally elongated square-pyramidal geometry around the
longer Cu–Cl (bridging) bonds (∼2.88 Å), which is facilitated
by additional coordination of solvent molecules.53 Thus, the Cu(II) center and comparable to the observed spectrum for the
type 2 center in the native pMMO (g = 2.24, g⊥ = 2.04 and
copper dimer complexes 1–4 are readily dissociable in solu-
tion. It is known that weakly exchange-coupled copper(II)
dimers54 display two signals and this is also true for the
present systems.
k
A (Cu) = 185 G).36
k
DFT OPTIMIZED STRUCTURES AND MOLECULAR ORBITALS. Geometric
parameters of di(μ-Cl) bridged complexes (1–4) have been fully
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optimized by DFT calculations in their triplet ground-state
configurations. The relevant computed bond lengths and bond
angles are summarized in Table 2 (in the square bracket). The
computed structures in the gas phase are in relatively good
agreement with the X-ray crystallographic SP-I type structures.
The two Cu metal ions have a nearly planar arrangement
within the N3Cl chromophore. The calculated Cu1–Cu1_a dis-
tances are in the 3.70–3.75 Å range and are larger than the
observed crystallographic data. The largest discrepancy of
0.345 Å is noted in complex 2 (LOMe) for which the crystallo-
graphic Cu1–Cu1_a distance is remarkably short in compari-
son to other complexes. The DFT optimized equatorial
Cu1–Cl1 bond lengths for all four complexes are around
2.315 Å and the discrepancies with the X-ray data are about
0.075–0.085 Å. The agreement for the axial Cu1–Cl1_a bond
distances is even better. It is worth mentioning that only for
complex 1 (LH), the calculated axial Cu1–Cl1_a bond is
shorter than the experimental one. In all the complexes, the
Cu–N bond for the tertiary amino nitrogen in the equatorial
plane is longer than the other two Cu–N(pyrazole) bonds. This
trend is in accordance with the X-ray crystallographic data.
However, the DFT optimized Cu–N bond distances are found
to be always larger by 0.04–0.10 Å. In contrast to the X-ray
structure with Cu1–Cl–Cu1_a angles always below 90°, these
bridging angles for the triplet ground state of the four DFT
optimized complexes are always above 90°. The computed
N–Cu–N and Cl–Cu–N bond angles agree well with the crystal-
lographic data and the discrepancies are less than 6–7°. Geo-
metries of all four complexes in their lowest singlet excited
states have also been optimized to ensure that the ground
state belongs to the triplet spin multiplicity. The singlet–triplet
energy separations at their optimized geometries lie in the
1.36–1.45 eV range.
Table 4 Experimental and DFT calculated values of J/K and their relations with
Φ/R
J/K (Calc.)
Φ/R
J/K
X
Φ/° (Exp.) R/Å (axial.Cu⋯Cl) (Exp.) (Exp.) X-Ray DFT
H
86.15
2.897
2.844
2.793
2.830
29.74 +0.37 +0.70 −5.70
29.90 +0.44 +0.90 −7.60
31.19 +0.47 +1.00 −6.00
30.85 +0.27 +1.30 −5.80
OMe 85.03
Me
Cl
87.11
87.30
Scheme 2 Products formed in the aerial oxidation of 4-TBC.
As seen in Table 4, there is excellent agreement between the
estimated magnetic interactions from magnetic measurements
and the single point DFT calculated coupling constants from
the X-ray crystal structure determination. The largest discre-
pancy is noted for complex 4 (LCl). The small ferromagnetic
coupling constants (J/kB < 1 K) are the result of small energy
separation between the triplet ground state and excited singlet
Fig. 7 ESI-MS+ (m/z) of the reaction products.
state. However, the coupling constants become negative (i.e. increases the extent of antiferromagnetic coupling between the
antiferromagnetic with J/kB ∼ −5.7 to −7.6 K) when calcu- two Cu(II) centers.
lations are carried out at the triplet ground-state geometry opti-
OXIDIZING PROPERTIES. In MeOH 4-tert-butyl-catechol (4-TBCH2)
mized at the UB3LYP/LANL2DZ level, which is explained by the undergoes oxidation catalyzed by complexes 1–3 in MeOH
fact that Cu1–Cl–Cu1_a bond angles increase from a ferro- saturated with O2 [[O2] ∼ 0.8 mM]56 under normal atmospheric
magnetic angle of <90° to an antiferromagnetic angle of >90° conditions. For the characterization of the oxidizing properties
in the gas phase. Therefore, the relaxation of the geometry in of the dinuclear copper(II) complexes, 4-tert-butyl-catechol
the gas phase favors antiferromagnetic coupling and thus a (4-TBCH2) was used as the substrate due to its comparatively
singlet ground state. However, the small J values indicate that higher redox potential value (Etcc = +0.312 V, E4-TBC = +0.03 V,
the two states are energetically very close. Fig. S3† shows two Ecatechol = −0.046 V, Edtbc = −0.222 V; all vs. Ag/AgCl).57 The
degenerated SOMOs (singly occupied molecular orbitals) for reactions in excess of 4-TBC with [Cu2(μ-Cl)2(LX)2](ClO4)2 in
four complexes obtained from the DFT optimized ground state MeOH lead to the formation of 4-TBQ, 5-MeO-4-TBQ along
geometries. Both SOMOs are not localized on the Cu(II) centers with other minor products (Scheme 2 and Fig. 7).
but are delocalized mostly on (i) the π-orbitals of the phenyl
Fig. 8 shows the typical time-resolved UV-Visible spectra
ring and (ii) the p-orbitals of the N-atom of one of the two pyr- obtained during the reaction of complex 1 with 4-TBCH2 that
azole rings. This delocalization from the Cu(II) centers thus clearly indicates the formation of 5-MeO-4-TBQ and 4-TBQ as
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Fig. 8 Time resolved absorption spectra in the visible region for the oxidation
of 4-TBCH2 by molecular oxygen with 1 in MeOH. The spectra were generated
at a time interval of 100 seconds. Conditions are [1] = 0.5 mM, [4-TBC] =
5.0 mM, Temp. = 25 °C.
Fig. 9 ki (in M s−1) vs. [4-TBCH2] plots for the oxidation of 4-TBCH2. Conditions
are [1] = 0.5 mM; T = 25 °C. Inset shows the Lineweaver–Burk plot of 1/ν vs. 1/
[S].
the major products. Reactions with 2 and 3 gave similar
results. However, it is interesting to note that when similar
reactions were carried out with complex 4 (X = Cl), no reprodu-
cible kinetic traces were found, probably due to the very fast
reaction. In order to analyse the oxidation product formed
during the reaction of copper(II) complexes with 4-TBCH2, the
following procedure was employed. 1.0 mM complex was
reacted with 1.0 mM 4-TBCH2 in MeOH under aerobic con-
ditions for 2 hours. The resulting solution was evaporated to
dryness under vacuum and the solid residue was then redis-
solved in a minimum volume of MeCN and finally chromato-
graphed on silica gel with MeCN as the eluent. Yellow-orange
and green bands corresponding to organic products and the
Cu(II) complex were collected separately and characterized
spectroscopically and by mass spectrometry. It is interesting to
note that the mononuclear species like [Cu(LX)Cl]+ (X = H;
FAB-mass) (422), [Cu(LX)Cl(H2O)]+ (ESI-MS+ (m/z) 440) and
[Cu(LX)Cl(MeOH)]+ [ESI-MS+ (m/z) 453] were detected in solu-
tion after the oxidation of 4-TBCH2. The different organic oxi-
dation products of 4-TBC are characterized by ESI-MS+ (m/z) as
depicted in Fig. 7. The preparative thin layer chromatography
(TLC) on the mass obtained in the yellow band after vacuum
evaporation using 1 : 10 v/v EtOAc–pet-ether (60–80 °C) as the
eluent led to isolation of 1 and 2 as major products (40 : 50)
and minor amounts of 3, 4, 5 and 6. The existence of 3 to 6
was confirmed only by ESI-MS+ analysis. Compounds 1 and 2
were further characterized by 1H NMR studies.
Table 5 Data for catecholase activities of complexes 1–3, [c] = 0.5 mM
Complexes
Kcat (h−1
)
vmax (Ms−1) × 104
KM (mM)
1
2
3
648
50.4
111
8.94 0.20
0.70 0.001
1.41 0.02
14.02 0.60
5.08 0.20
7.98 0.44
v being the velocity of the reaction, vmax the maximum velocity
that is attained when [CS], the adduct formed between S (sub-
strate, 4-TBCH2) and C (complex), is maximum in the presence
of a large excess substrate. Here, the Michaelis–Menten con-
stant is given as KM = (k2 + k−1)/k1 and the corresponding reac-
tion sequence is
k1
k2
ꢀ*
C þ S )ꢀ CS ꢀ! C þ P
ð6Þ
k1
Non-linear fitting of data to the Michaelis–Menten equation
leads to the evaluation of vmax, and KM from which the Kcat
rate constant for the dissociation of the complex–substrate
intermediate (i.e., the turnover number (TON), Kcat = vmax/[c])
was calculated. All these parameters are listed in Table 5. We
have also carried out the Lineweaver–Burk plot of 1/ν vs. 1/[S]
(inset Fig. 9) and linear dependence indicates the validity of
the Michaelis–Menten equation in this catalytic process.
In the oxidation of 4-TBCH2 there are parallel reactions
with contribution of 40 : 50 leading to the formation of
quinone and methoxylated quinone. So it can be concluded
that the Michaelis–Menten parameters (KM, Kcat) are the sum
of both the reactions.
Kinetic experiments were performed under pseudo-first-
order conditions, with the complex as a minor component and
the observed rate constants (ki) (Table S1†) were obtained from
a linear fit (initial-rate method) of the kinetic traces. All the plots
of ki vs. [4-TBCH2] yield non-linear curves of decreasing slope
(Fig. 9) indicating a first-order dependence of ki on [4-TBCH2] at
lower concentration while at higher concentration the rate
becomes almost independent of [4-TBCH2]. This type of rate
dependence is best described by the Michaelis–Menten equation:
We tried to correlate KM and Kcat with Hammett σ para-
meters and plots of log (Ki) (i = M or cat) against σ give straight
lines with ρ values of 1.61 0.14 and 4.18 0.44 for KM and
Kcat, respectively (Fig. 10). When we consider the E1/2 (Cu2+
Cu+) of the complexes we see the following order 1H > 3Me
→
>
2OMe which also corroborates the reactivity trend and is best
v ¼ vmax½Sꢂ=ðKM þ ½SꢂÞ
ð5Þ
explained by considering the substituent constants, although
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no linear correlation for the E1/2 vs. σ plot was observed. The place either through 2e− reduction to give H2O2 or through
E1/2 (Cu2+ → Cu+) for 4Cl does not correspond to this trend and 4e− reduction to give H2O (Scheme 3).59 It is worth mentioning
in solution some modified species may be formed which that the reduction of dioxygen to H2O2 upon catechol oxi-
behaves differently. In order to check the dependence of the dation by copper(II) complexes has already been observed in a
reaction rate on the complex concentration, experiments were few cases.58
58
carried out in the range 0.1 to 1.0 mM keeping [4-TBCH2] fixed
In order to test whether the catalytic oxidation of 4-TBCH2
at 1.0 mM (Table S2†). The plot of ki vs. [complex] (Fig. S4†) is occurs through the generation of H2O2, at least at the initial
perfectly linear indicating a first-order dependence of the rate stage of the reaction, an iodometric assay method60 based on
on the complex concentration.
the spectrophotometric determination of [I3−] was adopted. In
A significantly high TON indicates that the oxidation of the presence of H2O2, a characteristic absorption band is
4-TBCH2 by molecular oxygen occurs catalytically. The observed at 353 nm (ε = 26 000 M−1 cm−1 in water). The corres-
reduction of molecular oxygen in the catalytic cycle may take ponding amplitude of the absorption band is almost the same
as that found in the blank experiments carried out in
the absence of catechol but in the presence of 4-TBQ and the
active complex. So, it can be concluded that in the catalytic
cycle molecular oxygen undergoes 4e− reduction to H2O
instead of 2e− reduction to H2O2.
The following observations are pertinent for framing the
mechanism of the oxidation of 4-TBCH2 as outlined in
Scheme 4.
(i) Complexes in solution (MeOH) undergo rapid decompo-
sition to the corresponding mononuclear entities [Cu(LX)Cl]+
and support comes from the following observations: (a) The
EPR spectrum in MeOH at 77 K is typical for mononuclear
Fig. 10 Hammett plot for log KM and log Kcat
.
copper(II) complexes. (b) FAB-mass spectra of the reaction
immediately after the initiation of the reaction show a promi-
nent peak for [Cu(LH)Cl]+ or [Cu(LH)Cl]2 (422) while ESI-MS+
2+
(m/z) mass spectra reveal the presence of [Cu(LH)Cl(H2O)]+
(440) and [Cu(LH)Cl-(MeOH)]+ (453). The two copper(II) centers
may be equally coordinated by two solvent molecules to justify
the dinuclear structure with ESI-MS+ (m/z) 440 (H2O) and
Scheme 3 Oxidation of the catechol to 1,2-benzoquinone.
Scheme 4 Oxidation of the 4-TBCH2 to 5-methoxy-4-TBQ.
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453 (MeOH). But there is a problem because copper(II) centers
in the parent complexes are already in 5-coordinated distorted
square pyramidal geometry and due to the presence of two
bulky 3,5-dimethylpyrazole groups around each copper(II)
center it is very difficult to accommodate a solvent molecule to
get octahedral geometry. So one can safely assume that the
dimer splits in solution to give monomers (Scheme 5). More-
over, (c) conductivity measurements of the pure complex in
MeCN give an equivalent conductance of 150 (mho) which is
typical for a 1 : 1 electrolyte.
(ii) The dependence of initial rate (ki) on the concentration
of [Cu2(LH)2Cl2](ClO4)2 was checked by varying the concen-
tration in the range 0.01–0.50 mM, keeping 4-TBCH2 con-
centration fixed at 1 mM (Table S3†). A plot of ki vs.
[CuII2Cl2(LX)22+] or 2[CuLXCl]+ gives a straight line (Fig. S4†)
Fig. 12 Time resolved spectra for the reaction between [Cu2(LH)2(Cl)2]2+ and
4-tert-butylcatechol in MeCN. Conditions: [C] = 2.0 mM, [4-TBCH2] = 10.0 mM,
manifesting a first-order dependence on complex concen- T = 32 °C.
tration precluding the possibility of existence of di ⇌ mono
equilibrium, particularly in the experimental range of complex
concentration.
(vi) The appearance of a peak at 386 nm may be attributed
(iii) Oxidation of 4-TBCH2 occurs via 4e− reduction of O2 to the formation of a free or bound semiquinone radical.61
leading to the formation of H2O.
Inhibition of the reaction rate by the added Cl− concentration
(iv) The externally added LiCl was found to reduce the reac- was observed which indicates that there is a dissociation of the
tion rates of oxidation of 4-TBCH2 [Fig. S5†] supporting the longer Cu–Cl bridging bond to facilitate the coordination of
fission of the longer Cu–Cl (bridging) bond in a dimer to form catechol to one Cu-atom in the complex moiety. In order to get
monomers which is further facilitated by externally added Cl−. some idea that the semiquinone-bound Cu-species formed
The inner-sphere association between a complex and 4-TBCH2 during the course of the reaction, 4-TBCH2 was reacted with
is thought to be more facile in the case of [CuLX(Cl)(MeOH)]+ the complex at higher concentration ([complex] = 2 mM,
than [CuLX(Cl)2] clearly explaining the slight inhibition of reac- [4-TBCH2] = 10 mM) and a peak in the region 500–600 nm was
tion rates by externally added Cl−.
gradually built up (Fig. 12), which clearly indicates the for-
(v) The splitting of a dimer to monomers is further sup- mation of a metal-bound semiquinone radical anion as pre-
ported by considering mono-phenoxo-monochlorodicopper viously observed.62 We have also tried to check the formation
(MPD) as one of the reaction intermediates generated from of CuI-semiquinone species by an electrochemical method but
[Cu(LH)Cl]22+ and 4-TBCH2; but the optimized structures reveal failed to detect such species; this may be due to its very fast
its splitting into two isolated monomers (MPD′) ([Cu(LH)- decomposition on the electrode surface (beyond the diffusion
(4-TBC-2H)] and [Cu(LH)Cl]+) probably due to steric crowding limit) precluding its accumulation to the detection limit. But
around the metal centers (Fig. 11).
in the reaction mixture there is a definite signature of [Cu(LH)
Cl(4-TBCH2-H+)] in the FAB mass [(III) + K+ = 626].
All the foregoing observations lead us to frame a reaction
mechanism as outlined in Scheme 5. Here, the molecular
dioxygen binds to the semiquinone radical at the C5 position
and is transformed into a peroxyl radical, which in turn coordi-
nates to the metal center to form a seven-membered ring and
concomitantly oxidizes the metal center to Cu(II). Breakdown
of the peroxo-linkage as well as CuII–OAr may be anticipated to
occur through the formation of a five-membered ring in the
transition state which ultimately gives [CuII(LH)(OH)Cl]
[ESI-MS+ of CuII(L)(OH)Cl + H+ = 440.9], H2O and 5-methoxy-4-
TBQ. The [CuII(L)(OH)Cl] then reacts with catechol to regener-
ate the original catalyst [LXCu–Cl]+ and H2O to complete the
catalytic cycle. When we carried out the catalytic oxidation of
3,5-di-tert-butyl catechol (3,5-di-But-TBCH2) we did not get any
oxygenated product, as in the case of 4-TBCH2, instead 100%
3,5-di-tert-butyl-o-quinone is formed (Fig. S5†). The failure to
get the oxygenated product in the case of 3,5-di-But-TBCH2
may be attributed to the fact that electrophilic site C5 is
already occupied and does not allow the attack by molecular
Scheme 5 Splitting of mother complexes in solution phase.
Fig. 11 Splitting of [Cu2(μ-Cl)(4-TBCH2-H)(LH)2]2+ into [Cu (LH)2(μ-Cl)]+ and [Cu-
(LH)(4-TBCH2-2H)] revealed from DFT calculations.
1888 | Dalton Trans., 2013, 42, 1879–1892
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oxygen on to it. In the present case the attack of molecular type of mechanism has been previously proposed for the elec-
oxygen on C5 and Me transfer from the solvent (MeOH or trochemical oxidation of catechol.64 Formation of other
MeCN) are in line with the previous observations made by Ali organic products as minor components, as observed pre-
et al.63 The methyl group in MeO comes from the solvent, viously,63 is supported by the mass spectrometric methods.
which is verified by carrying out the same experiments in
The above mechanism is further supported by the DFT
MeCN, which also gives 5-MeO-4-TBQ in comparable yield as optimization of various probable intermediates and consider-
in the case of reactions in MeOH. Thus it manifests the fact ing their energies. The dichloro species [Cu2(LX)2(Cl)2]2+ splits
that the oxygen molecule attacks C5 to generate the transient into two monopositive monomers which get coordinated with
intermediate (V) which rapidly transforms to [Cu(LX)(OH)(Cl)].
water and/or methanol. Each of the three monomers was fully
The formation of 6,6′-But-biphenyl-3,4,3′,4′-tetraol as a optimized at the B3LYP/LANL2DZ level of calculations and was
minor product may be facilitated by the reaction of a part of 4- found to be stable as confirmed from their vibrational fre-
TBQ formed in the reaction with catechol (Scheme 6). This quency calculations. However, DFT calculations reveal that
such a splitting is energetically favorable by about −49.2 kcal
mol−1. It has been observed that when [Cu(LX)(Cl)]+ is coordi-
nated with H2O and MeOH, the stabilization energies are
−13.7 and −12.9 kcal mol−1, respectively. The reaction of the
solvent coordinated complex (I) with catechol (II) favors the
formation of (III). The DFT calculated energy of stabilization is
about −54 kcal mol−1. The calculations further establish that
other possible products IIIa, IIIb and IIIc are energetically less
stable. We have fully optimized the geometries of intermedi-
ates (VI) and (VII), which are found to be stable. In the pres-
ence of oxygen and methanol, the intermediate (III)
Scheme 6 Oxidation of the catechol by 4-But-1,2-benzoquinone to 6,6’-But-
biphenyl-3,4,3’,4’-tetraol.
transforms into (VI) and (VII) through a proposed transition
structure (V). The energy lowering during this process is
Fig. 13 DFT optimized structures proposed in framing the mechanism.
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calculated to be −43.9 kcal mol−1
.
Other intermediates
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tioned here that singlet oxygen is involved in this transform-
ation as is found in the biological systems. The formation of
[Cu(LH)(Cl)2] (X) from [Cu(LH)(Cl)]+ +Cl− (externally added) has
also been checked through DFT calculations and was found to
be favored by ∼ −116 kcal mol−1 and supports the inhibition
of the reaction in the presence of added Cl− (Fig. 13). The
details of DFT calculations and energies are listed in
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Acknowledgements
Financial support from CSIR (Ref. No. 01(2490)/11/EMR-II),
DST (Ref. SR/S1/IC-35/2006), and DRDO (Ref. No. ERIP/ER/ 24 H.-H. T. Nguyen, K. H. Nakagawa, B. Hedman, S. J. Eliot,
0503510/M/904) New Delhi, India is gratefully acknowledged.
Financial support received from the Centre for Advanced
M. E. Lidstrom, K. O. Hodgson and S. I. Chan, J. Am. Chem.
Soc., 1996, 118, 12766.
Studies (CAS) program of the Department of Chemistry, Jadav- 25 S. J. Elliott, D. W. Randall, R. D. Britt and S. I. Chan, J. Am.
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University of Bordeaux, the CNRS and the Conseil Régional 26 H.-H. T. Nguyen, S. J. Elliot, J. H.-K. Yip and S. I. Chan,
d’Aquitaine (GIS Advanced Materials in Aquitaine/COMET J. Biol. Chem., 1998, 273, 7957.
Project) for financial support. IB acknowledges a fellowship 27 R. L. Lieberman, D. B. Shrestha, P. E. Doan,
under the scheme “UGC Fellowship for Meritorious Students”.
B. M. Hoffman, T. L. Stemmler and A. C. Rosenzweig, Proc.
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28 J. S. Pap, J. Kaizer and G. Speier, Coord. Chem. Rev., 2010,
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29 C. Würtele, E. Gaoutchenova, K. Harms, M. Holthausen,
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