Unveiling the effects of the: In situ generated arene anion radical and imine radical on catecholase like activity: A DFT supported experimental investigation

Article abstract of DOI:10.1039/c7dt00733g

Two new dinuclear nickel(ii) complexes namely [Ni₂(L¹)₂(OAc)₂(H₂O)₂]·CH₃CN (1) and [Ni₂(L²)₂(SCN)₂(CH₃OH)₂]·CH

Full text of DOI:10.1039/c7dt00733g

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Unveiling the effects of in situ generated arene anion radical and imine radical on
catecholase like activity: A DFT supported experimental investigation
b
Sanchari Dasgupta,a Jaydeep Adhikary,a Sanjib Giri, a Antonio Bauza, Antonio Frontera, b
and Debasis Das*,a
aDepartment of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata – 700009,
India.
bDepartament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5,
07122, Palma (Baleares), Spain
Two new dinuclear nickel(II) complexes namely [Ni2(L1)2(OAc)2(H2O)2].CH3CN (1) and
[Ni2(L2)2(SCN)2(CH3OH)2].CH3OH (2) have been synthesized from designed Schiffꢀbase
ligand 4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀethylimino)ꢀmethyl]ꢀphenol (HL1) and its
reduced analogue 4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀethylamino)ꢀmethyl]ꢀphenol (HL2),
respectively. Both 1 and 2 have been characterised by usual physicochemical techniques
(UVꢀVis, FTꢀIR, ESIꢀMS study, Single crystal XRD) and their variable temperature magnetic
study has been performed. The nickel(II) centres in the dinuclear complexes 1 and 2 are
antiferromagnetically coupled through participation of bridging phenoxyl oxygen. In
acetonitrile solution both 1 and 2 retain their dinuclear structural integrity as is evident from
ESIꢀMS study. Catecholase like activity of 1 and 2 have been performed in acetonitrile
medium using 3, 5 ꢀdiꢀtertꢀbutylcatechol (3, 5ꢀDTBC) as model substrate. Complex 1 shows
higher catalytic activity than that of complex 2. ESIꢀMS study suggests that dinuclear species
undergo cleaving into mononuclear entity in presence of 3,5ꢀDTBC and that mononuclear
species supposed to act as active catalyst in the catalytic cycle. EPR study of catalytic
reactions confirms that organic radicals have been generated during catalysis in both cases.
However, in case of complex 1 catalyzed reaction a single isotropic signal at g = 1.97 is
obtained which is most likely to be due to imine radical formation. On the other hand, for
complex 2 catalyzed reaction the spectrum shows signal with hyperfine splitting (g = 2.11,
2.05, 1.9) and thereby suggesting the generation of a new radical i.e. arene anion radical in
catecholase activity study. Extensive DFT calculations have been performed to support the
experimental observation and thus to put forward the most probable mechanistic pathways
operating in the two cases. Higher efficiency of imine radical pathway over arene anion
radical has been rationalized by DFT calculations.
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Introduction
Catechol oxidase, a typeꢀIII copper protein, consisting of two antiferromagnetically coupled
copper(II) centres in its active site catalyzes the oxidation of ortho diphenols to the respective
1ꢀ7
ortho benzoquinones exclusively.
Over the years enormous numbers of complexes of
various ligand systems have been synthesized to investigate the mechanistic pathway of
catechol oxidation by the aforesaid protein. Two hypotheses are now nearly equally accepted
as the origin of catecholase activity; (I) via metal centered redox participation and (II)
through ligand centered redox participation particularly with redox nonꢀinnocent ligands. The
modelꢀcomplexes containing copper and manganese as the active centre generally show
8ꢀ17
activity as a consequence of metal centred redox participation.
On the other hand,
catecholase activity exhibited by nickel(II), zinc(II) etc. are supposed to be due to the ligand
centered redox participation where the coordinated imine ligand undergoes one electron
reduction during the elementary steps involved in the catecholase activity and the oxidation
state of the metal remains unaltered. Studies reported so far have established that for Schiffꢀ
base ligands imine bond is the actual site to take part in radical generation, whereas for
Mannich base ligands phenoxyl part has been considered as the susceptible site although the
possibility of benzene anion radical has not yet been explored.18ꢀ21 Recently, catecholase
active Ni(II) complexes of reduced Schiffꢀbase ligands have been reported by Ghosh et. al
and suggested radical pathway as the origin of that activity.22 However, they failed to assign
the probable site of the ligand where the radical generation is likely to take place. Moreover,
no comparative account on the radical path initiated catalytic efficacy of Schiffꢀbase and
reduced Schiffꢀbase ligand complexes is in literature till date. In order to get satisfactory
answers of all those queries we have undertaken this project. Here we have designed and
synthesized one Schiffꢀ base ligand, 4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀethylimino)ꢀ
methyl]ꢀphenol (HL1) and its reduced analogue,4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀ
ethylamino)ꢀmethyl]ꢀphenol (HL2) followed by their Ni(II) complexes namely
[Ni2(L1)2(OAc)2(H2O)2].CH3CN (1) and [Ni2(L2)2(SCN)2(MeOH)2]. CH3OH (2)(Scheme 1).
Both the complexes have been structurally and magnetically characterized. Detailed
catecholaseꢀlike activity of the complexes have been done and the origin of their activity
towards the aerobic oxidation of 3,5ꢀDTBC have been explored experimentally and DFT
calculations have been performed to rationalize the experimental fact. Combined
experimental and theoretical investigation help us to explore new findings in catecholase
activity study and all those findings we have vividly discussed herein.
2

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Scheme 1 Synthetic route of the complexes.
Results and discussion
Synthesis of the Ligands and Complexes.
Ligands were synthesized according to procedures as reported earlier.23
Complex 1 was prepared by adding a methanolic solution of nickel acetate tetrahydrate to a
methanolic solution of HL1 and the crude product thus obtained was recrystallised from
acetonitrile. Complex 2 was synthesized by addition of solid nickel thiocyanate to methanolic
solution of HL2.
FTꢀIR Spectra of Complexes.
All the complexes have been characterized by FTꢀIR and UVꢀVis Spectroscopy. The FTꢀIR
spectra of complex 1 (Fig. S1) and complex 2 (Fig. S2) consist of a sharp band at ~1648
ꢀ1
cmꢀ1 and ~1583 cm due to C=N stretching and CꢀN stretching respectively. The sharp
bands at ~1468ꢀ1471 cmꢀ1 are due to skeletal vibration for both complexes. The another two
bands at ~1383 cmꢀ1 and ~2113 cmꢀ1 are due to acetate ion for Complex 1 and thiocyanate ion
for Complex 2 respectively. 24
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UVꢀVis Spectra of Complexes.
According to Orgel diagram octahedral d8 system shows three spin allowed transition for
3
spinꢀallowed 3A2g→3T1g(P), A2g→3T1g(F), and 3A2g→3T2g(F) transitions in solution phase. 25
The acetonitrile solution of complexes showed weak absorption band at ~950ꢀ980 nm (ε= 14ꢀ
15 L mol−1 cm−1) and at ~610ꢀ690 nm (ε= 14ꢀ28 L mol−1 cm−1) are observed due to
3
3A2g→3T2g(F) and A2g→3T1g(P) transition respectively for both the complexes. The bands
3
due to A2g→3T1g(F) transition are observed at ~758 nm (ε= 2 L mol−1 cm−1) and ~857 nm
(ε= 2 L mol−1 cm−1) for complex 1 and complex 2 respectively. The UVꢀVis spectra of both
complexes suggest that they are present in octahedral environment. The bands at ~350ꢀ377
nm are attributed to metal to ligand charge transfer band (ε = 5940 L mol−1 cm−1for complex
1) which is weak in case of complex 2 (ε = 666 L mol−1 cm−1) due to absence of C=N imine
bond (Fig. S3).
ESIꢀMS Study of Complexes.
In order to have an idea about the solution phase structure of complex 1 and complex 2 the
ESIꢀMS study of both the complexes were performed in acetonitrile. The ESIꢀMS spectrum
of complex 1 consisted of a base peak at m/z = 658.8984 amu which is consistent with the
+
dinuclear species C22H25Br2N2Ni2O4 (calc. m/z= 658.8822amu). The acetonitrilic solution of
complex 2 showed a sharp peak at m/z = 662.9206 amu which may be assigned to
+
C22H29Br2N2Ni2O4 (calc.m/z = 662.9135 amu) (Fig. S4, S5)
Electrochemical Study of Complexes.
The cyclic voltammetry study of the complexes was carried out in acetonitrile medium using
tertiarybutylammonium perchlorate as supporting electrolyte under nitrogen atmosphere. The
irreversible reductive response at ꢀ1.00 V was observed for complex 1due to reduction of NiII
to NiI. The reductive response of NiII/ NiI couple was obtained at ꢀ0.9739 V for complex 2.
The oxidative response at 1.45 V observed in cyclic voltammogram of complex 2 which may
be tentatively assigned to ligand centre oxidation.17,22 ( Fig. S6)
Solid State UVꢀVis Spectra of Complexes.
The solid state UVꢀVis Spectra of complex 1 and 2 are very much similar to the solution
3
phase spectra. Complex 1 showed bands at λmax = 936 nm and 608 nm due to A2g→3T2g(F)
3
and A2g→3T1g(P) respectively. The bands at λmax= 964 nm and 615 nm were observed for
complex 2 which were assigned to 3A2g→3T2g(F) and 3A2g→3T1g(P) respectively (Fig. S7).
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Structural Description.
Xꢀray crystal structural analysis of complex 1 reveals that it comprises of a dinuclear
complex present on a crystallographic inversion center as depicted in Fig. 1. Nickel(II) ion in
complex 1is hexaꢀcoordinated. The complex is formed by two tridentate Schiffꢀbase ligand
HL1 coordinating the Nickel(II) ion through O,N,O donor site, thus forming a phenoxidoꢀ
bridged dimer. The basal plane of the nickel(II) ion is occupied by two phenoxyl O, imine N
and alcoholic O. The two axial positions are occupied by O atoms aqua molecule and acetate
anion. The bond lengths of two axial NiꢀO (NiꢀO(3) =2.097(2) Å and NiꢀO(1w) =2.104(2) Å)
atoms are comparatively longer than equatorial NiꢀO (NiꢀO(1) = 2.010 (2)Å, NiꢀO(2)
=2.130(2)Å)bonds (Table 1). Thus it can be concluded that geometry of complex 1 deviated
from ideal octahedral geometry and adopted an elongated distorted octahedral geometry.
Fig. 1 ORTEP view (ellipsoid at 50% probability) of complex 1 located about a centre of
symmetry. A CH3CN lattice molecule (per metal unit) is not shown.
Table 1 Coordination bond lengths (Å) and angles (°) for complex 1
NiꢀO(1)
2.010(2)
1.998(3)
2.130(2)
2.104(2)
92.20(9)
NiꢀO(3)
NiꢀO(1')
NiꢀNi'
2.097(2)
2.075(2)
3.117(1)
NiꢀN(1)
NiꢀO(2)
NiꢀO(1w)
O(1)ꢀNiꢀN(1)
O(2)ꢀNiꢀO(3)
88.50(8)
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N(1)ꢀNiꢀO(2)
O(1)ꢀNiꢀO(3)
N(1)ꢀNiꢀO(1w)
O(1)ꢀNiꢀO(2)
O(1)ꢀNiꢀO(1w)
O(3)ꢀNiꢀO(1w)
O(3)ꢀNiꢀN(1)
80.89(9)
O(1)ꢀNiꢀO(1')
O(2)ꢀNiꢀO(1')
N(1)ꢀNiꢀO(1')
O(3)ꢀNiꢀO(1')
O(2)ꢀNiꢀO(1w)
O(1')ꢀNiꢀO(1w)
80.52(8)
106.46(9)
172.63(9)
86.37(8)
97.61(8)
84.45(9)
88.33(8)
94.00(10)
172.14(9)
86.55(8)
170.11(9)
94.65(9)
Excluding two donor site, the four coordination sites of complex 2 are chelated by reduced
Schiffꢀbase ligand HL2 in an analogous manner as in complex 1. The vacant axial
coordination sites are coordinated by one molecule each of MeOH and SCN, O and N being
the respective donor atom. The axial NiꢀN(NiꢀN(2)=2.041(6)Å) bond length is shorter than
equatorial bond (NiꢀN(1)=2.063(4)Å) where as axial NiꢀO (NiꢀO(3)=2.161(4)Å) bond lengths
are longer than equatorial NiꢀO (NiꢀO(1)=2.015(3)Å and NiꢀO(2)=2.076(4)Å ). Therefore it
can be said that Nickel(II) ion in complex 2 is also present in distorted octahedral geometry
(Fig. 2).
Fig. 2 ORTEP view (ellipsoid at 50% probability) of centroꢀsymmetric complex 2. A CH3OH
lattice molecule (per metal unit) is not shown.
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Table 2 Coordination bond lengths (Å) and angles (°) for complex 2
NiꢀO(1)
NiꢀN(1)
NiꢀO(2)
NiꢀO(3)
2.015(3)
2.063(4)
2.076(4)
2.161(4)
NiꢀN(2)
NiꢀO(1')
NiꢀNi'
2.041(6)
2.038(3)
3.115(1)
O(1)ꢀNiꢀN(1)
N(1)ꢀNiꢀO(2)
O(1)ꢀNiꢀO(3)
N(1)ꢀNiꢀ N(2)
O(1)ꢀNiꢀO(2)
O(1)ꢀNiꢀN(2)
O(3)ꢀNiꢀN(2)
O(3)ꢀNiꢀN(1)
90.66(15)
79.81(16)
84.82(14)
98.02(19)
168.18(14)
93.81(17)
176.82(18)
84.88(15)
O(2)ꢀNiꢀO(3)
O(1)ꢀNiꢀO(1')
O(2)ꢀNiꢀO(1')
N(1)ꢀNiꢀO(1')
O(3)ꢀNiꢀO(1')
O(2)ꢀNiꢀN(2)
O(1')ꢀNiꢀ N(2)
87.37(16)
79.57(13)
108.60(14)
166.59(15)
85.10(14)
94.42(17)
91.83(17)
Magnetic Study
Magnetic Properties.
Temperature variation of the product of molar magnetic susceptibility and temperature (χMT)
for complex 1ꢀ2 are shown in Fig. 3. The complexes show room temperature χMT values of
ca. 2.21 cm3 K molꢀ1 for 1 and 2.11 cm3 K molꢀ1 for 2, which are in good agreement with a
magnetically non interacted nickel(II) dimer value of 2.0 cm3 K molꢀ1 for g=2 at 300K. In
both of the cases, the behaviour is very similar; a decrease of χMT value takes place upon
cooling and attains to almost zero (0.039 and 0.081 cm3 K mol−1 for 1 and 2) at 2K, which
suggests for strong antiferromagnetic coupling between pairs of nickel(II) ions in each of the
complexes, as mediated by double phenoxido bridges. To calculate the coupling constant (
the experimental data of complexes 1ꢀ2 have been fitted using the following VanꢀVleck
equation (Equation 1) which can be derived from the Hamiltonian −JS1S2 (S1 = S2 = 1).
J),
H
=
26ꢀ27 The constants N, β and k in Equation 2 have their usual meanings. The fitted curves were
obtained by minimizing the function R =Σ[(χMT)exp– (χMT)cal]2/Σ[(χMT)exp]2. The fits lead to
the following parameters:
J
= –12.48 (±0.02) cm–1, g = 2.156 (±0.07), R = 1.2×10ꢀ5 for
complex1 and
J
= –14.87 (±0.05) cm–1, g = 2.106 (±0.05), R = 3.6×10ꢀ5 for complex 2.
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Ng2β 2 2ex +10e3x
χ
MT =
×
Where x=J ꢁ kT
(1)
k
1+ 3ex + 5e3x
Fig. 3 Thermal variation of the χMT product for complexes1 and 2 is shown by circular
points. The solid lines represent the best fitted curves.
Magnetoꢀstructural Correlations.
In magnetoꢀstructural correlations of bis(phenoxido) bridged dinuclear system, it has been
reported that the coupling constant (J) has much dependence on phenoxido bridging angles.
For example, a bis(phenoxido) bridged NiII dimer can possibly show ferromagnetic coupling
if the angle is less than a critical value of ~97°; however, for larger angle an
antiferromagnetic coupling is observed. Those studies also clarify that the alteration of sign in
J is occurred at ca. 97°.28In this context, it should be mentioned that metal phenoxido
(M....O) bond length, outꢀofꢀplane shift of the phenoxido rings (τ) and hinge distortion of the
Ni2O2 core framework has also effects on J value. 29,30,31 Thus, the critical angle value of 97°
should be fluctuated based on these factors. As a consequence, another study on similar
complexes has reported that the critical angle value is 95ꢀ96° instead of 97°.42 Since the hinge
distortion for both the complexes (1 and 2) is zero (Table 3), it has been ignored. As the
Ni…O bond distances and τ angles are very close in both of the compound, its effect can also
be ignored. The Ni–O–Ni bond angle found in the present complexes 1ꢀ2 are reasonably
larger (99.74° and 100.51°) than the critical angle. Therefore, moderately strong
antiferromagnetic coupling is expected in both cases due to higher diphenoxido bridging
angles, which indeed agree with our experimental results and consistent with the recent
reports on similar structures. 26,27,32 To support the experimental J values,brokenꢀsymmetry
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DFT calculations has been carried out from the X–ray structures. The calculated values are in
good agreement in sign and magnitude with the experimentally obtained J values (Table 3).
Table 3 Magnetic and structural parameters for complexes 1ꢀ2
Parameters
Complex 1
2.156 (±0.07)
–12.48(±0.02)
1.2×10ꢀ5
–15.60
Complex 2
2.106 (±0.05)
–14.87(±0.05)
3.6×10ꢀ5
–16.44
g
J Exp. (cmꢀ1)
R
JDFT (cmꢀ1)
2.010
2.012
Ni–O (Å)
2.075
2.042
Ni–O–Ni (°)
99.74
100.51
Hinge distortion (°)
Outꢀofꢀplane shift of phenyl
rings (τ /°)
0
0
17.03
14.36
Catecholase Like Activity.
To compare the efficiency of complexes 1 and 2 as a suitable catalyst towards the oxidation
of catechol, the reactions were carried out in acetonitrile medium using 3,5ꢀDTBC as model
substrate and monitored by time dependent UVꢀVis spectroscopy. Due to oxidation of 3,5ꢀ
DTBC catalyzed by complex 1 an increment of peak at λmax= ~383 nm characteristics of 3,5ꢀ
diꢀtertꢀbutylbenzoquinone (3,5ꢀDTBQ) was observed (Fig. 4a). Whereas for complex 2 the
peak of 3,5ꢀDTBQ was observed at λmax =400 nm (Fig. 4b). Now it is important to detect the
species (H2O or H2O2) generated due to reduction of diꢀoxygen during oxidation of 3,5ꢀ
ꢀ
DTBC. There occurs oxidation of Iꢀ to I2 followed by formation of I3 as was detected in UVꢀ
Vis spectra of the reaction mixture of 3,5ꢀDTBC, complex and KI. The characteristics bands
ꢀ
of I3 were observed at 354 nm and 356 nm for complex 1 and 2 respectively. The observed
oxidation as is evident from spectroscopic data hints the releasing of H2O2 during catalytic
cycle (Fig. S8).
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(a)
(b)
Fig. 4 Oxidation of 3,5ꢀDTBC by (a) complex 1 and (b) complex 2 observed in UVꢀVis
spectroscopy with time.
Kinetics Study.
The kcat value calculated from Linewear Burk plot suggested that catalytic efficiency of
complex 1 (77.33 hꢀ1) is higher than complex 2(26.89hꢀ1) (Fig. S9). From kcat value it can be
said that oxidation of 3,5ꢀDTBC through imine bond radical formation is more feasible than
phenoxyl radical formation. All the kinetic parameters are given in table 4.
Table 4 Kinetic parameters for complexes 1 and 2
Catalyst
Vmax in Msꢀ1
2.14×10ꢀ6
7.47×10ꢀ7
Kmin M
1.21×10ꢀ3
1.25×10ꢀ3
kcat value in hꢀ1
Complex 1
77.33
Complex 2
26.89
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ESIꢀMS Study.
To evaluate the plausible mechanistic pathway of catalytic cycle ESIꢀMS study in positive
mode of mixture of complexes and 3,5ꢀDTBC were carried out in acetonitrile. Both complex
1 and 2 were present in dinuclear form as is evident from ESIꢀMS study of the complexes. It
was observed that both the complexes were converted to mono nuclear species upon addition
of 3,5ꢀDTBC.The base peak at m/z= 329.9406 amu (calc. 329.9463 amu) of molecular
formula C11H13NO2BrNi+obtained from complex 1 and for complex 2 the base peak for
mononuclear species C11H15NO2BrNi+ was observed at m/z= 331.9534 amu (calc. 331.9619
amu). The mononuclear species was active catalyst. The spectra taken at different time
interval showed that mononuclear species underwent 1:1 adduct formation with catecholate
anion. The peaks at m/z= 550.0912 amu (C25H33NO4BrNi, calc.550.9206 amu) and 600.9683
amu (C25H33NO4BrNi+3H2O, calc.600.1107 amu) were generated after 5 min of mixing of
complex 1 and DTBC and the peak was increased with time (Fig. S10). For complex 2 same
observations were noticed and a peak at m/z= 605.0010 amu (C25H35NO4BrNi+3H2O,
calc.605.1498 amu) was most probably due to 1:1 adduct formation between complex 2 and
catecholate anion (Fig. S11). The peak observed at m/z= 243.1246 amu corresponds to Naꢀ
3,5ꢀDTBQ (calc.243.1361 amu).
EPR Study.
The Xꢀband EPR spectra at 77 K of both the complexes 1 and 2 revealed that they are EPR
silent (Fig. S12). The mixture of complex 1 and 3,5ꢀDTBC in 1:10 molar ratio in acetonitrile
medium exhibited an isotropic signal at g = 2.05 which is a signature of organic free radical
formation and imine bond radical was generated in this case as reported earlier.17,18 In case of
reaction mixture of complex 2 and 3,5ꢀDTBC the pattern of EPR signal is completely
different. The spectrum consists of signal with hyperfine splitting (g = 2.11, 2.05, 1.9) (Fig.
5). Here we have two possibilities: (i) formation of aryl anion radical or (ii) phenoxyl radical.
However, the spectral patterns clearly indicate the formation of aryl anion radical. If it was
phenoxyl radical one could get a sharp EPR signal at g~2 as reported earlier. 21 In order to get
better understanding of our observed experimental facts we have performed DFT study
extensively (vide infra). 33ꢀ34
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(a)
(b)
Fig. 5 EPR spectra of mixture of (a) Complex 1 and 3,5ꢀDTBC, (b) Complex 2 and 3,5ꢀ
DTBC (red line is simulated spectrum) in acetonitrile medium.
Theoretical TDꢀDFT study
In the ground state (S0) of complex 1 the energy of HOMO is –4.390 eV,. It is basically
composed by an atomic d orbital of Ni with some participation of both oxygen directly
bonded to it. Similarly, the LUMO (–3.248 eV) is also basically composed by an atomic d
orbital of Ni with some participation of the four atoms directly bonded to it. The energy of
the LUMO+1 orbital is –2.580 eV and it is basically composed by the aromatic ligand and the
conjugated C=N bond. The energy difference between the HOMO and LUMO is 1.142 eV
(Fig. S13). To get better insight into the experimental absorption values TDDFT calculations
were done for the complex on the basis of the optimized geometry at the B3LYP/6ꢀ311+G*
level of theory. The calculated absorption energies associated with their oscillator strengths,
the main configurations and their assignments of the complex are given in Table S1. Here the
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lowest lying singlet → singlet absorption bands can be mainly attributed to one excitation. It
occurs at 1.4238 eV (λ = 871 nm and f = 0.0002) due to an HOMO → LUMO transition with
metalꢀmetal character, (see Fig. S13, highlighted in red). The following absorption band
consists of one excitation (1.8290 eV, 655 nm and f = 0.0045) that is attributed in a 96% to a
HOMO → LUMO+1, transition which can be assigned to Ni(d) → π*(L) with basically
MLCT character (see Fig. S13). The experimental values are in good agreement with the
theoretical ones, giving reliability to the optimized geometry and the level of theory.
In the ground state (S0) of reduced complex 2 the energy of HOMO, HOMO–1, HOMO–2
and HOMO–3 are –4.187 eV, –4.703 eV, –5.106 eV. –5.212 e, respectively). Apart from
HOMO–3 (centered in the aromatic ring), the HOMOs are basically composed by atomic d
orbitals of Ni with some little participation of the atoms directly bonded to it. Similarly, the
LUMO (–3.207 eV) is basically composed by an atomic d orbital of Ni with some
participation of the four atoms directly bonded to it. The energy difference between the
HOMO and LUMO is 0.980 eV (Fig. S14). The calculated absorption energies associated
with their oscillator strengths, the main configurations and their assignments of complex 2 are
given in Table S1. Here the lowest lying singlet → singlet absorption bands can be mainly
attributed to one excitation. It occurs at 1.2853 eV (λ = 965 nm and f = 0.0029) due to an
HOMO → LUMO transition with metalꢀmetal character, (Fig. S14, highlighted in red). The
following absorption band consists of one excitation (1.8290 eV, 655 nm and f = 0.0045) that
is attributed to several transitions (Table S1 and Figure S14), two transitions which can be
assigned to d(Ni) → d*(Ni) and one with some mixed character (metal–metal and ligand to
metal CT character). The experimental values are in good agreement with the theoretical
ones, giving reliability to the optimized geometry.
Theoretical study on the catecholase activity.
We have based our theoretical study in case of complex 1 derived from Schiffꢀbase ligand on
the catalytic cycle proposed in Scheme 2 and we have optimized all intermediates at the
M06ꢀ2X/def2ꢀTZVP level of theory. Based on the ESIꢀMS study, we have assumed that the
mononuclear complex is the predominant species in solution. In the first step of the
mechanism (Scheme 2) the acetonitrile molecule in B is replaced by the catechol. This
coordination increases the acidity of the phenolic group. Moreover, we have observed in this
optimized complex B an Hꢀbond interaction between the other phenolic group of catechol
and the alkoxido group of the ligand. We propose two intramolecular proton transfer
reactions and an electron transfer from the electron rich catecholate to the imine bond to yield
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intermediate E via intermediate D. It should be mentioned that it is not important the order of
these proton transfer reactions (even the proton transfer could also involve the solvent) since
the mechanism remains basically the same. The electron is transferred from the catechol
moiety to the C=N bond generating the semibenzoquinone. In the subsequent step the
molecular oxygen combines with the unpaired electron of the semibenzoquinone moiety
forming a C–O2˙ bond and yielding F. At this point, an electron transfer from the imine to the
terminal O atom of the peroxide yields F′). Finally two intramolecular proton transfers
facilitate the release of oꢀquinone and hydrogen peroxide.
Scheme 2 Catalytic cycle of catechol oxidation by complex 1 used in this study. One
molecule of catechol is oxidized coupled with the reduction of molecular oxygen to hydrogen
peroxide
The optimized geometries of the complexes A–G along with relative energies of
transformation at each of the reaction steps are given in Fig. 6. It was observed that the
coordination of catechol to the Ni is favourable by –2.5 kcal/mol. The subsequent
transformation of B to C has an energy cost of ꢂEC→D = 11.8 kcal mol−1 at the M06ꢀ2X/def2ꢀ
TZVP level of theory in acetonitrile. The rate determining step is the transformation of D to
E that involves the formation of the diradical species. The incorporation of molecular oxygen
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and the concomitant electron transfer and proton transfer steps are energetically favourable
(Fig. 6). The proton transfer is facilitated by the strong Hꢀbond between the O atom of the
peroxide bond and the phenolic group (2.02 Å) that is established in F. Finally, the formation
of hydrogen peroxide, oꢀquinone and coordination of the acetonitrile is also favorable ꢂEG→B
= −11.1 kcal mol−1.
Fig. 6 M06ꢀ2X optimized geometries, energies and some distances (in Å) of compounds Aꢀ
G.
We have analysed the HOMO/LUMO of D and singleꢀoccupied molecular orbitals (SOMOs)
of intermediates E and F to investigate the location of the unpaired electrons and support the
mechanistic proposal depicted in Scheme 2. The frontier orbitals of intermediate D are given
in Fig. 7 and it can be observed that the HOMO is located in the coordinated catecholate
moiety and the LUMO is composed by the Schiffꢀbase bond and the conjugated ring, thus
supporting the D→E transformation and the electron transfer from the catecholate to the
Schiff base. This electron transfer can be likely mediated by the Ni atom.
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Fig. 7 Frontier orbital plots for intermediates D.
In the intermediate E, where the proton transfer occurs, the highest SOMO is located in the πꢀ
system of the semiquinonate moiety with a minimal contribution of the d atomic orbitals of
the metal center (Fig. 8). Upon the coupling of O2 (formation of F), the SOMOs plots reveal
that one unpaired electron is located in the imine C=N bond and the conjugated arene
(highest SOMO). The other electron is distributed in the the πꢀsystem of catechol and the–
O2˙. This SOMO distribution also supports the mechanistic step F→G that is energetically
favoured.
Fig. 8 Frontier orbital plots for intermediates E and F.
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Mechanism for Complex 2
The mechanism proposed for the complex of reduced ligand (HL2) is given in Scheme 3 and
it similar to the proposed above for the unreduced one. The main difference is the coupled
proton and electron transfer to yield intermediate ER via intermediate DR. The one electron is
transferred from the catechol moiety to the arene generating the semibenzoquinone and a
concomitant anion radical in the arene. The subsequent steps are similar to those proposed for
complex 1, where the molecular oxygen combines with the unpaired electron of the
semibenzoquinone moiety forming a C–O2˙ bond and yielding FR. At this point, an electron
transfer from the imine to the terminal O atom of the peroxide yields F′). Finally two
intramolecular proton transfers facilitate the release of oꢀquinone and hydrogen peroxide.
Scheme 3 Catalytic cycle of catechol oxidation by complex 2 used in this study. One
molecule of catechol is oxidized coupled with the reduction of molecular oxygen to hydrogen
peroxide.
The optimized geometries of the complexes AR–GR along with relative energies of
transformation at each of the reaction steps are given in Fig. 9 for complex 2. It can be
observed that the coordination of catechol to the Ni is favourable by –3.5 kcal/mol. The
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subsequent transformation of BR to CR has an energy cost of ꢂEC→D = 11.7 kcal mol−1 at the
M06ꢀ2X/def2ꢀTZVP level of theory in acetonitrile. The rate determining step is the
transformation of DR to ER that involves the formation of the diradical species and it is 1
kcal/mol higher than that of complex 1, in good agreement with experiment that
demonstrated lower catalytic activity for complex 2 than 1. Similarly to complex 1, the
incorporation of molecular oxygen and the subsequent electron transfer and proton transfer
steps are energetically favourable (Fig. 8). Finally, the formation of hydrogen peroxide, oꢀ
quinone and coordination of the acetonitrile is also favourable ꢂEGR→BR = −12.8 kcal mol−1.
Fig. 9 M06ꢀ2X optimized geometries, energies and some distances (in Å) of compounds ARꢀ
GR.
We have analysed the HOMO/LUMO of DR and singleꢀoccupied molecular orbitals
(SOMOs) of intermediates ERand FRto investigate the location of the unpaired electrons and
support the mechanistic proposal depicted in Fig. 10. The frontier orbitals of intermediate DR
are given in Fig. 10 and it can be observed that the HOMO is located in the coordinated
catecholate moiety and the LUMO is located in the phenolic ring, thus supporting the
DR→ER transformation and the electron transfer from the catecholate to the aromatic ring in
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the hydrogenated ligand instead to the C=N bond. The LUMO of intermediate DR is higher in
energy than the LUMO of compound D (unreduced ligand) thus explaining the lower activity
of the reduced ligand.
Fig.10 Frontier orbital plots for intermediates DR and ER.
In the intermediate ER, where the proton transfer occurs, one SOMO is located in the πꢀ
system of the semiquinonate moiety with a minimal contribution of the d atomic orbitals of
the metal center. The other SOMO is located in the aromatic ring of the ligand. Upon the
coupling of O2 (formation of FR), the SOMOs plots (Fig. 11) reveal that one unpaired
electron is located in the aromatic ring (highest SOMO). The other electron is distributed in
the πꢀsystem of catechol and the–O2˙ (higher orbital coefficient). This SOMO distribution
also supports the mechanistic step FR→GR(electron transfer and proton transfer) that is
energetically favoured.
Fig. 11 Frontier orbital plots for intermediate FR.
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EXPERIMENTAL SECTION
Physical Methods and Materials.
Elemental analyses (carbon, hydrogen, and nitrogen) were carried out using a Perkin−Elmer
240C elemental analyzer. Fourier transform infrared spectra were performed on KBr disks
(400−4000 cm−1) with a PerkinꢀElmer RXI FTꢀIR spectrophotometer. Electronic spectra and
kinetic studies (200−1200 nm) were recorded at room temperature on a Shimadzu UVꢀ
1
3101PC and Hitachi Uꢀ3501 Spectrophotometer using dry acetonitrile as solvent. H NMR
spectra (300MHz) were recorded in CDCl3 solvents and TMSCl as standard at 25 °C on a
Bruker AV300 Superconductor NMR spectrometer using the solvent signal as the internal
standard in a 5 mm BBO probe. Electrospray mass spectra were recorded on a Waters Xevo
G2ꢀS QꢀTof mass spectrometer using HRMS grade acetonitrile as solvent. Molar magnetic
susceptibility (χM) was measured with a superconducting quantum interference device
vibrating sample magnetometer (SQUIDꢀVSM, Quantum Design). The electron paramagnetic
resonance (EPR) experiment was performed in pure acetonitrile at 77 K temperature using a
Bruker EMXꢀX band spectrometer and a JEOL JESꢀFA200 spectrometer.
2ꢀAminoꢀ2ꢀmethylꢀpropanol, 5ꢀBromoꢀ2ꢀhydoxybenzaldehyde and Sodium borohydride were
purchased from commercial sources (Alfa Aesar, Sigma Aldrich) and used as received.
Solvents are dried according to standard procedures and distilled. All other chemicals used
were of AR grade.
Synthesis of Schiffꢀbase ligand 4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀethylimino)ꢀ
methyl]ꢀphenol(HL1).
Schiffꢀbase ligand was synthesized and characterized as reported earlier.23
Synthesis of Reduced Schiffꢀbase ligand4ꢀBromoꢀ2ꢀ[(2ꢀhydroxyꢀ1,1ꢀdimethylꢀ
ethylamino)ꢀmethyl]ꢀphenol (HL2).
The reduced Schiffꢀbase ligand synthesized and characterized as reported earlier.23
Synthesis of [Ni2(L1)2(OAc)2(H2O)2].CH3CN(1) .
To a methanolic solution of Schiffꢀbase ligand (0.136 g , 0.5 mmol), methanolic solution of
nickel(II) acetate tetrahydrate (0.248 g ,1mmol) was added and refluxed for 30 min. The
light green solution was filtered and was kept in a CaCl2 desiccator. Light green product was
obtained and was recrystallised from acetonitrile. Light green coloured crystals suitable for
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Xꢀray data collection were obtained after three days (Yield: 72 %). Elemental analysis calcd
(%) for C26H38Br2N2Ni2O9: C 38.87; H 4.46; N 3.44; found : C 39.21; H 4.63 ;N 3.59; FTꢀIR
(KBr): ν(C=N) 1648 cm−1; ν(skeletal vibration) 1468 cm−1; ν(acetate) 1383 cm−1(Fig. S1,
Supporting Information); UVꢀVis (Acetonitrile): λmax(ε) = 351 nm(5940 L mol−1cm−1), 607
nm(13.5L mol−1 cm−1 ) ,758 nm(2L mol−1 cm−1 ) , 954 nm(14.8 L mol−1 cm−1 ) ( Fig. S3,
Supporting Information).
Synthesis of [Ni2(L2)2(SCN)2(MeOH)2] . CH3OH (2) .
To the methanolic solution of reduced Schiffꢀbase ligand (0.137g , 0.5 mmol), solid nickel
thiocyanate (0.174 g,1mmol) was added and was stirred for 15 min. The light green solution
was filtered and was kept in CaCl2 desiccator. Light green coloured crystals suitable for Xꢀ
ray data collection were obtained after two days. (Yield: 84 %). Elemental analysis calcd (%)
for C26H38Br2N4Ni2O6S2: C 37.00; H 4.54; N 6.64; found : C 37.43; H 4.31 ;N 6.67;FTꢀIR
(KBr): ν(CꢀN) 1583 cm−1; ν(skeletal vibration) 1471 cm−1; ν(SCN) 2113cm−1 (Fig.
S2,Supporting Information); UVꢀVis (Acetonitrile): λmax(ε) = 377 nm(666 L mol−1 cm−1), 686
nm(29L mol−1 cm−1) ,857 nm(12L mol−1 cm−1), 980 nm(14L mol−1 cm−1 ) (Fig. S3,
Supporting Information).
Xꢀray Crystallography.
Intensities data for crystal structure analyses of complexes 1and 2 were collected on a Bruker
Smart Apex diffractometer equipped with CCD. In all cases measurements were conducted at
room temperature with MoꢀKα radiation (λ = 0.71073 Å). Cell refinement, indexing, and
scaling of the data sets were performed using the program Bruker Smart Apex, and Bruker
Saint packages. 35All structures were solved by direct methods and subsequent Fourier
analyses36 and refined by the fullꢀmatrix leastꢀsquares method based on F2 with all observed
36
reflections.
Hydrogen atoms were placed at calculated positions and included in final
cycles of refinement. All calculations were performed using the WinGX System, Ver2013.3.
37 Crystal data and details of refinements are given in Table 5.
Table 5 Crystallographic data and details of refinements for complexes 1ꢀ2
1.(CH3CN)
2.(CH3OH)
empirical formula
Fw
C26H36Br2Ni2N2O10
895.86
C26H34Br2Ni2N2O4S2
904.07
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system
Monoclinic
Orthorhombic
P21/c
P
space group
a (Å)
b c a
13.050(2)
13.181(2)
11.5502(19)
90.0
10.556(3)
18.208(6)
20.193(6)
90.0
b (Å)
c (Å)
α (°)
112.782(2)
90.0
90.0
β (°)
γ (°)
90.0
V/(Å3)
1831.8(5)
2
3881(2)
4
Z
Dcalcd (g/cm3)
ꢃ (MoꢀKα) (mmꢀ1)
F(000)
1.621
1.547
3.266
3.181
912.0
1848.0
2.27 ꢀ 32.13
21316
2780
1.96 –26.612
12917
θ range (°)
collected reflections
indep reflections
Rint
3517
0.052
0.097
Obs reflections [I> 2678
2σ(I)]
2020
Parameters
232
218
R1 [I> 2σ(I)] [a]
wR2 [I> 2σ(I)] [a]
GOF on F2
0.0359
0.0785
1.03
0.0449
0.1146
1.02
residuals (e Å–3)
ꢀ0.67, 0.36
ꢀ0.56, 0.71
CCDC 1518430ꢀ1518431 contain the supplementary crystallographic data for this paper.
These data can be obtained free charge from the Cambridge Crystallographic Data Centre
viahttp://www.ccdc.cam.ac.uk/5 data_request/cif.
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Magnetic Study
Experimental Details.
Molar magnetic susceptibility (χM) for polycrystalline samples of complexes 1ꢀ2 were
measured with a superconducting quantum interference device vibrating sample
magnetometer (SQUIDꢀVSM, Quantum Design) with an applied field 750 Oe throughout the
temperature range 2–300 K. The susceptibility data were corrected for the diamagnetic
contributions of sample holder and then it has been further corrected by Pascal’s diamagnetic
contribution.
Computational Methodology.
The theoretical calculation based on broken symmetry DFT has been used to estimate the
29,38
coupling constant (J) of these complexes as proposed by Ruiz et al.
All the calculations
were accomplished with the consideration of experimental Xꢀray structures. The hybrid
B3LYP functional 39ꢀ41 and widely employed TZVP 42 basis set has been considered in all
calculations as implemented in the ORCA package. 43 RI approximation with auxiliary TZV/J
coulomb fitting basis set and tight SCF convergence criteria (Grid 4) have also been
incorporated. 44
45
For the catecholase mechanism study, we have used the M06ꢀ2X functional,
which
intrinsically includes dispersion correction,46 and the def2ꢀTZVP basis set using the
Turbomole 7.0 program.47 The optimized structures of complexes and intermediates are true
minima in the potential energy surface. Solvent effects (acetonitrile) have been considered
using the COSMO.48
Catecholase Like Activity and Kinetics Study of Complexes.
The catecholase like activity of these two complexes were monitored by treating 1×10ꢀ4(M)
complex solution with 1×10ꢀ2 (M) 3,5ꢀdiꢀtertꢀbutylcatechol (3,5ꢀDTBC) solution under
aerobic condition at 25 °C. The time dependent wavelength scan was performed in
acetonitrile medium to understand the potential of two complexes as catalyst towards
oxidation of 3,5ꢀDTBC.
The kinetic study of the oxidation of 3,5ꢀDTBC by two complexes were performed by initial
rate method at 25 °C. Solutions of substrates of concentration ranging from 0.001 to 0.05 (M)
were prepared from a 1×10ꢀ2 (M) substrate solution. The dependence of the initial rate on the
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concentration of the substrate was monitored by UVꢀVis spectroscopy at the respective
wavelength by maintaining the catalyst concentration 1×10ꢀ4(M) for each set. The complexes
showed saturation kinetics, and a treatment based on the MichaelisꢀMenten model was
applied. The binding constant (KM), maximum velocity (Vmax), and rate constant for
dissociation of the substrates (i.e., turn over number, kcat) were calculated for the complexes
1
using the Lineweaver−Burk graph of versus ʞ1ʟ, with the equation (2),
V
S
1
V
KM
Vmax
1
Vmax
1
ʞ ʟ
S
=ʠ
ʡ×
+
(2)
Detection of Hydrogen Peroxide in the Catalytic Reaction.
To detect the formation of hydrogen peroxide during the catalytic cycle reaction mixtures
were prepared as in kinetic experiments. After 1 hour of reaction equal volume of water was
added and 3,5ꢀDTBQ formed was extracted with dichloromethane. pH of the aqueous layer
was fixed to 2 with H2SO4. 1 mL of a 10% solution of KI and three drops of 3% solution of
ammonium molybdate were added. In the presence of hydrogen peroxide occurs the reaction
H2O2 + 2Iꢀ + 2H+→ 2H2O + I2, and with an excess of iodide ions, the triiodide ion is formed
ꢀ
according to the reaction I2(aq) + Iꢀ= I3 .The reaction rate is slow but increases with
increasing concentrations of acid, and the addition of an ammonium molybdate solution
ꢀ
renders the reaction almost instantaneous. The formation of I3 could be monitored
ꢀ
spectrophotometrically due to the development of the characteristic I3 band (λmax=353 nm, ε
=26000 L mol−1 cm−1).
Conclusions.
Model study on catechol oxidase makes it evident that radical pathway is a reasonable
alternative of metalꢀcentre redox participation. Our present investigation explores a new type
of radical participation, arene anion radical, which was not known to us earlier in catechol
oxidation study by model compounds. Experimental evidences followed by detailed DFT
calculations clearly suggest that in complex 2 where the ligand system is reduced Schiffꢀbase
ligand, the arene anion radical formation is the origin of catecholaseꢀlike activity of the
model compound. Judicial design of ligands in the present study gives us a golden
opportunity not only to explore a new radical generation in catecholase activity but also to
portrait a comparative account on catalytic efficiency between imine radical and arene anion
radical towards catechol oxidation. Since arene anion radical formation is energetically less
favourable than imine radical formation complex 1 shows higher catalytic efficiency than that
of 2 towards oxidation of 3,5ꢀDTBC.
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Acknowledgments
The authors wish to thank University of Calcutta for providing the facility of the singleꢀ
crystal Xꢀray diffractometer from the DST FIST program and ESIꢀMS spectrophotometer
from the DST PURSE and Indian Association for the Cultivation of Science for providing
EPR facility. S.D is thankful to UGC and University of Calcutta [UGC/485/Fellow (univ)]
for providing fellowship. Financial assistance from DSTꢀWB (Sanction memo no.
681(sanc)/ST/P/S&T/15Gꢀ8/2016 dt 09/11/2016) to DD has been duly acknowledged.
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Table of Contents
A comparative mechanistic account has been portrayed on the basis of arene anion and imine
radicals in catecholase activity study.