Ashish Kumar
Dhara
,
Udai P.
Singh
and
Kaushik
Ghosh
*
Department of Chemistry, Indian Institute of Technology Roorkee, Uttrakhand, India. E-mail: ghoshfcy@iitr.ernet.in
First published on 20th September 2016
Diphenoxo bridged dinuclear zinc complexes [Zn2(OMe-Phimp)2(Cl)2] (1) (OMe-PhimpH = (E)-4-methoxy-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn2(Me-Phimp)2(Cl)2] (2) (Me-PhimpH = (E)-4-methyl-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn2(N-Phimp)2(Cl)2]·CH3CN (3·CH3CN) (N-PhimpH = (E)-1-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)naphthalen-2-ol) and [Zn2(Phimp)2(Cl)2] (4) (PhimpH = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol) were synthesized and spectroscopically characterized. The molecular structures of 1 and 3·CH3CN were determined using X-ray crystallography. DFT and TD-DFT calculations were performed to optimize the molecular geometry, interpret the spectroscopic results and investigate the contribution of the ligands to the redox properties of the complexes. Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium nitrate (CAN) and the redox properties were examined through cyclic voltammetric measurements. All the dinuclear zinc complexes were found to be highly active in catalyzing the oxidation of the primary alcohols 3,5-di-tert-butylcatechol and o-aminophenol. The catalytic activity was found to be in the order of complex 1 > complex 2 > complex 4 > complex 3 for both calechol oxidation and o-aminophenol oxidation. An EPR experiment clearly hinted at the generation of a radical during the oxidation of catechol and o-aminophenol. Reaction models for these processes were proposed and theoretical studies were performed to support the proposed mechanism. ESI-MS spectra clearly indicate the formation of a catalyst–substrate adduct by removal of one Cl− ion.
Three important features of the reported complexes prompted us to perform the present study. First, these complexes could generate a stable phenoxyl radical (at room temperature) even though no substituent group was present on the phenyl ring containing the phenolato function. Second, the crystal structure of these complexes revealed a cis-arrangement of the ligand as well as chloride ions along the Zn–Zn axis, which could allow ligation of a bidentate ligand. The third important feature is the redox activity possessed by these complexes. Knowing the stability of the generated phenoxyl radical in these complexes, alcohol oxidation was investigated for functional mimicking of the GO enzyme.9b,c On the other hand, we were interested in studying the interactions of catechol (substrate for catecholase activity studies) and o-aminophenol (substrate for phenoxazinone synthase activity studies) with our complexes. Catechol oxidase is an enzyme with a type-3 active site that catalyzes the oxidation of catechols to the corresponding o-quinones with reduction of O2 to H2O.13–20 On the other hand, o-aminophenol oxidase is also a copper containing enzyme,21 which is responsible for the synthesis of actinomycin. Although –NH2 and –OH are two isoelectronic functional groups, investigation of the mechanisms clearly indicated a completely different fate of the two substrates via oxidation chemistry (shown in Scheme 2).
Investigation of the literature clearly exposed that there are very few reports22,23 on the catecholase activity of phenoxo-bridged dinuclear zinc complexes. To the best of our knowledge, systematic studies on catecholase-like processes as well as o-aminophenol oxidation with zinc complexes capable of generating a stable phenoxyl radical complex have not been reported in the literature. Herein, we report the synthesis and characterization of four zinc(II) complexes [Zn2(OMe-Phimp)2(Cl)2] (1), [Zn2(Me-Phimp)2(Cl)2] (2), [Zn2(N-Phimp)2(Cl)2]·CH3CN (3·CH3CN), and [Zn2(Phimp)2(Cl)2] (4) derived from the ligands OMe-PhimpH, Me-PhimpH, N-PhimpH and PhimpH respectively (shown in Scheme 3).24 The complexes were characterized using different spectroscopic studies. The molecular structures of complexes 1 and 3·CH3CN were determined using X-ray crystallography. We have investigated the oxidation of benzyl alcohol, catechol and o-aminophenol through experimental studies and theoretical calculations. Reaction pathways for these oxidations will be scrutinized and probable mechanisms are suggested.
Fig. 1 ORTEP diagram (40% probability level) of the complex [Zn2(OMe-Phimp)2(Cl)2] (1). Hydrogen atoms and solvent molecules are omitted for clarity. |
Fig. 2 ORTEP diagram (40% probability level) of the complex [Zn2(N-Phimp)2(Cl)2] (3·CH3CN). All hydrogen atoms and solvent molecules are omitted for clarity. |
1 | 3·CH3CN | ||
---|---|---|---|
Bond lengths [Å] | |||
Zn(1)–O(1) | 2.039(4) | Zn(1)–O(1) | 2.021(4) |
Zn(1)–O(2) | 2.044(4) | Zn(1)–O(2) | 2.081(4) |
Zn(1)–N(1) | 2.159(5) | Zn(1)–N(1) | 2.148(6) |
Zn(1)–N(3) | 2.119(5) | Zn(1)–N(3) | 2.071(7) |
Zn(1)–Zn(2) | 3.155(12) | Zn(1)–Zn(2) | 3.170(12) |
Zn(2)–O(1) | 2.039(4) | Zn(2)–O(1) | 2.042(4) |
Zn(2)–O(2) | 2.064(4) | Zn(2)–O(2) | 2.026(4) |
Zn(2)–N(4) | 2.149(5) | Zn(2)–N(4) | 2.135(6) |
Zn(2)–N(6) | 2.110(5) | Zn(2)–N(6) | 2.084(6) |
Zn(2)–Cl(1) | 2.231(2) | Zn(2)–Cl(2) | 2.230(2) |
Bond angles [°] | |||
O(1)–Zn(1)–O(2) | 78.59(16) | O(1)–Zn(1)–O(2) | 78.29(15) |
O(1)–Zn(1)–N(3) | 130.19(18) | O(1)–Zn(1)–N(3) | 131.7(3) |
O(2)–Zn(1)–N(3) | 97.31(19) | N(3)–Zn(1)–O(2) | 97.4(2) |
O(1)–Zn(1)–N(1) | 81.12(17) | O(1)–Zn(1)–N(1) | 80.7(2) |
O(2)–Zn(1)–N(1) | 144.49(17) | N(3)–Zn(1)–N(1) | 75.3(2) |
N(3)–Zn(1)–N(1) | 74.40(2) | O(2)–Zn(1)–N(1) | 143.6(2) |
O(1)–Zn(1)–Cl(2) | 117.66(12) | O(1)–Zn(1)–Cl(1) | 113.4(2) |
O(2)–Zn(1)–Cl(2) | 110.85(13) | N(3)–Zn(1)–Cl(1) | 112.79(18) |
N(3)–Zn(1)–Cl(2) | 110.19(15) | O(2)–Zn(1)–Cl(1) | 111.7(2) |
N(1)–Zn(1)–Cl(2) | 104.33(14) | N(1)–Zn(1)–Cl(1) | 103.82(17) |
O(1)–Zn(2)–O(2) | 78.13(16) | O(2)–Zn(2)–O(1) | 79.09(16) |
O(1)–Zn(2)–N(6) | 101.43(19) | O(2)–Zn(2)–N(6) | 130.0(3) |
O(2)–Zn(2)–N(6) | 136.93(18) | O(1)–Zn(2)–N(6) | 98.6(2) |
O(1)–Zn(2)–N(4) | 143.53(18) | O(2)–Zn(2)–N(4) | 81.7(2) |
O(2)–Zn(2)–N(4) | 80.54(18) | O(1)–Zn(2)–N(4) | 148.1(2) |
N(6)–Zn(2)–N(4) | 75.20(2) | N(6)–Zn(2)–N(4) | 75.0(2) |
O(1)–Zn(2)–Cl(1) | 110.86(12) | O(2)–Zn(2)–Cl(2) | 115.6(2) |
O(2)–Zn(2)–Cl(1) | 113.72(13) | O(1)–Zn(2)–Cl(2) | 112.8(2) |
N(6)–Zn(2)–Cl(1) | 106.64(15) | N(6)–Zn(2)–Cl(2) | 111.22(18) |
N(4)–Zn(2)–Cl(1) | 104.71(15) | N(4)–Zn(2)–Cl(2) | 98.41(18) |
Zn(1)–O(1)–Zn(2) | 102.03(18) | Zn(1)–O(1)–Zn(2) | 101.85(19) |
Zn(1)–O(2)–Zn(2) | 101.01(18) | Zn(2)–O(2)–Zn(1) | 100.39(17) |
Complex 1 adopted a dinuclear structure, consisting of two pentacoordinated zinc atoms bridged by two phenolato functional groups. Structural index parameter (τ)12,29 calculations for complex 1 afforded τ values of 0.23 and 0.11 for the Zn1 and Zn2 centers respectively. These values clearly indicate a distorted square pyramidal geometry of the ligands around both the zinc centres.
Similarly, in complex 3·CH3CN, the dinuclear unit consisted of two pentacoordinated zinc atoms bridged by the two phenolato moieties of two deprotonated OMe-PhimpH ligands. Structural index parameter (τ)12,29 calculations for complex 3·CH3CN afforded τ values of 0.19 and 0.30 for the Zn1 and Zn2 centers respectively, which clearly supports a distorted square pyramidal geometry of the ligands around both the zinc centres. Interestingly, in both metal complexes 1 and 3·CH3CN, two chloride ions (Cl1 and Cl2) were found to be on the same side of the Zn–Zn axis, and consequently the ligands (OMe-Phimp− and N-Phimp−) occupied the other side giving rise to an overall unique cis configuration in complexes 1 and 3·CH3CN respectively (shown in Fig. 3). The cis arrangement of chloride ions along the Zn–Zn axis is different to the structure reported by Reedijk and coworkers.29 The Zn–Cl bond distances in complexes 1 and 3·CH3CN are consistent with the literature values.12,31 Also, all the Zn–N (Nimine as well as Npyridine) distances were found to be consistent with the reported literature.12,31
Fig. 3 The cis configuration of complex 1, as viewed along the Zn–Zn axis, and a polyhedron representation of the Zn2O2 unit. |
We have reported12 that the Zn–Zn distance in these types of complexes was found to be in the range of 2.94 Å–3.24 Å. The Zn–Zn distances obtained for complex 1 (3.15 Å) and complex 3·CH3CN (3.17 Å) were found to be in the higher side of this range. In the case of complexes 1 and 3·CH3CN, the Zn–O (phenolato) bond lengths were found to be in a range from 2.064 Å to 2.039 Å and 2.079 Å to 2.022 Å respectively. These distances are similar to the data reported by Das and co-workers,22 and Reedijk and co-workers.29
Noncovalent interactions, like π–π stacking interactions and hydrogen bonding networks, play an important role in supramolecular chemistry, crystal engineering and biology.32 For complexes 1 and 3·CH3CN, intermolecular as well as intramolecular hydrogen bonding interactions were observed between the chloride ions and aryl hydrogens of the phenyl ring, and the interactions are depicted in Fig. S38 and S39 respectively in the ESI.†
From the crystal structure, complex 1 possessed intermolecular as well as intramolecular hydrogen bonding interactions between the chloride ions and aryl hydrogens of the phenyl ring with distances of 2.763 Å (intermolecular hydrogen bonding), and 2.799 Å and 2.791 Å (intramolecular hydrogen bonding interactions). Similarly, complex 3·CH3CN also possessed both intermolecular and intramolecular hydrogen bonding interactions between the chloride ions and aryl hydrogens of the phenyl ring with distances of 2.845 Å (intermolecular hydrogen bonding), and 2.769 Å and 2.840 Å (intramolecular hydrogen bonding interactions). The non-covalent interactions found in complexes 1 and 3·CH3CN are described in the ESI (Table S1†).
Orbital | Energy (eV) | % Contribution | Main bond type | |||
---|---|---|---|---|---|---|
Zn | O | Phenolato ring | Ligand | |||
Complex 1 | ||||||
L+4 | −1.1643 | 0 | 0 | 0 | 96 | π*(L) |
L+3 | −1.2650 | 0 | 1 | 11 | 100 | π*(L) + π*(phenolato ring) |
L+2 | −1.3194 | 0 | 0 | 10 | 100 | π*(L) + π*(phenolato ring) |
L+1 | −1.7729 | 1 | 2 | 25 | 98 | π*(L) + π*(phenolato ring) |
L | −1.7956 | 1 | 2 | 25 | 99 | π*(L) + π*(phenolato ring) |
H | −5.1985 | 0 | 19 | 73 | 99 | π (L) + π(phenolato ring) |
H−1 | −5.2787 | 1 | 15 | 69 | 97 | π (L) + π(phenolato ring) |
H−2 | −6.0020 | 0 | 1 | 30 | 84 | π(L) + π(phenolato ring) + n(Cl) |
H−3 | −6.0953 | 1 | 1 | 28 | 71 | π(L) + π(phenolato ring) + n(Cl) |
H−4 | −6.1231 | 4 | 0 | 2 | 6 | π(L)) + n(Cl) |
Complex 2 | ||||||
L+4 | −1.2465 | 0 | 0 | 0 | 100 | π*(L) |
L+3 | −1.3796 | 0 | 1 | 14 | 100 | π*(L) + π*(phenolato ring) |
L+2 | −1.4261 | 0 | 0 | 13 | 100 | π*(L) + π*(phenolato ring) |
L+1 | −1.8805 | 1 | 4 | 39 | 99 | π*(L) + π*(phenolato ring) |
L | −1.8969 | 1 | 3 | 38 | 99 | π*(L) + π*(phenolato ring) |
H | −5.3712 | 0 | 11 | 67 | 98 | π (L) + π(phenolato ring) |
H−1 | −5.4314 | 1 | 9 | 66 | 96 | π (L) + π(phenolato ring) |
H−2 | −6.1302 | 2 | 6 | 39 | 47 | π(L) + π(phenolato ring) + n(Cl) |
H−3 | −6.1800 | 5 | 1 | 2 | 3 | π(L) + π(phenolato ring) + n(Cl) |
H−4 | −6.2020 | 3 | 1 | 4 | 7 | π(L)) + n(Cl) + π(phenolato ring) |
The nature of the bonding interactions of all the dinuclear zinc complexes has been investigated (ESI, Tables S9, S10, S12, S13, S15, S16, S18 and S19†).57,59 The composition and occupancy of the calculated C–O, C–N and N–N natural bond orbitals (NBOS) for complex 1 are given in Table S9.† The data for complexes 2, 3·CH3CN and 4 are also given in the ESI (Tables S12, S15 and S18†).
On the basis of natural bond orbitals, the electronic arrangement of Zn(1) in complex 1 is: [Ar]4s0.303d9.984p0.425p0.01 with 10.70 valence electrons, while the electronic arrangement of Zn(2) in complex 1 is: [Ar]4s0.313d9.984p0.425p0.01 with 10.70 valence electrons. So both metal centres have the same electronic configuration. The occupancies for the zinc (filled d10 system having two electrons in each orbital) were found to be similar for both the metal centres in 1 and the electronic distribution could be written as: dxy1.996dyz1.997dzx1.997dx2−y21.996dz21.995. Similarly for complex 2 the electronic arrangement of the Zn centre is: [Ar] 4s0.303d9.984p0.425p0.01 with 10.70 valence electrons, which is similar to the electronic arrangement of complex 1. Complexes 3·CH3CN and 4 also have similar electronic arrangements. All complexes have fully filled d orbitals at the metal centre, in a similar fashion. Therefore, the zinc centre does not have any effective role, rather the participation of the ligand in catecholase and o-aminophenol oxidase type-processes is extremely important for the present study. The strength of the interactions between the donor and acceptor sites can be calculated using the second order perturbation theory (ESI, Tables S11, S14, S17 and S20†).33a,b A higher value of E2 (kcal mol−1) indicates a better interaction of the orbitals inside a molecule.33a,b For complex 1, the values of E2 for the interactions LP (3) O75 → BD*(2) C42–C66, LP (3) O76 → BD*(2) C12–C65, LP (1) N80 → BD*(2) C43–N79 and LP (1) N 80 → BD*(2) C56–C57 are 35.02 kcal mol−1, 33.46 kcal mol−1, 24.04 kcal mol−1 and 39.68 kcal mol−1 respectively. However, the values of E2 for the interactions LP (2)O 75 → LP*(7)Zn 2, LP (2) O75 → LP*(8)Zn 2, LP (2) O 76 → LP*(7)Zn 1 and LP (2)O 76 → LP*(8)Zn1 are 17.23 kcal mol−1, 10.27 kcal mol−1, 17.78 kcal mol−1 and 10.67 kcal mol−1 respectively. Comparison of the above two sets of E2 values for complex 1 indicates that the interactions between O and Zn are not effective and that the intraligand interactions are more effective. The rest of the data calculated for the other complexes are present in the ESI (Tables S11, S14, S17 and S20†). We have performed calculations through time dependent density functional theory (TD-DFT) at the ground state (S0). The data obtained for complexes 1 and 2 after the TD-DFT calculations are shown in Table S7,† while the TD-DFT data of complexes 3·CH3CN and 4 are also present in the ESI (Table S8†). Five main transitions were observed for complex 1. The main transitions are (1) S0 → S3 (H−1 → L), 412 nm, f = 0.0246, (2) S0 → S4 (H−1 → L+1), 405 nm, f = 0.1239, and (3) S0 → S7 (H−1 → L+2), 355 nm, f = 0.0470. The peak at ca. 412 nm was recognized as mostly due to an intraligand (ILCT) charge transfer transition with minor contributions from H−1 → L+1 and H → L+1.
The TD-DFT spectrum of complex 2 showed four transitions at 377 nm, 340 nm, 315 nm and 285 nm, with oscillator strengths of 0.1620, 0.0859, 0.3165 and 0.1209 respectively, which were assigned to H−1 → L+1, H → L+3, H−3 → L+1 and H−8 → L+1 respectively. These transitions in the spectrum can be assigned to ILCT (pπ → pπ*) transitions. The probable electronic transitions of complexes 1 and 2 are shown in Fig. 4 and 5. Similarly, complexes 3·CH3CN and 4 also showed three and four transitions respectively, which can also be assigned to ILCT transitions. In the case of complex 3, the three main transitions are (1) S0 → S4 (H−1 → L+1), 391 nm, f = 0.3822, (2) S0 → S15 (H−2 → L), 329 nm, f = 0.1934, and (3) S0 → S60 (H → L+11, H−1 → L+10), 273 nm, f = 0.0575. Similarly, complex 4 showed four transitions at 378 nm, 331 nm, 312 nm and 282 nm, with oscillator strengths of 0.1164, 0.0795, 0.3405 and 0.1441 respectively, which were assigned to H → L, H → L+3, H−5 → L+1 and H−1 → L+6 transitions respectively.
Fig. 7 Generation of phenoxyl radicals with complex 1 (10 × 10−5 M) after addition of CAN ( 1 equivalent, 2 equivalents, 3 equivalents). |
After the addition of an acetonitrile solution of CAN into a solution of complex 1, the band at 390 nm disappeared, and a new band at 410 nm and a broad band of low intensity at around 800 nm were observed, which clearly indicated the formation of phenoxyl radical (Fig. 7).
A similar absorption spectrum (414 and 800 nm) was also obtained during the oxidation of complex 2 (Fig. S23†). For complex 3·CH3CN, after the addition of CAN a band at 420 nm disappeared and a band at around 800 nm was observed, which shows the radical generation (Fig. S24†). However, for complex 4 the phenoxyl radical generation was performed using a procedure reported in our previous work12 (Fig. S25†). It is well known from the literature that an absorption band at around 420 nm can be assigned to a π–π* transition of the phenoxyl radical.12 In the case of complex 1, the oxidized species 1˙+ have filled d10 orbitals at the zinc centre, hence this oxidized species was found to be paramagnetic (EPR active) due to the presence of one unpaired electron in 1˙+. Interestingly, it was found that complex 1 is as such a radical complex. So complex 1 has a tendency to be oxidized due to the signature of the EPR signal. The EPR spectrum was also obtained, which exhibited a very sharp signal at g = 2.001 for radical complex 1˙+ and this gave rise to information on the generated phenoxyl radical (S = ½) species.12 The X-band EPR spectrum of the phenoxyl radical complex derived from 1 in CH3CN at room temperature is shown in the ESI (shown in Fig. S22†).
It was found that the contribution from the ring containing the phenolato function was 73% for the HOMO due to the +R effect of the –OMe group present in complex 1. This value is more than those obtained for complexes 2, 3·CH3CN and 4, and a high contribution from the phenolato ring in the HOMO gives rise to a higher electron density and consequently stabilization of the phenoxyl (cationic and electron deficient) radical.8 We performed TD-DFT calculations for the complex 1˙+ and we found that two main transitions in the NIR region are observed for complex 1˙+. The transitions are (1) (H−7 → L), 751 nm, f = 0.0068, and (2) (H−8 → L), 1119 nm, f = 0.0282, which was recognized as mostly due to an intraligand charge transfer (ILCT) transition.
Complex | Reactant | Oxidant | Product | TON | % Conversion |
---|---|---|---|---|---|
1 | Benzyl alcohol | TEMPO/TBHP | Benzaldehyde | 300 | 60 |
TBHP | Benzaldehyde | 260 | 52 | ||
p-Methoxy benzyl alcohol | TEMPO/TBHP | p-Methoxy benzaldehyde | 325 | 65 | |
TBHP | p-Methoxy benzaldehyde | 320 | 64 | ||
2 | Benzyl alcohol | TEMPO/TBHP | Benzaldehyde | 155 | 31 |
TBHP | Benzaldehyde | 130 | 26 | ||
p-Methoxy benzyl alcohol | TEMPO/TBHP | p-Methoxy benzaldehyde | 180 | 36 | |
TBHP | p-Methoxy benzaldehyde | 90 | 18 | ||
3 | Benzyl alcohol | TEMPO/TBHP | Benzaldehyde | 100 | 20 |
TBHP | Benzaldehyde | 115 | 23 | ||
p-Methoxy benzyl alcohol | TEMPO/TBHP | p-Methoxy benzaldehyde | 150 | 30 | |
TBHP | p-Methoxy benzaldehyde | 75 | 15 | ||
4 | Benzyl alcohol | TEMPO/TBHP | Benzaldehyde | 150 | 44 |
TBHP | Benzaldehyde | 180 | 36 | ||
p-Methoxy benzyl alcohol | TEMPO/TBHP | p-Methoxy benzaldehyde | 140 | 28 | |
TBHP | p-Methoxy benzaldehyde | 285 | 57 |
In the oxidation of the primary alcohols, complex 1 was found to be most effective. On the other hand, complex 3 was found to be least effective. These data clearly indicate the importance of generation of a stable phenoxyl radical in solution during the catalytic process.
The oxidation capability of complexes 1–4 was consistent with results reported by Wieghardt and coworkers.9b,c The addition of a base plays an important role in activation of the alcohol.9b,c In our study, potassium carbonate (K2CO3) was found to be effective for the alcohol oxidation, however other bases such as triethylamine can also be employed.9c
Complex 1 was found to be capable of oxidizing primary alcohols to the corresponding aldehyde, through a 2e− reduction and forming the 2e− reduced form. The mechanism of TEMPO-catalyzed oxidation of primary alcohols with zinc complexes could not be established. The reaction could progress via the mechanism proposed by Wieghardt and coworkers.9b,c
Fig. 9a shows the initial rate of the oxidation of 3,5 DTBC for complex 1 and the other spectra are shown in the ESI (Fig. S32, S33 and S34†). Lineweaver–Burk plots for complexes 1–4 are shown in Fig. 9b. All the dinuclear complexes showed saturation kinetics and a treatment based on the Michaelis–Menten model seemed to be appropriate. The values of the Michaelis binding constant (Km), the maximum velocity (Vmax), and the rate constant for dissociation of substrates (i.e., turnover number, kcat) were calculated for the complexes from the graphs of 1/V vs. 1/[S] (Fig. 9b), known as a Lineweaver–Burk graph, using the equation 1/V = {Km/Vmax}{1/[S]} + 1/Vmax, and the kinetic parameters are presented in Table 5.34
Complex | V max (M s−1) | K m (M) | k cat (h−1) |
---|---|---|---|
1 | (6.59 ± 0.08) × 10−4 | (8.4 ± 0.06) × 10−3 | 2.3 × 104 |
2 | (6.09 ± 0.13) × 10−4 | (6.3 ± 0.11) × 10−3 | 2.1 × 104 |
3 | (3.85 ± 0.16) × 10−4 | (2.2 ± 0.09) × 10−3 | 1.3 × 104 |
4 | (4.65 ± 0.22) × 10−4 | (1.3 ± 0.10) × 10−2 | 1.8 × 104 |
For this purpose, 1.0 × 10−4 M solutions of the complexes were mixed with a 1.0 × 10−2 M solution of OAPH, and spectra were recorded for up to 2 h in dioxygen saturated acetonitrile at 25 °C. For complex 1, the spectral changes after the addition of OAPH are shown in Fig. 10. However, for the other complexes, the plots are present in the ESI (Fig. S29–S31†). The spectral scans revealed a progressive increase of the peak intensity at ca. 425 nm, characteristic of a phenoxazinone chromophore, suggesting catalytic conversion of the OAPH to 2-aminophenoxazine-3-one under aerobic conditions. A blank experiment without the catalyst under identical conditions did not show significant growth of the band at 425 nm. The spectral results allowed us to conclude that all the complexes exhibit phenoxazinone synthase-like activity under aerobic conditions. Kinetic studies were performed to understand the extent of the catalytic activity. For this purpose, 1 × 10−4 M solutions of 1, 2, 3·CH3CN and 4 were treated with at least a 100-fold more concentrated substrate solution so as to maintain the pseudo-first-order conditions.
All the kinetic experiments were carried out at a constant temperature of 25 °C, and monitored with a UV-Visible spectrophotometer under aerobic conditions. For a particular complex–substrate mixture, timed scans of the maximum band of 2-aminophenoxazine-3-one were performed for a period of one hour. For complex 1, a plot of the initial rate of the reaction versus the concentration of the substrate showed rate saturation kinetics, as can be seen from Fig. 11a. While for the other complexes, the plots of initial rate versus concentration are present in the ESI (Fig. S35–S37†). This observation indicates that 2-aminophenoxazine-3-one formation proceeds through a relatively stable intermediate, a complex–substrate adduct, followed by redox decomposition of the intermediate. This type of saturation rate dependency can easily cause recall of the Michaelis–Menten model, originally developed for enzymatic kinetics, which on linearization provides a double reciprocal Lineweaver–Burk plot that can enable analysis of the values of various parameters like Vmax, Km, and kcat. The Lineweaver–Burk plots for complexes 1, 2, 3·CH3CN and 4 are shown in Fig. 11b. Analysis of the experimental data yielded the Michaelis binding constant Km and Vmax, and the turnover number (kcat) value was obtained by dividing the Vmax by the concentration of the complex used. These parameters are listed in Table 6. Moreover, for a particular substrate concentration, on varying the complex concentration a linear relationship for the initial rates was obtained, which shows a first-order dependence on the complex concentration. The kinetic parameters, such as the maximum velocity (Vmax), turnover number (kcat) and Michaelis binding constant (Km), of all the complexes are listed in Table 6. During investigation of the catecholase activity, it was found that complex 1, which contains a methoxy group on the phenolato donor, provided a stable phenoxyl radical, showing greater activity with a turnover number of 2.3 × 104. Similarly, for the phenoxazinone synthase activity, complex 1 was also found to be more active with a turnover number of 2.6 × 103. It was clearly indicated that the activity of the phenoxyl radical complexes follows the order OMe (1) > Me (2) > H (4) > Naph (3), due to the stability of the phenoxyl radical in both the catecholase as well as the phenoxazinone synthase activity investigations. This explanation is also supported by the data reported by Belle and co-workers in which the catecholase activity follows the order OMe > Me > F > CF3, with OMe showing the highest activity.35,36 Another group, Torelli and co-workers,36 also investigated the catecholase activity of copper complexes, and they found a drastic change in response to the effect of the para subtituents and observed that the activity was decreased by a small extent when the ligand had an electron withdrawing F atom as substituent.
Complex | V max (M s−1) | K m (M) | k cat (h−1) |
---|---|---|---|
1 | (7.3 ± 0.11) × 10−5 | (1.3 ± 0.08) × 10−2 | 2.6 × 103 |
2 | (5.78 ± 0.3) × 10−6 | (4.4 ± 0.17) × 10−3 | 2.0 × 102 |
3 | (2.7 ± 0.24) × 10−6 | (6.4 ± 0.19) × 10−3 | 0.97 × 102 |
4 | (4.9 ± 0.09) × 10−6 | (1.4 ± 0.06) × 10−3 | 1.76 × 102 |
Probably monoligation as well as bidentate ligation of catechol is happening during the reaction. The substrate, after bidentate chelation, would undergo oxidation due to the presence of a non-innocent and redox active ligand in these complexes. In the case of monodentate ligation, oxidation of the catechol could also occur and at the end of the reaction o-quinone would be produced (vide infra). Both the reaction models are shown in Schemes 4 and 5. We have performed EPR spectral studies of the dinuclear zinc complex as well as a mixture of zinc complex 1 and catechol. Complex 1 provided an EPR signal with a g value of 2.001. This data is consistent with the data reported by Reedijk and co-workers.29
We would like to mention here that the zinc complexes capable of generating stable phenoxyl radicals did not provide sharp NMR signals and broadening of the signals was observed.12 The EPR signal clearly indicates the presence of radical species in complex 1 due to the redox nature of the ligand. The other complexes did not provide this g ≈ 2 EPR signal. This may be due to inferior stabilization of the phenoxyl radical in these complexes. Addition of 3,5-DTBC to the zinc complexes provided an EPR signal with a g value of 2.00. We examined the EPR signals of complex 2 in the presence of catechol and in the absence of catechol. The generation of an EPR signal after catechol addition clearly indicated the formation of a ligand centered radical species in the solution mixture (vide infra).
To investigate the possible binding modes of 3,5-DTBC with the dinuclear zinc complexes, the Gibbs free energies were calculated and compared for the different binding modes of catechol to a dinuclear zinc complex (Fig. 12).58 The ΔG value was found to be 22.90 kcal mol−1 when the catechol binds to the dinuclear zinc complex in a monodentate fashion (I). However, in the case of bidentate chelation (II), the ΔG value was found to be 67.17 kcal mol−1. We concluded that in the case of a bidentate binding mode for catechol the value of the Gibbs free energy (ΔG) is three times more than the monodentate binding mode. This may be due to steric hindrance and suggested that the monodentate binding mode of catechol, as an intermediate, is more favourable during the oxidation of catechol. The energies for the possible binding modes of catechol with dinuclear zinc complex 4 and the possible intermediates are shown in Fig. 13.
Fig. 12 ΔG values and optimized structures of the possible intermediates for the catecholase reaction using complex 4. |
Fig. 13 Energy profile diagram for complex 4 during the catecholase reaction, with the possible intermediates. |
Our catecholase activity results are consistent with the data reported by Das and co-workers22 and Biswas and co-workers,23 and we speculate that such a conversion has a radical dependent pathway. The more facile the generation of the phenoxyl radical, the more easy oxidation of the catechol will be. We have investigated the phenoxazinone synthase activity of our zinc complexes. We would like to mention here that the rate of formation of phenoxazinone is greater with those zinc complexes capable of generating a stable phenoxyl radical. Consistent with our concept of phenoxyl radical complexes, we found out that a molecule capable of generating a stable phenoxyl radical is efficient at performing the catecholase reaction as well as the o-aminophenol oxidase reaction. The mechanistic pathway for the phenoxazinone synthase process clearly indicates the role of the radical generation, as shown in Scheme 6.
For authentication of the binding mode of the o-aminophenol with the dinuclear zinc complexes, Gibbs free energies were also calculated and compared for the different binding modes of o-aminophenol with a dinuclear zinc complex. The ΔG value was found to be 20.79 kcal mol−1 when the phenolic group (after deprotonation) binds to the zinc complex in a monodentate fashion (I, Fig. 14). However, the ΔG value calculated for deprotonation of the amine group in the same way (II, Fig. 14) was found to be 37.01 kcal mol−1. In the case of step (III), where the neutral amine of o-aminophenol is bound to the dinuclear zinc complex, the value of Gibbs free energy (ΔG) was found to be 161.88 kcal mol−1. Analysis of the above data clearly suggested that the monodentate binding mode of the o-aminophenol in step (I) is more favourable than steps (II) and (III) during the o-aminophenol oxidase reaction.
Fig. 14 ΔG values and optimized structures for the possible intermediates during the phenoxazinone synthase reaction using complex 4. |
Mechanistic investigation results with the possible binding modes of o-aminophenol with the dinuclear zinc complex are shown in Fig. 14 and a relative energy diagram for the possible intermediates during the o-aminophenol oxidase reaction with complex 4 is shown in Fig. 15.
Fig. 15 Energy profile diagram for complex 4 during the o-aminophenol oxidase reaction, with the possible intermediates. |
Molecules with a Zn2O2 moiety have a tendency to form phenoxyl radical complexes, and this was mentioned in our previous report.12 We also mentioned that the NMR spectra signals were found to be broad for the complexes. This prompted us to study the EPR spectra of the complexes. It is well known from the literature that a OMe group stabilizes phenoxyl radical complexes.34 Complex 1 by itself provided the EPR signal shown in Fig. 16a at 25 °C. On the other hand, complex 2 did not provide an EPR signal under the same conditions. However, an EPR study (at 25 °C) was performed immediately after mixing complex 2 and 3,5-DTBC under aerobic conditions, and the spectrum obtained is shown in Fig. 16b.
Fig. 16 EPR spectra of (a) complex 1, (b) complex 2 (solid line) and a mixture of complex 2 and 3,5-DTBC (dotted line) at 25 °C. |
These results clearly indicate the radical nature of the complex, as for complex 1. It is important to note that our data are consistent with the reported data by Reedijk and co-workers.29 To confirm the participation of radical species during the oxidation of 3,5-DTBC, the reaction was performed in the presence of a radical scavenger TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinoxyl).22 Oxidation of 3,5-DTBC was not observed in the presence of TEMPO (significant decrease in the reaction rate), which clearly indicated the generation of a radical during the catechol oxidation (Fig. 17a). Moreover, when this catalytic reaction was performed under an inert atmosphere, very little formation of 3,5-DTBQ was observed. Similarly, oxidation of o-aminophenol was investigated under similar conditions to those described above. The desired activity (oxidation of o-aminophenol) was not observed in the presence of TEMPO (Fig. 17b). All these results clearly express the role of dioxygen, as well as radical intermediates, in this catalytic reaction.
Fig. 18 Electrospray ionization mass spectrum (ESI-MS positive mode) of a 1:100 mixture of complex 4 and 3,5-DTBC in methanol, recorded after 30 minutes. |
Fig. 19 Electrospray ionization mass spectrum (ESI-MS positive mode) of a 1:100 mixture of complex 4 and o-aminophenol in methanol/acetonitrile, recorded after 30 minutes. |
Das and co-workers22 described a mechanism for zinc complexes derived from compartmental ligands that involved the reduction of an imine function during the catecholase activity study. We would like to mention that the following few observations prompted us to predict that the above reaction pathway occurs with our diphenoxo-bridged dinuclear zinc complexes. First, these complexes (1–4) have a tendency to be oxidised to generate phenoxyl radical complexes. EPR spectra of complex 1 authenticated the same. The complexes also provided broad NMR signals and we did not find generation of reduced species in the cyclic voltammetry studies. Second, the DFT calculations clearly depicted the presence of electron density close to the phenolato function. Third, these complexes are similar to the complexes reported by Reedijk and co-workers,29 which have a Zn2O2 moiety and provided oxidative activity.
(a) Diphenoxo bridged dinuclear zinc complexes derived from meridional tridentate ligands were synthesized and characterized. The molecular structures of representative complexes 1 and 3·CH3CN were determined using X-ray crystallography and a cis orientation of the chloride ions along the Zn–Zn axis gave rise to coordination of catechol and o-aminophenol and their oxidation.
(b) We performed theoretical calculations and discovered the non-innocent behavior of the ligands in these complexes. The HOMO and LUMO were found to have a negligible contribution from the zinc. TD-DFT calculations were used to explain the spectral bands obtained during the UV-Vis absorption spectral studies and the transitions were mainly of the intraligand charge transfer (ILCT) type.
(c) Electron donating substituent(s) (–OMe) on the phenyl ring containing phenolato function afforded a better stability of the phenoxyl radical complexes. Complex 1 gave rise to a stable phenoxyl radical complex and the order of the stability of the generated phenoxyl radicals was found to be complex 1 > complex 2 > complex 4 > complex 3.
(d) The complexes were utilized for catalytic oxidation of benzyl alcohol. A combination of the zinc complex and TBHP/TEMPO was found to be suitable for oxidation of the benzyl alcohol to benzaldehyde. Complex 1 was found to be most effective and complex 3 was found to be the least effective for such an oxidation.
(e) These complexes exhibited catechol oxidase and o-aminophenol oxidase activity and this was investigated. We proposed a possible mechanism for the catechol oxidase as well as the o-aminophenol oxidase reaction and performed theoretical calculations to support our mechanism. Participation of radical intermediate(s) during the oxidase activity studies was confirmed through EPR as well as ESI-MS spectral studies.
(f) The order of the activity was found to be complex 1 > complex 2 > complex 4 > complex 3 for both the catechol oxidase and o-aminophenol oxidase processes. Hence we found that the complex will be more active if the complex is capable of generating a more stable phenoxyl radical. Compared to reported results, complex 1 was found to be more efficient in terms of the catecholase activity when compared with the activity of other known complexes.22
Similarly for the o-aminophenol oxidase (phenoxazinone synthase) activity, o-aminophenol was used as the substrate. A 10−1 M stock solution of o-aminophenol in methanol was prepared. For this activity study, 10−4 M solutions of the complexes were added to 100 equivalents of the substrate in acetonitrile under aerobic conditions at 25 °C, and the activity was measured using a UV-vis spectrophotometer. After completion of the reaction, the solution of the mixture was passed through a silica gel column and then eluted with methanol solvent, and after that the product was quantified using the GC mass instrument. Similar to the catecholase activity study, the reactions were investigated in the presence of TEMPO and under anaerobic conditions for creation of the radical mechanism.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1419324 and 1419325. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00356g |
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