Catalytic Fate of Two Copper Complexes ... - Wiley Online Library

DOI: 10.1002/slct.201702113 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

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z Inorganic Chemistry

Catalytic Fate of Two Copper Complexes towards Phenoxazinone Synthase and Catechol Dioxygenase Activity Mamoni Garai,[a] Dhananjay Dey,[a] Hare Ram Yadav,[c] Angshuman Roy Choudhury,[c] Milan Maji,[d] and Bhaskar Biswas*[a, b]

This research work is dedicated to Dr. Niranjan Kole, retired Associate Professor of Department of Chemistry, Raghunathpur College, Purulia 723133 who remains a constant source of inspiration.

In this present work, we report the synthesis and structural characterization of two copper(II) complexes, [Cu(bpy)Cl2] (1) & [Cu(m-Cl)(phen)Cl]2 (2) [bpy = 2,2’-bipyridine; phen = 1,10-phenanthroline]. We have also studied their catalytic fate towards phenoxazinone synthase and catechol dioxygenase activity. Xray structural analyses revealed that 1 & 2 crystallize in triclinic & monoclinic system with P 1 and Cc space group respectively. The copper complexes catalyse the oxidative coupling of 2amino phenol (2-AP) to aminophenoxazin-3-one with significant turn over number, kcat(h1) = 2.08 3 103 & 2.16 3 103 for 1 & 2 resectively. During investigation of catechol dioxygenase

1. Introduction The thrust in modeling of metalloenzyme mimics appears from their potentiality in producing valuable mechanistic insights of important biological functions for different metalloenzymes.[1–3] Nature uses lots of metalloproteins and metalloenzymes for the catalytic oxidation of different organic molecules for its own sustainability.[4–6] Catalytic fate of copper ions towards molecular oxygen activation in selective and efficient oxidation reactions was paid substantial attention during last few decades for immense biological significance of copper ion in our living system.[7–9] Copper, an essential bio-metal, usually

[a] M. Garai, D. Dey, Dr. B. Biswas Department of Chemistry, Raghunathpur College, Purulia 723 133,West Bengal, India E-mail: [email protected] Homepage: www.raghunathpurcollege.in [b] Dr. B. Biswas Present Address: Department of Chemistry, Surendranath College, 24/2 M.G. Road, Kolkata 700009, West Bengal, India [c] Dr. H. R. Yadav, Dr. A. R. Choudhury Department of Chemical Sciences, Indian Institute of Science Education and Research, S.A.S. Nagar, Sector 81, Manauli PO, Mohali 140 306, India [d] Dr. M. Maji Department of Chemistry, National Institute of Technology, Durgapur 713209, West Bengal, India Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201702113

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activity, stoichiometric addition of 1 & 2 to 3,5-di-tertbutylcatechol (DTBC) in acetonitrile produce in situ catecholateto-Cu(II) absorption bands at 812 and 821 nm respectively. The in situ Cu(II)-catecholate species for both 1 & 2 react with molecular oxygen at the rate, kobs: 7.95 3 104 and 1.30 3 103 min1 respectively and produce intradiol cleavage products in exclusive amount. Minor amount of benzoquinone is also found in solution. Intradiol products are found as major product in solution and accounts in favour of substrate activation mechanism.

takes part as important oxygen carriers[10,11] and copper containing metalloenzymes in living system play fundamental role in revealing its catalytic role as dioxygen and/or substrates activators towards different bio-functions.[12–16] In this context, it shouldn’t be out of place to mention that chemists have significantly contributed in unveiling the mystery of nature, specially by designing different metal complexes which can potentially mimic the active site of different metalloenzymes in nature.[17–21] One of the important copper based metalloenzyme is Phenoxazinone synthase (PHS), which consists of pentanuclear copper core in its active site and catalyzes the oxidative coupling of aminophenol to its phenoxazinone chromophore during formation of an aromatic heterocyclic natural product, Actinomycin D.[22,23] Both xanthommatin and cinnabarin have also been found to contain phenoxazinone chromophore.[24,25] In general, between the two classes of catechol dioxygenase enzymes, intradiol dioxygenase catalyses the oxidative cleavage of carbon–carbon bond in catechols using non-heme iron(III) cofactor while the extradiol dioxygenase carries out cleavage of carbon–carbon bond in catechols employing an iron(II) cofactor.[26–28] Herein, we report the synthesis and structural characterization of two copper(II) complexes, [Cu(bpy)Cl2] (1) & [Cu(mCl)(phen)Cl]2 (2). The mono- and dinuclear copper(II) complexes have been testified as potential catalytic systems with an aim to mimic the functional sites of phenoxazinone synthase and catechol dioxygenase enzyme.

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2. Results and discussion 2.1. Synthesis and formulation The copper(II) complexes were synthesized following a simple reaction methodology. Addition of copper(II) chloride to 2,2’bipyridine or 1,10-phenanthroline in aqueous methanolic medium produces copper(II) compounds, 1 & 2. The structural formulation of 1 & 2 were confirmed by single crystal X-ray diffraction study and supported by other spectroscopic and analytical results.

Scheme 1. Preparative route for the copper(II) complexes (1 & 2)

2.2. Description of crystal structures The X-ray crystal structure analysis reveals that the copper complexes 1 & 2 crystallize in triclinic & monoclinic crystal system with P 1 & Cc space groups respectively. An ORTEP view of the asymmetric units of 1 & 2 are shown in Figures 1 & 2. The crystallographic structural parameters of 1 & 2 are listed in Table S1. Selected bond angles, and bond distances of 1 & 2 are given respectively in Table S2 & S3. Bond angles, [ffCl1-Cu1Cl2, 92.808;ffCl1-Cu1-N1, 93.458; ffCl1-Cu1-N2, 171.828; ffCl2-Cu1N1, 172.748; ffCl2-Cu1-N2, 93.628; ffN1-Cu1-N2, 80.58] and the coordination geometry are slightly distorted from ideal square planar geometry for 1 as evident from the deviation of bond angles from ideal tetrahedral geometry. This deviation from ideal structure is probably due to the free rotation for CC bond attached to the two pyridine rings in 2,2’-bipyridine. Though B. Luis et al., previously reported the molecular structure of 1 without X-ray structure but herein, we are able to isolate the single crystals of 1 with a presentation of X-ray structure.[29] Furthermore, we have attempted several ways to produce single crystals of mononuclear copper(II) complex containing phen ligand and dinuclear copper complex containing bpy ligand, but couldn’t be able to produce the compounds in the form of single crystals. The crystal structure analysis of 2 reveals that the coordination geometry for each of the copper(II) centres adopt distorted square pyramidal geometry, which is also evident from geometric index value, t = 0.276; [t = j b-a j /60], where a = 90.08 and b = 106.618; t is 1 for a perfect trigonalbipyramidal geometry and is zero for a perfect square pyramidal geometry.[30]The Cu1 & Cu2 centres in 2 are interconnected ChemistrySelect 2017, 2, 11040 – 11047

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Figure 1. An ellipsoid plot (30% probability) of [Cu(bpy)Cl2] (1) with atom numbering scheme.

through bridging chloride ions with internuclear distance of 3.5 8 A and bridging angles, [ffCu1-Cl2-Cu2, 88.118; ffCu1-Cl3-Cu2, 90.638]. Each of the square planes in this square pyramidal structure for Cu1 [N1,N2,Cl1,Cl2] & Cu2 [N3,N4,Cl3,Cl4] consist of two nitrogen atoms from phenanthroline ligand, one terminal chloride ion and one bridging chloride ion respectively. The other bridging chloride ion [Cl3 for 1 & Cl2 for 2] occupies at axial position in the square pyramid. 2.3. Solution properties of the copper(II) complexes (1 & 2) In order to obtain the structural aspects and solution properties of the copper complexes we have recorded electronic spectra and molar conductance values for 1 & 2 in methanol at room temperature. Further, we carried out EPR spectral analysis for the compounds in methanol to confirm the solution properties. In the electronic spectrum of 1 in methanol, characteristic bands at 300, 316 and 692 nm were observed and compound 2 exhibited characteristic bands at 296, 320, 728 nm (Figure 3). High energy sharp transitions correspond to n!p* transitions of ligand origin[31] while the broad absorption bands at 692 & 728 nm with molar absorption coefficients, 62 & 86 Lmol1cm1 imply d-d transitions[31,32] of Cu(II) centres for 1 & 2 respectively. The position of absorption bands for d-d transitions in 1 & 2 resembles well with other structurally related copper(II) complexes.[31]

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orientation resulting from interaction of unpaired electron with copper nuclei (I = 3/2). The structure of 2 displayed a squarepyramidal geometry (Figure S2) as evident from the spin Hamiltonian parameters as gx = gy = g? = 2.057; gz = g j j = 2.26 and A j j = 154 G in methanol. Hence, geometry of Cu(II) centres in the dimeric species indicates axial symmetry of the metal centres and is close to the square pyramid with the nitrogen, chlorine and the bridging Cl ion in the equatorial plane and a second Cl in the axial position. These EPR results remain consistent with the results obtained from EPR analysis of similar class of compounds in scientific literatures.[32] 2.4. Phenoxazinone Synthase activity of the copper(II) complexes (1 & 2) and their catalytic fate Figure 2. An ORTEP diagram of 2 (30% ellipsoid probability) with atom numbering scheme.

The phenoxazinone synthase mimicking activity of the copper (II) complexes (1 & 2) was studied using 2-aminophenol (2- AP) as a convenient substrate in methanol solvent under aerobic condition at room temperature (25 8C). To carry out this catalytic oxidation reaction, copper(II) complexes (1 3 104 M solution) were treated with 2-AP (1 3 10 2 M solution) and the course of catalytic oxidation of 2-AP were monitored with UV– Vis spectrophotometer and the spectrophotometric scan was performed at an interval of 12 min for 5 h, where 2-AP showed a single band at 267 nm. During the progress of the spectrophotometric titration with time, it is seen that characteristic band for 2-AP at 267 nm[33] gradually decreased in intensity and concomitantly initial new bands at 432 & 433 nm with increasing intensity appeared for both 1 & 2 respectively (Figure 4 & 5). The appearance of the bands at 432 & 433 nm

Figure 3. Electronic spectra of 1 & 2 in 104 (M) methanolic solution; Inset: Expanded region (500–900 nm) of the spectra. Green & blue colour indicate the UV-Vis spectra for complexes 1 & 2 respectively.

The electrical molar conductivity measurements of 1 and 2 in 104 M dry methanolic solution show a non-electrolytic behaviour with molar conductance values of 67 and 49 Scm2mol1 respectively. These observations provide additional support to the structural rigidity of the copper(II) complexes in solution. EPR spectra for the copper complexes (1 and 2; Figures S1 & S2) in frozen methanol at 77 K attested the existence of mononuclear Cu(II) unit and chloro-bridged dimeric Cu(II) unit respectively. The EPR spectrum of 1 exhibited an axially symmetric spectrum having quartet hyperfine structure only the parallel components (1, g j j = 2.2718, A j j = 150 G, g? = 2.0501). The typical EPR spectrum of 1 (Figure S1) showed the presence of nine line super hyperfine structure (14AN = 15 G) on the most intense component of the quartet hyperfine structure in second derivative presentation and confirmed a nearly square planar geometry for Cu centre. The frozen solution EPR spectrum of 2 (Figure S2) at 77 K displayed well defined resolution of hyperfine splitting of parallel ChemistrySelect 2017, 2, 11040 – 11047

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Figure 4. Incremental intensity of aminophenoxazinone band at 432 nm after addition of 100 equivalents of 2-AP to 1 in MeOH solution. The spectra were recorded after every 12 min. Inset: Plot of time vs Abs

for both the compounds is a definite signature for the generation of phenoxazinone species in solution.[34–37] We also carried out controlled experiments using 2-AP in methanol solvent under aerobic condition at room temperature and also

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performed same controlled experiments with an addition of bipyridine and phenanthroline separately to 2-AP under identical reaction conditions (Figures S3 & S4) upto 3 h but the yield of phenoxazinone compound was very little to consider. In fact the yield of phenoxazinone compound under controlled experiments is too small than the yield of catalytic oxidation, and can be neglected. This very little conversion of 2-AP to phenoxazinone in controlled experiments indicates auto oxidation of 2-AP under normal aerobic atmosphere. The absorbance values for the catalytically produced phenoxazinone compounds are little bit high and the titration curves are started to get saturation after 5 h of catalysis by both 1 & 2 respectively. In this context it would be better to mention that lowering of catalyst concentration from 104 M to 105 M for 1 & 2 with 102 M 2-AP, helps to maintain the absorbance values within 1 a.u. (Figures S5 & S6).

Figure 5. Increase of aminophenoxazinone band at 433 nm after addition of 100 equivalents of 2-AP to 2 solution in MeOH medium. The spectra were recorded after every 12 min. Inset: Plot of time vs Abs of the same solution

The phenoxazinone compound was separated in pure form by column chromatography using neutral alumina as column support and benzene-ethyl acetate as an eluant mixture. The compound was isolated in high yield (~ 78% & ~ 81% for 1 & 2) by slow evaporation of the eluant. The product was principally identified by 1H NMR spectroscopy (Figure S7). 1H NMR data for 2-amino-3H-phenoxazine-3-one (APX), (CDCl3, 400 MHz,) dH: 7.61 (m, 1H), 7.46 (m, 3H), 6.48 (s, 1H), 6.39 (s, 1H), 6.27 (s, 1H). The characterization of this phenoxazinone product was further consolidated from the appearance of base peak at m/z 213 in ESIMs spectrometry Kinetic studies of the catalytic oxidation of 2-AP were carried out to understand the catalytic efficacy for the copper (II) complexes. The kinetics of oxidative coupling of 2-AP were determined by following the method of initial rates and monitoring the growth of the phenoxazinone band at 432 & 433 nm as a function of time (Figures S8 & S9) [36, 37]. The rate constants versus concentration of the substrate plot showed ChemistrySelect 2017, 2, 11040 – 11047

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rate saturation kinetics and was analyzed following Michaelis Menten approach of enzymatic kinetics to get the values of the kinetics parameters for 1 & 2 and tabulated in Table 1. We have

Table 1. Kinetic parameters for the catalytic oxidation of 2-AP by 1 & 2 in MeOH at 25 8C Complex

Vmax (M s1)

Km (M)

Kcat ( h1) Ref

1* 2* 3b [CuL1Cl]Cl [Cu4(L)4]

1.50 3 104 4.71 3 104 3.40 3 103 1.92 3 102 3.36 3 103

6.41 3 103 1.96 3 103 1.44 3 102 1.95 3 103 2.89 3 103

5.40 3 103 Present 1.69 3 104 Present 3.40 3 102 [38a] 9.43 3 101 [38b] 1.21 3 105 [38c]

*Std. Error for 1, Vmax (MS1) = 2.69 3 105; Std. Error for Km (M) = 2.45 3 103 *Std. Error for 2, Vmax (MS1) = 2.25 3 105; Std. Error for Km (M) = 2.19 3 103

also made a comparison of reactivity towards phenoxazinone synthase activity between our copper complexes and few reported copper(II) complexes. The catalytic efficiency, kcat/KM = 1.88 3 105 & 1.86 3 106 for 1 & 2 towards phenoxazinone synthase activity was found high because both the copper complexes are labile in presence of 2AP, which also acts a very good chelating agent. Further, it is seen that compound 2 showed ~ 10 times more reactivity than 1. This may be explained by considering the fact that dimeric form of compound 2 actually contains two active sites in methanolic solution. Though this class of catalytic oxidative coupling reaction bears special significance but we could find few literature to make a comparison between phenoxazinone synthase activity of our Cu(II)-polypyridyl compounds and previously reported works. It is observed that our Cu(II)polyridyl catalytic systems exhibited better catalytic efficiency compare to other copper(II) complexes. We reviewed large number of scientific literatures to explore the mechanistic insights of catalytic phenoxazinone activity of the copper(II) complexes. One of the renowned scientists, P. Chaudhury et al.[39a] modeled a tetracopper complex for the mimicking of catalytic oxidation of 2-aminophenol to 2-amino-phenoxazine-3-one, and proposed an “on-off” mechanism of the radicals together with redox participation of the metal center behind the mimics of six-electron oxidative coupling in the catalytic function of the copper containing enzyme phenoxazinone synthase. Furthermore, another renowned scientist, T.P. Begley et al.[39b,c,d] suggested that 2aminophenoxazinone synthesis proceeds via a sequence of three consecutive 2-electron aminophenol oxidations and the aminophenol moiety is regenerated during the reaction sequence by facile tautomerization reactions. We have carried out ESI mass spectral (positive mode) analysis of the reaction mixture (catalyst + substrate) in methanol medium to evaluate the mechanistic aspects of the catalytic cycle by copper(II) complexes for the oxidative coupling of aminophenol to phenoxazinone product. ESI-MS provides valuable information regarding the reactive species and product formed in the reaction. The ESI mass spectral analysis of the reaction mixture (Figure S10) for 1 in methanol

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medium exhibits the characteristics peaks at m/z 214.34 and 351.63 with isotope distribution patterns. This mass spectral analysis further consolidated the corroboration at m/z 214.34 and 351.63 respectively with [(2-amino-3H-phenoxazine-3ones) + H + ] and [[Cu(bpy)(2-AP)] + Na + ]. The ESI mass spectrum of reaction mixture (Figure S11) for 2 shows characteristics peaks at m/z 214.21, and 376.43 with isotope distribution patterns which definitely indicates the existence of [(2-amino3H-phenoxazine-3-ones) + H + ] and [[Cu(phen)(2-AP)] + Na + ] species in solution respectively. Actually, the coordinative unsaturation at the copper centres and presence of lability at 1 & 2 enhances the easy formation of enzyme-substrate adduct towards 2-AP (strong chelator). In order to find deep insights of mechanistic routes, we have also carried out an X-band EPR study of the reaction mixture between 2-AP & Cu(II) complexes, but couldn’t able to detect any signal originated for the presence of for iminobenzoquinone radical at 77 K. In most of the cases it is observed that iminobenzoquinone radical can’t be easily detected by EPR analysis. Further, most of the model studies related to phenoxazinone synthase was described as the production of the 2-AP radical as the rate determining step.[40] Absence of an EPR signal in the present case may be considered similar to other reported cases and this is probably due to faster disproportionation of the 2-AP radical on the EPR time scale in presence of a metal catalyst. On the basis of our experimental results related to mechanistic aspects, we feel difficulty to comment on the mechanistic routes of the catalytic cycle for phenoxazinone synthase activity. But it can be confirmed that the catalytic oxidations are carried out by these copper(II) complexes through catalyst-substrate intermediate which is already consolidated from the ESI-MS spectra of the reaction mixture. Further experiments will need to carry out for evaluation of mechanistic routes in details for phenoxazinone synthase activity. We have also investigated the aspect of photostability for the copper(II) complexes in presence and absence of the substrate. During the spectrophotometric titration (after 30 min scan), we have collected the methanolic solution of the mixture (catalyst with substrate) from UV-Vis cuvette and recorded the molar conductance values for both the complexes. The molar conductance values were found as 131 and 159 Scm2mol1 for 1 & 2 respectively. But when we checked the photo-stability of pure copper complexes under identical conditions in absence of substrate, we found molar conductance values as 75 and 63 Scm2mol1 for 1 & 2 respectively. So conductivity measurements at room temperature helps to conclude that the pure complexes are photo-stable in dry methanol but ligand dissociation reactions are observed in presence of strong chelators (substrate) with cisoid binding motifs. Although these are preliminary results but detailed investigation of the photocatalysis by these copper(II) complexes will be carried out in future. We also tried to recycle the catalysts but the copper catalysts can’t be recycled in their original forms since cholide ligands are dissociated from metal centres.


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2.5. Catechol dioxygenase activity of copper(II) complexes (1 & 2) To investigate the capability for the activation of dioxygen, the oxygenation reactions for the copper complexes were carried out using 3,5-di-tert-butylcatechol (DTBC) as the model substrate in MeCN (Scheme 2).

Scheme 2. Catechol dioxygenase by copper(II) complexes

The DTBC2 adducts of Cu(II)-bpy/Cu(II)-phen complexes were generated in situ in MeCN and investigation of O2 activation by in situ DTBC2 adducts was performed by monitoring the decay of the low energy DTBC2-to-Cu(II) absorption bands at 812 & 821 nm (Figures 6 & 7) for the complexes 1 & 2 respectively. The lower energy visible bands with decreasing in absorbance may be attributed to DTBC2-toCu(II) absorption bands and accounts on the involvment of two different orbitals of catecholate ligand.[41–45] We have also investigated the effect of DTBC in absence of copper(II) complexes in molecular oxygen (Figures S12) at room temperature in acetonitrile. But no cleavage products were found in solution under identical reaction conditions. Further, to check the contribution of the ligands towards catechol dioxygenase activity, we have performed dioxygenase activity of DTBC in presence of the ligands (bpy & phen) separately in oxygen saturated acetonitrile under identical reaction condition. But, we couldn’t trace out any role from the ligands corner and found only the 3,5-di-tert-butylquinone as the oxidation product in the reaction medium. In general, nature of the ligands and Lewis acidity of the metal ion in metal complex strongly affects the electronic transitions.[41,42] The disappearance of the lower energy in situ catecholate-to-Cu(II) absorption bands (Figures 6 & 7) on oxygenation leads to important information about the reaction kinetics. In this case, the pseudo first-order kinetics is confirmed from the linearity of the plot [1 + log(Absorbance)] versus time[43,44] (Figure 8). Slope of the plot accounts for the pseudofirst order rate constant value, kobs. kobs values were determined as 7.95 3 104 and 1.30 3 103 min1 in acetonitrile medium for 1 & 2 respectively. The catechol derived cleavage products for the copper(II) complexes were identified and quantified by GC-MS analysis. 1 H NMR (Figure R3) further supports and consolidates the catechol cleavage products. GC-MS analysis confirms the exclusive production of intradiol cleavage product, 3,5-di-tertbutyl-5-(carboxymethyl)-2-furanone as major product and supports the production of 3,5-di-tert-butylbenzoquinone in minor amount. Very little amount of 4,6-di-tert-butyl-2-pyrone and

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Figure 6. Progress of the reaction between in situ catecholate adduct, [Cu(bpy)(DTBC)] and O2 in MeCN solution. Disappearance of the DTBC2 to Cu(II) absorption band at 812 nm is monitored.

Figure 8. [1 + log(absorbance)] versus time plot for the reaction of adduct, [Cu(bpy)(DBC)] & [Cu(phen)(DBC)] with O2 for 1 (top) and 2 (bottom) at 25 8C in MeCN solution.

Figure 7. Progress of the reaction between in situ catecholate adduct, [Cu(phen)(DTBC)] and O2 in MeCN solution. Disappearance of the DTBC2-toCu(II) charge absorption band at 821 nm is monitored.

3,5-di-tert-butyl-2-pyrone are also found as extradiol cleavage products for both 1 & 2 in solution. From GC-MS analysis it is revealed that both the copper complexes produced similar class of products but the chlorido bridged dicopper compound produced the intradiol cleavage product (71.6%) to a greater extent than monocopper species (63.8%) also consolidated the results. Probably restriction in CC free rotation in rigid phenanthroline in 2 kept itself in plane and facilitates the formation of in situ Cu(II)- catecholate adduct by attaching DTBC in a suitable conformation. On the other hand, free rotation along CC bond in bipyridine for 1 decreases the efficiency of the formation of in situ Cu(II)- catecholate adduct in solution. We have reviewed extensive scientific literatures to find out the proposed mechanistic routes for intradiol products and found that Funabiki et al,[46] developed a functional model with significant amounts of oxidative cleavage products ChemistrySelect 2017, 2, 11040 – 11047

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employing a FeCl2, pyridine, and bipyridine. Later, Que and coworkers carried out an important study by varying the tetradentate tripods on the basis of electron-donation properties of the ligand to modulate Lewis acidity of the iron(III) center.[47] They exhibited & suggested that Lewis acidic nature of the metal center may be conveniently followed by the positions of the two catecholate absorption bands in the UVVis-NIR spectra of the complexes.[41–43] Increasing the Lewis acidity of the metal centre actually cause a red shift of the transition maxima which indicates that the metal acceptor (t2g) orbitals decreased in energy to approach the ligand donor orbitals in energy. More the red shift of the catecholate absorption band, faster the reaction of the complex with O2 and consequently higher will be the yield of the intradiol cleavage product. For our case, production of substantial amount of intradiol catechol cleavage product reflects the formation of a Cu(II)-peroxo intermediate that underwent acyl migration through 1,2-Criegee rearrangement[41–45] in the catalytic route. In spectrophotometry, characteristic absorption bands at ~ 587 & 566 nm with decreasing in absorbance for both 1 & 2 indicate the presence of metal bound semiquinone species in solution which facilitates substrate activation mechanism to produce intradiol cleavage products in major amount. Origination of minor benzoquinone product also indicates aerobic oxidation of the substrate (DTBC) in solution.

3. Conclusions In summary, we have synthesized two copper(II) complexes, [Cu(bpy)Cl2] (1) & [Cu(m-Cl)(phen)Cl]2 (2) [bpy = 2,2’-bipyridine; phen = 1,10-phenanthroline] and structurally characterized by different spectroscopic methods including single crystal X-ray diffraction study. Investigation of catalytic fate of these copper complexes towards the oxidative coupling of 2-amino phenol (2-AP) in methanol suggests that both the compounds (1 & 2)

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produce aminophenoxazin-3-one species with significant turn over number, kcat(h1) = 5.40 3 103 & 1.69 3 104. Experimental results further confirm that the catalytic oxidation reactions proceed through Catalyst-Substrate intermediate. Preliminary study suggests that pure copper(II) complexes are photo-stable in dry methanol but undergo ligand dissociation reactions in presence of substrate (2-AP) with strong chelating ability during the course of catalytic oxidation reaction. Investigation of catechol dioxygenase activity by 1 & 2 towards 3,5-di-tertbutylcatechol (DTBC), it is revealed that DTBC effectively binds with the copper(II) complexes in solution and in situ catecholate-to-Cu(II) adducts react with molecular oxygen at the rate, kobs: 7.95 3 104 and 1.30 3 103 min1 in acetonitrile medium to afford exclusively intradiol cleavage products with little amount of benzoquinone. Intradiol products are found as major product in solution and accounts in favour of substrate activation mechanism.

[3]

[4]

[5]

[6] [7]

[8] [9] [10]

Supporting Information CCDC 1524681 & 1524680 contains the supplementary crystallographic data for 1 & 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223–336-033; or email: [email protected] information such as weak interactions, IR & UV-Vis spectra, ESI mass spectra, rate vs. [substrate] plot are given here.

[11] [12]

[13] [14] [15]

Acknowledgements BB gratefully acknowledges the financial support from Science and Engineering Research Board (SERB), a Statutory body under DST, New Delhi India under the FAST TRACK SCHEME for YOUNG SCIENTIST (No. SB/FT/CS-088/2013 dtd. 21/05/2014). ARC thanks the X-ray facility of the Department of Chemical Sciences, IISER Mohali for single crystal X-ray diffraction data collection. HRY thanks IISER Mohali for research fellowship.

Conflict of Interest

[16] [17] [18] [19] [20] [21] [22]

[23] [24]

The authors declare no conflict of interest. Keywords: Bio-mimicking study · Catechol dioxygenase activity · Copper(II) · Phenoxazinone Synthase activity · Synthesis · X-ray structure

[25] [26] [27]

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Submitted: September 9, 2017 Revised: November 11, 2017 Accepted: November 14, 2017

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