Accepted Manuscript Copper(II) Complexes as Catalyst for the Aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine Raghvi Khattar, Anjana Yadav, Pavan Mathur PII: DOI: Reference:

S1386-1425(15)00136-5 http://dx.doi.org/10.1016/j.saa.2015.01.115 SAA 13283

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

27 October 2014 21 January 2015 30 January 2015

Please cite this article as: R. Khattar, A. Yadav, P. Mathur, Copper(II) Complexes as Catalyst for the Aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.01.115

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Copper(II) Complexes as Catalyst for the Aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine Raghvi Khattar, Anjana Yadav and Pavan Mathur* Department of Chemistry, University of Delhi, Delhi, India

Abstract Two new mononuclear Copper(II) complexes [Cu (L) (NO3)2] (1) and [Cu (L) Br2](2) where (L=bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether)

are

synthesized

and

characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV-Visible, IR spectroscopy, EPR and Cyclic voltammetry. The complexes exhibit different coordination structures; The E1/2 value of the complex (1) is found to be relatively more cathodic than that of complex (2). X-Band EPR spectra at low temperature in DMF supports a tetragonally distorted complex (1) while complex (2) shows three different g values suggesting a rhombic geometry. These complexes were utilized as a catalyst for the aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was found to be higher for the nitrate bound complex, while presence of acetate anion acts as a mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of reaction. The inhibition suggests that the generated protons are further required in another important catalytic step.

Keywords: Bis-benzimidazoles, Copper(II) complex, aerobic oxidation, o-phenylenediamine, 2,3-diaminophenazine.

Corresponding Author: Tel.No:91-1127667725(1381), Fax: 91-1127666605 E-mail address: [email protected] (Pavan Mathur)

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1. Introduction Aerobic oxidation of aromatic amines catalyzed by copper(II) metal ion complexes is important as it develops an understanding into the mechanism involved in copper oxygenase/oxidases, as well as for synthetic applications. The oxidation of chromogenic o-phenylenediamine has been employed for assaying the activity of multi-copper containing oxidase (Laccase) [1] and also horseradish peroxidase [2]. Earlier oxidation of o-phenylenediamine has been reported to utilize molecular oxygen in the form of µ-peroxo-cobalt(III) and oxocobalt(IV) complexes [3] while cobaloxime(II) complexes have been shown to oxidize the diamine to 2,2-disubstituted-2H-benzimidazoles[4]. A bisimidazole copper(II) complex is reported to oxidize the diamine to 2,3-diaminophenazine via two one electron steps [5], while Co(III), Mn(III) and Cu(II) oxalate complexes are reported to oxidize the phenylenediamine via an inner sphere mechanism [6]. Oxidation of 2-aminophenol a close analougue of o-phenylenediamine to form 2-aminophenoxazin-3-one are also reported [7]. Benzimidazole based ligands have been extensively used for forming copper(II) complexes as they mimic the imidazole functionality found in native systems. Due to their bulky nature they cause steric crowding, generate unusual coordination geometries and affect redox properties of metal ion bound to them [8-11]. The present paper reports the synthesis, spectral and structural studies on two copper(II) complexes with N-picolyl substituted benzimidazolyl ligand having two imines nitrogen moieties and one ether oxygen as donor groups. Since reports of aerobic oxidation of o-phenylenediamine using copper(II) complexes are scarce, therefore the present complexes are used to study the kinetics of aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine. Earlier we have reported the oxidation of ophenylenediamine assisted by H2O2 using a Fe(III) bis-benzimidazolyl diamide complex [12].

2. Experimental 2.1. Analysis and physical measurements

Elemental analysis were obtained using Elemental Analyzer at USIC, University of Delhi, Delhi. X-ray diffraction data were collected on an Oxford Diffraction Xcalibur CCD diffractometer with graphite monochromated (Mo-Kα radiation, λ=0.71073 Aº), temperature of 298(2) K at IIT Bombay. The electronic spectra were recorded in HPLC grade DMF on a Shimadzu 1601 spectrophotometer in the region of 200-1100 nm. IR Spectra were recorded 2

in solid state as KBr Pellets on a Perkin Elmer FTIR-2000 spectrometer in the range 4004000 cm-1 in the department of chemistry, university of Delhi, Delhi. Cyclic Voltammetric measurements were carried out using a BAS CV 50W electrochemical analysis system. The cyclic voltammograms of the complexes were recorded in 3:2 DMSO: CH3CN with tetra-nbutylammonium perchlorate (TBAP) 0.1M as supporting electrolyte. A three electrode configuration composed of Pt working electrode 3.1 mm2 area, a Pt wire counter electrode, and an Ag/AgNO3 electrode reference electrode were used for the measurement. The reversible one electron Fc+/Fc couple in the above solvent system has an E1/2 of 59.5mV vs. Ag/AgNO3 electrode. Solution state EPR spectra were recorded on an X-Band Brukerspectrometer and X-band E-112 ESR Spectrometer with a variable temperature liquid Nitrogen cryostat at 120K at IIT-Kanpur, Kanpur, India and IIT- Bombay, India. 2.2.Materials All chemicals diglycolic acid (Aldrich), benzene-1, 2-diamine, 2-(chloromethyl) pyridine and spectroscopic solvents were obtained from commercial sources and used without further purification. Bis (2-benzimidazolyl methyl) ether (DGB) was prepared as described earlier [13]. 2.2.1. Synthesis of ligand (L)

The ligand (L) bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) was synthesized by N-picolylation of bis(2-benzimidazolyl methyl) ether as described earlier [14]. The ESI/M+1, M+2 peak is found to be at 461 and 462, respectively. 2.3. Preparation of Copper (II) complexes 2.3.1. Synthesis of complex [Cu (L) (NO3)2] (1)

A methanolic solution (10 mL) of Cu (NO3)2 .3H2O (52.4mg, 0.22 mmol) was added to a methanolic solution (15 mL) of the ligand (100mg, 0.22 mmol). The resulting bluish colored solution was stirred for 1 h. The reaction mixture was then concentrated on a rotatory evaporator to 2 mL. A sky blue colored product separated on cooling. The product was centrifuged and washed with small amounts of cold methanol. The product obtained was then recrystallized from water: methanol (1: 2) mixture, bluish colored compound crystallized on cooling and was dried over P2O5. Yield: 42%, Anal. found for C28H24N8O7Cu (648.09): C, 3

51.4; H, 3.8; N, 17.1%. Calcd: C, 51.8; H, 3.7; N, 17.3%. UV/Vis spectrum (DMF) λmax, nm (log ε, M-1 cm-1) = 269 (4.48), 274 (4.48), 281 (4.43), 722 (1.90). IR (KBr Pellets, cm-1): ν= 1594 (C=N benzimidazole and pyridine), 1458(C=N-C=C (benzimidazole)), 754 (C=C benzene), 1312 (C-H benzene),1002, 1312(O-N-O (sym)) and1478 (O-N-O(asym)). 2.3.2. Synthesis of complex [Cu (L) Br2] (2) The ligand (100mg, 0.22mmol) was dissolved in methanol (15 mL). A methanolic solution of CuBr2 (48.5mg, 0.22 mmols) (10 mL) was added to the ligand solution. A yellow colored solution was formed and was stirred for half hrs and the product was obtained upon reducing the volume to nearly half. The product so obtained was then recrystallized from water: methanol (1: 2) mixture, yellow colored compound crystallized on cooling and was dried over P2O5. Yield: 70%, Anal. found for

C28H24N6OBr2Cu (683.89): C, 49.4; H, 3.6; N, 11.9%. Calcd: C, 49.1; H, 3.5; N, 12.2%. UV/Vis spectrum (DMF) λmax, nm (log ε, M-1 cm-1) = 269 (4.26), 273 (4.24), 280 (4.20), 917(2.40) IR (KBr Pellets, cm-1): ν= 1595 (C=N benzimidazole and pyridine), 1478(C=NC=C (benzimidazole)), 751 (C=C benzene), 1323 (C-H benzene). 3. Results and Discussion Characterization of Copper(II) complexes 3.1. Crystal structure description of [Cu (L) (NO3)2] (1)

The complex 20mg was dissolved in 25 ml in HPLC grade methanol. Slow evaporation at room temperature, resulted in the formation of thin plates of bluish crystal. The ORTEP diagram and atom numbering scheme is shown in (Fig.1). Crystal data collection, refinement parameters and selected bond lengths and bond angles are given in (Table S1 and Table S2). A total of 9919 reflections were measured, of which 4831were unique and 2145 were considered as observed (I > 2σ (I)]. It crystallizes in the Triclinic with space group P-1. Cu(II) ion is coordinated in a distorted octahedral environment by two N atoms of benzimidazole (N1, N3), two nitrate groups and O1 atom of ether linkage in the ligand. One nitrate group is bidentate, O5 and O7 are involved in coordination and the other is monodentate, O2 is coordinated to Copper(II). The Cu-N bond distances of 1.959(4) Å (Cu– N1) and 1.974(5) Å (Cu–N3) are in the range found for similar benzimidazole ligated compounds [15-18]. (Cu –O5), (Cu –O2) and (Cu –O1) bond distances are 1.981(5), 1.980(4) and 2.342(4) Å. Bond angles C(7)-N(1)- Cu(1) ,C(10)-N(3)-Cu(1), N(1)-Cu(1)-N(3) and N(1)-Cu(1)-O(1) are 117.9(4) º,118.1(4) º, 150.47(18)º and 75.71(15)º respectively. These are 4

comparable to bond angle found with similar type of ligated complexes, as in [Cu(OBB)2]picrate-DMF [18] and [Cu(Meobb)2](NO3)2.2CH3OH [15]. 3.2. Crystal structure description of [Cu (L) Br2] (2)

The complex 20 mg was dissolved in warm 25 mL HPLC grade methanol. Slow evaporation at room temperature, resulted in the formation of orange crystal. The ORTEP diagram and atom numbering scheme is shown in (Fig.2). Crystal data collection, refinement parameters and selected bond lengths and bond angles are given in (Table S1 and Table S3). A total of 25414 reflections were measured, of which 4978 were unique and 3098 were considered as observed (I > 2σ (I)]. It crystallizes in the Monoclinic with space group P 21/c. Cu(II) ion is penta-coordinated by two N atoms of benzimidazole (N1, N4) , two bromide groups and O1 atom of ether linkage in the ligand. The Cu-N bond distances of 1.953(6) Å (Cu–N1) and 1.947(6) Å (Cu–N4) are in the range found for similar benzimidazole ligated compounds [16(a)]. (Cu –O1) bond distance is 2.253(5) Å. Bond angles C(7)-N(1)- Cu(1) and C(10)-N(4)-Cu(1) are 118.9(5)º and 118.7(5)º. Present complex has τ = 0.359, where α is the angle between N1-Cu-N4 (151.2(3) º) and β is the angle between O1-Cu-Br2 (129.64(14) º) indicating a highly distorted square pyramidal geometry [19]. The equatorial bond angles deviates from the expected value of 120º (O1-Cu (1) - Br1 = 116.00(14) º, Br1-Cu (1) - Br2 =114.36(5) º and O1-Cu-Br2 = 129.64(14) º. The deviation in equatorial bond angles suggests remarkable asymmetry within the trigonal plane. Sum of the equatorial bond angles is 360º indicating that Cu centre mainly resides in the equatorial plane. 3.3. Electronic Spectroscopy

Electronic spectra of the Cu(II) complexes were recorded in HPLC grade DMF. Three peaks in the range 265–285 nm are observed in the complexes and are assigned to intraligand π–π* transition of benzimidazole and pyridyl moiety present in the ligating system [20]. The [Cu (L) (NO3)2] complex has a d-d band at λmax 722 nm. This falls in the range of tetragonally distorted six coordinate complexes [21] while the [Cu (L) Br2] complex has a d-d band with λmax917nm, the relatively low energy of this band is in conformity with five coordinate copper(II) complexes.

5

3.4. IR spectral studies

Strong bands at 1616 and 1463 cm-1 for free ligand are assignable to the pyridyl (C=N) and benzimidazole (C=N-C=C) stretching mode [22]. These are shifted upon complexation by 515 cm-1 indicating the binding of imine nitrogen to Cu(II) as found for other metal complexes with benzimidazoles. In the nitrate complex bands appear at 1594 and 1458 cm-1 and in bromide complex bands appear at 1595 and 1478 cm-1. In the nitrate complex (1) three bands appear at 1002, 1312 and 1478 cm-1, the separation of two highest peak is around 165 cm-1 [21] indicating bidentate nitrate bound to copper(II). Presence of benzene ring is confirmed by a very strong peak in the region 750-755cm-1.

3.5. Cyclic voltammetry

The complex [Cu (L) (NO3)2](1) and [Cu (L) Br2](2)in (3:2) DMSO : MeCN displays a quasi reversible redox waves due to the Cu(II)/Cu(I) reduction process with E1/2 value at -193.8 and -124.0 mV, while the value of IPc/IPa ratio found to be 0.85 and 1.0 respectively (Fig.S1 and Fig.S2). The present E1/2 values of these complexes are relatively negative when compared to other benzimidazole bound Cu(II) complexes [23]. E1/2 value of nitrate bound complex (1) is more cathodic than the bromide bound complex (2), this indicates that the Cu(II) state is stabilized due to a six coordinate nitrate bound complex, relative to a bromide bound complex. The E1/2 value with N-pyridyl substituted ligand(L) is much more cathodic than the E1/2 values with un-substituted ligand (OBB) in [Cu(OBB)2](pic)2.4(DMF) [21(b)] and mixed ligand complex [Cu(L1)(L2)](ClO4) .mEt2O.nH2O [22(a)] and [Cu(Meobb)2](NO3)2.2CH3OH [20(a)]. 3.6. EPR data

The X-band EPR spectra of the Copper(II) complexes have been recorded at liquid nitrogen temperature in DMF, and EPR spectral parameters of the complex (1) are given in (Table S4). Complex (1) shows four g|| components and a broadening of g┴ component (Fig.S3). Such a broadening of the g┴ component is indicative of a lowered symmetry, and is reflected in a small value of A|| leading to large g||/A|| [24]. Nitrogen super hyperfine (N-SHF) structure is observed in the perpendicular region with a coupling constant of AN = 16±2G, typical of NSHF interaction [24(b)]. Whereas in complex (2) no four line hyperfine pattern is observed, and EPR spectra have been analyzed as given by Kneubuhl et al [25]. This shows three different g

6

values implying a rhombic geometry (Fig.S4). The values g1, g2 and g3 are reported in (Table S4). 4. Aerobic oxidation of o-phenylenediamine 4.1. General procedure:

(a) A 0.19 mM (2mL) methanolic solution of Cu(II) complexes [Cu(NO3)2L] or [CuBr2L] was added to a methanolic solution 5.7 mM (2 mL) of o-phenylenediamine and molecular oxygen was passed for 5min. The ratio of catalyst to substrate is 1:30. One ml of this reaction mixture is diluted by one ml of methanol and the absorption spectra of this solution were recorded with time in the range 300-1100 nm at room temperature, for a period of 40 minutes. A new band at 425-430 nm increases which confirms the formation of 2,3-diaminophenazine [12]. The concentration of product formed was calculated using the reported extinction coefficient of 2,3diaminophenazine [12]. Absorbance versus wavelength plot is shown in fig.3(a). The initial rates of reaction are tabulated in table.1 and table.3. (b) A blank experiment was also carried out wherein a methanolic solution of ophenylenediamine (5.7mM) (4mL) was bubbled with molecular oxygen for 5 minutes. One ml of this reaction mixture is diluted to make two ml with methanol and absorption spectra were recorded in the range of 300-1100 nm at room temperature, for a period of 60 min. A band at 425-430 nm was found to slowly increase during 60 minutes. Absorbance versus wavelength plot is shown in fig.3(b). The rate of aerial oxidation in the absence of catalyst is found to be tenfold lower than in the presence of catalyst, confirming the role of copper(II) complexes in the aerobic oxidation of ophenylenediamine. (c) An experiment was carried out to check the effect of acetate ion on the rate of formation of 2,3-diaminophenazine. For this experiment the concentration of ophenylenediamine and Cu(II) complex was kept at 0.19mM(2mL) and 5.7mM (2mL), 0.01 mL of 2M acetate anion was added so as to keep the ratio of catalyst: substrate: anion as 1:30:10.5. The oxidation reaction in presence of acetate anion shows a different behavior. Two new intermediate bands at 581 nm and 927 nm are generated simultaneously which are found to decay. After 4-5 minutes a band at 430 nm starts to appear and the two intermediate bands start to decrease in intensity. Absorption and concentration versus time plots are shown in fig.4 and fig.5.

7

4.2. Kinetic studies The kinetics of the oxidation reaction between o-phenylenediamine (OPD) in presence of catalytic amount of copper(II) complexes was performed at room temperature under the following conditions: 4.2.1. Substrate variation The amount of substrate (0.96, 1.93, 3.8, 5.7 mM) was varied while keeping the amount of catalyst fixed at 0.19mM at least two sets of data were taken. Plot of concentration of 2,3diaminophenazine(λmax ~ 430 nm) formed versus time is given in fig.6. The initial rates are given in table 1. The rate of oxidation reaction with varying substrate concentration is plotted in fig.S5.This shows that the initial rate of reaction increases as the concentration of the substrate increases at a fixed catalyst concentration. The logarithm of initial rate of reaction was plotted against the ln[substrate] while keeping the concentration of catalyst constant. This plot is shown in Fig.7 from where the slope is found to be 0.62 suggesting a pseudo first order dependence, on the substrate concentration. 4.2.2. Catalyst variation The amount of copper(II) catalyst (0.09, 0.19, 0.28, 0.38 mM) was varied while keeping the amount of o-phenylenediamine (substrate) fixed at 5.7 mM at least two sets of data were taken. Plot of concentration of 2,3-diaminophenazine formed (λmax~430 nm) versus time is given in fig.8. The initial rates are given in table 2. The initial rate of oxidation reaction with varying catalyst concentration is given in fig.S6. This shows that the rate of reaction increases as the concentration of the substrate increases at a fixed catalyst concentration, up to ratio of substrate: complex (30:1). The logarithm of initial rate of reaction was plotted against the ln[catalyst], while keeping the concentration of substrate constant. This plot is shown in Fig.9 from where the slope is found to be 0.61 suggesting a pseudo first order dependence on the catalyst concentration. The initial rate of reaction with [Cu(NO3)2(L)] complex is higher than that found for [CuBr2(L)] complex (ratio of catalyst: substrate being 1:30); the relative lowering could be due to a greater stability of the Cu(I) state for the Br- bound complex rather than the NO3bound as suggested by their E1/2. This may make a reoxidation step in the catalytic cycle relatively difficult.

8

The intermediate bands generated at λmax~581 nm and 930nm start to decay as soon as they are formed in the presence of acetate anion, while the 2,3-diaminophenazine band starts to grow (λmax~430 nm). An attempt was made to obtain the relative velocity of decay of the intermediate bands and there were compared with the velocity of formation of 2,3diaminophenazine band, using the absorption versus time plots (fig.4). It is found that the band at λmax~581 nm and 930 nm decays by almost three fold and tenfold slowly than the growth of the band at λmax~430 nm. This suggests that species formed at λmax~581 nm and 930 nm may not have any role to play in the formation of phenazine. The drop in rate of reaction in the presence of OAc- ions for both the complexes, suggests that protons generated during the course of reaction are further required in another catalytic step, OAc- anions, pick up these released H+ ions, causing their concentration to deplete and thus impeding an intermediate step. Based on above discussion a possible scheme of catalytic oxidation is given below: The first step involves the deprotonation of o-phenylenediamine in the presence of copper(II) catalyst, thus forming a weak coordination bond between deprotonated o-phenylenediamine and copper(II) complex. The Cu(II) complex than oxidizing o-phenylenediamine by one electron thereby reducing itself to Cu(I) species. The reduced catalyst Cu(I) is then reoxidised by molecular oxygen and the generated superoxide picks up a released proton to give the hydro superoxide radical (HO2-.) and Cu(II). The hydro superoxide further oxidizes the amine to o-quinonediimine and forming H2O2 (Catalytic step I). The presence of superoxide in the reaction was confirmed by carrying the oxidation reaction in the presence of ascorbic acid, which is known to be a superoxide quencher [26]. The final ratio were catalyst: substrate: quencher :: 1:30:1. It was found that in the presence of ascorbic acid the rate of reaction diminishes to nearly half the original value (table.1 fig. S7), confirming the role of superoxide in the oxidation reaction(scheme I) . The o-quinonediimine formed condenses with another molecule of o-phenylenediamine. Cyclization and oxidation by two molecules of H2 O2generated in the catalytic cycle are used to further give the 2,3-diaminophenazine (Catalytic step II)

9

Catalytic step –I NH2 HN

+

CuII

H2O2

HN

HO2H2N

HO2

.

NH2

.

-H+

+ Cu(II)

HN

CuII

H2N

-

N H

O2

CuI

H2N HN

Catalytic step- II

NH

H N

OPDA

NH2

+ H2O2

NH

NH2

NH

-2H+ -2e-

N

NH2

NH2

NH

+ 2H2O

Cyclization

2H2O

N

NH2

N

NH2

+

2,3-diaminophenazine

-2H+, -2e-

N

NH2

N H

NH2

Scheme.1 Reaction scheme for the oxidation of o-phenylenediamine to 2,3diaminophenazine. 10

+ H2O2

Conclusion Two new mononuclear copper(II) complexes [Cu (L) (NO3)2] and [Cu (L) Br2] are synthesized and characterized. These complexes carry out the aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine. Rate of oxidation in the absence of the catalyst shows a tenfold drop confirming the rate of Cu(II) complex as a catalyst in this reaction. Kinetic study shows that the reaction follows first order with respect to catalyst and substrate. Initial rate of reaction was found to be higher for nitrate bound complex. The presence of acetate anion causes a mild inhibition of reaction, suggesting the role as a proton sponge in the catalytic cycle. The catalytic oxidation proceeds via a copper(II)/ copper(I) cycle.

Acknowledgment We gratefully acknowledge the financial support from the University of Delhi, Delhi, India for the special grant. Anjana Yadav gratefully acknowledges JRF from UGC, India, and one of the authors, Pavan Mathur gratefully acknowledges the grant of sabbatical leave from the University of Delhi, Delhi, India.

Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre [Cu (L) (NO3)2] (1) and [Cu (L) Br2] (2) having CCDC 1010841 and 1010840 respectively. These data can be obtained free of charge from The Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif

supplementary material contain EPR, Cyclic voltammetry, Rate versus concentration plots, tables for crystal structure and refinement, table of bond angles and bond lengths.

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References [1] H. Huang, R. Cai, Y. Du, Z. Lin, Y. Zeng, Anal. Chim.Acta 318 (1995) 63-69. [2] S. Fornera, P. Walde, Anal. Biochem. 407 (2010) 293-295. [3] L.I. Simandi, T. barna, Z. Szeverenyi, S.Nemeth, Pure Appl. Chem. 64 (1992) 1511-1518. [4] L.I. Simandi, T.M.Simandi, Z. May, G. Besenyei, Coord. Chem. Rev. 245 (2003) 85-93. [5] A.L. Abuhijleh, polyhedron 15 (1996) 285-293. [6] A.B. Zaki, M.Y. El-Seikh, J. Evans, S.A. El-Safty, polyhedron 19 (2000) 1317-1328. [7](a) X. Li, W. Shi, Q. Cheng, L. Huang, M.Wei, L.Cheng, Q. Zeng, A.Xu, Appl. Catal. A 475(2014) 297-304. (b) M. Szavuly, R.Csonka, G. Speier, R. Barabas, M.Giorgi,J.Kaizer, J, Mol. Catal. A: Chemical 392(2014) 120-126. (c) A. Yadav, P. Mathur, Catal. Commun. 55 (2014) 1-5. [8] W.Kaim, J.Rall,Angew. Chem 35(2003)43-60. [9] J.P. Klinman, Chem Rev 96(1996)2541-2561. [10] a) F.Téllez, H.López-Sandoval, S. E.Castillo-Blum, N.Barba-Behrens, Arkivoc (2008) (v) 245- 275. (b) H.Kozłowski,T.Kowalik-Jankowska, M.Jezowska-Bojczuk,Coord.Chem Rev 249(2005)2323-2334. (c) P.Deschamps, P.P. Kulkarnia, M.Gautam-Basak, B Sarkar, Coord. Chem. Rev 249(2005)895-909.(d) R. Contreras A. Flores-Parra, E. Mijangos, F. Tellez, H.Lopez-Sandoval, N.Barba-Behrens Coord.Chem.Rev 253 (2009) 1979-1999. [11]B.J.Hathway,G.Wilkinson,R.D.Gillard,J.A.McClevertyComprehensive Coord.Chem.Pergamon Press, Oxford, 5(1987)558. [12] N. Tyagi, P. Mathur, Spectrochim. Acta 96 (2012) 759-767. [13] G.Batra, P.Mathur,Inorg.Chem 31(1992)1575-1580. [14] (a) R.Khattar, M. S. Hundal, P.Mathur, Inorg. Chim.Acta 390(2012)129-134. (b) R.Khattar, P.Mathur,Inorg. Chem. Comm 31(2013)37-43. [15] (a) H.Wu, F.Kou, F.Jia, B.Liu, J.Yuan, Y.Bai,J. Photochem. Photobio B: Biology 105(2011)190-197. (b)H.Wu, R.Yun, K.Wang, X.Huang, T.Sun, K.Li, Synth React. Inorg. Metal-Organic Nano-Metal Chem. 39(2009) 406-409. [16] (a) J.Liu, Z.Song, L.Wang,Trans Metal Chem. 24(1999)499-502. (b) M. Palaniandavar, R. J. Butcher,A. W.Addison, Inorg. Chem. 35(1996) 467-471. [17] Ö.Tamer, B.Sarıboga, I.Ucar, O.Büyükgüngör, Spectrochim. Acta 84(2011)168-177. [18] R.Yun, W.Ying, B.Qi, X.Fan, H.Wua, ActaCrystE64(2008)1529. [19] T. N. Rao, A.W. Addison, J. Chem. Soc., Dalton Trans. 1984, 1349. 12

[20] E.Monanzani, L.Quinti, A.Perotti, L.Casella, M.Gulloti, L.Randaccio, S.Geremia, G.Nardin, P. Faleschini, G.Tabbi, Inorg. Chem. 37(1998)553-562. [21] S. Tehlan, M.S. Hundal, P. Mathur, Inorg. Chem. 43(2004)6589-6595. [22] (a) M. Gupta, P. Mathur, R.J. Butcher. Inorg. Chem. 40(2001)878-885. (b) S.K. Upadhyay, S. Tehlan, P. Mathur, Spectrochim. Acta Part A 66(2007) 347-352. [23] (a) Y. Nakao, M. Onoda, T.Sakurai, A.Nakahara, L.Kinoshita, S.Ooi, Inorg. Chim. Acta 151(1988)55-59. (b) A.W. Addison, H.M.J. Hendricks, J. Reedijk, L. K. Thompson, Inorg. Chem. 20(1981)103-110 (c) U. Sivagnanam,M.Palaniandavar, J.Chem. Soc. Dalton Trans(1994)2277-2283. [24] (a) G.Ahuja, P. Mathur, Inorg. Chem. Comm 17(2012)42-48. (b) S.C. Mohapatra, S. Tehlan, M. S. Hundal, P. Mathur, Inorg. Chim. Acta. 361(2008)1897-1907. (c) R. Bakshi, M. Rossi, F. Caruso, P. Mathur, Inorg. Chim. Acta 376(2011)175-188. [25] F.K. Kneubuhl, J. Chem. Phys. 33(1960)1074-1078. [26] M. Nishikimi, Biochem. Biophys. Res. Commun 63(1975) 463.

Fig.1.ORTEP diagram of copper(II) complex(C28 H24 Cu N8 O7)(1) drawn in 20% thermal probability ellipsoids showing atomic numbering scheme. Hydrogens are omitted for the sake of clarity.

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Fig.2.ORTEP diagram of copper(II) complex(C28 H24 Br2 Cu N6 O)(2) drawn in 20% thermal probability ellipsoids showing atomic numbering scheme. Hydrogens are omitted for the sake of clarity.

Fig.3. (a)Time dependant UV-Visible spectral changes for the oxidation of ophenylenediamine catalysed by the copper(II)complex[CuL(NO3)2] in dioxygen at room 14

temperature for 40 min. (b) Blank experiment for the oxidation of o-phenylenediamine in dioxygen at room temperature in the absence of the catalyst for 60 min.

Fig.4.Time dependant UV-Visible spectral changes for the oxidation of o-phenylenediamine catalysed by the copper(II)complex[CuL(NO3)2] in dioxygen at room in presence of acetate anion.

Fig.5. Plot of formation of 2,3–diaminophenazine vs time while keeping: (A) Substrate concentration [0.19mM], catalyst concentration [CuL(NO3)2][5.79mM] and acetate anion[0.01ml of 2M] giving a ratio 1:30:10.5. (B) Substrate concentration [0.18mM], catalyst concentration [Cu(L)(Br)2][5.49mM] and acetate anion[0.007ml of 2M] giving a ratio 1:30:10.5.

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Fig.6. Plot(A-D): Formation of 2,3-diaminophenazine vs time while keeping catalyst [Cu(L)(NO3)2] concentration fixed[0.19M] and varying the substrate concentration. Plot(E): Formation of 2,3-diaminophenazine vs time while keeping catalyst[Cu(L)(Br)2] concentration [0.18M] and the substrate concentration [5.49M], giving a ratio of catalyst : substrate as (1:30).

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Fig.7. Plot of Ln[Initial Rate] of 2,3-diaminophenazine substrate].

formed vs Ln[concentration of

Fig.8. Plot of formation of 2,3-diaminophenazine vs time while keeping substrate concentration fixed and varying the catalyst concentration.

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Fig.9. Plot of Ln[Initial Rate] of 2,3-diaminophenazine catalyst].

formed vs Ln[concentration of

Table1. Rate of oxidation of o-phenylenediamine with varying concentration of substrate S.No. Concentration Concentration of Ratio Initial rate of reaction of catalyst(mM) substrate(mM) *10 -5mM(initial rates) 1. 0.19 0.96 1:5 237 2. 0.19 1.93 1:10 324 3. 0.19 3.86 1:20 450 4. 0.19 5.79 1:30 612 5. 0.19 5.79 + 0.19mM (ascorbic 1:30:1 355 acid) 6. 5.79 50.2 7. 0.19 5.79+0.01ml of 2M, OAc 1:30:10.5 510

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Table2. Rate of oxidation of orthophenylenediamine with varying concentration of catalyst [Cu(NO3)2(L)] S.No.

1. 2. 3. 4. 5.

Concentration of catalyst(mM) 0.09 0.19 0.28 0.38

Concentration of substrate(mM) 5.79 5.79 5.79 5.79

Ratio

0.5:30 1:30 1.5:30 2:30

Initial rate of reaction *10-5mM(initial rates) 340 612 650 857

Table3. Rate of oxidation of o-phenylenediamine in presence of a catalyst [CuBr2L] S.No.

1. 2.

Concentration of catalyst(mM) 0.18 0.18

Concentration of substrate(mM) 5.49 5.49+ 0.007ml of 2M OAc-

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Ratio

1:30 1:30:10.5

Initial rate of reaction *10-5mM(initial rates) 400 189

GRAPHICAL ABSTRACT

Copper(II) Complexes as Catalyst for the Aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine Raghvi Khattar, Anjana Yadav and Pavan Mathur* Department of Chemistry, University of Delhi, Delhi, India

Copper(II) complexes have been utilized as a catalyst for the aerobic oxidation of ophenylenediamine to 2,3-diaminophenazine. Rate of oxidation in the absence of the catalyst shows a tenfold drop confirming the role of Cu(II) complex as a catalyst in this reaction.

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Highlights






Copper(II) complexes of a bisbenzimidazolyl ligand are synthesised. Complexes catalyse the aerobic oxidation of o-phenylenediamine. Reaction follows first order kinetics with respect to substrate and catalyst. Rate of oxidation shows tenfold drop in absence of catalyst. Addition of acetate anion causes a mild inhibition of rate of reaction.

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