Mono and tri-nuclear cobalt(III) complexes with sterically constrained phenol based N2O2 ligand: Synthesis, structure and catechol oxidase activity

Journal Pre-proofs Mono and tri-nuclear cobalt(III) complexes with sterically constrained phenol based N2O2 ligand : Synthesis, Structure and Catechol oxidase activity Imran Ali, Bikramaditya Mandal, Rajat Saha, Rajarshi Ghosh, Mithun Chandra Majee, Dhrubajyoti Mondal, Partha Mitra, Debdas Mandal PII: DOI: Reference:

S0277-5387(20)30086-3 https://doi.org/10.1016/j.poly.2020.114429 POLY 114429

To appear in:

Polyhedron

Received Date: Revised Date: Accepted Date:

8 December 2019 31 January 2020 1 February 2020

Please cite this article as: I. Ali, B. Mandal, R. Saha, R. Ghosh, M. Chandra Majee, D. Mondal, P. Mitra, D. Mandal, Mono and tri-nuclear cobalt(III) complexes with sterically constrained phenol based N2O2 ligand : Synthesis, Structure and Catechol oxidase activity, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.2020.114429

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Mono and tri-nuclear cobalt(III) complexes with sterically constrained phenol based N2O2 ligand : Synthesis, Structure and Catechol oxidase activity Imran Ali a, Bikramaditya Mandal a, Rajat Saha b, Rajarshi Ghosh c, Mithun Chandra Majee d, Dhrubajyoti Mondal d, Partha Mitra d, Debdas Mandal a,* aDepartment

of Chemistry, Sidho-Kanho-Birsha University, Purulia - 723104, West Bengal,

India b Department

of Chemistry, Kazi Nazrul University , Asansol, Paschim Bardhaman-713340, West

Bengal, India c Department

of Chemistry, The University of Burdwan, Burdwan 713 104, India

dDepartment

of Inorganic Chemistry, Indian association for the cultivation of science, Kolkata – 700032, India * Corresponding author E mail – id: [email protected] (Debdas Mandal) Abstract Mono and tri- nuclear Co(III) complexes, [Co3(OMe)4L2]ClO4 (1) and

[CoL(acac)]

(2) have been synthesized using sterically constrained tetradentate ligand N,N/-dimethyl-N,N/bis(2-hydroxy-3,5-dimethylbenzyl)-ethylenediamine (H2L) with varying reaction condition. All complexes have been characterized by single crystal X-ray diffractometer and various spectroscopic tools. Complex 1 is tri-nuclear methoxo and phenoxo bridged cobalt complex with CoIII – CoIII – CoIII linear core, whereas complex 2 is mononuclear mixed ligand octahedral Co(III) complex. Both complexes show catechol oxidase activity in methanol as monitored by the UV–Vis spectroscopy of the aerial oxidation of 3,5-DTBC to 3,5-DTBQ. The kinetic parameters have been determined using Michaelis–Menten approach. The complexes are efficient catalysts with high turnover numbers 3.32 × 103 h-1 and 2.19 × 103 h-1 for 1 and 2, respectively. Mechanistic investigations of the catalytic behaviour by ESI-MS spectra and estimation of hydrogen peroxide formation indicates that the catalytic reaction occurs through the reduction of Co(III) to Co(II). Keywords: Mono and tri-nuclear Co(III) complexes, Tetradentate ligand, Syntheses, X-ray structures, Catecholase activity

1. Introduction Multinuclear transition-metal complexes are of current interest due to their wide range of applications in various fields such as catalysis, adsorption, storage, magnetism, molecular recognition, fluorescence, nonlinear optics, and sensors [1]. Bridging capability of phenolate donor site will be exploited judiciously to synthesize multinuclear metal complexes with aesthetically pleasing structural frame works [2]. Using phenol-based ligands, we would like to assemble more metal ions in close proximity. Inspired by the above knowledge, we would like to synthesize new type of ligands having pre-organized donor set with biological relevance. We design ligand in such a way so that the ligands possess flexibility character. Flexibility in ligand environment introduces structural diversity in metal complexes giving interesting property. Coordination complex with catecholase like activity plays a pivotal role for the development of new bio-inspired catalyst [3]. Catechol oxidase is a copper containing type III active site protein which catalyzes the oxidation of catechol to the corresponding quinone. The generated quinones are highly reactive compounds that undergo auto polymerization to produce melanin which is a brown polyphenolic pigments. Melanin may be responsible for protecting the tissues from damage against pathogens and insects [4]. The crystal structure of catechol oxidase, isolated from sweet potatoes was determined in 1998 which revealed that the active center of the catechol oxidase consists of a hydroxo bridged dicopper (II) center in which each copper(II) center is coordinated to three histidine ligands [5]. The ability of di-copper complexes to oxidize phenols and catechols is a topic of recent interest for the development of new bio inspired catalyst [6]. From the report of various research group, catechol oxidase activity of metal complexes with other metal ion such as Co(III), Fe(II), Ni(II), Mn(III) and Zn(II) is well established in recent time [7]. With the above mentioned two fold aims, namely, development of multinuclear transition metal complexes and catechol oxidase activity of cobalt(III) complexes derived from flexible phenol-based ligand, we have isolated two

new cobalt complexes of composition

[CoIII3(OMe)4L2]ClO4 (1) and [CoIIIL(acac)] (2). These complexes are structurally characterized by single crystal X –ray diffractometer. Tri-nuclear cobalt complex 1 has a unique

molecular structure with CoIII – CoIII – CoIII linear core and it is a rare type of methoxo- and phenoxo bridged cobalt complex with aesthetically pleasing structural framework. Complex 2 is a mixed ligand octahedral monomer cobalt(III) complex with sterically constrained phenol based tetradentate N2O2 ligand together with acetylacetone as ancillary ligand. All complexes act as an effective catalyst towards the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-ditert-butylbenzoquinone (3,5-DTBQ) with molecular oxygen in methanol at 25°C. 2. Experimental Section 2.1. Materials The

tetradentate

ligand

N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-

dimethylbenzyl)ethylenediamine (H2L) was prepared according to a reported method [8]. Solvents were purified by taking suitable

drying agents and distilled under nitrogen prior to

their use [9]. All other chemicals were collected from local market. The chemicals were of reagent grade and used as received without further purification. Caution! Perchlorate compounds are potentially explosive. Only a small amount of material should be prepared and handled with care. 2.2. Syntheses 2.2.1. Preparation of complexes [CoIII3(OMe)4L2]ClO4 (1). Ligand H2L (0.36 g, 1mmol) was dissolved in 25 ml methanol. Co(ClO4)2.6H2O (0. 54 g, 1.5mmol) was added to the solution. Resulting reaction mixture was stirred continuously for 1 hour. The solution was turned into black color and the solution was filtered. The filtrate was kept in open air at room temperature for crystallization. A brown crystalline complex along with single crystals was obtained. Yeild : 0.68 g (62 %). IR (KBr disk cm-1): 2925, 1477, 1274, 1232, 1095, 1047, 622. Anal. calcd for C48 H72 Cl Co3 N4 O12: C, 51.92; H, 6.49; N, 5.05. Found: C, 51.10; H, 6.85; N, 5.20%. ESI-MS in CH2Cl2: m/z 994 (M + H2O –ClO4-)+. UV- Vis (MeOH) [ λmax/nm] : 599, 402. [CoIIIL(acac)] (2). Co(acac)3 ( 0.35 g, 1 mmol ) was added to the a methanol solution (25 mL) of H2L (0.36 g, 1 mmol) and the resulting mixture was stirred. After 1h stirring, solution was turned into black color. Resulting solution was filtered. The filtrate was kept undisturbed in open

air at room temperature for crystallization. A brown crystalline complex along with single crystal was obtained. Yield: 0.38 g (74 %). IR (KBr disk, cm-1): 2908, 1593, 1517, 1469, 1392, 1261, 792. Anal. calcd for C27 H37 Co N2 O4: C, 63.21 ; H, 7.22; N, 10.93. Found: C, 62.93; H, 7.45; N, 10.20% . ESI-MS in CH2Cl2: m/z 535 (M + Na )+. 535. UV- Vis (MeOH) [λmax/nm] : 532, 435.

2.3. Physical Measurements UV-visible spectra in solution were taken on a Perkin-Elmer 950 UV/vis/NIR spectrophotometer and infrared spectra were recorded on a Nicolet Magna 750 FT-IR spectrometer, series II with samples prepared as KBr pellets. Mass spectra (ESI-MS in positive ion mode) were collected on a QTOF Model YA263 Micro Mass Spectrometer. 2.3.1. X-ray Crystallography The single crystals of 1 - 2 for X-ray diffraction study were obtained at room temperature by slow diffusion of methanol solution of the complex. Intensity data for the complexes were collected on a Bruker SMART 1000 CCD diffractometer using a graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K. Intensity data were taken within the θ range of

1.517 –

26.563° and 1.692 – 28.528° respectively for complex 1 and 2. The structures were solved by direct methods [10] and refined on F2 by a full-matrix least-squares procedure [11] using the program SHELXL 97. Relevant crystal data and refinement details are written in Table 1.

Table 1 Crystal data and structure refinement parameters of the complex 1 and 2. 1

2

Empirical formula

C48H72Cl Co3N4O12

C27H37 CoN2O4

Formula weight

1109.33

512.51

T (K)

293

293

λ (Mo Kα), Å

0.71073

0.71073

Space group

C2221

C 2/c

Crystal system

Orthorhombic

Monoclinic

a (Å)

12.651(4)

8.8971(7)

b (Å)

14.698(5)

24.0717(18)

c (Å)

26.854(9)

12.0486(8)

α

90

90

β

90

91.868(5)

γ

90

90

V (Å3)

4993(3)

2576.7(3)

Z

4

4

DCalcd. (g cm-3)

1.476

1.321

μ(mm_1)

1.103

0.701

F (000)

2328

1088

θ ranges (º)

1.517 – 26.5630

1.692 – 28.528

-15 ≤ h ≤ 15

-11 ≤ h ≤ 11

-18 ≤ k ≤ 18

-32 ≤ k ≤ 31

-33 ≤ l ≤ 33

-16 ≤ l ≤ 15

27210

17736

Index ranges

Reflections collected

Rint

0.0496

0.0896

Goodness of fit

1.040

1.266

No. of parameters

309

159

R1a (Fo), wR2b(Fo) (all data)

0.0477, 0.1243

0.0908, 0.2466

Largest diff. peak, deepest hole (eÅ-3)

0.856, -0.548

0.760, -0.860

aR

= ∑||Fo| - |Fc||/∑|Fo|. b wR = [∑[w((Fo2 - Fc2)2]/∑w(Fo2)2]

2.3.2. Catalytic oxidation of 3,5-DTBC The catechol oxidase activity of complexes 1 – 2 were observed by taking most familiar 3,5-di-tertbutylcatechol (3,5-DTBC) as the substrate in a methanol solution at room temperature in presence of air [12]. 3,5-di-tert-butylcatechol (3,5-DTBC) (100 equivalent) in methanol were added to 10-4 M solutions of 1 – 2 in methanol under aerobic condition. Absorbance of the resultant reaction mixture was plotted with respect to wavelength at 5 min intervals in spectrophotometer to observe the quinone band around 400 nm. In case of a blank experiment without catalyst, no quinone band was observed up to 6 h in MeOH. The dependence of the rate on various concentrations were obtained by treatment of a 10-4 M solution of different complexes with 100-500 equivalents of substrate and in each case, the reaction was observed spectro photometrically by monitoring the increase in the absorbance at 400 nm ( the peak corresponding to the quinone band maximum) as a function of time [13]. 2.3.3. Detection of Hydrogen Peroxide in the Catalytic Reactions Hydrogen peroxide was detected by modified iodometric method during catalytic reaction [14]. The catalyst and 3,5 DTBC was mixed as in the kinetic experiments. Equal amount of

water was added to the reaction mixture after 1 h of reaction. The quinone was separated from the reaction mixture by extracting with dichloromethane. Aqueous portion was acidified by few drops of H2SO4 to stop further oxidation. Reaction mixture is mixed with 1 mL of a 10% solution of KI and few drops of 3% solution of ammonium molybdate. Hydrogen peroxide obtained from catalytic reaction oxidizes I- to I2 which is stabilised as a I3- ion in the solution. Characteristic I3band at 353 nm was noticed by UV – vis to confirm the formation of I3- ion. 3. Results and Discussion 3.1.

Syntheses A sterically hindered tetradentate phenol based N2O2 ligand (H2L) has been used here to

synthesize mono and tri-nuclear cobalt complexes. Nuclearity of these complexes depends upon the nature of the metal salt used. When the ligand is treated with cobalt perchlorate hexahydrate as metal ion precursor in methanol, the product obtained is methoxo and phenoxo bridged trinuclear complex [CoIII3(OMe)4L2]ClO4 (1) with linear symmetric CoIII – CoIII – CoIII core as revealed from X-ray crystallography (see later). Here, ligand was added to the methanolic solution of Co(ClO4)2.6H2O in 2:3 mole ratio. Interestingly enough, when ligand is reacted with equimolar amount of Co(acac)3 ( acac = acetylacetonate) in methanol, a mononuclear octahedral mixed ligand complex [CoIIIL(acac)] (2) is produced. The detailed strategy is summarized in Scheme 1. Chaudhury et. al. also have reported the syntheses of new mixed-oxidation manganese (II/III) complexes namely, [Mn4(L)2(PhCOO)6] (1), [Mn3(L)2(CH3CH2COO)2(OMe)2] · H2O (2), and [Mn(L){(CH3)3CCOO}(CH3OH)] · CH3OH (3), based on a tetradentate N2O2 ligand H2L and various carboxylates, namely, benzoate, propionate, and pivalate, as ancillary ligands. The Prime ligand H2L has sufficiently flexible backbone and consequently it acts as a capping ligand in these complexes [15]. Characteristic bands of the ligand bound with complexes (1-2) are confirmed from IR spectra.

Both complexes show a prominent band around 1270 cm-1 for υ(C–O/phenolate)

stretching vibrations [16]. Spectrum of 1 contains a strong band at 1047 cm-1 due to bridging methoxy stretchings. Spectrum of 1 also exhibits two strong bands at ca. 1095 and 622 cm-1, confirming the presence of ionic perchlorate in this molecule [17]. Corresponding signature

vibrations for the β-diketonate moiety in complex 2 appear in the form of a twin band at ca. 1593 and 1517 cm-1 due to υ(C=C) and υ(C=O) stretchings, respectively [18].

2

1

Scheme1. Schematic pathway of synthesis of complexes 1 and 2. 3.2. Description of crystal structure The molecular structure of 1 is shown in Fig. 1 and the selected bond distances and angles are listed in Table 2. Complex 1 crystallizes in orthorhombic space group C2221. The asymmetric unit contains a perchlorate anion along with the monocation involving three Co(III) centers. The tri-nuclear complex has a grossly linear structure involving CoIII -·CoIII –CoIII core with aesthetically pleasing structural framework. The

central

crystallographic geometry

cobalt

inversion

comprised

of

atom

centre

four

Co(1), has

bridging

a

which

sits

distorted methoxy

on

the

octahedral

oxygen

together with a pair of bridging phenoxy oxygen atoms

atoms coming

from the tetradentate N2O2 ligands. Terminal Co(2) ions in this complex are six coordinated using a doubly deprotonated tetradentate ligand (L)2-, providing O(1), O(2), N(1) and N(2) donor sites and two methoxo O(3) and O(4) donor sites. Distance between two terminal cobalt atom is 5.195 Å while distance between one terminal and middle cobalt atom is 2.602 Å. The bond distances Co– O (1.849(4) – 1.970(4) Å) and Co – N (1.971(5) – 1.974(5) Å) are all in the expected ranges [19].

The trimeric units are further connected by supramolecular C-H···π interactions to form 1D supramolecular chain structure along crystallographic a-axis (Fig. 2). π- interaction parameters of complex 1 are given in Table 3. Single crystal X-ray diffraction analysis reveals that complex 2 crystallizes monoclinic system with centro-symmetric space group C2/c. The ORTEP diagram is presented in Fig.3. Its important inter atomic parameters are listed in Table 4. Complex 2 is monomer and the asymmetric unit contains one Co3+ ion, one doubly deprotonated ligand (L2-) and one acetylacetonate (acac) ligand. The CoIII center shows six-coordinated distorted octahedral geometry. Octahedral complex derived from salen type tetradentate ligand with N2O2 donor site normally shows two different arrangements of four donor atoms around the metal ion. Firstly, all donor atoms are situated in the four equatorial positions. In the second case, one oxygen atom is removed from the equatorial position and occupy in the axial position [20 -21]. In our complex, two imine nitrogen atoms of the deprotonated ligand L2- and two oxygen atoms of acac occupy the four equatorial sites and two phenoxide oxygen atoms occupy the two axial sites of the metal ion. The bond distances of Co–N (2.015(5) Å) and Co–O (1.886 (4) – 1.933(3) Å) are all in the expected ranges [19]. Complex 2 packed by using Van der Waals interaction to form 3D supramolecular structure.

Table 2 Selected bond distance (Å) and angles (deg) for complex 1. Bond Distance (Å) Co1 - O4 1.889(5) Co1 - O4 1.889(5) Co1 - O3 1.896(4) Co1 - O3 1.896(4) Co1 - O2 1.970(4) Co1 - O2 1.970(4) Co1 - Co2 2.6020(11) Co1 - Co2 2.6021(11) Co2 - O1 1.849(4) Co2 - O4 1.911(4) Co2 - O2 1.927(4) Co2 - O3 1.935(4)

Bond Angles (deg) O4 - Co1 - O4 O4 - Co1- O3 O4 - Co1 - O3 O4 - Co1- O3 O4 - Co1 - O3 O3- Co1 - O3 O4- Co1 - O2 O3- Co1- O2 O4 - Co1- O2 O4- Co1- O2 O3- Co1- O2 O3- Co1- O2

100.0(3) 97.89(19) 83.44(19) 83.44(19) 97.90(19) 177.9(3) 174.53(18) 100.42(17) 174.53(18) 75.76(17) 78.35(17) 100.42(17)

Co2 - N2 1.971(5) Co2 - N1 1.974(5)

O2O4O3 O2O2O4O4 O3O3O2O2Co2O1 O1 O4 O1O4 O2 O1 O4O2O3 O1O4 O2 O3N2 -

Co1Co1 Co1Co1Co1Co1Co1Co1Co1Co1Co1Co1Co2Co2Co2Co2Co2 Co2Co2Co2Co2 Co2Co2Co2 Co2Co2Co2-

O2 Co2 Co2 Co2 Co2 Co2 Co2 Co2 Co2 Co2 Co2 Co2 O4 O2 O2 O3 O3 O3 N2 N2 N2 N2 N1 N1 N1 N1 N1

Table 3 C-H···π interaction parameters of complex 1 C-H···Ri H···Cg/(Å) C2-> C3-> C4-> C5-> C6->

108.7(3) 47.13(13) 47.85(12) 47.40(11) 138.26(12) 127.15(14) 47.13(13) 47.85(12) 132.31(12) 138.26(12) 47.40(11) 173.18(6) 94.2(2) 167.20(19) 76.29(17) 91.79(19) 81.84(19) 78.48(18) 92.5(2) 172.4(2) 96.5(2) 94.3(2) 93.6(2) 94.9(2) 95.8(2) 173.93(18) 88.3(2)

Symmetry 1+x, y, z

Table 4 Selected bond distance(Å) and angles(deg) for complex 2. Bond Distance (Å) Co1 Co1 Co1 Co1 Co1 Co1 -

O1 1.886(4) O1 1.886(4) O3 1.933(3) O3 1.933(3) N1 2.015(5) N1 2.015(5)

Bond Angles (deg) O1- Co1 -O1 94.6(2) O1- Co1- O3 92.16(16) O1- Co1- O3 86.27(17) O1 -Co1 -O3 86.27(17) O1 - Co1 -O3 92.16(16) O3- Co1- O3 177.7(3) O1- Co1- N1 175.12(18) O1-Co1- N1 88.52(19) O3 -Co1- N1 91.81(17) O3- Co1- N1 89.84(17) O1 -Co1- N1 88.52(19) O1- Co1- N1 175.12(18) O3 -Co1- N1 89.84(17) O3- Co1- N1 91.81(17) N1-Co1- N1 88.7(3)

Fig.1. ORTEP drawing of the cation in [CoIII3(OMe)4L2]ClO4 (1) showing the atomic numbering scheme with 20% ellipsoid probability. Only coordinated atoms are numbered. The perchlorate anion and hydrogen atoms have been omitted for clarity.

Fig. 2. Supramolecular 1D chain formation through C-H···π interactions

Fig. 3.

ORTEP diagram of complex 2 with 20% ellipsoid probability (Symmetry* = -x, y, 3/2-

z). We have labeled only the coordinated donor atoms for clarity.

3.3. Mass Spectroscopy ESI–mass spectral data (in the positive ion mode) for the complexes 1 and 2 are listed in the experimental section. Complex 1 displays respective molecular ion peak at around m/z = 994 due to the [M + H2O –ClO4- ]+ ionic species whereas complex 2 shows a parent ion peak at around m/z = 535, corresponding to the ionic entity [M + Na+]. The isotope distribution pattern for the base peak of 2 is depicted in Fig.4. as a representative example together with its simulation pattern.

(A)

(B)

Fig. 4. (A) Dominant [M+Na+] peak cluster in the electrospray ionization mass spectrum of 2 (B) Comparison of the simulated isotope pattern for [M+Na+] of 2.

3.4.

Catecholase activity Catecholase activity of the complexes 1-2

were studied using 3,5-di-tert-butyl catechol

(3,5-DTBC) as a model substrate in methanol at room temperature in presence of air. 3,5-di-tertbutylcatechol (3,5 - DTBC) has been utilized as the substrate as it can be oxidised to 3,5-di-tertbutyl benzoquinone (3,5 - DTBQ) at ambient condition. The presence of Di-tertiary butyl groups attached with the catechol moeity is responsible for the oxidation of 3,5 Di-tertiary butyl catechol (3,5 – DTBC) to the corresponding 3,5 Di-tertiary butyl quinone (3,5 – DTBQ) easily. Inclusion of t-butyl group in the catechol ring prevent further oxidation reaction which leads to ring opening. To monitor the reaction, 10-4 M methanolic solutions of 1-2 were treated with 100 equivalent of 3,5- DTBC. After mixing 3,5 - DTBC into the complexes 1–2, sequentially, the progress of the reaction was recorded by UV-Vis spectra of the mixtures at 5 min time interval. The conversion of 3,5- DTBC to 3,5-DTBQ (Quinone band maxima) was watched with time at a wave length of 400 nm in methanol as shown in Fig. 5 and 6 respectively for complex 1 and 2. The rate constant for a given complex–substrate concentration ratio was obtained by change in absorbance versus time plot by employing initial rate method. The substrate concentration dependence of the 3,5 DTBC oxidation rate was examined in presence of air, taking 1 x 10-4 M solutions of 1-2 and increasing concentration of 3,5 DTBC from.1.4 x 10-3 M to 5 x 10-3 M. The observed rate vs. [substrate] plot in methanol solution, as well as Line weaver–Burk plot, are given in Fig. 7 and 8 respectively for complex 1 and 2. By taking rate constant versus substrate concentration data and applying the Michaelise-Menten approach of enzymatic kinetics, we get the Lineweaver Burk plot as well as the values of the parameters Vmax, KM and Kcat. The details of different enzyme kinetic parameters are tabulated on Table 5. Table 5 Kinetic parameters for the oxidation of 3,5-DTBC catalysed by 1 and 2. Solvent

Compounds

Vmax (M s-1)

Std. error

KM (M)

Std. error

Kcat(h-1)

MeOH

1

9.23 × 10-5

4.47 × 10-5

7.81 × 10-3

5.27 × 10-3

3.32 × 103

MeOH

2

6.09 × 10-5

3.13 × 10-5

9.44 × 10-3

6.58 × 10-3

2.19 × 103

Fig. 5.

Increase in absorbance after the addition of 3, 5- DTBC to an methanolic solution of

complex 1 at around 400 nm

Fig.6. Increase in the quinone absorption at around 400 nm upon addition of 3,5- DTBC to an methanolic solution of complex 2.

10.0

1/Rate X 10

4

-1

0.3

Rate X 10 (Ms )

0.4

-4

0.5

0.2 0.1

8.0 6.0 4.0 2.0 0

200

0.0 0.000

0.002

0.004

0.006

400 600 800 1/[Substrate]

0.008

1000

0.010

[Substrate] (M) Fig.7. Rate vs. substrate concentration plot for complex 1 in MeOH; inset represents Lineweaver –Burk plot

0.25 0.20

18.0

-4

-1

Rate X 10 (Ms )

0.30

15.0 1/Rate X 10

4

0.15 0.10 0.05

12.0 9.0 6.0 3.0 0

200

0.00 0.000

0.002

0.004

0.006

400 600 800 1/[Substrate]

0.008

1000

0.010

0.012

[Substrate] (M) Fig.8. Plot of rate vs. [substrate] in the presence of 2 in MeOH; inset represents Lineweaver – Burk plot

Table 6 Comparison of Kcat value for the oxidation of 3,5-DTBC to 3,5-DTBQ by different cobalt complexes

Solvent

Vmax(M min-1)

KM(M)

Kcat (h-1)

Ref.

[Co3(OMe)4L2]ClO4(1)

MeOH

9.23 × 10-5

7.81 × 10-3

3.32 × 103

Present work

[CoL(acac)] (2)

MeOH

6.09 × 10-5

2.19 × 103

Present work

cis-[Co3L12(MeOH)2(N3)2(µ1,1-N3)2]

MeCN

11.8 × 10-5

1.89 × 10-3

142

19

trans-[Co3L12(H2O)2(N3)2(µ1,1-N3)2]·(H2O)2

MeCN

7.17 × 10-5

1.44 × 10-3

85

19

[Co3L22(N3)3(µ1,3-N3)]

MeCN

8.29 × 10-5

1.65 × 10-3

99

19

[CoIIIL3(NCS)(H2O)].DMF.H2O

DMF

4.68 × 10-6

1.24 × 10-3

11.2

22

[CoIIIL3(N3)2(H3O+)].2MeOH

MeCN

4.18 × 10-6

1.26 × 10-3

10.0

22

[Co(HL4)2](OAc).H2O

MeOH

3.36 × 10-5

7.38 × 10-4

1.21 × 103

23

[Co(L5--N,N,O)(LK--N,O)(NCS)]·0.5H2O

MeCN

1.54 × 10-4

7.74 × 10-3

92.66

24

[Co2(HL6)(H2O)2(OAc)2](OAc)2

MeOH

1.24 × 10-4

2.45 × 10-3

4.47 × 102

25

[Co2(L7)(H2O)2(OAc)2](OAc)

MeOH

1.28 × 10-5

1.78 × 10-3

4.59 × 101

25

[Co2(L8)(H2O)2(OAc)2](OAc)

MeOH

1.19 × 10-5

2.39 × 10-3

4.29

25

[(CoIII2(H2L9)2(OAc)2)]CH3OH

MeCN

1.39 × 10-5

8.7 × 10-3

79

26

Complex

9.44 × 10-3

H2L1= N,N′-bis(salicylidene)-1,3-propanediamine, H2L2= N,N′-bis(2- hydroxybenzyl)-1,3propanediamine),

H2L3

=

H3L4

N,N'-ethylenebis(3-ethoxysalicylaldiimine),

hydroxyethyl)-3-methoxysalicylaldimine,

HL5

=

=

N-(2-

2((2-morpholinoethylimino)methyl)-6-

ethoxyphenol, HL6 = 2,6-bis((E)-(2-(piperazin-1-yl)ethylimino)methyl)-4-methylphenol, HL7 = 2,6-bis((E)-(2-(pyridin-2-yl)ethylimino)methyl)-4-methylphenol,

HL8

=

2,6-bis((E)-(2-

H2L9

(piperidin-1-yl)ethylimino)methyl)-4-methylphenol,

=

2-((E)-(1,3-dihydroxy-2-

(hydroxymethyl)propylimino)methyl)-4,6-di-tert-butylphenol, Table 6 depicts the catalytic activities of some mononuclear as well as some multi nuclear cobalt complexes. From this table we see that our cobalt complexes show very high TON (Kcat) for the catalytic oxidation of 3,5 DTBC to 3,5 DTBQ by comparing

with other reported cobalt

complexes [19, 22-26]. Coordination of catechol moiety to the metal center during the catalytic cycle by any possible way is an important criterion to show catecholase activity. We have investigated the possible complex-substrate intermediate in catalytic cycle by ESI –MS spectroscopy. We have recorded the ESI-MS spectrum of 1: 100 mixture of complex 1 and 3,5 DTBC in methanol after 15 minute of mixing. Complex 1 exhibits some characteristics peak at m/z = 243, 357, 463, 473 and 693 (Fig. S1). The peak around m/z = 243 can be assigned to a sodium aggregate of quinone [3,5 DTBC (Na)]+. The signal at m/z = 357 is due to the formation of the protonated ligand [(H2L)H]+. Another a peak at m/z = 463 is probably due to the 2:1 quinone sodium aggregate [(3,5- DTBC)2Na]+. The peak around m/z = 473 is due to the formation of a sodium aggregate of the intermediate species [CoL(OH)(H2O)Na]+.

Formation of a sodium aggregate of the

intermediate species [Co(L)(DTBC)(H2O)Na]+ is identified by a peak at m/z = 693. These results show the formation of catalyst-substrate intermediates which take part in substrate activation during the oxidation of 3, 5 DTBC to 3,5 DTBQ. We have also monitored ESI –MS spectrum of a mixture of 2 and 3, 5 DTBC in methanol to check if any complex–substrate aggregate could be identified. Unfortunately, we did not get any catalyst-substrate intermediate in ESI-MS spectrum. According to previous report, either water or hydrogen peroxide can be produced as a side product in the catalytic oxidation of catechol by model complexes [27-29]. Formation of H2O2 during catalytic oxidation procedure gives some idea about the possible mechanism for catechol oxidation. The di-oxygen of atmosphere is reduced to H2O2 during the oxidation process. To detect the formation of hydrogen peroxide during the catalytic reaction we obeyed the modified iodometric method as mentioned in reported paper [14]. Both complexes show positive result for the formation of H2O2 as the end product by monitoring the characteristic peak of 353 nm for I3ions generated by the reaction of the peroxide with potassium iodide (Fig. S2 and Fig. S3). The

estimation of H2O2 indicates that after 1 h of oxidation

nearly 80 % H2O2 is produced with

respect to the formation of 3,5 DTBQ in all cases . The results suggest that a nearly equimolar amount of hydrogen peroxide is formed with respect to 3,5- DTBQ. Such incident clearly suggests that in the catalytic cycle two electron reduction of molecular oxygen is occurred. We may assume that reaction proceeds through the formation of a Co(II) – semiquinonate radical intermediate

by comparing the mechanism of reported paper [19]. We have suggested a

plausible mechanishm for the aerobic oxidation of 3,5 DTBC to 3,5 DTBQ catalysed by complex 1 is depicted in the scheme 2. Here we believe that the initial step of the catalytic cycle involves the dissociation of complex 1 to produce active species 1a.

In the next step of the

catalytic cycle, active species 1a binds with the substrate molecule to form species 1b. Then the reaction proceeds through the formation of a Co(II)- semiquinonate radical intermediate 1c. The catalytic cycle is completed by the reaction of CoII - semiquinone species with di-oxygen leading to the re-oxidation of CoII and the release of the quinone molecule, with hydrogen peroxide as a by-product [19, 29]. We have investigated the probable complex-substrate aggregate through ESI –MS spectroscopy as mentioned earlier. The proposed mechanistic pathway matches well with the ESI-MS data. In case of complex 2, we did not get a similar peak as obtained in complex 1 in ESI – MS data due to the formation of catalyst-substrate intermediate which take part in the substrate activation during the oxidation of 3,5 – DTBC to 3,5 –DTBQ. However, H2O2 was produced as a side product in the catalytic oxidation of 3,5 DTBC to 3,5 DTBQ by complex 2 also. We may assume that in case complex 2, catalytic cycle also proceed similar way as expected for complex 1.

+

N

O

O

III Co N

O III Co

O III Co

O

O O

O

N

N

1 + OH O

N

Solvent

N

OH

III Co

O III Co

Solvent

N

O

N

O

Solvent + H+

Solvent HO

O

1b

1a

O O

+ H2O2

H+ O2 + Solvent + 2H+

N

O II Co

Solvent O O

N O

1c

Scheme 2. A plausible mechanistic pathways for the aerobic catechol oxidation catalyzed by complex 1.

4. Conclusion Sterically constrained tetradentate phenol based ligand H2L has been used here to synthesize mono- and tri- nuclear cobalt (III) complexes.

Resulting

spectroscopic diffractometer.

complexes

tools

as

well

are as

characterized single

by

crystal

various X-ray

Complex 1 is tri-nuclear methoxo and phenoxo bridged cobalt

complex with CoIII – CoIII – CoIII linear core, whereas 2 is mononuclear mixed ligand octahedral Co(III) complex supported by acetylacetonate ion as a co ligand. Recently, few tri-nuclear cobalt complexes containing phenoxo bridge [30], carboxylate bridge [31], azido bridge [ 19, 32], combination of both phenoxo and carboxylate bridges [33] have been reported. A rare example has been provided here of a linear tri-nuclear cobalt complex with Co(III) –Co(III)Co(III) core connected together by bridging methoxo and phenoxo moieties. Both 1 and 2 possess excellent catalytic activity towards oxidation of 3, 5-di-tertbutyl catechol to its corresponding quinone. The kinetic parameters have been determined using Michaelis–Menten approach. Each complexes show very high turnover numbers (3.32 × 103 h-1 for 1 and 2.19 × 103 h-1 for 2) in this catechol oxidation reaction. Mechanistic investigations of these complexes by

ESI-MS spectra and estimation of hydrogen peroxide formation indicate that oxidation reaction proceeds through participation of metal centre and Co(III/II) redox process is responsible for the oxidation of 3,5 di-tertiary butyl catechol to 3,5 di-tertiary butyl quinone. Appendix A. Supplementary data CCDC 1903233 and CCDC 1966105 contain the supplementary crystallographic data for complex 1 and 2 respectively. 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 e-mail: deposit@ ccdc.cam.ac.uk. Acknowledgment Financial support received from Department of Science and Technology, Government of West Bengal, Kolkata (Project memo no. 1071(Sanc)/ST/P/S&T/15G-4/2015 dated 23.02.2016)

is gratefully acknowledged. We are grateful for the instrumental support from the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India.

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The manuscript is original, not published and is not being consider publication for elsewhere. All the co-authors have significant contribution in preparing this manuscript.

Declaration of interest statement I hereby declare on behalf of myself and my co-author that The article submitted is an original work of our and has neither been published in any other peer reviewed journal nor is under consideration for publication by any other journal

Highlights

   

Mono and tri- nuclear Co(III) complexes, [Co3(OMe)4L2]ClO4 (1) and [CoL(acac)] (2) have been synthesized Nuclearity of these complexes depends upon the nature of the metal salt used. The structure of the complexes was characterized by single crystal X-ray crystallography and others spectroscopic techniques. Catechol oxidase activity of the complexes was observed.