Synthesis and structure of a cobalt(III) complex containing pendant Schiff base ligand: Exploration of its catechol oxidase and phenoxazinone synthase like activity

Accepted Manuscript Research paper Synthesis and structure of a cobalt(III) complex containing pendant Schiff base ligand: Exploration of its catechol oxidase and phenoxazinone synthase like activity Kousik Ghosh, Michael G.B. Drew, Shouvik Chattopadhyay PII: DOI: Reference:

S0020-1693(17)31935-7 https://doi.org/10.1016/j.ica.2018.05.025 ICA 18276

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

27 December 2017 14 May 2018 19 May 2018

Please cite this article as: K. Ghosh, M.G.B. Drew, S. Chattopadhyay, Synthesis and structure of a cobalt(III) complex containing pendant Schiff base ligand: Exploration of its catechol oxidase and phenoxazinone synthase like activity, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.05.025

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Synthesis and structure of a cobalt(III) complex containing pendant Schiff base ligand: Exploration of its catechol oxidase and phenoxazinone synthase like activity Kousik Ghosh,[a] Michael G. B. Drew,[b] Shouvik Chattopadhyay*[a] [a]

Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India

e-mail: [email protected] Tel. +91-33- 2414-2941 [b]

School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6

6AD, UK. E-mail: [email protected]

Abstract A new cobalt(III) complex, [Co(L--N,N,O)(L--N,O)(NCS)]·0.5H2O, with a pendant Schiff base {HL =2((2-morpholinoethylimino)methyl)-6-ethoxyphenol} has been synthesized and characterized by elemental and several spectral analyses. Single crystal X-ray diffraction studies confirmed its structure. Extended supra-molecular assemblies were generated in the complex through weak noncovalent interactions. The complex was found to exhibit catechol oxidase and phenoxazinone synthase mimicking activity. Keywords: Cobalt(III); Pendant Schiff base; Crystal structure; catechol oxidase mimicking activity; Phenoxazinone synthase mimicking activity.

1. Introduction

The efficiency and selectivity of copper containing metalloenzymes in processing molecular dioxygen have lead synthetic inorganic chemists and coordination chemists to investigate the structural and catalytic properties of these enzymes [1-7]. Two such important copper containing metalloenzymes are catechol oxidase and phenoxazinone synthase, both of which have important roles in the bio-degradation of aromatic molecules [8-10]. Catechol oxidase catalyzes the oxidation of catechols into o-quinones, whereas phenoxazinone synthase catalyzes the oxidative coupling of o-aminophenol derivatives to form the 2aminophenoxazinone chromophore, with concomitant reduction of molecular dioxygen to water in each case [11-13]. To investigate the ability of transition metal complexes to mimic the activity of these enzymes is an active area of research at the interface of chemistry and biology [14-18]. It is to be noted here that many transition metal complexes have already been synthesized and their ability to be used as functional models of these metalloenzymes have been examined extensively [19-21]. We prepared a series of mononuclear cobalt(III) Schiff base complexes with pseudohalide co-ligand and explored their catalytic activity towards the oxidation of 3,5-ditertbutylcatechol (DTBC) and o-aminophenol (OAPH) {catechol oxidase and phenoxazinone synthase mimicking activities} [22,23]. It was observed that bis-(Schiff base ligand) complexes were inactive towards the above catalysis. In the present work, our primary aim is to develop a new bis-(Schiff base ligand) cobalt(III) complex, which may have the potential to mimic the functions of these oxidase enzymes. For this purpose, a tridentate Schiff base, HL [2((2morpholinoethylimino)methyl)-6-ethoxyphenol], has been used to prepare a new cobalt(III) complex, [Co(L--N,N,O)(L--N,O)(NCS)]·0.5H2O. The complex is attractive in the sense that one molecule of the Schiff base is exhibiting tridentate bonding mode and the other molecule of

it is showing bidentate bonding mode keeping a pendant side arm. The structure of the complex has been confirmed by single crystal X-ray crystallography. The complex exhibits catechol oxidase and phenoxazinone synthase mimicking activities.

2. Experimental Section All chemicals and solvents used in this work were commercially available and used without further purification. Cobalt(II) acetate tetrahydrate and sodium thiocyanate were purchased from Mark, India. All other chemicals and solvents were purchased from SigmaAldrich. 2.1. Preparations 2.1.1. Preparation of [Co(L--N,N,O)(L--N,O)(NCS)]·0.5H2O A methanol solution (20 ml) of 4-(2-aminoethyl)morpholine (0.13 mL, 1 mmol) and 3ethoxysalicyldehyde (166 mg, 1 mmol) was refluxed for ca. 1 h to prepare the tridentate Schiff base ligand, HL. A methanol solution (5 ml) of cobalt(II) acetate tetrahydrate (1 mmol, 249 mg) was then added to it, followed by the addition of a methanol solution (5 ml) of sodium thiocyanate (1 mmol, 81 mg) with constant stirring. The stirring was continued for additional 2 h. Dark brown single crystals, suitable for X-ray diffraction, were started to separate after ca. 7 days. Yield: 286 mg (~42%); based on cobalt(III). Anal. Calc. for C31H43CoN5O6.5S (FW = 680.69): C, 54.65; H, 6.17; N, 10.28%. Found: C, 54.58; H, 6.12; N, 10.34%. ESI-MS (positive ion mode, acetonitrile) m/z: 613.1965 [Co-(L)2]+. FT-IR (KBr, cm-1): 1599, 1624, 1646, (υC=N); 2097 (υSCN); 2862-2967 (υC-H); 3440 (υO-H). UV-Vis, max (nm), [εmax (dm3 mol-1 cm-1)]

(CH3CN), 690 (1.81 x 102), 457 (1.03 × 103), 396 (6.28 × 103), 262 (5.06 × 104). Magnetic moment: 0.086 BM. 2.2. Physical measurements Elemental analysis (carbon, hydrogen and nitrogen) was carried out on a Perkin-Elmer 240C elemental analyzer. Infrared spectrum was measured with a PerkinElmer Spectrum Two FTIR spectrophotometer in the range of 4500-500 cm-1. Electronic spectra in acetonitrile medium (900-200 nm) was recorded in a PerkinElmer Lambda 35 UV-Vis spectrophotometer. The powder XRD data was collected on a Bruker D8 Advance X-ray diffractometer using copper Kα radiation (λ = 1.548 Å) generated at 40 kV and 40 mA. 1-D Lynxeye detector (5º < 2θ > 50º) was used at ambient conditions. The magnetic susceptibility measurement was performed with a magnetic susceptibility balance, made by Sherwood Scientific, Cambridge, UK at room temperature (300 K). The corrected magnetic susceptibility, m, is calculated using the relation: m = meas - D. Diamagnetic susceptibilities, D were calculated using Pascal's constants [24]. Effective magnetic moments were calculated using the formula, eff = 2.828(χm T)1/2, where χm was the corrected molar susceptibility. The instrument was calibrated with metallic nickel. Electrospray ionization mass spectra were recorded using Waters QTOF Micro YA263 mass analyzer. Electron paramagnetic resonance (EPR) spectra were recorded in standard quartz EPR tubes using JEOL JES-FA200 X-band spectrometer. 2.3. X-ray crystallography and Hirshfeld Surface Analysis X-ray diffraction intensities of a suitable single crystal were picked and mounted on glass fibre, and diffraction intensities were measured at 150 K with an Oxford Diffraction X Calibur diffractometer equipped with molybdenum Kα radiation (λ = 0.71073 Å), generated at 50 kV, 40

mA. Crysalis software was used for data collection and reduction [25]. The structure was solved by direct method and refined by full-matrix least squares on F2 using the SHELXL-2016/6 package [26,27]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on their parent atoms. The programs used included PLATON [28], DIAMOND [29], ORTEP [30] and MERCURY [31]. A lattice water molecule present in the unit cell has been refined with occupancy 0.5. Hydrogen atoms bonded to water oxygen atom could not be located and was not included into the refinement. Details of crystallographic and refinement data are given in Table 1. Crystal Explorer [32] was used to calculate Hirshfeld surfaces [33,34] and associated 2D fingerprint plots [35-37]. The more details of Hirshfeld surface analysis could be found in supplementary information part. 2.4. Catalytic activity and kinetic study The catechol oxidase and phenoxazinone synthase mimicking activity of the complex are monitored spectrophotometrically. The more experimental details of catalytic activity and kinetic study are given in supplementary information part.

3. Results and discussion 3.1. Synthesis The tridentate Schiff base ligand, HL, was prepared by facile condensation of 4-(2aminoethyl)morpholine with 3-ethoxysalicyldehyde in methanol following the literature method [38-41]. The ligand was not isolated and the yellow coloured methanol solution of the ligand was then reacted with a methanol solution of cobalt(II) acetate tetrahydrate. Addition of sodium thiocyanate into the solution produced a mononuclear Schiff base complex, [Co(L--N,N,O)(L-

-N,O)(NCS)]·0.5H2O. Cobalt(II) was converted into cobalt(III) by aerial oxidation in presence of (strong field) Schiff base ligand, as observed in many previous cases [42-44]. Use of anaerobic condition prevents the formation of this complex. The formation of the complex is shown in Scheme 1.

Scheme 1: Synthetic route to the complex. 3.2. Crystal structure description and Hirshfeld surface analysis Single crystal X-ray analysis reveals that the cobalt(III) Schiff base complex has 2:1 ligand to metal stoichiometry. The complex crystallizes in triclinic system with P space group. A perspective view of the complex together with the selective atom numbering scheme is shown in Figure 1. Structure of the complex consists of a hexa-coordinated cobalt(III) in a distorted octahedral geometry. The central cobalt(III) is coordinated by one amine nitrogen atom, N(22), two imine nitrogen atoms, N(19) and N(39), and two phenoxo oxygen atoms, O(11) and O(31), from two deprotonated pendent Schiff base ligand, (L-). The thiocyanate nitrogen atom, N(1), occupied the sixth coordination site of cobalt(III) to complete its distorted octahedral geometry. The equatorial plane of the octahedron is constructed by amine nitrogen atom, N(22), imine nitrogen atoms, N(19) and N(39), and phenoxo oxygen atom, O(11). Whereas phenoxo oxygen atoms, O(31) and the thiocyanate nitrogen atom, N(1) coordinate axially. One interesting observation is that some of the equatorial bond lengths are larger than that of axial bond lengths.

This happens possibly due to the steric congestion around the metal center. The most attractive structural feature of the complex is bidentate binding mode of a tridentate Schiff base. This makes the Schiff base to remain as pendant. Relevant bond lengths and angles are summarized in Tables 2 and 3, respectively. The bond angles deviate from the ideal values of 90 for the cis angles and 180 for the trans angles, which clearly indicate the distortion from octahedral geometry. The Co(III)-Namine bond length is slightly longer than the Co(III)-Nimine bond length, as were also observed in similar previously reported complexes [45,46]. The Co(III)-Nimine bond length in tridentate bonding Schiff base is somewhat shorter than Co-Nimine bond length in bidentate bonding Schiff base. The saturated five-membered chelate ring, [Co(1)-N(19)-C(20)C(21)-N(22)], has an envelope conformation with puckering parameters [47,48] Q = 0.437(5) Å and ϕ = 278.50(5)°. The N(19)-Co(1)-N(22) angle is 86.39(16)º and is characteristic of five membered chelate rings [49,50]. A comparative study of geometrical parameters of investigated complex with some similar type mononuclear cobalt(III) complexes has also been done. It is evident from the study that as the steric congestion increases around the metal center, the saturated chelate rings prefer envelope conformation. All these complexes are gathered in Table 4.

Figure 1: Perspective view of the complex with selective atom numbering scheme. The hydrogen atoms are omitted for clarity. The complex shows significant C-H interactions. The hydrogen atoms, H(13B) and H(14), attached to ethoxy carbon atom, C(132), and phenyl carbon atom, C(14), are participated in inter-molecular C-H interaction with the symmetry related (2-x,2-y,2-z) phenyl rings, Cg6 [C(12)-C(13)-C(14)-C(15)-C(16)-C(17)] and Cg7 [C(32)-C(33)-C(34)-C(35)-C(36)-C(37)], respectively. Other hydrogen atom, H(33D), attached to the carbon atom, C(333), is involved in another inter-molecular C-H interaction with the symmetry related (x,-1+y,z) phenyl ring, Cg6 [C(12)-C(13)-C(14)-C(15)-C(16)-C(17)]. Because of these types of inter-molecular C-H interactions, a 1D array has been formed in crystal packing of the complex (Figure 2). Geometric features of the C-H interactions are given in Table 5.

Figure 2: C-H···π interactions in the complex. Only relevant hydrogen atoms are shown for clarity. The nature of intermolecular interactions and packing modes of complex in the crystalline state may be examined by Hirshfeld surface analysis, which provides a visual picture of intermolecular interactions and of molecular shapes in crystalline environment. The Hirshfeld surface of the complex is exemplified in Figure 3, by showing dnorm, shape-index and curvedness surface. To visualize the molecular moiety, transparent mapped surfaces are shown. The dominant interactions are between oxygen and hydrogen and between sulphur and hydrogen. Red spots on the dnorm surface indicate these interactions (Figure 3). Additionally, the 2D fingerprint plots (Figure 4) illustrate the difference between the intermolecular interaction patterns and the relative contributions (in percentage) [64-66] for the major intermolecular interactions associated with the complex. In the 2D fingerprint plot (Figure 4) intermolecular interactions appear as distinct spikes.

Figure 3: Hirshfeld surfaces mapped with dnorm (left-side), shape index (middle) and curvedness (right-side).

Figure 4: Fingerprint plot: Full and resolved into H···H, C⋯H / H⋯C, N···H / H···N, O···H / H···O and S···H / H···S contact contributed to the total Hirshfeld Surface area of the complex.

3.3. IR and electronic spectroscopy

The IR and electronic spectra of the complex are in well agreement with its crystal structure. The condensation of the free amine group is conformed from the IR spectrum of the complex as no band due to υN-H was observed. The band corresponding to azomethine (C=N) group is distinct and occur at 1624 cm-1 [51-53]. The appearance of a strong band at 2097 cm-1 indicates the presence of N-coordinated thiocyanate in the complex [54,55]. The bands in the range of 2925-2940 cm-1 due to alkyl C-H stretching vibrations are routinely noticed [56,57]. The UV-Vis absorption spectrum of the complex in acetonitrile shows two d-d transition bands at 690 and 457 nm, as expected for a low spin cobalt(III) complex in octahedral geometry [58,59]. The intense absorption bands below 400 nm may be assigned to ligand to metal charge transfer bands (LMCT) and intra-ligand -* / n-* transitions [60-62]. IR, UV and electrospray ionization mass spectra (ESI-MS positive) of the complex are given in Supplementary information. The complex is diamagnetic as expected for low-spin cobalt(III) complex [63]. 3.4. X-ray diffraction of powdered material The purity of the bulk material can easily be cross-checked by comparing the patterns of experimental and simulated X-ray diffraction of powdered material. In our case, the experimental powder X-ray diffraction and simulated powder XRD patterns from single crystal X-ray diffraction are in excellent concurrence. The simulated pattern was calculated from the single crystal structural data (cif file) using the CCDC MERCURY software. Experimental and simulated powder XRD patterns of the complex are given in Figure S6 (Supplementary information).

3.5. Catalytic oxidation of 3,5-DTBC and OAPH (Catechol oxidase and phenoxazinone synthase mimicking activity) Catalytic properties of the complex for the oxidation of 3,5-DTBC to 3,5-DTBQ and OAPH to 2-aminophenoxazine-3-one with molecular dioxygen as the oxidant at room temperature were explored. The key focus of this experimental measurement was to check catechol oxidase and phenoxazinone synthase mimicking activity of the complex. The catalytic activities were monitored in acetonitrile medium because the complex, substrates and their products are highly soluble in acetonitrile. The catalytic reactions were carried out in absence of any supplementary base to minimize the possibility of oxidation of substrates by molecular dioxygen. The progress of the reactions was followed spectrophotometrically. Spectrophotometric scans revealed a gradual increase in intensity of the absorption band at ~ 400 nm (characteristic for 3,5-DTBQ) or ~ 433 nm (characteristic for phenoxazinone chromophore) for catechol oxidase or phenoxazinone synthase like activities respectively. The resulting solutions were also monitored spectrophotometrically after 24 hour which show the formation of 3,5-DTBQ or 2-aminophenoxazine-3-one respectively as the sole product. These results clearly indicate that the complex is active towards both catechol oxidase and phenoxazinone synthase mimicking activities. Blank experiments without catalyst for the oxidation of 3,5-DTBC and OAPH were also performed under identical conditions and in each case, any significant growth of the absorption bands around 400 or 433 nm could not be observed. The molar extinction coefficient value of 3,5-DTBQ and 2-aminophenoxazine-3-one are 8.630x103 and 1.093x104, respectively at 400 nm and at 433 nm in acetonitrile medium. The time dependent spectral growth for catalytic oxidations of 3,5-DTBC and OAPH are shown in Figure 6.The detail kinetic studies were performed to understand the extent of the catalytic efficiency.

Figure 6: The time dependent spectral growth of 3,5-DTBQ (a) and 2-aminophenoxazine-3-one (b), upon addition of 10-2 M substrate to 10-4 M of complex. The time interval between each reading of the catalytic cycles is 5 min. 3.6. Kinetic investigation through several enzyme kinetic plots Biochemical reactions involving a single substrate are often assumed to follow Michaelis-Menten kinetics, which is an important tool for treating this type of saturation rate dependency on substrate concentration. In this model, following equation (eq.1) is used to relate the rate of enzymatic reactions, V to the concentration of substrate, S.

V=

∙∙∙(1)

Here, Vmax represents the maximum rate achieved by the system, at saturating substrate concentration. KM is the Michaelis constant and it is equal to the substrate concentration at which the reaction rate is half of Vmax. Linearization of Michaelis-Menten equation produces a double reciprocal LineweaverBurk plot, which is used to analyze a variety of kinetic parameters, e.g. Vmax and KM. The Lineweaver-Burk Equation may be represented by equation 2.

+

∙∙∙(2)

On the other hand, rearrangement of Michaelis-Menten equation gives Hanes equation (eq. 3). In this case, the ratio of the substrate concentration (S) to the reaction rate (V) is plotted against substrate concentration (S).

=

+

∙∙∙(3)

Eadie-Hofstee equation (eq. 4) is sometimes used in biochemistry for a graphical representation of enzyme kinetics. Here reaction rate (V) is plotted as a function of the ratio between reaction rate (V) and substrate concentration (S):

−

∙∙∙(4)

All these enzyme kinetic plots are used to evaluate several kinetic parameters, including turnover number (kcat) and specificity constant (kcat/KM) for both catechol oxidase and phenoxazinone synthase mimicking activity of the complex. In enzyme kinetics, turnover number (also termed as kcat) is defined as the maximum number of catalytic conversions of substrate molecules per unit time that a single catalytic site will execute for a specific enzyme concentration. The specificity constant (also termed as kcat/KM ratio) is a measure of how efficiently a catalyst converts substrates into products. This ratio is a useful index for measuring the substrate specificity of catalyst. The higher the specificity constant, the more the enzyme prefers that substrate. Figures 7 and 8 represent the Michaelis-Menten plot, Lineweaver-Burk plot, Hanes-Woolf plot and Eadie-Hofstee plot for catalytic oxidation of 3,5-DTBC and OAPH. Tables 6 and 7 contain kinetic parameters for catechol oxidase and phenoxazinone synthase mimicking activities, respectively.

Figure 7: Michaelis-Menten plot (a), Lineweaver-Burk plot (b), Hanes-Woolf plot (c) and Eadie-Hofstee plot (d) for catalytic oxidation of 3,5-DTBC.

Figure 8: Michaelis-Menten plot (a), Lineweaver-Burk plot (b), Hanes-Woolf plot (c) and Eadie-Hofstee plot (d) for catalytic oxidation of OAPH.

3.7. Origin and mechanistic pathway of catalytic activity At this stage, it is very clear that the complex is reactive towards the aerial oxidation of 3,5-DTBC and OAPH. So as to confirm the involvement of molecular dioxygen in the catalytic oxidation of 3,5-DTBC and OAPH, electronic spectra of a mixture of a 10-4 M solution of the complex with large excess of substrates (3,5-DTBC or OAPH) were recorded under anaerobic atmosphere and no significant spectral growths for 3,5-DTBQ or 2-aminophenoxazine-3-one

were observed up to 24 h of mixing. Based on the above results, tentative catalytic cycles for the formation of the quinone and phenoxazinone chromophores are shown in Scheme 2 and 3, respectively.

Scheme 2: Plausible mechanistic pathway for the aerial oxidation of 3,5-DTBC.

Scheme 3: Proposed mechanistic pathway for the aerial oxidation of o-aminophenol.

Several factors may affect the catalytic activity, such as the variable oxidation state of the metal, coordination geometry around the metal center, metal-metal distance, ligand flexibility, exogenous bridging ligand etc. In several previous reports where mechanistic considerations of these types of catalytic oxidation are available, it is established that the oxidation reaction proceeds through involvement of molecular dioxygen [67-69]. At initial step, the substrate is

coordinated with the complex, by replacing monodentate thiocyanate co-ligand. This complex substrate intermediate degrades in the rate determining step by the production of substrate radical, which subsequently yields respective quinone or phenoxazinone chromophore in presence of molecular dioxygen via several oxidative dehydrogenation processes, as shown in Scheme 2 and 3. In our previous reports, we have also observed similar mechanistic pathway for the catalytic oxidations of 3,5-DTBC and OAPH [22,23,70]. The electrospray ionization mass spectra (ESI-MS positive) of 1 : 50 mixture of the complex and substrate (3,5-DTBC for catechol oxidase mimicking activity and OAPH for phenoxazinone synthase mimicking activity) were recorded after 10 minutes of mixing in acetonitrile medium in order to get an insight into the possible mechanistic pathways. ESI-MS positive spectra of catalytic oxidation of 3,5-DTBC and OAPH are given as Supplementary Figures S4 and S5, respectively. For catalytic oxidation of 3,5-DTBC, the mass spectrum exhibits a peak at m/z = 243.1187 corresponding to 3,5-DTBQ-sodium aggregate [3,5-DTBQNa]+, confirming the catechol oxidase mimicking activity of the complex. Similarly, for catalytic oxidation of OAPH, the mass spectrum exhibits a peak at m/z = 279.2084 which can be assigned to the sodium adduct of 2-aminophenoxazine-3-one, [2-aminophenoxazine-3-one-Na-CH3CN]+. This peak clearly confirms the phenoxazinone synthase mimicking activity of the complex. Peak observed at m/z ~ 613 arises due to the loss of monodentate thiocyanate co-ligand [Co-(L)2]+, which is the initial step of catalysis. This peak is also observed in the mass spectral analysis of the pure complex (given in Supplementary information, Figure S3). So it is clear that monodentate thiocyanate co-ligand can easily be replaced. The mechanistic pathways shown in Schemes 2 and 3 are therefore justified by the mass spectral analysis.

Electron paramagnetic resonance spectroscopy is a useful tool to monitor the generation of substrate radical during the catalytic cycles. To confirmed the formation of substrate radicals, EPR measurements have been performed on the reaction mixture of the complex and 3,5-DTBC and o-aminophenol separately. Mixture solutions (complex and substrate) shows EPR signal around 1.998 (g value) with a peak-to-peak line width of ca. 3400 G. The g value of the signal is closed to 2.0023 (value for a free electron), [71] this value is characteristic for organic radicals, such as a phenoxyl-type radical [72, 73]. Therefore it can be concluded that the catalytic cycles passed through a radical pathway, as shown in the Schemes 2 and 3.

Figure 9: EPR spectrum m obtained from the acetonitrile solution of the complex after addition of 3,5-DTBC, recorded at 77K.

A few similar type mononuclear cobalt(III) complexes showing catechol oxidase and phenoxazinone synthase mimicking activity already exist in literature. These previously reported

complexes exhibiting catechol oxidase and phenoxazinone synthase mimicking activity are gathered in Tables 8 and 9, respectively. As the turnover number (kcat) value of investigated complex lies in between the previously reported complexes turnover number value, it can be considered as moderate catalyst.

4. Conclusion In conclusion, a cobalt(III) complex with a pendant Schiff base ligand has been synthesized and characterized by elemental and several spectral analyses. Crystal structure determination shows that a tridentate Schiff base is acting as a bidentate ligand, keeping the remaining donor site pendant. Furthermore, the solid state structure of the complex shows participation of the organic ligand in several supramolecular interactions involving ethoxy arms and aromatic rings. The complex is found to be active towards catechol oxidase and phenoxazinone synthase mimicking activity. The mimicking activity was assessed by following conventional Michaelis-Menten enzymatic kinetics. Kinetic studies of the catalytic cycles have been performed in detail using a variety of enzyme kinetics plots to calculate different kinetic parameters, including the specificity constant.

Acknowledgement K.G. thanks UGC, India, for awarding a Senior Research Fellowship.

Appendix A. Supplementary data CCDC 1570018 contains the supplementary crystallographic data of the complex. This 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: [email protected].

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Table 1: Crystal data and refinement details of the complex. Formula

C31H43CoN5O6.5S

Formula Weight

680.69

Crystal System

Triclinic

Space group

P

a(Å)

9.6870(9)

b(Å)

10.359(1)

c(Å)

17.3597(15)

α(°)

101.621(7)

β(°)

96.025(8)

γ(°)

107.117(8)

d(calc) [g/cm3]

1.40

µ [ mm-1 ]

0.652

F(000)

716

Total Reflections

11392

Unique Reflections

8986

Observed data[I > 2 σ(I)]

6008

No of parameters

403

R(int)

0.065

R1, wR2(all data)

0.1277, 0.2572

R1, wR2([I > 2 σ(I)]

0.0932, 0.2352

Table 2: Selected bond lengths (Å) of the complex. Co(1)-N(1)

1.924(4)

Co(1)-N(19)

1.896(4)

Co(1)-N(22)

2.074(4)

Co(1)-N(39)

1.957(4)

Co(1)-O(11)

1.882(3)

Co(1)-O(31)

1.876(3)

Table 3: Selected bond angles (°) of the complex. N(22)-Co(1)-N(39)

96.63(16)

N(19)-Co(1)-N(39)

176.81(15)

N(19)-Co(1)-N(22)

86.39(16)

N(1)-Co(1)-N(39)

94.33(19)

N(1)-Co(1)-N(22)

92.65(15)

N(1)-Co(1)-N(19)

86.53(19)

O(31)-Co(1)-N(39)

94.46(17)

O(31)-Co(1)-N(22)

87.01(15)

O(31)-Co(1)-N(19)

84.66(17)

O(31)-Co(1)-N(1)

171.2(2)

O(11)-Co(1)-N(39)

82.49(14)

O(11)-Co(1)-N(22)

177.76(15)

O(11)-Co(1)-N(19)

94.46(15)

O(11)-Co(1)-N(1)

89.47(15)

O(11)-Co(1)-O(31)

91.01(14)

Table 4: Comparative study of geometrical parameters of some similar type mononuclear cobalt(III) complexes. Complexes

Saturated chelate ring

Q (Å)

θ (º)

ϕ (º)

Conformation

Ref

[Co(L1)(L2)(N3)]

Five membered

0.4282(14)

-

260.80(14)

Envelope

72

[Co(L3)(L4)(NCS)]

Six membered

0.6448(15)

6.11(13)

338.6(14)

Intermediate geometry between chair and half-chair

72

[Co(L5)(L6)(N3)]

Five membered

0.453(3)

-

113.1(3)

Envelope

42

[Co(L5)(L7)(N3)]

Five membered

0.445(3)

-

115.5(3)

Envelope

42

[Co(L8)2]ClO4

Six membered

0.697(8)

163.3(7)

170(3)

Half-chair

22

[Co(L8)(L9)(N3)]

Six membered

0.672(2)

10.6(3)

335.0(13)

Half-chair

22

[Co(L8)(L9)(NCS)]

Six membered

0.662(2)

169.25(17)

158.4(11)

Half-chair

22

[Co(L5)(L9)(N3)]

Five membered

0.460(3)

-

86.4(1)

Twisted

23

[Co(L10)(L11)(N3)]

Five membered

0.383(8)

-

63.8(10)

Envelope

23

Five membered

0.442(4)

-

258.1(4)

Envelope

[Co(L12)2]ClO4·2H2O

Five membered

0.453(4)

-

256.5(4)

Envelope

23

[Co(L--N,N,O)(L-N,O)(NCS)]·0.5H2O

Five membered

0.437(5)

-

278.50(5)

Envelope

This work

HL1 = 2-((2-(piperidin-1-yl)ethylimino)methyl)-6-ethoxyphenol, HL2 = 1-acetyl-2-naphthol, HL3 = 2-((3-(dimethylamino)propylimino)methyl)-6-methoxyphenol, HL4 = 2,4-pentanedione, HL5 = 1-((2-(diethylamino)ethylimino)methyl)naphthalene-2-ol, HL6 = 2-hydroxy-1-naphthaldehyde, HL7 = acetylacetone, HL8 = 2-(3-(dimethylamino)propyliminomethyl)-6-ethoxyphenol, HL9 = 1-

benzoylacetone, HL10 = 2((2(2-hydroxyethylamino)ethylimino)methyl)-6-ethoxyphenol, HL11 = 2-acetyl-1-naphthol, HL12 = 2((2(2-hydroxyethylamino)ethylimino)methyl)-6-methoxyphenol.

Table 5: Geometric features (distances in Å and angles in ) of the C-H···π interactions obtained for the complex. C-H···Cg(Ring)

H···Cg (Å)

C-H···Cg (°)

C···Cg (Å)

C(132)-H(13B)···Cg(6)a

2.67

143

3.493(6)

C(14)-H(14)···Cg(7)a

2.73

144

3.528(5)

C(333)-H(33D)···Cg(6)b

2.78

153

3.658(8)

Symmetry transformations: a = 2-X,2-Y,2-Z and b = x,-1+y, z; Cg(6) = Centre of gravity of the ring [C(12)-C(13)-C(14)-C(15)-C(16)-C(17)] and Cg(7) = Centre of gravity of the ring [C(32)-C(33)-C(34)-C(35)-C(36)-C(37)].

Table 6: Kinetic parameters of the complex for catechol oxidase mimicking activity using various enzyme kinetic plots at 25 ºC in acetonitrile medium. Enzyme

Vmax ± SE (M S-1)

KM ± SE (M)

kcat ± SE (S-1)

kinetic plots

kcat/KM ± SE (S-1 M-1)

Michaelis-

(2.509 ± 0.077) x

(73.700 ± 4.400)

(2.509 ± 0.077) (0.034 ± 0.002)

Menten Plot

10-6

x 10-4

x 10-2

Lineweaver-

(2.574 ± 0.540) x

(77.426 ± 2.461)

(2.574 ± 0.540) (0.033 ± 0.012)

Burk plot

10-6

x 10-4

x 10-2

Hanes-Woolf

(2.574 ± 0.322) x

(77.426 ± 7.038)

(2.574 ± 0.322) (0.033 ± 0.002)

plot

10-6

x 10-4

x 10-2

Eadie-

(2.617 ± 0.107) x

(79.700 ± 4.624)

(2.617 ± 0.107) (0.033 ± 0.001)

Hofstee plot

10-6

x 10-4

x 10-2

x 10+2

x 10+2

x 10+2

x 10+2

Table 7: Kinetic parameters of the complex for phenoxazinone synthase mimicking activity using various enzyme kinetic plots at 25 ºC in acetonitrile medium. Enzyme

Vmax ± SE (M S-1)

KM ± SE (M)

kcat ± SE (S-1)

kinetic plots

kcat/KM ± SE (S-1 M-1)

Michaelis-

(8.514 ± 0.263) x

(192.900 ± 8.300)

(8.514 ± 0.263)

(0.044 ± 0.001)

Menten Plot

10-6

x 10-4

x 10-2

x 10+2

Lineweaver-

(8.830 ± 0.569) x

(202.718 ± 3.582)

(8.830 ± 0.569)

(0.044 ± 0.004)

Burk plot

10-6

x 10-4

x 10-2

x 10+2

Hanes-Woolf

(8.830 ± 0.893) x

(208.900 ± 11.001) (8.830 ± 0.893)

(0.042 ± 0.004)

plot

10-6

x 10-4

x 10-2

x 10+2

Eadie-

(9.044 ± 0.413) x

(202.718 ± 6.142)

(9.044 ± 0.413)

(0.045 ± 0.007)

Hofstee plot

10-6

x 10-4

x 10-2

x 10+2

Table 8: A comparison of turnover number (kcat) values for catechol oxidase mimicking activity of some reported mononuclear cobalt(III) complexes. Complexes

Solvent

kcat (s-1)

References

[Co(L1)(L2)(N3)]

CH3CN

3.412 x 10-2

75

[Co(L3)(L4)(NCS)]

CH3CN

12.965 x 10-2

75

[Co(L13)2](ClO4)3

CH3OH

139.444 x 10-2

74

[Co(L14)2(Cl)2]+

CH3OH

26.806 x 10-2

76

[Co(L15)(N3)2(H3O+)]·2CH3OH

CH3CN

0.278 x 10-2

77

[Co(L15)(NCS)(H2O)]·DMF.H2O

DMF

0.311 x 10-2

77

DCM

40.556 x 10-2

CH3OH

33.611 x 10-2

CH3CN

-2

[Co(HL16)2(OAc)]·H2O

60.000 x 10

78

L13 = N1-(phenyl(pyridin-2-yl)methylene)propane-1,3-diamine, L14 = 1,10-phenanthroline, H2L15 = N,N'-ethylenebis(3-ethoxysalicylaldiimine), H2L16 = N-(2-hydroxyethyl)-3methoxysalicylaldimine.

Table 9: A comparison of turnover number (kcat) values for phenoxazinone synthase mimicking activity of some reported mononuclear cobalt(III) complexes. Complexes

Solvent

kcat (s-1)

References

[Co(L1)(L2)(N3)]

CH3CN

4.629 x 10-2

75

[Co(L3)(L4)(NCS)]

CH3CN

17.400 x 10-2

75

[Co(L5)(L7)(N3)]

CH3OH

13.867 x 10-2

42

[Co(L8)(L9)(N3)]

CH3CN

6.867 x 10-2

22

[Co(L8)(L9)(NCS)]

CH3CN

7.150 x 10-2

22

[Co(L10)(L11)(N3)]

CH3CN

2.153 x 10-2

23

[Co(L5)(L9)(N3)]

CH3CN

2.003 x 10-2

23

[Co(L17)(N3)3]

CH3OH

0.566 x 10-2

79

[Co(L18)(N3)3]

CH3OH

0.924 x 10-2

79

CH3OH

127.500 x 10-2

CH3CN

142.222 x 10-2

[Co(L19)(N3)2]·0.5CH3CN

CH3OH

1.500 x 10-2

80

[Co(L19)(NCS)2]·0.5H2O

CH3OH

1.350 x 10-2

80

13

[Co(L )2](ClO4)3

74

L17 = bis(2-pyridylmethyl)amine, L18 = (2-pyridylmethyl)(2-pyridylethyl)amine, HL19 = 2-((3(3(dimethylamino)propylamino)propylimino)methyl)-6-methoxyphenol.

Graphical abstract A new mononuclear octahedral cobalt(III) complex with pendant Schiff base has been synthesized and characterized by single crystal X-ray diffraction experiment. Solid state supramolecular interactions of the complex have been explored. The complex was found to be active towards catechol oxidase and phenoxazinone synthase mimicking activity.