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Synthesis and structural characterization of a new high-valent bis(oxo)-bridged manganese(IV) complex and its catechol oxidase activity
Inorganica Chimica Acta 465 (2017) 70–77
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Research paper
Synthesis and structural characterization of a new high-valent bis(oxo)-bridged manganese(IV) complex and its catechol oxidase activity Dhrubajyoti Mondal ⇑, Mithun Chandra Majee Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 26 January 2017 Received in revised form 2 May 2017 Accepted 30 May 2017 Available online 1 June 2017 Keywords: High valent metal-oxo Manganese(IV)-oxo Mannich reaction Catechol oxidase model
a b s t r a c t The work in this report presents the synthesis, structural and spectroscopic characterization of a new type of high-valent bis(oxo)-bridged dimanganese(IV) complex 1 supported by a pyridine and phenol based tetradentate flexible N3O donor HL ligand (where HL = 2-benzyl-6-((bis(pyridin-2-ylmethyl) amino)methyl)-4-chlorophenol). Complex 1 possess unique Mn2(l-O)2 core with short Mn Mn distances (2.65 Å) and Mn–O distance (1.80 Å). This dinuclear manganese(IV) complex show excellent catecholase-like activity for the aerobic oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone in acetonitrile at 25 °C. Kinetic measurements suggest that the rate of catechol oxidation follows saturation kinetics with respect to the substrate and first order kinetics with respect to the catalyst. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Metal–oxygen adducts (Mx-Ox, x = 1 or 2), such as metal–superoxo, –peroxo, and –oxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes [1,2]. Among these species di-l-oxo dimetal cores commonly found in metalloenzymes have attracted large attention as a structural motif to create artificial small-molecular systems. Among the M2-O2 adducts, di-l-oxo dimanganese(IV) complexes are extensively investigated as bioinorganic model in relation to the oxygen-evolving complex of manganese catalases and photosystem II [3–5]. Photosystem II (PSII), a tetrameric manganese cluster associated with Ca2+, Cl and a redox-active tyrosine is known to catalyze light-driven four-electron oxidation of water into molecular oxygen [3,4]. Recent progress in the X-ray crystallography of PSII has enabled the modeling of the OEC as a cuboidal Mn3CaO4 structure is made up of di-l-oxo dimeric Mn units [6–8]. EXAFS spectra [9,10] of the oxygen-evolving complex in a variety of oxidation states are consistent with short Mn-Mn (2.7 Å) and somewhat longer (3.3 Å) separations. For this reason synthesis of model complexes having [(l2-O)(Mn)]2 cores as dinuclear compounds is one of the important research topic because these complexes often exhibit both short Mn-Mn separations and relatively short Mn-O distances [11–21]. This type of unique di-l-oxo dimetal cores of iron and copper is also a key intermediate for distinct ⇑ Corresponding author. E-mail address: [email protected] (D. Mondal). http://dx.doi.org/10.1016/j.ica.2017.05.073 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
oxygenation reaction [22–24]. Some of the major metalloenzymes having binuclear bis(oxo)-bridged metal active site are manganese catalase (Cat), iron methane monooxygenase (MMO) [25,26], iron ribonucleotide reductase (RR) [27–29], copper tyrosinase (Tyr) and copper catechol oxidase (CO) enzyme [26,30–32]. The ubiquitous enzyme catechol oxidases (CO) [26,30–32], found in bacteria, fungi and plants, belong to the class of type 3 copper proteins and in contrast to tyrosinase catalyze the oxidation of a wide range of o-diphenols (catechols) to the corresponding oquinones coupled with 2e/2H+ reduction of O2 to H2O, without acting on monophenols. The generated highly reactive o-quinones are autopolymerized producing a brown polyphenolic pigment, i.e., melanin, a process which is considered to protect tissues against the possible damage caused by pathogens or insects. Although these two proteins exhibit different enzymatic functions, their spectroscopic properties and crystallographic structures in their active sites are similar [33–36]. The main active site of these two metalloenzymes contains a lower valent side-on peroxo-bridged dicopper(II) form which is in valence tautomeric equilibrium with high-valent bis(oxo)-bridged dicopper(III) form [37,38]. So bis(loxo)dicopper(III) core [39,40] plays a key role in the understanding the function of this class of enzymes. Several copper complexes [41] with suitable ligands have been designed to mimic this enzyme and probe its mechanism. Although the enzyme has copper at the active site, investigations have also been shown that other transition metal ions such as Mn [42–50], Co [51–54], or Ni [55–58] can also promote such activity as well.
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Generally the [MnIV(l2-O)(L)]2 complexes have been synthesized from their lower valent precursors using different types of oxidizing agent (e.g. hydrogen peroxide or tert-Butyl hydrogenperoxide). Here in this work we report the synthesis and characterization of a new pyridine and phenol based flexible N3O donor HL ligand. Then the HL ligand has been used to synthesis a new high valent [(l2-O)MnIV(L)]2(ClO4)2 complex 1 by treating Mn(ClO4)26H2O as an metal precursor and tert-butyl hydrogenperoxide as an oxidizing agent. Spectroscopic characterization and crystal structure of complex 1 have been reported in this paper. As high valent bis(l-oxo)dicopper(III) core is the main active intermediate in Catechol oxidase and Tyrosinase enzyme, here in this work we have used this high valent bis(l-oxo)dimanganese(IV) complex as a model complex to mimic the catechol oxidase activity. A detailed kinetic study on the aerobic oxidation of catechol has been also reported in this paper. 2. Experimental Section 2.1. Materials The solvents were reagent grade, dried by standard procedure [59] and distilled under nitrogen prior to their use. All other chemicals were of reagent grade, purchased from commercial sources and used without further purification. The N3O HL ligand was prepared using Mannich type condensation reaction. Although the ligand is new, several similar ligands with other phenol moieties have been reported before using different synthetic procedure [60]. While we have experienced no problems in working with perchlorate compounds, they are potentially explosive and care must be taken not to work with large quantities. 2.2. Syntheses 2.2.1. Synthesis of ligand 2-benzyl-6-((bis(pyridin-2-ylmethyl)amino) methyl)-4-chlorophenol (HL) The ligand HL was prepared by Mannich type condensation reaction of bis((pyridine-2-yl)methyl)amine (1.99 g, 10 mmol), paraformaldehyde (0.30 g, 10 mmol) and 2-benzyl-4-chlorophenol (2.18 g, 10 mmol) in methanol (30 mL). The solution was heated under reflux for 14 h and then cooled to ca. 4 °C. The resulting white compound was filtered off and recrystallized from acetonitrile solution to give a white crystalline product of HL. Yield: 1.75 g (82%). 1H NMR (Fig. S1) (CDCl3, 400 MHz, ppm) 3.76 (s, 2H), 3.86 (s, 4H), 4.025 (s, 2H), 6.917 (d, 2H), 7.149 (m, 2H), 7.24 (m, 7H), 7.594 (t, 2H), 8.563 (d, 2H). 13C NMR (CDCl3, 125 MHz, ppm): d 36.25, 56.50, 56.74, 124.35, 126.14, 128.49, 128.62, 128.71, 128.85, 129.02, 129.33, 130.43, 131.71, 140.62, 142.46, 143.57, 153.44, 155.14. Anal. Calc. for C26H24N3OCl: C, 72.63; H, 5.63; N, 9.77 Found: C, 72.10; H, 5.98; N, 9.74%. FT-IR bands (KBr pellets, cm1): 3045b, 1591s, 1475s, 1433s, 1369s, 1228s, 1000m, 972m, 865m, 757s, 702m. ESI-MS (positive) in CH3CN: m/z 430.23, 100%, (M + H)+. 2.2.2. Synthesis of complex [{MnIV(L)(l-O)}2][ClO4]22CH3CNCH3OH (1) To a stirred solution of HL (107 mg, 0.25 mmol) in methanol/ acetonitrile (1:1) was added 1 equivalents of triethylamine (25 mg, 0.25 mmol), followed by an addition of Mn(ClO4)2 6H2O (92 mg, 0.25 mmol) to get a light brown solution. It was stirred further for ca. 2 h. The resulting solution was cooled at 0 °C in an ice bath under constant stirring and 5 equivalent of tert-butyl hydrogenperoxide (5–6 M solution in decane) was added and the mixture was stirred for 1 h. The resulting dark brown solution was filtered over a celite bed; the filtrate volume was reduced to about 10 mL by rotary evaporation and then kept in a refrigerator at 4 °C.
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The product was obtained as a dark brown crystalline solid within 2–3 days. Some of these block-shaped crystals were of X-ray diffraction quality and used directly for crystal structure analysis. The compound is prone to solvent loss. Drying under vacuum for a long time afforded a fully desolvated sample, used for various measurements including microanalysis. Yield: 100 mg (30%). Anal. Calc. for C52H46N6O12Cl4Mn2: C, 52.11; H, 3.87; N, 7.01. Found: C, 52.01; H, 3.68; N, 6.99%. FT-IR bands (KBr pellets, cm1): 2923m, 1606m, 1446m, 1290m, 1224m, 1091s, 761m, 622m, 650m, 601m. UVVis (CH2Cl2) [kmax, nm (e, L mol1 cm1)] 384 (4200), 508 (2350), 577 (2550), 807 (810). l (at 295 K)/BM: 2.62. ESI-MS (positive) in CH3CN: m/z 499.04, 100%, [{MnIV(L)(l-O)}2]2+. 2.3. Physical measurements IR spectroscopic measurements were made on samples pressed into KBr pellets using a Shimadzu 8400S FT-IR spectrometer while for UV–visible spectral measurements; a PerkinElmer Lambda 950 UV/vis/NIR or an Agilent 8453 diode array spectrophotometer was employed. Elemental analyses (for C, H and N) were performed at IACS on a PerkinElmer model 2400 Series II CHNS Analyzer. The electrospray ionization mass spectra (ESIMS) in positive ion mode were measured on a Micromass QTOF model YA 263 mass spectrometer. The 1H NMR spectra were recorded on a Bruker model Avance DPX-400 spectrometer using SiMe4 as internal reference. Cyclic voltammetry (CV) in acetonitrile (ACN) was recorded on CH Instruments (CHI720D electrochemical analyzer) using a glassy carbon working electrode and a platinum wire counter electrode. Ag/AgCl was used for reference and Fc/Fc+ as the internal standard. Room temperature magnetic moment of 1 was measured by a magnetic susceptibility balance procured from Sherwood Scientific, UK. The diamagnetic correction was evaluated from Pascal’s constants. 2.4. X-ray crystallography Suitable crystals of 1 (brown block, 0.15 0.12 0.10 mm3) were mounted on glass fibers coated with perfluoropolyether oil before mounting. Intensity data for the aligned crystals were measured employing a Bruker SMART APEX II CCD diffractometer equipped with a monochromatized Mo Ka radiation (k = 0.71073 Å) source at 293(2) K. No crystal decay was observed during the data collection. In all cases, absorption corrections based on multiscans using the SADABS software [61] were applied. The structures were solved by direct methods [62] and refined on F2 by a full-matrix least-squares procedure based on all data minP P P P imizing wR = [ [w(F20 F2c )2]/ (F20)2]½, R = | |F0| |Fc| |/ |F0|, P ½ 2 2 2 and S = [ [w(F0 Fc ) ]/(n A p)] . SHELXL - 2013 was used for both structure solutions and refinements [63]. A summary of the relevant crystallographic data and the final refinement details are given in Table 1. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were calculated and isotropically fixed in the final refinement [d(CAH) = 0.95 Å, with the isotropic thermal parameter of Uiso(H) = 1.2 Uiso(C)]. The SMART and SAINT software packages [64] were used for data collection and reduction, respectively. Crystallographic diagrams were drawn using the DIAMOND software package [65]. 3. Results and discussion 3.1. Synthesis of complex (1) The mono-phenol N3O ligand (HL) has been prepared by Mannich type reaction of dipicolyl amine, 2-benzyl-4-chlorophenol
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Table 1 Summary of the crystallographic data for 1.
a b
Parameters
1
Composition Formula wt. Crystal System Space group a, Å b, Å c, Å a, deg b, deg c, deg V, Å3 qcalc, Mg m3 Temp, K k (Mo Ka), Å Z F(000)/l mm1 2hmax [°] Reflections Collected/Unique Rint/GOF on F2 No. of parameters R1a(F0), wR2b(F0) (all data) Largest diff. peak, Deepest hole, eÅ3
C57H52N8O13Cl4Mn2 1308.74 monoclinic C2/c 22.4384(19) 13.2040(12) 22.026(3) 90 113.6567(17) 90 5977.4(11) 1.454 293(2) 0.71073 4 2688/0.670 48.256 25404/4743 0.0615/1.463 375 0.0773, 0.2031 1.116, 0.473
P P R = ||Fo| |Fc||/ |Fo|. P P wR = [ [w((F2o F2c )2]/ w(F2o)2]1/2.
Compound 1 also displays two strong bands at 1091 and 622 cm1 due to the stretching and bending mode of perchlorate anions, respectively. In addition, a sharp strong band is also observed at 1606 and 1224 cm1 due to m(C@N/pyridine) and m(CAO/phenolate) vibration, respectively. UV–Vis spectrum is shown in Fig. 5 (Inset) and it exhibits bands at 384 (4200), 508 (2350), 577 (2550) and 807 (810) nm. Band at 384 nm appears due to intraligand p-p⁄ transition of HL ligand. The band with high e value at 508 and 577 nm can be assigned to charge transfer transition [67,68] from oxo to metal and phenolate to metal, respectively and the band around 807 nm with relatively low e can be assigned to the d-d transitions. 3.3. Mass spectrometry ESIMS spectra (in positive ion mode) for ligand HL and complex 1 have been recorded in acetonitrile solution and the spectra are shown in Figs. S4 and S5, respectively. Ligand HL exhibits a 100% molecular ion peak at m/z 430.23 due to the [M + H]+. Complex 1 demonstrates also a 100% molecular ion peak at m/z 499.042 due to 12+ and the isotope distribution patterns of this ion along with its simulation patterns are displayed in Fig. 1. Thus this result confirms the integrity of this binuclear oxo bridged compound in solution. 3.4. Description of the structure
and paraformaldehyde in methanol solvent by using 1:1:1 mixture. This facially coordinating sterically constrained pyridine and phenol based N3O ligand was then used to synthesis a new type of bis-(m-oxido)dimanganese(IV) complex 1. This complex 1 has been prepared in a single-pot reaction by using Mn(ClO4)26H2O as the metal ion precursor, HL ligand and tert-butyl hydrogenperoxide as an oxidizing agent in 1:1 solvent mixture of acetonitrile and methanol in presence of triethylamine. The reaction scheme is summarized in Scheme 1.
3.2. IR and UV–Vis spectra of the complex Selected IR band for HL ligand and compound 1 have been listed in the Section 2 and the spectra are shown in Figs. S2 and S3, respectively. It is well-known that IR spectra of the di-l-oxo dimanganese(IV) complexes exhibit sharp absorption bands characteristic for a {Mn2O2} core in the region 600–700 cm1 due to stretching vibrations of Mn-(oxo) bonds [66]. Here the complex 1 exhibit two sharp bands at 651 and 601 cm1 due to MnIV 2 O2 core.
The molecular structures and atom-labeling schemes for the dim-oxo dimanganese(IV) complex 1 are shown in Fig. 2 and its important interatomic parameters are listed in Table 2. This complex crystallizes in a monoclinic space group, C2/c, with four molecular weight units per unit cell. The dimer sits on a crystallographic inversion center and the asymmetric unit of this molecule contains half of the molecule. The Mn(IV) ions show a distorted octahedral geometry and this metal centers are coordinated by N3O3 donor atoms, N3O coordination atom from deprotonated ligand and two oxygen donor atoms from the bridging m-oxo atoms. The basal plane of Mn(1) are coordinated by phenolate O (1), amine N(2), bridging oxo O(2) and pyridine N(3) and the apical site is coordinated by remaining pyridine N(1) and bridging oxo O (20 ) atom. Within the planar Mn2O2 rhomb the Mn(1) Mn(10 ) and O(2) O(20 ) separation of this molecule are 2.6541(11) and 2.458 Å, respectively. Here in this work the Mn Mn separation is lower than reported by Pecoraro’s [14] (2.72 Å) and Fujii’s [13] (2.78 Å) di-l-oxo dimanganese(IV) complex. Here it is worth to mention that there are very few structures [20,21] that have been
Scheme 1. Synthetic scheme for the synthesis of complex 1.
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D. Mondal, M. Chandra Majee / Inorganica Chimica Acta 465 (2017) 70–77 Table 2 Selected bond distances (Å) and angles (°) for 1. Bond distances (Å)
Fig. 1. Molecular ion peaks in the ESI mass spectra (positive ion mode) for the complex 1 in acetonitrile with (a) simulated and (b) observed isotopic distribution patterns, respectively.
reported so far, containing lower MnIV MnIV separation [47] than this structure. The two short Mn(IV)-oxo distances of 1.793(3) and 1.824(3) Å, respectively and the acute O(2)–Mn(1)–O(20 ) angle of
Parameters Mn(1)-O(2) Mn(1)-O(20 ) Mn(1)-O(1) Mn(1)-N(1) Mn(1)-N(2) Mn(1)-N(3)
1 1.793(3) 1.824(3) 1.869(3) 2.083(3) 2.025(3) 2.034(3)
Bond angles (°) O(2)-Mn(1)-O(20 ) O(20 )-Mn(1)-O(1) O(2)-Mn(1)-O(1) O(20 )-Mn(1)-N(2) O(2)-Mn(1)-N(2) O(1)-Mn(1)-N(2) O(20 )-Mn(1)-N(3) O(2)-Mn(1)-N(3) O(1)-Mn(1)-N(3) N(2)-Mn(1)-N(3) O(20 )-Mn(1)-N(1) O(2)-Mn(1)-N(1) O(1)-Mn(1)-N(1) N(2)-Mn(1)-N(1) N(3)-Mn(1)-N(1)
85.59(12) 95.87(12) 95.58(12) 97.36(12) 173.99(12) 89.34(12) 92.46(13) 90.07(12) 170.26(13) 84.59(13) 171.15(12) 94.31(12) 92.95(13) 81.97(13) 78.69(14)
85.59(12)°, are entirely consistent with other di-(m-oxo)-bridged manganese(IV) complexes [11–21]. The trans angles at the manganese centers lying in the range of 170.26(13)–173.99(12)° are very close to 180°. The Mn(1)N(1) bond (2.083(3) Å) that is trans to the bridging O(2) atom is significantly elongated by 0.058 Å as compared to the Mn(1)N(2) bond (2.025(3) Å). The unit cell of this molecule (Fig. 3) contains eight cations at the center of eight edges, and four cations at the centers of four opposite faces of the cube and so the total cations are 4. Total anions in the unit cell are 8 and so for each cation there are two counter anions. The unit cell also contains eight acetonitrile and four methanol molecules as solvent of crystallization. Here it is important to mention that we are unable to isolate bis-(m-oxido)dimanganese(IV) complex with different other substituents on the phenol ring of the supporting ligand and also worth to be noted that such phenol based [N,N,N,O] ligands have not been applied so far in the synthesis of bis(oxo)-bridged dimanganese(IV) complexes [69–79]. This indicates that the benzyl and chloro group of the supporting ligand play an important role to stabilize the manganese atoms in +IV oxidation state. All the cations and anions of the complex 1 are participating in the formation of an extended system of p p stacking, CAH p interactions and C-H anions (Fig. 4) interactions which sustain the stability of the dinuclear cluster and the crystal structure packing as well.
Fig. 2. Structure of the cation [{MnIV(L)(l-O)}2+ 2 ] of 1, Hydrogen atoms are omitted for clarity. (a) Partially labeled POV-Ray (in ball and stick form) diagram (b) Space-filling representation.
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Fig. 3. Unit cell of the complex 1. Solvent molecules are omitted for clarity.
Fig. 5. Cyclic voltammogram of compound 1 in acetonitrile. Potentials vs. Ag/AgCl, 0.1 M TBAP at a glassy carbon working electrode, scan rate: 100 mV s1.
4. Catechol oxidase model studies of complex 1 and kinetics In order to study the catechol oxidase studies of complex 1, 3,5di-tert-butylcatechol (3,5-DTBC) is used as a substrate. 3,5-DTBC is
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in air-saturated acetonitrile solvent.
Fig. 4. (a) p p stacking and CAH p interactions between the cations (b) C-H anion interactions between the cations and anion of complex 1. Solvent molecules are omitted for clarity.
3.5. Electrochemistry Cyclic voltammogram of complex 1 (Fig. 5) has been recorded at a glassy carbon working electrode in acetonitrile solution (0.1 M TBAP) under an atmosphere of purified dinitrogen at 25 °C in the potential range from 1.5 V to +1.5 V vs. Ag/AgCl reference. The ligand is electrode active in this potential range, showing an irreversible oxidation wave at a potential of Epa +0.91 V (Fig. S6). The ill-defined CV of complex 1 exhibits four irreversible electrochemical processes I (MnIVMnIV ? MnIVMnIII), II (MnIVMnIII ? MnIIIMnIII), III (MnIIIMnIII ? MnIIIMnII) and IV (MnIIIMnII ? MnIIMnII) [80]. The first two irreversible reduction processes I and II are observed at the potentials of +0.55 and +0.37 V, respectively and are relatively comparable to those reported in literature [74,81,82]. The remaining two reduction processes III and IV are appeared at 0.34 and 0.79 V, respectively.
Fig. 6. Increase of the quinone band at around 400 nm after the addition of 100 equivalents of 3,5-DTBC to an acetonitrile solution with complex 1. The spectra were recorded at 2 min intervals. Inset: UV–Vis spectra of the complex 1.
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most widely used for catecholase activity of biomimicking coordination compounds because of its low reduction potential makes it easy to oxidize. The bulky tert-butyl substituents of this substrate prevent further overoxidation reactions such as ring-opening [83]. For these reasons it easily oxidized to the corresponding o-quinone, 3,5-DTBQ, is highly stable and exhibit a characteristic absorption band maxima at 402 nm (e = 1900 M1 cm1) in pure acetonitrile (Scheme 2). A 50 lM solution of 1 was treated with 100 equivalents of 3,5-DTBC under aerobic conditions at ambient temperature in acetonitrile solvent. The progress of the reaction was followed by recording the UVvis spectra of the mixture at 2 min interval times (Fig. 6). The colour of the solution gradually turned deep brown, indicative of a gradual conversion of 3,5-DTBC to 3,5-DTBQ. For this reason a gradual increase of an absorption band around 400 nm was observed in UV–Vis spectroscopy. The concentration of 3,5-DTBQ is measured after 4 h according to Lambert Beer law and it shows almost 85% conversion of 3,5-DTBC to 3,5-DTBQ, with
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turnover number (number of mol of product/number of mol of catalyst) 85. This result indicates that complex 1 is a good catalyst for catalytic conversion of 3,5-DTBC to 3,5-DTBQ under aerobic condition. At this point, it is worth to mention that blank experiment without complex 1 showed no formation of the quinone up to 12 h in MeCN. Detailed kinetic studies were also performed on this [MnIV(O) (L)]2 complex 1. In order to find out the comparative reaction rate between 3,5-DTBC and 1, the reaction kinetics between 1 and 3,5DTBC were studied by observing the growth of the quinone band at around 400 nm as a function of time. The difference in absorbance DA at 400 nm was plotted against time to obtain the initial rate for that particular catalyst to substrate concentration ratio. First-order catalytic reaction is observed, with an initial rate of 2.8 103 min1 (Fig. 7). The substrate concentration dependence of the oxidation rates and various kinetic parameters were determined by using 50 lM solution of 1 with different concentrations of 3,5DTBC (varying its concentration from 1 103 M to 1 102 M) under aerobic conditions. The average rate constant values show that the rate is first order at low concentrations of the 3,5-DTBC substrate but zero order at its higher concentrations. This type of saturation rate dependency on the concentration of the substrate may be explained by considering the Michaelis–Menten equation for enzymatic kinetics. The observed rate versus concentration of substrate was plotted to get the Lineweaver–Burk (double reciprocal) plot (Fig. 8) as well as the values of the various kinetic parameters, Vmax, KM and Kcat according to the well known equation.
1 KM 1 1 ¼ : þ V V max ½S V max
Fig. 7. A plot of the difference in absorbance (DA) vs. time to evaluate the initial rate of the catalysis by complex 1 in acetonitrile.
The Vmax and KM value are 7.245 103 (M min1) and 7.793 103 (M) respectively. Turnover number (Kcat) value was obtained by dividing the Vmax values with the concentration of the corresponding complexes. The Kcat value for this complex is 8.69 103 h1. It is noteworthy that in the literature there are a few examples of manganese complexes which mimic the activity of the catechol oxidase [Table S1]. To the best of our knowledge, there is so far only two reported work [45,47] on catecholase activity by higher-valent manganese(IV) complex. Here compound 1
Fig. 8. Plot of the rate vs. substrate concentration for complex 1. The inset shows the corresponding Lineweaver–Burk plot.
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Scheme 3. A plausible mechanistic pathway for the aerobic catechol oxidation catalyzed by complex 1.
has the higher catecholase activity in comparison to [MnIII 2 O2(tpa)2] (BPh4)2 [46] and (Ph4P)4[MnIV 2 O2(opbaMe2)2] [47] complexes and this may be due to its shorter Mn Mn distance. It is also important to mention that the bulky benzyl substituents do not allow the substrate molecule to bind strongly with the metal centers in the putative dimetal-catechol intermediate due to steric repulsion, thereby makes the substrate more prone to oxidation by O2. We have proposed a plausible mechanistic pathway for the aerobic catechol oxidation catalyzed by complex 1 and it is shown in Scheme 3. Here we believe that initially the catechol substrate is activated by coordination to the dimetal center through doubly deprotonation by one of the bridging oxo group with release of a water molecule, leading to a bridging catecholate moiety as proposed for the catalytic cycle of the enzymes Tyr and CO [26,30–32]. Subsequently, the oxidation to quinone is happened by molecular oxygen. As we are unable to synthesis the complex 1 by treating the depronated ligand with lower valent manganese ion in presence of dioxygen, so here in this model system we also believe that no intramolecular two-electron transfer is occurred to produce o-quinone product and to generate any lower valent dinuclear manganese centers. So here the role of Mn(IV) ion is to act as a lewis acid [47] to activate the catechol substrate for two electron oxidation to o-quinone by aerial oxygen.
5. Conclusion In this paper we present the synthesis of a pyridine and phenol based tetradentate facially coordinating N3O donor ligand. This ligand has been used to synthesis a stable high-valent bis(oxo)bridged dimanganese(IV) complex. This binuclear high valent oxo bridged compound is also stable in solution and exhibit at m/z 499.04 with a 100% peak corresponding to [{MnIV(L)(l-O)}2]2+ ion. The structural characterization is also discussed in this paper and it shows very short Mn(IV) Mn(IV) separation (2.65 Å). As this new complex 1 has structural similarity with high valent bis (oxo)-bridged dicopper(III) complex (active site of catecholase catalytic cycle of the enzymes Tyr and CO), catecholase activity has also been discussed in this paper. This complex 1 exhibit good catalytic activity with a Kcat value of 8.69 103 h1 for the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone and it obey the Michaelis–Menten mechanism for enzymatic kinetics, showing first-order rate dependence at lower substrate concentration and saturation kinetics at higher substrate concentrations. Therefore, the present study leads to the development of biomimetic manganese(IV)-oxo complexes that can catalyze the reaction performed by catechol oxidases.
Acknowledgments We thank Professor M. Chaudhury for many helpful discussions. We are also very much thankful to the reviewers at the revision stage for their valuable and most appropriate suggestions in improving the standard of the manuscript. This work was supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, India. We also thank the CSIR for the award of Research Fellowships. We are grateful for the instrumental support from the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science. The single-crystal X-ray diffraction data were recorded on an instrument supported by DST, New Delhi, as a National Facility at IACS under the IRHPA program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.05.073. References [1] A.J. Wu, J.E. Penner-Hahn, V.L. Pecoraro, Chem. Rev. 104 (2004) 903. [2] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104 (2004) 3981. [3] G.C. Dismukes, Chem. Rev. 96 (1996) 2909. [4] C.S. Mullins, V.L. Pecoraro, Coord. Chem. Rev. 252 (2008) 416. [5] C.W. Cady, R.H. Crabtree, G.W. Brudvig, Coord. Chem. Rev. 252 (2008) 444. [6] K.N. Ferreira, T.M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science 303 (2004) 1831. [7] B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature (London) 438 (2005) 1040. [8] A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, P. Orth, Nature (London) 409 (2001) 739. [9] G.N. George, R.C. Prince, S.P. Cramer, Science 243 (1989) 789. [10] J.E. Penner-Hahn, R. Fronko, V.L. Pecoraro, N.F. Bowlby, S.D. Betts, C.F. Yocum, J. Am. Chem. Soc. 112 (1990) 2549. [11] K. Wieghardt, Angew. Chem., Int. Ed. Engl. 28 (1990) 1153. [12] H. Chen, R. Tagore, S. Das, C. Incarvito, J.W. Faller, R.H. Crabtree, G.W. Brudvig, Inorg. Chem. 44 (2005) 7661. [13] T. Kurahashi, M. Hada, H. Fujii, Inorg. Chem. 53 (2014) 1070. [14] E.J. Larson, V.L. Pecoraro, J. Am. Chem. Soc. 113 (1991) 3810. [15] R. Manchanda, G.W. Brudvig, S.D. Gala, R.H. Crabtree, Inorg. Chem. 33 (1994) 5157. [16] K.O. Schalfer, R. Bittl, W. Zweygart, F. Lendzian, G. Haselhorst, T. Weyhermuller, K. Wieghardt, W. Lubitz, J. Am. Chem. Soc. 120 (1998) 13104. [17] A.S. Larsen, K. Wang, M.A. Lockwood, G.L. Rice, T.J. Won, S. Lovell, M. Sadilek, F. Turecek, J.M. Mayer, J. Am, Chem. Soc. 124 (2002) 10112. [18] H.Y. Chen, J.W. Faller, R.H. Crabtree, G.W. Brudvig, J. Am. Chem. Soc. 126 (2004) 7345. [19] J. Limburg, J.S. Vrettos, L.M. Liable-Sands, A.L. Rheingold, R.H. Crabtree, G.W. Brudvig, Science 283 (1999) 1524. [20] R. Ruiz, A. Aukauloo, Y. Journaux, I. Fernandez, J.R. Pedro, A.L. Rosello, B. Cervera, I. Castro, M.C. Munoz, Chem. Commun. (1998) 989. [21] N.A. Law, J.W. Kampf, V.L. Pecoraro, Inorg. Chim. Acta 297 (2000) 252. [22] L. Que Jr., W.B. Tolman, Angew. Chem., Int. Ed. 41 (2002) 1114.
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Research paper
Synthesis and structural characterization of a new high-valent bis(oxo)-bridged manganese(IV) complex and its catechol oxidase activity Dhrubajyoti Mondal ⇑, Mithun Chandra Majee Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 26 January 2017 Received in revised form 2 May 2017 Accepted 30 May 2017 Available online 1 June 2017 Keywords: High valent metal-oxo Manganese(IV)-oxo Mannich reaction Catechol oxidase model
a b s t r a c t The work in this report presents the synthesis, structural and spectroscopic characterization of a new type of high-valent bis(oxo)-bridged dimanganese(IV) complex 1 supported by a pyridine and phenol based tetradentate flexible N3O donor HL ligand (where HL = 2-benzyl-6-((bis(pyridin-2-ylmethyl) amino)methyl)-4-chlorophenol). Complex 1 possess unique Mn2(l-O)2 core with short Mn Mn distances (2.65 Å) and Mn–O distance (1.80 Å). This dinuclear manganese(IV) complex show excellent catecholase-like activity for the aerobic oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone in acetonitrile at 25 °C. Kinetic measurements suggest that the rate of catechol oxidation follows saturation kinetics with respect to the substrate and first order kinetics with respect to the catalyst. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Metal–oxygen adducts (Mx-Ox, x = 1 or 2), such as metal–superoxo, –peroxo, and –oxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes [1,2]. Among these species di-l-oxo dimetal cores commonly found in metalloenzymes have attracted large attention as a structural motif to create artificial small-molecular systems. Among the M2-O2 adducts, di-l-oxo dimanganese(IV) complexes are extensively investigated as bioinorganic model in relation to the oxygen-evolving complex of manganese catalases and photosystem II [3–5]. Photosystem II (PSII), a tetrameric manganese cluster associated with Ca2+, Cl and a redox-active tyrosine is known to catalyze light-driven four-electron oxidation of water into molecular oxygen [3,4]. Recent progress in the X-ray crystallography of PSII has enabled the modeling of the OEC as a cuboidal Mn3CaO4 structure is made up of di-l-oxo dimeric Mn units [6–8]. EXAFS spectra [9,10] of the oxygen-evolving complex in a variety of oxidation states are consistent with short Mn-Mn (2.7 Å) and somewhat longer (3.3 Å) separations. For this reason synthesis of model complexes having [(l2-O)(Mn)]2 cores as dinuclear compounds is one of the important research topic because these complexes often exhibit both short Mn-Mn separations and relatively short Mn-O distances [11–21]. This type of unique di-l-oxo dimetal cores of iron and copper is also a key intermediate for distinct ⇑ Corresponding author. E-mail address: [email protected] (D. Mondal). http://dx.doi.org/10.1016/j.ica.2017.05.073 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
oxygenation reaction [22–24]. Some of the major metalloenzymes having binuclear bis(oxo)-bridged metal active site are manganese catalase (Cat), iron methane monooxygenase (MMO) [25,26], iron ribonucleotide reductase (RR) [27–29], copper tyrosinase (Tyr) and copper catechol oxidase (CO) enzyme [26,30–32]. The ubiquitous enzyme catechol oxidases (CO) [26,30–32], found in bacteria, fungi and plants, belong to the class of type 3 copper proteins and in contrast to tyrosinase catalyze the oxidation of a wide range of o-diphenols (catechols) to the corresponding oquinones coupled with 2e/2H+ reduction of O2 to H2O, without acting on monophenols. The generated highly reactive o-quinones are autopolymerized producing a brown polyphenolic pigment, i.e., melanin, a process which is considered to protect tissues against the possible damage caused by pathogens or insects. Although these two proteins exhibit different enzymatic functions, their spectroscopic properties and crystallographic structures in their active sites are similar [33–36]. The main active site of these two metalloenzymes contains a lower valent side-on peroxo-bridged dicopper(II) form which is in valence tautomeric equilibrium with high-valent bis(oxo)-bridged dicopper(III) form [37,38]. So bis(loxo)dicopper(III) core [39,40] plays a key role in the understanding the function of this class of enzymes. Several copper complexes [41] with suitable ligands have been designed to mimic this enzyme and probe its mechanism. Although the enzyme has copper at the active site, investigations have also been shown that other transition metal ions such as Mn [42–50], Co [51–54], or Ni [55–58] can also promote such activity as well.
D. Mondal, M. Chandra Majee / Inorganica Chimica Acta 465 (2017) 70–77
Generally the [MnIV(l2-O)(L)]2 complexes have been synthesized from their lower valent precursors using different types of oxidizing agent (e.g. hydrogen peroxide or tert-Butyl hydrogenperoxide). Here in this work we report the synthesis and characterization of a new pyridine and phenol based flexible N3O donor HL ligand. Then the HL ligand has been used to synthesis a new high valent [(l2-O)MnIV(L)]2(ClO4)2 complex 1 by treating Mn(ClO4)26H2O as an metal precursor and tert-butyl hydrogenperoxide as an oxidizing agent. Spectroscopic characterization and crystal structure of complex 1 have been reported in this paper. As high valent bis(l-oxo)dicopper(III) core is the main active intermediate in Catechol oxidase and Tyrosinase enzyme, here in this work we have used this high valent bis(l-oxo)dimanganese(IV) complex as a model complex to mimic the catechol oxidase activity. A detailed kinetic study on the aerobic oxidation of catechol has been also reported in this paper. 2. Experimental Section 2.1. Materials The solvents were reagent grade, dried by standard procedure [59] and distilled under nitrogen prior to their use. All other chemicals were of reagent grade, purchased from commercial sources and used without further purification. The N3O HL ligand was prepared using Mannich type condensation reaction. Although the ligand is new, several similar ligands with other phenol moieties have been reported before using different synthetic procedure [60]. While we have experienced no problems in working with perchlorate compounds, they are potentially explosive and care must be taken not to work with large quantities. 2.2. Syntheses 2.2.1. Synthesis of ligand 2-benzyl-6-((bis(pyridin-2-ylmethyl)amino) methyl)-4-chlorophenol (HL) The ligand HL was prepared by Mannich type condensation reaction of bis((pyridine-2-yl)methyl)amine (1.99 g, 10 mmol), paraformaldehyde (0.30 g, 10 mmol) and 2-benzyl-4-chlorophenol (2.18 g, 10 mmol) in methanol (30 mL). The solution was heated under reflux for 14 h and then cooled to ca. 4 °C. The resulting white compound was filtered off and recrystallized from acetonitrile solution to give a white crystalline product of HL. Yield: 1.75 g (82%). 1H NMR (Fig. S1) (CDCl3, 400 MHz, ppm) 3.76 (s, 2H), 3.86 (s, 4H), 4.025 (s, 2H), 6.917 (d, 2H), 7.149 (m, 2H), 7.24 (m, 7H), 7.594 (t, 2H), 8.563 (d, 2H). 13C NMR (CDCl3, 125 MHz, ppm): d 36.25, 56.50, 56.74, 124.35, 126.14, 128.49, 128.62, 128.71, 128.85, 129.02, 129.33, 130.43, 131.71, 140.62, 142.46, 143.57, 153.44, 155.14. Anal. Calc. for C26H24N3OCl: C, 72.63; H, 5.63; N, 9.77 Found: C, 72.10; H, 5.98; N, 9.74%. FT-IR bands (KBr pellets, cm1): 3045b, 1591s, 1475s, 1433s, 1369s, 1228s, 1000m, 972m, 865m, 757s, 702m. ESI-MS (positive) in CH3CN: m/z 430.23, 100%, (M + H)+. 2.2.2. Synthesis of complex [{MnIV(L)(l-O)}2][ClO4]22CH3CNCH3OH (1) To a stirred solution of HL (107 mg, 0.25 mmol) in methanol/ acetonitrile (1:1) was added 1 equivalents of triethylamine (25 mg, 0.25 mmol), followed by an addition of Mn(ClO4)2 6H2O (92 mg, 0.25 mmol) to get a light brown solution. It was stirred further for ca. 2 h. The resulting solution was cooled at 0 °C in an ice bath under constant stirring and 5 equivalent of tert-butyl hydrogenperoxide (5–6 M solution in decane) was added and the mixture was stirred for 1 h. The resulting dark brown solution was filtered over a celite bed; the filtrate volume was reduced to about 10 mL by rotary evaporation and then kept in a refrigerator at 4 °C.
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The product was obtained as a dark brown crystalline solid within 2–3 days. Some of these block-shaped crystals were of X-ray diffraction quality and used directly for crystal structure analysis. The compound is prone to solvent loss. Drying under vacuum for a long time afforded a fully desolvated sample, used for various measurements including microanalysis. Yield: 100 mg (30%). Anal. Calc. for C52H46N6O12Cl4Mn2: C, 52.11; H, 3.87; N, 7.01. Found: C, 52.01; H, 3.68; N, 6.99%. FT-IR bands (KBr pellets, cm1): 2923m, 1606m, 1446m, 1290m, 1224m, 1091s, 761m, 622m, 650m, 601m. UVVis (CH2Cl2) [kmax, nm (e, L mol1 cm1)] 384 (4200), 508 (2350), 577 (2550), 807 (810). l (at 295 K)/BM: 2.62. ESI-MS (positive) in CH3CN: m/z 499.04, 100%, [{MnIV(L)(l-O)}2]2+. 2.3. Physical measurements IR spectroscopic measurements were made on samples pressed into KBr pellets using a Shimadzu 8400S FT-IR spectrometer while for UV–visible spectral measurements; a PerkinElmer Lambda 950 UV/vis/NIR or an Agilent 8453 diode array spectrophotometer was employed. Elemental analyses (for C, H and N) were performed at IACS on a PerkinElmer model 2400 Series II CHNS Analyzer. The electrospray ionization mass spectra (ESIMS) in positive ion mode were measured on a Micromass QTOF model YA 263 mass spectrometer. The 1H NMR spectra were recorded on a Bruker model Avance DPX-400 spectrometer using SiMe4 as internal reference. Cyclic voltammetry (CV) in acetonitrile (ACN) was recorded on CH Instruments (CHI720D electrochemical analyzer) using a glassy carbon working electrode and a platinum wire counter electrode. Ag/AgCl was used for reference and Fc/Fc+ as the internal standard. Room temperature magnetic moment of 1 was measured by a magnetic susceptibility balance procured from Sherwood Scientific, UK. The diamagnetic correction was evaluated from Pascal’s constants. 2.4. X-ray crystallography Suitable crystals of 1 (brown block, 0.15 0.12 0.10 mm3) were mounted on glass fibers coated with perfluoropolyether oil before mounting. Intensity data for the aligned crystals were measured employing a Bruker SMART APEX II CCD diffractometer equipped with a monochromatized Mo Ka radiation (k = 0.71073 Å) source at 293(2) K. No crystal decay was observed during the data collection. In all cases, absorption corrections based on multiscans using the SADABS software [61] were applied. The structures were solved by direct methods [62] and refined on F2 by a full-matrix least-squares procedure based on all data minP P P P imizing wR = [ [w(F20 F2c )2]/ (F20)2]½, R = | |F0| |Fc| |/ |F0|, P ½ 2 2 2 and S = [ [w(F0 Fc ) ]/(n A p)] . SHELXL - 2013 was used for both structure solutions and refinements [63]. A summary of the relevant crystallographic data and the final refinement details are given in Table 1. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were calculated and isotropically fixed in the final refinement [d(CAH) = 0.95 Å, with the isotropic thermal parameter of Uiso(H) = 1.2 Uiso(C)]. The SMART and SAINT software packages [64] were used for data collection and reduction, respectively. Crystallographic diagrams were drawn using the DIAMOND software package [65]. 3. Results and discussion 3.1. Synthesis of complex (1) The mono-phenol N3O ligand (HL) has been prepared by Mannich type reaction of dipicolyl amine, 2-benzyl-4-chlorophenol
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Table 1 Summary of the crystallographic data for 1.
a b
Parameters
1
Composition Formula wt. Crystal System Space group a, Å b, Å c, Å a, deg b, deg c, deg V, Å3 qcalc, Mg m3 Temp, K k (Mo Ka), Å Z F(000)/l mm1 2hmax [°] Reflections Collected/Unique Rint/GOF on F2 No. of parameters R1a(F0), wR2b(F0) (all data) Largest diff. peak, Deepest hole, eÅ3
C57H52N8O13Cl4Mn2 1308.74 monoclinic C2/c 22.4384(19) 13.2040(12) 22.026(3) 90 113.6567(17) 90 5977.4(11) 1.454 293(2) 0.71073 4 2688/0.670 48.256 25404/4743 0.0615/1.463 375 0.0773, 0.2031 1.116, 0.473
P P R = ||Fo| |Fc||/ |Fo|. P P wR = [ [w((F2o F2c )2]/ w(F2o)2]1/2.
Compound 1 also displays two strong bands at 1091 and 622 cm1 due to the stretching and bending mode of perchlorate anions, respectively. In addition, a sharp strong band is also observed at 1606 and 1224 cm1 due to m(C@N/pyridine) and m(CAO/phenolate) vibration, respectively. UV–Vis spectrum is shown in Fig. 5 (Inset) and it exhibits bands at 384 (4200), 508 (2350), 577 (2550) and 807 (810) nm. Band at 384 nm appears due to intraligand p-p⁄ transition of HL ligand. The band with high e value at 508 and 577 nm can be assigned to charge transfer transition [67,68] from oxo to metal and phenolate to metal, respectively and the band around 807 nm with relatively low e can be assigned to the d-d transitions. 3.3. Mass spectrometry ESIMS spectra (in positive ion mode) for ligand HL and complex 1 have been recorded in acetonitrile solution and the spectra are shown in Figs. S4 and S5, respectively. Ligand HL exhibits a 100% molecular ion peak at m/z 430.23 due to the [M + H]+. Complex 1 demonstrates also a 100% molecular ion peak at m/z 499.042 due to 12+ and the isotope distribution patterns of this ion along with its simulation patterns are displayed in Fig. 1. Thus this result confirms the integrity of this binuclear oxo bridged compound in solution. 3.4. Description of the structure
and paraformaldehyde in methanol solvent by using 1:1:1 mixture. This facially coordinating sterically constrained pyridine and phenol based N3O ligand was then used to synthesis a new type of bis-(m-oxido)dimanganese(IV) complex 1. This complex 1 has been prepared in a single-pot reaction by using Mn(ClO4)26H2O as the metal ion precursor, HL ligand and tert-butyl hydrogenperoxide as an oxidizing agent in 1:1 solvent mixture of acetonitrile and methanol in presence of triethylamine. The reaction scheme is summarized in Scheme 1.
3.2. IR and UV–Vis spectra of the complex Selected IR band for HL ligand and compound 1 have been listed in the Section 2 and the spectra are shown in Figs. S2 and S3, respectively. It is well-known that IR spectra of the di-l-oxo dimanganese(IV) complexes exhibit sharp absorption bands characteristic for a {Mn2O2} core in the region 600–700 cm1 due to stretching vibrations of Mn-(oxo) bonds [66]. Here the complex 1 exhibit two sharp bands at 651 and 601 cm1 due to MnIV 2 O2 core.
The molecular structures and atom-labeling schemes for the dim-oxo dimanganese(IV) complex 1 are shown in Fig. 2 and its important interatomic parameters are listed in Table 2. This complex crystallizes in a monoclinic space group, C2/c, with four molecular weight units per unit cell. The dimer sits on a crystallographic inversion center and the asymmetric unit of this molecule contains half of the molecule. The Mn(IV) ions show a distorted octahedral geometry and this metal centers are coordinated by N3O3 donor atoms, N3O coordination atom from deprotonated ligand and two oxygen donor atoms from the bridging m-oxo atoms. The basal plane of Mn(1) are coordinated by phenolate O (1), amine N(2), bridging oxo O(2) and pyridine N(3) and the apical site is coordinated by remaining pyridine N(1) and bridging oxo O (20 ) atom. Within the planar Mn2O2 rhomb the Mn(1) Mn(10 ) and O(2) O(20 ) separation of this molecule are 2.6541(11) and 2.458 Å, respectively. Here in this work the Mn Mn separation is lower than reported by Pecoraro’s [14] (2.72 Å) and Fujii’s [13] (2.78 Å) di-l-oxo dimanganese(IV) complex. Here it is worth to mention that there are very few structures [20,21] that have been
Scheme 1. Synthetic scheme for the synthesis of complex 1.
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D. Mondal, M. Chandra Majee / Inorganica Chimica Acta 465 (2017) 70–77 Table 2 Selected bond distances (Å) and angles (°) for 1. Bond distances (Å)
Fig. 1. Molecular ion peaks in the ESI mass spectra (positive ion mode) for the complex 1 in acetonitrile with (a) simulated and (b) observed isotopic distribution patterns, respectively.
reported so far, containing lower MnIV MnIV separation [47] than this structure. The two short Mn(IV)-oxo distances of 1.793(3) and 1.824(3) Å, respectively and the acute O(2)–Mn(1)–O(20 ) angle of
Parameters Mn(1)-O(2) Mn(1)-O(20 ) Mn(1)-O(1) Mn(1)-N(1) Mn(1)-N(2) Mn(1)-N(3)
1 1.793(3) 1.824(3) 1.869(3) 2.083(3) 2.025(3) 2.034(3)
Bond angles (°) O(2)-Mn(1)-O(20 ) O(20 )-Mn(1)-O(1) O(2)-Mn(1)-O(1) O(20 )-Mn(1)-N(2) O(2)-Mn(1)-N(2) O(1)-Mn(1)-N(2) O(20 )-Mn(1)-N(3) O(2)-Mn(1)-N(3) O(1)-Mn(1)-N(3) N(2)-Mn(1)-N(3) O(20 )-Mn(1)-N(1) O(2)-Mn(1)-N(1) O(1)-Mn(1)-N(1) N(2)-Mn(1)-N(1) N(3)-Mn(1)-N(1)
85.59(12) 95.87(12) 95.58(12) 97.36(12) 173.99(12) 89.34(12) 92.46(13) 90.07(12) 170.26(13) 84.59(13) 171.15(12) 94.31(12) 92.95(13) 81.97(13) 78.69(14)
85.59(12)°, are entirely consistent with other di-(m-oxo)-bridged manganese(IV) complexes [11–21]. The trans angles at the manganese centers lying in the range of 170.26(13)–173.99(12)° are very close to 180°. The Mn(1)N(1) bond (2.083(3) Å) that is trans to the bridging O(2) atom is significantly elongated by 0.058 Å as compared to the Mn(1)N(2) bond (2.025(3) Å). The unit cell of this molecule (Fig. 3) contains eight cations at the center of eight edges, and four cations at the centers of four opposite faces of the cube and so the total cations are 4. Total anions in the unit cell are 8 and so for each cation there are two counter anions. The unit cell also contains eight acetonitrile and four methanol molecules as solvent of crystallization. Here it is important to mention that we are unable to isolate bis-(m-oxido)dimanganese(IV) complex with different other substituents on the phenol ring of the supporting ligand and also worth to be noted that such phenol based [N,N,N,O] ligands have not been applied so far in the synthesis of bis(oxo)-bridged dimanganese(IV) complexes [69–79]. This indicates that the benzyl and chloro group of the supporting ligand play an important role to stabilize the manganese atoms in +IV oxidation state. All the cations and anions of the complex 1 are participating in the formation of an extended system of p p stacking, CAH p interactions and C-H anions (Fig. 4) interactions which sustain the stability of the dinuclear cluster and the crystal structure packing as well.
Fig. 2. Structure of the cation [{MnIV(L)(l-O)}2+ 2 ] of 1, Hydrogen atoms are omitted for clarity. (a) Partially labeled POV-Ray (in ball and stick form) diagram (b) Space-filling representation.
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Fig. 3. Unit cell of the complex 1. Solvent molecules are omitted for clarity.
Fig. 5. Cyclic voltammogram of compound 1 in acetonitrile. Potentials vs. Ag/AgCl, 0.1 M TBAP at a glassy carbon working electrode, scan rate: 100 mV s1.
4. Catechol oxidase model studies of complex 1 and kinetics In order to study the catechol oxidase studies of complex 1, 3,5di-tert-butylcatechol (3,5-DTBC) is used as a substrate. 3,5-DTBC is
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in air-saturated acetonitrile solvent.
Fig. 4. (a) p p stacking and CAH p interactions between the cations (b) C-H anion interactions between the cations and anion of complex 1. Solvent molecules are omitted for clarity.
3.5. Electrochemistry Cyclic voltammogram of complex 1 (Fig. 5) has been recorded at a glassy carbon working electrode in acetonitrile solution (0.1 M TBAP) under an atmosphere of purified dinitrogen at 25 °C in the potential range from 1.5 V to +1.5 V vs. Ag/AgCl reference. The ligand is electrode active in this potential range, showing an irreversible oxidation wave at a potential of Epa +0.91 V (Fig. S6). The ill-defined CV of complex 1 exhibits four irreversible electrochemical processes I (MnIVMnIV ? MnIVMnIII), II (MnIVMnIII ? MnIIIMnIII), III (MnIIIMnIII ? MnIIIMnII) and IV (MnIIIMnII ? MnIIMnII) [80]. The first two irreversible reduction processes I and II are observed at the potentials of +0.55 and +0.37 V, respectively and are relatively comparable to those reported in literature [74,81,82]. The remaining two reduction processes III and IV are appeared at 0.34 and 0.79 V, respectively.
Fig. 6. Increase of the quinone band at around 400 nm after the addition of 100 equivalents of 3,5-DTBC to an acetonitrile solution with complex 1. The spectra were recorded at 2 min intervals. Inset: UV–Vis spectra of the complex 1.
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most widely used for catecholase activity of biomimicking coordination compounds because of its low reduction potential makes it easy to oxidize. The bulky tert-butyl substituents of this substrate prevent further overoxidation reactions such as ring-opening [83]. For these reasons it easily oxidized to the corresponding o-quinone, 3,5-DTBQ, is highly stable and exhibit a characteristic absorption band maxima at 402 nm (e = 1900 M1 cm1) in pure acetonitrile (Scheme 2). A 50 lM solution of 1 was treated with 100 equivalents of 3,5-DTBC under aerobic conditions at ambient temperature in acetonitrile solvent. The progress of the reaction was followed by recording the UVvis spectra of the mixture at 2 min interval times (Fig. 6). The colour of the solution gradually turned deep brown, indicative of a gradual conversion of 3,5-DTBC to 3,5-DTBQ. For this reason a gradual increase of an absorption band around 400 nm was observed in UV–Vis spectroscopy. The concentration of 3,5-DTBQ is measured after 4 h according to Lambert Beer law and it shows almost 85% conversion of 3,5-DTBC to 3,5-DTBQ, with
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turnover number (number of mol of product/number of mol of catalyst) 85. This result indicates that complex 1 is a good catalyst for catalytic conversion of 3,5-DTBC to 3,5-DTBQ under aerobic condition. At this point, it is worth to mention that blank experiment without complex 1 showed no formation of the quinone up to 12 h in MeCN. Detailed kinetic studies were also performed on this [MnIV(O) (L)]2 complex 1. In order to find out the comparative reaction rate between 3,5-DTBC and 1, the reaction kinetics between 1 and 3,5DTBC were studied by observing the growth of the quinone band at around 400 nm as a function of time. The difference in absorbance DA at 400 nm was plotted against time to obtain the initial rate for that particular catalyst to substrate concentration ratio. First-order catalytic reaction is observed, with an initial rate of 2.8 103 min1 (Fig. 7). The substrate concentration dependence of the oxidation rates and various kinetic parameters were determined by using 50 lM solution of 1 with different concentrations of 3,5DTBC (varying its concentration from 1 103 M to 1 102 M) under aerobic conditions. The average rate constant values show that the rate is first order at low concentrations of the 3,5-DTBC substrate but zero order at its higher concentrations. This type of saturation rate dependency on the concentration of the substrate may be explained by considering the Michaelis–Menten equation for enzymatic kinetics. The observed rate versus concentration of substrate was plotted to get the Lineweaver–Burk (double reciprocal) plot (Fig. 8) as well as the values of the various kinetic parameters, Vmax, KM and Kcat according to the well known equation.
1 KM 1 1 ¼ : þ V V max ½S V max
Fig. 7. A plot of the difference in absorbance (DA) vs. time to evaluate the initial rate of the catalysis by complex 1 in acetonitrile.
The Vmax and KM value are 7.245 103 (M min1) and 7.793 103 (M) respectively. Turnover number (Kcat) value was obtained by dividing the Vmax values with the concentration of the corresponding complexes. The Kcat value for this complex is 8.69 103 h1. It is noteworthy that in the literature there are a few examples of manganese complexes which mimic the activity of the catechol oxidase [Table S1]. To the best of our knowledge, there is so far only two reported work [45,47] on catecholase activity by higher-valent manganese(IV) complex. Here compound 1
Fig. 8. Plot of the rate vs. substrate concentration for complex 1. The inset shows the corresponding Lineweaver–Burk plot.
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Scheme 3. A plausible mechanistic pathway for the aerobic catechol oxidation catalyzed by complex 1.
has the higher catecholase activity in comparison to [MnIII 2 O2(tpa)2] (BPh4)2 [46] and (Ph4P)4[MnIV 2 O2(opbaMe2)2] [47] complexes and this may be due to its shorter Mn Mn distance. It is also important to mention that the bulky benzyl substituents do not allow the substrate molecule to bind strongly with the metal centers in the putative dimetal-catechol intermediate due to steric repulsion, thereby makes the substrate more prone to oxidation by O2. We have proposed a plausible mechanistic pathway for the aerobic catechol oxidation catalyzed by complex 1 and it is shown in Scheme 3. Here we believe that initially the catechol substrate is activated by coordination to the dimetal center through doubly deprotonation by one of the bridging oxo group with release of a water molecule, leading to a bridging catecholate moiety as proposed for the catalytic cycle of the enzymes Tyr and CO [26,30–32]. Subsequently, the oxidation to quinone is happened by molecular oxygen. As we are unable to synthesis the complex 1 by treating the depronated ligand with lower valent manganese ion in presence of dioxygen, so here in this model system we also believe that no intramolecular two-electron transfer is occurred to produce o-quinone product and to generate any lower valent dinuclear manganese centers. So here the role of Mn(IV) ion is to act as a lewis acid [47] to activate the catechol substrate for two electron oxidation to o-quinone by aerial oxygen.
5. Conclusion In this paper we present the synthesis of a pyridine and phenol based tetradentate facially coordinating N3O donor ligand. This ligand has been used to synthesis a stable high-valent bis(oxo)bridged dimanganese(IV) complex. This binuclear high valent oxo bridged compound is also stable in solution and exhibit at m/z 499.04 with a 100% peak corresponding to [{MnIV(L)(l-O)}2]2+ ion. The structural characterization is also discussed in this paper and it shows very short Mn(IV) Mn(IV) separation (2.65 Å). As this new complex 1 has structural similarity with high valent bis (oxo)-bridged dicopper(III) complex (active site of catecholase catalytic cycle of the enzymes Tyr and CO), catecholase activity has also been discussed in this paper. This complex 1 exhibit good catalytic activity with a Kcat value of 8.69 103 h1 for the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone and it obey the Michaelis–Menten mechanism for enzymatic kinetics, showing first-order rate dependence at lower substrate concentration and saturation kinetics at higher substrate concentrations. Therefore, the present study leads to the development of biomimetic manganese(IV)-oxo complexes that can catalyze the reaction performed by catechol oxidases.
Acknowledgments We thank Professor M. Chaudhury for many helpful discussions. We are also very much thankful to the reviewers at the revision stage for their valuable and most appropriate suggestions in improving the standard of the manuscript. This work was supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, India. We also thank the CSIR for the award of Research Fellowships. We are grateful for the instrumental support from the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science. The single-crystal X-ray diffraction data were recorded on an instrument supported by DST, New Delhi, as a National Facility at IACS under the IRHPA program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.05.073. References [1] A.J. Wu, J.E. Penner-Hahn, V.L. Pecoraro, Chem. Rev. 104 (2004) 903. [2] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104 (2004) 3981. [3] G.C. Dismukes, Chem. Rev. 96 (1996) 2909. [4] C.S. Mullins, V.L. Pecoraro, Coord. Chem. Rev. 252 (2008) 416. [5] C.W. Cady, R.H. Crabtree, G.W. Brudvig, Coord. Chem. Rev. 252 (2008) 444. [6] K.N. Ferreira, T.M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science 303 (2004) 1831. [7] B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature (London) 438 (2005) 1040. [8] A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, P. Orth, Nature (London) 409 (2001) 739. [9] G.N. George, R.C. Prince, S.P. Cramer, Science 243 (1989) 789. [10] J.E. Penner-Hahn, R. Fronko, V.L. Pecoraro, N.F. Bowlby, S.D. Betts, C.F. Yocum, J. Am. Chem. Soc. 112 (1990) 2549. [11] K. Wieghardt, Angew. Chem., Int. Ed. Engl. 28 (1990) 1153. [12] H. Chen, R. Tagore, S. Das, C. Incarvito, J.W. Faller, R.H. Crabtree, G.W. Brudvig, Inorg. Chem. 44 (2005) 7661. [13] T. Kurahashi, M. Hada, H. Fujii, Inorg. Chem. 53 (2014) 1070. [14] E.J. Larson, V.L. Pecoraro, J. Am. Chem. Soc. 113 (1991) 3810. [15] R. Manchanda, G.W. Brudvig, S.D. Gala, R.H. Crabtree, Inorg. Chem. 33 (1994) 5157. [16] K.O. Schalfer, R. Bittl, W. Zweygart, F. Lendzian, G. Haselhorst, T. Weyhermuller, K. Wieghardt, W. Lubitz, J. Am. Chem. Soc. 120 (1998) 13104. [17] A.S. Larsen, K. Wang, M.A. Lockwood, G.L. Rice, T.J. Won, S. Lovell, M. Sadilek, F. Turecek, J.M. Mayer, J. Am, Chem. Soc. 124 (2002) 10112. [18] H.Y. Chen, J.W. Faller, R.H. Crabtree, G.W. Brudvig, J. Am. Chem. Soc. 126 (2004) 7345. [19] J. Limburg, J.S. Vrettos, L.M. Liable-Sands, A.L. Rheingold, R.H. Crabtree, G.W. Brudvig, Science 283 (1999) 1524. [20] R. Ruiz, A. Aukauloo, Y. Journaux, I. Fernandez, J.R. Pedro, A.L. Rosello, B. Cervera, I. Castro, M.C. Munoz, Chem. Commun. (1998) 989. [21] N.A. Law, J.W. Kampf, V.L. Pecoraro, Inorg. Chim. Acta 297 (2000) 252. [22] L. Que Jr., W.B. Tolman, Angew. Chem., Int. Ed. 41 (2002) 1114.
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