Mononuclear manganese(III) complexes of bidentate NO donor Schiff base ligands: synthesis, structural characterization, magnetic and catecholase studies

Abstract

We have synthesized four mononuclear manganese(III) complexes (1–4) of four closely related bidentate NO donor Schiff-base ligands, out of which three (2–4) were structurally characterized. Crystal structure determination reveals that all these complexes are in octahedral geometries. Magnetic studies have been carried out on complexes 2, 3 and 4 in the temperature range 2–300 K under a magnetic field of 0.1 T which yielded negative ZFS parameters of −2.96, –3.51 and −3.72 cm⁻¹ respectively. The catecholase activities of complexes 1–4 have been investigated following the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) with molecular oxygen in DCM at 25 °C, which were found to follow the Michalis–Menton type relation giving the highest TON (K_cat) for the so far reported Mn(III) complexes.

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Introduction

It is now well established that a number of Mn-containing biomolecules are found to contain manganese as mononuclear (superoxide dismutase¹ and manganese dioxygenase²), dinuclear (catalases,^3–7 ribonucleotide reductase,⁸ arginase⁹) and also as tetranuclear (oxygen evolving complex¹⁰) species. A significant advancement has been achieved in the biomimetic structural and functional modelling of manganese containing enzymes by synthetic chemists and biochemists, which have added a wealth of knowledge to our understanding of various aspects of manganese cluster chemistry with respect to structural, electrochemical and magnetic properties. In addition, these structural or functional models are also important in catalyzing a number of organic reactions.^11–15 Though structural and functional models of manganese containing metalloenzymes have been extensively explored^16,17 this area is still under continuous development.^18,19

Catechol oxidase (CO) is a member of the type-III copper proteins²⁰ which catalyzes exclusively the oxidation of catechols to the corresponding quinones, highly reactive intermediates that undergo auto-polymerization to produce melanin, a brown pigment responsible for protecting damaged tissues against pathogens and insects of the higher plants. Although CO is believed to be a copper containing protein and many dicopper complexes are found to act as successful models for such metalloenzymes; there are only few reports where manganese^21–36 complexes were found to display such activities as well, despite the fact that no such native enzymes having metal ions other than copper is known. In these complexes tetradentate tripodal ligands, several pyridine derivatives, azametallacrown and compartmental Schiff base ligands have been utilized but there is no report on the mononuclear octahedral Mn(III) complexes of bidentate Schiff base ligands that display catecholase activities. Therefore, design and synthesis of functional model of CO containing metal ions other than copper, say manganese, seems to be interesting and at the same time challenging.

High-spin manganese(III) complexes with ⁵D ground term splits in octahedral crystal fields into ⁵T_2g, and ⁵E_g, terms. Non-cubic symmetry and/or Jahn–Teller distortions are responsible to lift the orbital degeneracy of the ⁵E_g, ground term to orbital singlet lowest, either ⁵A_1g or ⁵B_1g, (in D_4h symmetry). The spin degeneracy of the ground state is further lifted by spin–orbit coupling to give the so-called zero-field splitting (ZFS).

The exact nature of the ground state depends critically on the symmetry of the ligand field and the nature of metal ion. If the d_x²−y² orbital is unoccupied, then the complex is expected to be axially elongated, with the ⁵B_1g level lying lowest. Such a ground term is expected³⁷ to have principal susceptibilities in the order K_‖ > K_⊥, and hence D will be negative. Conversely, compression of the octahedron results in the ⁵A_1g, lying lowest with positive D.

The zero-field splitting in Mn(III) is typically of the order of a few wavenumbers, and so the magnetic properties are expected to be Curie-like and close to spin only value (4.90μ_B), except at very low temperatures where a rapid decrease in μ_Mn occurs. In fact, a number of studies on hexacoordinated Mn(III) Schiff-base complexes showed that these complexes displayed Curie-like behaviour in the temperature range 80–300 K with magnetic moments of about 4.90μ_B.^38–40 Owing to small size of the ZFS, together with the possibilities of weak magnetic exchange, exact interpretation of the data was not always possible, and the possibility of relatively large errors in the magnitude of such interactions existed. More recently, a number of structural studies on Mn(III) Schiff-base complexes have been reported,⁴¹ mostly showing the presence of quite appreciable distortions about the Mn(III) center. Therefore, axially elongated configuration attracted considerable attention in the search for new Mn(III) SIMs with negative D values. In the present paper we are going to describe the catecholase activities and magnetic properties of some monomeric octahedral Schiff-base complexes of the type [Mn(NO)₃].

Experimental section

Materials and reagents

The starting materials for the synthesis of the ligand H₂Lⁱ (i = 1, 2, 3, and 4) like 5-bromo-salicylaldehyde, hydroxyl-amine hydrochloride, Na₂SO₄, tetrabutylammonium hydroxide, sodium azide, Mn(ClO₄)₂·6H₂O are of reagent grade and used as received. Solvents like methanol, diethyl ether, dichloromethane (DCM), acetonitrile were of reagent grade and dried before use.

Physical measurements

Elemental analyses were carried out using a Perkin-Elmer 240 elemental analyzer. Infrared spectra (400–4000 cm⁻¹) were recorded from KBr pellets on a Nicolet Magna IR 750 series-II FTIR spectrophotometers. ¹H-NMR spectra were recorded in CDCl₃ on a Bruker 300 MHz NMR Spectrophotometer using tetramethylsilane (δ = 0) as an internal standard. UV-vis spectra and kinetic studies were performed on Agilent 8453 UV-vis spectrophotometer. TOF-MS⁺ spectra were generated on a waters HRMS instrument (model: XEVO G2QTOF).

Crystallography

Single crystal X-ray data of 2–4 were collected at room temperature on a Bruker SMART APEX-II CCD diffractometers using graphite monochromated MoK_α radiation (λ = 0.71073 Å). Data integration and reductions were processed with SAINT+ software.⁴² Structures were solved by the direct method and then refined on F² by the full matrix least square technique with SHELX-97 software.⁴³ The crystallographic data for 2–4 are given in Table 1.

Table 1 Crystal data and details of the structure determination

Formula	C42H33Br3MnN3O3	C90H79Br6Mn2N6O6	C42H30Br3Cl3MnN3O3
Formula weight	922.35	1929.87	1025.68
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P1̄ (no. 2)	P1̄ (no. 2)	P1̄ (no.2)
a, b, c [Å]	8.246(2) 12.765(4) 18.256(6)	11.5135(4) 14.5392(6) 25.8603(10)	8.8296(15) 12.345(2) 20.440(4)
α, β, γ [°]	70.607(9) 84.283(9) 84.456(8)	103.534(3) 90.011(3) 90.013(3)	76.581(5) 85.576(6) 70.170(5)
V [Å^3]	1799.5(9)	4208.7(3)	2038.7(6)
Z	2	2	2
D(calc) [g cm^−3]	1.702	1.523	1.671
μ(MoKα) [mm^−1]	3.743	3.205	3.503
F(000)	920	1938	1016
Crystal size [mm]	0.08 × 0.10 × 0.12	0.09 × 0.15 × 0.23	0.10 × 0.12 × 0.20
Temperature (K)	120	296	120
Radiation [Å MoKα]	0.71073	0.71073	0.71073
θmin, θmax [°]	1.7, 25.7	0.8, 27.5	1.8, 24.8
Dataset	−6: 10; −14: 14; −21: 20	−14: 14; −18: 18; −33: 33	−6: 10; −14: 14; −24: 24
Tot., Uniq. Data, R(int)	11577, 6805, 0.030	64441, 19368, 0.154	13165, 6902, 0.032
Observed data [I > 2.0σ(I)]	4184	5262	4265
Nref, Npar	6805, 469	19368, 997	6902, 496
R, wR2, S	0.0390, 0.1275, 0.98	0.0804, 0.1650, 0.992	0.0689, 0.1786, 1.01

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Magnetic Measurements

Magnetic measurements were performed using a Quantum Design SVSM (Squid-Vibrating Sample Magnetometer) magnetometer. The static susceptibility measurement were performed in 300–1.8 K temperature range with an applied field of 5 kOe. Measurement of magnetization at different fields and at a given temperature confirm the absence of ferromagnetic impurities. Data were corrected for the sample holder and diamagnetism was estimated from Pascal constants.

Syntheses and characterization

Preparation of Schiff base ligands. All four ligands (HL¹–HL⁴) were synthesized following the same procedure as described below in details for HL¹, except using the appropriate aldehyde and amine (Scheme 1).

Scheme 1 Schematic presentation of preparation of complexes 1–4.

Synthesis of ligand HL¹. 0.136 g (1 mmol) of 2-hydroxy-5-methyl benzaldehyde was dissolved in 20 mL ethanol to which 0.110 g (1 mmol) of benzylamine was added drop wise and the solution was refluxed for 3–4 hours. It was then cooled and solvent was removed under reduced pressure to get solid product which was recrystallized from MeOH to get pure product. This protocol was adopted for the synthesis of other related ligands viz. HL², HL³ and HL⁴.

Analyses.
HL¹. ¹H-NMR: δ in ppm 2.247 (3H, s, –CH₃), 4.73 (2H, s, –CH₂), 6.7–7.3 (8H, m, aromatic protons), 8.45 (1H, s, –N=CH), 13.41 (1H, s, –OH). ¹³C-NMR: δ in ppm 18.96 (s, Me Carbon), 62.20 (s, C7), 116.36–159.41 (m, aromatic carbons), 165.94 (s, C8). Elemental analysis: Calculated, C, 80.00; H, 6.67; N, 6.22; found, C, 81.20; H, 6.33; N, 6.41.
HL². ¹H-NMR: δ in ppm 4.82 (2H, s, –CH₂), 6.85–7.40 (8H, m, aromatic protons), 8.35 (1H, s, –N=CH), 13.42 (1H, s, –OH). ¹³C-NMR: δ in ppm 62.99 (s, C7), 109.94–160.11 (m, aromatic carbons), 164.20 (s, C8). Elemental analysis: calculated, C, 57.93; H, 4.14; N, 4.83; found, C, 58.32; H, 4.39; N, 5.08.
HL³. ¹H-NMR: δ in ppm 2.36 (3H, s, –CH₃), 4.77 (2H, s, –CH₂), 6.85–7.40 (7H, m, aromatic protons), 8.31(1H, s, –N=CH), 13.49 (1H, s, –OH). ¹³C-NMR: δ in ppm 21.13 (s, Me Carbon), 62.77 (s, C7), 109.99–160.29 (m, aromatic carbons), 164.07 (s, C8). Elemental analysis: calculated, C, 59.21; H, 4.61; N, 4.61; found, C, 60.02; H, 4.35; N, 5.25.
HL⁴. ¹H-NMR: δ in ppm 4.78 (2H, s, –CH₂), 6.87–7.43 (7H, m, aromatic protons), 8.36 (1H, s, –N=CH), 13.25 (1H, s, –OH). ¹³C-NMR: δ in ppm 62.34 (s, C7), 110.08–159.99 (m, aromatic carbons), 164.54 (s, C8). Elemental analysis: calculated, C, 51.77; H, 3.39; N, 4.31; found, C, 52.52; H, 3.63; N, 5.02.

Preparation of complexes 1–4. All complexes were synthesized following the same procedure as described below in details for complex 1. Bright amorphous solid product was obtained for 1 while shiny single crystals, suitable for single crystal X-ray diffraction were obtained for complexes 2–4.

Preparation of [Mn(L¹)₃] (1). 3.0 mmol (0.87 g) ligand (HL¹) was dissolved in minimum volume of acetonitrile. To this solution an equimolar amount of (0.303 g) TEA (triethylamine) was added followed by 1 mmol (0.254 g) Mn(ClO₄)₂·6H₂O. It was then refluxed for 1 hour. The resulting greenish brown solution was cooled to room temperature, filtered and kept in rack where upon dark brown amorphous solid product was obtained within 2–3 days. Yield: 0.472 g (65%).

Elemental analyses.
[Mn^III(L¹)₃] (1). C₄₅H₄₂MnN₃O₃ (MW = 727.77); ESI-MS (+ve): M + H⁺ = 728.81, elemental analysis: calculated, C, 74.27; H, 5.82; N, 5.77; found, C, 75.10; H, 5.45; N, 5.12.
[Mn^III(L²)₃] (2). Color: dark brown, C₄₂H₃₃Br₃MnN₃O₃ (MW = 922.35); yield: 0.599 g (65%), calculated, C, 54.68; H, 3.60; N, 4.56; found, C, 55.38; H, 3.85; N, 4.95.
[Mn^III(L³)₃] (3). Color: dark green, C₄₅H₃₉Br₃MnN₃O₃ (per unit cell) (MW = 964.43); yield: 0.655 g (68%), calculated, C, 56.04; H, 4.08; N, 4.36; found, C, 56.54; H, 4.39; N, 4.77.
[Mn^III(L⁴)₃] (4). Color: dark brown, C₄₂H₃₀Br₃Cl₃MnN₃O₃ (MW = 1025.68); yield: 0.717 g (70%), calculated, C, 49.18; H, 2.95; N, 4.10; found, C, 50.11; H, 3.15; N, 4.32.

Kinetic studies. All kinetic experiments were carried out under pseudo-first-order conditions, with the Mn(III) complexes as the minor component. All measurements were performed in an Agilent diode-array UV-vis spectrophotometer. Detail kinetic procedures involve the preparation of stock solutions of complexes (1.0 × 10⁻³ M) and the substrate, 3,5-DTBC, at higher concentrations in pure DCM. From this stock solutions a set of 12 solutions of [3,5-DTBC] = 0.0001–0.01 M were prepared. A 2 ml portion of each solution was pipetted out into a quartz cell and equilibrated for 15 min at 25 °C by inserting into the shell holder which is attached to a Peltier temperature controller system. Now 20 μL of stock solution of the complex was added to it to achieve the ultimate concentration of the complex 1.0 × 10⁻⁵ M.

Results and discussion

Synthesis and structural descriptions of complexes 2–4

Bidentate NO donor ligands HLⁱ (i = 1–4) were synthesized by the simple Schiff base condensation between appropriate aldehydes and amines in EtOH under refluxing conditions. All the four Mn(III) complexes were synthesized by the reaction between Mn(ClO₄)₂·6H₂O and appropriate ligand in MeCN at ambient temperature which afford single crystals suitable for X-ray diffraction studies with the exception of complex 1. Several trials to get single crystals of 1 were not successful.

The single crystal X-ray diffraction studies showed that complexes 2–4, depicted in Fig. 1–3, are crystallized in triclinic system with space group P1̄. All these complexes are monomeric in nature surrounded by three bidentate NO donor Schiff base ligands. The central manganese atom in each complex is in +3 oxidation state. In case of complexes 2 and 4 there are only one complex unit in the unit cell and the Mn atoms are coordinated by N1, O1, N2, O2 and N3, O3 atom from three ligands while in complex 3 two complex species (A and B) are present in the unit cell. In A the central manganese(III) ion (Mn1) are coordinated by N1, O1, N2, O2 and N3, O3 and Mn2 atom in B is coordinated by N4, O4, N5, O5 and N6, O6, which differ only in bond angles and bond lengths. Mn–O_i distances fall in the range 1.858–1.930 Å for 2, 1.877–1.908 Å (A) and 1.881–1.929 Å (B) for 3, whereas 1.862–1.933 Å for 4. The Mn–N_i bond distances are: 2.063–2.30 Å for 2, 2.091–2.277 Å (A) and 2.087–2.252 Å (B) for 3 whereas 2.082–2.310 Å for 4. Some selected bond distances and angles for complexes 2–4 are summarized in Table 2. The non-covalent interactions e.g. hydrogen bonding, π⋯π and C–H⋯π stacking have been taken into account to clarify the possible supramolecular topologies.

Fig. 1 The molecular view of complex 2. All H-atoms are omitted for clarity.

Fig. 2 The molecular view of complex 3. All H-atoms are omitted for clarity.

Fig. 3 The molecular view of complex 4. All H-atoms are omitted for clarity.

Table 2 Selected bond distances and bond angles of complexes 2–4

2		3		4
Mn1–O1	1.858(3)	Mn1–O2	1.909(8)	Mn1–O1	1.866(5)
Mn1–O2	1.931(3)	Mn1–N2	2.277(10)	Mn1–O2	1.934(6)
Mn1–O3	1.879(3)	Mn1–O3	1.877(6)	Mn1–O3	1.862(5)
Mn1–N1	2.300(3)	Mn1–N1	2.091(10)	Mn1–N1	2.281(7)
Mn1–N2	2.234(3)	Mn1–O1	1.905(6)	Mn1–N2	2.082(7)
Mn1–N3	2.063(3)	Mn1–N3	2.244(8)	Mn1–N3	2.310(7)
		Mn2–O5	1.893(6)
		Mn2–O6	1.881(6)
		Mn2–O4	1.930(8)
		Mn2–N5	2.253(10)
		Mn2–N6	2.245(10)
		Mn2–N4	2.087(10)
2		3		4
O1–Mn1–O2	90.63(12)	O1–Mn1–O2	91.2(3)	O1–Mn1–O2	88.1(2)
O1–Mn1–O3	175.94(13)	O1–Mn1–O3	178.5(3)	O1–Mn1–O3	179.8(3)
O1–Mn1–N1	87.57(13)	O1–Mn1–N1	90.4(3)	O1–Mn1–N1	94.3(2)
O1–Mn1–N2	86.65(13)	O1–Mn1–N2	91.4(3)	O1–Mn1–N2	88.9(3)
O1–Mn1–N3	90.03(14)	O1–Mn1–N3	86.8(3)	O1–Mn1–N3	95.6(3)
O2–Mn1–O3	90.40(13)	O2–Mn1–O3	89.7(3)	O2–Mn1–O3	91.8(2)
O2–Mn1–N1	82.65(12)	O2–Mn1–N1	177.4(3)	O2–Mn1–N1	92.2(3)
O2–Mn1–N2	95.26(12)	O2–Mn1–N2	83.6(3)	O2–Mn1–N2	175.1(3)
O2–Mn1–N3	170.65(13)	O2–Mn1–N3	88.8(3)	O2–Mn1–N3	83.1(3)
O3–Mn1–N1	96.46(14)	O3–Mn1–N1	88.8(3)	O3–Mn1–N1	85.5(2)
O3–Mn1–N2	89.35(14)	O3–Mn1–N2	87.5(3)	O3–Mn1–N2	91.2(3)
O3–Mn1–N3	89.60(15)	O3–Mn1–N3	94.5(3)	O3–Mn1–N3	84.6(2)
N1–Mn1–N2	173.83(15)	N1–Mn1–N2	94.3(3)	N1–Mn1–N2	91.9(3)
N1–Mn1–N3	88.07(12)	N1–Mn1–N3	93.4(3)	N1–Mn1–N3	168.9(2)
N2–Mn1–N3	94.09(13)	N2–Mn1–N3	172.1(3)	N2–Mn1–N3	93.3(2)
		O5–Mn2–N6	87.2(3)
		O6–Mn2–N4	88.5(3)
		O6–Mn2–N5	87.8(3)
		O6–Mn2–N6	93.5(3)
		N4–Mn2–N5	94.2(3)
		N4–Mn2–N6	93.7(3)
		N5–Mn2–N6	172.0(3)
		O4–Mn2–O5	91.0(3)
		O4–Mn2–O6	89.8(3)
		O4–Mn2–N4	177.4(3)
		O4–Mn2–N5	83.8(3)
		O4–Mn2–N6	88.4(3)
		O5–Mn2–O6	178.9(3)

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In complex 2 the supramolecular interactions between: (i) aromatic hydrogen and one phenoxido oxygen (green dotted) (ii) π(aromatic)⋯π(aromatic) (magenta dotted lines) and (iii) C–H⋯π (green dotted lines) lead to the formation of supramolecular 1D chains along the crystallographic c axis (Fig. 4) and these 1D chains are further extended to 2D network (Fig. S9†) via inter-locking Br⋯H interactions among themselves in the crystallographic ab plane. Similarly, complex 3 exhibits the same supramolecular topologies (Fig. 5 and S10†) via hydrogen bonding as well as C–H⋯π interactions in crystallographic bc plane. The hydrogen bonding (Br⋯H and Cl⋯H) and C–H⋯π interactions for complex 4 also play a crucial role for the formation of H-bonded supramolecular network (Fig. S11†).

Fig. 4 The supramolecular interactions: H-bonding (green dotted lines), CH⋯π(phenyl) (blue lines) and π(phenyl)⋯π(phenyl) (magenta lines) are combined to form 1D network along crystallographic c-axis in 2. All H-atoms except those involved in interactions are omitted for clarity.

Fig. 5 The supramolecular interactions: H-bonding (green dotted line), CH⋯π (phenyl) (blue dotted line) are combined to form 1D framework along crystallographic bc plane in 3.

Electronic spectra of complexes

The electronic spectra of all the four complexes were recorded in DCM, the details of which are incorporated in Table 3. Bands at ∼333 and 389 nm correspond to the LMCT transitions (Fig. 6).⁴⁴

Table 3 Summary of electronic spectra of Mn(III) complexes in DCM

Complexes	λmax (nm)	ε (dm^3 mol^−1 cm^−1) × 10^4	λmax (nm)	ε (mol^−1 dm^3 cm^−1) × 10^5
1	389	6.82	324	1.32
2	385	1.67	327	0.38
3	388	1.06	329	0.20
4	389	2.69	328	0.43

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Fig. 6 Electronic spectra of manganese(III) complexes 1–4 in DCM.

Catecholase activity

General features. The reaction of excess 3,5-ditertiarybutyl catechol (3,5-DTBC) with [Mn(Lⁱ)₃] in DCM leads to the formation of 3,5-ditertiarybutyl quinone (3,5-DTBQ) as shown in eqn (1). Fig. 7 and S12, S13† show time resolved spectra of the reaction of Mn(III) complexes (1–4) with 3,5-DTBC. When the complexes, 1–4 (∼1.0 × 10⁻⁵ M) were treated with 3,5-DTBC in DCM, under aerobic conditions there was a gradual increase in absorbance at 403 nm, characteristics of the formation of 3,5-DTBQ.

Fig. 7 Time-resolved spectra for the reaction of 1 and 2 with 5-DTBC in DCM.

Kinetics. The formation of 3,5-DTBQ was monitored with time at a wavelength of ∼400 nm (Fig. 7). The observed rate constants (k_i) were extracted by the initial rate method. Plots of k_i vs. [3,5-DTBC] gave non-linear curve of decreasing slope (Fig. 8) which are best described by eqn (2). A reasonable rate law can be derived as described in eqn (3), where V = initial rate constant (k_i); [S] = concentration of the substrate, 3,5-DTBC; K_M = (k₂ + k₃)/k₁, Michaelis–Menten constant for the metal complex and V_max = maximum initial rate attained for a particular concentration of the metal complex in the presence of a large excess of the 3,5-DTBC. The liner form of the Michaelis–Menten equation is given in eqn (4), known as Lineweaver–Burk equation and corresponding fitting is shown in Fig. S14.† Non-linear fitting of data to Michaelis–Menten eqn (3) leads to the evaluation of V_max, K_M and K_cat = K_M/[complex] and all these parameters are listed in Table 4.

Fig. 8 Plots of k_i vs. [3,5-ditertiarybutyl catechol] for the reaction of 3,5-DTBC with molecular oxygen catalyzed by manganese(III) complexes.

Table 4 Summary of catecholase activity of 1–4 with 3,5-ditertiarybutyl catechol in DCM. Conditions: [c] = 1.0 × 10⁻⁵ M

Complex	10^4 × KM	10^3 × Vmax (M s^−1)	Kcat (s^−1)
1	1.79 ± 0.28	9.84 ± 3.27	17.9
2	3.48 ± 0.48	5.69 ± 0.22	34.8
3	1.70 ± 0.63	3.39 ± 0.27	17.0
4	2.51 ± 0.45	7.51 ± 0.33	25.1

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It is interesting to note that there are only few examples of where manganese complexes were claimed to display CO activities.^{21,25,45–49} As for example, radical Mn(IV) complexes catalyzed the oxidation of 3,5-DTBC to 3,5-DTBQ in presence of molecular oxygen under mild conditions.^44,45 Some mononuclear Mn(III) complexes also found to favour the oxidation of catechol to quinone where direct involvement of Mn(III) was suggested.⁴⁵ Table 5 displays the catalytic activities of some mononuclear Mn(III) as well as some multinuclear complexes. It is worth mentioning that our complexes showed the highest TON (K_cat, h⁻¹) for the catalytic oxidation of 3,5-DTBC to 3,5-DTBQ under mild conditions by molecular oxygen and the existence of 3,5-DTBQ was identified from ESI-MS⁺ (m/z) study (Fig. 9).

Table 5 Catalytic activity of synthetic catecholase mimic

Compound	Kcat (h^−1)	10^4 × KM (M)	Kcat/KM (s^−1 M^−1)	Ref.
1	6.44 × 10^4	1.79	9.84	This work
2	1.25 × 10^5	3.48	5.69	This work
3	6.12 × 10^4	1.70	3.39	This work
4	9.04 × 10^4	2.51	7.51	This work
[Mn(bpia)(OAc)(OCH3)](PF6)	86	15.0	16	21
[Mn(bipa)(OAc)(OCH3)](PF6)	101	12.0	23	21
[Mn(bpia)(Cl)2](ClO4)	230	13.0	49	21
[Mn(bipa)(Cl)2](ClO4)	130	8.0	45	21
[Mn(diclofenac)2(H2O)]	225			46
[Mn(tpa)2](ClO4)2	4			47
[MnL^1(OOCH)(OH2)]	936.64	5.50	1.606 × 10^−4	25
[MnL2(OH2)^2][Mn2L2^2(NO2)3]	365.34	6.41	6.089 × 10^−5	25
[Mn2L2^1(NO2)2]	1432.74	49.17	2.388 × 10^−4	25
[Mn2^IIIMn4^IIO2(pyz)2(C6H5CH2COO)10]n	2.547 × 10^3	1.75	7.076 × 10^−5	48
Mn2L^8Cl4·4H2O	3.60 × 10^3	9.00	1.00 × 10^−4	26

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Fig. 9 ESI-MS⁺ (m/z) spectrum for the ultimate catecholase product during the reaction of 2 with 3,5-DTBC.

Magnetic study. Magnetic measurements have been carried out on all three mononuclear crystalline complexes 2–4. Variable-temperature DC magnetic susceptibility (χ_M) of complexes 2–4 is investigated in the temperature range of 2–300 K under a magnetic field of 0.1 T. Fig. 10 shows the variation of susceptibility in the form of χ_MT vs. T. The observed χ_MT values (2.95 and 2.65 cm³ K mol⁻¹) for complexes 2 and 3 at 300 K are close to the value obtained for uncoupled high spin S = 2 Mn(III) system (g = 2), while the same is quite low for complex 4 (1.21 cm³ K mol⁻¹). Upon lowering the temperature for complexes 2 and 3, the χ_MT product remains almost constant between 300 to 25 K and then decreases sharply to lower temperature regime down to 2 K, signifying the presence of magnetic anisotropy (zero-field splitting). For complex 4, the χ_MT product starts increasing from 300 to 25 K and follows the same path at lower temperatures similar to complexes 2 and 3. This anomalous behavior is due to the presence of strong supra-molecular π⋯π stacking interaction (intermolecular distance: 3.890 Å) in complex 4 which is weak in complex 3 and almost absent in complex 2. This mediates the ferromagnetic exchange coupling (+0.16 cm⁻¹) between the two Mn(III) ions of the adjacent mononuclear units in complex 4. The above susceptibility behavior is fitted using the following axial spin Hamiltonian (5).where, the specific terms have their usual meaning.

Fig. 10 The variation of χ_MT (experimental points) with temperature for the three complexes 2, 3 and 4. Solid lines represent the theoretical curve and the points are the experimental data.

Weak intermolecular interaction (J) acting between two Mn(III) ions is incorporated in the expression (5) with zero field splitting (D) parameter to obtain the best fit parameters for g (Lande factor) and D (ZFS). The χ_MT data of the complexes were fitted assuming the same values of Lande g factors (g₁ and g₂) and the ZFS terms (D₁ and D₂) for the two exchange spin systems (S₁ and S₂). The magnetization data were fitted by taking the same g values obtained from the fitted plots of χ_MT vs. T (Fig. 11 and S15, S16†). This also leads to the same values of exchange and ZFS parameters, summarized in Table 6. In each case, the best fitted curve was obtained by the negative values of zero-field splitting (D) parameter.

Fig. 11 M vs. H plots collected at 2, 4 and 5 K for complex 2. Solid lines represent the theoretical curve and the points are the experimental data.

Table 6 Results of fitting of dc magnetic data and comparison with related systems^a

Complex	D (cm^−1)	J (cm^−1)	g	10^4 × R^2
a Estimated errors: D (= ±0.02), J (= ±0.01); g (= ±0.01).b Ref. 49. Dbm = dibenzoylmethane.
Mn(L^2)3 (2)	–2.96	–0.08	1.98	1.89
Mn(L^3)3 (3)	–3.51	–0.06	1.97	3.78
Mn(L^4)3 (4)	–3.72	+0.16	1.98	5.34
[Mn(dbm)3]^b	−4.52		2.03	1.30
[Mn(dbm)2(DMSO)2](ClO4)^b	−3.42		1.92	4.10
[Mn(dbm)2(py)2](ClO4)^b	−4.46		1.97	0.47

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Table 6 displays ZFS values (D) for some recently reported octahedral Mn(III) complexes along with our results. The large negative D values are consistent with the Jahn–Teller axis elongation of the octahedral geometry.

Conclusion

We have synthesized four mononuclear Mn(III) complexes of bidentate Schiff base ligands out of which three (2–4) were characterized structurally and found to have octahedral geometries which showed excellent catechol oxidase activity with very high, probable the highest, turn over number (TON) of the so far reported Mn(III) complexes. Another interesting aspect of this study is that these complexes being in regular octahedral geometry and devoid of any labile ancillary ligand which encourages the coordination of catechol to the metal center also showed CO activity. Cryomagnetic studies on complexes 2–4 gives negative ZFS values, comparable to recently reported values.

Acknowledgements

Financial support from CSIR (ref. 02(2490)/11/EMR-II) and UGC [39-735/2010(SR)], New Delhi, are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. CCDC [1041929(2), 1041930(3), 1041931(4)]. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra00239g

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