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. 2024 Dec 5;9(50):49953–49965. doi: 10.1021/acsomega.4c09254

Supramolecular Dimeric MnIII Complexes: Synthesis, Structure, Magnetic Properties, and Catalytic Oxidation Studies

Narayan Ch Jana †, Zvonko Jagličić ‡, Paula Brandão §, Anangamohan Panja †,∥,*
PMCID: PMC11656400  PMID: 39713639

Abstract

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In this study, a tetradentate Schiff-base ligand (H2L), synthesized by the condensation of ethylenediamine with 2-hydroxy-3-methoxy-5-methylbenzaldehyde, was reacted with either manganese salts or manganese salts in the presence of various pseudohalides in methanol. This reaction resulted in the formation of five mononuclear MnIII complexes: [Mn(L)(H2O)2](NO3)·1/2H2O·1/2CH3OH (1), [Mn(L)(H2O)2](ClO4)·H2O (2), [Mn(L)(N3)(H2O)]·1/3H2O (3), [Mn(L)(NCS)(H2O)] (4), and [Mn(L)(H2O)2](dca) (5) (where dca is dicyanamide ion). X-ray crystallography revealed that the MnIII centers adopt a hexa-coordinate pseudo-octahedral geometry, where the equatorial plane is constructed with phenoxo oxygen and imine nitrogen atoms from the Schiff base ligand, while the axial positions are occupied by water molecules or a combination of water and pseudohalides. Supramolecular interactions, primarily π–π stacking and hydrogen bonding, contribute to the formation of pseudodimeric structures in the solid state. Magnetic susceptibility measurements indicated antiferromagnetic coupling within quasi-dimers, primarily through hydrogen bonds. Catalytic studies showed that the complexes effectively catalyze the aerobic oxidation of substrates such as 2-aminophenol and 3,5-di-tert-butylcatechol to yield 2-aminophenoxazin-3-one and 3,5-di-tert-butylquinone, respectively. They also catalyze the oxidation of styrene to its corresponding oxirane, demonstrating their versatile catalytic proficiency. Mechanistic insights, supported by ESI mass spectrometry and EPR studies, suggest that catalysis involves the formation of a complex–substrate aggregate, followed by an intramolecular electron transfer.

Introduction

The exploration of manganese coordination chemistry with a diverse range of ligands has long been of interest to both chemists and physicists.1 This interest primarily stems from the potential use of manganese complexes in molecular magnetism, with promising applications in areas such as quantum computing and high-density data storage.2−8 The field gained significant attention after the accidental discovery of slow magnetization relaxation originating purely at the molecular level in a mixed-valent Mn12 cluster.9 The Jahn–Teller distortion associated with the MnIII ion generates substantial easy-axis magnetic anisotropy, a feature prominently observed in numerous single-molecule magnets (SMMs) reported over the past two decades.10−22 From a biological perspective, the redox flexibility of manganese, allowing it to switch between different oxidation states, has been exploited by nature.23−25 For example, manganese ions play a critical role in the active sites of several redox metalloenzymes, including catalase, superoxide dismutase, ribonucleotide reductase, arginase, bacterial thiosulfate oxidases, and the oxygen-evolving complex (OEC) in photosystem II (PS-II).26−28 Bioinorganic chemists have consequently focused on understanding the structural, spectroscopic, and mechanistic properties of these metalloenzymes by modeling their active sites.29 This research aids in the development of efficient catalysts to replace traditional toxic industrial catalysts. Furthermore, the rich electrochemical properties and structural diversity of manganese compounds have made them indispensable in industrial catalysis, such as in epoxidation, C–H activation,30,31 and oxidative cyclization reactions.32,33

MnIII complexes featuring salen-type tetradentate Schiff bases, derived from the condensation of various diamines with salicylaldehyde or its derivatives, have significantly contributed to the development of molecular magnetism.2,3,34−37 These complexes typically fall into three categories: (i) anion-bridged coordination chains, which frequently display magnetic anisotropy and easy-axis magnetization resulting from Jahn–Teller elongation at the MnIII centers, with some examples identified as low-temperature single-chain magnets (SCMs) due to their slow magnetization relaxation35,37; (ii) phenoxido-bridged dimeric structures, where the nature of magnetic exchange interactions depends on the bridge geometry, with some acting as SMMs at low temperatures,2,3 and (iii) mononuclear complexes, where the axial positions are usually occupied by solvent molecules, leading to weak magnetic coupling through intermolecular contacts.2,3 Additionally, the latter two classes of compounds have been employed as catalysts in the epoxidation of olefins,38 C–H activation, and oxidative cyclization.30−33 In this context, catalytic oxidation of styrene using Mn-salen complexes is popular in which transformation of styrene into various oxygenated products was observed.38−40 This reaction is significant in the chemical industry due to the value of the resulting compounds, such as styrene oxide, benzaldehyde, and phenylacetaldehyde, which serve as precursors in the production of polymers, resins, and pharmaceuticals. Thus, the development of various manganese complexes in different oxidation states, including these salen-type MnIII complexes, has been motivated by the desire to mimic the structural, spectroscopic, and catalytic properties of native enzymes.

Our research group has been actively engaged in developing synthetic model complexes that mimic the catalytic functions of the copper-containing enzymes phenoxazinone synthase (PHS) and catechol oxidase (CAO) (Scheme 1).41−55 CAO catalyzes the oxidation of catechols to their corresponding quinones, which serve as key intermediates in the autopolymerization reaction that produces melanin,56 a natural antiseptic pigment protecting plant tissues from pathogens and insects. In contrast, PHS catalyzes the oxidative coupling of o-aminophenols to produce the phenoxazinone chromophore, a crucial component in the biosynthesis of actinomycin D,57 an antitumor drug used in the treatment of various cancers.58−60 While several multinuclear copper complexes have successfully modeled these metalloenzymes, there is a need to develop manganese complexes as they are considered excellent alternatives to copper complexes for these activities.56 In line with our interest in molecular magnetism and bioinorganic chemistry, we aim to develop MnIII salen-type Schiff base complexes using our recently synthesized Schiff base, N,N′-bis(3-methoxy-5-methylsalicylidene)-ethane-1,2-diamine (H2L) (Scheme 2),54 as these complexes are potential sources of homogeneous catalysts and may exhibit interesting magnetic properties due to the inherent easy-axis magnetic anisotropy at MnIII centers arising from Jahn–Teller elongation. Accordingly, in this study, we allowed ligand H2L to react with various MnIII salts under aerobic conditions, both in the presence and absence of pseudohalides, resulting in the successful isolation of five supramolecular dimeric MnIII complexes: [Mn(L)(H2O)2](NO3)·1/2H2O·1/2CH3OH (1), [Mn(L)(H2O)2](ClO4)·H2O (2), [Mn(L)(N3)(H2O)]·1/3H2O (3), [Mn(L)(NCS)(H2O)] (4), and [Mn(L)(H2O)2](dca) (5). Magnetic measurements were performed on all the synthesized compounds to explore their magnetic exchange interactions and anisotropy. Temperature- and field-dependent magnetic data were further analyzed simultaneously, considering isotropic exchange constants (J) and single-ion zero-field splitting parameters (D). Moreover, all the complexes exhibited catalytic activity in the aerobic oxidation of various substrates. EPR spectroscopy and mass spectrometry were employed to gain a deeper understanding of the oxidation mechanism.

Scheme 1. Phenoxazinone Synthase and Catechol Oxidase Mimetic Activities, As Well As Styrene Oxidation.

Scheme 1

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Scheme 2. Scheme Illustrating the Synthesis of Schiff Base H2L and the Formation of Its MnIII Complexes.

Scheme 2

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Experimental Section

Materials and Physical Measurements

Schiff base N,N′-bis(3-methoxy-5-methylsalicylidene)-ethane-1,2-diamine (H2L) was reproduced following a standard reported method.54 All other reagents were purchased from commercial sources and used as received. Caution! Metal perchlorates and azides can be highly explosive, particularly when combined with organic ligands, and should be handled with extreme care.

Elemental analysis for C, H, and N was performed using a PerkinElmer 240C CHNS Organic Elemental Analyzer. Infrared (IR) spectra for all complexes were recorded in the range of 500–4000 cm–1. Electronic absorption spectra of the complexes were obtained at ambient temperature using an Agilent Cary-60 diode array spectrophotometer. Electrospray ionization mass spectrometry (ESI-MS) in positive mode was carried out on a Micromass Q-Tof-Micro Quadrupole mass spectrometer. X-band EPR spectra of selected complexes, both with and without substrates, were measured in methanol at ambient temperature using a JEOL JES-FA 200 instrument. Magnetic susceptibility measurements were performed on a Quantum Design MPMS-XL-5 magnetometer over the temperature range of 2–370 K in a constant magnetic field of 1 kOe. Diamagnetic corrections for both the samples and the sample holder were applied to the obtained data.

Synthesis of [Mn(L)(H2O)2](NO3)·1/2H2O·1/2CH3OH (1)

Mn(NO3)2·4H2O (125 mg, 0.5 mmol) and Schiff base ligand H2L (178 mg, 0.5 mmol) were dissolved in 30 mL of methanol with stirring, followed by the addition of triethylamine (1.0 mmol). The resulting solution was then refluxed for 1 h, during which the solution turned dark brown. After cooling to room temperature, the reaction mixture was filtered, and the filtrate was left to evaporate slowly at ambient temperature, yielding dark-brown crystals with distinct shapes, suitable for single-crystal X-ray diffraction analysis. These crystals were collected by filtration, washed with a methanol/ether solution, and air-dried. Yield: 0.229 g (85%). Anal. calcd. for C21H30N3O10Mn: C 46.76%; H 5.61%; N 7.79%. Found: C 46.51%, H 5.47%, N 7.56%. FTIR (KBr, cm–1): ν̅ (O–H) 3404 br; ν̅ (C=N) 1618 s; ν̅ (NO3) 1384 s.

Synthesis of [Mn(L)(H2O)2](ClO4)·H2O (2)

Complex 2 was synthesized by following a very similar reaction protocol as that adopted for complex 1, with the exception that Mn(ClO4)2·6H2O was used instead of Mn(NO3)2·4H2O. Yield: 0.242 g (89%). Anal. calcd. for C40H56N4O22Cl2Mn2: C 42.68%; H 5.01%; N 4.98%. Found: C 42.95%, H 4.89%, N 4.75%. FTIR (KBr, cm–1): ν̅ (O–H) 3392 br; ν̅ (C=N) 1619 s; ν̅ (ClO4) 624 br, 1078 br.

Synthesis of Complexes 3, 4, and 5

Complexes 3, 4, and 5 were isolated in good yields using a procedure similar to that for complex 1, but with Mn(OAc)2·4H2O and the respective sodium salts of pseudohalides instead of Mn(NO3)2·4H2O, and without the addition of base to deprotonate the ligand.

[Mn(L)(N3)(H2O)]·1/3H2O (3)

Yield: 0.197 g (81%). Anal. calcd. for C20H26MnN5O6: C 49.29%; H 5.38%; N 11.27%. Found: C 49.05%, H 5.23%, N 11.43%. FTIR (KBr, cm–1): ν̅ (O–H) 3442 br; ν̅ (N3–) 2025 s; ν̅ (C=N) 1621 s.

[Mn(L)(NCS)(H2O)] (4)

Yield: 0.201 g (83%). Anal. calcd. for C21H24N3O5SMn: C 51.96%; H 4.98%; N 8.66%. Found: C 51.77%, H 5.13%, N 8.48% FTIR (KBr, cm–1): ν̅ (O–H) 3406 br; ν̅ (NCS–) 2046 s; ν̅ (C=N) 1615 s.

[Mn(L)(H2O)2](dca) (5)

Yield: 0.196 g (77%). Anal. calcd. for C22H26N5O6Mn: C 51.67%; H 5.12%; N 13.69%. Found: C 51.35%, H 5.28%, N 13.51%. FTIR (KBr, cm–1): ν̅ (O–H) 3398 br; ν̅ (N(CN)2–) 2183 s; ν̅ (C=N) 1617 s.

X-ray Crystallography

Single-crystal X-ray crystallographic studies of complexes 1–5 were conducted using a Bruker Kappa Apex-II CCD diffractometer equipped with a graphite monochromator (Mo–Kα, λ = 0.71073 Å). The collected data were processed with the Bruker APEX-II software suite, including absorption corrections using the multiscan method in SADABS,61 and intensity corrections for Lorentz-polarization effects. The structures were solved using direct methods with SHELXT 2014/5 and refined by weighted full-matrix least-squares fitting on F2 using SHELXL 2018/3.62 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms on oxygen atoms of solvent molecules were located from the difference Fourier map and refined with constraints where necessary, while hydrogen atoms on carbon were placed at idealized positions with fixed thermal parameters. Crystal structures and packing diagrams were generated using MERCURY 4.3.1 and POV-ray software. Additional crystallographic information and refinement parameters are assembled in Table S1.

Biorelevant Catalytic Studies

The aerobic oxidation of substrates, 3,5-di-tert-butylcatechol and o-aminophenol, was catalytically carried out by reacting 1 × 10–4 or 5 × 10–5 M of the complexes with 1.0 × 10–2 M substrates in acetonitrile (for 3,5-di-tert-butylcatechol) and methanol (for o-aminophenol) at room temperature. The reaction kinetics were studied by monitoring the increase in absorbance over time at the respective absorption maxima of the products. To study the effect of substrate concentration on the reaction rate and determine the kinetic parameters, solutions of the complexes at a concentration of 1 × 10–5 M were reacted with a minimum of 10 equiv of substrate, maintaining pseudo-first-order conditions. All kinetic studies were performed over a 10 min period, and the initial reaction rate was calculated by performing linear regression of the absorbance versus time data.

Procedure for Styrene Oxidation

Styrene (0.052 g, 0.5 mmol), along with x mol % of manganese complexes 1–5 and y mol % of oxidant PHIO, was weighed and placed in a 10 mL reaction tube. Then, 2 mL of solvent was added to the tube, and the mixture was stirred for a specified time. Gas chromatography measurements were conducted to measure the products conversion at specific time intervals.

Results and Discussion

Synthesis and Characterization of 1–5

The tetradentate Schiff-base ligand (H2L) was synthesized by combining ethylenediamine with 2-hydroxy-3-methoxy-5-methylbenzaldehyde in a 1:2 ratio in methanol, as described in our recently reported paper.54 Thereafter, this Schiff base ligand was reacted either with MnII salts or combination of MnII salts and pseudohalide ions in methanol in aerobic condition, yielding dark brown crystals of hydrogen-bonded supramolecular dimeric MnIII complexes: [Mn(L)(H2O)2](NO3)·1/2H2O·1/2CH3OH (1), [Mn(L)(H2O)2](ClO4)·H2O (2), [Mn(L)(N3)(H2O)]·1/3H2O (3), [Mn(L)(NCS)(H2O)] (4), and [Mn(L)(H2O)2](dca) (5), all obtained in high yield (Scheme 2). In each complex, the MnII ion is air oxidized in the presence of the deprotonated Schiff base ligand. Triethylamine is required for the deprotonation of the Schiff base in complexes 1 and 2, while in complexes 3–5, the acetate ions from Mn(OAc)2·4H2O facilitated the deprotonation during the complexation process. Initial characterization of all these complexes was performed by elemental analysis and IR spectroscopic studies (Figure S1). The IR spectra of complexes 1–5 display a strong band in the range of 1615–1621 cm–1, corresponding to the azomethine C=N bond of the Schiff base. The O–H stretching vibrations of aqua ligands and lattice water molecules are observed between 3392 and 3442 cm–1. Additionally, the bending and stretching vibrations of perchlorate counterions in complex 2 are observed at 624 and 1078 cm–1, respectively. In complex 1, the stretching band for the nitrate counterion is observed at 1384 cm–1, while the characteristic stretching vibrations of pseudohalide ions in complexes 3–5 are found at 2025, 2046, and 2183 cm–1, respectively.

Description of the Crystal Structures

X-ray crystallographic studies showed that complexes 1 and 2 crystallized in the orthorhombic crystal system, with space groups Pccn and Pna21, respectively. Complexes 3 and 5 were found to crystallize in the monoclinic P21/c space group, while complex 4 crystallized in the triclinic P1̅ space group. The crystal structures with selected atom labeling for complexes 1–5 are shown in Figures 1 and S2, with selected bond parameters listed in Table 1. All complexes feature similar mononuclear MnIII structures, with differences arising from the coordination of solvents or pseudohalides at the axial positions of the MnIII ions. Additionally, the crystal structures accommodate varying types and quantities of lattice solvent molecules and counterions, if necessary, to balance the overall charge of the system. In complex 2, the asymmetric unit consists of two crystallographically independent complex molecules along with two highly disordered perchlorate ions for electroneutrality. In contrast, the other complexes contain only a single complex cation or molecule in the asymmetric unit. The highly disordered perchlorate ions may contribute to the loss of additional symmetry in the solid state in complex 2.

Figure 1.

Figure 1

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Crystal structures (ellipsoid plots with 30% probability) of complexes 1 (left) and 3 (right) with selective atom numbering schemes.

Table 1. Selected Bond Parameters of Complexes 1–5.

bonds 1 2 3 4 5
A B
Mn–O (phenolate) 1.8856(10) 1.889(3) 1.889(3) 1.882(3) 1.8832(10) 1.8783(19)
1.8806(10) 1.884(3) 1.883(3) 1.888(3) 1.8872(9) 1.884(2)
Mn–O (water) 2.2569(11) 2.250(4) 2.252(4) 2.324(4) 2.2979(10) 2.378(2)
2.2285(13) 2.235(4) 2.254(4) 2.328(2)
Mn–N (imine) 1.9852(13) 1.980(4) 1.982(4) 1.972(4) 1.9834(11) 1.972(2)
1.9854(12) 1.981(4) 1.983(4) 1.994(4) 1.9746(13) 1.968(2)
Mn–N (pseudohalide)       2.242(5) 2.2216(14)  
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In all the structures, the MnIII centers exhibit a hexa-coordinate pseudo-octahedral geometry, with an equatorial plane formed by two phenoxido oxygen atoms and two imine nitrogen atoms from the doubly deprotonated ligand, a typical coordination pattern for N2O2 donor Schiff base-metal complexes. The axial sites of the MnIII centers are coordinated by different ligands across the complexes: in 1, 2, and 5, they are coordinated by aqua ligands, while in complexes 3 and 4, one of the axial sites is coordinated by an aqua ligand and the other by pseudohalides (azide ion in 3 and thiocyanate ion in 4). The Mn–O and Mn–N bond lengths of the Schiff base fall within the typical ranges of 1.8783(19)–1.889(3) and 1.968(2)–1.994(4) Å, respectively, while the axial Mn–O bond lengths (2.2285(13)–2.378(2) Å) for the coordinated aqua ligands in 1–5 and the axial Mn–N lengths of 2.242(5) and 2.2216(14) Å for the coordinated azide and thiocyanate ions in 3 and 4, respectively, are significantly longer. This elongation is typical for Jahn–Teller effect observed in high-spin MnIII complexes.63−66 The equatorial N2O2 donors-set from the Schiff base ligand are almost coplanar, with minimal deviation of the MnIII centers from the planes.

Inspection of crystal packing reveals that adjacent mononuclear units are linked in an antiparallel arrangement through π–π interactions, leading to a supramolecular dimeric structure. These dimers are further stabilized by hydrogen bonding interactions (Table S2), where the coordinated water molecules from adjacent complex molecules are trapped inside the O2O′2 core of the Schiff base, as depicted in Figures 2 and S3–S6. These supramolecular dimers experience various noncovalent interactions for further stability in the solid state, depending on the coordinated ligands at the other axial positions and the availability of counterions and solvent molecules at lattice sites. For instance, in complex 1, both counteranions and lattice solvent molecules contribute to formation of a hydrogen-bonded hexagonal ring structure involving two coordinated H2O molecules, two methanol molecules, and two nitrate ions, resulting in the formation of a hydrogen-bonded chain of supramolecular dimers along the crystallographic b axis (Figure 3 and Table S2). A similar hydrogen-bonded chain along the crystallographic b axis is observed in complex 2 (Table S2). However, in this case, the hydrogen-bonded hexagonal ring structure is formed by the participation of two coordinated and two lattice H2O molecules, along with two perchlorate counterions, as shown in Figure S3. On the other hand, in the absence of any lattice solvent molecules in complex 5, only the counter dicyanamide ion participates in hydrogen bonding interactions (Table S2), forming an extensive network of hydrogen-bonded sheet structures in the bc plane (Figure S6). In contrast, the coordination of pseudohalide ions (azide in complex 3 and thiocyanate in complex 4) at the other axial positions of the MnIII centers blocks further propagation via conventional hydrogen bonding interactions in these two complexes. Consequently, the C–H···π and/or π–π interactions, along with nonconventional C–H···O/N hydrogen bonds bring the solid-state stability in these complexes (Figures S4 and S5).

Figure 2.

Figure 2

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Hydrogen-bonded and π–π stacked supramolecular dimers of 2 (or 5) and 4.

Figure 3.

Figure 3

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Hydrogen-bonded hexagonal ring structure involving two coordinated H2O, two methanol molecules, and two nitrate ions, resulting in the formation of a hydrogen-bonded chain of supramolecular dimers of 1 along the crystallographic b axis.

Magnetic Studies

The magnetic susceptibility of polycrystalline samples of compounds 1–5 was measured in a direct current (dc) magnetic field of 1000 Oe over a temperature range of 2–300 K, with the data presented as χT versus T (where χ represents the dc magnetic susceptibility per formula unit), as shown in Figure 4. At room temperature, the χT values for compounds 1–5 range from 2.91 to 3.03 emu K/mol, which are close to the theoretical value for a high-spin, noninteracting MnIII ion with S = 2 and g = 2.0. As cooled, χT remains nearly constant down to 50 K, after which it abruptly decreases to values between 0.29 and 0.42 emu K/mol for compounds 1–5. In all the studied complexes, the formation of quasi-dimers between [Mn(L)(H2O)2]+ subunits for 1, 2, and 5 or [Mn(L)(X)(H2O)] subunits for 3 (X = azide ion) and 4 (X = thiocyanate ion), held together by hydrogen bonding interaction involving coordinated aqua ligands and O2O′2 core of the Schiff base. Within these quasi-dimers, the Mn···Mn distances range narrowly from 4.73 to 4.92 Å, while the separations between dimers are slightly larger, ranging from 6.76 to 8.16 Å. These structural features indicate that the superexchange mechanism mainly operates through hydrogen bonds. The magnetic exchange interaction can be assessed by examining the temperature dependence of susceptibility and the χT products of these compounds. The observed decrease in χT values below 50 K, along with the presence of a susceptibility maximum at temperatures below 10 K, indicates the presence of antiferromagnetic coupling between the homospin MnIII ions in the quasi-dimers. Additionally, the influence of zero-field splitting on the magnetic properties must be considered, particularly for MnIII ions with a tetragonally elongated geometry. The sigmoidal shape of the isothermal magnetization curve, along with the far-from-saturation values at the highest applied field, further supports the idea of antiferromagnetic coupling within the pseudodimers and the zero-field splitting effect on the MnIII ions, which contribute to the overall magnetic behavior of these systems. The DC magnetic data for these complexes were quantitatively analyzed using the following spin Hamiltonian:

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Figure 4.

Figure 4

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Magnetic data for 1–5 are presented, with each plot displaying the temperature dependence of the χT product at a 1.0 kOe magnetic field and the isothermal magnetization curves measured at T = 2.0, 5.0, and 10 K for 1–3 and at T = 2.0 K for 4 and 5. The empty symbols represent the experimental data, while the full lines correspond to the best fit obtained using the PHI program, based on the equations outlined in the text, and the best fit parameters for 1–5 are provided in tabular form.

In this equation, the first term represents the isotropic exchange (J), the second term accounts for zero-field splitting (D), the third term is the Zeeman effect, and the final term includes a molecular-field correction parameter (zj) that arises from weak interdimer interactions. The simultaneous simulation of both temperature- and field-dependent magnetic data using the above Hamiltonian with the PHI program provided the best-fit parameters, as shown in Figure 4, which are in good agreement with literature-reported values for analogous systems.2,3,64,66,67 The dynamic magnetic studies reveal that a representative complex 1 does not display significant amount out-of-phase susceptibly signals, related to slow relaxation of magnetization, presumably due to small zero-field splitting parameters in these systems (Figure S7).

Cyclic Voltammetry

Cyclic voltammetry is a commonly employed electrochemical technique for studying the redox behavior of various compounds, including metal complexes. In this context, MnIII Salen complexes are known for their intriguing electrochemical behavior, which helps in evaluating their potential for catalytic applications. With this intend, we conducted a cyclic voltammetry experiment on a representative complex 4 in acetonitrile, with TBAP (0.1 M) as the supporting electrolyte. The experiment was carried out using an Ag/AgCl reference electrode, a glassy carbon working electrode, and a platinum wire as the auxiliary electrode. The cyclic voltammogram reveals characteristic redox peaks corresponding to the MnIII/MnII and MnIV/MnIII redox couples, as depicted in Figure 5. The MnIII/MnII couple shows a quasi-reversible behavior with a well-defined anodic and cathodic peak at −0.14 and −0.23 V, indicating a one-electron transfer process.68 Conversely, the MnIV/MnIII redox process is observed within the potential range of 0.75–0.45 V.68 This process is often less quasi-reversible and can involve more complex electron transfer mechanisms, possibly coupled with ligand oxidation or rearrangement. The electrochemical studies indicate that the current MnIII complexes are capable of supporting multiple oxidation states.69,70

Figure 5.

Figure 5

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Cyclic voltammogram of complex 4, measured in acetonitrile with TBAP as the supporting electrolyte at a 100 mV s–1 scan rate and room temperature, referenced against an Ag/AgCl electrode.

Catalytic Studies

Aminophenol oxidation is a crucial chemical process with significant implications across various fields, particularly in pharmaceuticals, dye manufacturing, and environmental chemistry. Hence, the catalytic oxidation of o-aminophenol (OAPH) to 2-aminophenoxazin-3-one was carried using all five manganese(III) complexes as catalysts. The reaction was conducted in air-saturated methanol at room temperature without an external base, and its progress was monitored via UV–vis spectrophotometry. All complexes (1–5) exhibited a consistent increase in absorption at 435 nm in the UV–vis spectra, as shown in Figure 6. Afterward, we conducted kinetic studies to precisely determine various kinetic parameters and evaluate the catalytic efficiency of the complexes. These studies revealed that complexes 1–5 exhibit first-order kinetics for the oxidation of OAPH at lower substrate concentrations, transitioning to rate-saturated kinetics at higher substrate concentrations (see Figure 6). Employing the Michaelis–Menten equation on these data yielded Vmax, KM, and Kcat values for 1–5, as presented in Table 2.

Figure 6.

Figure 6

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Time-course UV–vis spectral profile for the oxidation of o-aminophenol to 2-aminophenoxazin-3-one catalyzed by complexes 1–5 shows a steady increase in absorption at 435 nm.

Table 2. Kinetic Parameters for Substrate Oxidation Catalyzed by MnIII Complexes.

substrate catalyst Vmax(M s–1) KM(M) kcat(h–1)
OAPH 1 (9.40 ± 0.28) × 10–8 (1.12 ± 0.12) × 10–3 33.85
2 (8.77 ± 0.32) × 10–8 (1.59 ± 0.18) × 10–3 31.59
3 (1.30 ± 0.04) × 10–7 (1.15 ± 0.13) × 10–4 46.81
4 (1.30 ± 0.05) × 10–7 (1.47 ± 0.16) × 10–4 46.88
5 (1.14 ± 0.04) × 10–7 (1.11 ± 0.12) × 10–4 41.15
3,5-H2DTBC 3 (5.72 ± 0.08) × 10–8 (7.36 ± 0.38) × 10–4 20.60
4 (5.25 ± 0.08) × 10–8 (7.03 ± 0.41) × 10–4 18.92
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Catalytic experiments with o-aminophenol as the substrate showed that complexes 3 and 4 exhibited the highest catalytic efficiency. To further explore their catalytic potential, the oxidation of 3,5-di-tert-butylcatechol (3,5-H2DTBC) by complexes 3 and 4 was studied by observing the increase in absorbance around 400 nm, indicating the formation of 3,5-di-tert-butylquinone (3,5-DTBQ). This reaction was conducted in air-saturated acetonitrile at room temperature, with no oxidation observed when methanol was used as the solvent. Time-resolved spectral data for complexes 3 and 4 demonstrated a significant enhancement of the band at 400 nm, confirming the effective transformation of the substrate into the product (Figure 7). Similar to the oxidation of o-aminophenol, the reaction exhibited rate saturation kinetics in both cases (Figure 7). Comprehensive kinetic analysis revealed the rate saturation kinetics, suggesting that the reaction proceeds through the formation of a catalyst-substrate intermediate. The kinetic parameters such as KM, Vmax, and kcat were determined using the Michaelis–Menten equation and are summarized in Table 2.

Figure 7.

Figure 7

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Time-course UV–vis spectral profile for the oxidation of 3,5-H2DTBC catalyzed by complexes 3 and 4 exhibits a consistent increase in the absorption band at 400 nm, corresponding to the formation of 3,5-DTBC.

Styrene Oxidation

Inspired by the excellent catalytic activities of these complexes in the oxidation of aminophenol and catechol substrates, we extended our investigation to evaluate their catalytic efficiency in the oxidation of styrene (Table 3). Initially, we checked the possibility of styrene oxidation in acetonitrile at room temperature in the presence of iodosobenzene (PHIO).71 In the presence of 10 mol % of manganese complexes 1–5 as precatalysts and 2 equiv of oxidant PHIO as an oxidant, we observed complete conversion of styrene in 1 h (Table 3: entries 1–5). A half an hour of reaction resulted incomplete conversion (60–75%) of styrene under similar reaction condition (entries 1–5). GC analysis of the reaction mixture confirmed that the major product in this oxidation process is styrene oxide. Very trace amount of benzaldehyde (2–3%) is formed in this oxidation process. At this point we can conclude that, all the catalyst is active for the oxidation of styrene and complex 4 showed the best catalytic activity. However, if we use tert-butyl hydroperoxide (TBHP) as an oxidant in the presence of 10 mol % of complex 4, we observed poor conversion of styrene (only 26%) under similar reaction condition with different selectivity (entry 6). Hydrogen peroxide as an oxidant is found to be inactive in this oxidation process (entry 7). Thereafter, decreasing the catalyst loading to 5 mol % in the presence of 2 equiv of PHIO, we observed 62% conversion of styrene in 1 h (entry 8). Increasing the time to 3 and 6 h, we found 86% (entry 9) and complete conversion (entry 10) of styrene, respectively. Changing the solvent form MeCN to MeOH, DCM, and THF, we could not observe any improvement in catalytic activity (entries 11–13). Moreover, an elevated reaction temperature (70 °C) also resulted very similar reactivity as that of room temperature (entry 14). Reducing the oxidant loading (1 equiv), we found much less conversion of the styrene (57%; entry 15) under similar reaction conditions. Thus, we consider entries 4 and 10 as preferable reaction conditions for the oxidation of styrene to styrene oxide as the final product.

Table 3. Catalytic Performance of Complexes 1–5 for Styrene Oxidationa.

graphic file with name ao4c09254_0013.jpg

ent. cat. (mol %) oxidant (eqv) time (h) temp (°C) solvent conv.b of S (%) GC yield (%)
P1 P2
1 1 (10) PHIO (2) 0.5/1 r.t. MeCN 61/>99 58/95 2/2
2 2 (10) PHIO (2) 0.5/1 r.t. MeCN 60/>99 57/94 2/3
3 3 (10) PHIO (2) 0.5/1 r.t. MeCN 68/>99 65/96 2/2
4 4 (10) PHIO (2) 0.5/1 r.t. MeCN 75/>99 73/97 2/2
5 5 (10) PHIO (2) 0.5/1 r.t. MeCN 66/>99 62/95 3/2
6 4 (10) TBHP (2) 1 r.t. MeCN 26 5 19
7 4 (10) H2O2 1 r.t. MeCN <10    
8 4 (5) PHIO (2) 1 r.t. MeCN 62 58 3
9 4 (5) PHIO (2) 3 r.t. MeCN 86 83 1
10 4 (5) PHIO (2) 6 r.t. MeCN >99 96 3
11 4 (5) PHIO (2) 6 r.t. MeOH 32 30 1
12 4 (5) PHIO (2) 6 r.t. DCM 65 60 4
13 4 (5) PHIO (2) 6 r.t. THF 43 40 2
14 4 (5) PHIO (2) 3 70 °C MeCN 88 83 5
15 4 (5) PHIO (1) 6 r.t. MeCN 57 54 2
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a

The reactions were carried out in a 10 mL vial containing a mixture of 0.25 mmol styrene, x mol % of complexes 1–5, and y equivalents of PHIO, dissolved in 2 mL of solvent at room temperature.

b

The styrene conversions (Conv.) were analyzed using GC.

Substrate Scope

Thereafter, we extended the scope of the oxidation process to other styrene derivatives (Figure 8). Under optimized conditions, 4-methyl styrene was successfully converted to the corresponding oxirane in a quantitative yield (O1: 96%). Styrene derivatives featuring electron-withdrawing groups, including chloro, bromo, ester, and nitro groups, were efficiently transformed into their corresponding epoxide products with high yields (O2: 88%, O3: 85%, O4: 73%, and O5: 81%). Aliphatic cyclic alkenes, including cyclopentene, cyclohexene, cyclohex-2-en-1-one, and cyclooctene, were converted to their corresponding cyclic epoxides under optimized conditions (O6: 71%, O7: 75%, O8: 79%, and O9: 86%). Additionally, acyclic alkenes such as 1-hexene, 3-bromo-1-hexene, and 6-bromo-1-hexene underwent the epoxidation process, yielding their respective epoxides in good to excellent yields (O10: 93%, O11: 87%, and O12: 90%). Long-chain alkenes also underwent the oxidation process, resulting in the corresponding oxiranes (O13: 84%, O14: 77%, and O15: 75%). Finally, two more aromatic alkenes, diphenylethene, and vinylnaphthalene, yielded their respective oxiranes in good yields (O15: 83%, and O16: 89%)

Figure 8.

Figure 8

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Oxidation of various alkene derivatives into their corresponding alkane oxide catalyzed by complex 4 under optimized conditions. Percentage of the product yield is given within the first bracket.

Mechanism

Catalytic efficiency is typically attributed to the presence of vacant coordination sites or labile groups within the metal coordination sphere, which facilitate the formation of complex–substrate aggregates. In our current systems, the structures of mononuclear manganese(III) complexes are very similar, with the only variation occurring at one of the axial sites around the manganese center. Both catalytic processes (CAO or PHS) are expected to proceed via a similar mechanistic pathway for substrate oxidations. However, a schematic representation of the catalytic pathway is provided specifically for the oxidation of OAPH as a substrate using complex 4 as the catalyst. Mass spectrometry serves as a valuable tool for identifying various intermediate species involved in the catalytic cycle. Consequently, ESI mass spectrometry was conducted on complex 4 alone and in the presence of the substrate (Figures S8 and S9). The ESI mass spectrum of complex 4 in methanol revealed the base peak with m/z value of 409.0775, corresponding to the complex cation [Mn(L) + H]+ (calculated m/z = 410.1179 for C20H23MnN2O4). This species is formed following the removal of the thiocyanate ion and aqua ligand from the coordination sphere. This study demonstrates that the metal center creates a vacant coordination site at the axial position in solution, enabling the formation of complex-substrate adducts. Initially, the dissociation of the thiocyanate ion from the MnIII coordination sphere generates species A, which acts as a potential active catalyst in the catalytic cycle (Scheme 3I). Subsequently, the dissociated thiocyanate ion or a water molecule functions as a base, abstracting a proton from the substrate OAPH and forming the complex–substrate adduct B. This adduct undergoes intramolecular electron transfer (IMET) from the substrate to the MnIII center, producing intermediate species C with simultaneous reduction of the metal center. Support for the formation of the complex–substrate adduct is provided by ESI (positive) mass spectrometry of a mixture containing complex 4 and 30 equiv of o-aminophenol. Notably, a peak at m/z 688.1853 corresponds to a 1:2 complex–substrate aggregate of the monocationic species [NaKMnII(L)(OAP)2 + H]+ (calculated m/z = 688.1472). Evidence for the reduction of the MnIII center is further supported by EPR spectroscopy, which shows six-line EPR signals characteristic of the reduction of MnIII to MnII upon substrate addition to the complex solution (Figure 9). In subsequent catalytic steps, dioxygen reacts with intermediate species C to regenerate initial active species A, accompanied by the formation of o-benzoquinone monoamine (BQMI) as an intermediate product and an equivalent amount of hydrogen peroxide. Under aerobic conditions, BQMI couples with another equivalent of OAPH to form the final product, 2-aminophenoxazin-3-one. For styrene oxidation, a mechanistic pathway is proposed based on literature reports (Scheme 3II). In the first step, the MnIII complex is oxidized to MnIV by PHIO, in line with the electrochemical data of the complexes. This oxidation step leads to the formation of a MnIV-oxo species D, which plays a key role in the catalytic cycle. The alkene substrate then interacts with the oxygen atom, forming an oxygen-bridge species E. The oxygen atom is transferred to the alkene carbons, resulting in the formation of species F, and the simultaneous reduction of MnIV to MnII occurs. The appearance of six-line spectrum is again consistent with the above fact (Figure 9). Finally, the oxirane product is released, regenerating the active catalytic species A.

Scheme 3. Proposed Catalytic Cycles Showing Oxidation of o-Aminophenol to 2-Aminophenoxazin-3-one and Styrene to Styrene Oxide Catalyzed by Complexes 1–5.

Scheme 3

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Figure 9.

Figure 9

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EPR spectra of complex 4 alone and in the presence of OAPH and 3,5-H2DTBC (left) and styrene (right).

Conclusions

This study presents the synthesis, magnetic, and catalytic properties of five new mononuclear MnIII complexes with a N2O2 donor Schiff base ligand derived from ethylenediamine and 2-hydroxy-3-methoxy-5-methylbenzaldehyde. The synthesized complexes exhibit diverse structural configurations influenced by the incorporation of pseudohalide ions and coordinated solvents. These complexes display six-coordinate Jahn–Teller distorted octahedral geometry, typical for MnIII complexes. Supramolecular interactions, particularly π–π stacking and hydrogen bonding, play a significant role in stabilizing these complexes in the solid state. Magnetic studies reveal antiferromagnetic coupling within the quasi-dimers formed by these complexes, with significant contributions from zero-field splitting effects. The catalytic activity of these complexes was evaluated for the oxidation of 2-aminophenol and 3,5-di-tert-butylcatechol, with kinetic studies indicating first-order kinetics at low substrate concentrations and rate saturation observed at higher concentrations. Complexes 4 and 5 demonstrated superior catalytic activity. Additionally, these complexes efficiently catalyzed the oxidation of styrene to styrene epoxide as a major product. Mechanistic investigations, supported by ESI mass spectrometry and EPR spectroscopy, indicate that the catalytic cycle involves the formation of complex–substrate adducts and subsequent intramolecular electron transfer. Overall, the combination of structural, magnetic, and catalytic analyses provides a comprehensive understanding of these systems, paving the way for future exploration and optimization of Schiff base MnIII complexes in catalysis.

Acknowledgments

A.P. gratefully acknowledges the financial support for this work from the Council of Scientific and Industrial Research (CSIR), New Delhi, India (Sanction no. 01/3118/23/EMR-II dated 08.07.23). Z.J. acknowledges the financial support from the Slovenian Research Agency (Grant No. P2-0348). P.B. is grateful for the financial support from the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020, and LA/P/0006/2020, financed by National Funds through Fundação para a Ciência e a Tecnologia (FCT), MCTES (PIDDAC).

Data Availability Statement

IR and EPR spectroscopic data, UV–vis and ESI mass spectral data, X-ray crystallographic data, electrochemical data, and the data related to catalytic and magnetic studies.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c09254.

  • IR spectra of 1–5; X-ray crystallographic information; ac magnetic data; crystal packing; ESI mass spectra; and CCDC numbers 2378212–2378216 for 1–5, respectively, and 2377040 for a catalytically oxidized epoxide product containing the supplementary crystallographic data for this paper (PDF)

Author Contributions

This manuscript was written with contributions from all authors, and all have approved the final version.

The authors declare no competing financial interest.

Supplementary Material

ao4c09254_si_001.pdf (1.1MB, pdf)

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Associated Data

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Supplementary Materials

ao4c09254_si_001.pdf (1.1MB, pdf)

Data Availability Statement

IR and EPR spectroscopic data, UV–vis and ESI mass spectral data, X-ray crystallographic data, electrochemical data, and the data related to catalytic and magnetic studies.

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