Crystal structure, magnetic and catalytic oxidation properties of manganese(III) tetrakis(ethoxycarbonyl)porphyrin

Abstract

A new meso-tetraalkyl porphyrin manganese complex, 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrinatomanganese (Mn^IIITECPCl), had been prepared and characterized by X-ray structure determination. Mn^IIITECPCl exists as a coordinated dimer in its crystal structure with a weak antiferromagnetic coupling between two Mn(III) ions. The catalytic oxidation of styrene by Mn^IIITECPCl was carried out in acetonitrile. Mn^IIITECPCl was found to be recyclable with a high conversion efficiency when using TBHP as an oxidant and the major product was benzaldehyde. Mn^IIITECPCl was also reusable by using an PhIO oxidant, but the major products turned out to be phenyl acetaldehyde and styrene epoxide.

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Introduction

From a structural viewpoint, synthetic manganese porphyrins resemble the active centre of heme-containing biological systems. Thus, manganese porphyrins are also widely used in the biomimetic studies of cytochrome P-450,¹ catalases² or other enzymes.³ However, manganese porphyrins usually undergo oxidative destruction in homogeneous catalytic oxidation systems.^4–6 The oxidative destruction of a manganese porphyrin can be significantly improved with electron withdrawing substituents such as β-position halogenation.^7–9 The other way to avoid oxidative destruction is by immobilizing the catalysts onto a rigid, inert support such as montmorillonite,¹⁰ multiwalled carbon nanotube,¹¹ magnetite nanoparticles,¹² metahalloysite,¹³ resin¹⁴ and silica gel.¹⁵ Assembled manganese porphyrin crystalline micro-sized porous coordination polymers¹⁶ and metal–organic framework (MOF)^17,18 are also proved robust catalysts. Although these heterogeneous catalysts are recyclable and stable, the synthetic procedure is complicated with limited type of ligands and the catalytic oxidation is time consuming. On the other hand, lots of oxidants such as t-butyl hydroperoxide (TBHP),¹⁹ iodosylbenzene (PhIO),²⁰ iodobenzene diacetate (PhI(OAc)₂),²¹ m-chloroperbenzoic acid (m-CPBA),²² dioxygen²³ and hydrogen peroxide²⁴ are effective in the manganese porphyrin catalyzed oxidation reaction. Manganese(V)-oxo,^25–27 manganese(IV)-oxo^28–30 and Mn^III–HOOR^31–33 are suggested as the active intermediates. Interestingly, trans-dioxo-Mn^V-porphyrin is found inert.²⁷

Meso-substitution has significant effects on the reactivity of metal porphyrin, especially free radicals involved reactions.^34,35 Meso-tetraalkyl porphyrins are useful probes for understanding the structure–property relationship of porphyrin molecules.³⁶ Although tetraalkylporphyrins may be prepared with good yields,^37–39 there is the less reports on the application of manganese tetraalkylporphyrin in catalytic oxidation.^40,41 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin (TECP) is a convenient precursor for porphine.⁴² Recently, we have prepared β-fluorinated TECP.⁴³ Here, we wish to report the preparation and catalytic styrene oxidation of manganese(III) TECP (Scheme 1, Mn^IIITECPCl). The X-ray crystal structure showed that Mn^IIITECPCl exists as a dimer with plenty of hydrogen bonding interactions. There is a weak antiferromagnetic coupling between two Mn(III) ions in the dimer. The catalytic oxidation of styrene by Mn^IIITECPCl was carried out in acetonitrile solution at room temperature using TBHP and PhIO as oxidants. Mn^IIITECPCl was found recyclable with high conversion efficiency when using these oxidants. Plausible mechanism has been proposed to explain this observation.

Scheme 1 Molecule structure of Mn^IIITECPCl and Mn^IIITPPCl.

Experimental

Materials and methods

All reagents were purchased from commercial sources and used without further purification, unless otherwise mentioned. Styrene was passed through silica gel column prior to use. Electronic spectra were recorded on Hitachi 3900H UV-vis Spectrophotometer in 1 cm optical path length quartz cell at room temperature. Fourier translation infrared spectra (FT-IR) were recorded on a Perkin-Elmer spectrophotometer by using KBr pellets. HR-MS spectra were recorded on Bruker maxis impact mass spectrometer with an ESI source. Single-crystal X-ray diffraction data were recorded on a Rigaku R-AXIS SPIDER IP diffractometer with Cu K_α radiation. All cyclic voltammograms (CV) were performed in N,N-dimethylformamide (DMF) solution containing 0.1 M tetrabutylammonium perchlorate (TBAP) and Mn^IIITECPCl (3 mM) using CHI-660E electrochemical analyzer under nitrogen atmosphere at ambient temperature. The scan rate was 100 mV s⁻¹. A three-electrode system consisting of a glassy carbon working electrode, a platinum wire counter electrode and saturated Ag/AgNO₃ electrode as the reference electrode were employed. The Ag/AgNO₃ electrode contained 1 M TBAP in DMF. Half-wave potentials (E_1/2) for reversible or quasi-reversible redox processes were calculated as E_1/2 = (E_pa + E_pc)/2, where E_pa and E_pc represent the anodic and cathode peak potentials, respectively. The E_1/2 value for the ferrocene couple under these conditions was 0.47 V. Analysis of oxidation products was recorded by gas chromatograph (Echrom A90) equipped with an Agilent HP-5 capillary column (30.0 m × 320 μm ID: 0.25 μm film thickness) coupled with FID detector. The carrier gas was nitrogen and the chromatographic conditions were as follows: the oven temperature was retained at 60 °C for 4 min, and then increased at a rate of 10 °C min (from 60 to 250 °C); the injector temperature was set 230 °C while the detector temperature was kept 250 °C. The injection volume was 1.0 μL and the products were confirmed by the retention time using standard samples under the same GC conditions. The yields of products were reported with respect to the amount of oxidant used. The solid-state magnetic susceptibility was measured under helium on a Quantum Design PPMS-9 magnetometer from 2 to 300 K at a field of 1 T. The powder sample was installed in the parafilm and the magnetic moment was obtained after background correction. The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal's table).

Synthesis of catalysts

The metalloporphyrin catalysts 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin manganese(III) chloride (Mn^IIITECPCl) and 5,10,15,20-tetraphenylporphyrin manganese(III) chloride (Mn^IIITPPCl) (Scheme 1) were prepared using literature procedures.^44,45 The synthesis of 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin ligand (TECP) was performed by our recently reported procedure.⁴³

Mn^IIITECPCl was obtained by refluxing TECP with Mn(OAc)₂·4H₂O in DMF solution for 2 h. After that it was diluted with CH₂Cl₂ and washed with saturated aqueous solution of NaCl and HCl several times. The organic layer was collected and dried over anhydrous Na₂SO₄. The filtrate was concentrated and the crude product was purified on silica gel (100–200 mesh) using CH₂Cl₂/CH₃OH (95 : 5 v/v) as eluent. Black solid was obtained after recrystallization from CH₂Cl₂/hexane (1 : 4 v/v). Yield: 85%. UV-vis (CH₂Cl₂) λ_max (log ε): 364 (4.80), 474 (4.90), 570 (4.07); HR-MS (ESI) ([M − Cl]⁺): calcd for C₃₂H₂₈MnN₄O₈ 651.1282, found 651.1285, with an isotope distribution pattern the same as the calculated one.

X-ray diffraction studies of Mn^IIITECPCl

A suitable crystal was selected and measurement on an Xcalibur, Sapphire3, Gemini ultra diffractometer using Cu K_α radiation. The crystal was kept at 150(5) K during data collection. Using Olex2,⁴⁶ the structure was solved with the Olex2.solve structure solution program using charge flipping and refined with the XL⁴⁷ refinement package using least squares minimization. All non-hydrogen atoms of the complex were refined anisotropically. The hydrogen atoms were generated by the riding mode. Data collection and structural refinement parameters are given in Table 1. CCDC 1059156 contained the supplementary crystallographic data for Mn^IIITECPCl.†

Table 1 Crystal data and structure refinement for Mn^IIITECPCl

Compound	Mn^IIITECPCl
Empirical formula	C32H28N4O8MnCl
Formula weight	686.97
Temperature/K	150(5)
Crystal system	Monoclinic
Space group	P21/c
a/Å	14.9631(3)
b/Å	9.6880(3)
c/Å	21.4177(5)
α/°	90.00
β/°	100.638(2)
γ/°	90.00
Volume/Å^3	3051.41(13)
Z	4
ρcalc g cm^−3	1.495
μ/mm^−1	4.832
F(000)	1416.0
Radiation	Cu Kα (λ = 1.54184 Å)
Reflections collected	10 067
Independent reflections	4591 [Rint = 0.0372, Rsigma = 0.0453]
Data/restraints/parameters	4591/0/527
Goodness-of-fit on F^2	1.049
Final R indexes [I ≥ 2σ(I)]	R1 = 0.0400, wR2 = 0.1002
Final R indexes [all data]	R1 = 0.0447, wR2 = 0.1044
Largest diff. peak/hole/e Å^−3	0.50/−0.42

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General procedure for catalytic oxidation

A mixture of organic substrate (1.0 mmol), oxidant (0.1 mmol) and catalyst (1.0 μmol) in 2 mL acetonitrile was stirred in a 10 mL glass flask at room temperature. After an appropriate reaction time, chlorobenzene (5.0 μL) was added to this reaction mixture as internal standard. The conversion efficiency and selectivity were monitored by GC analysis.

Typical procedure for the reuse of catalyst in the catalytic oxidation of styrene

A mixture of catalyst (1.0 μmol), styrene (1 mmol) and TBHP (0.1 mmol) in CH₃CN (2 mL) was sealed in a 10 mL glass flask, evaporated under vacuum after stirring at room temperature for 2.5 h. The resulting solid was washed with hexane for several times, and detected by GC analysis. The recovered catalyst solid was used in the successive run.

The stability of catalyst

A mixture of Mn^IIITECPCl and oxidant in 2 mL acetonitrile was stirred in a 10 mL glass flask at room temperature. After an appropriate reaction time, 100 μL of the mixture was diluted to 3 mL acetonitrile solution in a quartz cell with 1 cm optical path length and UV-vis spectrum was obtained.

Results and discussion

X-ray structure of Mn^IIITECPCl

Single crystal of Mn^IIITECPCl, suitable for X-ray diffraction analysis, was obtained by slow evaporation of dichloromethane/hexane solution. Single crystal of Mn^IIITECPCl was monoclinic in the space group P2₁/c, with nonplanar porphyrin core, like Mn(III) tetraphenylporphyrin.⁴⁸ The molecular structure of Mn^IIITECPCl is depicted in Fig. 1a. The 24-membered macrocyclic core of the porphyrin is coplanar, and the displacement of each atom in the equatorial mean plane is within 0.15 Å. The distance between the Mn³⁺ ion to the mean plane is 0.1701 Å. The six-coordinate manganese atom located at the centre of this porphyrin macrocycle and is bonded to four nitrogen, one chlorine and one oxygen atom to construct a distorted octahedral. Two manganese porphyrin molecules combined in a dimer (Fig. 1b) and two manganese porphyrin core planes are parallel with a distance of 3.2845 Å, indicating the existence of π–π stacking, in addition to coordination interactions. The short intermacrocycle distances are the consequence of two supplementary structural features. The first one is weak coordination bond between manganese ion and oxygen atom from another macrocycle. The second one is the nearly coplanar orientation of the meso-5,10,15,20-tetra-ethoxycarbonyl groups towards the macrocycle, which facilitates the intermolecular approach, despite their spatial proximity. This distance is comparable to the same meso-ethoxycarbonyl groups corrole free base.⁴⁹

Fig. 1 ORTEP plots of Mn^IIITECPCl. (a) Monomer. (b) Dimer (showing 50% probability thermal displacement ellipsoids, hydrogen atoms are omitted for clarity).

Selected bond distances and angles for Mn^IIITECPCl are listed in Table 2. The Mn–N bond length range from 2.0159(2) Å to 2.0275(2) Å with an average value of 2.0224(9) Å, and the Mn–Cl bond length is 2.4235(8) Å, both of the bond length were comparable to 5,10,15,20-tetrakis(4-hydroxyphenyl) porphyrin manganese chloride (Mn^IIITOHPPCl) or Mn^IIITPPCl found in the literature.^48,50 The Mn–O bond length is 2.6290(2) Å, and is weaker than covalent bond (2.16 Å).

Table 2 Selected bond distances (Å) and bond angles (°) for Mn^IIITECPCl

Bond distances (Å)
Mn1–Cl1	2.4235(7)	Mn1–N1	2.0270(2)
Mn1–N2	2.0275(19)	Mn1–N3	2.0200(19)
Mn1–N4	2.0160(2)	Mn1–O5A	2.6290(2)
Bond angles (°)
N3–Mn1–Cl1	94.65(6)	N3–Mn1–N1	169.66(8)
N3–Mn1–N2	89.33(8)	N1–Mn1–Cl1	95.69(6)
N1–Mn1–N2	88.91(8)	N4–Mn1–Cl1	90.72(6)
N4–Mn1–N3	89.67(8)	N4–Mn1–N1	90.38(8)
N4–Mn1–N2	170.48(8)	N2–Mn1–Cl1	98.79(6)
C14–N3–Mn1	127.23(16)	C11–N3–Mn1	126.80(15)
C1–N1–Mn1	127.91(16)	C4–N1–Mn1	126.14(17)
C6–N4–Mn1	125.65(17)	C9–N4–Mn1	126.80(16)
C19–N2–Mn1	127.32(15)	C16–N2–Mn1	126.55(16)

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Hydrogen bonding plays an important role in constructing multi-dimensional structure as well as in catalytic oxidation.⁵⁰ Fig. 2 shows the packing of Mn^IIITECPCl constructed from hydrogen bonding. Apparently, there are two types of hydrogen bonding i.e. intramolecular and intermolecular bonding. The former exists between oxygen of carbonyl and hydrogen of pyrrole, which was also observed in the meso-ethoxycarbonyl groups corroles.⁴⁹ While the latter exists between the oxygen of the ester group from one manganese porphyrin and hydrogen of pyrrole or ethyl group from another manganese porphyrin.

Fig. 2 Unit cell of Mn^IIITECPCl show hydrogen bonding interaction. Hydrogen atoms not involved in these contacts are omitted for clarity.

Magnetic properties

Magnetic data for Mn^IIITECPCl are reported in Fig. 3 in the form of χ_m and μ_eff versus T. The value of μ_eff varies from 4.66 μ_B at 300 K to 3.35 μ_B at 2 K. The magnetic moment clearly shows a platform (∼4.64 μ_B) at high temperatures (300–38 K). With decreasing the temperature, the μ_eff decreases slowly and reaches its terminal value of 4.48 μ_B at about 12 K, and after this the μ_eff decrease rapidly and reaches its lowest value of ∼3.35 μ_B at 2 K. The abrupt μ_eff rise in the range of temperature 2 < T < 12 K is characteristic of a compound with significant zero-field splitting (ZFS).⁵¹ The effective magnetic moment of 4.66 μ_B at room temperature is lower than the spin-only moment of 4.9 μ_B for an S = 2 system, consistent with other high spin Mn(III) complexes in which g < 2. The magnetic susceptibilities of Mn^IIITECPCl in the whole temperature range obeys Curie–Weiss law with small negative Weiss constant θ = −0.91 K and Curie constant C = 2.71 emu K mol⁻¹, indicating the occurrence of weak antiferromagnetic interactions in Mn^IIITECPCl.

Fig. 3 Temperature dependence of magnetic susceptibility in the forms of μ_eff and χ_m versus T for Mn^IIITECPCl at 1 T between 2 and 300 K. The red solid lines represent the best-fitting results. Inset: Temperature dependence of the magnetic susceptibility in the form of χ_m⁻¹ versus T for Mn^IIITECPCl between 2 and 300 K. The solid line was obtained from the best fit by the Curie–Weiss expression.

The molecular structure of Mn^IIITECPCl shows that the metal (Mn) centers are linked by weak O5A–Mn coordination bonds to form a dimer (Fig. 1), and such interactions are able to transmit magnetic interactions. On the basis of the binuclear Mn^III–Mn^III (S = 2 and 2) model, the magnetic susceptibilities for Mn^IIITECPCl can be fitted by eqn (2) derived from the isotropic exchange spin Hamiltonian (eqn (1)).^52,53

Ĥ = −2JŜ_MnŜ_Mn + DŜ_zMn² (1)

where, , and .The term p (or q) is the fraction of Mn(III) [or Mn(III)–Mn(III) dimer], g_eff is the effective g value, TIP is the temperature independent paramagnetism. J is the coupling exchange parameter and D is the parameter which describes the effects of the axial field. The best fits, as represented in Fig. 3, gave the values g_eff = 1.75, p = 0.70, D = 7.34 cm⁻¹, q = 0.08, 2J = −0.84 cm⁻¹ and TIP = −2.7 × 10⁻⁴ cm³ mol⁻¹, with an agreement factor of 0.61%, showing the high reliability of data. The R value is defined as⁵²

R = Σ(χ_obsdT − χ_caldT)²/Σ(χ_obsdT)² × 100% (3)

The small negative J value (|J| < 20 cm⁻¹)⁵³ suggests a weak antiferromagnetic coupling between the Mn(III) and Mn(III) connected by the coordination bridge. Within the dimer, Mn⋯Mn separation of 6.56 Å in Mn^IIITECPCl precludes direct metal–metal bonding, the superexchange through O5A–Mn coordination bonding in the Mn(III)⋯Mn(III) dimer must be responsible for this antiferromagnetic interaction. The shortest interdimer Mn⋯Mn distance of 10.53 Å in Mn^IIITECPCl indicates that interdimer interactions are negligible. This is similar to the previous observation for Mn(2-NC₃H₅NCTPP)Br complex, in which the antiferromagnetic coupling constant J between Mn(III)–Mn(III) ions is −0.02 cm⁻¹ with an intradimer Mn⋯Mn separation of 8.177 Å linked by C–H⋯Br hydrogen bonding.⁵²

Electrochemistry

The electrochemical characteristics of Mn^IIITECPCl and Mn^IIITPPCl were investigated by cyclic voltammetry in DMF solutions. Fig. 4 represents the cyclic voltammograms of Mn^IIITECPCl and Mn^IIITPPCl in DMF containing 0.1 M TBAP and ferrocene as internal standard. Manganese porphyrins show a single one-electron reversible or quasi-reversible oxidation and a series of irreversible reductions within the scan window (−2.0 to 2.0 V). Mn^IIITECPCl exhibits four redox couples located at E_1/2 = −1.831, −1.531, −0.96 and 0.021 V. The voltammogram is in close resemblance to that of Mn^IIITPPCl, which exhibits the same redox couples located at E_1/2 = −1.948, −1.836, −1.310, −0.19 V respectively. The electrochemical properties of manganese porphyrins are well documented in the literature.^54–56 Here, the assignments of Mn^IIITECPCl redox couples are made accordingly. The redox couple at E_1/2 = 0.021 V may be assigned to Mn(III)/Mn(II). The other more negative potential redox couples at E_1/2 = −0.96, −1.53 and −1.83 V are assigned to porphyrin ring-centered redox processes related to formation of Mn(II) π-anion radical, π-dianion and π-trianions radicals species. The Mn(III)/Mn(II) redox couple of Mn^IIITPPCl appeared at more negative E_1/2 (−0.19 V). This suggests Mn^IIITECPCl may have a higher stability and catalytic oxidation activity than Mn^IIITPPCl. In addition, an additional oxidation process was observed at E_1/2 = 1.64 V for Mn^IIITPPCl, which is assigned to the formation of the porphyrin radical cation.^57,58 This redox peak was not observed for Mn^IIITECPCl within the same electrochemical window, possibly because of the electron-deficient virtue of Mn^IIITECPCl.

Fig. 4 Cyclic voltammograms of Mn^IIITECPCl and Mn^IIITPPCl in DMF containing 0.1 M TBAP, scan rate = 0.1 V s⁻¹ (ferrocene as internal standard).

Catalytic oxidation activity

It is well documented that high-valent metalloporphyrins having electron-withdrawing groups at the four meso positions are efficient oxidation catalysts.⁵⁹ Styrene is a commonly used substrate.^23,60,61 Previous literatures and our group studies showed that acetonitrile is an ideal solvent for manganese catalysed oxidation of organic substrate.^19,45,62 Here, the catalytic oxidation of styrene was carried out in acetonitrile with different oxidants at room temperature. The main oxidation products are benzaldehyde (BA), phenyl acetaldehyde (PA) and styrene epoxide (SO).

The nature and the relative yields of the products formed by catalytic oxidation of styrene using porphyrins vary considerably depending on the catalyst and oxidant. In this work, three oxidants TBHP, m-CPBA and PhIO were used, and the experimental conditions were kept the same in all catalytic reactions. In the absence of oxidant, no any products could be detected (Table 3, entry 1). When using m-CPBA or PhIO as oxidant, the main products are phenyl acetaldehyde (PA) and styrene epoxide (SO) (Table 3, entry 2 to 5), similar to the previously reported catalytic oxidation of styrene by β-brominated manganese porphyrins.⁷ PhIO or m-CPBA is a single-oxygen donor and favors the formation of the high-valent (oxo)manganese species Mn^IV=O,²⁹ which is responsible for oxygen insertion into the carbon–carbon double bond of styrene. When using TBHP as oxidant, the main product was benzaldehyde (Table 3, entry 6), which may be attributed to free radical involved oxidation.^63–65 The yield of benzaldehyde and total oxidation yield was significantly lowered under argon (Table 3, entry 7), further confirming Mn^IIITECPCl/TBHP system inclines to free radical involved oxidation.⁴⁵ For all tested oxidants, the catalytic activity of Mn^IIITECPCl was observed much higher than control Mn^IIITPPCl, this may be rationalized by the electron-withdrawing effect of the four meso-ethoxycarbonyl groups in Mn^IIITECPCl, which leads to a higher catalytic activity. This may also be confirmed from the anticathode shift of its Mn³⁺/Mn²⁺ couple as compared to Mn^IIITPPCl (Fig. 4).

Table 3 Catalytic oxidation of styrene by manganese porphyrins

Entry^a	Oxidant	Catalyst	Product yield %
			BA	PA	SO	Total
a Reaction conditions: catalyst (0.001 mmol), oxidation (0.1 mmol), styrene (1 mmol) in 2 mL acetonitrile stirring at room temperature. Yield and selectivity of product were detected by GC, chlorobenzene as internal standard.b Under argon atmosphere.
1	None	Mn^IIITECPCl	0	0	0	0
2	m-CPBA	Mn^IIITECPCl	0.72	14.00	17.43	32.15
3		Mn^IIITPPCl	1.32	2.62	6.48	10.42
4	PhIO	Mn^IIITECPCl	4.67	37.56	33.31	75.54
5		Mn^IIITPPCl	3.20	29.68	41.37	74.25
6	TBHP	Mn^IIITECPCl	71.47	3.25	14.73	89.45
7^b		Mn^IIITECPCl	37.01	1.63	11.07	49.71
8		Mn^IIITPPCl	42.40	1.57	3.68	47.65

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It is known the periphery electron-withdrawing group will not only enhance the catalytic activity of metalloporphyrin, but also its anti-oxidation stability.⁵⁹ To check the anti-oxidation stability of Mn^IIITECPCl, direct reaction of catalysts with PhIO and TBHP were carried out and the reaction were monitored by UV-vis spectroscopy. As shown in Fig. 5A, Mn^IIITECP displays classical electronic spectra of manganese porphyrin spectrum. Its UV-vis characteristics may well be depicted using Gouterman's four-orbital model.⁶⁶ Electronic spectra of Mn^IIITPPCl (Fig. 5B) has been assigned in earlier publication.⁶⁷ Soret peak VI (372 nm) is originated from a_1u, a_2u → e_g* and a′_2u, b_2u → e_g transitions. Soret peak V_a (398 nm) is LMCT band from a′_2u → a_1g transition. The sharp absorption peak V (474 nm) is the typical LMCT band assigned to a′_2u, b_2u → e_g transition. The less intense Q bands IV (580 nm) is from a_1u, a_2u → e_g* transition, and III (617 nm) is LMCT band from a_1u, a_2u → e_g transition. Mn^IIITECPCl exhibits similar absorption peaks at 358 nm (VI), 468 nm (V) and 568 nm (IV) respectively. Due to the substituent effect, V_a and III peaks could not be resolved clearly in the UV-vis spectra of Mn^IIITECPCl. Which is quite similar to the case of chloro(aquo)etioporphyrinI manganese(III) complex.⁶⁷ As compared to Mn^IIITPPCl, the electronic spectrum of Mn^IIITECPCl is obviously blue-shifted. This may be rationalized by the electron-deficient property of porphyriniod macrocycle.⁶⁸ When PhIO oxidant was added to the catalyst solution, Mn^IIITPPCl was completely destructed after 2.5 hours as indicated by UV-vis spectrum (Fig. 5B). While only about 12% Mn^IIITECPCl decomposed at the same condition (Fig. 5A). Interestingly, when using TBHP as oxidant, nearly no decomposition of Mn^IIITECPCl could be observed even after 12.5 hours with [TBHP]/[catalyst] = 500 : 1 (Fig. 6A). In this situation, although the stability of Mn^IIITPPCl was also observed greatly enhanced, the oxidative destruction Mn^IIITPPCl was still very obvious as indicated by the intensity loss of the absorption peaks (Fig. 6B).

Fig. 5 UV-vis spectra changes of Mn^IIITECPCl (A) and Mn^IIITPPCl (B) upon the addition of PhIO in CH₃CN solution (reaction time 2.5 h. [PhIO]/[catalyst] = 100 : 1).

Fig. 6 UV-vis spectra changes of Mn^IIITECPCl (A) and Mn^IIITPPCl (B) upon the addition of TBHP in CH₃CN solution (Reaction time 12.5 h. [TBHP]/[catalyst] = 500 : 1.).

These observations prompted us to explore the recovery or reuse of Mn^IIITECPCl under the catalytic reaction conditions using TBHP and PhIO oxidatns (Fig. 7). After five cycles, the total oxidation products yield by Mn^IIITECPCl was kept above 70% for both oxidants. This indicates meso-ethoxycarbonyl groups were a good alternative to stabilize manganese porphyrin in resisting oxidation. The IR spectra of recovered Mn^IIITECPCl is the same to original catalyst (Fig. 8), suggesting Mn^IIITECPCl is very stable in the catalytic reactions.

Fig. 7 The total oxidation products yield changes of Mn^IIITECPCl catalyst with the different oxidants and cycles (reaction condition: catalyst/oxidant/styrene = 1 : 100 : 1000 in 2 mL CH₃CN, catalyst: 1 μM, products were analyzed by GC after 2.5 h).

Fig. 8 FT-IR spectra (KBr pellets) of recovered and original Mn^IIITECPCl catalyst.

Mechanistic considerations

Manganese porphyrins possess high catalytic activity in alkene epoxidation using PhI(OAc)₂,²¹ PhIO⁷ and m-CPBA²² oxidants. Epoxides are generally considered from the oxygen atom transfer (OAT) reaction of high-valent (oxo)manganese porphyrin and alkene.²⁷ It has been observed manganese porphyrin catalyzed oxidation of styrene by TBHP oxidant gave benzaldehyde as main product.¹⁹ Three types of reactive species have been postulated for manganese porphyrin catalyzed oxidation reactions: porphyrin manganese(IV)-oxo,²⁸ porphyrin manganese(V)-oxo²⁶ and porphyrin Mn^III–HOOR.^31–33 Porphyrin manganese(V)-oxo is a transient species, it return back to manganese(III) via manganese(IV)-oxo in a few minutes.^21,25,69

To check the possible active species involved, direction reaction the manganese porphyrin with TBHP and PhIO were monitored by UV-vis spectra (Fig. 9). In the presence of TBHP, there was no obvious change in the spectrum, except a small new shoulder peak appeared at Soret band and obvious decrease in the absorption intensity. Similar spectral changes were observed for Mn^IIITPPCl too. This spectra change may be ascribed to the formation of Mn^III–HOOR. In the presence of PhIO, the UV-vis spectra of Mn^IIITECPCl exhibited a new Soret band at 387 nm and the LMCT (ligand to metal charge transfer) band at around 475 nm disappeared, indicating the formation of high-valent porphyrin Mn^IV=O intermediate.³¹ Similar phenomena were observed for Mn^IIITPPCl. From these observations, it can be concluded that Mn^IIITPPCl and Mn^IIITECPCl react with PhIO yield high-valent Mn^IV=O intermediate, while TBHP coordinates to the porphyrin manganese metal centre to form axial ligated complexes. As the main product is benzaldehyde in styrene/Mn^IIITECPCl/TBHP system and there is no spectroscopic sign that the reaction of Mn^IIITECPCl and TBHP will generate porphyrin Mn^IV=O intermediate. We suggest here the catalytic oxidation of styrene by TBHP is a free-radical involved process.⁷⁰ p-Benzoquinone (p-BQ) is a well-known reactive oxygen radical species (RORS) trapping agent.⁷¹ When p-BQ was added to the styrene/Mn^IIITECPCl/TBHP system, the catalytic oxidation reaction was totally suppressed (Fig. 10), indicating RORS were indispensable active intermediate. When using PhIO as oxidant, the addition of p-BQ will also lower the total product yield remarkably. This suggests RORS is also involved in the oxidation of styrene by PhIO. Interestingly, p-BQ cannot suppress the styrene oxidation completely when using Mn^IIITPPCl catalyst and TBHP oxidant. When using PhIO as oxidant, the catalytic activity of Mn^IIITPPCl was not inhibited as much as that of Mn^IIITECPCl. This shows the catalytic pathway not only depends on the oxidant, but the virtue of the catalyst also. Detailed mechanism still needs further investigation.

Fig. 9 UV-vis spectra changes of Mn^IIITECPCl (left) and Mn^IIITPPCl (right) upon the addition of TBHP and PhIO in acetonitrile [Mn porphyrin]/[Oxd] = 1/10.

Fig. 10 Change in the yield of products after adding p-BQ in different catalysts/oxidants systems (+ = present; − = absent).

The plausible mechanism of styrene oxidation catalyzed by manganese porphyrin is depicted in Scheme 2. The reaction between TBHP and Mn^IIITECPCl or Mn^IIITPPCl gives Mn^III-HOOR intermediate, which undergo heterolysis to form RORS which oxidizes styrene to give BA, PA and SO. When using PhIO as oxidant, porphyrin Mn^IV=O formed and subsequently transfers its oxygen to the carbon–carbon double bond of styrene to form SO and PA. An alternate pathway is that Mn^IV=O reacts with styrene to give radical intermediate which will initiate oxygen involved free radical chain propagation reaction.^28,29

Scheme 2 Plausible mechanism for oxidation of styrene by Mn^IIITECPCl.

Conclusions

In summary, we have reported the synthesis, structure characterization, magnetic properties of Mn^IIITECPCl, and its catalytic activities for styrene in CH₃CN in homogenous system. In its crystal, Mn^IIITECPCl is linked through O–Mn coordination bonds to form dimer, and displays weak antiferromagnetic intradimer coupling between Mn(III) ions. As compared to tetra-aryl manganese porphyrin Mn^IIITPPCl, Mn^IIITECPCl exhibits better catalytic activity and anti-oxidation capacity. Mn^IIITECPCl can be recycled with high catalytic activity even after five cycles in the oxidation of styrene by TBHP and the plausible reactive intermediate is reactive oxygen radical species generated from the heterolysis of porphyrin Mn^III–HOOR. Mn^IIITECPCl was also reusable in the oxidation of styrene by using PhIO oxidant.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21171057, 21371059).

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Footnote

† CCDC 1059156. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra15943a

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