Electrochemical Conversion of 5-Hydroxymethylfurfural to 2,5-Furandicarboxaldehyde Using Mn(III)–Schiff Base Catalysts

Abstract

2,5-furandicarboxaldehyde (DFF) is one of the most promising biomass-based building blocks for the synthesis of biobased polymers. DFF can be obtained from 5-hydroxymethylfurfural (HMF), a fructose derivate, and it is a key molecule in the sequence of reactions of furan chemistry to develop biobased plastics. In this frame, four manganese(III)–Schiff base complexes 1–4 have been obtained. The general formula for the complexes, MnLⁿ(OCN)(H₂O/CH₃OH)_m (Lⁿ being the Schiff base ligands L¹–L⁴, formed as the result of the condensation of different substituted hydroxybenzaldehydes with diverse diamines, and m = 1–3), has been confirmed by characterization through different analytical and spectroscopic techniques. X-ray crystallographic studies for 1 and 2 showed tetragonally distorted octahedral structures, where the Schiff base was placed in the equatorial coordination positions of the Mn(III) ion. Complexes 1 and 2 behaved as efficient catalysts in the oxidation of HMF to DFF in an electrolytic reaction at pH 8.5, with phosphate buffer at room temperature, with conversion rates of 70–80%. On the other hand, complexes 3 and 4, where the axial position was sterically less accessible, yielded only an 11% conversion of HMF to DFF. The results indicate that a correct selection of metal complexes allows the development of a new efficient way to obtain DFF.

1. Introduction

The development of new biodegradable polymers obtained from renewable sources is a challenge for our society, which needs alternatives to petroleum-derived plastic polymers as these are non-renewable materials with high persistence in the environment [1,2,3,4,5,6]. The harmful effects caused by pollution with these petroleum-derived plastics, such as polyethylene terephthalate (PET), are diverse and pose significant health risks at different levels, such as various endocrine problems or reproductive issues [7,8]. Without a doubt, lignocellulosic biomass represents a renewable source to produce polymers in the context of implementing a circular economy approach that avoids polymers from fossil sources [9,10].

Among the different ways to valorize this biomass is its conversion into monomers as a preliminary step to the orderly manufacture of new uniform polymers with the desired properties. In biorefineries, where catalytic fractionation is carried out, the aim is to optimize the production of different monomers and the conversion reactions of some compounds into others to finally obtain the constituent blocks for these polymers. We can highlight the 5-hydroxymethylfurfural (HMF) monomer compound, a furan formed during the thermal decomposition of carbohydrates, and among the key conversion reactions are the transformation of HMF into 2,5-furandicarboxylic acid (FDCA) and the transformation of HMF into 2,5-furandicarboxaldehyde (DFF) [11,12,13,14,15,16,17,18,19,20,21,22,23].

In the recent literature, there have been numerous proposals to optimize the oxidation of HMF to DFF, using both natural enzymes [24,25,26] and various synthetic catalytic systems, both homogeneous and heterogeneous, using different metal centers like manganese [27], vanadium [28], ruthenium [29], niobium [30], molybdenum [31], zinc [32], cobalt [33], or iron [34].

Electrocatalytic conversion represents another environmentally sustainable alternative for this oxidation [35] since it can be carried out under mild conditions (at atmospheric pressure and, typically, at room temperature), avoids adding oxidants to the reaction medium, and generates a reduced quantity of waste products. The inherent advantages of this route have driven research in recent years, in many cases coupling the furfural oxidation reaction with hydrogen production from the electrolytic decomposition of water [36]. Different catalysts have been successfully used as catalysts for these electrochemical conversions of HMF, including those derived from noble metals, such as Pd, Pt, Ru, and Au [37,38], as well as those obtained through synthesis involving heterojunction manufacturing (NiSeO₃@(CoSeO₃)₄ [39] or Sc-NiFe-LDH [40]) and those resulting from complex synthetic routes [41]. However, for the electrocatalytic process to be economically viable, there is a need to develop new catalysts from non-noble metals that can be obtained through simple syntheses.

In this work, manganese(III)–Schiff bases are studied as electrocatalysts for the conversion of HMF to DFF. These types of complexes are models for a variety of redox enzymes such as catalases or peroxidases [42], and they can also act as effective catalysts for the oxygenation of both saturated and unsaturated hydrocarbons [43,44,45]. In addition, we have previously reported their ability to catalyze the electrochemical conversion of the lignin model veratryl alcohol to veratryl aldehyde [46]. Four new manganese complexes, incorporating the ligands H₂L¹–H₂L⁴ (Scheme 1), were prepared and characterized, and their peroxidase-like activity and their effect in catalyzing the electrochemical oxidation of HMF are reported.

2. Results

2.1. Characterization of the Manganese Complexes

The manganese(III)–Schiff base complexes 1–4 were prepared as detailed in the experimental section and all of them were obtained with high purity. The use of different characterization techniques allowed us to establish the formulae of the metal complexes and confirm the coordination of both the H₂Lⁿ Schiff base and the cyanate to the metal ion. Elemental analysis data, mass spectrometry, conductivity measurements, and IR spectroscopy point to a general formula: MnLⁿ(OCN)(H₂O/CH₃OH)_m (where n = 1–4 and m = 1–3). Regarding IR spectroscopy, the spectra of the metal complexes show the bands corresponding to the Schiff bases, with shifts in the C=N and C-O bands compared to the spectra of the free ligands (Figures S1–S4) [45,46,47,48], due to the coordination of the metal ion to the donor atoms of these groups. The band of the cyanate group is also identified at 2160–2190 cm⁻¹, slightly shifted from that of the free ion (2220 cm⁻¹). The non-electrolytic behavior shown by the conductivity values is also consistent with the coordination of the cyanate ion to the metal center. Finally, ESI mass spectra (Figures S5 and S6) collect, in all cases, the peaks corresponding to the fragment [MnL]⁺, as well as other minor signals, in some of the spectra, that point to the presence of the cyanate ion in the first coordination sphere of the complex.

The oxidation state of the manganese was established by the use of a magnetic balance to measure the magnetic moments at room temperature, with values close to the theoretical 4.9 B.M. expected for a high-spin magnetically diluted d⁴ manganese(III) ion. Also, the UV spectra are consistent with d⁴ metal complexes with broad bands at about 540–575 nm, attributable to ⁵E_g → ⁵T_2g spin-allowed transitions for a Jahn–Teller distorted manganese(III) ion [49].

The proposed structure for the complexes was confirmed by X-ray diffraction thanks to the obtaining of single crystals suitable for the resolution by this technique in the cases of complexes 1a and 2. Complex 1a was obtained by the recrystallization of 1 in methanol, and it incorporated a molecule of this solvent in its coordination sphere. The geometry of the manganese ion was a distorted octahedral for 1a and 2, with the Schiff base ligand acting as a tetradentate through the ONNO inner cavity and occupying the equatorial plane of the octahedral environment of the metal ion.

Figure 1 shows the coordination environment of each of the two complexes. In the case of compound 1a, the axial positions are occupied by the OCN^- ion and a methanol molecule. In the two axial ligands of 1a, the donor atom is oxygen, unlike the situation in compound 2. In this case, the axial positions are occupied by a water molecule and another cyanate, but the latter binds to the metal ion through the nitrogen atom. Thus, these two crystal structures provide a clear example of the ambidentate nature of this ligand [50,51], which can bind to the metal ion through the donor electron pairs of both oxygen and nitrogen.

In the resolution of the structure of complex 1a, a degree of disorder was manifested in some of the atoms, particularly for the cyanate ligand and the methyl groups of the ethylene chain between the imine groups. In this complex, some asymmetry was observed, revealed in slight variations in bond distances and angles within the molecule (Table S1). The Mn-O_phenolic distances in the equatorial plane were Mn1-O2 1.892(4) Å and Mn1-O24 1.879(4) Å while the Mn-N_imine distances Mn1-N12 1.977(5) Å and Mn1-N16 1.99(3) Å were slightly longer, as is common in this type of manganese(III) complex [45,46,47,48,52,53,54]. The axial distances were significantly longer, with values of 2.138(10) Å for Mn1-O27 and 2.367(4) Å for Mn1-O30, indicating axial elongation.

In this complex 1a, chains were formed by hydrogen bonds involving free water molecules, coordinated methanol molecules, phenolic oxygens, and methoxide oxygens of the Schiff base ligand, as well as cyanate groups. The set of these intermolecular forces generated a packing of the structure, which is shown in Figure 2. Table S2 collects the hydrogen bond scheme for 1a.

In the case of the unit cell of complex 2, two MnL²(NCO)(H₂O) molecules could be observed, which differed slightly in bond distances (Figure S7). This phenomenon of the co-crystallization of molecules is not so unusual for this type of complex and is a sign of their sensitivity to small variations leading to different structures.

As can be seen in Table S3, the Mn-O_phenolic distances differed slightly, with values between 1.8749(12) Å and 1.8922(12) Å. The Mn-N_imine distances were also slightly different, ranging from 1.9814(15) Å to 1.9875(15) Å. The distances were of the same order, approximately, as those mentioned for complex 1a. In the axial positions of complex 2, coordination to the metal ion through the nitrogen of the OCN⁻ group could be observed with values of 2.1876(15)–2.2008(15) Å (depending on the two molecules co-crystallizing in the unit cell) and the coordination of a water molecule with longer Mn-O distances (2.3140(13)–2.3407(13) Å). The axial water molecules were involved in the formation of dimeric structures through hydrogen bonds with the phenolic and ethoxide oxygens of the Schiff base of the neighboring molecule (Figure 3). As a result of these interactions, the Mn-Mn distances in complex 2 were 4.888 Å, significantly shorter than the Mn-Mn distances of 6.864 Å in complex 1a. Table S4 collects the hydrogen bond scheme for 2.

2.2. Electrochemical and Catalytic Properties

The electrochemical behavior of the complexes was studied by cyclic voltammetry. Figure 4 shows the cyclic voltammograms for complexes 1–4 at a scan rate of 0.02 V s⁻¹, and electrochemical data are shown in Table 1 and other data in the experimental section of the manuscript.

The four complexes underwent a one-electron quasi-reversible reduction process from manganese(III) to manganese(II) with voltages ranging from −74 mV (complex 1) to −300 (complex 4). The electron-donor character of the alkoxy substituents (methoxy/ethoxy groups) on the phenyl rings of the Schiff base played a key role in the achievement of such negative values and facilitated achieving higher oxidation states for the manganese ion during catalysis [47]. The peak-to-peak separation was ranging from 193 mV in 3 to 347 mV in 4, which had the less reversible character of the different redox processes.

The peroxidase-like activity of complexes 1–4 was studied. Peroxidases catalyze bisubstrate redox reactions using hydrogen peroxide as an oxidant and a second substrate with reducing characteristics that is oxidized by the peroxide [42]. This makes them interesting systems as catalysts for industrial processes that can be developed using environmentally friendly catalytic methods.

In our studies, the diammonium salt of 2,2′-azinobis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) was used as the substrate to be oxidized by hydrogen peroxide [55,56]. During the process, the oxidized radical cation, ABTS⁺, was formed, and this presented distinct absorption bands in ultraviolet-visible spectroscopy, allowing for the quantitative monitoring of the catalysis. In the present assays, absorption at a wavelength of 650 nm was chosen to monitor the reaction (ε = 12,000 M⁻¹ cm⁻¹) as we observed the incidence of undesired photochemical processes due to the generation of OH radicals from hydrogen peroxide, which can lead to erroneous measurements [57]. These photochemical interferences can be avoided by using filters that block radiation with wavelengths below 455 nm.

Table 1 shows the results of ABTS^·+ ion radical formation in these experiments with complexes 1–4, with values of approximately 50–65% for tests with complexes 1 and 2 and no significant activity for complexes 3 and 4. Hydrogen peroxide, which acts as a sacrificial oxidant, was in excess during the experiments. The turnover numbers were approximately 22–30 for 1 and 2, respectively.

The oxidation of HMF to DFF was carried out following the procedure detailed in the experimental section, and the results, expressed as a percentage of conversion, are shown in Table 1. Complexes 1 and 2 acted as catalysts with high conversion percentages under mild catalytic conditions, at room temperature, at pH 8.5 (regulated with phosphate buffer solution), and with catalyst loadings of 1%. The electrochemical efficiency of the cell, defined as the amount of HMF converted to DFF per Faraday charge, was around 0.3–0.4 mol F⁻¹ for 1 and 2, being less than 0.12 mol F⁻¹ for 3 and 4.

3. Discussion

In its structural approach, this work shows the ease with which the cyanate ion can behave as an ambidentate ligand, being able to bind through either oxygen or nitrogen atoms. In the literature, we can find examples of cyanate coordination in both modes to the manganese ion [50,51], but in the present work it has been shown, to the authors’ knowledge, for the first time, that both types of coordination are exhibited in basically similar environments, attached to analogous manganese–Schiff base entities.

The choice of this cyanate ion in the present study was made with the aim of inducing a tetragonal elongation in the octahedral environment of manganese(III) coordinated to the Schiff base through the ONNO donor set, seeking a similar effect to that previously achieved with the dicyanamide ion or the thiocyanate ion [46,47,48]. In these previous cases, greater catalytic activity in different redox processes was observed for complexes with greater tetragonal elongation, which was interpreted as the ease of generating a vacancy in the coordination environment to accommodate a substrate molecule.

The crystallographic data of complexes 1 and 2 reaffirm the hypothesis that the cyanate ion favors such tetragonal elongation while the results of the peroxidase-like activity test are consistent with this potential capacity to coordinate the hydrogen peroxide molecule [46,47,48]. Methoxy and ethoxy groups of the phenyl rings of the Schiff base ligands can also act as an internal base that can assist proton transfer during catalysis. Another factor reported as facilitating the formation of vacancies around the metal ion in this type of complex is the length of the spacer between the imine groups of the Schiff base. When we have a short spacer, as is the case with complexes 1 and 2, with two carbon atoms between the imine groups, the clamp effect of this type of spacer causes greater tetragonal elongation in the structure and higher catalytic activities. Conversely, with spacers with longer chain lengths, as in the case of complexes 3 and 4, with three and four carbon atoms between the imine groups, more symmetrical octahedral structures are favored, with less tetragonal elongation, and consequently, it is more difficult to form vacancies and subsequently coordinate the substrate molecule [48]. This structural behavior can justify the different peroxidase activities found for complexes 1 and 2, which were much higher than those presented by complexes 3 and 4. The catalytic reaction for the transformation of HMF into DFF followed the same pattern regarding the activity of the metal complexes, with higher activities for 1 and 2 and reduced activities for 3 and 4, suggesting that it also occurred through an inner-sphere mechanism favored by the possibility of generating vacancies in the coordination environment of the metal ion. During the electrochemical process, the pH was adjusted to 8.5, a basic medium in which the equilibrium of the saline solution shifted towards the formation of hypochlorite [52], thus avoiding the presence of chlorine in the reaction medium. Chlorine was the predominant species at an acidic pH, along with hypochlorous acid, but it undergoes disproportionation in a basic medium to generate hypochlorite and chloride [58,59,60]. In this way, under the experimental conditions, hypochlorite could act as a sacrificial oxidant in the reaction medium, and the behavior consistent with an inner-sphere mechanism in the catalysis with complexes 1 and 2 could be justified. In this mechanism, hypochlorite would coordinate to manganese during catalysis in a manner similar to that previously proposed by other authors for Mn(III)-salen complexes [61], analogous to those used in the present study. Previous mechanistic studies [62,63] supported the formation of transient Mn^V = O species by the oxidation of the initial Mn^III complex by hypochlorite in basic media. The high-valent oxo complex could then oxidize the organic substrate, in this case HMF to DFF, through a fast reaction (see Scheme 2).

4. Materials and Methods

4.1. Materials

All solvents, 3-methoxy-2-hydroxybenzaldehyde, 3-ethoxy-2-hydroxybenzaldehyde, 1,2-diaminepropane, 1,2-diamineethane, 1,3-diaminepropane, 1-4-diaminebutane, manganese(II) acetate tetrahydrate, potassium cyanate, hydrogen peroxide, 5-hydroxymethylfurfural, and sodium chloride, were commercial and used without further purification. Experimental details concerning instruments used for characterization of the compounds are available in the Supplementary Materials section.

Ligands H₂L¹–H₂L⁴ were prepared as reported [64] by condensation of the methoxy/ethoxy substituted 2-hydroxybenzaldehyde and the corresponding diamine.

4.2. Synthesis of the Complexes

[MnL¹(OCN)(H₂O)] (1): To 30 mL of a methanolic solution of H₂L¹ (0.29 mmol, 100 mg), 0.29 mmol (72 mg) of Mn(CH₃COO)₂·4H₂O was added. The initial yellow solution changed its color into brown. The mixture was gently heated and stirred for half an hour, and then, 0.29 mmol (24 mg) of KOCN in 10 mL of methanol was added. The brown solution was concentrated by slow evaporation to afford brown crystals, which were isolated by filtration, washed with diethyl ether, and dried in air. Yield: 79 mg (60%). Anal. calcd. for C₂₀H₂₂MnN₃O₆ (455.3): C, 52.8; H, 4.9; N, 9.2. Found: C, 52.0; H, 5.0; N, 9.1%. MS ES (m/z): 395 [MnL¹]⁺. IR (KBr, cm⁻¹): ν(O-H) 3421 (m), ν(C=N) 1624 (vs.), ν(C-O) 1250 (s), ν(OC≡N) 2181 (vs.). UV (in MeOH, nm): λ = 540 (ε = 440 M⁻¹ cm⁻¹). μ = 4.7 BM. Conductivity (in MeOH) Λ_M = 38 S cm²/mol. E_ox = 27 mV; E_red = −175 mV. Slow evaporation of the mother liquors yielded crystals of [MnL¹(OCN)(CH₃OH)]·H₂O (1a).

[MnL²(OCN)(H₂O)] (2): Synthetic procedure similar to 1 using 0.28 mmol (100 mg) of H₂L², 0.28 mmol (69 mg) of Mn(CH₃COO)₂·4H₂O, and 0.28 mmol (22.7 mg) of KOCN. Yield: 72 mg (55%). Anal. calcd. for C₂₁H₂₄MnN₃O₆ (469.4): C, 53.7; H, 5.2; N, 9.0. Found: C, 53.5; H, 5.1; N, 8.9%. MS ES (m/z): 409.2 [MnL²]⁺; 507.2 [MnL² + OCN + 3H₂O]⁺. IR (KBr, cm⁻¹): ν(O-H) 3439 (m), ν(C=N) 1624 (vs.), ν(C-O) 1254 (s), ν(OC≡N) 2168 (vs.). UV (in MeOH, nm): λ = 566 (ε = 550 M⁻¹ cm⁻¹). μ = 5.0 BM. Conductivity (in MeOH) Λ_M = 23 S cm²/mol. E_ox = −10 mV; E_red = −247 mV. Slow evaporation of the mother liquors yielded crystals of [MnL²(NCO)(H₂O)].

[MnL³(OCN)(H₂O)]·2H₂O (3): Synthetic procedure similar to 1 using 0.29 mmol (100 mg) of H₂L³, 0.29 mmol (72 mg) of Mn(CH₃COO)₂·4H₂O, and 0.29 mmol (24 mg) of KOCN. Yield: 93 mg (65%). Anal. calcd. for C₂₂H₂₆MnN₃O₈ (491.4): C, 48.9; H, 5.3; N, 8.6. Found: C, 48.8; H, 5.3; N, 8.9%. MS ES (m/z): 396 [MnL³]⁺; 493 [MnL³ + OCN + 3H₂O]⁺. IR (KBr, cm⁻¹): ν(O-H) 3436 (m), ν(C=N) 1616 (vs), ν(C-O) 1250 (s), ν(OC≡N) 2170 (vs.). UV (in MeOH, nm): λ = 574 (ε = 200 M⁻¹ cm⁻¹). μ = 4.9 BM. Conductivity (in MeOH) Λ_M = 37 S cm²/mol. E_ox = −87 mV; E_red = −280 mV.

[MnL⁴(OCN)(H₂O)₂(CH₃OH)] (4): Synthetic procedure similar to 1 using 0.20 mmol (100 mg) of H₂L⁴, 0.20 mmol (50 mg) of Mn(CH₃COO)₂·4H₂O, and 0.20 mmol (16.5 mg) of KOCN. Yield: 66 mg (60%). Anal. calcd. for C₂₄H₃₄MnN₃O₈ (547.5): C, 52.7; H, 6.3; N, 7.7. Found: C, 52.3; H, 6.4; N, 8.0%. MS ES (m/z): 440 [MnL⁴]⁺. IR (KBr, cm⁻¹): ν(O-H) 3439 (m), ν(C=N) 1614 (vs.), ν(C-O) 1250 (s), ν(OC≡N) 2191 (vs.). UV (in DMF, nm): λ = 575 (ε = 370 M⁻¹ cm⁻¹). μ = 4.9 BM. Conductivity (in MeOH) Λ_M = 24 S cm²/mol. E_ox = −127 mV; E_red = −474 mV.

4.3. Peroxidase-Like Function of the Complexes

Peroxidase-like activity of the complexes was tested using hydrogen peroxide as oxidant and the diammonium salt of the 2,2′-azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) as organic substrate. In a typical experiment, an aqueous solution of ABTS (0.009 M; 50 μL; 4.5 × 10⁻⁷ mol) and a methanolic solution of the manganese complex (0.001 M; 10 μL; 10⁻⁸ mol) were added to 3 mL of water in a quartz cuvette. The oxidation of ABTS took place after the addition of an aqueous solution of hydrogen peroxide (10 M; 50 μL; 5 × 10⁻⁴ mol). From the different absorbances of the oxidized ABTS radical, values for 650 nm were selected to follow the peroxidase-like function.

4.4. Electrochemical Oxidations of 5-Hydroxymethylfurfural

Oxidations of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxaldehyde (DFF) were performed under mild conditions in saline media, following a procedure developed by us [46]. In a typical experiment, a buffered solution (pH 8.5) containing the manganese complex (1.35 × 10⁻⁶ mol) and sodium chloride (2.1 g) was thermostatted at 40 °C. Then HMF (1.35 × 10⁻⁵ mol) was added and the preset current was applied using an Iso-tech laboratory DC power supply (model IPS 303DD) at a fixed potential for 2 h. After electrochemical reaction, catalyst was removed by filtrating the solution through a short silica gel plug. The filtered solution was then separated in dichloromethane in the presence of saturated aqueous sodium chloride solution. The dichloromethane layer was dried over magnesium sulfate. This solution was directly used for the analysis of product by GC or evaporated under reduced pressure for ¹H NMR analysis.

4.5. Crystallographic Studies

Single crystals of 1a and 2, suitable for X-ray diffraction studies, were obtained by slow evaporation as detailed in their synthesis section. Diffraction data were collected on a Bruker Smart CCD 1000 diffractometer at 100 K using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Crystal data collection and refinement are summarized in Table 2. The two structures were solved by direct methods [65] and finally refined by full-matrix least-squares base on F². SADABS [66] was used to apply an empirical absorption correction. Molecular graphics were generated with PLATON [67] and Mercury [68].

5. Conclusions

This work presents a new pathway for the oxidation of HMF to DFF through an environmentally sustainable electrolytic procedure catalyzed by manganese complexes. The catalysts use the Schiff base and cyanate as ligands. The results show different catalytic activities for the tested complexes and propose their ability to coordinate substrate molecules as the determining factor for achieving efficient catalysts. The work also demonstrates the versatility of these types of metal complexes to act as catalysts in different redox processes using various sacrificial oxidants like hydrogen peroxide or hydroperoxide ions. In the present study, the highest efficiency for the two oxidations studied (peroxidase activity and HMF oxidation) coincided with the two complexes (1 and 2) with a short carbon chain between the imine groups of the Schiff base ligand. These results are consistent with previous studies and suggest that catalysis is favored in those complexes with an octahedral structure with greater tetragonal distortion since this type of structure will facilitate the formation of vacancies in the coordination environment to accommodate the substrate molecule in an inner-sphere mechanism.

Author Contributions

Conceptualization, M.M.; methodology, M.I.F.-G., A.M.G.-N., and M.M.; formal analysis, U.B.-S., S.F.-F., I.V.-H., and M.M.; investigation, U.B.-S.; S.F.-F., M.I.F.-G., I.V.-H., and M.M.; resources, M.I.F.-G., A.M.G.-N., and M.M.; data curation, U.B.-S.; S.F.-F., M.I.F.-G., I.V.-H., and M.M.; writing—original draft preparation, U.B.-S.; S.F.-F., M.I.F.-G., I.V.-H., and M.M.; writing—review and editing, U.B.-S.; S.F.-F., M.I.F.-G., A.M.G.-N., I.V.-H., and M.M.; supervision, M.I.F.-G., A.M.G.-N., and M.M.; project administration, S.F.-F., M.I.F.-G., A.M.G.-N., and M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Footnotes

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Figures, Schemes and Tables

Scheme 1. Structures of the ligands H2L1–H2L4.

Figure 1. Ball and stick diagrams showing crystal structures for 1a (a) and 2 (b) with the atom numbering scheme (manganese ions in purple; nitrogen in blue; oxygen in red; carbon in grey; hydrogen in white). Uncoordinated water molecules are omitted in 1a. No solvent molecules are present in 2.

Figure 2. Stick diagram showing the packing arrangement for 1a viewed along the a-axis direction. The intermolecular hydrogen bonds are shown by red and light blue dashed lines.

Figure 3. Stick diagram for 2 showing the µ-aquo dimeric unit (manganese ions in purple; nitrogen in blue; oxygen in red; carbon in grey; hydrogen in white).

Figure 4. Cyclic voltammetry at 0.02 V scan rate for 1 (a), 2 (b), 3 (c), and 4 (d).

Scheme 2. Proposed mechanism for oxidation of HMF to DFF. In the center of the cycle is shown the scheme of the proposed inner sphere mechanism for the catalytic activity of metal complexes 1–2.

Table 1

HMF to DFF conversion, peroxidase activity, and redox potentials for the complexes.

Compound	HMF to DFF a	Peroxidase Activity b	E1/2 (mV) c	ΔE (mV) d
1	78 ± 4	65 ± 4	−74	202
2	70 ± 3	49 ± 3	−128	237
3	10 ± 1	3 ± 0.5	−183	193
4	11 ± 1	2 ± 0.5	−300	347

^a Conversion rates of HMF to DFF. ^b Peroxidase activity expressed as percentage of conversion of ABTS to ABTS·+ measured 10 min after mixing the solutions. ^c Half-wave potential. ^d Peak-to-peak separation.

Table 2

Crystal data and structure refinement for 1a and 2.

	1a	2
Empirical formula	C21H24MnN3O6·H2O	C21H24MnN3O6
Formula weight	487.39	469.37
Temperature [K]	100(2)	100(2)
Wavelength [Å]	0.71073	0.71073
Crystal system	Monoclinic	Triclinic
Space group	P 21/c	P-1
a [Å]	12.2376(4)	12.5606(5)
b [Å]	14.3006(6)	13.3548(5)
c [Å]	13.6772(5)	13.7492(5)
α [°]	90	88.901(2)
β [°]	116.362(2)	86.201(2)
γ [°]	90	63.478(2)
Volume [Å3]	2144.66(14)	2058.96(14)
Z	4	4
Density (calculated) [g cm−3]	1.509	1.514
Absorption coefficient [mm−1]	0.665	0.686
Theta range for data collection [°]	1.86 to 26.47	1.48 to 26.42
Reflections collected	21229	50020
Independent reflections	4414	8413
Final R indices [I > 2sigma(I)]	R1 = 0.0582; wR2 = 0.1403	R1 = 0.0319; wR2 = 0.068
R indices (all data)	R1 = 0.127; wR2 = 0.1721	R1 = 0.0456; wR2 = 0.0735

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13020030/s1, Figure S1: IR spectra for H₂L¹ and 1; Figure S2: IR spectra for H₂L² and 2; Figure S3: IR spectra for H₂L³ and 3; Figure S4: IR spectra for H₂L⁴ and 4; Figure S5: ESI mass spectrum for 1; Figure S6: ESI mass spectrum for 2; Figure S7: ORTEP crystal structure view for 2 showing the two molecules in the unit cell; Table S1: Selected bond lengths (Å) and angles (°) for 1a; Table S2: Hydrogen bond scheme showing distances (Å) and angles (°) for 1a; Table S3: Selected bond lengths (Å) and angles (°) for 2; Table S4: Hydrogen bond scheme showing distances (Å) and angles (°) for 2; Table S5: Physical measurements details. Checkcifs for 1a and 2.

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