Tuning Ni-pyrazolate frameworks by post-synthetic Fe-incorporation for oxidase-mimicking H2O2 Activation

Tuning Ni-Pyrazolate Frameworks by Post-Synthetic Fe-Incorporation for Oxidase-Mimicking H2O2Activation Nuria Martín+,[a] Francisco G. Cirujano+,[a] Eduardo García-Verdugo,[a] Jordi Llorca,[b] Enrique del Río,[c] Ignacio Jiménez-Morales,[c] Adrián Bogeat-Barroso,[c] Elena López-Maya,*[c] and Mayra G. Álvarez*[c] The introduction of iron ionic sites by metal exchange of defective homometallic nickel pyrazolate frameworks generates non-precious, Earth-abundant, first-row heterometallic Fe/Ni-pyrazolate frameworks. The Fe incorporation at the Ni nodes of the framework allows to control the hydrogen peroxide activation, minimizing its decomposition and O2liberation, occurring at the homometallic Ni nodes. The generation of FeOH reactive oxygen species at the heterometallic Fe/Ni nodes is demonstrated by the higher activity in the proof-of-concept oxidation of 1-phenylethanol to acetophenone in an aqueous medium. Introduction Metal–Organic Frameworks (MOFs) are excellent candidates to bridge homogeneous, bio-, and heterogeneous catalysis because they present high surface areas, a high proportion of open metal sites, accessibility to active sites through their intrinsic ordered porosity, and an unprecedented degree of tunability that enables the integration of a large selection of catalytic sites by different modifications.[1–3] A novel strategy developed to incorporate several active sites within the metal-organic framework, mimicking the complexity of enzymes, is the post-synthetic modification (PSM). PSM presents several advantages: i) inclusion of a wide range of different active catalytic groups into the framework; ii) simple isolation of the multi-functionalized MOF; and iii) the same MOF material can be modified several times, thus obtaining a system with a different functionality each time.[4,5] Among the developed PSM methodologies, the post-synthetic exchange (PSE) is a simple strategy by which a MOF can be functionalized to alter its chemical and physical properties. The ionic species involved in the exchange can be ligands, metals or guest ions. Particularly, the post-synthetic metal exchange is a process where the MOF is exposed to solutions containing metal ions to carry out ion exchange, resulting in the incorporation of a secondary metal by substitution of the parent one.[6] By PSE metal ions can be effectively incorporated in the secondary building unit (SBU), without significant changes in the material structure, but also as extraframework charge balancing cations in anionic MOFs. In this sense, some of the authors have shown that the post-synthetic treatment of the [Ni8(OH)4(H2O)2(BDP)6] (H2BDP=1,4-bis(pyrazol-4-yl) systems (usually referred to as NiBDP) with KOH leads to the formation of the defective material K[Ni8(OH)6(BDP)5.5] (NiBDP@K).[7] This MOF contains anionic moieties, which are balanced by extra-framework potassium cations. A second post-synthetic modification with Cu(ClO4)2allows to incorporate Cu(II) both as extraframework cations and into the cluster replacing Ni(II) ions.[8] Since NiBDP@K defective materials have opened the way to post-synthetic exchange,[9,10] in the present work we have tested the iron incorporation into a robust nickel pyrazolate framework by exposing the NiBDP@K systems to solutions containing iron ions. Following our previous studies on the reactivity of Cu and Pd transition metal-exchanged Ni-pyrazolates in CH activations of hydrocarbons,[7,8] herein we report the incorporation of iron sites in the above-mentioned platform for OH activations of peroxides and alcohols. The reactive oxygen species (ROS) generated by transition metal ions in the presence of oxidants have been employed as catalytic active sites in the oxidation of alcohol to carbonyl groups. A wide variety of catalysts have been proposed and studied [a] Dr. N. Martín,+Dr. F. G. Cirujano,+Prof. E. García-Verdugo Department of Inorganic and Organic Chemistry Universitat Jaume I. Av. Vicent Sos Baynat, s/n 12006 Castelló de la Plana (Spain) [b] Prof. J. Llorca Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering Eduard Maristany 10–14 08019 Barcelona (Spain) [c] Dr. E. del Río, Dr. I. Jiménez-Morales, Dr. A. Bogeat-Barroso, Dr. E. LópezMaya, Dr. M. G. Álvarez Department of Inorganic Chemistry University of Salamanca GIR-QUESCAT Group Pl. Caídos, s/n 37008 Salamanca (Spain) E-mail: [email protected] [email protected] [+]These authors contributed equally to this work. Supporting information for this article is available on the WWW under https://doi.org/10.1002/cplu.202300447 Part of a Special Collection: “Ibero-American Women in Chemistry” © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Wiley VCH Montag, 27.11.2023 2312 / 325122 [S. 161/167] 1 ChemPlusChem 2023,88, e202300447 (1 of 7) © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH ChemPlusChem www.chempluschem.org Research Article doi.org/10.1002/cplu.202300447 for this oxidation reaction,[11] which often suffer from corrosiveness, toxicity oxidants, and/or the use of halogenated solvents.[12] Several metal complexes have been also employed as homogeneous catalysts for oxidation reactions.[13,14] In order to overcome the limitations of homogeneous catalysis, heterogeneous catalysts containing noble metals have been traditionally employed in the oxidation of alcohol to carbonyl compounds.[15,16] Recently, Earth-abundant metals, such as Fe, have been well-isolated in silicates as high-performant heterogeneous catalysts for alcohol oxidations using aqueous H2O2solutions.[17,18] Moreover, iron (III)-MOFs, such as the MIL-100(Fe), have been also tested for alcohol oxidation in different works[19,20] and FeMOF-74 was proved for phenol hydroxylation reaction.[21] In contrast to traditional inorganic heterogeneous catalysts, MOFs have arisen as very promising biomimetic materials for enzyme-mimic catalysis due to versatile, flexible, and confined metal-organic sites.[22,23] In particular, MOFs containing hydroxo iron(III) sites have been studied either as alcohol oxidase-mimic catalysts for several alcohol oxidation reactions,[24,25] or as peroxidase-mimic catalysts in the decomposition of H2O2.[26] Those examples serve as a precedent of the proposed synthetic design of heterometallic pyrazolate crystalline porous frameworks obtained by post-synthetic Fe(II)-Ni(II) exchange. This novel approach allows to tune the reactivity of the metal nodes towards aqueous H2O2solutions. As a result, the iron-azolate metalorganic sites obtained resemble those Zn(II) or Fe(II) sites coordinated to the nitrogen atoms of histidine residues or porphyrin present in alcohol dehydrogenase or peroxidase/ catalase enzymes, respectively.[27] In this work, we show how both Ni-pyrazolate and Ni/Fepyrazolate sites are able to activate the hydrogen peroxide molecule in water, exhibiting “heme-based enzyme-like activity”.[28] On the one hand, the homometallic parent framework favors the disproportionation of H2O2into dioxygen gas and water, i.e., catalase-mimic catalytic activity (see the top part of Scheme 1). On the other hand, the heterometallic framework is able to activate and transfer ROS from H2O2to alcohol substrates (e.g., 1-phenylethanol), highlighting the key role of the PSM Fe incorporation in the MOF for stabilizing reactive Fe-OH species (see the bottom part of Scheme 1). Results and Discussion In order to tune the metal composition of the inorganic nodes in the highly robust and defective nickel pyrazolate framework, we have post-synthetically modified the nodes with iron sites. The synthesis of both the parent NiBDP and defective NiBDP@K materials is described in the supporting information. Briefly, the introduction of iron (II) ions into the nickel MOF is carried out by exposing the defective NiBDP@K materials to an 0.1 M Fe(ClO4)2ethanolic solution (Figure 1). The successful ion-exchange process is confirmed by different characterization techniques. The Ni:Fe ratio of 2.7:1 and traces of K (Ni:K ratio=94:1) found with inductively coupled plasma optical emission spectroscopy (ICP-OES) indicate not only a replacement of the extraframework potassium ions by iron ions, but also the partial incorporation of iron in the octanuclear metal hydroxide cluster. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies were carried out to confirm the uniformity of the NiBDP@Fe sample. Figure S2a shows a representative HAADF-STEM image of this material. The sample homogeneity (made up of nanoparticles smaller than 100 nm) displays different contrasts, which points out a different distribution of the elements present in the sample. The inset shows an Energy Dispersive X-Ray (EDX) spectrum recorded in the indicated area (ca. 50×50 nm). The Ni:Fe atomic ratio obtained by this technique is 3.3:1, similar to that measured by ICP-OES. This fact suggests that there is no surface segregation of any metal. Figure S1b corresponds to another HAADF-STEM image of this sample, which further corroborates its homogeneity. An EDX profile analysis was carried out along the line shown in image S2b, and the chemical signals corresponding to Ni, Fe, and Cl are shown in the inset. The presence of chlorine is attributed to a partial encapsulation of Fe(ClO4)3ion pairs.[8] It is noteworthy that the intensity of the Ni and Fe signals follow the same pattern, suggesting the simultaneous presence of both metals throughout the region measured. In addition, the intensity of the Ni and Fe Scheme 1. PSM of NiBDP@K into NiBDP@Fe changes the reactivity towards H2O2, passing from a catalase (green Ni sites) to a peroxidase (orange Fe sites) -mimic catalytic activity. Figure 1. Schematic representation of the post-synthetic modification of K[Ni8(OH)6(BDP)5.5] (NiBDP@K) by ion exchange with Fe(ClO4)2 to produce [Fe0,33(Ni6,8Fe1,2(OH)6(H2O)2(C12H8N4)5.5] (Fe(ClO4)3). For simplicity, only the SBU is shown. Ni (green); K (violet); Fe (orange); N (blue); O (red). Wiley VCH Montag, 27.11.2023 2312 / 325122 [S. 162/167] 1 ChemPlusChem 2023,88, e202300447 (2 of 7) © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH ChemPlusChem Research Article doi.org/10.1002/cplu.202300447 21926506, 2023, 12, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cplu.202300447 by Readcube (Labtiva Inc.), Wiley Online Library on [02/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License signals coincide with the brightest parts of the sample, as expected from the higher contrast exhibited by the elements with a higher atomic number according to the HAADF-STEM imaging mode. Figure S2c corresponds to a High Resolution Transmission Electron Microscopy (HRTEM) image of the NiBDP@Fe nanoparticle aggregates. As expected, a homogeneous distribution is observed and different contrasts are clearly visible. The dark areas in HRTEM are associated to electron-dense parts, where both Ni and Fe are located. An enlargement of the selected area in Figure S2c is depicted in Figure S2d. To obtain the quantitative composition of the new ironexchanged material, X-ray fluorescence (XRF) measurements were also carried out on the NiBDP@Fe system. Based on these analyses, the following molecular formula has been calculated: [Fe0,33(Ni6,8Fe1,2(OH)6(H2O)2(C12H8N4)5.5] (Fe(ClO4)3), in good agreement with ICP-OES and elemental analysis (see Supporting Information). Moreover, the oxidation state of iron ions was confirmed by X-ray photoelectron spectroscopy (XPS). The position of the Fe 2p1/2 and Fe 2p3/2 bands at 724.6 and 711.7 eV, respectively, are ascribed to Fe(III) (Figure S3a).[29] The structure, composition, and stability of the new MOF have been corroborated by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), and N2adsorption isotherms. The stable framework preserves its structure after the multistep post-synthetic modifications, as proven by the similar PXRD patterns (Figure 2). TGA results show that NiBDP@Fe has similar thermal stability than both NiBDP and NiBDP@K, starting to decompose above 310°C, with a mass loss exclusively corresponding to the combustion of the BDP linker (Figure S4). To examine the permanent porosity, the N2adsorption isotherm of NiBDP@Fe was collected at 196°C. After incorporation of iron sites, NiBDP@Fe still exhibits a BET surface area of 940 m2·g1(Figure S5). This value is lower than that measured for the parent NiBDP@K (1385 m2·g1), which is attributed to the presence of occluded Fe(ClO4)3ion pairs species, as indicated by the Cl detected in the EDX analysis. In fact, the FTIR spectrum of NiBDP@Fe shows the most characteristic band of perchlorate anion (1080 cm1), confirming the presence of the ClO4group (Figure S6).[30] The selective oxidation of alcohols is still a challenge, even more if water is used as a solvent to comply with the precepts of green chemistry. For this reason, all the prepared MOFs were tested as catalysts in the oxidation of a secondary alcohol (i.e., 1-phenylethanol) with H2O2in an aqueous medium to assess their suitability in that sort of reaction. Experimental results, which are summarized in Table S2, provide interesting behavior of the different synthesized materials. In a first attempt, all the materials – i.e., NiBDP, NiBDP@K, and NiBDP@Fe – were tested in the oxidation of 1-phenylethanol using H2O2(30 wt.%) as the oxidant and (solvent-free) reaction medium at room temperature and in an open system (Table S2, entries 1–3). All three materials yielded a negligible conversion of 1-phenylethanol to phenylacetone after 24 h of reaction. However, a strong oxygen evolution was observed for all the systems at the very first minutes of the reaction, especially for NiBDP and NiBDP@K catalysts. This observation clearly indicates a competitive H2O2decomposition reaction, which occurs over the oxidase-mimic catalysts at room temperature. In view of these results, the catalyzed H2O2decomposition was studied at room temperature by gravimetric analysis (see Supporting Information Section IV-a for details). The addition of either NiBDP or NiBDP@K to the H2O2aqueous solution revealed an abrupt catalase-like activity of these materials as compared to NiBDP@Fe. Interestingly, the addition of 30 mg of NiBDP or NiBDP@K catalyst to ca. 1.30 g of a H2O2aqueous solution (50 wt.%) decreases the initial mass -due to H2O2degradation into dioxygen gasfrom 1.29 g to 0.63 g (retains 49% of the initial mass, see right axis of Figure 3) and from 1.30 g to 0.81 g (retains 62% Figure 2. PXRD patterns of samples NiBDP (grey), NiBDP@K (blue), and NiBDP@Fe (green). Figure 3. Peroxidase (left blue axis) vs. catalase (right red axis)-mimic catalytic activity in the 1-phenylethanol oxidation to acetophenone (oxidation rate expressed as mmol of ketone produced per gram of MOF and hour) and H2O2degradation (percentage of the initial mass of a 50 wt.% H2O2aqueous solution after 10 min), respectively, for NiBDP, NiBDP@K, and NiBDP@Fe isoreticular MOFs (as-prepared for the H2O2decomposition and pretreated with H2O2for the alcohol oxidation at 90°C). Wiley VCH Montag, 27.11.2023 2312 / 325122 [S. 163/167] 1 ChemPlusChem 2023,88, e202300447 (3 of 7) © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH ChemPlusChem Research Article doi.org/10.1002/cplu.202300447 21926506, 2023, 12, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cplu.202300447 by Readcube (Labtiva Inc.), Wiley Online Library on [02/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License of the initial mass) after 10 minutes of reaction, respectively. These final masses did not significantly vary at increasing reaction times, thus suggesting that H2O2decomposition occurs with a high initial rate. Furthermore, those values are in line with the mass decrease from 100% to 35% recently reported elsewhere for the FeBDC MOF.[26] Meanwhile, the reaction over NiBDP@Fe only resulted in a mass decrease from 1.31 g to 1.04 g (maintains 79% of the initial mass), a value significantly lower than those observed for the ironfree catalysts (see right axis of Figure 3). Therefore, this is a simple method to prove that H2O2decomposition occurs at a faster rate and to a higher extent for the NiBDP and NiBDP@K materials. Such rapid H2O2decomposition would prevent the formation of ROS responsible for the 1-phenylethanol oxidation, accounting for the observed low conversion to the ketone (see left axis of Figure 3). In view of these results, the materials were recovered from the reaction media by centrifugation followed by partial drying with a flow of air. The resulting partially wet materials are named NiBDP-p, NiBDP@K-p, and NiBDP@Fe-p (p:pre-treated with H2O2). The H2O2activated materials maintain the crystalline structure (Figure S7) and the XPS measurement of NiBDP@Fe-p reveals the coexistence of Fe2 +/Fe3+ion pairs as indicated by the Fe 2p3/2 signal, which shows two components at 711.5 and 709.5 eV (Figure S8). These pre-treated materials were reused in the alcohol oxidation reaction in a closed glass vessel at 90°C (reaction conditions are given in Experimental Section and SI). Interestingly, the NiBDP@Fe-p MOF presented a notable improvement in the 1-phenylethanol conversion of 17% after 30 min of reaction as compared to NiBDP-p and NiBDP@K-p, which still led to negligible conversions (Table S2, entries 4–6). It is worth mentioning that acetophenone was the only oxidation product detected in all cases. In addition, Table S2 (entry 7) shows the heterogeneous nature of NiBDP@Fe-p, which maintains the conversion of acetophenone for three consecutive cycles. To clarify the effect of the H2O2pretreatment, the freshly prepared catalysts were tested under the same reaction conditions. However, NiBDP@Fe decreased its activity from 17 to 5%, while the iron-free catalysts exhibited the same low yields to acetophenone (Table S2, entries 9–11). These results indicate that pretreatment with H2O2has an important influence on the activity of NiBDP@Fe. An analysis of the reaction conditions and yields to acetophenone suggest that different phenomena are taking place simultaneously in the reaction system (Scheme 2): i) a fast decomposition of H2O2, which must be prevented since it avoids the formation of active oxidant species (ROS); ii) the formation of the oxidant species, which would yield the oxidation of 1-phenylethanol, and iii) the catalyst activation with H2O2, which seems to be a vital step to achieve significant alcohol conversion to phenylacetone. This would explain why the catalyst reused after the H2O2decomposition experiment (which can be considered at this point as an activation pretreatment) exhibits significantly higher activity than the fresh catalyst in the oxidation of 1-phenylethanol. Accordingly, if the fresh catalyst is directly introduced in the reaction system, an induction period corresponding to its activation occurs, thus leading to lower acetophenone yields (Table S2, entry 9). When ROS start to be produced in the reaction medium, most of the H2O2has been already decomposed since this reaction has a faster initial rate. Then, the concentration of ROS formed in the reaction medium is very low and the yield to the oxidation product markedly decreases. As described in the literature for this kind of MOF-catalyzed Fenton chemistry, the interaction of H2O2with the active sites and the reversible electron transfer between H2O2and the Fe(II)/Fe(III) active sites in the MOF is crucial for the generation of ROS, such as·OH or OOH radicals.[31,32] Moreover, the presence of the H2O2in the reaction medium may account for the reoxidation of the Fe(II) sites into catalytically active Fe(III) sites, so that the lifetime of the catalyst increases due to this regeneration through the Haber–Weiss reaction.[33] In order to corroborate the proposed key mechanistic steps of H2O2activation by the novel Fe-exchanged material, the FT-IR spectra of the H2O2-pretreated and non-pretreated MOFs before and after the oxidation reaction was measured and are displayed in Figure 4 and Figure S9. The FT-IR analysis of the NiBDP@Fe-p before the catalytic reaction shows a significant absorption at ca. 3200 cm1. This suggests the presence of OH terminated reactive oxygen species, most likely FeOH/OH2from the decomposed H2O2, as reported previously in the literature.[26] After the alcohol oxidation, this band is sharper and more intense in NiBDP@Fe-p than in NiBDP@Fe most likely due to the formation of new (Ni)FeOH/OH2groups from the MOFactivated H2O2. Meanwhile, the spectrum of NiBDP@Fe after 1-phenylethanol oxidation displays similar peaks with respect to the fresh material, with no evident transformation of FeN/O coordination, FeOH/OH2or coordinated Fe(O)x species, which suggests a poor interaction with H2O2, well in agreement with our previous hypothesis. In fact, UV-Vis analysis of NiBDP@Fe-p suggest the appearance of a band around 310 nm (Figure S10), probably due to hydroxylated iron species, e.g., Fe(OH)2+and Fe(OH)2+, present in the iron MOF after the H2O2treatment.[34] More relevant is the Scheme 2. Proposed reaction mechanism for the competitive peroxide degradation (favored at the nickel sites) and alcohol oxidation (favored at the iron sites). Wiley VCH Montag, 27.11.2023 2312 / 325122 [S. 164/167] 1 ChemPlusChem 2023,88, e202300447 (4 of 7) © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH ChemPlusChem Research Article doi.org/10.1002/cplu.202300447 21926506, 2023, 12, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cplu.202300447 by Readcube (Labtiva Inc.), Wiley Online Library on [02/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License appearance of a new absorption peak at about 1800 cm1in the region of C=O groups, which can be attributed to the occluded acetophenone oxidation product. In addition, in order to prove the formation of radical species an additional experiment in the presence of pbenzoquinone, a radical scavenger selective to OH[34] has been performed. When the oxidation reaction was carried out in the presence of 20%mol of p-benzoquinone with respect to the 1-phenylethanol substrate, the conversion decreased to 6%. This value was much lower than the 17% obtained in the absence of the radical scavenger, indicating the formation of ROS as intermediates of the reaction, which should be quenched by the radical scavenger. The PXRD recorded for the NiBDP@Fe-p and NiBDP@Fe catalysts after the oxidation reaction confirmed that the original structure of the MOF was preserved, although with a reduction in the long-range order for NiBDP@Fe-p (Figures S7 and S11). Conclusions Post-synthesis modification of NiBDP@K MOF has led to new first-row, heterometallic, crystalline porous Fe/Ni pyrazolate framework. This tuned NiBDP@Fe material is able to activate aqueous H2O2into ROS, as revealed by its significant activity in the oxidation of 1-phenylethanol as proof-of concept reaction, having a peroxidase-mimic catalytic activity. Contrarily, homometallic NiBDP and defective NiBDP@K present catalase-like activity, thus leading to rapid H2O2decomposition into dioxygen gas and water at the Ni(II) sites. The introduction of Fe ions in the Ni nodes of the MOF allows the formation of active oxidant species, responsible for the oxidation of 1-phenylethanol through an electron transfer mechanism, probably involving Fe(II)/Fe(III) species. This finding will contribute to develop novel reaction pathways and unprecedented selectivities in MOFs by PSM strategies, with direct applications in enzyme mimetic, catalysis and bioremediation. Experimental Section Characterization Thermogravimetric analyses were performed on a ShimadzuTGA-50H equipment, using a reactive air atmosphere and at a heating rate of 5°Cmin1. Powder X-ray diffraction patterns were registered on a D4 Endeavor, Bruker-AXS diffractometer. N2adsorption isotherms were measured at 77 K for on a Micromeritics Tristar 3000 volumetric instrument. Prior to starting the measurements, powder samples were outgassed and heated overnight at 150°C. Fourier Transform Infrared (FT-IR) spectra were acquired with a Pike single-reflection ATR diamond/ZnSe accessory in a JASCO FT/IR-4700 spectrophotometer. Gas chromatography (GC) analyses were carried out in a Varian 3900 gas chromatograph, using a CyclodexB column (length 30 m, i.d. 0.25 mm, film 0.25 μm). Transmission electron microscopy was performed on a FEI Tecnai F20 electron microscope equipped with a field emission electron gun operated at an accelerating voltage of 200 kV. Xray photoelectron spectroscopy (XPS) analyses were carried out using a SPECS system, equipped with a XR50 source operating at 150 W and a Phoibos 150 MCD-9 detector. No sample charging compensation was applied. X-ray fluorescence (XRF) analyses were carried out using a Bruker M4 Tornado model micro–fluorescence spectrometer. The content of carbon, hydrogen, and nitrogen was determined in a THERMO SCIENTIFIC Model Flash 2000 elemental analyzer. Inductively coupled plasma-optical emission spectroscopy (ICPOES) analyses were performed in a PERKIN-ELMER OPTIMA 8300 spectrometer. Materials and Methods p-Phenylenediacetic acid (99%) and phosphorus(V) oxychloride were purchased from Merck. Nickel(II) acetate tetrahydrate (98%), N,N-dimethylformamide, ethanol and acetone were Figure 4. FT-IR spectra of the a) as synthesized NiBDP@K (black), NiBDP@K after alcohol oxidation (grey) and NiBDP@K-p after alcohol oxidation (green); b) as synthesized NiBDP@Fe (red) NiBDP@Fe after alcohol oxidation (brown), as synthesized NiBDP@Fe-p (dark blue) and NiBDP@Fe-p after alcohol oxidation (light blue). Wiley VCH Montag, 27.11.2023 2312 / 325122 [S. 165/167] 1 ChemPlusChem 2023,88, e202300447 (5 of 7) © 2023 The Authors. ChemPlusChem published by Wiley-VCH GmbH ChemPlusChem Research Article doi.org/10.1002/cplu.202300447 21926506, 2023, 12, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cplu.202300447 by Readcube (Labtiva Inc.), Wiley Online Library on [02/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License purchased from Scharlab. All reagents and solvents were used without any previous purification. Synthesis of [Ni8(OH)4(H2O)2(1,4-bis(1H-pyrazol4-yl)benzene)6] (NiBDP) The starting ligand was synthesized according to a previously reported procedure[33] (see SI for details). The synthesis of MOFs samples NiBDP and NiBDP@K were prepared according to the procedure reported by our group,[6] with some modifications as follows. In a typical synthesis of NiBDP, 378 mg of 4,4’-benzene1,4-diylbis(1H-pyrazole) were dissolved in 96 mL of N,N’-dimethylformamide and 595 mg of Ni(CH3COO)2·4 H2O were dissolved in 24 mL of H2O. The two solutions were mixed and refluxed for 8 h under stirring. The solid obtained was filtered off and washed with N,N’-dimethylformamide, ethanol and acetone, obtaining the desired MOF. Synthesis of K[Ni8(OH)3(C2H5O)3(H2O)2(C12H8N4)5.5] (NiBDP@K) Postsynthetical modification of NiBDP system was carried out with thermal activation of the as-synthesized MOF at 150°C and outgassing (101Pa) for 8 h. Afterwards, 450 mg of NiBDP was suspended in 45 mL of 0.35 M KOH absolute ethanol solution. The resulting suspension was stirred overnight under an inert N2atmosphere, filtered off and washed thoroughly with absolute ethanol yielding the NiBDP@K system. Preparation of ion exchanged materials [Fe0.33(Ni6.8Fe1.2(OH)6(H2O)2(1,4-bis(1H-pyrazol-4-yl)benzene)5.5] (Fe(ClO4)3) [NiBDP@Fe] Approximately, 120 mg of NiBDP@K was suspended in 15 mL of 0.1 M absolute ethanol solution of Fe(ClO4)2with stirring at room temperature overnight. The resulting material. Postmodified materials were subsequently filtered off, washed thoroughly with absolute ethanol and dried in air. Afterwards, the solid was suspended in 60 mL of ethanol for 3 h to remove the eventual absorbed ion pairs. Finally, the suspension was filtered off and washed with ethanol to obtain the NiBDP@Fe material. Anal. calc. for material Fe0.33(Ni6.8Fe1.2(OH)6(H2O)2(C12H8N4)5.5) (Fe- (ClO4)3)(H2O)18 (NiBDP@Fe): C, 32.47; H, 3.69; N, 12.62. Anal. found: C, 32.36; H, 3.73; N, 12.57. Calculated residue from TGA for NiBDP@Fe: (NiO)6.8(Fe2O3)- 1.26:33.0%. Found: 32.8%. ICP-OES composition for NiBDP@Fe: Ni, 45.0 ppm; Fe, 16.9 ppm. Catalytic tests The oxidation of 1-phenylethanol was carried out following 2 procedures: 1) At room temperature (using 10 μL H2O2·mg-1 MOF). 10 mg of each as-synthesized material were suspended in 100 μL of H2O2. Afterwards, 10 μL of 1-phenylethanol were added to the suspension. The evolution of the reaction was followed at room temperature by means of Gas Chromatography and taking 50 μL aliquots of the supernatant solution but significant differences were not observed at different reaction times. 2) At 90°C (using 6 μL H2O2·mg1MOF). In these experiments, a solution of 1-phenylethanol (10 μL) in 200 μL of water was added to a vessel containing 10 mg of MOF (either assynthesized or pretreated in H2O2). H2O2(30 wt.% in water) was added in 3 portions of 20 μL every 10 min (from reaction starting until 20 min reaction time). The acetophenone yield was determined (after 0.5 h, 1 h and 24 h) using gas chromatography in the following manner: an aliquot of 50 μL from the reaction was taken after 0.5 h, diluted in 0.5 mL of methanol and the solid was separated by centrifugation. The liquid phase was injected into the gas chromatograph and no significant difference in yield was observed after 0.5 h of reaction. Details on H2O2decomposition reaction, radical scavenging and recycling can be found in the SI (Section IV) Supporting Information Additional figures and information are given in the Supporting Information. Acknowledgements Authors acknowledge the Research Program I, C2 call of University of Salamanca (PIC2-2022-08). J. L. is a Serra Húnter Fellow and is grateful to ICREA Academia program, MICINN/FEDER PID2021-124572OBC31, and 2021 SGR 01061. F.G.C. and N.M. acknowledge the “Ramon y Cajal” contract with codes RYC2020-028681-I and RYC2021033167-I funded MCIN/AEI/10.13039/501100011033 and by “ESF investing in your future”/“European Union NextGenerationEU/PRTR”. Conflict of Interests The authors declare no conflict of interest. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Keywords: catalase-like activity ·heterometallic MOFs ·iron catalysis ·oxidase mimic [1] J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C. Y. Su, Chem. Soc. Rev. 2014, 43, 6011–6061. [2] C. Dey, T. Kundu, B. P. Biswal, A. Mallick, R. Banerjee, Acta Crystallogr. Sect. B Struct. Sci. Cryst. Eng. Mater. 2014,70, 3–10. [3] K. K. Tanabe, S. M. Cohen, Chem. Soc. Rev. 2011,40, 498–519. [4] Z. Wang, S. M. Cohen, Chem. Soc. 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