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Article Cite This: Inorg. Chem. 2018, 57, 1171−1183

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Fe(III) Protoporphyrin IX Encapsulated in a Zinc Metal−Organic Framework Shows Dramatically Enhanced Peroxidatic Activity Nicola A. Dare,† Lee Brammer,‡ Susan A. Bourne,† and Timothy J. Egan*,† †

Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom

‡

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S Supporting Information *

ABSTRACT: Two MOFs, [H 2 N(CH 3 ) 2 ][Zn3 (TATB)2 (HCOO)]·HN(CH3)2·DMF·6H2O (1) and Zn-HKUST-1 (2), were investigated as potential hosts to encapsulate Fe(III) heme (Fe(III) protoporphyrin IX = Fe(III)PPIX). Methyl orange (MO) adsorption was used as an initial model for substrate uptake. MOF 1 showed good adsorption of MO (10.3 ± 0.8 mg g−1) which could undergo in situ protonation upon exposure to aqueous HCl vapor. By contrast, MO uptake by 2 was much lower (2 ± 1 mg g−1), and PXRD indicated that structural instability on exposure to water was the likely cause. Two methods for Fe(III)PPIX-1 preparation were investigated: soaking and encapsulation. Encapsulation was verified by SEM-EDS and showed comparable concentrations of Fe(III)PPIX on exposed interior surfaces and on the original surface of fractured crystals. SEM-EDS results were consistent with ICP-OES data on bulk material (1.2 ± 0.1 mass % Fe). PXRD data showed that the framework in 1 was unchanged after encapsulation of Fe(III)PPIX. MO adsorption (5.8 ± 1.2 mg g−1) by Fe(III)PPIX-1 confirmed there is space for substrate diffusion into the framework, while the UV−vis spectrum of solubilized crystals confirmed that Fe(III)PPIX retained its integrity. A solid-state UV−vis spectrum of Fe(III)PPIX-1 indicated that Fe(III)PPIX was not in a μ-oxo dimeric form. Although single-crystal XRD data did not allow for full refinement of the encapsulated Fe(III)PPIX molecule owing to disorder of the metalloporphyrin, the Fe atom and pyrrole N atoms were located, enabling rigid-body modeling of the porphine core. Reaction of 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) with H2O2, catalyzed by Fe(III)PPIX-1 and -2, showed that Fe(III)PPIX-1 is significantly more efficient than Fe(III)PPIX-2 and is superior to solid Fe(III)PPIX-Cl. Fe(III)PPIX-1 was used to catalyze the oxidation of hydroquinone, thymol, benzyl alcohol, and phenyl ethanol by tert-butyl-hydroperoxide with t1/2 values that increase with increasing substrate molecular volume.

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entrapped in sol−gels,7 hydrogels,8 zeolites,9−12 intercalated clays, 13,14 detergent micelles, 15 polymer films,16,17 graphene,18−22 carbon nanotubes,23,24 nanosheets,25 nanoparticles,26 hybrid materials,27−29 and metal−organic frameworks (MOFs).30 To date, limited success has been achieved probably because, in contrast to the original protein hosts, few of the materials produce well-defined active sites.30,31 MOFs consist of metalcontaining building blocks which serve as nodes that are connected into a two- or three-dimensional network by multitopic organic ligands.32−35 These permanently porous materials afford a large degree of structural versatility, and by altering the ligand and metal combination, it is possible to tune the dimensions and chemical composition of the pores.36,37 Thus, MOFs have the potential to overcome the problems identified with other solid supports by providing a well-defined environment for catalyst confinement in isolated active sites in

INTRODUCTION Hemoproteins are a prominent subclass of metalloproteins employing iron protoporphyrin IX (FePPIX) as a cofactor and are responsible for a variety of essential metabolic functions including gas binding, electron transport, and redox catalysis.1,2 Their versatility and ability to function with high stereo-, chemo-, and regioselectivity render them extremely effective catalysts.3,4 The efficiency of these systems is due, in part, to the versatility of FePPIX, and consequently, there has been much interest in harnessing the catalytic power of this prosthetic group outside of its protein environment. This approach is challenging, as FePPIX loses its catalytic potency when removed from a protein binding pocket. A number of factors including low solubility, aggregation, and irreversible oxidation in aqueous solution account for this loss of catalytic activity.5,6 In an attempt to circumvent these complications, one can envisage immobilizing FePPIX on a solid support in order to prevent aggregation and irreversible oxidation while still allowing substrate access to the catalytic center. Several approaches have been investigated in this regard in an attempt to develop efficient heterogeneous catalysts: FePPIX has been © 2018 American Chemical Society

Received: October 10, 2017 Published: January 8, 2018 1171

DOI: 10.1021/acs.inorgchem.7b02612 Inorg. Chem. 2018, 57, 1171−1183

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Inorganic Chemistry

material.36 This is a problem that is yet to be addressed in the field of metalloporphyrin@MOF catalysis. When designing a metalloporphyrin@MOF catalyst, this, together with other factors, needs to be taken into account. Thus, an ideal MOF for FePPIX encapsulation requires the following features: (i) cavities large enough to encapsulate the metalloporphyrin, (ii) enough space for diffusion of small molecules (reactants and products) through the framework, (iii) ability to withstand catalyst leaching, and (iv) a framework that maintains its structural and chemical integrity under catalytic conditions.69 An additional criterion for investigation of redox catalysis is that the MOF itself should not be redox active. Herein we report the preparation and characterization of a Fe(III)PPIX-containing MOF constructed using redox inactive Zn(II) ions, namely, [H2N(CH3)2][Zn3(TATB)2(HCOO)] HN(CH3)2·DMF·6H2O (1). This MOF has wide helical channels of 43 crystallographic symmetry and a very large calculated solvent-accessible surface area.70 We demonstrate the following: there is no change in the structure of the framework upon encapsulation of the metalloporphyrin; Fe(III)PPIX is distributed throughout the crystal and not merely confined to the surface; it occupies well-defined positions in the framework; it is not present as a μ-oxo dimer; the Fe(III)PPIX-loaded framework retains efficient substrate diffusion through the framework; this system is substantially more peroxidatically active than the corresponding Zn(II)-HKUST-1 system (2) encapsulating Fe(III)PPIX; and finally Fe(III)PPIX-1 is able to catalyze oxidation of various substrates showing a dramatic improvement compared to native solid hemin.

which substrate access to the catalytic sites is also controlled, thereby creating robust and efficient catalysts.31,38−40 Two approaches can be adopted in the synthesis of metalloporphyrin-containing MOFs: (i) the metalloporphyrin can be incorporated into the framework of the MOF itself by acting as a linker between metal nodes;41−44 or (ii) metalloporphyrins can be included as guests in pores within the MOF.30,36,45 The first approach relies on the use of a metalloporphyrin with peripheral functional groups in a suitable geometric arrangement to serve as a multidentate ligand in the formation of a MOF. Thus, only symmetric synthetic metalloporphyrins such as iron(III) tetrakis(4-carboxyphenyl) porphyrin can be used in this approach. On the other hand, asymmetric metalloporphyrins, including the natural prosthetic group FePPIX, can only be included in a MOF using the second approach. This method is thus much more versatile and can be used to address questions surrounding the reactivity of specific metalloporphyrins. For example, it has been established that the vinyl groups on FePPIX play an important role in its catalytic activity in enzymes, but direct comparison with synthetic analogues that lack these groups is difficult. As noted above, in aqueous solution such a comparison is prevented by aggregation and irreversible oxidation,46,47 but incorporation of synthetic porphyrin analogues into enzymes is not generally feasible. Including FePPIX and other synthetic iron porphyrins as guests in a suitable MOF may be a viable strategy to make such direct comparisons. There are a number of reports of the inclusion of synthetic iron porphyrins in MOFs.45,48−58 Most have made use of an HKUST-1 framework ([M3(BTC)2]) constructed using various metal ions including Zn(II), Cu(II), Co(II), Ni(II), Mg(II), and Cd(II). There are several reports of FePPIX-containing MOFs used in composite materials for the construction of electrochemical sensors.59−63 For example Chai and co-workers used FePPIX encapsulated in Fe-MIL-88 functionalized with gold nanoparticles to construct an electrochemical aptasensor for thrombin detection.59 All of these composite materials using hemin employed redox active MOFs such as Fe-MIL-88 and Cu-HKUST-1 for hemin encapsulation. Recently, Cheng et al. used nonredox active ZIF-8 with hemin and glucose oxidase guests as an integrated nanozyme or INZyme to measure glucose via oxidation of a synthetic substrate to form a chromophoric product.64 No loading was reported for hemin in this system. Luo et al. made use of the Cu-HKUST-1 framework to encapsulate Fe(III)PPIX which was applied with limited success as a glucose sensor by coupling the peroxidase reaction with a glucose oxidase enzyme.65 The CuHKUST-1 system is, however, poorly suited for investigating the redox activity of metalloporphyrins because Cu(II) is itself redox active, and thus, the MOF exhibits inherent peroxidatic activity.66,67 Qin et al. used the MIL-101-NH2(Al) framework. Although this framework is not redox active, it undergoes a structural and compositional change upon inclusion of hemin as seen by PXRD.68 Finally, a drawback of both MOF CuHKUST1 and MIL-101-NH2(Al) was that they could encapsulate only a small quantity of Fe(III)PPIX. Despite the versatility in choice of metalloporphyrin afforded by its inclusion as a guest in a MOF framework, a serious drawback of this approach has been identified: namely, poor access of substrates to the active site.30 In order to be catalytically active, substrates require rapid access to the metalloporphyrin to allow for efficient interaction and for catalysis not to be limited to sites at the surface of the

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RESULTS AND DISCUSSION Synthesis of Two Potential MOF Host Systems for Fe(III)PPIX Encapsulation. The two MOFs 1 and 2 were selected as potential Fe(III)PPIX hosts based on their large pore sizes, solvent-accessible surface areas, and redox inactive metal centers. Their key properties are given in Table 1. Table 1. Properties of Frameworks 1 and 2 Pertinent to Substrate Accessibility

pore diameter (Å) pore topology % solvent accessible solvent-accessible volume per channel (Å3)

[H2N(CH3)2] [Zn3(TATB)2(HCOO)] (1)

Zn-HKUST-1 (2)

21.9a 43 infinite helical channelsc 50.7d 4245d

9.1,a 5.9a square channelsb 64.2d 3002d

a

Calculated with the Mercury software package, excluding van der Waals radii.71,72 bData from previously reported crystal structure.73 c Data from previously reported crystal structure.70 dCalculated using the Mercury software package with a probe radius of 1.2 Å.71,72

Framework 1, which has previously been shown to be highly porous, contains infinite 43 helical channels with permanent porosity and has good gas adsorption properties.70 It also has very large diameter solvent-accessible pores indicating that it has the necessary room for Fe(III)PPIX encapsulation with substrate accessibility. Cu-HKUST-1 has previously been used as a host for metalloporphyrins but, although highly porous, is a poor choice for studying the role of the FePPIX guest in redox catalysis since the Cu(II) ions in the framework are redox active and have been shown to exert peroxidatic activity themselves.66,67 Replacing Cu with redox inactive Zn affords Zn1172

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Scheme 1. Synthesis of (a) [H2N(CH3)2][Zn3(TATB)2(HCOO)]·HN(CH3)2·DMF·6H2O (1) and (b) ZnHKUST-1 (2) from Zn(NO3)·6H2O and Their Respective Ligands in DMFa

a

Crystal structures of 1 and 2 are shown viewed along the a-axis.70,73.

Figure 1. Adsorption of methyl orange by MOF crystals. (a) Methyl orange adsorption by 1 (black) and 2 (purple) shown as adsorption quotient (qt) of MO adsorbed from a 40 μM MO solution in the presence of 2.5 mg of the MOF; (b) crystals of 1, (c) exposed to methyl orange and then (d) HCl vapors; (e) crystals of 2 exposed to methyl orange.

unstable MOF which loses its crystallinity rapidly after removal from solvent.70 The presence of the formate ion was observed in the single-crystal structure of synthesized 1, and the high thermal stability was confirmed by TG analysis (Figure S3). The composition of the product was confirmed by combustion analysis. The ability of substrate to diffuse into the two structures, essential for efficient MOF-assisted heterogeneous catalysis, was investigated by monitoring the uptake of methyl orange (MO) into the crystals. MO was chosen as a model substrate as it is of a similar size to typical organic substrates and it is stable under the conditions used to probe peroxidatic catalysis by FePPIX. Framework 1 showed a substantial uptake of MO (10.3 ± 0.8 mg g−1 in 3 h, Figure 1a). MO uptake follows a two-step process: an initial rapid step that is attributed to the adsorption of the dye onto the crystal surface, followed by the slower step of MO diffusion into the framework until equilibrium is reached, a process similar to that previously

HKUST-1 (2), an isostructural analogue with the same (3,4)connected tbo net topology and similar properties (Table 1). Frameworks 1 and 2 without Fe(III)PPIX were synthesized according to previously reported methods (Scheme 1) and confirmed as phase-pure materials by Pawley fitting of powder X-ray diffraction patterns (Figure S1).70,73 Single crystals of 1 were obtained and it was confirmed that they had the same asymmetric unit as reported by Sun et al. (Figure S2).70 Location of solvent molecules in the channels was not possible due to extensive disorder. It is important to note that the addition of HNO3 in the synthesis of 1 was essential for the in situ hydrolysis of solvent dimethylformamide molecules to form both the formate ions that terminally coordinate to the zinc oxygen clusters in the MOF and the dimethylammonium counterions. Formate coordination causes increased stability and results in the formation of infinite helical channels, producing chiral crystals that crystallize as a racemic mixture. Omission of HNO3 results in the formation of a thermally 1173

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Inorganic Chemistry observed with the adsorption of MO into a Fe-HKUST-1 system.31 This diffusion of MO into 1 is clearly evidenced by the dramatic color change from colorless to orange (Figure 1b,c) which shifts to red when the MO is protonated by exposing crystals to vapor emanating from aqueous HCl (Figure 1d). This latter chromism indicates that the substrate can readily undergo chemical changes while contained within the pores of the framework. By contrast, crystals of 2 show very little orange color after exposure to MO for the same length of time, indicating that minimal penetration occurs through this framework. This was supported by the observed MO uptake (Figure 1a) which displays only an initial fast step representative of surface adsorption and a correspondingly low adsorption quotient (2 ± 1 mg g−1). It has been reported that a densified layer forms on the surface of 2 which limits porosity and has been shown to result in very poor gas sorption properties.73,74 This may account for the observed low adsorption of MO. A further consideration was whether the framework maintained its crystallinity after being exposed to aqueous solution during MO adsorption. In the case of 1, crystallinity was retained and Pawley fitting of the PXRD pattern demonstrated that the material did not undergo any phase changes upon exposure to water (Figure S4). Crystals of 2 immediately went opaque when exposed to water (Figure 1e). PXRD analysis revealed that a phase change occurred upon exposure to water (Figure S5), but the new phase could not be identified because the pattern could not be indexed. An extensive search of the literature did not yield a match for this new phase. This change of phase upon exposure to aqueous solution which may account for the poor MO adsorption data indicated that 1 was more suitable for Fe(III)PPIX encapsulation, and therefore, this framework was prioritized for further investigation. Synthesis and Characterization of Fe(III)PPIX-1. There are two main synthetic methodologies for noncovalent encapsulation of guests in MOFs: (i) diffusion by soaking the MOF in a solution of the guest,75 and (ii) encapsulation by including the guest in the MOF reaction mixture and allowing the framework to form around it.48 The latter method has been used with great success in the field of porphyrin MOF catalysis. Both strategies were investigated to determine the optimal method of Fe(III)PPIX inclusion into our chosen MOF (1). Energy dispersive X-ray spectroscopy (EDS) analysis of the original surface and exposed interior surface of fractured crystals prepared by both methods was used to determine the iron distribution in the crystals. Preparation of Fe(III)PPIX-1 via soaking was found to be unsuitable as there was no detectable Fe on the newly exposed interior surface of crystals broken open after completion of the preparation (Figure 2). Instead Fe was localized at the original surface of the crystals, indicating that only surface adsorption of Fe(III)PPIX had occurred. The observation that very little, if any, Fe was present within the crystal core suggests that Fe(III)PPIX is likely too large to diffuse through the framework channels. By contrast, Fe(III)PPIX-1 synthesized through encapsulation displayed a uniform distribution of Fe throughout the crystal as there was no statistically significant difference in the iron content between the original surface and a newly exposed interior surface formed by crystal cleavage (Figure 2). The loading of Fe(III)PPIX into framework 1 prepared by encapsulation was determined by ICP-OES on digested crystals (1.2 ± 0.1 mass % Fe) and was

Figure 2. (a) Loading of Fe in crystals prepared via soaking and encapsulation. Representative SEM images of fractured crystals prepared via (b) soaking and (c) encapsulation. Red boxes indicate interior surfaces of the crystal exposed upon cleavage, and black boxes indicate unbroken original surface areas that were analyzed with EDS as presented in part a. ICP-OES refers to the ICP-OES measurement of bulk material prepared via the encapsulation method.

not significantly different from that determined using EDS analysis. The observation that Fe(III)PPIX uptake into 1 occurs through encapsulation but not diffusion suggests that, once included, catalyst leaching is not a concern. This was confirmed by UV−vis spectroscopy, which showed an absence of the characteristically intense Soret band of Fe(III)PPIX for an aqueous solution in which Fe(III)PPIX-1 crystals were soaked for over 3 h (Figure S6). Pawley fitting of the powder X-ray diffraction pattern of framework 1 following encapsulation of Fe(III)PPIX indicates that the framework retained its integrity (Figure 3a and Figure S7), with only a slight decrease in the length of the c-axis upon formation of Fe(III)PPIX-1. The crystals showed no change in morphology, although a dramatic color change due to the presence of Fe(III)PPIX was observed (Figure 3b). This dark color persisted throughout the crystal as observed upon cleaving crystals. Furthermore, an experiment with differing concentrations of Fe(III)PPIX in the reaction mixture revealed using ICP-OES that a loading in the MOF of approximately 1.2 mass % Fe was the maximum that could be achieved (Figure 3c). MO uptake experiments confirmed that porosity was retained in the Fe(III)PPIX-1 crystals, albeit to a lesser extent than the empty framework itself (Figure 3d) with an adsorption quotient for MO of 5.8 ± 1.2 mg g−1 (56% of that observed for the empty framework). This reduced capacity for MO uptake is still nearly 3 times greater than the uptake for empty framework 2. Retention of porosity of 1 after encapsulation of Fe(III)PPIX was verified using N2 adsorption isotherms. This experiment also further confirmed encapsulation with a reduction of the Langmuir surface area from 1398 to 1015 m2 g−1 (Figure S8). The reported Langmuir surface area of 1 is close to that reported previously,70 and the reduction after encapsulation corresponds to a 30% reduction in Langmuir surface area. It was also found that the material was able to adsorb and desorb water vapor without loss of porosity (Figure S9), therefore indicating that the material retains its stability in the presence of water. Evidence that Fe(III)PPIX had not degraded under the harsh crystallization conditions employed was obtained by solubilizing crystals in basic DMF and recording the resulting UV−vis 1174

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Figure 3. (a) PXRD of 1 (black) and Fe(III)PPIX-1 (red) showing no significant change after encapsulation. (b) Images of 1 (top left) and Fe(III)PPIX-1 (top right) showing the dramatic color change as well as the dark color present (bottom) within cleaved crystals. (c) Dependence of loading of Fe(III)PPIX into 1 on concentration of Fe(III)PPIX in the reaction mixture. (d) Adsorption of MO into 1 (black) and Fe(III)PPIX-1 (red). (e) UV−vis spectra of solubilized 1 (black) and Fe(III)PPIX-1 (red) showing the presence of the Soret band at 403 nm in the latter. (f) Solution-state UV−vis spectra of μ-[Fe(III)PPIX]2O (green) and H2O−Fe(III)PPIX (blue) compared to the solid-state UV−vis spectrum of Fe(III)PPIX-1 (red).

spectrum. This revealed the characteristic Soret band of Fe(III)PPIX at 400 nm (Figure 3e). Further information regarding Fe(III)PPIX speciation in the framework was obtained from solid-state optical absorption spectroscopy. The absorption spectrum of Fe(III)PPIX in Fe(III)PPIX-1 clearly shows that it does not exhibit the spectroscopic features of the μ-oxo dimer, since it lacks the characteristic feature near 600 nm. Instead, the broad envelope observed around 650 nm more closely resembles the predominantly charge-transfer band of H2O−Fe(III)PPIX (Figure 3f).76,77 The fact that Fe(III)PPIX remains intact after encapsulation and does not form a μoxo dimer within the framework highlights the potential for catalytic activity of Fe(III)PPIX when encapsulated in 1. Crystal Structure of Fe(III)PPIX-1. Further insight into the structure of Fe(III)PPIX-1 was obtained through single-crystal X-ray diffraction analysis. The crystal studied was found to be a three-component twin with approximately equal contributions from each component. The framework was found to be isostructural with empty framework 1. Interestingly, the bridging formate group present in the empty framework could not be located in the crystal structure of Fe(III)PPIX-1, and only a bridging oxygen atom probably attributable to a hydroxide ion between the secondary building units (SBUs)

was located. The crystals form as a racemic mixture and therefore belong either to the space group P43 or P41. It should be noted that previous studies of the host framework (1) reported crystals of higher symmetry (P4322 or P4122).70 However, the presence of the asymmetric guest lowers the symmetry of the system.70 The asymmetric unit contains an hourglass-shaped SBU, or pinwheel SBU, which consists of two tetrahedral terminal zinc ions and a central octahedral zinc ion (Figure 4a). The central zinc is coordinated to six carboxylate oxygen atoms of the TATB3− ligand while both tetrahedral zinc ions are coordinated to three carboxylate oxygen atoms and a bridging oxygen atom, linking the SBUs. The TATB3− ligands π-stack with each other at a distance of 3.36 Å between the triazine rings and form a D3 Piedfort unit (θ ≈ 30°).70,78 The framework is a noninterpenetrated (10,3)-a network with chiral 41 (or 43) helical channels, creating a chiral environment within the channels. These channels are the hypothesized pathways through which substrates diffuse into the framework. Despite exhaustive attempts to fully model the Fe(III)PPIX guest within the framework, this was not possible, probably on account of its extensive disorder within the channels, exacerbated by the fact that the crystal was twinned and the 1175

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either the use of very low concentrations of both MO and H2O2, resulting in slow reaction, or requiring an inconvenient out of cell reaction. For this reason, the chemically and structurally similar 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) was chosen as a substrate for further investigation. This substrate shows an increased absorption of a band at 660 nm, which results from oxidation to the radical cation (ABTS•+).80 To compare the activity of the two porphyrin@MOF systems, Fe(III)PPIX-2 was synthesized in a similar manner to Fe(III)PPIX-1 using the encapsulation method. Retention of the framework structure of 2 on forming Fe(III)PPIX-2 was confirmed by PXRD and Pawley fitting (Figure S11), and the Fe content was determined by ICP-OES as 1.039 ± 0.0004 mass % Fe (compared to 1.2 ± 0.1 mass % Fe in Fe(III)PPIX1). The MO adsorption quotient of Fe(III)PPIX-2 was found to be 0.99 ± 0.28 mg g−1, 50% of the adsorption quotient for 2 (2 ± 1 mg.g−1). This decrease in adsorption quotient was proportionate to that observed upon encapsulation of Fe(III)PPIX into 1. The much larger adsorption quotient in Fe(III)PPIX-1 suggests that substrates would have better access to catalytic sites in this material, being able to access sites both on the surface and in the interior of the crystal. On the other hand, for Fe(III)PPIX-2 it is likely that only surface sites can be accessed. This suggests that Fe(III)PPIX-1 is likely to be more suitable for MOF-encapsulated Fe(III)PPIX catalysis. The catalytic activity of Fe(III)PPIX-1 and Fe(III)PPIX-2 was also further compared to solid hemin, Fe(III)PPIX-Cl (Figure 5). All reactions were conducted with [ABTS] ≫

Figure 4. Crystal structure of Fe(III)PPIX-1. (a) Coordination environment and Piedfort unit of the framework as seen in the asymmetric unit of Fe(III)PPIX-1 showing the located Fe and N atoms of Fe(III)PPIX in 1 and (b) packing of Fe(III)PPIX-1 viewed along the b-axis with a rigid-body model of the porphine core of Fe(III)PPIX in the channels of the framework (shown as space-fill; orange = Fe, blue = nitrogen).

loading low. Nonetheless, the central Fe(III) ion of Fe(III)PPIX could be located on the basis of residual electron density. This was refined isotropically, and the site occupancy of the Fe ions was refined using a combination of the ICP-OES loading data and the magnitude of the observed electron density. The nitrogen atoms coordinated to the central iron ion could be located and were also refined isotropically using distance restraints (Figure 4a). The occupancy of these nitrogen atoms was lower than that of the iron, as they represent one orientation of the disordered porphyrin center. Further residual electron density in the vicinity of the iron center could not be satisfactorily modeled. Location of the first coordination sphere of Fe(III)PPIX, however, did allow its position in the framework channels to be determined. It was not found to πstack with the TATB3− ligands along the walls of the channels, but rather was present nearer the center of the channels. This channel-center location of the heme could contribute to the orientational disorder observed, as the interactions between Fe(III)PPIX and the framework are likely to be weak. Rigid-body modeling of the porphine core confirmed that there is sufficient space for this substructure of Fe(III)PPIX in the located position (Figure 4b). The solvent-accessible volume after encapsulation is 3520 Å3 (calculated using the Mercury software package with a probe radius of 1.2 Å),72 which is approximately 40% of the unit cell volume. This confirms that typical organic substrates, such as MO,71,72 have sufficient space to diffuse into the channels. Catalytic Activity of Fe(III)PPIX-1. To investigate whether the Fe(III)PPIX-1 system is suitable as an efficient heterogeneous catalyst and whether it shows improved catalytic activity over solid-state Fe(III)PPIX itself, its peroxidatic activity was investigated. MO can undergo oxidation in the presence of peroxides and a suitable catalyst,79 and it was indeed found to react with H2O2 in the presence of Fe(III)PPIX-1 with 86.6% conversion to its oxidized product (Figure S10). It is not, however, a convenient substrate for kinetic studies owing to its intense color which bleaches in this reaction. This necessitates

Figure 5. Initial rates of ABTS oxidation by H2O2 catalyzed by Fe(III)PPIX-1 (red), Fe(III)PPIX-2 (orange), and solid Fe(III)PPIXCl (blue). Inset: Initial rate with Fe(III)PPIX-1 (red) and solution state Fe(III)PPIX (pink). Reaction conditions are given in Table 2.

[H2O2] ≫ [Fe(III)PPIX]. Under these reaction conditions one H2O2 molecule will oxidize two ABTS molecules to ABTS•+ through the formation of a high valent iron-oxo intermediate.81,82 It was found that, in the presence of 1, no catalytic activity was observed (Figure S12), confirming that 1 is not itself catalytically active. The initial rates as well as the percentage conversion of ABTS in each system are presented in Table 2. It is important to note that the yield observed experimentally is not a true reflection of the number of ABTS molecules oxidized owing to disproportionation of ABTS•+.83 The ABTS•+ cation exists in equilibrium with unreacted ABTS and an azodication. This reaction is slow and will therefore not affect initial rates; however, it will affect the experimentally 1176

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Inorganic Chemistry Table 2. Summary of Kinetic Results for Oxidation of ABTS by H2O2 d

catalyst loading initial rate (mM ABTS•+ h−1) max [ABTS•+] (mM) conversion %e TONf

Fe(III)PPIX-1a

Fe(III)PPIX-2b

Fe(III)PPIX-Clc

1.36 mol % 0.2380 ± 0.0009 0.22 ± 0.02 10.9 ± 1.2 3.4 ± 0.3

1.28 mol % 0.0826 ± 0.0003 0.075 ± 0.003 1.86 ± 0.06 1.2 ± 0.1

12.24 mol % 0.0062 ± 0.0006 0.011 ± 0.001 0.55 ± 0.09 0.0202 ± 0.0001

≈0.17 μmol of Fe(III)PPIX in 2.5 mL of TRIS reaction buffer. b≈0.16 μmol of Fe(III)PPIX in 2.5 mL of TRIS reaction buffer. c≈1.59 μmol of Fe(III)PPIX. dMol of Fe(III)PPIX per mol of H2O2. eCalculated using H2O2 as the limiting reagent with a maximum ABTS•+ yield of 2 mM: conversion % = [ABTS•+] (mM)/2 × 100. fTON = [ABTS•+] (M)/[Fe(III)PPIX] (M). All reactions were conducted under the following conditions: 5 mM ABTS, 1 mM H2O2 in 0.01 M TRIS (pH 7.4), 37.00 °C, 2 h. a

Figure 6. (a) Yield of ABTS•+ (relative to cycle 1) and (b) initial rate of ABTS oxidation represented as mM ABTS•+ h−1 μmol−1 of Fe(III)PPIX catalyzed by Fe(III)PPIX-1 through five successive cycles of catalyst recovery and reuse.

observed yield of ABTS•+ and therefore the experimentally observed TON.83 Both Fe(III)PPIX-1 and Fe(III)PPIX-2 showed a dramatic increase in yield and conversion of ABTS relative to solid Fe(III)PPIX-Cl in spite of the fact that the quantity of Fe(III)PPIX was more than 10-fold higher in the Fe(III)PPIXCl experiments. This is a clear indication that the Fe(III)PPIX centers are more accessible in the metalloporphyrin@MOF materials compared to Fe(III)PPIX-Cl. Fe(III)PPIX-1, however, exhibited an approximately 3-fold faster rate and 6-fold higher conversion than Fe(III)PPIX-2 confirming that the former is a more efficient catalyst than the latter. This is consistent with the ability of substrates to access the Fe(III)PPIX sites in the interior of 1 by diffusion and not only surface sites as seems to be the case for framework 2 as well as the fact that Fe(III)PPIX-1 is structurally stable in water, while Fe(III)PPIX-2 is not. It is noteworthy that, at the same substrate concentration, the initial rate of reaction catalyzed by dissolved Fe(III)PPIX-OH (0.125 μmol Fe(III)PPIX; 1 mol %) in aqueous solution was only about 6 times faster (1.34 ± 0.06 mM ABTS.hr−1) than that for Fe(III)PPIX-1 (Figure 5, inset). On the other hand, this fast reaction in solution quickly dropped off so that the conversion of ABTS was only 5.67% compared to 10.91% for Fe(III)PPIX-1 after reaction completion. This suggests the rate of the reaction is limited by the diffusion of ABTS into the frameworks. The increased yield can probably be attributed to the framework preventing catalyst aggregation and protecting the metalloporphyrin from oxidative degradation, both of which contribute to limited FePPIX catalyst lifetime in solution. Interestingly, when comparing the oxidation of ABTS catalyzed by Fe(III)PPIX-1 and a metalloporphyrin@MOF with porphyrin linkers (MMPF-6) reported by Chen et al., the initial rates are comparable (0.2380 ± 0.0009 mM ABTS•+ h−1 mM−1 H2O2 for Fe(III)PPIX-1 and 0.2945 mM ABTS•+ h−1 mM−1 H2O2 for MMPF-6).41 This indicates that Fe(III)PPIX-1

reaches the efficiency of catalysts in which the metalloporphyrin forms part of a highly ordered framework. The reusability of Fe(III)PPIX-1 was investigated by conducting recycling experiments. The catalyst maintained its structural integrity through several cycles. PXRD analysis showed only changes in intensity of the peaks (Figure S13) and can be attributed to differing solvent content in the material as residual DMF leaves the pores and is replaced by water. Pawley fitting confirmed that the unit cell remained the same after three successive cycles of ABTS oxidation with a slight expansion along the a- and b-axes from 18.0993 to 18.15374 Å (Figure S13). Fe(III)PPIX-1 showed no loss of catalytic activity through five successive cycles, demonstrating that this catalyst has excellent recyclability (Figure 6). Thus, by encapsulating Fe(III)PPIX into the MOF, the catalytic center becomes stable in an aqueous medium and, being heterogeneous, can be recovered and reused. This is different from previously reported recyclability for a synthetic tetraphenyl porphyrin in Cu-HKUST-148 and for Fe(III)PPIX encapsulated in MIL-101-NH2(Al)68 which both showed a decrease in activity after each successive cycle. The versatility of Fe(III)PPIX-1 as an oxidation catalyst was investigated by studying oxidation of hydroquinone (HQ) to its corresponding benzoquinone (BQ). The reaction was initially carried out in water with H2O2 as the oxidant; however, formation of side products was observed. Changing to acetonitrile (AcN) solvent with t-butyl-hydroperoxide (tBuOOH) as oxidant resulted in a clean reaction which was also faster. Indeed, it was found that the oxidation of ABTS by t BuOOH in water was also faster than that by H2O2, with the initial rate going from to 0.2380 ± 0.0009 mM ABTS•+ h−1 to 0.44 ± 0.07 mM ABTS•+ h−1 (Figure S14). The oxidation of HQ by tBuOOH in AcN was readily monitored by UV−vis spectroscopy. The decrease in λmax of HQ (280 nm) as well as an increase in the λmax of BQ (240 nm) showed complete conversion of HQ to BQ within 20 h 1177

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Figure 7. Oxidation of HQ by tBuOOH catalyzed by Fe(III)PPIX-1: (a) UV−vis spectra of the reaction mixture at different time points (* denotes an isosbestic point); (b) increasing concentration of BQ (green) and decreasing concentration of HQ (blue) as a function of time; and (c) 1H NMR spectra of the reaction mixture at 0, 2, and 24 h.

Table 3. Conversion, Half-Lives, and Molar Volumes of Substrates Oxidized by tBuOOH in the Presence of Fe(III)PPIX-1

a

Relative concentrations of substrate and product were determined by NMR at each time point by integrating each peak relative to an internal standard (tBuOOH + in situ formed tert-butanol [tBuOH]). All reactions were carried out with 15 mg of substrate in 1 mL of d3-AcN and initiated by addition of 50 μL of 70% (w/w) tBuOOH in water at 25 °C. Characterization of products is presented in Supporting Information (Figures S18− S27). bHalf-lives were determined from UV−vis spectroscopy data obtained at 37.00 °C, fitted with a nonlinear regression model in GraphPad Prism (Figure S16).84 cMolecular volume was calculated using Chemicalize.85

reaction was further monitored by 1H NMR spectroscopy at 0, 2, and 24 h. The disappearance of the peak corresponding to the CH protons (δ 6.63 ppm) of HQ and the appearance of the peak corresponding to the CH protons of BQ (δ 6.77 ppm) further confirmed 100% conversion of HQ to BQ (Figure 7c).

(Figure 7a). The presence of an isosbestic point confirmed there were only two chromophoric components present in solution. The half-life of the reaction was determined by monitoring either the increase in concentration of BQ or the decrease in concentration of HQ (Figure 7b, Table 3). The 1178

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Inorganic Chemistry In light of the instability of framework 2 in aqueous media, the oxidation of HQ by Fe(III)PPIX-2 in AcN by tBuOOH was investigated and compared to that by Fe(III)PPIX-1. It was found that there was incomplete conversion (41.5%) compared to Fe(III)PPIX-1 (100%) and a longer half-life (2.56 h compared to 2.07 h) indicating that even in an organic solvent this material is considerably less efficient as an oxidation catalyst (Figure S15). In view of the 100% conversion of HQ in the presence of Fe(III)PPIX-1, the oxidation of other substrates, thymol (TH), benzyl alcohol (BN), and phenyl ethanol (PH), was investigated in the presence of this catalyst (Table 3, Figure S16). All reactions were monitored in the same manner as HQ, with an added 1H NMR spectrum at 48 h. These reactions did not achieve full conversion, and it was found that as the molecular volume of the substrate increased, the half-life of the reaction increased, suggesting that the larger molecules diffuse through the pores more slowly and this therefore affects the reaction rate. This hypothesis was supported by the fact that the same oxidation reactions catalyzed by Fe(III)PPIX in solution did not show a similar trend (Table S1, Figure S17). Rather, it was found that the half-lives for the oxidation of TH, BN, and PH were approximately equal. This points toward the possible use of Fe(III)PPIX-1 as a size-selective oxidation catalyst and emphasizes the importance of the catalytic sites in the interior of the MOF. Fe(III)PPIX-1 has been shown to be both effective as a heterogeneous oxidation catalyst and very much more efficient than solid hemin. Owing to the focus of previous studies on applications as functional materials, it is not possible to directly compare the catalytic activity of Fe(III)PPIX-1 to the previously synthesized FePPIX@MOFs. Luo et al. synthesized FePPIX@Cu-HKUST-1 and studied the oxidation of luminol by H2O2 using chemiluminescence. Unfortunately, no kinetics data were reported since the study focused on detection of H2O2 and glucose by coupling the FePPIX@MOF with glucose oxidase.65 Furthermore, since copper is redox active,66,67 it is not certain whether the activity can be directly ascribed to the metalloporphyrin. In the study by Qin et al., FePPIX was encapsulated in MIL−101(Al)−NH2 which converted to MIL− 53(Al)−NH2 after encapsulation, and a loading of FePPIX into the MOF of 1.02% was found using the Al:Fe ratio.68 This loading was similar to that of Fe(III)PPIX in 1 found in this study. The catalytic oxidation of 3,3,5,5-tetramethylbenzidine (TMB) by H2O2 was investigated, and the material was found to be catalytically active, but no initial rates were reported, so direct comparison to Fe(III)PPIX-1 again could not be made. Cheng et al. also encapsulated FePPIX in a redox inactive MOF, ZIF-8.64 In this study no initial rates or loading of FePPIX in ZIF-8 were reported. The focus was on utilizing the MOF with the enzyme glucose oxidase and FePPIX guests for the purpose of glucose detection. This system was quite robust in that it maintained activity through four cycles, but it was apparently less stable than Fe(III)PPIX-1 in that there was about a 20% reduction in activity by the fourth cycle. In addition, in all reports to date, no single-crystal X-ray data of Fe(III)PPIX encapsulated within the framework were reported, and there is no confirmation that Fe(III)PPIX was distributed throughout the MOF crystal and not confined to the surface except in the case of the ZIF-8 system.

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CONCLUSION

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EXPERIMENTAL SECTION

In this study, we reported that encapsulation of Fe(III)PPIX in a MOF affords an effective and long-lived oxidation catalyst for a variety of substrates. We have directly shown access of substrates into the framework of [H2N(CH3)2][Zn3(TATB)2(HCOO)]·HN(CH3)2·DMF·6H2O (1) and the corresponding catalytically active material Fe(III)PPIX-1, which most probably occurs by diffusion through the infinite 43 helical channels. The distribution and accessibility of catalytic sites in Fe(III)PPIX-1 have been confirmed by synchrotron singlecrystal X-ray diffraction, and this is the first report of singlecrystal X-ray data for encapsulation of Fe(III)PPIX into a MOF. By contrast, Zn-HKUST-1 (2) shows poor substrate uptake, and Fe(III)PPIX-2 shows correspondingly poorer catalytic performance than Fe(III)PPIX-1. Furthermore, the observed aqueous instability of 2 reinforces its unsuitability as a peroxidation catalyst in this system. Inclusion of Fe(III)PPIX into 1 drastically increases both the initial rate of peroxidatic oxidation of ABTS compared to solid Fe(III)PPIX-Cl and dramatically increases the yield compared to both solid-state Fe(III)PPIX-Cl and solution-state Fe(III)PPIX. What is more, Fe(III)PPIX-1 was shown to catalyze ABTS oxidation by H2O2 at an initial rate less than an order of magnitude slower than Fe(III)PPIX in solution. The catalyst can be readily recovered and recycled without loss in catalytic activity. Furthermore, the ability of Fe(III)PPIX-1 to oxidize both H2O2 as well as a variety of other substrates further demonstrates its efficacy as a heterogeneous oxidation catalyst. This emphasizes the success of the Fe(III)PPIX-1 system in harnessing the power of Fe(III)PPIX in a synthetic heterogeneous MOF host. This MOF system therefore provides a suitable support for comparing the reactivity of Fe(III)PPIX with synthetic porphyrins. Detailed mechanistic studies on this system are currently underway with both Fe(III)PPIX and Fe(III) tetraphenylporphine.

General. With the exception of 4,4′,4″-s-triazine-2,4,6-triyltribenzoic acid (CGene Tech. Inc.), all chemicals were purchased from Sigma-Aldrich and used without further purification. Doubledistilled deionized water was provided by a Millipore Direct-Q3 water purification system. The solvent used for synthesis was anhydrous N,N-dimethylformamide (DMF, 99.8% purity) which was stored over activated molecular sieves (3 Å). All glassware was washed scrupulously as reported previously in order to prevent build-up of Fe(III)PPIX.86 TGA was performed on a TA-Q500 TA, and data were analyzed using the Universal Analysis 2000 program. Synthesis of [H2N(CH3)2][Zn3(TATB)2(HCOO)]·HN(CH3)2·DMF· 6H2O (1). Single crystals of 1 were prepared by dissolving 4,4′,4″-striazine-2,4,6-triyl-tribenzoic acid (H3TATB) (5 mg, 0.0113 mmol) and Zn(NO3)2.6H2O (27 mg, 0.0907 mmol) in DMF (2 mL) with 30 μL of 0.5 M HNO3 in a glass vial. The vial was sealed with a lid and covered with Parafilm to ensure a tighter seal, sonicated to ensure the dissolution of all material, and placed in an oven at 105 ± 2 °C for 16 h. The oven was then allowed to cool to room temperature over 4 h; the crystals were collected and washed extensively with fresh DMF. The product was identified and phase purity confirmed by Pawley fitting of the PXRD pattern, elemental combustion analysis, and TGA. The dimethylammonium ([H2N(CH3)2]+) counterion and formate ions present in the structure are produced by acid hydrolysis of DMF. Combustion analysis provided evidence for further uncoordinated dimethylamine, DMF, and H2O in the channels of the MOF. These solvent molecules were identified using a combination of TGA (Figure S28) and elemental analysis and were in good agreement with previously reported results.70 Anal. Calcd for C56H59N9O21Zn3: C, 1179

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Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Samples were ashed in a furnace at 600 °C overnight before digestion with 4 mL of 4:1 conc HF and HNO3 at 130 °C for 48 h (Warning: HF is extremely dangerous and should be handled with caution.) These samples were then dried and washed twice with 2 mL of conc HNO3 with drying between washes. The solid material remaining was diluted in a 5% HNO3 solution before analysis by ICPOES using a Varian 730-ES spectrometer. The mass % of Fe was calculated from the data obtained. All ICP-OES measurements of each sample were done on three preparations of the same material which were then combined in order to obtain an accurate mass % across differing preparations. Gas Sorption. Both N2 and water vapor sorption experiments were conducted using a Micromeritics 3Flex surface area analyzer. Approximately 100 mg of oven-dried 1 and Fe(III)PPIX-1 were further dried under dynamic vacuum for 24 h at 80 °C prior to being treated in a Micromeritics Flowprep with a constant flow of nitrogen gas over the sample at 60 °C for 24 h. The samples were further heated at 60 °C under vacuum in situ on the Micromeritics 3Flex surface area analyzer in order to ensure all solvent was evacuated from the pores of the MOF. Adsorption isotherms for N2 were measured at −196 °C and water vapor at 25 °C. Data were analyzed using 3Flex Version 3.01 to obtain the calculated Langmuir surface area. X-ray Diffraction. Single-Crystal X-ray Diffraction. Single crystals of Fe(III)PPIX-1 were removed from the mother liquor and immediately placed under Paratone oil in order to prevent solvent loss and crystal degradation. Due to the dark color of the crystals, light polarization could not be used to select a sample of suitable quality for data collection. Instead, crystals were screened on a Bruker KAPPA APEX II Duo diffractometer using Mo Kα radiation (λ = 0.71073 Å) to find suitable crystals. Full data collection using synchrotron radiation was conducted at the I19 small-molecule single-crystal diffraction beamline at Diamond Light Source (λ = 0.6889(3) Å), which is equipped with a Pilatus 2 M detector.88 An Oxford Cryosystems Cryostream device was used to cool the crystal to −173.15(2) °C using dry N2 gas. Data were collected as a series of four sequences of frames with 0.1 s exposure time per frame with no beam attenuation. The best data were obtained for a crystal determined to be a three-component twin. Threecomponent twin data integration and reduction were performed using ChrysAlisPro, and data were corrected for absorption using multiscan empirical absorption correction.89 Unit cell and space group were determined using ChrysAlisPro. Structure solution was conducted using direct methods with SHELXS-97 which located all nonhydrogen atoms of the MOF, while SHELXL-97 was used in a refinement by full matrix least-squares on F2, with anisotropic displacement parameters.90 Hydrogen atoms were placed with geometric constraints and were refined with isotropic displacement parameters. All structure determination was completed using the Olex2 interface.91 The iron center of the porphyrin core was located using residual electron density and refined isotropically with a fixed site occupancy based on the calculated loading from the ICP-OES data. The nitrogen atoms forming the first coordination sphere of the porphyrin were also located and their positions refined using distance and angle restraints. These atoms were modeled with a lower site occupancy than the iron as a consequence of orientational disorder of the porphine core.90 Crystal structure data are presented in Table S2. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) was used to confirm the structure of the bulk material. All patterns were collected at 25 °C on a Bruker D8 Advance diffractometer with a Lynxeye detector using Cu Kα radiation (λ = 1.5406 Å) and operating at 40 mA and 30 kV. A receiving slit of 0.6 mm and primary and secondary slits of 2.5 mm were used. All samples were measured using a zero-background sample holder and were lightly ground to a fine powder before collection of data. The samples were scanned over a 2θ range of 4° to 40° using a step size of 0.02° (1760 steps) with 2 s exposure time. Pawley fitting92 was conducted using TOPAS (v 4.2) in order to determine unit cells from the PXRD patterns.93

48.38%; H, 4.28%; N, 9.07%. Found: C, 48.65%; H, 4.12%; O, 9.10%. Solvent mass % from TGA: 20.9 ± 1.1%. Synthesis of Zn-HKUST-1 (2). Single crystals of 2 were prepared by dissolving benzene-1,3,5-tricarboxylic acid (28 mg, 0.133 mmol) and Zn(NO3)2·6H2O (62 mg, 0.208 mmol) in DMF (2 mL) in a glass vial. The vial was sealed with a lid and covered with Parafilm, sonicated to ensure all material was dissolved, and placed in an oven at 85 ± 2 °C for 20 h. The oven was then allowed to cool to room temperature over 3 h, and the crystals were collected and washed extensively with fresh DMF. The product was identified and phase purity confirmed by Pawley fitting of the PXRD. TGA was used to determine the solvent content (Figure S28). Solvent mass % from TGA: 32.58 ± 0.60%. Synthesis of Fe(III)PPIX-1 and Fe(III)PPIX-2. The same procedures used for synthesis of 1 and 2 were used for the synthesis of Fe(III)PPIX-1 and Fe(III)PPIX-2, respectively, with the exception that 5 mg (0.0079 mmol) of hematin (Fe(III)PPIX-OH) was included in each reaction mixture. The crystals obtained from each mixture were washed extensively with fresh DMF in order to remove excess surfaceadsorbed Fe(III)PPIX-OH. The products were identified, and phase purity was confirmed by Pawley fitting of PXRD patterns of the bulk materials. TGA was used to determine the solvent content (Figure S28). The amount of Fe(III)PPIX encapsulated within the framework was determined using ICP-OES. Solvent mass % from TGA: Fe(III)PPIX-1, 18.17 ± 0.28%; Fe(III)PPIX-2, 27.22 ± 0.73%. Ultraviolet−Visible (UV−vis) Spectroscopy. Solution-state UV−vis spectra were recorded on a Shimadzu UV-1800 spectrophotometer over the range 200−800 nm. Quartz cuvettes (Hellma, Suprasil, 1 cm path length) were maintained at a constant temperature of 25.00 ± 0.02 °C by a TCC-100 thermoelectrically temperaturecontrolled cell holder accessory. Solid-state spectra were recorded at room temperature on a Cary 5000 UV−vis spectrophotometer with a Harrick Praying Mantis diffuse reflectance accessory. Methyl Orange (MO) Adsorption. Prior to the MO adsorption experiments, single crystals of 1, 2, Fe(III)PPIX-1, and Fe(III)PPIX-2 were kept in an oven at 50 °C for 14 h. The remaining DMF mass was determined by TGA and accounted for to obtain an accurate mass of material used for MO adsorption experiments in order to calculate the adsorption quotient. All TGA experiments were performed with a dry N2 gas flow rate of 50 cm3 min−1 and were monitored from room temperature to 400 °C at a heating rate of 10 °C min−1. A 40 μM aqueous methyl orange (MO) solution was prepared by diluting a 1 mM aqueous stock solution of MO (sodium 4-{[4-(dimethylamino)phenyl]diazenyl}benzene-1-sulfonate) in 0.01 M TRIS buffer (pH 7.4). Approximately 2.5 mg of the appropriate dried MOF material was added, and the decrease in concentration of MO was monitored at 465 nm using UV−vis spectroscopy over 3 h. (A Beer’s law plot was used to determine the extinction coefficient of the MO.) All experiments were conducted with constant magnetic stirring until equilibrium was reached. The TG data were used to calculate an accurate mass of crystals used in the experiments without framework-associated solvent for the adsorption quotient calculation. Data were analyzed using nonlinear regression analysis in GraphPad Prism v6.05.84 The adsorption quotient was calculated from eq 1:

qt =

(ci − ct )V m

(1)

where qt is the mass of dye adsorbed at time t (mg·g−1), ci and ct are the initial and time t solution concentrations respectively (mg·L−1), V is the volume of the aqueous dye solution added (L) and m is the mass of material studied (g).87 Scanning Electron Microscopy (SEM). A Leo 1450 LaB6 scanning electron microscope with a Bruker XFlash EDS Si Drift Detector was used to obtain micrographs of crystals. Quantification of the zinc and iron content on the exterior and exposed interior surfaces of Fe(III)PPIX-1 crystals prepared by soaking and encapsulation methods was performed using energy dispersive X-ray spectroscopy (EDS). Prior to visualization, samples were sputter-coated with carbon. Statistical significance was determined using an unpaired twotailed t test. 1180

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Research Foundation for financial support via a PhD studentship. We are grateful to the Diamond Light Source (beamline I19) for beamtime and to Dr. Sarah Barnett for conducting the experiments at Diamond Light Source. We thank Dr. Scott Bohle (McGill University, Canada) for use of the solid-state UV−vis spectrometer. We would also like to thank Dr. Rebecca Smith (University of Sheffield) for her contribution to early efforts in crystallographic characterization of Fe(III)PPIX-1.

Kinetics Measurements. The chromophoric substrate 2,2′-azinobis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) was used to monitor the progress of the peroxidatic reaction catalyzed by Fe(III)PPIX-Cl or the Fe(III)PPIX-MOF systems. In a typical reaction, 1 mg of catalyst was added to 2.375 mL of 5 mM aqueous ABTS solution (0.01 M TRIS, pH 7.4). The reaction was initiated by adding 125 μL of 20 mM H2O2 (final concentration of 1 mM H2O2 and total volume of 2.5 mL), and the rate of ABTS•+ formation was monitored by the intensity of its absorption band at 660 nm on a Shimadzu UV-1800 spectrometer at 37.00 ± 0.02 °C with constant magnetic stirring. Data were analyzed using the one-phase exponential function in GraphPad Prism in order to obtain maximal yield (Ymax) and a straight-line function to obtain initial rates.84 The recycling experiments were conducted in the same manner as the previous kinetics measurements in triplicate (cycles four and five were single measurements). Crystals were collected and dried after each successive cycle. The solvent content was determined using TGA analysis before each cycle, and PXRD was conducted to ensure that crystallinity was retained. Oxidation of Additional Substrates. Hydroquinone (15 mg, 0.1362 mmol), benzyl alcohol (15 μL, 0.1443 mmol), thymol (15 mg, 0.0995 mmol), or phenyl ethanol (15 μL, 0.1226 mmol) was each added to 1 mL of acetonitrile (AcN) containing either Fe(III)PPIX-1 (5 mg, 0.89 μmol Fe(III)PPIX), Fe(III)PPIX-2 (5 mg, 0.64 μmol), or 178 μL of 5 mM hemin solution (0.89 μmol in 1 mL). The reaction was started by adding 50 μL of tert-butyl hydroperoxide. The reaction was monitored spectrophotometrically on a Shimadzu UV-1800 by reading aliquots of the reaction mixture at time intervals. All spectra were recorded over a range 200−800 nm. Nuclear magnetic resonance (NMR) spectra of in situ reactions under the same conditions were obtained in acetonitrile-d3 to confirm the reaction product. NMR spectra were recorded on a Bruker 400 MHz Ultrashield 400 Plus NMR spectrometer, and chemical shifts (δ) were recorded relative to acetonitrile-d3 (δ 1.94 ppm in 1H NMR, δ 118.26 ppm in 13C NMR). All chemical shift values are reported in ppm.

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(1) Groves, J. T. Models and Mechanisms of Cytochrome P450 Action. In Cytochrome P450; Springer US: Boston, MA, 2007; pp 1− 43. (2) Poulos, T. L. Heme Enzyme Structure and Function. Chem. Rev. 2014, 114, 3919−3962. (3) Schmid, A.; Dordick, J. S.; Hauer, B.; Kiener, A.; Wubbolts, M.; Witholt, B. Industrial biocatalysis today and tomorrow. Nature 2001, 409, 258−268. (4) Reedy, C. J.; Gibney, B. R. Heme Protein Assemblies. Chem. Rev. 2004, 104, 617−649. (5) Bruice, T. C. Reactions of hydroperoxides with metallotetraphenylporphyrins in aqueous solutions. Acc. Chem. Res. 1991, 24, 243−249. (6) Guo, C.-C.; Song, J.-X.; Chen, X.-B.; Jiang, G.-F. A new evidence of the high-valent oxo−metal radical cation intermediate and hydrogen radical abstract mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyrins. J. Mol. Catal. A: Chem. 2000, 157, 31−40. (7) Battioni, P.; Cardin, E.; Louloudi, M.; Schollhorn, B.; Spyroulias, G. a.; Mansuy, D.; Traylor, T. G. Metalloporphyrinosilicas: a new class of hybrid organic-inorganic materials acting as selective biomimetic oxidation catalysts. Chem. Commun. 1996, 2037−2038. (8) Wang, Q.; Yang, Z.; Ma, M.; Chang, C. K.; Xu, B. High Catalytic Activities of Artificial Peroxidases Based on Supramolecular Hydrogels That Contain Heme Models. Chem. - Eur. J. 2008, 14, 5073−5078. (9) Liu, C.-J.; Li, S.-G.; Pang, W.-Q.; Che, C.-M. Ruthenium porphyrin encapsulated in modified mesoporous molecular sieve MCM-41 for alkene oxidation. Chem. Commun. 1997, 65−66. (10) Holland, B. T.; Walkup, C.; Stein, A. Encapsulation, Stabilization, and Catalytic Properties of Flexible Metal Porphyrin Complexes in MCM-41 with Minimal Electronic Perturbation by the Environment. J. Phys. Chem. B 1998, 102, 4301−4309. (11) Cady, S. S.; Pinnavaia, T. Porphyrin intercalation in mica-type silicates. Inorg. Chem. 1978, 17, 1501−1507. (12) Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. A functional zeolite analogue assembled from metalloporphyrins. Nat. Mater. 2002, 1, 118−121. (13) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. Spectroscopic characterization of porphyrin monolayer assemblies. J. Am. Chem. Soc. 1989, 111, 1344−1350. (14) Barloy, L.; Battioni, P.; Mansuy, D. Manganese porphyrins supported on montmorillonite as hydrocarbon mono-oxygenation catalysts: particular efficacy for linear alkane hydroxylation. J. Chem. Soc., Chem. Commun. 1990, 1365−1367. (15) van den Broeke, L. J. P.; de Bruijn, V. G.; Heijnen, J. H. M.; Keurentjes, J. T. F. Micellar Catalysis for Epoxidation Reactions. Ind. Eng. Chem. Res. 2001, 40, 5240−5245. (16) Nestler, O.; Severin, K. A Ruthenium Porphyrin Catalyst Immobilized in a Highly Cross-linked Polymer. Org. Lett. 2001, 3, 3907−3909. (17) Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y.; Che, C.-M. PolymerSupported Ruthenium Porphyrins: Versatile and Robust Epoxidation Catalysts with Unusual Selectivity. J. Am. Chem. Soc. 2000, 122, 5337− 5342. (18) Xue, T.; Jiang, S.; Qu, Y.; Su, Q.; Cheng, R.; Dubin, S.; Chiu, C.Y.; Kaner, R.; Huang, Y.; Duan, X. Graphene-Supported Hemin as a Highly Active Biomimetic Oxidation Catalyst. Angew. Chem. 2012, 124, 3888−3891.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02612. Pawley fitting of PXRDs, additional data, and characterization of catalytic products (PDF) Accession Codes

CCDC 1577957−1577958 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lee Brammer: 0000-0001-6435-7197 Susan A. Bourne: 0000-0002-2491-2843 Timothy J. Egan: 0000-0001-7720-8473 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of South Africa (Grant Number CPRR13082330465). N.A.D. acknowledges the National 1181

DOI: 10.1021/acs.inorgchem.7b02612 Inorg. Chem. 2018, 57, 1171−1183

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