Probing Framework-Restricted Metal Axial Ligation and Spin State

Letter pubs.acs.org/JPCL

Probing Framework-Restricted Metal Axial Ligation and Spin State Patterns in a Post-Synthetically Reduced Iron-Porphyrin-Based Metal−Organic Framework Pavel Kucheryavy,† Nicole Lahanas,† Ever Velasco,† Cheng-Jun Sun,‡ and Jenny V. Lockard*,† †

Department of Chemistry, Rutgers University, Newark, New Jersey 07102, United States X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

S Supporting Information *

ABSTRACT: An iron-porphyrin-based metal organic framework PCN-222(Fe) is investigated upon postsynthetic reduction with piperidine. Fe K-edge X-ray absorption and Kβ mainline emission spectroscopy measurements reveal the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. Analysis and fitting of these data confirm the binding pattern predicted by a space-filling model of the structurally constrained pore environments. These results are further supported by UV−vis diffuse reflectance, IR, and resonance Raman spectroscopy data.

M

etalloporphyrins are attractive building blocks for bioinspired solid-state materials designed to harness their potential for small-molecule binding and catalytic behavior. Porous solid-state networks of these metal complexes are commonly targeted to promote high densities of metal sites and their controlled availability for axial binding through host− guest interactions.1,2 The incorporation of metalloporphyrins in metal−organic frameworks (MOFs) is a relatively recent design approach that has shown great promise not only for producing structurally robust porous networks but for imparting additional properties such as pore shape and size selectivity.3−6 MOFs are self-assembled solid-state materials comprised of metal ions connected through coordination bonds with organic linker molecules. Notably, these crystalline architectures exhibit permanent microporosity upon solvent removal, making them particularly attractive for adsorption-based applications.7,8 In porphyrin-based MOFs, pre- or postmetalated porphyrin linkers can be connected through highly coordinated metal nodes to form porous networks. In this motif, pore-wallaccessible metals that are prone to accommodate coordination and oxidation state changes can be included without compromising the overall structural integrity of the framework. In this work, we explore a highly stable iron-porphyrin-based MOF called PCN-222(Fe)3 (Figure 1) and the ramifications of its postsynthetic reduction. The framework, also reported as MOF-5454 or MMPF-6,9 contains nodes of Zr6 clusters connected by carboxyphenyl meso-substituted iron porphyrins to form a 3D architecture with two types of 1D channels along the c-axis (diameters of ∼37 and ∼10 Å). This MOF, which has already shown promising catalytic behavior,3,9 serves as an ideal © XXXX American Chemical Society

Figure 1. (Top) FeCl-PCN-222 crystal structure. (Bottom) Spacefilling model of the trigonal channel before and after postsynthetic reduction with piperidine.

platform for exploring framework-imposed structural restriction on substrate binding patterns. The vastly different dimensions of the two types of channels in PCN-222 effectively create unique pore environments around the two axial binding sites of the metalloporphyrin channel walls. Here, we focus on the postsynthetic reduction of the iron centers in this framework with piperidine. Detailed local electronic and structural consequences of this reaction under the constraints of the Received: February 10, 2016 Accepted: March 7, 2016

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the MOF iron porphyrin sites is provided by analysis of the XANES measurements, as discussed below. Reduction of free iron(III) porphyrin complexes in excess of amine leads to formation of hexacoordinated iron(II) products through stabilizing axial ligation of the metal center.12 However, with its vastly different pore sizes (diameters of 37 vs 10 Å), the PCN-222 environment poses restrictions on the number of feasibly occupied axial ligation sites. A simple space-filling model (Figure 1) shows that while the large hexagonal channels can easily accommodate a coordinating amine at each iron site, this axial ligation is spatially limited on the side facing the much smaller trigonal channels. In these smaller pores, only one out of the three iron porphyrin faces can reasonably accommodate a piperidine molecule. This simple model therefore predicts a ratio of one saturated hexacoordinated to two unsaturated pentacoordinated iron centers in the reduced Fe-PCN-222-pip MOF. Axial ligation of Fe(II) porphyrin systems is well-known to influence the spin state of the metal.20 Focusing on the coordination environments relevant to this study, the hexacoordinated FePy2TPP complex, with medium-field pyridine ligands, is reported to have spin state S = 0.20 With a similar coordination environment, the doubly ligated Fe-pip2 sites expected in Fe-PCN-222-pip should also yield this lowspin state. There are no simple, easily obtained iron(II) porphyrin reference complexes that replicate the unsaturated pentacoordinated iron centers predicted for the piperidinetreated MOF. However, basket-handle21 and picket-fence22 iron(II) porphyrin complexes as well as naturally occurring heme centers have similar square-pyramidal geometry when singly ligated. With intermediate field axial ligands like imidazole, these porphyrin systems with approximate C4v symmetry were all reported to have high-spin (S = 2) states;23 therefore, the same is expected for the analogous fivecoordinate environment within Fe-PCN-222-pip. Differences in metal coordination geometry and accompanying spin state changes provide convenient handles for assessing the axial ligation pattern of iron porphyrin sites in the MOF as long as precaution is taken to avoid other influences on spin state, such as temperature. In fact, spin state changes at low temperatures, notably in five-coordinate iron(II) porphyrin systems, have been documented.24 To avoid complications from low-temperature-induced spin-crossover effects, roomtemperature measurements are desired. This precludes the use of Mössbauer spectroscopy because the low iron content in FePCN-222 (∼2−4%) would inevitably require low-temperature data collection to achieve an adequate signal-to-noise ratio for analysis. In this study, we employed several electronically and structurally sensitive spectroscopy methods to track the iron coordination and spin state properties strictly at room temperature. Among them, vibrational spectroscopy methods can provide some insight. Following well-established literature precedent on porphyrin vibrational spectroscopy,17−19 spin state marker bands in the IR and resonance Raman spectra of PCN-222(Fe) before and after reduction in comparison with those of the Fe(III) and Fe(II) porphyrin reference complexes indicate the presence of both high-spin and low-spin iron(II) centers (and therefore a mixture of hexa- and pentacoordination) in Fe-PCN-222-pip. Specifically, the oxidation- and spinstate-sensitive IR-active mode (Figure S5), commonly labeled band II, splits into two components at 792 and 798 cm−1. Similarly, the spin-state-sensitive mode (band I) appears as two poorly resolved shoulders at 1338 and 1348 cm−1 due to

framework are determined using Fe K-edge X-ray absorption (XAS) and emission spectroscopy (XES) and other supplemental methods such as UV−vis diffuse reflectance, resonance Raman, and IR spectroscopy. Together, these results produce an accurate picture of the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. The precursor MOF FeCl-PCN-222 was synthesized according to literature procedures.3 Fe(III)ClTPP, Fe(III)ClTCPP, and Fe(II)Py2TPP (TPP = tetraphenylporphyrin, TCPP = tetrakis(4-carboxypheny)porphyrin, Py = pyridine) were used as reference complexes for comparison with the MOF metalloporphyrin sites before and after reduction. A variety of reducing agents can be used for the preparation of Fe(II) porphyrin complexes in solution,10−14 but in a MOF environment, the number is limited due to the pore size restrictions and the effect on the framework stability. Mild reducing agents, such as piperidine,13 proved to be successful in reducing the iron sites without destroying the framework in the process. The reduced form of PCN-222(Fe) was therefore prepared by mixing preactivated FeCl-PCN-222 with piperidine and soaking for 24 h at 60 °C under inert atmosphere (see the Supporting Information for a detailed description of synthetic procedure). The retention of crystallinity in the product, FePCN-222-pip, was confirmed by powder X-ray diffraction (XRD) (Figure S1). Confirmation of the complete one-electron reduction of the Fe sites was provided by several spectroscopic methods. The porphyrin Soret and Q-bands in the UV−vis diffuse reflectance spectra (Figure S2) of the MOF materials compared to those of the analogous ferrous and ferric porphyrin reference complexes follow the well-established spectral trends for iron porphyrin systems in these two oxidation states.14−16 Specifically, the blue shift of the Soret band and distinctly altered Q-bands at around 513 and 687 nm indicate the conversion from Fe(III) to Fe(II) porphyrin sites in the MOF. Moreover, the oxidation state marker bands at 380 and 1361 cm−1 in the resonance Raman spectra of FeCl-PCN222 systems are each shifted to lower frequency by ∼3 cm−1 in comparison with the Fe-PCN-222-pip spectrum (Figure 2).

Figure 2. Resonance Raman spectra highlighting oxidation and spin state marker bands for the as-synthesized FeCl-PCN-222 (green) and reduced Fe-PCN-222-pip MOF (black).

While these bands also show sensitivity to the presence of the peripheral carboxylate functionality on the phenyl groups (Figure S4), particularly the low-frequency Fe−Npor breathing mode, the observed frequency trend upon piperidine treatment indicates the presence of Fe(II) upon reduction of the MOF samples.17−19 Further evidence for the successful reduction of 1110

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Figure 3. (A) XANES region with 10× expansion of the pre-edge region and (B) k-space and (C) R-space EXAFS spectra of FeClTPP (dark green), FePy2TPP (red), FeCl-PCN-222 (light green), and Fe-PCN-222-pip (black).

Table 1. Local Iron Environments and Experimental X-ray Spectroscopy Data for the Iron Porphyrin Model Complexes and MOFs XESb

XANES iron porphyrin system

ox. state

spin state

local symmetry

edge,a eV

pre-edge, eV

Kβ1,3, eV

Kβ′, eV

ΔE, eV

IKβ′ /IKβ1,3

FeClTPP FeCl-PCN-222 FePy2TPP Fe-PCN-222-pip

+3 +3 +2 +2

5/2 5/2 0 0, 2

C4v C4v D4h D4h/C4v

7122.1 7122.3 7120.6 7120.6

7113.3 7113.3 7112.1 7112.1, 7113.2

7058.3 7058.2 7056.9 7057.6

7044.3 7043.2 7043.6 7044.3

14.0 14.0 13.3 13.3

0.35 0.33 0.19 0.25

Edge energy determined from the first inflection point of the rising edge. bPeak energies and intensity ratios determined by pseudo-Voigt curve fits to experimental data.25 a

lower energy edge position at 7120.6 eV. These values are consistent with those previously reported in the literature for this complex.27,28 XANES analysis for Fe-PCN-222-pip presents a slightly more complicated picture. While the edge position closely matches that of the Fe(II) reference complex, signifying complete reduction of the iron sites, a low-intensity, broad pre-edge feature is observed with two partially resolved components at around 7112.1 and 7113.2 eV. The first component occurs at the same low energy as the pre-edge peak in the spectrum of the FePy2TPP complex, consistent with the presence of similar hexacoordinated Fe(II)porphyrin sites in the MOF. The higher-energy feature, however, which is absent from the FePy2TPP spectrum, indicates that other coordination environments are involved. The pre-edge intensity contribution from residual square-pyramidal Fe(III) porphyrin sites in the framework is an unlikely possibility because in that case, the XANES edge position would be expected at a higher energy, closer to those of the Fe(III) systems. Four-coordinate Fe(II) sites without any axial ligation can be excluded as well because the signature, intense “shakedown” feature at around 7115.4 eV for this square-planar geometry28 is not observed in the spectrum. A more likely contributor to the pre-edge region of the Fe-PCN-222-pip spectrum is the presence of fivecoordinate C4v Fe(II) porphyrin sites. XANES spectra obtained for heme proteins28 and basket-handle porphyrins29 containing iron(II) in square-pyramidal environments showed similarly broad pre-edge features with low intensity spanning the same energy range. Moreover, DFT calculations of XAS transitions carried out for simple Fe(II) complexes with square-pyramidal geometry produced features at around 7111.5 and 7113.3 eV,27 with the relative intensity depending on the strength of the axial ligand. As in the case of high-spin ferric complexes with C4v

overlap with the more intense symmetric (CO) stretching mode. In the resonance Raman spectrum of Fe-PCN-222-pip, the spin-state-sensitive band at around 1554 cm−1 is broadened, and two bands at 1527 and 1535 cm−1 are present. These measurements provide intriguing hints on the spin composition and therefore mixed ligation of the Fe(II) sites in the reduced MOF, but their indirect nature and vibrational mode sensitivity to other structural factors complicates the evaluation of the iron centers with different coordination environments and therefore spin states. In order to get a clearer picture of the iron coordination environments in these systems, we turned to Fe K-edge X-ray spectroscopy, which directly probes core electron transitions that are sensitive to the local metal electronic and structural environment. X-ray absorption near-edge structure (XANES) spectra obtained for the MOF and reference complexes at room temperature are shown in Figure 3a, and the relevant edge and pre-edge energy values are summarized in Table 1. The preedge region contains 1s to 3d transition features that are formally dipole-forbidden but gain intensity through metal 3d− ligand 4p orbital mixing or quadrupolar coupling. Consequently, these transitions exhibit strong sensitivity to both the electronic and geometric structure of the absorbing metal. The edge energy is also informative as it is sensitive to oxidation and spin state. The spectra of FeCl-PCN-222 and FeClTPP are nearly identical, indicating comparable square-pyramidal Fe(III) coordination environments, as expected. The prominent pre-edge feature at 7113.3 eV and the rising edge energy at around 7122.2 eV in each case correlate well with the literature data for high-spin five-coordinate iron(III) porphyrins.26 The centrosymmetric FePy2TPP complex displays a less intense quadrupole-allowed pre-edge feature shifted to 7112.1 eV and a 1111

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Figure 4. (a) (top) Experimental Kβ XES spectra of FeClTPP (dark green), FeCl-PCN-222 (light green), FePy2TPP (red), and Fe-PCN-222-pip (black) normalized to unit area and (bottom) HS(E) − LS(E) (blue) and MS(E) − LS(E) (cyan) difference spectra. (b) Calculated fits (dashed lines) of FeClTPP and FePy2TPP Kβ XES spectra. (c) Linear combination fit (dashed gray line) of Fe-PCN-222-pip using FePy2TPP (red) and high-spin Fe(II) C4v model (magenta) theoretical spectra. See the text for details.

geometry, the intensity of the first feature arises from the partially dipole-allowed transition due to 4p mixing. The preceding analysis of the observed pre-edge features in the Fe-PCN-222-pip spectrum indicates the likely presence of both five- and six-coordinated Fe(II) porphyrin environments. Analysis of the EXAFS region provides additional information on the local coordination number and distances. The EXAFS spectra of the Fe porphryin reference complexes and MOFs are shown in Figure 3. The strong resemblance between the FeClTPP and FeCl-PCN-222 EXAFS spectra reflects the similarity of the iron coordination environments. Fits to these spectra using the Fe(III)ClTPP crystal structure30 as a model (see Supporting Information) further confirm this observation. The pyridine axial ligand environment of FePy2TPP provides a measurably different spectrum as confirmed by the spectral fit using a model derived from its crystal structure.31 The Fe-PCN222-pip EXAFS spectrum reveals that while the first shell peak occurs at a similar R value, its amplitude is drastically lower than that of the FePy2TPP spectrum. This comparison indicates that the iron sites in Fe-PCN-222-pip involve a similar ligand environment (i.e., Fe−N ligation with distances indistinguishable by EXAFS) but a lower average coordination number. These observations are consistent with a mixed single and double piperidine ligation pattern of the Fe porphryin sites. EXAFS fitting has limited utility for accurately estimating the ratio because other variable parameters, in addition to coordination number, affect the EXAFS amplitude (see the Supporting Information for more details). Fortunately, a quantitative route for assessing this ratio is available through Fe Kβ XES. Kβ main line X-ray emission involves 3p → 1s transitions resulting in two main features, Kβ1,3 and Kβ′, that arise from the exchange interaction between the 3p core hole and the partially filled 3d orbitals in the final state.32−34 The energy splitting and intensity ratio of the two features are sensitive to the number of unpaired electrons and therefore the spin state of the absorbing metal. In general, a decrease in Kβ′ intensity and in energy splitting between the two features occurs with reduced spin at the metal. This trend, which has been welldocumented in numerous first-row transition-metal-containing systems with varied spin states,25,33,35−37 including iron

porphryin complexes,38 is also illustrated by the Kβ XES spectra of the high-spin S = 5/2 FeClTPP and low-spin S = 0 FePy2TPP reference complexes (Figure 4a, top). The Kβ peak splittings and intensity ratios for all systems are summarized in Table 1. Comparison with the MOF spectra reveals that the FeCl-PCN-222 spectrum has a nearly identical Kβ peak energy splitting and intensity ratio as that of the FeClTPP reference, confirming the matching high-spin Fe(III) environments. The FePy2TPP reference complex spectrum presents a smaller intensity ratio as well as smaller splitting, which is consistent with low-spin Fe(II) configurations.25 In contrast, the Fe-PCN-222-pip spectrum substantially deviates from that of either reference complex (Figure 4a, top). While the energy spitting is approximately the same as that of the lowspin Fe(II) reference, within the energy resolution of the experiment, the intensity ratio of the bands appears in between those of the pure high- and low-spin reference complexes, qualitatively suggesting the presence of a mixture of spin states. The spectral differences can be quantitatively analyzed to estimate the ratio of the two spin states and therefore the two different axial ligation scenarios in Fe-PCN-222-pip. Several quantitative methods have been established for evaluating relative spin state contributions to Kβ XES, namely, for spincrossover systems, magnetic materials, and mixed-spin state compounds.32,39−42 One approach utilizes the integrated area of the absolute values of difference spectra (IADs) as a means for extracting the relative spin state contribution.32,43 This line shape analysis method takes advantage of the finding that IAD values scale linearly with the difference in number of unpaired electrons (ΔS) associated with the spectra used to generate the difference spectra. The effective or average spin state of a mixed-spin system of unknown composition compared to a reference with known spin state (e.g., a low-spin reference system), ΔSML, can be derived using the following equation ΔSML =

IADML ΔSHL IADHL

(1)

where ΔSHL is the difference in spin state between two reference systems with known high- and low-spin states, IADHL = ∫ [HS(E) − LS(E)] dE and IADML = ∫ [MS(E) − LS(E)] dE. In these IAD integrals, HS(E), LS(E), and MS(E) represent the 1112

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The Journal of Physical Chemistry Letters spectra of a high-spin reference, low-spin reference, and mixedspin system, respectively, each normalized to unit area. One of the advantages of this method is its dependence on the effective number of unpaired d electrons, rather than the oxidation state of the metal.33,43 In our case, therefore, the spectra of FeClTPP (S = 5/2) and FePy2TPP (S = 0) serve as the HS(E) and LS(E) references, respectively, to generate IADHL, with ΔSHL = 5/2. IADML is then generated using the spectra of Fe-PCN-222-pip (MS(E)) and FePy2TPP (LS(E)). The difference spectra are presented in Figure 4a (bottom). Following the abovedescribed analysis, ΔSML is calculated to be 1.42. In terms of the relative Fe(II) spin state contributions to the Fe-PCN-222pip spectrum, (i.e., S = 2 and 0), this ΔSML value translates to a 71% high-spin Fe(II) and 29% low-spin Fe(II) composition. As a separate independent route for estimating the highspin/low-spin Fe(II) fractions in the MOF, linear combination fitting of the Kβ XES spectrum was also performed using theoretical spectra generated using crystal field multiplet theory.44,45 These calculations allow the inclusion of Coulomb and exchange integrals along with ligand field effects that cause further splitting of the free ion states,32 which are needed to reproduce the experimental spectra. Using crystal field parameters reported in the literature for similar iron porphyrin environments,23 rather good agreement with experiment was achieved for the two reference complexes (Figure 4b). The FePCN-222-pip Kβ spectrum was fit using a linear combination of the two theoretical spectra that model the proposed Fe(II) coordination environments in the reduced MOF (Figure 4c), in other words, the simulated spectrum of the FePy2TPP low-spin Fe(II) reference and a theoretical spectrum generated using a high-spin Fe(II) model with C4v symmetry. Crystal field parameters determined for deoxymyoglobin, which also has known high-spin Fe(II) square-pyramidal geometry, were used to generate the latter component.46 All spectra (both theoretical and experimental) were normalized to unit area for best comparison of relative intensities. Using this linear combination analysis, the best fit to the experiment yielded a high-spin fraction of 68% and a low-spin fraction of 32%. In terms of axial coordination, this result and that of the IAD analysis are both in concert with the space-filling model presented at the outset (i.e., 66% high-spin five-coordinate and 33% low-spin six-coordinate Fe(II) sites), indicating that approximately only one small pore-facing iron center out of three has a coordinated piperidine. In conclusion, new methods for producing and probing reduced iron porphyrin sites in PCN-222(Fe) are presented. Several complementary spectroscopy methods not only verified the oxidation state change but also revealed the axial binding and spin state pattern enforced by the structural constraints of the framework. This work demonstrates the utility of these methods, in particular, Kβ XES, for quantitatively assessing the local coordination and electronic structure of the metal sites at room temperature and lays the foundation for using this approach to study other metalloporphyrin-based MOFs with catalytically relevant guest species and by in situ means.

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and XANES data. EXAFS and XES fitting procedures and results (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Sector 20 facilities at the APS, and research at these facilities, are supported by the DOE - Basic Energy Sciences, the Canadian Light Source and its funding partners, the University of Washington, and the APS. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank Dr. Nebojsa Marinkovic for beamline support at SSRL. Use of SSRL Beam Line 2-2 is coordinated with the National Synchrotron Light Source II, Brookhaven National Laboratory, under DOE Contract No. DE-SC0012704. The beamline 2-2 equipment is supported by the U.S. Department of Energy Grant No. DE-SC0012335. We thank Prof. Frieder Jakle for use of his glovebox for sample preparation, Prof. Elena Galoppini for use of her FT-IR instrument, and Dr. Ilya Stavitski for help with prior XES experiments carried out at APS. J.V.L. would like to acknowledge support by the National Science Foundation under Grant No. DMR-1455127.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00302. Details on materials synthesis and characterization including PXRD, IR, Raman, UV−vis diffuse reflectance, 1113

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Letter

The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.6b00302 J. Phys. Chem. Lett. 2016, 7, 1109−1115

Letter

The Journal of Physical Chemistry Letters Spectroscopy of Myoglobin Complexes and Photolysis Products. J. Am. Chem. Soc. 1997, 119, 4921−4928.

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DOI: 10.1021/acs.jpclett.6b00302 J. Phys. Chem. Lett. 2016, 7, 1109−1115