A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal–organic framework

Within a MOF, a side-on peroxomanganese(iv) porphyrin has been isolated and comprehensively examined.


Table of Contents
Experimental Section S3 Figure S1: Powder X-ray Diffraction of PCN-224Mn II S6 Figure S2: N 2 adsorption data for PCN-224Mn II S7 Figure S3: IR spectra for PCN-224Mn II before and after the addition of O 2 S8 Figure S4: Thermal ellipsoid plots for PCN-224Mn II and PCN-224MnO 2 S9 Table S1: Crystallographic table for PCN-224Mn II S10 Table S2: Crystallographic table for PCN-224MnO 2 S11 Figure S5: Variable temperature EPR spectra for PCN-224MnO 2 S12 Table S3: Selected spin-Hamiltonian parameters obtained from EPR simulations S13 Figure S6: EPR spectra demonstrating the reversibility of O 2 binding S14 Figure S7: Comparison of O 2 adsorption data for PCN-224 and PCN-224Mn II S15 Figure S8: O 2 adsorption and desorption data for PCN-224Mn II S16 Table S4: Langmuir fit parameters for PCN-224Mn II plus O 2 S17 Figure S9: Characteristic X-ray diffraction pattern of PCN-224Mn II S18 References S19 . Note that all subsequent manipulations of this compound were carried out in a solvent-free atmosphere glovebox. The experimental BET surface area of 2455 m 2 /g is close to the accessible surface area reported for other metalated variants of PCN-224, which confirms material porosity and successful removal of solvent from the pores (see Figure S1). 1,2, 3 , 4 Complete metalation was confirmed by ICP-AES (Zr:Mn mass ratio: expected 6.6; found 7.0) and UV/Visible spectroscopy (see Figure 1).

Synthesis of PCN-224MnO 2 (2).
For bulk measurements, this compound was generated by exposing freshly-activated 1 (100 mg, 0.0229 mmol) to ca. 1 atm of dry O 2 . Samples for singlecrystal X-ray diffraction were prepared by cooling activated crystals of 1 to −78 °C and then introducing 1 atm of O 2 for 10 min on a Schlenk line. After this time, the crystals were coated in Paratone-N oil with a backflow of O 2 , transported while cold, and rapidly mounted on the diffractometer.
Diffuse-Reflectance UV/Visible Spectroscopy. The sample was prepared as a 10-fold dilution of 1 in KBr and pulverized with a mortar and pestle to make a smooth, homogenous powder. The samples were then transferred to a Harrick Praying Mantis TM reaction chamber under a N 2 atmosphere. Solid-state UV/Visible spectra were obtained using an Agilent Cary 5000 spectrophotometer at room temperature. The data were treated with a background correction of KBr and the spectra are reported as Kubelka-Munk transform.
Solution UV/Visible Spectroscopy. A sample of 1 was pulverized with a mortar and pestle and transferred as a toluene suspension to a 3 mL (1 cm pathlength) quartz UV/VIS cuvette under a N 2 atmosphere. UV/Visible spectra were obtained using an Agilent Cary 5000 spectrophotometer S3 at room temperature. A baseline of toluene was taken prior to the measurement of the sample of interest.

Diffuse-Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).
The sample was prepared as a 50-fold dilution of 1 in KBr and pulverized with a mortar and pestle to make a smooth, homogenous powder. The sample was then transferred to a Harrick Praying Mantis TM low-temperature reaction chamber equipped with a gas inlet and outlet, allowing Ar and O 2 to flow through the sample. Data collections were performed on a Thermo Nicolet 6700 FT-IR spectrometer operating at a resolution of 4 cm -1 at 298 K. Compound 1 was purged at 298 K for 20 min with Ar and then exposed to an O 2 /N 2 gas mixture for 20 min. Three iterative Ar and O 2 /N 2 cycles were performed to demonstrate the reversibility of the oxygenation (see Figure S2). The collected data were treated with a background correction of the reaction chamber without a sample. Data were collected at the Northwestern University Clean Catalysis (CleanCat) Core Facility.
X-ray Structure Determination. Single crystals of 1 and 2 suitable for X-ray analysis were coated in Paratone-N oil and mounted on a Micro Mounts™ rod attached to a goniometer head. The crystallographic data were collected at 100 K on a Bruker APEX II diffractometer equipped with CuKα microsource. Raw data were integrated and corrected for Lorentz and polarization effects using Bruker APEX2 v. 2009.1.4 Absorption corrections were applied using SADABS. 5 Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. Structures were solved and refined with SHELXL 6 operated within the Olex2 interface with the aid of standard restraints. 7,8 Disorder in 1 and 2 was modeled by splitting the atomic coordinates, and residual electron density found in the difference Fourier map was removed using the solvent mask protocol included in Olex2. This residual electron density likely arises from partial occupation of a separate morphology of the closely related MOF-525. 9 Due to the disorder of the O 2 adduct in 2, the O1-O1 and O2-O2 bond distances were restrained to refine to expected O-O bond distances of other previously characterized transition metal peroxo complexes. 10,11,12 Specifically, this distance was fixed to a target value of 1.40 ± 0.02 Å using the soft restraint DFIX, and subsequent refinement gave an O-O distance of 1.39(2) Å. The Mn-O1 and Mn-O2 as well as the Mn-N3 distances were also constrained to equal each other to ensure physical correctness of the system. The values reported in the manuscript are the mean distances obtained from these bonds. Note that the atoms comprising the metalloporphyrin unit are elongated out of the plane of the porphyrin due to the intrinsic disorder of the system. See Figure S7 for a characteristic diffraction image of PCN-224 that highlights the diffuse scattering of the Bragg peaks due to the inherent disorder of the system and gives rise to the elongated thermal parameters for some of the atoms. See Figure S3 for thermal ellipsoid plots and Tables S1 and S2 for experimental crystallographic information.

Electron Paramagnetic Resonance (EPR)
Spectroscopy. An activated sample of PCN-224Mn II was loaded into a quartz EPR tube under a N 2 atmosphere and capped with a septum. The quartz EPR tube was then evacuated on a Schlenk line for 12 h at 150 °C, an EPR spectra was then collected on the evacuated sample revealing the formation of 1. The same sample was then dosed with ca. 1 atm of dry O 2 at ambient temperature with the EPR spectrum revealing the formation of 2. The same sample was then purged with Ar and evacuated with a corresponding S4 EPR spectrum revealing the restoration of the deoxygenated manganese(II) porphyrin center, 1. Continuous-wave EPR spectra were collected on 1 at 77 K and 2 at 298, 77, 20, and 4.2 K. An activated sample of 1 (30 mg, 0.000749 mmol) was loaded into a Vac Suprasil ® EPR tube equipped with a J Young valve under a N 2 atmosphere followed by the addition of toluene (1 mL) to yield PCN-224Mn II •(C 7 H 8 ). To the same sample, a solution of pyridine (5.2 mg, 0.066 mmol) dissolved in toluene (0.50 mL) was added to yield PCN-224Mn II •(C 5 H 5 N). Continuouswave EPR spectra were collected on these two samples at 77 K. Measurements were performed using a Bruker ESP 300 X-band EPR spectrometer. Temperatures were held constant using an Oxford Instruments ESR 900 continuous-flow helium cryostat. The spectrometer was equipped with a dual mode cavity, operating in perpendicular mode. Data were collected using the following instrumental parameters: radiation frequency = 9.3684 GHz; microwave power = 2 mW; modulation amplitude = 13 G; modulation frequency = 100 kHz. Spectral simulations were carried out using the program Easyspin 5.0.16. 13 Gas Adsorption Measurements. Crystalline samples of PCN-224 or 1 were transferred into a pre-weighed analysis tube, which was then sealed with a Micromeritics TranSeal TM . Activation and analysis was then performed on a Micromeritics ASAP 2020 instrument. The samples were activated at 150 °C until an outgas rate of less than 1 mTorr/minute was observed. After activation, the samples were weighed to determine the final mass of analyte. The sample was checked to ensure the outgas rate remained below 1 mTorr/minute. O 2 uptake was measured using volumetric methods and the free space of all samples was determined with UHP He prior to analysis. The analysis was performed on both PCN-224 and 1 to demonstrate that the manganese center is responsible for the strong, initial uptake of O 2 (see Figure S6). Temperature control was provided with a variety of cold baths: dry ice/acetonitrile for 226 K, ice bath for 273 K, and no bath was used for the 298 K data collection.

Differential Enthalpies of Adsorption Calculations.
The variable-temperature O 2 adsorption isotherms at 226, 273 and 298 K were independently fit with a dual-site Langmuir-Freundlich model (Eqn 1), where n is the amount adsorbed in mmol/g, P is the pressure in bar, n sat,i is the saturation capacity in mmol/g, v i is the Freundlich parameter, and b i is the Langmuir parameter in bar −v for two sites 1 and 2.
(1) Note that the Langmuir-Freundlich model was simply used to mathematically fit each adsorption isotherm in preparation for isosteric heat of adsorption calculations using the Clausius-Clapeyron equation, and we do not intend to attribute any physical meaning to the obtained parameters. The fitted parameters for each adsorption isotherm can be found in Table S4.
The Clausius-Clapeyron equation (Eqn 2) was used to calculate the differential enthalpies of adsorption, -h ads , using the dual-site Langmuir-Freundlich fits at each temperature. Here, P is the pressure, n is the amount of gas adsorbed, T is the temperature, R is the universal gas constant, and C is a constant.
(2) The isosteric heats of adsorption were obtained from the slope of plots of (ln P) n versus 1/T. An error in the isosteric heat for a given loading can be calculated from the standard error in the S5 slope of the best-fit line. Fundamentally, this error describes the quality of agreement between the fitted isotherms and the Clausius-Clapeyron relation.
Other Physical Measurements. Inductively coupled plasma atomic emission (ICP-AE) spectra were collected with a Thermo iCAP 7600 ICP-OES instrument.
Powder X-ray Diffraction (PXRD) Analysis. An activated powder sample of PCN-224Mn II under a N 2 atmosphere was loaded into two 0.7 mm boron-rich X-ray capillaries (Charles-Supper Company) and sealed with Kapton tape. PXRD data were collected at room temperature on a STOE-STADIMP powder diffractometer equipped with an asymmetric curved Germanium monochromator (Cu Ka1 radiation, l = 1.54056 Å). The samples were measured in a transmission geometry utilizing a rotating capillary holder. See Figure S6 for the diffractogram of PCN-224Mn II . S6 Figure S1 | Powder X-ray diffraction analysis of PCN-224 (green) and PCN-224Mn II (purple) demonstrating the retention of bulk-phase purity after metalation.   (2). Ellipsoids shown at 30% probability level. Vertices of the green octahedra are Zr atoms; purple, blue, red, and gray spheres represent Mn, N, O, and C atoms, respectively; H atoms are omitted for clarity. S10  where P = (Fo 2 +2Fc 2 )/3 S12 Figure S5 | EPR spectra 2 (black) at various temperatures with the corresponding spectra of 1 (purple) highlighting changes to the electronic structure of the Mn ion upon the addition of O 2 . Selected EPR simulation parameters for 1 and 2 are provided adjacent to the spectra. S13    Figure S9 | A characteristic image of the X-ray diffraction data for 1, illustrating the presence of diffuse scattering along the Bragg peaks due to the inherent disorder of the system. The crystal structures have been modeled to account for this disorder, which gives rise to elongated thermal parameters for some of the atoms.