Understanding disorder and linker deficiency in porphyrinic zirconium-based metal-organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221

Abstract

Porphyrin-based metal-organic frameworks (MOFs), exemplified by MOF-525, PCN-221, and PCN-224, are promising systems for catalysis, optoelectronics, and solar energy conversion. However, subtle differences between synthetic protocols for these three MOFs give rise to vast discrepancies in purported product outcomes and description of framework topologies. Here, based on a comprehensive synthetic and structural analysis spanning local and long-range length scales, we show that PCN-221 consists of Zr₆O₄(OH)₄ clusters in four distinct orientations within the unit cell, rather than Zr₈O₆ clusters as originally published, and linker vacancies at levels of around 50%, which may form in a locally correlated manner. We propose disordered PCN-224 (dPCN-224) as a unified model to understand PCN-221, MOF-525, and PCN-224 by varying the degree of orientational cluster disorder, linker conformation and vacancies, and cluster-linker binding. Our work thus introduces a new perspective on network topology and disorder in Zr-MOFs and pinpoints the structural variables that direct their functional properties.

Fig. 1. Published crystal structures of the cubic porphyrinic MOFs.

PCN-221, MOF-525, and PCN-224 built of tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers and Zr-cluster (teal) with unit cells highlighted, viewed along the c-axis, and their corresponding Zr₈O₆, straight Zr₆O₄(OH)₄, and tilted Zr₆O₄(OH)₄ cluster, respectively^,,.

Fig. 2. Powder X-ray diffraction (PXRD) patterns for samples and models.

Comparison of measured PXRD patterns (Cu Kα₁) of MOF_ZrCl₄, MOF_ZrOCl₂, MOF_Zr₆, and experimental PCN-224 and simulated PXRD patterns (lit.) of PCN-221, MOF-525, and PCN-224^,,. The gray bars highlight the superstructure reflections of PCN-224 at 3.2 and 5.5° 2θ.

Fig. 3. Local structures of MOF samples and models.

a Experimental pair distribution functions (PDFs) of MOF_ZrCl₄, MOF_ZrOCl₂, MOF_Zr₆, and experimental PCN-224; b experimental PDF of MOF_ZrCl₄ compared to the simulated PDFs (lit.) of the published MOF structures (solid line) and SBUs only (dashed line) of PCN-224, MOF-525, and PCN-221^,,. The star highlights the Zr–Zr distance of 2.69 Å in PCN-221.

Fig. 4. Cluster orientations from single-crystal X-ray diffraction (SCXRD).

a Zr₈ cluster (teal) determined from SCXRD data, with a flat disk-shaped ellipsoid of the Zr atom (left) and a truncated cube created by the overlap of four different orientations of Zr₆O₄(OH)₄ clusters (blue, green, red, and orange), each occupied by 25% (right); b Fourier map of the (111) plane through the truncated corners of the cube showing the electron density map of three Zr atoms (positions indicated in white).

Fig. 5. Structure models with corresponding pair distribution function (PDF) and Rietveld refinements.

a MOF structures shown with Zr (teal) and tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers with 100% (red), 50% (pink), and 0% (gray) occupancy: (1) MOF-525 with planar TCPP linkers and no vacancies, (2) MOF-525 structure with twisted phenyl rings in the TCPP linker and no linker vacancies, (3) PCN-221 (Zr₈ clusters) with equally distributed linker vacancies, (4) PCN-224 structure with ordered TCPP vacancies, (5) dPCN-224-1 built of Zr₆ clusters with four different orientations and equally distributed linker vacancies, and (6) dPCN-224-2 with relative cluster orientations and linker occupation refined separately for each individual linker and cluster site, allowing for different possible local environments (the linker colors of (6) are an artistic representation and not specific to occupancies of the refined model). b PDF fits compared to experimental dPCN-224 (blue, PDF of MOF_ZrOCl₂ measured using synchrotron radiation), showing poor agreement between the local environments in structures 1–3 and good agreement with the local structure of PCN-224 (structure 4). c Improvement of the Rietveld refinements of experimental dPCN-224 (pink, also sample MOF_ZrOCl₂) for models shown. The extra intensities generated by the 2 × 2 × 2 supercell model of dPCN-224 are highlighted (*). Rw is the residual value (see Supplementary Information “Pair distribution function analysis”).

Fig. 6. Solid-state nuclear magnetic resonance (ssNMR) spectroscopic analysis of dPCN-224 (MOF_ZrOCl₂) and PCN-224.

a ¹H high-resolution magic angle spinning (MAS) NMR spectra recorded at a field of 23.48 T (ν₀(¹H) = 1 GHz). Characteristic resonances for the aromatic and NH protons of the tetrakis(4-carboxyphenyl)porphyrin (TCPP) linker, the µ3-OH groups of the Zr₆O₄(OH)₄ cluster, hydroxy groups, and water molecules^, are highlighted by color-coded regions. b ¹³C cross polarization magic angle spinning (CPMAS) spectra with assignment of the aromatic carbons of the TCPP linkers and benzoic acid (Bz), and chemically inequivalent carboxylic acid groups, characteristic of chelating and bridging binding motifs for the carboxylic acid groups to the cluster. c Main coordination motifs for the Zr₆O₄(OH)₄ clusters, from left to right: hydroxy water pairs and bridging and chelating carboxylate groups, which were found in decreasing proportions along this sequence. d ⁹¹Zr quadrupolar Carr-Purcell-Meiboom-Gill variable offset cumulative spectroscopy (qCPMG VOCS)^– consisting of at least four different quadrupolar lineshapes (blue), typical for quadrupolar coupling constants between 20 and 25 MHz. Together with the non-axial symmetry of the quadrupolar coupling tensors, an environment with a low symmetry and complex coordination of the Zr atoms is revealed within the cluster. Individual lineshapes and spectral fits are illustrated in blue and red, respectively.