2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units

Abstract

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the Cu^II complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H₂L-Cu^II, where H₂ refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D Mn^II- or Co^II-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Keywords: 2D MOF; 2D materials; MOF; Porphyrin MOF; Porphyrinoids; coordination chemistry; metal-organic framework; rod MOF.

Figure 1

H₂L-Cu^II was used to prepare two 2D porphyrinic rod MOFs, PROD-1 and PROD-2. Insets show the 1D chain SBUs of PROD-1 and PROD-2. H atoms and solvent molecules are omitted for clarity. Colour scheme: C white, N blue, O red, Cu orange, Mn pink, Co violet. Mn^II and Co^II coordination environments are represented by blue and red polyhedra, respectively.

Figure 2

View of the structure of PROD-1 in the crystal, showing (a) the MOF’s rod-shaped SBU which comprises an infinite 1D chain of octahedrally coordinated Mn^II ions bridged by the syn–syn coordinating carboxylate functionalities of (L-Cu^II)²⁻ linkers, (b) the linear metalloporphyrin ligand (L-Cu^II)²⁻ and (c,d) the packing arrangement of PROD-1, highlighting the MOF’s layered architecture. H atoms and solvent molecules are omitted for clarity. Colour scheme: C white, N blue, O red, Mn pink, Cu orange. Mn^II coordination environments are shown as blue polyhedra.

Figure 3

(a) The coordination environment of the Mn^II ion Mn(1) within the 1D chain SBU of PROD-1, which includes two MeOH solvent-derived O-donors O(2) and O(4), and (b) a wireframe representation highlighting π–π stacking interactions which stabilize PROD-1. H atoms have been omitted for clarity. Colour scheme: C white, N blue, O red, Cu orange, Mn pink. Blue polyhedra represent Mn^II coordination environments. π–π interactions are highlighted using dashed red lines.

Figure 4

View of the structure of PROD-2 in the crystal, highlighting (a) the framework’s 1D rod-shaped SBU comprising an infinite chain of alternating tetrahedrally and octahedrally coordinated Co^II ions, (b) the linker (L-Cu^II)²⁻ and (c) the packing arrangement of PROD-2. Hydrogen atoms and solvent molecules are omitted for the purpose of clarity. Colour scheme: C white, O red, N blue, Cu orange. Red polyhedra represent Co^II coordination environments.

Figure 5

(a) Coordination environments of the Co^II ions within PROD-2′s 1D SBU, featuring two DEA-derived O-donors O(5) and O(5′), and (b) selected π–π stacking interactions that stabilize PROD-2. H atoms are omitted for clarity. Colour scheme: C white, N blue, O red, Cu orange, Co violet. Red polyhedra represent Co^II coordination spheres and dashed red lines show π–π stacking.