Assembling Coordination Frameworks of Tetrakis[meso-(3,5-biscarboxyphenyl)]-Metalloporphyrins with Polynuclear Metallic Nodes: Mechanistic Insights into the Synthesis and Crystallization Process

Abstract

The article provides a deep insight into an improved synthetic methodology of crystalline framework solids with octa-topic tetrakis(3,5-dicarboxyphenyl)-zinc/cobalt-porphyrin linkers and a variety of metallic nodes, using NaOH as a modulator to enhance crystal growth. In the given conditions of the supramolecular reaction polynuclear metallic clusters were formed in situ, leading to the construction of metalloporphyrin frameworks (MPFs) of diverse architectures perforated by wide intralattice voids. Synthetic procedures with Pr3+ ions yielded two different frameworks, labeled as PrMPF-1 (1) and PrMPF-2 (2). The former is sustained by heterometallic {NaPr(COO)4}n pillared synthons, revealing tetragonal interporphyrin channel voids. In the latter the porphyrin units are intercoordinated by heterometallic tetranuclear {PrNa3(H2O)6(COO)9}3– clusters and the resulting structure exhibits hexagonal voids. Additional crystalline materials could be obtained with In3+ and Ga3+ cations as the inorganic component, yielding open 3D framework-structures InMPF (3) and GaMPF (4). These were found to be stabilized by the heterometallic In2Na(μ3-H2O)(COO)7 cluster-type and {NaGa(COO)4}n chain-type interaction synthons, respectively. When the transition metal ions Fe3+ or Mn3+ were used as the connecting reagents, homometallic trinuclear oxo-centered clusters (Fe/Mn)3(μ3-O) (COO)7 were formed in the corresponding polymeric structures FeMPF (5) and MnMPF (6). In 6, adjacent trinuclear clusters are further interconnected by a formate bridge to form covalently linked [Mn3(μ3-O)]2 dimers. Similar reactions in the presence of the NaOH additive, but with Co2+ or Zr4+, yielded two isomorphous 3D supramolecular architectures (7; NaMPF-1 and 8; NaMPF-2) with tetragonally shaped interporphyrin channel voids. Yet, in these structures the porphyrin-bridging {Na(COO)2}−n-chains were found to contain Na+ ions only. The solvent (DMF, water, and possibly dissociation products of hydrolyzed DMF) accessible voids in the metal–organic frameworks 1–8 account for >50% of the crystal volume. Mechanistic aspects of the NaOH-modulated synthetic approach are discussed, characterizing the products at each step in the synthetic course and proposing a plausible reaction mechanism.