Novel: Meso-substituted trans -A₂B₂ porphyrins: Synthesis and structure of their metal-mediated supramolecular assemblies

Five novel trans-A₂B₂ porphyrins bearing 4-pyridyl groups at 5- and 15-meso-positions ('A' substituent) and different functional groups at 10- and 20-meso-positions ('B' substituent) have been synthesised [5,15-di(4′-pyridyl)-10,20-di(thienyl) porphyrin (DPyDThP, 1), 5,15-di(4′-pyridyl)-10,20-di(4′-nitrophenyl) porphyrin (DPyDNO₂P, 2), 5,15-di(4′-pyridyl)-10,20-di(phenanthryl) porphyrin (DPyDPhnP, 3), 5,15-di(4′-pyridyl)-10,20-di(pyrenyl) porphyrin (DPyDPyrP, 4) and 5,15-di(4′-pyridyl)-10,20-di(3′,5′-dicarboxy) zinc porphyrin (DPyTcZP, 5)]. The compounds are fully characterised by NMR spectroscopy and mass spectrometry. The absorbance and emission studies of the porphyrins have been conducted, revealing the effect of the substituents at the 10,20-positions on the optical properties of the compounds. The porphyrins (1-5) were further reacted with transition metals (Mn^II, Co^II and Zn^II) under solvothermal conditions that led to the generation of coordination polymers (CPs) 1a-5a [Co-DPyDThP CP (1a), Zn-DPyDThP CP (1b), Co-DPyDNO₂P CP (2a), Mn-DPyDNO₂P CP (2b), Co-DPyDPhnP CP (3a), Zn-DPyDPhnP CP (3b), Co-DPyDPyrP CP (4a), Mn-DPyDPyrP CP (4b), Zn-DPyDPyrP CP (4c), Zn-DPyTcZP CP (5a)]. The CPs were characterized by single crystal X-ray diffraction studies, manifesting the influence of the substituents on their structural patterns. It has been observed that CP 1a adopts a 1D double chain structure while 1b and 2a are isostructural with a 3D framework, containing large hexagonal channels. Under the same reaction conditions as for 2a, CP 2b assumes a 1D-chain structure unlike 2a. Both the CPs 3a and 3b exhibit 2D square-grid type architecture whereas CPs 4a, 4b and 4c represent 1D zigzag polymeric chain structures. CP 5a is rather different, due to the active coordination of both the substituents (4-pyridyl and 3,5-dicarboxyphenyl), and reveal an attractive 2D grid-like architecture containing two kinds of voids.