Synthesis and versatile supramolecular self-assembly of the 5,15-bis(4-hydroxyphenyl)-10,20-bis(4-carboxyphenyl)porphyrin scaffold

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Abstract
This study reports the synthesis of the 5,15-bis(4-hydroxyphenyl)-10,20-bis(4-carboxyphenyl)porphyrin (BBP) moiety, and its versatile self-assembly into multiporphyrin hydrogen-bonding ‘polymers’ in four different reaction and crystallization conditions. In two cases the polymers are sustained by characteristic intermolecular cyclic dimeric (COOH)2 hydrogen-bond synthons between the carboxyphenyl functions of neighboring species, which face one another along a common equatorial axis of the porphyrin building blocks. The hydroxyphenyl substituents, which are directed perpendicular to the porphyrin chains, form either O–H⋯O(dioxane) or O–H⋯N(pyridine) weak hydrogen bonds by donating their protons to the solvent guest species. In the third example, the multiporphyrin chains are tessellated by rather unusual pairs of OH⋯OC(OH) hydroxyl–carboxyl hydrogen bonds between cis-related arms of adjacent porphyrin species along opposite directions of the polymer. Here the carboxylic protons are donated to the carbonyl site of the ethyl benzoate guest, thus linking the latter to the side surfaces of the polymeric chain. The fourth structure, which involves chain polymers of deprotonated porphyrin building blocks, is sustained by strong symmetric [–COO⋯H⋯OOC–]− hydrogen bonds along the chains. These chains aggregate further into complex three-dimensional architecture by weaker hydrogen bonds through bridging water molecules. The observed diversity of the intermolecular binding patters suggests that the BBP scaffold is not well suited for a programmed construction of network solids.

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