Facile Synthesis and Self‐Assembly of Zinc (2‐Diethoxyphosphorylethynyl)porphyrins

Abstract

The synthesis of zinc (2‐diethoxyphosphorylethynyl)porphyrins and their self‐organization in solid state and in solution are reported. Copper‐cocatalyzed Sonogashira reaction involving bromoporphyrins and diethyl ethynylphosphonate was used to prepare meso‐substituted derivatives Zn4 and Zn5 bearing one or two ethynylphosphonate substituents at the macrocycle. In contrast, copper‐free Sonogashira coupling of 2‐bromoporphyrin with diethyl ethynylphosphonate is more convenient to prepare porphyrin Zn6 bearing this substituent at the β‐pyrrolic position of the macrocycle. The structures of complexes Zn4a·C7H8, Zn4a·CHCl3, Zn4c and Zn5b·CH2Cl2 in crystals were elucidated by X‐ray diffraction analysis. The supramolecular self‐assembly through weak coordinative binding of the phosphoryl group of one porphyrin molecule to the zinc(II) ion located in the center of another macrocyclic cavity is observed in all crystals. However, ZnII‐porphyrinates Zn4a and Zn5b exist in the solid state as mutually coordinated dimers while complex Zn4c displays a 1D polymer structure. In all dimer complexes, neighboring porphyrin rings are spatially separated, with Zn–O and P=O bond lengths being similar, while the interplanar porphyrin distance is variable, depending on weak intermolecular interactions in the crystals. The self‐organization of (phosphorylethynyl)porphyrins is also observed in weakly polar chloroform and detectable by NMR and UV/Vis spectroscopy. The mutually coordinated dimer of electron‐deficient porphyrin Zn5b was observed in 5 × 10–3 m chloroform solution even at room temperature. The kinetic lability of this supramolecular complex was proven by NMR spectroscopy. The associates of more electron‐donating porphyrins Zn4a and Zn4c are less stable, and self‐assembled dimers become the predominant species in 5 × 10–3 m chloroform solution only below 225 K. The self‐assembly of porphyrin Zn4a and Zn5b in more diluted solutions (up to 10–6 m) was also investigated by UV/Vis spectroscopy.