Acetylacetone as an axial ligand in metalloporphyrin: its first crystal structure, coordination chemistry, and potential application as an efficient photosensitizer

Abstract

Acetylacetone, a diketone, is a very common bidentate ligand and has been used in chemistry laboratory classes for teaching the synthesis of different types of metal coordination compounds for a long time. It is a versatile organic compound and a large number of coordination and heterocyclic compounds based on it are known. This work demonstrates the first single crystal structure of an acetylacetone-coordinated metalloporphyrin, [MgT(4-Cl)PP(Hacac)], 1 [T(4-Cl)PP = 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin, Hacac = acetylacetone]. Compound 1 was characterized by UV-visible, infrared, NMR, and mass spectroscopy as well as single-crystal and powder XRD. Compound 1 is penta-coordinated with axial ligation of an acetylacetone molecule via the keto oxygen atom of a more stable enolic form. The redox properties of 1 were studied by cyclic voltammetry. Its photophysical properties were investigated by the fluorescence emission spectral analysis. The synthesized porphyrin compounds exhibited good photostability and singlet oxygen generation ability. Compound 1 was used as a photosensitizer for the degradation of two common water contaminants, methylene blue (MB) and crystal violet (CV). Studies were conducted in water under heterogeneous conditions using sunlight, and 1 was found to be efficient in the photodegradation of MB and CV dyes, following pseudo-first-order kinetics with 100% degradation of MB and CV dyes after 20 and 15 minutes, respectively. Quenching experiments in the presence of scavengers confirmed the mechanism of degradation via singlet oxygen. FTIR spectral analyses verified the degradation of dyes and the multiple reaction intermediates due to the breaking of the bonds during the photocatalytic degradation process, which was confirmed by mass spectral analyses. Moreover, from the photostability experiment, it was found that compound 1 could be used up to 6 cycles without appreciable activity loss. Theoretical calculations like optimization of geometry, the energy of frontier molecular orbitals, simulation of electronic spectra, and molecular electrostatic potential (MEP) analysis were performed to support the experimental results.