Synthesis, characterization and computational insight into the reactivity of chromium isoporphyrins

Abstract

The synthesis, characterization and theoretical studies of three new chromium isoporphyrins are reported because a comprehensive study on chromium isoporphyrin is not known in the literature. On treatment with ammonium ceric nitrate as an oxidizing agent in a methanol medium chromium porphyrins easily form compounds which have strong absorbance at lower energy region (800 to 900 nm) which is the unique indication of isoporphyrin formation. Based on the standard spectroscopic (UV–Visible, Mass, PXRD and IR spectroscopy) analysis used for isoporphyrin characterization it is reasonable to conclude that the synthesized compounds are most probably methoxy chromium isoporphyrins, and accordingly named as chloro[5-(methoxy)-5,10,15,20-tetrakis-5,21H porphinato]chromium(III) nitrate,(1b), chloro[5-(methoxy)-5,10,15,20-tetrakis(4-methyl)- 5,21H-porphinato]chromium(III) nitrate,(2b), chloro[5-(methoxy)-5,10,15,20-tetrakis(4- methoxy)-5,21H-porphinato]chromium(III) nitrate, (3b). The parent compounds for the isoporphyrins are respectively [Cr^IIITPPCl] (1a), [Cr^III(4-MeTPP)Cl](2a) and [Cr^III(4- OMeTPP)Cl](3a). We have also performed theoretical studies at DFT levels, such as geometry optimization, energy calculation, simulation of UV–Vis spectra, NBO, and global reactivity indices to understand the distinctive properties of the chromium isoporphyrins for the first time. The theoretical UV-visible spectra and HOMO-LUMO energy gap supported experimental results. Moreover, Rietveld refinement showed that the experimental PXRD plots closely match the theoretical plots supporting that the experimental structures are similar to the theoretically optimized structures. Chromium isoporphyrins are found to have a lower HOMO-LUMO energy gap than their parent porphyrins, which accounts for their ease of reduction and the appearance of UV–Vis spectra at lower energy regions. Furthermore, we investigated the antioxidant properties of chromium methoxy isoporphyrins and compared them to their parent porphyrins and observed that complex 3b (77 %) has the highest antioxidant activity.