Structural and spectroscopic investigation of 5-hydroxyflavone and its complex with aluminium

This study constitutes the third step in a program aimed to investigate the chelation of hydroxyflavones. After the 3-hydroxyflavone and 3 ' ,4 ' -dih ydroxyflavone, we now study the complexation of 5-hydroxyflavone (5HF) with AI(III). A stoichiometry of 1:1 was obtained from spectrophotometric metho ds for the complex in methanol. Important structural modifications of 5HF h ave been observed by density functional theory (DFr) calculation between fr ee and complexed molecules. Whereas, 5HF adopts a staggered conformation, t he complexed molecule is rather flat, involving participation of pyronium a nd cinnamoyl forms. Both gamma -pyrone and B-rings are affected by complexa tion, and changes in the inter-ring linkage are observed. UV-vis and Raman spectroscopies have permitted to confirm the structural models of free and complexed 5HF as well as the modifications produced by complexation. The ba thochromic shift of band I observed in the electronic spectrum of complex r elative to the 5HF one, has been attributed to pronounced changes in the pa ttern of frontier orbitals involved in this transition. (C) 2001 Elsevier S cience B.V. All rights reserved.