3,3′-bis(dipyrrolylmethenes) as new chelating ligands: Synthesis and spectral properties

Hydrobromides of three new alkyl-substituted tetrapyrrole ligands with an open chain, in which the dipyrrolylmethene fragments are linked by a CH2-spacer at the 3,3′-pyrrole carbon atoms, were synthesized and studied by IR, 1H NMR, and electronic absorption spectroscopy. As compared to the 2,2′ isomers (alkyl derivatives of biladiene-a,c) and monomers (2,2′-, 2,3′-, and 3,3′-dipyrrolylmethenes, the effect of structural factors is manifested in a considerable (up to 19–31 nm) bathochromic shift of the strong band in the electronic spectrum, an increase in the N-H stretching vibration frequency in the IR spectra (by more than 30 cm−1), and a decrease in the stability of 3,3′-bis(dipyrrolylmethene) salts. The solvent effect is manifested in small changes in the quantitative characteristics of the electronic absorption spectra of 3,3′-tetrapyrrole hydrobromides in C6H6, CCl4, CH2Cl2, CHCl3, and alcohols. In DMF, DMSO, and C5H5N, the salts undergo solvolytic dissociation to the free ligands and HBr, which accelerates in dilute solutions (<10−4 M) and with an increase in the electron-donor power of the solvent. The auxochromic effects of protons in the electronic absorption spectra of the salts, compared to ligands, were estimated quantitatively.