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

Hydrobromides of' thre-- new alkyl-substituted tetrapyrrole ligands with an open chain, in which the dipyrrolylmethenc 1ragments are linked by a CH-spacer at the 3,3'-pyrrole carbon atoms, were synthesized and studied by IR, 1H NMR, and el( ctronic absorption spectroscopy. As compared to the 2,2' isorners (alkyl derivatives of' biladiene-a,c) and inonorners (2,2'-, 2,3'-, and 3,Y-dipyrrolylinethenes, the ef't'ect of' structural t'actors is mandested in a considerable (up to 19-3 1 nm) bathochromic shi 11 ol'the strong band in the electronic spectrum, an increase in the N-H stretching vibration 1requency in the IR spectra (by more than 30 crn'). and a decrease in the stability of' 3,3'-bis(dipyrrolylinethene) salts. The solvent effect is manit'ested in small changes in the quantitative characteristics of the electronic absorption spectra o4' 3,3'-tetrapyrrole hydrobromidcs in C,H,, CC14, CHCl,. CHC13, anJ alcohols. In DMF, DMSO, and C5H5N, the salts undergo solvolytic dissociation to the 1ree ligands and HBr, which accelerates in dilute solutions (<10 -4 M) and with an increase in the electron-donor power of' the solvent. The auxochrornic effects of' protons in the electronic absorption spectra ol'the salts, compared to ligands, were estimated quantitatively.