Synthesis and characterization of dimeric mutually coordinated magnesium meso-2-pyridylporphyrins

The synthesis and characterization of a series of meso-2-pyridylporphyrins and their Mg2+ complexes are reported. Condensation of 4-alkylbenzyl-2,2'-dipyrromethanes (alkyl = Me, n-Pr, or n-Bu) with 2-pyridinecarboxaldehyde yielded a series of free-base meso-2-pyridylporphyrins. Insertion of Mg2+ into the free-base porphyrins yielded the respective magnesium complexes. These compounds were characterized using 1D (H-1 and C-13) and 2D (H-1-H-1 COSY) NMR methods, UV-visible absorption spectroscopy, fluorescence spectroscopy, and mass spectrometry. The interplanar spacing of the dimers is sufficiently small that there is excitonic coupling of the constituent chromophores. The overall dissociation constant of these dimers is estimated at 2 x 10(-6) M. Addition of donor ligands such as acetone, DMF, DMSO, or pyridine converts the dimeric species to their respective constituent monomers. Titration of the dimeric complex with pyridine-d(5) shows that disaggregation requires coordination of two pyridine molecules at independent binding sites. Tracking of the pyridine coordination by H-1 NMR spectroscopy allowed for determination of the equilibrium constant for the pyridine-induced disaggregation reaction (2.1 x 10(-3) M-2). Both the spontaneous dissociation and the pyridine-induced disaggregation reactions occur by two steps.