BICYCLO[2.2.2]OCTANE-CAPPED PORPHYRINS .1. SYNTHESIS, CHARACTERIZATION, AND SOLUTION 3-DIMENSIONAL STRUCTURE

Three novel capped porphyrins {bicyclo[2.2.2]octane-capped porphyrins, VIAH2 and VIBH2, and bicyclo[2.2.2]oct-7-ene-capped porphyrin, VICH2} have been successfully synthesized by the Lewis acid (BF3.Et2O) catalyzed condensation of pyrrole with tetraacetals 6, 12, and 15, respectively. H-1 and C-13 1D and H-1-H-1 2D NMR (DQF-COSY and ROESY) of porphyrins VIAH2 and VIBH2 and H-1 1D and 2D NMR of VICH2 established that the bridges of the bicyclo[2.2.2]octane-caps are pointing outside the cavity formed between the cap and the porphyrin plane. The three-dimensional structure of VIAH2 in solution has been determined using ROESY and Distance Geometry calculations, and the size of the cavity formed between porphyrin and cap has been shown to be only large enough to accommodate a single H2O. The corresponding iron(III) complexes of VIAH2 {(VIA)Fe(III)(X) where X = Cl- or SbF6-} have been prepared and characterized. Equilibrium binding experiments of N-methylimidazole to (VIA)Fe(III)(SbF6) show that only one nitrogen base can be ligated.