Subporphyrinoid systems: a theoretical study of the effects of the diheteroatom substitution in pyrrole subunits and of the nature of the bridging meso linkages.

A theoretical study of the geometry, the electronic structure, the electronic absorption spectra, and (1)H and (13)C NMR spectra of the [14]subporphine(1.1.1)-hydroxyboron(III) complex, free-base subporphyrin, and its dioxygen and dithio pyrrole substituted derivatives using CH, N, and P as bridging meso linkages was performed at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) theory level. The geometrical structure of these systems is mainly determined by the internal area delimited by the meso atoms and the alpha-carbon atoms of the pyrrolic rings, and by the number and nature of the atoms located on this area. All the hydroxyboron subporphyrins and dioxo and dithio subporphyrins with CH meso connectors display a conical shaped geometry. The presence of strong repulsions between the atoms on the central zone of the remaining systems provokes a correlative tilting of one of the three rings with loss of the conical shape with important consequences on spectroscopic properties. A particularly interesting case is the dioxosubporphyrin with P connectors in which the large area of the central zone determined by these connectors allows for an almost planar geometry that endows it with special features. The molecules presenting a tilted ring display weak absorption bands. Generally, the intensity of the bands moderately increases when the geometry is cone shaped. The dioxo heterosubporphyrins with CH (conical shape) and P (almost planar) connectors present strong absorption bands. (1)H and (13)C chemical shifts clearly reflect the effect of geometry distortion provoked by the repulsion among the atoms of the central area of the system indicating a deep perturbation of the pi system of the molecules.