Molecular structures and energetics of corrole isomers: A comprehensive local density functional theoretical study

By using geometry optimizations with local density functional theory and double-zeta plus polarization basis sets, an extensive study has been carried out on the molecular structures and stabilities of free-base and metal-complexed corrole isomers. The optimized structures of normal metallocorroles have been found to agree well with crystallographic results. For both free-base and metal-complexed derivatives, the [1.1.1] ring system is found to be the most stable. The [2.0.1]- and [2.1.0]corrole isomers are unequivocally predicted to exist as stable materials. Of these, the [2.0.1] ring system, known as isocorrole, has been recently synthesized. Various derivatives of these two ring systems lie only about 10-20 kcal mol(-1) above analogous derivatives of normal corrole. In general, the cis- and trans-[3.0.0]corrole derivatives are predicted to be significantly less stable than the other metallocorroles. These differences in core corrole isomers. However, the Sc-III complexes of these two stereoisomeric ring systems are surprisingly stable. Direct C-alpha-C-alpha linkages between pyrrole rings are identified as a principal source of strain in the molecular structures of corrole isomers. The N-4 cores of [1.1.1]- and [2.0.1]corrole isomers are significantly smaller than the porphyrin core. Thus, these corrole isomers are predicted to have a strong preference for smaller metal ions such as Ga-III. The [2.1.0] core is somewhat larger, as evidenced by metal-nitrogen distances in [2.1.0] geometry account for an interesting reversal of the relative stabilities of [2.0.1]-] and [2.1.0]metallocorroles with increasing ionic radius of the complexed metal ion. Analogous to porphyrin isomer chemistry, the trans stereoisomer of [3.0.0]corrole is found to be more stable than the cis stereoisomer for the free-base and for the Sc-III and In-III derivatives. For the free bases of any particular isomer, the tautomers are quite similar in energy, differing by only 2-7 kcal mol(-1). This, together with the presence of short, strong N-H ... N hydrogen bonds, suggests that N-H tautomerization in at least some free-base corrole isomers should be considerably faster than that in porphyrins. Overall, it has been possible in most cases to establish a good correlation between the energetics trends and structural differences among molecules.