Molecular Structures and Energetics of Corrole Isomers: A Comprehensive Local Density Functional Theoretical Study

Abstract

AbstractBy using geometry optimizations with local density functional theory and double‐π 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 corrole isomers. However, the ScIII complexes of these two stereoisomeric ring systems are surprisingly stable. Direct C−α‐Cα linkages between pyrrole rings are identified as a principal source of strain in the molecular structures of corrole isomers. The N4 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 GaIII. The [2.1.0] core is somewhat larger, as evidenced by longer metal‐nitrogen distances in [2.1.0]‐metallocorroles. These differences in core 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 ScIII and InIII 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.