Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom

A careful density functional theory reexamination of the geometric and electronic structures of reduced main-group porphyrin complexes E­(Por)­L<sub>2</sub> (E = Si or Ge; L = pyridine or tetrahydrofuran), B<sub>2</sub>(Por), and C<sub>2</sub>(Por) has confirmed these as pure isophlorin derivatives with normal-valent coordinated central atoms. Only axially unligated Ge­(Por) and the dications [B<sub>2</sub>(Por)]<sup>2+</sup> and [C<sub>2</sub>(Por)]<sup>2+</sup> feature aromatic porphyrin ligands. The calculations faithfully reproduce the strong bond-length alternation along the outer rim of the macrocycle in the reduced complexes, consistent with antiaromatic character, as well as much stronger ruffling in the reduced group 14 complexes relative to their nonreduced counterparts such as E­(Por)­X<sub>2</sub> (E = Si or Ge; X = F or Cl). The latter is thought to reflect the lower barrier to nonplanar deformation for the antiaromatic systems. In addition, unlike B<sub>2</sub>(Por) and its dication, which are planar, C<sub>2</sub>(Por) and its dication are predicted to be strongly ruffled, reflecting the smaller size of the central C<sub>2</sub> unit. The calculations also predict characteristically low ionization potentials and singlet–triplet gaps for the antiaromatic complexes. A brief exploratory study of the as-yet-unknown group 15 complexes E­(TPP)­(Ph)­(py), where E = P and As, also indicated an antiaromatic isophlorin macrocycle coordinated to a pentavalent group 15 center.