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Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom
journal contribution
posted on 2019-03-15, 14:19 authored by Jeanet Conradie, Penelope J. Brothers, Abhik GhoshA careful density
functional theory reexamination of the geometric and electronic structures
of reduced main-group porphyrin complexes E(Por)L2 (E =
Si or Ge; L = pyridine or tetrahydrofuran), B2(Por), and
C2(Por) has confirmed these as pure isophlorin derivatives
with normal-valent coordinated central atoms. Only axially unligated
Ge(Por) and the dications [B2(Por)]2+ and [C2(Por)]2+ 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)X2 (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 B2(Por) and
its dication, which are planar, C2(Por) and its dication
are predicted to be strongly ruffled, reflecting the smaller size
of the central C2 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.