Chiral Interconversions of Pd and/or Au Bis-Metalated [32]Octaphyrins(1,0,1,0,1,0,1,0) Involving Hückel and Möbius Macrocyclic Topologies: A Theoretical Prediction

Abstract

Several Pd and/or Au bis-metalated [32]octaphyrins(1,0,1,0,1,0,1,0) were theoretically designed with rich conformations of Hückel or Möbius topology. The conformations and hence properties of macrocycles were tuned by twisting the active pyrrolic ring either clockwise (through multistep reactions with several Hückel and Möbius macrocyclic intermediates) or anticlockwise (via a direct Hückel-Hückel chiral interconversion). The encapsulated metal atoms, M(1), M(2) = Pd, Au, give different impacts on these two reaction processes. Facile occurrences of chiral interconversions between two enantiomers of bis-metalated octaphyrins were predicted with the largest activation barrier less than 40 kcal/mol. Some Au-coordinated octaphyrins (M(1) = Au) were demonstrated to be thermodynamically stable with large negative nucleus-independent chemical shift (NICS) values, which are comparable to those of the synthetic Pd-coordinated complexes. The free-base [32]octaphyrins(1,0,1,0,1,0,1,0) display the characteristic absorption spectra with distinct sharp Soret-like bands. After metalation, the Soret-like bands are red-shifted in different degrees along with the appearance of rather weak Q-like band. The heterometal-coordinated complexes (i.e., M(1) ≠ M(2)) show stronger and more splitting absorptions than the homometal-coordinated ones with M(1) = M(2). The hyperpolarizabilities sharply augment with the metalation in Hückel systems due to the destruction of the centrosymmetry and the increase in polarizability by coordinated metal atoms.

Aromaticity of Hückel and Möbius Topologies Involved in Conformation Conversion of Macrocyclic [32]Octaphyrin(1.0.1.0.1.0.1.0): Refined Evidence from Multiple Visual Criteria

“…They confirmed that the complex adopts a figure-eight structure with weak antiaromaticity by spectroscopic analyses and density functional theory (DFT) calculations. Our previous theoretical prediction about the aromaticity of this compound is basically in agreement with their assertion . Moreover, using free-based [32]­octaphyrin(1.0.1.0.1.0.1.0) with multiple conformers involving Hückel and Möbius topologies, we have demonstrated that the conclusions from some numerical indexes of aromaticity, such as the Julg parameter (Julg A), harmonic oscillator model for aromaticity (HOMA), magnetic susceptibility exaltations (Λ), and nucleus-independent chemical shift (NICS), are perfectly matched with each other.…”

Optical Properties of Novel Conjugated Nanohoops: Revealing the Effects of Topology and Size

“…The equilibrium constants ( K 298.15 ) of Hückel–Möbius isomerization for three systems with different sizes are calculated to be 0.18, 3.95, and 3.68, respectively. The large reaction rate and the comparable population of the isomers in Hückel-to-Möbius transformations indicate that the two conformers are easily converted to each other and can maintain dynamic balance under normal conditions, as observed in macrocyclic expanded porphyrins. ,− , …”

How do the Hückel and Baird Rules Fade away in Annulenes?

“…Some recent works have studied Hückel's rule from the perspective of electron delocalization [58,59]. Since the studies of Hückel on organic molecules, the concept of aromaticity has extended importantly including all sorts of new aromatic molecules such as metalloaromatic molecules [4,51,52,60,61], fullerens [62], nanotubes [63], porphyrins [64,65] with a Möbius-like structure [66][67][68][69][70][71][72][73], and all-metal clusters [4,5], among others.…”