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

Several Pd and/or Au bis-metalated [32]-octaphyrins(1,0,1,0,1,0,1,0) were theoretically designed with rich conformations of Huckel or Mobius topology. The conformations and hence properties of macrocycles were tuned by twisting the active pyrrolic ring either clockwise (through multistep reactions with several Huckel and Mobius macrocyclic intermediates) or anticlockwise (via a direct Huckel-Huckel 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 not equal 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 Huckel systems due to the destruction of the centrosymmetry and the increase in polarizability by coordinated metal atoms.