Synthesis, characterization, and spectroscopic analysis of antiaromatic benzofused metalloporphyrins

A two-electron oxidation of the Cu ^II (9) and Zn ^II (12) complexes of tetraphenyltetrabenzoporphyrin (TPTBP) results in the formation of stable antiaromatic [(TPTBP)Cu ^II(H ₂O)] ²⁺·2 [SbF ₆] ^- (10) and [(TPTBP)Zn ^II(H ₂O) ₂] ²⁺·2 [SbF ₆] ^- (13) with 16π electrons on the inner ligand perimeter. X-ray structures of the parent TPTBP complexes, the dications, and singly oxidized species [(TPTBP)Cu ^II] ^·+[SbF ₆] ^- (11) reveal that the use of TPTBP rather than a porphyrin ligand reduces the degree of nonplanarity in the 16π-electron species relative to the parent 18π complex. Significant high-field shifts of the ¹H NMR signals of the outer ring protons and large positive values in calculations of nucleus-independent chemical shifts on the central cavity of the porphyrin ring provide unambiguous evidence for the antiaromatic character of the 16π Zn ^II species. A combination of magnetic circular dichroism spectroscopic studies and TD-DFT calculations on both the Zn ^II and Cu ^II species demonstrates that the main electronic bands of the dicationic species can be readily assigned by using Michl's 4N perimeter model. Femtosecond transient absorption studies clearly demonstrated that the number of π electrons on the inner ligand perimeter and the configuration of the central metal ion play a critical role in the excited-state relaxation dynamics. Redox potentials for conversion between the 16π, 17π, and 18π systems were measured by cyclic voltammetry in dichloromethane and benzonitrile, and UV/Vis spectra of each oxidation/reduction product were monitored by thin-layer spectroelectrochemistry.