Metal-Stabilized Quinoidal Dibenzo[ g, p]chrysene-Fused Bis-dicarbacorrole System

We report here a metal complexation-based strategy that permits access to a highly stable expanded porphyrin-type quinoidal polycyclic aromatic hydrocarbons (PAH). Specifically, double insertion of Pd(II) ions into a dibenzo[ g, p]chrysene-fused bis-dicarbacorrole (bis-H₃) gives rise to a bis-metalated species (bis-Pd) that undergoes a facile benzenoid-quinonoid transformation. In contrast to what is true for the corresponding mono-Pd(II) complex, which has organic radical character, well resolved ¹H NMR and ¹⁹F NMR spectra are seen for bis-Pd. This complex is also electron paramagnetic resonance (EPR) silent over a range of temperatures. On the basis of crystallographic analyses, Raman spectroscopic studies, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) calculations, we suggest that the dibenzo[ g, p]chrysene bridge in bis-Pd has quinoidal character and that the system as a whole is a closed shell species. As expected for a quinoidal system, bis-Pd is characterized by a lowest energy absorption band that is shifted into the NIR (λ_max = ca. 1420 nm (ε > 1.5 × 10⁵ M^-1 cm^-1) for bis-Pd vs 780 nm (ε < 5.0 × 10³ M^-1 cm^-1) for bis-H₃). On the other hand, bis-Pd displays solvent dependent ground state and transient absorption spectral features. Such findings provide support for a zwitterionic resonance contribution to what is a predominantly a quinonoid-type ground state. The use of specific metalation to fine-tune the electronic features of polytopic ligands, as reported here, opens the door to what might be a potentially generalizable approach to the design of quinoidal PAH structures with long wavelength solvatochromic absorption features.