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 (λmax = 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.
Bibliographical noteFunding Information:
We thank the National Science Foundation (Grant CHE-1402004 to J.L.S.) and the Robert A. Welch Foundation for support (F-0018). The work at Yonsei University was supported by the Strategic Research program (NRF-2016R1E1A1A01943379) administered through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, Information and Communication Technologies and Future Planning (to D.K.). We would like to thank Mr. Zhu-Lin Xie for help with the EPR measurements and Dr. Tianhan Kai for helpful discussions.
© 2018 American Chemical Society.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry