Xanthene-bridged cofacial bisporphyrins

The synthesis and characterization of cofacial bisporphyrins juxtaposed by xanthene-bridged pillars are presented. The one-pot preparation of the xanthene dialdehyde avoids the lengthy bridge synthesis accompanying other cofacial porphyrin systems, thus allowing for the facile preparation of homobimetallic zinc (10), copper (11), and nickel (12) complexes. The cofacial orientation of the two porphyrin macrocycles was confirmed by X-ray crystallography. Structural data are provided for bisporphyrins 10-12: 10 (C₇₉H₈₂N₈OZn₂), triclinic, space group P1̄, a = 11.2671(2) Å, b = 14.9809(2) Å, c = 20.4852(2) Å, α = 101.6680(10)°, β = 100.8890(10)°, γ = 101.8060(10)°, Z = 2; 11 (C₇₉H₈₂N₈OCu₂), triclinic, space group P1̄, a = 11.21410(10) Å, b = 14.9539(5) Å, c = 20.6915(7) Å, α = 101.810(2)°, β = 101.044(2)°, γ = 101.722(2)°, Z = 2; 12 (C₇₉H₈₂N₈ONi₂), monoclinic, space group C2/c, a = 24.1671(4) Å, b = 10.669 Å, c = 50.5080(9) Å, β = 99.553(2)°, Z = 8. Exciton interactions between the porphyrin tings are apparent in electronic spectra, consistent with the cofacial superstructure. The combination of structural and spectroscopic data provides a basis for the design of additional metal derivatives for the activation of dioxygen and other small molecules.