Structural, spectroscopic, and reactivity comparison of xanthene- and dibenzofuran-bridged cofacial bisporphyrins

A comparison of the structure, spectroscopy, and oxygen atom-transfer reactivity of cofacial bisporphyrins anchored by xanthene (DPX) and dibenzofuran (DPD) pillars is presented. The synthesis and characterization of dicopper(II) and dinickel(II) complexes of DPD completes a homologous series of homobimetallic zinc(II), copper(II), and nickel(II) complexes for both cofacial platforms. X-ray crystallographic analysis of the parent free-base porphyrins H₄DPX (1) and H₄DPD (5) confirms the face-to-face arrangement of the two porphyrin macrocycles with a large available range of vertical pocket sizes: 1 (C₈₀H₉₂Cl₂N₈O), triclinic, space group P, a = 13.5167(12) Å, b = 21.7008(18) Å, c = 23.808(2) Å, α = 80.116(2)°, β = 76.832(2)°, γ = 80.4070(10)°, Z = 4; 5 (C₈₀H₈₃N₈O₂), monoclinic, space group C2/c, a = 22.666(2) Å, b = 13.6749(14) Å, c = 42.084(4) Å, β = 94.554(2)°, Z = 8. EPR spectroscopy of dicopper(II) derivatives Cu₂DPX (3) and Cu₂DPD (7) complements the crystallographic studies by probing intramolecular metal-metal arrangements in frozen solution. Exciton interactions between the porphyrin subunits in fluid solution are revealed by steady-state and time-resolved electronic absorption and emission spectroscopy. The resulting compilation of structural and spectroscopic data provides a benchmark for the use of these and related platforms for the activation of small-molecule substrates. A structure-function relation is developed for the photoinduced oxygen atom-transfer reactions of bisiron(III) μ-oxo derivatives of DPX and DPD. The efficiency of the photochemical process is markedly dependent (∼10⁴-fold) on the vertical flexibility of cofacial architecture provided by the spacer.