¹⁹ NMR and Structural Evidence for Spin-State Modulation of Six-Coordinate Cobalt(II) in a Weak Field Porphyrin Ligand

The syntheses and characterization of [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]cobalt, Co(F₈TPP), and [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl) porphinato]cobalt, Co(F₂₈TPP), are reported. Co(F₂₈TPP)·2tol (tol = toluene) crystallizes in the monoclinic space group C2/c-C_h (No. 15) with a = 22.1616(5) Å, b = 12.0274(3) Å, c = 19.9159(2) Å, β = 110.645(1)°, V = 4967.6(2) ų, and Z = 4 {d_calcd = 1.818 g/cm³; μ_a(Mo Kα) = 0.50 mm^-1}, and Co(F₂₈TPP)·2THF crystallizes in the triclinic space group P1̄ (No. 2) with a = 11.0691(1) Å, b = 12.0451(1) Å, c = 12.9558(2) Å. α = 62.531(1)°, β = 69.544(1)°, γ = 76.181(1)°, V = 1429.71(3) ų and Z = 1 {d_calcd = 1.700 g/cm³; μ_a(Mo Kα) = 0.45 mm^-1}. A comparison of the X-ray crystal structure data from Co(F₂₈TPP)·2tol and Co(F₂₈TPP)·2THF indicates that the porphyrin core expands dramatically (0.08 Å) in the six-coordinate complex. Optical and ¹⁹F NMR spectroscopic studies of Co(F₂₈TPP) in the presence of added ligand demonstrate that spin-state modulation of the six-coordinate Co(II) center is facile. Partial population of the ⁴E_g state is accessed upon coordination of the cobalt center with THF σ-donor ligands, while six-coordinate complexes with 1-methylimidazole result in complete conversion to the high spin state, as evinced by 280 ppm downfield chemical shifts for the β-fluorine resonances in the ¹⁹F NMR spectrum. Co(F₂₈TPP) is the first example of a porphyrin which supports a high-spin cobalt ion.