Electrochemistry of nonplanar zinc(II) tetrakis(pentafluorophenyl)porphyrins

The reduction potentials for the zinc derivatives of three nonplanar tetrakis(pentafluorophenyl)porphyrins, TFPPX{sub 8} (X = Cl, Br, Me), have been measured by cyclic voltammetry in 0.1 M (TBA)PF{sub 6}/CH{sub 2}Cl{sub 2} solution. E{sup 0{prime}} values (V vs AgCl/Ag): ZnTFPPCl{sub 8} (2+/+ = 1.57; +/0 = 1.63; 0/- = -0.47; -/2- = -0.75); ZnTFPPBr{sub 8} (2+/+ = 1.53; +/0 = 1.57; 0/- = -0.48; -/2- = 0.76); ZnTFPPMe{sub 8} (2+/+ = 0.99; +/0 = 0.97; 0/- = -1.14; -/2- = -1.52 (Ep)). The experimentally determined trends in the electrochemical behavior of these porphyrins are in agreement with the prediction that the D{sub 4h} {yields} D{sub 2} distortion of the macrocycle destabilizes the HOMOs (a{sub 1u}, a{sub 2u} {yields} a, b{sub 1}) more than the LUMOs (e{sub g} {yields} b{sub 2}, b{sub 3}). The instabilities of the {pi}-radical cations, ZnTFPPX{sub 8}{sup +}, relative to ZnTFPPX{sub 8} and ZnTFPPX{sub 8}{sup 2+} (K{sub disp} = 12 {+-} 5 (X = Cl); 5 {+-} 3 (X = Br); 0.4 {+-} 1 (X = Me)), are related to the effects of electron withdrawal at the meso positions as well as porphyrin distortion on the relative energies of the a(a{sub 1u}) and b{sub 1}(a{sub 2u}) orbitals.