Electronic effects in transition metal porphyrins. 2. The sensitivity of redox and ligand addition reactions in para-substituted tetraphenylporphyrin complexes of cobalt (II).

摘要: Electrochemical oxidation, by cyclic voltammetry, of a series Co"(p-X)TPP complexes yields first (Co"'- (p-X)TPP)+, followed the K cation radical (CO"'(~-X)TPP)~+ and then dication (Co"l(p-X)TPPI3+. in discrete re- versible one-electron transfer steps. Reduction same compounds (Co'(p-X)TPP)-, anion (Co1(p-X)TPPI2-, again In each case, Hammett relationship is observed be- tween reversible Eli2 values u constants for four substituents, X, on phenyl rings (Cob-X)TPP) fl*. Ring oxidations or reductions are considerably more sensitive to substituents than metal oxidation reduction. addi- tion, results an investigation five solvent systems indicate that reaction constant reduction less effects indicating axial ligation important Co(lI)/Co(III) system Co(II)/Co(I) system. Elp negative those Schiffs' base 912 model vitamin itself, but cobalt( 11) tetradehydrocorrins. The equilibrium addition one molecule Lewis (electron pair donor) (CO"(~-X)TPP)~ toluene solution also follow relationship, with p = 0.168 (pyridine) 0.145 (piperi- dine) at 25 OC. Equilibrium molecular oxygen Co"(p-X)TPP-py have been measured as function temperature. Again, was observed, temperature dependent. extrapolated value 20 OC -0.056 02 form dioxygen complex. comparison, (calculated rrom AGO,) yield (Co"I(p-X)TPP)+ -0.3 1 (Me2SO) -0.79 (benzonitrile). derived dissociation CoTPP.b.02 s CoTPP.9 + 02- estimated be range 10-ll.9to 10-16.7 M-I, degree mode transmission electron density from various points porphyrin ring through o T orbitals nitrogens central ion long interest who investigated physical properties chemical reactions metallo- porphyrin~.~-~ Because conjugated nature por- phyrin system, donating withdrawing sub- stituents periphery shown affect basicity nitrogen^.^,^ This, turn, often affects visible absorption spectra, redox potentials, free bases and/or their spective metalloporphyrin c~mplexes.~~~-~ One best means quantifying elec- tron-donating -withdrawing groups metalloporphyrins lationship.8 A log (KX/KH) up Here, characteristic donating/with- drawing substituent measures sen- sitivity under study substituents. defined ionization substituted benzoic acids, which .O equation la.8 This valid both rate con- stants, and, suitably modified, can applied electro- potentials.'