Slow Electron Transfer Rates for Fluorinated Cobalt Porphyrins: Electronic and Conformational Factors Modulating Metalloporphyrin ET Haoran Sun, Valeriy V. Smirnov, and Stephen G. DiMagno* Department of Chemistry, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588-0304 Received June 20, 2003 The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro- 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF 28 TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20- tetraphenyl)porphyrinato]cobalt, CoF 8 TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF 20 TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV-vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, 19 F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as supporting electrolyte, the ET rate constants for the Co 2+/ 3+ redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k el ) varied by a factor of 10 4 , and the ET self-exchange rate constants (k ex ) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k el values for all compounds. UV-vis and IR spectroelectrochemistry, bulk electrolysis, and 19 F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (λ i ) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF 8 TPP, CoF 20 TPP, and CoF 28 TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C-C, C-N, and N-M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k ET . The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature’s exploitation of this design strategy. Introduction Owing to their utility as functional models for naturally occurring hemes, 1-4 cobalt porphyrins have been subjected to a wide range of electrochemical and electron transfer (ET) studies. 5 It has long been noted that the metal-centered oxidation of cobalt (Co 2+/3+ ) in porphyrins exhibits sluggish heterogeneous and homogeneous ET rates in comparison to porphyrin ring oxidation, to ring or metal reduction, or to the analogous Fe 2+/3+ oxidation in iron porphyrins. 5 For example, self-exchange ET rate constants (k ex ) for 5,10,15,20- tetraarylmetalloporphyrins fall in the range 10 -2 to 10 4 M -1 s -1 for the Co 2+/3+ process, 6 and 10 7 to 10 8 M -1 s -1 for the Fe 2+/3+ process. 7-10 As studies of cobalt cytochrome c have demonstrated, the reduction in k ex for the oxidation of Co 2+ porphyrins is primarily due to large inner sphere reorganiza- tion energies (λ i ∼ 2.4 eV). 11 Because Co 2+ porphyrins are (1) Chien, J. C. W.; Gibson, H. L.; Dickinson, L. C. Biochemistry 1978, 17, 2579-84. (2) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. ReV. 1979, 79, 139-79. (3) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437- 444. (4) Collman, J. P.; Fu, L.; Herrmann, P. C.; Zhang, X. Science (Wash- ington, D.C.) 1997, 275, 949-951. (5) Fukuzumi, S. In The Porphyrin Handbook; Guilard, R., Ed.; Academic Press: San Diego, CA, 2000; Vol. 8, pp 115-151. (6) Chapman, R. D.; Fleischer, E. B. J. Am. Chem. Soc. 1982, 104, 1575- 82. (7) Dixon, D. W.; Woehler, S.; Hong, X.; Stolzenberg, A. M. Inorg. Chem. 1988, 27, 3682-5. (8) Shirazi, A.; Barbush, M.; Ghosh, S.; Dixon, D. W. Inorg. Chem. 1985, 24, 2495-502. (9) Dixon, D. W.; Barbush, M.; Shirazi, A. Inorg. Chem. 1985, 24, 1081- 7. (10) Dixon, D. W.; Barbush, M.; Shirazi, A. J. Am. Chem. Soc. 1984, 106, 4638-9. Inorg. Chem. 2003, 42, 6032-6040 6032 Inorganic Chemistry, Vol. 42, No. 19, 2003 10.1021/ic034705o CCC: $25.00 © 2003 American Chemical Society Published on Web 08/28/2003