(F8TPP)FeII/O2 reactivity studies {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2-)}:: Spectroscopic (UV-visible and NMR) and kinetic study of solvent-dependent (Fe/O2=1:1 or 2:1) reversible O2-reduction and ferryl formation

In this report, we describe in detail the O-2-binding chemistry of the metalloporphyrin (F8TPP)Fe-II (1). This complex was synthesized from aqueous dithionite reduction of (F8TPP)Fe-III-Cl (X-ray structure reported: C55H36CIF8FeN4O; a = 13.6517(2) Angstrom, b = 13.6475(2) Angstrom, c = 26.3896(4), alpha = 90 degrees, beta = 99.9776(4)degrees, gamma = 90 degrees; monoclinic, P2(1)/c, Z = 4). Complex I crystallizes from toluene/heptane solvent system as a bis(toluene) solvate, (F8TPP)Fe-II.(C7H8)(2), with ferrous ion in the porphyrin plane (C58H36F8FeN4; a = 20.9177(2) Angstrom, b = 11.7738(2) Angstrom, c 19.3875(2), alpha = 90 degrees, beta = 108.6999(6)degrees, gamma = 90 degrees; monoclinic, C2/c, Z = 4; Fe-N-4(av) = 2.002 Angstrom; N-Fe-N (all) = 90.0 degrees). Close metal-arene contacts are also observed at 3.11-3.15 Angstrom. Upon oxygenation of I at 193 K in coordinating solvents, UV-visible and H-2 and F-19 NMR spectroscopies revealed the presence of a reversibly formed dioxygen adduct, formulated as the heme-superoxo complex (S)(F8TPP)Fe-III-(O-2(-)) (2) (S = solvent) [(i) tetrahydrofuran (THF) solvent: UV-visible, 416 (Soret), 536 nm; H-2 NMR: delta (pyrrole) 8.9 ppm; (ii) EtCN solvent:, UV-visible, 414 (Soret), 536 nm; (iii) acetone solvent: UV-visible, 416 (Soret), 537 rim; H-2 NMR: delta (pyrrole) 8.9 ppm]. Dioxygen-uptake manometry (THF, 193 K) revealed an O-2:1 oxygenation stoichiometry of 1.02:1, consistent with the heme-superoxo formulation of 2. Stopped-flow UV-visible spectrophotometry studies of the (F8TPP)FeII (1)/O-2 reaction in EtCN and THF solvents were able to provide kinetic and thermodynamic insight into the reversible formation of 2 [(i) EtCN: DeltaH degrees = -40 +/- 5 kJ/mol; DeltaS degrees = -105 +/- 23 J(K mol); k(1) = (5.57 +/- 0.04) x 10(3) M-1 S-1 (183 K); DeltaH(double dagger) = 38.6 +/- 0.2 kJ/mol; DeltaS(double dagger) = 42 +/- 1 J/(K mol); (ii) THF: DeltaH degrees = -37.5 +/- 0.4 kJ/mol,. DeltaS degrees = -109 +/- 2 J/(K mol)). The (F8TPP)Fe-II (1)/O-2 reaction was also examined at reduced temperatures in noncoordinating solvents (toluene, CH2Cl2), where UV-visible and H-2 and F-19 NMR spectroscopies also revealed the presence of a reversibly formed adduct, formulated as the peroxo-bridged dinuclear complex [(F8TPP)Fe-III](2)-(O-2(2-)) (3) [CH2Cl2: UV-visible, 414 (Soret), 535 nm; H-2 NMR, delta (pyrrole) 17.5 ppm]. Dioxygen-uptake spectrophotometric titrations revealed a stoichiometry of 2 (F8TPP)Fe-II (1) per O-2 Upon full formation of 3. Addition of a nitrogenous base, 4-(dimethylamino)pyridine, to a cold solution of 3 in dichloromethane gave rapid formation of the iron(IV)-oxo ferryl species (DMAP)(F8TPP)Fe-IV=O, (4), based upon UV-visible [417 (Soret), 541 nm] and H-2 NMR (delta (pyrrole) = 3.5 ppm) spectroscopic characterization. These detailed investigations into the O-2-adducts and "ferryl" species formed from (F8TPP)Fell (1) may be potentially important for a full understanding of our ongoing heme-copper oxidase model studies, which employ I or similar "tethered" (i.e., covalently attached Cu-chelate) porphyrin analogues in heme/Cu heterobinuclear systems.