(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
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Abstract
In this report, we describe in detail the O(2)-binding chemistry of the metalloporphyrin (F(8)TPP)Fe(II) (1). This complex was synthesized from aqueous dithionite reduction of (F(8)TPP)Fe(III)-Cl (X-ray structure reported: C(55)H(36)ClF(8)FeN(4)O; a = 13.6517(2) A, b = 13.6475(2) A, c = 26.3896(4), alpha = 90 degrees, beta = 89.9776(4) degrees, gamma = 90 degrees; monoclinic, P2(1)/c, Z = 4). Complex 1 crystallizes from toluene/heptane solvent system as a bis(toluene) solvate, (F(8)TPP)Fe(II).(C(7)H(8))(2), with ferrous ion in the porphyrin plane (C(58)H(36)F(8)FeN(4); a = 20.9177(2) A, b = 11.7738(2) A, 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 A; N-Fe-N (all) = 90.0 degrees ). Close metal-arene contacts are also observed at 3.11-3.15 A. Upon oxygenation of 1 at 193 K in coordinating solvents, UV-visible and (2)H and (19)F NMR spectroscopies revealed the presence of a reversibly formed dioxygen adduct, formulated as the heme-superoxo complex (S)(F(8)TPP)Fe(III)-(O(2)(-)) (2) (S = solvent) [(i) tetrahydrofuran (THF) solvent: UV-visible, 416 (Soret), 536 nm; (2)H NMR: delta(pyrrole) 8.9 ppm; (ii) EtCN solvent: UV-visible, 414 (Soret), 536 nm; (iii) acetone solvent: UV-visible, 416 (Soret), 537 nm; (2)H 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 (F(8)TPP)Fe(II) (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: Delta H degrees = -40 +/- 5 kJ/mol; Delta S degrees = -105 +/- 23 J/(K mol); k(1) = (5.57 +/- 0.04) x 10(3) M(-)(1) s(-)(1) (183 K); Delta H(++) = 38.6 +/- 0.2 kJ/mol; Delta S(++) = 42 +/- 1 J/(K mol); (ii) THF: Delta H* = -37.5 +/- 0.4 kJ/mol; Delta S* = -109 +/- 2 J/(K mol)]. The (F(8)TPP)Fe(II) (1)/O(2) reaction was also examined at reduced temperatures in noncoordinating solvents (toluene, CH(2)Cl(2)), where UV-visible and (2)H and (19)F NMR spectroscopies also revealed the presence of a reversibly formed adduct, formulated as the peroxo-bridged dinuclear complex [(F(8)TPP)Fe(III)](2)-(O(2)(2)(-)) (3) [CH(2)Cl(2): UV-visible, 414 (Soret), 535 nm; (2)H NMR, delta(pyrrole) 17.5 ppm]. Dioxygen-uptake spectrophotometric titrations revealed a stoichiometry of 2 (F(8)TPP)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)(F(8)TPP)Fe(IV)==O (4), based upon UV-visible [417 (Soret), 541 nm] and (2)H NMR (delta(pyrrole) = 3.5 ppm) spectroscopic characterization. These detailed investigations into the O(2)-adducts and "ferryl" species formed from (F(8)TPP)Fe(II) (1) may be potentially important for a full understanding of our ongoing heme-copper oxidase model studies, which employ 1 or similar "tethered" (i.e., covalently attached Cu-chelate) porphyrin analogues in heme/Cu heterobinuclear systems.
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