Synthesis, Characterization, and Laser Flash Photolysis Reactivity of a Carbonmonoxy Heme Complex

We present here the synthesis, characterization, and flash photolysis study of [(F₈TPP)Fe^II(CO)(THF)] (1) {F₈TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2−)}. Complex 1 crystallizes from THF/heptane solvent system as a tris-THF solvate, [(F₈TPP)Fe^II(CO)(THF)]·3THF (1·3THF), with ferrous ion in the porphyrin plane (C₆₁H₅₂F₈FeN₄O₅; a = 11.7908(2) Å, b = 20.4453(2) Å, c = 39.9423(3), α = 90°, β = 90°, γ = 90°; orthorhombic, P2₁2₁2₁, Z = 8; Fe−N₄(av) = 2.00 Å; N−Fe−N (all) = 90.0°). This complex (as 1·THF) has also been characterized by ¹H NMR {six-coordinate, low-spin heme; CD₃CN, RT, δ 8.82 (s, pyrrole-H, 8H), 7.89 (s, para-phenyl-H, 8H), 7.46 (s, meta-phenyl-H, 4H), 3.58 (s, THF, 8H), 1.73 (s, THF, 8H)}, ²H NMR (pyrrole-deuterated analogue) [(F₈TPP-d₈)Fe^II(CO)(THF)] {THF, RT, δ 8.78 ppm (s, pyrrole-D)}, ¹³C NMR (on ¹³CO-enriched adduct) {THF-d₈, RT, δ 206.5 ppm; CD₂Cl₂, RT, δ 206.1 ppm}, UV−vis {THF, RT, λ_max, 411 (Soret), 525 nm}, and IR {293 K, solution, ν_CO 1979 cm^-1 (THF), 1976 cm^-1 (acetone), 1982 cm^-1 (CH₃CN)} spectroscopies. In order to more fully understand the intricacies of solvent−ligand binding (as compared to CO rebinding to the photolyzed heme), we have also synthesized the bis-THF adduct [(F₈TPP)Fe^II(THF)₂]. Complex 2 also crystallizes from THF/heptane solvent system as a bis-THF solvate, [(F₈TPP)Fe^II(THF)₂]·2THF (2·2THF), with ferrous iron in the porphyrin plane (C₆₀H₅₂F₈FeN₄O₄; a = 21.3216(3) Å, b = 12.1191(2) Å, c = 21.0125(2) Å, α = 90°, β = 105.3658(5)°, γ = 90°; monoclinic, C2/c, Z = 4; Fe−N₄(av) = 2.07 Å; N−Fe−N (all) = 90.0°). Further characterization of 2 includes UV−vis {THF, λ_max, 421 (Soret), 542 nm} and ¹H NMR {six-coordinate, high spin heme; THF-d₈, RT, δ 56.7 (s, pyrrole-H, 8H), 8.38 (s, para-phenyl-H, 8H), 7.15 (s, meta-phenyl-H, 4H)} spectroscopies. Flash photolysis studies employing 1 were able to resolve the CO rebinding kinetics in both THF and cyclohexane solvents. In CO saturated THF {[CO] ∼ 5 mM} and at [1] ≅ 5 μM, the conversion of [(F₈TPP)Fe^II(THF)₂] (produced after photolytic displacement of CO) to [(F₈TPP)Fe^II(CO)(THF)] was monoexponential, with k_obs = 1.6 (±0.2) × 10⁴ s^-1. Reduction in [CO] by vigorous Ar purging gave k_obs ≅ 10³ s^-1 in cyclohexane. The study presented in this report lays the foundation for applying fast-time scale studies based on CO flash photolysis to the more complicated heterobimetallic heme/Cu systems.