Synthesis, structure and spectroscopic properties of two models for the active site of the oxygenated state of cytochrome P540

Two dioxygen adducts of thiolato‐iron(II) porphyrins, [K(⊂ 222)][Fe(TP_pivP)(SC₆HF₄)(O₂)] 1a and [Na(⊂ 18c.6)][Fe(TP_pivP)(SC₆HF₄)(O₂)] 2 were synthesized by reaction of O₂ with five‐coordinate, high‐spin, cryptated alkali metal thiolato‐iron(II) ‘picket fence’ porphyrinate. They were characterized by visible and infrared spectroscopy: λ_max (log ɛ) = 360 nm (4), 427 nm (4.69), 560 nm (3.69), 610 nm (3.40) for both compounds; v(¹⁶O‐¹⁶O) = 1139 cm⁻¹ in chlorobenzene and fluorobenzene for 1a and 2. Single crystals of composition [K(⊂ 222)][Fe(TP_pivP)(SC₆HF₄)(O₂)] · [K(⊂ 222)](SC₆HF₄)(C₆H₅Cl)(H₂O) 1b were obtained by diffusion of pentane/xylene mixtures into chlorobenzene solutions of 1a at – 5°C. Single crystals of composition [Na(⊂ 18c.6)][Fe(TP_pivP)(SC₆HF₄)(O₂)] were obtained by slow diffusion of pentane into benzene solutions of 2. Structures of 1b and 2 were studied at 20°C (1b) and – 100°C (1b and 2). 1b: space group P2₁/c (monoclinic), a= 16.806(5) Å (1.6806 nm), b= 14.331(4) Å (1.4331 nm), c= 52.000(15) Å (5.2000 nm), β= 92.95(2)°, V= 12507 ų (12.507 nm³), Z= 4, D_cal= 1.28 g · cm⁻³ (t= 20°C). The final R₁ factor was 0.085 for 5238 reflections having I > 3 σ(I). 2: space group P2₁/c (monoclinic), a= 13.107(3) Å (1.3107 nm), b= 27.055(4) Å (2.7055 nm), c= 25.029(4) Å (2.5029 nm), β= 96.84(2)°, V= 8812 ų (8.812 nm³), Z= 4, D_cal= 1.18 g · cm⁻³ (t=−100°C). The final R₁ factor was 0.088 for 6587 reflections having I > 3 σ(I). The iron atom is, in both compounds, bonded to the four porphyrinato nitrogens (Np), the sulfur atom of the axial thiolate and one oxygen atom of the axially end‐on bonded dioxygen molecule. The average Fe‐Np distance found in 1b [1.994(4) Å, 0.1994 nm] is not significantly different from that found in 2 [1.993(3) Å, 0.1993 nm]. The Fe‐S bond length is 2.367(3) Å (0.2367 nm) in 1b and 2.365(2) Å (0.2365 nm) in 2. The Fe‐O1 distances with the oxygen atom of O₂ bonded to iron are respectively 1.837(9) Å (0.1837 nm) and 1.850(4) Å (0.1850 nm). The end‐on bonded O₂ molecule is disordered in both complexes 1b and 2. This disorder is, however, slightly different from one structure to the other. Whereas the terminal oxygen atom of O₂ occupies two positions in 1b [τ(O2A) = 78%, τ(O2B) = 22% at 20°C], it occupies three sites in 2 [τ(O2A) = 51%, τ(O2B) =τ(O2C) = 24.5% at – 100°C] (τ= occupancy). The estimate of the magnitude of the non‐bonded interactions between O₂ and its environment located in a sphere of 7‐Å (0.7‐nm) radius in 1b and 2 is consistent with a static disorder of O₂ in 1b and a dynamic distribution in 2. Mössbauer measurements were performed with polycrystals of 1b and 2 in the temperature ranges 5 – 197 K and 4.2 – 295 K, respectively, and under externally applied magnetic field (6.76 T) at 4.2 K (1b and 2) and at 173 K (2). The temperature variation of the Lamb‐Mössbauer factor indicates that the iron atom in these oxycytochrome P450 models is not very firmly bonded, more like that in pentacoordinated than in other hexacoordinated hemes. From the specific temperature behavior of quadrupole splitting, asymmetry parameter and line width obtained for 2, the structural disorder of the terminal oxygen atom of O₂ is identified as dynamic distribution over three different sites. Corresponding values obtained for 1b are consistent with a static disorder of O₂. Magnetic spectra reveal that the oxycytochrome P450 models are purely diamagnetic.