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Relevant academic research and scientific papers

Synthesis, characterization, and laser flash photolysis reactivity of a carbonmonoxy heme complex

We present here the synthesis, characterization, and flash photolysis study of [(F8TPP)FeII(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 (C61H52F8FeN4O 5; a = 11.7908(2) A, b = 20.4453(2) A, c = 39.9423(3), α = 90°, β = 90°, γ = 90°; orthorhombic, orthorhombic, P212121, Z = 8; Fe-N 4(av) = 2.00 A; 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) [(F 8TPP-d8)FeII(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, λmax, 411 (Soret), 525 nm}, and IR {293 K, solution, vCO 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 (C60H52F8FeN 4O4; a = 21.3216(3) A, b = 12.1191(2) A, c = 21.0125(2) A, α = 90°, β = 105.3658(5)°, γ = 90°; monoclinic, C2/c, Z = 4; Fe-N4(av) = 2.07 A; 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 [(F8-TPP)FeII(CO)(THF)] was monoexponential, with kobs = 1.6 (±0.2) × 104 s-1. Reduction in [CO] by vigorous Ar purging gave kobs ? 10 3 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.

New heme-dioxygen and carbon monoxide adducts using pyridyl or imidazolyl tailed porphyrins

Inspired by the chemistry relevant to dioxygen storage, transport and activation by metalloproteins, in particular for heme/copper oxidases, and carbon monoxide binding to metal-containing active sites as a probe or surrogate for dioxygen binding, a series of heme derived dioxygen and CO complexes have been designed, synthesized, and characterized with respect to their physical properties and reactivity. The focus of this study is in the description and comparison of three types heme-superoxo and heme- CO adducts. The starting point is in the characterization of the reduced heme complexes, [(F 8)FeII], [(PPy)FeII] and [(P Im)FeII], where F8, PPy and P Im are iron(II)-porphyrinates and where PPy and P Im possess a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of these complexes vary with solvent. The low temperature reactions between O2 and these reduced porphyrin FeII complexes yield distinctive low spin heme-superoxo adducts. The dioxygen binding properties for all three complexes are shown to be reversible, via alternate argon or O2 bubbling. Carbon monoxide binds to the reduced heme-FeII precursors to form low spin heme-CO adducts. The implications for future investigations of these heme O2 and CO adducts are discussed.

Reactivity studies on FeIII-(O22-)-Cu II compounds: Influence of the ligand architecture and copper ligand denticity

Heme-Cu/O2 adducts are of interest in the elucidation of the fundamental metal-O2 chemistry occurring in heme-Cu enzymes which effect reductive O-O cleavage of dioxygen to water. In this report, the chemistry of four heme-peroxo-copper [FeIII-(O2 2-)-CuII]+ complexes (1-4), varying in their ligand architecture, copper-ligand denticity, or both and thus their structures and physical properties are compared in their reactivity toward CO, PPh 3, acids, cobaltocene, and phenols. In 1 and 2, the copper(II) ligand is N4-tetradentate, and the peroxo unit is bound side-on to iron(III) and end-on to the copper(II). In contrast, 3 and 4 contain a N 3-tridentate copper(II) ligand, and the peroxo unit is bound side-on to both metal ions. CO displaces the peroxo ligand from 2-4 to form reduced CO-FeII and CO-CuI species. PPh3 reacts with 3 and 4 displacing the peroxide ligand from copper, forming (porphyrinate)FeIII-superoxide plus CuI-PPh3 species. Complex 2 does not react with PPh3, and surprisingly, 1 reacts neither with PPh3 nor CO, exhibiting remarkable stability toward these reagents. The behavior of 1 and 2 compared to that of 3 and 4 correlates with the different denticity of the copper ligand (tetra vs tridentate). Complexes 1-4 react with HCl releasing H2O2, demonstrating the basic character of the peroxide ligand. Cobaltocene causes the two-electron reduction of 1-4 giving the corresponding μ-oxo [Fe III-(O2-)-CuII]+ complexes, in contrast to the findings for other heme-peroxo-copper species of different design. With t-butyl-substituted phenols, no reaction occurs with 1-4. The results described here emphasize how ligand design and variations influence and control not only the structure and physical properties but also the reactivity patterns for heme-Cu/O2 adducts. Implications for future investigations of protonated heme/Cu-peroxo complexes, low-spin analogues, and ultimately O-O cleavage chemistry are discussed.