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

Abstract

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, [(F8)FeII], [(PPy)FeII] and [(PIm)FeII], where F8, PPy and PIm are iron(II)-porphyrinates and where PPy and PIm possess a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of these complexes vary with solvent. The low temperature reaction between O2 and these reduced porphyrin FeII complex 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.

Transition Metal Complexes and the Activation of Dioxygen

“…Manometry confirmed the formation of a 1:1 O 2 /Fe adduct in tetrahydrofuran; however, in non-coordinating solvents the reversible formation of a μ-peroxo dimer was observed, as supported by a 1:2 O 2 /Fe stoichiometry [21]. More recently, compounds 10-12 have also been synthesized in which the axial solvent molecule has been replaced by a pendant imidazole or pyridyl ligand [22,23]. Compounds 9, 10, and 12 have been assigned as low-spin Fe(III)-superoxide species, primarily on the basis of their 1 H NMR and UV-vis spectra, as well as resonance Raman (rR) data in the case of 9 [22].…”

“…More recently, compounds 10-12 have also been synthesized in which the axial solvent molecule has been replaced by a pendant imidazole or pyridyl ligand [22,23]. Compounds 9, 10, and 12 have been assigned as low-spin Fe(III)-superoxide species, primarily on the basis of their 1 H NMR and UV-vis spectra, as well as resonance Raman (rR) data in the case of 9 [22]. The assignment of 11 was inferred based on its conversion to an Fe(III)-peroxo intermediate upon one electron reduction [23].…”

Reactions of a heme-superoxo complex toward a cuprous chelate and •NO(g):CcOand NOD chemistry

“…Earlier work in our group [31] described the synthesis and characterization of (P Py )Fe II ( 1 ) [(P Py ) = pyridyl tailed porphyrinate (2-)] (λ max = 416, 525, 554 (sh) nm) which reacts reversibly with dioxygen to give a diamagnetic iron(III)-superoxo species (P Py )Fe III -(O 2 •- ) ( 2 ) (UV-vis, λ max = 419, 535 nm; EPR, silent). This is stable in solution below −30 °C in coordinating solvents such as tetrahydrofuran (THF), acetone, or acetonitrile as well as in non-coordinating solvents like dichloromethane (DCM).…”

“…(P Py )Fe II /( d 8 -P Py )Fe II ( 1 ) and [Cu I (AN)]BAr F were synthesized as previously described [31, 32]. …”

“…A ferric superoxo species is the most well studied dioxygen intermediate generated upon initial O 2 -reaction with the fully reduced activesite heme Fe(II)–Cu(I) center [28]. This species forms prior to O–O bond cleavage and as such has attracted considerable interest [29–31]. Here, we describe the reactivity of an iron(III)-superoxo species ( 2 ) towards (i) a cuprous-chelated complex [CuI(AN)][B(C6F5)4] and (ii) •NO, these reactions representing synthetic functional models for C c O and NOD, respectively.…”