Mononuclear (Nitrido)iron(V) and (Oxo)iron(IV) Complexes via Photolysis of [(cyclam-acetato)Fe^III(N₃)]⁺ and Ozonolysis of [(cyclam-acetato)Fe^III(O₃SCF₃)]⁺ in Water/Acetone Mixtures

Abstract

Reaction of the monoanionic, pentacoordinate ligand lithium 1,4,8,11-tetraazacyclotetradecane-1-acetate, Li(cyclam-acetate), with FeCl3 yields, upon addition of KPF6, [(cyclam-acetato)FeCl]PF6 (1) as a red microcrystalline solid. Addition of excess NaN3 prior to addition of KPF6 yields the azide derivative [(cyclam-acetato)FeN3]PF6 (2a) as orange microcrystals. The X-ray crystal structure of the azide derivative has been determined as the tetraphenylborate salt (2b). Reaction of 1 with silver triflate yields [(cyclam-acetato)Fe(O3SCF3)]PF6 (3), which partially dissociates triflate in nondried solvents to yield a mixture of triflate and aqua bound species. Each of the iron(III) derivatives is low-spin (d5, S = 1/2) as determined by variable-temperature magnetic susceptibility measurements, Mössbauer and EPR spectroscopy. The low-spin iron(II) (d6, S = 0) complexes 1red and 2ared have been prepared by electrochemical and chemical methods and have been characterized by Mössbauer spectroscopy. Photolysis of 2a at 419 nm in frozen acetonitrile yields a nearly colorless species in approximately 80% conversion with an isomer shift delta = -0.04 mm/s and a quadrupole splitting delta EQ = -1.67 mm/s. A spin-Hamiltonian analysis of the magnetic Mössbauer spectra is consistent with an FeV ion (d3, S = 3/2). The proposed [(cyclam-acetato)FeV=N]+ results from the photooxidation of 2a via heterolytic N-N cleavage of coordinated azide. Photolysis of 2a in acetonitrile solution at -35 degrees C (300 nm) or 20 degrees C (Hg immersion lamp) results primarily in photoreduction via homolytic Fe-Nazide cleavage yielding FeII (d,6 S = 0) with an isomer shift delta = 0.56 mm/s and quadrupole splitting delta EQ = 0.54 mm/s. A minor product containing high-valent iron is suggested by Mössbauer spectroscopy and is proposed to originate from [((cyclam-acetato)Fe)2(mu-N)]2+ with a mixed-valent (FeIV(mu-N)FeIII))4+S = 1/2 core. Exposure of 3 to a stream of oxygen/ozone at low temperatures (-80 degrees C) in acetone/water results in a single oxidized product with an isomer shift delta = 0.01 mm/s and quadrupole splitting delta EQ = 1.37 mm/s. A spin-Hamiltonian analysis of the magnetic Mössbauer yields parameters similar to those of compound II of horseradish peroxidase which are consistent with an FeIV=O monomeric complex (S = 1).

An Fe IV O complex of a tetradentate tripodal nonheme ligand

“…In contrast, [Fe IV 2 ( -O) 2 (BPMCN) 2 ](OTf) 4 can be obtained only at Ϫ80°C and decomposes readily on warming (17). The cyclam-acetate complex appears similarly unstable, but its behavior is not well characterized (20). The relative stabilities of the TPA, TMC, and BPMCN complexes are inversely correlated with their oxidative reactivity, that is, the less stable iron(IV) complexes are the stronger oxidants.…”

Oxidative dealkylation DNA repair mediated by the mononuclear non-heme iron AlkB proteins

“…Model compounds also provide strong evidence for the existence of Fe IV =O in proteins. Discrete mononuclear Fe IV =O species have been synthesized and characterized recently [54][55][56][57]. These complexes serve as models for the intermediate species that may be generated in mononuclear non-heme iron-containing enzymes.…”

The biology and chemistry of high-valent iron–oxo and iron–nitrido complexes

“…8), the original report of a non-haem oxoiron(IV) complex appeared almost two decades later in 2000 (ref. 40). The biggest impediment to progress in identifying and trapping a non-haem oxoiron(IV) species was the lack of a convenient spectroscopic signature that would readily signal its presence in a reaction mixture.…”

“…Grapperhaus et al 40 were the first to obtain a major breakthrough in this regard. They generated a terminal non-haem oxoiron(IV) species by the ozonolysis of [(cy-ac)Fe III (CF 3 23,24 .…”