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

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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 1 red and 2a red 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 δ = −0.04 mm/s and a quadrupole splitting ΔEQ = −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)FeVN]+ results from the photooxidation of 2a via heterolytic N−N cleavage of coordinated azide. Photolysis of 2a in acetonitrile solution at −35 °C (300 nm) or 20 °C (Hg immersion lamp) results primarily in photoreduction via homolytic Fe−Nazide cleavage yielding FeII (d,6S = 0) with an isomer shift δ = 0.56 mm/s and quadrupole splitting ΔEQ = 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(μ-N)]2+ with a mixed-valent {FeIV(μ-N)FeIII}4+S = 1/2 core. Exposure of 3 to a stream of oxygen/ozone at low temperatures (−80 °C) in acetone/water results in a single oxidized product with an isomer shift δ = 0.01 mm/s and quadrupole splitting ΔEQ = 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 FeIVO monomeric complex (S = 1).