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).

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO4(2-). We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet d2(xy) ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

Reaction of cis-[Fe III (cyclam)Cl 2 ]Cl in acidic H 2 O/CH 3 OH or CH 3 CN/H 2 O mixtures with NaN 3 at 50 °C produced upon addition of NaClO 4 or NaPF 6 the complex trans-[Fe III (cyclam)(N 3 ) 2 ]ClO 4 (1a) or the hexafluorophosphate salt 1b, whereas at -18 °C the same reaction produced cis-[Fe III 4,8,11-tetraazacyclotetradecane). The crystal structures of 1b and 2 were determined by singlecrystal X-ray crystallography. Complexes 1a, b, contain a low-spin (S ) 1 / 2 ) and 2, a high-spin ferric ion (S ) 5 / 2 ) as was established by variable-temperature magnetic susceptibility measurements and Mo ¨ssbauer and X-band EPR spectroscopy. The low-spin trans-[(cyclam)Fe II (N 3 ) 2 ] and high-spin cis-[(cyclam)Fe II (N 3 ) 2 ] species were generated electrochemically in CH 3 CN solution and were characterized by Mo ¨ssbauer spectroscopy. Photolysis of 1a in CH 3 CN at -35 °C and 20 °C with a Hg immersion lamp generated within 15 min a yellow solution. EPR and Mo ¨ssbauer spectra show that a single high-valent species with an S ) 1 / 2 ground state is formed: [{trans-(cyclam)Fe(N 3 )} 2 (µ-N)] 2+ . The Mo ¨ssbauer spectrum at 80 K exhibits two quadrupole doublets (ratio 1:1), indicating the presence of low-spin Fe IV (S ) 1) with isomer shift, δ, at 0.11 mm s -1 and quadrupole splitting, ∆E Q , at 0.97 mm s -1 and intermediate-spin Fe III (S ) 3 / 2 ) with δ ) 0.20 mm s -1 and ∆E Q ) 2.09 mm s -1 . The valencies in this mixed valent [Fe III (µ-N)Fe IV ] 4+ core (S t ) 1 / 2 ) are localized on the Mo ¨ssbauer time scale. In addition 18% of a low-spin Fe II complex (S ) 0) was found with δ ) 0.54 mm s -1 , ∆E Q ) 0.65 mm s -1 . Photolysis of 2 at -35 °C in CH 3 CN produced a yellow mixed valent dinuclear species with an S ) 3 / 2 ground state. For this species we propose the structure [{cis-(cyclam)Fe III (N 3 )}(µ-N){trans-(cyclam)Fe IV -(N 3 )}] 2+ with an [Fe III (µ-N)Fe IV ] 2+ core (S t ) 3 / 2 ). The Mo ¨ssbauer spectrum at 80 K clearly shows that the valencies are localized: two subspectra are detected (ratio 1:1) at (a) δ ) 0.14 mm s -1 , ∆E Q ) 0.81 mm s -1 (Fe IV ; S ) 1) and (b) δ ) 0.50 mm s -1 , ∆E Q ) 1.89 mm s -1 (Fe III ; S ) 5 / 2 ). Strong intramolecular antiferromagnetic coupling between the two iron sites produces the observed S t ) 3 / 2 ground state. The proposed spin states and spin-coupling schemes for both photolysis products were proven by magnetic Mo ¨ssbauer and EPR spectra. The formation of the dinuclear species requires the primary formation of a nitridoiron(V) species: trans-[NdFe V (cyclam)(N 3 )] + . This intermediate has been detected during photolysis of frozen CH 3 CN solutions of 1a at 4 and 77 K by EPR and Mo ¨ssbauer spectroscopy. The species contains an Fe V ion (d 3 , S ) 3 / 2 ) with an isomer shift δ ) -0.04 mm s -1 and a quadrupole splitting ∆E Q ) -1.90 mm s -1 at 80 K. In addition, a five-coordinate ferrous species has been identified as a primary photolysis product which is formed via photolytic homolysis of the Fe III -N 3 bond. Thus, simultaneous photooxidation and -re...