Photolysis ofcis- andtrans-[FeIII(cyclam)(N₃)₂]⁺Complexes:  Spectroscopic Characterization of a Nitridoiron(V) Species

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 (cyclam)(N 3 ) 2 ](ClO 4 ) ( 2 ) (cyclam = 1,4,8,11-tetraazacyclotetradecane). The crystal structures of 1b and 2 were determined by single-crystal 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 Mössbauer 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 Mössbauer 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 Mössbauer 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 Mössbauer 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, Δ E Q, at 0.97 mm s - 1 and intermediate-spin Fe III ( S = 3 / 2 ) with δ = 0.20 mm s - 1 and Δ 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 Mössbauer time scale. In addition 18% of a low-spin Fe II complex ( S = 0) was found with δ = 0.54 mm s - 1, Δ 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 Mössbauer 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, Δ E Q = 0.81 mm s - 1 (Fe IV; S = 1) and (b) δ = 0.50 mm s - 1, Δ 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 Mössbauer and EPR spectra. The formation of the dinuclear species requires the primary formation of a nitridoiron(V) species: trans -[N Fe 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 Mössbauer 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 Δ 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 photo oxidation and -reduction processes occur during the photolysis of 1a in frozen solution. The electronic structure of the Fe V species was investigated with EPR, magnetic Mössbauer spectroscopy, and spin-Hamiltonian analyses.