Photolysis of cis- and trans-[FeIII(cyclam)(N3)2]+ complexes:: Spectroscopic characterization of a nitridoiron(V) species

Reaction of cis-[Fe-III(cyclam)Cl-2]C1 in acidic H2O/CH3OH or CH3CN/H2O mixtures with NaN3 at 50 degrees C produced upon addition of NaClO4 or NaPF6 the complex trans-[Fe-III(cyclam)(N-3)(2)]ClO4 (1a) or the hexafluorophosphate salt 1b, whereas at -18 degrees C the same reaction produced cis-[Fe-III(cyclam)(N-3)(2)](ClO4) (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 Mossbauer 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 CH3CN solution and were characterized by Mossbauer spectroscopy. Photolysis of 1a in CH3CN at -35 degrees C and 20 degrees C with a Hg immersion lamp generated within 15 min a yellow solution. EPR and Mossbauer spectra show that a single high-valent species with an S = 1/2 ground state is formed: [{trans-(cyclam)Fe(N-3)}(2)(mu-N)(2+). The Mossbauer spectrum at 80 K exhibits two quadrupole doublets (ratio 1:1), indicating the presence of low-spin Fe-IV (S = 1) with isomer shift, delta, at 0.11 mm s(-1) and quadrupole splitting, Delta E-Q, at 0.97 mm s(-1) and intermediate-spin Fe-IV (S = 3/2) With delta = 0.20 mm S-1 and Delta E-Q = 2.09 mm s(-1). The valencies in this mixed valent [Fe-III(mu-N)Fe-IV](4+) core (S-t = 1/2) are localized on the Mossbauer time scale. In addition 18% of a low-spin Fe-II complex (S = 0) was found with delta = 0.54 mm s(-1), Delta E-Q = 0.65 mm s(-1). Photolysis of 2 at -35 degrees C in CH3CN 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)}(mu-N){trans-(cyclam)Fe-IV(N3)}](2+) With an [Fe-III(mu-N)Fe-IV](2+) core (S-t = 3/2). The Mossbauer spectrum at 80 K clearly shows that the valencies are localized: two subspectra are detected (ratio 1:1) at (a) delta = 0.14 mm s(-1), Delta E-Q = 0.81 mm s(-1) (Fe-IV; S = 1) and (b) delta = 0.50 mm s(-1), Delta E-Q = 1.89 mm s(-1) (FeIII; S = 5/2). Strong intramolecular antiferromagnetic coupling between the two iron sites produces the observed St = 3/2 ground state. The proposed spin states and spin-coupling schemes for both photolysis products were proven by magnetic Mossbauer 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 CH3CN solutions of 1a at 4 and 77 K by EPR and Mossbauer spectroscopy. The species contains an Fe-V ion (d(3), S = 3/2) with an isomer shift delta = -0.04 mm s(-1) and a quadrupole splitting Delta 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 -reduction processes occur during the photolysis of 1a in frozen solution. The electronic structure of the FeV species was investigated with EPR, magnetic Mossbauer spectroscopy, and spin-Hamiltonian analyses.