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

Abstract

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...

On the feasibility of N 2 fixation via a single-site Fe I /Fe IV cycle: Spectroscopic studies of Fe I (N 2 )Fe I , Fe IV N, and related species

“…The Tris(amido)amine Fe IV OCN complex of Schrock has a significantly negative isomer shift and large quadrupole splitting. Wieghardt and coworkers (42,43) have characterized two S ϭ 3͞2 Fe(V)Onitrido complexes at very low temperatures with ␦ ϭ Ϫ0.04 mm͞s, ⌬E Q values Ϫ1.90 to Ϫ1.0 mm͞s, and -N bridged complexes having an S ϭ 1 Fe(IV) center ␦ ϭ ϩ0.04 to ϩ0.14 mm͞s and ⌬E Q values between 0.79 and 1.13 mm͞s. Interestingly, the large quadrupole-splitting parameter observed for 3a and 3b is larger than those of any other Fe IV species and, to our knowledge, the largest of any known diamagnetic Fe complex.…”

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

“…In 1999, Meyer et al 81 81 . On the basis of EPR and applied-field Mössbauer spectroscopy (Table 1), the total spin state (S t ) in these mixed-valent complexes, S t = 1/2 for trans/trans and S t = 3/2 for trans/cis-isomer, was explained based on the assumption of strong antiferromagnetic coupling of an intermediate-spin Fe III (S = 3/2 in trans/trans), or a high-spin Fe III (S = 5/2 in trans/cis) with a low-spin Fe IV (S = 1) metal centre and localized valencies in the [Fe IV = N-Fe III ] 4 + core 93 .…”

“…a number of transition metal nitrido complexes have been synthesized via deoxygenation of no, reductive decarbonylation of isocyanate, N-N bond cleavage in n 2 o reduction, inter-metal n-atom transfer from nitrido complexes, and metathesis of nitrile from metal alkylidyne and a m-m multiple bonded complex. in contrast, fewer synthetic routes are known for iron-nitrido complexes: the photo-and thermolysis of azide precursors 80,81 , n-atom transfer via strain release by anthracene elimination from 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene 85 , and, only recently, with elemental dinitrogen (synthesis not depicted) 100 .…”

The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy

“…13,22,23 Irradiation of a precursor complex containing an axial azide ligand leads to heterolytic N−N bond cleavage and elimination of dinitrogen thereby formally photooxidizing the metal center: (1)…”