ACTIVATION OF DIAZENE AND THE NITROGENASE PROBLEM - AN INVESTIGATION OF DIAZENE-BRIDGED FE(II) CENTERS WITH SULFUR LIGAND SPHERE .2. VIBRATIONAL PROPERTIES

Resonance Raman and IR spectroscopic measurements of the two diazene-b ridged systems [{Fe'NHS4'}(2-)(N2H2)] ('NHS4'(2-) = 2,2'-bis(2-mercapt ophenylthio)diethylamine(2-)) (I) and [{Fe'S-4'(PPr3)}(2)(N2H2)] ('S-4 '(2-) = 1,2-bis(2-mercaptophenylthio)ethane(2-)) (II) including H-2- a nd N-15-isotopomers are combined with a normal coordinate analysis (NC A) in order to describe the vibrational properties of diazene coordina ted to Fe(II)-sulfur centers. Eight of the 12 normal modes of the Fe-N 2H2-Fe unit have been identified by their isotope shifts. Most of the Raman-active diazene vibrations are resonance enhanced with respect to the 600 nm transition providing further support to the assignment of this band as Fe(d) to diazene(pi) charge-transfer (MLCT) transition. The force constant for the N-N bond of 7.7 mdyn/Angstrom corresponds t o a N-N bond order of about 1.5 and indicates that the diazene unit is moderately activated. The force constant of the Fe-N bond of 1.8 mdyn /Angstrom is about 2.5 times as large as for a simple sigma-bond. Thes e values confirm the sigma-donor pi-acceptor formulation of the Fe-dia zene bond presented in part 1 of this paper. The splitting of the diaz ene N-H and N-D vibrations observed in the Raman spectra of II is ascr ibed to a photoisomerization process taking place upon irradiation int o the MLCT band.