Synthesis and relative stability of a series of compounds of type [Fe(II)(bztpen)X]+, where bztpen=pentadentate ligand, N5, and X-=monodentate anion.

The structural and solution characterization of novel Fe(II) compounds of the general formula [Fe(bztpen)X]PF6 and [Fe(bztpen)CH3CN](PF6)2 is presented, where bztpen is the pentadentate ligand N-benzyl-N,N',N'-tris(2-methylpyridyl)ethylenediamine and X- is a monodentate ligand. All complexes were characterized in solution and in the solid state, employing the usual techniques and single-crystal X-ray diffraction. The results obtained are discussed in terms of the existing information for some previously reported analogous compounds to arrive at a rationalization regarding the influence of a variation in the coordination environment of all compounds and to evaluate their relative stability. The observed magnetic response in the solid state is paramagnetic in the entire temperature range for the Cl-, Br-, I-, OCN-, and SCN- derivatives, while the N(CN)2-, CH3CN, and CN- derivatives are diamagnetic. The diamagnetic character of these last two compounds is confirmed in acetonitrile solution, while a spin transition step is observed for the N(CN)2- derivative. Diffraction data for all compounds as hexafluorophosphates shows that the I-, Br-, and OCN- derivatives crystallize in the orthorhombic space group Pbca, while the CN-, SCN-, and CH3CN compounds crystallize in the triclinic space group P. Average bond lengths and the trigonal distortion parameter can be correlated to the observed magnetic susceptibility depending on the coordinated monodentate ligand. Solution measurements of electronic properties for the compounds follow the trend established by the spectrochemical series. The relative stability of the Fe(II) complexes can be established in terms of the percentage of dissociation from the voltammetry and conductivity results, which are consistent with those obtained spectrophotometrically, mainly, the larger stability for the CN- derivative and the lower for the I- derivative. The redox potential and percentage of dissociation values allow for the estimation of the relative stability constants for the Fe(II) and Fe(III) complexes.