Six-coordinate high-spin iron(II) complexes with bidentate PN ligands based on 2-aminopyridine – new Fe(II) spin crossover systems
Several new octahedral iron(II) complexes of the type [Fe(PNR-Ph)2X2] (X = Cl, Br; R = H, Me) containing bidentate PNR-Ph (R = H, Me) (1a,b) ligands based on 2-aminopyridine were prepared. 57Fe Mössbauer spectroscopy and magnetization studies confirmed in all cases their high spin nature at room temperature with magnetic moments very close to 4.9μB reflecting the expected four unpaired d-electrons in all these compounds. While in the case of the PNH-Ph ligand an S = 2 to S = 0 spin crossover was observed at low temperatures, complexes with the N-methylated analog PNMe-Ph retain an S = 2 spin state also at low temperatures. Thus, [Fe(PNH-Ph)2X2] (2a,3a) and [Fe(PNMe-Ph)2X2] (2b,3b) adopt different geometries. In the first case a cis-Cl,P,N-arrangement seems to be most likely, as supported by various experimental data derived from 57Fe Mössbauer spectroscopy, SQUID magnetometry, UV/Vis, Raman, and ESI-MS as well as DFT and TDDFT calculations, while in the case of the PNMe-Ph ligand a trans-Cl,P,N-configuration is adopted. The latter is also confirmed by X-ray crystallography. In contrast to [Fe(PNMe-Ph)2X2] (2b,3b), [Fe(PNH-Ph)2X2] (2a,3a) is labile and undergoes rearrangement reactions. In CH3OH, the diamagnetic dicationic complex [Fe(PNH-Ph)3]2+ (5) is formed via the intermediacy of cis-P,N-[Fe(κ2-P,N-PNH-Ph)2(κ1-P-PNH-Ph)(X)]+ (4a,b) where one PN ligand is coordinated in a κ1-P-fashion. In CH3CN the diamagnetic dicationic complex cis-N,P,N-[Fe(PNH-Ph)2(CH3CN)2]2+ (6) is formed as a major isomer where the two halide ligands are replaced by CH3CN.