Six-coordinate high-spin iron(ii) complexes with bidentate PN ligands based on 2-aminopyridine – new Fe(ii) spin crossover systems

Abstract

Several new octahedral iron(II) complexes of the type [Fe(PN^R-Ph)₂X₂] (X = Cl, Br; R = H, Me) containing bidentate PN^R-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 4.9μ_B reflecting the expected four unpaired d-electrons in all these compounds. While in the case of the PN^H-Ph ligand an S = 2 to S = 0 spin crossover was observed at low temperatures, complexes with the N-methylated analog PN^Me-Ph retain an S = 2 spin state also at low temperatures. Thus, [Fe(PN^H-Ph)₂X₂] (2a,3a) and [Fe(PN^Me-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 PN^Me-Ph ligand a trans-Cl,P,N-configuration is adopted. The latter is also confirmed by X-ray crystallography. In contrast to [Fe(PN^Me-Ph)₂X₂] (2b,3b), [Fe(PN^H-Ph)₂X₂] (2a,3a) is labile and undergoes rearrangement reactions. In CH₃OH, the diamagnetic dicationic complex [Fe(PN^H-Ph)₃]²⁺ (5) is formed via the intermediacy of cis-P,N-[Fe(κ²-P,N-PN^H-Ph)₂(κ¹-P-PN^H-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(PN^H-Ph)₂(CH₃CN)₂]²⁺ (6) is formed as a major isomer where the two halide ligands are replaced by CH₃CN.