The Nature of Spin Crossover and Coordination Core Distortion in a Family of Binuclear Iron(II) Complexes with Bipyridyl‐Like Bridging Ligands

Abstract

Abstract The synthesis and characterization of two new binuclear compounds [{Fe(dpia)(NCS) 2 } 2 (bpac)] · n CH 3 OH [ n = 0 ( 1 ) and 2 ( 2 ), dpia = bis(2‐picolyl)amine, bpac = 1,2‐bis(4‐pyridyl)ethyne] are reported. The magnetic susceptibility measurements of the compounds revealed different types of magnetic behavior. Complex 1 displays a two‐step spin crossover (SCO) that suggests the occurrence of a mixed [HS–LS] (HS = high spin, LS = low spin) pair at the plateau temperature (145 K), at which about 50 % of the complexes undergo a thermal spin conversion. The high‐temperature step is very gradual, whereas the low‐temperature step shows a first‐order spin transition with a hysteresis width equal to 17 K. Compound 2 manifests a relatively abrupt one‐step spin change with a narrow hysteresis of 4 K. The single‐crystal X‐ray structure of 1 was solved for the [HS–HS] complex at room temperature. Compound 2 was structurally characterized in both [HS–HS] and [LS–LS] spin states at 290 and 110 K. The magnetic and structural characteristics of 1 and 2 were compared with previously reported related binuclear compounds with 4,4′‐bipyridine and 1,2‐bis(4‐pyridyl)ethene spacer ligands. This analysis confirms a recently proposed idea about the key role of the [FeN 6 ] core structural distortions, which are caused by crystal packing and strain effects of terminal and/or bridging ligands, in the spin‐crossover behavior of binuclear complexes. This study also revealed the predominant role of inter‐ vs. intramolecular interactions in the spin transition cooperativity.