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

The synthesis and characterization of two new binuclear compounds [{Fe(dpia)(NCS)(2)}(2)(bpac)]center dot nCH(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 [FeN6] 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.