Spin crossover in dinuclear iron(II) complexes bridged by bis-bipyridine ligands: dimer effects on electronic structure, spectroscopic properties and spin-state switching.

Inspired by the well-studied mononuclear spin crossover compound [Fe(HB(pz))(bipy)], the bipyridine-based bisbidentate ligands 1,2-di(2,2'-bipyridin-5-yl)ethyne (ac(bipy)) and 1,4-di(2,2'-bipyridine-5-yl)-3,5-dimethoxybenzene (Ph(OMe)(bipy)) are used to bridge two [Fe(HB(pz))] units, leading to the charge-neutral dinuclear iron(II) compounds [{Fe(HB(pz))} μ-(ac(bipy))] (1) and [{Fe(HB(pz))} μ-(Ph(OMe)(bipy))] (2), respectively. The spin-crossover properties of these molecules are investigated by temperature-dependent PPMS measurements, Mössbauer, vibrational and UV/Vis spectroscopy as well as X-ray absorption spectroscopy. While compound 1 undergoes complete SCO with = 125 K, an incomplete spin transition is observed for 2 with an inflection point at 152 K and a remaining high-spin fraction of 40% below 65 K. The spin transitions of the dinuclear compounds are also more gradual than for the parent compound [Fe(HB(pz))(bipy)]. This is attributed to steric hindrance between the molecules, limiting intermolecular interactions such as π-π-stacking.