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(H 2 B(pz) 2 ) 2 (bipy)], the bipyridine-based bisbidentate ligands 1,2-di(2,2'-bipyridin-5-yl)ethyne (ac(bipy) 2 ) and 1,4-di(2,2'-bipyridine-5-yl)-3,5-dimethoxybenzene (Ph(OMe) 2 (bipy) 2 ) are used to bridge two [Fe(H 2 B(pz) 2 ) 2 ] units, leading to the charge-neutral dinuclear iron(II) compounds [{Fe(H 2 B(pz) 2 ) 2 } 2 μ-(ac(bipy) 2 )] (1) and [{Fe(H 2 B(pz) 2 ) 2 } 2 μ-(Ph(OMe) 2 (bipy) 2 )] (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 T 1/2 = 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(H 2 B(pz) 2 ) 2 (bipy)]. This is attributed to steric hindrance between the molecules, limiting intermolecular interactions such as π-π-stacking.