Strong Cooperative Spin Crossover in 2D and 3D FeII–MI,II Hofmann-Like Coordination Polymers Based on 2-Fluoropyrazine

Self-assembling iron(II), 2-fluoropyrazine (Fpz), and [M II (CN) 4 ] 2– (M II = Ni, Pd, Pt) or [Au I (CN) 2 ] − building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ 4 -[M II (CN) 4 ] 2– groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe(Fpz) 2 ([M II (CN) 4 ]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by T 1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe(Fpz)[Pt(CN) 4 ]}·1/2H 2 O and {Fe(Fpz)[Au(CN) 2 ] 2 }, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine.