Strong Cooperative Spin Crossover in 2D and 3D Fe^II–M^I,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 μ₄-[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 <i>T</i>_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]}·1/2H₂O 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