Symmetry Breaking and Cooperative Spin Crossover in a Hofmann-Type Coordination Polymer Based on Negatively Charged {Fe(II)(μ(2)-[M(II)(CN)(4)])(2)}(n)(2n–) Layers (M(II) = Pd, Pt)

[Image: see text] We report herein the synthesis and characterization of two unprecedented isomorphous spin-crossover two-dimensional coordination polymers of the Hofmann-type formulated {Fe(II)(Hdpyan)(2)(μ(2)-[M(II)(CN)(4)])(2)}, with M(II) = Pd, Pt and Hdpyan is the in situ partially protonated form of 2,5-(dipyridin-4-yl)aniline (dpyan). The Fe(II) is axially coordinated by the pyridine ring attached to the 2-position of the aniline ring, while it is equatorially surrounded by four [M(II)(CN)(4)](2–) planar groups acting as trans μ(2)-bidentate ligands defining layers, which stack parallel to each other. The other pyridine group of Hdpyan, being protonated, remains peripheral but involved in a strong [M(II)-C≡N···Hpy(+)] hydrogen bond between alternate layers. This provokes a nearly 90° rotation of the plane defined by the [M(II)(CN)(4)](2–) groups, with respect to the average plane defined by the layers, forcing the observed uncommon bridging mode and the accumulation of negative charge around each Fe(II), which is compensated by the axial [Hdpyan](+) ligands. According to the magnetic and calorimetric data, both compounds undergo a strong cooperative spin transition featuring a 10–12 K wide hysteresis loop centered at 220 (Pt) and 211 K (Pd) accompanied by large entropy variations, 97.4 (Pt) and 102.9 (Pd) J/K mol. The breaking symmetry involving almost 90° rotation of one of the two coordinated pyridines together with the large unit-cell volume change per Fe(II) (ca. 50 Å(3)), and subsequent release of significantly short interlayer contacts upon the low-spin → high-spin event, accounts for the strong cooperativity.