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 2 n - Layers (M II = Pd, Pt).

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.