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)

• Published in: Inorganic chemistry 2023-08, Vol.62 (32), p.12783-12792
• Main Authors: Orellana-Silla, Alejandro, Meneses-Sánchez, Manuel, Turo-Cortés, Rubén, Muñoz, M Carmen, Bartual-Murgui, Carlos, Real, José Antonio
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.3c01332

We report herein the synthesis and characterization of two unprecedented isomorphous spin-crossover two-dimensional coordination polymers of the Hofmann-type formulated {Fe (Hdpyan) (μ -[M (CN) ]) }, with M = Pd, Pt and Hdpyan is the partially protonated form of 2,5-(dipyridin-4-yl)aniline (dpyan). The Fe is axially coordinated by the pyridine ring attached to the 2-position of the aniline ring, while it is equatorially surrounded by four [M (CN) ] planar groups acting as μ -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 -C≡N···Hpy ] hydrogen bond between alternate layers. This provokes a nearly 90° rotation of the plane defined by the [M (CN) ] 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 , 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 ( . 50 Å ), and subsequent release of significantly short interlayer contacts upon the low-spin → high-spin event, accounts for the strong cooperativity.