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

We report hereinthe synthesis and characterization of two unprecedentedisomorphous spin-crossover two-dimensional coordination polymers ofthe Hofmann-type formulated {Fe-II(Hdpyan)(2)(& mu;(2)-[M-II(CN)(4)])(2)}, with M-II = Pd, Pt and Hdpyan is the in situ partiallyprotonated form of 2,5-(dipyridin-4-yl)aniline (dpyan). The Fe-II is axially coordinated by the pyridine ring attached tothe 2-position of the aniline ring, while it is equatorially surroundedby four [M-II(CN)(4)](2-) planargroups acting as trans & mu;(2)-bidentateligands defining layers, which stack parallel to each other. The otherpyridine group of Hdpyan, being protonated, remains peripheral butinvolved in a strong [M-II-C VN & BULL;& BULL;& BULL;Hpy(+)] hydrogen bond between alternate layers. This provokes anearly 90 & DEG; rotation of the plane defined by the [M-II(CN)(4)](2-) groups, with respect to theaverage plane defined by the layers, forcing the observed uncommonbridging mode and the accumulation of negative charge around eachFe(II), which is compensated by the axial [Hdpyan](+) ligands. According to the magnetic and calorimetric data, both compoundsundergo a strong cooperative spin transition featuring a 10-12K wide hysteresis loop centered at 220 (Pt) and 211 K (Pd) accompaniedby large entropy variations, 97.4 (Pt) and 102.9 (Pd) J/K mol. Thebreaking symmetry involving almost 90 & DEG; rotation of one of thetwo coordinated pyridines together with the large unit-cell volumechange per Fe-II (ca. 50 & ANGS;(3)), and subsequent release of significantly short interlayer contactsupon the low-spin & RARR; high-spin event, accounts for the strongcooperativity. The [M-II(CN)(4)](2-) building blocks of the reported compounds(M-II = Pd,Pt) act as trans & mu;(2)-bridging ligandslinking the Fe-II ions through the four equatorial sites.The accumulated negative charge is canceled by in situ protonation of the axial Hdpyan(+) ligands. This unprecedented2D Hofmann-type structure, featuring strong intermolecular interactions,provides cooperative hysteretic spin transitions coupled with theloss & LRARR; gain of an inversion center through the rotation ofa pyridine ring.