Spin-crossover behavior in cyanide-bridged iron(II)-silver(I) bimetallic 2D Hofmann-like metal-organic frameworks

Two new series of compounds formulated {Fe(3-Xpyridine)(2)[Ag(CN)(2)](2)} (X := F (1), Cl (2), Br (3), 1 (4)) and {Fe(3-Xpyridine)(2)[Ag(CN)(2)][Ag(3-Xpyridine)(CN)(2)]}center dot 3-Xpy (X = Br (5), I (6)) have been synthesized and characterized. The six compounds are made up of stacking of slightly corrugated two-dimensional coordination polymers defined by sharing {Fe-4[Ag(CN)(2)](4)}(n) motifs. The stacking is different for the two families. In compounds 1-4 the layers are organized by pairs displaying argentophilic interactions; the Ag center dot center dot center dot Ag distance was found to be in the interval 3.0-3.3 angstrom, while the Ag center dot center dot center dot Ag separation between two consecutive layers belonging to different pairs was found to be around 6 A. Compounds 5 and 6 are isostructural with a crystal packing defined by an almost homogeneous distribution of layers separated by around 8.3 angstrom (referred to the Fe center dot center dot center dot Fe interlayer distance). Between the layers an uncoordinated 3-Xpyridine molecule is included. Another 3-Xpyridine molecule, which remains in the plane defined by the {Fe-4[Ag(CN)(2)](4)}(n) windows, coordinates one silver atom. Both series display quite different properties; at 300 K, 1-4 are pale-yellow and display similar distorted [FeN6] octahedron cores characteristic of the iron(II) ion in the high-spin state. 1 and 2 undergo a two-step (T-1/2((1)) = 96 K and T-1/2((2)) = 162 K) and a 50% spin transition (T-1/2 = 106 K), respectively. Compounds 3 and 4 are high-spin compounds at ambient pressure. 5 and 6 are deep red in color at 300 K and undergo spin-crossover behavior at significantly higher temperatures T1/2 = 306 and 261 K, respectively.