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), I (4)) and
{Fe(3-Xpyridine)2[Ag(CN)2][Ag(3-Xpyridine)(CN)2]}·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 {Fe4[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···Ag distance was found to be in the interval 3.0−3.3 Å, while the Ag···Ag separation between two consecutive layers belonging to different pairs was found to be
around 6 Å. Compounds 5 and 6 are isostructural with a crystal packing defined by an almost homogeneous
distribution of layers separated by around 8.3 Å (referred to the Fe···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 {Fe4[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)1/2 = 96 K and T(2)1/2 = 162 K) and a 50% spin transition
(T1/2 = 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.