Spin-Crossover Behavior in Cyanide-bridged Iron(II)−Gold(I) Bimetallic 2D Hofmann-like Metal−Organic Frameworks

The synthesis and characterization of new two-dimensional (2D) cyanide-bridged iron(II)−gold(I) bimetallic coordination polymers formulated, {Fe(3-Xpy)₂[Au(CN)₂]₂} (py = pyridine; X = F (1), Cl (2), Br (3), and I (4)) and the clathrate derivative {Fe(3-Ipy)₂[Au(CN)₂]₂}·1/2(3-Ipy) (5), are reported. The iron(II) ion lies in pseudoctahedral [FeN₆] sites defined by four [Au(CN)₂]⁻ bridging ligands and two 3-Xpy ligands occupying the equatorial and axial positions, respectively. Although only compounds 2 and 4 can be considered strictly isostructurals, all of the components of this family are made up of parallel stacks of corrugated {Fe[Au(CN)₂]₂}_n grids. The grids are formed by edge sharing of {Fe₄[Au(CN)₂]₄} pseudosquare moieties. The stacks are constituted of double layers sustained by short aurophilic contacts ranging from 3.016(2) to 3.1580(8) Å. The Au···Au distances between consecutive double layers are in the range of 5.9562(9)−8.790(2) Å. Compound 5, considered a clathrate derivative of 4, includes one-half of a 3-Ipy molecule per iron(II) atom between the double layers. Compound 1 undergoes a half-spin transition with critical temperatures T_c↓ = 140 K and T_c↑ = 145 K. The corresponding thermodynamic parameters derived from differential scanning calorimetry (DSC) are ΔH = 9.8 ± 0.4 kJ mol⁻¹ and ΔS = 68.2 ± 3 J K mol⁻¹. This spin transition is accompanied by a crystallographic phase transition from the monoclinic P2₁/c space group to the triclinic P1̅ space group. At high temperatures, where 1 is 100% high-spin, there is only one crystallographically independent iron(II) site. In contrast, the low temperature structural analysis shows the occurrence of two crystallographically independent iron(II) sites with equal population, one high-spin and the other low-spin. Furthermore, 1 undergoes a complete two-step spin transition at pressures as high as 0.26 GPa. Compounds 2−4 are high-spin iron(II) complexes according to their magnetic and [FeN₆] structural characteristics. Compound 5, characterized for having two different iron(II) sites, displays a two-step spin transition with critical temperatures of T_c¹ = 155 K, T_c²↓ = 97 K, and T_c²↑ = 110 K. This change of spin state takes place in both sites simultaneously. All of these results are compared and discussed in the context of other {Fe(L)_x[M^I(CN)₂]} coordination polymers, particularly those belonging to the homologous compounds {Fe(3-Xpy)₂[Ag(CN)₂]₂} and their corresponding clathrate derivatives.