Spin-Crossover Behavior in Cyanide-Bridged Iron(II)-Copper(I) Bimetallic 1-3D Metal-Organic Frameworks

The synthesis and characterization of a series of 1-3D cyanide-bridged iron(II)-copper(I) bimetallic coordination polymers formulated as {Fe(3-Xpy)(2)[Cu(3-Xpy)ACN)(2)](2)}, where Mpy is a 3-halogenpyridine ligand with X = F (z = 1.5, 1), Cl (z = 1, 2 and 3), Br (z = 1, 4), and I (z = 1, 5), are reported. In all derivatives, the Fell ion lies in pseudoctahedral [FeN6] sites defined by four in situ formed [Cu(3-Xpy)(z)(CN)(2)](-) bridging ligands and two Mpy terminal ligands occupying the equatorial and axial positions, respectively. 1 consists of stacks of corrugated grids whose square windows are defined by pseudotrigonal and pseudotetrahedral [Cu(3-Fpy)(CN)(2)](-) and [Cu(3Fpy)(2)(CN)(2)](-) units, respectively. 2 is a 3D coordination polymer with the topology of the open-framework CdSO4. The [Cu(3-CIpy)(CN)(2)](-) rods connecting the pseudooctahedral Fe-II sites are arranged in such a way that interpenetration is avoided. 3, an architectural isomer of 2, is defined by arrays of linear chains. 4 and 5 are isostructural to 3. Polymer 1 is essentially a low-spin (LS) compound with ca. 19% of residual Fe-II ions in the high-spin (HS) state at 293 K. It undergoes an irreversible spin transition at T, = 356 K. Subsequent cooling-warming cycles give a new spin-crossover behavior characterized by T-c = 187 K. The structural analysis at 130 and 293 K and at 293 K after irreversible transformation (293 K*) reveals a large unit cell volume variation of 67 angstrom(3) per Fe atom. In addition to the volume change associated with the spin-state conversion, remarkable bond and angle modifications around the Cu-I sites account for the high flexible nature of the crystal. 2 displays a complete not well-resolved two-step spin conversion, T-c1 = 169 K and T-c2 = 210 K, reflecting the occurrence of two distinct crystallographically Fe-II sites. The large unit cell volume variation per Fe atom in 2, 59 angstrom(3), has been rationalized in terms similar to those for 1. 1D polymers 3-5 are HS compounds.