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)z(CN)2]2}, where 3-Xpy 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 FeII ion lies in pseudoctahedral [FeN6] sites defined by four in situ formed [Cu(3-Xpy)z(CN)2]− bridging ligands and two 3-Xpy 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(3-Fpy)2(CN)2]− units, respectively. 2 is a 3D coordination polymer with the topology of the open-framework CdSO4. The [Cu(3-Clpy)(CN)2]− rods connecting the pseudooctahedral FeII 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 FeII ions in the high-spin (HS) state at 293 K. It undergoes an irreversible spin transition at Tc = 356 K. Subsequent cooling−warming cycles give a new spin-crossover behavior characterized by Tc = 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 Å3 per Fe atom. In addition to the volume change associated with the spin-state conversion, remarkable bond and angle modifications around the CuI sites account for the high flexible nature of the crystal. 2 displays a complete not well-resolved two-step spin conversion, Tc1 = 169 K and Tc2 = 210 K, reflecting the occurrence of two distinct crystallographically FeII sites. The large unit cell volume variation per Fe atom in 2, 59 Å3, has been rationalized in terms similar to those for 1. 1D polymers 3−5 are HS compounds.