High Spin and Spin-Crossover Two-Dimensional Coordination Polymers Containing FeII(tetrazol-2-yl)4(solv)2 (solv = Ethanol, Acetonitrile) Cores Linked by Flexible/Elastic Spacers

1,6-Di(tetrazol-2-yl)hexane (1, hbtz) in the reaction with Fe(ClO4)(2)center dot 6H(2)O performed in acetonitrile or ethanol forms complexes {[Fe(hbtz)(2)(CH3CN)(2)](ClO4)(2)}(infinity) (2) and {[Fe(hbtz)(2)(C2H5OH)(2)](ClO4)(2)}(infinity) (3), respectively. Both compounds crystallize in the triclinic system and P (1) over bar space group. In 2 and 3 the basal plane of the coordination octahedron is formed by four tetrazole rings coordinated through N4 nitrogen atoms. The coordination spheres in 2 and 3 are completed by axially coordinated CH3CN or C2H5OH molecules, respectively. In 2 and 3 ligand molecules bridge neighboring iron(II) ions in two directions resulting in two-dimensional (2D) networks. 3 is paramagnetic in the range 5-300 K whereas 2 undergoes the thermally induced HS reversible arrow LS transition (SCO) with T-1/2(up arrow) =T-1/2(down arrow) = 128 K. Both complexes have very similar architecture with (4,4) network topology; however, the temperature dependence of ligand bridge parameters in 2 differs markedly from the ones observed for 3. In 2 a lowering of temperature from 293 to 100 K causes a reduction of the distances between bridged iron(II) ions at 0.26 and 0.30 angstrom accompanied by the shortening of the distances between N4,N4' donor atoms at about 0.04 and 0.09 angstrom indicating elasticity of the ligand molecules. In 3 at 250 K iron(II) ions are linked by ordered and disordered over two positions hbtz bridges. Cooling to 100 K involves a shortening of Fe center dot center dot center dot Fe and N4 center dot center dot center dot N4' distances of the ordered bridge at 0.09 and 0.07 angstrom, respectively. The previously disordered bridge is ordered at 100 K, and only one conformer is present. In this case conformational changes enable a reduction of Fe center dot center dot center dot Fe and N4 center dot center dot center dot N4' distances at about 0.14 and 0.14 angstrom, respectively.