Iron(II) Spin Crossover Polymers of Planar N2O2 Schiff Base Templates and 4,4’-bis(pyridyl)urea Bridges

Introduction: The synthesis of four new iron(II) coordination polymers [Fe(L1a)(bpua)] (1), [Fe(L1b)(bpua)](0.5bpua) (2), [Fe(L2a)(bpua)] (3), [Fe(L1b)(bpua)](yEtOH) (5) and one trinuclear complex [{Fe(L1a)(bpua)(MeOH)}2-µ{Fe(L1a)}](xMeOH) (4) with Schiff base-like N2O2coordinating equatorial ligands (L1a, L1b and L2a) and 4,4’-bis(pyridyl)urea (bpua) as bridging axial ligand is described. Materials and Methods: Single crystal X-ray structure elucidation of the trinuclear module4and of the coordination polymer5reveals the presence of HS-LS-HS chains and all-HS infinite 1-D strands, respectively. As anticipated the presence of the bridging urea supports the supramolecular concatenation within an extended hydrogen-bonding network. Magnetic measurements reveal spin crossover behavior for four of the five complexes (1–4) that is strongly solvent dependent. Results and Conclusion: Interestingly, in two cases, complete removal of the solvent from the crystal packing leads to wider thermal hysteresis loops.