Complete and incomplete spin transitions in 1D chain iron(II) compounds.

The synthesis and characterisation of two new octahedral iron(II) SCO coordination polymers [FeL1(bimm)] (1) and [FeL2(bppa)](MeOH)(0.5) (2) (L1 = [3,3']-[1,2-phenylenebis-(iminomethylidyne)bis(4-phenyl-2,4-butanedionato)(2-)-N,N',O-2,O-2'], L2 = [E, E]-[{diethyl 2,2'- 1,2- phenylenebis(iminomethylidyne) bis(3-oxo-3-phenylpropanato)}(2-)- N,N',O3,O3'], bimm = bis(1H- imidazol-1-yl) methane and bppa = 1,3-bis(pyridine-4-yl)propane) is presented. Results from X-ray structure analysis at different temperatures revealed in the case of 1 that the transition from a gradual to a cooperative SCO with a 5 K wide hysteresis is due to an increase of the short intermolecular contacts, which exceed a certain threshold for the cooperative effect. In the case of compound 2 an incomplete spin transition with a 4 K wide hysteresis was observed. The low temperature chi T-M product remains constant at a value typical for a mixed HS/LS state in stepwise spin transitions. A quantitative correlation between the cooperative effects of 12 monomer and polymer iron(II) SCO complexes and their structural properties derived from X-ray structure analysis, the so-called crystal contact index, CCI, is introduced.