Reversible Guest Binding in a Non-Porous FeII Coordination Polymer Host Toggles Spin Crossover

Formation of either a dimetallic compound or a 1D coordination polymer of adiponitrile adducts of [Fe(bpte)](2+) (bpte=[1,2-bis(pyridin-2-ylmethyl)thio]ethane) can be controlled by the choice of counteranion. The iron(II) atoms of the bis(adiponitrile)-bridged dimeric complex [Fe-2(bpte)(2)((2)-(NC(CH2)(4)CN)(2)](SbF6)(4) (2) are low spin at room temperature, as are those in the polymeric adiponitrile-linked acetone solvate polymer {[Fe(bpte)((2)-NC(CH2)(4)CN)](BPh4)(2)Me2CO} (3Me(2)CO). On heating 3Me(2)CO to 80 degrees C, the acetone is abruptly removed with an accompanying purple to dull lavender colour change corresponding to a conversion to a high-spin compound. Cooling reveals that the desolvate 3 shows hysteretic and abrupt spin crossover (SCO) S=0S=2 behaviour centred at 205K. Non-porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3Me(2)CO concurrently reinstating a low-spin state.