Reversible Guest Binding in a Non‐Porous Fe II Coordination Polymer Host Toggles Spin Crossover

Formation of either a dimetallic compound or a 1 D 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 (μ 2 ‐(NC(CH 2 ) 4 CN) 2 ](SbF 6 ) 4 ( 2 ) are low spin at room temperature, as are those in the polymeric adiponitrile‐linked acetone solvate polymer {[Fe(bpte)(μ 2 ‐NC(CH 2 ) 4 CN)](BPh 4 ) 2 ⋅ Me 2 CO} ( 3⋅ Me 2 CO). On heating 3⋅ Me 2 CO to 80 °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 =0↔ S =2 behaviour centred at 205 K. Non‐porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3⋅ Me 2 CO concurrently reinstating a low‐spin state.