Guest‐, Light‐ and Thermally‐Modulated Spin Crossover in [Fe II 2 ] Supramolecular Helicates

A new bis(pyrazolylpyridine) ligand (H 2 L) has been prepared to form functional [Fe 2 (H 2 L) 3 ] 4+ metallohelicates. Changes to the synthesis yield six derivatives, X@[Fe 2 (H 2 L) 3 ]X(PF 6 ) 2 ⋅ x CH 3 OH ( 1 , x =5.7 and X=Cl; 2 , x =4 and X=Br), X@[Fe 2 (H 2 L) 3 ]X(PF 6 ) 2 ⋅ y CH 3 OH⋅H 2 O ( 1 a , y =3 and X=Cl; 2 a , y =1 and X=Br) and X@[Fe 2 (H 2 L) 3 ](I 3 ) 2 ⋅3 Et 2 O ( 1 b , X=Cl; 2 b , X=Br). Their structure and functional properties are described in detail by single‐crystal X‐ray diffraction experiments at several temperatures. Helicates 1 a and 2 a are obtained from 1 and 2 , respectively, by a single‐crystal‐to‐single‐crystal mechanism. The three possible magnetic states, [LS–LS], [LS–HS], and [HS–HS] can be accessed over large temperature ranges as a result of the structural nonequivalence of the Fe II centers. The nature of the guest (Cl − vs. Br − ) shifts the spin crossover (SCO) temperature by roughly 40 K. Also, metastable [LS–HS] or [HS–HS] states are generated through irradiation. All helicates (X@[Fe 2 (H 2 L) 3 ]) 3+ persist in solution.