Guest-, Light- and Thermally-Modulated Spin Crossover in [FeII2] Supramolecular Helicates

A new bis(pyrazolylpyridine) ligand (H2L) has been prepared to form functional [Fe2(H2L)3]4+ metallohelicates. Changes to the synthesis yield six derivatives, X@[Fe2(H2L)3]X(PF6)2⋅xCH3OH (1, x=5.7 and X=Cl; 2, x=4 and X=Br), X@[Fe2(H2L)3]X(PF6)2⋅yCH3OH⋅H2O (1 a, y=3 and X=Cl; 2 a, y=1 and X=Br) and X@[Fe2(H2L)3](I3)2⋅3 Et2O (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 FeII 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@[Fe2(H2L)3])3+ persist in solution.