Guest-, Light- and Thermally-Modulated Spin Crossover in [Fe2II] Supramolecular Helicates

A new bis(pyrazolylpyridine) ligand (H2L) has been prepared to form functional [Fe-2(H2L)(3)](4+) metallohelicates. Changes to the synthesis yield six derivatives, X@[Fe-2(H2L)(3)]X(PF6)(2)center dot xCH(3)OH (1, x = 5.7 and X = Cl; 2, x = 4 and X = Br), X@[Fe-2(H2L)(3)]X(PF6)(2)center dot yCH(3)OH center dot H2O (1a, y = 3 and X = Cl; 2a, y = 1 and X = Br) and X@[Fe-2(H2L)(3)](I-3)(2)center dot 3Et(2)O (1b, X = Cl; 2b, X = Br). Their structure and functional properties are described in detail by single-crystal X-ray diffraction experiments at several temperatures. Helicates 1a and 2a 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 non-equivalence 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@[Fe-2(H2L)(3)])(3+) persist in solution.