Halobenzene Clathrates of the Porous Metal–Organic Spin-Crossover Framework [Fe(tvp)(2)(NCS)(2)](n). Stabilization of a Four-Step Transition

[Image: see text] Here we show that the porous metal–organic spin crossover (SCO) framework [Fe(tvp)(2)(NCS)(2)]@4(CH(3)CN·H(2)O) [1@4(CH(3)CN·H(2)O)] is an excellent precursor material for the systematic synthesis, via single-crystal to single-crystal transformation, of a series of halobenzene clathrates. Immersion of samples constituted of single crystals of 1@4(CH(3)CN·H(2)O) in the liquid halobenzenes PhX(n), X = F (n = 1–6), X = Cl (n = 1, 2), and X = Br (n = 1) at room temperature induces complete replacement of the guest molecules by PhX(n) to afford 1@2PhX(n). Single-crystal analyses of the new clathrates confirm the integrity of the porous framework with the PhX(n) guests being organized by pairs via π-stacking filling the nanochannels. The magnetic and calorimetric data confirm the occurrence of practically complete SCO behavior in all of the clathrates. The characteristic SCO equilibrium temperatures, T(1/2), seem to be the result of a subtle balance in the host–guest interactions, which are temperature- and spin-state-dependent. The radically distinct supramolecular organization of the PhCl(2) guests in 1@2PhCl(2) affords a rare example of four-step SCO behavior following the sequence [HS(1):LS(0)] ↔ [HS(2/3):LS(1/3)] ↔ [HS(1/2):LS(1/2)] ↔ [HS(1/4):LS(3/4)] ↔ [HS(0):LS(1)], which has been structurally characterized.