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

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 ] ↔ [H S 1/2 :L S 1/2 ] ↔ [HS 1/4 :LS 3/4 ] ↔ [HS 0 :LS 1 ], which has been structurally characterized.