Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior.

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt 2 p) of formulas [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·1.5MeNO 2 ( 1 ), [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·MeNO 2 ( 2 ), and [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·2MeNO 2 ( 3 ) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2 , a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·MeNO 2 ( 2·MeNO 2 ), [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·MeCN ( 2·MeCN ), [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·0.5Me 2 CO (2·Me 2 CO ), and [Fe(bpCOOEt 2 p) 2 ](ClO 4 ) 2 ·MeCOOH ( 2·MeCOOH ) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me 2 CO and the high-spin state in 2·MeCN . Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.