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

[Image: see text] 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.