Four new spin-crossover salts of [Fe(3-bpp)2]2+ (3-bpp = 2,6-bis[1H-pyrazol-3-yl]pyridine)

Abstract

Four new salts of the well-known spin-crossover centre [Fe(3-bpp)2]2+ are described: [Fe(3-bpp)2][M(CN)2]2 (M=Ag, 1; M=Au, 2), [Fe(3-bpp)2][Au(SCN)2]2 (3) and [Fe(3-bpp)2][BPh4]2 (4) are reported. Monohydrate crystals of 1 and 2 are isostructural, and are better formulated as [Fe(3-bpp)2]2[M(CN)2][M3(CN)6]·2H2O (M=Ag or Au) with the mononuclear and trinuclear anion sites being disordered within each anion dyad. These groups are linked into a 2D network topology through N–H⋯X (X=N or O) hydrogen bonding between the cations and the cyanometallate anions, and the lattice water. In contrast, the complex cations in the solvate 4·2CH3NO2·(C2H5)2O are completely encapsulated by phenyl groups from the BPh4− anions. Dried powder samples of 1–4 all exhibit very gradual thermal spin-state transitions centred at 198⩽T½⩽291K. ES mass spectrometry of 1–3 implies that extensive ligand exchange between the iron and coinage metal ions occurs in solution. Solid 1 and 2 are not emissive at room temperature upon irradiation in the UV.