Co-crystallising two functional complex molecules in a terpyridine embrace lattice

Abstract

Two series of solid solutions based on the [Fe(bpp)₂][BF₄]₂ (bpp = 2,6-di(pyrazol-1-yl)pyridine) spin-crossover lattice are described. Materials of formula [Fe_yNi_1−y(bpp)₂][BF₄]₂ (0.95 ≥ x ≥ 0.15) are all phase-pure. In contrast, [Fe(bpp)₂]_x[Ru(terpy)₂]_1−x[BF₄]₂ (0.95 ≥ x ≥ 0.28) crystallise as a mixture of the neat [Fe(bpp)₂][BF₄]₂ (space group P2₁) and [Ru(terpy)₂][BF₄]₂ (Cc) phases as x approaches 0.5, although there is no evidence of segregation of the iron and ruthenium centres between the two. Spin-crossover in [Fe_yNi_1−y(bpp)₂][BF₄]₂ shows a normal variation in T_½, ΔH, ΔS and lattice elasticity with decreasing y, for an isomorphously doped material. However, T_½ in [Fe(bpp)₂]_x[Ru(terpy)₂]_1−x[BF₄]₂ shows an unusual dependence on x, which decreases slightly for 1.00 ≥ x ≥ 0.57 but increases as x is reduced further. We attribute this anomaly to the change in structure of the solid solutions, from predominantly the P2₁ lattice when x ≥ 0.47 to the Cc lattice when x = 0.28. The ruthenium centres in those compounds retain their low-temperature fluorescence, albeit with strongly reduced emission intensities. An unusual dependence of ΔH and ΔS for the spin transition in [Fe(bpp)₂][BF₄]₂ on the crystallisation solvent used is also noted.