Doping ruthenium complexes into a molecular spin-crossover material

Two ruthenium compounds, [Ru(κ3C,N,C-bip)2][BF4]2 (bip = 2,6-di{1-methylimidazol-2-ylidene-3-yl}pyridine) and [Ru(terpy)(κ3N,N,C-terpy∗)][BF4]2 (terpy = 2,2′:6′,2″-terpyridine and terpy∗ = 2,2′:6′,4″-terpyridine), have been investigated as dopants for the spin-crossover lattice [Fe(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine). While [Fe(bpp)2][BF4]2 and [Ru(bip)2][BF4]2 did not co-crystallize, five different compositions of solid solutions [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1–x[BF4]2 were prepared, with 0.96 ⩾ x ⩾ 0.13. The materials with intermediate compositions (0.58 ⩾ x ⩾ 0.28) contained a mixture of crystalline and amorphous material by powder diffraction. The spin-crossover midpoint temperature (T½) in [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1–x[BF4]2 decreases smoothly with x, as the larger ruthenium dopant expands the host lattice and stabilizes its high-spin state. That contrasts with our previously published materials [Fe(bpp)2]z[Ru(terpy)2]1–z[BF4]2, which show a more complicated relationship between T½ and their composition.