Solid Versus Solution Spin Crossover and the Importance of the Fe–N≡C(X) Angle

A new family of mononuclear [FeII(Rdpt)2(NCE)2] complexes (E = S, Se, or BH3) is formed by 1:2 reaction of [FeII(pyridine)4(NCE)2] with the monotopic pyridyl triazole ligand 4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-1,2,4-triazole (tolpyph). The three complexes are obtained as six different solvatomorphs: [FeII(tolpyph)2(NCS)2]·H2O (1·H2O), 1·1.5CH3OH·0.5H2O, [FeII(tolpyph)2(NCSe)2] (2), 2·1.5H2O, [FeII(tolpyph)2(NCBH3)2] (3), and 3·H2O. Single-crystal X-ray diffraction reveals that 1·1.5CH3OH·0.5H2O and 2 are high-spin (HS) at 100 K, while 3 is low-spin (LS) at 100 K and HS at 373 K. Compound 3 is the first structurally characterized example of an [FeII(Rdpt)2(NCE)2]-type complex with NCBH3 co-ligand: the crystal packing is dominated by aromatic stacking interactions. Solid-state magnetic measurements show that 1·H2O and 2·1.5H2O remain HS down to 50 K, whereas 3·H2O undergoes spin crossover (SCO) with a T1/2 of 309 K, slightly above room temperature. A literature survey of analogous trans-[FeII(Rdpt)2(NCX)2]-type complexes (53 distinct crystal structures) shows that for the complexes that are SCO active in the solid state the Fe-N≡C(X) angle is usually close to straight, 162-178°, whereas it is usually lower, 142-159°, for the complexes that remain HS. UV-vis studies in CHCl3 solution show that in each case the use of a 6:1 ratio of tolpyph/Fe(II) is required to ensure the iron(II) is present in solution as [FeII(tolpyph)2(NCE)2]. Interestingly, using this ratio, all three compounds are SCO-active in CDCl3 solution-in dramatic contrast to the solid-state findings. Specifically, while compounds 1 and 2 are not SCO-active in the solid state (they remain HS), they undergo gradual SCO in CDCl3 solution, with T1/2 values of 290 and 310 K, respectively. In CDCl3 solution, compound 3 has a T1/2 value of 288 K, which is 21 K lower than in the solid state. These results highlight the differences between solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on SCO, with the latter studies revealing room-temperature SCO for all three of these complexes.