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

A new family of mononuclear [Fe II ( Rdpt ) 2 (NCE) 2 ] complexes (E = S, Se, or BH 3 ) is formed by 1:2 reaction of [Fe II (pyridine) 4 (NCE) 2 ] with the monotopic pyridyl triazole ligand 4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4 H -1,2,4-triazole ( tolpyph ). The three complexes are obtained as six different solvatomorphs: [Fe II ( tolpyph ) 2 (NCS) 2 ]·H 2 O ( 1 ·H 2 O), 1 ·1.5CH 3 OH·0.5H 2 O, [Fe II ( tolpyph ) 2 (NCSe) 2 ] ( 2 ), 2 ·1.5H 2 O, [Fe II ( tolpyph ) 2 (NCBH 3 ) 2 ] ( 3 ), and 3 ·H 2 O. Single-crystal X-ray diffraction reveals that 1 ·1.5CH 3 OH·0.5H 2 O 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 [Fe II ( Rdpt ) 2 (NCE) 2 ]-type complex with NCBH 3 co-ligand: the crystal packing is dominated by aromatic stacking interactions. Solid-state magnetic measurements show that 1 ·H 2 O and 2 ·1.5H 2 O remain HS down to 50 K, whereas 3 ·H 2 O undergoes spin crossover (SCO) with a T 1/2 of 309 K, slightly above room temperature. A literature survey of analogous trans -[Fe II ( 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 CHCl 3 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 [Fe II ( tolpyph ) 2 (NCE) 2 ]. Interestingly, using this ratio, all three compounds are SCO-active in CDCl 3 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 CDCl 3 solution, with T 1/2 values of 290 and 310 K, respectively. In CDCl 3 solution, compound 3 has a T 1/2 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.