Structural Transformations and Spin‐Crossover in [FeL2]2+ Salts (L=4‐{tert‐Butylsulfanyl}‐2,6‐di{pyrazol‐1‐yl}pyridine): The Influence of Bulky Ligand Substituents

Abstract 4‐( tert ‐Butylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine ( L ) was obtained in low yield from a one‐pot reaction of 2,4,6‐trifluoropyridine with 2‐methylpropane‐2‐thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [Fe L 2 ][BF 4 ] 2 ⋅solv ( 1[BF 4 ] 2 ⋅solv) and [Fe L 2 ][ClO 4 ] 2 ⋅solv ( 1[ClO 4 ] 2 ⋅solv; solv=MeNO 2 , MeCN or Me 2 CO) exhibit a variety of structures and spin‐state behaviors including thermal spin‐crossover (SCO). Solvent loss on heating 1[BF 4 ] 2 ⋅ x MeNO 2 ( x ≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a “reverse‐SCO” step in its warming cycle at a scan rate of 5 K min −1 . The reverse‐SCO is not observed in a slower 1 K min −1 measurement, however, confirming its kinetic nature. The final product [Fe L 2 ][BF 4 ] 2 ⋅0.75 MeNO 2 was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF 4 ] 2 ⋅ y Me 2 CO ( y ≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low‐spin 1[ClO 4 ] 2 ⋅0.5 Me 2 CO is not isostructural with its BF 4 − congener, and undergoes single‐crystal‐to‐single‐crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low‐temperature phases when the materials are fully low‐spin.