Structural Transformations and Spin‐Crossover in [FeL2]2+ Salts (L = 4‐{tertbutylsulfanyl}‐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 [FeL2][BF4]2·solv (1[BF4]2·solv) and [FeL2][ClO4]2·solv (1[ClO4]2·solv; solv = MeNO2, MeCN or Me2CO) exhibit a variety of structures and spin‐state behaviors including thermal spin‐crossover (SCO). Solvent loss on heating 1[BF4]2·xMeNO2 (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 Kmin‒1. The reverse‐SCO is not observed in a slower 1 Kmin‒1 measurement, however, confirming its kinetic nature. The final product [FeL2][BF4]2·0.75MeNO2 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[BF4]2·yMe2CO (y ≈ 1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low‐spin 1[ClO4 ]2·0.5Me2CO is not isostructural with its BF4− congener, and undergoes single‐crystal‐to‐single‐crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P ‐1 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