Spin Crossover in Dinuclear N4S2 Iron(II) Thioether-Triazole Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States

Abstract

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PSiBuT, PSt-BuPhT, and PSMePhT, with R = Ph, iBu, t-BuPh, and MePh, respectively. Three dinuclear colorless to pale green iron(II) complexes, [FeII2(PSRT)2](BF4)4·solvent, featuring N4S2 donor sets, were prepared. Single-crystal structure determinations on [FeII2(PSPhT)2](BF4)4·2MeCN·H2O, [FeII2(PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF, [FeII2(PSMePhT)2](BF4)4·2MeCN, and [FeII2(PSiBuT)2](BF4)4·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [FeII2(PSPhT)2](BF4)4·21/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [FeII2(PSMePhT)2](BF4)4·11/2MeCN·2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [FeII2(PSiBuT)2](BF4)4·MeCN·H2O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [FeII2(PSPhT)2](BF4)4·1/2CHCl3·21/2H2O (solvatomorph A) reveals a trace of LS character (572 nm band 1A1g → 1T1g). Evans' 1H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.