Synthesis, chemical behaviour, and structure (crystal and solution) of a fluorouranocene(IV) tetrafluoroborate; X-ray crystal structure of [{Ucp″2(µ-BF4)(µ-F)}2][cp″=η-C5H3(SiMe3)2]

Abstract

Reaction of Ag[BF₄] in OEt₂ with either [Ucp″₂Cl₂] or [Ucp″₂(CH₂R)₂][cp″=η-C₅H₃(SiMe₃)₂, R = SiMe₃ or Ph] affords the monofluoro (tetrafluoroborato)uranocene(IV), which in the crystal is a dimmer (1) with a pair of bridging bidentate BF₄^– ligand and a (µ-F)₂^– arrangement, with mean U–FB, U–F, and B–FU bond lengths of 2.41, 2.31, and 1.34 Å respectively, and B–F (terminal) of 1.38(4) and 1.23(2)Å in CDCl₃ or CD₃C₆D₅(1) is in equilibrium with the monomer (2), with ΔH= 54.2 kJ mol^–1 and ΔS ca. 200 J mol^–1 K^–1 for the dissociation, and for each of (1) and (2) there is a temperature-dependent dynamic process, with ΔG^‡_{263 or 288 K}=ca. 50 kJ mol^–1, which may relate to U–cp″ rotations.