Uranium(iii) complexes supported by hydrobis(mercaptoimidazolyl)borates: synthesis and oxidation chemistry

Abstract

The reaction of [UI₃(thf)₄] with the sodium or lithium salts of hydrobis(2-mercapto-1-methylimidazolyl)borate ligands ([H(R)B(tim^Me)₂]⁻) in a 1 : 2 ratio, in tetrahydrofuran, gave the U(III) complexes [UI{κ³-H,S,S′-H(R)B(tim^Me)₂}₂(thf)₂] (R = H (1), Ph (2)) in good yields. Crystals of [UI{κ³-H,S,S′-H(Ph)B(tim^Me)₂}₂(thf)₂] (2) were obtained by recrystallization from a tetrahydrofuran/acetonitrile solution, and the ion-separated uranium complex [U{κ³-H,S,S′-H(Ph)B(tim^Me)₂}₂(CH₃CN)₃][I] (3-I) was obtained by dissolution of 2 in acetonitrile followed by recrystallization. One-electron oxidation of 2 with AgBPh₄ or I₂ resulted in the formation of the cationic U(IV) complexes [U{κ³-H,S,S′-H(Ph)B(tim^Me)₂}₃][X] (X = BPh₄ (6-BPh₄), I (6-I)), due to a ligand redistribution process. These complexes are the first examples of homoleptic poly(azolyl)borate U(IV) complexes. Treatment of complex 2 with azobenzene led to the isolation of crystals of the U(IV) compound [UI{κ³-H(Ph)B(tim^Me)₂}₂(κ²-tim^Me)] (7). Treatment of 2 with pyridine-N oxide (pyNO) led to the formation of the uranyl complex [UO₂{κ²-S,S′-H(Ph)B(tim^Me)₂}₂] (8) and of complex 6-I, while from the reaction of [U{κ³-H(Ph)B(tim^Me)₂}₂(thf)₃][BPh₄] (5) with pyNO, the oxo-bridged U(IV) complex [{U{κ³-H(Ph)B(tim^Me)₂}₂(pyNO)}₂(μ-O)][BPh₄]₂ (9) was also obtained. In the U(III) and U(IV) complexes, the bis(azolyl)borate ligands bind to the uranium center in a κ³-H,S,S′ coordination mode, while in the U(VI) complex the ligands bind to the metal in a κ²-S,S′ mode. The presence of U⋯H–B interactions in the solid-state, for the nine-coordinate complexes 1, 2, 3, 6 and 7 and for the eight-coordinate complex 9, was supported by IR spectroscopy and/or X-ray diffraction analysis.