Keywords: Uranium(III) and uranium(IV) complexes / Uranium metal / Hydrides / Chlorides / Triflate complexes / X-ray crystal structure
Reactions of pure triflic acid (TfOH) with UH 3 at 20 °C and with The yields were in excess of 90%. The crystal structure of [U(OTf) 2 (OPPh 3 ) 4 ][OTf] (3) was determined, which revealed a U or UCl 3 at 120 °C afforded U(OTf) 3 (1), which was transformed into U(OTf) 4 (2) by reaction with TfOH at 180 °C; seven-coordinate uranium(III) centre bound by monodentate and bidentate triflate ligands. 2 was also synthesized by treating UCl 4 with TfOH at 120 °C.
The widespread interest in metal triflates in both organic isolated in quantitative yield. Of course, 2 could be prepared directly, without isolating 1, by heating U, UH 3 or and inorganic chemistry [1] prompted us to consider the potential of such complexes of uranium. We recently reported UCl 3 in TfOH at 180°C. To the best of our knowledge, these reactions of U or UH 3 with TfOH represent the first on the first uranium(IV) triflates, U(OTf) 4 (NC 5 H 5 ) (OTf ϭ OSO 2 CF 3 ), as well as on some organometallic derivatives. syntheses of metal triflates from the metal or metal hydride. [1a,3] Alternatively, compound 2 could be synthesized These were prepared by a novel synthetic route, namely the protonolysis of alkyl and amide precursors with pyridinium by reaction of TfOH with UCl 4 at 120°C; rapid HCl evolution occurred and the pale green-blue powder of 2 was triflate. [2] Following these first results, it soon became apparent that further studies of uranium triflates would be recovered in 91% yield after evaporation of the excess TfOH. considerably expedited if we had at our disposal sufficient quantities of the base-free complexes U(OTf) 3 (1) and Irrespective of their mode of preparation, compounds 1 U(OTf) 4 (2). Here, we present simple, large-scale syntheses and 2 were obtained in a pure state, as established from of 1 and 2 from classical starting materials; Lewis base their elemental analyses. However, the samples retained a adducts of 1 and 2 have also been prepared and the trace of triflic acid, even if they were thoroughly dried in crystal structure of one of the first uranium(III) triflates, vacuo at 150°C for several days; this was evidenced by the [U(OTf) 2 (OPPh 3 ) 4 ][OTf] (3), has been determined. formation of pyridinium triflate (< 1%) upon their dissol-Syntheses of the U III and U IV triflates are summarized in ution in pyridine. The strong Lewis acids 1 and 2 were Scheme 1. 1 and 2 were prepared by treating triflic acid found to be quite soluble in coordinating solvents such as (TfOH) with either U, UH 3 or UCl 3 in the absence of solpyridine and dimethoxyethane (DME), giving stable advent; 2 was also synthesized from UCl 4 . It was essential that ducts. Use of tetrahydrofuran (THF) as a solvent was not the acid was freshly distilled in order to achieve a clean practical since it was readily polymerized at ambient temand straightforward reaction. Treatment of UH 3 with excess perature. TfOH at 20°C afforded 1 as an ochre powder within a few Addition of stoichiometric amounts of OPPh 3 to soluminutes, the reaction being accompanied by the vigorous tions of 1 and 2 in DME led to the immediate formation of evolution of dihydrogen gas. Reactions of activated uran-U(OTf) 3 (OPPh 3 ) 4 (3) and U(OTf) 4 (OPPh 3 ) 2 (4), which were ium turnings or uranium trichloride were much slower, reisolated in yields of 85% as red and pale-green powders, quiring 3 d at 120°C to reach completion. After evaporrespectively. These OPPh 3 adducts, as well as the pyridine ation of the excess TfOH and drying in vacuo, 1 was obadducts of 1 and 2, were found to be soluble in THF and tained in almost quantitative yield as a very fine green powno polymerization of the solvent was observed. der. Uranium(III) triflate (1) proved to be remarkably stable X-ray analysis showed the crystals of 3 to be composed at 120°C in suspension in pure TfOH and its oxidation, of discrete pairs of OTf anions and U(OTf) 2 (OPPh 3 ) 4 catsignified by a colour change from ochre to pale green-blue, ions; such a dissociation of OTf ligands has frequently been occurred only when the temperature was raised to encountered, in particular from lanthanide triflates. [4] The 170Ϫ180°C; after 10 h, the acid was evaporated and 2 was structure of 3 is shown in Figure 1, together with selected bond lengths and angles. The uranium atom is surrounded [a] Laboratoire de Chimie de lЈUranium, Service de Chimie by seven oxygen atoms, which form a distorted pentagonal Mole ´culaire, DSM, DRECAM, CNRS URA 331, CEA Saclay, bipyramid. The apical positions are occupied by two OPPh 3