A number of years ago, we reported a brief investigation of the thermally unstable products of the reaction of uranium tetrachloride with alkyl lithium reagents [1] (eqn. (1)). The purpose of this investigation was to ascertain whether β-hydride elimination might occur in a uranium hydrocarbyl with potential coordinate unsaturation. This being the object of the investigation, no attempt was made to characterize the intermediate organometallics nor was any structure or stoichimetric formulation specifically claimed for them (historically such species have been presumed to be tetrahydrocarbyls [2, 3]). Subsequent to this work, several groups have reported the successful use of eqn. (1) to generate finely divided uranium metal for synthetic purposes [4, 5], and evidence has been presented that, under certain conditions [6], greater than four alkyl groups may coordinate to uranium. In hydrocarbon or ether solvents, eqn. (1) is obviously a highly complex, heterogeneous reaction, and the course of the transformation should be critically dependent on the state and history of the UCl4. During a recent study of ‘stabilized’ actinide tetrahydrocarbyls [7], the sensitivity of reactions such as eqn. (1) to parameters involving the heterogeneity became apparent and stimulated a brief reinvestigated of our earlier work, using improved analytical techniques and a wider range of reaction conditions. We report here for two representative lithium reagents and the ‘innocent’ solvent heptane, further observations of eqn. (1) as regards optimization of RLi-derived products and, ultimately, metallic uranium. Results The principal goal of this investigation was to determine how, for constant solvent and lithium reagent, the course of eqn. (1) depends on the exact state of the UCl4 and the reaction conditions. In Table I are compiled data for the gaseous organic products of eqn. (1) as a function of these t001. Gases Evolved in the UCl4 + 4RLi Reaction. Experiment Lithium Reagent UCl4 Treatmenta Time (h) Agitation Cumulative Yield Rh + (RH)-H (%)b Butene:Butane 1 R = n-C4H9 none 153 stirring 20 0.7 ultrasound 34 105 stirring 44 52:48c 2 R = n-C4H9 grinding, SOCl2 110 stirring 48 26 ultrasound + 9 stirring 77 38 ultrasound 90 55 ultrasound 90 55:45c 3 R = t-C4H9 none 87 stirring 11 2.3 ultrasound + 33 stirring 28 3 ultrasound 30 30 stirring 30 56:44d 4 R = t-C4H9 SOCl2 90 stirring 38 1 ultrasound + 24 stirring 52 1 ultrasound + 48 stirring 60 1 ultrasound + 38 stirring 63 1 ultrasound + 24 stirring 63 62:38d 5 R = t-C4H9 grinding, SOCl2 118 stirring 58 1 ultrasound + 9 stirring 68 8 ultrasound + 37 stirring 81 21 ultrasound + 59 stirring 96 35 ultrasound + 47 stirring 98 47 ultrasound 98 60:40d a None indicates that UCl4 was employed as obtained from the procedure of ref. 9. b Estimated uncertainty: ±5%. c 1-Butene: n-butane. Estimated uncertainty in yields: ±3%. d Isobutene:isobutane. Estimated uncertainty in yields: ±3%. Full-size table Table options View in workspace Download as CSV parameters. For all experiments, the distribution of organic products (n-butane:1-butene, isobutane: isobutene) is similar to that reported in eqn. (1) for the longest reaction periods. The nature of the products is evidence that the intermediate uranium hydrocarbyls readily suffer β-hydride elimination. The yields are, however, found to be quite sensitive to the history of UCl4 and the agitation procedure. Thus, for practical reaction times, yields of butane and butene are significantly below stoichiometric if UCl4 is employed as obtained from the synthesis, and simple stirring is carried out. These low yields are due to the heterogeneous nature of the conditions and incomplete reaction. Thus, finely pulverizing the UCl4 and drying it with SOCl2 both increase the hydrocarbon yields substantially1 Furthermore, the use of the ultrasonic agitation, which is known to accelerate many types of heterogeneous reactions [8], increases the butane:butene yield to near quantitative. In the case of experiment 4, where the butane:butene yield is not quantitative, the additional equivalents of t-butyl functionality can be readily accounted for an unreacted lithium reagent.
Read full abstract