Uranium borohydride

Finally, uranium borohydride could be produced by the method we had originally used (7). 

Uranium Borohydride [Uranium (III) tetrahydro-borate]. U(BH4)3 mw 28258 green pyrophoric crysts, volatile at RT mp >70° (22mm Hg) bp, explds on heating. Reacts with water and methanol. Prepn is by reacting U (IV) fluorides with an excess of A1 borohydride... 

Schlesinger H.C. Brown, Uranium Borohydride and its Alkyl Derivatives , in J J. Katz E. Rabinowitch, Eds, Chemistry of Uranium-Collected Papers , TID 5290, Book 1, USAEC, TISE (1958), 145-46 3) K.N. Semenenko... 

Phosphines. A limited number of trivalent uranium borohydride phosphine complexes have been reported. The complexes U(BFl4)3(dmpe)2 (dmpe = bis(l,2-dimethylphosphino)ethane)" and U(BH4)3(n-PPh2(C6F[4N))2" " are prepared from the reaction of U(BH4)3(THF)x and the corresponding ligand, while U(MeBFl3)3(dmpe)2" is generated when the tetravalent precursor U(BH4)4(dmpe) is heated in the presence of excess dmpe. 

X-ray crystal structure determination of the bis(pentamethylcyclopentadienyl)uranium borohydride Cp 2U(BH4)2 has been reported.57,132... 

Uranium borohydride itself, U(BH4)4 [229], and its adducts with dimethyl and diethyl ether [230], with diisopropyl ether [231], and with tetrahydrofuran [232] contain double and triple hydrogen bridges between uranium and boron. Usually the U atom achieves coordination 14, mostly with H atoms but including up to two O atoms. Polymers are common since BH4 molecules are frequently bridged to two U atoms. The average U-H bond length (from neutron difiraction) in U(BH4)4 is 2.38 A, close to that in UH3. 

These methods deal with specific cases. The list of examples is not exhaustive. The low-T (200-300°C) decomposition of the transition-metal borohydrides M(BH4> , e.g., leads to titanium, zirconium, halfnium, uranium and thorium borides . Alternatively, the uranium diboride may be obtained by reacting uranium hydride with diborane in hydrogen at 200-400°C. 

In the usual high-vacuum apparatus I placed aluminum borohydride onto a sample of uranium tetrafluoride. I observed an immediate reaction, with formation of green crystals which could be moved about the vacuum apparatus. As it happened, on the very day I was performing the synthesis, we had a visit from Professor H. C. Urey, who was in charge of the overall study. He immediately asked us to increase our research effort and undertake to prepare uranium tetraborohydride on a relatively large scale, adequate for testing. 

This was obviously a major improvement over the original procedure, which involved many more steps, some of them quite difficult. We happily reported this improved synthesis and undertook to use it to produce uranium(IV) borohydride for large-scale testing. 

Borohydrides, 13 613-621 lithium, 13 620-621 potassium, 13 620 sodium, 13 614—620 in uranium systems, 25 440 Boron (B), 4 132-138. See also Boranes Hydroboration MgB2 entries addition to aluminum wrought alloys, 2 327... 

The only examples of compounds of this type are the borohydrides, MIV(BH4)4 (MIV = Th-Pu) and MIV(MeBH3)4 (MIV = Th, U, Np). These compounds are conveniently prepared by reaction, for example, of the metal tetrafluoride with A1(BH4)3 in a sealed tube, followed by vacuum sublimation of the product.159 The neptunium and plutonium compounds are liquids at room temperature and are more volatile than the thorium, protactinium or uranium analogues. 

Uranium and thorium seldom accept monodentate phosphine ligands. However, U(III) borohydride attaches two P(C2Hs)3 or PC6H5(C2H5)2 molecules. The red-brown complexes were prepared in toluene and excess ligand and solvent removed in vacuo. Satisfactory analyses and IR spectra were reported for the first of these complexes, and detailed H, B, and P NMR confirmed the identity of both adducts (351). 

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