Uranium borohydride U₄ is a volatile uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 complex with borohydride. This green-coloured compound is polymeric in the solid but evaporates to a tetrahedral monomer

Monomer

A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

.

Preparation

This compound was first prepared by reacting uranium tetrafluoride

Uranium tetrafluoride

Uranium tetrafluoride is a green crystalline solid compound of uranium with an insignificant vapor pressure and very slight solubility in water. Uranium in its tetravalent state is very important in different technological processes...

 with aluminium borohydride

Aluminium borohydride

Aluminium borohydride, also known as aluminium tetrahydroborate, is the chemical compound with the formula Al3. It is a volatile pyrophoric liquid which is used as rocket fuel or jet fuel, or as a reducing agent in laboratories...

:

UF₄ + 2 Al(BH₄)₃ → U(BH₄)₄ + 2 Al(BH₄)F₂

It may also be prepared by the solid state reaction of uranium tetrachloride

Uranium tetrachloride

Uranium tetrachloride is compound of uranium in oxidation state +4. It was used in the electromagnetic isotope separation process of uranium enrichment. It is one of the main starting materials for organouranium chemistry.- Synthesis :...

 with lithium borohydride

Lithium borohydride

Lithium borohydride is a tetrahydroborate and known in organic synthesis as a reducing agent for esters. Although less common than the related sodium borohydride, the lithium salt offers some advantages of being highly soluble in ethers and being a stronger reducing agent but still safer to...

 in vacuo:

UCl₄ + 4 LiBH₄ → U(BH₄)₄ + 4 LiCl

Although U(BH₄)₄ is polymeric in the solid state, U(BH₃CH₃)₄ is monomeric and hence more volatile

History

During the Manhattan Project

Manhattan Project

The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...

 the need arose to find volatile compounds of uranium suitable for use in the diffusion separation

Isotope separation

Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...

 of uranium isotopes. Uranium borohydride is, after uranium hexafluoride

Uranium hexafluoride

Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...

, the most volatile compound of uranium known with a vapor pressure

Vapor pressure

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

 of 4 mmHg (533.3 Pa) at 60 °C. Uranium borohydride was discovered by Hermann Irving Schlesinger

Hermann Irving Schlesinger

Hermann Irving Schlesinger was an American inorganic chemist, working in boron chemistry.He and Herbert C. Brown discovered sodium borohydride in 1940 and both were involved in further development of borohydride chemistry....

 and Herbert C. Brown

Herbert C. Brown

Herbert Charles Brown was a chemist and Nobel Prize in Chemistry laureate for his work with organoboranes....

, who also discovered sodium borohydride.

Uranium hexafluoride is very corrosive

Corrosive

A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...

, which presented serious handling difficulties, leading to serious consideration of the borohydride. However by the time the synthesis method was finalized, the uranium hexafluoride related problems were already solved. Borohydrides are nonideal ligands for isotope separations since boron occurs naturally with two abundant isotopes, ¹⁰B (20%) and ¹¹B (80%).

Enrico Fermi

Enrico Fermi

Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

's purported comment when he observed the neutron cross section for boron—"My God! It's as big as the side of a barn!"—not only gave a name to the unit of cross section (the barn

Barn (unit)

A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...

), it also put an end to using uranium borohydride in the diffusion process.