Transition metal borohydrides

Satoh, T., Suzuki, S., Suzuki, Y., Miyaji, Y., and Imai, Z., Reduction of organic compound with sodium borohydride-transition metal salt systems reduction of organic nitrile, nitro and amide compounds to primary amines, Tetrahedron Lett., 10, 4555 4558, 1969. 

Quinoxaline derivatives give, after reduction with sodium borohydride-transition-metal salt system, e.g. nickel(II) chloride, 1,2,3,4-tetrahydroquinoxaline derivatives. ... 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

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. 

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

Borohydride, Hydridoborate, and Related Complexes of the Transition Metals 134... 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

Studies on borohydride complexes of post-transition metals are as follows. Theoretical calculations on M(BH4) (M = Cu, Ag, Au) at the ab initio level indicate that Cu(i) and Ag(i) congeners have tetrahydridoborate structures, while the gold complex adopts a hydridoborane structure HAu(BH3).37 MeZn 2 /-BH4) 30 is prepared by addition of B2H6 to ZnMe2 or MeZnCl and Li[BH4]. The X-ray structure shows a polymeric helical arrangement of ZnMe + and [BH4]- fragments—similar to Be(BH4) or H2Ga(BH4) 50.38... 

In the reduction of dienes and polyenes, combinations of a metal hydride and transition metal halides can also be used. Sodium borohydride and cobalt(II) halides were applied in the selective reduction of unsaturated carbon-carbon double bonds93. LiAlH4, in the presence of Zrlv-, TiIV- or Vlv-halides, is a selective reducing agent of dienes94,95. The following reactions were carried out with sodium borohydride and iodine (equation 28)96. 

Lippard, S.J., Ucko, D.A. 1968. Transition metal borohydride complexes. II. Th reaction of copper(I) compounds with boron hydride anions. Inorg Chem 7 1051-1058. 

Treatment of halides or sulfonates with hydride donors such as tetrabutylammonium borohydride,38 lithium aluminium hydride,39 lithium triethylborohydride46 or sodium borohydride generate deoxy sugar derivatives (Scheme 3.8c).41 When sodium borohydride is employed, a transition metal catalyst (PdCk or NiC-b) may be added. 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

General articles concerning transition metal hydrides,366 their crystallography,368 and on the preparation and properties of borohydride complexes369 are available. An overview on the use of HCo(CO)4 and related cobalt hydrides as catalysts in the hydroformylation of alkenes is available.367... 

Zinc borohydride as an example of a transition metal compound contains 8.5 wt.% H2. The decomposition temperature of about 85 °C is quite low and is associated with the simultaneous melting of the compound. Between 85 and 140 °C an endothermic decomposition starts which is accompanied by the liberation of hydrogen and significant amounts of diborane. This excludes the use of Zn(BH4)2 for solid hydrogen storage [51]. 

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