Nickel boride

Nickel borides are inorganic compounds with the chemical formula NixBy, where x and y vary. A common formula is Ni2B, which is available in two forms, known as P−1 and P−2. Other less common borides of nickel are NiB, Ni3B, o-Ni4B3 and m-Ni4B3 (o for orthogonal, m for metastable). Nickel borides are inorganic compounds with the chemical formula NixBy, where x and y vary. A common formula is Ni2B, which is available in two forms, known as P−1 and P−2. Other less common borides of nickel are NiB, Ni3B, o-Ni4B3 and m-Ni4B3 (o for orthogonal, m for metastable). This article focuses mainly on the most common nickel boride, Ni2B. Ni2B has been suggested to be an amorphous compound, composed of nickel bonded to individual boron centres. However it contains nanoparticles of nickel which on heating under inert conditions become more crystaline. The two forms P−1 and P−2 differ in terms of amount of their contamination by NaBO2 adsorbed on the surface. P−1 Ni2B has an oxide to boride ratio of 1:4, whereas that of P−2 Ni2B is 10:1. Their properties differ in terms of catalytic efficiency and substrate specificity. The preparation of amorphous nickel boride is simple compared with other borides which requires high temperatures, special techniques and equipment. The P−1 form of Ni2B can be generated by mixing nickel(II) sulfate and sodium borohydride in alkaline aqueous solutions. The product is not nickel boride but nanoparticles of nickel disperses in a boron compound matrix. The P−2 form is prepared similarly from nickel(II) acetate and sodium borohydride in ethanol. The product precipitates as a fine, black amorphous powder. These catalysts were usually generated in situ, which involves the use of NiCl2/NaBH4 mixture system. Nickel boride is in the form of black amorphous powder or black granules. It is insoluble in all solvents, but reacts with concentrated mineral acids. The solid is air stable. As expected for a boride, it has a high melting point. Ni2B is an efficient catalyst and reducing agent. It is used as a heterogeneous hydrogenation catalyst. The catalytic activity of P−1 is insensitive to steric hindrance of side chains on the substrate and thus more active, and seldom affects protecting groups. In contrast, P−2 is very sensitive to steric factors. For these reasons, P−1 is usually used for the complete reduction of unsaturated hydrocarbons under mild conditions, while P−2 is useful in partial reductions such as converting alkynes to alkenes in high yields: The H2/Ni2B system will not hydrogenolyse ethers, alcohols, aldehydes, amines and amides as it reduces alkenes in preference, even under forcing conditions. It leaves epoxides unaffected, but affects cyclopropanes occasionally. Most esters are stable to Ni2B, except for benzylic, allylic and propargylic esters which are cleaved by hydrogenolysis: