Nickel boride catalyst

Last updated

Nickel boride is the common name of materials composed chiefly of the elements nickel and boron that are widely used as catalysts in organic chemistry. [1] [2] Their approximate chemical composition is Ni2.5B, [3] and they are often incorrectly denoted "Ni
2B" in organic chemistry publications.

Contents

Nickel boride catalysts are typically prepared by reacting a salt of nickel with sodium borohydride. The composition and properties vary depending on the specific preparation method. The two most common forms, described and evaluated in detail by Herbert C. Brown and Charles Allan Brown in 1963, are known as P−1 nickel [4] [5] and P−2 nickel. [6] [7]

These catalysts are usually obtained as black granules (P−1) or colloidal suspensions (P−2). [8] They are air-stable, non-magnetic and non-pyrophoric, [5] but slowly react with water to form nickel hydroxide Ni(OH)
2. [3] They are insoluble in all solvents, but react with concentrated mineral acids. [1] They are claimed to be more effective hydrogenation catalysts than Raney nickel. [5]

History

These catalysts originate during World War II with the work of a research group led by Hermann I. Schlesinger, discoverer of borohydrides. They noted that reaction of NaBH
4 with salts of certain transition metals yielded black precipitates, and that the cobalt product catalyzed the decomposition of borohydride. However, their research was focused on war-related applications, and the black precipitate was not investigated further. [9]

In 1951, Raymond Paul and others investigated the reaction of NaBH
4 with nickel chloride, sulfate, and acetate in various solvents and measured their performance as hydrogenation catalysts. [10]

In 1963, H. C. Brown and Charles A. Brown reported the synthesis and performance of two similar catalysts, which they denoted by "P-1" (the same as Paul's) and "P-2", obtained by reacting sodium borohydride with nickel acetate in water and ethanol, respectively. [4] [6]

Preparation

In contrast with other borides, which require high temperatures, preparation of these nickel boride catalysts can be carried out at ambient temperature, without special equipment. [11] Indeed, they are usually generated in situ. [1]

The P−1 catalyst can be generated by reacting a nickel(II) salt, such as sulfate, chloride, nitrate, or acetate, and sodium borohydride in alkaline aqueous solutions. [5] The product precipitates as a fine, black granular powder. [5] [3] The chemistry is very similar to that of electroless nickel-boron plating, and yields hydrogen gas and the corresponding sodium salt as byproducts. [5] The borohydride must be added gradually to the nickel salt solution, not the other way around, because the product catalyzes the hydrolysis of borohydride to hydrogen and hypoborate BO−
2. [10] The catalytic activity of P-1 is enhanced by adding small amount of salts of other metals (but not cobalt) to the nickel salt during preparation. [10] Benzene however reduces its activity somewhat. [5]

The P−2 form is prepared similarly from nickel(II) acetate and sodium borohydride in ethanol. An inert atmosphere was found necessary for maximum catalytic activity. The result was an almost colloidal suspension of the black catalyst. [7] Another method uses nickel chloride NiCl2 instead of acetate. [1]

Structure and composition

The P-1 and P-2 "nickel boride" catalyst have been suggested to be amorphous compounds, composed of nickel bonded to individual boron centres. [11] However, that structure was later found to be incorrect.

An X-ray diffraction analysis of P-1 by L. Hofer and others in 1964 indicated that the nickel and boron contents were in 2.5:1 ratio, but the solid contained 11% of strongly bound water and other compounds. was amorphous when freshly prepared (with crystalline nanoparticles about 1.5 nm across), but even heating at 90 °C caused the formation of some crystalline nickel. Heating at 250 °C caused it to separate into two phases: metallic nickel, and crystalline trinickel boride Ni
3B with the cementite structure, stable at least up to 750 C. No trace of the true dinickel boride Ni
2B was seen. The authors concluded that P-1 was an intimate mixture of metallic nickel and some amorphous boron-containing compound. [3]

The true structure of these "nickel borides" was elucidated only in 2007. They consist of small grains of crystalline nickel boride embedded in an amorphous nickel matrix. [12]

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. [1]

Applications

Ni2B is an efficient catalyst and reducing agent. It is used as a heterogeneous hydrogenation catalyst.

Catalytic hydrogenation

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. [1] 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: [13]

Reduction of hex-3-yne to hex-3-ene, using Nickel boride.png

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: [1]

Hydrogenolysis of a benzylic ester by Nickel boride.png

Desulfurization

The NiCl2/NaBH4 system desulfurizes thioamides, thioethers, thioesters, thiols and sulfides. Organic sulfides, disulfides, thiols, and sulfoxides are reduced by NiCl2/NaBH4 to hydrocarbons. Illustrated is the reduction of phenothiazine to diphenylamine:

Desulfurization of phenothiazine to diphenylamine by nickel boride.png

Ni2B can also be used to cleave thioacetals. Since Ni2B is non-pyrophoric, stable in air, and give high yields in many cases, it is proposed as a safer alternative to Raney Nickel for removal of cyclic thioacetals. Desulfurization catalyzed by Ni2B proved to occur with retention of configuration by isotopic labeling. [1]

Reduction of nitrogenous groups

The NiCl2/NaBH4 system reduces aliphatic nitro groups, nitriles and oximes completely to amines. For aryl amines, nitrobenzenes are converted to anilines, and azoxybenzenes to azobenzenes. Azides are cleanly reduced to amines in preference to sterically hindered aliphatic nitro groups: [1]

Reduction of an aliphatic azide to an amine by nickel boride.png

Dehalogenation

Most organic fluorides and chlorides are unaffected by Ni2B, bromides show variable reactivity, and iodides are often completely reduced to hydrocarbons. With Ni2B in dimethylformamide, α-bromoketones are reduced to the parent ketones. Vicinal bromides are dehalogenated to alkenes:

Dehalogenation of a vic-bromide into alkene by nickel boride.png

For aryl bromides, the modified system Ni(PPh3)3Cl2/NaBH4 in dimethylformamide is used for clean debromination. Reductive cleavage of iodides occurs with retention of configuration. [1]

Safety

Nickel compounds are possible carcinogens and contact with skin should be avoided. Particular care should be taken whenever NiCl2/NaBH4 is used in dimethylformamide as sodium borohydride may spontaneously ignite in dimethylformamide.

See also

Related Research Articles

<span class="mw-page-title-main">Hydride</span> Molecule with a hydrogen bound to a more electropositive element or group

In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen atom with two electrons. The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

<span class="mw-page-title-main">Lithium aluminium hydride</span> Chemical compound

Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[AlH4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.

<span class="mw-page-title-main">Sodium borohydride</span> Chemical compound

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4. It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodium borohydride is a reducing agent that finds application in papermaking and dye industries. It is also used as a reagent in organic synthesis.

<span class="mw-page-title-main">Nickel(II) chloride</span> Chemical compound

Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.

<span class="mw-page-title-main">Raney nickel</span> Chemical compound

Raney nickel, also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium alloy. Several grades are known, of which most are gray solids. Some are pyrophoric, but most are used as air-stable slurries. Raney nickel is used as a reagent and as a catalyst in organic chemistry. It was developed in 1926 by American engineer Murray Raney for the hydrogenation of vegetable oils. Raney is a registered trademark of W. R. Grace and Company. Other major producers are Evonik and Johnson Matthey.

<span class="mw-page-title-main">Borazine</span> Boron compound

Borazine, also known as borazole, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene”. Like benzene, borazine is a colourless liquid with an aromatic odor.

<span class="mw-page-title-main">Ammonia borane</span> Chemical compound

Ammonia borane (also systematically named Ammoniotrihydroborate), also called borazane, is the chemical compound with the formula H3NBH3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source of hydrogen fuel, but is otherwise primarily of academic interest.

<span class="mw-page-title-main">Borohydride</span>

Borohydride refers to the anion [BH4]−, which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing [BH4−nXn]−, where n is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydroborate [BH3(CN)]− and triethylborohydride or triethylhydroborate [BH(CH2CH3)3]−. Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known. Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.

<span class="mw-page-title-main">Lithium borohydride</span> Chemical compound

Lithium borohydride (LiBH4) is a borohydride 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, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than lithium aluminium hydride.

The reduction of nitro compounds are chemical reactions of wide interest in organic chemistry. The conversion can be effected by many reagents. The nitro group was one of the first functional groups to be reduced. Alkyl and aryl nitro compounds behave differently. Most useful is the reduction of aryl nitro compounds.

<span class="mw-page-title-main">Sodium metaborate</span> Chemical compound

Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula NaBO2. However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is Na3B3O6 or (Na+)3[B3O6]3−. The formula can be written also as Na2O·B2O3 to highlight the relation to the main oxides of sodium and boron. The name is also applied to several hydrates whose formulas can be written NaBO2·nH2O for various values of n.

In nitrile reduction a nitrile is reduced to either an amine or an aldehyde with a suitable chemical reagent.

<span class="mw-page-title-main">Aluminium borohydride</span> Chemical compound

Aluminium borohydride, also known as aluminium tetrahydroborate, is the chemical compound with the formula Al(BH4)3. It is a volatile pyrophoric liquid which is used as a reducing agent in laboratories. Unlike most other metal–borohydrides, which are ionic structures, aluminium borohydride is a covalent compound.

<span class="mw-page-title-main">Carbonyl reduction</span> Organic reduction of any carbonyl group by a reducing agent

In organic chemistry, carbonyl reduction is the organic reduction of any carbonyl group by a reducing agent.

<span class="mw-page-title-main">Sodium triacetoxyborohydride</span> Chemical compound

Sodium triacetoxyborohydride, also known as sodium triacetoxyhydroborate, commonly abbreviated STAB, is a chemical compound with the formula Na[(CH3COO)3BH]. Like other borohydrides, it is used as a reducing agent in organic synthesis. This colourless salt is prepared by protonolysis of sodium borohydride with acetic acid:

Cobalt borides are inorganic compounds with the general formula CoxBy. The two main cobalt borides are CoB and Co2B. These are refractory materials.

Nickel compounds are chemical compounds containing the element nickel which is a member of the group 10 of the periodic table. Most compounds in the group have an oxidation state of +2. Nickel is classified as a transition metal with nickel(II) having much chemical behaviour in common with iron(II) and cobalt(II). Many salts of nickel(II) are isomorphous with salts of magnesium due to the ionic radii of the cations being almost the same. Nickel forms many coordination complexes. Nickel tetracarbonyl was the first pure metal carbonyl produced, and is unusual in its volatility. Metalloproteins containing nickel are found in biological systems.

<span class="mw-page-title-main">Dinickel boride</span> Chemical compound

Dinickel boride is a chemical compound of nickel and boron with formula Ni
2B. It is one of the borides of nickel.

Trinickel boride is a compound of nickel and boron with chemical formula Ni
3B. It is one of the borides of nickel.

References

  1. 1 2 3 4 5 6 7 8 9 10 Steven D. Burke; Rick L. Danheiser (1999). "Nickel boride". Handbook of Reagents for Organic Synthesis, Oxidizing and Reducing Agents. Wiley. p. 246. ISBN   978-0-471-97926-5.
  2. ↑ Robert A. Scott (2011). "Boron: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry. Wiley. p. 401. ISBN   9780470862100.
  3. 1 2 3 4 L. J. E. Hofer, J. F. Shultz, R. D. Panson, and R. B. Anderson (1964): "The nature of the nickel boride formed by the action of sodium borohydride on nickel salts". Inorganic Chemistry, volume 3, issue 12, pages 1783–1785. doi : 10.1021/ic50022a031
  4. 1 2 Charles A. Brown and Herbert C. Brown (1963): "The reaction of sodium borohydride with nickel acetate in aqueous solution—a convenient synthesis of a nickel hydrogenation catalyst of low isomerization tendency". Journal of the American Chemical Society (Communications to the Editor), volume 85, issue 7, pages 1003-1005. doi : 10.1021/ja00890a040
  5. 1 2 3 4 5 6 7 Charles Allan Brown (1970): "Catalytic hydrogenation. V. Reaction of sodium borohydride with aqueous nickel salts. P-1 nickel boride, a convenient, highly active nickel hydrogenation catalyst". The Journal of Organic Chemistry , volume 35, issue 6, pages 1900–1904. doi : 10.1021/jo00831a039
  6. 1 2 Herbert C. Brown and Charles A. Brown (1963): "The reaction of sodium borohydride with nickel acetate in ethanol solution: a highly selective nickel hydrogenation catalyst". Journal of the American Chemical Society (Communications to the Editor), volume 85, issue 7, pages 1005-1006. doi : 10.1021/ja00890a041
  7. 1 2 Charles Allan Brown and Vijay K. Ahuja (1973): "Catalytic hydrogenation. VI. Reaction of sodium borohydride with nickel salts in ethanol solution. P-2 Nickel, a highly convenient, new, selective hydrogenation catalyst with great sensitivity to substrate structure". Journal of Organic Chemistry, volume 38, issue 12, pages 2226–2230. doi : 10.1021/jo00952a024
  8. ↑ Chemicals & Reagents, 2008-2010
  9. ↑ Hermann Irving Schlesinger, Herbert C. Brown, A. E. Finholt, James R. Gilbreath, Henry R. Hoekstra, and Earl K. Hyde (1953): "Sodium borohydride its hydrolysis and its use as a reducing agent and in the generation of hydrogen". Journal of the American Chemical Society, volume 75, issue 1,pages 215-219. doi : 10.1021/ja01097a057
  10. 1 2 3 Raymond Paul, Paul Buisson, and Nicole Joseph (1952): "Catalytic activity of nickel borides". Industrial and Engineering Chemistry, volume 44, issue 5, pages 1006-1010. doi : 10.1021/ie50509a029
  11. 1 2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 147. ISBN   978-0-08-037941-8.
  12. ↑ Geng, J.; Jefferson, D.A.; Johnson, B.F.G. (2007). "The unusual nanostructure of nickel–boron catalyst". Chemical Communications (9): 969–971. doi:10.1039/B615529D. PMID   17311137.
  13. ↑ T. W. Graham Solomons; Craig Fryhle (2007). Organic Chemistry, 9th Edition. Wiley. p. 361. ISBN   978-0-471-68496-1.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.