{{about|the catalyst comonly called "nickel boride" or "Ni<sub>2</sub>B"|true Ni<sub>2</sub>B|[[dinickel boride]]|other compounds of nickel and boron|nickel boride (disambiguation)}}

'''Nickel boride''' is the common name of materials composed chiefly of the elements [[nickel]] and [[boron]] that are widely used as [[catalyst]]s in [[organic chemistry]].<ref name=burk1999/><ref name=scott2011/> Their approximate [[molecular formula|chemical composition]] is Ni<sub>2.5</sub>B,<ref name=hofer1964/> and they are often incorrectly denoted "'''{{chem|Ni|2|B}}'''" in organic chemistry publications.

Nickel boride catalysts are typically prepared by reacting a [[salt (chemistry)|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]] and [[Vijay K. Ahuja]] in 1963, are known as '''P−1'''<ref name=brown1970/> and '''P−2'''.<ref name=brown1973/>

These catalysts are usually obtained as black granules (P−1) or colloidal suspensions (P−2).<ref name=merck2008> They are air-stable, non-[[magnetism|magnetic]] and non-[[pyrophoricity|pyrophoric]],<ref name=brown1970/> but slowly react with water to form [[nickel hydroxide]] {{chem|Ni(OH)|2}}.<ref name=hofer1964/>  They are insoluble in all solvents, but react with concentrated [[mineral acids]].<ref name=burk1999/>  They are claimed to be more effective [[hydrogenation]] catalysts than [[Raney nickel]].<ref name=brown1970/>

==History==

These catalysts originate with the work of . However, the black precipitate was not investigated further. In 1963, [[Herbert C. Brown|H. C. Brown]] and Charles A. Brown reported the synthesis and performance of the catalyst from nickel acetate in ethanol (what would be later named "P-2").<ref name=brown1963a/><ref name=brown1963b/>

==Preparation==
In contrast with other [[boride]]s, which require high temperatures, preparation of these nickel boride catalysts can be carried out at ambient temperature, without special equipment.<ref name=green1997/> Ineded, they are usually generated ''in situ''.<ref name=burk1999/>

The P−1 catalyst can be generated by reacting a nickel(II) salt, such as [[nickel(II) sulfate|sulfate]], [[nickel(II) chloride|chloride]], [[nickel(II) nitrate|nitrate]], or [[nickel(II) acetate|acetate]], and [[sodium borohydride]] in alkaline aqueous solutions.<ref name=brown1970/>  The product precipitates as a fine, black granular powder.<ref name=brown1970/><ref name=hofer1964/>  The chemistry is very similar to that of [[electroless nickel-boron plating]], and yields [[hydrogen]] gas and the corresponding [[sodium]] salt as byproducts.<ref name=brown1970/> The catalytic activity of P-1 is enhanced by adding small amount of salts of other metals to the nickel salt during preparation.  Benzene however reduces its activity somewhat.<ref name=brown1970/>

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.<ref name=brown1973/>Another method uses [[nickel chloride]] NiCl<sub>2</sub> instead of acetate.<ref name=burk1999/>

==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.<ref name=green1997/>  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 [[nanoparticle]]s about 1.5 [[nanometre|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]] {{chem|Ni|3|B}} with the [[cementite]] structure, stable at least up to 750 C.  No trace of the true [[dinickel boride]] {{chem|Ni|2|B}} was seen. The authors concluded that P-1 was an intimate mixture of metallic nickel and some amorphous boron-containing compound.<ref name=hofer1964/>

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.<ref name=johns2007/> 

Another article claimed that the precipitate obtained by reacting {{chem|NiCl|2}} and {{chem|NaBH|4}} in water, after heating to 400 C, consisted of  {{chem|Ni|4|B|3}}.<ref name=zain2017/>

The two forms P−1 and P−2 differ in terms of amount of their contamination by NaBO<sub>2</sub> [[adsorption|adsorbed]] on the surface. P−1 Ni<sub>2</sub>B has an oxide to boride ratio of 1:4, whereas that of P−2 Ni<sub>2</sub>B is 10:1. Their properties differ in terms of catalytic efficiency and substrate specificity.<ref name=burk1999/>

==Applications==
Ni<sub>2</sub>B is an efficient [[catalyst]] and [[reducing agent]]. It is used as a [[Hydrogenation#Heterogeneous catalysts|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.<ref name=burk1999/> For these reasons, P−1 is usually used for the complete reduction of [[Saturated and unsaturated compounds|unsaturated]] [[hydrocarbons]] under mild conditions, while P−2 is useful in partial reductions such as converting [[alkyne]]s to [[alkene]]s in high yields:<ref name=solo2007/>

[[File:Reduction of hex-3-yne to hex-3-ene, using Nickel boride.png|none|550px]]

The  H<sub>2</sub>/Ni<sub>2</sub>B system will not [[hydrogenolysis|hydrogenolyse]] [[ether]]s, [[alcohol]]s, [[aldehyde]]s, [[amine]]s and [[amide]]s as it reduces alkenes in preference, even under forcing conditions. It leaves [[epoxide]]s unaffected, but affects [[cyclopropane]]s occasionally. Most [[ester]]s are stable to Ni<sub>2</sub>B, except for benzylic, allylic and propargylic esters which are cleaved by hydrogenolysis:<ref name=burk1999/>

[[File:Hydrogenolysis of a benzylic ester by Nickel boride.png|none|550px]]

===Desulfurization===

The NiCl<sub>2</sub>/NaBH<sub>4</sub> system [[desulfurization|desulfurize]]s [[thioamide]]s, [[thioether]]s, [[thioester]]s, [[thiol]]s and [[Sulfide#Organic chemistry|sulfide]]s. Organic sulfides, [[disulfide]]s, thiols, and [[sulfoxide]]s are reduced by NiCl<sub>2</sub>/NaBH<sub>4</sub> to [[hydrocarbon]]s. Illustrated is the reduction of [[phenothiazine]] to [[diphenylamine]]:

[[File:Desulfurization of phenothiazine to diphenylamine by nickel boride.png|none|550px]]

Ni<sub>2</sub>B can also be used to cleave [[thioacetal]]s. Since Ni<sub>2</sub>B is [[pyrophoric|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 [[thioacetal|cyclic thioacetals]].Desulfurization catalyzed by Ni<sub>2</sub>B proved to occur with retention of configuration by [[isotopic labeling]].<ref name=burk1999/>

===Reduction of nitrogenous groups===

The NiCl<sub>2</sub>/NaBH<sub>4</sub> system reduces aliphatic [[nitro group]]s, [[nitrile]]s and [[oxime]]s completely to [[amine]]s. For aryl amines, [[nitrobenzene]]s are converted to [[aniline]]s, and [[azoxy]]benzenes to [[azobenzene]]s. [[Azide]]s are cleanly reduced to amines in preference to [[steric effects|sterically hindered]] aliphatic nitro groups:<ref name=burk1999/>

[[File:Reduction of an aliphatic azide to an amine by nickel boride.png|none|500px]]

===Dehalogenation===

Most organic [[fluoroalkane|fluorides]] and [[chloroalkane|chlorides]] are unaffected by Ni<sub>2</sub>B, [[bromoalkane|bromides]] show variable reactivity, and [[iodoalkane|iodides]] are often completely reduced to hydrocarbons. With Ni<sub>2</sub>B in [[dimethylformamide|DMF]], α-bromoketones are reduced to the parent ketones. Vicinal bromides are [[dehalogenation|dehalogenated]] to alkenes:

[[File:Dehalogenation of a vic-bromide into alkene by nickel boride.png|none|550px]]

For aryl bromides, the modified system Ni([[triphenylphosphine|PPh<sub>3</sub>]])<sub>3</sub>Cl<sub>2</sub>/NaBH<sub>4</sub> in DMF is used for clean debromination. Reductive cleavage of iodides occurs with retention of configuration.<ref name=burk1999/>

==Safety==

Nickel compounds are possible carcinogens and contact with skin should be avoided. Particular care should be taken whenever NiCl<sub>2</sub>/NaBH<sub>4</sub> is used in [[Dimethylformamide|DMF]] as [[sodium borohydride]] may spontaneously ignite in DMF.

==See also==
*[[Cobalt boride]]
*[[Urushibara nickel]]

==References==
<references>

<ref name=brown1963a>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}}</ref>

<ref name=brown1963b>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}}</ref>

<ref name=hofer1964>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}}</ref>

<ref name=brown1970>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}}</ref>

<ref name=brown1973>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}}</ref>

<ref name=green1997>{{Greenwood&Earnshaw2nd|page=147}}</ref>

<ref name=burk1999>{{cite book|title = Handbook of Reagents for Organic Synthesis, Oxidizing and Reducing Agents| chapter = Nickel boride| author1 = Steven D. Burke| author2 = Rick L. Danheiser|authorlink2=Rick L. Danheiser| publisher = Wiley| year = 1999| isbn = 978-0-471-97926-5| page = 246}}</ref>

<ref name=johns2007>{{cite journal | title=The unusual nanostructure of nickel–boron catalyst | first1= J. | last1= Geng | first2=D.A. | last2=Jefferson | first3=B.F.G. | last3=Johnson | journal=[[Chemical Communications]] | year=2007 | pages=969-971 | doi=10.1039/B615529D}}</ref>

<ref name=solo2007>{{cite book | title = Organic Chemistry, 9th Edition | chapter = | author1 = T. W. Graham Solomons | author2 = Craig Fryhle | publisher = Wiley | year = 2007 | isbn = 978-0-471-68496-1 | page = 361 }}</ref>

<ref name=merck2008>Chemicals & Reagents, 2008-2010</ref>

<ref name=scott2011>{{cite book| title = Encyclopedia of Inorganic Chemistry| chapter = Boron: Inorganic Chemistry| author1 = Robert A. Scott| publisher = Wiley| year = 2011| isbn = 9780470862100| page = 401}}</ref>

<ref name=zain2017> Omar Ali Al-Zain, and Reem Soliyman Al-Masoudi (2017): "Nano-metal borides of cobalt, nickel and copper". ''Journal of Nanomedicine & Nanotechnology'', volume 8, issue 6, pages 477–. {{doi|10.4172/2157-7439.1000477}}</ref>

<ref name=schle1953>Herbert Irving S. H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R. Hoekstra, and E. 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, pages 215-. </ref>

<ref name=paul1952>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}}</ref>


</references>

[[Category:Borides]]
[[Category:Nickel compounds]]
[[Category:Hydrogenation catalysts]]