tsathoggua1
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Storage of nickel boride Ni2B and Ni2B-NaBH4 reductions
Of late, I've been experimenting a little with nickel boride, of the nominal stoichiometry Nii2B , of the P2 type (there are two variants of this
catalyst, Ni2B-P1 and Ni2B-P2, the former being prepared in aqueous media the latter in ethanolic media, although I've been using methanol for cost
reasons and the methanolic P2 type catalyst seems alright thus far)
It is prepared by addition of borohydride solution (either lithium or sodium borohydride, although cyanoborohydride cannot be used, uncertain about
STAB) to a solution of a nickel (II) salt, and it appears to be a very easily prepared hydrogenation catalyst, desulffurization catalyst, capable of
some dehalogenations, hydrogenolysis and also reduces both aliphatic and aromatic nitros, in some cases partial reduction of alkynes to alkenes and
partial reductions of cyclohexadienes, cyclopentadienes etc. reduction of alkenes to saturated hydrocarbons and de-chalcogenation of phenylselenides
and phenyltellurides in the presence of sulfide groups. For reduction of nitro groups at least, further borohydride is required, in a Ni2B-NaBH4
system, which runs at atmospheric pressure or with a balloon taped over the mouth of the flask. Oximes are reportedly reduced to amines in excellent
yields, typically 80-90%, sometimes even quantitatively.
Apparently the Ni (II) salt used has some effect on selectivity although I haven't as of yet had chance to probe this particular aspect of these
catalysts, I am just beginning in my studies of them. Nickel (II) acetate is reportedly, of several salts tested, the most acceptable with regards to
quality of the produced catalyst.
I've been experimenting so far with the P2 type, which has more borate adsorbed onto the surface of the nickel boride particles. Formation of the
catalyst on addition of 1M borohydride in MeOH to a solution of nickel (II) acetate is rapid, despite the paper that originally inspired me to start
tinkering with Ni2B, the authors stated that addition would take a half hour due to foaming, using EtOH, and that the less polar iPA produces an
inferior catalyst. Although less active could also mean more selective. Need to get more iPA and perhaps test it with some tertiary alcohols as well.
Addition of NaBH4 in MeOH (not anhydrous, but very dry, 99.9something %, not about to walk back inside through the pissing rain and get soaked to read
that last .01 of a % ) results in a vigorous
but not violent evolution of hydrogen gas, and the borohydride must be added portion-wise.
The result is a very fine precipitate (some papers also use sonication, however I lack the equipment to do so, although P2 nickel boride w/
borohydride seems to reduce aliphatic nitro groups judging by the rapid discharge of yellow coloration of a nitroalkene. This needs working up and a
sample testing with dilute iodine/KI solution to test for discharge of the color of the halogen and determine the presence or absence of the double
bond (in the case of the substrate in question, the nitro group and the double bond are on adjacent carbons, which may be important, particularly with
type P2 nickel boride since P2 Ni2B is reportedly quite sensitive both to steric constraints and the nature of the alkene.
Anybody else interested in this easy to prepare, really rather interesting looking catalytic reduction system? since of the hydride reducing agents,
borohydride is probably the easiest of all to obtain, this could be of use to many here.
What I'd like to know, is how re-usable a given quantity of catalyst is, before it begins to lose catalytic activity, whether or not when/if it does
so, it can be re-activated by any means, risk of evolution of borane or diborane gas on exposure to acid during workup since the particle size is
extremely fine when prepared in MeOH, the selectivity of Ni2B prepared not in alcohol alone or H2O alone but 50-50, the effects of temperature and
stoichiometry on the activity and selectivity of the catalysts and, whether or not they must be freshly prepared or if the catalyst can be prepared,
permitted to settle out, the solvent decanted and if washing the catalyst with H2O or/and C1-C4 alcohols has any effect on activity. And if it can be
prepared, dried in the vacuum dessicator and stored long term, and if so, how long is 'long term'?
Also, if storable, aside from just the shelf life of the reagent, whats the best way to store it? under alcohol or H2O, or as dried powder?
reportedly, despite at least in the case of P1 type nickel boride its being a more powerful, and more rapid catalytic reducing agent than some raney
nickels (W2 is stated where this reference is made, and specifically in relation to time course of reduction of H2C=CH2 to CH3-CH3) apparently these
boride catalysts are non-pyrophoric.
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Melgar
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Posts: 2004
Registered: 23-2-2010
Location: Connecticut
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If I remember correctly, nickel boride loses catalytic activity over time, so it's usually prepared in situ. Plus, it's pretty cheap and easy to
prepare.
If you keep using methanol as your solvent, you're going to burn through your sodium borohydride a lot faster. Try a 2:1 ratio of isopropanol to
water, or just use 70% isopropanol directly for some of your reactions. Sodium borohydride works perfectly fine in the presence of water, and isn't
decomposed as readily by water as it is methanol.
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