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Author: Subject: Experimental: Alternative to Al/Hg - the Al/Cu
TheCopperMan
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[*] posted on 4-5-2014 at 10:51
Experimental: Alternative to Al/Hg - the Al/Cu


References
There has been some talks before about using copper to activate Al:

- Woelens site talks about activating Al using CuSO4 and NaCl.
- Klute successfully reduced nitromethane to methylamine with a solution of CuSO4 and NaCl.
- On the hyperlab forums, it has been used for reductions of other compounds than nitromethane, but generally the results were unconclusive and/or low yielding.

Now, after lots of experimenting with this method (Lots of time, resources, and at least 40 test reactions ran), a general method has been found that produces approx. the same yields as one would expect from an Al/Hg. So far it's been ran successfully on a various set of nitrostyrenes. (including aromatic nitroalkenes)

Experimental
15g regular Al-foil (folded 4x, in flat strips, 2in long, 0.8in wide) was added into a 2-necked, 2L RBF. The Al-strips were made as flat as possible to avoid them floating to the surface later. 300ml EtOH, 120ml water, 45ml GAA was added in. Lastly 20g of nitrostyrene was added in. A condenser was mounted to the flask, along with a stirring rod attached on the 2nd neck, with a PTFE seal. Contents of the flask were heated to 60C. A solution of 10g CuSO4, 20g NaCl and 40ml hot water was mixed together separately. This was then added all at once to the 2L RBF, and the flask was immediately swirled around to ensure even distribution of the copper solution. Immediately, a vigorous reaction set in. During this, some Al floated to the surface. The stirring rod was lifted up by hand (without moving the PTFE seal) then pushed down on the Al in the flask, in order to keep the Al below the surface as best as possible. After 10 minutes or so, the reaction slowed down, and some Al was still left. Reflux was turned on for 1 hr, under overhead stirring. (If one does not have an overhead stirring motor, simply give the stirring rod an occasional swirl with your hands, to move the flask contents around. If one stirrs enough, then after 1 hrs, most of the Al should be gone.) The reaction was now finished and 300-400ml hot water was added whilst it was still at reflux temperature. It was then quickly re-rigged for distillation, and the alcohol was taken off. The solution was then cooled, ~130g NaOH added under mixing, and it was extracted 3x with DCM (or any solvent of choice). One can also do an in-situ steam distillation, then add acid of choice directly to the distilled water, and evaporate it in the open to be left with the salt. No need to worry about mercury fumes.

Some notes
• Generally, this reaction is a bit milder than the Al/Hg. So a larger surface area of Al can be used, and the reaction contents need to be heated a bit before the copper is added, to make it vigorous.
• It can be scaled up easily, and the amounts of CuSO4 / NaCl needed do not increase.
• It is efficient and environmentally friendly in terms of chemicals used. The alcohol is recycled, the amount of GAA used is small (45ml per 20g styrene), even the water can be recycled. If it's steam distilled, it's not neccessary to add anymore NaOH than what it takes to reach the correct PH, and no solvents would need to be used either. A lot of this possible, because one needs not worry about mercury which would otherwise contaminate everything.
• It is believed (although not 100% sure) that most copper will remain as an insoluble solid.
• It can probably be used for reductive aminations, but the CuSO4 and NaCl would possibly react with methylamine. Use of CuCl2 in place of CuSO4 and NaCl is possible.

Lastly, it should be noted that the reaction seems very tolerant and consistently produces the same yields, despite large changes in variables. For instance, in the beginning, this reaction was ran with 20g styrene in 250ml water, 250ml GAA, 250ml EtOH in an open beaker, with 50g CuSO4 and 100g NaCl dissolved in. This reaction was a mess, and there was a large amount of green precipitate at the bottom even before it began. Then 5g Al foil at a time, packed together by hand, was held into the reaction with a metal tool. The aluminum was eaten up with vengance, and the solution was boiling hard and solvent actively escaped into the open, and had to be replaced constantly. Nontheless, a total of 40g Al was consumed. And even here, yields were the same (~70%) as with the above reaction.

[Edited on 4-5-2014 by TheCopperMan]

[Edited on 4-5-2014 by TheCopperMan]

[Edited on 4-5-2014 by TheCopperMan]
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[*] posted on 4-5-2014 at 11:45


Welcome.

That is a very nice write-up but to the uninitiated like me it isn't clear what you've produced?




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[*] posted on 4-5-2014 at 12:09


Very interesting, I have a few questions:
Exactly what are all the reactions happening here?
Excuse my apparent ignorance, but what is GAA?
What is the main reaction product? You just said reduction of aluminum, (Al → Al3- + 3e-) what is the anion?



[Edited on 4-5-2014 by Zyklonb]




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[*] posted on 4-5-2014 at 12:44


Quote: Originally posted by Zyklonb  
Very interesting, I have a few questions:
Exactly what are all the reactions happening here?
Excuse my apparent ignorance, but what is GAA?
What is the main reaction product? You just said reduction of aluminum, (Al → Al3- + 3e-) what is the anion?



[Edited on 4-5-2014 by Zyklonb]

Please see the reference to Woelens site, where he explains the reactions between aluminum and the tetrachlorocuprate.

In short:

2Al(s) + 3[CuCl4]2-(aq) → 2[AlCl4]–(aq) + 4Cl–(aq) + 3Cu(s)

2Al(s) + 6H2O(aq) → 2Al(OH)3(s) + 3H2(g)

The exact mechanism isn't too well understood. It seems that aluminum and copper also works as an electrolytic couple, which oxidizes the aluminum and produces hydrogen from the water at the copper site.


GAA is glacial acetic acid.


As for the last one, the main text has been corrected. Aluminum was not reduced obviously, but consumed in the reaction...


Quote: Originally posted by blogfast25  
Welcome.

That is a very nice write-up but to the uninitiated like me it isn't clear what you've produced?


Amines, following the reduction from various nitrostyrenes...

[Edited on 4-5-2014 by TheCopperMan]

[Edited on 4-5-2014 by TheCopperMan]
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[*] posted on 4-5-2014 at 13:18


What's the reduction half reaction of the nitrate?

R-NO<sub>2</sub> + 6 H<sup>+</sup> + 6 e<sup>-</sup> === > R-NH<sub>2</sub> + 2 H<sub>2</sub>O

... by any chance?




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[*] posted on 4-5-2014 at 13:58


blogfast25: Think so

Al oxidation half-reaction:
2Al ------> 2Al3+ + 6e-


Also there might be reactions between any unreacted CuSO4 and the activated Al itself:

2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
Ox: Al(0) → Al(2+) + 2e-
Red: Cu(2+) + 2e- → Cu(0)


Also in solution, Na2SO4 formed from the reaction between CuSO4 and NaCl.

Not sure if CuCl2 and CuCl3- might also be present.


To be fair, these reactions between copper salts and aluminum have always been confusing...there is a lot going on.

[Edited on 4-5-2014 by TheCopperMan]
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[*] posted on 5-5-2014 at 02:32


Forgive my ignorance, and the noobishness of the question, but what is the purpose of the glacial acetic acid? I've seen it used before in reactions, but I can never understand its purpose. Is acting as a solvent? If so, why ethanol and glacial acetic acid?



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[*] posted on 5-5-2014 at 04:20


By nitrostyrene, must we understand 2-nitrovinylbenzene?

Did you characterise your product (presumably 2-aminovinylbenzene)?

Have you tried other reductions with this tetrachlorocuprate/Al reducing agent?

[Edited on 5-5-2014 by blogfast25]




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[*] posted on 5-5-2014 at 04:47


I would think it would be reduced all the way to 2-phenylethylamine.

Wouldn't 2-aminovinylbenzene tautomerize to the imine?




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[*] posted on 6-5-2014 at 09:50


Thank you for you contribution. Besides the reports in the long thread at the Hyperlab, I think this is the only other such report confirming this system is able to reduce beta-nitrostyrenes.
Quote: Originally posted by TheCopperMan  

• It can be scaled up easily, and the amounts of CuSO4 / NaCl needed do not increase.

In the experimental example, you used a huge and over-stoichiometric amount of CuSO4 (pentahydrate, I assume). Here you imply that this amount could be reduced (without affecting yields?). Did you evaluate the effect of the CuSO4 loading during your experiments?
In the reductions of nitrostyrenes with Hg activated aluminium, reducing the amount of Hg(MeCOO)2 or other Hg salts (e.g., <15 mg / g Al) can result in the nitrostyrenes or oximes not being effectively reduced all the way to the amines (losing yields and build up of the N-hydroxyamines). Did you observe any such lower limit for Cu(II) that would dramatically affect yields and purity?
Quote:
Lastly, it should be noted that the reaction seems very tolerant and consistently produces the same yields, despite large changes in variables.

Did you try different substrates, ring substituted beta-nitrostyrenes, beta-substituted nitrostyrenes, oximes, or others? In what range were the yields?

I'm moving this thread to the Organic chemistry section. Though the main issue is about an alternative aluminium surface activation, the discussion is about a new organic synthetic method.




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[*] posted on 6-5-2014 at 14:08


Quote: Originally posted by blogfast25  
By nitrostyrene, must we understand 2-nitrovinylbenzene?

I'm assuming it's actually a phenethylamine product, possibly an amphetamine. Maybe it's because I get paid to think about this kind of thing all day, but it seems very similar to some clandestine writeups with a new catalyst.
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[*] posted on 6-5-2014 at 18:45


Sorry for the late replies,

Regarding the exact needed amounts of CuSO4, it has been researched a bit but no sure conclusion yet. What is interesting is that after the initial reaction is over, if one is to add more CuSO4 / NaCl solution, this will not speed up the reaction substantially. It was at some points tried adding at least 10g more CuSO4 and 20g more NaCl, after the initial reaction with 10g CuSO4 / 20g NaCl, and this did not change the speed at all (okay maybe a tiny bit, but nothing compared to the speed of the initial reaction).

However, the initial amounts added seem to matter to the initial speed. It was tried adding as little as 2g CuSO4 / 4g NaCl one time, and the initial speed was substantially less than from 10g CuSo4 / 20g NaCl. If more is added initially, this seems to make it more vigorous.

As for scaling, the only thing certain is that the amnt. of CuSO4 / NaCl needed do not increase proportionaly to scale. A 10x scale was ran at least once, and it was was ran with around 25g CuSO4 / 50g NaCl. It created a dangerous situation where BOTH the ice-cooled Allihn condensers attached clogged with alcohol halfway up. Here, the amounts of copper solution could preferrably be reduced, or perhaps the initial temperature of the reaction could be reduced.


Regarding substrates, the entire list won't be posted. Nitroalkanes (from NaBH4 C=C red) were reduced. Aromatic nitroalkenes were reduced. At least one oxime of an aromatic nitroalkene was reduced successfully. (Oxime prepared previously with SnCl2 in ethyl acetate). Substituted, yes. The bottom line is, everything that an Al/Hg is expected to go work for, the Al/Cu worked for in similar or slightly lower yields.

An interesting sidenote to this, is that SnCl2 was also tried as an aluminum activating agent (SnCl2 has very good solubility in EtOH unlike CuSO4) and in fact it works. It was tried for only one experiment, which did indeed produce an amine. However the amounts needed seem to be much greater than with the copper. In the future, gallium chlorides will be tried as well to activate the aluminum.





[Edited on 7-5-2014 by TheCopperMan]

[Edited on 7-5-2014 by TheCopperMan]

[Edited on 7-5-2014 by TheCopperMan]
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[*] posted on 6-5-2014 at 20:34


I'd expect copper(II) chloride to be a bit more soluble in alcohol than the sulphate- perhaps that would work.

I'm not much into organic chemistry; would this work for reducing aldehydes to alcohols?




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[*] posted on 6-5-2014 at 23:41


Quote: Originally posted by DraconicAcid  

I'm not much into organic chemistry; would this work for reducing aldehydes to alcohols?

I would expect it to as Al/Hg can produce alcohols and pinacols from aldehydes and ketones, respectively.
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[*] posted on 7-5-2014 at 00:41


Quote: Originally posted by DraconicAcid  
I'd expect copper(II) chloride to be a bit more soluble in alcohol than the sulphate- perhaps that would work.

I'm not much into organic chemistry; would this work for reducing aldehydes to alcohols?


The CuSO4 and NaCl already produces copper chlorides though. Unsure if it produces just [CuCl4]2- or also CuCl3- and CuCl2. It seems these complexes exist in an equilibrum according to:

CuCl2 + Cl− <--> CuCl3-
CuCl2 + 2Cl- <--> [CuCl4]2-
Source

Unsure of the solubility of CuCl3- and [CuCl4]2- in alcohol, but it seems these are formed either way also when using just CuCl2 instead of CuSO4 and NaCl.

Copper(II) chloride would still be the best choice, at least for reductive aminations, because the less salts in solution the better or methylamine might get salted out.
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[*] posted on 7-5-2014 at 05:49


Quote: Originally posted by Chemosynthesis  
Quote: Originally posted by DraconicAcid  

I'm not much into organic chemistry; would this work for reducing aldehydes to alcohols?

I would expect it to as Al/Hg can produce alcohols and pinacols from aldehydes and ketones, respectively.

Aluminum amalgam has been used sporadically to reduce aldehydes and ketones, but it has a relatively narrow scope. Many ketones are quite inert toward it while some give almost exclusively the pinacol coupling product.
A good study on this topic is DOI: 10.1016/S0040-4039(00)80139-2. It was found that simple aliphatic ketones do not reduce significantly (only few % conversions), while 4-7 membered cycloaliphatic ketones do reduce as long as there is no significant steric hindrance at the carbonyl group. Unhindered cyclohexanones are reduced with best yields. Aromatic ketones and benzaldehydes give pinacols only and in excellent yields. One aliphatic aldehyde was tried (n-heptanal) and it reduced with only 20% yield.

Of course, it would be a nice scientific contribution, if someone applied this Al-Cu system on ketones or aldehydes. It might be quite suitable for pinacol couplings.

I should add that all this is quite new and not yet described in the scientific literature - an excellent achievement by the amateur chemistry community and is now becoming a combined Hyperlab-Sciencemadness effort.




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[*] posted on 7-5-2014 at 11:05


Quote: Originally posted by Nicodem  

I should add that all this is quite new and not yet described in the scientific literature - an excellent achievement by the amateur chemistry community and is now becoming a combined Hyperlab-Sciencemadness effort.

Really fascinating. If anyone has spare time and interest, they may track down citations 82-84 of Organic Electrochemistry, Fourth Edition edited by Ole Hammerich and Henning Lund, page 1161. They appear relevant.
It was reviewed here: J. Am. Chem. Soc., 2001, 123 (36), pp 8880–8880 DOI: 10.1021/ja015214a
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[*] posted on 11-5-2014 at 14:42


Perhaps using this method one can produce aluminium isopropoxide for the Meerwein-Ponndorf-Verley reduction from the activated aluminium and isopropanol. CuCl2 can be used instead CuSO4+NaCl and is more soluble in alcohols.
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[*] posted on 12-5-2014 at 12:48


Quote: Originally posted by Chemosynthesis  
Quote: Originally posted by blogfast25  
By nitrostyrene, must we understand 2-nitrovinylbenzene?

I'm assuming it's actually a phenethylamine product, possibly an amphetamine. Maybe it's because I get paid to think about this kind of thing all day, but it seems very similar to some clandestine writeups with a new catalyst.


I had the same thought. Then I looked at the number of posts and saw only 5 and all about the same thing. Interesting. Didn't the beta-nitrostyrene reduction and the nitromethane to methylamine reduction in the same post seem funny? Once you have the beta nitrostyrene you don't need MeNH2 unless you were going to go back to phenylacetone (from the styrene) first? Of course, depends if you want the monomethyl or are happy with the plain amine. Or, just maybe he is after various substituted phenethylamines? Maybe he's the new Shulgin? There's talent there for sure.

[Edited on 12-5-2014 by Dan Vizine]
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[*] posted on 12-5-2014 at 16:20


A little update, the same exact reaction was ran as in the original post, but this time using 1.5g CuCl2 instead. That was the only changing variable. It gave the exact same results as with the 10g CuSO4 and 20g NaCl, in fact it seemed to react even more vigorous and Al dissolved more easily. It's possible the amount of CuCl2 can be reduced even further. Note that this has only been tried for one reaction, but so far yields are the same as with the CuSO4 and NaCl. I see no reason why CuCl2 shouldn't produce the same results on all substrates.

CuCl2 also has very good alcohol solubility, so it's fully possible that one can use regular Al/Hg solvent ratios and writeups, and in them, simply just substitute CuCl2 for HgCl2 and get the same results.

[Edited on 13-5-2014 by TheCopperMan]

[Edited on 13-5-2014 by TheCopperMan]
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[*] posted on 13-5-2014 at 13:43


Something interesting to read:
Reductive Ring Cleavage of Nonconjugated Δ²-Isoxazolines to β-Hydroxy Ketones with Aluminum and Copper(II) Chloride
Very a propos :)



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[*] posted on 14-5-2014 at 04:58


Would this system (Al/Cu) stand even the remotest chance of reducing phenol to benzene? My guess, based on the bond strength of C-O, would be 'no'. In the Clemmensen reduction Zn/Hg reduces ketones to alkanes but the C=O bond is weaker than the C-O bond.



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[*] posted on 14-5-2014 at 07:47


Usually to do that you have to turn the phenol into an aryl triflate, oxidatively add it onto a metal, and throw a proton source at it.

Nice find kmno4. I'd like to point out (for those who only skim articles) that they randomly include two examples of reduction of nitroarenes at the end and report good yields.

This is a method that deserves further investigation. The biggest gap in the amateur chemist's arsenal has long been access to selective reducers, ones that won't convert nearby lakes into toxic dumps as mercury so easily can.

Have you tried running this reaction in neutral media, as they did in the SynLett posted by kmno4? (Or gradually adding AcOH merely to maintain acidic pH). Extractions are (obviously) easier with less AcOH to deal with. Might not be such a bad idea either to quench with aqueous ammonia, to displace any of your product that may be coordinating to copper and complicating extraction.

Quote: Originally posted by Jesse Pinkman  
Perhaps using this method one can produce aluminium isopropoxide for the Meerwein-Ponndorf-Verley reduction from the activated aluminium and isopropanol. CuCl2 can be used instead CuSO4+NaCl and is more soluble in alcohols.


Definitely worth a shot.

Regarding CuCl<sub>2</sub>, this is trivial to prepare: CaCl<sub>2</sub> is available in 50lb bags for melting ice, and CaSO<sub>4</sub> is almost completely insoluble in water. Hence, all you have to do is mix equamolar quantities in aqueous solution and filter out the precipitate, and you will have a solution of CuCl<sub>2</sub>.

CuSO4 + CaCl<sub>2</sub> ----> CuCl<sub>2</sub> + CaSO<sub>4</sub> (insoluble)

[Edited on 14-5-2014 by madscientist]




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[*] posted on 14-5-2014 at 10:00


A bit off topic, but would the Al/Cu approach be an environmentally friendly method for generating LARGE quanties of H2 gas?
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[*] posted on 14-5-2014 at 10:32


Better than many of the alternatives, that's for sure. But don't let any copper salts go down the drain - they're rather toxic.

Aluminum sulfate is a lot less soluble in alcohols than aluminum chloride, so that's another possible answer to why chloride seems to be needed.

I'm not convinced aluminum is actually effecting the reduction directly - it may be, but there's reason to believe otherwise. I think it would be worth looking at a few other metals, such as cobalt and nickel - looking at the SI from the third reference below suggests they may prove to be superior. It's also worth considering running these reactions in straight alcohol, or even aprotic solvents such toluene with the alcoholic proton source being added gradually.

I'm wondering if an alternative reagent could be used to generate copper nanoparticles - maybe ascorbic acid would do the trick? It's certainly effective at generating copper colloids (which soon begin dumping out precipitate). And of course, it's probably worth trying Zn or Mg dust as well.

The first paper attached reports reduction of a lot of functional groups with Cu(II) + NaBH<sub>4</sub> systems that don't occur easily with NaBH<sub>4</sub> alone. Hence my thoughts on this matter. Nitriles, esters, nitro groups can all be reduced (they make phenethylamine, for example). Amides seem to be inert.

Highly Chemoselective Reduction of Aromatic Nitro Compounds by Copper Nanoparticles/Ammonium Formate
A highly chemoselective reduction of aromatic nitro compounds to the corresponding amino derivatives has been achieved by a combination of copper nanoparticles and ammonium formate in ethylene glycol at 120 °C. The reductions are successfully carried out in presence of a wide variety of other reducible functional groups in the molecule, such as Cl, I, OCH2Ph, NHCH2Ph, COR, COOR, CN, etc. The reactions are very clean and high yielding.
http://pubs.acs.org/doi/abs/10.1021/jo800863m?journalCode=jo...

Highly Chemo- and Regioselective Reduction of Aromatic Nitro Compounds Catalyzed by Recyclable Copper(II) as well as Cobalt(II) Phthalocyanines
Copper/cobalt phthalocyanines were established for the first time as catalysts for the very efficient chemo- and regioselective reduction of aromatic nitro compounds to generate the corresponding amines. The selective reduction of nitro compounds was observed in the presence of a large range of functional groups such as aldehyde, keto, acid, amide, ester, halogen, lactone, nitrile and heterocyclic functional groups. Furthermore, the present method was found to be highly regioselective towards the reduction of aromatic dinitro compounds in a short time with high yields. In most of the cases the conversion and selectivity were >99% as monitored by GC-MS. The reduction mechanism was elucidated by UV-vis and electrospray ionization quadrupole time-of-flight tandem mass spectrometry.
http://onlinelibrary.wiley.com/doi/10.1002/adsc.201000191/ab...

Room-Temperature Chemoselective Reduction of Nitro Groups Using Non-noble Metal Nanocatalysts in Water
Purely aqueous-phase chemoselective reduction of a wide range of aromatic and aliphatic nitro substrates to the respective amines has been achieved in the presence of inexpensive Ni and Co metal nanocatalysts, with a high tolerance to other highly reducible groups present in close proximity to the targeted nitro groups, using hydrous hydrazine as a reducing agent at room temperature.
http://pubs.acs.org/doi/abs/10.1021/ic402674z

Preparation of fine Ni powders from nickel hydrazine complex
Fine nickel powders with narrow size distribution have been prepared from the reduction of nickel hydrazine complexes in aqueous solution. The pure nickel hydrazine complexes, [Ni(N2H4)3]Cl2 were prepared with the molar ratio of N2H4/Ni2+ = 4.5, while a mixture of complexes, such as Ni(N2H4)2Cl2, [Ni(N2H4)3]Cl2, and [Ni(NH3)6]Cl2 were formed with N2H4/Ni2+ < 4.5. By the X-ray diffraction (XRD), FT-IR, and scanning electron microscopy (SEM) analyses, it was found that the reduction of Ni2+ to metallic Ni powder proceeded via the formation of nickel hydroxide which was reduced by hydrazine liberated from the ligand exchange reaction between the nickel hydrazine complex and NaOH. The standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex while the mean particle size increased. As the amount of hydrazine increased, the surface roughness of the particles was improved significantly due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle. It was found that average particle size could be controlled from 150 to 380 nm by adjusting the reaction molar ratio and temperature.
http://www.sciencedirect.com/science/article/pii/S0254058405...

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[Edited on 14-5-2014 by madscientist]




I weep at the sight of flaming acetic anhydride.
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