Jor
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low yield Mn(acac)3 ?
Does anyone have experience with making Mn(acac)3 (acac=acetylacetonate ligand) ?
I have done the following:
Dissolve 1,30g MnSO4.H2O in 25mL H2O, add 5ml acetylacetone and 3g sodium acetate in 15mL H2O (made by mixing acetic acid and NaOH-solution in right
proportions). Next 0,30g KMnO4 in 15mL H2O together with 3 more grams in 15mL H2O were added and the solution was warmed. The purple colour
disappeared and a brown suspension was formed. Some more KMnO4-solution was added and the colour still vanished even though I added more than the
theoretical amount. The suspension was filtered on a glas Buchner with suction, washed with water and dried on the Buchner. The dried solid weighed
1,35g, wich is not very much (only a little more than 50%). The filtrate still had a very dark color, adding solid 0,10g KMnO4 to this did not produce
more precitipate.
As indicated in my procedure the solid still has to be purified from MnO2. The solid was dissolved in boiling toluene (I need about 17-18mL total) and
filtered hot. To the brown solution was added 55mL of ligroin (wich is 100-140C pet. ether right?). This should precitipate the complex, but it did
not precitipate after 10 minutes. I will keep waiting a little more, otherwise I will start boiling away the solvent, until no more toluene is left
wich should precitipate it.
My question was, did anyone else make these complexes with similar yield? I also experienced similar yields when making the beautiful dark blue
VO(acac)2. And does it matter I substituted pet ether 40-60C for ligroin. I have hexane but don't want to use so much of it and ligroin is much
cheaper.
[Edited on 25-6-2011 by Jor]
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blogfast25
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It seems your permanganate is being reduced to MnO2, that’s quite usual in alkaline conditions. Was that on purpose?
Mn2(SO4)3 can also be obtained by dissolving MnO2 in strong H2SO4.
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Jor
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Yes probably some might be reduced to MnO2, but it shouldn't be much. Remember the solution is not or is hardly akaline and maybe even acidic when the
rxn completes because the acetate deprotonates the acetylacetone, of wich the anion is the actual ligand.
1 eq. of permanganate reacts with 4 eq. of Mn(II) to form 5 eq. of Mn(III) wich bonds to acetylacetonate-ions to form the insoluble complex. I wonder
how insoluble it is, as my solution was still highly colored after filtration:
The solution in the Buchner is the filtrate. The other filtrate in the grav. filtration is the toluene solution of the complex. A bit crowded pic
though sorry for that, pipettes , beakers and a small brown bottle of methyl iodide 
But then, why would the procedure (on a university page for students) use so much water? Maybe because it's easier to work with and they only want a
usable sample to obtain spectra from?
After 2 hours I filtered the solution of Mn(acac)3 in toluene/pet ether (100-140C), wich was much less intensely colored by now, and was left with
beautiful pure crystals of the wanted compound. I washed the crystals with some hexane to remove the high boiling solvent and left them to dry on the
filter with air sucked through them. Final yield only 0,55-0,60g  Some MnO2
remained in the filter used for filtering the hot toluene before adding pet ether, but it seems also quite some product was still in the organic
solvent after adding pet ether and filtering. The filtrate was washed with dil. H2SO4 to remove the manganese and poured in the organic waste bottle.
The crystals:
Hmm, so I used up a lot of solvent, 5mL of acetylacetone and manganese salts to make only just a little more than half a gram of the complex. Not to
talk about the ~100mL of generated organic waste (toluene,pet ether, hexane and acetone washings of used glassware)
http://i405.photobucket.com/albums/pp133/Joris12345/P1150813...
[Edited on 25-6-2011 by Jor]
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blogfast25
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Still... nice crystals!
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