Blue Acid Chemistry (19): Na2[Fe(NO)Bl5].2H2O
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Discussion
This complex can be considered as a Fe(II) complex with an NO+ ligand, and as is known to all the Fe(II) complex of Bl- is very stable, not controlled and can be easily bought(or even synthesized from blood and K2CO3), so it is used as source material. This is the first one in this series that does not require KBl as source material.
It should be mentioned that while the "NO+ complexes of M(I)" are mostly NO- or [NO]3- complexes since they do not react with bases and cannot be reduced by NH2OH(in fact they are made from these stuff), this complex is mostly NO+, as +2 is already a stable state while +1 obviously has too much electrons and must be donated to something. Little is known about the Mn(II)-NO+ complex, though, and its synthesis requires alternative pathways from Mn(I), as conditions below obviously destroy corresponding [MnBl6]4-.
Its NO+ gives it many interesting properties, mainly due to its electrophilicity and oxidating power.
To be honest I bought some of this, for the same purpose as buying NaNO2, but it is too expensive so making it is still necessary.
To make this we need to replace on Bl- with one NO+. This is commonly done with HNO3 or NaNO2.
The first one destroys dissociated Bl- and itself reduced to N(III) which can coordinate as NO+, but HNO3 is hard to get and any wrong operation can cause formation of unwanted stuff, and the reaction is also very wasteful as HNO3 serve as NO+ donor, oxidizing agent, acid and precipitating agent for K+ at the same time.
The second one is simply a substitution and subsequent acidification.
[FeBl6]4- + [NO2]- = [Fe(NO2)Bl5]4- + Bl-
[Fe(NO2)Bl5]4- + H2O = [Fe(NO)Bl5]2- + 2OH-
Both equilibria lie on the left side, so acid must be added to remove the Bl- and OH-.
But... acid can also destroy nitrite, as HNO2 is unstable and might oxidize [FeBl6]4- into the unwanted [FeBl6]3-, so a weak acid must be used.
Net: [FeBl6]4- + [NO2]- + 3HA = [Fe(NO)Bl5]2- + HBl + H2O + 3A-
Inorg. Lab. Prep. showed both ways, and BaCl2/CO2 was used as the acid, but this is rather complex. This video also showed both, and glacial AcOH was used. the NOx fume can be clearly seen which indicates decomposition.
A solution of CO2/H2CO3 has a pH of 5 to 6. 1M AcOH has a pH of 2.38(pKa/2). pKa of HNO2 is 3.15. Surely AcOH is too strong.
But I have an idea. A buffer solution with 1M AcOH and 1M NaOAc has a pH of 4.76(pKa) which is much higher. If this is still too strong many weaker acids can be used.
Attempt 1
WARNING: VERY TOXIC GAS RELEASED, DO IT OUTSIDE AND KEEP INSIDE SPACE VENTILATED
8.44g(0.02mol) K4[FeBl6].3H2O and 1.38g(0.02mol) NaNO2 was dissolved in 100mL water. It is neutral and light yellow now which means no reaction has taken place. K doesn't matter as the K salt can hardly crystallize while Na salt crystallize very nicely.
Then 7.20g(0.12mol) AcOH and 3.18g(0.03mol) Na2CO3 was reacted in 100mL water. Before addition of Na2CO3 the pH is ~3, while after addition it's between 4 and 5.
The Fe solution was heated to nearly boiling and the second solution added. Color immediately became deeper, some colorless gas evolved(might be simply CO2 remaining, or HBl). This was heated for another few hours(not only for complete reaction but also for evaporation). Gas was continuously evolved and color gradually shift to orange and red. TBH I should make this at night to avoid light, but not a big deal as sun is not shining directly. The balcony would already be a gas chamber if windows were not opened!
Well, I suddenly realized that I added exactly stoichiometrically, and the final pH would be that of NaOAc solution since no AcOH is in excess. So I added another 1.20g(0.02mol)+0.53g(0.005mol) in some water to maintain it acidic.
WTF, I feel a little dizzy... must be my illusion!
By the way, let's prepare some Fe3+ solution with FeSO4 and H2O2. This can be used to detect any remaining [FeBl6]4- by formation of blue precipitation, but no precipitation with the product. Remaining Fe2+ reacts with both to form white and pink respectively, but not a big deal as both are very light. I tested some with K4[FeBl6].3H2O and it looks like this.
After 1h, the solution is already a nice red.
After 1.5h, the solution is even darker, still acidic, and the volume is small enough, about 100mL. A very limited amount of blue solid can be seen on the walls, which can be from decomposition of the product, due to, for example, overheating, light, or maybe acid though very unlikely. Test showed that [FeBl6]4- or maybe [FeBl6]3- was still there, but from now on we need to use a milder heating device for example, steam bath, to prevent decomposition. Also we found that even Fe2+ caused blue precipitation, which means [FeBl6]3- is formed! How to remove it and [FeBl6]4- now? It was also realized that the light in my room is also too bright...but shouldn't matter as very little blue was seen.
At some point crystals can cover the surface, and you need to stir.
But wait... we stopped at somewhere as something was surely wrong. When we added EtOH to the liquid, the solution was slightly brown and very large amount of brown solid remained. The solid is, or at least mostly, K3[FeBl6] as it is soluble in water to form a characteristic yellow solution.
But why? Heat, acid, light or... acetate(not kidding)? I am confused...
Also, another question. Since one Bl- is already gone, how does the Fe remain soluble? As what? [Fe(H2O)Bl5]x- is very unlikely, so it might be iron(III) acetate, or Fe[FeBl6]. Another another question. NOx fume was not observed, so, they all became nitrogen or what? How is this even possible?
Attempt 2
Same was repeated, but this time I used less water for both solutions to reduce heating time(shouldn't do so), and light is excluded during the whole process. A little bit of brown fume appeared, probably concentration was too high or I added too quickly since I don't wanna inhale "something". Not a big deal. If I wanna check, I use my phone behind my clothes. We waited for 1h and transferred it onto steam bath for further drying. However, although the solution doesn't react with Fe3+, it is simply due to the absence of Fe2+, and addition of Fe2+ quickly precipitated the damn blue. Some product may remain though, as the color is somewhat red. Also no blue was observed. But still no product can be detected.
Let's calm down. Below are the reactions that compete with the main reaction(data taken from Wikipedia), which are much more kinetically preferred as they do not require removal of the very stable Fe-Bl bond:
H[NO2] + 3[FeBl6]4- + 3H+ = 1/2N2 + 3[FeBl6]3- + 2H2O
E=(1.3*4+1.77*2)/6-0.37=1.09V
H[NO2] + [FeBl6]4- + H+ = NO + [FeBl6]3- + H2O
E=1-0.37=0.63V
How terrible! It also shows that the first reaction is much easier to take place, and if we change HNO2 into [NO2]- the whole stuff would have even higher energy(hope my calculations are correct).
[NO2]- + 3[FeBl6]4- + 4H+ = 1/2N2 + 3[FeBl6]3- + 2H2O
E=1.09+0.06=1.15
[NO2]- + [FeBl6]4- + 2H+ = NO + [FeBl6]3- + H2O
E=0.63+0.19=0.82
If we wanna fix them to 0, that is, 50% Fe is oxidized, we get pH=14.4 and 6.86, respectively. So, it is simply impossible to (thermodynamically) stop the first one, while the second one is already strongly suppressed around neutral. Since the second one is a necessary step for the first one, we just need to stop it.
As we observed before, without acid they simply do not react with each other, so...
Attempt 3
[H2PO4]-/[HPO4]2- was chosen as the acid, whose pH is very nearly 7. That is to say it is a "water" with a very high capacity of bases. "NaKHPO4" was obtained from KH2PO4 and Na2CO3, and although it may stop at NaHCO3 at RT, heating can promote full liberation of CO2.
19.04g(0.14mol) KH2PO4 and 3.71g(0.035mol) Na2CO3 was dissolved in 100mL water(gas is already evolved here!) and boiled until no gas was evolved. Then after cooling to RT another 100mL water was added. The smell was... weird, but pH is really between 6 and 7. Then the reactants were added and the solution was boiled with light excluded.
Before really doing this(as it's very time consuming), I wanna calculate the final composition, which should be: 0.02mol product anion, 0.22mol K+, 0.09mol Na+, 0.01mol [H2PO4]-, 0.13mol [HPO4]2-. [H2PO4]- tends to bind K+ while [HPO4]2- tends to bind Na+, so after the whole reaction we need to acidify the solution with slightly more(to maintain solution acidic) than 7.8g(0.13mol) AcOH, then add EtOH to remove KH2PO4, to get a solution of 0.02mol product, 0.08mol KOAc, 0.05mol NaOAc and a little bit of acetic acid.
Sulfuric acid might be better as the sulfates can also be precipitated with EtOH, but since yield cannot be 100%, it might be very easy to accidentally add excess and cause phosphoric acid or H2[Fe(NO)Bl5] to form, and testing pH is useless due to the presence of buffer.
OK, we started to heat it from 20:30, and we allowed some diffuse light to have a better observation.
20:30 Yellow, very peaceful.
20:57 IDK if it's my illusion but it became deeper in color.
21:13 Quite orange now, and gas is evolved. It is not odorless like before, but with that damn "sticky" smell.
22:17 Water was added since it's about to dry, and the color is nice red now. Fe3+ test still gives blue color.
23:04 Fe3+ test gives only a "grey" color and is not a precipitate. Fe2+ also gives similar result, so [FeBl6]x- are absent. We added 8.0g AcOH. Much unknown "white" precipitation appeared and solution became "yellow" but since it still gives no blue with Fe2+/Fe3+ we can just ignore it.
23:55 Finally the volume is small enough(~30mL), much "yellow" solid appeared on the wall, and we cooled it. Too much solid appeared so we added some more water(~20mL), and then ~70mL EtOH was added. Very large amount(if it's really KDP then the amount should be the same as added, and NaDP should be way too soluble to precipitate) of disgusting "grey" solid separated, and solution remained red, so the discoloration of solution was just due to solid. We shake the mixture harshly and transferred everything into PTFE beaker and freeze it for a few hours to promote precipitation of KDP. A little bit of red crystals appeared so we added a little more water and EtOH.
03:00 Solution was quickly filtered, and we got a nice orange-red solution and some slightly green solid.
The KDP was discarded as it doesn't worth recycling, and the color might be due to trace of KFe[FeBl6] or something. Solution was poured into a crystallization dish and put into a dark but open place to let EtOH evaporate, as my only heating device is alcohol lamp, which would surely ignite the EtOH vapor.
When EtOH was almost gone we heated on a steam bath with vigorous stirring until it became almost dry(surely not completely dry as trace of AcOH is still there!), then cooled. The slurry was filtered, and from the color we can see that loss is negligible, that is to say, NaOAc strongly suppressed the solubility.
This was dried between tissues, and then repeatedly shaken with EtOH until no foggy stuff was released into EtOH. Then this was dried in a sealed box with CaCl2 in a dark place. Its solubility in EtOH is quite low, and although color can be somewhat observed it should not matter.
When cleaning containers we made an interesting experiment. To the dilute solution of the product, which is pink, KOH was added. Solution immediately became a nice yellow which should be [FeBl5(NO2)]4-.
Then NH3OHCl was added. Gas was released and it became red. This should be [FeBl5(H2O)]3-.
Then acid was added. Gas was released further(probably because NH3OHCl was in excess) and the mixture became green or brown. When looked carefully it's some micro particles floating in it, maybe K2Fe[FeBl6] or even Fe metal.
Yield: 2.09g(40%).
Hmm... still something wrong, as I cannot see where did the remaining Fe go. Next time we should optimize something.
Attempt 4
あーもういいや
——化けの花 by なきそ
↑ignore this
New ingredient: 19.04g(0.14mol) KDP, 3.45g(0.025mol) K2CO3, 1.06g(0.01mol) Na2CO3, boil and cool then 8.44g(0.02mol) complex, 1.38g(0.02mol) NaNO2, boil and concentrate to minimal volume, finally 17.03g(0.065mol) 37.4% H2SO4 when cooled. This is because, Na2HPO4 is not very soluble and will make concentrating hard. Addition of acid when hot may cause decomposition. H2SO4 can in fact be used as there is much KDP, and H3PO4 is weak, cannot protonate product whose conjugate acid is a strong acid.
By the way, during the first step ammonia was released, that is to say, KDP was contaminated by ADP. IDK what will this affect our experiment. A loss of "alkali" may cause the final solution to be over-acidified. However due to the presence of phosphate buffer it is almost impossible to test anything about ammonium and pH. The pH after the first step is correct, around 7.
My fault, I added complex far before reaction, and some part decomposed(since dissociated Bl- would be removed by buffer)! This should not be a problem though.
This time we performed it during day as I must sleep at night, and we only turned off the light inside. Also we decided not to test anything as heating for 2.5h should be enough as shown before. Both K4[FeBl6].3H2O and K3[FeBl6] would be precipitated by EtOH, so the first one doesn't matter and the latter one can immediately indicate failure.
When H2SO4 was added the mixture immediately became very dark but no gas was evolved. After addition of EtOH and filtration we found that solution was brown or red and solid was somewhat green or blue. However solid was nicely soluble to form a very slightly green solution so we decided that this should be due to decomposition of remaining [FeBl5(H2O)]3- which is from a very slight mismatch of stoichiometry or decomposition due to light or incomplete reaction, and we can continue. Solution was extremely acidic, at a pH of about 2, but this might still be due to the same reasons. Although the color is somewhat wrong, when added into FeSO4 solution only a very slightly blue color appeared, and it's not a precipitation, so K3FeBl6 or K4FeBl6 is negligible.
After freezing for 2h we filtered to get green solid and brown-red solution. Solid was dissolved and filtrated to give little but non-negligible blue solid and strongly yellow solution.
Well, I admit defeat, and next time we should be more patient, and it's time to sleep. Maybe we can explore later...
By the way I took some pure product and treated it with H2SO4 and NaNO2 but nothing happened, and then addition of FeSO4 gave no even trace of blue.
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