Sciencemadness Discussion Board

Why do catalytic transfer hydrogenations with ammonium formate + metal always require excess formate?

Sidmadra - 14-8-2017 at 22:06

Specifically with reactions of using ammonium formate and a metal such as palladium, platinum, magnesium, zinc, and a handful of others, a 3-10x excess of ammonium formate is often employed.

I feel like a fool for asking this but the answer has persistently eluded me in spite of how much I have investigated it. Over the years I've used various transfer hydrogenations utilizing ammonium formate for a plethora of different reactions, I am yet still in the dark on why an excess of formate is needed.


My assumption is that the metal takes the hydrogen from the formate, releases the carbon dioxide and ammonia, and then possibly some of the hydrogen gets released (wasted), instead of being transferred to a reagent. Then again, I doubt this because an excess is often still needed in sealed reactions, and I never see anyone give "hydrogen warnings" for these reactions when conducted open.

CRUSTY - 15-8-2017 at 05:42

I haven't done much with this particular reaction, but with such a massive excess as that (the molar ratio for the ideal reaction is 1:1 formate to target molecule), I'd imagine that it is just to push the equilibrium to the products side. Were it a 2x molar excess, I'd maybe say that it would be something else, but 3-10 is pretty large.

Is there a reasonable amount of ammonium formate left over after the reaction?

[Edited on 15-8-2017 by CRUSTY]

wg48 - 15-8-2017 at 06:59

Perhaps the efficiency of the transfer is low for some substrates/catalysts and most of the formic acid is decomposed without reducing anything. For example nickel and palladium can catalyse the decomposition of formic acid without anything to reduce.

Sidmadra - 15-8-2017 at 09:48

Quote: Originally posted by CRUSTY  
I'd imagine that it is just to push the equilibrium to the products side.
[Edited on 15-8-2017 by CRUSTY]


These sorts of reactions aren't reversible usually, at least not in the case of alkenes or imines.
Quote: Originally posted by wg48  
Perhaps the efficiency of the transfer is low for some substrates/catalysts and most of the formic acid is decomposed without reducing anything. For example nickel and palladium can catalyse the decomposition of formic acid without anything to reduce.


That is what I am wondering, what is the decomposition reaction, if this is the case? I understand CO2 can be liberated, but is the hydrogen also wasted?

wg48 - 15-8-2017 at 10:35

Sidmadra:

I thought I knew it can decomposes to CO2. Of cause your right: what then happens to the hydrogen if it does not reduce anything it must be given off as a gas.

I did a quick search; apparently it can also decompose to CO and H2O. I think I have read precautions about the release of CO. Its probably ph dependent along with other factors.

Sidmadra - 15-8-2017 at 10:41

I have never seen any warnings about CO in papers on hydrogenations using ammonium formate. I imagine if that was a potential threat then every paper would make haste in mentioning it. I saw a thread here recently about the decomposition of formic acid into CO, and one poster said that metal alone would not generate CO, but it has to be a dehydrating acid like sulfuric acid.


I thought, maybe the metal can decompose the ammonium formate into ammonia, carbon dioxide, and water, but the stoichiometry doesn't work out here.

zed - 15-8-2017 at 18:34

I try to assume the worst.

If CO is a possible product, I'd run the reaction out-of doors, or in a well ventilated hood.

I tend towards paranoia, myself.

Still, I'm pretty old, and I don't seem to have Parkinson's, or any other form of the heebie-jeebies.

So, maybe a lifetime of being a scaredy- cat, was really OK..

Sidmadra - 15-8-2017 at 18:48

I would be surprised if CO was a byproduct in such reactions with ammonium formate. I've read several dozens of papers on the topic over the years and not one of them made any sort of mention of that.

Melgar - 25-8-2017 at 12:26

Palladium catalyzes the decomposition of formic acid into CO2 and H2. I'm 100% sure about this. To get carbon monoxide, you need an exceptionally strong, exceptionally acidic desiccant, which palladium is not.