Sidmadra
Hazard to Others
Posts: 129
Registered: 17-2-2017
Member Is Offline
Mood: No Mood
|
|
Alternative reducing agents to Carbon Monoxide when preparing dimeric metal complexes?
I've been trying to make palladium and other dimer catalysts such as this:
https://en.wikipedia.org/wiki/Allylpalladium_chloride_dimer
The reaction is fairly straightforward, in that you dissolve palladium and the ligand in a methanol/water solution and bubble in carbon monoxide, with
CO dissolving in methanol. Then just let it stir:
http://cssp.chemspider.com/50
I don't have a CO Cylinder + Regulator, which costs some $500+, and in doing this reaction with a CO Generator (Formic Acid + Sulfuric Acid 2:1), the
reaction proves to be extremely inefficient (yields below 7% over 3 days with 10x excess theoretical CO).
In this reaction the CO serves as as a reducing agent, so I figure there must be some alternative I can use. I can't find any alternative literature
references for this particular reaction. Too strong of a reducing agent, such as borohydride, and the palladium will be reduced, which should be
avoided.
Does anyone have any suggestions?
[Edited on 15-5-2017 by Sidmadra]
|
|
|
Melgar
Anti-Spam Agent
Posts: 2004
Registered: 23-2-2010
Location: Connecticut
Member Is Offline
Mood: Estrified
|
|
The thing is, you'd need a reducing agent that's also a stable radical, and carbon monoxide is the only one I'm aware of that can be used at those
temperatures. Normally, I'd think about recommending trying hydrogen, because palladium works so well to catalyze hydrogen reductions. But then,
that would probably be the first thing that researchers would try, for that reason.
Of course, if you're using a 2:1 ratio of formic to nitric acid, well, that's your problem right there. You need H2SO4's strong dehydrating powers,
not its acid powers, for generating carbon monoxide from formic acid. And sulfuric acid only can really do that when it's above 80% or so, and is
much better at it above 90% or so. Each molecule of formic acid adds a molecule of water to your sulfuric acid, diluting it. So you need a large
excess of sulfuric acid to generate carbon monoxide fast enough to do the reaction in an hour like the paper you cited. Ideally, you'd have oleum, or
a mixture of P2O5 and H2SO4. If you're in the US, P2O5 can be purchased online, and it's actually not that expensive. Or you can make it yourself by
burning phosphorus, if you have access to that.
|
|
|
Sidmadra
Hazard to Others
Posts: 129
Registered: 17-2-2017
Member Is Offline
Mood: No Mood
|
|
I was using 98% Sulfuric Acid and 97% Formic Acid. The dehydration was definitely taking place, but it doesn't go to anywhere close to completion at
room temperature. I had to heat it periodically with a torch to really keep the CO generating. My guess is that because the CO gas flow is not
controlled in any manner, it was just spurting more CO through the needle than the solution could absorb. If I were to use a lecture bottle with a
pressure regulator, I could use an extremely slow yet consistent stream of CO to get it most of it absorbed.
Is the radical action of CO that you suggest taking place in this reaction? I wasn't able to find any information on the mechanism at play here in the
many papers I've read.
I've also wondered... is there someway I might be able to use formic acid directly as the reducing agent? If I understand the CO generation mechanism
right, formic acid is just, hydrated carbon monoxide. Formic acid is known to be used directly as a reducing agent in other reactions. There is
palladium in the reaction so maybe that could activate the formic acid in some respect.
|
|
|
Melgar
Anti-Spam Agent
Posts: 2004
Registered: 23-2-2010
Location: Connecticut
Member Is Offline
Mood: Estrified
|
|
Right, what I'm telling you is that your formic acid, when it turns into water and carbon monoxide, leaves the water behind in the sulfuric acid,
eventually diluting it to a level where it makes a very poor dehydrating agent for the rest of the acid that hasn't reacted yet. Thus, you need to
use a large excess of sulfuric acid, so that the water from the formic acid isn't enough to get the concentration down to below 90% or so. If you
want the reaction to go more slowly, P2O5 turns into a thick, viscous liquid as it absorbs water (eventually turning into phosphoric acid), and this
transformation limits its ability to react, resulting in a slow, gradual reaction. But you shouldn't need P2O5 if you just use a lot of sulfuric acid
and add the formic acid slowly. Remember, you're only adding water to the acid, ultimately, so it can be concentrated again and reused.
Palladium undergoes a totally different reaction with formic acid, resulting in CO2 and H2. The CO2 goes away, and the H2 is unlikely to work for
reasons stated earlier. However, this reaction is extensively used for catalytic transfer hydrogenation, allowing hydrogen to be added without high
pressures to a palladium catalyst.
|
|
|
Sidmadra
Hazard to Others
Posts: 129
Registered: 17-2-2017
Member Is Offline
Mood: No Mood
|
|
So is it accurate to say that the dimer forming reaction takes place through the Carbon Monoxide radical which you suggested in the previous post?
|
|
|
Melgar
Anti-Spam Agent
Posts: 2004
Registered: 23-2-2010
Location: Connecticut
Member Is Offline
Mood: Estrified
|
|
Quote: Originally posted by Sidmadra  | | So is it accurate to say that the dimer forming reaction takes place through the Carbon Monoxide radical which you suggested in the previous post?
|
I'd imagine that any reaction that starts with a radical, stable or otherwise, (like carbon monoxide, or O2 for that matter) and ends with
non-radicals takes place through a radical mechanism. That doesn't always mean you need a radical mechanism to produce it, but if the standard
procedure to make it uses carbon monoxide, that probably means that there have been quite a few things they tried that didn't work, before finally
trying carbon monoxide.
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
