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Sun Jun 25, 2017, 02:19 PM

Neil Bartlett's superpowerful oxidants NiF6- and AgF4- and the preparation of RhF6.

In recent years I've been interested in the inorganic room temperature molten salt formed by cesium fluoride and hydrogen fluoride, in which liquid is formed with a ratio (at the eutectic point) in a ratio of 2.3 molecules of HF to one molecule of CsF.

Generally fluorine gas, an important industrial agent for a variety of reasons, is formed by electrolysis of a related system in which potassium fluoride KF, complexes similarly with HF. However this system requires significant heat to melt, thus raising the energy cost associated with the preparation of F[sub]2[/sub] gas.

This is discussed in a nice paper in the Journal of Fluorine Chemistry published in 2006: Cesium fluorohydrogenate, Cs(FH)2.3F (Journal of Fluorine Chemistry Volume 127, Issue 10, October 2006, Pages 1339-1343)

In the introduction the authors write:

"1. Introduction

It has been known that alkaline metal fluorides (MF, M = alkaline metal) form complex salts with HF [1,2]. These salts are composed of M+ cations and fluorohydrogenate ((FH)nF−, where n is an integer) anions. KFHF system is used as an electrolyte for electrochemical synthesis of elemental fluorine and many studies have been made on its physical, chemical and electrochemical properties [35]. A similar application was also examined for CsFHF system, although it was not applied to industrial electrolysis due to the high cost of CsF [2,6]. The advantage of CsFHF system is its low melting point compared to KFHF system which enables the electrolysis to be performed at lower temperatures to save energy and give broader choice of materials of electrolytic cells. According to the phase diagram [2], the CsFHF system has a eutectic point below room temperature at the composition of CsF:HF of 1 to 2.3 (m.p. 16.9 C), corresponding to the formula, Cs(FH)[sub]2.3[/sub]F."


As I was poking around looking for this paper, I came across a paper by Neil Bartlett who is famous among chemists for his discovery, in 1962, that xenon, until then thought to be totally inert, could form compounds. Since 1962 many xenon compounds, most often fluorides but also oxides and other complex compounds, have been discovered, some by Bartlett himself.

The Barlett paper to which I refer, published just two years before his death in 2008 is this one:

Low temperature preparation and uses of potent oxidizers (Journal of Fluorine Chemistry Volume 127, Issue 10, October 2006, Pages 1285-1288)

Normally silver exhibits one oxidation state, 1+, although the 2+ state, analogous to that of its cogener copper, is well known.

Nickel's most common oxidation state is 2+, although a 3+ oxidation state is well known.

The Barlett chemistry is described in the graphic of his reactions found in the paper which may be seen by simply accessing the abstract. In this case, however, Barlett has prepared silver in the 3+ oxidation state, and nickel in the 4+ state, extremely unusual oxidation states.

These compounds, which are fluorine complexes are powerful oxidizing agents, and, as detailed in the paper, they are useful to prepare the hexafluorides of both rhodium and ruthenium (as well as platinum.)

Why is this important?

In less than 15 years the world supply of rhodium from ores, an important industrial catalyst and material with important technological implications, is expected be smaller than the supply available in used nuclear fuels. It is not clear that the element will be available from geological sources at economically recoverable levels at all in the next 20 to 50 years.

At that point it may become necessary to secure rhodium from used nuclear fuels, since that supply from ores will either have been depleted, or insufficient to meet demand for the metal.

Hexafluorides are known for 18 elements, and for all of them, the compounds are either gases or low boiling liquids. Historically used nuclear fuels have been recycled using complexes formed in solvents - the Purex process - which leaves a fair amount of chemical waste and other by products.

A far superior approach will be pyroprocessing including electrolytic recovery, perhaps in molten salts, including, but not limited to inorganic molten salts. (Organic room temperature molten salts - aka "ionic liquids" - are well know and vastly discussed compounds also of potential utility in the recovery of metals from complex mixtures like used nuclear fuels.)

Access to the hexafluorides of ruthenium and rhodium - both elements fairly large constituents of the fission products in used nuclear fuels - will allow for their recovery by simple distillation from the salts, a very clean process compared to extraction. In the case of rhodium in particular, although ruthenium is also an important and valuable element.

I note that uranium, neptunium, and plutonium, which are also valuable constituents of used nuclear fuels also form hexafluorides. The oxidation of americium to its highest state - AmF[sub]5[/sub] - also makes its separation from the lanthanide fission products and curium much easier to perform, although, regrettably, not as a gas.

Interesting and important implications I think, exciting chemistry to chemists of a certain type.

Have a nice Sunday afternoon.

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Reply Neil Bartlett's superpowerful oxidants NiF6- and AgF4- and the preparation of RhF6. (Original post)
NNadir Jun 2017 OP
eppur_se_muova Jun 2017 #1
NNadir Jun 2017 #2

Response to NNadir (Original post)

Sun Jun 25, 2017, 02:56 PM

1. Neil Bartlett got 20 nominations for a Nobel Prize. Never awarded.

https://www.nobelprize.org/nomination/archive/show_people.php?id=13013

I remember when I was only 8-10 years old and learned for the first time (several years after the discovery -- I wasn't really keeping up with the literature )that noble gas compounds had been discovered for the first time. That was perhaps my first real awareness that chemistry wasn't just a bunch of facts you could look up in books, but a still-growing science, with much left to discover. Although I eventually became an organic chemist -- somewhat to my surprise -- I always maintained a minor interest in the chemistry of noble gases and fluorination (applied for a job at Daikin, didn't get it). I remember reading a paper in grad school in which Bartlett used XeF4 as a solvent to prepare salts of AuF6- -- I believe (FXeFXeF)AuF6 -- which decomposed to AuF5 on warming. Bartlett was aiming for AuF6 (neutral) but seemed to have convinced himself that it wasn't possible as a result of that research. Worth remembering that it was his study of PtF6 which originally led to the discovery of xenon compounds, using textbook chemical logic and a little accidental contamination by O2. (Tried to track down a quote, supposedly due to Fermi, that "Sometimes the hardest part of making a discovery is recognizing that you have made a discovery." Couldn't locate it. Probably in a GAUSSIAN printout somewhere.)

I can't help but wonder how many other future chemists were influenced by that discovery and might not have become chemists otherwise. Seems at least an honorary Nobel is in order for that alone.

(Looks like we lost sub and sup after the hack. No more neat chemical formulas now. C'est la vie.)


ETA: Oops, just noticed that should be AgF4- in the title. Ag(V) still awaits discovery.

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Response to eppur_se_muova (Reply #1)

Sun Jun 25, 2017, 03:25 PM

2. I always felt he deserved the prize. Like you, I think most young future chemists of that time...

...were inspired.

When I opened the journal article, his name immediately caught my eye.

I'm in one of the Princeton libraries this afternoon; he was here in the late 60's, but ultimately moved on to UC Berkeley and Lawrence Livermore. He probably knew Seaborg and a whole bunch of other Nobel Laureates well. I imagine he experienced a good deal of kidding about that from the Laureates themselves, and hopefully he didn't experience any bitterness about it.

Some very important and transcendent scientists didn't get the prize. Most notably there's Lise Meitner. (She did get an element named after her though.)

I've been thinking a lot about volatile fluorides in recent years as well as their complexes with cesium fluorides.

The chemistry of nuclear fuels is very much involved with that, and I've been studying that chemistry - to the point of obsession I must confess - for thirty years now, ever since Chernobyl blew up.

Like you, I may have started with Bartlett as being the first chemist of whom I was aware, became an organic chemist and finally - to my unending surprise - an analytical chemist, or a management person associated with analytical chemists anyway. (I only go to the lab to talk to people, I don't actually do chemistry anymore.)

Bartlett will always have a special place in my heart; because, as you say, he taught us that we don't, and never will, know everything that is possible.

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