Polyoxometalates - Science topic

Thank you Ioannis Samaras . I will share the results with you soon.

I wonder if 1.7 V is too high a voltage and can cause water splitting due to aqueous electrolyte.

You will get excellint result is you can try MALDI.

Dear Aranya Kar thank you for sharing this very interesting technical question with the RG community. For some basic information about the formation of toroid structures please have a look at the following potentially useful article which might help you in your analysis:

Unfortunately this article has not yet been posted by the authors as public full text on RG. However, the paper isn't that old (published 2013) and three of the authors have RG profiles. Thus you can easily contact one of them directly via RG and request the full text. You can also discuss your question directly with them as they are real experts in this field.

I hope this helps. Good luck with your work and best wishes, Frank Edelmann

Dear Parul Sood many thanks for sharing this very interesting technical question with the RG community. Although the most common coordination number for nickel are 4 and 6, a coordination number of 8 is not that unusual. High coordination numbers are often the result of the presenec of multidentate (chelating) ligands in which the donor atoms are in close proximity, e.g. chelating nitrate or sulfate ligands. If, in your case, the coordination number 8 has been determined by X-ray crystallography, it needs no particular explanation or "justification". For example, vanadate, molybdate and tungstate anions coordinated to other transition metals also have several oxygen donor atoms in close proximity and can be the cause of higher than usual coordination mumbers.

Good luck with your research and best wishes, Frank Edelmann

First, increase the window potential based on the solvent used, then run a blank to check that your electrode configuration is okay with the electrolyte used. Just post this first so that we can check and then if it is okay, one can add polyoxometalate and check if it is redox-active in that potential window?

Yes

Latif Ullah There is no doubt that Toshio Okuhara was a distinguished chemist, but in 1980s he worked in prof. Makoto Misono's group. The POMs in crystal form are indeed not a porous material. Commonly, POMs are unstable at elevated pH. Some POM have enough room in their interior to accommodate counter cations. The titled question was not about an encapsulation of POM in porous materials. I would downvote your post

Polyoxometalates (POMs) are a certain type of metal complexes. You can't say that POMs are different than metal complexes

Try this: https://www.tandfonline.com/doi/abs/10.1080/08927028908021961?journalCode=gmos20

Dear Pedram,

If you have CrystalMaker software you can calculate the center of the selected pore and then place a dummy atom like Si from the "edit-structure" panel. Later you can increase the radius of this dummy atom to fill the pore.

The hydrolytic stability of polyoxometalates varies in a very wide range depending on their structure. One of the best known PW12O40(3-) is stable only at pH <1.8, AlW12O40(5-) is stable at pH <8, but becomes protonated at low pH.

Thiometalates are generally formed in strongly alkaline solution for example (MoS4)2- is formed by dissolution of MoO3 in ammonia followed by reaction with H2S gas. The same reaction is true for thiotungstate, thiovanadate, thioantimonate etc. The sulfur rich compounds like [Mo2S12]2-, [Mo3S13]n- etc are also formed by reaction using polysulfide (Note that the medium is alakaline). In acidic medium insoluble metal sulfides are obtained.

Howevr polyoxometalates aregenerally formed in acidic medium. The following paper titled "

can be useful.

Also, hyper chem

Can you clarify your needs? e.g.: Is the POM in a salt or in solution, or in vacuum? Are biomolecules around? What properties are you interested in? Polyoxometalates are typically very heavy and can be challenging to model, even by DFT, if you go for "ab initio" modelling. But since you are in structural biology perhaps you rather work with some kind of force fields? Giving more details about the question would help people reply with more precise answers too.

Dear waqas, It will be better if you attach the voltammogram of MOF as well as POM based MOF. There maybe several reasons e.g. change in redox potential because of electron donation or subtraction effect, Peak overlapping, and third and major reason maybe the POM BASED MOF redox potentials may not be in the potential window you are using

Have you read the review? 

"The average dimensions of the Anderson–Evans anion measures approximately 8.6 × 8.6 × 2.7 Å (Fig. 1B)."

Such database does not exist. You need to specify which of lacunary POMs you are interested in. Details are very different for different POMs

Thousand papers have been published on photo- and  electrochemical properties of POMs. Your question is too broad to answer. You have to be more specific. The good reviews on POM are in special issue:

Chemical Reviews, 1998, Vol. 98, No. 1

Your best bet is to look through a crystal structure database and generate calculated powder patterns. The peaks in a PXRD spectrum will vary depending on how the Keggin units pack.

Mr. Suxiaofang,

For two layer ONIOM try with:

#p opt freq oniom(mp2/genecp:PM6/auto) test

-link-

#p freq oniom(mp2/chkbas:PM6) test geom=allcheck guess=read

Generally, however, I have found that in your input P is in +1 oxidations state, but the geometry corresponds to phosphate anion {[PO4]3-}. Is it P in +1 or this is crystallographic input?! There is better to localize the changes in your structure. Otherwise FF gives you a picture to your system shown as attachment, which should correspond to minimum of the energy. But I think that this is not the picture that you expect. There are not shown changes in the metal centers as well.

FF provides adequate description in systems with strong electrostatic interactions, but in your cases the atoms are shown as neutral particles, with a total charge -28 which presumably should be distributed within the frame of these atoms. But in the real case you have interactions between (metal) ions with fixed oxidation states. This oxidation state should be an integrity number in your initial state. For P, +1 is not typical oxidation state.

Thanks Fabrizio, I really appreciate your help.

Hello, I am stadying the "giant" POM containing more 100 atoms of Mo. In these structures there is the basic unit formed round the MoO7 building block. It is link to research group which discovered this POM: http://www.uni-bielefeld.de/chemie/emeriti/ac1-mueller//

And one review-article about its structure. Good luck!

Generally speaking, the question is about distinguishing the chiral material (even when it is represented by a racemic mixture) from non-chiral or meso-form. The most direct method would be x-ray analysis if it is possible.

Other possibilities may include an action of a chiral agent; we may use the fact that enantiomers of a chiral material show identical properties unless they are interacting with another chiral agent or put into a chiral environment.

In the mentioned case I assume that the POMs are soluble in water or some other solvents. So one option is the use of a chral solvent or some enantiomerically pure additive, that may interact with the POMs enantiomers (if present), and induce the non-equivalency of some sort; the induced non-equivalency could be detected by, let's say, NMR. Another option could be the HPLC analysis on a column with the chiral stationary phase.

In any case, it will be difficult to get a positive confirmation of the chirality present in the material. The absence of changes in the chiral environment for your sample could be due to very small changes, that just remain undetected.

Bojidarka B. Ivanova Thank you so much for such an elaborate  helpful answer.

 i am using avogadro package to draw the molecule, its hard to do it cause i dont view the inside phosphorus atom, if you want we collaborate toghether.

You are not supposed to see any oxidation peak of your POM at high positive potentials. What you see is likely to be an oxidation of organics:ethanol/methanol/formic acid. Do you see peaks related to the reduction of Mo(VI)?

Sure, there is. Of course the efficiency and extent of adsorption depends strongly both on the nature of the surface and the POM. See some examples below: http://pubs.rsc.org/en/Content/ArticleLanding/2012/SC/c2sc20210g#!divAbstract ; http://www.quimica.urv.es/w3qf/superficies/poms.html ; http://www.researchgate.net/publication/229064946_Functionalization_and_post-functionalization_A_step_towards_polyoxometalate-based_materials?enrichId=rgreq-e18da8a4-1753-496a-948b-6c5b11f7000a&enrichSource=Y292ZXJQYWdlOzIyOTA2NDk0NjtBUzo5OTQ0MDk1ODM3Nzk5NEAxNDAwNzE5OTgwMzcx&el=1_x_3 ; https://books.google.hu/books?id=FUqzig2p4PAC&pg=PA268&lpg=PA268&dq=adsorption+of+polyoxometalates+in+macroporous+materials&source=bl&ots=3-KuXznqbF&sig=ll0WD_7LZ_2haaKf9RO3AY1_jUY&hl=en&sa=X&ved=0CF0Q6AEwCGoVChMIy5bZlbSKxwIVyxEsCh1T6wML#v=onepage&q=adsorption%20of%20polyoxometalates%20in%20macroporous%20materials&f=false

The Keggin POM with formula H12PW12O44 does not exist. You might have H3PW12O40(4H2O) with 4 coordinated water molecules

It could be coated on electrode or you can made an electrode of POM.

1.take 10mg POM, 80microliter ethanol and 20microlitre nefion solution. sonicate the mixture for 8 hours, take the the mixture with the hrlp of micropipette and deposite on the surface of electrode, wait till it get dried completely, now it is ready to use.

2. take a mixture of POM and graphite, fill it in a teflon tube with a bore size of 2mm, put a Cu wire at one end of the tube into the mixture, connect this wire with crocodile of CV instrument. Other end of the tube having exposed material acts as an electrode.

Dear Shafi,

I agree with everyone's comments above. However, when you drop the pH to evaluate the release I suggest doing a control in your standard release media (maybe PBS, pH 7.4?). This way you can say more or less release as well. From my experience, just dropping the pH to below the pKa of carboxylic acid groups did not instantly dump the drug off from my polymer. The drug binding happens in hours so this is unlikely explained by diffusion limitations of my drug out of the polymer network. Dropping the pH did increase how much came off, but not all came off immediately. This may be due to (1) my system itself and yours could behave differently, (2) strong ionic interaction that slows protonation, (3) or something else (opinions, thoughts, etc. would be great).

Mr. Li,

Which values of g-tensor components you would like to compute - theoretical or experimental ones?

If you would like to compute g experimentally, that can be performed by EPR, using Eq. (1) (attachment), where when you have carried out measurements in (i) field domain, you can calculate frequency, using ge value. While when you have operated in (ii) frequency domain, vise versa you can obtain the field (H₀). These values, H or frequency, are taken at inflex point of the first derivative of the EPR-signal. When you have determined them, than you can receive g-factor, again using same equation, transformed towards g = f(H₀,freq.), however.

If you would like to compute g-tensor components theoretically, than you can use, for example, DFT approaches. Ref. [1] has provided details about the accuracy of set of widely omplemented DFT approachs. Particularly, towards accuracy there has been shown differences between theoretical and experimental values D(Dg_par/perp) from 0.1 to 16.4, depending on the system studied.

Towards a physical meaning of g-tensor components for different transition metal ion/s containing systems you should recognize further from Drago's books (refs. [2,3]), for example, because of there have numerouse studies in a large set of transition metal ion containing complexes, diverse number of coordination environment, geometries of metal the chromophores, oxidation states of metal centres and the fourth.

[1] M. Kaupp, M. Buehl, V. Malkin, Calculation of NMR and EPR parameters, Wiley, 2004, Weinheim, pp. 1- 603.

[2] Physical methods for chemists, Drago, R., Saunders, 1992, pp. 1-750

[3] Physical methods in inorganic chemistry, Drago, R., Reinhold, 1965, pp 1-430.

Dear Aleks,

my question was about metal-containing polyoxo-cations. In any case thank you for comment and interesting links!

Good luck and all the best,

Denis

Hi !

In our lab, we have a long-time experience of 183W NMR, in particular with R. Thouvenot who is now retired. Please find some "classical" parameters we used in routine :

"The 183W NMR spectra were recorded in 10 mm o.d. tubes at 20.8 MHz on a Bruker DRX500 spectrometer equipped with a broadband VSP probehead. Chemical shifts are referenced with respect to external 85% H3PO4 (31P), and to an external alkaline 2M Na2WO4 aqueous solution (183W) respectively and were measured by the substitution method. For 183W a saturated aqueous solution of the dodecatungstosilicic acid (H4SiW12O40) was used as secondary standard (δ = -103.8 ppm). The 183W NMR spectrum of the species was recorded at room temperature from a solution (about 0.3 to 0.5 g in 2.5 mL) in which 0.2 mL of D2O was added."

However it depends on several parameters :

i) the power of your spectrometer. The concentration needed can be decreased when you work with a 600 MHz spectrometer.

ii) the time of acquisition. If you can perform an acquisition during two or three days, so do it !

iii) probably the more important point : the symmetry of your POM ! you don't have to think in terms of concentration of species, but in terms of concentration of different W atoms (I mean chemically different) in your compounds. The more symmetric your compound, the less the concentration...

see for instance this ref :

Richard Villanneau,Séverine Renaudineau, Patrick Herson, Kamal Boubekeur,

René Thouvenot and Anna Proust. Eur. J. Inorg. Chem. 2009, 479–488.