I have (at least) one organic compound in my aqueous samples which has a marked peak at ~0.1 ppm (see attached spectrum). I need to find out what it is. It is not TMS, since I use another internal standard at ~7.5 ppm. It's not a contamination from silicon grease (as some suggested in my previous question) since extensive negative controls performed on my experimental setup show absence of this peak. Other blanks show no other reagents I use have this peak.
CH4 is one of the potential products of my CO2-reduction (with H2) experiments. I did a spike test with this sample, and when I dissolve commercial methane into it the peak at 0.08 ppm does indeed become larger; supporting the idea that it's methane. I did GC-FID to see if I saw methane (see attached chromatogram). We don't have a suitable column for GC-MS unfortunately. The FID results show that whatever the peak at 1.35 min is, it isn't methane which has a peak at 1.52 min. The peak at 1.35 is absent in room air blanks, and I presume it's the same organic I see in the 1H-NMR results.
Another possible product of my experiments are (Ni or Fe) bound methyl groups: e.g. Fe-CH3, Fe-C3H9, etc. Which chemical shift should I expect from methyl protons attached to a metal atom? I suspect it would be similar to TMS, since that's exactly what TMS is, right? See the attached example of a Pt organometal chemical shift showing at 0.6 ppm.
Dear Eloi Camprubí-Casas thank you for your very interesting technical question. In fact, 1H NMR resonances for organometallic complexes comprising Fe-CH3 and Ni-CH3 moieties can be in the range of 0 ppm. To my knowledge, methyl resonances of Fe-CH3 complexes are normally observed in the range of ca. 0.07 to –3.47 ppm (cf. L. D. Field et al., Inorganic. Chem. 2015, 54, 4768−4776; see Ref. 20). As a typical example of a nickel(II) complex containing sigma-bonded methyl groups see e.g. the compound (tmeda)Ni(CH3)2 (tmeda = N,N,N′,N′- tetramethylethylene diamine) which has been published in Organometallics 2020, 39, 3433−3440. The CH3 resonance in the 1H NMR spectrum of this complex is observed at −0.470 ppm. This means that it cannot be ruled out that in your system metal species containing sigma-CH3 bonds are present. What is unusual, however, is to see such species formed in aqueous solution. Normally such complexes are quite air- and moisture-sensitive.
Good luck with your work and best wishes!
Thanks Frank T. Edelmann . Makes sense. Some of these chemical shifts are acquired in solvents other than H2O (as you point out). Would the chemical shifts change much if they were analysed in water + 10% D2O (let's assume they are stable in water for this)?
Dear Eloi, no guarantee, but I assume that the chemical shifts will not change to a great extent. However, I have no idea if the NMR spectra of the complexes can be measured in water / D2O mixtures. Surprisingly there are rare publications describing water-stable and (partially) water soluble nickel(II)-sigma-methyl complexes. For example, please have a look at the paper entitled "Nickel(II)-Methyl Complexes with Water-Soluble Ligands L [(salicylaldiminato-K2N,O)NiMe(L)] and Their Catalytic Properties in Disperse Aqueous Systems", S. Mecking et al., Organometallics 2007, 26, 1311-1316. In this paper the authors report the synthesis of Ni-CH3 complexes stabilized by bulky, water-soluble ligands. The CH3 resonances in the 1H NMR spectra were found in the range of –1.00 to –1.50 ppm. The complexes are reported to be stable towards water. Three of the complexes were either insoluble or slightly soluble in water. One of the compounds was even found to be water-soluble. The 1H NMR spectra were measured in C6D6, CD3OD, and acetone-d6.
Thanks again Frank T. Edelmann . This is extremely useful information. Which type of technique would you recommend to confirm (or not) that I have similar types of organometallic compounds in my aqueous samples? Perhaps Raman or IR spectroscopy?
They are quite dilute (
Dear Eloi, thank you for your response and the additional question. For a typical organometallic chemist it is pretty unusual to work with aqueous solutions. Normally all new organometallic compounds we prepare in out lab are highly air- and moisture sensitive. Thus I'm not an expert enough to suggest to you the best spectroscopic methods to characterize sigma-methyl complex species in dilute aqueous solutions. After all, I have no idea what else is present in your reaction mixtures. It might be worth a try to isolate the organometallic species by some sort of precipitation reactions. Let's assume that your sigma-CH3 complexes are present in cationic form (which seems quite possible in aqueous solutions). Then it might be possible to isolate them as salts with bulky anions such as tetraphenylborate, [BPh4]–. Thus you could take a sample of your reaction mixture and add an aqueous solution of Na[BPh4]. If you are lucky, the cationic organometallic species will form a precipitate of the teteraphenylborate salt, which can then be fully characterized. With best wishes, Frank Edelmann
I only have experience of Platinum methyl complexes and they are quite stable (though not water soluble) I used CDCl3. The chemical shift is around 1 ppm and (as your spectrum above showed) they show coupling to 195Pt which has spin 1/2. From the NMR point of view Ni does not have any isotopes (of reasonable abundance) which are NMR active, but you should see small side peaks for a Fe complex due to 57Fe which has a spin of 1/2 (2% abundant).
Maybe IR would be useful but the metal-carbon vibration will be quite low frequency (maybe out of the range of standard Ir spectrometers).
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