Date: Tue, 26 Apr 2016 17:43:11 +0800
Dear All
?
Thank you very much for so many response. I have learned a lot from your
advice or comments. The list below are the summary of the cooments:
?
(1) ?Possible chemical exchange:
extreme broadening of the signals because you are close to the coalescence
point. Depending on the reaction entropie this can be streched over a very
broad temperature region (so changing the temp. Wont bring a lot). There are a
lot of weak Mg-N bonds which can be the cause of that.
(2)?Paramagnetism. Grignard compounds are known to do
SET type of mechanism. This can be perhaps checked by a paramagnetic balance
(see Bergers 200 and more experiments)
(3)
Aggregation. The Mg can bridge two ligands and build a massive complex, which
enlarge the T2 and your signals get very broad.
(4)?some of the Mg is actually
substituted by paramagnetic metal contamination? (e.g. Cu++ or Fe+++).?
Only a trace would be enough to considerably broaden the spectrum.
(5)??conformational heterogneity, each aromatic proton will be
subject to variety of ring-current shifts that might dramatically splinter /
broaden the resonances.?
(6)??Chemical exchange between Mg-bound and
free ligand or???Conformational exchange within the
complex or?? ??Both.
?
According to your comments, I have asked the Ph D candidate to check the epr. I have found that there were a strong epr signal in the spectrum which the mid point located at ?3389.15 Gauss. Based on this result, I thought paramagnetism is the main reason for the abnormal proton spectrum of Mg complexes.
I am really sorry for the long delay to post the summary.
Best regards
JunfengICCAS, China
??
Original question:
Recently, a Ph D candidate just consulted with me for the proton spectrum of a Mg complex. It is very surpising that the Mg complex is NMR inactive in CD2Cl2, CD3CN, CD3COCD3?at room temperature and low temperature (insoluble in C6D6), but the ligand showed a NMR spectra in C6D6?and CD2Cl2?at low temperature. For the clarity at the beginning, I am a little worried about the success of this complex because Mg is not a strong metal ion. I have asked him to check the proton, carbon and nitrogen contribution by elementary analysis. It turned out the complex indeed contain these elements. Then the problem come: Why I cannot observe any NMR signals for this compound? Any suggestion is appreciated.
The possible molecular structure is listed below:
(image/png attachment: clip_image002_04-26-17-17-29_.png)