I have prepared a Co(III)corrole, and the NMR in DCCl3 shows broad signals between 7.30 and 9 ppm. I've read that Co(III)corroles are paramagnetic with S = 1. So I attempted to add MeCN to coordinate the Co ion. It didn't change anything. Then excess triethylammine and although the broad signals move slightly, and some fine signals appear at ~10 ppm, there is still no big change. I thought that maybe I had aggregation, so I greatly diluted the same sample, and although I obtained much weaker signals, they were still broad.
I also prepared the analogous Al(III)corrole, and the NMR is similarly broad, with signals in the same region. That one threw me off.
Does anyone know what could be happening?
Thank you very much!
Hello
Look at your FID, if there is a fast decay of your signal then reduce your t1 (t=0.2 s for example).
Thank you, I will check this, but there are other signals which are sharp, so I think it is a problem with the compound, not the whole sample.
Update: Most people do NMR with PPh3 coordinating the Co(III) ion, which likely gives a low spin configuration. Maybe ammines are not strong enough ligands, or maybe there are very low energy excited paramagnetic states. I will have to keep trying ligands.
Check your elemental analysis and other spectroscopic properties to identify the formulation of the isolated compound. Make sure that you are examining the nmr of pure sample. As noted before Co3 is d5 and in the presence of your ligand depending on the reaction condition and starting material you may have CoIII(k4-n,n,n,n-corrole-H2)XH2O (X could be any other anion), for that you need to firmly confirm the composition of your sample in the absence of that it is just hypothesis. In the case you have k4-N,N,N,N-correle-H2 coordinated sample then it is low -spin paramagnetic sample and that may cause line broadening in the nmr but under certain conditions you may see sharpe lines. Before going any further, measure the magnetic moment of the sample using Evans method (i.e. use nmr to measure the magnetic moment of your sample). There is more to add but at this time you need to make sure that you have pure sample before doing any measurements.
Thank you to the people who answered. Just in case someone searches for this question, I later found that the free base corrole oligomerizes when left at RT for some time, and causes broadening in the same signals that were sharp when the corrole was first purified. It's possible that the cobalt corrole was already partially oligomerized, or it did so after metallation. I haven't checked this yet, but I will do preparative TLC on the cobalt corrole samples and try to characterize each fraction as thoroughly as possible and post the findings. Maybe it will be useful for people working with corroles.
I think that your complex is noninnocent low-spin CoII–corrole•2– electronic configuration. Either convert your cobalt corrole to bis(pyridine) hexacoordinated complex to get diamagnetic NMR or measure NMR in Pyridine-d5 solvent.
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