I synthesized some paramagnetic cobalt and nickel complexes (expected to be Co(II) and Ni(II)) using a bidentate ligand - N of pyridine ring (aromatic) and O of P=O (aliphatic), and I obtained HNMR and CNMR analyses from these complexes.
The issue I'm facing is in the CNMR, where there are no peaks in the aromatic (downfield) area, but the peaks in the aliphatic (upfield) area (0-50 ppm) are good and very similar to the ligands' peaks in that area. In the HNMR, it's the same as CNMR, but there are broad weak peaks in the downfield too.
I'm aware that two ligands of the same type coordinated with the metal, but I don't know which part of the ligand coordinated with the metal (N of pyridine ring or O of P=O).
Can these NMR spectra confirm the coordination sites' type?
Thanks in advance for your suggestions.
If avaialble get the P31 NMR of ligand before and after complexation. If you see the difference or shift in chemical shift value of P31 signal, its the P linking to metal if not than Oxygen.
I hope it would help:)
Thanks Aaliya, I have the PNMR results for complexes, and they are the same as the ligand with an aromatic shift.
My ligands can't coordinate with P. They can coordinate from the Oxygen of the phosphoryl group or the Nitrogen of the pyridine ring.
My question is, can I use NMR data to find out from which of these two sites my ligand is coordinated?
It seems the paramagnetic electrons are significantly increasing the relaxation rates of the pyradine-ring carbon signals. In the simplest case the increase in T2 is proportional to 1/r6 where r is the distance between the paramagnetic electron(s) and the carbon atom. So, the affect on the aliphatic carbons’ relaxation is small. I assume the 13C spectra signal-to- noise ratios are much lower than the 1H spectra you mentioned.
If the aromatic carbon linewidths are broad, they will be difficult to observe unless their signal-to-noise ratios are high. To make matters worse, the usual 13C peak amplitude increase due to C-H NOE is no longer present when 13C relaxation is dominated by unpaired electrons.
T1 relaxation is also strongly affected by paramagnetic metals. This means the increasing the pulse repetition rate can result in significantly increase the signal averaging efficiency for the aromatic 13C peaks.
I suggest you measure a 13C spectrum using 90 degree pulses and a very short data acquisition time (0.10 sec or less) with no other delays. You can signal-average about an order of magnitude faster. Hopefully the signal-to-noise ratio for the aromatic peaks will increase to the point where broad peaks are observed. The short acquisition time will be not be an issue for the aromatic carbons since we expect to have broad lines. I don’t know if it’s possible the aromatic carbons could have a large change in chemical shift. But I would also double the spectrum width.
Dr. Hutton, thank you very much for your valuable guidance.
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