H1 NMR Spectra of inorganic complex, Cu4OCl6(2-mepy)4

Question

I'm having difficult interpreting the following H¹ NMR I have been given for Cu₄OCl₆(2-mepy)₄.

The NMR spectra obtained with integration trace beneath. I'm not too sure how to rationalise the 4.5:1 ratio between the peaks nor the cause of the small broad peak at 115ppm (well to the left of the spectra).

I'm also not too sure why the H1 NMR range is so vast. I appreciate being bonded to metals the electron density is changed across the relevant proton environments but by that much?

Thanks a lot

Answer 1

What you have there is a typical example of an H NMR spectrum of a paramagnetic compound. The formal oxidation state of the Cu here is Cu(II) resulting in d9 configuration. This means that there will be an unpaired d electron making the Cu atoms paramagnetic.

The lone electron in the Cu atoms disturbs the magnetic field the Hs' feel resulting in extreme chemical shifts and also provide a very efficient relaxation mechanism resulting in extreme peak broadening (in some cases you cant even see the resonances at all). The highest paramagnetic shift I have encountered so far was around 250 ppm for an Fe(II) complex I made. In another occasion we had resonances at 1200 ppm!

As for the peak broadening I had peaks of ν1/2 of a few thousands of Hz. The integration wont always make much sense with paramagnetic compounds either.