1H NMR study of pyridine-type ligands coordinated to the paramagnetic polyoxometalates, [CoIIW11MO39]n− (M=CoII, NiII, CuII, or FeIII)

1H NMR spectra of pyridine, 3-, and 4-methylpyridine coordinated to the paramagnetic polyoxometalates, [CoIIW11CoIIO39]8− (CoCo), [CoIIW11NiIIO39]8− (CoNi), [CoIIW11CuIIO39]8− (CoCu), and [CoIIW11FeIIIO39]7− (CoFe) are reported. For these complexes the ligand exchange is slow on the NMR time-scale, and separate resonance lines have been observed for the complexes and the free ligands. Since no NMR signal is observed for [SiW11CuII(ptl)O39]6− and [SiW11FeIII(ptl)O39]5− (ptl=pyridine-type ligand), appearance of good NMR spectra for the CoCu and CoFe complexes indicates that magnetic coupling with the fast relaxing CoII ion shortens the electronic relaxation times of the CuII and FeIII ions by one or two orders of magnitude. The isotropic shifts for the CoNi and CoCu complexes parallel those of the [SiW11NiIIO39]6− (SiNi) complexes which come mainly from the contact shifts. CoNi and CoCu complexes, which exhibit sharper NMR lines than SiNi complexes, may be useful in measuring the contact shifts for complex systems. The isotropic shifts for the CoCo and CoFe complexes contain both contact and pseudocontact shifts. Analysis of the isotropic shifts for the CoCo complex of pyridine shows that exchange coupling between the two CoII ions reduces the contact shifts and reverses the signs of the pseudocontact shifts.