H-1 NMR study of pyridine-type ligands coordinated to the paramagnetic polyoxometalates, [(CoW11Mo39)-W-II](n-) (M = Co-II, Ni-II, Cu-II, or Fe-III)

H-1 NMR spectra of pyridine, 3-, and 4-methylpyridine coordinated to the pa ramagnetic polyoxometalates, [(CoW11CoO39)-W-II-O-II](8-) (CoCo), [(CoW11Ni O39)-W-II-O-II](8-) (CoNi), [(CoW11CuO39)-W-II-O-II](8-) (CoCu), and [(CoW1 1FeO39)-W-II-O-III](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 i s observed for [SiW11CuII(ptl)O-39](6-) and [SiW11FeIII(ptl)O-39](5-) (ptl = pyridine-type ligand), appearance of good NMR spectra for the CoCu and Co Fe complexes indicates that magnetic coupling with the fast relaxing Co-II ion shortens the electronic relaxation times of the Cu-II and Fe-III ions b y one or two orders of magnitude. The isotropic shifts for the CoNi and CoC u complexes parallel those of the [(SiW11NiO39)-O-II](6-) (SiNi) complexes which come mainly from the contact shifts. CoNi and CoCu complexes, which e xhibit sharper NMR lines than SiNi complexes, may be useful in measuring th e 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 c oupling between the two Co-II ions reduces the contact shifts and reverses the signs of the pseudocontact shifts. (C) 2001 Elsevier Science B.V. All r ights reserved.