1H NMR study of pyridine-type ligands coordinated to the paramagnetic polyoxometalates, [Co IIW 11MO 39] n− (M=Co II, Ni II, Cu II, or Fe III)

Abstract

1H NMR spectra of pyridine, 3-, and 4-methylpyridine coordinated to the paramagnetic polyoxometalates, [Co IIW 11Co IIO 39] 8− (CoCo), [Co IIW 11Ni IIO 39] 8− (CoNi), [Co IIW 11Cu IIO 39] 8− (CoCu), and [Co IIW 11Fe IIIO 39] 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 [SiW 11Cu II(ptl)O 39] 6− and [SiW 11Fe III(ptl)O 39] 5− (ptl=pyridine-type ligand), appearance of good NMR spectra for the CoCu and CoFe 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 by one or two orders of magnitude. The isotropic shifts for the CoNi and CoCu complexes parallel those of the [SiW 11Ni IIO 39] 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 Co II ions reduces the contact shifts and reverses the signs of the pseudocontact shifts.