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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 18, Number 4
BKCSDE 18(4)
April 20, 1997 

Effects of Axial Ligand Basicity on the Isotropic NMR Shifts in Pyridine-Type Ligands Coordinated to the Paramagnetic Polyoxometalate, [SiW11CoⅡO39]6-
Ji Young Kim, Suk Min Park, Hyunsoo So
When 1H NMR spectra of pyridine, 4-amino-, 4-methyl-, and 4-cyanopyridine coordinated to the paramagnetic polyoxometalate, [SiW11CoⅡO39]6- in D2O are compared, both α- and β-proton peaks are shifted upfield as the basicity of the ligand decreases. The isotropic shifts are separated into contact and pseudocontact contributions by assuming that the contact shifts are proportional to the isotropic shifts of the same ligands coordinated to [SiW11NiⅡO39]6-. This separation reveals that the shift variations with the axial ligand basicity are dominated by changes in the magnetic anisotropy (pseudocontact shift) of [SiW11CoⅡ(ptl)O39]6- (ptl=pyridine-type ligand). The magnitude of the magnetic anisotropy in a series of pyridine-type ligands increases linearly as the pKa of their conjugate acids decreases.
369 - 373
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