SOLVENT EFFECTS ON THE ISOTROPIC NMR SHIFTS IN QUINUCLIDINE AND PYRIDINE-TYPE LIGANDS COORDINATED TO THE PARAMAGNETIC POLYOXOMETALATE, [(SIW11COO39)-O-II](6-)

The solvent effects on the isotropic NMR shifts in conformationally ri gid ligands such as quinuclidine, pyridine, and 4-aminopyridine coordi nated to the paramagnetic polyoxometalate, [(SiW11CoO39)-O-II](6-)(SiW 11Co), are reported. For these complexes the ligand exchange is slow o n the NMR time scale and pure H-1 NMR signals have been observed at ro om temperature. The signals for the SiW11Co complexes are shifted upfi eld when dimethyl sulfoxide-d(6) (DMSO) is added to a D2O solution. Th e isotropic shifts are separated into contact and pseudocontact contri butions by assuming that the contact shifts are proportional to the is otropic shifts of the same ligands coordinated to [(SiW11NiO39)-O-II]( 6-). It is shown that both the contact and pseudocontact shifts decrea se (the absolute values of the pseudocontact shifts increase), when D2 O is replaced by DMSO. It is suggested that D2O, a strong hydrogen bon d donor, withdraws electron density from [(SiW11CoO39)-O-II](6-),incre asing the acidity of the cobalt ion toward the axial ligand. When D2O is replaced by DMSO, the acidity of the cobalt ion in SiW11Co decrease s, weakening the Co-N bond. Then both the contact and pseudocontact sh ifts are expected to decrease in agreement with the observed solvent e ffects.