NMR Studies of Various Ligands Coordinated to Paramagnetic Polyoxometalates

Abstract

1H NMR spectroscopy was used to study various ligands coordinated to some paramagnetic polyoxometalates (POMs). Pure signals of the complexes were observed, which indicates that ligand exchange is slow on the NMR time scale. Mono- and diprotonated species of [CoW 11 CoO 39 ]8− and [SiW 9 Cu 3 O 37 ]10− were detected from the spectra of pyridine coordinated to these POMs. 2-Aminopyridine binds to [SiW 11 CoO 39 ]6−(SiW 11 Co) whereas 2-methylpyridine does not. This indicates that hydrogen bonding between the amine group and a bridging oxygen atom on SiW 11 Co plays an important role in complex formation. 4- Aminopyrimidine forms two linkage isomers, a and b, binding to SiW 11 Co via N(1) and N(3), respectively. The relative amount of isomer b increases, when D2O is replaced by DMF, indicating that hydrogen bonding between the amine group and SiW 11 Co is more favorable in DMF than in D 2 O. 3,3-Dimethylpiperidine undergoes rapid chair-chair interconversion at room temperature. When it is coordinated to SiW 11 Co in D 2 O the conformation is frozen even at room temperature. When DMSO is added to a D 2 O solution, the spectral change indicates that another conformation is stabilized in DMSO.Key wordsParamagnetic NMRpolyoxometalatesprotonationhydrogen bondinglinkage isomersconformations