H-1-NMR STUDY OF IMIDAZOLE, L-HISTIDINE, AND THEIR DERIVATIVES COORDINATED TO THE PARAMAGNETIC UNDECATUNGSTOCOBALTO(II)SILICATE AND UNDECATUNGSTONICKELO(II)SILICATE ANIONS

H-1 NMR spectra of imidazole, 2- and 4(5)-methylimidazole, histamine, L-histidine, L-histidine methyl ester, N alpha-acetyl-L-histidine, and L-carnosine coordinated to the paramagnetic undecatungstocobalto(II)s ilicate (SiW11Co) and undecatung-stonickelo(II)silicate (SiW11Ni) anio ns are reported. For these complexes the ligand exchange is slow on th e NMR time scale and the pure resonance lines of the free ligand and t he complexes have been observed separately at room temperature. Two di fferent complexes are formed, depending upon which nitrogen atom of th e imidazole ring is coordinated to the cobalt or nickel ion of SiW(11) M. Thus the NMR spectrum of a D2O solution containing a ligand and SiW (11)M consists of three sets of lines originating from the free ligand and two complexes. All NMR lines of the SiW11Co complexes have been a ssigned unequivocally using the saturation transfer technique. The tem perature dependence of some spectra are also reported. The NMR spectra of some complexes show that the internal rotation of the substituent on the imidazole ring is hampered by the heteropolyanion moiety even a t room temperature.