Trimetallo Derivatives of Lacunary 9-Tungstosilicate Heteropolyanions. 2. Isotropic NMR Shifts in Pyridine-Type Ligands Coordinated to the Paramagnetic 9-Tungsto-3-cuprio(II)silicate Anion

1 H NMR spectra of pyridine and α-, β-, and γ-picoline coordinated to the paramagnetic heteropolyanion [H m SiW 9 O 37 Cu 3 ] (10 - m ) - (SiW 9 Cu 3 ) are reported. NMR lines are assigned to [H m SiW 9 O 37 Cu 3 (ptl) n ] (10 - m ) - ( n = 1, 2 or 3; m = 0, 1 or 2; ptl = pyridine-type ligands) on the basis of their pH and [SiW 9 Cu 3 ]/[ptl] dependence. The isotropic NMR shifts come mainly from the contact shifts, which for some protons exceed 70% of those in analogous nickel complexes. The nuclear longitudinal relaxation times are 0.006, 0.06, and 0.1 s for α-, β-, and γ-H in pyridine coordinated to SiW 9 Cu 3 . The electronic correlation time calculated from the T 1 values is in the order of 10 - 11 or 10 - 12 s, which is two or three orders of magnitude smaller than those of ordinary copper complexes. It is suggested that spin frustration causes rapid electronic relaxation in these complexes in which the three Cu 2+ ions form an equilateral triangle.