FT-IR, UV-VIS and EPR investigations of multicopper polyoxotungstates with BiIII as heteroatom

Three Cu-II-polyoxotungstates Na-8[Bi2W20Cu2O68(OH)(2)(H2O)(6)]. 26H(2)O (1), Na-12[Cu-3(H2O)(3)(BiW9O33)(2)]. 29H(2)O (2) and Na-10[Cu-4(H2O)(2)(BiW9O33)(2)]. 43H(2)O (3) were prepared and investigated by spectroscopic (FT-IR, UV-VIS, EPR) methods. The Cu-II coordination to beta -B-BiW9O33 units (in 1) and alpha -B-BiW9O33 units (in 2 and 3) arises from the shift of nu (as)(W-O-b.c-W) and nu (as)(Bi-O-a) stretching vibration bands. The lone-pair electrons on each bismuth atom is sterically-inert in 2 but sterically-active in 3. The p(pi)(Od) --> d(pi)(W) charge transfer bands of the Cu-II-compounds are shifted towards higher frequencies compared to the ligands bands (with 350-950 cm(-1)), while the bands for p(pi)(O-b.c) --> d(pi)(W) transitions are splitted and generally redshifted (with 800-5150 cm(-1)). The broad d --> d transition bands in the visible region of the electronic sprctra correspond to Cu-II ions in C-4 nu symmetry (for 2 with CuO4O* chromophore) or distorted O-h symmetry (for 1 with CuO3O3* local unit and for 3 with CuO6 and CuO5O* local units). The axial powder EPR spectrum of compound 1 (g(parallel to) = 2.267, g(perpendicular to) = 2.013) was simulated by considering noninteracting Cu-II ions. The copper ions are antifferomagneticaly coupled in compounds 2 (g(parallel to) = 2.080, g, = 2.237, D = 0.0214 cm(-1)) and 3 (g(parallel to) = 2.071,g(perpendicular to) = 2.216, D = 0.0-127 cm(-1)). (C) 2001 Elsevier Science B.V, All rights reserved.