Reactions of hydrated aqua halo cluster complexes [(M6X12)(H2O)4X2].4H2O (M = niobium, tantalum; X = chloride, bromide) in methanol. Substitution of coordinated water and halogen by methanol as determined by proton NMR spectra and conductance data

Abstract

From an aqueous solution of [(Ta{sub 6}Br{sub 12})(H{sub 2}O){sub 4}Br{sub 2}]{center_dot}4H{sub 2}O at room temperature 2.0 Br{sup {minus}} per cluster were easily precipitated with AgNO{sub 3} solution. The same result was obtained by the titration of acidic (H{sub 2}SO{sub 4}) aqueous or water-methanol solutions of [(M{sub 6}Br{sub 12})(H{sub 2}O){sub 4}Br{sub 2}]{center_dot}4H{sub 2}O and [(M{sub 6}Br{sub 12})(H{sub 2}O){sub 4}Cl{sub 2}]{center_dot}4H{sub 2}O (M = Nb, Ta) with CH{sub 3}COOAg, whereas the results for [(M{sub 6}Cl{sub 12})(H{sub 2}O){sub 4}Cl{sub 2}]{center_dot}4H{sub 2}O were approximately 25% lower than expected.{sup 6} Most of these differences can be explained by slower ionization of terminal chloride{sup 7} and slower replacement of ligand water with methanol molecules from the solvent. This idea is supported by the results described here of the study of {sup 1}H NMR spectra of methanolic solutions, of specific conductance measurements, and finally of free chloride and bromide using {sup 35}Cl and {sub 81}Br NMR spectroscopy.