Tantalum(V) alkoxides : Electrochemical synthesis, mass-spectral investigation and oxoalkoxocomplexes

Abstract

Ta(OR) 5 (R = Me( 1), Et( 2), Bu n( 3)) and Ta 2O(OPr i) 8·Pr iOH ( 4) were obtained by the anodic oxidation of metallic tantalum in absolute alcohols in the presence of LiCl (electroconductive additive). Ta(OPr i) 5 ( 5) was isolated by distillation in the course of thermolysis of 4 under reduced pressure. Ta(OC 2H 4OMe) 5 ( 6) was obtained by reaction of 5 with 2-methoxyethanol. Compounds 1 and 5 are isostructural with the corresponding [Nb(OR) 5] 2 (R = Me,Pr'); their molecules involve two [M0 61 octrahedra sharing a common edge. Recrystallization of 1 from McOH at < 20°C resulted in the isolation of a monosolvate Ta(OMe) 5·MeOH. The solubility of Ta(OMe) 5 in . was studied in the temperature range of −80°C to 50°C. The dinuclear molecule 4 has one, μ-O(oxo) and one μ-OR group, ROH is in an axial position. Compound 4 is the first example of a stable oxoalkoxide (in solution and gas phase) with as few as two metal nuclei, in contrast to most M x O x (OR) z , which are usually of higher nuclearity. Crystalline Ta709(OPr i) 17 ( 7) was obtained by the uncontrolled hydrolysis of 4 and 5. Its molecule is a heptanuclear metal oxoalkoxide of a new type containing a metal core consisting of tetrahedra [Ta 4] with a common vertex. In the mass spectra of 1–3 and 5, along with the ions obtained on fragmentation of Ta(OR) 5, fragments of Ta 20(OR) 8 (R = Me, Et, Bu, Pr') molecules were recorded, the latter are formed as a result of elimination of R 2O from Ta(OR) 5.