Antimony-substituted Keggin-type polyoxomolybdates: Polar crystals and catalytic oxidative dehydrogenation of alcohols

An antimony-substituted polyoxomolybdate, [PSb^III(H₂O) -Mo₁₁O₃₉]^4-, of the α-Keggin structure, has been synthesized. Although the molecular structure is typical, we have found from the analysis of the X-ray diffraction data that the crystal is anisotropic with alignment of the molecular units so that the substituted antimony atoms are all directed along the polar c axis and on the same side of the molecular unit. UV/Vis, ESR and magnetic susceptibility data support a formulation of [PSb^III(H₂O)Mo₁₁O₃₉]^4- whereby there is partial electron delocalization with some formation (10-20%) of Mo^V. Oxidation of [PSb^III(H₂O)Mo₁₁O₃₉]^4- with ozone or bromine yields the oxidized species [PSb^V(O)Mo₁₁O₃₉]^4- and [PSb^V(Br)Mo₁₁O₃₉]^3-. In the anaerobic oxydehydrogenation of alcohols, [PSb^V(O)Mo₁₁O₃₉]^4- is more active than [PSb^V(Br)Mo₁₁O₃₉]^3-. Catalytic aerobic oxidation of benzylic alcohols to aldehydes was also possible with no over-oxidation and formation of carboxylic̈ acids.