Catalytic Photochemical Dehydrogenation of Organic Substrates by Polyoxometalates

Article abstract of DOI:10.1021/ja00304a019

The photochemical behavior of the polyoxometalates based on W, Mo, V, Nb, and Ta in the presence of water or one of a variety of organic substrates including alcohols, amides, ethers, aldehydes, carboxylic acids, nitriles, ketones, and ureas is examined.Irradiation of the charge-transfer bands of polyoxometalates dissolved in organic media at 25 deg C leads in most cases to the oxidation of the organic substrate and reduction of the polyoxometalate.The polyoxometalates fall into three categories defined by their thermal and photochemical redox chemistry in the presence of organic substrates.Type I complexes exemplified by those of Nb and Ta do not appear to photooxidize any organic substrate upon iradiation.Type II complexes, exemplified by decavanadate and most heteropoly- and isopolymolybdates, and type III complexes, exemplified by most heteropoly- and isopolytungstates, do oxidize a wide range of organic substrates upon irradiation.Reoxidation of the reduced forms of the type II complexes either by reaction with O2 or by evolution of H2 is kinetically or thermodynamically unfavorable; reoxidation of the reduced forms of the type III complexes either by reaction with O2 or by evolution of H2 is not.Several factors affecting the quantum yields for production of reduced polyoxometalates are outlined, and the energetic features regarding hydrogen evolution are discussed.The infrared, electronic, 31P NMR, 183W NMR, and 17O NMR spectral properties of α-H3PW12O40*6H2O, 1, and other polyoxometalates remain the same before and after catalytic photochemical dehydrogenation of representative alcohol, ether, or amide substrates.These results indicate that little if any polyoxometalate decomposition occurs during the photoredox chemistry.Interactions between organic substrates and polyoxometalates have profound effects on the electronic structure of the polyoxometalates.The charge-transfer transitions of polyoxometalate 1 display different sensitivities to medium in the low-energy (λ>300 nm) vs. the high-energy region of the ultraviolet-visible spectral range.The highest sensitivities of the quantum yields for photoredox chemistry involving organic substrates and 1 to medium are observed in the low-energy or absorption tail region of the spectrum.One possible model explaining the wavelength dependence of the absorption and photochemical action spectra is discussed.A general mechanism in agreement with all the experimental data is proposed for organic substrate oxidation and the effective capture of light energy in these polyoxometalate-organic substrate systems.