Toward a qualitative understanding of the initial electron transfer site in Dawson-type heteropolyanions

Abstract

EPR, NMR, optical spectra and electrochemistry experiments on an extended series of one-electron reduced Dawson-type heteropolyanions converge to support the conclusion that the equatorial (α₁) tungsten atoms are first reduced in [P₂W₁₈O₆₂]⁶⁻. This conclusion is reinforced, for instance, by the interpretation of the electrochemical behaviours of a series of [P₂W_18 − xMo_xO₆₂]⁶⁻ derivatives. In search for a rationale, the study of the electronic structure of such Dawson-type heteropolyanions by means of extended Hückel calculations provides a qualitative understanding of this reduction process. It is found that the lowest unoccupied molecular orbital (LUMO), which is likely to be involved in the initial electron transfer, is essentially “belt”-centred in [P₂W₁₈O₆₂]⁶⁻. Also, the change of the reduction site (α₁ → α₂) upon substitution in the “cap” region (α₂ site) by a more electronegative metal centre (Mo for instance) is consistent with the localization of the LUMO on the substituted centre(s).