Reduced polyoxomolybdates with the Keggin and Dawson structures: preparation and crystal structures of two-electron reduced [K(18-crown-6)]2[N(PPh3)2]2[HPMo12O40]·8MeCN·18-crown-6 and four-electron reduced [NBun4]5[H3S2Mo18O62]·4MeCn (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane)

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Abstract
By reaction of [K(18-crown-6)][N(PPh3)2]2[PMo12O40]·2MeCN 1 and [NBun 4]4[S2Mo18O62] with PPh3 in acetonitrile the new compounds [K(18-crown-6)]2[N(PPh3)2]2[HPMo12O40]·8MeCN·18-crown-6 2(18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) and [NBun 4]5[H3S2Mo18O62]·4MeCN 3 were prepared. Compound 2 contains the two-electron reduced [HPMo12O40]4– ion having the α-Keggin structure, 3 the four-electron reduced [H3S2Mo18O62]5– ion having the α-Dawson structure. Compounds 1–3 were characterised by means of IR and NMR spectroscopy and X-ray diffraction. The structures of 1 and 2 are disordered in space group P. The heteropolyanions show apparent Th symmetry; their actual symmetry is T. Upon reduction the Mo ⋯ Mo distances increase slightly. The strongly alternating ‘short’(mean 1.814 Å) and ‘long’(1.990 Å) Mo–O–Mo bonds in the unreduced compound 1 become more equal in 2. The structural changes upon reduction observed in 3 were analysed in detail. The most significant alterations are an increase of the Mo ⋯ Mo distances between corner-sharing MoO6 octahedra in the equatorial belt by 0.066 Å, and a decrease of the Mo–O–Mo bond lengths connecting the two halves of the anion. The alternation of the Mo–O(bridge) bonds is nearly evened out after reduction. All structural changes observed in 2 and 3 are consistent with the description that the additional electrons occupy delocalised molecular orbitals extending over the Mo–Ob framework.

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