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)

Abstract

By reaction of [K(18-crown-6)][N(PPh₃)₂]₂[PMo₁₂O₄₀]·2MeCN 1 and [NBuⁿ₄]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 [NBuⁿ₄]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 T_h 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–O_b framework.