Hydrothermal synthesis and characterization of hexavanadium polyoxo alkoxide anion clusters: crystal structures of the vanadium(IV) species Ba[V6O7(OH)3{(OCH2)3CCH3}3].3H2O and Na2[V6O7{(OCH2)3CCH2CH3}4], of the mixed-valence complex (Me3NH)[VIV5VVO7(OH)3{(OCH2)3CCH3}3], and of the fluoro derivative