Coordination compounds of polyoxovanadates with a hexametalate core. Chemical and structural characterization of [VV6O13[(OCH2)3CR]2]2-, [VV6O11(OH)2[(OCH2)3CR]2], [VIV4VV2O9(OH)4[(OCH2)3CR]2]2-, and [VIV6O7(OH)6](OCH2)3CR]2]2-