Inorganic Borate Functionalized {Ni6SiW9} Polyoxometalates: From Cluster to Cage Cluster

In aqueous solution systems, an all-inorganic Ni6-added polyoxometalate (Ni6AP) [Ni6(μ3-OH)3(H2O)12(B-α-SiW9O34)]1- ({Ni6SiW9}, here, {Ni6} = Ni6(μ3-OH)3(H2O)12) has been made via a hydrothermal method. By introducing inorganic boron sources, a B3O3(OH)6 ({B3}) cluster was precisely anchored onto {Ni6} to form a [B3O3(OH)6Ni6(μ3-OH)2(H2O)8(B-α-SiW9O34)]3- ({B3Ni6(OH)2(H2O)8SiW9}) through replacing four terminal water molecules and condensing with one hydroxyl of the {Ni6} cluster. Subsequently, the addition of further boron sources facilitated the in situ growth of B-O clusters on the basis of {B3Ni6(OH)2(H2O)8SiW9}, which resulted in the successful synthesis of {[B7O8O2/3(OH)6Ni6(μ3-OH)2(H2O)8(B-α-SiW9O34)]3}7- ({B7Ni6(OH)2(H2O)8SiW9}3 or {B21}@{Ni6(OH)2(H2O)8SiW9}3). It is the first all-inorganic cage cluster constructed by NiAPs and B-O clusters, which successfully realizes the inorganic borate functionalized POMs from a single cluster to a cage cluster. The central {B7O8O2/3(OH)6}3 ({B21}) cluster exhibits an uncommon cavity configuration. Meanwhile, we have conducted a discussion of the possible formation mechanism of B-O functionalized NiAPs. Furthermore, these three compounds efficiently accelerate the oxidative transformation of sulfides, and the number of B atoms in compounds relatively affects the selectivity of the products.