Three Double-Layered {Ni₈}@{B _n }₂ (n=3, 4, 6) Cluster Sandwiched Polyoxometalates

The reaction of [A-alpha-GeW9O34](10-) with Ni2+ in the presence of inorganic boron sources yielded three unprecedented sandwiched Ni-added polyoxometalates (NiAPs): K6Na7H7[({Ni-8(mu(6)-O)(OH)(2)}@{B3O6(OH)(3)}(2))@(B-alpha-GeW9O34)(2)]16H(2)O (1), K4Na4H12[({Ni-8(mu(6)-O)}@{B4O8(OH)(3)}(2))@(B-alpha-GeW9O34)(2)]16H(2)O (2), and K10Na6[({Ni-8(mu(6)-O)}@ {B6O9(OH)(5)}(2))@(B-alpha-GeW9O34)(2)]12H(2)O (3). The common feature of 1-3 is that a rare double-layered {Ni-8}@{B-n}(2) (n = 3,4,6) cluster is inlaid in their sandwich belts. The {Ni-8} cluster is composed of two cubane {Ni-4} clusters by six bridging oxygen atoms and sharing a mu(6)-O atom. The numbers of boron atoms in the {Ni-8}@{B-n}(2) (n = 3,4,6) clusters of 1-3 are different, namely, {Ni-8}@{B-3}(2), {Ni-8}@{B-4}(2), and {Ni-8}@{B-6}(2) clusters. To the best of our knowledge, such architectures containing a double-layered {Ni-8}@{B-n}(2) (n = 3,4,6) cluster in the sandwich belts are the first observed in POM chemistry. Furthermore, 2 was investigated as an efficient heterogeneous catalyst for the Knoevenagel condensation of various aldehydes with malononitrile at room temperature.