Three Double-Layered {Ni 8 }@{B n } 2 ( n = 3, 4, 6) Cluster Sandwiched Polyoxometalates.

The reaction of [ A -α-GeW 9 O 34 ] 10- with Ni 2+ in the presence of inorganic boron sources yielded three unprecedented sandwiched Ni-added polyoxometalates (NiAPs): K 6 Na 7 H 7 [({Ni 8 (μ 6 -O)(OH) 2 }@{B 3 O 6 (OH) 3 } 2 )@( B-α- GeW 9 O 34 ) 2 ]·16H 2 O ( 1 ), K 4 Na 4 H 12 [({Ni 8 (μ 6 -O)}@{B 4 O 8 (OH) 3 } 2 )@( B-α- GeW 9 O 34 ) 2 ]·16H 2 O ( 2 ), and K 10 Na 6 [({Ni 8 (μ 6 -O)}@ {B 6 O 9 (OH) 5 } 2 )@( B-α- GeW 9 O 34 ) 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 μ 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.