2:2-Type complexes of zirconium(IV)/hafnium(IV) centers with mono-lacunary Keggin polyoxometalates: Syntheses and molecular structures of [(α-SiW11O39M)2(μ-OH)2]10− (M = Zr, Hf) with edge-sharing octahedral units and [(α-SiW11O39M)2(μ-OH)3]11− with face-sharing octahedral units

Abstract This paper describes the syntheses and crystal structures of a set of novel 2:2-type Zr IV /Hf IV -containing Keggin complexes with edge-sharing and face-sharing linkages prepared under different pH conditions, each consisting of di-Zr IV /Hf IV cluster cations sandwiched between two mono-lacunary α-Keggin polyoxometalates (POMs) with an Si heteroatom, i.e., (Et 2 NH 2 ) 10 [(α-SiW 11 O 39 M) 2 (μ-OH) 2 ]·4H 2 O (M = Zr, Zr-edge ; Hf, Hf-edge ) and (Et 2 NH 2 ) 11 [(α-SiW 11 O 39 M) 2 (μ-OH) 3 ]·15H 2 O (M = Zr, Zr-face ; Hf, Hf-face ). The two central Zr and Hf atoms of the Zr-edge and Hf-edge are linked by two μ-OH groups, i.e., an edge-sharing linkage, whereas those of the Zr-face and Hf-face are linked by three μ-OH groups, i.e., a face-sharing linkage. Previously reported Zr/Hf-containing POM dimers were generally linked by edge-sharing M(OH) 2 M (M = Zr, Hf) bonds. Therefore, the present Zr-face and Hf-face are the first examples of Zr/Hf-containing POM dimers linked in a face-sharing fashion.