Sandwich-type Hf(IV) and Zr(IV)complexes composed of tri-lacunary Keggin polyoxometalates: structure of [M(3)(mu-OH)(3)(A-alpha-PW(9)O(34))(2)](9-) (M = Hf and Zr)

Abstract

The syntheses and structures of hafnium(iv) and zirconium(iv) complexes with Keggin tri-lacunary polyoxometalates (POM) [A-alpha-PW(9)O(34)](9-), i.e., (Et(2)NH(2))(7)H(2)[Hf(3)(micro-OH)(3)(A-alpha-PW(9)O(34))(2)].11H(2)O (Et(2)NH(2)-) and (Et(2)NH(2))(7)H(2)[Zr(3)(micro-OH)(3)(A-alpha-PW(9)O(34))(2)].12H(2)O (Et(2)NH(2)-) are described. The two POMs, Et(2)NH(2)- and Et(2)NH(2)-, were obtained as analytically pure, colorless crystals in 30.0 and 40.9% yields, respectively, by 1:3-molar ratio reactions of [A-PW(9)O(34)](9-) with Hf(SO(4))(2) and Zr(SO(4))(2), respectively, in aqueous solutions at 80-90 degrees C, followed by refluxing. The two POMs were characterized by elemental analysis, TG/DTA, FTIR, solution ((31)P and (183)W) NMR and X-ray crystallography. X-Ray structure analysis of Et(2)NH(2)- revealed that in polyoxoanion , the three 6-coordinate prismatic Hf(IV) ions linked with the three bridging OH groups, i.e., the [Hf(3)(micro-OH)(3)](9+) cluster cation was sandwiched between two alpha-Keggin tri-lacunary POMs. The molecular structure of was isostructural with that of polyoxoanion in Et(2)NH(2)-. X-Ray crystallography also showed that the crude crystals of Et(2)NH(2)- and Et(2)NH(2)-, which were first formed without refluxing, contained the two isomeric crystalline species in a 7:3 ratio with the alpha,alpha- and alpha,beta-junctions of the two Keggin tri-lacunary POMs. These facts were also confirmed by (31)P NMR in D(2)O. By refluxing the solutions containing the crude crystals, only the alpha,alpha-isomers of Et(2)NH(2)- and Et(2)NH(2)- were finally isolated.