Tetravalent Metal Complexation by Keggin and Lacunary Phosphomolybdate Anions

We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between CeIV and ThIV with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo11O39]7–, results in the formation of compounds containing the anions [Ce(PMo11O39)2]10− and [Th(PMo11O39)2]10−, respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs10[Ce(PMo11O39)2]·20H2O and (NH4)10[Th(PMo11O39)2]·22H2O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo11O39]7− anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably wide pH range. Density functional analysis suggests that the Ce−O bond strength in [Ce(PMo11O39)2]10− is greater than the Th−O bond strength in [Th(PMo11O39)2]10−, with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state ZrIV and HfIV complexes formed contain the anions [Zr(PMo12O40)(PMo11O39)]6− and [Hf(PMo12O40)(PMo11O39)]6−, respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo11O39]7− ligand and to four bridging oxygens of a plenary Keggin anion, [PMo12O40]3−. In addition, (NH4)5{Hf[PMo12O40][(NH4)PMo11O39]}·23.5H2O can be crystallized as a minor product. The structure of the anion, {Hf[PMo12O40][(NH4)PMo11O39]}5−, reveals coordination of the central HfIV cation via four bridging oxygens on both the coordinated [PMo11O39]7− and [PMo12O40]3− anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation. 31P NMR indicates that complexation of the Keggin anion, [PMo12O40]3−, to HfIV and ZrIV will stabilize the Keggin anion to a much higher pH than usually observed.