Size and charge effect of guest cations in the formation of polyoxopalladates: a theoretical and experimental study

Abstract

The development of rational synthetic procedures with desired nuclearity and high selectivity is a critical issue in inorganic chemistry. Here we demonstrate a comprehensive understanding of the template effect induced by metal cations in the formation mechanism of the class of polyoxopalladates ({MPd₁₂L₈} nanocube and {MPd₁₅L₁₀} nanostar) by combining computational and experimental techniques. The capture of a Mⁿ⁺ guest ion by a peripheral palladium(II)-oxo shell leads to a competition between the parent Pd²⁺ addenda ion and the respective guest metal ion. The present study reveals that (i) the selection of the incorporated guest ion has a thermodynamic control, (ii) the main factors governing the formation of a particular polyanion are the charge and size of the guest cation, (iii) the electrostatic interaction between the cation and the surrounding oxo ligands and (iv) the dehydration ability of the cation. As expected from the number of observed {Mⁿ⁺Pd₁₂L₈} species, trivalent cations M³⁺ were found to be good templates resulting in several examples of {M³⁺Pd₁₂L₈}, whereas monovalent cations M⁺ are much less prone to form {M⁺Pd₁₂L₈}. For tetravalent cations the dehydration energies are very large, however, the formation of {M⁴⁺Pd₁₂L₈} nanocubes is found to be still energetic favourable. Fully consistent with computational predictions, four novel polyoxo-12-palladates were synthesized: the La³⁺-centered nanocube [LaPd₁₂O₈(PhAsO₃)₈]⁵⁻ (LaPd₁₂-closed), the La³⁺-centered “open” nanocube [LaPd₁₂O₆(OH)₃(PhAsO₃)₆(OAc)₃]³⁻ (LaPd₁₂-open), the Ga³⁺-centered [GaPd₁₂O₈(PhAsO₃)₈]⁵⁻ (GaPd₁₂), and the In³⁺-analogue [InPd₁₂O₈(PhAsO₃)₈]⁵⁻ (InPd₁₂). All four compounds were fully characterized in the solid state and in solution by a multitude of physicochemical techniques, including ⁷¹Ga and ¹¹⁵In NMR as well as mass spectrometry. The experimentally observed selective incorporation of only In³⁺ ions in the presence of Ga³⁺ and In³⁺ confirmed the thermodynamic control of the formation mechanism, which we had predicted by theory.