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.