Asymmetric dinuclear, hexanuclear and octanuclear oxovanadium citrates with triazolates: novel mixed-ligands and mixed-valence complexes

Abstract

The triazolate-assisted asymmetric dinuclear oxovanadium(IV) citrate [V₂O₂(cit)(Hdatrz)₃]·5H₂O (1, H₄cit = citric acid, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) and its additive salt [V₂O₂(cit)(Hdatrz)₃][V₂O₂(cit)₂]_½·2H₂datrz·9.5H₂O (2) and the polymerized hexanuclear product [V₆O₆(μ₃-O)₂(cit)₂(Hdatrz)₄]·4H₂O (3) have been isolated at different temperatures, respectively. Adduct 2 shows strong evidence for the conversion of a symmetric dinuclear oxovanadium(IV) citrate to a mixed-ligand asymmetric oxovanadium(IV) citrate. Moreover, a fully oxidized trinuclear vanadium(V) species [V₃O₆(μ₂-OH)(μ₃-O)(Hdatrz)₂]·4.5H₂O (4) has also been isolated as a quasi-intermediate product of 3 without the coordination of citrate. Intriguingly, an octanuclear mixed-valence oxovanadium(V/IV) citrate K₂{[V^IV/V₂O₂(cit)(Hdatrz)(datrz)]₂[V^IV₂O₂(cit)(Hdatrz)(datrz)]₂}·27.5H₂O (5) has been obtained with different vanadium units, where dinuclear mixed-ligands and mixed-valence oxovanadium(IV/V) citrates [V^IV/V₂O₂(cit)(Hdatrz)(datrz)] (5a) and [V^IV₂O₂(cit)(Hdatrz)(datrz)] (5b) have been trapped. Citrate adopts a μ₂-η¹:η¹:η¹:η² coordination mode in 1, 2 and 5, while a μ₃-η¹:η¹:η¹:η² fashion has been observed in 3. Unlike 1–4, complex 5 contains both protonated and deprotonated triazolates simultaneously, where four triazolates further coordinate in a μ₃-η¹:η¹:η¹ manner to construct an octanuclear unit. These different structural features in 1–5 are dominated by flexible multidentate citrates and protonated/deprotonated triazolates, showing their synergistic effects. Furthermore, 1 exhibits a rectangular channel, showing preferential adsorption of O₂ and CO₂ over gases N₂, H₂, and CH₄.