Influence of metal ions, coligands and reaction conditions on the structural versatility and properties of 5-pyrimidyl-tetrazolate containing complexes

Abstract

Eight new complexes containing the versatile multidentate ligand 5-pyrimidyl-tetrazolate (pmtz), namely [Cu(μ₃-pmtz)]_n (1), [Pd₂(μ-pmtz)₂Cl₂] (2), [Ni(pmtz)(en)₂]Cl·2H₂O (3), [Cu(tren)(pmtz)(ClO₄)] (4), [Co₂(tren)₂(μ-pmtz)(μ-O₂)](ClO₄)₃·3H₂O (5), [Fe₂(pmtz)₄(H₂O)₂(μ-O)]·5H₂O (6), [(UO₂)₄(μ-pmtz)₂(μ-OH)₂(μ₃-O)₂(H₂O)₄]·6H₂O (7) and [Dy₂(pmtz)₄(μ-pmtz)(H₂O)₆](pmtz)·6H₂O (8), have been prepared by either conventional solution or hydrothermal techniques and characterised by single-crystal X-ray diffraction. Those compounds prepared under hydrothermal conditions ((1), (2), (7) and (8)) exhibit chelating/bridging coordination modes that connect two or three metal ions generating polynuclear species. Thus, compound 1 is a 2D polymer where the ligand exhibits a new chelating/bridging κ²N₁,N₇:κN₂:κN₃ coordination mode, whereas complexes (2) and (7) are dinuclear and tetranuclear species, respectively, in which the ligand displays a κ²N₁,N₇:κN₂ coordination mode. The increase of the metal size favours the adoption of the κ²N₁,N₇:κ²N₄,N₁₁ bis(chelating)bridging mode. This is the case of compound (8) that contains bulky Dy^III metal ions. Despite the large size of the UO²⁺ metal ion, the pmtz ligand in (7) does not adopt the expected bis(chelating)bridging mode because the uranyl cation hydrolyzes to the highly stable bis(μ₃-oxo) tetranuclear unit. Conventional mild solution conditions lead to complexes containing monodentate or chelating coordination modes. Thus, compounds (3) and (6) are mononuclear and μ-oxo dinuclear species, respectively, which contain κ²N₁,N₇ chelating pmtz ligands. Compound (4) is mononuclear with the ligand acting in a new monodentate form through the N₁ atom of the tetrazolato ring, whereas (5) is a dinuclear complex where pmtz adopts a new exo-bidentate N₂,N₃ bridging coordination mode. Variable-temperature magnetic measurements of (6) indicate that the oxo-bridge mediates a very strong antiferromagnetic interaction between the iron(III) ions with a J = −222.7 cm⁻¹. Luminescence measurements show that the pmtz ligands act as “antenna” for energy absorption and transfer to the uranyl emissive state.