Polynuclear lanthanide complexes of a series of bridging ligands containing two tridentate N,N′,O-donor units: structures and luminescence properties

Abstract

A set of three potentially bridging ligands containing two tridentate chelating N,N′,O-donor (pyrazole–pyridine–amide) donors separated by an o, m, or p-phenylene spacer has been prepared and their coordination chemistry with lanthanide(III) ions investigated. Ligand L¹ (p-phenylene spacer) forms complexes with a 2 : 3 M : L ratio according to the proportions used in the reaction mixture; the Ln₂(L¹)₃ complexes contain two 9-coordinate Ln(III) centres with all three bridging ligands spanning both metal ions, and have a cylindrical (non-helical) ‘mesocate’ architecture. The 1 : 1 complexes display a range of structural types depending on the conditions used, including a cyclic Ln₄(L¹)₄ tetranuclear helicate, a Ln₂(L¹)₂ dinuclear mesocate, and an infinite one-dimensional coordination polymer in which metal ions and bridging ligands alternate along the sequence. ESMS studies indicate that the 1 : 1 complexes form a mixture of oligonuclear species {Ln(L¹)}_n in solution (n up to 5) which are likely to be cyclic helicates. In contrast, ligands L² and L³ (with o- and m-phenylene spacers, respectively) generally form dinuclear Ln₂L₂ Ln(III) complexes in which the two ligands may be arranged in a helical or non-helical architecture about the two metal ions. These complexes also contain an additional exogenous bidentate bridging ligand, either acetate or formate, which has arisen from hydrolysis of solvent molecules promoted by the Lewis-acidity of the Ln(III) ions. Luminescence studies on some of the Nd(III) complexes showed that excitation into ligand-centred π–π* transitions result in the characteristic near-infrared luminescence from Nd(III) at 1060 nm.