Synthesis and spectroscopic analysis of an extended series of hetero dinuclear complexes containing two different lanthanides in 1:1 stoichiometry

An extended series of air and moisture stable and thermally volatile hetero dinuclear complexes of general composition [Ln1Ln2(fod)6(μ-bpm)] (where Ln1–Ln2 = Pr–Nd (1); Nd–Sm (2); Dy–Ho (3); Ho–Er (4); Er–Tm (5); Tm–Yb (6); Yb–Lu (7) and Eu–Tb (8); fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate; bpm = 2,2′-bipyrimidine) have been synthesized and characterised. To explore the stoichiometry of the two different lanthanides in these complexes, we performed a thorough analysis of the f–f absorption spectra of the complexes in solution and found that the hetero dinuclear complexes contain two different lanthanides in 1:1 stoichiometric ratio. Furthermore, the complexes display intense f–f hypersensitive transitions as compared to Ln3+ aqua-ions. In pyridine (a strong coordinating solvent) the oscillator strength and band shape of the f–f hypersensitive transitions show distinct variations as compared to the oscillator strength and band shape in chloroform (non-coordinating solvent). We attribute this change to a change in asymmetry of the field around Ln3+ ion due to the coordination of the solvent (pyridine). A thorough investigation of the steady-state emission and excitation spectra of the complexes 3 and 8 in chloroform and pyridine further establishes that pyridine competes for coordination with weak bpm bridge and eventually replaces it to generate two different species in solution, i.e. [Ln1(fod)3(py)n] and [Ln2(fod)3(py)n] (where py = pyridine).