Synthesis and luminescence studies of mono- and C3-symmetric, tris(ligand) complexes of Sm(iii), Y(iii) and Eu(iii) with sulfur-bridged binaphtholate ligands

Abstract

A series of trivalent mono- and tris(ligand) lanthanide complexes of a sulfur-bridged binaphthol ligand [1,1′-S(2-HOC₁₀H₄Bu^t₂-3,6)₂] H₂L^SN, have been prepared and characterised both structurally and photophysically. The H₂L^SN ligand provides an increased steric bulk and offers an additional donor atom (sulfur) as compared with 1,1′-binaphthol (BINOL), a ligand commonly used to complex Lewis acidic lanthanide catalysts. Reaction of the diol H₂L^SN with [Sm{N(SiMe₃)₂}₃] affords silylamido- and amino- derivatives [Sm(L^SN){N(SiMe₃)₂}{HN(SiMe₃)₂}] and the crystallographically characterised [Sm(L^SN){N(SiMe₃)₂}(thf)₂] with different degrees of structural rigidity, depending on the presence of coordinating solvents. The binaphthyl groups of the L^SN ligand act as sensitisers of the metal centred emission, which is observed for the Eu(III) and Sm(III) complexes studied. We have therefore sought to use emission spectroscopy as a non-invasive technique to monitor a monomer–dimer equilibrium in these complexes. A dramatic difference between the emission properties of the unreactive dimeric Sm(III) aryloxide complex, the solvated monomeric analogues and the amido adduct demonstrated the potential use of such a technique. For a few representative lanthanides (Ln = Sm, Eu and Y) the reaction of the dilithium salt Li₂L^SN with either [Ln{N(SiMe₃)₂}₃] or [LnCl₃(thf)₃] affords only the homoleptic complex [Li(S)₃][LnL^SN₃] (S = thf or diethyl ether); we report the structural characterisation of the Sm complex. However, the reactions of this dipotassium salt K₂L^SN with [Sm{N(SiMe₃)₂}₃] or [SmCl₃(thf)₃] give only [SmL^SNN(SiMe₃)₂], or intractable mixtures respectively, in which no (tris)binaphtholate is observed. The only isolable lanthanide-L^SN halide adduct so far is [YbL^SNI(thf)].