Modular Assembly, fluorescent and magnetic properties of a series of non-symmetric binuclear lanthanide complexes

Abstract

As a kind of lanthanide complex with special molecular structures, non-symmetric binuclear lanthanide complexes ({Ln₂}) feature two Ln centers with distinct coordination geometries, and have showed some unique properties that common symmetric {Ln₂} could not achieved. Yet, there is still great difficult in their direct synthesis, and most of them were obtained through a serendipitous way. In this work, a series of non-symmetric {Ln₂}, namely, [Ln₂(bbpen)(hfac)₄] [Ln = Eu (1), Tb (2) and Dy (3); H₂bbpen = N, N′-bis(2-hydroxybenzyl)-N, N′-bis(2-picolyl)ethylene diamine; hfac⁻ = 1,1,1,5,5,5-hexafluoroacetylacetonate ion], have been synthesized by a modular assembly approach successfully. This is the first application of modular assembly approach in the synthesis of non-symmetric {Ln₂}. A [Ln(hfac)₃(H₂O)₂] complex precursor and a mononuclear Ln module built by a bbpen²⁻ ligand were assembled together by coordination substitution reaction between them, forming a kind of non-symmetric {Ln₂} structure where two Ln ions featuring distinct coordination geometries are dual-bridged by two O_phenolato atoms. The fluorescence spectra of 1 and 2 show characteristic emission peaks belonging to Eu- and Tb-based transitions, respectively. In particular, 1 presents a bright red emission. The static and dynamic magnetic properties of 3 were investigated. It is revealed that 3 possesses typical dual-relaxation behavior of non-symmetric {Ln₂} single molecule magnets (SMMs).