Anion-Induced Self-Assembly of Luminescent and Magnetic Homoleptic Cyclic Tetranuclear Ln₄(Salen)₄ and Ln₄(Salen)₂ Complexes (Ln = Nd, Yb, Er, or Gd)

Unique homoleptic cyclic tetranuclear Ln4(Salen)4 complexes [Ln4(L)2(HL)2(μ3-OH)2Cl2]·2Cl (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) or Ln4(Salen)2 complexes [Ln4(L)2(μ3-OH)2(OAc)6] (Ln = Nd, 5; Ln = Yb, 6; Ln = Er, 7; Ln = Gd, 8) have been self-assembled from the reaction of the hexadentate Salen-type Schiff-base ligand H2L with LnCl3·6H2O or Ln(OAc)6·6H2O (Ln = Nd, Yb, Er, or Gd), respectively (H2L: N,N′-bis(salicylidene)cyclohexane-1,2-diamine). The result of their photophysical properties shows that the strong and characteristic NIR luminescence for complexes 1−2 and 5−6 with emissive lifetimes in microsecond ranges are observed, and the sensitization arises from the excited state (both LC and LC) of the hexadentate Salen-type Schiff-base ligand with the flexible linker. Temperature dependence (1.8−300 K) magnetic susceptibility studies of the eight complexes suggest the presence of an antiferromagnetic interaction between the Ln ions. ■ INTRODUCTION Polynuclear Ln complexes with distinct luminescent and magnetic properties are currently of interest because of their potential applications in the preparation of new optical or magnetic materials and ideal probes in biology. However, the control of structures of polynuclear Ln complexes is often problematic due to the small energy differences, the high coordination numbers, and the flexible coordination geometries adopted by the Ln ions. In fact, the challenge to resolve the problem is strengthened because that construction of the polynuclear Ln complexes is distinctively affected by other factors, such as the character of organic ligands, the nature of counterions, and the reaction conditions. Moreover, from the viewpoint of the enhancement of their photophysical properties, it is required that the strong light-harvesting of the organic chromophores, the effective energy transfer from the chromophores to the Ln ion, and the minimization of nonradiative processes of the Ln ion are achieved, besides complete avoiding or decreasing the luminescent quenching effect arising from OH-, CH-, or NH-oscillators around the Ln ion. On the other hand, as to the discrete Ln-based clusters as the promising compounds for the development of single-molecule magnets (SMMs), the promotion of magnetic-exchange interactions between Ln ions through the overlap of bridging ligands orbitals with their “contracted” 4f orbitals is also a difficult task. Compared to the amount of efforts on the photophysical or magnetic behavior of 3d−4f heteronuclear complexes from the compartmental Salen-type Schiff-base ligands, the research on luminescent and magnetic polynuclear Ln Salen complexes has not been researched nearly as extensively, especially the limited single-crystal X-ray diffraction studies have been reported for those classic complexes. Nonetheless, for the typical quadridentate Salen-type Schiff-base ligands, binuclear triple-decker and trinuclear triple-decker Ln complexes are obtained, in which the Salen-type Schiff-base ligand with the rigid linker has been used, their photophysical properties should be further enhanced due to the mismatch of energy levels, despite the chromophores with rigid linkers absorbing at longer wavelength. As to the pure quadridentate Salen-type Schiff-base ligands with flexible linkers, anionReceived: May 5, 2012 Published: October 8, 2012 Article