The effect of the lanthanide contraction on coordination with the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine (dpp)

Attempts to make homodinuclear lanthanide complexes using the polyazine bridging ligand 2,3-bis(2-pyridyl) pyrazine (dpp) with Pr3+, Nd3+, Eu3+, and Gd3+ are presented. The ionic radius of the Ln(3+) ion plays a crucial role in complex formation, with the larger lanthanides forming mononuclear complexes, and the smaller lanthanides forming dinuclear complexes. The structural and temperature dependent luminescent properties of [Eu(tta)(3)](2)dpp; where tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione, are reported. The beta-diketonate ligand (tta) is used as a capping ligand in this bonding motif. Both Eu3+ ions are eight-coordinate with six O atoms from the beta-diketones and two N atoms from the polyazine bridging ligand. Repulsion of the pyridyl hydrogens causes a distortion in the bridging ligand. Excitation of the beta-diketonate ligand tta at ca. 340 nm in methanol solutions results in the characteristic Eu3+ emission in the visible region of the spectrum. The emission intensity, associated with the Eu3+ centers, decreases as the temperature of the solution is increased. Evaluation of the data in comparison to data obtained from a monometallic Eu3+ analog reveals very similar photoluminescent properties. This suggests little electronic communication between Eu3+ ions via the polyazine dpp bridging ligand. (C) 2012 Elsevier B.V. All rights reserved.