Structural Characterization of Shielded Isomeric Europium Complexes with Metal-Metal Contact Ga-Lai Law, ² Ka-Leung Wong,* ,‡ Yang-Yi Yang, § Qiao-Yuan Yi, § Guohua Jia, ‡ Wing-Tak Wong, ² and Peter A. Tanner* ,‡ Department of Chemistry, The UniVersity of Hong Kong, Hong Kong S.A.R., People’s Republic of China, Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong S.A.R., People’s Republic of China, and MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry & Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, People’s Republic of China Received June 14, 2007 New luminescent isomeric europium(III) complexes with carboxylic carbonyl group coordination (I and II) have been prepared by solvothermal synthesis using the ligand 2,2′-bipyridine-4,4′-dicarboxylic acid (bpdc), with the nonradiatively shielded Eu 3+ coordination sphere completed by dimethyl sulfoxide ligands. The room temperature IR spectra and Eu 3+ luminescence spectra do not provide a definitive distinction between I and II, but low-temperature luminescence can give a clear identification. Introduction The choice of ligand for complexation plays a key role in constructing efficient luminescent lanthanide complexes. Two of the common requirements when choosing a ligand for coordination with lanthanide ions are the binding strength and the ultraviolet (UV) absorption properties of the ligand entity. Aromatic carboxylate ligands fulfill the above two basic criteria 1-2 as sensitizing agents for enhancing the luminescence of lanthanide ions because they can chelate the lanthanide metal ions tightly as well as display strong UV absorption. In particular, 2,2′-bipyridine-6,6′-dicarboxylic acid is a good antenna to sensitize emission from Tb 3+ . 3 The aqua complexes of the isomer 2,2′-bipyridine-4,4′-dicar- boxylic acid (bpdc) with the three lanthanide ions Eu, Tb, and Gd have been characterized by luminescence spectros- copy, although a structural investigation was not carried out. 4 Because this ligand appears to be a promising antenna, we have further shielded the environment of Eu 3+ by removing the OH 2 ligands in order to minimize nonradiative channels. Herein, we report the X-ray structure of europium(III) complexes with carbonyl group coordination using the ligand bpdc and demonstrate the subtle differences between the two isomers. The distinction between the isomers is readily accomplished from the low-temperature electronic emission spectra. Experimental Section Synthesis. Direct reaction by solvothermal synthesis of europium trichloride with the ligand in dimethyl sulfoxide (DMSO) gave the polymeric europium complex with the two isomers [Eu 2 (bpdc) 3 - (DMSO) 4 ] n ‚6nH 2 O(I) and [Eu 2 (bpdc) 3 (DMSO) 4 ] n ‚2nDMSO‚2nH 2 O (II) (Figure 1). A mixture of EuCl 3 ‚6H 2 O, 2,2′-bipyridine-4,4′- dicarboxylic acid (bpdc; Figure S1 in the Supporting Information), and DMSO with a mole ratio of 1:1:1500 was stirred for 0.5 h and then sealed in a Teflon-lined autoclave. The mixture was heated at 100 °C for 2 days. After cooling to room temperature, colorless crystals were obtained on the walls of the flask and on the bottom, together with a small amount of precipitate (hereafter labeled as PPT). The crystals were removed, recrystallized, and then air-dried (isomer I). Isomer II was obtained from the mother solution after filtering out the mixture to remove the small amount of precipitate, and it also crystallized as a colorless block crystal (yield: I, 46%; II, 28%). The reaction was then repeated over the longer time period of 5 days. This resulted in a mixture without a precipitate, where I was again found on the container walls. Isomer II was obtained from slow evaporation of the mother solution over a few days because the crystal contains a higher proportion of DMSO. This reaction yielded different ratios of the two isomers (I, 23%; II, * To whom correspondence should be addressed. E-mail: klwong@ cityu.edu.hk (K.-L.W.), bhtan@cityu.edu.hk (P.A.T.). ² The University of Hong Kong. ‡ City University of Hong Kong. § Sun Yat-Sen University. (1) Lukes ˇ, I.; Kotek, J.; Vojtisek, P.; Hermann, P. Coord. Chem. ReV. 2001, 216-217, 287. (2) Beerby, A.; Bushby, L. M.; Maffeo, D.; Williams, J. A. J. Chem. Soc., Dalton Trans. 2002, 48. (3) Bu ¨nzli, J.-C. G.; Charbonnie `re, L. J.; Ziessel, R. F. J. Chem. Soc., Dalton Trans. 2000, 1917. (4) Calefi, P. S.; Ribeiro, A. O.; Pires, A. M.; Serra, O. A. J. Alloys Compds. 2002, 344, 285. Inorg. Chem. 2007, 46, 9754-9759 9754 Inorganic Chemistry, Vol. 46, No. 23, 2007 10.1021/ic701162j CCC: $37.00 © 2007 American Chemical Society Published on Web 09/28/2007