Lanthanide contraction and pH value controlled structural change in a series of rare earth complexes with p-aminobenzoic acid Hao-Ling Sun a , Chao-Hong Ye a , Xin-Yi Wang a , Jun-Ran Li a, * , Song Gao a, * , Kai-Bei Yu b a State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Chengdu Center for Analysis and Measurement, Academia Sinica, Chengdu 610041, China Received 2 May 2004; revised 6 June 2004; accepted 9 June 2004 Abstract A series of rare earth complexes with p-aminobenzoic acid (HL) have been synthesized: [RE 2 L 6 (H 2 O) 2 ] n [RE ¼ La (1), Ce(2), Pr(3), Sm(4), Eu(5), Tb(6), Dy(7), Er(9)] and [RE 2 L 6 (H 2 O) 4 ]·2H 2 O [RE ¼ Tb(6 0 ), Ho(8), Yb(10), Lu(11), Y(12)]. The crystal structures of 1, 2, 6, 6 0 , 7, 9 and 12 have been determined and the isomorphous relationships of the others have been identified. Their structures change from two- dimensional (2D) array (the coordination number of the metal ions is nine for 1 and eight for 2–7 and 9) to double-nuclear structure (the metal ions are eight-coordinated) for 6 0 , 8 and 10–12, as controlled by lanthanide contraction. The structural type has been found influenced by the pH value of the reaction mixtures. q 2004 Elsevier B.V. All rights reserved. Keywords: Rare earth; Structure; p-aminobenzoic acid; Lanthanide contraction; pH value 1. Introduction p-Aminobenzoic acid (HL), a biologically important ligand, can form complexes with various metal ions. In order to gain some understanding on the interaction between biological ligands and metal ions, various complexes using HL as ligand have been synthesized, and their crystal structures studied. In complexes containing Na þ or K þ , the ligand L 2 is not directly coordinated to the metal ion, which is surrounded by coordination H 2 O molecules [1]. In complexes of Ba 2þ , Ag þ , Zn 2þ , Cd 2þ and Ni 2þ [1–5], both the carboxylic group and the amino group of L 2 are coordinated to the metal. However, in complexes of Sr 2þ , Mg 2þ and Co 2þ with L 2 [1,6,7], only the oxygen atoms of the carboxylic groups are coordinated to the metal ion and the amino groups of the L 2 ligands are not coordinated. Therefore, the effect of metal ions on the structural type of their HL complexes is apparent. Structural investigations of HL rare earth (RE) complexes are sparse. Based on their X-ray powder diffraction patterns, Brzyska and Rzaczyn´ska suggested that the HL complexes of RE elements La – Tm, Y and Yb – Lu are isostructural [8]. Nevertheless, a systematic structural investigation on complexes of the lanthanide series with this ligand has not existed to date. The p-aminobenzoic acid complexes of some RE metals Nd, Dy and Yb have been synthesized and their crystal structures determined [9–11]. The results show that the crystal structures of the Dy 3þ and Yb 3þ complexes are alike, whereas the crystal structure of the Nd 3þ complex is clearly different from the two complexes mentioned above. The effect of lanthanide contraction on the structure of RE complexes has been reported earlier [12–18] that, using 3,5- pyrazoldicarboxylic acid (H 3 pdc) as the ligand, lanthanide complexes (Ln ¼ La, Ce, Eu, Er and Lu) of three different structural types (two-dimensional single layer, two-dimen- sional double layer and three dimensional array) can be obtained [18]. Nitrilotriacetic acid (H 3 NTA) is an important derivative of glycine. A series of Ln–Cu–NTA complexes have been synthesized in our laboratory. Crystal structures of these complexes indicate that their dimensional diversity is controlled by lanthanide contraction [17]. As an extension of this work, we herein report the synthesis and structural diversity of a series of RE-L 2 complexes. 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.06.013 Journal of Molecular Structure 702 (2004) 77–83 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: þ 86-10-62753517; fax: þ 86-10- 62751708. E-mail addresses: lijunran@pku.edu.cn (J.-R. Li), gaosong@pku.edu. cn (S. Gao).