Lamellar and three-dimensional hybrid compounds formed by cyclohexene- and cyclohexanedicarboxylates of Pb, La, and Cd

Abstract

To establish factors that determine the formation of three-dimensional hybrid structures of metal dicarboxylates involving metal–oxygen–metal linkages, we have investigated metal dicarboxylates derived from 1,2-cyclohexene as well as 1,2-, 1,3-, and 1,4-cyclohexane dicarboxylic acids. Thus, we have synthesized a 1,2-cyclohexenedicarboxylate of Cd, [Cd(1,2-CHeDC)(H₂O)] (I), a 1,2-cyclohexanedicarboxylate of Pb, [Pb(1,2-CHDC)] (II), and three 1,4-cyclohexanedicarboxylates of La [La₂(1,4-CHDC)₃(H₂O)₄] (III), [La₃(1,4-HCHDC)₂(1,4-CHDC)₅(H₂O)₂]·H₂O (IV) and [La₂(1,4-CHDC)₃(H₂O)]·2.5 H₂O (V) under hydrothermal conditions and determined their structures. A mixed dicarboxylate involving both 1,3- and 1,4-cyclohexenedicarboxylates of Pb, [Pb₃O(1,3-CHDC)(1,4-CHDC)]·0.5 H₂O (VI) and a 1,4-cyclohexanedicarboxylate of Pb, [Pb₆O₂(1,4-CHDC)₃(1,4-HCHDC)₂], have also been synthesized and characterized. While the 1,2-dicarboxylates have layered structures, the 1,4-dicarboxylates and the mixed dicarboxylates possess three-dimensional structures. Interestingly, both the 1,2 and 1,4-dicarboxylates are true hybrid compounds composed of infinite M-O-M linkages. The equatorial–equatorial (e,e) conformation is adopted commonly in all these compounds, although less stable conformations are encountered occasionally. The formation of the layered and the three-dimensional structures can be understood based on the relative disposition of the two carboxylic groups, the 1,4-isomer favoring the three-dimensional structure. Based on the results of the present study along with the available literature, we conclude that in order to obtain three-dimensional hybrid structures with metal-oxygen-metal networks, it appears necessary to make use of the 1,4-cyclohexanedicarboxylic acid.