Synthesis, Structures, and Magnetic Properties of Carboxylate-Bridged Tetranuclear Copper(II)−Lanthanoid(III) Complexes [Cu2Ln2(betaine)10(H2O)8](ClO4)10·2H2O and [Cu2Ln2(betaine)12(ClO4)2](ClO4)8

Synthesis, crystal structures, and magnetic properties of two types of carboxylate-bridged Cu II 2 Ln III 2 complexes, formulated as [Cu 2 Ln 2 (bet) 10 (H 2 O) 8 ](ClO 4 ) 10 ·2H 2 O ( 1; Ln III = La III, Ce III, or Gd III; bet = betaine) and [Cu 2 Ln 2 (bet) 12 (ClO 4 ) 2 ](ClO 4 ) 8 ( 2; Ln III = Gd III or Sm III ), have been described. 1 ·La features a carboxylate-bridged, tetranuclear [Cu 2 La 2 (bet) 10 (H 2 O) 8 ] 10+ cation, crystallizing in the triclinic space group P 1̄, with a = 12.778(4) Å, b = 15.553(5) Å, c = 16.041(5) Å, α = 110.18(2)°, β = 100.72(2)°, γ = 105.03(2)°, and Z = 1; 1 ·Ce is isomorphous with 1 ·La, crystallizing in the triclinic space group P 1̄, with a = 12.730(2) Å, b = 15.489(2) Å, c = 15.987(3) Å, α = 110.28(1)°, β = 100.70(1)°, γ = 105.00(2)°, and Z = 1. In both 1 ·La and 1 ·Ce, a Cu II atom is quadruply bridged to a Ln III atom by the μ 2 -carboxylate groups into a dinuclear subunit, and a pair of such dinuclear subunits is bridged by two carboxylate groups to form a tetranuclear cation. In the tetranuclear cation, each Cu II atom is coordinated in a square pyramid by four carboxy oxygen atoms at the basal positions and by one aqua ligand at the apical position whereas each Ln III atom is coordinated in a monocapped square antiprism by six carboxy oxygen atoms and three aqua ligands. 1 ·Gd has been characterized to be an analogue of 1 ·La. 2 ·Gd consists of a different tetranuclear [Cu 2 Gd 2 (bet) 12 (ClO 4 ) 2 ] 8+ cation, crystallizing in the orthorhombic space group Pmcb with a = 12.323(3) Å, b = 17.119(4) Å, c = 26.381(7) Å, and Z = 2; 2 ·Sm is isomorphous with 2 ·Gd, crystallizing in the orthorhombic space group Pmcb, with a = 12.496(2) Å, b = 17.378(3) Å, c = 26.767(5) Å, and Z = 2. In both 2 ·Gd and 2 ·Sm, a pair of dinuclear subunits analogous to those in 1 ·La are linked by a quadruple carboxylate bridge between the pair of Ln III atoms into a tetranuclear cation, where the apical ligand of the square pyramid about each Cu II atom is a perchlorate and each Ln III atom is coordinated in a square antiprism by eight carboxy oxygen atoms. The magnetic behavior of these complexes obeys the Curie−Weiss law, showing very weak antiferromagnetic interaction in the solid. Therefore, it may be concluded that shielding of the 4f electrons by the outer shell electrons very effectively precludes significant coupling interactions between the lanthanoid 4f electrons and copper 3d electrons in a carboxylate-bridged system.