Synthesis, Crystal Structure and Magnetic Characterization of a Series of CuII–LnIII Heterometallic [Ln = La, Ce, Pr, Nd and Sm) Metal–Organic Compounds with an Unusual Single Crystal to Single Crystal Phase Transition

The synthesis and structural characterization of five Cu­(II)–Ln­(III) heteronuclear metal–organic frameworks of formula {[Ln4Cu4(H2O)26(bta)5]·mH2O}n and {[Ln4Cu4(H2O)24(bta)5]·pH2O}n [Ln = LaIII (1A/1B), CeIII (2A/2B), PrIII (3A/3B), NdIII (4A/4B) and SmIII (5A/5B) with m/p = 20 (1A)/16 (1B), 18 (2A)/16 (2B), 14 (3A)/16 (3B), 22 (4A)/16 (4B) and 21 (5A)/14 (5B); H4bta =1,2,4,5-benzenetetracarboxylic acid (1–5)] have been performed. These compounds present a single-crystal to single-crystal phase transition from expanded A phases toward the B shrinking networks, which is triggered only in the presence of a dry environment. This phase transition is accompanied by a compression of the crystallographic b-axis in the range 2.4 to 2.8 Å with the consequent decrease of the unit cell volume from 9.5% to 12%. The isomorphous crystal structures of 1A–5A can be described as two crystallographically independent [Cu­(II)–Ln­(III)] heterometallic dinuclear units which are connected through two crystallographically independent bta4– ligands in the ac-plane, leading to 4,4-rectangular grids. These layers are connected along the crystallographic b-axis, through a pillaring bta4– group. The phase transition implies a change of the coordination mode of the bta4– pillar from bis-monodentate (1A–5A) to tetrakis-monodentate (1B–5B). Magnetic susceptibility measurements of polycrystalline samples of 1A–5A in the temperature range 2.0–300 K have in common the decrease of the χMT product with T which in the case of 1A is due to weak antiferromagnetic interactions between the copper­(II) ions through the bta4– skeleton, the LaIII cation being diamagnetic [J = −3.5 cm–1 with the Hamiltonian defined H = −JSCu1·SCu2]. For the 2A–5A compounds, the additional exchange interaction between CuII and the paramagnetic LnIII is masked by the crystal field effects (which partially removes the 2J + 1 degeneracy of the 2S+1LJ free-ion ground state in zero magnetic field) (2A–5A) and the thermal population of excited free-ion states (5A).