Synthesis, crystal structure and magnetic characterization of a series of Cu_(II)-Ln_(III) heterometallic [Ln = La, Ce, Pr, Nd and Sm) metal-organic compounds with an unusual single crystal to single crystal phase transition

摘要

The synthesis and structural characterization of five Cu(II)-Ln(III) heteronuclear metal-organic frameworks of formula {[Ln_4Cu_ 4(H_2O)_(26)(bta)_5]·mH_ 2O}_n and {[Ln_4Cu_4(H_ 2O)_(24)(bta)_5]·pH_2O}_ n [Ln = La~(III) (1A/1B), Ce~(III) (2A/2B), Pr~ (III) (3A/3B), Nd~(III) (4A/4B) and Sm~(III) (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); H_4bta =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 ? 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 bta~(4-) ligands in the ac-plane, leading to 4,4-rectangular grids. These layers are connected along the crystallographic b-axis, through a pillaring bta~(4-) group. The phase transition implies a change of the coordination mode of the bta~(4-) 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 χ_MT product with T which in the case of 1A is due to weak antiferromagnetic interactions between the copper(II) ions through the bta~ (4-) skeleton, the La~(III) cation being diamagnetic [J = -3.5 cm~(-1) with the Hamiltonian defined H = -JS_(Cu1)·S_ (Cu2)]. For the 2A-5A compounds, the additional exchange interaction between Cu_(II) and the paramagnetic Ln_(III) is masked by the crystal field effects (which partially removes the 2J + 1 degeneracy of the~ (2S+1)L_J free-ion ground state in zero magnetic field) (2A-5A) and the thermal population of excited free-ion states (5A).