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

• The synthesis and structural characterization of five Cu(II)-Ln(III) heteronuclear metal-organic frameworks of formula {[Ln(4)Cu(4)(H2O)(26)(bta)(5)]center dot mH(2)O} and {[Ln(4)Cu(4)(H2O)(24)(bta)(5)]center dot pH(2)O} [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 (3016 (3B), 22 (4A)/16 (4B) and 21 (5A)/14 (5B); H(4)bta =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 angstrom 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 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 chi T-M 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 Lam cation being diamagnetic [J = -3.5 cm(-1) with the Hamiltonian defined H = -JS(Cu1)center dot S-Cu2,]. For the 2A-5A compounds, the additional exchange interaction between Cu-II and the paramagnetic Ln(II) is masked by the crystal field effects (which partially removes the 2J + 1 degeneracy of the L-2S+1(J) free-ion ground state in zero magnetic field) (2A-5A) and the thermal population of excited free-ion states (5A).

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