Synthesis and magneto-structural studies on a new family of carbonato bridged 3d-4f complexes featuring a [Co3IILn3III(CO3)] (Ln = La, Gd, Tb, Dy and Ho) core: slow magnetic relaxation displayed by the cobalt(II)-dysprosium(III) analogue

A new family of [3 + 3] hexanuclear 3d-4f complexes [(mu(3)-CO3){Co(II)Ln(III)L(mu(3)-OH)(OH2)}(3)](ClO4)center dot mC(2)H(5)OH center dot nH(2)O (1-5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62-78%) following a self-assembly reaction between the ligand 6,6', 6''-(nitrilotris (methylene)) tris-(2-methoxy-4-methylphenol) (H3L), Co(OAc)(2)center dot 4H(2)O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare eta(2):eta(2):eta(2)-mu(3) mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the Gd-III analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the Co(3)(II)Gd6(3)(III) complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the (CoGdIII)-Gd-II superexchange, mediated via the bridging oxygen atoms. Only the Co-II-Dy-III compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/k(B) = 9.2 K and the relaxation time constant tau(0) = 1.0 x 10(-7) s.