Synthesis and magneto-structural studies on a new family of carbonato bridged 3d-4f complexes featuring a [CoII3LnIII3(CO3)] (Ln = La, Gd, Tb, Dy and Ho) core: slow magnetic relaxation displayed by the cobalt(ii)-dysprosium(iii) analogueElectronic supplementary information (ESI) available: Details of DFT calculations; relevant metrical parameters for compounds 1-5 (Table S1); the basic experimental magnetic data for the complexes 1-5 (Table S2); the calculated (DFT) values of exchange coupling c

A new family of [3 + 3] hexanuclear 3d-4f complexes [(μ 3 -CO 3 ){Co II Ln III L(μ 3 -OH)(OH 2 )} 3 ]-(ClO 4 )· m C 2 H 5 OH· n H 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) (H 3 L), Co(OAc) 2 ·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 η 2 :η 2 :η 2 -μ 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 II 3 Gd III 3 complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the Co II -Gd III 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 τ 0 = 1.0 × 10 −7 s. A new series of carbonato-bridged complexes containing a CoII3LnIII3 core have been synthesized.