Investigating the Role of Cobalt(II) in the Single-Molecule Magnets Behavior of 2p-3d-4f clusters

International audience1,5-dimethyl-3-(3'-(hydroxymethyl)-2'-pyridine)-6-oxotetrazane (H3vdpyCH2OH) or its oxidized verdazyl form (vdpyCH2OH) are reacted with transition metal and/or lanthanide acetates to yield [(vdpyCH2O)2Co2Ln2(acO)8] (Ln=Y(III): ICo,Y; Gd(III): ICo,Gd), [(vdpyCH2O)2M3(acO)4] (M=Zn(II): IIZn; Co(II): IICo) and [(vdpyCH2OH)Zn(acO)2] (IIIZn) through self-assembly implying a complex-asligand intermediate. Single-crystal diffraction reveals that IMT,Ln are composed of 2p-3d-4f centrosymmetric clusters with verdazyl radicals at the two ends coordinated to the transition metal ion in a tridentate mode and to the {Ln2(acO)4} lanthanide central core in a monodentate mode through its alkoxo moiety. In ICo,Gd, the transition metal ions adopt an irregular octahedral environment and the {Ln2(acO)4} core a paddlewheel motif whereas in ICo,Y, the transition metal is pentacoordinated and the central core contains only two acetate bridges. Going from ICo,Y to IICo, the central {Y2(acO)4} core is replaced by an axially compressed octahedral cobalt(II) center whereas the outer parts of the molecule remain still. The dc magnetic studies revealed that the alternate p-stacking of the verdazyl radicals in IIZn led to the formation of alternate antiferromagnetically coupled 1D chains with Jvd-vd =-8.2(1) cm-1 and Jvd-vd' =-7.6(1) cm-1 (H=-2JS1.S2). In ICo,Y, a complex fitting procedure allowed to retrieve a complete set of magnetic parameters to take into account both the magnetic anisotropy of the cobalt(II) centers and intra-and inter-molecular exchange effects. For ICo,Y, it led to gCo = 2.13(4), DCo= 100(2) cm-1 , ECo= 19.9(5) cm-1 , JCo-vd = +26.5(4) cm-1 and Jvd-vd =-7.95(4) cm-1. ac magnetic susceptibility of ICo,Y, ICo,Gd and IICo did not reveal any slow relaxation of the magnetization even when a dc external magnetic field up to 2000 Oe was applied