Impact of 3d Metal Ions on the Magnetic Dynamics in Isostructural Tetranuclear Butterfly {Ni2 IIDy2 III} and {Co2 IIDy2 III} Complexes

This paper describes the synthesis, structures, and magnetic properties of four isostructural butterfly-type heterometallic tetranuclear 3d-4f complexes, [Ni2Dy2(HL1)2(mu(3)-OH)(2)(piv)(6)]<middle dot>2CH(3)CN (1), [Ni2Dy2(HL2)(2)(mu(3)-OH)(2)(piv)(6)]<middle dot>2CH(3)CN (2), [Co2Dy2(HL1)2(mu(3)-OH)(2)(piv)(6)]<middle dot>2H(2)O (3), and [Co2Dy2(HL2)(2)(mu(3)-OH)(2)(piv)(6)]<middle dot>2CH(3)CN (4), derived from two closely related Schiff base ligands H2L1 and H2L2 in the presence of pivalate (piv) ions. X-ray crystallographic studies of 1-4 revealed that all of the complexes are constructed with a type-II butterfly core in which Dy-III ions are located at body positions, while the 3d metal ions (Ni-II and Co-II) occupy the wingtip positions. It is noteworthy that all reported systems containing Ni(II )and Co-II have been isolated as type-I butterfly structures, with only one report each on the (Ni2Dy2)-Dy- II (III )and Co-2 (II)Dy(2)( III )type-II butterfly core systems, which have been both structurally and magnetically characterized. Direct current magnetic susceptibility measurements performed on 1-4 revealed that the susceptibility response is dominated by the depopulation of Stark sublevels of the Dy-III ions and the ZFS of the Co-II cations in the case of 3 and 4 and the weak intramolecular superexchange interactions have been calculated by means of CASSCF and CASSCF/NEVPT2 calculations. Dynamic magnetic studies revealed that only Co-II analogues (3 and 4) display single-molecule magnet behaviors in which the additional anisotropy of the Co-II centers is responsible for the occurrence of the out-of-phase response in contrast to 1 and 2 with poorly anisotropic Ni(II )cations.