Single-Molecule Magnetism in a Family of {Co^III₂Dy^III₂} Butterfly Complexes: Effects of Ligand Replacement on the Dynamics of Magnetic Relaxation

The synthesis and structural characterization of four related heterometallic complexes of formulas [DyIII2CoIII2(OMe)2(teaH)2(O2CPh)4(MeOH)4](NO3)2·MeOH·H2O (1a) and [DyIII2CoIII2(OMe)2(teaH)2(O2CPh)4(MeOH)2(NO3)2]·MeOH·H2O (1b), [DyIII2CoIII2(OMe)2(dea)2(O2CPh)4(MeOH)4](NO3)2 (2), [DyIII2CoIII2(OMe)2(mdea)2(O2CPh)4(NO3)2] (3), and [DyIII2CoIII2(OMe)2(bdea)2(O2CPh)4(MeOH)4](NO3)2·0.5MeOH·H2O (4a) and [DyIII2CoIII2(OMe)2(bdea)2(O2CPh)4(MeOH)2(NO3)2]·MeOH·1.5H2O (4b) are reported (teaH3 = triethanolamine, deaH2 = diethanolamine, mdeaH2 = N-methyldiethanolamine, and bdeaH2 = N-n-butyldiethanolamine). Compounds 1 (≡ 1a and 1b) and 4 (≡ 4a and 4b) both display two unique molecules within the same crystal and all compounds display a butterfly type core, with the DyIII ions occupying the central body positions and the diamagnetic CoIII ions the outer wing-tip sites. Compounds 1–4 were investigated via direct current and alternating current magnetic susceptibility measurements, and it was found that each complex displayed single-molecule magnet (SMM) behavior. All four compounds display unique coordination and geometric environments around the DyIII ions and it was found that each displays a different anisotropy barrier. Ab initio calculations were performed on 1–4 and these determined the low lying electronic structure of each DyIII ion and the magnetic interactions for each cluster. It was found that there was a strong correlation between the calculated energy gap between the ground and first excited states of the single-ion ligand-field split DyIII levels and the experimentally observed anisotropy barrier. Furthermore, the transverse g factors found for the DyIII ions, defining the tunnelling rates within the ground Kramers doublets, are largest for 1, which agrees with the experimental observation of the shortest relaxation time in the high-temperature domain for this complex. The magnetic exchange between the DyIII ions revealed overall antiferromagnetic interactions for each compound, derived from the dominant dipolar exchange resulting in nonmagnetic ground states for 1–4. The diamagnetic ground states coupled with small tunneling gaps resulted in quantum tunneling time scales at zero field of between 0.1 and >1.5 s.