Tuning the Raman/Orbach Relaxation Ratio in Co2IIIDyIII Single-Molecule Magnets through Acetate-to-Nitrate Substitution

To precisely regulate Orbach and Raman processes in polynuclear complexes, four novel Co2IIILnIII complexes with the molecular formula of [Co2IIILn(HL)2{(py)2CO2}(OAc)3]·2CH3CN·1.5H2O (Ln = Tb (1Tb), Dy (2Dy), and Ho (3Ho); (py)2CO2H2 = the gem-diol form of di-2-pyridyl ketone(dpk); OAc– = acetate) and [Co2IIIDy(HL)2{(py)2CO2}(OAc)2(NO3)]·2CH3OH·H2O (4Dy) have been obtained. The main difference between 1Tb–3Ho and 4Dy is that one μ1-κ1-O,O′-OAc– ligand coordinates with LnIII in 1Tb, 2Dy, and 3Ho, while one μ1-κ1-O,O′-NO3– coordinates with DyIII in 4Dy. Alternating current (ac) susceptibility studies revealed that 1Tb and 3Ho show field-induced slow magnetic relaxation signatures, while 2Dy and 4Dy both exhibit field-induced single-molecule magnet behavior. Additionally, the Raman/Orbach ratios are about 18 for 2Dy and about 5480 for 4Dy, indicating that the relative weighting of the Raman and the Orbach relaxation channels can be markedly modulated by OAc–/NO3– substitution. Ab initio calculations analyzed the difference in the magnetic relaxation behavior of 2Dy and 4Dy and revealed that the directions of the main magnetic axes of DyIII are sensitive to the coordinated OAc–/NO3– ions, namely, the main magnetic axis of DyIII in 2Dy is almost parallel to the Dy1–O2 bond, while the axis of DyIII in 4Dy deviates from all Dy–O bond directions.