Modulation of the Coordination Environment around the Magnetic Easy Axis Leads to Significant Magnetic Relaxations in a Series of 3d-4f Schiff Complexes

A series of Salen-type Zn(II) Dy(III) complexes [(LZn)-Zn-1(II)CIDy(III)(acac)(2)]center dot H2O (1), [(LZn)-Zn-1(II)BrDy(III)(acac)(2)]-H2O (2), [LIZn(II)(H2O)Dy(III)(acac)(2)]center dot CH2C2 center dot PF6 (3), [(LZn)-Zn-2(II)(H2O)Dy(III)(acac)(2)]center dot PF6 (4), and Co(III) Dy(III) complexes [(LCo)-Co-1(III)Br2Dy(III)-(acac)(2)]CH2Cl2 (5), [(LCo)-Co-2(III)Cl2Dy(III)(acac)Cl(MeO)] (6), [(LCo)-Co-2(III)Cl2Dy(III)(acac)Cl(H2O)] (7), and [L2Co-(III)Cl2Dy(III)(NO3)(2)(MeO)] (8) heterobinuclear single molecule magnets (SMMs) were synthesized and magnetically characterized. These complexes were constructed by incorporating diamagnetic Zn(II) and Co(III) ions with acetylacetone (acac) and compartmental Schiff-base ligands (H2L1 = N,N'-bis(2-oxy-3-methoxybenzylidene)-1,2-phenylenediamine; H2L2 = N,N'-bis(2-oxy-3-methoxybenzylidene)-1,2-diaminocyclohexane). In the Zn(II) Dy(III) (1-4) system, the coordination environments of the Dy(III) ions are nearly identical, but the apical coordination atom to the Zn(II) ion is different. Complexes 1, 2, and 4 displayed no magnetic relaxation in the absence of external magnetic field, but complex 3 displayed more pronounced SMM behavior with a relaxation energy barrier U-eff/k(B) 38 K and magnetic hysteresis at 1.8 K. The SMM performances of 5, 6, and 7 were enhanced significantly by incorporating an octahedral Co(III) instead of square pyramidal Zn(II) and replacing one of acac(-) group around Dy(III) ion by a neutral 0 atom, displaying [U-eff of 167, 118, and 75 K as well as magnetic hysteresis up to 3.5 K. These studies indicated that the remote diamagnetic Zn(II) and Co(III) ions played a key role, and the SMM properties were very strongly related to the special coordination atoms configuration around Dy(III) ion. When this coordination configuration around was broken as 8 exhibited, however, it resulted in a dramatically decreased SMM performance. From this work, the key factors that significantly affect the SMM performance of these heteronuclear Zn(II)/Co(III)-Dy(III) SMMs are unambiguously presented.