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 [L1Zn(II)ClDy(III)(acac)2]·H2O (1), [L1Zn(II)BrDy(III)(acac)2]·H2O (2), [L1Zn(II)(H2O)Dy(III)(acac)2]·CH2Cl2·PF6 (3), [L2Zn(II)(H2O)Dy(III)(acac)2]·PF6 (4), and Co(III)-Dy(III) complexes [L1Co(III)Br2Dy(III)(acac)2]·CH2Cl2 (5), [L2Co(III)Cl2Dy(III)(acac)Cl(MeO)] (6), [L2Co(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 Ueff/ kB 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 O atom, displaying Ueff 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.