Exchange Interactions Switch Tunneling: A Comparative Experimental and Theoretical Study on Relaxation Dynamics by Targeted Metal Ion Replacement

The magnetic relaxation and magnetization blocking barriers of tailor‐made homo‐ and heterodinuclear compounds [Dy2(opch)2(OAc)2(H2O)2]⋅MeOH (1) and [DyMn(opch)2(OAc)(MeOH)(H2O)2] (2), where H2opch is (E)‐N′‐(2‐hydroxy‐3‐methoxybenzylidene)pyrazine‐2‐carbohydrazide, were systematically investigated and the change in single‐molecule magnet behavior originating from targeted replacement of one dysprosium site in the Dy2 compound with manganese was elucidated through a combination of experimental and theoretical studies. A detailed comparative study on these closely related model compounds revealed remarkable changes of the crystal‐field splitting and anisotropy of the Dy site and the total exchange spectrum due to the replacement of Dy by Mn. The blocking barriers of these two compounds, which explain their different relaxation behaviors, were analyzed. The two Ising doublets arising from the magnetic interaction in the case of 1 are strongly uniaxial, with tunneling splittings smaller than 10−6 cm−1, and this leads to magnetic relaxation at temperatures exceeding the exchange energy (2.14 cm−1), which involves transition via the excited states corresponding to local transitions on the excited doublet at the Dy site. The third and fourth exchange doublets in 2 (located at 2.16 and 3.25 cm−1, respectively) show much larger tunneling splittings (of 10−4 and 10−3 cm−1, respectively), and thus open an important path for magnetic relaxation. A tailor‐made pair: Centrosymmetric Dy2 complex 1 and its DyMn derivative 2 were developed to probe systematically the magnetic relaxation mechanism. Complex 1 shows typical single‐molecule magnet behavior, whereas no out‐of‐phase ac signal was observed for 2. The Ising interaction in 1 efficiently suppresses the quantum tunneling, whereas in 2, the third and fourth exchange doublets show much larger tunneling splittings, and thus an important path for magnetic relaxation in this compound is switched on (see figure).