Combined experimental and theoretical studies on a series of mononuclear LnIII single-molecule magnets: dramatic influence of remote substitution on the magnetic dynamics in Dy analogues

A series of Ln(III) complexes of general formula [Ln(H2L1)(2)(NO3)(2)(H2O)](NO3) (1-5) [Ln = Dy (1), Tb (2) Ho (3), Er (4), and Yb (5)] and an analogous Dy-III complex with ligand H2L2, [Dy(H2L2)(2)(NO3)(3)(H2O)](NO3) (6), where H2L1 and H2L2 stand for (E)-2-[(2-hydroxyphenyl)iminomethyl]-6-methoxy-4-methylphenol and (E)-2-[(2-hydroxy-5-methylphenyl)iminomethyl]-6-methoxy-4-methylphenol, respectively, have been synthesized and magneto-structurally characterized. All these complexes are isostructural and isomorphous, in which the zwitterionic form of the ligands predominantly coordinate the metal centers. The magnetic study revealed that complex 3 displays negligible SMM behaviour, while 1 and 6 are zero field SMMs, the performance of which can largely be improved in the presence of an applied dc field by lowering under barrier relaxation processes, and finally 2, 4, and 5 are field-induced SMMs. The most remarkable observation in the present study is the dramatically-enhanced SMM performance in 6 compared to 1, achieved by only a remote methyl substitution at the ligand framework to increase the intermolecular separation. Although SINGLE_ANISO ab initio calculations for 1 and 6 are very similar, the POLY_ANISO module revealed weak dipolar interactions in both the compounds but significant antiferromagnetic interaction in 1, thereby justifying the experimental fact. The present work discloses that even a small substitution such as a methyl group can adequately increase the intermolecular separation, leading to several-fold enhanced effective energy barrier.