Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less M_S mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [Mn^III ₆ Cr^III ]³⁺ and the triplesalalen-based *[Mn^III ₆ Cr^III ]³⁺ . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism. However, we found weak antiferromagnetic couplings, that we assigned to an inefficient spin-polarization by a heteroradialene formation. To prevent this heteroradialene formation, the triplesalalen ligand H₆ talalen $$ was designed. Here, we present the building block [(talalen $$ )Mn^III ₃ ]³⁺ and its application for the assembly of [{(talalen $$ )Mn^III ₃ }₂ {Cr^III (CN)₆ }]³⁺ (=*[Mn^III ₆ Cr^III ]³⁺ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld $$ )Mn^III ₃ ]³⁺ proves the absence of heteroradialene character and the enforcement of ferromagnetic Mn^III -Mn^III interactions in the (talalen $$ )^6- complexes. This results in an increase of the barrier for spin reversal U_eff from 25 K in the triplesalen-based [Mn^III ₆ Cr^III ]³⁺ SMMs to 37 K in the triplesalalen-based *[Mn^III ₆ Cr^III ]³⁺ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.