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 MS 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 [MnIII6 CrIII ]3+ and the triplesalalen-based *[MnIII6 CrIII ]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 tBu2 was designed. Here, we present the building block [(talalen tBu2 )MnIII3 ]3+ and its application for the assembly of [{(talalen tBu2 )MnIII3 }2 {CrIII (CN)6 }]3+ (=*[MnIII6 CrIII ]3+ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld tBu2 )MnIII3 ]3+ proves the absence of heteroradialene character and the enforcement of ferromagnetic MnIII -MnIII interactions in the (talalen tBu2 )6- complexes. This results in an increase of the barrier for spin reversal Ueff from 25 K in the triplesalen-based [MnIII6 CrIII ]3+ SMMs to 37 K in the triplesalalen-based *[MnIII6 CrIII ]3+ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.