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