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 H ₆ talalen t Bu 2 was designed. Here, we present the building block [(talalen t Bu 2 )Mn ^III ₃ ] ³⁺ and its application for the assembly of [{(talalen t Bu 2 )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 t Bu 2 )Mn ^III ₃ ] ³⁺ proves the absence of heteroradialene character and the enforcement of ferromagnetic Mn ^III -Mn ^III interactions in the (talalen t Bu 2 ) ^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.
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