Ferromagnetic 2p-2p and 4f-2p Couplings in a Macrocycle from Two Biradicals and Two Gadolinium(III) Ions

A new ground triplet biradical 2′,4′,6′-triisopropylbiphenyl-3,5-diyl bis(tert-butyl nitroxide) (iPr(3)BPBN) was prepared and characterized by means of room-temperature ESR spectroscopy displaying a zero-field splitting pattern together with a half-field signal. Complex formation with gadolinium(III) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate (hfac) afforded a macrocycle [{Gd(hfac)(3)(μ-iPr(3)BPBN)}(2)]. As the X-ray crystallographic analysis clarified, the biradical serves as a bridging ligand, giving a 16-membered ring, where each nitroxide radical oxygen atom is directly bonded to a Gd(3+) ion. The magnetic study revealed that the iPr(3)BPBN bridge behaved as a practically triplet biradical and that the Gd(3+)-radical magnetic coupling was weakly ferromagnetic. The exchange parameters were estimated as 2j(rad-rad)/k(B) > 300 K and 2J(Gd-rad)/k(B) = 1.2 K in the H = −2J S(1)•S(2) convention. The DFT calculation based on the atomic coordinates clarified the ground triplet nature in metal-free iPr(3)BPBN and the enhanced triplet character upon coordination. The calculation also suggests that ferromagnetic coupling would be favorable when the Gd-O-N-C(sp(2)) torsion comes around 100°. The present results are compatible with the proposed magneto-structure relationship on the nitroxide-Gd compounds.