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 2 j rad-rad / k B > 300 K and 2 J Gd-rad / k B = 1.2 K in the H = -2 J   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.