Ferromagnetic versus antiferromagnetic exchange interactions in tetrathiafulvalene-based 3d/4f heterobimetallic complexes

(TTF-salphen)M compounds (TTF-salphen^2-=4,5-bis(propylthio) tetrathiafulvalene-N,N′-phenylenebis(salicylideneimine) dianion; M=Cu ^II and Ni^II) have been treated with Ln(hfac) ₃·2 H₂O precursors (hfac^-=1,1,1,5,5,5- hexafluoroacetylacetonate anion; Ln=Gd^III, Tb^III, and Dy^III) to elaborate unprecedented 3d/4f TTF-based heterobimetallic complexes of formula [(TTF-salphen)MLn(hfac)₃]. All the structures of these compounds have been resolved by X-ray diffraction on single crystals. The structures of these complexes are formed by a TTF-salphen^2- ligand coordinated to the 3d metal ions in the inert tetradentate N₂O ₂ site. The Ln(hfac)₃ fragment is coordinated to the (TTF-salphen)M one through the two phenolate bridges. Even if the complexes are similar in both Cu^II and Ni^II families, the crystal packing is different. In the first case, dimers of TTF-salphen^2- donors constitute the organic network. In the other case, a reminiscent organic network is observed with S⋯S contacts. The photophysical properties of [(TTF-salphen)CuDy(hfac)₃] (3) in chloroform solution highlight the redshift of the TTF→salphen charge transfer (400 cm^-1) relative to the analogue excitations in (TTF-salphen)Cu, which attest to the stability of these structures in solution. Static magnetic measurements have allowed us to quantify the ferromagnetic interactions (J=+1.29 cm^-1) between Cu^II and Gd^III in the [(TTF-salphen)CuGd(hfac) ₃] complex. Finally, an empirical method that consists of the comparisons of the magnetic properties of [(TTF-salphen)CuTb(hfac)₃] with [(TTF-salphen)NiTb(hfac)₃] and [(TTF-salphen)CuDy(hfac) ₃] with [(TTF-salphen)NiDy(hfac)₃] has established that ferromagnetic interactions take place between Cu^II and Tb ^III ions, whereas unusual antiferromagnetic interactions have been identified between Cu^II and Dy^III ions. Perfect partners: Tetrathiafulvalene-based 3d/4f heterobimetallic complexes have been crystallized. Their photophysical properties demonstrate the coordination effect of the 4f lanthanides and the stability of the complexes in solution. Empirical methods have been used to determine that ferromagnetic interactions take place in the Cu/Gd and Cu/Tb pairs, while antiferromagnetic interactions are found for Cu/Dy pairs (see picture).