Structural, electronic and magnetic properties of Cu(ii) complexes of 2-substituted tropones bearing a ferrocenyl group at 5-position

Heterotrinuclear Fe(ii)-Cu(ii)-Fe(ii) complexes [Cu(FcTropOMe) ₂(H ₂O) ₂](OTf) ₂ (FcTropOMe = 5-ferrocenyl-2-methoxytropone) (1), [Cu(FcTropNEt ₂) ₂](OTf) ₂ (FcTropNEt ₂ = 2-(N,N-diethylamino)-5- ferrocenyltropone) (2) and [Cu(FcTropNEt) ₂] (FcTropNEt = 2-(N-ethylamino)-5-ferrocenyltroponate) (3) were synthesized. In addition, a hexafluorophosphate salt of heterotrinuclear Fe(iii)-Cu(ii)-Fe(iii) complex [Cu(FcTropNEt) ₂] ²⁺ (3 ²⁺) was successfully obtained as single crystals by electrochemical oxidation of 3. By comparing the X-ray structures and absorption spectra of dicationic complexes 1 and 2, the 2-(diethylamino)tropone ligand was found to induce a greater intramolecular charge transfer (CT) from ferrocenyl to tropone-Cu(ii) moieties than the 2-methoxytropone ligand. On the other hand, 3 ²⁺ showed a broad CT band in the near-infrared (NIR) region similar to 2, which can be assigned to a transition from troponato-Cu(ii) to ferrocenium moieties. As for the magnetic properties of 3 ²⁺(PF ₆ ^-) ₂, measurements of temperature dependence of magnetic susceptibility and ESR on the solid state and in solution revealed the presence of a strong ferromagnetic interaction (J _Fe-Cu = +12.0 cm ^-1) between the low spin Fe(iii) ion with S = 1/2 and Cu(ii) ion with S = 1/2 despite a long distance pathway via the aminotroponato and cyclopentadienyl moieties. DFT calculations supported this intramolecular ferromagnetism, which is induced by a spin polarization mechanism through the π-spacers.