Trinuclear Copper Complexes with Triplesalen Ligands: Geometric and Electronic Effects on Ferromagnetic Coupling via the Spin‐Polarization Mechanism
Authors: Glaser Thorsten , Heidemeier Maik , Strautmann Julia B. H. , Bögge Hartmut , Stammler Anja , Krickemeyer Erich , Huenerbein Robert , Grimme Stefan , Bothe Eberhard , Bill Eckhard 
Published: October 2007

AbstractA series of trinuclear CuII complexes with the tris(tetradentate) triplesalen ligands H6talen, H6talen, and H6talen, namely [(talen)CuII3] (1), [(talen)CuII3] (2), and [(talen)CuII3] (3), were synthesized and their molecular and electronic structures determined. These triplesalen ligands provide three salen‐like coordination environments bridged in a meta‐phenylene arrangement by a phloroglucinol backbone. The structure of [(talen)CuII3] (1) was communicated recently. The structure of the tert‐butyl derivative [(talen)CuII3] (2) was established in three different solvates. The molecular structures of these trinuclear complexes show notable differences, the most important of which is the degree of ligand folding around the central CuII–phenolate bonds. This folding is symmetric with regard to the central phloroglucinol backbone in two structures, where it gives rise to bowl‐shaped overall geometries. For one solvate two trinuclear triplesalen complexes form a supramolecular disk‐like arrangement, hosting two dichloromethane molecules like two pearls in an oyster. The FTIR spectra of these complexes indicate the higher effective nuclear charge of CuII in comparison to the trinuclear NiII complexes by the lower CO and higher CN stretching frequencies. The UV/Vis/NIR spectra of 1–3 reflect the stronger ligand folding in the tert‐butyl complex 2 by an intense phenolate‐to‐CuII LMCT. This absorption is absent in 1 and is obscured by the nitro chromophore in 3. The more planar molecular structures cause orthogonality of the CuII d orbital and the phenolate O pz orbital, which leads to small LMCT dipole strengths. Whereas 1 and 3 exhibit only irreversible oxidations, 2 exhibits a reversible one‐electron oxidation at +0.26 V, a reversible two‐electron oxidation at +0.59 V, and a reversible one‐electron oxidation at +0.81 V versus Fc+/Fc. The one‐electron oxidized form 2+ is strongly stabilized with respect to reference mononuclear salen‐like Cu complexes. Chemical one‐electron oxidation of 2 to 2+ allows the determination of its UV/Vis/NIR spectrum, which indicates a ligand‐centered oxidation that can be assigned to the central phloroglucinol unit by analogy with the trinuclear Ni triplesalen series. Delocalization of this oxidation over three CuII–phenolate subunits causes the observed energetic stabilization of 2+. Temperature‐dependent magnetic susceptibility measurements reveal ferromagnetic couplings for all three trinuclear CuII triplesalen complexes. The trend of the coupling constants can be rationalized by two opposing effects: 1) electron‐withdrawing terminal substituents stabilize the central CuII–phenolate bond, which results in a stronger coupling, and 2) ligand folding around the central CuII–phenolate bond opens a bonding pathway between the magnetic CuII d orbital and the phenolate O pz orbital, which results in a stronger coupling. Density functional calculations indicate that both spin‐polarization and spin‐delocalization are operative and that slight geometric variations alter their relative magnitudes.