Jean-Claude Bunzli - Molecular Magnetism and Iron(Ii) Spin-State Equilibrium As Structural Probes in Heterodinuclear D-F Complexes

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  Piguet,C., RivaraMinten,E., Hopfgartner,G., Bunzli,J.C.G.   Helvetica Chimica Acta 1995 78  (7) 1651-1672

  Abstract: Fe(ClO4)(2) reacts with the segmental   ligand   2-{6-[1-(3,5-dimethoxybenzyl)-1H-benzimidazol-2-yl]pyridin-2-yl}-1,1'-dimethyl- 5,5'-methylene-2'-(5-methylpyridin-2-yl)bis[1H-benzimidazole]   (L(2)) in MeCN to give the diamagnetic deep violet complex   [Fe(L(2))(2)](2+) where the metal is pseudo-octahedrally   coordinated by two perpendicular tridentate binding units. When   L(2) reacts with an equimolar mixture of Ln(ClO4)(3) (Ln = La,   Ce, Pr, Nd, Sm, Eu) and Fe(ClO4)(2), electrospray-mass   spectrometric, spectrophotometric, and H-1-NMR data in MeCN show   the selective formation of the deep red heterodinuclear   C-3-cylindrical complexes [LnFe(L(2))(3)](5+) where the three   ligands L(2) are wrapped about the metal-metal axis. Fe-II   occupies the pseudo-octahedral capping site produced by the three   bidentate units and Ln(III) lies in the resulting 'facial'   pseudo-tricapped trigonal prismatic site defined by the three   remaining tridentate coordinating units. The heterodinuclear   complexes [LnFe(L(2))(3)](5+) display spin-state equilibrium   ((1)A reversible arrow(5)T) and thermochromism in MeCN between   243 and 333 K. Detailed H-1-NMR, UV/VIS, and magnetic   measurements in solution show that the partial spin-crossover   behavior of [LnFe(L(2))(3)](5+) occurs for Ln = La-Eu with   similar thermodynamic parameters (Delta H-sc = 20-23 kJ . mol(-1)   and Delta S-sc = 55-66 J . mol(-1). K-1) indicating that the size   of Ln(III) has a negligible influence on the spin-state   equilibrium. However, the smaller Ln(III) ions have less affinity   for the pseudo-tricapped trigonal prismatic coordination site in   the heterodinuclear complexes as demonstrated by the partial   decomplexation of [YFe(L(2))(3)](5+) to give [Fe(L(2))(2)](2+)   and the absence of the heterodinuclear complex   [LuFe(L(2))(3)](5+) under the same conditions, The crucial role   played by the sterically demanding Fe-II in the assembly   processes is discussed together with the use of the efficient   combination of lanthanide probes with magnetic d-block probes for   the design and investigation of luminescent and magnetic   materials with controlled structural and physical properties.   Photophysical measurements reveal that efficient ligand -->   metal and Eu --> Fe energy transfers occur in   [EuFe(L(2))(3)](5+) which strongly quench both the ligand and the   Eu-centered luminescence

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