Molecular Magnetism and Iron(II) Spin‐State Equilibrium as Structural Probes in Heterodinuclear d–f Complexes

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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] (L2) in MeCN to give the diamagnetic deep violet complex [Fe(L2)2]2+ where the metal is pseudo‐octahedrally coordinated by two perpendicular tridentate binding units. When L2 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 1H‐NMR data in MeCN show the selective formation of the deep red heterodinuclear C3‐cylindrical complexes [LnFe(L2)3]5+ where the three ligands L2 are wrapped about the metal‐metal axis. FeII occupies the pseudo‐octahedral capping site produced by the three bidentate units and LnIII lies in the resulting ‘facial’ pseudo‐tricapped trigonal prismatic site defined by the three remaining tridentate coordinating units. The heterodinuclear complexes [LnFe(L2)3]5+ display spin‐state equilibrium (1A ⇄ 5T) and thermochromism in MeCN between 243 and 333 K. Detailed 1H‐NMR, UV/VIS, and magnetic measurements in solution show that the partial spin‐crossover behavior of [LnFe(L2)3]5+ occurs for Ln = LaEu with similar thermodynamic parameters (ΔHsc = 20–23 kJ·mol−1 and ΔSsc = 55–66 J·mol−1·K−1) indicating that the size of LnIII has a negligible influence on the spin‐state equilibrium. However, the smaller LnIII ions have less affinity for the pseudo‐tricapped trigonal prismatic coordination site in the heterodinuclear complexes as demonstrated by the partial decomplexation of [YFe(L2)3]5+ to give [Fe(L2)2]2+ and the absence of the heterodinuclear complex [LuFe(L2)3]5+ under the same conditions. The crucial role played by the sterically demanding FeII 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 transfer occur in [EuFe(L2)3]5+ which strongly quench both the ligand and the Eu‐centered luminescence.

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