Ferrocenyl-based di- and trinuclear lanthanide complexes: solid state structures, (spectro)electrochemical and DFT studies

Abstract

Dinuclear and trinuclear ferrocenylcarboxylato-bridged lanthanide complexes of type [Ln(μO:κ²OO′-O₂CFc)(O₂CFc)₂(H₂O)(dmf)]₂·(dmf)₂ (Ln = Sm (2), Eu (3), Gd (4), Tb (5); Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)), and novel [Bu₄N][Ln₃(μ-O₂CFc)₃(μO:κ²OO′-O₂CFc)₃(O₂CFc)₃(μ₃-OH)]·[Bu₄N]Cl (Ln = Gd (6), Tb (7)) were prepared by the reaction of [LnCl₃·6H₂O] (synthesis of 2–5) or LnCl₃ (synthesis of 6, 7) with FcCO₂H (1) in the ratio of 1 : 3. As evidenced by single crystal X-ray structure determination, in 2–5 the lanthanide ions are connected by symmetric FcCO₂ units. In addition, two ferrocenylcarboxylato groups are μ-bridged to Ln^III. Each Ln^III ion is coordinated by nine oxygen donor atoms derived from one H₂O, one dmf and three carboxylates. The latter are found in chelating κ² and bridging μ,κ³ coordination modes. Complexes 6 and 7 assemble three Ln^III cores around a central μ₃-netting hydroxide and nine FcCO₂ entities. A combination of κ², μ,κ² and μ,κ³ coordination modes results in an eight-fold coordination sphere for each metal, which is best described as bicapped trigonal prismatic. IR spectroscopy confirms the chelating and bridging motifs. Electrochemical studies of complexes 2–7via cyclic voltammetry (CV) and square-wave voltammetry (SWV) showed one redox event between E°′ = 250 and 260 mV vs. FcH/FcH⁺ for 2–5 with all six FcCO₂ redox events superimposed. Complexes 6 and 7 show a total of three events in the CV with the oxidations of the nine FcCO₂ units occurring in close proximity. Deconvolution of individual redox events correlates well with the mononuclear complex [Bu₄N][Gd(O₂CFc)₄]. UV–Vis/NIR spectroelectrochemical measurements of 7 did not reveal electron transfer between either Fc units, nor the coordinated lanthanides and resembled the absorption behavior of [Bu₄N][Tb(O₂CFc)₄]. DFT (Density Functional Theory) calculations on the B3LYP def2-TZVP level of theory were carried out to assign the order of redox events in 6 showing that the spatial distance towards the most recent redox center, instead of the binding mode, is decisive.