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Kovalski, Eduard
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article
Ferrocenyl-based di- and trinuclear lanthanide complexes
Abstract
<p>Dinuclear and trinuclear ferrocenylcarboxylato-bridged lanthanide complexes of type [Ln(μO:κ<sup>2</sup>OO′-O<sub>2</sub>CFc)(O<sub>2</sub>CFc)<sub>2</sub>(H<sub>2</sub>O)(dmf)]<sub>2</sub>·(dmf)<sub>2</sub> (Ln = Sm (2), Eu (3), Gd (4), Tb (5); Fc = Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)), and novel [Bu<sub>4</sub>N][Ln<sub>3</sub>(μ-O<sub>2</sub>CFc)<sub>3</sub>(μO:κ<sup>2</sup>OO′-O<sub>2</sub>CFc)<sub>3</sub>(O<sub>2</sub>CFc)<sub>3</sub>(μ<sub>3</sub>-OH)]·[Bu<sub>4</sub>N]Cl (Ln = Gd (6), Tb (7)) were prepared by the reaction of [LnCl<sub>3</sub>·6H<sub>2</sub>O] (synthesis of 2-5) or LnCl<sub>3</sub> (synthesis of 6, 7) with FcCO<sub>2</sub>H (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 FcCO<sub>2</sub> units. In addition, two ferrocenylcarboxylato groups are μ-bridged to Ln<sup>III</sup>. Each Ln<sup>III</sup> ion is coordinated by nine oxygen donor atoms derived from one H<sub>2</sub>O, one dmf and three carboxylates. The latter are found in chelating κ<sup>2</sup> and bridging μ,κ<sup>3</sup> coordination modes. Complexes 6 and 7 assemble three Ln<sup>III</sup> cores around a central μ<sub>3</sub>-netting hydroxide and nine FcCO<sub>2</sub> entities. A combination of κ<sup>2</sup>, μ,κ<sup>2</sup> and μ,κ<sup>3</sup> 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<sup>+</sup> for 2-5 with all six FcCO<sub>2</sub> redox events superimposed. Complexes 6 and 7 show a total of three events in the CV with the oxidations of the nine FcCO<sub>2</sub> units occurring in close proximity. Deconvolution of individual redox events correlates well with the mononuclear complex [Bu<sub>4</sub>N][Gd(O<sub>2</sub>CFc)<sub>4</sub>]. 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 [Bu<sub>4</sub>N][Tb(O<sub>2</sub>CFc)<sub>4</sub>]. 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.</p>