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

Dinuclear and trinuclear ferrocenylcarboxylato-bridged lanthanide complexes of type [Ln(μ O :κ 2 OO ′-O 2 CFc)(O 2 CFc) 2 (H 2 O)(dmf)] 2 ·(dmf) 2 (Ln = Sm ( 2 ), Eu ( 3 ), Gd ( 4 ), Tb ( 5 ); Fc = Fe(η 5 -C 5 H 4 )(η 5 -C 5 H 5 )), and novel [Bu 4 N][Ln 3 (μ-O 2 CFc) 3 (μ O :κ 2 OO ′-O 2 CFc) 3 (O 2 CFc) 3 (μ 3 -OH)]·[Bu 4 N]Cl (Ln = Gd ( 6 ), Tb ( 7 )) were prepared by the reaction of [LnCl 3 ·6H 2 O] (synthesis of 2-5 ) or LnCl 3 (synthesis of 6 , 7 ) with FcCO 2 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 2 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 H 2 O, one dmf and three carboxylates. The latter are found in chelating κ 2 and bridging μ,κ 3 coordination modes. Complexes 6 and 7 assemble three Ln III cores around a central μ 3 -netting hydroxide and nine FcCO 2 entities. A combination of κ 2 , μ,κ 2 and μ,κ 3 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 2-7 via 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 FcCO 2 redox events superimposed. Complexes 6 and 7 show a total of three events in the CV with the oxidations of the nine FcCO 2 units occurring in close proximity. Deconvolution of individual redox events correlates well with the mononuclear complex [Bu 4 N][Gd(O 2 CFc) 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 [Bu 4 N][Tb(O 2 CFc) 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. Reaction of ferrocenyl carboxylic acid with lanthanide salts gave di- and trimeric complexes bearing 6 and 9 organometallics. Square wave voltammetry and DFT studies showed that electrostatic repulsion determines the distribution of redox events.