Synthesis, Characterization, Electrochemistry, and Computational Studies of Ferrocenyl-Substituted Siloles

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(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 LnIII. Each LnIII 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 LnIII 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.

Electronic Coupling in 1,2,3-Triazole Bridged Ferrocenes and Its Impact on Reactive Oxygen Species Generation and Deleterious Activity in Cancer Cells

Mixed-valence (MV) binuclear ferrocenyl compounds have long been studied as models for testing theories of electron transfer and in attempts to design molecular-scale electronic devices (e.g., molecular wires). In contrary to that, far less attention has been paid to MV binuclear ferrocenes as anticancer agents. Herein, we discuss the synthesis of six 1,2,3-triazole ferrocenyl compounds for combined (spectro)electrochemical, electron paramagnetic resonance (EPR), computational, and anticancer activity studies. Our synthetic approach was based on the copper-catalyzed 1,3-dipolar azide–alkyne cycloaddition reaction and enabled us to obtain in one step compounds bearing either one, two, or three ferrocenyl entities linked to the common 1,2,3-triazole core. Thus, two series of complexes were obtained, which pertain to derivatives of 3′-azido-3′-deoxythymidine (AZT) and 3-azidopropionylferrocene, respectively. Based on the experimental and theoretical data, the two mono-oxidized species corresponding to binuclear AZT and trinuclear 3-azidopropionylferrocene complexes have been categorized as class II mixed-valence according to the classification proposed by Robin and Day. Of importance is the observation that these two compounds are more active against human A549 and H1975 non-small-cell lung cancer cells than their congeners, which do not show MV characteristics. Moreover, the anticancer activity of MV species competes or surpasses, dependent on the cell line, the activity of reference anticancer drugs such as cisplatin, tamoxifen, and 5-fluorouracil. The most active from the entire series of compounds was the binuclear thymidine derivative with the lowest IC₅₀ value of 5 ± 2 μM against lung H1975 cancer cells. The major mechanism of antiproliferative activity for the investigated MV compounds is based on reactive oxygen species generation in cancer cells. This hypothesis was substantiated by EPR spin-trapping experiments and the observation of decreased anticancer activity in the presence of N-acetyl cysteine (NAC) free-radical scavenger.

The 1,2,3-triazole bridged bi- and triferrocenyl compounds were prepared via a “click” reaction. Their corresponding mono-oxidized forms have been categorized as class II MV species. The biferrocenyl thymidine derivative showed remarkable anticancer activity against human A549 and H1975 cancer cells and negligible activity against nonmalignant human BEAS-2B cells. The anticancer activity mechanism is mainly due to ROS generation, and it originates from the combination of electronic coupling and the thymidine moiety, combined all together in one molecular scaffold.

Planar-chiral 1,1'-diaminoferrocenes

Planar-chiral homologues of 1,1'-diaminoferrocene, which bear a single additional substituent adjacent to one of the amino groups, are prepared as racemic mixtures in a few steps and in good yields from ferrocene. Various substituents relevant to steric shielding, coordination and further functionalisation are used, giving access to ferrocene-based planar-chiral diimines and diamines as well as stable N-heterocyclic carbenes and tetrylenes by transformations analogous to procedures established for 1,1'-diaminoferrocene.

Ferrocenyl-Pyrenes, Ferrocenyl-9,10-Phenanthrenediones, and Ferrocenyl-9,10-Dimethoxyphenanthrenes: Charge-Transfer Studies and SWCNT Functionalization

The synthesis of 1-Fc- (3), 1-Br-6-Fc- (5 a), 2-Br-7-Fc- (7 a), 1,6-Fc₂ - (5 b), 2,7-Fc₂ -pyrene (7 b), 3,6-Fc₂ -9,10-phenanthrenedione (10), and 3,6-Fc₂ -9,10-dimethoxyphenanthrene (12; Fc=Fe(η⁵ -C₅ H₄ )(η⁵ -C₅ H₅ )) is discussed. Of these compounds, 10 and 12 form 1D or 2D coordination polymers in the solid state. (Spectro)Electrochemical studies confirmed reversible Fc/Fc⁺ redox events between -130 and 160 mV. 1,6- and 2,7-Substitution in 5 a (E°'=-130 mV) and 7 a (E°'=50 mV) influences the redox potentials, whereas the ones of 5 b and 7 b (E°'=20 mV) are independent. Compounds 5 b, 7 b, 10, and 12 show single Fc oxidation processes with redox splittings between 70 and 100 mV. UV/Vis/NIR spectroelectrochemistry confirmed a weak electron transfer between Fe^II /Fe^III in mixed-valent [5 b]⁺ and [12]⁺ . DFT calculations showed that 5 b non-covalently interacts with the single-walled carbon nanotube (SWCNT) sidewalls as proven by, for example, disentangling experiments. In addition, CV studies of the as-obtained dispersions confirmed exohedral attachment of 5 b at the SWCNTs.

Diaqua-β-octaferrocenyltetraphenylporphyrin: a multiredox-active and air-stable 16π non-aromatic species

Herein the synthesis and properties of the first β-octaferrocenyltetraphenylporphyrin, {TPPFc₈(H₂O)₂}, in its extraordinary stable and non-aromatic 16π form are reported, showing seven separate reversible redox events.

Ferrocenyl naphthalenes: substituent- and substitution pattern-depending charge transfer studies

The synthesis and characterisation of a series of ferrocenyl-functionalized naphthalenes is reported, whereby the electrochemical behaviour and charge transfer properties depend on the substitution pattern.

The anionic Fries rearrangement: a convenient route to ortho-functionalized aromatics

The scope and mechanism of anionic (hetero-) Fries rearrangements are summarized for various migrating groups and arenes, including applications and computational studies.

Sandwich and half-sandwich metal complexes derived from cross-conjugated 3-methylene-penta-1,4-diynes

Mono-, di- and trimetallic derivatives of the cross-conjugated 3-methylene-penta-1,4-diyne moiety have been prepared, and studied by (spectro)electrochemical methods.

Direct, Sequential, and Stereoselective Alkynylation of C,C-Dibromophosphaalkenes

The first direct alkynylation of C,C-dibromophosphaalkenes by a reaction with sulfonylacetylenes is reported. Alkynylation proceeds selectively in the trans position relative to the P substituent to afford bromoethynylphosphaalkenes. Owing to the absence of transition metals in the procedure, the previously observed conversion of dibromophosphaalkenes into phosphaalkynes through the phosphorus analog of the Fritsch-Buttenberg-Wiechell rearrangement is thus suppressed. The bromoethynylphosphaalkenes can subsequently be converted to C,C-diacetylenic, cross-conjugated phosphaalkenes by following a Sonogashira coupling protocol in good overall yields. By using the newly described method, full control over the stereochemistry at the P=C double bond is achieved. The substrate scope of this reaction is demonstrated for different dibromophosphaalkenes as well as different sulfonylacetylenes.

Crystal structure of 3-ferrocenyl-1-phenyl-1H-pyrrole, [Fe(η(5)-C5H4 (c) C4H3 NPh)(η(5)-C5H5)]

The mol-ecular structure of the title compound, [Fe(C5H5)(C15H12N)], consists of a ferrocene moiety with an N-phenyl-pyrrole heterocycle bound to one cyclo-penta-dienyl ring. The 1,3-disubstitution of the pyrrole results in an L-shaped arrangement of the mol-ecule with plane inter-sections of 2.78 (17)° between the pyrrole and the N-bonded phenyl ring and of 8.17 (18)° between the pyrrole and the cyclo-penta-dienyl ring. In the crystal, no remarkable inter-molecular inter-actions are observed.

Synthesis and (spectro)electrochemistry of mixed-valent diferrocenyl–dihydrothiopyran derivatives

Cationic cyclic diferrocenyl–methane complexes display characteristic IVCT absorptions. These spectral features result from effective “through space” electronic communication between the metal centre in these mixed-valent dimetallic species.

Negishi coupling in the synthesis of advanced electronic, optical, electrochemical, and magnetic materials

Negishi coupling is an efficient and versatile tool for selective C–C bond formation in the synthesis of organic electronic, optical, electrochemical, and magnetic materials.