Are Redox-Active Centers Bridged by Saturated Flexible Linkers Systematically Electrochemically Independent?

The extent to which electrophores covalently bridged by a saturated linker are electrochemically independent was investigated considering the charge/spin duality of the electron and functionality of the electrophore as a spin carrier upon reduction. By combining computational modeling with electrochemical experiments, we investigated the mechanism by which tethered electrophores react together within 4,4'-oligo[n]methylene-bipyridinium assemblies (with n=2 to 5). We show that native dicationic electrophores (redox state Z=+2) are folded prior to electron injection into the system, allowing the emergence of supra-molecular orbitals (supra-MOs) likely to support the process of the reductive σ bond formation giving cyclomers. Indeed, for Z=+2, London Dispersion (LD) forces contribute to flatten the potential energy surface such that all-trans and folded conformers are approximately isoenergetic. Then, upon one-electron injection, for radical cations (Z=+1), LD forces significantly stabilize the folded conformers, except for the ethylene derivative deprived of supra-MOs. For radical cations equipped with supra-MOs, the unpaired electron is delocalized over both heterocycles through space. Cyclomer completion (Z=0) upon the second electron transfer occurs according to the inversion of redox potentials. This mechanism explains why intramolecular reactivity is favored and why pyridinium electrophores are not independent.

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.

A decacationic ferrocene-based metallostar

Decacationic metallostars have been prepared by the reaction of permercurated ferrocene FeC₁₀(HgO₂CCF₃)₁₀ with superacidic (C₅F₅NH)(SbF₆) (pK _a = -11 estimated in H₂O) in multigram scale. In the resulting compound, [FeC₁₀Hg₁₀(NC₅F₅) _n ][SbF₆]₁₀, the labile pentafluoropyridine ligands are readily displaced by acetonitrile (MeCN) or tetrahydrothiophene (THT). In the X-ray structure of [FeC₁₀Hg₁₀(THT)₁₀][SbF₆]₁₀·24 MeCN no cation-anion contacts between mercury and fluorine were observed. Moreover, cyclic voltammetry measurements of [FeC₁₀(Hg(MeCN))₁₀]¹⁰⁺ and [FeC₁₀(Hg(THT))₁₀]¹⁰⁺ revealed a (quasi)reversible one-electron oxidation of Fe(ii) to Fe(iii). From the reaction of [FeC₁₀(Hg(MeCN))₁₀]¹⁰⁺ with MoF₆ as oxidant the ferrocenium cation [FeC₁₀(Hg(MeCN))₁₀]¹¹⁺ was obtained and characterized via single crystal XRD. These electrophilic metallostars are promising potential building blocks for the synthesis of dendritic architectures containing a robust, tenfold functionalized ferrocene core.

How to Control the Rate of Heterogeneous Electron Transfer across the Rim of M6L12 and M12L24 Nanospheres

Catalysis in confined spaces, such as those provided by supramolecular cages, is quickly gaining momentum. It allows for second coordination sphere strategies to control the selectivity and activity of transition metal catalysts, beyond the classical methods like fine-tuning the steric and electronic properties of the coordinating ligands. Only a few electrocatalytic reactions within cages have been reported, and there is no information regarding the electron transfer kinetics and thermodynamics of redox-active species encapsulated into supramolecular assemblies. This contribution revolves around the preparation of M6L12 and larger M12L24 (M = Pd or Pt) nanospheres functionalized with different numbers of redox-active probes encapsulated within their cavity, either in a covalent fashion via different types of linkers (flexible, rigid and conjugated or rigid and nonconjugated) or by supramolecular hydrogen bonding interactions. The redox probes can be addressed by electrochemical electron transfer across the rim of nanospheres, and the thermodynamics and kinetics of this process are described. Our study identifies that the linker type and the number of redox probes within the cage are useful handles to fine-tune the electron transfer rates, paving the way for the encapsulation of electroactive catalysts and electrocatalytic applications of such supramolecular assemblies.

Multivalent Lactose-Ferrocene Conjugates Based on Poly (Amido Amine) Dendrimers and Gold Nanoparticles as Electrochemical Probes for Sensing Galectin-3

Galectin-3 is considered a cancer biomarker and bioindicator of fibrosis and cardiac remodeling and, therefore, it is desirable to develop convenient methods for its detection. Herein, an approach based on the development of multivalent electrochemical probes with high galectin-3 sensing abilities is reported. The probes consist of multivalent presentations of lactose–ferrocene conjugates scaffolded on poly (amido amine) (PAMAM) dendrimers and gold nanoparticles. Such multivalent lactose–ferrocene conjugates are synthesized by coupling of azidomethyl ferrocene–lactose building blocks on alkyne-functionalized PAMAM, for the case of the glycodendrimers, and to disulfide-functionalized linkers that are then used for the surface modification of citrate-stabilized gold nanoparticles. The binding and sensing abilities toward galectin-3 of both ferrocene-containing lactose dendrimers and gold nanoparticles have been evaluated by means of isothermal titration calorimetry, UV–vis spectroscopy, and differential pulse voltammetry. The highest sensitivity by electrochemical methods to galectin-3 was shown by lactosylferrocenylated gold nanoparticles, which are able to detect the lectin in nanomolar concentrations.

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.

Iron, iron everywhere: synthesis and characterization of iron 5,10,15-triferrocenylcorrole complexes

A new series of iron triferrocenylcorroles with three different axial ligands, NO, Cl⁻and σ-Ph, is synthesized and characterized using¹H NMR, electrochemical and spectroelectrochemical techniques in nonaqueous media.

Mono-DMSO ligated cobalt nitrophenylcorroles: electrochemical and spectral characterization

Four mono-DMSO ligated cobalt corroles with one or threemeso-nitrophenyl substituents on the macrocycle were synthesized and investigated as to their electrochemical and spectroscopic properties in CH₂Cl₂and DMSO.

Synthesis, characterization, optical and electrochemical properties and antifungal and anticancer activities of ferrocenyl conjugated novel dendrimers

A new class of triazoloferrocenyl conjugates was prepared by copper(i) catalyzed click chemistry, which shows good antifungal activity against fungal pathogens, and also shows excellent anticancer activity against MCF-7 cells.

Interactions between the trianionic ligand-centred redox-active metalloligand [Cr(III)(perfluorocatecholato)3](3-) and guest metal ions

The redox-active metalloligand (RML) (Et3NH)3[Cr(III)(F4Cat)3] (F4Cat = perfluorocatecholato) () was synthesized and its interactions with guest metal ions Li(+), Mn(2+), Fe(2+), Co(2+), Cu(2+), and Zn(2+) were examined. Cyclic voltammetry measurements and spectroelectrochemical studies revealed that complex shows three-step ligand-centred one-electron oxidation to consecutively generate [Cr(III)(F4Cat)2(F4SQ)](2-) (F4SQ = perfluorosemiquinonato), [Cr(III)(F4Cat)(F4SQ)2](-), and [Cr(III)(F4SQ)3] at -0.12, 0.23, and 0.53 V vs. Ag/Ag(+) in dichloromethane, or at -0.21, 0.08, and 0.50 V in acetonitrile (MeCN), respectively. Titration experiments in MeCN revealed that treatment of with Cu(2+) leads to the formation of [Cr(III)(F4Cat)2(F4SQ)](2-) and Cu(+)via a redox reaction. However, when was treated with Li(+), Mn(2+), Fe(2+), Co(2+), and Zn(2+), further titration experiments revealed that these metal ions coordinated via the lone pairs on the coordinating oxygen atoms of the F4Cat(2-) moieties in a one-to-one ratio, and binding constants of 3.7 (±0.3) × 10(4) (Li(+)), 1.5 (±0.2) × 10(5) (Mn(2+)), 2.2 (±0.4) × 10(5) (Fe(2+)), 1.9 (±0.2) × 10(5) (Co(2+)), and 3.8 (±0.4) × 10(5) M(-1) (Zn(2+)) were established. Moreover, the oxidation potentials of were positively shifted by 0.08-0.33 V upon addition of guest metal ions. Spectroelectrochemical studies of in the presence of guest metal ions suggested that ligand-centred one- and two-electron oxidation of the RML occurred for Li(+), Mn(2+), Co(2+), and Zn(2+), respectively, while guest metal-centred one-electron oxidation was observed for Fe(2+). Considering all the aforementioned results, this study demonstrated for the first time the ability of [Cr(III)(F4Cat)3](3-) to act as a RML in solution.

Organometallic rotaxane dendrimers with fourth-generation mechanically interlocked branches

Mechanically interlocked molecules, such as catenanes, rotaxanes, and knots, have applications in information storage, switching devices, and chemical catalysis. Rotaxanes are dumbbell-shaped molecules that are threaded through a large ring, and the relative motion of the two components along each other can respond to external stimuli. Multiple rotaxane units can amplify responsiveness, and repetitively branched molecules--dendrimers--can serve as vehicles for assembly of many rotaxanes on single, monodisperse compounds. Here, we report the synthesis of higher-generation rotaxane dendrimers by a divergent approach. Linkages were introduced as spacer elements to reduce crowding and to facilitate rotaxane motion, even at the congested periphery of the compounds up to the fourth generation. The structures were characterized by 1D multinuclear ((1)H, (13)C, and (31)P) and 2D NMR spectroscopy, MALDI-TOF-MS, gel permeation chromatography (GPC), and microscopy-based methods including atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM and TEM studies of rotaxane dendrimers vs. model dendrimers show that the rotaxane units enhance the rigidity and reduce the tendency of these assemblies to collapse by self-folding. Surface functionalization of the dendrimers with ferrocenes as termini produced electrochemically active assemblies. The preparation of dendrimers with a well-defined topological structure, enhanced rigidity, and diverse functional groups opens previously unidentified avenues for the application of these materials in molecular electronics and materials science.

Sandwich complex-containing macromolecules: property tunability through versatile synthesis

Sandwich complexes feature unique properties as the physical and electronic properties of a hydrocarbon ligand or its derivative are integrated into the physical, electronic, magnetic, and optical properties of a metal. Incorporation of these complexes into macromolecules results in intriguing physical, electrical, and optical properties that were hitherto unknown in organic-based macromolecules. These properties are tunable through well-designed synthetic strategies. This review surveys many of the synthetic approaches that have resulted in tuning the properties of sandwich complex-containing macromolecules. While the past two decades have seen an ever-growing number of research publications in this field, gaps remain to be filled. Thus, we expect this review to stimulate research interest towards bridging these gaps, which include the insolubility of some of these macromolecules as well as expanding the scope of the sandwich complexes.

Synthesis of novel ferrocenyl N/O-heterocycles, chiral P,N-ligand and α-dehydro-β-amino acid derived short peptides from Morita–Baylis–Hillman adducts of ferrocenealdehyde

The golden triangle of Fc, OH/NH, COO moieties created by classical/aza-MBH reaction of ferrocenealdehyde has been exploited for the first time for the synthesis of the title compounds.

Heptametallic, octupolar nonlinear optical chromophores with six ferrocenyl substituents

New complexes with six ferrocenyl (Fc) groups connected to Zn(II) or Cd(II) tris(2,2'-bipyridyl) cores are described. A thorough characterisation of their BPh4(-) salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe(II))→π* metal-to-ligand charge-transfer (MLCT) bands accompany the π→π* intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe(III/II) wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β0 reaching as high as approximately 10(-27)  esu and generally increasing with π-conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections σ2 of up to 5400 GM in one case. DFT calculations reproduce the π-conjugation dependence of β0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.

Multielectron-transfer templates via consecutive two-electron transformations: iron-sulfur complexes relevant to biological enzymes

[FeFe] hydrogenase mimics: two polynuclear iron-sulfur complexes (1 and 2) were prepared and structurally characterized. They are potentially effective and stable multielectron-transfer relays for mediating four- and six-electron transformations via a cascade of reversible two-electron redox steps with relatively narrow potential spans.

Electrochemistry and DFT simulation of optical spectra of N-ferrocenoyl-N'-ω-decenoyl-ethylenediamine

A new hybrid organic-inorganic chromophore based on ferrocene was synthesized. In addition to a detailed synthesis description the article describes complex quantum chemical and experimental (UV-VIS and electrochemical) studies of the redox and chromophore characteristics. The properties studied include the diffusion coefficient as well as solvatochromic effects in solvents differing in polarity, namely dichloromethane, propylene carbonate and acetonitrile. The experimental results are discussed within a framework of theoretical description at the DFT and TDDFT (6-31G(df,p)) levels of theory, with the PCM solvation model. Accuracy of B3LYP and BLYP functionals in reproducing orbital energy levels and optical transitions of the title molecule is briefly discussed.

1,2,3-Triazole Ferrocenyldendrimers Through Click Chemistry Approach and their Optical and Electrochemical Properties

Novel ferrocenyl dendrimers with ferrocene as a surface group and with triazole as a bridging unit have been synthesised through click chemistry. The increasing numbers of triazole and ferrocenyl units at the antenna increase the light absorbing ability. The electrochemical behaviour changes with increasing ferrocenyl and triazole units.