Ferrocene Analogues Research Articles

Pnictogen bonding at the service of gold catalysis: the case of a phosphinostiborane gold complex.

The search for alternative gold catalyst activators has led us to consider the design of platforms in which a phosphine gold chloride moiety could be activated via formation of a pnictogen bond with a neighboring antimony unit. Here, we describe that such a system can be accessed from 4-(diphenylphosphino)-5-(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene, by oxidation of the stibine with 3,5-di-tert-butyl-o-benzoquinone and by coordination of an AuCl unit to the phosphine. This strategy affords a complex in which a Lewis acidic or pnictogen-bond donor catecholatostiborane unit flanks the adjacent gold chloride moiety. This design impacts the catalytic reactivity of the gold center, as reflected by the ability of this complex to catalyze propargyl amide cyclization reactions. Comparisons with a phosphinostiborane ferrocene analog and computations point to the formation of an intramolecular Au-Cl → Sb(V) interaction as responsible for the observed catalytic activity.

Ferrocene-Based Drugs, Delivery Nanomaterials and Fenton Mechanism: State of the Art, Recent Developments and Prospects.

Ferrocene has been the most used organometallic moiety introduced in organic and bioinorganic drugs to cure cancers and various other diseases. Following several pioneering studies, two real breakthroughs occurred in 1996 and 1997. In 1996, Jaouen et al. reported ferrocifens, ferrocene analogs of tamoxifen, the chemotherapeutic for hormone-dependent breast cancer. Several ferrocifens are now in preclinical evaluation. Independently, in 1997, ferroquine, an analog of the antimalarial drug chloroquine upon the introduction of a ferrocenyl substituent in the carbon chain, was reported by the Biot-Brocard group and found to be active against both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. Ferroquine, in combination with artefenomel, completed phase IIb clinical evaluation in 2019. More than 1000 studies have been published on ferrocenyl-containing pharmacophores against infectious diseases, including parasitic, bacterial, fungal, and viral infections, but the relationship between structure and biological activity has been scarcely demonstrated, unlike for ferrocifens and ferroquines. In a majority of ferrocene-containing drugs, however, the production of reactive oxygen species (ROS), in particular the OH. radical, produced by Fenton catalysis, plays a key role and is scrutinized in this mini-review, together with the supramolecular approach utilizing drug delivery nanosystems, such as micelles, metal-organic frameworks (MOFs), polymers, and dendrimers.

Ferrocene modified analogues of imatinib and nilotinib as potent anti-cancer agents.

The unique features of ferrocene and the need for development of targeted anticancer drugs inspired the design, synthesis and biological evaluation of ferrocenyl modified tyrosine kinase inhibitors by replacing the pyridyl moiety in imatinib and nilotinib generalized structures with a ferrocenyl group. A series of seven new ferrocene analogues were synthesized and evaluated for their anticancer activity in a panel of bcr-abl positive human malignant cell lines using imatinib as a reference drug. The metallocenes exhibited a dose-dependent inhibition on malignant cell growth with varying antileukemic activity. The most potent analogues were compounds 9 and 15a showing comparable or even superior efficacy to the reference. Their cancer selectivity indices suggest a favorable selectivity profile, indicating a 250 times higher preferential activity of 15a towards malignantly transformed K-562 cells and an even twice greater one (500) of 9 in the LAMA-84 leukemic model as compared to the normal murine fibroblast cell line.

Ferrocene analogs of Brooker's merocyanine: a new library of organometallic inverted solvatochromic dyes

Four organometallic dyes with ferrocene as the electron donor group and pyridinium or quinolinium acceptor groups were synthesized and their spectral behavior was recorded in various solvents with different polarities.

Synthesis, Coordination and Electrochemistry of a Ferrocenyl‐Tagged Aminobisphosphane Ligand

AbstractIntroducing a ferrocene moiety into a molecule results in the incorporation of a metal center and a redox active moiety. This contribution describes the synthesis of FcN(CH2PPh2)2 (1; Fc=ferrocenyl), a ferrocene analog of the widely studied bis‐phosphane pincer ligands, RN(CH2PPh2)2. Compound 1 was studied as a ligand in complexes of Group 10 and 11 metal ions. The following compounds were synthesized and characterized using a combination of spectroscopic methods and single‐crystal X‐ray diffraction analysis: the square planar complexes [MX2(1‐κ2P,P’)] (M/X=Ni/Cl, Pd/Cl, Pd/Br, Pd/I and Pt/Cl) and the tetrahedral complexes [M(1‐κ2P,P’)2]X (M/X=Cu/PF6, Ag/SbF6 and Au/SbF6). Furthermore, a pair of open and cyclic digold(I) complexes containing bis‐phosphane 1 as a P,P‐bridging ligand, viz. [(μ(P,P’)‐1)(AuCl)2] and [(μ(P,P’)‐1)2Au2][SbF6]2, were isolated. Ligand 1, the corresponding phosphane selenide 1‐Se and all complexes (except for the poorly soluble [(μ(P,P’)‐1)2Au2][SbF6] and unstable [Ag(1‐κ2P,P’)2][SbF6]) were further analyzed by cyclic voltammetry.

Mechanochemistry of Cationic Cobaltocenium Mechanophore.

Recent research on the mechanochemistry of metallocene mechanophores has shed light on the force-responsiveness of these thermally and chemically stable organometallic compounds. In this work, we report a combination of experimental and computational studies on the mechanochemistry of main-chain cobaltocenium-containing polymers. Ester derivatives of the cationic cobaltocenium, though isoelectronic to neutral ferrocene, are unstable in the nonmechanical control experimental conditions that were accommodated by their ferrocene analogs. Replacing the electron withdrawing C-ester linkages with electron-donating C-alkyls conferred the necessary stability and enabled the mechanochemistry of the cobaltocenium to be assessed. Despite their high bond dissociation energy, cobaltocenium mechanophores are found to be selective sites of main chain scission under sonomechanical activation. Computational CoGEF calculations suggest that the presence of a counterion to cobaltocenium plays a vital role by promoting a peeling mechanism of dissociation in conjunction with the initial slipping.

Metalla-aromatics: Planar, Nonplanar, and Spiro.

ConspectusThe concept of aromaticity is one of the most fundamental principles in chemistry. It is generally accepted that planarity is a prerequisite for aromaticity, and typically the more planar the geometry of an aromatic compound is, the stronger aromatic it is. However, it is not always the case, particularly when transition metals are involved in conjugation and electron delocalization of aromatic systems, i.e., metalla-aromatics. Because of the intrinsic nature of transition-metal orbitals, besides planar geometries, the most stable molecular structures of metalla-aromatic compounds could take nonplanar and even spiro geometries. In this Account, we outline several unprecedented types of metalla-aromatics developed recently in our research group.Around seven years ago, we found that 1,4-dilithio-1,3-butadienes, dilithio reagents with π-conjugation, could function as non-innocent ligands and react with low-valent transition-metal complexes, generating monocyclic metalla-aromatic compounds. Later on, by taking advantage of the unique behavior of dilithio reagents and the intrinsic nature of different transition metals, we have synthesized a series of metalla-aromatic compounds, of which four types are discussed here, and each of them represents the first of its kind. First, nearly planar aromatic dicupra[10]annulenes, a 10 π-electron aromatic system with two bridging Cu atoms participating in the orbital conjugation and electron delocalization, are synthesized by annulating two dilithio reagents with two Cu(I) complexes.Second, four kinds of spiro metalla-aromatics, featuring planar (with Pd, Pt, or Rh as the spiro atom) geometry with a whole 10π aromatic system, octahedral (tris-spiro metalla-aromatics with V as the spiro atom) geometry with an entire 40π Craig-Möbius aromatic system, tetrahedral (with Mn as the spiro atom) geometry having two independent and perpendicular 6π planar aromatic rings, and tetrahedral (with Mn as the spiro atom) geometry with one planar and one nonplanar 6π aromatic rings, respectively, are generated. In sharp contrast to spiroaromaticity with carbon acting as the spiro atom described in Organic Chemistry, the metal spiro atom herein takes part in orbital conjugation and electron delocalization.Third, nonplanar aromatic butadienyl diiron complexes are realized. Different from planar aromatic systems featuring delocalized π-type overlap, this nonplanar metalla-aromaticity is achieved by the novel σ-type overlap between the two Fe 3dxz orbitals and the butadienyl π orbital, forming a 6π aromatic system. Fourth, dinickelaferrocene, a ferrocene analogue with two aromatic nickeloles, is synthesized from our monocyclic aromatic dilithionickelole and FeBr2. The aromaticity of dinickelaferrocene and its nickelole ligands is realized by electron back-donation from the Fe 3d orbital to the π* orbital of nickeloles, which also deepens our understanding of the origin of aromaticity.The search for unprecedented and exciting aromatic systems, particularly with transition metals being involved, will continue to drive this intriguing research field forward. Given the synthetic strategies and various types of metalla-aromatics developed and described, diversified metalla-aromatics of interesting structures and reaction chemistry, novel chemical bonding modes, and useful functions can be expected.

Synthetic, spectroscopic, structural, and electrochemical investigations of ferricenium derivatives with weakly coordinating anions: ion pairing, substituent, and solvent effects.

A facile and effective strategy for the preparation of a series of ferricenium complexes bearing either electron-donating or electron-withdrawing substituents with weakly coordinating anions such as [B(C6F5)4]- or SbF6- is reported. These systems were thoroughly investigated for their ground state electronic structures in both solution and solid states using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies as well as single crystal X-ray crystallography and electrochemical measurements. The X-ray structures of the six electron-deficient ferricenium derivatives are of particular interest as only a handful (∼5) of such derivatives have been structurally characterized to date. Comparison of the structural data for both neutral and oxidized derivatives reveals that the nature of the substituents on the cyclopentadienyl (Cp) ligands displays a more significant impact on the metal-ligand separations (FeCt) in the oxidized species than in their neutral analogs. Our 1H-NMR measurements corroborate that in the neutral ferrocene derivatives, electron-donating ring substitutions lead to a greater shielding of the ring protons while electron-withdrawing groups via induction deshield the nearby ring protons. However, the data for the paramagnetic ferricenium derivatives reveals that this substitutional behavior is more complex and fundamentally reversed, which is further supported by our structural studies. We ascribe this reversal of behavior in the ferricenium derivatives to the δ back-donation from the iron atom into the Cp rings which can lead to the overall shielding of the ring protons. Interestingly, our NMR results for the electron-deficient ferricenium derivatives in solution also indicate a direct correlation between the solvent dielectric constant and the energy barrier for rotation around the metal-ligand bond in these systems, whereas such a correlation is absent or not significant in the case of the electron-rich ferricenium species or the corresponding neutral ferrocene analogs. In this work, we also present the electrochemical behavior of the corresponding ferricenium/ferrocene redox couples including potential values (E1/2), peak-to-peak separation (ΔE1/2), and diffusion coefficients (D) of the redox active species in order to provide a concise outline of these data in one place. Our electrochemical studies involved three different solvents and two supporting electrolytes. Notably, our findings point to the significant effect of ion-pairing in lowering the energy necessary for reduction of the ferricenium ion and E1/2 in lower-polarity media. This has significant implications in applications of the ferrocene or ferricenium derivatives as redox agents in low-polarity solvents where an accurate determination of redox potential is critical.

Dinickelaferrocene: A Ferrocene Analogue with Two Aromatic Nickeloles Realized by Electron Back‐Donation from Iron

AbstractThe first example of ferrocene analogues with two transition‐metal metallole ligands of the general formula (η5‐C4R4M)2Fe in a sandwich structure are reported. Specifically, dinickelaferrocene 2, a type of dimetallametallocene, is efficiently synthesized from the reaction of dilithionickelole 1 with FeBr2 or FeCl2, presumably via a redox process, and is subjected to detailed experimental (single‐crystal X‐ray structural analysis, ICP‐OES, magnetometry, 57Fe Mössbauer, XPS) and theoretical (MOs, CDA, NICS, ICSS, and AICD) characterizations. Unlike ferrocene and its Cp ligands, the aromaticity of dinickelaferrocene and its nickelole ligands is accomplished by electron back‐donation from the Fe 3d orbitals to the π* orbitals of nickelole. Taken together, this work describes a new class of metallaferrocene sandwich complexes and provides a novel approach to effect aromaticity that will contribute to further development of metallocene chemistry.

Dinickelaferrocene: A Ferrocene Analogue with Two Aromatic Nickeloles Realized by Electron Back-Donation from Iron.

The first example of ferrocene analogues with two transition-metal metallole ligands of the general formula (η5 -C4 R4 M)2 Fe in a sandwich structure are reported. Specifically, dinickelaferrocene 2, a type of dimetallametallocene, is efficiently synthesized from the reaction of dilithionickelole 1 with FeBr2 or FeCl2 , presumably via a redox process, and is subjected to detailed experimental (single-crystal X-ray structural analysis, ICP-OES, magnetometry, 57 Fe Mössbauer, XPS) and theoretical (MOs, CDA, NICS, ICSS, and AICD) characterizations. Unlike ferrocene and its Cp ligands, the aromaticity of dinickelaferrocene and its nickelole ligands is accomplished by electron back-donation from the Fe 3d orbitals to the π* orbitals of nickelole. Taken together, this work describes a new class of metallaferrocene sandwich complexes and provides a novel approach to effect aromaticity that will contribute to further development of metallocene chemistry.

ELECTROCHEMICAL, ESR, THEORETICAL STUDIES AND in vitro ANTI-T. cruzi ACTIVITY OF 2-ORGANOMETALLIC-5-NITRO-BENZIMIDAZOLES

The bioorganometallic compounds 2-cyrhetrenyl-5-nitro-benzimidazole (Bzn-1) and 2-ferrocenyl-5-nitro-benzimidazole (Bzn-2) have been proposed as potential anti-Trypanosoma cruzi agents. On this regard, electrochemical, electron spin resonance and biological studies were carried out. Cyclic voltammetry experiments showed the generation of nitro anion radical derivatives and a self-protonation process was observed. Nitro anion radicals generated were characterized and analyzed using electron spin resonance spectroscopy. The compounds were tested in vitro against Trypanosoma cruzi (Dm28c strain). Biological evaluation display that 2-cyrhetrenyl-5-nitro-benzimidazole (Bzn-1) was more active than its ferrocene analogue (Bzn-2) and purely organic derivate (Bzn-3), associated with the electron-withdrawing properties of the (η5-C5H4)Re(CO)3 moiety. Finally, theoretical studies were carried out in order to elucidate the correlation between organometallic fragment, nitro-reduction potentials and trypanocidal activity.

Regorafenib analogues and their ferrocenic counterparts: synthesis and biological evaluation

New ferrocene analogues of regorafenib have been prepared and their biological activity was evaluated in kinase and cellular assays.

Synthesis and Biological Investigation of (+)-JD1, an Organometallic BET Bromodomain Inhibitor

(+)-JD1, a rationally designed ferrocene analogue of the BET bromodomain (BRD) probe molecule (+)-JQ1, has been synthesized and evaluated in biophysical, cell-based assays as well as in pharmacokinetic studies. It displays nanomolar activity against BRD isoforms, and its cocrystal structure was determined in complex with the first bromodomain of BRD4 and compared with that of (+)-JQ1, a known BRD4 small-molecule probe. At 1 μM concentration, (+)-JD1 was able to inhibit c-Myc, a key driver in cancer and an indirect target of BRD4.

Synthesis of ferrocene annulated trifluoromethylated heterocycles with crispine and lamellarin skeletons

Ferrocene analogues of pyrrolo[2,1-a]isoquinoline alkaloids, 2-(trifluoromethyl)-5,6-dihydroferroceno[g]indolizine and 6-(trifluoromethyl)-8,9-dihydro-6H-chromeno[4,3-b]ferroceno[g]indolizines, were obtained in 44–65% yield via the Grob reaction between 1-nitro-1-(2,2,2-trifluoroethylidene)alkanes or 3-nitro-2-(trifluoromethyl)-2H-chromenes and 1-alkyl-3,4-dihydroferroceno[c]pyridines in 2-propanol at reflux.

New ethylferrocenyl based pyranylidene bridge dyads: Synthesis, characterization and investigation of electrochemical and optical properties

In this research, we designed and synthesized six new pyranylidene backbone end-capped with donor and acceptor groups. Ethylferrocene was presented as new donor to evaluate the effect of alkyl chain substituted in ferrocene unit on linear optical and electrochemical properties of push-pull systems and compare it with their unsubstituted ferrocene analogues. Proposed structures were confirmed via FT-IR, 1H NMR, 13C NMR, elemental analyses and mass spectra. Optical band gaps were determined in the range of 1.85–2.24 using UV–vis spectroscopy for synthesized chromophores. Electrochemical studies were performed by cyclic voltammetry and show the reversible redox behavior with ΔEp=Epa−Epc≤0.08V, and diffusion limited for the redox process.

Mixed-Valent Ruthenocene-Vinylruthenium Conjugates: Valence Delocalization Despite Chemically Different Redox Sites.

Ruthenocene-vinylruthenium conjugates Rc/Rc*-CH═CH-Ru(CO)(L)(P iPr3)2 (Rc = (η5-C5H5)Ru(η5-C5H4); Rc* = (η5-C5Me5)Ru(η5-C5H4); L = Cl or κ O, O' -acetylacetonato) have been prepared and investigated in their neutral, mono-, and dioxidized states by cyclic voltammetry, IR and UV/vis/NIR spectroelectrochemistry, and EPR spectroscopy. Their corresponding radical cations are (almost) completely delocalized mixed-valent systems as indicated by the low half-widths, the absence of solvatochromism, and the low-energy cutoff of their IVCT bands in the near-infrared (NIR) and their IR and EPR spectroscopic signatures. The degree of electronic coupling even exceeds that of their ferrocene analogs despite comparable differences between the intrinsic half-wave potentials of the vinylruthenium and the metallocenyl entities and substantially smaller half-wave potential splittings, Δ E1/2, in the ruthenocene congeners. All experimental results are backed by quantum chemical calculations.

Ferrocene-appended pharmacophores: an exciting approach for modulating the biological potential of organic scaffolds.

Two exemplary contributions of organometallics in medicinal chemistry, ferroquine and ferrocifen, which exhibit excellent anti-plasmodial and anti-cancer activities, respectively, have opened a new field called medicinal organometallic chemistry. This field has been gaining significant interest due to the recent upsurge in ferrocene-linked organic frameworks with promising biological potential. The success of ferrocene is due to the sustained efforts by organic medicinal chemists and its inherent stability in air, heat and light, low toxicity, low cost and reversible redox properties. The replacement of the aryl/heteroaryl core with a ferrocene nucleus in organic molecules imparts a significant change not only in their molecular properties, such as solubility and hydro-/lipophilicity, but also improves the activities of bioactive compounds. Ferrocifen (ferrocene analogue of hydroxytamoxifen) possesses the remarkable feature of being anti-proliferative against both the MCF-7 (hormone dependent) and MDA-MB-231 (hormone independent) breast cancer cell lines. Accordingly, this review article is aimed at updating researchers on the recent developments (2014-18) on the synthesis and evaluation of ferrocene-containing bio-active pharmacophores with emphasis on their structure-activity relationship and mechanism of action.

Probing the Anticancer Action of Novel Ferrocene Analogues of MNK Inhibitors.

Two novel ferrocene-containing compounds based upon a known MNK1/2 kinase (MAPK-interacting kinase) inhibitor have been synthesized. The compounds were designed to use the unique shape of ferrocene to exploit a large hydrophobic pocket in MNK1/2 that is only partially occupied by the original compound. Screening of the ferrocene analogues showed that both exhibited potent anticancer effects in several breast cancer and AML (acute myeloid leukemia) cell lines, despite a loss of MNK potency. The most potent ferrocene-based compound 5 was further analysed in vitro in MDA-MB-231 (triple negative breast cancer cells). Dose–response curves of compound 5 for 2D assay and 3D assay generated IC50 values (half maximal inhibitory concentration) of 0.55 µM and 1.25 µM, respectively.

Application of One-Pot Three-Component Condensation Reaction for the Synthesis of New Organophosphorus–Sulfur Macrocycles

An efficient approach has been developed for the synthesis of new phosphorus–sulfur heterocycles by a one-pot three-component condensation reaction of a four-membered-ring thionation reagent [Lawesson’s reagent or its ferrocene analogue (2,4-diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide)], an alkane- or arenedithiol, and a dihaloalkane at room temperature in the presence of triethylamine. The simple synthesis method with mild conditions (room temperature and normal reactant concentrations) enhances further the application of the multicomponent reaction in the preparation of novel phosphorus–sulfur heterocycles. Six representative X-ray structures confirmed the formation of these macrocycles.

Synthesis and biological evaluation of ferrocene-based cannabinoid receptor 2 ligands.

Ferrocene analogs of known fatty acid amide hydrolase inhibitors and CB2 ligands have been synthesized and characterized spectroscopically and crystallographically. The resulting bio-organometallic isoxazoles were assayed for their effects on CB1 and CB2 receptors as well as on fatty acid amide hydrolase. None had any fatty acid amide hydrolase activity but compound 3, 5-(2-(pentyloxy)phenyl)-N-ferrocenylisoxazole-3-carboxamide, was found to be a potent CB2 ligand (Ki=32.5nM).