Direct measurement of the diamagnetic anisotropy of the ferrocenyl moiety: The origin of unusual 1H NMR shifts in ferrocenyl-triptycenes and barrelenes

Measuring the Magnetic Anisotropy of Metal-Metal Multiple Bonds: The Importance of Correcting for Ligand Effects

We describe the synthesis and characterization of the quadruply-bonded dimer Mo2(CH2NMe2BH3)4 in which each molybdenum(II) center is bound to two chelating boranatodimethylaminomethyl (BDAM) ligands. The BDAM anions bind to the metal at one end by a metal-carbon σ bond and at the other by a three-center M-H-B interaction. Each BDAM ligand chelates to a single Mo atom so that the metal-metal bond is unbridged; the Mo-Mo distance is 2.114(2) Å. Structural and solution NMR data, analyzed via McConnell's equation and supported by DFT calculations, show that the magnetic anisotropies associated with highly polarizable and π-bonding ligands (such as chloride groups and aryl rings) can greatly affect the NMR chemical shifts of reporter groups, so that ignoring their contributions leads to significant overestimates of the anisotropy due just to the metal-metal bond. We propose a method to quantify and correct for the magnetic anisotropy effects arising from the ligands. Application of this method to Mo2(BDAM)4 indicates that the magnetic anisotropy of the Mo-Mo quadruple bond in this molecule is about -800 × 10-36 m3 molecule-1. Anisotropies significantly higher than this value (as sometimes reported in the prior literature) are most likely incorrect.

The average magnetic anisotropy of polystyrene in polymersomes self-assembled from poly(ethylene glycol)-b-polystyrene

Using the diamagnetic anisotropy of polymers for the characterization of polymers and polymer aggregates is a relatively new approach in the field of soft-matter and polymer research. So far, a good and thorough quantitative description of these diamagnetic properties has been lacking. Using a simple equation that links the magnetic properties of an average polymer repeating unit to those of the polymer vesicle of any shape, we measured, using magnetic birefringence, the average diamagnetic anisotropy of a polystyrene (PS) repeating unit, ΔχPS, inside a poly(ethylene glycol)-polystyrene (PEG-PS) polymersome membrane as a function of the PS-length and as a function of the preparation method. All obtained values of ΔχPS have a negative sign which results in polymers tending to align perpendicular to an applied magnetic field. Combined, the same order of magnitude of ΔχPS (10-12 m3 mol-1) for all polymersome shapes proves that the individual polymers are organized similarly regardless of the PS length and polymersome shape. Furthermore, the value found is only a fraction (∼1%) of what it can maximally be due to the random coiling of the polymers. We, therefore, predict that further ordering of the polymers within the membrane could lead to similar responses at much lower magnetic fields, possibly obtainable with permanent magnets, which would be highly advantageous for practical applications.

New Iron Twist to Chloroquine─Upgrading Antimalarials with Immunomodulatory and Antimicrobial Features

Herein, upgraded chloroquine (CQ) derivatives capable of overcoming Plasmodium resistance and, at the same time, suppressing excessive immune response and risk of concurrent bacteremia were developed. Twelve new ferrocene-CQ hybrids tethered with a small azathia heterocycle (1,3-thiazolidin-4-one, 1,3-thiazinan-4-one, or 5-methyl-1,3-thiazolidin-4-one) were synthesized and fully characterized. All hybrids were evaluated for their in vitro antiplasmodial, antimicrobial, and immunomodulatory activities. Additional assays were performed on selected hybrids to gain insights into their mode of action. Although only hybrid 4a was more potent than the parent drug toward CQ-resistant Dd2 Plasmodium falciparum strain, several other hybrids (such as 6b, 6c, and 6d) manifested substantially improved antimicrobial and immunomodulatory properties. Interesting structure-activity relationship data were obtained, hinting at future research for the development of new multitarget chemotherapies for malaria and other infectious diseases complicated by drug resistance, bacterial co-infection, and immune-driven pathology issues.

2,3-Dihydroferroceno[3,4]pyrrolo[2,1-b]thiazol-5(8b)-ones: Synthesis, Structure and DFT Study on the Mechanism of Chemo- and Diastereoslective Annulations of (Sp)-2-Formylferrocenecarbonyl Fluoride and (Sp)-2-Formylferrocenecarboxylic Acid

By means of the annulation of easy-to-handle yet sufficiently reactive (S_p)-2-formylferrocenecar- bonyl fluoride with the hydrochlorides of cysteamine and the methyl esters of enentiomer cysteines conducted in dichloromethane at room temperature in the presence of pyridine, the first members of 2,3-dihydroferroceno[3,4]pyrrolo[2,1-b]thiazol-5(8bH)-ones with the elements of planar- and central chirality were prepared as single enantiomers. An atom economic procedure was also elaborated for the synthesis of these organometallic heterocycles directly exploring (S_p)-2-formylferrocenecarboxylic acid in situ activated by CDI and TFA, sequentially added to the reaction mixture. The relative and consequently, the absolute, configuration of the isolated diastereomers was determined by NMR measurements supported by DFT structural optimization. On the basis of the results of synthetic control experiments and a series of further DFT modelling studies, including energetic and MO analysis of the iminium intermediates, we propose a mechanism for the thiazolidine-forming annulations that proceed via primary N-acylation followed by proton-mediated cyclocondensation and subsequent diastereoselective sulfhydryl-attack on the resulting iminium center.

Palladium-Catalysed Coupling Reactions En Route to Molecular Machines: Sterically Hindered Indenyl and Ferrocenyl Anthracenes and Triptycenes, and Biindenyls

Pd-catalysed Stille and Suzuki cross-couplings were used to prepare 9-(3-indenyl)-, 6, and 9-(2-indenyl)-anthracene, 7; addition of benzyne led to the 9-Indenyl-triptycenes, 8 and 9. In 6, [4 + 2] addition also occurred to the indenyl substituent. Reaction of 6 through 9 with Cr(CO)₆ or Re₂(CO)₁₀ gave their M(CO)₃ derivatives, where the Cr or Re was complexed to a six- or five-membered ring, respectively. In the 9-(2-indenyl)triptycene complexes, slowed rotation of the paddlewheel on the NMR time-scale was apparent in the η⁵-Re(CO)₃ case and, when the η⁶-Cr(CO)₃ was deprotonated, the resulting haptotropic shift of the metal tripod onto the five-membered ring also blocked paddlewheel rotation, thus functioning as an organometallic molecular brake. Suzuki coupling of ferrocenylboronic acid to mono- or dibromoanthracene yielded the ferrocenyl anthracenes en route to the corresponding triptycenes in which stepwise hindered rotations of the ferrocenyl groups behaved like molecular dials. CuCl₂-mediated coupling of methyl- and phenyl-indenes yielded their rac and meso 2,2′-biindenyls; surprisingly, however, the apparently sterically crowded rac 2,2′-Bis(9-triptycyl)biindenyl functioned as a freely rotating set of molecular gears. The predicted high rotation barrier in 9-phenylanthracene was experimentally validated via the Pd-catalysed syntheses of di(3-fluorophenyl)anthracene and 9-(1-naphthyl)-10-phenylanthracene.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.