Teaching Ferrocenium How to Relax: A Systematic Study on Spin-Lattice Relaxation Processes in tert -Butyl-Substituted Ferrocenium Derivatives

Why charging Li-air batteries with current low-voltage mediators is slow and singlet oxygen does not explain degradation

Although Li-air rechargeable batteries offer higher energy densities than lithium-ion batteries, the insulating Li₂O₂ formed during discharge hinders rapid, efficient re-charging. Redox mediators are used to facilitate Li₂O₂ oxidation; however, fast kinetics at a low charging voltage are necessary for practical applications and are yet to be achieved. We investigate the mechanism of Li₂O₂ oxidation by redox mediators. The rate-limiting step is the outer-sphere one-electron oxidation of Li₂O₂ to LiO₂, which follows Marcus theory. The second step is dominated by LiO₂ disproportionation, forming mostly triplet-state O₂. The yield of singlet-state O₂ depends on the redox potential of the mediator in a way that does not correlate with electrolyte degradation, in contrast to earlier views. Our mechanistic understanding explains why current low-voltage mediators (<+3.3 V) fail to deliver high rates (the maximum rate is at +3.74 V) and suggests important mediator design strategies to deliver sufficiently high rates for fast charging at potentials closer to the thermodynamic potential of Li₂O₂ oxidation (+2.96 V).

Spin-Phonon Coupling and Slow-Magnetic Relaxation in Pristine Ferrocenium

We report the spin dynamic properties of non‐substituted ferrocenium complexes. Ferrocenium shows a field‐induced single‐molecule magnet behaviour in DMF solution while cobaltocene lacks slow spin relaxation neither in powder nor in solution. Multireference quantum mechanical calculations give a non‐Aufbau orbital occupation for ferrocenium with small first excitation energy that agrees with the relatively large measured magnetic anisotropy for a transition metal S=1/2 system. The analysis of the spin relaxation shows an important participation of quantum tunnelling, Raman, direct and local‐mode mechanisms which depend on temperature and the external field conditions. The calculation of spin‐phonon coupling constants for the vibrational modes shows that the first vibrational mode, despite having a low spin‐phonon constant, is the most efficient process for the spin relaxation at low temperatures. In such conditions, vibrational modes with higher spin‐phonon coupling constants are not populated. Additionally, the vibrational energy of this first mode is in excellent agreement with the experimental fitted value obtained from the local‐mode mechanism.

The Redox Journey of Iconic Ferrocene: Ferrocenium Dications and Ferrocenate Anions

The recent discoveries of both dicationic and monoanionic ferrocene derivatives throw light on the effect of the substituents on the C₅ ring as well as the choice of redox agents and solvent system in the preparation of previously believed to be difficult synthetic targets. These oxidized and reduced forms of ferrocene are of interest to spectroscopists, magnetochemists, and theoreticians.

Electronic Structure of a Diiron Complex: A Multitechnique Experimental Study of [(dppf)Fe(CO) 3]+/0

Here we explore the electronic structure of the diiron complex [(dppf)Fe(CO)₃]^0/+ [1^0/+; dppf = 1,1'-bis(diphenylphosphino)ferrocene] in two oxidation states by an advanced multitechnique experimental approach. A combination of magnetic circular dichroism, X-ray absorption and emission, high-frequency electron paramagnetic resonance (EPR), and Mössbauer spectroscopies is used to establish that oxidation of 1⁰ occurs on the carbonyl iron ion, resulting in a low-spin iron(I) ion. It is shown that an unequivocal result is obtained by combining several methods. Compound 1⁺ displays slow spin dynamics, which is used here to study its geometric structure by means of pulsed EPR methods. Surprisingly, these data show an association of the tetrakis[3,5-bis(trifluoromethylphenyl)]borate counterion with 1⁺.

Isolation and electronic structures of derivatized manganocene, ferrocene and cobaltocene anions

The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry. Although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low-temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3d metallocenates, which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulky Cp^ttt ligands (where Cp^ttt is {1,2,4-C₅H₂ ^tBu₃}). These thermally and air-sensitive complexes decompose rapidly above -30 °C; however, we were able to characterize all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S = 3/2 ground state for the 19e^- derivatized ferrocene anion.

Ferro-self-assembly: magnetic and electrochemical adaptation of a multiresponsive zwitterionic metalloamphiphile showing a shape-hysteresis effect

Metallosurfactants are molecular compounds which combine the unique features of amphiphiles, like their capability of self-organization, with the peculiar properties of metal complexes like magnetism and a rich redox chemistry. Considering the high relevance of surfactants in industry and science, amphiphiles that change their properties on applying an external trigger are highly desirable. A special feature of the surfactant reported here, 1-(Z)-heptenyl-1′-dimethylammonium-methyl-(3-sulfopropyl)ferrocene (6), is that the redox-active ferrocene constituent is in a gemini-position. Oxidation to 6⁺ induces a drastic change of the surfactant's properties accompanied by the emergence of paramagnetism. The effects of an external magnetic field on vesicles formed by 6⁺ and the associated dynamics were monitored in situ using a custom-made optical birefringence and dual dynamic light scattering setup. This allowed us to observe the optical anisotropy as well as the anisotropy of the diffusion coefficient and revealed the field-induced formation of oriented string-of-pearls-like aggregates and their delayed disappearance after the field is switched off.

The self-organization properties of a stimuli responsive amphiphile can be altered by subjecting the paramagnetic oxidized form to a magnetic field of 0.8 T and monitored in real time by coupling optical birefringence with dynamic light scattering.

Polymorphous Indium Metal-Organic Frameworks Based on a Ferrocene Linker: Redox Activity, Porosity, and Structural Diversity

The metallocene-based linker molecule 1,1'-ferrocenedicarboxylic acid (H₂FcDC) was used to synthesize four different polymorphs of composition [In(OH)(FeC₁₂H₈O₄)]. Using conventional solvent-based synthesis methods and varying the synthetic parameters such as metal source, reaction temperature, and solvent, two different MOFs and one 1D-coordination polymer denoted as CAU-43 (1), In-MIL-53-FcDC_a (2), and In-FcDC (3) were obtained. Furthermore, thermal treatment of CAU-43 (1) at 190 °C under vacuum yielded a new polymorph of 2, In-MIL-53-FcDC_b (4). Both MOFs 2 and 4 crystallize in a MIL-53 type structure, but in different space groups C2/m for 2 and P1̅ for 4. The structures of the four title compounds were determined by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), or a combination of three-dimensional electron diffraction measurements (3D ED) and PXRD. N₂ sorption experiments of 1, 2, and 4 showed specific surface areas of 355 m² g^-1, 110 m² g^-1, and 140 m² g^-1, respectively. Furthermore, the electronic properties of the title compounds were characterized via Mössbauer and EPR spectroscopy. All Mössbauer spectra showed the characteristic doublet, proving the persistence of the ferrocene moiety. In the cases of 1, 3, and 4, appreciable impurities of ferrocenium ions could be detected by electron paramagnetic resonance spectroscopy. Cyclovoltammetric experiments were performed to demonstrate the accessible redox activity of the linker molecule of the title compounds. A redox process of FcDC^2- with oxidation (between 0.86 and 0.97 V) and reduction wave (between 0.69 and 0.80 V) was observed.

Observation of three different linker conformers in a scandium ferrocenedicarboxylate coordination polymer

In the coordination polymer CAU-50 based on 1,1′-ferrocenedicarboxylate and scandium, three different conformers of the same linker molecule are observed.

Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]₂ (1-M, M = Fe, Co; Cp' = η⁵-1,2,4-tri-tert-butylcyclopentadienyl) with [Im^DippNLi]₂ (Im^DippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NIm^Dipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe₃)₂}] (3-Co) and Im^DippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by ⁵⁷Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Tunable Redox Potential, Optical Properties, and Enhanced Stability of Modified Ferrocene-Based Complexes

We report a series of ferrocene-based derivatives and their corresponding oxidized forms in which the introduction of simple electron donating groups like methyl or tert-butyl units on cyclopentadienyl-rings afford great tunability of Fe^+III/Fe^+II redox potentials from +0.403 V down to -0.096 V versus saturated calomel electrode. The spin forbidden d-d transitions of ferrocene derivatives shift slightly toward the blue region with an increasing number of electron-donating groups on the cyclopentadienyl-rings with very little change in absorptivity values, whereas the ligand-to-metal transitions of the corresponding ferricinium salts move significantly to the near-IR region. The electron-donating groups also contribute in the strengthening of electron density of Fe^+III d-orbitals, which therefore improves the chemical stability against the oxygen reaction. Further, density functional theory calculations show a reducing trend in outer shell reorganization energy with an increasing number of the electron donating units.

Magnetization Slow Dynamics in Ferrocenium Complexes

The single-molecule magnet (SMM) properties of a series of ferrocenium complexes, [Fe(η⁵ -C₅ R₅ )₂ ]⁺ (R=Me, Bn), are reported. In the presence of an applied dc field, the slow dynamics of the magnetization in [Fe(η⁵ -C₅ Me₅ )₂ ]BAr_F are revealed. Multireference quantum mechanical calculations show a large energy difference between the ground and first excited states, excluding the commonly invoked, thermally activated (Orbach-like) mechanism of relaxation. In contrast, a detailed analysis of the relaxation time highlights that both direct and Raman processes are responsible for the SMM properties. Similarly, the bulky ferrocenium complexes, [Fe(η⁵ -C₅ Bn₅ )₂ ]BF₄ and [Fe(η⁵ -C₅ Bn₅ )₂ ]PF₆ , also exhibit magnetization slow dynamics, however an additional relaxation process is clearly detected for these analogous systems.

The Irony of Manganocene: An Interplay between the Jahn-Teller Effect and Close-Lying Electronic and Spin States

Although the unusual structural, magnetic, electronic, and spin characteristics of manganocene has intrigued scientists for decades, a unified explanation and rationalization of its properties has not yet been provided. Results obtained by Multideterminantal Density Functional Theory (MD-DFT), Energy Decomposition Analysis (EDA), and Intrinsic Distortion Path (IDP) methodologies indicate how this uniqueness can be traced back to the manganocene's peculiar electronic structure, mainly, the degenerate ground state and close-lying electronic and spin states.

A porous and redox active ferrocenedicarboxylic acid based aluminium MOF with a MIL-53 architecture

A new redox active MOF Al-MIL-53-FcDC based on 1,1′-ferrocenedicarboxylate exhibits reversible electrochemical activity and permanent porosity.

Monomeric Fe(iii) half-sandwich complexes [Cp'FeX2] - synthesis, properties and electronic structure

The half-sandwich complex [Cp'Fe(μ-I)]₂ (1; Cp' = η⁵-1,2,4-(Me₃C)₃C₅H₂) is cleaved when heated in toluene to form a cation-anion pair [{Cp'Fe(η⁶-toluene)}⁺{Cp'FeI₂}^-] (2), in which the two Fe(ii) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C₂H₄I₂, the thermally stable 15VE species [Cp'FeI₂] (3) can be isolated, in which the Fe(iii) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH₂SiMe₃)₂ to form the unprecedented Fe(iii) (S = 3/2) bis(alkyl) complex [Cp'Fe(CH₂SiMe₃)₂] (4). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field ⁵⁷Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(ii) alkyl species [Cp'FeCH(SiMe₃)₂], which resists the reaction with H₂ as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H₂ (8 bar) in cyclohexane to yield iron hydrides [{Cp'Fe}₂(μ-H)₃] (5) and [Cp'Fe(μ-H)₂]₂ (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp'Fe(η⁶-C₆H₆)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp'Fe)₂(μ₂-η⁵:η⁵-C₁₂H₁₂)] (7). Further evidence for the intermediacy of [Cp'Fe(η⁶-C₆H₆)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF₆ proceeded via C-C bond cleavage instead of metal oxidation to form [Cp'Fe(C₆H₆)][SbF₆] (8).