Isolation and structural and electronic characterization of salts of the decamethylferrocene dication

Understanding the Decamethylferrocene FeIII/IV Oxidation Process in Tris(pentafluoroethyl)trifluorophosphate-Containing Ionic Liquids at Glassy Carbon and Boron-Doped Diamond Electrodes

Under voltammetric conditions, the neutral decamethylferrocene ([Me10Fc]) to cationic ([Me10Fc]+) FeII/III process is a well-known reversible outer-sphere reaction. A companion cationic [Me10Fc]+ to dicationic [Me10Fc]2+ FeIII/IV process has been reported under direct current (DC) cyclic voltammetric conditions at highly positive potentials in liquid SO2 at low temperatures and in a 1.5:1.0 AlCl3/1-butylpyridinium chloride melt. This study demonstrates that in room-temperature ionic liquids containing the hard to oxidize and hydrophobic tris(pentafluoroethyl)trifluorophosphate anion, the [Me10Fc]+/2+ process can be detected as a quasi-reversible reaction at glassy carbon (GC) and boron-doped diamond (BDD) electrodes. Large amplitude Fourier-transformed alternating current (FT-AC) voltammetry minimizes background current contributions occurring at potentials similar to those of the FeIII/IV process in the second and higher-order harmonics. This enables a straightforward determination of the thermodynamics and kinetics for both the FeII/III and FeIII/IV processes. Unlike the ideal outer-sphere FeII/III process, the parameters of the FeIII/IV process may be impacted by ion-interaction effects. For the faster FeII/III process, heterogeneous rate constants are approximately 10 times smaller at BDD than those at GC electrodes. This electrode dependence is less pronounced for the slower FeIII/IV process. The slower BDD kinetics may be attributed in part to a density of states lower than that at GC.

A lithium-aluminium heterobimetallic dimetallocene

Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η5 bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al-Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium-aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al-Li bond is easily cleaved.

Ultrafast photogeneration of a metal-organic nitrene from 1,1'-diazidoferrocene

Ferrocene and its derivatives have fascinated chemists for more than 70 years, not least due to the analogies with the properties of benzene. Despite these similarities, the obvious difference between benzene and ferrocene is the presence of an iron ion and hence the availability of d-orbitals for properties and reactivity. Phenylnitrene with its rich photochemistry can be considered an analogue of nitrenoferrocene. As with most organic and inorganic nitrenes, nitrenoferrocene can be obtained by irradiating the azide precursor. We study the photophysical and photochemical processes of dinitrogen release from 1,1'-diazidoferrocene to form 1-azido-1'-nitrenoferrocene with UV-pump-mid-IR-probe transient absorption spectroscopy and time-dependent density functional theory calculations including spin-orbit coupling. An intermediate with a bent azide moiety is identified that is pre-organised for dinitrogen release via a low-lying transition state. The photochemical decay paths on the singlet and triplet surfaces including the importance of spin-orbit coupling are discussed. We compare our findings with the processes discussed for photochemical dinitrogen activation and highlight implications for the photochemistry of azides more generally.

Manganese(III) Nitrate Complexes as Bench-Stable Powerful Oxidants

We report herein a convenient one-pot synthesis for the shelf-stable molecular complex [Mn(NO3)3(OPPh3)2] (2) and describe the properties that make it a powerful and selective one-electron oxidation (deelectronation) reagent. 2 has a high reduction potential of 1.02 V versus ferrocene (MeCN) (1.65 vs normal hydrogen electrode), which is one the highest known among readily available redox agents used in chemical synthesis. 2 exhibits stability toward air in the solid state, can be handled with relative ease, and is soluble in most common laboratory solvents such as MeCN, dichloromethane, and fluorobenzene. 2 is substitutionally labile with respect to the coordinated (pseudo)halide ions enabling the synthesis of other new Mn(III) nitrato complexes also with high reduction potentials ranging from 0.6 to 1.0 V versus ferrocene.

Doping Ferrocene-Based Conjugated Microporous Polymers with 7,7,8,8-Tetracyanoquinodimethane for Efficient Photocatalytic CO2 Reduction

The design and synthesis of organic photocatalysts remain a great challenge due to their strict structural constraints. However, this could be mitigated by achieving structural flexibility by constructing permanent porosity into the materials. Conjugated microporous polymers (CMPs) are an emerging class of porous materials with an amorphous, three-dimensional network structure, which makes it possible to integrate the elaborate functional groups to enhance photocatalytic performance. Here, we report the synthesis of a novel CMP, named TAPFc-TFPPy-CMP, constructed by 1,1'3,3'-tetra(4-aminophenyl)ferrocene (TAPFc) and 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) monomers. The integration of the p-type dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ) into the TAPFc-TFPPy-CMP improved the light adsorption performance, leading to a decrease in the optical bandgap from 2.00 to 1.43 eV. The doped CMP (TCNQ@TAPFc-TFPPy-CMP) exhibited promising catalytic activity in photocatalytic CO2 reduction under visible light, yielding 546.8 μmol g-1 h-1 of CO with a selectivity of 96% and 5.2 μmol g-1 h-1 of CH4. This represented an 80% increase in the CO yield compared to the maternal TAPFc-TFPPy-CMP. The steady-state photoluminescence (PL) and fluorescence lifetime (FL) measurements reveal faster carrier separation and transport after the doping. This study provides guidance for the development of organic photocatalysts for the utilization of renewable energy.

Ruptured liquid metal microcapsules enabling hybridized silver nanowire networks towards high-performance deformable transparent conductors

Extensive studies have been carried out on silver nanowires (AgNWs) in view of their impressive conductivity and highly flexible one-dimensional structure. They are seen as a promising choice for producing deformable transparent conductors. Nonetheless, the widespread adoption of AgNW-based transparent conductors is hindered by critical challenges represented by the significant contact resistance at the nanowire junctions and inadequate interfacial adhesion between the nanowires and the substrate. This study presents a novel solution to tackle the aforementioned challenges by capitalizing on liquid metal microcapsules (LMMs). Upon exposure to acid vapor, the encapsulated LMMs rupture, releasing the fluid LM which then forms a metallic overlay and hybridizes with the underlying Ag network. As a result, a transparent conductive film with greatly enhanced electrical and mechanical properties was obtained. The transparent conductor displays negligible resistance variation even after undergoing chemical stability, adhesion, and bending tests, and ultrasonic treatment. This indicates its outstanding adhesion strength to the substrate and mechanical flexibility. The exceptional electrical properties and robust mechanical stability of the transparent conductor position it as an ideal choice for direct integration into flexible touch panels and wearable strain sensors, as evidenced in this study. By resolving the critical challenges in this field, the proposed strategy establishes a compelling roadmap to navigate the development of high-performance AgNW-based transparent conductors, setting a solid foundation for further advancement in the field of deformable electronics.

Electronic Structure and Photophysics of Low Spin d5 Metallocenes

The electronic structure and photophysics of two low spin metallocenes, decamethylmanganocene (MnCp*2) and decamethylrhenocene (ReCp*2), were investigated to probe their promise as photoredox reagents. Computational studies support the assignment of 2E2 ground state configurations and low energy ligand-to-metal charge transfer transitions for both complexes. Weak emission is observed at room temperature for ReCp*2 with τ = 1.8 ns in pentane, whereas MnCp*2 is not emissive. Calculation of the excited state reduction potentials for both metallocenes reveal their potential potency as excited state reductants (E°'([MnCp*2]+/0*) = -3.38 V and E°'([ReCp*2]+/0*) = -2.61 V vs Fc+/0). Comparatively, both complexes exhibit mild potentials for photo-oxidative processes (E°'([MnCp*2]0*/-) = -0.18 V and E°'([ReCp*2]0*/-) = -0.20 V vs Fc+/0). These results showcase the rich electronic structure of low spin d5 metallocenes and their promise as excited state reductants.

Synthesis and Characterization of a Stable Nickelocenium Dication Salt

We report the synthesis and characterization of the nickelocenium cations [NiCp2 ]⋅+ and [NiCp2 ]2+ as their [F-{Al(ORF )3 }2 ]- (Cp = C5 H5 ; RF =C(CF3 )3 ) salts. Diamagnetic [NiCp2 ]2+ represents the first example for the isolation of an unsubstituted parent metallocene dication. Both salts were generated by reacting neutral NiCp2 with [NO]+ [F-{Al(ORF )3 }2 ]- in 1,2,3,4-tetrafluorobenzene (4FB). The salts were characterized by single crystal X-ray diffraction (XRD), indicating shorter metal-ligand bond lengths for the higher charged salt. Powder XRD shows the salts to be phase pure, cyclic voltammetry in 4FB gave quasi reversible redox waves at -0.44 (0→1) and +1.17 V (1→2) vs Fc/Fc+ . The 1 H NMR of [NiCp2 ]2+ is a singlet at 8.6 ppm, whereas paramagnetic [NiCp2 ]⋅+ is significantly shifted upfield to -103.1 ppm.

The Renaissance of Ferrocene-Based Electrocatalysts: Properties, Synthesis Strategies, and Applications

The fascinating electrochemical properties of the redox-active compound ferrocene have inspired researchers across the globe to develop ferrocene-based electrocatalysts for a wide variety of applications. Advantages including excellent chemical and thermal stability, solubility in organic solvents, a pair of stable redox states, rapid electron transfer, and nontoxic nature improve its utility in various electrochemical applications. The use of ferrocene-based electrocatalysts enables control over the intrinsic properties and electroactive sites at the surface of the electrode to achieve specific electrochemical activities. Ferrocene and its derivatives can function as a potential redox medium that promotes electron transfer rates, thereby enhancing the reaction kinetics and electrochemical responses of the device. The outstanding electrocatalytic activity of ferrocene-based compounds at lower operating potentials enhances the specificity and sensitivity of reactions and also amplifies the response signals. Owing to their versatile redox chemistry and catalytic activities, ferrocene-based electrocatalysts are widely employed in various energy-related systems, molecular machines, and agricultural, biological, medicinal, and sensing applications. This review highlights the importance of ferrocene-based electrocatalysts, with emphasis on their properties, synthesis strategies for obtaining different ferrocene-based compounds, and their electrochemical applications.

Synthesis and characterization of a formal 21-electron cobaltocene derivative

Metallocenes are highly versatile organometallic compounds. The versatility of the metallocenes stems from their ability to stabilize a wide range of formal electron counts. To date, d-block metallocenes with an electron count of up to 20 have been synthesized and utilized in catalysis, sensing, and other fields. However, d-block metallocenes with more than formal 20-electron counts have remained elusive. The synthesis and isolation of such complexes are challenging because the metal-carbon bonds in d-block metallocenes become weaker with increasing deviation from the stable 18-electron configuration. Here, we report the synthesis, isolation, and characterization of a 21-electron cobaltocene derivative. This discovery is based on the ligand design that allows the coordination of an electron pair donor to a 19-electron cobaltocene derivative while maintaining the cobalt-carbon bonds, a previously unexplored synthetic approach. Furthermore, we elucidate the origin of the stability, redox chemistry, and spin state of the 21-electron complex. This study reveals a synthetic method, structure, chemical bonding, and properties of the 21-electron metallocene derivative that expands our conceptual understanding of d-block metallocene chemistry. We expect that this report will open up previously unexplored synthetic possibilities in d-block transition metal chemistry, including the fields of catalysis and materials chemistry.

Magnetically Induced Binary Ferrocene with Oxidized Iron

Ferrocene is perhaps the most popular and well-studied organometallic molecule, but our understanding of its structure and electronic properties has not changed for more than 70 years. In particular, all previous attempts of chemically oxidizing pure ferrocene by binding directly to the iron center have been unsuccessful, and no significant change in structure or magnetism has been reported. Using a metal organic framework host material, we were able to fundamentally change the electronic and magnetic structure of ferrocene to take on a never-before observed physically stretched/bent high-spin Fe(II) state, which readily accepts O2 from air, chemically oxidizing the iron from Fe(II) to Fe(III). We also show that the binding of oxygen is reversible through temperature swing experiments. Our analysis is based on combining Mößbauer spectroscopy, extended X-ray absorption fine structure, in situ infrared, SQUID, thermal gravimetric analysis, and energy dispersive X-ray fluorescence spectroscopy measurements with ab initio modeling.

Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP^2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)₂Fe^II]^2- leads to the octahedral [(TSMP)₂Fe^III]^-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)₂Fe^III]^- can be reversibly oxidized to the stable high-valent [(TSMP)₂Fe^IV]⁰ complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (g_iso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm^-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Dominant changes in centre Fe atom of decamethyl-ferrocene from ferrocene in methylation

Staggered decamethyl-ferrocene (*Fc) becomes the lower energy conformer at low temperature, whereas the eclipsed conformer of ferrocene (Fc) is more stable. The powerful infrared (IR) spectroscopy which has remarkably provided signatures of ferrocene (Fc) in eclipsed and staggered conformers recently is employed to investigate methylation of Fc. The most significant consequences of the full methylation of Fc in the IR spectra are the blue shift of the band at ~ 800 cm−1 in Fc to ~ 1500 cm−1 in *Fc, and the enhancement of the C–H stretch band at ~ 3200 cm−1 region in *Fc. Further analysis reveals large impact of Fc methylation on core electron energies of the centre Fe atom (1s22s22p63s23p6). The Fe core electron energy changes can be as large as ~ 10 kcal mol−1 and are directional—the Fe 2pz and 3pz orbitals along the *Cp–Fe–*Cp axis (Cp centroids, vertical) change more strongly than other Fe core electrons in px and py orbitals. The directional inner shell energy changes are evidenced by larger inner shell reorganization energy. Energy decomposition analysis (EDA) indicates that methyl groups in *Fc apparently change the physical energy components with respect to Fc. The large steric energy of *Fc evidences that the closest hydrogens on adjacent methyl groups of the same *Cp ring in crystal structure are 0.2–0.4 Å closer than the hydrogens on nearest-neighbour methyl groups on opposing rings in *Fc. A significant increase in Pauli repulsive energy contributes to the large repulsive steric energy in *Fc.

Metal-organic framework (MOF) facilitated highly stretchable and fatigue-resistant ionogels for recyclable sensors

Ionogel-based flexible sensors are widely applied in wearable biomedical devices and soft robots. However, the abandoned ionic sensors are rapidly turning into e-waste. Here, we harness the porosity and the coordination of metal sites of metal-organic frameworks (MOFs) to develop physically crosslinked ionogels, which are composed of polymer chains that coordinate with the MOF metal sites. The covalent crosslinking of the host material transformed into reversible bond interactions that significantly enhance the mechanical properties of the MOF-ionogels. The obtained ionogels can endure an 11 000% stretch and exhibit Young's modulus and toughness of 58 MPa and 25 MJ m^-3, respectively. In addition, the fracture energy is as high as 125 kJ m^-2, outperforming most reported ionogels. Furthermore, the UiO-66-ionogels are fully recyclable and both the mechanical and electrical properties can be restored. The results of this work provide a new vision for the development of future "green" sensors.

Bis(imidazolium)-1,3-diphosphete-diide: A Building Block for FeC2 P2 Complexes and Clusters

Reaction of the 6π-electron aromatic four-membered heterocycle (IPr)₂ C₂ P₂ (1) (IPr=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) with [Fe₂ CO₉ ] gives the neutral iron tricarbonyl complex [Fe(CO)₃ -η³ -{(IPr)₂ C₂ P₂ }] (2). Oxidation with two equivalents of the ferrocenium salt, [Fe(Cp)₂ ](BArF₂₄ ), affords the dicationic tricarbonyl complex [Fe(CO)₃ -η⁴ -{(IPr)₂ C₂ P₂ }](BArF₂₄ )₂ (4). The one-electron oxidation proceeds under concomitant loss of one CO ligand to give the paramagnetic dicarbonyl radical cation complex [Fe(CO)₂ -η⁴ -{(IPr)₂ C₂ P₂ }](BArF₂₄ ) (5). Reduction of 5 allows the preparation of the neutral dicarbonyl complex [Fe(CO)₂ -η⁴ -{(IPr)₂ C₂ P₂ }] (6). An analysis by various spectroscopic techniques (⁵⁷ Fe Mössbauer, EPR) combined with DFT calculations gives insight into differences of the electronic structure within the members of this unique series of iron carbonyl complexes, which can be either described as electron precise or Wade-Mingos clusters.

Introducing the Perfluorinated Cp* Ligand into Coordination Chemistry

The reaction of AgBF₄ and [Rh(COD)Cl]₂ (COD=1,5-cyclooctadiene) in presence of [NEt₄ ][C₅ (CF₃ )₅ ] afforded the fluorocarbon soluble complex [Rh(COD)(C₅ (CF₃ )₅ )] by salt metathesis. This complex represents the first example for a successful coordination of the weakly basic [C₅ (CF₃ )₅ ]^- ligand, since its first synthesis in 1980. In addition to [Rh(COD)(C₅ (CF₃ )₅ )] also the byproduct [Rh(COD)(C₅ (CF₃ )₄ H)] was isolated and fully characterized. Accompanying DFT studies showed that the interaction energy of the [C₅ (CF₃ )₅ ]^- ligand towards the 12-electron fragment [Rh(COD)]⁺ is ≈70 kcal mol^-1 lower in comparison to [C₅ (CH₃ )₅ ]^- due to reduced electrostatic interactions and weaker π-donor properties of the ligand. The quantitative but reversible substitution of the [C₅ (CF₃ )₅ ]^- ligand by toluene, converting it into a weakly coordinating anion, experimentally proved the extraordinary weak bonding interaction.

Pentacoordinate Carbon Atoms in a Ferrocene Dication Derivative—[Fe(Si2-η5-C5H2)2]2+

Pentacoordinate carbon atoms are theoretically predicted here in a ferrocene dication derivative in the eclipsed-(1; C2v), gauche-(2; C2) and staggered-[Fe(Si2-η5-C5H2)2]2+(3; C2h) forms for the first time. Energetically, the relative energy gaps for 2 and 3 range from −3.06 to 16.74 and −2.78 to 40.34 kJ mol−1, respectively, when compared to the singlet electronic state of 1 at different levels. The planar tetracoordinate carbon (ptC) atom in the ligand Si2C5H2 becomes a pentacoordinate carbon upon complexation. The ligand with a ptC atom was predicted to be both a thermodynamically and kinetically stable molecule by some of us in our earlier theoretical works. Natural bond orbital and adaptive natural density partitioning analyses confirm the pentacoordinate nature of carbon in these three complexes (1–3). Although they are hypothetical at the moment, they support the idea of “hypercoordinate metallocenes” within organometallic chemistry. Moreover, ab initio molecular dynamics simulations carried out at 298 K temperature for 2000 fs suggest that these molecules are kinetically stable.

Persistent Radicals Derived from N-Heterocyclic Carbenes for Material Applications

Persistent radicals are potential building blocks of novel materials in many fields. Recently, highly stable persistent radicals are considered to be within reach, thanks to several radical stabilization strategies such as spin delocalization and steric protection. N-Heterocyclic carbene (NHC)-derived substituents can be attached to a radical center for these purposes, as illustrated by numerous NHC-stabilized radicals reported in the last two decades.This Account describes our recent work on developing NHC-derived persistent radicals, as well as their prospective applications. Considering that NHCs not only stabilize radicals but also reversibly interact with gas molecules, in 2015 our group reported NHC-nitric oxide (NHC-NO) radicals produced by reversibly trapping nitric oxide (NO) radical gas in NHCs. The resultant compounds were loaded into biocompatible poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL) micelles and injected into tumor-bearing mice. Then, NO release was triggered by high-intensity focused ultrasound irradiation of the tumor tissue. Furthermore, the NHC-NO radicals could also serve as a platform to generate other organic radicals such as oxime ether or iminyl radicals. Apart from medicine-related applications, radicals stabilized by NHCs can be used as energy storage materials. In this context, the triazenyl radical containing two NHC units reported by our laboratory could be a cathode active material in batteries, as an organic alternative to LiCoO₂. The subsequently prepared unsymmetrical triazenyl radical derivatives were applied as anolytes in nonaqueous all-organic redox flow batteries. In addition, a ferrocene-based redox flow battery anolyte was obtained by introducing NHC-derived substituents that effectively stabilize the ferrocenate derivatives previously reported only at low temperatures. The batteries containing NHC-supported radicals exhibited high energy efficiency and insignificant radical decomposition over multiple cycles. Finally, toward developing air-persistent organic radicals for flexible devices and MRI contrasting agents, we also highlight our recent air- and physiologically stable organic radicals derived from NHCs. Coordination of tris(pentafluorophenyl)borane to the NHC-NO radical produced a new radical cation that is stable in an organic solvent under air for several months. The readily accessible 1,2-dicarbonyl radical cations generated by the reaction of NHCs with oxalyl chloride are remarkably persistent even in an aqueous solution for several months. They are also highly stable even under physiological conditions, making them particularly attractive potential candidates for organic MRI contrast agents. We hope that this Account will serve as a guide for the future development of stable NHC-derived organic radicals and draw the attention of the synthetic community to their potential applications in material science.

Increasing the oxidation power of TCNQ by coordination of B(C6F5)3

The oxidation power of the cyanocarbon TCNQ (tetracyano-quinodimethane) can be significantly increased to approximately E = +0.9 V vs. Cp₂Fe by coordination of up to four equivalents of the strong fluorinated Lewis acid B(C₆F₅)₃, resulting in a highly reactive but easy-to-use oxidation system. Thianthrene and tris(4-bromophenyl)amine were oxidized to the corresponding radical cations. Dianionic [TCNQ·4 B(C₆F₅)₃]^2- was formed upon reduction with two equivalents of ferrocene or decamethylcobaltocene. [TCNQ·4 B(C₆F₅)₃]^- and [TCNQ·4 B(C₆F₅)₃]^2- are rare cases of redox-active weakly-coordinating anions.

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

Ferrocene functionalized enantiomerically pure Schiff bases and their Zn(ii) and Pd(ii) complexes: a spectroscopic, crystallographic, electrochemical and computational investigation

A combination of X-ray diffraction, IR, UV-vis and NMR spectroscopy together with computational methods was used to characterize and study the properties of the title compounds.

Nickel-Borolide Complexes and Their Complex Electronic Structure

Borolides (BC₄^2-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr₂NBC₄Ph₂)₂ (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni²⁺ species bound to two monoanionic radical (BC₄^•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Pyrrolinium-Substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and ⁵⁷Fe Mössbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the pyrrolinium-substituted ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

Selective dimerization of a trinuclear mixed-metal sandwich complex: construction of an axially chiral metal skeleton

Certain metal sandwich complexes undergo dimerization through metal-metal bond formation. Here, we found that a reductive dimerization of mixed-metal Pd₂Pt or PdPt₂ sandwich complexes proceeds through selective Pt-Pt bond formation. A restricted rotation at the Pt-Pt bond of the PdPt₂ dimer gave a unique axially chiral structure derived from a heterometal arrangement in a mixed-metal cluster.

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.

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.

Revealing the Structure of Transition Metal Complexes of Formaldoxime

Aerobic reactions of iron(III), nickel(II), and manganese(II) chlorides with formaldoxime cyclotrimer (tfoH₃) and 1,4,7-triazacyclononane (tacn) produce indefinitely stable complexes of general formula [M(tacn)(tfo)]Cl. Although the formation of formaldoxime complexes has been known since the end of 19th century and applied in spectrophotometric determination of d-metals (formaldoxime method), the structure of these coordination compounds remained elusive until now. According to the X-ray analysis, [M(tacn)(tfo)]⁺ cation has a distorted adamantane-like structure with the metal ion being coordinated by three oxygen atoms of deprotonated tfoH₃ ligand. The metal has a formal +4 oxidation state, which is atypical for organic complexes of iron and nickel. Electronic structure of [M(tacn)(tfo)]⁺ cations was studied by XPS, NMR, cyclic (CV) and differential pulse (DPV) voltammetries, Mössbauer spectroscopy, and DFT calculations. Unusual stabilization of high-valent metal ion by tfo^3- ligand was explained by the donation of electron density from the nitrogen atom to the antibonding orbital of the metal-oxygen bond via hyperconjugation as confirmed by the NBO analysis. All complexes [M(tacn)(tfo)]Cl exhibited high catalytic activity in the aerobic dehydrogenative dimerization of p-thiocresol under ambient conditions.

Eightfold Electrophilic Methylation of Octacyanotungstate [W(CN)8]4-/3-: Preparation of Homoleptic, Eight-Coordinate Methyl Isocyanide Complexes [W(CNMe)8]4+/5

Homoleptic eightfold coordinated methyl isocyanide complexes of W(IV) and W(V) have been prepared for the first time. The reaction of [NBu₄]₄[W(CN)₈] with methyl triflate (MeOTf) gives [W(CNMe)₈][OTf]₄. The even stronger methylating mixture of methyl fluoride (MeF) and arsenic pentafluoride (AsF₅) in liquid sulfur dioxide (SO₂) is able to fully alkylate both [NBu₄]₄[W(CN)₈] and [NBu₄]₃[W(CN)₈]. The paramagnetic octakis(methyl isocyanide)tungsten(V) complex [W(CNMe)₈][AsF₆]₅ is thermally highly unstable above -30 °C. All compounds have been characterized via single-crystal X-ray diffraction and IR, Raman, and NMR or EPR spectroscopy.

Ring rotation of ferrocene in interlocked molecules in single crystals

This work describes unique molecular motions of ferrocene-containing interlocked molecules observed by single-crystal X-ray crystallography. The rotational flexibility of ferrocene is achieved using combinations of ferrocene-tethered ammonium and 30-membered ring dibenzo-crown ether. By contrast, ferrocene was locked in the complex with an 18-membered ring dibenzo-crown ether and CH2Cl2. When the complex was heated at 358 K, CH2Cl2 was removed from the complex, which led to drastic structural changes, including a semieclipsed-to-disordered transition of ferrocene and flipping of the dibenzo-crown ether.

Low-spin 1,1'-diphosphametallocenates of chromium and iron

We report two anionic diphosphametallocenates, [K(2.2.2-crypt)][M(PC4Me4)2] (M = Cr, 2-Cr; Fe, 2-Fe). Both are low-spin (S = ½) by EPR spectroscopy and SQUID magnetometry. This contrasts the high-spin (S = 3/2) ferrocenate, [K(2.2.2-crypt)][Fe(C5H2-1,2,4-tBu)2] (4-Fe). Quantum chemical calculations suggest this is due to significant differences in ligand field splitting of the d-orbitals which also explain structural features in the 2-M complexes.

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.

A Stable Homoleptic Organometallic Iron(IV) Complex

A homoleptic organometallic Fe^IV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)₂ ](PF₆ )₂ (phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate]^- ). This Fe^IV N-heterocyclic carbene (NHC) complex was characterized by ¹ H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)₂ ](PF₆ )₂ is a triplet Fe^IV low-spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe^IV complex was studied and revealed a ≈0.8 ps lifetime of the ³ LMCT excited state of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile.

The [2+2] cycloaddition product of perhalogenated cyclopentadienyl cations: structural characterization of salts of the [C10Cl10]2+ and [C10Br10]2+ dications

Instead of monomeric cyclopentadienyl cations, the low-temperature reaction of hexachloro- and hexabromocyclo-pentadiene (C5Cl6 and C5Br6) with powerful Lewis acids SbF5 and AsF5 in SO2ClF yields salts of perhalogenated dications [C10Cl10][Sb3F16]2 and [C10Br10][As2F11]2 which are characterized via single crystal X-ray diffraction and NMR spectroscopy. Additionally, this reactivity is rationalized by quantum-chemical calculations.

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

While the icosahedral closo-[B₁₂H₁₂]^2- cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B₁₂(O-3-methylbutyl)₁₂ (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br₂C₆H₃)₃]^•+ afforded the isolable [1]^•+ cluster, which is the first example of an open-shell cationic B₁₂ cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]^•+ with ferrocene resulted in its reduction back to 1. The identity of [1]^•+ is supported by EPR, UV-vis, multinuclear NMR (¹H, ¹¹B), and X-ray photoelectron spectroscopic characterization.

Synthesis and Application of a Perfluorinated Ammoniumyl Radical Cation as a Very Strong Deelectronator

The perfluorinated dihydrophenazine derivative (perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine) ("phenazine^F ") can be easily transformed to a stable and weighable radical cation salt by deelectronation (i.e. oxidation) with Ag[Al(OR^F )₄ ]/ Br₂ mixtures (R^F =C(CF₃ )₃ ). As an innocent deelectronator it has a strong and fully reversible half-wave potential versus Fc⁺ /Fc in the coordinating solvent MeCN (E°'=1.21 V), but also in almost non-coordinating oDFB (=1,2-F₂ C₆ H₄ ; E°'=1.29 V). It allows for the deelectronation of [Fe^III Cp*₂ ]⁺ to [Fe^IV (CO)Cp*₂ ]²⁺ and [Fe^IV (CN-^t Bu)Cp*₂ ]²⁺ in common laboratory solvents and is compatible with good σ-donor ligands, such as L=trispyrazolylmethane, to generate novel [M(L)_x ]ⁿ⁺ complex salts from the respective elemental metals.