Tuning a metal's oxidation state: the potential of clathrochelate systems

Effects of solvatomorphism, the nature of a chelating ligand synthon and a counterion on the single crystal XRD structure and SMM properties of paramagnetic monocapped cobalt(II) tris-pyrazoloximates

A series of monocapped cobalt(II) tris-pyrazoloximates was obtained through the template condensation of the corresponding pyrazoloxime, phenylboronic acid and a suitable cobalt(II) halogenide. Comparing 3-acetylpyrazoloxime versus its methine-containing homolog, the former produced cobalt(II) clathrochelates in substantially higher yields due to the electron donating effect of the methyl substituent, increasing the N-donor ability of its oxime group. Their less N-donor analog with the electron acceptor trifluoromethyl group did not form cobalt(II) complexes of this type. In all their solvent-free and solvent-containing crystals, the encapsulated cobalt(II) ion adopted a high-spin state, as gauged by the Co-N bond lengths of 2.112(4)-2.188(9) Å, and was located almost in the center of its CoN6-coordination polyhedron. Their CoN6-polyhedra had an almost ideal trigonal-prismatic (TP) geometry with distortion angles φ below 4°. This TP-like geometry was assisted by hydrogen bonding between their NH groups and the apical counterion. The absence of methyl groups makes them close to an ideal TP. In contrast, stronger N-H⋯Cl hydrogen bonds occurred in the methyl-containing complex, while the Co-N bond lengths stayed the same at 2.144(2) Å on average. In its solvates with benzene, chloroform and acetone, there is a clear tendency for φ to decrease from 2.7(3)° to 0.47(13)°. The comparable effects of the ribbed methyl substituents, the cross-linking counterion and the lattice solvent on their molecular geometry were observed; the larger the distortions from an ideal TP geometry, the stronger the hydrogen bonds to the corresponding apical halogenide anion. The analysis of the experimental AC- and DC-magnetometry data for their fine-crystalline samples suggests that the passing from the derivative of the methyl-substituted synthon to that of its methine-containing homolog caused a substantial decrease in the magnetic susceptibility value χT and an increase in the QTM contribution to the magnetic relaxation. The effect of a cross-linking halogenide counteranion on the Orbach remagnetization barrier is greater than that of the solvatomorphism of their crystals.

Precise Fingerprint Determination of Vibrational Infrared Spectra in a Series of Co(II) Clathrochelates through Experimental and Theoretical Analyses

The energetic demands of modern society for clean energy vectors, such as H2, have caused a surge in research associated with homogeneous and immobilized electrocatalysts that may replace Pt. In particular, clathrochelates have shown excellent electrocatalytic properties for the hydrogen evolution reaction (HER). However, the actual mechanism for the HER catalyzed by these d-metal complexes remains an open debate, which may be addressed via Operando spectroelectrochemistry. The prediction of electrochemical properties via density functional theory (DFT) needs access to thermodynamic functions, which are only available after Hessian calculations. Unfortunately, there is a notable lack in the current literature regarding the precise evaluation of vibrational spectra of such complexes, given their structural complexity and the associated tangled IR spectra. In this work, we have performed a detailed theoretical and experimental analysis in a family of Co(II) clathrochelates, in order to establish univocally their IR pattern, and also the calculation methodology that is adequate for such predictions. In summary, we have observed the presence of multiple common bands shared by this clathrochelate family, using the B3LYP functional, the LANL2DZ basis, and effective core potentials (ECP) for heavy atoms. The most important issue addressed in this article was therefore related to the detailed assignment of the fingerprint associated with cobalt(II) clathrochelates, which is a challenging endeavor due to the crowded nature of their spectra.

Synthesis, crystal polymorphism and spin crossover behavior of adamantylboron-capped cobalt(II) hexachloroclathrochelate and its transformation into the CoIIICoIICoIII-bis-macrobicyclic derivative

Fast crystallization of the monoclathrochelate cobalt(II) intracomplex [Co(Cl₂Gm)₃(BAd)₂] (where Cl₂Gm^2- is a dichloroglyoxime dianion and BAd is an adamantylboron capping group, 1), initially obtained by the direct template condensation of the corresponding chelating α-dioximate and cross-linking ligand synthons on the Co²⁺ ion as a matrix, from benzene or dichloromethane afforded its structural triclinic and hexagonal polymorphs. Its prolonged recrystallization from dichloromethane under air atmosphere and sunlight irradiation unexpectedly gave the crystals of the Co^IIICo^IICo^III-trinuclear dodecachloro-bis-clathrochelate intracomplex [[Co^III(Cl₂Gm)₃(BAd)]₂Co^II] (2), the molecule of which consists of two macrobicyclic frameworks with encapsulated low-spin (LS) Co³⁺ ions, which are cross-linked by a μ₃-bridging Co²⁺ ion as a bifunctional Lewis-acidic center. The most plausible pathway of such a 1 → 2 transformation is based on the photoinitiated radical oxidation of dichloromethane with air oxygen giving the reactive species. Cobalt(II) monoclathrochelate 1 was found to undergo a temperature-induced spin crossover (SCO) both in its solutions and in the solid state. In spite of the conformational rigidity of the corresponding quasiaromatic diboron-capped tris-α-dioximate framework, the main parameters of this SCO transition (i.e., its completeness and gradual character) are strongly affected by the nature of the used solvent (in the case of its solutions) and by the structural polymorphism of its crystals (in the solid state). In the latter case, the LS state (S = 1/2) of this complex is more thermally stable and, therefore, the cobalt(II)-centered 1/2 → 3/2 SCO is more gradual than that in solutions.

Expanding manganese(IV) aqueous chemistry: unusually stable water-soluble hexahydrazide clathrochelate complexes

Mn cage complexes are rare, and the ones successfully isolated in the solid state are not stable in water and organic solvents. Herein, we present the first report of mononuclear Mn clathrochelates, in which the encapsulated metal exists in the oxidation state +4. The complexes are extremely stable in the crystalline state and in solutions and show rich redox chemistry.

Intrinsic hydroquinone-functionalized aggregation-induced emission core shows redox and pH sensitivity

Aggregation-induced emission (AIE) fluorophores exhibit strong fluorescence in an aggregated state but emit no or weak fluorescence in dilute solutions. This emerging class of AIE optical materials comprise a variety of functionalities. Here an AIE luminescence core, 1-hydroquinol-1,2,2-triphenylethene (HQTPE), has been designed and synthesized. This AIE core is simple but is fundamentally important to chemistry because of its intrinsic redox and pH activities. The incorporation of hydroquinone (HQ) moiety into a common AIE core tetraphenylethene (TPE) yields HQTPE with unique fluorescent properties like nonlinear self-quenching over most other AIE-active fluorophores (AIEgens) so far reported. There are differences of photochemical properties between HQTPE, 1-benzoquinol-1,2,2-triphenylethene (QTPE, the oxidized counterpart) and its anions. Interestingly, as the solution concentration is increased, AIEgen HQTPE shows stronger fluorescence but QTPE exhibits rapid quenching of fluorescence in a nonlinear fashion, which are in agreement with theoretical studies. The fluorescence of HQTPE is also highly dependent on the pH value of media. We have further explored HQTPE as an ultrasensitive redox probe and efficient deoxidizer, which could lead to potential applications in health care, food security, environmental monitoring, optic and electronic devices.

New types of the hybrid functional materials based on cage metal complexes for (electro) catalytic hydrogen production

Successful using of cage metal complexes (clathrochelates) and the functional hybrid materials based on them as promising electro- and (pre)catalysts for hydrogen and syngas production is highlighted in this microreview. The designed polyaromatic-terminated iron, cobalt and ruthenium clathrochelates, adsorbed on carbon materials, were found to be the efficient electrocatalysts of the hydrogen evolution reaction (HER), including those in polymer electrolyte membrane (PEM) water electrolysers. The clathrochelate-electrocatalayzed performances of HER 2H+/H2 in these semi-industrial electrolysers are encouraging being similar to those for the best known to date molecular catalysts and for the promising non-platinum solid-state HER electrocatalysts as well. Electrocatalytic activity of the above clathrochelates was found to be affected by the number of the terminal polyaromatic group(s) per a clathrochelate molecule and the lowest Tafel slopes were obtained with hexaphenanthrene macrobicyclic complexes. The use of suitable carbon materials of a high surface area, as the substrates for their efficient immobilization, allowed to substantially increase an electrocatalytic activity of the corresponding clathrochelate-containing carbon paper-based cathodes. In the case of the reaction of dry reforming of methane (DRM) into syngas of a stoichiometry CO/H2 1:1, the designed metal(II) clathrochelates with terminal polar groups are only the precursors (precatalysts) of single atom catalysts, where each of their catalytically active single sites is included in a matrix of its former encapsulating ligand. Choice of their designed ligands allowed an efficient immobilization of the corresponding cage metal complexes on the surface of a given highly porous ceramic material as a substrate and caused increasing of a surface concentration of the catalytically active centers (and, therefore, that of the catalytic activity of hybrid materials modified with these clathrochelates). Thus designed cage metal complexes and hybrid materials based on them operate under the principals of “green chemistry” and can be considered as efficient alternatives to some classical inorganic and molecular (pre)catalysts of these industrial processes.

Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above -2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of -C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of "metric oxidation state" (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal-ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.

Clathrochelate Metalloligands in Supramolecular Chemistry and Materials Science

The term "clathrochelate" describes a complex in which a coordinatively saturated metal ion is surrounded by a macropolycyclic ligand. First examples of clathrochelate complexes were reported 50 years ago. Meanwhile, the synthesis and reactivity of clathrochelates have been investigated in detail, and numerous applications have been explored. In this Account, we summarize work on the utilization of transition metal clathrochelates as metalloligands in supramolecular chemistry and materials science, with special focus on results from our group. First, we discuss the chemistry of boron-capped clathrochelates. These complexes are facile to synthesize by metal-templated condensation reactions. The synthesis is modular, and it is straightforward to implement structural variations. Importantly, it is possible to attach functional groups such as amines, pyridines, or carboxylic acids to the ligand periphery. Other noteworthy features of boron-capped clathrochelates are high thermodynamic and kinetic stability, tunable redox potential, and good solubility. Next, we show that clathrochelate-based metalloligands can be used to build molecularly defined metal-ligand assemblies of nanoscale dimensions. Different molecular architectures are described, including coordination cages with unusual gyrobifastigium or square orthobicupola-like structures. Metalloligands containing multiple clathrochelate complexes are particularly well suited to build large metal-ligand assemblies (>3 nm) with minimal synthetic efforts. Boron-capped clathrochelates have also been investigated in the context of materials chemistry. Linear or cross-linked clathrochelate polymers were found to display permanent porosity. Furthermore, such polymers were used to prepare conducting films on electrodes. Clathrochelate metalloligands are well suited to prepare metal-organic frameworks (MOFs). The high stability of clathrochelates ensures compatibility with harsh reaction conditions, and it mitigates potential problems such as exchange reactions. Boron-capped clathrochelates can be decorated with functional groups in lateral and apical position, and it is possible to use these complexes as multiconnected nodes in polymeric structures. Overall, we hope to convey the utility of clathrochelate complexes in supramolecular chemistry and materials science. The work published thus far gives a first glimpse of the potential of these compounds, but there are other directions, which are waiting to be explored. For example, it will be interesting to study the properties of nanostructures based on chiral clathrochelate complexes. Furthermore, the redox and magnetic properties of clathrochelates may give rise to novel functional materials. Given that clathrochelates are straightforward to prepare, we hope that others will join the efforts to explore the supramolecular and materials chemistry of these interesting molecular building blocks.

Synthesis and characterization of an Fe(i) cage complex with high stability towards strong H-acids

The new iron(i) dioximate showed an unrivaled stability towards strong acids. This calls for a reassessment of the electrocatalytic activity of similar low-valent Co and Fe cage complexes, which have shown to be effective HER electrocatalysts.

A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents

The study tackles one of the challenges in developing platinum-free molecular electrocatalysts for hydrogen evolution, which is to seek for new possibilities to ensure large turnover numbers by stabilizing electrocatalytic intermediates. These species are often much more reactive than the initial electrocatalysts, and if not properly stabilized by a suitable choice of functionalizing substituents, they have a limited long-time activity. Here, we describe new iron and cobalt(II) cage complexes (clathrochelates) that in contrast to many previously reported complexes of this type do not act as electrocatalysts for hydrogen evolution. We argue that the most probable reason for this behavior is an excessive stabilization of the metal(I) species by perfluoroaryl ribbed groups, resulting in an unprecedented long-term stability of the metal(I) complexes even in acidic solutions.

Indefinitely stable iron(IV) cage complexes formed in water by air oxidation

In nature, iron, the fourth most abundant element of the Earth's crust, occurs in its stable forms either as the native metal or in its compounds in the +2 or +3 (low-valent) oxidation states. High-valent iron (+4, +5, +6) compounds are not formed spontaneously at ambient conditions, and the ones obtained synthetically appear to be unstable in polar organic solvents, especially aqueous solutions, and this is what limits their studies and use. Here we describe unprecedented iron(IV) hexahydrazide clathrochelate complexes that are assembled in alkaline aqueous media from iron(III) salts, oxalodihydrazide and formaldehyde in the course of a metal-templated reaction accompanied by air oxidation. The complexes can exist indefinitely at ambient conditions without any sign of decomposition in water, nonaqueous solutions and in the solid state. We anticipate that our findings may open a way to aqueous solution and polynuclear high-valent iron chemistry that remains underexplored and presents an important challenge.

Cytotoxicity of electrophilic iron(II)-clathrochelates in human promyelocytic leukemia cell line

We observed that electrophilic iron(II)-clathrochelates exhibit significant cytotoxicity in human promyelocytic leukemia cells (IC50=6.5±4.6μM), which correlates with the enhancement of intracellular oxidative stress (17-fold increase with respect to the cells treated with the solvent only). Based on in vitro studies we suggested that this effect is caused by alkylation of glutathione leading to inhibition of the cellular antioxidative system and by catalytic generation of reactive oxygen species by products of the alkylation reaction.

First iron and cobalt(II) hexabromoclathrochelates: structural, magnetic, redox, and electrocatalytic behavior

Template condensation of dibromoglyoxime with n-butylboronic acid on the corresponding metal ion as a matrix under vigorous reaction conditions afforded iron and cobalt(ii) hexabromoclathrochelates. The paramagnetic cobalt clathrochelate was found to be a low-spin complex at temperatures below 100 K, with a gradual increase in the effective magnetic moment at higher temperatures due to the temperature 1/2↔3/2 spin crossover and a gap caused by the structure phase transition. The multitemperature X-ray and DSC studies of this complex and its iron(ii)-containing analog also showed temperature structural transitions. The variation of an encapsulated metal ion's radius, electronic structure and spin state caused substantial differences in the geometry of its coordination polyhedron; these differences increase with the decrease in temperature due to Jahn-Teller distortion of the encapsulated cobalt(ii) ion with an electronic configuration d(7). As follows from CV and GC data, these cage iron and cobalt complexes undergo both oxidation and reduction quasireversibly, and showed an electrocatalytic activity for hydrogen production in different producing systems.

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10(-32) m(3) at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

Copper-promoted reductive homocoupling of quasi-aromatic iron(II) clathrochelates: boosting the inhibitory activity in a transcription assay

A copper-promoted reductive homocoupling reaction, for the first time used for a metal complex, allowed obtaining a new kind of complexes with the encapsulated metal ions, C-C conjugated bis-clathrochelates. These compounds demonstrate extremely high transcription inhibition activity in the T7 RNA polymerase system with values of IC50 reaching as low as the submicromolar range, which places them among the most potent metal-based transcription inhibitors.

Spin-Crossover Anticooperativity Induced by Weak Intermolecular Interactions

As a rule, rational design of cooperative spin-crossover (SCO) molecular switches is largely based on consideration of sizes and structures of individual building blocks, whereas a meticulous analysis of crystal packing, including the weakest intermolecular interactions, is often assumed to play a secondary role or is even fully neglected. By investigating cobalt(II) clathrochelates, which do not change the molecular volume upon SCO, we showed that even weak (1.2 kcal/mol) π···Cl intermolecular interactions can cause a pronounced anticooperativity of SCO, being more gradual in the solid state than in solution. Our results clearly demonstrate that the "chemical pressure" concept is not as general as it is thought to be, and the successful design of molecular switches requires in-depth analysis of intermolecular interactions, however weak they seem.

Nano-sized heterometallic macrocycles based on 4-pyridinylboron-capped iron(II) clathrochelates: syntheses, structures and properties

Ir-Fe heterometallic macrocycles from clathrochelate-based bipyridyl and bis(amidinate) ligands with controllable cavity size have been prepared and characterized.

Size matters, so does shape: Inhibition of transcription of T7 RNA polymerase by iron(II) clathrochelates

Coordination and organoelement compounds are rarely proposed as the drug candidates despite their vast potential in the area owing to their strictly controlled geometry and rather extensive surface. This is the first example of the inhibition of transcription in the system of T7 RNA polymerase by cage metal complexes. Their IC50 values reach as low as the nanomolar range, placing them among the most potent metal-based transcription inhibitors.

Efficient electrocatalytic hydrogen production from H+ ions using specially designed boron-capped cobalt clathrochelates

Specially designed hexachlorine-containing cobalt(II) tris-dioximate clathrochelates were found to efficiently electrocatalyze the production of molecular hydrogen from H(+) ions without the overpotential of this process.