Asymmetric Hydroformylation Using a Rhodium Catalyst Encapsulated in a Chiral Capsule

Supramolecular capsules can be used to change the activity and selectivity of a catalyst through the influence of the second coordination sphere, reminiscent of how enzymes control the selectivity of their processes. In enzymes, this approach is used to also control the enantioselectivity of reactions in which the active catalytic site is often not chiral but the second coordination sphere is. We are interested in the possibility to generate a chiral second coordination sphere around an otherwise achiral transition metal complex for asymmetric catalysis. In this paper we show that the ligand template approach can be used to generate a chiral second coordination sphere around a rhodium complex, which is used in asymmetric hydroformylation.     

Functional Hydride Transfer by a Thiolate‐Containing Model of Mono‐Iron Hydrogenase featuring an Anthracene Scaffold

We report the synthesis, X‐ray structure and functional biomimetic activity of a model complex of mono‐iron hydrogenase (Hmd). To achieve the desired biomimetic fac‐CNS(thiolate) ligation motif, an anthracene framework is used to provide the requisite donors in a single chelate. A bulky aryl thiolate (ortho dimethylphenyl) is included to achieve mononuclearity. In addition to exhibiting structural (X‐ray) and spectroscopic (NMR, IR) similarity to the enzyme, the complex is competent for H₂ activation (heterolysis) and hydride transfer to a model substrate—mimicking the functional behavior of the enzyme in a biomimetic CNS coordination sphere for the first time.     

Tetragonal-pyramidal complex of copper nitrate with 2-Amino-5-ethyl-1,3,4-thiadiazole

A complex compound of Cu(II) nitrate with 2-amino-5-ethyl-1,3,4-thiadiazole was synthesized and its structure was studied by the methods of IR spectroscopy and X-ray crystal analysis. The complex has the composition Cu(NO₃)₂(2-amino-5-ethyl-1,3,4-thiadiazole)₄ with four molecules of the heterocyclic ligand (coordination through nitrogen atoms of thiadiazole rings) and one of nitrate ions (the other is replaced in the second sphere) entering into the coordination sphere of the complex polyhedron. The internal coordination sphere of the complex has the form of a tetragonal pyramid with 2-amino-5-ethyl-1,3,4-thiadiazole ligands in the sites of its base and the oxygen atom of the nitrate ion in a slightly distorted vertex of the pyramid.     

Enhanced Electrocatalytic Activity of a Zinc Porphyrin for CO2 Reduction: Cooperative Effects of Triazole Units in the Second Coordination Sphere

The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO₂) to carbon monoxide (CO). This electrocatalyst converted CO₂ to CO with a Faradaic efficiency of 99 % and a current density of −6.2 mA cm⁻² at −2.4 V vs. Fc/Fc⁺ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO₂-catalyst adduct in order to facilitate intramolecular proton transfer. Our findings demonstrate that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO₂ conversion.     

Magnetic anisotropy in Ni(II)-Y(III) binuclear complexes: on the importance of both the first coordination sphere of the Ni(II) ion and the Y(III) ion belonging to the second coordination sphere

The synthesis of a new Ni(II)-Y(III) binuclear complex with a marked elongation axis in the first coordination sphere of the Ni(II) ion is presented. Its zero-field splitting (ZFS) is studied by means of magnetic data and state-of-the-art ab initio calculations. A good agreement between the experimental and theoretical ZFS parameter values is encountered, validating the whole approach. The magnetic anisotropy axes are extracted from the ab initio calculations, showing that the elongation axis around the Ni(II) ion corresponds to the hard axis of magnetization and that the sign of the axial D parameter is imposed by this axis. The Ni-Y axis is found to be an easy axis of magnetization, which is, however, not significant according to the sign of D. The already reported [(H(2)O)Ni(ovan)(2)(μ-NO(3))Y(ovan)(NO(3))]·H(2)O (ovan = o-vanillin) complex is then revisited. In this case, the elongation axis in the Ni(II) coordination sphere is less marked and the ZFS is dominated by the effect of the Y(III) ion belonging to the second coordination sphere. As a consequence, the D parameter is negative and the low-temperature behavior is dominated by the Ni-Y easy axis of magnetization. A competition between the first coordination sphere of the Ni(II) ion and the electrostatic effect of the Y(III) ion belonging to the second coordination sphere is then evidenced in both complexes, and the positive and negative D parameters are then linked to the relative importance of both effects in each complex.     

Single‐Molecule Spin Switch Based on Voltage‐Triggered Distortion of the Coordination Sphere

Here, we report on a new single‐molecule‐switching concept based on the coordination‐sphere‐dependent spin state of Fe^II species. The perpendicular arrangement of two terpyridine (tpy) ligands within heteroleptic complexes is distorted by the applied electric field. Whereas one ligand fixes the complex in the junction, the second one exhibits an intrinsic dipole moment which senses the E field and causes the distortion of the Fe^II coordination sphere triggering the alteration of its spin state. A series of complexes with different dipole moments have been synthesized and their transport features were investigated via mechanically controlled break‐junctions. Statistical analyses support the hypothesized switching mechanism with increasing numbers of junctions displaying voltage‐dependent bistabilities upon increasing the Fe^II complexes’ intrinsic dipole moments. A constant threshold value of the E field required for switching corroborates the mechanism.     

5-Methylcytosine is Oxidized to the Natural Metabolites of TET Enzymes by a Biomimetic Iron(IV)-Oxo Complex

Ten-eleven-translocation (TET) methyl cytosine dioxygenases play a key role in epigenetics by oxidizing the epigenetic marker 5-methyl cytosine (5mC) to 5-hydroxymethyl cytosine (5hmC), 5-formyl cytosine (5fC), and 5-carboxy cytosine (5cC). Although much of the metabolism of 5mC has been studied closely, certain aspects—such as discrepancies among the observed catalytic activity of TET enzymes and calculated bond dissociation energies of the different cytosine substrates—remain elusive. Here, it is reported that the DNA base 5mC is oxidized to 5hmC, 5fC, and 5cC by a biomimetic iron(IV)-oxo complex, reminiscent of the activity of TET enzymes. Studies show that 5hmC is preferentially turned over compared with 5mC and 5fC and that this is in line with the calculated bond dissociation energies. The optimized syntheses of d₃-5mC and d₂-5hmC are also reported and in the reaction with the biomimetic iron(IV)-oxo complex these deuterated substrates showed large kinetic isotope effects, confirming the hydrogen abstraction as the rate-limiting step. Taken together, these results shed light on the intrinsic reactivity of the C−H bonds of epigenetic markers and the contribution of the second coordination sphere in TET enzymes.     

Second sphere interaction in fluoroanion binding: Synthesis, spectroscopic and X-ray structural study of trans-dichlorobis(ethylenediamine) cobalt(III) terafluoroborate

Trans-dichlorobis(ethylenediamine) cobalt(III) terafluoroborate was synthesised and detailed packing analysis was undertaken to delineate the topological complimentarity of [trans-Co(en)₂Cl₂]⁺ and BF₄⁻ ions by second sphere coordination. The complex was completely characterised by elemental analyses, solubility product measurement and spectroscopic studies (IR, UV–Vis, multinuclear NMR). In the crystal lattice, discrete ions [trans-Co(en)₂Cl₂]⁺ and BF₄⁻ are arranged in A–B–A–B pattern (in both a and c directions of the lattice) forming columns of anions and cations. Crystal lattice is stabilized by electrostatic forces of attraction and hydrogen bonding interactions, i.e. N–HF⁻ and N–HCl involving second sphere coordination. It appears that the topological features of [trans-Co(en)₂Cl₂]⁺ are conducive for generating second sphere interactions. This strategy may be used as a viable method for the capture of other fluoroanions.     

Modifying the Steric Properties in the Second Coordination Sphere of Designed Peptides Leads to Enhancement of Nitrite Reductase Activity

Protein design is a useful strategy to interrogate the protein structure‐function relationship. We demonstrate using a highly modular 3‐stranded coiled coil (TRI‐peptide system) that a functional type 2 copper center exhibiting copper nitrite reductase (NiR) activity exhibits the highest homogeneous catalytic efficiency under aqueous conditions for the reduction of nitrite to NO and H₂O. Modification of the amino acids in the second coordination sphere of the copper center increases the nitrite reductase activity up to 75‐fold compared to previously reported systems. We find also that steric bulk can be used to enforce a three‐coordinate Cu^I in a site, which tends toward two‐coordination with decreased steric bulk. This study demonstrates the importance of the second coordination sphere environment both for controlling metal‐center ligation and enhancing the catalytic efficiency of metalloenzymes and their analogues.     

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.     

Catalytic reduction of dioxygen to water with a monomeric manganese complex at room temperature

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.     

A five-coordinate complex of copper chloride with 2-amino-5-ethyl-1,3,4-thiadiazole

A complex of Cu(II) chloride with 2-amino-5-ethyl-1,3,4-thiadiazole (AET) was prepared, and its structure was studied by IR spectroscopy and single crystal X-ray diffraction. The complex has the composition CuCl₂(AET)₄. The coordination sphere of the copper atom includes four molecules of the heterocyclic ligand coordinated via N atoms of thiadiazole rings and one of Cl^? anions; the second Cl^? anion is in the outer sphere.     

X-ray diffraction and EXAFS studies of hydroxo-Cu(II) complexes based on a calix[6]arene-N3 ligand: evidence for a mononuclear-dinuclear equilibrium controlled by supramolecular features

The formation of hydroxo complexes based on a calix[6]trisimidazole ligand is described. Deprotonation of the mononuclear Cu(II)-aqua complex gives rise to a dinuclear bis(mu-hydroxo) complex that has been characterized by X-ray diffraction analysis. Spectroscopic studies (EPR and UV-vis), conducted in dichloromethane solutions in the presence of various coordinating cosolvents (DMF, EtOH, or RCN) or with acetamide, revealed the coexistence of a mononuclear hydroxo species. The latter could be trapped by acetic acid to provide an acetato-Cu(II) complex that presents close spectroscopic features. An EXAFS study first conducted on the hydroxo-Cu(II) complex led to an excellent fit for the dinuclear core. It then allowed for the characterization of the mononuclear acetato complex with an acetamide guest included in the calixarene cavity. Hence, this study illustrates the flexibility of calixarene-based ligands and the role of the second coordination sphere in the stabilization of acidic or basic Cu(II) complexes.     

Formation of High‐Valent Iron–Oxo Species in Superoxide Reductase: Characterization by Resonance Raman Spectroscopy

Superoxide reductase (SOR), a non‐heme mononuclear iron protein that is involved in superoxide detoxification in microorganisms, can be used as an unprecedented model to study the mechanisms of O₂ activation and of the formation of high‐valent iron–oxo species in metalloenzymes. By using resonance Raman spectroscopy, it was shown that the mutation of two residues in the second coordination sphere of the SOR iron active site, K₄₈ and I₁₁₈, led to the formation of a high‐valent iron–oxo species when the mutant proteins were reacted with H₂O₂. These data demonstrate that these residues in the second coordination sphere tightly control the evolution and the cleavage of the O? O bond of the ferric iron hydroperoxide intermediate that is formed in the SOR active site.     

Energetics of the first and second coordination spheres of transition metal ions

For complexes of transition metals (manganese, iron, cobalt, nickel) with monodentate ligands, equilibrium metal-ligand distances and ligand bond energies in the first and second coordination spheres have been calculated by the CNDO method. Some effects of ligand bond energies in different coordination spheres are analyzed. These effects significantly differ between the first and second coordination spheres. In the first sphere, the ligand bond energy is mainly determined by the nature of the central ion and the type of donor atom of the ligand, but weakly depends on the structure of the ligand. Conversely, in the second coordination sphere, the ligand bond energy weakly depends on the nature of the central ion and the type of donor atom, but considerably depends on the structure of the ligands in the first coordination sphere. In the second coordination sphere, ligand binding is determined by ligand interactions with both the central ion and the ligands of the first sphere. In the general case, when strong specific interactions between ligands are absent, the energetics of the second sphere is determined by the size of the inner-spheric ligands, which may be considered to be a specific steric effect. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 370–374, March–April, 1995. Translated from L. Smolina     

Hydrogenase Mimics in M12L24 Nanospheres to Control Overpotential and Activity in Proton-Reduction Catalysis

Hydrogenase enzymes are excellent proton reduction catalysts and therefore provide clear blueprints for the development of nature-inspired synthetic analogues. Mimicking their catalytic center is straightforward but mimicking the protein matrix around the active site and all its functions remains challenging. Synthetic models lack this precisely controlled second coordination sphere that provides substrate preorganization and catalyst stability and, as a result, their performances are far from those of the natural enzyme. In this contribution, we report a strategy to easily introduce a specific yet customizable second coordination sphere around synthetic hydrogenase models by encapsulation inside M₁₂L₂₄ cages and, at the same time, create a proton-rich nano-environment by co-encapsulation of ammonium salts, effectively providing substrate preorganization and intermediates stabilization. We show that catalyst encapsulation in these nanocages reduces the catalytic overpotential for proton reduction by 250 mV as compared to the uncaged catalyst, while the proton-rich nano-environment created around the catalyst ensures that high catalytic rates are maintained.     

Ethylene biosynthesis by 1-aminocyclopropane-1-carboxylic acid oxidase: a DFT study

The reaction catalyzed by the plant enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) was investigated by using hybrid density functional theory. ACCO belongs to the non-heme iron(II) enzyme superfamily and carries out the bicarbonate-dependent two-electron oxidation of its substrate ACC (1-aminocyclopropane-1-carboxylic acid) concomitant with the reduction of dioxygen and oxidation of a reducing agent probably ascorbate. The reaction gives ethylene, CO(2), cyanide and two water molecules. A model including the mononuclear iron complex with ACC in the first coordination sphere was used to study the details of O-O bond cleavage and cyclopropane ring opening. Calculations imply that this unusual and complex reaction is triggered by a hydrogen atom abstraction step generating a radical on the amino nitrogen of ACC. Subsequently, cyclopropane ring opening followed by O-O bond heterolysis leads to a very reactive iron(IV)-oxo intermediate, which decomposes to ethylene and cyanoformate with very low energy barriers. The reaction is assisted by bicarbonate located in the second coordination sphere of the metal.     

Bis(2-(1H-imidazol-2-yl)-pyridine)copper(II): Effects of counteranions on the molecular and supramolecular structures

The synthesis and physical properties of bis(2-(1H-imidazol-2-yl)-pyridine)copper(II) with chloride, nitrate and perchlorate as counteranions have been described. Microanalysis, magnetic susceptibility, conductivity and various spectroscopic measurements have been used for the characterization of the complexes. The crystal structures of all three complexes have been determined. Intermolecular hydrogen-bonding interactions and the resulting self-assembly patterns for each of the species have been scrutinized. The chloride containing complex crystallizes as a trihydrate, where the metal ion is in a tetragonally elongated cis-N₄Cl₂ coordination sphere. This complex provides a three-dimensional honeycomb-like structure through N–H?Cl, O–H?Cl and O–H?O hydrogen bonds. In the nitrate containing species, one of the two counteranions coordinates to the metal centre to provide an irregular N₄O₂ coordination sphere, while the other counteranion, with the help of a lattice water molecule, assembles a ladder-like structure via N–H?O and bifurcated O–H?O,O hydrogen bonds. A one-dimensional polymeric species has been formed when perchlorate is the counteranion. Here one of the two perchlorates acts as a bridge between the metal centres that are in tetragonally elongated trans-N₄O₂ coordination spheres. This polymeric chain, together with the second perchlorate and a water molecule, form a ribbon-like structure due to N–H?O and O–H?O hydrogen bonds.     

Second-sphere effects on H2O2 activation by non-heme FeII complexes: role of a phenol group in the [H2O2]-dependent accumulation of FeIVO vs. FeIIIOOH

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O₂ or H₂O₂ as oxidants and release harmless side-products like water. Their oxidation selectivity is intrinsically linked to the control of the oxidizing species generated during the catalytic cycle. To do so, a second coordination sphere is used in order to create a pull effect during the activation of O₂ or H₂O₂, thus ensuring a heterolytic O–O bond cleavage. Herein, we report the synthesis and study of a new non-heme Fe^II complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H₂O₂ generates the classical Fe^IIIOOH species at high H₂O₂ loading. But at low H₂O₂ concentrations, an Fe^IVO species is generated instead. The formation of the latter is directly related to the presence of the 2nd sphere phenol group. Kinetic, variable temperature and labelling studies support the involvement of the attached phenol as a second coordination sphere moiety (weak acid) during H₂O₂ activation. Our results suggest a direct Fe^II → Fe^IVO conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the Fe^II/H₂O₂ adduct leading to a heterolytic O–O bond cleavage.

A new Fe^II complex with a phenol group attached as a second coordination sphere moiety activates H₂O₂ to yield Fe^IVO following a mechanism reminiscent of peroxidase enzymes.     

Comparative review of structural parameters of the nearest surrounding of monoatomic cations in water and methanol media

Literature data on structure of coordination sphere of selected monoatomic cations in aqueous and methanol solutions have been reviewed and compared. The following structural parameters have been considered: coordination number, interparticle distances, the second coordination sphere features, and ionic association type. It has been revealed that for doubly charged ions the parameters of the first coordination sphere are similar in the both solvents. The second coordination sphere of the same cation consists of fewer solvating molecules located farther from the ion in the case of methanol as compared to water.