Metal-ligand Coordination Bonds Research Articles

Unfolding and Mechanochemical Scission of Supramolecular Polymers Containing a Metal–Ligand Coordination Bond

Mechanochemical scission of supramolecular polymer complexes by ultrasound is investigated using viscosity measurements and molecular dynamics (MD) simulations combined with constrained geometry optimization (COGEF) calculations. The supramolecular polymers used in this study consist of a poly(tetrahydrofuran) (PTHF) backbone that contains a silver(I)–NHC (NHC = N-heterocyclic carbene) coordination complex in the chain center. The limiting molecular weight (Mlim) for mechanochemical chain scission is lower for supramolecular polymers than for their covalent analogues. The longest characteristic relaxation times of the supramolecular polymers were obtained from viscosity measurements, and they confirmed that the criterion for coil-to-stretch is fulfilled in typical sonication experiments. A model DFT study was performed to estimate the value of external force that is required to break a silver(I)–NHC coordination bond. A combination of ab initio MD simulations and COGEF provided an atomistic insight into the response of the supramolecular center of the polymer chain to external force. The calculations indicated that the force required to break the chain is between 400 and 500 pN. This is significantly lower than the force of several nN that is typically required to break e.g. covalent C–C bonds in polymer backbones. These results confirm that the reduction in Mlim is due to the lower bond strength of the metal–ligand coordination bonds as compared to covalent bonds.

Use of PM6 quantum-chemical calculations in studying the properties of antioxidants in the biological environment

The energies of formation, enthalpies, and entropies of the conformers of 1-(carboxy)-1-(N-methylamide)-2-(3′,5′-di-tert-butyl)-4-hydroxyphenyl)-propionic acid and sodium and potassium 1-(carboxy)-1-(N-methylamide)-2-(3′,5′-di-tert-butyl)-4-hydroxyphenyl)-propionates are calculated by quantum-chemical methods in the PM6 approximation. A doubling of signals in the 1H NMR spectrum of the first conformer is observed, which merge into singlets when the compound is heated. Changes in the structure of the conformers and donor-acceptor complexes (solvates) occur with the preservation of the metal-ligand coordination bond. Calculations of the characteristics of 1-(carboxy)-1-(N-methylamide)-2-(3′,5′-di-tert-butyl)-4-hydroxyphenyl)-propionic acid and sodium and potassium 1-(carboxy)-1-(N-methylamide)-2-(3′,5′-di-tert-butyl)-4-hydroxyphenyl)-propionates in the PM6 approximation make it possible to predict the structure and properties of the solvated structures. The energies of homolysis of the H-O bond D(OH) are calculated, and a linear dependence of the antioxidant activity on D(OH) for the structures of the studied compounds is demonstrated. The results make it possible to predict the properties of antioxidants in the biological environment.

Structural Alteration of Hybrid Supramolecular Capsule Induced by Guest Encapsulation

Heterofunctionalized C(2v) symmetrical cavitand 1 with 4-pyridylethynyl and 3-carbamoylphenyl groups in alternating arrangement was designed and synthesized. A 1:1 mixture of the cavitand 1 and a cis-coordinated palladium(II) or platinum(II) complex self-assembled into a hybrid supramolecular capsule via both metal-ligand coordination bonds and hydrogen bonds. Formation of the capsular assembly was confirmed by NMR spectroscopy and mass spectrometry. The hybrid capsule encapsulated the appropriate guest, the molecular sizes of which fit the size of the capsular cavity. Structural alteration of the hybrid capsule was induced by the guest encapsulation. A C(2h) structure for the encapsulation complex was assigned by 2D NMR spectra analysis. Thermodynamic and kinetic properties of the guest encapsulation were investigated. The kinetics of in/out guest exchange was strongly influenced by hydrogen bonding in the hybrid capsule.

Selective Hydrolysis of Phosphate Monoester by a Supramolecular Phosphatase Formed by the Self-Assembly of a Bis(Zn2+-cyclen) Complex, Cyanuric Acid, and Copper in an Aqueous Solution (Cyclen = 1,4,7,10-Tetraazacyclododecane)

In Nature, organized nanoscale structures such as proteins and enzymes are formed in aqueous media via intermolecular interactions between multicomponents. Supramolecular and self-assembling strategies provide versatile methods for the construction of artificial chemical architectures for controlling reaction rates and the specificities of chemical reactions, but most are designed in hydrophobic environments. The preparation of artificial catalysts that have potential in aqueous media mimicking natural enzymes such as hydrolases remains a great challenge in the fields of supramolecular chemistry. Herein, we describe that a dimeric Zn(2+) complex having a 2,2'-bipyridyl linker, cyanuric acid, and a Cu(2+) ion automatically assembles in an aqueous solution to form a 4:4:4 complex, which is stabilized by metal-ligand coordination bonds, π-π-stacking interactions, and hydrogen bonding and contains μ-Cu(2)(OH)(2) cores analogous to the catalytic centers of phosphatase, a dinuclear metalloenzyme. The 4:4:4 complex selectively accelerates the hydrolysis of a phosphate monoester, mono(4-nitrophenyl)phosphate, at neutral pH.

Towards Modular Design of Chiroptically Switchable Molecules Based on Formation and Cleavage of Metal-Ligand Coordination Bonds

The results presented in this paper demonstrate that the proposed design of C 2 -symmetric, pentadentate achiral or chiral ligands 8a-d and 15 allows to generate, upon coordination with Ni(II) and Pd(II), the corresponding diastereomeric complexes possessing three new elements of chirality: stereogenic axis, center, and helix. Of particular importance is that due to the specific steric characteristics of the designed ligands the formation of the corresponding diastereomeric products is highly stereoselective allowing preparation of only two out of four possible stereochemical combinations. For instance, each diastereoisomeric product (R a ',P h ',S c ')-9a and (R a ',M h ',R c ')-12a can be selectively prepared and characterized in solid state, simply by the choice of the chelating metal [Ni(II) or Pd(II)]. Furthermore, introduction of stereochemical information into the ligands design with application of a simple chiral 'Amine Module' allows for complete transfer of the corresponding stereochemistry to the newly generated axial, helical, and central chirality. For example, starting with chiral ligand (R)-15, out of eight possible products, only a single product of (R c ,R a ,P h ,S c ) absolute configuration was obtained in the solid state. Taking into account the modular nature of this design, one may agree that modification of the three major 'phenone', 'acid', and 'amine' modules, or application of different metals, will allow for virtually unlimited structural and functional flexibility in fine-tuning the diastereomeric relationships of this type of complexes making them more selective and controllable by an external stimulus.

Two-dimensional assembly of the type Cd10Te4 thiolate cluster with 4,4′-trimethylenedipyridine

Abstract Cadmium tellurium cluster molecule [Cd10Te4(SC6H4Me-p)12] and bifunctioanal organic ligand 4,4′-trimethylenedipyridine (tmdp) are joined together through metal–ligand coordination bonds, forming a new inorganic–organic hybrid polymeric cluster {Cd10Te4(SC6H4Me-p)12(tmdp)2}n (1) with a unique two-dimensional super-structure. The room temperature absorption and the diffuse reflectance UV–Vis spectra along with the photo-luminescence behavior were determined in the solid state. The thermal stability of the polymeric cluster was studied by thermal gravimetric analysis.

Hybrid Cavitand Capsule with Hydrogen Bonds and Metal−Ligand Coordination Bonds: Guest Encapsulation with Anion Assistance

Multifunctionalized C(2v)-cavitand 1 was synthesized in six steps from a tetrabromo cavitand. A 1:1 mixture of 1 and 2 in CDCl(3) or CD(2)Cl(2) quantitatively self-assemble into a supramolecular hybrid capsule 3 through hydrogen bond and metal-ligand coordination bond. The cavity of the capsule 3 is filled with triflate ion and penetrated alkyl groups of ureides. Capsule 3 encapsulates a neutral guest molecule such as 4,4'-diiodobiphenyl (4) with the assistance of an anion. The encapsulation complex was identified by an NMR technique. The kinetics of guest exchange is controllable by the amounts and/or types of anions or other influences such as the polarity of the solvent.

Complexes of Ag(i), Hg(i) and Hg(ii) with multidentate pyrazolyl-pyridine ligands: from mononuclear complexes to coordination polymers via helicates, a mesocate, a cage and a catenate

The coordination chemistry of a series of di- and tri-nucleating ligands with Ag(I), Hg(I) and Hg(II) has been investigated. Most of the ligands contain two or three N,N'-bidentate chelating pyrazolyl-pyridine units pendant from a central aromatic spacer; one contains three binding sites (2 + 3 + 2-dentate) in a linear sequence. A series of thirteen complexes has been structurally characterised displaying a wide range of structural types. Bis-bidentate bridging ligands react with Ag(I) to give complexes in which Ag(I) is four-coordinate from two bidentate donors, but the complexes can take the form of one-dimensional coordination polymers, or dinuclear complexes (mesocate or helicate). A tris-bidentate triangular ligand forms a complicated two-dimensional coordination network with Ag(I) in which Ag...Ag contacts, as well as metal-ligand coordination bonds, play a significant role. Three dinuclear Hg(I) complexes were isolated which contain an {Hg2}2+ metal-metal bonded core bound to a single bis-bidentate ligand which can span both metal ions. Also characterised were a series of Hg(II) complexes comprising a simple mononuclear four-coordinate Hg(II) complex, a tetrahedral Hg(II)4 cage which incorporates a counter-ion in its central cavity, a trinuclear double helicate, and a trinuclear catenated structure in which two long ligands have spontaneously formed interlocked metallomacrocyclic rings thanks to cyclometallation of two of the Hg(II) centres.

Discrete and Infinite Cyclic Dimer M(II) Complexes (M = Co, Zn) Based on Angular Dipyridyl Ligands Incorporating an Amide Spacer

Crystal structure, Looped coordination polymer, Co(II), Zn(II), Hydrogen bondingAn increasing interest has been directed toward the synthesis of new inorganic/or ganic hybrid compounds which involve the self-assembly of structural motifs into well-defined supramolecules via hydrogen bonds, metal-ligand coordination bonds, and donor-acceptor interactions. In particular, the formation of cyclic nanostructures with large cavity makes these materials potential candidates not only for the catalysis

Nature of the coordination bond in metal complexes of substituted pyridine derivatives. II. The far infrared spectra and metal–ligand force constants of copper complexes of 4-substituted pyridines

Infrared spectra, including the lower frequency region, of copper (II) complexes of the type [CuL2Cl2], where L = 4-methylpyridine, 4-carbinylpyridine, pyridine, 4-acetylpyridine, 4-pyridinecarboxamide, 4-carbomethoxypyridine, and 4-cyanopyridine have been measured. The substituent effect upon the copper–nitrogen (ligand) and copper–chlorine stretching frequencies has also been examined. It was found that the substituents on the pyridine ring not only affected the nature of copper–nitrogen bond, but also the copper–chlorine bond. It appears that an entirely delocalized system exists, involving the chlorine atoms and the substituents in the complexes. Force constants associated with the coordination bond between copper and the nitrogen of the ligands have been calculated by means of an IBM 1620 computer. By comparison of the copper–nitrogen stretching frequencies with the corresponding force constants, and the heats of coordination, it may be concluded that the magnitude of the metal–ligand stretching frequency is directly related to the metal–ligand coordination bond strength for the systems under study.