Flexible bidentate pyridine and chiral ligands in the self-assembly of supramolecular 3-D cages

Discrete, nanoscopic 3-D cages are prepared in high yield via coordination-driven self-assembly from a variety of building blocks, including bidentate 3-substituted pyridines, chiral, and silicon-based tripods. All are characterized by NMR ((31)P, (1)H) and electrospray ionization mass spectrometry.     

Rh(I) complexes with new C2‐symmetric chiral diphosphoramidite ligands: Catalytic activity for asymmetric hydrogenation of olefins

The design and synthesis of three new C ₂‐symmetric chiral diphosphoramidite ligands starting from simple and cheap building blocks have been developed. Rhodium(I) cationic complexes bearing these chelate ligands have been prepared and applied in asymmetric hydrogenation of model olefins. A rhodium complex with a diphosphoramidite containing a chiral diamine configurationally stable and two fluxional chiral biphenyl units gave higher enantioselectivity with increasing hydrogen pressure (87% ee) in the hydrogenation of dimethyl itaconate.     

Guest Binding Drives Host Redistribution in Libraries of CoII4L4 Cages

Two Co^II₄L₄ tetrahedral cages prepared from similar building blocks showed contrasting host–guest properties. One cage did not bind guests, whereas the second encapsulated a series of anions, due to electronic and geometric effects. When the building blocks of both cages were present during self‐assembly, a library of five Co^II L^A _x L^B _4?x cages was formed in a statistical ratio in the absence of guests. Upon incorporation of anions able to interact preferentially with some library members, the products obtained were redistributed in favor of the best anion binders. To quantify the magnitudes of these templation effects, ESI‐MS was used to gauge the effect of each template upon library redistribution.     

Size selective self-sorting in coordination-driven self-assembly of finite ensembles

Size selective self-sorting in the coordination-driven self-assembly of two-dimensional (2-D) polygons and three-dimensional (3-D) cages is presented. Two types of polygons (rectangular and triangular) of different size are formed spontaneously from within mixtures of a molecular "clip" or a 60 degrees organoplatinum acceptor with dipyridyl linkers of different lengths via self-sorting. Furthermore, two different sized 3-D supramolecular cages are formed upon mixing one ditopic organoplatinum acceptor and two different sized tritopic donors from the self-sorting process. The formation of these polygons and cages is characterized using NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). In all cases, the self-sorting process is directed by the size of the donor building blocks and a dynamic, thermodynamically driven self-correction process resulting in the formation of discrete products from complex mixtures.     

Ambidentate pyridyl-carboxylate ligands in the coordination-driven self-assembly of 2D Pt macrocycles: self-selection for a single isomer

The coordination-driven self-assembly of discrete 2D macrocyclic species from ambidentate pyridyl-carboxylate-based donor ligands and platinum-containing acceptors is presented. All these species are characterized by electrospray ionization mass spectrometry (ESIMS), multinuclear NMR, and in one example, X-ray crystallography. In each case only one isomeric assembly is selectively formed in high yield, despite the potential for more than one product as a consequence of differences in connectivity.     

Structural Tailoring Effects on the Magnetic Behavior of Symmetric and Asymmetric Cubane‐Type Ni complexes

Using two kinds of carboxylate ligands with small but significant differences in steric size, symmetric and asymmetric Fe^II and Ni^II cubanes have been synthesized in a controlled fashion. Fast sweeping pulsed field measurements showed magnetization hysteresis loops for two cubane‐type molecular complexes, [Ni₄(μ‐OMe)₄(O₂CAr^4F‐Ph)₄(HOMe)₈] and [Ni₄(μ‐OMe)₄(O₂CAr^Tol)₄(HOMe)₆], thus suggesting single‐molecule magnet behavior. To differentiate the magnetic properties between the symmetric and asymmetric cubanes, detailed electron paramagnetic resonance (EPR) measurements were performed. From the EPR data, taken at various frequencies and temperatures, zero‐field splitting parameters D, E, and other higher‐order parameters for both cubane samples were extracted. Compared to the symmetric Ni‐cubane, the asymmetric one shows an increase in the D and E values by about 20 %, thereby suggesting structural engineering effects on the magnetic properties. By using the magnetic parameters determined by EPR, a static magnetization curve at 2 K and a temperature dependence of the magnetic susceptibility were simulated. A good agreement between theoretical and experimental data confirms the validity of the values obtained from EPR measurements.     

Synthesis,Characterization, and Properties of Organometallic Molecular Cylinders Bearing Bulky Imidazo[1,5-a]pyridine-Based N-Heterocyclic Carbene Ligands

The metal-controlled self-assembly of organometallic molecular cylinders from a series of imidazo[1,5-a]pyridine-based tris-NHC ligands is described in this report. The imidazo[1,5-a]pyridinium salts H₃- L (PF₆)₃ ( L = 4 a – 4 c ) were treated with 1.5 equivalents of Ag₂O to yield the trinuclear Ag^I hexacarbene cages [Ag₃( L )₂](PF₆)₃ ( L = 4 a – 4 c ), in which three Ag^I are sandwiched between the two tricarbene ligands. The silver(I) complexes [Ag₃( L )₂](PF₆)₃ underwent a facile transmetalation reaction in the presence of 3 equivalents of [AuCl(tht)] (tht=tetrahydrothiophene) to furnish the trinuclear Au^I cylinder-like cages [Au₃( L )₂](PF₆)₃ ( L = 4 a – 4 c ) without destruction of the metallosupramolecular structure. The new hexacarbene assemblies feature a large cavity that can easily accommodate a molecule of dimethyl sulfoxide as molecular guest. This is the first study of a unique “host–guest” system containing an organometallic cylinder-like cage derived exclusively from poly-NHC ligands.     

Kinetics of the reaction of 4,4′‐biphenol with bromoethane and 1‐bromobutane by phase‐transfer catalysis

The reaction of 4,4′‐biphenol and two species of bromoalkanes (e.g., bromoethane and 1‐bromobutane) to synthesize two symmetric products (4,4′‐diethanoxy biphenyl and 4,4′‐dibutanoxy biphenyl) and one asymmetric product (4‐ethanoxy, 4′‐butanoxy biphenyl) was successfully carried out under two‐phase phase‐transfer catalysis conditions. A rational mechanism and kinetic model were built up by considering the reactions both in aqueous phase and in organic phase. The first active catalyst (QO(Ph)₂OQ) was also synthesized under two‐phase reaction and was identified by instruments. The experimental data were explained satisfactorily by the pseudo‐steady‐state hypothesis. Two sets of rate constants of organic reactions, i.e. primary (k₁ and k₂) and secondary (k₁₁, k₁₂, k₂₁, and k₂₂) rate constants participate in the kinetic model. The two primary rate constants were obtained individually via experimental data for synthesizing the symmetric products. The ratios of the other four secondary rate constants were obtained from the reaction of synthesizing asymmetric products and determined from the initial yield rates of symmetric products. The effects of the ratio of bromoethane and 1‐bromobutane, temperature, organic solvents, amount of catalyst, and amount of sodium hydroxide on the reaction rate and the selectivity of products were investigated in detail. The results were explained satisfactorily by the interaction between the reactants and the environmental species. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 139–153, 2003     

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a – h were prepared by the reaction of readily available N,N′-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a – h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC-gold(I) complexes 3 – 10 . These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded-ring architecture.     

Synthesis,characterization and first application of chiral C2‐symmetric bis(phosphinite)–Pd(II) complexes as catalysts in asymmetric intermolecular Heck reactions

A series of new chiral C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes have been synthesized and for the first time used as catalysts in the palladium‐catalysed asymmetric intermolecular Heck coupling reactions of 2,3‐dihydrofuran with iodobenzene or aryl triflate. Under optimized conditions, products were obtained with high conversions and moderate to good enantioselectivities. The new C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes were characterized using multinuclear NMR and Fourier transform infrared spectroscopies and elemental analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle

Spherical assemblies of the type [Pd_nL_2n]²ⁿ⁺ can be obtained from Pd^II salts and curved N-donor ligands, L. It is well established that the bent angle, α, of the ligand is a decisive factor in the self-assembly process, with larger angles leading to complexes with a higher nuclearity, n. Herein, we report heteroleptic coordination cages of the type [Pd_nL_nL′_n]²ⁿ⁺, for which a similar correlation between the ligand bent angle and the nuclearity is observed. Tetranuclear cages were obtained by combining [Pd(CH₃CN)₄](BF₄)₂ with 1,3-di(pyridin-3-yl)benzene and ligands featuring a bent angle of α=120°. The use of a dipyridyl ligand with α=149° led to the formation of a hexanuclear complex with a trigonal prismatic geometry; for linear ligands, octanuclear assemblies of the type [Pd₈L₈L′₈]¹⁶⁺ were obtained. The predictable formation of heteroleptic Pd^II cages from 1,3-di(pyridin-3-yl)benzene and different dipyridyl ligands is evidence that there are entire classes of heteroleptic cage structures that are privileged from a thermodynamic point of view.     

Very Facile Polarity Umpolung and Noncovalent Functionalization of Inorganic Nanoparticles: A Tool Kit for Supramolecular Materials Chemistry

The facile assembly of shell‐by‐shell (SbS)‐coated nanoparticles [TiO₂?PAC₁₆]@shell 1 – 7 (PAC₁₆=hexadecylphosphonic acid), which are soluble in water and can be isolated as stable solids, is reported. In these functional architectures, an umpolung of dispersibility (organic apolar versus water) was accomplished by the noncovalent binding of ligands 1 – 7 to titania nanoparticles [TiO₂?PAC₁₆] containing a first covalent coating with PAC₁₆. Ligands 1 – 7 are amphiphilic and form the outer second shell of [TiO₂?PAC₁₆]@shell 1 – 7 . The tailor‐designed dendritic building blocks 3 – 5 contain negative and positive charges in the same molecule, and ligands 6 and 7 contain a perylenetetracarboxylic acid dimide (PDI) core ( 6 / 7 ) as a photoactive reporter component. In the redox and photoactive system [TiO₂?PAC₁₆]@shell 7 , electronic communication between the inorganic core to the PDI ligands was observed.     

Self-recognition in the coordination-driven self-assembly of three-dimensional M3L2 polyhedra

[reaction: see text] Self-recognition in the coordination-driven self-assembly of three-dimensional (3-D) polyhedra is described. Multiple discrete 3-D polyhedra were formed in one vessel through the self-recognition of a mixture of subunits. The dynamic self-recognition process is determined by the information stored within the geometry and directionality of multiple rigid building blocks as well as the thermodynamic stability of the discrete products.     

Synthesis and characterization of rhodium(III), iridium(III), ruthenium(III) and osmium(III) complexes containing sterically hindered heterocyclic ligands

Summary Some thiazolidine-2-thione and thiomorpholin-3-one complexes of rhodium(lll), iridium(III), ruthenium(III) and osmium(III) have been prepared and characterized by chemical analysis, conductivity measurements, room temperature magnetic moment studies, electronic, i.r. and far i.r. spectra and n.m.r. measurements. From the magnetic properties it was concluded that the above ligands form low-spin complexes with all the metal ions. The position and multiplicity of the metal-halogen stretching modes in the far-i.r. region have been extensively investigated and discussed; the results are particularly useful in distinguishing between themer- andfac-isomers in the octahedral compounds of the ML₃ X₃ type. The wavelengths of the principal electronic absorption peaks have been accounted for quantitatively in terms of the crystal field theory and the various parameters have been calculated. On the basis of the electronic spectra a trigonal bipyramidal geometry,D _3h, has been established for the Ru(tm)₂Cl₃ complex; the Ir(rm)₂Cl₃ · H₂O complex has also been prepared. It is penta-coordinated and a trigonal bipyramidal environment is suggested for the iridium(III) ion.     

Synthesis of Citronellal by RhI‐Catalysed Asymmetric Isomerization of N,N‐Diethyl‐Substituted Geranyl‐ and Nerylamines or Geraniol and Nerol in the Presence of Chiral Diphosphino Ligands,under Homogeneous and Supported Conditions

For the asymmetric isomerization of geranyl‐ or neryldiethylamine ((E)‐ or (Z)‐ 1 , resp.) and allyl alcohols geraniol or nerol ((E)‐ or (Z)‐ 2 , resp.) to citronellal ( 4 ) in the presence of a [Rh^I(ligand)cycloocta‐1,5‐diene)]⁺ catalyst, the atropic ligands 5 – 11 are compared under homogeneous and polymer‐supported conditions with the non‐C₂‐symmetrical diphosphino ferrocene ligands 12 – 16 . The ^tBu‐josiphos ligand 13 or daniphos ligand 19 , available in both antipodal series, already catalyse the reaction of (E)‐ 1 at 20° (97% e.e.) and favourably compare with the binap ligand 5 (see Table 1). Silica‐gel‐ or polymer‐supported diphosphino ligands usually afford similar selectivity as compared to the corresponding ligands applied under homogeneous conditions, but are generally less reactive. In this context, a polymer‐supported ligand of interest is the polymer‐anchored binap (R)‐ 6 , in terms of reactivity, selectivity, and recoverability, with a turnover of more than 14400.     

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high‐resolution neutron powder diffraction. Three dihydrogen molecules (H₂, D₂, and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen‐doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut‐like motion of the HD guest in the center of anisotropic sII‐S (sII‐S=small cages of structure II hydrates). However, we failed to observe a mass‐dependent relationship when comparing D₂ with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.     

Balancing Ligand Flexibility versus Rigidity for the Stepwise Self-Assembly of M12L24 via M6L12 Metal–Organic Cages

Non-covalent interactions are important for directing protein folding across multiple intermediates and can even provide access to multiple stable structures with different properties and functions. Herein, we describe an approach for mimicking this behavior in the self-assembly of metal–organic cages. Two ligands, the bend angles of which are controlled by non-covalent interactions and one ligand lacking the above-mentioned interactions, were synthesized and used for self-assembly with Pd²⁺. As these weak interactions are easily broken, the bend angles have a controlled flexibility giving access to M₂( L1 )₄, M₆( L2 )₁₂, and M₁₂( L2 )₂₄ cages. By controlling the self-assembly conditions this process can be directed in a stepwise fashion. Additionally, the multiple endohedral hydrogen-bonding sites on the ligand were found to play a role in the binding and discrimination of neutral guests.     

A novel five-coordinated silver complex {[Ag(HPPz)4(NO3)](NO3)4} [PPz=trans-2-(2-phenylethenyl)pyrazine]: Self-assembly of supramolecular architecture based on anions

The title complex {[Ag(HPPz)₄(NO₃)](NO₃)₄}, 1, has been prepared from the reaction of trans-2-(2-phenylethenyl)pyrazine (PPz) with AgNO₃ at room temperature in 10% HNO₃ and structurally determined by single-crystal X-ray diffraction analysis. The result shows that the Ag^I center is five-coordinated with one anion and four mono-protonated HPPz⁺ ligands. The anions display versatility not only participating in coordination, but also linking the HPPz⁺ ligands by electrostatic and hydrogen bond interactions. The final framework can be considered as a 3D supramolecular architecture built up by the self-assembly of the building blocks ([Ag(HPPz)₄(NO₃)]³⁺, ), driven by diverse interactions based on the anions.     

Mechanistic Insight into the NN Bond‐Cleavage of Azo‐Compounds that was Induced by an Al?Al‐bonded Compound [L2−AlII?AlIIL2−]

An α‐diimine‐stabilized Al? Al‐bonded compound [L^2?Al^II? Al^IIL^2?] (L=[{(2,6‐iPr₂C₆H₃)NC(Me)}₂]; 1 ) consists of dianionic α‐diimine ligands and sub‐valent Al²⁺ ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo‐compounds afforded phenylimido‐bridged products [L^?Al^III(μ₂‐NPh)(μ₂‐NAr)Al^IIIL^?] ( 2 – 4 ). During the reaction, the dianionic ligands and Al²⁺ ions were oxidized into monoanions and Al³⁺, respectively, whilst the [NAr]^2? imides were produced by the four‐electron reductive cleavage of the N?N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L^2?)₂Al^III₂(μ₂‐NPh)₂Na₂(thf)₄] ( 5 ). Interestingly, when asymmetric azo‐compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4 ). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet‐state monoanionic ligands could result from an exothermic singlet‐to‐triplet conversion during the reaction process.