Structure, adsorption and magnetic properties of chiral metal-organic frameworks bearing linear trinuclear secondary building blocks

The reactions of new chiral organic ligands trimesoyltri(L-alanine) (L-TMTAH(3)) or trimesoyltri(D-alanine) (D-TMTAH(3)) with transition metal salts in the presence of an ancillary ligand of 4,4'-bipyridine gave two pairs of three dimensional frameworks [Co(3)(L-TMTA)(2)(4,4'-bpy)(4)]·28H(2)O (1), [Co(3)(D-TMTA)(2)(4,4'-bpy)(4)]·28H(2)O (2) [Ni(3)(L-TMTA)(2)(4,4'-bpy)(4)]·2C(2)H(5)OH·14H(2)O (3) and [Ni(3)(D-TMTA)(2)(4,4'-bpy)(4)]·2C(2)H(5)OH·14H(2)O (4). These compounds were characterized by elemental analysis, IR, and X-ray powder diffraction analysis and the structures of 1-3 were determined from X-ray single crystal diffraction analysis. Complexes 1-4 feature linear trinuclear secondary building blocks [M(3)(COO)(4)](2+) formed via the connection of three metal ions by four carboxylato groups from four TMTA(3-) ligands. Every adjacent two linear trinuclear secondary building blocks are linked by one and three 4,4'-bipyridine molecules along the a and c axis, respectively, to form two-dimensional sheets, which are further connected by TMTA(3-) ligands to construct a porous three dimensional framework with one-dimensional channels. Compound 3 was taken as an example to investigate the adsorption properties of compounds 1-4. It revealed a saturated hydrogen uptake of 216.6 cm(3) g(-1) (2.0 wt%) at 11.1 atm measured at 77 K, a maximum CO(2) uptake of 119.4 cm(3) g(-1) (23.5 wt%) at 19.5 atm measured at 298 K and a saturated CH(4) uptake of 77.8 cm(3) g(-1) (5.6 wt%) at 27.1 atm measured at 298 K. The magnetic studies of complexes 1 and 3 indicate the presence of antiferromagnetic interactions between the metal ions in the two compounds.     

Interpenetrating metal-organic frameworks formed by self-assembly of tetrahedral and octahedral building blocks

To investigate the relationship between topological types and molecular building blocks (MBBs), we have designed and synthesized a series of three-dimensional (3D) interpenetrating metal-organic frameworks based on different polygons or polyhedra under hydrothermal conditions, namely [Cd(bpib)_0.5(L¹)] (1), [Cd(bpib)_0.5(L²)]·H₂O (2), [Cd(bpib)_0.5(L³)] (3) and [Cd(bib)_0.5(L¹)] (4), where bpib=1,4-bis(2-(pyridin-2-yl)-1H-imidazol-1-yl)butane, bib=1,4-bis(1H-imidazol-1-yl)butane, H₂L¹=4-(4-carboxybenzyloxy)benzoic acid, H₂L²=4,4′-(ethane-1,2-diylbis(oxy))dibenzoic acid and H₂L³=4,4′-(1,4-phenylenebis(methylene))bis(oxy)dibenzoic acid, respectively. Their structures have been determined by single crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric (TG) analyses. Compounds 1–3 display α-Po topological nets with different degrees of interpenetration based on the similar octahedral [Cd₂(–COO)₄] building blocks. Compound 4 is a six-fold interpenetrating diamondoid net based on tetrahedral MBBs. By careful inspection of these structures, we find that various carboxylic ligands and N-donor ligands with different coordination modes and conformations, and metal centers with different geometries are important for the formation of the different MBBs. It is believed that different topological types lie on different MBBs with various polygons or polyhedra. Such as four- and six-connected topologies are formed by tetrahedral and octahedral building blocks. In addition, with the increase of carboxylic ligands’ length, the degrees of interpenetration have been changed in the α-Po topological nets. And the luminescent properties of these compounds have been investigated in detail.     

A rare chiral self-catenated network formed by two cationic and one anionic frameworks

A chiral heterometallic complex, obtained from the solvothermal reaction of [Zn(4-pytpy)2](BF4)2 [4-pytpy=4'-(4-pyridyl)-2,2':6',2'-terpyridine] and CuCN, exhibits a rare self-catenated network formed by two 3D cationic and one 3D anionic frameworks.     

Microporous metal-organic frameworks formed in a stepwise manner from luminescent building blocks

A series of trivalent lanthanides (Sm, Eu, Gd, Tb, Dy) have been complexed to the dianionic ligand, 4,4'-disulfo-2,2'-bipyridine-N,N'-dioxide, L, in a 3:1 ratio to form trianionic complex building blocks. These units were then cross-linked into a network solid by addition of BaCl2 to form mixed-metal networks of formula {Ba2(H2O)4[LnL3(H2O)2](H2O)nCl}infinity, Ln = Sm3+ (1), Eu3+ (2), Gd3+ (3), Tb3+ (4), Dy3+ (5). The networks were isostructural and contained open channels which readily absorbed and desorbed water accompanied by a spongelike shrinkage and expansion of the host. CO2 sorption measurements confirmed microporosity giving a DR surface area of 718 m2/gm and an average pore size of 6.4 A. Ligand L sensitized all the lanthanide ions with the exception of Gd3+. Studying the series of Ln complexes allowed the determination of the triplet state energy of L which is itself a new ligand for sensitization purposes. The luminescent properties of the lanthanide building blocks were retained in the porous network solid. From the luminescence data, it was possible to attribute the spongelike properties of the network to the Ba2+ coordination sphere rather than the Ln3+ center. Networks were characterized by X-ray crystallography, PXRD, DSC/TGA, water vapor and gas sorption, and luminescence spectroscopy.     

Acetate aldol reactions of chiral oxocarbenium ions

Chiral oxocarbenium ions have been exploited to carry out highly diastereoselective and enantioselective acetate aldol addition reactions. The chiral auxiliary has been optimized to give the product with good diastereoselectivity.     

Homochiral oligopeptides by chiral amplification: interpretation of experimental data with a copolymerization model

We present a differential rate equation model of chiral polymerization based on a simple copolymerization scheme in which the enantiomers are added to, or removed from, the homochiral or heterochiral chains (reversible stepwise isodesmic growth or dissociation). The model is set up for closed systems and takes into account the corresponding thermodynamic constraints implied by the reversible monomer attachments, while obeying a constant mass constraint. In its simplest form, the model depends on a single variable rate constant, the maximum chain length N, and the initial concentrations. We have fit the model to the experimental data from the Rehovot group on lattice-controlled chiral amplification of oligopeptides. We find in all the chemical systems employed, except for one, that the model fits the measured relative abundances of the oligopeptides with higher degrees of correlation than from a purely random polymerization process.     

Pfeiffer effect optical activity developed in lanthanide tris(pyridine-2,6-dicarboxylate) complexes by chiral tartrate substrates

Optical activity has been induced in the title complexes through outer-sphere complexation with L-tartaric acid, and with several carboxyl and hydroxyl esters. The optical activity was studied in the Tb(III) and Eu(III) complexes by means of circularly polarized luminescence (CPL) spectroscopy, and the symmetry changes associated with the adduct formation were followed by examining the hypersensitive absorption of the Ho(III) complexes. Optical activity was found to be induced by each substrate, although the nature of the outer-sphere interaction was necessarily different in the various systems. It was found in every case that the presence of a (R,R)-tartrate substrate led to enrichment of the A-isomer of the lanthanide complexes.     

Organocatalysis: asymmetric cascade reactions catalysed by chiral secondary amines

The utilisation of chiral secondary amines as promoters for asymmetric cascade reactions has been subject of intensive research in asymmetric organocatalysis in the past few years. Key developments in this area are highlighted in this review. As shown, these powerful synthetic methodologies serve as efficient approaches to the construction of complex chiral molecular architectures from simple achiral materials in one-pot transformations under mild conditions with high stereocontrol.     

Three-component reactions leading to 2D and 3D metal-organic frameworks assembled on dinickel-carboxylate secondary building units

Three-component reactions involving Ni(II) ions and dicarboxylate and bipyridyl ligands under hydrothermal conditions produce two novel metal-organic coordination polymers formulated empirically as [Ni(PDA)(BPE)] (1) and [Ni₂(PDA)₂(BPP)(H₂O)]·2.5H₂O (2), where PDA = 1,4-phenylenediacetate, BPE = 1,2-bis(4-pyridyl)ethane, and BPP = 1,3-bis(4-pyridyl)propane. Both compounds possess 2D or 3D metal-organic frameworks (MOFs) that are assembled on dinickel-carboxylate secondary building units. Compound 1 has a condensed 3D MOF, whereas 2 contains void between 2D MOFs where guest water molecules reside. Both compounds demonstrate antiferromagnetic coupling between Ni(II) ions.     

Synthesis, structure, and luminescent properties of microporous lanthanide metal-organic frameworks with inorganic rod-shaped building units

A series of microporous lanthanide metal-organic frameworks, Tb3(BDC)(4.5)(DMF)2(H2O)3.(DMF)(H2O) (1) and Ln3(BDC)(4.5)(DMF)2(H2O)3.(DMF)(C2H5OH)(0.5)(H2O)(0.5) [Ln = Dy (2), Ho (3), Er (4)], have been synthesized by the reaction of the lanthanide metal ion (Ln3+) with 1,4-benzenedicarboxylic acid and triethylenetetramine in a mixed solution of N,N'-dimethylformamide (DMF), water, and C(2)H(5)OH. X-ray diffraction analyses reveal that they are extremely similar in structure and crystallized in triclinic space group P. An edge-sharing metallic dimer and 4 metallic monomers assemble with 18 carboxylate groups to form discrete inorganic rod-shaped building units [Ln6(CO2)18], which link to each other through phenyl groups to lead to three-dimensional open frameworks with approximately 4 x 6 A rhombic channels along the [0,-1,1] direction. A water sorption isotherm proves that guest molecules in the framework of complex 1 can be removed to create permanent microporosity and about four water molecules per formula unit can be adsorbed into the micropores. These complexes exhibit blue fluorescence, and complex 1 shows a Tb3+ characteristic emission in the range of 450-650 nm.     

Highly enantioselective asymmetric reactions involving zinc ions promoted by chiral aziridine alcohols

Enantiomerically pure, chiral secondary and tertiary aziridine alcohols (including the aziridine analogue of ProPhenol—AziPhenol) have proven to be highly effective catalysts for enantioselective asymmetric reactions in the presence of zinc ions, including arylation of aromatic aldehydes, epoxidation of chalcone and addition of diethylzinc to aldehydes, leading to the desired chiral products in high chemical yields (up to 90%) and with ee’s up to 90%. A higher catalytic activity of Prophenol-type bis(aziridine alcohol) in the aforementioned asymmetric transformations has been demonstrated.     

Homochiral crystallization of microporous framework materials from achiral precursors by chiral catalysis

While it is not uncommon to form chiral crystals during crystallization, the formation of bulk porous homochiral materials from achiral building units is rare. Reported here is the homochiral crystallization of microporous materials through the chirality induction effect of natural alkaloids. The resulting material possesses permanent microporosity and has a uniform pore size of 9.3 A.     

Regio- and diastereoselective reactions of chiral secondary alkylcopper reagents with propargylic phosphates: preparation of chiral allenes

The diastereoselective S_N2′-substitution of secondary alkylcopper reagents with propargylic phosphates enables the preparation of stereodefined alkylallenes. By using enantiomerically enriched alkylcopper reagents and enantioenriched propargylic phosphates as electrophiles anti-S_N2′-substitutions were performend leading to α-chiral allenes in good yields with excellent regioselectivity and retention of configuration. DFT-calculations were performed to rationalize the structure of these alkylcopper reagents in various solvents, emphasizing their configurational stability in THF.

The diastereoselective S_N2′-substitution of secondary alkylcopper reagents with propargylic phosphates enables the preparation of stereodefined alkylallenes.     

Complexes of lanthanide ions with dibenzodioxatetraazamacrocyclic ligands

Efficiency of luminescence of europium and ytterbium ions in complexes with three 16-membered dibenzodioxatetraazamacrocycles was estimated.     

Microporous 3-D chiral metal-organic framework with a quartzlike topology based on an achiral building unit

A zinc coordination polymer, Zn(IN)₂ · 2H₂O (1) (HIN = isonicotinic acid), has been synthesized and characterized by IR, elemental analysis, thermogravimetric analysis, and single crystal X-ray diffraction. This compound crystallizes in the chiral space group P6₂ with a = 15.512(3), b = 15.512(3), c = 6.262(2) Å, V = 1304.9(5) ų, and Z = 3. The O and N atoms of the IN ligand are coordinated to a Zn^II ion, locking the asymmetry of the IN ligand and transferring this asymmetry throughout the crystal structure to “direct” the formation of a chiral network. The structure of 1 is characteristic of a 3-D quartz-like framework, which contains hetero-chiral helical channels (pseudo-trigonal and hexagon channels) alternately along the c-axis. Compound 1 exhibits high stability and intense fluorescent emission at room temperature.     

Asymmetrically decorated POM framework formed by a chiral polyanion

Reactions of NaH₂AsO₄ with C-type polyanion precursor lead to a new compound, (NH₄)₁₅[As₂Fe₆Mo₂₂O₈₅AsO₂(OH)₂]·31.5H₂O (1). Compound 1 was characterized by elemental analysis, infrared spectra, thermogravimetric analysis, and single-crystal X-ray diffraction. Structural analysis reveals that AsO₂(OH)^2? asymmetrically decorated the POM framework, constituting the first inorganic anion asymmetrically decorating a POM. AC measurements of the frequency-dependent χ′′ below 5 K indicated a slow magnetization relaxation in 1, resembling typical single-molecule-magnet behavior. Photocatalytic experiments indicated that introduction of AsO₂(OH)^2? may offer new active sites and enhanced the overall photocatalytic activity compared with the C-type precursor.     

Photochromic coumarin spiropyranes with switching of optical properties by lanthanide ions

Russian Chemical Bulletin - Coumarin spiropyrans with terminal carbohydrazide substituents were synthesized. UV irradiation of their solutions was found to initiate rearrangement into a deeply...     

Axially chiral amidinium ions as inducers of enantioselectivity in Diels-Alder reactions

[reaction: see text] Enantioselective catalysis of Diels-Alder reactions is mostly achieved by coordinating the dienophile to relatively strong chiral Lewis acids. Here we report on a novel approach employing the hydrogen-bond-mediated association of dienophiles to chiral host molecules. In a reaction forming the steroid skeleton of norgestrel, chiral amidinium ions induce 5:ent-5 ratios of up to 2.5:1. Improved and simplified amidinium catalysts may become interesting candidates to perform stereoselective transformations.