Tetrahedral Centers Research Articles

Tetrahedral M(II) based binuclear double-stranded helicates: single-ion-magnet and fluorescence behaviour.

A rare class of dinuclear double-stranded helicates having tetrahedral metal centres with formulae [Co2(L(1))2]·2(CH3CN) (1), [Co2(L(2))2]·6(CH3CN) (2), [Zn2(L(1))2]·2(CH3CN)·(CH3OH) (3) and [Zn2(L(2))2]·4(CH3CN) (4) were synthesized and characterized. Detailed dc and ac magnetic susceptibility measurements reveal the presence of field induced slow magnetic relaxation behaviour in high spin tetrahedral Co(II) centres with an easy-plane magnetic anisotropy. Complexes 1 and 2 are the rare examples of transition metal based helicates showing such behaviour. Furthermore, 3 and 4 exhibit fluorescence emission in different solvents that are analyzed in terms of fluorescence quantum yields and lifetimes.

Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and their Relevance in 16O2/18O2 Isotopic Exchange, deN2O, and deCH4 Processes—A Theoretical and Experimental Account

Periodic spin unrestricted, gradient corrected DFT calculations joined with atomistic thermodynamic modeling and experiment were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel (100) surface. The surface state diagram of oxygen in a wide range of pressures and temperatures was constructed for coverage varying from ΘO = 1.51 atom·nm–2 to ΘO = 6.04 atom·nm–2. A large variety of the unraveled surface ROS includes diatomic superoxo (CoO–O2––CoO), peroxo (CoT–O22––CoO), and spin paired (CoO–O2–CoO) adducts along with monatomic metal-oxo (CoT–O+, CoO–O2+) species, where CoT and CoO stand for the tetrahedral and octahedral cobalt surface centers, respectively. There are also two kinds of peroxo species associated with surface oxygen ions connected with 3CoO or 2CoO and 1CoT cations ((O2O,1T–O)2– and (O3O–O)2–), respectively). The results revealed that in the oxygen pressure range of typical catal...

A sheet structured MOF magnet: Poly[(1,10-phenanthroline)tetrakis(imidazolato)diiron(II)

We report the synthesis and structural and magnetic characterization of poly[(1,10-phenanthroline)tetrakis(imidazolato)diiron(II)], [Fe2(imid)4(phen)]x, 1. The extended structure of 1 is that of a metal organic framework (MOF) involving double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands. The octahedral iron centers are each additionally coordinated by phenanthroline ligands. Mössbauer spectroscopy confirms the presence of octahedral and tetrahedral metal centers. A combination of DC susceptibility, ZFCM and Mossbauer studies reveals a magnetic phase transition to long-range order at ∼7K in 1. Magnetic hysteresis is observed at low temperatures, with coersive fields of approximately 360 and 650G and remnant magnetizations of approximately 450 and 720cm3Gmol−1 at 5 and 2.5K respectively.

Reaction kinetics of isopropyl palmitate synthesis

In this study, the kinetics of isopropyl palmitate synthesis including the reaction mechanism was studied based on the two-step noncatalytic method. The liquid-phase diffusion effect on the reaction process was eliminated by adjusting the stirring rate. The results showed that the two-step reaction followed a tetrahedral mechanism and conformed to second-order reaction kinetics. Nucleophilic attack on the carbonyl carbon afforded an intermediate, containing a tetrahedral carbon center. The intermediate ultimately decomposed by elimination of the leaving group, affording isopropyl palmitate. The experimental data were analyzed at different temperatures by the integral method. The kinetic equations of the each step were deduced, and the activation energy and frequency factor were obtained. Experiments were performed to verify the feasibility of kinetic equations, and the result showed that the kinetic equations were reliable. This study could be very significant to both industrial application and determining the continuous production of isopropyl palmitate.

Self-Assembly of Metal Boron Imidazolate Cages

To study the synthetic strategy from cages to extended frameworks is very important to elucidate and mimic cage-based self-assembly behavior in zeolites and other open frameworks. Here, two dodecahedral cages are rationally designed by using tridentate boron imidazolate ligands to link tetrahedral or square metal centers. The self-assembly of metal boron imidazolate cages is successfully realized. The cage-based materials display good properties on gas adsorption and reducing noble metals. This work may help us to understand the formation mechanism of extended metal–organic frameworks.

Solid state 13C-NMR, infrared, X-ray powder diffraction and differential thermal studies of the homologous series of some mono-valent metal (Li, Na, K, Ag) n-alkanoates: A comparative study

A comparative study of the molecular packing, lattice structures and phase behaviors of the homologous series of some mono-valent metal carboxylates (Li, Na, K and Ag) is carried out via solid state FT-infrared and 13C-NMR spectroscopes, X-rays powder diffraction, density measurements, differential scanning calorimetry, polarizing light microscopy and variable temperature infrared spectroscopy. It is proposed that, for lithium, sodium and potassium carboxylates, metal-carboxyl coordination is via asymmetric chelating bidentate bonding with extensive intermolecular interactions to form tetrahedral metal centers, irrespective of chain length. However, for silver n-alkanoates, carboxyl moieties are bound to silver ions via syn–syn type bridging bidentate coordination to form dimeric units held together by extensive head group inter-molecular interactions. Furthermore, the fully extended hydrocarbon chains which are crystallized in the all-trans conformation are tilted at ca. 30°, 27°, 15° and 31° with respect to a normal to the metal plane, for lithium, sodium, silver and potassium carboxylates, respectively. All compounds are packed as lamellar bilayer structures, however, lithium compounds are crystallized in a triclinic crystal system whilst silver, sodium and potassium n-alkanoates are all monoclinic with possible P1 bravais lattice. Odd–even alternation observed in various physical features is associated with different inter-planar spacing between closely packed layers in the bilayer which are not in the same plane; a phenomenon controlled by lattice packing symmetry requirements.All compounds, except silver carboxylates, show partially reversibly first order pre-melting transitions; the number of which increases with increasing chain length. These transitions are associated, for the most part, with lamellar collapse followed by increased gauche-trans isomerism in the methylene group assembly, irrespective of chain length. It is proposed that the absence of mesomorphic transitions in their phase sequences is due to a lack of sufficient balance between attractive and repulsive electrostatic and van der Waals forces during phase change. The evidence presented in this study shows that phase behaviors of mono-valent metal carboxylates are controlled, mainly, by head group bonding.

Vanadyl calix[6]arene complexes: synthesis, structural studies and ethylene homo-(co-)polymerization capability.

Treatment of p-tert-butylcalix[6]areneH6 (L(6)H6) with in situ [LiVO(Ot-Bu)4] afforded, after work-up, the dark green complex [Li(MeCN)4][V2(O)2Li(MeCN)(L(6)H2)2]·8MeCN (1·8MeCN). On one occasion, the reaction led to the formation of a mixture of products, the bulk of which differing from 1 only in the amount of solvate, viz.2·9.67MeCN. The second minor, yellow product has the formula {[(VO2)2(L(6)H2)(Li(MeCN)2)2]·2MeCN}n (3·2MeCN), and comprises a 1D polymeric structure with links through the L(6)H2 ligand and Li2O2 units. When the reverse order of addition was employed such that lithium tert-butoxide (7.5 equivalents) was added to L(6)H6, and subsequently treated with VOCl3 (2 equiv.), the complex {[VO(THF)][VO(μ-O)]2Li(THF)(Et2O)][L(6)]}·2Et2O·0.5THF (4·2Et2O·0.5THF), which contains a trinuclear motif possessing a central, octahedral vanadyl centre linked via oxo bridges to two tetrahedral (C3v) vanadyl centres, was isolated. The calix[6]arene in 4 is severely twisted and adopts a ‘down, down, down, down, out, out’ conformation. Use of excess lithium tert-butoxide led to a complex very similar to 4, differing only in the solvent of crystallization, namely 5·Et2O·2THF. The ability of 1 and 5 to act as pre-catalysts for ethylene polymerization in the presence of a variety of co-catalysts and under various conditions has been investigated. Co-polymerization of ethylene with propylene and with 1-hexene have also been conducted; results are compared versus VO(OEt)Cl2.

Synthesis and structure of zinc dichloride bis(t-butylhydrazine) monohydrate

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Syntheses, structures and properties of three 4-fold interpenetration coordination polymers based on two different dia interpenetrating modes

Three new 4-fold interpenetrated coordination polymers based on two different dia interpenetrating modes have been prepared successfully, namely [Cu0.5(bib)0.5(tftpa)0.5]n (1), {[Zn0.5(bib)0.5(tftpa)0.5]·0.5H2O}n (2) and [Cd(bib)(tftpa)(EtOH)]n (3) (bib=1,4-bis(2-methyl-imidazol-1-yl)butane, H2tftpa=tetrafluoroterephthalic acid). Complexes 1 and 3, with the same octahedral metal center, are obtained as 4-fold interpenetrated dia coordination frameworks; however, complex 2, with a tetrahedral metal center, presents a 4-fold dia network with an uncommon [2+2] mode of interpenetration. All of the polymers contain right- and left-handed helices that are constructed alternatively and each helix is built up from [M2(tftpa)(bib)]n. Additionally, thermogravimetric curves and luminescent emissions of 2 and 3 have been studied.

Nickel Complexes Featuring Iminophosphorane–Phenoxide Ligands for Catalytic Ethylene Dimerization

A series of bidentate ligands associating an iminophosphorane and a phenoxide were synthesized and coordinated to nickel(II), leading initially to bimetallic KNi adducts. Replacement of the potassium by another metal allowed the isolation and characterization of bimetallic LiNi and AlNi complexes, while addition of one equivalent of triphenylphosphine gave access to monometallic complexes. The same type of complex was obtained with the coordination of a tridentate ligand incorporating a supplementary amine donor. These paramagnetic complexes were characterized by elemental analysis, and some of them by X-ray diffraction, evidencing a tetrahedral nickel center. They were shown to efficiently catalyze the oligomerization of ethylene in the presence of Et2AlCl (Al/Ni = 22.5) with TOF up to 72 000 mol(C2H4)/mol(Ni)/h, giving selectively butene (more than 97%) with at best 93% of 1-C4.

Amorphous Metal-organic Frameworks: Structure, Properties and Application

A small but growing number of cases of MOFs lacking the long-range order characteristic of crystalline structures are steadily capturing scientific interest (though they remain woefully under represented), despite the phenomenon being known in inorganic zeolites. The majority of work has been performed on a sub-family of MOFs called zeolitic imidazolate frameworks (ZIFs), which consist of tetrahedral metal centers (typically Zn2+ or Co2+), connected by Imidazolate-type (C3H3N2-) organic ligands in zeolitic topologies. Recently, we discovered that ZIFs could be amorphized by ball-milling and heating, and that the resultant amorphous material has a similar topology to that of silica glass.(Bennett et al., 2010, Bennett et al., 2011) Similarly, they can also be amorphized by pressure, though there is considerable debate about the reversibility of the process. Amorphous MOFs, whilst significantly harder to characterise, are more mechanically stable than their well-known crystalline counterparts. They also offer routes to functional glass materials, through melting of functionalised parent frameworks (incoporating lanthanide ions or chiral ligands). Some of these glasses have been shown to be topologically `perfect' with fragilities lower than that of aSiO2. Recently, we have shown that the release in-vitro of ibuprofen and caffeine stored within ZIF-8 can be tailored by ball-milling inducing collapse of the framework, whilst I2 has also been irreversible captured using similar methodology.(Bennett et al., 2013) Significant challenges exist in exploring the field of amorphous hybrid frameworks, though access to a multitude of functional materials is indeed a worthwhile reward.

3-(4-pyridyl)-acetylacetone--a fully featured substituted pyridine and a flexible linker for complex materials.

3-(4-Pyridyl)-acetylacetone (HacacPy) acts as a pyridine-type ligand towards CdX2 (X = Cl, Br, I). Chain polymers with six-coordinated metal cations are obtained from CdCl2 and with alternating five- and six-coordinated Cd centers from CdBr2. In either case, the formation of these compounds does not depend on the precise stoichiometry. In contrast, two different reaction products form with the heavier congener CdI2, namely a ligand-rich molecular complex CdI2(HacacPy)2 and a ligand-deficient one-dimensional polymer [CdI2(HacacPy)](1)(∞). Interconversion between these two iodo derivatives is possible via thermal degradation and mechanochemical synthesis. The acetylacetone moiety in HacacPy may be deprotonated and chelated to Fe(III), and the resulting complex Fe(acacPy)3 reacts analogously to a bridging polypyridine ligand towards the same Cd halides as the molecule HacacPy itself. With CdCl2 and CdBr2, isomorphous chain polymers are obtained in which the Cd cations adopt distorted octahedral coordination and one of the peripheric pyridyl groups remains uncoordinated. With CdI2, the iron complex acts as a mu3Fe(acacPy)3 bridge between tetrahedral Cd centers and gives rise to a ladder structure.

Prodigiosin analogue designed for metal coordination: stable zinc and copper pyrrolyldipyrrins.

The pyrrolyldipyrrin motif is found in several naturally occurring prodigiosin pigments. The potential roles of the interactions of prodigiosins with transition metals and the properties of metal-bound pyrrolyldipyrrins, however, have been difficult to assess because of the very limited number of well-characterized stable complexes. Here, we show that the introduction of a meso-aryl substituent and an ethyl ester group during the sequential assembly of the three heterocycles affords a pyrrolyldipyrrin of enhanced coordinating abilities when compared to that of natural prodigiosins. UV–visible absorption studies indicate that this ligand promptly binds Zn(II) ions with 2:1 ligand-to-metal stoichiometry and Cu(II) ions with 1:1 stoichiometry. Notably, no addition of base is required for the formation of the resulting stable complexes. The crystal structures reveal that whereas the tetrahedral zinc center engages two nitrogen donors on each ligand, the pseudosquare planar copper complex features coordination of all three pyrrolic nitrogen atoms and employs the ester group as a neutral ligand. This first example of coordination of a redox-active transition metal within a fully conjugated pyrrolyldipyrrin framework was investigated spectroscopically by electron paramagnetic resonance to show that the 1:1 metal-to-ligand ratio found in the crystal structure is also maintained in solution.

Theoretical studies of the spin-Hamiltonian parameters for the rhombic Mo5+ tetrahedral centres in ZrSiO4 crystal from a two-mechanism model

The high-order perturbation formulas of spin-Hamiltonian parameters (g factors gi and hyperfine structure constants Ai, i = x, y, z) based on the two-mechanism model for d1 ions in rhombic tetrahedral clusters with the ground state are derived in this paper. In the model, besides the contributions to spin-Hamiltonian parameters due to the crystal-field (CF) mechanism in the widely applied CF theory, those due to the charge-transfer (CT) mechanism are taken into account. From these formulas, the six spin-Hamiltonian parameters gi and Ai for the rhombic Mo5+ tetrahedral centres in ZrSiO4 crystal are calculated with four adjustable parameters. The calculated results are in reasonable agreement with the experimental values. The calculations show that for the exact and complete calculations of spin-Hamiltonian parameters of the high valence state dn ions in crystals, the contributions due to CT mechanism should not be neglected. Some information on the defect structure of Mo5+ impurity centres in ZrSiO4 crystal is also acquired from the calculations. The results are discussed.

Thio-, seleno- and telluro-ether complexes of aluminium(iii) halides: synthesis, structures and properties

The reaction of AlCl3 with Me2E (E = S, Se or Te) or (n)Bu2E (E = Se or Te) in CH2Cl2 under rigorously anhydrous conditions gave the pseudo-tetrahedral complexes [AlCl3(R2E)]. The [AlX3(Me2E)] (X = Br or I, E = S; X = Br, E = Te) were made from toluene solution since attempted syntheses in CH2Cl2 resulted in substantial chloride incorporation. The synthesis of [(AlCl3)2{o-C6H4(CH2SEt)2}], in which the ligand bridges two tetrahedral aluminium centres, and of the six-coordinate trans-[AlX2{MeE(CH2)2EMe}2][AlX4] (X = Cl or Br, E = S, and X = Cl, E = Se) and cis-[AlI2{MeS(CH2)2SMe}2][AlI4] are reported. The tripodal thioether forms [AlCl3{MeC(CH2SMe)3}], which is a chain polymer with κ(2)-coordinated ligand and a tbp arrangement at Al(iii). Chalcogenoether macrocycle complexes [AlCl3([9]aneS3)], [AlCl2([14]aneS4)][AlCl4] and [AlCl2([16]aneSe4)] [AlCl4] are also described. All complexes were characterised by microanalysis, IR and multinuclear NMR ((1)H, (27)Al, (77)Se or (125)Te) spectroscopy as appropriate. In CH2Cl2 solution [AlCl3(Me2S)] with added Me2S forms [AlCl3(Me2S)2], and the [AlX2{MeS(CH2)2SMe}2][AlX4] exist as mixtures of cis and trans isomers which undergo rapid exchange at ambient temperatures. X-Ray crystal structures are reported for [AlCl3(Me2Se], [AlX3(Me2Te)] (X = Cl or Br), trans-[AlCl2{MeE(CH2)2EMe}2][AlCl4] (E = S or Se), cis-[AlI2{MeS(CH2)2SMe}2][AlI4], [AlCl3{MeC(CH2SMe)3}], and for the sulfonium salt [Me2SH][AlCl4]. The aluminium halide chalcogenoether chemistry is compared with the corresponding gallium and indium systems, and the relative Lewis acidities of the metals discussed. Attempts to use [AlCl3((n)Bu2E)] (E = Se or Te) as LPCVD reagents to form aluminium chalcogenide films were unsuccessful.

Rare-earth transition-metal gallium chalcogenides RE3MGaCh7 (M=Fe, Co, Ni; Ch=S, Se)

Six series of quaternary rare-earth transition-metal chalcogenides RE3MGaCh7 (M=Fe, Co, Ni; Ch=S, Se), comprising 33 compounds in total, have been prepared by reactions of the elements at 1050°C (for the sulphides) or 900°C (for the selenides). They adopt noncentrosymmetric hexagonal structures (ordered Ce3Al1.67S7-type, space group P63, Z=2) with cell parameters in the ranges of a=9.5–10.2Å and c=6.0–6.1Å for the sulphides and a=10.0–10.5Å and c=6.3–6.4Å for the selenides as refined from powder X-ray diffraction data. Single-crystal structures were determined for five members of the sulphide series RE3FeGaS7 (RE=La, Pr, Tb) and RE3CoGaS7 (RE=La, Tb). The highly anisotropic crystal structures consist of one-dimensional chains of M-centred face-sharing octahedra and stacks of Ga-centred tetrahedra all pointing in the same direction. Magnetic measurements on the sulphides reveal paramagnetic behaviour in some cases and long-range antiferromagnetic behaviour with low Néel temperatures (15K or lower) in others. Ga L-edge XANES spectra support the presence of highly cationic Ga tetrahedral centres with a tendency towards more covalent Ga–Ch character on proceeding from the sulphides to the selenides. Band structure calculations on La3FeGaS7 indicate that the electronic structure is dominated by Fe 3d-based states near the Fermi level.

Effects of molecular and lattice structure on the thermal behaviours of some long chain length potassium(I) n-alkanoates

Lattice structures and thermal behaviours for some long chain potassium carboxylates (nc=8–18, inclusive) are investigated using Fourier Transform Infrared spectroscopy, X-ray Powder Diffraction, Solid State spin decoupled 13C NMR spectroscopy, Differential Scanning Calorimetry and Thermogravimetry. The measurements show that the carboxyl groups are coordinated to potassium atoms via asymmetric chelating bidentate bonding, with extensive carboxyl intermolecular interactions to yield tetrahedral metal centers, irrespective of chain length. Furthermore, the hydrocarbon chains are crystallized in the fully extended all-trans configuration and are arranged as non-overlapping lamellar bilayer structures with closely packed methyl groups from opposite layers. Additionally, odd–even alternation, observed in density and methyl group chemical shift, is ascribed to the relative vertical distances between layers in the bilayer, that are not in the same plane. Therefore, for even chain homologues, where this distances is less than for odd chain adducts, more intimate packing is indicated. The phase sequences for all compounds show several reversible crystal–crystal transition associated with kinetically controlled gauche–trans isomerism of the polymethylene chains which undergo incomplete fusion when heated to the melt. The compounds degrade above 785K to yield carbon dioxide, water, potassium oxide and an alkene.

Porous ctn‐Type Boron Imidazolate Framework for Gas Storage and Separation

BIFing it up: The assembly between triangular HB(mim)3 units and tetrahedral Zn centers (see graphic) leads to the formation of a ctn-type BIF material with high porosity for gas storage and separation (mim = 2-methylimidazolate).

PtS-Related {[CuI(F4TCNQII–)]−}∞ Networks

A series of compounds of composition A[CuI(F4TCNQII–)] (A = a quaternary ammonium or phosphonium cation, F4TCNQII– = the dianionic form of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) have been synthesized and structurally characterized. In each structure, an anionic [CuI(F4TCNQII–)]− framework possessing the topology of PtS is formed with the Cu(I) center serving as a tetrahedral 4-connecting center and the F4TCNQ2– anion acting as a planar 4-connecting unit. Although a PtS topology is observed for six different compounds, the anionic framework shows significant geometric variation depending upon the identity of the cation. Very similar structures are obtained when the organic cation is NMe4+, NMe2Pr2+, or NMe2Bu2+. A distorted anionic structure possessing the same connectivity is generated when the cation is NEt4+, and anionic frameworks with a different connectivity, but still related to PtS, are obtained when the much larger quaternary phosphonium cations are employed. Of interest in the stru...

Research on the g factors and tetragonal distortion of the Cu2+ tetrahedral center in ZnS:Cu2+ nanocrystals

The EPR g factors g∥ and g⊥ for the tetragonal Cu2+ tetrahedral clusters in ZnS:Cu2+ nanocrystals are calculated from two theoretical methods, the complete diagonalization (of the energy matrix) method (CDM) and the perturbation theory method (PTM). Both methods are based on the cluster approach in which not only the contribution due to the spin–orbit parameter of the central metal (3dn) ion but also that of ligand ions via the covalence effect are considered. The calculated results from both methods are in reasonable agreement with the experimental values, suggesting that either CDM or PTM can be used to explain the g factors of d9 ions in tetrahedral clusters. The calculations indicate that the Cu2+ tetrahedra in ZnS:Cu2+ nanocrystals are tetragonally compressed (with the ground singlet 2B2 ()) rather than tetragonally elongated. This point is consistent with the expectation based on the Jahn–Teller effect.