Chelate ring geometry,and the metal ion selectivity of macrocyclic ligands. Some recent developments

Abstract

The idea (Hancock, 1992) that the dominant architectural feature in controlling metal ion selectivity in both open-chain and macro-cyclic ligands is the size of the chelate ring is pursued further. It is shown that when more than one or two six-membered chelate rings are present in the complex of a nitrogen donor macrocycle, the steric requirements of the six-membered chelate ring of a M-N bond length of 1.6 Å and N-M-N angle of 109.5° become particularly severe, and can only be met by a small tetrahedral metal ion. Thus, the ligand 16-aneN₄ (1,5,9,13-tetraazacyclohexadecane) forms complexes of low stability with all metal ions studied to date, but a conformer of 16-aneN₄ is identified by MM calculation which is predicted to form complexes of high stability with very small tetrahedral metal ions. The question of the M-O bond length and O-M-O angles that will produce minimum strain in chelate rings containing neutral oxygen donor is addressed. The observation (Hay, 1993) that the geometry around an ethereal oxygen coordinated to a metal ion approximates to trigonal planar rather than tetrahedral leads to ideal M-O-C angles of about 126°, which leads to minimum strain energy with much longer M-L lengths in chelate rings containing neutral oxygen donors than neutral nitrogen donors. It is suggested that this fact accounts for the general tendency of crown ethers to form their most stable complexes with potassium out of the alkali metal ions, and also accounts for the very small macrocyclic effect observed in complexes of macrocycles containing mixed nitrogen and oxygen donor groups. The preferred geometry of four-membered chelate rings is discussed, and it is shown that higher coordination numbers of metal ions are associated with four membered chelate rings, and that four membered chelate rings may be used to engineer preference for larger metal ions. Very rigid reinforced chelate rings are discussed, and it is shown that open-chain ligands with reinforced bridges between the donor atoms can display all the thermodynamic and kinetic aspects associated with macrocyclic ligands.     

Enhanced metal ion selectivity of 2,9-di-(pyrid-2-yl)-1,10-phenanthroline and its use as a fluorescent sensor for cadmium(II)

The metal ion complexing properties of the ligand DPP (2,9-di-(pyrid-2-yl)-1,10-phenanthroline) were studied by crystallography, fluorimetry, and UV-visible spectroscopy. Because DPP forms five-membered chelate rings, it will favor complexation with metal ions of an ionic radius close to 1.0 A. Metal ion complexation and accompanying selectivity of DPP is enhanced by the rigidity of the aromatic backbone of the ligand. Cd2+, with an ionic radius of 0.96 A, exhibits a strong CHEF (chelation enhanced fluorescence) effect with 10(-8) M DPP, and Cd2+ concentrations down to 10(-9) M can be detected. Other metal ions that cause a significant CHEF effect with DPP are Ca2+ (10(-3) M) and Na+ (1.0 M), whereas metal ions such as Zn2+, Pb2+, and Hg2+ cause no CHEF effect with DPP. The lack of a CHEF effect for Zn2+ relates to the inability of this small ion to contact all four donor atoms of DPP. The structures of [Cd(DPP)2](ClO4)2 (1), [Pb(DPP)(ClO4)2H2O] (2), and [Hg(DPP)(ClO4)2] (3) are reported. The Cd(II) in 1 is 8-coordinate with the Cd-N bonds to the outer pyridyl groups stretched by steric clashes between the o-hydrogens on these outer pyridyl groups and the central aromatic ring of the second DPP ligand. The 8-coordinate Pb(II) in 2 has two short Pb-N bonds to the two central nitrogens of DPP, with longer bonds to the outer N-donors. The coordination sphere around the Pb(II) is completed by a coordinated water molecule, and two coordinated ClO4(-) ions, with long Pb-O bonds to ClO4(-) oxygens, typical of a sterically active lone pair on Pb(II). The Hg(II) in 3 shows an 8-coordinate structure with the Hg(II) forming short Hg-N bonds to the outer pyridyl groups of DPP, whereas the other Hg-N and Hg-O bonds are rather long. The structures are discussed in terms of the fit of large metal ions to DPP with minimal steric strain. The UV-visible studies of the equilibria involving DPP and metal ions gave formation constants that show that DPP has a higher affinity for metal ions with an ionic radius close to 1.0 A, particularly Cd(II), Gd(III), and Bi(III), and low affinity for small metal ions such as Ni(II) and Zn(II). The complexes of several metal ions, such as Cd(II), Gd(III), and Pb(II), showed an equilibrium involving deprotonation of the complex at remarkably low pH values, which was attributed to deprotonation of coordinated water molecules according to: [M(DPP)(H2O)]n+ <==> [M(DPP)(OH)](n-1)+ + H+. The tendency to deprotonation of these DPP complexes at low pH is discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing the hydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complex because of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.     

DFT B3LYP calculation of the spatial structure of Co(II), Ni(II), and Cu(II) template complexes formed in ternary systems metal(II) ion-dithiooxamide-formaldehyde

The hybrid density functional theory B3LYP method with the 6-31G(d) basis set and the Gaussian 98 program has been used for calculating the geometric parameters of the Mn(II), Co(II), Ni(II), and Cu(II) complexes with NNSS-donor macrocyclic ligands forming in the course of template processes in the M(II)-dithiooxamide-formaldehyde systems. The bond lengths and bond angles in the complexes with the MN₂S₂ metal chelate core are reported. For all M(II) ions, the extra six-membered chelate ring that form as a result of template assembly is rotated through a rather large angle with respect to two five-membered rings and the ring itself is not planar.     

Iridium-Catalyzed Silylation of Five-Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl-Imidazoline Ligand

The steric effects of substituents on five-membered rings are less pronounced than those on six-membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions of five-membered heteroarenes that occur with selectivities dictated by steric effects, such as the borylation of C−H bonds, have been poor in many cases. We report that the silylation of five-membered-ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]₂ (cod=1,5-cyclooctadiene) and a phenanthroline ligand or a new pyridyl-imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C−H bonds of these rings under conditions that the borylation of C−H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross-coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl, and perfluoroalkyl substituents.     

Gas-phase reactions of divalent Ni complex ions with acetonitrile: Chelate ring size,inductive, and steric effects

Ni(II) complexes of a series of pentadentate polyamine ligands have been reacted with CH₃CN in the gas phase using a modified quadrupole ion trap mass spectrometer. The ligands have structural features such that upon complexation, chelate ring size, sterics, and inductive effects can be evaluated in the gas phase. Rate and equilibrium constants for CH₃CN addition to the metal complexes show that there is a general decrease in the gas-phase reactivity as the chelate ring size is increased. Density functional theory calculations at the B3LYP/LANL2DZ level of theory have been used to obtain minimum energy structures and Mulliken charges for the complexes. The decreased reactivity observed as the chelate ring size is increased correlates with a decrease in the atomic charge on the metal. A larger chelate ring size enhances ligand flexibility and improves the overlap of the ligand’s donor atoms with the metal center. Adding methyl groups adjacent to or on the nitrogen donor groups of a ligand also decreases the rate and equilibrium constants for the reactions of a given complex with CH₃CN. Analysis of Mulliken charges for these complexes indicates that both inductive and steric effects are responsible for lower complex reactivity. These results suggest that while the gas-phase reactivity of a metal complex with CH₃CN is very dependent on the functional groups directly bound to the metal, in some cases steric effects can conceal the correlation between reactivity and coordination structure.     

Thin-layer chroamtography of pt-metals complexes with ketoanils: Analysis and effect of chelate ring contraction on RF values

Summary Thin-Layer chromatogrtaphy on silica gel was used for the separation of platinum group metal complexes using one-, two-, and three-component solvent systems. In octahedral and square planar, cationic complexes, containing two identical five-membered chelate rings formed by three different ketoanils, a linear dependence on chelate ring contraction (and also metal-ligand bond frequencies and ligand field parameters) of their R_F values has been established in one-component solvent systems.     

Calculation of geometric parameters and energies of macrocyclic metal chelates in the ternary M(II) ion-thiocarbamoylmethanamide-formaldehyde systems

The thermodynamic and geometric parameters of isomeric macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes that can, in principle, be the products of the reaction of corresponding hexacyanoferrates(II) with thiocarbamoylmethanamide H₂N-C(=S)-C(=O)-NH₂ and formaldehyde in gelatin-immobilized matrix implants have been calculated by the hybrid B3LYP density functional theory method with the use of the 6–31G(d) basis set and the Gaussian 98 program package. For any of the M(II) ions under consideration, the most stable complexes have the MN₄ metal chelate cores. The bond lengths and bond angles in these complexes have been reported, and it has been stated that the five-membered chelate rings in the complexes are not strictly planar. The additional six-membered chelate ring resulting from template cross-linking is also nonplanar and, in some cases, is turned at a rather large angle to the five-membered rings.     

Aromatic properties of 8-hydroxyquinoline and its metal complexes

Chelatoaromaticity (aromaticity of chelate complexes) has been recently recognized as an important property influencing the stability of chelate compounds. In this paper, aromaticity of various forms of 8-hydroxyquinoline (anion, neutral molecule, zwitterion and cation) as well as its chelate complexes with magnesium and aluminium ions are investigated. Aromatic properties of these compounds are analyzed using several aromaticity indices based on energetic, geometric, magnetic and electronic physical manifestations of this phenomenon. Results of performed calculations have shown different aromatic properties for the two rings (pyridine and benzene) occurring in the studied ligand. Aromaticity of these rings in metal complexes of 8-hydroxyquinoline is significantly higher than that in corresponding ligand anion. This means that during complexation the aromaticity of the ligand increases and the chelatoaromatic effect stabilizes the studied metal complexes. In contrast, metallocyclic rings of studied metal complexes have non-aromatic properties, and, consequently, the metallocyclic ring is not stabilized by chelatoaromaticity. We conclude that, in the complex, every 8-hydroxyquinoline unit and the metal ion are separated p-electronic systems.      

Molecular structures and spectroscopic characterization of cobalt(III) and nickel(II) complexes of <Emphasis Type="Italic">N</Emphasis>-(2-hydroxyethyl)-2-(thiophene-2-ylmethylene)-hydrazinecarbothioamide

Two complexes of a thiosemicarbazone ligand, namely N-(2-hydroxyethyl)-2-(thiophene-2-ylmethylene)-hydrazinecarbothioamide (HL), have been synthesized. The complexes have been characterized by physico-chemical and spectroscopic methods. The crystal and molecular structures of [CoL₃]·2MeOH (1) and [NiL₂] (2) have been determined by X-ray diffraction studies. For both complexes, the metal is coordinated through the sulfur and azomethine nitrogen atoms of the thiosemicarbazone. The ligand exists in its thiolate tautomeric form, and the central Co(III) and Ni(II) atoms have distorted octahedral and square planar geometries, respectively, with five-membered chelate rings formed by the ligand. The lattice of 1 shows infinite oxygen donor/acceptor hydrogen bonds in the ab plane and weak interactions between rings along the c axis, respectively, giving a supramolecular network. The molecular units in 2 are linked together by hydrogen bonds between the hydroxyl oxygen and hydrazone N proton, giving rise to an infinite ribbon extended along the c-axis. These chains are connected by N3–H3···O1 interactions that form a sheet within the ac plane.     

非水体系中磺酸铜型树脂对苯胺衍生物的配位吸附

研究了磺酸铜型树脂在乙醇和正己烷中对3种芳胺的吸附行为，结果表明：磺酸铜型树脂在乙醇和正己烷中对苯胺，N－甲基苯胺和N，N－二甲基苯胺具有较好的吸附性能，且随着吸附质N原子上取代基的增加，吸附的亲和性逐渐减小，表现出明显的吸附选择性，因此，磺酸铜型树脂有望应用于在非小体系中吸附分离或去除某些难溶于水的天然产物如生物碱等物质。     

Application of chitosan and its derivatives for solid-phase extraction of metal and metalloid ions: a mini-review

Here we review chitosan-based materials for solid-phase extraction of metal and metalloid ions prior to their determination by atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, mass spectrometry, and some other spectrometric techniques. We show that nearly zero affinity of chitosan and its derivatives to alkali and alkali-earth metal ions is very beneficial for separation of analytes from the salt matrix, which is always present in natural waters, waste streams, and geological samples and interferes with analytical signals. Applicability of chitosan to selective recovery of different metal and metalloid ions can be significantly improved via functionalization with N-, S-, and O-containing groups imparting chitosan with additional electron donor atoms and capability to form stable five- and six-membered chelate rings with metal ions. Among most promising materials for analytical preconcentration, we discussed chitosan-based composites; carboxyalkyl chitosans; chitosan derivatives containing residues of aminoacids, iminodiacetic acid, ethylenediaminetetraacetic and diethylenetriaminepentaacetic acids; chitosans modified with aliphatic and aromatic amines, heterocyclic fragments (pyridyl, imidazole), and crown ethers. We have shown that most chitosan derivatives provide only group selectivity toward metal ions; however, optimization of recovery conditions allows metals and metalloids speciation and efficient separation of noble and transition metal ions.     

Equilibrium and structure of the Al(III)-ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP) complex. A multi-method study by potentiometry, NMR, ESI MS and X-ray diffraction

The equilibrium and structure of the complex formed by Al(III) and ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP2-) have been studied using pH-potentiometry, 1H and 27Al NMR, ESI MS and single crystal X-ray diffraction methods. The EDBHP ligand is a strong Al-binder in aqueous solution for pH between 4 and 8 and for c(Al) = c(EDBHP)> or = 0.1 mmol dm(-3). The dominating complex identified by ESI MS and potentiometry is a neutral dimer, Al2L2(OH)2, with logbeta(22-2) = 14.16 +/- 0.03. In the solid Al2(EDBHP)2(OH)2.2H2O the Al(III) ions are connected through a double hydroxo bridge. Both four-dentate organic ligands are coordinated terminally through two carboxylate groups and two N-donors forming three five-membered chelate rings. The hydroxyl groups of the ligand EDBHP remain protonated and are not coordinated to the aluminium ions. The structure and composition of the dimer are very likely the same in solution and the solid state.     

Synthesis and Characterization of Nitrato-Triamine-Metal(II) Complexes. Conglomerate Crystallization Part 55. Crystal Structure of [Ni(dien)(O2NO)(ONO2)] (I), {[Cu(dien)-(μ-ONO2)]NO3}∞ (II), [Zn(dien)(O2NO)(ONO2)] (III), [Ni(Medpt)(O2NO)(ONO2)] (IV) and [Cu(Medpt)(ONO2)2] (V)

In absolute ethanol and in the presence of triethylorthoformate, reactions of metal(II) nitrates with linear tridentate amines afforded metal complexes of the formula M(NNN)(NO₃)₂, where M = Ni²⁺, Cu²⁺ and Zn²⁺, and NNN = dien and Medpt. The compounds fall into three categories in accordance with their stereochemistry and mode of binding of the nitrato ligands. Compounds I, [Ni(dien)(O₂NO)(ONO₂)] and III, [Zn(dien)(O₂NO)(ONO₂)] are isomorphous and isostructural. They crystallize in the monoclinic space group P2₁/n with nearly identical cell constants. The stereochemistry of these two compounds is such that the terdentate dien ligand forms a fac MN₃ moiety with the two oxygens of the bidentate nitrato ligand trans to the terminal NH₂. These ligands form the base of the octahedral arrangement in which the sixth position, trans to the secondary nitrogen of the dien, is an oxygen of the monodentate nitrato ligand. Compound IV, [Ni(Medpt)(O₂NO)(ONO₂)] falls into the same category as I and III despite the fact that the two rings in the Ni-Medpt moiety are six-membered rings, unlike those in compounds I and III which are five-membered rings. Nevertheless, the nickel-amine arrangement is fac. The bidentate nitrato-oxygens are trans to the terminal NH₂ of the amine ligand, and the oxygen of the monodentate nitrato ligand is trans to the tertiary amine-nitrogen. Such stereochemistry is prevalent for nickel and zinc compounds. Interestingly, compound IV crystallizes as a conglomerate (space group P2₁2₁2₁). Compound II, {[Cu(dien)(μ-ONO₂)]NO₃}_∞ belongs to the second category and has a polymeric structure. The repeating fragment in the polymeric chain is a Cu(dien)-O fragment with the monodentate nitrato ligand occupying an equatorial position of the base. A second oxygen of the equatorial nitrate becomes an axial ligand for an adjacent Cu-N₃O fragment. In this way the substance propagates into an infinite chain. The repeating unit has an effective square pyramidal, five-coordinate, configuration. Finally, the compound crystallizes as a racemate. The second nitrate necessary for charge compensation of this copper(II) compound is ionic and its function is to hold the infinite chains of the lattice. The third category represented by compound V, [Cu(Medpt)(ONO₂)₂] contains two molecules in the asymmetric unit of the racemic lattice (monoclinic, space group P2₁/a). The structure of Cu-Medpt is unlike that of IV in that both species present in the asymmetric unit have the amine ligand in a mer configuration which together with a monodentate oxygen of a nitrato ligand form a base plane of a square pyramid. The fifth ligand of both Cu²⁺ ions is a second monodentate nitrato ligand. The stereochemical differences between the two Cu²⁺ ions are insignificant for the Cu-Medpt fragment, which share the same conformation and configuration. The major difference between the two species is the torsional angles defined by the Cu-O-N-O angles. The difference arises from variation in the hydrogens of the primary amine moieties selected by nitrato-oxygens to form intramolecular hydrogen bonds. Finally, there is a little variation in the equatorial Cu-ONO₂ stereochemistry because of steric hindrance, imposed by the Medpt, preventing large torsional angles by these nitrato ligands. This is evident by comparing the two copper species shown in Finally, nitrate-to-Br ligand exchange was found to take place when KBr pellets are prepared for IR spectral measurements.     

Calculation of geometric parameters of macrocyclic metal chelates formed by template synthesis in tertiary systems M(II) ion-ethanedithioamide-formaldehyde-ammonia

The geometric parameters of macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes with 2,8-dithio-3,5,7-triazanonanedithioamide-1,9 with the (N,N,S,S) coordination of the chelant donor centers (formed by template synthesis in the M(II)-ethanedithioamide-formaldehyde-ammonia system) have been calculated by the hybrid B3LYP density functional theory method with the use of the 6-31G(d) basis set and the Gaussian 98 program package. The bond lengths and bond angles in the complexes with the MN₂S₂ coordination core have been reported. Calculations demonstrated that in none of the complexes are the five-membered chelate rings planar and that these rings in the Zn(II) complex are significantly different. For all M(II) ions under consideration, an additional six-membered chelate ring resulting from template cross-linking is turned at a rather large angle to the two five-membered rings and this ring itself is nonplanar.     

2-Pyridylaldehydebenzoylhydrazone derivatives as highly selective precolumn chelating reagents for nickel (II) ion in kinetic differentiation mode high-performance liquid chromatography

2-Pyridylaldehydearoylhydrazones have been examined as reagents for precolumn derivatization of metal ions in the HPLC-spectrophotometry system. With the simplest analog, 2-pyridylaldehydebenzoylhydrazone (PAB), among 11 metal ions only Ni(II) ion gives the peak while the other metal chelates seem to be dissociated on an HPLC column where no added PAB is present in the eluent solutions. All other PAB analogs exhibit the peak for Ni(II) ion as well as Co(III) ion. In one reagent system, V(V) and Fe(II) chelates also appear in the chromatograms. It has been stressed that the selectivity principle is the kinetic differentiation (KD) towards metal chelates associated with the HPLC processes. The specificity for Ni(II) ion is in close relation to a key structure of the ligand molecules which provides an N,N,O coplanar coordination environment to form two five-membered chelate rings. An extremely selective and sensitive KD-HPLC method for the quantitation of Ni(II) ion at the ultra-trace levels was assessed; the detection limit (3 _Blank) for Ni(II) ion was down to 5.34 × 10^–9 mol l^–1 (31.5 pg in a 100 l injection) and the excellent applicability was checked using coal fly ash samples.     

Hydridotris(thioxotriazolyl)borate (Tt), an ambidentate (N(3)/S(3)) tripodal ligand. X-ray crystal structures of sodium, bismuth(III), tin(IV), and manganese(I) complexes

Treatment of the heterocycle 5-thioxo-4,5-dihydro-3,4-dimethyl-1,2,4-triazole (thioxotriazole) with sodium tetrahydroborate at 210 degrees C provides the new [N(3)/S(3)] ambidentate tripod ligand hydridotris(thioxotriazolyl)borate (Tt) as its sodium complex salt. Complexes of this ligand with sodium, bismuth(III), tin(IV), and manganese(I) have been synthesized and characterized by X-ray crystallography. The structures of these complexes illustrate the ambidentate character of the ligand with the softer metals bismuth and tin exhibiting sulfur coordination, while sodium and manganese(I) bond via the ligand nitrogen donors. In the [S(3)] coordination mode the ligand creates eight-membered chelate rings with the metal with the consequence that the metal ligand unit adopts a propeller-type conformation with C(3)-symmetry. However, in the [N(3)] mode six-membered chelate rings are formed analogous to the familiar hydrotris(pyrazolyl)borate (Tp) ligand.     

Metal-ion selectivity produced by C-alkyl substituents on the bridges of chelating ligands: the importance of short H-H nonbonded van der Waals contacts in controlling metal-ion selectivity. A thermodynamic, molecular mechanics, and crystallographic study

The effect on metal-ion selectivity of the use of cyclohexenyl bridges in ligands in place of ethylene bridges is examined (selectivity is defined as the difference in log K1 for one metal ion relative to that of another with the same ligand). The syntheses of N,N'-bis(2-hydroxycyclohexyl)ethane-1,2-diamine (Cy2-en), N,N'-bis(2-hydroxycyclohexyl)propane-1,3-diamine (Cy2-tn), and 1,7-bis(2-hydroxycyclohexyl)-1,4,7-triazaheptane (Cy2-dien) are reported. The crystal structures of [Cu(Cy2-tn)(H2O)](ClO4)2 (1) and [Cu(Cy2-dien)](ClO4)2 (2) are reported. Characteristics of 1: monoclinic, Pn space group, a=11.627(2) A, b=7.8950(10) A, c=12.737(8) A, beta=98.15(3) degrees, Z=2, R=0.0524. Characteristics of 2: orthorhombic, Pbca space group, a=21.815(16) A, b=8.525(7) A, c=25.404(14) A, Z=8, R=0.0821. Structure 1 has the Cu(II) atom coordinated in the plane of the ligand to the two N donors and two O donors, with a long bond to an axially coordinated water molecule. 2 has three N donors, and one hydroxyl O donor from the ligand is coordinated in the plane around the Cu(II) atom, with the second hydroxyl O donor of the ligand occupying an axial site with a long Cu-O bond. The salient feature of both structures is the short H-H nonbonded distance between H atoms on the cyclohexenyl bridges and H atoms on the ethylene bridges of the ligand. These short contacts are important in explaining the metal-ion selectivities of these ligands. Formation constants, determined by glass-electrode potentiometry, for the Cy2-en (Cu(II), Ni(II), Zn(II), Cd(II), Pb(II)), Cy2-dien (Cu(II), Zn(II), Cd(II), Pb(II)), and Cy2-tn (Cu(II), Zn(II), Cd(II)) complexes are reported. These all show a strong shift in selectivity toward smaller metal ions compared with the analogous ligands, where ethylene bridges are present in place of the cyclohexenyl bridges of the ligands studied here. Molecular mechanics (MM) calculations are used to analyze these changes in selectivity. These calculations show that the short H-H contacts become shorter with increasing metal-ion size, which is suggested as the cause of the shift in the selectivity of ligands in favor of smaller metal ions when ethylene bridges are replaced with cyclohexenyl bridges. MM calculations are also used to rationalize, in terms of short H-H contacts, the fact that when the chelate ring contains two neutral O donors, more stable complexes result with cis placement of the donor atoms on the cyclohexenyl bridge, but with two N donors, trans placement of the donor atoms results in more stable complexes.     

Crystal structure of (2,2'-bipyridine-N,N)(2,2',2'-triaminotriethylamine-N,N',N',N')nickel(II) diperchlorate.

An X-ray structure determination shows that the Ni(II) ion is a distorted six-coordinated octahedron by four nitrogen atoms of the tetradentate tren ligand constituting the equatorial square base, and by two nitrogen atoms of the bidentate bpy ligand in a cis position. The two six-membered rings of bpy are coplanar and almost pararell. The tetradentate ligand consists of three five-membered chelate rings in gauche coformations. The Ni-N(tren) bond lengths of this complex are almost equivalent to the reported values.     

Linear Conjugated Coordination Polymers Containing Eight-Coordinate Metal Centers

Two linear coordination polymers are reported in which conjugated organic ligands and nonrigid eight-coordinate metal centers are linked to form macromolecules with molecular weights of greater than 10⁴ Daltons. The tungsten(IV) chelate is an inert low-spin d² species with four oxygen and four nitrogen donors per metal with two bidentate blocking ligands and one bis-bidentate bridging ligand. The zirconium(IV) chelate is rendered inert through the use of a bis-quadridentate Schiff-base ligand. Future possibilities and potential uses are also discussed.     

Catalytic behavior of copper(II) chelate complexes sterically held in zeolite large cavities and fixed on its outer surface by a topological anchor

Catalytic properties of copper(II) tricyclic chelate compounds based on enamino ketone of 3-acetyl-2,4-pentanedione and 1,6-hexametylenediamine fixed on NaY and CaA zeolites by the methods of topological and topologically-anchor retention, respectively, were compared. The mode of fixation of chelates on a support affects the character of the liquid-phase catalytic oxidation of cyclohexene by molecular oxygen. At the topological fixation by a steric retention of a chelate compound in a large cavity of NaY zeolite, the reaction rate related to one reaction center of a metal complex is not proportional to the filling degree that points to inaccessibility for a substrate of catalyst molecules localized in inner cavities of crystallites. In an alternative mode of the catalyst topologically-anchor fixation, when only a ligand fragment is held in CaA zeolite, and catalytically active metal center is turned to the side of the reaction medium, the linear dependence of the cyclohexene oxidation rate on the amount of the chelate on the support is retained within the whole studied range of the surface catalyst concentrations. The catalytic activity of the topologically-anchor fixed Cu(II) chelate compound coincides with its activity in the zeolite absence, which points to a pseudo-homogeneous mode of cyclohexene oxidation. At topologically-anchor fixation of a Cu(II) chelate compound on CaA zeolite diffusion limitations characteristic for the fixation by steric retention are completely eliminated.