Small Metal Ions Research Articles

Structural Characterization of a Calix[4]arene Constrained to a 1,3-Alternate Conformation by Polyether Bridging

Despite evidence of coordinative interactions with alkali metal picrates in solution, attempts to crystallize Li+, Na+, K+, Rb+ or Cs+ complexes of a 'doubly crowned' calix [4] arene (obtained by reaction of tetraethylene glycol ditosylate with p-t- butylcalix [4] arene ) have resulted only in the deposition of crystals of the free ligand . A room-temperature single-crystal X-ray study has shown the crystals to be hexagonal, space group P6422, a (= b) 17.092(5), c 20.147(7) Ǻ, V 5097(5) Ǻ3, Z 3, conventional R (on |F|) 0.082 for 1397 'observed' reflections [I > 3σ(I)]. The highly symmetrical oblate-globular shape of this doublecrowncalix allows the enclosure of an extended narrow cavity possibly better suited to the binding of two small metal ions rather than one large.

Metal-ion assisted, radiation-induced conversion of B- to Z-DNA in poly (dGdC) and two natural DNAs.

Metal-ion assisted, gamma-radiation-induced B-Z conformation changes have been observed with UV and circular dichroism spectroscopy for poly (dGdC), calf thymus and herring testis DNA. These conformational changes are similar to those induced by increasing multivalent metal ion concentrations in DNA containing alternating purine and pyrimidine base sequences. In both the metal-ion-induced and the metal-ion-assisted, radiation-induced conformation changes, the conversions were from the right-handed B-DNA to the left-handed Z-DNA conformation. It is proposed that radiation-induced DNA strand breaks markedly reduce the high activation energy barrier in the metal ion-driven B-Z conformation conversion and allow much smaller metal ion concentrations to induce this conversion than in the absence of strand breaks. The biological importance of such radiation-induced conformational changes is discussed in terms of the potential significance of the Z-DNA conformation in the control of the DNA transcription process.

The neutral oxygen donor and macrocyclic chemistry

Abstract

Metal-ion complexation in aqueous solutions of 1 - thia-4,7-diazacyclononane-, 1-thia-4,8-diazacyclodecane and 2,5-diazahexane-N,N′-diacetic acid

The stability constants of the macrocyclic 1-thia-4,7-diazacyclononane-and 1-thia-4,8-diazacyclodecane-N,N′-diacetic acid (H2L1 and H2L2) and of the open-chain 2,5-diazahexane-N,N′-diacetic acid (H2L3) with MgII, CaII, SrII, BaII, MnII, CoII, NiII, CuII, ZnII, CdII, PbII and LaIII have been determined in aqueous solution (25 °C, 0.1 mol dm–3 KNO3) by pH potentiometry, and in some cases in combination with visible absorption spectrophotometry. The complexation enthalpies with CuII have been determined by adiabatic calorimetry. The electronic absorption spectra of the complexes of CuII and NiII were also recorded. All metal-ion complexes with H2L1 are stronger than with H2L3, even for the harder metal ions. The presence of the thioether donor particularly enhances covalent bonding with CuII as indicated by a higher heat of complexation, a more favourable entropy change and a stronger ligand-field strength. The change in stability of the complexes upon replacing H2L1 with a five-membered chelate ring between the metal ion and the two tertiary nitrogens, by H2L2 having a six-membered one, is dependent on the metal-ion size. The larger metal ions are destabilised relative to the small metal ions: the metal ion size-based selectivity for this pair of ligands is controlled by the chelate-ring size. Binding of CuII by H2L2 is sterically much more efficient than by H2L1 and is evidenced by a higher heat of complexation and a stronger ligand field.

Atomic size effects on local coordination and medium-range order in molten trivalent metal chlorides

Structural correlations in molten trivalent metal chlorides are evaluated as functions of the metal ion size RM across the range from LaCl3 (RM approximately=1.4 AA) to AlCl3 (RM approximately=0.8 AA), using a charged soft-sphere model and the hypernetted chain approximation. Main attention is given to trends in the local liquid structure (partial radial distribution functions, coordination numbers and bond lengths) and in the intermediate range order (first sharp diffraction peak in the number-number and partial structure factors). The trend towards fourfold local coordination of the metal ions, the stabilization of their first-neighbour chlorine cage and the growth of medium-range order are found to proceed in parallel as the size of the metal ion is allowed to decrease at constant number density and temperature. A tendency to molecular-type local structure and liquid-vapour phase separation is found within the hypernetted chain scheme at small metal ion sizes corresponding to AlCl3 and is emphasized by decreasing the number density of the fluid.

A Molecular‐Mechanics Analysis of Complexes of the Sexidentate Macrocycles cis‐ and trans‐6,13‐Dimethyl‐1,4,8,11‐tetraazacyclotetradecane‐6,13‐diamine

AbstractMolecular‐mechanics calculations of the sexidentate coordinated complexes of the pendant arm macrocyclic hexaamines trans‐ (L1) and cis‐6,13‐dimethyl‐3,4,8,11‐tetraazacyclotetradecane‐6,13‐diamine (L2) reveal that the trans‐isomer shows a preference toward small metal ions, whereas the L2 may coordinate a wider range of metal ions without significant intramolecular strain. Conformational disorder in previously reported crystal structures of complexes of L1 is interpreted, and predictions for some, as yet, unknown complexes of L2 are made.

Stabilities of the Fe(III), Ga(III) and In(III) chelates of N, N′, N″-triazacyclononanetriacetic acid

The stability constants of the complexes of Fe(III), Ga(III) and In(III) ions with the sexadentate macrocyclic ligand 1,4,7-triazacyclonane- N, N′, N″-triacetic acid (NOTA) have been determined in KCl supporting electrolyte (0.100 M) at 25.0 °C. The stability constants (K ML=[ML]/[L 3−][M 3+] of the Ga(III) and Fe(III) complexes with NOTA are high, 10 30.98 and 10 28.3, respectively, and indicate preferential interaction of these small metal ions with NOTA. The stability constant of the In(III)NOTA complex is somewhat lower (10 26.2). Species distribution curves are presented to illustrate the conversion of the Ga(III) and In(III) chelates to the monohydroxo forms at p[H] 9.3 and 6.5, respectively. The Fe(III)NOTA complex dissociates to form ferric hydroxide at and above p[H] 7.5, while the hydroxo In(III)NOTA complex is converted to indium hydroxide at and above p[H] 7.5. The Ga(III)NOTA system is soluble at all p[H] values, as a consequence of conversion of the hydroxo complex to the tetrahydroxo gallate ion at p[H] 10.4 and above.

Complex Formation Reaction of Polypyrazolylborates with Divalent Metal Ions in Aqueous Solution Studied by Solvent Extraction Technique

Extraction behavior of divalent transition and group IIA metals with five polypyrazolylborates, H4-nB(pz*)n¯ (pz*: 1-pyrazolyl or 3,5-dimethyl-l-pyrazolyl; n = 2, 3 and 4) has been studied. Bidentate bispyrazolylborate ligands extract divalent transition metals but do not group IIA metals. The selectivity of dihydrobis(l-pyrazolyl)borate for transition metals resembles that of 8-hydroxyquinoline. Methyl substituents in the pyrazole ring increase the stability of chelates through inductive effect and decrease that for small metal ions through steric hindrance. Hydrotris- and tetrakis(l-pyrazolyl)borate, which can act as a tridentate ligand, are highly strong extracting ligands for divalent transition metals. Moreover, these ligands extract Be(II), Mg(II) and Ca(II). Steric characteristics of the ligands contribute to anomalously high stability of the chelates and selectivity for the metal ions.

Powerful New Metal-Chelating Agents Developed

For years chemists have considered EDTA (ethylenediaminetetraacetic acid) the classic metal-chelating agent. But EDTA and similar compounds employ a flawed strategy for binding cations, work by University of Pittsburgh researchers now makes clear. New chelating agents designed by chemistry professor Julius Rebek Jr. and his coworkers offer dramatic improvements over classic chelators in tying up metal atoms. EDTA and similar compounds capture small metal ions by forming noncovalent bonds to the cations with unshared electrons on nitrogen atoms and carboxylate oxygens. Because of the molecular shapes of the chelating agents, only the anti lone pairs of the carboxylate oxygens—that is, the unshared electrons on the side opposite the carbonyl—contact the metal ions in the complex. Rebek's new chelating agents are constructed so they are forced to form complexes with the more basic syn lone pairs, those on the same side of the carbonyl oxygen. The result is molecules that are extraordinarily effective in ...

ChemInform Abstract: Metal Ion Size Selectivity in Ligands with Groups Containing the Neutral Oxygen Donor Atom. A Crystallographic and Thermodynamic Study

Abstract The coordinating properties of open-chain ligands containing alcoholic or ethereal oxygen donors are examined. Addition of oxygen donors usually leads to complex stabilisation for large metal ions (Pb2+, Cd2+) and to less favourable effects on complex stability for small metal ions (Cu2+, Ni2+). The formation constants of these metal ions with the set of ligands RN(CH2CHOH·CH3)2 where R is H, CH2CHOH·CH3, CH2CH2OCH3, CH2CH2OCH2CH2OH, and CH2CHOCH2CH2CH2 are reported. The largest stabilisation for each case where R is an O-donor group relative to R = H occurs for Pb2+, the largest metal ion, while Cu2+, the smallest metal ion, shows the smallest stabilisation. The crystal structure of [Ni(HOCH2CH2NHCH2CH2NH2)2] (NO3)2 is reported. The space group is P 1 , with cell constants a = 13.098(3), b = 8.737(4), and c = 7.746(3) A, β = 112.66(3), β = 90.65(3), and γ = 85.03(2), and Z = 2. Disorder of the nitrate anions hindered refinement, with the result that a final conventional R factor of 0.0903 was achieved. The NiN bond lengths average 2.06(1) (secondary nitrogen) and 2.10(2) (primary nitrogen). The NiO bond lengths are rather long, averaging 2.15(1) A, which is used to support the idea that the steric effects are responsible for destabilising the complexes of small metal ions such as Ni(11) when neutral oxygen donors are present.

Metal ion size selectivity of 1-Thia4, 7-diazacyclononane (9-aneN:S), and other tridentate macrocycles. A study by molecular mechanics calculation, structure determination, and formation constant determination of complexes of 9-aneN 2S

Several aspects of the coordination chemistry of the tridentate cyclononane type macrocycles are examined using molecular mechanics calculations, crystallography and formation constant determinations. The molecular mechanics calculations show that small metal ions coordinate best to these ligands, such that metal ions with a covalent radius of 1.25 A fit best into 9-aneS3 and 1.40 Ä fit best into 9-aneN3 (9-aneS3 = 1,4,7-trithiacyclononane, 9-aneN3 = 1,4,7-triazacyclononane). For mixed donor members of the series such as 9-aneN:S (1-thia4,7-diaza- cyclononane) the disparity in M-L bond length between the M-N and M-S bond lengths leads to a much higher strain situation than expected from the strain energies of the 9-aneN3 and 9-aneS3 complexes. This accounts for the order of ligand field strength in complexes of these ligands of 9-aneS 3 > 9-aneN 3 > 9-aneN:S. It is concluded that in the absence of the strain effects encountered in mixed donor ligands containing the thioether donor group, the latter group should always be higher in the spectrochemical series than ligands containing the secondary nitrogen donor. The formation constants of 9-aneN:S with Ni(II), Zn(II), Cd(II), Co(II), Fe(II), and Pb(II) are reported. Comparison of these with the formation constants for the 9-aneN:O and 9-aneN3 complexes shows that the macrocyclic effect (the difference in stability between the complex of the macrocycle and of its open chain analogue) is much higher for small metal ions, and small with large metal ions, in agreement with the molecular mechanics calculations which show that the cyclononane macrocycles coordinate best with small metal ions. The crystal structure of the complex [Cu(9-aneN:S)Br:] is reported: monoclinic, space group P2 1/n, with cell dimensions a = 7.603(1), b = 13.167(2), and c -- 10.873(2) Å, and β = 91.94- (1)°, Z = 4. Final conventional R = 0.061. The un-

Metal ion size selectivity in ligands with groups containing the neutral oxygen donor atom. A crystallographic and thermodynamic study

The coordinating properties of open-chain ligands containing alcoholic or ethereal oxygen donors are examined. Addition of oxygen donors usually leads to complex stabilisation for large metal ions (Pb 2+, Cd 2+) and to less favourable effects on complex stability for small metal ions (Cu 2+, Ni 2+). The formation constants of these metal ions with the set of ligands RN(CH 2CHOH·CH 3) 2 where R is H, CH 2CHOH·CH 3, CH 2CH 2OCH 3, CH 2CH 2OCH 2CH 2OH, and CH 2CHOCH 2CH 2CH 2 are reported. The largest stabilisation for each case where R is an O-donor group relative to R = H occurs for Pb 2+, the largest metal ion, while Cu 2+, the smallest metal ion, shows the smallest stabilisation. The crystal structure of [Ni(HOCH 2CH 2NHCH 2CH 2NH 2) 2] (NO 3) 2 is reported. The space group is P 1 , with cell constants a = 13.098(3), b = 8.737(4), and c = 7.746(3) Å, β = 112.66(3), β = 90.65(3), and γ = 85.03(2), and Z = 2. Disorder of the nitrate anions hindered refinement, with the result that a final conventional R factor of 0.0903 was achieved. The NiN bond lengths average 2.06(1) (secondary nitrogen) and 2.10(2) (primary nitrogen). The NiO bond lengths are rather long, averaging 2.15(1) Å, which is used to support the idea that the steric effects are responsible for destabilising the complexes of small metal ions such as Ni(11) when neutral oxygen donors are present.

Thermoelectric Power Measurements on Some Chevrel Phase Superconductors

The absolute thermoelectric power measurements on AgMo6S8, Mo6S8, pbMo6S8, SnMo6S8, SnMo6S8 were made from 4.2 to 300 k. All the compounds have positive thermoelectric power near room temperature. Mo6S8 and the compounds containing small metal ion such as Cu and Ag show a negative peak and the compounds containing large metal ion such as Pb, Sn show a positive peak in the thermopower vs. temperature graph. There seems to be a correlation between low temperature peak in the thermopower and delocalisation of the metal ion in the compound. Two mechanisms may account for the peaks in the thermopower viz. phonon drag effect and the enhancement of thermopower caused by the renormalisation of energy, velocity and relaxation time due to electron-phonon interaction. In the present case the mechanism mentioned later aoes not seem to provide an explanation for the negative peaks in the thermopower.

The effect of increase in chelate ring size beyond six-membered on the metal ion size selectivity patterns of tetra-aza macrocycles

The complex-formation constants and protonation constants of the ligand 1,4,7,10-tetra-aza-cyclopentadecane, which forms three five- and one eight-membered chelate ring, are reported. The protonation constants measured by glass-electrode potentiometry in 0.1 mol dm–3 NaNO3 at 25 °C are pK1 9.83, pK2 8.95, pK3 5.40, and pK4 1.6, while the complex-formation constants, log K1, were 19.25 (Cu2+), 11.74 (Ni2+), 10.70 (Zn2+), 10.18 (Cd2+), and 9.50 (Pb2+). These constants were compared to those of other tetra-aza macrocycles with three five-membered chelate rings plus a fourth chelate ring which varied regularly from five- to seven-membered, as well as of series of open-chain ligands in whose complexes a single chelate ring varied in size in the same manner. It was concluded that (a) the size of the macrocyclic ring plays very little part in determining complex stability, (b) the main determining factor in controlling complex stability is the size of the chelate ring formed, (c) an increase in chelate ring size from five- to six-membered produces a differentiation in complex stability dependent on the size of the metal ion, such that complexes of larger metal ions are destabilized relative to those of small metal ions, and (d) an increase in chelate ring size beyond six-membered produces a fairly uniform decrease in complex stability which does not show any strong dependence on the size of the metal ion.

ChemInform Abstract: Small Macrocyclic Ligands with Mixed Nitrogen‐ and Oxygen‐Donor Atoms.

AbstractThe stabilities of some of the complexes of the metal ions Cu2+, Ni2+, Zn2+, Cd2+, Pb2+, Ca2+, and Hg2+ with the ligands (I)‐(V) are reported. It is shown that addition of neutral O donors to existing amine ligands leads to an increase in complex stability for large metal ions (Pb2+, Ca2+) and a decrease for small metal ions (Cu2+, Zn2+).

Macrocycles and their selectivity for metal ions on the basis of size

Abstract

Die Sattelkonformation der hydroporphinoiden Nickel(II)‐Komplexe: Struktur, Ursprung und stereochemische Konsequenzen

The Saddle Conformation of Hydroporphinoid Nickel(II) Complexes: Structure, Origin, and Stereochemical ConsequencesThirteen crystal structures document the general phenomenon of coordination‐hole contraction in hexahydro‐ and tetrahydroporphinoid ligands of complexes with small metal ions such as low‐spin Ni(II). The contraction is characterized by a deformation of the ligand system towards a saddle‐shaped, ruffled conformation of approximate S4 symmetry. The central metal ion is coplanar with the four coordinating N‐centers whereas the four C(meso)‐atoms are situated alternately above and below this coordination plane. Increasing steepness of the saddle (parameter dm) is associated with decreasing metal‐N distances. For metal pyrrocorphinates, dm increases in the order Cu(II) < (pyridine) Co(II) < Ni(II), for Ni(II) complexes it does so in the order porphyrin < chlorin < bacteriochlorin < isobacteriochlorin < pyrrocorphin. In the saddle conformation of hydroporphinoid Ni(II) complexes, the hydropyrrole rings assume half‐chair conformations whereby the individual half‐chairs are conformationally constrained in such a way that the inclination of their peripheral single bond parallels the inclination of the ligand saddle (W‐conformation of the ensemble of 5‐membered ring half‐chairs). There are only two such conformations available for a given complex; they interrelate by saddle inversion with concomitant inversion of the ensemble of half‐chairs. The coordination‐hole contraction of hydroporphinoid ligands is expected and observed experimentally to exert control on the stereochemistry and reactivity of the ligand periphery as well as on the axial electrophilicity of the central metal ion. Tetracoordinate nickel(II) pyrrocorphinates are found to favor the tctct configuration of substituents at the ligand periphery, nickel(II) isobacteriochlorinates the tct configuration, whereas nickel(II) bacteriochlorinates are expected to favor the ttt configuration. Relative rates and regioselectivities of autoxidation of nickel(II) pyrrocorphinates to corresponding bacterio‐and isobacteriochlorinates depend on the configuration and conformation of the ligand periphery. The residual axial electrophilicity of the metal ion in tetracoordinate Ni(II) complexes of the octaethyl series appears to increase in the order chlorin < isobacteriochlorin ≲ bacteriochlorin < pyrrocorphin. All hydroporphinoid metal complexes used in the X‐ray structure studies were prepared as part of exploring the porphyrinogen → pyrrocorphin tautomerization, a novel structural transformation of porphyrinogens.

Positive ion composition and electron density in a combined auroral and NLC event

The positive ion composition and electron density were measured in the lower ionosphere above Kiruna in salvo A of CAMP (Cold Arctic Mesopause Project). The CAMP/P (S37/P) payload carrying a magnetic ion spectrometer, positive ion and electron probes, and propagation experiments was launched on 3 August 1982 2332 UT during extended Noctilucent Clouds (NLC) and auroral activities over Kiruna. The measured electron density was 5×10 3cm −3 at 80 km and 2.5×10 5cm −3 at 90 km. The increase of ion and electron densities in the D- and E-region during twilight was caused by precipitating auroral particles. The height distribution of the positive ions measured by the mass spectrometer in the mass range 19–280 amu is different from a winter flight with similar auroral conditions. Below 85.5 km proton hydrates H +(H 2O) 3 − H +(H 2O) 8 were the dominant ions. The heaviest proton hydrates H +(H 2O) 7 and H +(H 2O) 8 were most abundant at 82–85.5 km, the altitude of visible NLC. Above 85.5 km O 2 + and NO + became dominant. A small metal ion layer was observed between 90.5–93 km with a maximum ion density of 10% of the total positive ion density at 91 km altitude. The metal ion density disappeared within about a km below 90.5 km.

The influence of small monovalent cations on the hydrogen bonds of base pairs of DNA

The influence of small monovalent metal ions on the multiple hydrogen bonds of Watson-Crick DNA base pairs has been theoretically studied, using ab initio calculations with minimal GLO basis set, and combined with the Boys-Bernardi counterpoise method to eliminate the basis set superposition error. The results were compared with the available experimental data. Only metal ion binding to N3 or N7··O6 of guanine seems to lead to a hydrogen bond stabilization of the GC pair, whereas in the case of AT pair no such effect could be observed.

The regulation of ionic nickel uptake and cytotoxicity by specific amino acids and serum components

The effects of serum components and amino acids on the uptake and cytotoxicity of NiCl2 were examined in cultured Chinese hamster ovary (CHO) cells. CHO cells maintained in a minimal salts/glucose medium accumulated 10-fold more(63)Ni than did cells maintained in complete medium supplemented with 10% fetal bovine serum. Cell-surface binding of(63)Ni appeared to account for the majority of this increased accumulation of cell-associated nickel observed in the simple maintenance medium since such increases were reduced 70% by trypsin treatment. The addition of the Ni(2+)-binding amino acids cysteine or histidine to the salts/glucose medium markedly decreased(63)Ni accumulations, an effect not observed following addition of any of several amino acids that do not bind Ni(2+). Supplementation of the salts/glucose medium with fetal bovine serum decreased in a concentration dependent fashion both the(63)Ni(2+) uptake and cell detachment caused by Ni(2+), while dialyzed (amino acid-free) serum was 3-5-fold less effective than undialyzed serum at reducing(63)Ni(2+) uptake and similarly exhibited only a slight protective effect against nickel-induced cytotoxicity. Supplementation of dialyzed serum with cysteine at levels approximating those in whole serum partially restored its inhibitory activity toward nickel uptake by cells and restored completely its inhibition of nickel's cytotoxicity, indicating the predominant role of specific amino acids over serum proteins in regulating the uptake and subsequent cytotoxicity of Ni(2+). Addition of cysteine to the salts/glucose medium during a 2 h exposure of cells to either 100 μM HgCl2 or 1 mM NiCl2 masked the cytotoxic effects of these metal ions. These results demonstrate the importance of extracellular small molecular weight metal ion chelators in altering the biological effects of metal ions at the level of metal uptake.