Carboxylate Groups Of Ligands Research Articles

Enthalpic (electrostatic) contribution to the chelate effect: a correlation between ligand binding constant and a specific hydrogen bond strength in complexes of glycopeptide antibiotics with cell wall analogues

The 1H NMR chemical shift of amide protons in the binding pocket of glycopeptide antibiotics has been used to monitor the interaction of these amide protons with the carboxylate group of cell wall analogues and related ligands. A good correlation is observed between overall ligand binding energy (ΔG°) and amide NH chemical shift. We conclude that the strength of the electrostatic interaction of the carboxylate group, which is crucial to recognition and binding by the antibiotics, is cooperatively enhanced by adjacent functional groups on the same ligand template. Hydrogen bonding and burial of hydrocarbon in adjacent sites produce an enhancement of electrostatic binding of the carboxylate group. The data provide experimental evidence for an enthalpic contribution to the chelate effect that is distinct from, and works in addition to, the classic entropic chelate effect. The correlation between amide NH chemical shift and overall binding energy has been used to show binding affinity for eremomycin and chloroeremomcin by di-N-Ac-Lys-D-Ala-D-Lac (Lac = lactate), which is a cell wall analogue of bacteria which exhibit vancomycin resistance. Binding constants for this ligand have also been determined by UV difference spectrophotometry (70 dm3 mol–1 and 240 dm3 mol–1 respectively).

Tetraaqua(3,4-toluenediamine-N,N,N',N'-tetraacetato)dizinc(II)–Water (1/2)

The simplest unit of the title compound, [Zn 2 (C 15 H 14 N 2 O 8 )(H 2 O) 4 ].2H 2 O, is a dinuclear Zn II complex bridged by the O atoms of one carboxylate moiety of the 3,4-toluenediamine-N,N,N',N'-tetraacetate ligand (3,4-tdta). The coordination geometry of the zinc-3,4-tdta moiety is close to trigonal prismatic, whereas the other zinc ion is in a nearly O h environment comprising two O atoms from carboxylate groups of different ligands and four water molecules. The other two water molecules are waters of crystallization in the polynuclear complex [Zn-μ-(3,4-tdta)-Zn(H 2 O) 4 ] n .2H 2 O n .

Synthesis and Characterisation of Some New Ruthenium (III) Complexes Containing a Ferrocenecarboxylate

Abstract Under aerobic conditions, the ruthenium (II) triphenylphosphine complex RuCl2(PPh3)3 reacts with a few ferrocene carboxylic acids in a mixture of THF and benzene to afford ruthenium(III) (carboxylato) complexes of the type, RuCl2(O2CR)(PPh3)2, where R = alkyl or aryl derivatives of bis (cyclopentadienyl) iron, the title complexes have been confirmed through element analytical, infrared and ESR spectral studies. Infrared spectra of the complexes suggest that the carboxylate groups of ligands coordinate to ruthenium (III) through the two oxygen donor atoms.

Chromium(III) complexes ofd(−)tartaric andl(−) mandelic acids

The binary complexes of anhydrous chromium(III) chloride withd(−) tartaric acidl(−) mandelic acids have been characterized by elemental analyses, magnetic susceptibility, vibrational, electronic and circular dichroism spectra. The magnetic susceptibility data are close to the spin only value for a d3 chromium(III) ion. Three (Cr−Cl) vibrational modes in the region 420–290 cm−1 are observed for the formed complexes indicatingC2 local symmetry of ligand atoms around the chromium(III) rather thanC3, which would allow two modes. In the visible spectra, two peaks in the 21052–22222 and 15384–16129 cm−1 range are observed and are assigned to the4A2g→4T1g(F) and4A2g→4T2g transitions. The parameters (Dq, B,β35) place the ligands in the higher end of the spectrochemical series and provide reassurance that the hydroxy acid oxygen complexes to chromium(III) ion. The Cotton effects observed in the spin-forbidden band are assigned to the2E(2Eg),2A2(2T1g) and2E(2T1g), while that in the spin-allowed band are a results of the splitting of the4A2g(4T2g) to4A1(4T2g) and4E(4T2g) transitions. The tartaric acid chelates are likely to befac in terms of ligand carboxylate and/or hydroxy groups since stronger and better defined Cotton effects are observed while mandelic acid chelates are weak suggesting formation of themer structure.

The first structurally characterized silver/lanthanide heterometallic complex; synthesis and crystal structure of Ag[Gd(dipic) 2(H 2O) 3]·3H 2O (where H 2dipic = 2,6- pyridinedicarboxylic acid)

Reaction of the silver(I) complex Ag(Hdipic)(H 2dipic)·H 2O, where H 2dipic = 2,6-pyridinedicarboxylic acid, with gadolinium(III) nitrate in aqueous methanol gave the heterometallic complex Ag[Gd(dipic) 2(H 2O) 3]·3H 2O ( 1), which has been characterized by X-ray diffraction. Each gadolinium atom is nine-coordinate and has two tridentate dipic 2− ions and three water molecules bonded in a tricapped trigonal prismatic geometry. The silver atoms bind to the carboxylate groups of both dipic ligands forming polymeric Gd-carboxyl-Ag-carboxyl-Gd chains.

Nuclear magnetic resonance studies of diamagnetic metal-diethylenetriaminepentaacetate complexes

The 'H NMR and I3C NMR spectra of the alkaline earth and diamagnetic rare earth metal complexes of DTPA are discussed in terms of the labilities of the metal-oxygen and metal-nitrogen bonds. The various resonance splitting patterns observed in the alkaline earth spectra are correlated with the patterns predicted for possible hexadentate structures involving all three nitrogen donors and averaging for the other three coordination sites by four of the ligand carboxylate groups. A heptadentate structure is proposed for the complexes of DTPA with the lanthanides in which the unbound ligand group is the lone acetate group of the middle nitrogen.

The complexation of lanthanides by aminocarboxylate ligands—II: Thermodynamic parameters

The values of the free energy, enthalpy and entropy of complexation are reported for a series of aminocarboxylate ligands. For all the systems, a region of “compensation” between ΔH1, and ΔS1 exists between Nd(III) and Ho(III) which is related to dehydration effects. The entropies are a function of the number of ligand carboxylate groups coordinated. Correction of the enthalpy for the coordination of the carboxylate groups gives a residual enthalpy which is a linear function of the basicity of the nitrogen groups. In the HEDTA complexes, the data are consistent with outer sphere coordination of the alcohol group in the light lanthanides and with inner sphere coordination in the heavy members.