Stabilization of Copper(II) Thiosulfonate Coordination Complexes Through Cooperative Hydrogen Bonding Interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me3tren)(OH2)](ClO4)2 (Me3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO2S−, where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu−S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N−H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log Kf = 0.3−0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding

Stabilization of Copper(II) Thiosulfonate Coordination Complexes Through Cooperative Hydrogen Bonding Interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me3tren)(OH2)](ClO4)2 (Me3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO2S−, where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu−S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N−H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log Kf = 0.3−0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding

Stabilization of Copper(II) Thiosulfonate Coordination Complexes Through Cooperative Hydrogen Bonding Interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me3tren)(OH2)](ClO4)2 (Me3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO2S−, where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu−S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N−H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log Kf = 0.3−0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding

Stabilization of Copper(II) Thiosulfonate Coordination Complexes Through Cooperative Hydrogen Bonding Interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me3tren)(OH2)](ClO4)2 (Me3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO2S−, where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu−S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N−H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log Kf = 0.3−0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding

Synthesis, Characterization, and Structures of Copper(II)−Thiosulfate Complexes Incorporating Tripodal Tetraamine Ligands

The reaction of [Cu(L)(H2O)]2+ with an excess of thiosulfate in aqueous solution produces a blue to green color change indicative of thiosulfate coordination to Cu(II) [L = tren, Bz3tren, Me6tren, and Me3tren; tren = tris(2-aminoethyl)amine, Bz3tren = tris(2-benzylaminoethyl)amine, Me6tren = tris(2,2-dimethylaminoethyl)amine, and Me3tren = tris(2-methylaminoethyl)amine]. In excess thiosulfate, only [Cu(Me6tren)(H2O)]2+ promotes the oxidation of thiosulfate to polythionates. Products suitable for single-crystal X-ray diffraction analyses were obtained for three thiosulfate complexes, namely, [Cu(tren)(S2O3)]·H2O, [Cu(Bz3tren)(S2O3)]·MeOH, and (H3Me3tren)[Cu(Me3tren)(S2O3)]2(ClO4)3. Isolation of [Cu(Me6tren)(S2O3)] was prevented by its reactivity. In each complex, the copper(II) center is found in a trigonal bipyramidal (TBP) geometry consisting of four amine nitrogen atoms, with the bridgehead nitrogen in an axial position and an S-bound thiosulfate in the other axial site. Each structure exhibits H bonding (involving the amine ligand, thiosulfate, and solvent molecule, if present), forming either 2D sheets or 1D chains. The structure of [Cu(Me3tren)(MeCN)](ClO4)2 was also determined for comparison since no structures of mononuclear Cu(II)−Me3tren complexes have been reported. The thiosulfate binding constant was determined spectrophotometrically for each Cu(II)−amine complex. Three complexes yielded the highest values reported to date [Kf = (1.82 ± 0.09) × 103 M-1 for tren, (4.30 ± 0.21) × 104 M-1 for Bz3tren, and (2.13 ± 0.05) × 103 M-1 for Me3tren], while for Me6tren, the binding constant was much smaller (40 ± 10 M-1)

Kinetics and Mechanism of Hydrolysis of a Model Phosphate Diester by [Cu(Me₃tacn)(OH₂)₂]²⁺ (Me₃tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane)

The kinetics of hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) by [Cu(Me3tacn)(OH2)2]2+ has been studied by spectrophotometrical monitoring of the release of the p-nitrophenylate ion from BNPP. The reaction was followed for up to 8000 min at constant BNPP concentration (15 μM) and ionic strength (0.15 M) and variable concentration of complex (1.0−7.5 mM) and temperature (42.5−65.0 °C). Biphasic kinetic traces were observed, indicating that the complex promotes the cleavage of BNPP to NPP [(p-nitrophenyl)phosphate] and then cleavage of the latter to phosphate, the two processes differing in rate by 50−100-fold. Analysis of the more amenable cleavage of BNPP revealed that the rate of BNPP cleavage is among the highest measured for mononuclear copper(II) complexes and is slightly higher than that reported for the close analogue [Cu(iPr3tacn)(OH2)2]2+. Detailed analysis required the determination of the pKa for [Cu(Me3tacn)(OH2)2]2+ and the constant for the dimerization of the conjugate base to [(Me3tacn)Cu(OH)2Cu(Me3tacn)]2+ (Kdim). Thermodynamic parameters derived from spectrophotometric pH titration and the analysis of the kinetic data were in reasonable agreement. Second-order rate constants for cleavage of BNPP by [Cu(Me3tacn)(OH2)(OH)]+ and associated activation parameters were obtained from initial rate analysis (k = 0.065 M-1 s-1 at 50.0 °C, ΔH⧧ = 56 ± 6 kJ mol-1, ΔS⧧ = −95 ± 18 J K-1 mol-1) and biphasic kinetic analysis (k = 0.14 M-1 s-1 at 50.0 °C, ΔH⧧ = 55 ± 6 kJ mol-1, ΔS⧧ = −92 ± 20 J K-1 mol-1). The negative entropy of activation is consistent with a concerted mechanism with considerable associative character. The complex was found to catalyze the cleavage of BNPP with turnover rates of up to 1 per day. Although these turnover rates can be considered low from an application point of view, the ability of the complexes to catalyze phosphate ester cleavage is clearly demonstrated

Strong and Selective Ni(II) Extractants Based on Synergistic Mixtures of Sulfonic Acids and Bidentate N-Heterocycles

Bidentate 5,5ʹ-alkyl-3,3ʹ-bi-1H-pyrazole and 2-(5-alkyl-1H-pyrazol-3-yl)pyridine ligands, L5 and L6, have been shown to be stronger synergists for the solvent extraction of Ni(II) from sulfate solutions by dinonylnaphthalene sulfonic acid (DNNSAH) than the structurally related tridentate ligand 2,6-bis-[5-n-nonylpyrazol-3-yl]pyridine, L1, previously reported by Zhou and Pesic. The bidentate ligands are highly selective, providing the option of sequential recovery of Ni(II) and Co(II) and rejection of other metals commonly found in the liquors resulting from the acidic sulfate leaching of laterite ores. They were the strongest synergists identified in a screening carried out on 18 types of bidentate and tridentate N-heterocyclic ligands, including the recently reported 2-(2ʹ-pyridyl)imidazoles, L9−11. X-ray crystal structures of Ni(II) complexes of model ligands for L5 and L6, having t-butyl rather than long-chain alkyl groups and with 2-naphthalene sulfonate rather than DNNSA− as counteranions, show that the [Ni(L)3]2+ complexes form strong H-bonds from the pyrazolyl NH groups to the oxygen atoms of the sulfonate groups, an arrangement that will stabilize [Ni(L)3·(DNNSA)2] assemblies and shield their polar functionalities from diluent molecules of the water-immiscible phase. UV–visible spectra and mass spectrometry provide evidence for the strong synergists displacing all water molecules from the inner coordination sphere of the Ni(II) ions.</p