Kinetics Of Ligand Exchange Reactions Research Articles

Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation.

Polyaluminum cations, such as the MAl12 Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al3+, Ga3+, and Ge4+ have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three ε-analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl12 species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl12 nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al3+-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl12 species, such as FeAl12 7+. Based upon these results, we evaluated the Al3+/Zn2+/Cr3+ system and determined that substitution of Cr3+ is unfavorable in the heteroatom site but is preferred for Zn2+, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr3+-substituted δ-Keggin species where Cr3+ substitution occurs only in the octahedral positions. The isolated structures Na[AlO4Al9.6Cr2.4(OH)24(H2O)12](2,6-NDS)4(H2O)22 (δ-CrnAl13-n-1) and Na[AlO4Al9.5Cr2.5(OH)24(H2O)12](2,7-NDS)4(H2O)18.5 (δ-CrnAl13-n-2) are the first pieces of evidence of mixed Al3+/Cr3+ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-CrnAl13-n-2 structure also exhibits a unique placement of the bound Na+ cation, which may indicate that Cr3+ substitution can alter the surface reactivity of Keggin-type species.

Determination of Labile Fe(II) Species Complexed with Seawater Extractable Organic Matter Under Seawater Conditions Based on the Kinetics of Ligand-exchange Reactions with Ferrozine

A fertilizer, comprised of a mixture of steel slag and compost, was used to restore seaweed beds in barren coastal areas. Complex Fe(II) species, supplied by steel slag, play a significant role in supplying Fe(II) to coastal areas and stimulating seaweed growth. Seawater extractable organic matter (SWEOM) from compost is generally assumed to serve as a chelator of Fe(II) in the fertilizer. It is considered that the bioavailability of Fe(II)-SWEOM complexes is higher in the dissociable (labile) species. In the present study, a method for determining labile species of Fe(II)-SWEOM complexes in seawater (pH 8.0, I = 0.7) was developed. The method is based on a ligand-exchange reaction between SWEOM and ferrozine (FZ). Because Fe(II) is readily oxidized to Fe(III) under normal seawater conditions, ascorbic acid was added as an antioxidant. The coloring for the Fe-FZ complex in the presence of SWEOM was retarded. This retarding can be attributed to a ligand-exchange reaction between FZ and labile Fe(II)-SWEOM complexes. Conditional binding constants for the labile Fe(II)-SWEOM complexes and binding capacities of labile sites in SWEOM to Fe(II) were evaluated for a variety of total Fe(II) concentrations.

Kinetics of the Exchange Reactions between Gd(DTPA)2−, Gd(BOPTA)2−, and Gd(DTPA-BMA) Complexes, Used As MRI Contrast Agents, and the Triethylenetetraamine-Hexaacetate Ligand

The kinetics of ligand exchange reactions occurring between the Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) complexes, used as contrast agents in MRI, and the ligand TTHA, have been studied in the pH range 6.5-11.0 by measuring the water proton relaxation rates at 25 °C in 0.15 M NaCl. The rates of the reactions are directly proportional to the concentration of TTHA, indicating that the reactions take place with the direct attack of the H(i)TTHA((6-i)-) (i = 0, 1, 2 and 3) species on the Gd(3+) complexes, through the formation of ternary intermediates. The rates of the exchange reactions of the neutral Gd(DTPA-BMA) increase when the pH is increased from 6.5 to 9, because the less protonated H(i)TTHA((6-i)-) species can more efficiently attack the Gd(3+) complex. The rates of the exchange reactions of [Gd(DTPA)](2-) and [Gd(BOPTA)](2-) also increase from pH 8.5 to 11, but from 6.5 to 8.5 an unexpected decrease was observed in the reaction rates. The decrease has been interpreted by assuming the validity of general acid catalysis. The protons from the H(i)TTHA((6-i)-) species (i = 2 and 3) can be transferred to the coordinated DTPA or BOPTA in the ternary intermediates when the dissociation of the Gd(3+) complexes occurs faster. The kinetic inertness of Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) differs very considerably; the rates of the ligand exchange reactions of Gd(DTPA-BMA), thus the rates of its dissociation, are 2 to 3 orders of magnitude higher than those of Gd(DTPA) and Gd(BOPTA). The rates of the ligand exchange reactions increase with increasing concentration of the endogenous citrate, phosphate, or carbonate ions at a pH of 7.4, but the effect of citrate and phosphate is negligible at their physiological concentrations. The increase in the reaction rates at the physiological concentration of the carbonate ion is significant (20-60%), and the effect is the largest for the Gd(DTPA-BMA) complex.

Investigation of the kinetics of aquation of the 1:2 complex between Cr III and nitrilotriacetic acid

Aquation of the 1:2 complex between Cr III and nitrilotriacetic acid (NTA) was monitored using a combination of capillary electrophoresis (CE), ultraviolet–visible (UV–vis) spectrophotometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. To our knowledge, this is the first published report of the use of either CE or ATR-FTIR to monitor the kinetics of ligand exchange reactions of Cr III–aminocarboxylate complexes. The aquation products were identified as the 1:1 Cr III complex with NTA and “free” NTA. The 1:1 complex dimerized to form a 2:2 complex in a slower subsequent reaction step. Rates of disappearance of the 1:2 complex were first-order under all experimental conditions. First-order rate constants for aquation, k obs (h −1), measured using all three techniques were similar at equivalent pH conditions, and with values reported previously in the literature. Measured k obs values exhibited a complicated pH dependence with three distinct regions: (i) at pH < 6.5, k obs values increased with decreasing pH, (ii) between pH 6.5 and 8.0, k obs values were relatively constant, and (iii) at 8.0 < pH < 10.0, k obs increased with increasing pH and then leveled off. A kinetic model incorporating five distinct aquation pathways was successfully employed to model the pH dependence of k obs from 0.0 < pH < 10.0. These results show that CE and ATR-FTIR can be used as tools for better understanding ligand exchange processes occurring in aqueous solution.

High pressure NMR studies on ligand exchange reactions in the equatorial sites of [VO(L) 5] 2+ (L=acetonitrile, N, N-dimethylformamide, and N, N-dimethylacetamide)

Kinetics of ligand exchange reactions in the equatorial sites of [VO(L) 5] 2+ [L=acetonitrile, N, N-dimethylformamide (DMF), and N, N-dimethylacetamide (DMA)] have been studied by using high pressure NMR method. The values of activation volume (Δ V ≠, cm 3 mol −1) are −1.0±0.6 for [VO(CH 3CN) 5] 2+, −6.8±0.4 for [VO(DMF) 5] 2+, and −9.7±0.9 for [VO(DMA) 5] 2+. On the basis of the Δ V ≠ values obtained in the present and previous studies, the mechanism for the ligand exchange reactions in the equatorial sites of [VO(L) 5] 2+ (L=unidentate oxygen or nitrogen donor ligands) was discussed.

Molecular Structures of Zinc Complexes with Bisbenzimidazole Ligands in Crystals and the Kinetics of Ligand-Exchange Reactions in Their Solutions

Abstract Zinc complexes with bisbenzimidazole, [ZnL(PhCOO)2] 1, [ZnLCl2] 2, and [ZnL2]2+ 3, have been synthesized, where L = 2,2′-thiodimethylenebis(benzimidazole). Their molecular structures were determined based on X-ray crystal structure and extended X-ray absorption fine structure (EXAFS) analyses. The molecular structure of 1·1.5DMF shows a distorted tetrahedral geometry around Zn(II) coordinated by each nitrogen atom of two benzimidazolyl groups and each oxygen atom of two benzoate ions. In solutions of 2 or 3·(ClO4)2 the same chemical species, [ZnLCl2] or [ZnL2]2+, exist as in the coordination structure around zinc in each crystal. The combination of 13C NMR data with the most probable molecular structures of zinc complexes in 1, 2, and 3, determined by X-ray crystal structure and EXAFS analyses, allowed us to assign the two different chemical species, [ZnL(PhCOO)2] and [ZnL2]2+, as the major and minor components, respectively, in a solution of 1. A kinetic study of the ligand-exchange reaction between PhCOO− and L was provided us with the lifetimes of the chemical species of [ZnL(PhCOO)2] and [ZnL2]2+: 66 ms and 10 ms at −10 °C, respectively. The major species produced by a zinc-assisted supramolecular process in solution is the same type of chemical species [ZnL(PhCOO)2] as in the coordination structure around zinc in the crystal of 1·1.5DMF. The ESI mass spectra of a solution of 1, 2, and 3·(ClO4)2 supported the chemical species of [ZnL(PhCOO)2], [ZnL2]2+, and [ZnLCl2] in solution.

Kinetics of ligand exchange reaction of Cu(OH) 2-poly(vinyl alcohol) complex with potassium cyanide

The kinetics of the ligand exchange reaction between the green complex of Cu(OH) 2 included in poly(vinyl alcohol) (PVA) and potassium cyanide (CN −) was studied by a stopped-flow method at pH 11.6–12.60, at ionic strength μ = 0.1 (KCl) and at 25°C. The Cu(OH) 2 is believed to be of type Cu n (OH) 2+ 2 n − 2 . Therefore the polynuclear complex of the Cu n (OH) 2+ 2 n − 2 included in helical PVA chains can be expressed as PVA-Cu n (OH) 2+ 2 n − 2 . The rate law, written as: r = − d[ PVA− C n( OH) 2+ 2n−2] dt can be explained by four different reaction paths. In paths 1 and 2, the reactions are initiated by attack of H + on the polynuclear copper complex Cu n (OH) 2+ 2 n − 2 included inside the PVA helical chains. In path 1, the intermediate, Cu n (OH) 3+ 2 n − 3 is formed in the bulk water. However, in path 2, the intermediate, PVA-Cu n (OH) 3+ 2 n − 2 (H 2O), is formed inside the PVA helix. In the highly alkaline region, paths 3 and 4 involve the ligand substitution reaction. In path 3, the Cu n (OH) 2+ 2 n − 2 is believed to be released from PVA helical chain and combines with CN − in the exterior bulk water. In path 4, the rate-determining step is the bimolecular ligand exchange reaction between PVA-Cu n (OH) 2+ 2 n − 2 and CN −. A possible mechanism for the substitution reaction is discussed.

A nuclear magnetic resonance study of the kinetics of ligand exchange reactions in uranyl complexes. VII. Acetylacetonate exchange in bis(acetylacetonato)( N,N-dimethylformamide)dioxouranium(VI)

The exchange reaction of acac(acetylacetonate) in UO 2(acac) 2dmf (dmf= N,N-dimethylformamide) in o-dichlorobenzene has been studied by the NMR line-broadening method. The exchange rate depends on the concentration of the enol isomer of acetylacetone in its low region, and approaches the limiting value in its high region. It is proposed that the exchange reaction proceeds through the mechanism in which the dissociation of one end of the chelated acac is the rate-determining step. The kinetic parameters for this step are as follows: k (25 °C)=5.03 × 10 −3 s −1, Δ H‡=91.6 ± 3.8 kJ mol −1, and Δ S‡ =17.2 ± 10.5 JK −1 mol −1. The exchange rate becomes slower by the addition of free DMF. This may be due to the competition of DMF with the enol isomer of acetylacetone in attacking a four-coordinated intermediate in the equatorial plane.

Ligand exchange reactions between copper(II)- and nickel(II)-chelates of different sulfur- and selenium-containing ligands. VI [1]. Kinetics of ligands exchange reactions studied by stopped-flow ESR

Abstract The kinetics of ligand exchange reactions studied by means of stopped-flow ESR measurements are reported. The concentrations-time curves recorded for all reactions investigated correspond ton simple second-order rate laws. The rate constants of the reactions between copper(II) dithiolenes and Cu(Et 2 dsc) 2 (Et 2 dsc = diethyldiselenocarbamate) or Ni(Et 2 -dsc) 2 were found to be: [Cu(mnt) 2 ] 2– (mnt = maleo-nitriledithiolate) + Cu(Et 2 dsc) 2 , k 2 = (2.3 ±0.3)· 10 2 1 mol −1 s −1 ; [Cu(mnt) 2 ] 2− + Ni(Et 2 dsc) 2 , k 2 = (2.1 ± 0.1)· 10 2 1 mol −1 s −1 and [Cu(dmit) 2 ] 2− (dmit = isotrithione-3.4-dithiolate) + Ni(Et 2 dsc) 2 , k 2 = (2.5 ± 0.3)· 10 2 1 mol −1 s −1 . A rate constant of k 2 = (2.0 ± 0.3)· 10. 2 1 mol −1 s −1 was determined for the reaction between (Cu(Et 2 dsc) 2 and Cu(Et 2 -dtc) 2 (Et 2 dtc = diethyldithiocarbamate). Additional experiments suggest a chain mechanism for the ligand exchange reactions in which the mono-chelates e.g. [Cu(mnt)] s react as active sites.

Elastomers from Polymeric Chelates

Abstract In the previous work, we demonstrated that gels may be formed from polymeric acetylacetonate chelates. Now it has been shown that useful elastomers can be prepared from these derivatized polymers as well. Stress-strain measurements have been performed on these Pd(II) crosslinked SIR elastomers, and the results demonstrate that the materials are useful for physical testing. Because of unforeseen complications, we do not yet have a perfectly ideal system to solve fully the problems mentioned in the Introduction. The observations reported here are more than sufficient to encourage us to pursue this research vigorously. The difficult synthesis of ACAC-endcapped PDMS that has been achieved provides a viable route to model networks. What is required now is that we focus attention on the inorganic chemistry in an effort to better understand such things as the kinetics of ligand exchange reactions and the structure of ACAC complexes in low polarity media.

A Nuclear Magnetic Resonance Study of the Kinetics of Ligand Exchange Reactions in Uranyl Complexes. VI. N,N-Dimethylformamide Exchange in Bis(acetylacetonato) (N,N-dimethylformamide)dioxouranium(VI)

Abstract The exchange reaction of N,N-dimethylformamide(dmf) in [UO2(acac)2dmf](acac=acetylacetonato) has been studied by the NMR line-broadening method in CD3COCD3 and CD2Cl2. In CD3COCD3, the rate of dmf exchange is independent of the free DMF concentration. The rate constants(s−1) at 25 °C and activation parameters ΔH\eweq(kJ mol−1) and ΔS\eweq(JK−1 mol−1) are 1.19×103, 42.0±1.3, and −45.8±5.5, respectively. In CD2Cl2, the exchange rate depends on the free DMF concentration. The exchange rate constant (kex) is expressed by kex=k1+k2[DMF], where k1(25 °C)=1.90×102s−1, ΔH\eweq=32.8±1.7 kJ mol−1, and ΔS\eweq=−92.0±5.9 JK−1 mol−1, and k2 (25 °C)=5.66×103 M−1s−1(1M=1 mol dm−3), ΔH\eweq=37.0±2.1 kJ mol−1, and ΔS\eweq=−49.6±8.8 JK−1 mol−1. The D and I mechanisms are proposed for the dmf exchange in [UO2(acac)2 dmf].

Kinetics of ligand-exchange reactions of macrobicyclic cryptands

The rate constants for exchange reactions between metal cryptate complexes, MCryn+1, and a free cryptand, Cry2, have been determined in several solvents. Two mechanisms could be distinguished: a reaction proceeding via the free metal cation following dissociation of MCryn+1, and a direct, bimolecular cation exchange between the two ligands. In solvents such as water, dimethylsulphoxide and dimethylformamide, which interact strongly with cations, the former pathway predominates for alkali-metal and alkaline-earth-metal cryptates. In poorly solvating media such as methanol and propylene carbonate, where dissociation rate constants are very low, bimolecular pathways make an important contribution to the overall exchange pathway. Exchange reactions of Ag+ cryptates show ligand-dependent rates and strong saturation effects in both methanol and dimethylsulphoxide.

Kinetics of Ligand Exchange Reaction of Cu(II)–Poly(vinyl alcohol Complex with Ethylenedianune-N,N,N′,N′-tetraacetic Acid

Kinetics of Ligand Exchange Reaction of Cu(II)–Poly(vinyl alcohol Complex with Ethylenedianune- N , N , N ′, N ′-tetraacetic Acid

A Nuclear Magnetic Resonance Study of the Kinetics of Ligand Exchange Reactions in Uranyl Complexes. III. Dimethyl Sulfoxide Exchange in Bis(β-diketonato)(dimethyl sulfoxide)dioxouranium(VI)

Abstract The exchange reactions of dimethyl sulfoxide (DMSO) with UO2(acac)2dmso and UO2(dbm)2dmso (acac=acetylacetonate, dbm=dibenzoylmethanate) have been studied by the NMR method in CD3COCD3 and CD2Cl2. In CD3COCD3, rates of DMSO exchange for these complexes are independent of the free DMSO concentrations. Rate constants (s−1) at 25 °C and activation parameters ΔH\eweq (kJ mol−1) and ΔS\eweq (J K−1 mol−1) are 2.36×102, 45.8±0.8, −46.6±2.9 for UO2(acac)2dmso, and 3.92×102, 44.5±0.4, −46.2±2.1 for UO2(dbm)2dmso, respectively. In CD2Cl2, the exchange rates are dependent on the free DMSO concentration. For UO2(acac)2dmso, rate=(kd+kIKOS[DMSO])[UO2(acac)2dmso]/(1+KOS[DMSO]), where kd(25 °C=1.02×102 s−1, ΔH\eweq=47.9±5.2 kJ mol−1, and ΔS\eweq=−46.6±11.2 J K−1 mol−1, and kI(25 °C)=1.01×103 s−1, ΔH\eweq=35.4±12.4 kJ mol−1, and ΔS\eweq=−69.7±20.5 J K−1 mol−1, and KOS(10 °C)=4.6±2.1 M−1 (M=mol dm−3). For UO2(dbm)2dmso, rate=(k1+k2[DMSO]) [UO2(dbm)2dmso], where k1(25 °C)=1.74×102 s−1, ΔH\eweq=29.6±2.2 kJ mol−1, ΔS\eweq=−104±9 J K−1 mol−1, and k2(25 °C)=1.62×103 M−1 s−1, ΔH\eweq=34.9±0.6 kJ mol−1, and ΔS\eweq=−67.2±2.4 J K−1. The D and I mechanisms are proposed for the DMSO exchange in these complexes.

Kinetics of ligand exchange reaction of Cu(II)-ammine complex with poly(vinyl alcohol) in aqueous solution

The kinetics of the ligand exchange reaction of the Cu(II)-ammine complex with poly(vinyl alcohol) (PVA) has been studied by a stopped-flow method at pH 9–10, at μ=0.1 (NH 4Cl) and at 25°C. The reaction is initiated by the formation of unstable [Cu(NH 3) 3] 2+ by the attack of H + on Cu(II)-ammine complex, and proceeds through the mixed complex {[Cu(NH 3) 3(O−PVA)] 2+}. This step may be rate-determining, followed by a rapid reaction. Finally, the Cu(II) ion is taken up by PVA. The rate is given by d[Cu(II)−PVA]/d t=k[H +]{[Cu(NH 3) 4] 2+}[PVA]/[NH 4Cl], where k=k 1 + k′ 2[H +], k 1=4.25× 10s −1 and k′ 2=5.20× 10 11l mol −1s −1.

Study of equilibria and kinetics of ligand exchange reactions of coordination compounds in solution by electron spin resonance

Abstract

Kinetics of ligand exchange and dissociation reactions of the calcium(II)-EGTA complex investigated by the NMR technique

The kinetics of the ligand exchange reactions occurring in solutions containing calcium(II) ion and ethylenebis(oxyethylenedinitrilo)-tetraacetic acid (EGTA) have been studied by computer analysis of the nuclear magnetic resonance (NMR) spectra recorded at 35°C over a wide range of ligand and metal ion concentrations, in the pH range 5.0–9.5. The acidic dissociation constants of EGTA and the formation constants of its complexes formed with the calcium(II) ion have been obtained by alkalimetric potentiometric titrations in the same experimental conditions as the kinetic analysis. The experimental results are consistent with the hypothesis that the ligand exchange may occur either by dissociation reactions (with a first order dependance on the complex concentration and a zero order dependance on the incoming ligand concentration) or by symmetrical exchange reactions (with a first order dependance on both ligand and complex concentrations). Rate constant values have been obtained for the exchange reactions of the unprotonated complex with the un-, mono- and diprotonated ligand anions (20 ± 1 M −1 sec −1, 16 ± 1 M −1 sec −1, 7.0 ± 0.5 M −1 sec −1, respectively) and for the exchange reaction of the protonated complex with the diprotonated ligand anion ((9.8 ± 0.1) × 10 3 M −1 sec −1). The rate constant of the proton assisted dissociation reaction of the unprotonated CaEGTA complex has also been evaluated ((8.0 ± 0.1) × 10 6 M −1 sec −1), whereas only an upper limit has been estimated for the not catalyzed dissociation reaction of the same species. A discussion is given in which the experimental results are analyzed in order to ascertain whether EGTA may utilize its either oxygen atoms in binding to the metal ion.

Ligand exchange reactions of vanadium(III) β diketonate complexes

The kinetics of ligand exchange reactions of V(III) β diketonates have been studied. The replacement of acetylacetone by hexafluoroacetylacetone involves a rate law with both first- and second-order terms in the incoming ligand. The replacement of acetylacetone by deuteroacetyl-acetone involves terms zero-order and first-order in the incoming ligand. Both reactions are markedly catalyzed by acids and inhibited by bases. A mechanism involving a dangling ligand intermediate is suggested. The rates of ligand exchange are much less than the rates of isomerization.

Kinetics of ligand exchange reactions of copper(II) complexes

The kinetics of the reactions of the copper (II) complexes of pyridine-2-aldehyde-2′pyridylhydrazone (PAPHY) with ethylenediaminetetraacetic acid (EDTA) and triethylenetetramine (trien) have been studied at 25 °C and 0.1 ionic strength. the reaction is first order in copper complex and first order in the incoming ligand. The rate constant is pH dependent reaching a maximum at pH 7.5 and 9 for EDTA and trien respectively. The rate determining step in the reaction with trien is the loss of a water molecule from the CuPAPHY complexes. The rate determining step in the reaction with EDTA occurs after the coordination of the EDTA to the complex. The effect of deprotonation of the ligand in the copper complex on the rate of exchange of the coordinated water molecules is discussed.

Studies on the sulfur-containing chelating agents. XLV. Kinetics of ligand exchange reaction of mercury-monothiodibenzoylmethane complex with some thiol compounds by stripping solvent extraction.

The kinetics of ligand exchange reaction of mercury-monothiodibenzoylmethane complex with various thiol compounds (THIOL) is studied by stripping solvent extraction method. Mercaptoethylamine (MEA), penicillamine (Pen), cysteine (Cys), glutathione (GSH), α-mercaptopropionic acid (α-MPA), β-mercaptopropionic acid (β-MPA), N-acetylcysteine (NAC) and thiomalic acid (TMA) are used as thiol compounds. The reaction rate is determined by the measurements of the radioactivity of 203Hg in the aqueous phase. Thiol compounds were classified into two types based on their reaction rates, and in these reactions the formations of the mixed ligand complexes are considered to be involved. The observed reaction rates at physiological pH are distinctly higher in the thiols with amino group than in other thiols.