Coordination and ligand exchange dynamics of solvated metal ions

Recent developments in computer speed and capacity have opened the access to highly accurate molecular dynamics simulations based on quantum mechanically calculated forces for the chemically relevant region around ions in solution (QM/MM formalism). This accuracy, although still extremely consuming (30-300 days of CP time per simulation), is needed for reliable structural details and ligand exchange rates. A large number of main group and transition metal ions have been investigated by this approach, giving very detailed insight into the properties of these ions in solution and allowing to classify the ions by various characteristics. Most first-row transition metal ions have a very stable first hexa-coordinated solvation shell, whose vibrational distortions, however, strongly influence the dynamics of the second shell. The dynamical Jahn-Teller effect - shown to be a femto- and picosecond phenomenon - can strongly influence ligand coordination and exchange dynamics. A large number of ions with very labile solvation shell such as most main group ions, but also transition metal ions, e.g. Ag(I) and Hg(II), can change their coordination within the picosecond scale, leading to an almost simultaneous presence of several species hardly accessible by present experimental techniques. Among these ions, the structure breakers are of particular interest, and it could be shown that there are two types of them, one with a large and very labile first coordination shell such as Cs(I), the other characterised by a small first but an unusually large second solvation shell such as Au(I). Investigations of metal ions coordinated to ammonia ligands have shown that coordination to hetero-atoms can accelerate the ligand exchange reaction rates by several orders of magnitude, e.g. for Cu(II) and Ni(II). Simulations of ions in aqueous ammonia gave a very detailed picture of the complexity of species almost simultaneously present and illustrate the enormous difficulties encountered when trying to fit X-ray or neutron diffraction data for such systems. In general, ligand exchange rates situated in the picosecond range are far below the NMR scale, and as femtosecond laser pulse spectroscopy could not be applied so far to ionic solutions, accurate simulations have become a very important tool to access structure and dynamics of solvated ions. A number of VIDEO clips supplied on the Web as supporting material illustrates the processes occurring in solutions of the metal ions.     

Coordination polymers undergoing spin crossover and reversible ligand exchange in the solid

Here we report the synthesis and characterisation of a polymer made up of a system of parallel 2-D grids of Fe(II) ions linked by [Au(CN)2]- bridges and its transformation into a new system of three interpenetrated 3-D coordination open frameworks with the NbO topology. Reversibility of this crystal-to-crystal transformation is evidenced by X-ray crystallographic data and from their spin crossover properties.     

Role of the ligand density in cation exchange materials for the purification of proteins

The performance of functionalized materials, such as cation exchange resins, is dependent not only on the ligand type and ligand density, but also on the pore accessibility of the target molecule. In the case of large molecules such as antibodies this latter parameter becomes crucial, because the size of such molecules falls somewhere inside the pore size distribution of the resin. The influence of the ligand density and accessibility on the overall performance of the material is explored systematically. Five different materials, having the same chemistry as the strong cation exchange resin Fractogel EMD SO₃⁻ (M) , have been analyzed. These materials only differ in the ligand density. It is shown that the ligand density directly influences the porosity of the materials as well as the pore diffusivity and the dynamic binding capacity. For a given purification problem an optimal ligand density can be found. Based on the above results a new material is proposed, showing superior properties in terms of dynamic binding capacity. This is achieved by an optimization of the ligand density and by a decrease of the particle size of the stationary phase. The material properties are modeled with a general rate model. Further simulations were conducted to evaluate the performance of the new material in comparison with a conventional resin.     

Kinetics of ligand exchange in uranyl complexes in non-aqueous solvents

Stopped flow and T-jump experiments were carried out to study the kinetics of the reactions leading to the formation of complexes of the forms UPL and UPL₂ where U stands for uranyl, P for TBP and L for 8-hydroxyquinoline or diphenylcarbazone. The experimental results, for CCl₄ and CH₂ClCH₂Cl solutions, provided considerable information on the mechanisms of these reactions as well as numerical values for some of the rate constants and activation energies and entropies.     

Catalysis of ligand exchange in uranyl complexes in non-aqueous solvents

Catalysis of the exchange reaction between TBP and diphenyl carbazone, as ligands bound to uranyl, as well as catalysis of exchange between bound and free TBP was studied in a number of solvents. Catalysts were alcohols and water. A mechanism for the catalytic action is proposed and rate constants are reported. The implication of the results to the mechanism of uranium transfer through solvent membranes is discussed.     

The use of ligand exchange for the separation of aromatic acids on synthetic cation exchangers

Summary A very good separation of the mixture of eight aromatic acids has been obtained by ion-exchange chromatography using the ligand-exchange technique, on a column containing a synthetic cation exchanger and trivalent metal ion (Al³⁺, Fe³⁺, or Ce³⁺).     

Coordination chemistry and reactivity of zinc complexes supported by a phosphido pincer ligand

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P?NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.     

Structures and ligand exchange of N-confused porphyrin dimer complexes with group 12 metals

N-confused 5,20-diphenylporphyrin (NCDPP, 1) formed 2:2 dimer complexes with group 12 metals both in the solid state and in solution. X-ray single-crystal analyses of the Zn(II) and Cd(II) complexes (7, 8) revealed that each metal ion is coordinated with three inner core nitrogens and a peripheral nitrogen of the other NCDPP in the pair. In the (1)H NMR spectra of 7, 8, and the Hg(II) complex (9), the outer alpha-H signals of the confused pyrrole ring appeared in the upfield region at 2.57, 3.44, and 3.60 ppm, respectively, due to the ring current effect by the coordinated porphyrins. In the case of the Cd(II) and Hg(II) complexes (8, 9), additional magnetic couplings with the metal nuclei of the partner rings were observed. The equilibrium constants (K) of the monomer exchange reaction at 25 degrees C were determined to be 2.5, 1.3, and 0.6 for the (Zn-Cd), (Cd-Hg), and (Zn-Hg) heterodimer complexes, respectively, from the (1)H NMR spectra of a solution containing two different dimers. Furthermore, a metal-transfer reaction from a Zn(II) NCP dimer complex to the free base porphyrin was demonstrated.     

Iodo-methyl ligand exchange reaction in platinum complexes: A density functional study

A theoretical investigation at the gradient-corrected density functional (BP86) level of theory on the iodo-methyl ligand exchange reaction in platinum-diphosphine complexes is discussed. The reaction consists of two elementary steps: the oxidative addition of methyl-iodide, and reductive elimination of ethane from the intermediate Pt(bdpp)(CH₃)₃I complex which is the rate determining step with a free energy of activation of 19.5 kcal/mol in acetonitrile phase. The oxidative addition step takes place with S_N2 mechanism via a transition state with a collinear arrangement of the I-CH₃-Pt moiety.     

Separation of amines by ligand exchange : Part III. A comparison of different cation exchangers

The elution behavior of 4 amines, ethanolamine, diethanolanine, dimethylamine and n-butylamine, was studied on 4 cation-exchange resins and zirconium phosphate, all loaded with nickel ions. Aqueous ammonia was used for clution. Different selectivity orders were found with exchangers of different types. The carboxylic cation-exchange resin gave the sharpest bands, but 2% cross-linked sulfonic resin gave the best separation of these amines.     

Investigation of ligand exchange reactions in aqueous uranyl carbonate complexes using computational approaches

Carbonate anion exchange reactions with water in the uranyl-carbonate and calcium-uranyl-carbonate aqueous systems have been investigated using computational methods. Classical molecular dynamics (MD) simulations with the umbrella sampling technique were employed to determine potentials of mean force for the exchange reactions of water and carbonate. The presence of calcium counter-ions is predicted to increase the stability of the uranyl-carbonate species in accordance with previous experimental observations. However, the free energy barrier to carbonate exchange with water is found to be comparable both in the presence and absence of calcium cations. Possible implications of these results for uranyl adsorption on mineral surfaces are discussed. Density functional theory (DFT) calculations were also used to confirm the trends observed in classical molecular dynamics simulations and to corroborate the validity of the potential parameters employed in the MD scheme.     

HPLC enantioseparation of racemic bupropion,baclofen and etodolac: modification of conventional ligand exchange approach by pre‐column formation of chiral ligand exchange complexes

Separation of racemic mixture of (RS)‐bupropion, (RS)‐baclofen and (RS)‐etodolac, commonly marketed racemic drugs, has been achieved by modifying the conventional ligand exchange approach. The Cu(II) complexes were first prepared with a few l ‐amino acids, namely, l ‐proline, l ‐histidine, l ‐phenylalanine and l ‐tryptophan, and to these was introduced a mixture of the enantiomer pair of (RS)‐bupropion, or (RS)‐baclofen or (RS)‐etodolac. As a result, formation of a pair of diastereomeric complexes occurred by ‘chiral ligand exchange’ via the competition between the chelating l ‐amino acid and each of the two enantiomers from a given pair. The diastereomeric mixture formed in the pre‐column process was loaded onto HPLC column. Thus, both the phases during chromatographic separation process were achiral (i.e. neither the stationary phase had any chiral structural feature of its own nor did the mobile phase have any chiral additive). Separation of diastereomers was successful using a C₁₈ column and a binary mixture of MeCN and TEAP buffer of pH 4.0 (60:40, v/v) as mobile phase at a flow rate of 1 mL/min and UV detection at 230 nm for (RS)‐Bup, 220 nm for (RS)‐Bac and 223 nm for (RS)‐Etd. Baseline separation of the two enantiomers was obtained with a resolution of 6.63 in <15 min. Copyright © 2016 John Wiley & Sons, Ltd.     

Coordination of 2,2'-bipyridyl and 1,10-phenanthroline to yttrium and lanthanum complexes based on a scorpionate ligand

Reaction of yttrium and lanthanum trichloride with 1 equiv of sodium or potassium hydrotris(3,5-dimethylpyrazolyl)borate and 1 equiv of 2,2'-bipyridine gives good yields of the complexes [MCl(2)(Tp(Me2))(C(10)H(8)N(2))] (M = Y (1), La (2)). The analogous compounds with 1,10-phenanthroline, [MCl(2)(Tp(Me2))(C(12)H(8)N(2))] (M = Y (3), La (4)), have been obtained by a similar procedure. The solid-state structures of 2-4 were determined by single-crystal X-ray diffraction and revealed that the compounds are all seven-coordinate with capped octahedral geometry. In contrast, reaction of yttrium trichloride with 1 equiv of sodium hydrotris(3,5-dimethylpyrazolyl)borate in the presence of 1 equiv of neocuproine affords [YCl(3)(Tp(Me2))][Na(neoc)(3))] (5). Compounds 1 and 2 provide an entry for the synthesis of complexes containing the bipyridyl ligand in a radical anionic form or in a dianionic form. Reaction of 1 and 2 with an excess of sodium amalgam gives [Y(Tp(Me2))(bipy)(THF)(2)] (6) and [La(Tp(Me2))(2)(bipy)] (7), respectively. The structures of both compounds have been determined by X-ray crystallography. Compound 7 can be oxidized with iodine to give [La(Tp(Me2))(2)(bipy)]I (8).     

Tuning tetranuclear manganese-oxo core electronic properties: adamantane-shaped complexes synthesized by ligand exchange

A series of adamantane-shaped [Mn4O6]4+ aggregates has been prepared. Ligand substitution reactions of [Mn4O6(bpea)4](ClO4)4 (1) with tridentate amine and iminodicarboxylate ligands in acetonitrile affords derivative clusters [Mn4O6(tacn)4](ClO4)4 (4), [Mn4O6(bpea)2(dien)2](ClO4)4)(5), [Mn4O6(Medien)4](ClO4)4 (6), [Mn4O6(tach)4](ClO4)4 (7), [Mn4O6(bpea)2(me-ida)2] (8), [Mn4O6(bpea)2(bz-ida)2] (9), [Mn4O6(bpea)2((t)bu-ida)2] (10), and [Mn4O6(bpea)2((c)pent-ida)2] (11) generally on the order of 10 min with retention of core nuclearity and oxidation state. Of these complexes, only 4 had been synthesized previously. Characterization of two members of this series by X-ray crystallography reveals that compound 7 crystallizes as [Mn4O6(tach)4](ClO4)4 x 3CH3CN x 4.5H2O in the cubic space group Fmm and compound 11 crystallizes as [Mn4O6(bpea)2((c)pent-ida)2].7MeOH in the monoclinic space group C2/c. The unique substitution chemistry of 1 with iminodicarboxylate ligands afforded asymmetrically ligated complexes 8-11, the mixed ligand nature of which is most likely unachievable using self-assembly synthetic methods. A special feature of the iminodicarboxylate ligand complexes 8-11 is the substantial site differentiation of the oxo bridges of the [Mn4O6]4+ cores. While there are four site-differentiated oxo bridges in 8, the solution structural symmetry of 8H+ reveals essentially a single protonation isomer, in contrast to the observation of two protonation isomers for 1H+, one for each of the site-differentiated oxo bridges in 1. Magnetic susceptibility measurements on 4, 7, 8, and 9 indicate that each complex is overall ferromagnetically coupled, and variable-field magnetization data for 7 and 9 are consistent with an S = 6 ground state. Electrochemical analysis demonstrates that ligand substitution of bpea affords accessibility to the Mn(V)(Mn(IV))3 oxidation state.     

The gas-phase ligand exchange reactions of cobalt and zinc acetylacetonate, hexafluoroacetylacetonate, and trifluorotrimethylacetylacetonate complexes

The gas-phase ligand exchange reactions between Co(II) and Zn(II) complexes containing the acetylacetonate (acac), hexafluoroacetylacetonate (hfac), and trifluorotrimethylacetylacetonate (tftm) ligands were investigated using a triple quadrupole mass spectrometer. The gas-phase mixed ligand products of [Cu(acac)(tftm)](+), [Ni(acac)(tftm)](+), [Cu(hfac)(tftm)](+), and [Ni(hfac)(tftm)](+) were formed following the co-sublimation of either homo-metal or hetero-metal precursors and are reported herein for the first time. The fragmentation patterns of these mixed ligand species along with those of Cu(tftm)(2) and Ni(tftm)(2) are also presented. The collision cell of the instrument was utilized to examine the gas-phase reactions between mass-selected ions and specific neutral target compounds.     

Effect of electronic structure on the photoinduced ligand exchange of Ru(II) polypyridine complexes

The series of complexes [Ru(bpy)(2)(L)](2+), where bpy = 2,2'-bipyridine and L = 3,6-dithiaoctane (bete, 1), 1,2-bis(phenylthio)ethane (bpte, 2), ethylenediamine (en, 3), and 1,2-dianilinoethane (dae, 4), were synthesized, and their photochemistry was investigated. Photolysis experiments show that the bisthioether ligands in 1 and 2 are more easily photosubstituted by chloride ions, bpy, and H(2)O than the corresponding diammine complexes in 3 and 4 to generate the bis-substituted products. Electronic structure calculations show that bond elongation in the lowest energy triplet metal-to-ligand charge transfer ((3)MLCT) state compared to the ground state is greater for complexes containing bisthioether ligands than those with coordinated bidentate nitrogen atoms. This elongation in the excited state is attributed to Ru-S π-bonding character of the highest occupied molecular orbitals, which is not present in the diamine complexes. In the Ru→bpy (3)MLCT state, the lower electron density on the metal-centered highest occupied molecular orbital (HOMO) weakens the Ru-S bond and results in the greater photoreactivity of 1 and 2 relative to that of 3 and 4. The more efficient photoinduced ligand exchange of the complexes possessing thioether ligands results in binding of 1 and 2 to DNA upon irradiation.     

Coordination diversity in palladium(II)-picolinamide ligand complexes: structural and quantum chemical studies

The separation of different metal ions can be successfully accomplished by using picolinamide-based ligands. We herein report the first X-ray structure of picolinamide-based ligands of the type C₅H₄NCONR₂ (where R=ⁱC₃H₇ (L1) and ⁱC₄H₉ (L2)) and C₅H₄NCONHR (R=^tC₄H₉ (L3)) with palladium(II) ion. We have synthesized and characterized the structures of two palladium complexes, [PdCl₂(L1)₂] (1) and [PdCl₂L3] (3). In 1, ligand L1 forms a 2?:?1 complex with palladium(II) chloride, whereas in 3, the ligand L3 forms a 1?:?1 complex. Further, in 1, the ligand L1 acts as a monodentate ligand and is bound only through pyridyl-N atom, whereas in 3, the ligand L3 acts as a bidentate chelating ligand and is bound through both the pyridyl-N and amido-O atoms to the Pd(II) center. Electronic structure calculations are carried out to understand the experimental coordination diversity in the Pd complexes. Our calculations clearly suggest that a combination of steric hindrance of the ligand and the electronic effect of metal ions may modulate the coordination preferences.     

Spectroscopic and electrochemical characterization of an aqua ligand exchange and oxidatively induced deprotonation in diiron complexes

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl(2)) with Fe(H(2)O)(6)(ClO(4))(2) in the presence of disodium m-phenylenedipropionate (Na(2)(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe(2)(L-Bn)(mpdp)(H(2)O)](ClO(4))(2) and [Fe(2)(L-BnCl(2))(mpdp)(CH(3)OH)](ClO(4))(2), respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, alpha = 90 degrees, beta = 94.471(2) degrees, gamma = 90 degrees, V = 5202.6(9) A(3), and Z = 4. The structure of the compound is very similar to that of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm(-)(1)). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, M?ssbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by M?ssbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe(2)(L-Bn)(mpdp)(H(2)O)](2+) shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by M?ssbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q) >or= 1.0 mm.s(-)(1)) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.