Ligand Substitution Reactions Research Articles

Electrochemical Proton Reductions Catalyzed by the Simpler Hexacoordinate Iron Compounds as Functional Mimics of the Active Site in [FeFe] Hydrogenase

Ligand-substitution reaction of [(μ-S2C4N2H2)Fe2(CO)6] (1) generated a normal mono-substituted diiron dithiolate derivative [(μ-S2C4N2H2)Fe2(CO)5(PMe3)] (1P) and a simpler hexacoordinate mononuclear compound [(μ-S2C4N2H2)Fe([CO)2(PMe3)2] (2) via distinct reactivity pathways under identical conditions. 1P and 2 could act as the electrochemically functional mimics of the [2Fe] sub-cluster and the distal Fe moiety of the active site of [FeFe] hydrogenase. The electrochemical investigations showed that 2 catalyzed the production of hydrogen from weak acid (acetic acid, HOAc) via two catalytic processes with an initial metal-orbital based reduction. In contrast, the electrocatalytic reaction of 2 with stronger acid (trifluoroacetic acid, TFA) occurred via an initial ligand protonation, and proceeded through different pathways that involved distinct oxidation states of the catalyst. The most striking result obtained in this study was that the hydrogen formation by 2 from TFA occurred at a relatively low overpotential as small as −0.28 V. It could be rationalized by the exclusive employment of iron(II) and iron(I) redox levels in the catalytic cycle, which was consistent with the enzymatic process. The observations might shed light on some aspects of ways by which the model compounds catalyzed the reduction of protons.

Kinetics and mechanism for ligand substitution reactions of some square-planar platinum(II) complexes: Stability and reactivity correlations

ABSTRACTA brief overview of mechanistic studies on the reactions of cis-[Pt(dap)(H2O)2]2+ (1) and cis-[Pt(pipen)(H2O)2]2+ (2) (where dap and pipen are 1,3-diaminopropane and 2-aminomethylpiperidine respectively) with 2-thiouracil (LH), a nitrogen and sulfur-donor bio molecule, is presented. All the substitution reactions have been monitored at 327(for 1) and 326 (for 2) nm respectively where the differences in abosorbances between the reactants and products are maximum. At pH 4.0, the kinetics shows two consecutive processes, i.e., they show a non linear dependence on the concentration of ligand: first process is [ligand] dependent but the second step is [ligand] independent. The activation parameters and the thermodynamic parameters were also calculated, which supports the spontaneous formation of an outer sphere association complex. The product of this reaction has been characterized with the help of Job's method, IR and ESI-MS spectroscopic analysis.The purpose of this work is to improve the understanding of the mechanism of action of Pt(II) complexes as potential anti-tumour drugs in the human body.

1700657: Impact of chloride concentration on ligand substitution reactions of zinc(II) complexes with biologically relevant nitrogen nucleophiles-ESI

1700657: Impact of chloride concentration on ligand substitution reactions of zinc(II) complexes with biologically relevant nitrogen nucleophiles-ESI

Spectroscopic Evidence for Ligand Substitution Reactions at the Solid-Liquid Interface of a Sub-micrometer Gold(I) Carbene Complex.

Colloidal coordination compounds in the sub-micrometer range were synthesized from a chloro gold(I) carbene complex via the anti-solvent procedure and stabilized by a surfactant shell of Tween 20. This compound was successfully applied as model system to monitor heterogeneous multiphase ligand substitution reactions at the solid-liquid interface.

Spectroscopic Evidence for Ligand Substitution Reactions at the Solid–Liquid Interface of a Sub‐micrometer Gold(I) Carbene Complex

AbstractColloidal coordination compounds in the sub‐micrometer range were synthesized from a chloro gold(I) carbene complex via the anti‐solvent procedure and stabilized by a surfactant shell of Tween 20. This compound was successfully applied as model system to monitor heterogeneous multiphase ligand substitution reactions at the solid–liquid interface.

Kinetics and mechanism of ligand substitution reactions in [cis-M(CO)4(amine)(EPh3)] complexes (M = Mo, W; amine = pyridine, piperidine; E = As, Sb)

Group 6 metal carbonyls [cis-M(CO)4(amine)(EPh3)] (M = Mo, W; amine = piperidine (pip), pyridine (py); E = As, Sb) have been prepared and characterized. These complexes react thermally in chlorobenzene solutions with phosphine or phosphite ligands (= L) to give cis- and trans-M(CO)4(L)(EPh3) products. Kinetics of amine substitution by L in these complexes, under pseudo-first-order conditions, indicate that these reactions proceed by a rate law that is first-order in concentration of the metal complex. Rate constants and activation parameters for these reactions have been determined and are discussed. Competition studies for the [M(CO)4(EPh3)] intermediates show that these intermediates are highly reactive and react almost indiscriminately with various incoming nucleophiles with slight preference for more basic ones.

Axial-ligand substitution reactions of a head-to-head pivalamidato-bridged Pt(III) binuclear complex bearing equatorial bromide ligands: A mechanistic study

Axial ligand-substitution reactions of head-to-head (HH) a binuclear pivalamidato-bridged platinum(III) complex bearing equatorial bromide ligands, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(Br)2(OH2)]2+ (1), with chloride and bromide ions were thermodynamically and kinetically investigated in acidic aqueous solutions. Reactions of 1 with p-styrenesulfonate and 4-penten-1-ol were also kinetically investigated in order to clarify the reaction mechanisms involving olefins. In contrast to the reactions of the HH tetraammine pivalamidato-bridged platinum(III) binuclear complex, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(NH3)2(OH2)]4+ (2′), all substitution processes on 1 involve single-step reactions. The effect of the equatorial halide ligands on the axial ligand substitution reactions of 1 is discussed in relation to 2′; the rate-determining first substitution of H2O by X− at the Pt(Br2O2) site in 1 is followed by a fast second substitution of H2O by X− at Pt(N4). In contrast, axial ligand substitution on 2′ by X− proceeds in two consecutive steps. Reactions of 1 with p-styrenesulfonate or 4-penten-1-ol proceed in one step to form only mono-π complexes that are in rapid equilibrium with σ complexes, whereas reactions of 2′ with these olefins proceed in three consecutive steps via di-π complexes. These mechanistic differences are interpreted in terms of enhanced charge localization in the mono-π complexes of 1 ([PtII(NH3)2(μ-pivalamidato)2PtIV(Br)2(p-styrenesulfonate or 4-penten-1-ol)]2+).

Tunable Magnetization Dynamics through Solid-State Ligand Substitution Reaction.

The dimeric molecule [Dy2(acac)6(MeOH)2(bpe)]·bpe·2MeOH (1, acac = acetylacetonate, bpe = 1,2-bis(4-pyridyl)ethylene) undergoes a solid-state ligand substitution reaction upon heating, leading to the one-dimensional chain [Dy(acac)3(bpe)]n (2). This structural transformation takes advantage of the potential coordination of the guest bpe molecules present in 1. In both complexes the Dy(III) ions adopt similar octacoordinated D4d geometries. However, the different arrangement of the negatively charged and neutral ligands alters the direction of magnetic anisotropy axis and the energy states, thus resulting in largely distinct magnetization dynamics, as revealed by the CASSCF/RASSI calculations.

Hetero Face-to-Face Porphyrin Array with Cooperative Effects of Coordination and Host-Guest Complexation.

We successfully synthesized a hetero face-to-face porphyrin array composed of ZnTPP and RuTPP(DABCO)2 (TPP: 5, 10, 15, 20-tetraphenylporphyrin, DABCO: 1,4-diazabi-cyclo[2.2.2]octane) in 2:1 molar ratio. A cyclic Zn porphyrin dimer (ZnCP) was also used as the host molecule for the Ru porphyrin. In the latter, the Ru-DABCO bonding in RuTPP(DABCO)2 was stabilized by the host-guest complexation. Reaction progress kinetic analysis of the ligand substitution reaction of RuTPP(DABCO)2 and that in ZnCP revealed the stabilization mechanism of the Ru-DABCO bonding. Photoinduced electron transfer (PET) from the Zn porphyrin to the Ru porphyrin was observed in the porphyrin array. The host-guest stabilization of unstable complex for construction of a donor-acceptor-donor structure is expected to be a new method for an artificial photosynthesis.

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.

Reversing the metal-to-ligand charge transfer (3 MLCT)/metal-centered (3 MC) excited state order in iron(II) complexes is a challenging objective, yet would finally result in long-sought luminescent transition-metal complexes with an earth-abundant central ion. One approach to achieve this goal is based on low-energy charge-transfer absorptions in combination with a strong ligand field. Coordinating electron-rich and electron-poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and a strong ligand field. Variations of the electron-rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affecting the optical properties. On the other hand, N-deprotonation of the ligand backbone is feasible and reversible, yielding deep-green complexes with charge-transfer bands extending into the near-IR region. Time-dependent density functional theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charge transfer character. This unique geometric and electronic situation establishes a further regulating screw to increase the energy gap between potentially emitting charge-transfer states and the non-radiative ligand field states of iron(II) dyes.

Stoppering/unstoppering of a rotaxane formed between an N-hetorycle ligand containing surfactant: β-cyclodextrin pseudorotaxane and pentacyanoferrate(II) ions.

The assembly of a surfactant-based rotaxane by adding the labile aquopentacyanoferrate(II) ion to the previously formed pseudorotaxane between the surfactant 11-(isonicotinoyloxy)-N,N,N-triethyl-1-undecanaminium bromide and β-cyclodextrin was investigated by 1H NMR and kinetic measurements. NMR spectroscopy has showed that the rotaxane can be formed through two different mechanisms. The rotaxane can be unstoppered by using the pyridine ligand substitution reaction by the high-field cyanide ligand. In this work a new method is developed for the preparation of several new surfactant-based rotaxanes by changing the hydrophilic and hydrophobic regions of the surfactants and the nature of the macrocycle.

Photochemical Processes in a Rhenium(I) Tricarbonyl N-Heterocyclic Carbene Complex Studied by Time-Resolved Measurements.

We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)3(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)3(PyImPh)(MeCN)]+, is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand.

Chemoselective Hydrogenation of Nitroarenes Catalyzed by Molybdenum Sulphide Clusters

AbstractHerein, we describe an atom efficient and general protocol for the chemoselective hydrogenation of nitroarenes to anilines catalyzed by well‐defined diimino and diamino cubane‐type Mo3S4 clusters. The novel diimino [Mo3S4Cl3(dnbpy)3]+ ([5]+) (dnbpy=4,4′‐dinonyl‐2,2′‐dipyridyl, L1) trinuclear complex was synthesized in high yields by simple ligand substitution reactions starting from the thiourea (tu) [Mo3S4(tu)8(H2O)]Cl4⋅4 H2O (3) precursor. This strategy has also been successfully adapted for the isolation of the diamino [Mo3S4Cl3(dmen)3](BF4) ([6](BF4)), (dmen=N,N′‐dimethylethylenediamine) salt. Applying these catalysts, high selectivity in the hydrogenation of functionalized nitroarenes has been accomplished. Over thirty anilines bearing synthetically functional groups have been synthesized in 70 to 99 % yield. Notably, the integrity of the cluster core is preserved during catalysis. Based on kinetic studies on the hydrogenation of nitrobenzene and other potential reaction intermediates, the direct reduction to aniline is the preferential route.

Synthesis, structures and photophysical properties of Cu(I) phosphine complexes with various diimine ligands

A series of triphenylphosphine Cu(I) complexes with various diimine ligands, [Cu(N–N)(PPh3)2](ClO4) [N–N=dmp (2); Ph2Phen (3); Ph2dmp (4); dpq (5); dppz (6); phenCN (7); phenOH (8)] are synthesized by the ligand substitution reactions of [Cu(MeCN)4](ClO4) with 2moleequiv. of PPh3 ligand and 1.2moleequiv. of diimine ligand. {Cu(dpq)[P(XPh)3]2}(ClO4) [X=Me (5a); OMe (5b); Cl (5c)] and {Cu(phenOH)[P(XPh)3]2}(ClO4) [X=Me (8a); OMe (8b)] are also obtained by using various substituted phosphine ligands. These mixed-ligand complexes have been characterized using 1H and 31P NMR spectroscopy, IR spectroscopy, mass spectrometry and elemental analysis. Three of these complexes have been structurally characterized by X-ray crystallography. The photophysical and electrochemical properties of these complexes have been studied. These complexes exhibit MLCT phosphorescence with emission energy, lifetimes and quantum yields showing strong dependence on the nature of diimine and phosphine ligands. In particular, both the electronic and steric effects of the substituting groups in phosphine ligands are also found to affect the emission properties of the complexes. Our study provides structural-properties information for the future design of luminescent Cu(I) complexes.

A ruthenium tellurocarbonyl (CTe) complex with a cyclopentadienyl ligand: systematic studies of a series of chalcogenocarbonyl complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te).

The first tellurocarbonyl complex with a half-sandwich structure [CpRuCl(CTe)(H2IMes)] was synthesized by a ligand substitution reaction. The practically complete series of the CpCE complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te) were systematically explored. The tellurium atom in the CTe complex could be smoothly replaced with lighter chalcogen atoms.

Structure and reactivity of [RuII(terpy)(N^N)Cl]Cl complexes: consequences for biological applications.

The crystal structures of [RuII(terpy)(bipy)Cl]Cl·2H2O and [RuII(terpy)(en)Cl]Cl·3H2O, where terpy = 2,2':6',2''-terpyridine, bipy = 2,2'-bipyridine and en = ethylenediamine, were determined and compared to the structure of the complexes in solution obtained by multi-nuclear NMR spectroscopy in DMSOd-6 as a solvent. In aqueous solution, both chlorido complexes aquate fully to the corresponding aqua complexes, viz. [RuII(terpy)(bipy)(H2O)]2+ and [RuII(terpy)(en)(H2O)]2+, within ca. 2 h and ca. 2 min at 37 °C, respectively. The spontaneous aquation reactions can only be suppressed by chloride concentrations as high as 2 to 4 M, i.e. concentrations much higher than that found in human blood. The corresponding aqua complexes are characterized by pKa values of ca. 10 and 11, respectively, which suggest a more labile coordinated water molecule in the case of the [RuII(terpy)(en)(H2O)]2+ complex. Substitution reactions of the aqua complexes with chloride, cyanide and thiourea show that the [RuII(terpy)(en)(H2O)]2+ complex is 30-60 times more labile than the [RuII(terpy)(bipy)(H2O)]2+ complex at 25 °C. Water exchange reactions for both complexes were studied by 17O-NMR and DFT calculations (B3LYP(CPCM)/def2tzvp//B3LYP/def2svp and ωB97XD(CPCM)/def2tzvp//B3LYP/def2svp). Thermal and pressure activation parameters for the water exchange and ligand substitution reactions support the operation of an associative interchange (Ia) process. The difference in reactivity between these complexes can be accounted for in terms of π-back bonding effects of the terpy and bipy ligands and steric hindrance on the bipy complex. Consequences for eventual biological application of the chlorido complexes are discussed.

Tantallacyclopentadiene as a unique metal-containing diene ligand coordinated to nickel for preparing tantalum-nickel heterobimetallic complexes.

A mononuclear tantallacyclopentadiene complex, TaCl3(C4H2tBu2) (3), serves as a unique ligand to nickel: the addition of Ni(COD)2 to 3 selectively afforded heterobimetallic Ta-Ni complex 4. The cyclooctadiene ligand bound to the nickel center in complex 4 was readily substituted by monodentate and bidentate phosphine ligands, such as dimethylphenylphosphine, 1,2-bis(diphenylphosphino)ethane, and 1,2-bis(diethylphosphino)ethane, to give the corresponding phosphine complexes 5, 6a, and 6b. We also examined a ligand substitution reaction with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) to produce the corresponding Ta-Ni complex 7. These newly prepared Ta-Ni heterobimetallic complexes were characterized spectroscopically together with the crystal structures of 4, 6a, and 7.

Ruthenium complexes bearing a tridentate polypyridyl ligand with non-coordinating donor atoms: Construction of a specific coordination environment involving noncovalent interactions

We have prepared carbonyl (CO) ruthenium complexes which contained the tridentate polypyridyl type ligand (2,6-di(1,8-naphthyridin-2-yl)pyridine; dnp) with non-coordinating nitrogen atoms, for the development of specific reactions using noncovalent interactions. Similar to the aqua precursors that display intramolecular hydrogen bonds between the aqua and dnp ligands, a noncovalent interaction is formed between the terminal carbonyl ligand and non-coordinating nitrogen atoms of dnp; its strength depends on the identity of the axial ligand (triphenylphosphine or pyridine). Additionally, we investigated the reactivity of dnp-containing complexes bearing carbonyl ligands in both the equatorial and axial positions. As a result, it is suggested that the noncovalent interaction between dnp and CO ligands in positions equatorial to the dnp ligand is relatively strong, because the axial carbonyl ligand dissociates preferentially in ligand substitution reactions using organic solvents. Furthermore, in the ligand substitution reaction with a nitrile derivative, the coordination space constructed by the {Ru-dnp}2+ unit enables regulation of not only the ligand substitution reaction rate but also the identity of the donor atom. The present complex also leads to activation and cleavage of inert CCl and CBr bonds.

Silver Ion Polyelectrolyte Container as a Sensitive Quartz Crystal Microbalance Gas Detector.

A polyelectrolyte film containing metastable silver ions was applied as a quartz crystal microbalance (QCM) gas detector. The polyelectrolyte film was obtained by immersing a polyelectrolyte with numerous amine groups in a metal ion solution. The QCM detector with silver ions responded to a very low methylmercaptan gas concentration (20 ppb) but did not respond to ammonia, volatile amines, aromatic compounds, or alcohols. The response speed of the QCM detector increased gradually with increasing methylmercaptan concentrations. The highly sensitive and selective response is promoted by a ligand substitution reaction caused by the formation of coordinative bonds between a metastable silver ion and amine groups in the polyelectrolyte film. To the best of our knowledge, this system has the highest sensitivity among reported QCM gas detectors. Such high-sensitivity among reported QCM gas detectors. Such high-sensitivity gas detectors for volatile sulfur compounds have wide ranging applications in areas such as volcanic eruption prediction, food inspection, environmental analysis, and medical diagnostics.

Ligand-Substitution Reactions of the Tellurium Compound AS-101 in Physiological Aqueous and Alcoholic Solutions.

Since its first crystallization, the aqueous structure of the tellurium-containing experimental drug AS-101 has never been studied. We show that, under the aqueous conditions in which it is administered, AS-101 is subjected to an immediate ligand-substitution reaction with water, yielding a stable hydrolyzed oxide anion product that is identified, for the first time, to be TeOCl3-. Studying the structure of AS-101 in propylene glycol (PG), an alcoholic solvent often used for the topical and oral administration of AS-101, revealed the same phenomenon of ligand-substitution reaction between the alcoholic ligands. Upon exposure to water, the PG-substituted product is also hydrolyzed to the same tellurium(IV) oxide form, TeOCl3-.