Incoming Ligand Research Articles

On the Size Evolution of Gold-Monolayer-Protected Clusters by Ligand Place-Exchange Reactions: The Effect of Headgroup−Gold Interactions

To functionalize Au MPCs (monolayer-protected gold clusters), ligand place-exchange reactions provide a convenient route in which the initial capping ligands are displaced by mixing the MPCs with an excess amount of incoming ones. However, literature reports show that the diameters of the modified products do not always stay unchanged. Because of the diversity of experimental conditions carried out in the documented studies, there is still a lack of comprehensive understanding concerning the size evolution. Herein, carefully controlled and examined parameters include the initial size of MPCs [Au(101)(PPh(3))(21)Cl(5)], the reaction time, the concentration of incoming ligands, a homologous series of incoming ligands, and the anchoring headgroups. The results show that the final particle size is determined by the strength of headgroup-gold adsorption which is correlated with the curvature of the final product in a thermodynamic model derived from Gibbs-Thomson equation.

Experimental and Density Functional Theory Analysis of Serial Introductions of Electron-Withdrawing Ligands into the Ligand Shell of a Thiolate-Protected Au25 Nanoparticle

The progress of ligand exchange reactions between the ligands of Au25(SR)18− nanoparticles (SR = S(CH2)2Ph) and thiols with electron-withdrawing substituents (HSPh-p-X; X = Br, NO2) was monitored using 1H nuclear magnetic resonance. As the reactions proceed, the introduction of the electron withdrawing −SPhX ligands into the nanoparticle ligand shell causes a shift of the nanoparticle redox waves (Au251+/0 and Au250/1−) to more positive potentials. Combining the NMR and electrochemical results reveals a nearly linear shift of the redox formal potentials as a function of the average number of exchanged ligands: ∼42 and 25 mV/ligand for X = −NO2 and −Br, respectively. Using a simple model electron-withdrawing ligand (−SCH2Cl), density functional theory (DFT) was used to study in detail the effects on the nanoparticle electronic structure caused by exchange of this ligand for −SCH3. The calculations show how the electronegative −X group changes the polarization of the nanoparticle and the charge distribution among the ligands, the protecting (−SR−Au−SR−Au−SR−) semirings, and the Au13 core. The HOMO−LUMO gap is unchanged by the ligand exchanges; both states are equally stabilized by the presence of each incoming ligand, by ∼60 mV/ligand. Charge analysis suggests no significant changes in the Au13 core, even after complete exchange. Rather, the charge is transferred inside the ligands, mostly from nearest-neighbor atoms of the semirings.

Rhomboidal Heterometallic Alkynyl Based Pt2Cd2 Clusters: Structural, Photophysical, and Theoretical Studies

Reactions between [Pt(C[triple bond]CR)(4)](2-) (R = Tol a, C(6)H(4)OMe-4 b, C(6)H(4)OMe-3 c) and Cd(2+) depend on the media and the alkynyl substituent, leading to the formation of yellow tetranuclear solvate complexes [Pt(C[triple bond]CR)(4)Cd(acetone)](2) 1a,b(acetone)(2) and [Pt(C[triple bond]CC(6)H(4)OMe-3)(4)Cd(dmso)](2) 1c(dmso)(2) or white polymeric solvate-free species [Pt(C[triple bond]CR)(4)Cd](x) 1'a-c. Treatment of 1a,b(acetone)(2) or 1'a-c with N-donor ligands affords a series of tetranuclear clusters [Pt(C[triple bond]CR)(4)CdL](2) (L = py; 2a-c. R = Tol; L = NC(5)H(4)CH(3)-4 3, NC(5)H(4)CF(3)-4 4, pzH 5). X-ray crystallographic studies reveal that, in the tolyl complexes (2a, 4, and 5), the Cd-L(2+) unit is closely bonded to one Pt-C(alpha)(acetylide) bond (Pt-Cd = 2.7, Cd-C(alpha) approximately 2.48 A), and the resulting "Pt(C[triple bond]CTol)(4)CdL" unit dimerizes by two additional eta(2)-Cd-acetylide and a weaker Pt...Cd bonding interaction leading to a planar unsymmetrical rhomboidal metal core. By contrast, the m-methoxyphenyl derivatives (2c, 1c(dmso)(2)) form symmetrical Pt(2)Cd(2) cores, with each Cd bonded (coordination number, C.N. = 5) to the incoming ligand (pyridine 2c, dmso 1c(dmso)(2)) and four Pt-C(alpha) bonds (Pt-Cd approximately 2.85; Cd-C(alpha) 2.470(10)-2.551(5) A) of different Pt(II) fragments. Evidence of ligand dissociation was found for the solvate (1a,b(acetone)(2), 1c(dmso)(2)) and NC(5)H(4)CF(3)-4 (4) derivatives by NMR and UV-vis absorption spectra. All tetranuclear aggregates exhibit bright blue to green luminescence in the solid state. Time-dependent density functional theory (TD-DFT) calculations were performed to shed light on the nature of the electronic transitions. In the solvate 1a,b(acetone)(2) and 1c(dmso)(2), emissions have been assigned to a platinum-alkynyl to cadmium charge transfer ((3)MLM'CT), mixed with some intraligand (3)IL(C[triple bond]CR) character. In the imine derivatives 2-5, they are suggested to come from an excited state of large Pt(d)/piC[triple bond]CR-->pi*(imine) MLL'CT character, mixed with some Pt(d)/pi(C[triple bond]CR)-->Pt(2)Cd(2)/pi*C[triple bond]CR (ML'M'CT) contribution.

Reinvestigation of the formation of a mononuclear Fe(iii) hydroperoxido complex using high pressure kinetics

Previous stopped flow kinetic experiments suggested an interchange associative mechanism for the ligand substitution reaction, [Fe(bztpen)(OMe)](2+) + H(2)O(2)--> {[Fe(bztpen)(OMe)(HOOH)](2+)}(++)--> [Fe(bztpen)(OOH)](2+) + MeOH (bztpen = N-benzyl-N, N',N'tris(2-methylpyridyl)-ethylenediamine). Thus a seven-coordinate transition state containing both the leaving methoxide and the incoming hydrogen peroxide ligands was proposed. On the basis of high pressure kinetic data we can now conclude that this is not the case since the rate of the reaction is independent of pressure for the formation of the purple low spin transient hydroperoxido complex, [Fe(bztpen)(OOH)](2+). [Fe(bztpen)(OOH)](2+) has so far proved to be too short-lived for solid state isolation. As part of our ongoing pursuit of this elusive species we have structurally characterised the nitrosyl and acetate iron(II) complexes, [Fe(bztpen)(NO)](OTf)(2) and [Fe(bztpen)(OAc)](BPh(4)), as well as the air stable Co(II) complexes [Co(bztpen)Cl)]BF(4), [Co(metpen)Cl]SbF(6) and [Co(bztpen)(OAc)]BPh(4). We did not realise our aim of accessing stable Co(III) hydroperoxido or peroxido complexes by reaction of the cobalt complexes with H(2)O(2).

Dinitrogen cleavage and functionalization by carbon monoxide promoted by a hafnium complex

Molecular nitrogen (N(2)) and carbon monoxide (CO) have the two strongest bonds in chemistry and present significant challenges in developing new transformations that exploit these two abundant feedstocks. At the core of this objective is the discovery of transition-metal compounds that promote the six-electron reductive cleavage of N(2) at ambient temperature and pressure and also promote new nitrogen-element bond formation. Here we show that an organometallic hafnium compound induces N(2) cleavage on the addition of CO, with a simultaneous assembly of new nitrogen-carbon and carbon-carbon bonds. Subsequent addition of a weak acid liberates oxamide, which demonstrates that an important agrochemical can be synthesized directly from N(2) and CO. These studies introduce an alternative paradigm for N(2) cleavage and functionalization in which the six-electron reductive cleavage is promoted by both the transition metal and the incoming ligand, CO, used for the new bond formations.

Kinetics and equilibria for the axial ligation of bromomethyl (aqua)cobaloxime with pyridines — Isolation characterization and DNA binding

The kinetics and equilibria for the axial ligation of pyridine and substituted pyridines to bromomethyl(aqua)cobaloxime have been measured spectrophotometrically in aqueous solutions of ionic strength l·0 M (KC1) at 25°C as a function of pH. The binding constants and rate of formation increase in the order 4-NH2Py > 4-EtPy > 4-MePy > Py > 2-NH2Py > 2-EtPy. The data have been interpreted based on the basicity of the ligand, π-back bonding from Co(III) → L and hard and soft interactions. The rate of substitution of H2O varies with the pKa of the incoming ligand, thus establishing the existence of nucleophilic participation of the ligand in the transition state. We have investigated the DNA binding of bromomethyl(aqua)cobaloxime with DNA. Bromomethyl(ligand)cobaloximes were isolated and characterized by elemental analysis, IR and NMR (1H, 13C) spectra.

Influence of Sulf-Oxygenation on CO/L Substitution and Fe(CO)3 Rotation in Thiolate-Bridged Diiron Complexes

Kinetic studies of CO/L substitution reactions of the well-known organometallic complex (mu-pdt)[Fe(CO)(3)](2) (pdt = 1,3-propanedithiolate), complex 1, and its sulfur-oxygenated derivative (mu-pst)[Fe(CO)(3)](2) (pst = 3-sulfenatopropane-1-thiolate), 1-O, have been carried out with the goal of understanding the influence of the sulfenato ligand on the activation barrier to ligand substitution in such diiron carbonyl complexes which consists of two components: intramolecular structural rearrangement (or fluxionality) and nucleophilic attack by the incoming ligand. The CO/PMe(3) substitution reactions of complex 1 follow associative mechanisms in both the first and the second substitutions; the second substitution is found to have a higher activation barrier for the overall reaction that yields 1-(PMe(3))(2). Despite the increased electrophilicity of the Fe(CO)(3) unit in 1-O versus 1, the former reacts more sluggishly with PMe(3), where practical kinetic measurements are at such high temperatures that CO dissociation parallels the associative path. Kinetic studies have established that in complex 1-O both the first and the second CO/CN(-) substitutions proceed via associative paths with higher E(act) barriers than the analogous reactions with complex 1. Theoretical calculations (density functional theory) have been used in conjunction with variable temperature (13)C NMR spectral studies to examine the energy barriers associated with rotation of the Fe(CO)(3) unit. The activation energy required for rotation is higher in the sulfenato than in the analogous thiolato complexes. Thus, the greater barrier to structural deformation in 1-O inhibits its ability to expand its coordination number as compared to the thiolate, 1, resulting in slower reaction rates of both PMe(3) and CN(-) substitution reactions.

Thallium(I) as a Coordination Site Protection Agent: Preparation of an Isolable Zero‐Valent Nickel Tris‐Isocyanide

Blocking the pass: Low-valent Ni centers readily bind Tl(I) ions in a synthetically reversible fashion. The Tl units, in turn, serve as coordination site protection agents for Ni with respect to incoming Lewis basic ligands. This synthetic sequence allows for the isolation of a reactive zero-valent Ni tris-isocyanide complex.

Hard/soft selectivity in ligand substitution reactions of β-diketonate platinum(II) complexes

The reactivity of platinum(II) complexes of the type [PtCl(O,O-acac)(L)] (1) and [Pt(O,O-acac)(gamma-acac)(L)] (2) (L = DMSO, a; DMS, b), with a range of hard and soft nucleophiles such as dimethylsulfide (DMS, b), triphenylphosphine, (PPh3, c), ethylene (eta2-C2H4, d), carbon monoxide (CO, e), pyridine (py, f), and guanosine (Guo, g) has been investigated. Interestingly, the complexes 1a and 1b undergo selective substitution of the chloro or sulfur ligand depending on the hard/soft character of the incoming nucleophile. The soft incoming ligand replaces the softer one and the hard ligand replaces the harder one, giving [PtCl(O,O'-acac)(L)] complexes (1b, 1c, 1d and 1e in the reaction of 1a with L = DMS, PPh3, eta2-C2H4, CO, respectively), and [Pt(O,O'-acac)(DMSO)(L')] (3f, 3g) and [Pt(O,O'-acac)(DMS)(L')] (4f, 4g) species in the reaction of 1a and 1b with L' = py and guo, respectively. In the cases of 2a and 2b complexes, where the pi-bonded acac (gamma-acac) replaces the chloro ligand, only in the presence of an incoming soft nucleophile substituting the soft sulfur ligand the reaction occurs. Equilibrium constants for the substitution reactions were measured by 1H NMR spectroscopy. Variable temperature 1H NMR spectroscopy studies, performed for the reaction of 1a and 2a complexes with DMS, revealed that the selective substitution of DMSO with DMS takes place in both cases, according to a second-order kinetic law. The calculated values of DeltaH++ and DeltaS++ are consistent with an associative mechanism. NMR spectroscopic characterization (1H, 13C, 195Pt, 31P) for the complexes and crystal structures of isolated complexes ([PtCl(O,O'-acac)(L)] (1) and [Pt(O,O'-acac)(gamma-acac)(L)] (2), L = DMSO, 1a and 2a; L = DMS, 1b and 2b; L = PPh3, 1c and 2c) are herein reported and discussed.

Mass Spectrometrically Detected Statistical Aspects of Ligand Populations in Mixed Monolayer Au25L18 Nanoparticles

Ligand exchange reactions of Au25(SCH2CH2Ph)18 with hexanethiol (HSC6) and thiophenol (HSPh) as incoming ligands, and Brust reaction nanoparticle syntheses using mixtures of thiols (HSCH2CH2Ph and HSC6), produce nanoparticles having different, ideally statistically determined, relative populations of the two thiolate ligands (X and Y), i.e., Au25(X)m(Y)m′, where m and m′ vary but always sum to 18. By choice of reactant concentrations, the exchange reaction can reach an equilibrium state or a near-complete exchange of ligands or be at a kinetically determined (nonequilibrium) mixed population, at the time of reaction quenching and subsequent matrix-assisted laser desorption ionization−time-of-flight (MALDI-TOF) mass spectrometric examination. With the assumption that the reactivities of the 18 ligand sites are identical and independent, the equilibrium distributions of ligand populations of the mixed monolayer exchange products should adhere to a binominal distribution. A simulated kinetic model for ligand...

Reaction of 6-Methyl-2,2′-bipyridine with 1,2-Os3(CO)10(MeCN)2: Syntheses, Reductive Elimination/Ligand Displacement Kinetics, and X-ray Diffraction Structures of the Isomeric Clusters HOs3(CO)9(μ2-N2C11H9) and H2Os3(CO)8(μ3-N2C11H8)

Treatment of Os3(CO)10(MeCN)2 (1) with the heterocyclic ligand 6-methyl-2,2′-bipyridine (6-Me-2,2′-bpy) at room temperature leads to the formation of the isomeric hydride-bridged clusters HOs3(CO)9(μ2-CH2N2C10H7) (2) and HOs3(CO)9(μ2-N2C11H9) (3). The cyclometalation of the ancillary 6-Me group in 2 and the ortho metalation of the nonsubstituted pyridyl ring in 3 have been confirmed by spectroscopic and crystallographic methods. Thermolysis of 2 leads to the formation of 3 and the dihydride cluster H2Os3(CO)8(μ3-N2C11H8) (4); the latter cluster, whose structure has been crystallographically determined, derives from a formal loss of CO and C−H bond activation of the methylene moiety in 2. Heating 2 in the presence of ligand-trapping agents proceeds with the release of the 6-Me-2,2′-bpy ligand and formation of Os3(CO)9L3 [where L = CO, P(OMe)3]. The kinetics for the reaction between 2 and added ligand have been investigated by UV−vis and NMR spectroscopies and found to be first-order in starting cluster and independent of the incoming ligand. Parallel kinetic experiments employing the deuterated cluster DOs3(CO)9(μ2-CD2N2C10H7) (2-d3), which was prepared from cluster 1 and 6-Me-d3-2,2′-bpy, confirm the existence of a primary kinetic isotope effect (KIE) of 1.78 at 323 K. The KIE data and the calculated activation parameters [ΔH⧧ = 21.7(4) kcal/mol; ΔS⧧ = −13(1) eu] are strongly suggestive of a reaction scheme involving a rate-limiting reductive coupling of the bridging hydride ligand and cyclometalated alkyl moiety in 2 to furnish a putative sigma complex containing an intact methyl group bound to the Os3 cluster, prior to the generation of the unsaturated cluster Os3(CO)9(μ-N2C11H10). Thermolysis of 3 in the presence of added P(OMe)3 does not furnish free 6-Me-2,2′-bpy but proceeds by a ligand-induced displacement of the methyl-substituted pyridyl ring and formation of the cluster compound HOs3(CO)9[P(OMe)3](μ2-N2C11H9) (5). The kinetics for the reaction between 3 and P(OMe)3 have been studied over the temperature range 333−356 K, and on the basis of the observed activation parameters [ΔH⧧ = 13.0(3) kcal/mol; ΔS⧧ = −30(1) eu] and the first-order dependence on the cluster and ligand, an associative process that involves P(OMe)3 ligand attack on the cluster and release of the methyl-substituted pyridyl ring in the rate-limiting step is proposed.

Substitution of trans ligands in μ-oxo-bis(μ-acetato)diruthenium(III) complexes: Synthesis and kinetic studies

[Ru 2O(L) 6(acetate) 2](PF 6) 2 {L = pyridine 1; 4-picoline 2} undergo aquation in acetone–water (60:40 v/v) mixed solvent to form diaquo complexes in solution as shown by proton NMR studies. Ligands trans to the μ-oxo group are substituted. These diaquo complexes react with substituted pyridines and imidazoles to form respective disubstituted complexes. Rate constants for aquation and complexation under pseudo first order conditions of ligand are reported. Rate constants increase with increase in the basicity of incoming ligand. Disubstituted complexes proposed to be formed in solution have been isolated and characterized by elemental analyses, visible spectra, proton NMR. Single crystal X-ray structures of 4-picoline and 4-methylimidazole disubstituted complexes are reported. All the isolated complexes exhibit a strong peak between 570 and 585 nm in their visible absorption spectra. λ max varies linearly with ∑p k a of terminal ligands. In disubstituted complexes of 1 with 2-methyl and 4-methyl imidazole deprotonation of N(1)H of methylimidazoles takes place in solution.

Chemistry of Bridging Phosphanes: CuI Dimers Bearing 2,5‐Bis(2‐pyridyl)phosphole Ligands

Bis(2-pyridyl)phosphole 1 reacted with Cu(I) sources giving rise to dicationic or neutral dimers 2,3. In these derivatives, 1 acts as a 1kappaN:1,2kappaP:2kappaN donor with a symmetrically bridging P centre. X-ray diffraction studies of these species revealed no constraint due to the unusual coordination mode of the P donor. A comparative study with a monometallic Cu(I) complex in which 1 acts as a P,N chelate is presented. The acetonitrile ligands of the dicationic complex 2 can be displaced by a variety of donors. Bipyridine (bipy) acts as a chelating donor, while 1,1'-bis(diphenylphosphino)methane (dppm) and bis(2-pyridyl)phosphole 1 behave as bridging ligands. By using dppm and 1, the complexes arising from the stepwise displacement of the acetonitrile ligands of complex 2 can be isolated. X-ray diffraction studies performed on these novel complexes revealed that the P centre can easily switch from a bridging to a semibridging coordination mode. Of particular interest, within the same unit cell, complexes with P centres exhibiting bridging and semibridging coordination modes are observed. This switching can be induced by weak effects such as a different conformation of the incoming ligand. Cu(I) dimers assembled by 1 are air-stable derivatives that are not water sensitive. Hydrolysis of the PF(6) (-) counterion occurs under drastic conditions and results in the formation of a PO(2)F(2) fragment coordinated to a Cu(I)-1 fragment.

Orthopalladation of iminophosphoranes: synthesis, structure and study of stability

The reaction of Pd(OAc)(2) with polyfunctional iminophosphoranes Ph(3)P=NCH(2)CO(2)Me (1a), Ph(3)P=NCH(2)C(O)NMe(2) (1b), Ph(3)P=NCH(2)CH(2)SMe (1c) and Ph(3)P=NCH(2)-2-NC(5)H(4) (1d), gives the orthopalladated dinuclear complex [Pd(mu-Cl){C(6)H(4)(PPh(2)=NCH(2)CO(2)Me-kappa-C,N)-2}](2) (2a) and the mononuclear derivatives [PdCl{C(6)H(4)(PPh(2)=NCH(2)CONMe(2)-kappa-C,N,O)-2}] (2b), [PdCl{C(6)H(4)(PPh(2)=NCH(2)CH(2)SMe-kappa-C,N,S)-2}] (2c) and [PdCl{C(6)H(4)(PPh(2)=NCH(2)-2-NC(5)H(4)-kappa-C,N,N)-2}] (2d). The reaction implies the activation of a C-H bond in a phenyl ring of the phosphonium group, this fact being worthy of note due to the strongly deactivating nature of the phosphonium unit. The palladacycle containing the metallated carbon atom is remarkably stable toward the coordination of incoming ligands, while that formed by the iminic N atom and another heteroatom (O, 2a and 2b; S, 2c; N, 2d) is less stable and the resulting complexes can be considered as hemilabile. The X-ray crystal structures of the cyclopalladated [Pd(mu-Cl){C(6)H(4)(PPh(2)=NCH(2)CO(2)Me-kappa-C,N)-2}](2) (2a), [PdCl{C(6)H(4)(PPh(2)=NCH(2)-2-NC(5)H(4)-kappa-C,N,N)-2}] (2d), [Pd{C(6)H(4)(PPh(2)=NCH(2)CONMe(2)-kappa-C,N,O)-2}(NCMe)](ClO(4)) (7b) and [Pd{C(6)H(4)(PPh(2)NCH(2)CONMe(2)-kappa-C,N,O)-2}(py)](ClO(4)) (3b), and the coordination compound cis-[Pd(Cl)(2)(Ph(3)P=NCH(2)CH(2)SMe-kappa-N,S)] (8) are also reported.

Screening of the active site from water by the incoming ligand triggers catalysis and inhibition in serine proteases

The pKa of the catalytic His57 N(epsilon)H in the tetrahedral complex (TC) of chymotrypsin with trifluoromethyl ketone inhibitors is 4-5 units higher relative to the free enzyme (FE). Such stable TC's, formed with transition state (TS) analog inhibitors, are topologically similar to the catalytic TS. Thus, analysis of this pKa shift may shed light on the role of water solvation in the general base catalysis by histidine. We applied our QM/SCRF(VS) approach to study this shift. The method enables explicit quantum mechanical DFT calculations of large molecular clusters that simulate chemical reactions at the active site (AS) of water solvated enzymes. We derived an analytical expression for the pKa dependence on the degree of water exposure of the ionizable group, and on the total charge in the enzyme AS, Q(A) and Q(B), when the target ionizable functional group (His57 in this study) is in the acidic (A) and basic (B) forms, respectively. Q2(B) > Q2(A) both in the FE and in the TC of chymotrypsin. Therefore, water solvation decreases the relative stability of the protonated histidine in both. Ligand binding reduces the degree of water solvation of the imidazole ring, and consequently elevates the histidine pKa. Thus, the binding of the ligand plays a triggering role that switches on the cascade of catalytic reactions in serine proteases.

Displacement of a cis-Olefin from a trans-Olefin Complex: CpRu(CO)2(trans-olefin)+

Ruthenium(II)−olefin complexes CpRu(CO)2(η2-trans-olefin)+ (Cp = η5-C5H5; olefin = trans-3-hexene, trans-2-pentene, trans-3-octene, trans-4-octene, trans-5-decene) have been synthesized and characterized by IR, 1H NMR, and 13C NMR spectroscopies. The reactions of these complexes with a wide range of ligands (L) result in the formation of CpRu(CO)2(L)+ and the release of both cis- and trans-olefins: [CpRu(CO)2(trans-olefin)]BF4 + L → [CpRu(CO)2(L)]BF4 + cis/trans-olefin. The relative amounts of cis- and trans-olefin released are controlled by several factors: identity and amount of the incoming ligand L, identity of the olefin, temperature, and solvent. For 4-substituted pyridines, the cis/trans ratio increases as the electron-donating ability of the 4-substituent increases: F3C (18/82) < H (67/33) < CH3 (74/26) < CH3O (76/24). Increases in temperature, solvent polarity, and olefin side-chain length reduce the cis/trans ratio. A mechanism is proposed to account for the isomerization of trans-olefin ligands to their cis isomers during the substitution process.

Transport of Cu(II) from an albumin mimic peptide, GlyGlyHisGly, to histidine and penicillamine

Cu in blood has been believed to transport into cell via albumin and some amino acids. To shed light on the Cu transport process we studied the reaction of the Cu(II)-peptide with the amino acid by absorption and CD spectra. Albumin mimic peptides GlyGly- l-HisGly (GGHG) and penta-Gly(G5) formed stable 4N coordinated Cu(II) complexes, but in the reaction with histidine (His) and penicillamine (Pes) the ternary Cu(II) complex formations were observed different by the kinetic study. Cu(II)-G5 complexes reacted with Pes to form the ternary complex Cu(H −1G5)(Pes −) which was subsequently transformed to the binary complex Cu(Pes −) 2. In the system with GGHG the Cu(II) was also transported from GGHG to Pes, but the ternary Cu(H −1GGHG)(Pes −) complex as the intermediate was detected a trace. The ternary complex would be spontaneously transformed to Cu(Pes −) 2 upon forming, because the rate constant of the ternary complex formation k 1+ = ∼2 M −1 s −1 was less than k 2+ = ∼5 × 10 2 M −1 s −1 for the Cu(Pes −) 2 formation at physiological pH. In the Cu(II)-GGHG-His system the ternary Cu(H −1GGHG)(His) complex was also hardly identified because the formation constant K 1 and k 1+ were very small and the equilibrium existed between Cu(H −2GGHG) and Cu(His) 2 and its overall equilibrium constant β 2 for Cu(His) 2 was very small to be 1.00 ± 0.05 M −1 at pH 9.0. These results indicated that the ternary complex is formed in the Cu transport process from the albumin to the amino acid, but His imidazole nitrogen in the fourth-binding site of Cu(II) strongly resists the replacement by the incoming ligand.

Determination of Equilibrium Constants and Computational Interaction Energies for Adducts of [Rh2(RCO2)4-n(PC)n] (n = 0−2) with Lewis Bases

Properties of dirhodium catalysts with cyclometalated aryl phosphine ligands have been studied. We report here the study of the acid-base reaction of Rh2(RCO2)2(PC)2(H2O)2 catalysts (PC = cyclometalated aryl phosphine) with different Lewis bases. The determination of the equilibrium constants of these reactions can be used to study to which extent the properties of the axial coordination site of the catalyst, considered the active site, are affected by modification of the metalated phosphines, the carboxylate ligands, or the incoming axial ligand. The trends in the computational density functional theory interaction energies show good agreement with the major trends in the equilibrium constants, thus enabling a further study of the influence of the modification of the ligand core.

Biomimetic Model of Coenzyme B12: Aquabis(ethane‐1,2‐diamine‐κN,κN′)ethylcobalt(III) – Its Kinetic and Binding Studies with Imidazoles and Amino Acids and Interactions with CT DNA

AbstractPseudo‐first‐order reaction kinetics and binding studies of trans‐[Co(en)2(Et)H2O] complex with 1H‐imidazole, substituted 1H‐imidazoles, histidine, histamine, glycine and glycine ethyl ester were investigated by means of spectrophotometric techniques. Equilibrium constants were determined as a function of pH at 25°. Binding and kinetic studies were correlated to basicity and steric hindrance. From the equilibrium data, it was found that the entering nucleophile is participating in the transition state, an Id mechanism is proposed. The effect of the incoming ligands on the complex was studied by molecular mechanics. The interaction of trans‐[Co(en)2(Et)H2O] with CT DNA was studied spectrophotometrically.

Theoretical Investigation of the Stepwise Hydrolysis of the [Re3(μ-Cl)3Cl9]3- Anion

A detailed study of the stepwise substitution of the chloride ligands in the [Re3(mu-Cl)3Cl9](3-) (1) anion by water molecules is presented using theoretical methods. Ligand lability as well as the structure and relative stability of the various mono-[Re3(mu-Cl)3Cl8(H2O)](2-) (2a,b) and dihydro-[Re3(mu-Cl)3Cl7(H2O)2](-) (3a-f) conformers is examined. Clear preferences for the positions of the incoming water ligands are proposed based on calculated energy and vibrational data, which fully agree with the experimental results.