Hydroxo Ligands Research Articles

Ligand Sequential Replacement on Chromium(III)-Aqua Complexes by l-Alanine and Other Biological Amino Acids: A Kinetic Perspective.

The ligand sequential replacement on chromium(III)-aqua complexes by l-alanine in slightly acidic aqueous solutions (pH range: 3.55-5.61) has been kinetically followed by means of UV-vis spectrophotometry. A two rate constant model has been applied to fit the absorbance-time data, corresponding to the formation ( k1) and decay ( k2) of an intermediate not reactive enough to be in steady state (long-lived intermediate). The kinetic orders of the amino acid were fractional (0.40 ± 0.03 for k1 and 0.40 ± 0.02 for k2). The two steps showed base catalysis, and the activation energies were 60 ± 3 (for k1) and 83 ± 6 (for k2) kJ mol-1. The rate constants for the coordination of the first l-alanine ligand followed the sequence CrOH2+ < Cr(OH)2+ < Cr(OH)3, Cr3+ being almost inactive. This suggests that the increase in the reaction rate with increasing pH was caused by the enhancement of the lability of the Cr(III)-aqua bonds induced by the presence of hydroxo ligands. The activation parameters for a series of ligand substitution on Cr(III)-aqua complexes by organic molecules yielded a statistically significant enthalpy-entropy linear plot with an isokinetic temperature of 296 ± 21 K.

Chromotropism studies on copper(II) compounds. Part II. Dinuclear copper(II) complexes with triply-bridged hydroxo, acetate, and halo ligands

Two triply-bridged dinuclear copper(II) complexes of formula [LCu(μ-OH)(μ-OAc)(μ-X)CuL]X⋅0.5H2O where L is a bidentate ligand of N-(pyridine-2-ylmethyl)propane-2-amine and X=Cl, 1 and Br, 2 were synthesized and characterized by elemental analyses, spectroscopic techniques (IR, UV–Vis, EPR), thermal analysis, conductance measurements, and single-crystal X-ray structure determination. The structures of both complexes are similar. The complexes show a distorted square-pyramidal arrangement around each copper(II) ion with a CuN2O2X chromophore in which both copper(II) ions are connected by a hydroxo bridge and a triatomic syn-syn carboxylato bridge in equatorial positions and a halide ion bridge at the axial site. The chromotropism behavior of the complexes, including solvato-, thermo-, and halochromism, were investigated in detail. Their halochromism was investigated in the pH range of 2.0–11.0 by visible absorption spectroscopy. The reversible color variations from blue to colorless are attributable to deprotonation and protonation of the ligands. The complexes show reversible thermochromism in solution due to dissociation and recombination of ligands to copper ions.

Computational study of acetylene hydration by bio-inspired group six catalyst models

A combination of density functional theory and natural bond orbital analysis was employed to study two bio-inspired catalyst models that mimic the acetylene hydratase active site. The Group 6 metals (Cr, Mo, W) and substituents (CN, H, CH3) on the dithiolate supporting ligands were used to evaluate the effect of metal identity and supporting ligand on free energy barriers and the thermodynamics of the reaction. In catalyst model 1, a metal-bound hydroxo ligand (MIV-OH) acts as a general base, while in catalyst model 2, a propionate plays the role of general base. The calculations showed that in both chromium complexes 1 and 2, acetylene does not bind to the metal in the reactant catalysts, suggesting that Cr is not a soft enough acid in the 4+ formal oxidation state to bind acetylene. The comparison of calculated free energies for molybdenum and tungsten complexes reveals that for models 1 and 2, Mo complexes have lower free energy barriers and are more exergonic as compared to the W variant. Comparing the energy profiles of complexes 1 and 2 indicates that on average, complex 2 has lower barrier energies in comparison to 1, while model 1 has a more exergonic ΔGrxn for acetylene hydration. NBO analysis suggests a metallocyclopropene constitutes the more faithful representation of the Mo and W reactants rather than a π-acetylene. Furthermore, calculations indicate that for both complexes 1 and 2, using electron withdrawing substituents such as CN on dithiolate supporting ligands lowers the activation free energies compared to CH3 and H substituents. Analysis of the impact of polarity through use of different continuum solvents on the reaction coordinate suggest that a less polar active site is more favorable for acetylene hydration.

Structural determinants of ligand binding in truncated hemoglobins: Resonance Raman spectroscopy of the native states and their carbon monoxide and hydroxide complexes

The ligand binding characteristics of heme-containing proteins are determined by a number of factors, including the nature and conformation of the distal residues and their capability to stabilize the heme-bound ligand via hydrogen-bonding and electrostatic interactions. In this regard, the heme pockets of truncated hemoglobins (TrHbs) constitute an interesting case study as they share many common features, including a number of polar cavity residues. In this review, we will focus on three proteins of group II TrHbs, from Thermobifida fusca (Tf-HbO) and Pseudoalteromonas haloplanktis TAC125 (Ph-HbO). Although the residues in positions G8 (Trp) and B10 (Tyr) are conserved in all three proteins, the CD1 residue is a Tyr in T. fusca and a His in P. haloplanktis. Comparison of the ligand binding characteristics of these proteins, in particular the hydroxo and CO ligands by means of resonance Raman spectroscopy, reveals that this single difference in the key heme cavity residues markedly affects their ligand binding capability and conformation. Furthermore, although the two Ph-HbOs (Ph-HbO-2217 and Ph-HbO-0030) have identical key cavity residues, they display distinct ligand binding properties.

Cu(II)‐catalyzed hydrolysis of tris‐2‐pyridyl phosphate assisted by sodium dodecyl sulfate micelles

AbstractHydrolysis of the phosphate triester tris‐2‐pyridyl phosphate is catalyzed upon complexation with Cu2+, with enhancements of the order of 107‐fold in comparison to the spontaneous hydrolysis of the substrate. Experimental and theoretical results suggest that Cu2+ most probably coordinates to the nitrogen atoms of two of the pyridyl substituents disposing a metallo‐bound water or hydroxo ligand at appropriate distance for intramolecular attack on phosphorus. Micelles of the anionic sodium dodecyl sulfate remarkably accelerate the reactions, acting as nanoreactors that concentrate in the micellar microenvironment both the hydrophobic substrate and the positively charged metal ion, favoring complexation between tris‐2‐pyridyl phosphate and Cu2+.

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex.

The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pKa for the bound H2 O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [FeIV (O)(TMC)(OH2 )]2+ (1-OH2 ; TMC=1,4,8,11-tetramethylcyclam) by determining the pKa of the H2 O ligand. We establish that 1-OH2 has a pKa of 6.9±0.5, a value that falls in between those found for [FeIII (OH2 )6 ]3+ and [FeII (OH2 )6 ]2+ . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH2 , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.

Effect of proton concentration upon two-electron oxidation of water to hydrogen peroxide using GeIV-porphyrin-sensitized photovoltaic cell

The dependence of proton concentration (pH) in aqueous electrolyte solution upon two-electron oxidation of water to form hydrogen peroxide (H2O2) was investigated in a photovoltaic cell (PVC) composed of an FTO electrode coated by GeIV-porphyrin complexes having a phenyl boronic acid groups (1)-adsorbed titanium oxide (TiO2) particles as an anode (1/TiO2/FTO) and a Pt electrode as a cathode. The short-circuit photocurrent density (J), amount of H2O2 and faradaic efficiency FE strongly depended on pH, respectively. Both J and amount of H2O2 decreased with the increase of pH simply in the range from pH=2 to pH=10, suggesting that the photoinduced electron transfer (PET) from excited 1 to TiO2 conduction band (CB) should be suppressed due to the negative shift of CB by the increase of pH. On the other hand, the recovers of both J and amount of H2O2 were observed in 1-PVC system at pH=13. Especially, the FE of 1b became 91%, although it was 60% at pH=6. These results indicate an existence of different species with 1 under basic condition. The chemical species can be 4, in which a proton of axial ligand in 1 is dissociated under strong basic condition on the TiO2 surface. 4 can have a lower oxidation potential than that of 1, because 4 has a negative charge by the proton dissociation. Therefore, the observed recovers can be explained by both promotion of PET from excited 4 to TiO2 and the direct formation of Ge-oxyl complex from the PET of 4, which is a key intermediate for a two-electron activation of water to form hydrogen peroxide, not through a proton dissociation process of axial hydroxo ligand.

Tetrathiafulvalene Schiff base Cu(II) 1D coordination polymer upon a metallo-ligand approach

TTF-salicylaldehyde 1 has been prepared by Stille coupling between TTF-SnMe3 and 5-bromo-salicylaldehyde and characterized by single crystal X-ray structure analysis. A set of S⋯S, π-π and donor–acceptor intermolecular interactions is observed in the solid state architecture of this precursor. Its condensation reaction with 2-ethanolamine provided the chelating electroactive ligand 4-TTF-2-((2-hydroxyethyl)iminomethyl)-phenol 2 (H2L1). Reaction with Cu(OAc)2 in a 2:1 stoichiometry afforded the one-dimensional coordination chain-like polymer [Cu(HL1)2]n 3 for which the solid state structure has been determined. The Cu(II) coordination sphere contains in the equatorial plane a N2O2 motif arising from two chelating imino-phenol units, while the axial positions are occupied by hydroxo ligands from the pendant hydroxy-ethyl arms of top and bottom neighbouring intra-chain complexes with quite larger Cu–O distances indicative of Jahn-Teller elongation. The TTF units form regular stacks within the chains and further interact laterally through short S⋯S contacts. Cyclic voltammetry measurements on ligand 2 and complex 3 indicate excellent electron donating properties of these multifunctional precursors.

The influence of the metal cations and microhydration on the reaction trajectory of the N3 ↔ O2 thymine proton transfer: Quantum mechanical study.

This study involves the intramolecular proton transfer (PT) process on a thymine nucleobase between N3 and O2 atoms. We explore a mechanism for the PT assisted by hexacoordinated divalent metals cations, namely Mg2+ , Zn2+ , and Hg2+ . Our results point out that this reaction corresponds to a two-stage process. The first involves the PT from one of the aqua ligands toward O2. The implications of this stage are the formation of a hydroxo anion bound to the metal center and a positively charged thymine. To proceed to the second stage, a structural change is needed to allow the negatively charged hydroxo ligand to abstract the N3 proton, which represents the final product of the PT reaction. In the presence of the selected hexaaqua cations, the activation barrier is at most 8 kcal/mol. © 2017 Wiley Periodicals, Inc.

Spectroscopic and DFT Study of RhIII Chloro Complex Transformation in Alkaline Solutions.

The hydrolysis of [RhCl6]3- in NaOH-water solutions was studied by spectrophotometric methods. The reaction proceeds via successive substitution of chloride with hydroxide to quantitatively form [Rh(OH)6]3-. Ligand substitution kinetics was studied in an aqueous 0.434-1.085 M NaOH matrix in the temperature range 5.5-15.3 °C. Transformation of [RhCl6]3- into [RhCl5(OH)]3- was found to be the rate-determining step with activation parameters of ΔH† = 105 ± 4 kJ mol-1 and ΔS†= 59 ± 10 J K-1 mol-1. The coordinated hydroxo ligand(s) induces rapid ligand substitution to form [Rh(OH)6]3-. By simulating ligand substitution as a dissociative mechanism, using density functional theory (DFT), we can now explain the relatively fast and slow kinetics of chloride substitution in basic and acidic matrices, respectively. Moreover, the DFT calculated activation energies corroborated experimental data that the kinetic stereochemical sequence of [RhCl6]3- hydrolysis in an acidic solution proceeds as [RhCl6]3- → [RhCl5(H2O)]2- → cis-[RhCl4(H2O)2]-. However, DFT calculations predict in a basic solution the trans route of substitution [RhCl6]3- → [RhCl5(OH)]3- → trans-[RhCl4(OH)2]3- is kinetically favored.

Interfacial Interaction of Titania Nanoparticles and Ligated Uranyl Species: A Relativistic DFT Investigation.

To understand interfacial behavior of actinides adsorbed onto mineral surfaces and unravel their structure-property relationship, the structures, electronic properties, and energetics of various ligated uranyl species adsorbed onto TiO2 surface nanoparticle clusters (SNCs) were examined using relativistic density functional theory. Rutile (110) and anatase (101) titania surfaces, experimentally known to be stable, were fully optimized. For the former, models studied include clean and water-free Ti27O64H20 (dry), partially hydrated (Ti27O64H20)(H2O)8 (sol) and proton-saturated [(Ti27O64H20)(H2O)8(H)2]2+ (sat), while defect-free and defected anatase SNCs involving more than 38 TiO2 units were considered. The aquouranyl sorption onto rutile SNCs is energetically preferred, with interaction energies of -8.54, -10.36, and -2.39 eV, respectively. Energy decomposition demonstrates that the sorption is dominated by orbital attractive interactions and modified by steric effects. Greater hydrogen-bonding involvement leads to increased orbital interactions (i.e., more negative energy) from dry to sol/sat complexes, while much larger steric interaction in the sat complex significantly reduces the sorption interaction (i.e., more positive energy). For dry SNC, adsorbates were varied from aquo to aquo-carbonato, to carbonato, to hydroxo uranyl species. Longer U-Osurf/U-Ti distances and more positive sorption energies were calculated upon introducing carbonato and hydroxo ligands, indicative of weaker uranyl sorption onto the substrate. This is consistent with experimental observations that the uranyl sorption rate decreases upon raising solution pH value or adding carbon dioxide. Anatase SNCs adsorbing aquouranyl are even more exothermic, because more bonds are formed than in the case of rutile. Moreover, the anatase sorption can be tuned by surface defects as well as its Ti and O stoichiometry. All the aquouranyl-SNC complexes show similar character of molecular orbitals and energetic order although differing in highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps and orbital energy levels, but changes can be accomplished by adding carbonato and hydroxo ligands.

Protonation and Sulfido versus Oxo Ligation Changes at the Molybdenum Cofactor in Xanthine Dehydrogenase (XDH) Variants Studied by X-ray Absorption Spectroscopy.

Enzymes of the xanthine oxidase family are among the best characterized mononuclear molybdenum enzymes. Open questions about their mechanism of transfer of an oxygen atom to the substrate remain. The enzymes share a molybdenum cofactor (Moco) with the metal ion binding a molybdopterin (MPT) molecule via its dithiolene function and terminal sulfur and oxygen groups. For xanthine dehydrogenase (XDH) from the bacterium Rhodobacter capsulatus, we used X-ray absorption spectroscopy to determine the Mo site structure, its changes in a pH range of 5-10, and the influence of amino acids (Glu730 and Gln179) close to Moco in wild-type (WT), Q179A, and E730A variants, complemented by enzyme kinetics and quantum chemical studies. Oxidized WT and Q179A revealed a similar Mo(VI) ion with each one MPT, Mo═O, Mo-O-, and Mo═S ligand, and a weak Mo-O(E730) bond at alkaline pH. Protonation of an oxo to a hydroxo (OH) ligand (pK ∼ 6.8) causes inhibition of XDH at acidic pH, whereas deprotonated xanthine (pK ∼ 8.8) is an inhibitor at alkaline pH. A similar acidic pK for the WT and Q179A variants, as well as the metrical parameters of the Mo site and density functional theory calculations, suggested protonation at the equatorial oxo group. The sulfido was replaced with an oxo ligand in the inactive E730A variant, further showing another oxo and one Mo-OH ligand at Mo, which are independent of pH. Our findings suggest a reaction mechanism for XDH in which an initial oxo rather than a hydroxo group and the sulfido ligand are essential for xanthine oxidation.

Multilinear analysis of Time-Resolved Laser-Induced Fluorescence Spectra of U(VI) containing natural water samples

Natural waters’ uranium level monitoring is of great importance for health and environmental protection. One possible detection method is the Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS), which offers the possibility to distinguish different uranium species. The analytical identification of aqueous uranium species in natural water samples is of distinct importance since individual species differ significantly in sorption properties and mobility in the environment. Samples originate from former uranium mine sites and have been provided by Wismut GmbH, Germany. They have been characterized by total elemental concentrations and TRLFS spectra. Uranium in the samples is supposed to be in form of uranyl(VI) complexes mostly with carbonate (CO3 2− ) and bicarbonate (HCO3 − ) and to lesser extend with sulphate (SO4 2− ), arsenate (AsO4 3− ), hydroxo (OH− ), nitrate (NO3 − ) and other ligands. Presence of alkaline earth metal dications (M = Ca2+ , Mg2+ , Sr2+ ) will cause most of uranyl to prefer ternary complex species, e.g. Mn (UO2 )(CO3 )3 2n-4 (n ∊ {1; 2}). From species quenching the luminescence, Cl− and Fe2+ should be mentioned. Measurement has been done under cryogenic conditions to increase the luminescence signal. Data analysis has been based on Singular Value Decomposition and monoexponential fit of corresponding loadings (for separate TRLFS spectra, the “Factor analysis of Time Series” (FATS) method) and Parallel Factor Analysis (PARAFAC, all data analysed simultaneously). From individual component spectra, excitation energies T 00 , uranyl symmetric mode vibrational frequencies ω gs and excitation driven U-Oyl bond elongation ΔR have been determined and compared with quasirelativistic (TD)DFT/B3LYP theoretical predictions to cross -check experimental data interpretation.

Geometrical properties of the manganese(iv)/iron(iii) cofactor of Chlamydia trachomatis ribonucleotide reductase unveiled by simulations of XAS spectra.

Using simulations of Mn/Fe K-edge X-ray absorption spectroscopy (XAS), combined with DFT-optimized structural models and direct comparisons with available experimental data, we determine geometrical and electronic properties of the Mn-Fe active site of Chlamydia trachomatis (Ct) of ribonucleotide reductase (RNR). In the post-edge XAS energy range, we use extended X-ray absorption fine structure (EXAFS) data, to acquire absorber-scatterer geometrical information around each absorber metal center. For this task, we apply a protocol that evaluates Debye-Waller factors in scattering paths instead of scattering shells to fit the experimental EXAFS. The model of the manganese(iv)/iron(iii) cofactor that best fit Mn and Fe K-edge EXAFS experimental data is a structure with Mn at metal site 1 (proximal to Phe-127), a μ-oxo/μ-hydroxo/μ-1,3-carboxylato core, and a terminal hydroxo ligand, i.e. OH--Mn1(iv)-(μ-O)(μ-OH-)-Fe2(iii).

Chemoselective guest-triggered shaping of a polynuclear CuII calix[6]complex into a molecular host.

A new calix[6]arene scaffold bearing a tris-imidazole binding site at the small rim and three tetradentate aza ligands at the large rim was synthesized. The system binds three CuII ions at the large rim sites and is unable to bind a fourth one, which remains in solution. The charge repulsion between the complexes, together with the flexibility of the scaffold, disorganizes the small rim site for binding and prevents its use for host-guest studies. Although the presence of MeCN or DMF guests does not alter this state, the addition of a heptylamine guest, which further displays Brønsted basicity, restores its receptor ability by stabilizing the extra CuII ion at the tris-imidazole site with concomitant guest encapsulation and binding of an exo hydroxo ligand. This chemoselective nuclearity switch yields a tetranuclear complex in which the guest backbone is preorganized in front of three potentially reactive Cu(ii) complexes, reminiscent of polynuclear CuII enzyme active sites.

Preparation of bis(5-phenyltetrazolato) Pt(II) and Pt(IV) analogues of transplatin and in vitro evaluation for antitumor activity

To evaluate the 5-substituted tetrazolate (5R-tetrazolate) ligand for the development of novel antitumor active Pt(II) and Pt(IV) diamines, eight analogues of transplatin bearing 5-phenyltetrazolato and 5-(phenol)tetrazolato ligands were prepared by reacting transplatin with the 5-substituted tetrazolate in DMF in the presence of trimethylamine. Coordination to platinum was through the N2 position of the tetrazolato ligand, as evidenced by a X-ray crystal structure of trans-diamminobis[5-(phenyl-4-ol)tetrazolato-κN2]platinum(II). Attempts to synthesize the corresponding cis-coordinated analogues from cisplatin were unsuccessful due to isomerization to the more stable trans isomers. Hydrogen peroxide was used to oxidize trans-diamminobis(5-phenyltetrazolato) and trans-diamminobis(5-phenoltetrazolato)platinum(II) complexes to the trans-dihydroxo-Pt(IV) analogues; the hydroxo ligands were then further acylated. The binding of Pt to calf thymus DNA for trans,trans,trans-diamminodihydroxidobis(5-phenyltetrazolato-κN2)platinum(IV) was dependent on the presence of ascorbate; however, the Pt(II) analogue trans-diamminobis(5-phenyltetrazolato-κN2)platinum(II) itself bond only very weakly to DNA, indicating that this is not the product of the ascorbate induced reduction of the Pt(IV) complex. Evaluation of the antiproliferative activity on eight human cancer cells in vitro showed the Pt(IV) complexes to be more potent than the Pt(II) analogues but all complexes were still much less active than cisplatin. No cross resistance to cisplatin was observed in the A2870 ovarian cancer cell line.

The secondary coordination sphere and axial ligand effects on oxygen reduction reaction by iron porphyrins: a DFT computational study.

Oxygen reduction reaction (ORR) catalyzed by a bio-inspired iron porphyrin bearing a hanging carboxylic acid group over the porphyrin ring, and a tethered axial imidazole ligand was studied by DFT calculations. BP86 free energy calculations of the redox potentials and pK a's of reaction components involved in the proton coupled electron transfer (PCET) reactions of the ferric-hydroxo and -superoxo complexes were performed based on Born-Haber thermodynamic cycle in conjunction with a continuum solvation model. The comparison was made with iron porphyrins that lack either in the hanging acid group or axial ligand, suggesting that H-bond interaction between the carboxylic acid and iron-bound hydroxo, aquo, superoxo, and peroxo ligands (de)stabilizes the Fe-O bonding, resulting in the increase in the reduction potential of the ferric complexes. The axial ligand interaction with the imidazole raises the affinity of the iron-bound superoxo and peroxo ligands for proton. In addition, a low-spin end-on ferric-hydroperoxo intermediate, a key precursor for O-O cleavage, can be stabilized in the presence of axial ligation. Thus, selective and efficient ORR of iron porphyrin can be achieved with the aid of the secondary coordination sphere and axial ligand interactions.

Structural and Electronic Effects on the Properties of Fe2(dobdc) upon Oxidation with N2O.

We report electronic, vibrational, and magnetic properties, together with their structural dependences, for the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its derivatives, Fe2(O)2(dobdc) and Fe2(OH)2(dobdc)-species arising in the previously proposed mechanism for the oxidation of ethane to ethanol using N2O as an oxidant. Magnetic susceptibility measurements reported for Fe2(dobdc) in an earlier study and reported in the current study for Fe(II)0.26[Fe(III)(OH)]1.74(dobdc)(DMF)0.15(THF)0.22, which is more simply referred to as Fe2(OH)2(dobdc), were used to confirm the computational results. Theory was also compared to experiment for infrared spectra and powder X-ray diffraction structures. Structural and magnetic properties were computed by using Kohn-Sham density functional theory both with periodic boundary conditions and with cluster models. In addition, we studied the effects of different treatments of the exchange interactions on the magnetic coupling parameters by comparing several approaches to the exchange-correlation functional: generalized gradient approximation (GGA), GGA with empirical Coulomb and exchange integrals for 3d electrons (GGA+U), nonseparable gradient approximation (NGA) with empirical Coulomb and exchange integrals for 3d electrons (NGA+U), hybrid GGA, meta-GGA, and hybrid meta-GGA. We found the coupling between the metal centers along a chain to be ferromagnetic in the case of Fe2(dobdc) and antiferromagnetic in the cases of Fe2(O)2(dobdc) and Fe2(OH)2(dobdc). The shift in magnetic coupling behavior correlates with the changing electronic structure of the framework, which derives from both structural and electronic changes that occur upon metal oxidation and addition of the charge-balancing oxo and hydroxo ligands.

Theoretical Proposal for the Whole Phosphate Diester Hydrolysis Mechanism Promoted by a Catalytic Promiscuous Dinuclear Copper(II) Complex.

The catalytic mechanism that involves the cleavage of the phosphate diester model BDNPP (bis(2,4-dinitrophenyl) phosphate) catalyzed through a dinuclear copper complex is investigated in the current study. The metal complex was originally designed to catalyze catechol oxidation, and it showed an interesting catalytic promiscuity case in biomimetic systems. The current study investigates two different reaction mechanisms through quantum mechanics calculations in the gas phase, and it also includes the solvent effect through PCM (polarizable continuum model) single-point calculations using water as solvent. Two mechanisms are presented in order to fully describe the phosphate diester hydrolysis. Mechanism 1 is of the S(N)2 type, which involves the direct attack of the μ-OH bridge between the two copper(II) ions toward the phosphorus center, whereas mechanism 2 is the process in which hydrolysis takes place through proton transfer between the oxygen atom in the bridging hydroxo ligand and the other oxygen atom in the phosphate model. Actually, the present theoretical study shows two possible reaction paths in mechanism 1. Its first reaction path (p1) involves a proton transfer that occurs immediately after the hydrolytic cleavage, so that the proton transfer is the rate-determining step, which is followed by the entry of two water molecules. Its second reaction path (p2) consists of the entry of two water molecules right after the hydrolytic cleavage, but with no proton transfer; thus, hydrolytic cleavage is the rate-limiting step. The most likely catalytic path occurs in mechanism 1, following the second reaction path (p2), since it involves the lowest free energy activation barrier (ΔG(⧧) = 23.7 kcal mol(-1), in aqueous solution). A kinetic analysis showed that the experimental k(obs) value of 1.7 × 10(-5) s(-1) agrees with the calculated value k1 = 2.6 × 10(-5) s(-1); the concerted mechanism is kinetically favorable. The KIE (kinetic isotope effect) analysis applied to the second reaction path (p2) in mechanism 1 was also taken into account to assess the changes that take place in TS1-i (transition state of mechanism 1) and to perfectly characterize the mechanism described herein.

Deprotonation of Water Ligands in V, Cr, Mn, Fe, and Co Complexes Reduces Oxidation-Driven Carboxylate Ligand Frequency Shifts.

In Mn complexes, it has been shown that oxidation-driven changes in carboxylate ligand vibrations are suppressed, if a water or hydroxo ligand is simultaneously deprotonated. Deprotonation with oxidation has also been shown to greatly reduce the dependence of Mn complex redox energies on the oxidation state of the metal. We have here investigated the effect of oxidation with deprotonation on the carboxylate ligand frequencies of V, Cr, Mn, Fe, and Co complexes. The effects of anionic ligand substitution (instead of deprotonation) and solvent dielectric were also investigated to determine the mechanism that drives carboxylate frequency shifts. It is shown that the effect of deprotonation was similar for all of the metals tested in this study. C-O bond lengths and O-C-O angle changes in the carboxylate ligand were also reduced by deprotonation. Furthermore, the effect of anionic ligand substitution was similar to deprotonation in the suppression of carboxylate frequency shifts. These shifts were also reduced by increases in the solvent dielectric, in the absence of charge conservation through deprotonation. Therefore, we conclude that carboxylate frequency shifts are largely driven by electrostatic effects.