Transition metal oxo complexes

Kinetics and mechanistic investigations on antiviral drug-valacyclovir hydrochloride by heptavalent alkaline permanganate

Abstract

Kinetics of Permanganate (MnO₄⁻) oxidation of antiviral drug, valacyclovir hydrochloride (VCH) has been studied spectrophotometrically at a constant ionic strength of 0.1 mol dm⁻³. The reaction exhibiting a 2:1 stoichiometry (MnO₄⁻:VCH) has been studied over a wide range of experimental conditions. It was found that the rate enhancement was associated with an increase in concentrations of alkali, reductant and temperature. A plausible mechanism involving an intermediate Mn(VII)-VCH complex (C) was expected and rate law is derived accordingly. Calculated activation parameters also supported the anticipated mechanism.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12039-021-01969-4.

On the Origins of Some Spectroscopic Properties of "Purple Iron" (the Tetraoxoferrate(VI) Ion) and Its Pourbaix Safe-Space

In this work, the authors attempt to interpret the visible, infrared and Raman spectra of ferrate(VI) by means of theoretical physical-inorganic chemistry and historical highlights in this field of interest. In addition, the sacrificial decomposition of ferrate(VI) during water treatment will also be discussed together with a brief mention of how Rayleigh scattering caused by the decomposition of Fe^VIO₄²⁻ may render absorbance readings erroneous. This work is not a compendium of all the instrumental methods of analysis which have been deployed to identify ferrate(VI) or to study its plethora of reactions, but mention will be made of the relevant techniques (e.g., Mössbauer Spectroscopy amongst others) which support and advance this overall discourse at appropriate junctures, without undue elaboration on the foundational physics of these techniques.

Dioxo-vanadium(v), oxo-rhenium(v) and dioxo-uranium(vi) complexes with a tridentate Schiff base ligand

The complexation of a tridentate Schiff base ligand with three oxo-metal ions lead to the formations of four new complexes.

Mononuclear anionic AO2X3 compounds with non-VSEPR structure

Mononuclear (MoO₂X₃)⁻ compounds exhibit edge-capped tetrahedral structures, as has been theoretically calculated for X = H, Cl and CN and experimentally found for X = C₆F₅.

Role of the secondary coordination sphere in metal-mediated dioxygen activation

Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining the physical properties of transition-metal complexes. We now know that the secondary coordination sphere impacts nearly all aspects of transition-metal chemistry, including the reactivity and selectivity in metal-mediated processes. These features are highlighted in the binding and activation of dioxygen by transition-metal complexes. There are clear connections between control of the secondary coordination sphere and the ability of metal complexes to (1) reversibly bind dioxygen or (2) bind and activate dioxygen to form highly reactive metal-oxo complexes. In this Forum Article, several biological and synthetic examples are presented and discussed in terms of structure-function relationships. Particular emphasis is given to systems with defined noncovalent interactions, such as intramolecular H-bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage of C-H bonds by metal-oxo complexes with basic oxo ligands is described.

Large effect of a small substitution: competition of dehydration with charge retention and coulomb explosion in gaseous [(bipy(R))Au(mu-O)2Au(bipy(R))]2+ dications

Dinuclear gold(III) clusters with a rhombic Au(2)O(2) core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy(R)) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy(R))Au(mu-O)(2)Au(bipy(R))](2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(mu-O)(2)Au(bipy)](2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H(2)O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy(R) with R = CH(3), CH(CH(3))(2), CH(2)C(CH(3))(3), and 2,6-C(6)H(3)(CH(3))(2)) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au(2)O(2) core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.

Characterization of the structure and reactivity of monocopper-oxygen complexes supported by β-diketiminate and anilido-imine ligands

Copper-oxygen complexes supported by beta-diketiminate and anilido-imine ligands have recently been reported (Aboelella et al., J Am Chem Soc 2004, 126, 16896; Reynolds et al., Inorg Chem 2005, 44, 6989) as potential biomimetic models for dopamine beta-monooxygenase (DbetaM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM). However, in contrast to the enzymatic systems, these complexes fail to exhibit C--H hydroxylation activity (Reynolds et al., Chem Commun 2005, 2014). Quantum chemical characterization of the 1:1 Cu-O(2) model adducts and related species (Cu(III)-hydroperoxide, Cu(III)-oxo, and Cu(III)-hydroxide) indicates that the 1:1 Cu-O(2) adducts are unreactive toward substrates because of the weakness of the O--H bond that would be formed upon hydrogen-atom abstraction. This in turn is ascribed to the 1:1 adducts having both low reduction potentials and basicities. Cu(III)-oxo species on the other hand, determined to be intermediate between Cu(III)-oxo and Cu(II)-oxyl in character, are shown to be far more reactive toward substrates. Based on these results, design strategies for new DbetaM and PHM biomimetic ligands are proposed: new ligands should be made less electron rich so as to favor end-on dioxygen coordination in the 1:1 Cu-O(2) adducts. Comparison of the relative reactivities of the various copper-oxygen complexes as hydroxylating agents provides support for a Cu(II)-superoxide species as the intermediate responsible for substrate hydroxylation in DbetaM and PHM, and suggests that a Cu(III)-oxo intermediate would be competent in this process as well.

Synthesis, Structure, and Reactivity of O-Donor Ir(III) Complexes: C−H Activation Studies with Benzene

Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)2Ir(R)(L), R-Ir-L (acac-O,O = kappa2-O,O-acetylacetonate, -Ir- is the trans-(acac-O,O)2Ir(III) motif, R = CH3, C2H5, Ph, PhCH2CH2, L = Py) have been synthesized using the dinuclear complex [Ir(mu-acac-O,O,C3)-(acac-O,O)(acac-C3)]2, [acac-C-Ir]2, or acac-C-Ir-H2O. The dinuclear Ir (III) complexes, [Ir(mu-acac-O,O,C3)-(acac-O,O)(R)]2 (R = alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-Ir-Py (deltaH++ = 22.8 +/- 0.5 kcal/mol; deltaS++ = 8.4 +/- 1.6 eu; deltaG++298 K) = 20.3 +/- 1.0 kcal/mol) and CH3-Ir-Py (deltaH++ = 19.9 +/- 1.4 kcal/mol; deltaS++ = 4.4 +/- 5.5 eu; deltaG++298 K) = 18.6 +/- 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative, intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation parameters (deltaS++ =11.5 +/- 3.0 eu; deltaH++ = 41.1 +/- 1.1 kcal/mol; deltaG++298 K = 37.7 +/- 1.0 kcal/mol) for CH activation were obtained using CH3-Ir-Py as starting material at a constant ratio of [Py]/[C6D6] = 0.045. Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B) unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-R-Ir- square; (C) rate-determining coordination of this species to benzene to generate a discrete benzene complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with mixtures of C6H6/C6D6 (KIE = 1) and with 1,3,5-C6H3D3 (KIE approximately 3.2 at 110 degrees C) are consistent with this reaction mechanism.

Stepwise Oxidation of Anilines by cis-[RuIV(bpy)2(py)(O)]2+

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (bpy is 2,2'-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H(2)O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 degrees C, CH(3)CN) = (2.05 +/- 0.18) x 10(2) M(-1) s(-1) (DeltaH(++) = 5.0 +/- 0.7 kcal/mol; DeltaS(++) = -31 +/- 2 eu). On the basis of competition experiments, k(H)2(O)/k(D)2(O) kinetic isotope effects, and the results of an (18)O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (Ru(IV)=O(2+)). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH(+)) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by Ru(IV)=O(2+) to PhNO and PhNHNHPh to PhN=NPh. The rate laws for these reactions are first order in Ru(IV)=O(2+) and first order in reductant with k(14.4 degrees C, H(2)O/(CH(3))(2)CO) = (4.35 +/- 0.24) x 10(6) M(-1) s(-1) for PhNHOH and k(25.1 degrees C, CH(3)CN) = (1.79 +/- 0.14) x 10(4) M(-1) s(-1) for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO(2) and PhN=NPh to PhN(O)=NPh. The oxidation of PhNO is first order in PhNO and in Ru(IV)=O(2+) with k(25.1 degrees C, CH(3)CN) = 6.32 +/- 0.33 M(-1) s(-1) (DeltaH(++) = 4.6 +/- 0.8 kcal/mol; DeltaS(++) = -39 +/- 3 eu). The reaction occurs by O-atom transfer, as shown by an (18)O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.

Alkane C−H Activation and Catalysis by an O-Donor Ligated Iridium Complex

The first examples of well-defined, O-donor ligated, late-metal complexes that are competent for alkane C-H activation are reported. These complexes exhibit thermal and protic stability and are efficient catalysts for H/D exchange reactions with alkanes.

Solution chemistry of element 106: theoretical predictions of hydrolysis of group 6 cations Mo, W, and Sg

Fully relativistic molecular density-functional calculations of the electronic structure of hydrated and hydrolyzed complexes have been performed for the group 6 elements Mo, W, and element 106, Sg. By use of the electronic density distribution data, relative values of the free energy changes and constants of hydrolysis reactions were defined. The results show hydrolysis of the cationic species with the formation of neutral molecules to decrease in the order Mo > W > Sg, which is in agreement with experiments for Mo, W, and Sg. For the further hydrolysis process with the formation of anionic species, the trend is reversed: Mo > Sg > W. A decisive energetic factor in the hydrolysis process proved to be a predominant electrostatic metal-ligand interaction.