Kinetic UV-Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [OsVIIIO4(OH)n]n- (n = 1, 2) and Methanol in a Basic Aqueous Matrix.

This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by OsVIII oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of OsVII species via a comproportionation reaction between OsVIII and OsVI. UV-Vis spectroscopy kinetic analyses with either CH3OH or the deuterated analogue CD3OH as a reducing agent revealed that transfer of α-carbon-hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ⧧H° = 14.4 ± 1.2 kcal mol-1 (CH3OH) and 14.5 ± 1.3 kcal mol-1 (CD3OH)] but lowered activation entropy (Δ⧧S°) from -12.5 ± 4.1 cal mol-1 K-1 (CH3OH) to -17.1 ± 4.4 cal mol-1 K-1 (CD3OH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and OsVIII first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon-hydrogen transfer (not O-H transfer) from methanol to OsVIII by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and OsVI dissociate from each other. DFT-calculated Δ⧧H° is within 5 kcal mol-1 of the experimentally obtained value, while the DFT Δ⧧S° is three times larger than that found from the experiment.

New VIVO-complexes for oxidative desulfurization of refractory sulfur compounds in fuel: synthesis, structure, reactivity trend and mechanistic studies.

A series of 5-coordinate oxidovanadium(iv) complexes based on 2-(2'-hydroxyphenyl)imidazole (HPIMH), with substituent groups of different electronegativities on the phenolic para position (HPIMX; X = -H, -Br, -OMe and -NO2), were synthesized and characterized. Three of these complexes were characterized by single crystal X-ray diffraction, [VIVO(PIMH)2], [VIVO(PIMBr)2] and [VIVO(PIMNO2)2], as well as a dioxidovanadium(v) compound ([VVO2(PIMH)(PIMH2)]). The complexes were tested for their catalytic activities in the oxidation of dibenzothiophene (DBT), the major refractory organosulfur compound found in fuel. The nitro substituted compound [VIVO(PIMNO2)2] had the highest catalytic oxidation activity followed by: [VIVO(PIMH)2] > [VIVO(PIMBr)2] > [VIVO(PIMMeO)2]. The decrease in activity is attributed to the different electronegativities of the substituent groups, which influence the electron density on the metal center, the V[double bond, length as m-dash]O bond distances and infrared stretching bands. Geometry index (τ) values calculated from single crystal X-ray diffraction (SC-XRD) data and DFT studies provided further insights on the trend in activity observed. SC-XRD, EPR, 51V NMR and UV-Vis spectroscopies, and DFT studies were instrumental in studying the mechanism of the catalyzed reaction and proposal of intermediate species. Both radical and non-radical pathways are plausible for the catalytic oxidation and participation of reactive oxygen species in both pathways is also postulated.

A DFT Mechanistic Study of the trans-[OsVIO2(OH)4]2- and [OsVIIIO4(OH) n] n- ( n = 1, 2 cis) Comproportionation Proton-Coupled Electron Transfer Reaction.

Herein, we present a DFT computational study of the trans-[OsVIO2(OH)4]2- and [OsVIIIO4(OH) n] n- ( n = 1, 2 cis) comproportionation reaction mechanism that occurs in a basic aqueous matrix. The reaction pathway where [OsVIIIO4(OH)]- reacts with trans-[OsVIO2(OH)4]2- via an intermediate mediated concerted electron-proton transfer yielded the best agreement with experiment (Δ‡ H°, Δ‡ S° and Δ‡ G° experimental data for the forward reaction are 10.3 ± 0.5 kcal mol-1, -2.6 ± 1.6 cal mol-1 K-1, and 11.1 ± 0.9 kcal mol-1 and for the reverse reaction are -6.7 ± 1.0 kcal mol-1, -63.6 ± 3.4 cal mol-1 K-1, and 12.2 ± 2.0 kcal mol-1, respectively, where at the PBE-D3 level for the forward reaction are 11.3 kcal mol-1, -9.8 cal mol-1 K-1, and 14.2 kcal mol-1 and for the reverse reaction are -11.8 kcal mol-1, -80.7 cal mol-1 K-1, and 12.3 kcal mol-1, respectively) and consists of (i) formation of a (singlet spin state) noncovalent adduct, [OsVIII═O···HO-OsVI]3-, (ii) spin-forbidden, concerted electron-proton transfer (i-EPT) from the trans-[OsVIO2(OH)4]2- donor to the OsVIII acceptor to form a second (triplet spin state) noncovalent adduct, [OsVII-OH···O═OsVII]3-, (iii) separation of the OsVII monomers, and finally (iv) interconversion of the separated species to form trans-[OsVIIO3(OH)2]- and mer-[OsVIIO3(OH)3]2- stereoisomer species. i-EPT from OsVI to the OsVIII species was found to be the rate-determining step, which corroborated the experimental evidence (kinetic isotope effect) that the rate-determining step involves the transfer of a proton.

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.

A DFT study to unravel the ligand exchange kinetics and thermodynamics of Os(VIII) oxo/hydroxido/aqua complexes in aqueous matrices.

The Os(VIII) oxo/hydroxido complexes that are abundant in mild to relatively concentrated basic aqueous solutions are Os(VIII)O4, [Os(VIII)O4(OH)](-) and two cis-[Os(VIII)O4(OH)2](2-) species. Os(VIII) complexes that contain water ligands are thermodynamically unfavoured w.r.t. the abovementioned species. Os(VIII)O4 reacts with hydroxide in two, consecutive, elementary coordination sphere expansion steps to form the [Os(VIII)O4(OH)](-) complex and then the cis-[Os(VIII)O4(OH)2](2-) species. The Gibbs energy of activation for both reactions, in the forward and reverse direction, are in the range of 6-12 kcal mol(-1) and are relatively close to diffusion-controlled. The thermodynamic driving force of the first reaction is the bonding energy of the Os(VIII)-OH metal-hydroxido ligand, while of the second reaction it is the relatively large hydration energy of the doubly-charged cis-[Os(VIII)O4(OH)2](2-) product compared to the singly-charged reactants. The DFT-calculated (PBE-D3 functional) in the simulated aqueous phase (COSMO) is -2.4 kcal mol(-1) for the first reaction and -0.6 kcal mol(-1) for the second reaction and agree to within 1 kcal mol(-1) with reported experimental values, at -2.7 and 0.3 kcal mol(-1) respectively. From QTAIM and EDA analyses it is deduced that the Os(VIII)[double bond, length as m-dash]O bonding interactions are ionic (closed-shell) and that Os(VIII)-OH bonding interactions are polar covalent (dative). In contrast to QTAIM, NCI analysis allowed for the identification of relatively weak intramolecular hydrogen bonding interactions between neighbouring oxo and hydroxido ligands in both [Os(VIII)O4(OH)](-) and cis-[Os(VIII)O4(OH)2](2-) complexes.

Cyclometallated platinum(II) complexes of benzylidene-2,6-di-isopropylphenylamine containing bidentate phosphines: synthesis, structural properties and reactivity studies.

The reaction of the cyclometallated complex [PtCl(N^C)(dmso)], 1 (N^C represents the cyclometallated Schiff base, benzylidene-2,6-diisopropylphenylamine), with 1,1'-bis(diphenylphosphino)ferrocene, dppf, bis(diphenylphosphino)methane, dppm, or 1,2-bis(diphenylphosphino)ethane, dppe, in a 2 : 1 ratio or an equimolar ratio using acetone as the solvent produced the corresponding binuclear or mononuclear diphosphine platinum complexes. In the case of the mononuclear complexes, the diphosphines act as either a bidentate ligand or a monodentate ligand depending on the size of the bite angle of the diphosphines, while in the case of the binuclear complexes, the diphosphines act as a bridging ligand between the two metal centres. The solid state structures of some of the binuclear as well as mononuclear species are reported. The mononuclear derivatives were found to show different behaviour in solution and in the solid state when compared to the binuclear analogues. This behaviour is also influenced by the nature of the diphosphine ligands employed.

Cation-π induced aggregation of water-soluble [Pt(II)(diimine)(L(n)-S,O)]+ complexes studied by 1H DOSY NMR and TEM: from 'dimer aggregates' in acetonitrile to nano-aggregates ('metallogels') in water.

(1)H NMR chemical shift concentration dependence as well as the diffusion coefficients from DOSY NMR of mixed ligand [Pt(II)(1,10-phenanthroline)(N-pyrrolidyl-N-(2,2-dimethylpropanoyl)thiourea)]Cl ([Pt(II)(phen)(L(1)-S,O)]Cl) dissolved in mixtures of acetonitrile-water in the range 0-30% (v/v) D(2)O-CD(3)CN shows that the complex cation (M(+) = [Pt(II)(phen)(L(1)-S,O)](+)) aggregates to form dimers, 2M(+) ⇌ {M(+)}(2), with association constants ranging from K(D)(CD(3)CN) = 17 ± 2 M(-1) to K(D)(30% (v/v) D(2)O-CD(3)CN) = 71 ± 8 M(-1) at 299.3 K, presumably via non-covalent cation-π interactions. Experimental data are consistent with an 'offset' face-to-face cation-π stacking arrangement of the planar cation. However in water-rich solvent mixtures from >30% (v/v) D(2)O-CD(3)CN to pure D(2)O, the extent of aggregation significantly increases until a critical aggregation concentration (CAC) is reached, estimated to be 9.6 and 10.3 mM from (1)H NMR chemical shift concentration dependence and DOSY NMR measurements respectively. Above the CAC the formation of nano-structures formulated as {[Pt(II)(phen)(L(1)-S,O)](+)}(n)Cl(-)(y) (n, y > 2) is indicated. DOSY studies show a significant decrease of the average diffusion coefficient D(obs) as a function of increasing concentration of [Pt(II)(phen)(L(1)-S,O)]Cl in D(2)O. The aggregation number (N) estimated from hydrodynamic volumes of the mononuclear [Pt(II)(phen)(L(1)-S,O)](+) cation (V(H)(0)), and those V(H) estimated from D(obs) (N = V(H)/V(H)(0)) as a function of total complex concentration, ranges from ~2 to ~735 in pure D(2)O. Above the CAC well defined nano-structures which may be loosely termed "metallogels" could be characterized by means of transmission electron microscopy. As expected the addition of NaCl appears to increase the extent of aggregate formation, by presumably stabilizing the formation of nano-sized {[Pt(II)(phen)(L(1)-S,O)](+)}(n)Cl(-)(y) aggregates preventing excessive positive electrostatic charge build-up.

(35/37)Cl and (16/18)O isotope resolved 195Pt NMR: unique spectroscopic 'fingerprints' for unambiguous speciation of [PtCl(n)(H2O)(6-n)](4-n) (n = 2-5) complexes in an acidic aqueous solution.

At high magnetic fields the 128.8 MHz (195)Pt NMR of all the species in the series [PtCl(n)(H(2)O)(6-n)](4-n) (n = 2-6) display unique (35/37)Cl isotope effects resulting in a unique 'fine-structure' of each individual resonance, which constitutes an unambiguous spectroscopic 'fingerprint' characteristic of the structure of the octahedral platinum(IV) complex, provided (195)Pt NMR are recorded at optimum magnetic field homogeneity and carefully controlled temperature (293 ± 0.1 K). The detailed (195)Pt resonance fine-structure observed experimentally can readily be accounted for by an isotopologue and isotopomer model for each complex, showing particularly noticeable differences between stereoisomer pairs such as the cis/trans- and fac/mer-complexes. Moreover partial isotopic (18)O enrichment of the coordinated water molecules in the series [Pt(35/37)Cl(n)(H(2)(16/18)O)(6-n)](n-2) (n = 2-6) confirms this model. This technique can thus be considered a novel, direct spectroscopic method of chemical speciation of appropriate platinum(IV) complexes in solution without reference to accurate chemical shifts of authentic members of such a series. These effects are interpreted qualitatively in terms of the high sensitivity of (195)Pt NMR shielding to very small and subtle Pt-(35/37)Cl and Pt-(16/18)OH(2) bond displacements. Preliminary work shows this also applied to the corresponding bromido-complexes.

35Cl/37Cl isotope effects in 103Rh NMR of [RhCl(n)(H2O)(6-n)](3-n) complex anions in hydrochloric acid solution as a unique 'NMR finger-print' for unambiguous speciation.

A detailed analysis of the (35)Cl/(37)Cl isotope effects observed in the 19.11 MHz (103)Rh NMR resonances of [RhCl(n)(H(2)O)(6-n)](3-n) complexes (n=3-6) in acidic solution at 292.1K, shows that the 'fine structure' of each (103)Rh resonance can be understood in terms of the unique isotopologue and in certain instances the isotopomer distribution in each complex. These (35)Cl/(37)Cl isotope effects in the (103)Rh NMR resonance of the [Rh(35/37)Cl(6)](3-) species manifest only as a result of the statistically expected (35)Cl/(37)Cl isotopologues, whereas for the aquated species such as for example [Rh(35/37)Cl(5)(H(2)O)](2-), cis-[Rh(35/37)Cl(4)(H(2)O)(2)](-) as well as the mer-[Rh(35/37)Cl(3)(H(2)O)(3)] complexes, additional fine-structure due to the various possible isotopomers within each class of isotopologues, is visible. Of interest is the possibility of the direct identification of stereoisomers cis-[RhCl(4)(H(2)O)(2)](-), trans-[RhCl(4)(H(2)O)(2)](-), fac-[RhCl(3)(H(2)O)(3)] and mer-[RhCl(3)(H(2)O)(3)] based on the (103)Rh NMR line shape, other than on the basis of their very similar δ((103)Rh) chemical shift. The (103)Rh NMR resonance structure thus serves as a novel and unique 'NMR-fingerprint' leading to the unambiguous assignment of [RhCl(n)(H(2)O)(6-n)](3-n) complexes (n=3-6), without reliance on accurate δ((103)Rh) chemical shifts.

A robust method for speciation, separation and photometric characterization of all [PtCl(6-n)Br(n)]2- (n=0-6) and [PtCl(4-n)Br(n)]2- (n=0-4) complex anions by means of ion-pairing RP-HPLC coupled to ICP-MS/OES, validated by high resolution 195Pt NMR spectroscopy.

A robust reversed phase ion-pairing RP-HPLC method has been developed for the unambiguous speciation and quantification of all possible homoleptic and heteroleptic octahedral platinum(IV) [PtCl(6-n)Br(n)](2-) (n=0-6) as well as the corresponding platinum(II) [PtCl(4-n)Br(n)](2-) (n=0-4) complex anions using UV/Vis detection. High resolution (195)Pt NMR in more concentrated solutions of these Pt(II/IV) complexes (≥50 mM) served to validate the chromatographic peak assignments, particularly in the case of the possible stereoisomers of Pt(II/IV) complex anions. By means of IP-RP-HPLC coupled to ICP-MS or ICP-OES it is possible to accurately determine the relative concentrations of all possible Pt(II/IV) species in these solutions, which allows for the accurate determination of the photometric characteristics (λ(max) and ɛ) of all the species in this series, by recording of the UV/Vis absorption spectra of all eluted species, using photo-diode array, and quantification with ICP-MS or ICP-OES. With this method it is readily possible to separate and estimate the concentrations of the various stereoisomers which are present in these solutions at sub-millimolar concentrations, such as cis- and trans-[PtCl(4)Br(2)](2-), fac- and mer-[PtCl(3)Br(3)](2-) and cis- and trans-[PtCl(2)Br(4)](2-) for Pt(IV), and cis- and trans-[PtCl(2)Br(2)](2-) in the case of Pt(II). All mixed halide Pt(II) and Pt(IV) species can be separated and quantified in a single IP-RP-HPLC experiment, using the newly obtained photometric molar absorptivities, ɛ, determined herein at given wavelengths.

A kinetic and thermodynamic study of the unexpected comproportionation reaction between cis-[Os(VIII)O4(OH)2](2-) and trans-[Os(VI)O2(OH)4](2-) to form a postulated [Os(VII)O3(OH)3](2-) complex anion.

A kinetic study of [OsO(4)] reduction by aliphatic alcohols (MeOH and EtOH) was performed in a 2.0 M NaOH matrix at 298.1 K. The rate model that best fitted the UV-VIS data supports a one-step, two electron reduction of Os(VIII) (present as both the [Os(VIII)O(4)(OH)](-) and cis-[Os(VIII)O(4)(OH)(2)](2-) species in a ratio of 0.34:0.66) to form the trans-[Os(VI)O(2)(OH)(4)](2-) species. The formed trans-[Os(VI)O(2)(OH)(4)](2-) species subsequently reacts relatively rapidly with the cis-[Os(VIII)O(4)(OH)(2)](2-) complex anion to form a postulated [Os(VII)O(3)(OH)(3)](2-) species according to: cis-[Os(VIII)O(4)(OH)(2)](2-) + trans-[Os(VI)O(2)(OH)(4)](2-) (k+2) <−> (k-2) 2[Os(VII)O(3)(OH)(3)](2-). The calculated forward, k(+2), and reverse, k(-2), reaction rate constants of this comproportionation reaction are 620.9 ± 14.6 M(-1) s(-1) and 65.7 ± 1.2 M(-1) s(-1) respectively. Interestingly, it was found that the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion does not oxidize MeOH or EtOH. Furthermore, the reduction of Os(VIII) with MeOH or EtOH is first order with respect to the aliphatic alcohol concentration. In order to corroborate the formation of the [Os(VII)O(3)(OH)(3)](2-) species predicted with the rate model simulations, several Os(VIII)/Os(VI) mole fraction and mole ratio titrations were conducted in a 2.0 M NaOH matrix at 298.1 K under equilibrium conditions. These titrations confirmed that the cis-[Os(VIII)O(4)(OH)(2)](2-) and trans-[Os(VI)O(2)(OH)(4)](2-) species react in a 1:1 ratio with a calculated equilibrium constant, K(COM), of 9.3 ± 0.4. The ratio of rate constants k(+2) and k(-2) agrees quantitatively with K(COM), satisfying the principle of detailed balance. In addition, for the first time, the molar extinction coefficient spectrum of the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion is reported.

Separation and quantification of [RhCln(H2O)(6-n)](3-n) (n=0-6) complexes, including stereoisomers, by means of ion-pair HPLC-ICP-MS.

A hyphenated ion-pair (tetrabutylammonium chloride-TBACl) reversed phase (C(18)) HPLC-ICP-MS method (High Performance Liquid Chromatography Inductively Coupled Plasma Mass Spectroscopy) for anionic Rh(III) aqua chlorido-complexes present in an HCl matrix has been developed. Under optimum chromatographic conditions it was possible to separate and quantify cationic Rh(III) complexes (eluted as a single band), [RhCl(3)(H(2)O)(3)], cis-[RhCl(4)(H(2)O)(2)](-), trans-[RhCl(4)(H(2)O)(2)](-) and [RhCl(n)(H(2)O)(6-n)](3-n) (n=5, 6) species. The [RhCl(n)(H(2)O)(6-n)](3-n) (n=5, 6) complex anions eluted as a single band due to the relatively fast aquation of [RhCl(6)](3-) in a 0.1 mol L(-1) TBACl ionic strength mobile phase matrix. Moreover, the calculated t(1/2) of 1.3 min for [RhCl(6)](3-) aquation at 0.1 mol kg(-1) HCl ionic strength is significantly lower than the reported t(1/2) of 6.3 min at 4.0 mol kg(-1) HClO(4) ionic strength. Ionic strength or the activity of water in this context is a key parameter that determines whether [RhCl(n)(H(2)O)(6-n)](3-n) (n=5, 6) species can be chromatographically separated. In addition, aquation/anation rate constants were determined for [RhCl(n)(H(2)O)(6-n)](3-n) (n=3-6) complexes at low ionic strength (0.1 mol kg(-1) HCl) by means of spectrophotometry and independently with the developed ion-pair HPLC-ICP-MS technique for species assignment validation. The Rh(III) samples that was equilibrated in differing HCl concentrations for 2.8 years at 298K was analyzed with the ion-pair HPLC method. This analysis yielded a partial Rh(III) aqua chlorido-complex species distribution diagram as a function of HCl concentration. For the first time the distribution of the cis- and trans-[RhCl(4)(H(2)O)(2)](-) stereoisomers have been obtained. Furthermore, it was found that relatively large amounts of 'highly' aquated [RhCl(n)(H(2)O)(6-n)](3-n) (n=0-4) species persist in up to 2.8 mol L(-1) HCl and in 1.0 mol L(-1) HCl the abundance of the [RhCl(5)(H(2)O)](2-) species is only 8-10% of the total, far from the 70-80% as previously proposed. A 95% abundance of the [RhCl(6)](3-) complex anion occurs only when the HCl concentration is above 6 mol L(-1). The detection limit for a Rh(III) species eluted from the column is below 0.147 mg L(-1).

(195)Pt NMR isotopologue and isotopomer distributions of [PtCl(n)(H(2)O)(6 - n)](4 - n) (n = 6,5,4) species as a fingerprint for unambiguous assignment of isotopic stereoisomers.

A detailed analysis of the (35)Cl/(37)Cl isotope shifts induced in the 128.8 MHz (195)Pt NMR resonances of [PtCl(n)(H(2)O)(6 - n)](4 - n) complexes (n = 6,5,4) in acidic solution at 293 K, shows that the unique isotopologue and isotopomer distribution displayed by the resolved (195)Pt resonances, serve as a fingerprint for the unambiguous identification and assignment of the isotopic stereoisomers of [PtCl(5)(H(2)O)](-) and cis/trans-[PtCl(4)(H(2)O)(2)].