Hard Soft Acid Base Theory Research Articles

Nanostructured Semiconducting Metal Oxides for Ammonia Sensors. A Novel HSAB Sensing Paradigm

The paper demonstrates how the Hard-Soft Acid Base (HSAB) theory can be used as a valuable criterion in the selection process of semiconducting metal oxides (MOX) suitable as sensing layers for ammonia detection. Six different cases of ammonia detection performed by chemiresistive sensors employing MOX and related nanocomposites as sensing layers are identified and discussed. The role of HSAB as an efficient selection tool for appropriate sensing layer (any type of gas), is further reinforced by analyzing and discussing literature results on MOX-based trimethylamine sensing layers. By analyzing the operation of a fiber-optic ammonia sensor, we demonstrate that the HSAB principle can be also successfully applied to the selection of sensing layers for detectors employing other sensing principles, different than the chemiresistive one. Changing the sensing paradigm (i.e., the amino groups-based compounds are part of the sensing layer, rather than part of the analyte), the paper shows that these types of molecules (polymers, carbon nanotubes, ionic liquids) are appropriate constituents of a CO2 sensing layer, in full accordance to the HSAB criteria.

Controlling Reversible Expansion of Li2O2 Formation and Decomposition by Modifying Electrolyte in Li-O2 Batteries

Summary The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(II) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries.

Elucidating the Structure and Electrochemical Properties of Dimethylacetamide-Based Electrolytes for Lithium-Air Batteries

Improving the performance and rechargeability of Lithium-air batteries, which offer up to ten times more energy density than Li-ion batteries, has been the focus of much research in the last decade.1, 2 One of the ways to improve these batteries is to gain a deeper understanding of the oxygen reduction reactions (ORR) in non-aqueous electrolytes in order to find an optimum combination of Li salt and solvent that provides chemical stability and rechargeability. In this paper, we report the results of a study of the physical and electrochemical properties of N,N-dimethylacetamide (DMAc)-based electrolytes containing either lithium or tetrabutylammonium (TBA) salts (LiX and TBAX, where X= PF6 -, CF3SO3 -, ClO4 - and NO3 -). The chemical structures of the salt solutions were determined from NMR and FTIR spectroscopic data. The spectra revealed that DMAc (CH3CON(CH3)2) solvates the lithium salts via bonding between its carbonyl (C=O) group and Li+ to form ion pairs. In the case of Li salts with the anions PF6 -, ClO4 - or NO3, the ion pairs are solvent separated while in the case of CF3SO3 -, it is a contact ion pair. On the other hand, the nature of the interactions between DMAc and the TBA+ ions appeared to be dipolar in agreement with its soft Lewis acidity.3 The conductivities of these electrolytes were measured as a function of temperature from -20 to 40 oC and correlated with their concentrations and viscosities. The conductivities plotted versus temperature showed a linear fit for Vogel-Tamman-Fulcher (VTF) relationship indicating a solvent-assisted ion conduction mechanism. Diffusion coefficient and solubility of O2 in the various electrolytes measured using a microelectrode technique are reported in Table 1. Cyclic voltammograms (CV) for ORR on microelectrode in TBA/DMAc electrolytes (Figure 1A) showed steady-state limiting current behavior indicating that the ORR process is electrochemically reversible. In all Li/DMAc electrolytes the CVs (Figure 1B) on microelectrodes showed an electrochemical irreversible behavior. The irreversibility of the ORR process in Li/DMAc electrolytes is primarily caused by the deposition of insoluble reduction products on the electrode surface. The results obtained in this study correlates with the high donor number of DMAc (27.8) and comply with the Hard Soft acid base (HSAB) theory.3, 4

Construction of heterometallic M2Pd3 supramolecular cages via a metalloligand strategy as heterogeneous catalyst for Suzuki–Miyaura coupling reaction

On the basis of hard soft acid base theory, two neutral heterometallic M2Pd3 supramolecular cages, [Al2Pd3(L)6Cl6] (1) and [Fe2Pd3(L)6Cl6] (2) (where HL is 1-(4-(1H-imidazol-1-yl)phenyl)butane-1,3-dione), have been constructed by using the tripodal M(L)3 as metalloligands in combination with PdCl2 or Pd(MeCN)2Cl2. The supramolecular cage can be described as a trigonal bipyramid with two M3+ ions occupying at the two axial sites in ΔΔ or ΛΛ configurations and three Pd2+ ions sitting in the three equatorial positions with square planar geometry in trans configuration. The supramolecular cages exhibit high catalytic activity for the Suzuki–Miyaura coupling with arylbromide or arylchrolide and phenylboronic acid in biorenewable solvent glycerol under mild conditions. Moreover, the catalyst can be readily recycled and reused at least five times without loss of catalytic activity. The high catalytic performance may be attributed to the distinct properties of the cage structure with uniformly distributive and well-defined Pd active centres on the cage surfaces. The high stability in the cage structure also prevents Pd leaching out and agglomeration.

Unraveling the Correlation between Solvent Properties and Sulfur Redox Behavior in Lithium-Sulfur Batteries

Systematic understanding of how solvent property influences Li-S redox chemistry is required to develop an effective electrolyte for Li-S batteries. In this study, we investigate the correlation between solvent property and Li-S redox chemistry in nine non-aqueous electrolyte solvents that cover a wide range of three main solvent physiochemical properties, namely dielectric constant (ɛ), Gutmann donor number (DN), and acceptor number (AN). We exploit various analytical techniques including cyclic voltammetry, rotating ring disk electrode technique, UV-Vis spectroscopy and galvanostatic measurement in a two-compartment cell. We show that the potential of S8-reduction increases with increasing AN and that the polysulfide-reduction/oxidation is strongly influenced by the DN. The common discrepancy in the literature on the role of dielectric constant and donor number is addressed by examining the redox reactions, polysulfide stability, and the effect of salt concentration in acetonitrile - a solvent with high dielectric constant and low DN. We show that the DN is the primary descriptor for polysulfide redox reactions, as it controls the effective charge density of the solvated cation (Li+), which affects the stability of polysulfides with different charge density via Pearson's Hard Soft Acid Base theory.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals.

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Influence of counterion on the methylation of some ambident nucleophiles: DFT study

The reactions of ambident anions YCHX− and nucleophiles YCHXNa (X, Y = CH2, O, and S) with CH3F were investigated in the gas phase and THF solution. For this purpose the M06-2X/6-311+G(d,p) theoretical model was engaged in combination with the CPCM solvation model. The goal of the work was to examine the influence of counterion (Na+ in this case) to the reaction systems. It was found that the order of reactivity of non-ambident anions in both media is as follows: $${\text{CH}}_{2} {\text{CHCH}}_{2}^{ - }$$ > OCHO− > SCHS−. On the other side, the reactivity of non-ambident nucleophiles follows different order: CH2CHCH2Na > SCHSNa > OCHONa. The latter order of reactivity is in agreement with chemical intuition as it is inversely proportional to the order of hardness of the respective nucleophilic atoms: C < S < O. Ambident nucleophiles behave differently in the two media, thus indicating that solvents also influence the examined reactions. Namely, HOMO–LUMO gaps are smaller in the gas-phase, whereas charge distribution is more favorable in THF solution. Owing to these facts, C-methylation is more favorable than S- and O-methylations, and S-methylation is less unfavorable than O-methylation in the gas-phase. In THF solution O- and S-methylations are kinetically controlled, and competitive C-methylations are thermodynamically controlled. In this way, the applicability of the hard–soft acid–base theory to the examined reaction systems is confirmed. Furthermore, the findings of this work offer robust confirmation to our approach which involves counterions in examinations. Counterions do exist in reaction systems, and their influence should not be neglected.

Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films

Iron doping of nickel oxide films results in enhanced activity for promoting the oxygen evolution reaction (OER). Whereas this enhanced activity has been ascribed to a unique iron site within the nickel oxide matrix, we show here that Fe doping influences the Ni valency. The percent of Fe3+ doping promotes the formation of formal Ni4+, which in turn directly correlates with an enhanced activity of the catalyst in promoting OER. The role of Fe3+ is consistent with its behavior as a superior Lewis acid.

Native Serine Peptide Assembly: Scope and Utility.

This work develops serine peptide assembly (SPA), which complements and contrasts with classic native chemical ligation (NCL). Advances in reagent-less peptide bond formation have been applied to serine (and serine models) and a range of C-terminal amino acids, including bulky residues that are not amenable to NCL. The particular appeal of SPA is preparative-scale segment condensations with zero racemization risk and favourable process mass intensity (PMI). Mechanistic studies support a previously proposed reaction pathway via an initial trans-esterification step. An understanding of the factors favouring this pathway relies on hard-soft acid-base theory, where mildly activated esters with the largest carbonyl positive charge are most reactive with hydroxy amines. Novel C-terminal activators have been discovered that enhance reactivity and give harmless by-products.

Inorganic anion-dependent assembly of zero-, one-, two- and three-dimensional Cu(II)/Ag(I) complexes under the guidance of the HSAB theory: Synthesis, structure, and magnetic property

To systematically explore the influence of inorganic anions on building coordination complexes, five novel complexes based on 1-(benzotriazole-1-methyl)−2-propylimidazole (bpmi), [Cu(bpmi)2(Ac)2]·H2O (1), [Cu(bpmi)2(H2O)2]·2NO3·2H2O (2), [Cu(bpmi)(N3)2] (3), [Ag(bpmi)(NO3)] (4) and [Cu3(bpmi)2(SCN)4(DMF)] (5) (Ac−=CH3COO−, DMF=N,N-Dimethylformamide) are synthesized through rationally introducing Cu(II) salts and Ag(I) salt with different inorganic anions. X-ray single-crystal analyses reveal that these complexes show interesting structural features from mononuclear (1), one-dimensional (2 and 3), two-dimensional (4) to three-dimensional (5) under the influence of inorganic anions with different basicities. The structural variation can be explained by the hard-soft-acid-base (HSAB) theory. Magnetic susceptibility measurement indicates that complex 3 exhibits an antiferromagnetic coupling between adjacent Cu(II) ions.

Metathesis reactivity of bis(phosphinite) pincer ligated nickel chloride, isothiocyanate and azide complexes

A series of nickel pincer complexes of the type [4-Z-2,6-(R2PO)2C6H2]NiX (R = tBu, iPr, Ph; Z = H, CO2Me; X = NCS, N3) have been synthesized from the reactions of the corresponding nickel chloride complexes [4-Z-2,6-(R2PO)2C6H2]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A comparable Ni–N bond length (approx. 1.87 Å for the isothiocyanate complexes and 1.91 Å for the azide complexes) and an almost identical Ni–Cipso bond length (approx. 1.89 Å) have been observed for these complexes. Metathesis reactivity of [4-Z-2,6-(R2PO)2C6H2]NiCl and ligand exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center. The thermodynamic stability of these nickel complexes has been rationalized using hard-soft acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These experimental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in SCN– and N3−.

How heavy metals impact on flocculation of combined pollution of heavy metals–antibiotics: A comparative study

Previous research found that certain types of heavy metals could play a “bridge” role in promoting removal of antibiotics in flocculation, but details (i.e. under what conditions and how heavy metals wielded this effect) were not clear. To investigate this point, two sorts of combined pollution [Cu(II)–tetracycline and Zn(II)–sulfadiazine] were selected in synthetic wastewaters for flocculation, to give a comparative study. A flocculant with high capacity to coordinate with heavy metals was applied. The flocculation performance declared that, Cu(II) improved removal of coexisted antibiotic molecules whereas Zn(II) did not. Analyses of macro- and micro-scopic properties of flocs demonstrated that, (i) tightly binding of the flocculant with heavy metals, and (ii) strong coordination of heavy metals–antibiotics when the two contaminants were suitably matched according to Hard–Soft-Acid–Base theory and steric effect, were two factors for heavy metals to perform the “bridge” role, and to achieve high co-removal efficiencies of both contaminants. The finding had operational significance for both promoted removal of antibiotics with the coexistence of heavy metals and mutual promotion in the removal of the combined contaminants from water.

Some insights into the chemistry of gold adsorption by thiol and amine functionalized mesoporous silica in simulated thiosulfate system

Organic functional groups such as amines and thiols have been widely used to functionalize pore wall surfaces of mesoporous silicas for a variety of applications relying on adsorption of metals. One of the promising applications of such hybrid materials is in hydrometallurgical separation of metals in a leach solution. Toward this aim, mesoporous MSU silica was prepared and functionalized with short-chain thiol and primary amine organic groups. Adsorption of gold from simulated thiosulfate leach solutions was investigated with respect to the adsorption chemistry. Adsorption capacity of thiol-bearing silica was well showing the monolayer behavior approaching an ultimate metal:ligand=1:1 capacity. Higher capacities were obtained in either alkaline condition (pH~10.5) or at higher oxidation potential (Eh~300mV) of the solution. Adsorption capacity of amine groups was to some extent proportional to their surface molar concentration, but it was also beyond the monolayer capacity of chemisorption, besides not being explicable by hard–soft acid–base theory. Despite considerable gold recovery by amine functional groups, gold was not detected in XPS analysis unless adsorption was from pure solutions. Partial electrochemical reduction to metallic gold and formation of dispersed tiny nanoparticles during interaction of gold thiosulfate species in solution with amine ligands were proposed.

Crystallization of FeOOH via iron salts: an anion-chemoaffinity controlled hydrolysis toward high performance inorganic pseudocapacitor materials

In this work, we designed a simple hydrolysis reaction to study the chemical roles of anions with different chemoaffinity abilities to iron ions on the crystallization of FeOOH. FeOOH products with different morphologies and sizes can be crystallized by using commercial iron salts such as FeSO4, FeCl2, FeCl3 and Fe(NO3)3. Their crystallization mechanisms can be evidenced in terms of hard–soft acid–base theory, which can control the chemical reaction and crystallization process. Herein, FeOOH particles with high electroactivity were successfully synthesized at 80 °C via the use of a single-source of iron salt in each corresponding experiment without adding any other reagents. The as-obtained FeOOH samples exhibit high specific capacitances of 441.2, 385.7, 577.3 and 619.6 F g−1, which show good potential to be candidates as high performance electrode materials for supercapacitors. Moreover, this facile synthesis method provides a huge opportunity for future practical applications of supercapacitors.

Electrolyte-Directed Reactions of the Oxygen Electrode in Lithium-Air Batteries

Non-aqueous electrolytes play a prominent role in the redox reactions of the oxygen electrode in the non-aqueous Li-air battery. In all electrolytes the initial O2 reduction reaction (ORR) product is superoxide, O2−, whose stability is determined by the Lewis acidity of the ion-pairing cations present in the medium as governed by the Hard Soft Acid Base (HSAB) theory. Our results suggest that depending on the basicity of the solvent as measured by its Donor Number (DN), the superoxide will be stabilized to varying lengths of time before transforming to O22− via a chemical or an electrochemical reaction. Organic solvents decrease the Lewis acidity of Li+ through the formation of solvates, Li+(solvent)n. As a result, high DN solvents such as dimethyl sulfoxide (DMSO), which decrease Li+ acidity more than low DN solvents, provide longer life-time for the soft base O2− by forming the ion pair, Li+(DMSO)n–O2−. In low DN solvents such as 1, 2-dimethoxy ethane (DME) and tetraethyleneglycol dimethyl ether (TEGDME), the LiO2 rapidly decomposes to the peroxide, Li2O2. As the ionic bond strength between the O2 reduction product and the conducting salt cation becomes stronger, its rechargeability becomes poorer. Solvent electron donor property affects ORR catalysis also.

Removal of brownish-black tarnish on silver–copper alloy objects with sodium glycinate

This article has the principal aim of presenting a new method of chemical cleaning of tarnished silver–copper alloy objects. The chemical cleaning must be harmless to the health, selective to tarnish removal, and easy to use. Sodium glycinate was selected for the study. The reactions of sodium glycinate with tarnish and the silver–copper alloy were evaluated. Products of the reaction, the lixiviated material, and the esthetics of silver–copper alloy coins (used as prototypes) were studied to evaluate if the proposed method can be applied to the cleaning of silver objects.Silver–copper alloys can be deteriorated through a uniform and superficial corrosion process that produces brownish-black tarnish. This tarnish alters the esthetic of the object. The cleaning of artistic and archeological objects requires more caution than regular cleaning, and it must take into account the procedures for the conservation and restoration of cultural heritage. There are different methods for cleaning silver–copper alloy objects, chemical cleaning is one of them. We studied two chemical cleaning methods that use sodium glycinate and sodium acetylglycinate solutions. Silver–copper alloy coins were artificially corroded in a basic thiourea solution and immersed in solutions of sodium glycinate and sodium acetylglycinate. After immersion, optical microscopy and scanning electron microscopy of the surfaces were studied. The sodium glycinate solution was shown to be very efficient in removing the brownish-black tarnish. Absorption spectroscopy measured the percentage of silver and copper lixiviated in immersion baths, and very small quantities of these metals were detected. Infrared absorption spectroscopy and X-ray fluorescence characterized the obtained products. The greater efficiency of the sodium glycinate solution compared to the sodium acetylglycinate solution was explained by chelation and Hard–Soft Acid–Base Theory with the aid of quantum chemical calculations.

Identifying Descriptors for Solvent Stability in Non-Aqueous Li-O2 Batteries

A crucial challenge in developing rechargeable Li-O2 batteries is the identification of stable solvents that are resistant to decomposition in the electrochemical environment of Li-O2 discharge products. Recent theoretical studies using higher order quantum chemical methods have offered excellent insights into various possible degradation mechanisms such as nucleophilic attack/substitution and hydrogen atom abstraction in the presence of O2, O2 − and O2 2− anions [1-2]. Similarly, some experimental studies have analyzed various classes of solvents based on the reversibility of the O2 ↔ O2 − ↔ O2 2− oxygen reduction reactions in them [3-4]. These investigations, however, do not explicitly take into account either the effect of the electrode potential, or any direct electrochemical interaction of the solvent with the deposited Li-O2 discharge products. In this work, we attempt to identify suitable descriptor that can be used to test for solvent stability by building a unified picture of the energy levels of the solvent molecules, Li-O2 discharge products and the electrode potential. Based on recent experimental results [5-7], we first hypothesize that although solvent degradation affects the yield of Li2O2 during discharge, the majority of solvent degradation occurs via a process of decomposition of solvent into its oxidized products during charging, which eventually leads to CO2 evolution at higher potentials in non-aqueous Li-O2 batteries. Detailed analyses of these findings lead us to conclude the presence of a near universal mechanism of solvent degradation during charging. Further, we create the case for number of electrons per O2 evolved during the charging process to be the marker of cell performance, the ideal being 2e-/O2. Based on these concepts, we propose that the HOMO level would be a good descriptor for solvent stability and demonstrate that this correlates well the degree of rechargeability. Taking this analysis a step further, we demonstrate that Gutmann donor number is also a good empirical descriptor for solvent stability, which has been explained earlier in terms of Hard Soft Acid Base Theory. Employing these descriptors, we identify several solvents which could enhance the rechargeability of non-aqueous Li-O2 batteries.

Ratiometric fluorescence imaging of lysosomal Zn2+ release under oxidative stress in neural stem cells.

Zinc dyshomeostasis is a major mechanism of neuronal death, which is involved in many different neuropathological conditions. Lysosomal membrane permeabilisation has an important function in zinc-induced neuronal death under oxidative stress. To investigate lysosomal zinc functions in neurons with high spatial and temporal reliability, we report a ratiometric probe, LysoZn-1. It is derived from the styryl-BODIPY-DPA scaffold with a lysosome-targeted 2-morpholinoethylamine moiety to allow localisation in lysosomes. The electron donor at the meso-position of the BODIPY fluorophore makes the present probe prefer complexing with Zn2+ rather than Cd2+, which can be explained by HSAB (Hard-Soft Acid-Base) theory and was confirmed by Gaussian calculation. Upon Zn2+ binding, LysoZn-1 exhibits obvious fluorescence enhancement (F578 nm) and ratio (F578 nm/F680 nm) changes. The emission intensities of LysoZn-1 and LysoZn-1+Zn2+ do not change significantly under lysosomal pH ranging from 4.5 to 6.0. Confocal imaging experiments indicate that LysoZn-1 is able to localise to lysosomes in neural stem cells (NSCs), MCF-7 and Hela cells and detect exogenous Zn2+ levels in NSCs and MCF-7 cells. LysoZn-1 function is not disturbed by chloroquine in living cells. Furthermore, increases in lysosomal Zn2+ concentration upon H2O2 stimulation in NSCs are observed using LysoZn-1.

Mercuric chloride adsorption on sulfur-containing BC2N nanotube: toward HSAB concept

Electronic sensitivity of pristine and sulfur-containing BC2N nanotubes to mercuric chloride (HgCl2) molecule was investigated by utilizing density functional theory. It was found that HgCl2 is weakly adsorbed on the tube, releasing energy of 6.4–8.1 kcal/mol, and electronic properties of the tube are not significantly changed. To overcome this problem, carbon atoms of the tube were substituted by sulfur atom. Of interest are sulfur derivative functional adsorbents because of their strong affinity toward mercury. This observation may be explained based on the Pearson’s hard–soft acid–base theory. It was demonstrated that the S-doped tube can not only strongly adsorb the HgCl2 molecule, but also may effectively detect its presence due to drastic increment of the tube electrical conductivity.

Oxorhenium Complexes Bearing the Water-Soluble Tris(pyrazol-1-yl)methanesulfonate, 1,3,5-Triaza-7-phosphaadamantane, or Related Ligands, as Catalysts for Baeyer–Villiger Oxidation of Ketones

New rhenium(VII or III) complexes [ReO3(PTA)2][ReO4] (1) (PTA = 1,3,5-triaza-7-phosphaadamantane), [ReO3(mPTA)][ReO4]I (2) (mPTA = N-methyl-1,3,5-triaza-7-phosphaadamantane cation), [ReO3(HMT)2][ReO4] (3) (HMT = hexamethylenetetramine), [ReO3(η(2)-Tpm)(PTA)][ReO4] (4) [Tpm = hydrotris(pyrazol-1-yl)methane, HC(pz)3, pz = pyrazolyl], [ReO3(Hpz)(HMT)][ReO4] (5) (Hpz = pyrazole), [ReO(Tpms)(HMT)] (6) [Tpms = tris(pyrazol-1-yl)methanesulfonate, O3SC(pz)3(-)] and [ReCl2{N2C(O)Ph}(PTA)3] (7) have been prepared from the Re(VII) oxide Re2O7 (1-6) or, in the case of 7, by ligand exchange from the benzoyldiazenido complex [ReCl2{N2C(O)Ph}(Hpz)(PPh3)2], and characterized by IR and NMR spectroscopies, elemental analysis and electrochemical properties. Theoretical calculations at the density functional theory (DFT) level of theory indicated that the coordination of PTA to both Re(III) and Re(VII) centers by the P atom is preferable compared to the coordination by the N atom. This is interpreted in terms of the Re-PTA bond energy and hard-soft acid-base theory. The oxo-rhenium complexes 1-6 act as selective catalysts for the Baeyer-Villiger oxidation of cyclic and linear ketones (e.g., 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone, cyclopentanone, cyclobutanone, and 3,3-dimethyl-2-butanone or pinacolone) to the corresponding lactones or esters, in the presence of aqueous H2O2. The effects of a variety of factors are studied toward the optimization of the process.