Hydrogen Peroxide and Di(hydroperoxy)propane Adducts of Phosphine Oxides as Stoichiometric and Soluble Oxidizing Agents

Identifying Radical Pathways for Cu(I)/Cu(II) Relay Catalyzed Oxygenation via Online Coupled EPR/UV-Vis/Near-IR Monitoring

Copper-catalyzed C─H oxygenation has drawn considerable attention in mechanistic studies. However, a comprehensive investigation combining radical pathways with a metal-catalytic cycle is challenged by the intricate organic radicals and metallic intermediates. Herein, an online coupled EPR/UV-vis/near-IR detecting method is developed to simultaneously monitor both reactive radical species and copper complex intermediates during the reaction. Focusing on copper-catalyzed phenol oxygenation with cumene hydroperoxide, the short-lived alkylperoxyl radical (EPR signal at g = 2.0143) as well as the unexpected square planar Cu(II)-alkoxyl radical complex (near-IR signal at 833 nm) are unveiled during the reaction, in addition to the observable phenoxyl radical in EPR, quinone product in UV-vis, and Cu(II) center in EPR. With a comprehensive picture of diverse intermediates evolving over the same timeline, a novel Cu(I)/Cu(II) proposed relay-catalyzed sequential radical pathway. In this sequence, Cu(II) activates hydroperoxide through Cu(II)-OOR into the alkylperoxide radical, while the reaction between Cu(I) and hydroperoxide leads to Cu(II)(•OR)OH with high H-atom abstracting activity. These results provide a thorough understanding of the Cu(I)/Cu(II) relay catalysis for phenol oxygenation, setting the stage for mechanistic investigations into intricate radical reactions promoted by metallic complexes.

Oxidation-dependent Lewis acidity in chalcogen adducts of Sb/P frustrated Lewis pairs

The reactions of the frustrated Lewis pair (F5C2)2SbCH2P(tBu)2 with oxygen, sulphur, selenium and tellurium led to the mono-oxidation products (F5C2)2SbCH2P(E)(tBu)2 (E = O, S, Se, Te). Further oxidation of these chalcogen adducts with tetrachloro-ortho-benzoquinone (o-chloranil) gave (F5C2)2Sb(CH2)(μ-E)P(tBu)2·CatCl (CatCl = o-O2C6Cl4) with a central four-membered ring heterocycle for E = O, S, and Se. For E = Te the elimination of elemental tellurium led to an oxidation product with two equivalents of o-chloranil, (F5C2)2SbCH2P(tBu)2·2CatCl, which is also accessible by reaction of (F5C2)2SbCH2P(tBu)2 with o-chloranil. The synthesised compounds were characterised by NMR spectroscopy and X-ray structure analyses, and the structural properties were analysed in the light of the altered Lewis acidity due to the oxidation of the antimony atoms.

Noncovalent and Covalent O-H···O Interactions in PPh3O Cocrystals: A Correlation Study Involving QTAIM, SAPT, NBO, and IBSI Methods

PPh3O.hemihydrate polymorphs and 11 assorted PPh3O cocrystals collectively constitute a reliable stock to pursue a systematic analysis aiming to investigate the impacts of some vital issues on the TPPO.H-bond donor aggregates. The issues highlighted herein are (i) effect of varying acidity of H-bond donors on the degeneracy of lone pairs of the H-bond acceptor (PPh3O), (ii) effectiveness of the |V(r)|/G(r) and H(r)/ρ(r) parameters as a covalency metric, (iii) 3c-4e bonding in the covalent PPh3O.nitric acid cocrystal, (iv) salient features of H-bond interaction energy and an interplay of its components, (v) an intrinsic bond strength scale for the PPh3O cocrystals, and (vi) reliable empirical relations between several bond descriptors for a quick estimation of interaction energy. To be specific about point (vi), we have propounded two promising avenues for a fast semiquantitative calculation of interaction energy from an endearing nonenergetic parameter, viz., bond length: dO-H···O → ρBCP (MAPE = 2.36%) → ESAPT0 (MAPE = 9.26%), and dO-H···O → IBSI (MAPE = 1.87%) → ESAPT0 (MAPE = 9.66%). All the aforesaid issues have been explored in detail through the QTAIM, NBO, and IBSI analyses (M06-2X-D3/def2-TZVP level), as well as by the SAPT study at the SAPT0/aug-cc-pVDZ platform. The statistically valid correlation studies can be particularly conducive for practical purposes as a transformative extension of the established facts into postulates for the unknown cocrystals.

Tri-methyl-phosphine oxide dihydrate

The title hydrate, Me₃PO·2H₂O, crystallizes in the ortho-rhom-bic space group Pbca with eight formula units per unit cell. The extended structure displays O-H⋯O hydrogen bonding, with Me₃PO mol-ecules as acceptors and water mol-ecules acting as donors and acceptors of hydrogen bonds, forming hydrogen-bonded layers, which propagate in the ac plane.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au₁₃ unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Molecular Dynamics and Surface Interactions of Nickelocene Adsorbed on Silica: A Paramagnetic Solid-State NMR Study

When grinding nickelocene with silica in the absence of a solvent at room temperature, it adsorbs on the surface within the pores. This has also been demonstrated visually by adsorbing green nickelocene in the pores of a large colorless silica gel specimen. While this dry adsorption and translational mobility of nickelocene within the pores is proven visually, the site-to-site mobility of the nickelocene molecules and their orientation toward the surface are not yet understood. In this contribution, mesoporous silica is used as the support material for a systematic solid-state NMR study of these issues. Paramagnetic ¹H VT solid-state NMR and T₁ relaxation times have been powerful tools for studying the dynamics of nickelocene on the silica surface. Herewith, the mobility of the surface-adsorbed nickelocene molecules in the pores could be quantified on the molecular scale. According to the obtained data, the nickelocene molecules move like a liquid on the surface. Isotropically moving molecules exchange places rapidly with surface-attached molecular states of nickelocene in a sample with submonolayer surface coverage. This finding is corroborated by a macroscopic visualization experiment. The states of the surface-attached horizontally oriented nickelocene molecules that are prevalent at temperatures below 200 K have been quantified. The temperature dependencies of the rate k in coordinates of ln(k) versus 1/T and ln(k/T) versus 1/T form ideal straight lines that allow the determination of the kinetic parameters E_act = 5.5 kcal/mol, A = 1.1 × 10¹⁰, ΔH^‡ = 5.0 kcal/mol, and ΔS^‡ = -15 eu. Investigating a sample with equal amounts of nickelocene and ferrocene in a submonolayer amount of 80% overall surface coverage shows that the different metallocenes mix on the molecular level on the silica surface.

Comparison of Proton Acceptor and Proton Donor Properties of H2O and H2O2 in Organic Crystals of Drug-like Compounds: Peroxosolvates vs. Crystallohydrates

Two new peroxosolvates of drug-like compounds were synthesized and studied by a combination of X-ray crystallographic, Raman spectroscopic methods, and periodic DFT computations. The enthalpies of H-bonds formed by hydrogen peroxide (H₂O₂) as a donor and an acceptor of protons were compared with the enthalpies of analogous H-bonds formed by water (H₂O) in isomorphic (isostructural) hydrates. The enthalpies of H-bonds formed by H₂O₂ as a proton donor turned out to be higher than the values of the corresponding H-bonds formed by H₂O. In the case of H₂O₂ as a proton acceptor in H-bonds, the ratio appeared reversed. The neutral O∙∙∙H-O/O∙∙∙H-N bonds formed by the lone electron pair of the oxygen atom of water were the strongest H-bonds in the considered crystals. In the paper, it was found out that the low-frequency Raman spectra of isomorphous crystalline hydrate and peroxosolvate of N-(5-Nitro-2-furfurylidene)-1-aminohydantoin are similar. As for the isostructural hydrate and peroxosolvate of the salt of protonated 2-amino-nicotinic acid and maleic acid monoanion, the Raman spectra are different.

Non-covalent interactions of the hydroperoxo group in crystalline adducts of organic hydroperoxides and their potassium salts

X-ray diffraction of three new stable cocrystals of potassium salts of organic hydroperoxides with molecular hydroperoxides reveals strong charge-assisted ROO−⋯HOOR H-bonds.

Novel peroxosolvates of tetraalkylammonium halides: the first case of layers containing hydrogen-bonded peroxide molecules

The structures of Et4N+Cl−·2(H2O2) and Et4N+Br−·2(H2O2) formed by globose Et4N+ cations are the first examples of peroxosolvates with 2D layers containing H-bonded peroxides.

An H2 O2 Molecule Stabilized inside Open-Cage C60 Derivatives by a Hydroxy Stopper

An H₂ O₂ molecule was isolated inside hydroxylated open-cage fullerene derivatives by mixing an H₂ O₂ solution with a precursor molecule followed by reduction of one of carbonyl groups on its orifice. Depending on the reduction site, two structural isomers for H₂ O₂ @open-fullerenes were obtained. A high encapsulation ratio of 81 % was attained at low temperature. The structures of the peroxosolvate complexes thus obtained were studied by ¹ H NMR spectroscopy, X-ray analysis, and DFT calculations, showing strong hydrogen bonding between the encapsulated H₂ O₂ and the hydroxy group located at the center of the orifice. This OH group was found to act as a kinetic stopper, and the formation of the hydrogen bonding caused thermodynamic stabilization of the H₂ O₂ molecule, both of which prevent its escape from the cage. One of the peroxosolvates was isolated by HPLC, affording H₂ O₂ @open-fullerene with 100 % encapsulation ratio, likely due to the intramolecular hydrogen-bonding interaction.

Actual Symmetry of Symmetric Molecular Adducts in the Gas Phase, Solution and in the Solid State

This review discusses molecular adducts, whose composition allows a symmetric structure. Such adducts are popular model systems, as they are useful for analyzing the effect of structure on the property selected for study since they allow one to reduce the number of parameters. The main objectives of this discussion are to evaluate the influence of the surroundings on the symmetry of these adducts, steric hindrances within the adducts, competition between different noncovalent interactions responsible for stabilizing the adducts, and experimental methods that can be used to study the symmetry at different time scales. This review considers the following central binding units: hydrogen (proton), halogen (anion), metal (cation), water (hydrogen peroxide).

Modeling of Solute-Solvent Interactions Using an External Electric Field-From Tautomeric Equilibrium in Nonpolar Solvents to the Dissociation of Alkali Metal Halides

An implicit account of the solvent effect can be carried out using traditional static quantum chemistry calculations by applying an external electric field to the studied molecular system. This approach allows one to distinguish between the effects of the macroscopic reaction field of the solvent and specific solute-solvent interactions. In this study, we report on the dependence of the simulation results on the use of the polarizable continuum approximation and on the importance of the solvent effect in nonpolar solvents. The latter was demonstrated using experimental data on tautomeric equilibria between the pyridone and hydroxypyridine forms of 2,6-di-tert-butyl-4-hydroxy-pyridine in cyclohexane and chloroform.

Efficient capturing of hydrogen peroxide in dilute aqueous solution by co-crystallization with amino acids

X-ray structure analyses of co-crystals of H2O2 and l-Phe, dl-Phe, or dl-Asp prepared in a dilute aqueous solution (30 wt%) indicated that multi-layer motifs including water molecule is important for highly efficient H2O2 capture in dilute solutions.

Crystalline Peroxosolvates: Nature of the Coformer, Hydrogen-Bonded Networks and Clusters, Intermolecular Interactions

Despite the technological importance of urea perhydrate (percarbamide) and sodium percarbonate, and the growing technological attention to solid forms of peroxide, fewer than 45 peroxosolvates were known by 2000. However, recent advances in X-ray diffractometers more than tripled the number of structurally characterized peroxosolvates over the last 20 years, and even more so, allowed energetic interpretation and gleaning deeper insight into peroxosolvate stability. To date, 134 crystalline peroxosolvates have been structurally resolved providing sufficient insight to justify a first review article on the subject. In the first chapter of the review, a comprehensive analysis of the structural databases is carried out revealing the nature of the co-former in crystalline peroxosolvates. In the majority of cases, the coformers can be classified into three groups: (1) salts of inorganic and carboxylic acids; (2) amino acids, peptides, and related zwitterions; and (3) molecular compounds with a lone electron pair on nitrogen and/or oxygen atoms. The second chapter of the review is devoted to H-bonding in peroxosolvates. The database search and energy statistics revealed the importance of intermolecular hydrogen bonds (H-bonds) which play a structure-directing role in the considered crystals. H2O2 always forms two H-bonds as a proton donor, the energy of which is higher than the energy of analogous H-bonds existing in isostructural crystalline hydrates. This phenomenon is due to the higher acidity of H2O2 compared to water and the conformational mobility of H2O2. The dihedral angle H-O-O-H varies from 20 to 180° in crystalline peroxosolvates. As a result, infinite H-bonded 1D chain clusters are formed, consisting of H2O2 molecules, H2O2 and water molecules, and H2O2 and halogen anions. H2O2 can form up to four H-bonds as a proton acceptor. The third chapter of the review is devoted to energetic computations and in particular density functional theory with periodic boundary conditions. The approaches are considered in detail, allowing one to obtain the H-bond energies in crystals. DFT computations provide deeper insight into the stability of peroxosolvates and explain why percarbamide and sodium percarbonate are stable to H2O2/H2O isomorphic transformations. The review ends with a description of the main modern trends in the synthesis of crystalline peroxosolvates, in particular, the production of peroxosolvates of high-energy compounds and mixed pharmaceutical forms with antiseptic and analgesic effects.

For Whom a Puddle Is the Sea? Adsorption of Organic Guests on Hydrated MCM-41 Silica

Thermal and hydration effects on the mobility of compact and branched organic molecules and a bulky pharmaceutical substance loaded in submonolayer amounts onto mesoporous silica have been elucidated using ¹H and ³¹P solid-state NMR. In all cases, the ambient hydration has a stronger effect than an increase in temperature to 370 K for water-free silica. The effect of hydration depends on the guest and ranges from complete solvation to a silica-water-guest sandwich structure to a silica-guest/silica-water pattern. The mobility of the guests under different conditions has been described. The specific structure of the MCM-41 surface allows one to study very slow surface diffusion, a diffusivity of about 10^-15-10^-16 m²/s. The data reported are relevant to any nonfunctionalized silica, while the method used is applicable to any phosphor-containing guest on any host.

Di(hydroperoxy)cycloalkane Adducts of Triarylphosphine Oxides: A Comprehensive Study Including Solid-State Structures and Association in Solution

Four new di(hydroperoxy)cycloalkane adducts (Ahn adducts) of p-Tol₃PO (1) and o-Tol₃PO (2), namely, p-Tol₃PO·(HOO)₂C(CH₂)₅ (3), o-Tol₃PO·(HOO)₂C(CH₂)₅ (4), p-Tol₃PO·(HOO)₂C(CH₂)₆ (5), and o-Tol₃PO·(HOO)₂C(CH₂)₆ (6), have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The ³¹P NMR data are in accordance with hydrogen bonding of the di(hydroperoxy)alkanes to the P═O groups of the phosphine oxides. Due to their high solubility in organic solvents, natural abundance ¹⁷O NMR spectra of 1-6 could be recorded, providing the signals for the P═O groups and additionally the two different oxygen nuclei in the O-OH groups in the adducts 3-6. The association and mobility of 3-6 were explored by ¹H DOSY (diffusion ordered spectroscopy) NMR, which indicated persistent hydrogen bonding of the adducts in solution. Competition experiments with phosphine oxides allowed ranking of the affinities of the di(hydroperoxy)cycloalkanes for the different phosphine oxide carriers. On the basis of variable temperature ³¹P NMR investigations, the Gibbs energies of activation ΔG^‡ for the adduct dissociation processes of 3-6 at different temperatures, as well as the enthalpy ΔH^‡ and entropy ΔS^‡ of activation, have been determined. IR spectroscopy of 3-6 corroborated the hydrogen bonding, and in the Raman spectra, the ν(O-O) stretching bands have been identified, confirming the presence of peroxy groups in the solid materials. The high solubilities in selected organic solvents have been quantified.

Disentangling different modes of mobility for triphenylphosphine oxide adsorbed on alumina

Triphenylphosphine oxide (TPPO, 1) has been adsorbed on neutral alumina by dry grinding of the components in the absence of a solvent. The adsorption proves translational mobility of 1 on the surface of alumina. Different surface coverages from a densely packed monolayer (99% coverage) to a dilute sub-monolayer (25%) have been produced. The samples have been studied by diverse multinuclear ¹H, ¹³C, and ³¹P variable temperature solid-state nuclear magnetic resonance (NMR) techniques. The interactions of 1 with the surface are determined by hydrogen bonding of the P=O group to OH groups on the surface. The ³¹P solid-state NMR spectra prove that even at low temperatures, the molecules of 1 are highly mobile on the surface. Using T₁ and T₂ relaxation time analyses of the ³¹P resonance in the solid state at variable temperatures allowed the identification and quantification of two different modes of mobility. Besides the translational mobility that consists of jumps from one hydrogen-bonding OH site on the surface to an adjacent one, a rotational movement around the axis defined by the P=O group of 1 occurs.

Hydrogen peroxide adducts of triarylphosphine oxides

Five new hydrogen peroxide adducts of phosphine oxides (p-Tol₃PO·H₂O₂)₂ (1), (o-Tol₃PO·H₂O₂)₂ (2), (o-Tol₂PhPO·H₂O₂)₂ (3), (p-Tol₃PO)₂·H₂O₂ (4), and (o-TolPh₂PO)₂·H₂O₂ (5), and the water adduct (o-Tol₂PhPO·H₂O)₂ (6) have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The IR and ³¹P NMR data are in accordance with strong hydrogen bonding of the hydrogen peroxide. The mono- versus dimeric nature of the adduct assemblies has been investigated by DOSY NMR experiments. Raman spectroscopy of the symmetric adducts and the ν(O-O) stretching bands confirm the presence of hydrogen-bonded hydrogen peroxide in the solid materials. The solubilities in organic solvents have been quantified. Due to the high solubilities of 1-6 in organic solvents their ¹⁷O NMR spectra could be recorded in natural abundance, providing well-resolved signals for the P[double bond, length as m-dash]O and O-O groups. The adducts 1-5 have been probed regarding their stability in solution at 105 °C. The decomposition of the adduct 1 takes place by loss of the active oxygen atoms in two steps.

Structures and Dynamics of Secondary and Tertiary Alkylphosphine Oxides Adsorbed on Silica

The three secondary phosphine oxides [CH₂ =CH(CH₂ )₄ ]₂ HPO (1), [CH₂ =CH(CH₂ )₅ ]₂ HPO (2), and [CH₂ =CH(CH₂ )₆ ]₂ HPO (3), and two diphosphine dioxides, {[CH₂ =CH(CH₂ )₆ ]₂ PO(CH₂ )₇ }₂ (4) and {[CH₂ =CH(CH₂ )₆ ]₂ PO(CH₂ )₄ }₂ (5), incorporating long methylene chains, are described. The single crystal X-ray structures of 1, 2, and 5 have been determined. The phosphine oxides 3, 4, and 5 have been adsorbed on silica in submonolayer quantities to give 3 a-5 a. The ¹ H, ¹³ C, and ³¹ P solid-state NMR spectra of polycrystalline 3-5 have been analyzed and compared with those of 3 a-5 a. The changes of the solid-state NMR characteristics upon adsorption and the surface mobilities of the phosphine oxides are discussed.

Selective synthesis and stabilization of peroxides via phosphine oxides

MEKPO (methyl ethyl ketone peroxide) and other peroxides can be synthesized selectively and stabilized as hydrogen-bonded phosphine oxide adducts.

Unusual isomorphism in crystals of organic solvates with hydrazine and water

Unusual isomorphism and isomorphous substitution in crystals of organic solvates with hydrazine and water are observed for the first time.

2,2′-Disulfanediylbis(pyridineN-oxide)–hydrogen peroxide (1/1)

In the title co-crystal, C10H8N2O2S2·H2O2, both molecules are generated by crystallographic twofold symmetry; the dihedral angle between the pyridine rings is 101.16 (9)°. In the crystal, the components are linked by O—H...O hydrogen bonds to generate [010] chains of alternating 2,2′-dithiobis(pyridineN-oxide) and hydrogen peroxide molecules. The structure was refined as a two-component inversion twin.

Hydrogen Peroxide Insular Dodecameric and Pentameric Clusters in Peroxosolvate Structures

Peroxosolvates of 2-aminonicotinic acid (I) and lidocaine N-oxide (II) including the largest insular hydrogen peroxide clusters were isolated and their crystal structures were determined by single-crystal X-ray diffraction. An unprecedented dodecameric hydrogen peroxide insular cluster was found in I. An unusual cross-like pentameric cluster was observed in the structure of II. The topology of the (H₂ O₂ )₁₂ assembly was never observed for small-molecule clusters. In I and II new double and triple cross-orientational disorders of H₂ O₂ were found. Cluster II is the first example of a peroxosolvate crystal structure containing H₂ O₂ molecules with a homoleptic hydrogen peroxide environment. In II, a hydrogen bond between an H₂ O₂ molecule and a peptide group -CONH⋅⋅⋅O₂ H₂ was observed for the first time.

Crystal structure of (Z)-N-benzyl-idene-1-phenyl-methanamine oxide hydrogen peroxide monosolvate

The title structure consists of a (Z)-N-benzyl­idene-1-phenyl­methanamine oxide and a hydrogen peroxide mol­ecule linked through both O—H groups into a one-dimensional chain structure.

The title adduct, C₁₄H₁₃NO·H₂O₂, consists of (Z)-N-benzyl­idene-1-phenyl­methanamine oxide and hydrogen peroxide mol­ecules in a 1:1 ratio. The organic coformer adopts a skew geometry with an inter-aryl-ring dihedral angle of 81.9 (2)°. In the crystal, the organic and peroxide mol­ecules are linked through both peroxide O—H donor groups to oxide O-atom acceptors, giving one-dimensional chains extending along the b axis. Present also are weak inter­molecular C—H⋯O hydrogen-bonding inter­actions.

Crystal structure of 2,3,5,6-tetra-kis-(pyridin-2-yl)pyrazine hydrogen peroxide 4.75-solvate

The structure of title co-crystal consists of a 2,3,5,6-tetra­kis­(pyridin-2-yl)pyrazine coformer and hydrogen peroxide solvent mol­ecules in a ratio of 1:4.75.

The structure of the title co-crystal, C₂₄H₁₆N₆·4.75H₂O₂, consists of a 2,3,5,6-tetra­kis­(pyridin-2-yl)pyrazine coformer and hydrogen peroxide solvent mol­ecules in an overall ratio of 1:4.75. Three of the six H₂O₂ mol­ecules modelled in the structure were found to be cross-orientationally disordered over two positions with occupancy ratios 0.846 (9):0.154 (9), 0.75 (2):0.25 (2), and 0.891 (9):0.109 (9). In the crystal, all of the peroxide mol­ecules are linked into hydrogen-bonded chains that propagate parallel to the a axis. These chains are further linked by O—H⋯N hydrogen bonds to the pyridine groups of the main mol­ecule.