Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, FeIII(BNPAPh2O)(Lax)(Leq) (Lax/eq = N3-, NCS-, NCO-, and Cl-) have been synthesized using the previously reported BNPAPh2O- ligand. The ferrous analogs FeII(BNPAPh2O)(Lax) (Lax = N3-, NCS-, and NCO-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the FeIII complexes are six-coordinate, with one anionic ligand (Lax) in the H-bonding axial site and the other anionic ligand (Leq) in the equatorial plane, cis to the Lax ligand. The reaction of FeIII(BNPAPh2O-)(Lax)(Leq) with Ph3C• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows •N3 is the most favored for transfer in each case to the carbon radical, whereas Cl• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N3 > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPAPh2O-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.

Design and Preparation of Ethylene Fluorescence Probes Based on Arylolefins and Grubbs Catalysts

To detect the plant hormone ethylene, three arylolefins were employed to react with ethylene based on olefin metathesis. In this study, three fluorescence probes were successfully prepared using a first-generation Grubbs catalyst (G-1) and arylolefin with terminal vinyl groups. The probes were characterized using various techniques, including UV-vis, fluorescence, FT-IR, ¹H NMR, ¹³C NMR, and ³¹P NMR spectroscopies and HRMS. The probes exhibited an emission maximum at 394 nm and showed excellent ethylene response. The detection limits for the probes were calculated to be 0.128, 0.074, and 0.188 μL/mL (3σ), respectively, based on fluorescence stimulation by ethylene gas. Additionally, the YGTZ-2 probe was used to detect ethylene gas during the storage process of tomatoes. This work expands the application of arylolefin in ethylene detection and provides a foundation for the development of economic, rapid, and convenient photosensitive sensors for ethylene in the future.

The pKBHX Hydrogen-Bond Basicity Scale: From Molecules to Anions

The pK_BHX (logarithm of complexation constant K of 4-fluorophenol with bases) hydrogen-bond basicity scale of neutral hydrogen-bond acceptors (HBAs) is extended to anionic HBAs. The scale is constructed for 26 anions through (i) the infrared measurement of K on NBu₄⁺X^- ion pairs in CCl₄, (ii) the estimation of K from linear free energy relationships between measured K values and literature K values for various phenols in polar solvents, and (iii) the computation of K at the density functional theory level in CCl₄. The scale extends on a 9.4 pK unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their pK_BHX values can be related to the Hammett-Taft substituent constants σ. Unipolar substituents (O^- and S^-) obey the same pK_BHX versus σ relationships as dipolar ionic (N^-N⁺R₃) and dipolar (OH, CF₃, NR₂, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl, and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate by field-inductive and resonance effects, whereas substituents at sulfur and phosphorus operate only by the field-inductive effect.

Di(hydroperoxy)adamantane adducts: synthesis, characterization and application as oxidizers for the direct esterification of aldehydes

The di(hydroperoxy)adamantane adducts of water (1) and phosphine oxides p-Tol₃PO·(HOO)₂C(C₉H₁₄) (2), o-Tol₃PO·(HOO)₂C(C₉H₁₄) (3), and Cy₃PO·(HOO)₂C(C₉H₁₄) (4), as well as a CH₂Cl₂ adduct of a phosphole oxide dimer (8), have been created and investigated by multinuclear NMR spectroscopy, and by Raman and IR spectroscopy. The single crystal X-ray structures for 1-4 and 8 are reported. The IR and ³¹P NMR data are in accordance with strong hydrogen bonding of the di(hydroperoxy)adamantane adducts. The Raman ν(O-O) stretching bands of 1-4 prove that the peroxo groups are present in the solids. Selected di(hydroperoxy)alkane adducts, in combination with AlCl₃ as catalyst, have been applied for the direct oxidative esterification of n-nonyl aldehyde, benzaldehyde, p-methylbenzaldehyde, p-bromobenzaldehyde, and o-hydroxybenzaldehyde to the corresponding methyl esters. The esterification takes place in an inert atmosphere, under anhydrous and oxygen-free conditions, within a time frame of 45 minutes to 5 hours at room temperature. Hereby, two oxygen atoms per adduct assembly are active with respect to the quantitative transformation of the aldehyde into the ester.

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.

Controlling the outcome of SN2 reactions in ionic liquids: from rational data set design to predictive linear regression models

Rate constants for a bimolecular nucleophilic substitution (SN2) process in a range of ionic liquids are correlated with calculated parameters associated with the charge localisation on the cation of the ionic liquid (including the molecular electrostatic potential). Simple linear regression models proved effective, though the interdependency of the descriptors needs to be taken into account when considering generality. A series of ionic liquids were then prepared and evaluated as solvents for the same process; this data set was rationally chosen to incorporate homologous series (to evaluate systematic variation) and functionalities not available in the original data set. These new data were used to evaluate and refine the original models, which were expanded to include simple artificial neural networks. Along with showing the importance of an appropriate data set and the perils of overfitting, the work demonstrates that such models can be used to reliably predict ionic liquid solvent effects on an organic process, within the limits of the data set.

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.

H-Bond donor parameters for cations

UV/Vis absorption and NMR spectroscopy titrations have been used to investigate the formation of complexes between cations and neutral H-bond acceptors in organic solvents. Complexes formed by two different H-bond acceptors with fifteen different cations were studied in acetone and in acetonitrile. The effects of water and ion pairing with the counter-anion were found to be negligible in the two polar solvents employed for this study. The data were used to determine self-consistent H-bond donor parameters (α) for a series of organic and inorganic cations; guanidinium, primary, tertiary and quaternary ammonium, imidazolium, methylpyridinium, lithium, sodium, potassium, rubidium and caesium. The results demonstrate the transferability of α parameters for cations between different solvents and different H-bond acceptor partners, allowing reliable prediction of cation recognition properties in different environments. Lithium and protonated nitrogen cations form the most stable complexes, but the α parameter is only 5.0, which is similar to the neutral H-bond donor 3-trifluoromethyl,4-nitrophenol (α = 5.1). Quaternary ammonium is the weakest H-bond donor investigated with an α value of 2.7, which is comparable to an alcohol. The α parameters for alkali metal cations decrease down the group from 5.0 (Li⁺) to 3.5 (Cs⁺).

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.