Migration Tendencies of Group 14 Element Ligands in the Coordination Sphere of Cationic Phosphenium Iron Complexes

Structural and bonding features of the model iron−phosphenium cation complexes of the type (E = group 14 element; R = H and CH3) as well as the migration of the EH2R group from iron to phosphorus (1,2-migration) and that of a CH3 group from E to phosphorus (1,3-migration) have been studied using the hybrid DFT-B3LYP method. The remarkable stability of these complexes is due to the conjugation around P, which can be partitioned into N−P+−N lone pair π-conjugation and the Fe−P+ π bond. Both together contribute 61.9 kcal/mol to its stability. The calculations suggest that all the 1,2-migrations are feasible reactions and the CH2R migration has the highest tendency because of its small activation barrier (1.3 kcal/mol) and the exothermicity. However, the charge distribution on the structures involved in these reactions points out that external factors such as solvent and the counteranion can greatly influence the courses of the reactions. Compared to the 1,2-migrations, the 1,3-migrations of a methyl group from C, Si, and Sn need much higher activation energies (51.7, 42.7, and 32.7 kcal/mol, respectively), and all are endothermic. A rationalization for the reaction energies and the activation barriers is obtained by bond strength analysis. The bond strengths obtained in this study for C−C, C−Si, C−Sn, P−C, P−Si, P−Sn, Fe−C, Fe−Si, Fe−Sn, FeC, FeSi, and FeSn are 90.4, 86.6, 69.6, 93.7, 81.4, 72.1, 31.8, 41.7, 36.3, 77.4, 82.3, and 69.9 kcal/mol, respectively. Further, in the presence of a water molecule as a model base the 1,3-methyl migrations from Si to P and Sn to P become exothermic, and their activation barriers were lowered to a significant amount. An explanation for this behavior is given based on the Fisher-type and Schrock-type metal−ligand double-bond formation in these complexes. When E = Sn and R = CH3, a realistic base molecule could favor the 1,3-migration over the 1,2-migration, consistent with the experimental facts

Quantifying the Electronic Effect of Substituted Phosphine Ligands via Molecular Electrostatic Potential

Values of the molecular electrostatic potential minimum (Vmin) corresponding to the lone pair region of several substituted phosphine ligands (PR3) have been determined at the DFT level. The Vmin value is proposed as a quantitative measure of the electronic effect of the PR3 ligands. Good linear correlation between Vmin and Tolman electronic parameter of PR3 has been obtained. Vmin is also proportional to the pKa values of the conjugate acids of PR3, viz., [PR3H]+. Further, the ΔE values of the reaction Ni(CO)3 + PR3 → Ni(CO)3PR3 and ScH3 + PR3 → ScH3PR3 are also linearly proportional to the Vmin values. However, if there is a strong metal to phosphorus π-back-bonding, the ΔE and Vmin do not fit to a line. It is also found that the standard reduction potential as well as the enthalpy change corresponding to the electrochemical couple η-Cp(CO)(PR3)(COMe)Fe+/η-Cp(CO)(PR3)(COMe)Fe0 is linearly proportional to the Vmin values of PR3. These correlations suggest that Vmin is a quantitative measure of the σ-donating ability of the phosphine. It is hoped that, in phosphine−metal coordination chemistry, the Vmin based electronic parameter could be more advantageous than ν-CO and pKa based electronic parameters as it solely represents the inherent electronic property of the ligand

Quantifying the Electronic Effect of Substituted Phosphine Ligands via Molecular Electrostatic Potential

Values of the molecular electrostatic potential minimum (Vmin) corresponding to the lone pair region of several substituted phosphine ligands (PR3) have been determined at the DFT level. The Vmin value is proposed as a quantitative measure of the electronic effect of the PR3 ligands. Good linear correlation between Vmin and Tolman electronic parameter of PR3 has been obtained. Vmin is also proportional to the pKa values of the conjugate acids of PR3, viz., [PR3H]+. Further, the ΔE values of the reaction Ni(CO)3 + PR3 → Ni(CO)3PR3 and ScH3 + PR3 → ScH3PR3 are also linearly proportional to the Vmin values. However, if there is a strong metal to phosphorus π-back-bonding, the ΔE and Vmin do not fit to a line. It is also found that the standard reduction potential as well as the enthalpy change corresponding to the electrochemical couple η-Cp(CO)(PR3)(COMe)Fe+/η-Cp(CO)(PR3)(COMe)Fe0 is linearly proportional to the Vmin values of PR3. These correlations suggest that Vmin is a quantitative measure of the σ-donating ability of the phosphine. It is hoped that, in phosphine−metal coordination chemistry, the Vmin based electronic parameter could be more advantageous than ν-CO and pKa based electronic parameters as it solely represents the inherent electronic property of the ligand

A Novel Electrostatic Approach to Substituent Constants: Doubly Substituted Benzenes

The most negative-valued molecular electrostatic potential (MESP) minimum (Vmin) observed over the benzene ring is proposed as a sensitive quantity for the analysis of the electronic perturbations due to the substituents attached to it. MESP topography of 45 doubly substituted benzenes is mapped at HF/6-31G** level for an appraisal of this proposition. The Vmin values are seen to clearly reflect the changes due to the different orientations (para, meta, and ortho) and conformations of the substituents and different mechanisms of electron donation or withdrawal. Good linear correlations are obtained with Vmin and the experimentally determined σ values of the ortho- and meta-disubstituted benzenes. New quantities, Dp, Dm, and Do, introduced in this work as the substituent pair-constants respectively for the para, meta, and ortho arrangements, provide a quantitative measure of the simultaneous effect of two substituents over the aromatic nucleus. The predictive power of these quantities is checked in the case of some triply substituted benzenes using an equation Vmin = Vben + ∑ΔVmin(mono) − ∑Dp − ∑Dm − ∑Do where Vben is the MESP minimum of benzene and ΔVmin(mono) is the difference between the monosubstituted benzene Vmin and Vben. These predicted values are in fairly good agreement with the MESP values obtained at HF/6-31G** level

Revisiting Markovnikov Addition to Alkenes via Molecular Electrostatic Potential

Molecular electrostatic potentials (MESP) surrounding the π-region of several substituted ethylenes (CH2CHR) have been characterized by locating the most negative-valued point (Vmin) in that region. The substituents have been classified as electron donating and withdrawing on the basis of the increase or decrease in the negative character of Vmin in these systems as compared to ethylene. The values of Vmin show a good linear correlation with the Hammett σp constants, suggesting that the substituent electronic effects in substituted ethylenes and substituted benzenes are basically similar. With electron-donating substituents, the position of MESP minimum is closer to the unsubstituted carbon facilitating the π-complex formation of it with HCl at this site. Such a regiospecific π-complex formation is found to favor the formation of Markovnikov-type transition state for the addition of HCl to CH2CHR. For the electron-withdrawing substituents, the Vmin location is almost equidistant and farther from the ethylenic carbon atoms. This and the less negative Vmin values account for the less regiospecific CH2CHR···HCl π-complexes as well as the transition states for the HCl addition to CH2CHR when R is an electron-withdrawing group. The interaction energy (Eint) between CH2CHR and HCl for the formation of the CH2CHR···HCl π-complex shows a good linear correlation with the corresponding Vmin value

Oxidation Reactions of 2‑Thiouracil: A Theoretical and Pulse Radiolysis Study

The reaction of hydroxyl radical (^•OH) with the nucleic acid base analogue 2-thiouracil (<b>1</b>) has been studied by pulse radiolysis experiments and DFT. The generic intermediate radicals feasible for the ^•OH reactions with <b>1</b>, namely, one electron oxidation product (<b>1</b>^•+), ^•OH-adducts (<b>3</b>^•, <b>4</b>^•, and <b>5</b>^•), and H-abstracted radicals (<b>6</b>^• and <b>7</b>^•), were characterized by interpreting their electronic and structural properties along with calculated energetics and UV–vis spectra. Pulse radiolysis experiments showed that the transient formed in the reaction of ^•OH with <b>1</b> in water at pH 6.5 has λ_max at 430 nm. A bimolecular rate constant, <i>k</i>₂ of 9.6 × 10⁹ M^–1 s^–1, is determined for this reaction via competition kinetics with 2-propanol. The experiments suggested that the transient species could be a dimer radical cation <b>2</b>^•+, formed by the reaction of <b>1</b> with the radical cation <b>1</b>^•+. For this reaction, an equilibrium constant of 4.7 × 10³ M^–1 was determined. The transient formed in the reaction of <b>1</b> with pulse radiolytically produced Br₂^•– at pH 6.5 as well as Cl₂^•– at pH 1 has also produced λ_max at 430 nm and suggested the formation of <b>2</b>^•+. The calculated UV–vis spectra of the transient species (<b>1</b>^•+, <b>3</b>^•, <b>4</b>^•, <b>5</b>^•, <b>6</b>^•, and <b>7</b>^•) showed no resemblance to the experimental spectra, while that of <b>2</b>^•+ (λ_max = 420 nm) agreed well with the experimental value and thus confirmed the formation of <b>2</b>^•+. The 420 nm peak was due to σ → σ* electronic excitation centered on a 2-center–3-electron (2c–3e) sulfur–sulfur bond [−S∴S−]. <b>2</b>^•+ is the first reported example of a dimer radical cation in a pyrimidine heterocyclic system. Further, 5-C and 6-C substituted (substituents are −F, −Cl, −NH₂, −N­(CH₃)₂, −OCH₃, −CF₃, −CH₃, −CH₂CH₃, <i>n</i>-propyl, phenyl, and benzyl) and 5,6-disubstituted 2-thiouracil systems have been characterized by DFT and found that the reaction (<b>1</b> + <b>1</b>^•+ → <b>2</b>^•+) is exergonic (1.12–13.63 kcal/mol) for many of them

Correlation between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives

The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor−acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3-dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images

Correlation between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives

The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor−acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3-dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images

Photophysical and Theoretical Investigations of Oligo(<i>p</i>-phenyleneethynylene)s: Effect of Alkoxy Substitution and Alkyne−Aryl Bond Rotations

The unique photophysical, conformational, and electronic properties of two model phenyleneethynylene-based rigid rod molecular systems, possessing dialkoxy substitutions, are reported in comparison with an unsubstituted system. Twisting of the phenyl rings along the carbon−carbon triple bond is almost frictionless in these systems giving rise to planar as well as several twisted ground-state conformations, and this results in broad structureless absorption in the spectral region of 250−450 nm. In the case of 1,4-bis(phenylethynyl)benzene, a broad absorption band was observed due to the HOMO−LUMO transition, whereas dialkoxy-substituted compounds possess two well-separated bands. Dialkoxy substitution in the 2,5-position of the phenyl ring in phenyleneethynylenes alters its central arene π-orbitals through the resonance interaction with oxygen lone pairs resulting in similar orbital features for HOMO and HOMO−1/HOMO−2. Electronic transition from the low-lying HOMO−1/HOMO−2 orbital to LUMO results in the high-energy band, and the red-shifted band originates from the HOMO−LUMO transition. The first excited-state transition energies at different dihedral angles, calculated by the TDDFT method, indicate that the orthogonal conformation has the highest excitation energy with an energy difference of 15 kcal/mol higher than the low-lying planar conformation. The emission of these compounds originates preferentially from the more relaxed planar conformation resulting in well-defined vibronic features. The fluorescence spectral profile and lifetimes were found to be independent of excitation wavelengths, confirming the existence of a single emitting species

Self-Organization of Phenyleneethynylene into Wire-Like Molecular Materials on Surfaces

A model phenyleneethynylene, which does not possess any functional groups, self-organizes into wire-like structures on 2D surfaces. High-resolution STM imaging revealed that the molecules are arranged in a skewed 1D fashion. Analysis of various domains indicated the existence of two types of molecular packing arising from different modes of alkyl CH···π interaction, which was further supported by theoretical calculations