Specific and non-specific influence of the environment on dihydrogen bonding and proton transfer to [RuH2{P(CH2CH2PPh2)3}]

Dichotomous Si-H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes

The activation of silanes in dehydrogenative coupling with alcohols under general base catalysis was studied experimentally (using multinuclear NMR, IR, and UV-visible spectroscopies) and computationally (at DFT M06/6-311++G(d,p) theory level) on the example of Ph_4-nSiH_n (n = 1-3) interaction with (CF₃)₂CHOH in the presence of Et₃N. The effect of the phenyl groups' number and H^- substitution by the electron-withdrawing (CF₃)₂CHO^- group on Si-H bond hydricity (quantified as hydride-donating ability, HDA) and Lewis acidity of silicon atom (characterized by maxima of molecular electrostatic potential) was accessed. Our results show the coordination of Lewis base (Y = Me₃N, ROH, OR^-) leads to the increased hydricity of pentacoordinate hypervalent Ph_4-nSi(Y)H_n complexes and a decrease of the reaction barrier for H₂ release. The formation of tertiary complexes [Ph_4-nSi(Y)H_n]···HOR is a critical prerequisite for the dehydrocoupling with alkoxides being ideal activators. The latter can be external or internal, generated by in situ HOR deprotonation. The mutual effect of tetrel interaction and dihydrogen bonding in tertiary complexes (RO^-)Ph_4-nSiH_n···HOR leads to dichotomous activation of Si-H bond promoting the proton-hydride transfer and H₂ release.

Adduct under Field-A Qualitative Approach to Account for Solvent Effect on Hydrogen Bonding

The location of a mobile proton in acid-base complexes in aprotic solvents can be predicted using a simplified Adduct under Field (AuF) approach, where solute–solvent effects on the geometry of hydrogen bond are simulated using a fictitious external electric field. The parameters of the field have been estimated using experimental data on acid-base complexes in CDF₃/CDClF₂. With some limitations, they can be applied to the chemically similar CHCl₃ and CH₂Cl₂. The obtained data indicate that the solute–solvent effects are critically important regardless of the type of complexes. The temperature dependences of the strength and fluctuation rate of the field explain the behavior of experimentally measured parameters.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Dihydrogen bond intermediated alcoholysis of dimethylamine-borane in nonaqueous media

Dimethylamine-borane (DMAB) acid/base properties, its dihydrogen-bonded (DHB) complexes and proton transfer reaction in nonaqueous media were investigated both experimentally (IR, UV/vis, NMR, and X-ray) and theoretically (DFT, NBO, QTAIM, and NCI). The effects of DMAB concentration, solvents polarity and temperature on the degree of DMAB self-association are shown and the enthalpy of association is determined experimentally for the first time (-ΔH°assoc = 1.5-2.3 kcal/mol). The first case of "improper" (blue-shifting) NH···F hydrogen bonds was observed in fluorobenzene and perfluorobenzene solutions. It was shown that hydrogen-bonded complexes are the intermediates of proton transfer from alcohols and phenols to DMAB. The reaction mechanism was examined computationally taking into account the coordinating properties of the reaction media. The values of the rate constants of proton transfer from HFIP to DMAB in acetone were determined experimentally [(7.9 ± 0.1) × 10(-4) to (1.6 ± 0.1) × 10(-3) mol(-1)·s(-1)] at 270-310 K. Computed activation barrier of this reaction ΔG(‡theor)298 K(acetone) = 23.8 kcal/mol is in good agreement with the experimental value of the activation free energy ΔG(‡exp)270 K = 21.1 kcal/mol.

First Example of Hydrogen Bonding to Platinum Hydride

The interaction between various proton donors (indole, CF3CH2OH, (CF3)2CHOH, (CF3)3COH) and (NNC)PtH hydrido complex (NNC-H=6-(1,1'-dimethylbenzyl)-2,2'-bipyridine) was investigated through low-temperature IR and NMR spectroscopy in combination with density functional theory calculations at the M06 level of theory. The experiment shows formation of very weak hydrogen bonded complexes (Δ HºHB ca. -1.0 kcal mol-1), which undergo subsequent proton transfer surprisingly easy. Computational analysis of the hydrogen bonded complexes geometry, electronic parameters (obtained by NBO and AIM analysis), and orbital interaction energies shows that all the complexes are better described as bonded to the metal atom. At that in case of weak alcohols (CH3OH, TFE) there is also the additional interaction with the hydride ligand. These computational results allow explaining the observed experimental trends and give the first example of hydrogen bonding to a metal atom in the presence of hydride ligand.

Theoretical Study of Mixed Hydrogen and Dihydrogen Bond Interactions in Clusters

Ab initio calculations were used to analyze interactions of with 1–4 molecules of NH3at the MP2/6-311++G(d,p) and the B3LYP/6-311++G(d,p) computational levels. In addition to H3B–H⋯H–NH2dihydrogen bond, the H2N–H⋯NH3hydrogen bonds were also predicted in clusters. Negative cooperativity in clusters constructed from mixed H3B–H⋯H–NH2dihydrogen and H2N–H⋯NH3hydrogen bonds are more remarkable. The negative cooperativity increases with size and number of hydrogen bonds in cluster. The B–H stretching frequencies show blue shifts with respect to cluster formation. Greater blue shift in stretching frequencies was predicted for B–H bonds which did not contribute to dihydrogen bonding with NH3molecules. The structures were analyzed with the atoms in molecules (AIM) methodology.

Studies on tautomerism in the triazoline dione

The chemical properties of the triazoline dione have been extensively studied. The relative stability of three tautomers in the gas phase and solution (using PCM model) are evaluated using HF and DFT methods and compared with MP2 level using different basis sets. The results of the calculations indicate that 4H-1,2,4-triazole-3,5-dione (K form) is more stable in the gas phase than in solution. In addition, the transition states of proton transfer reaction is calculated. The vibrational frequencies and corresponding normal mode assignments of 4H-1,2,4-triazole-3,5-dione and 5-hydroxy-3H-1,2,4-triazol-3-one (E1 form) are also theoretically examined. In addition, the variation of dipole moments and charges on atoms with change of solvent is studied. Specific solvent effects with addition of one molecule of water near the electrophilic centers of three tautomers and the transition state of proton transfer assisted by a water molecule was investigated. Aggregation with water molecules does not change the order of stability of isomers, but proton transfer reaction assisted by a water molecule needs less energy than the intramolecular process.