Theoretical insights into the properties of the dihydrogen-bonded HXH···HCCH complexes (X=Be, Mg, and Ca)

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.

Competition between dihydrogen bond and beryllium bond in complexes between HBeH and HArF: a huge blue shift of distant H-Ar stretch

A novel interaction mechanism between HArF and BeH(2) has been validated and characterized with quantum chemical calculations at the MP2/aug-cc-pVQZ level. They can interact through beryllium bonding formed between the positively charged Be atom in BeH(2) and the negatively charged F atom in HArF, besides through dihydrogen bonding. The former (61.3 kcal/mol) is much stronger than the latter (5.9 kcal/mol). The red shift is found for the associated H-Ar stretch in the dihydrogen bonding, whereas the big blue shift is observed for the distant H-Ar stretch in the beryllium bonding. The blue shift of the distant H-Ar stretch is affected greatly by computational methods. It is calculated to be 712 cm(-1) at the CCSD(T)/6-311++G(3df,2p) level, which appears to be the largest blue shift validated for any weakly bound complex yet. The substitution effect on the beryllium bond is similar to that on hydrogen bonds. The Kr atom makes the beryllium bond weaken and the distant blue shift decrease. The nature and properties of beryllium bond have been analyzed with natural bond orbital (NBO), atoms in molecules (AIM), and energy decomposition.

The B-H...H-P dihydrogen bonding in ion pair complexes [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)] (n = 0-3) and its implication in H(2) elimination and activation reactions

The B-H(delta-)...(delta+)H-P dihydrogen bonding (DHB) in ion pair complexes [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)] (n = 0-3) and its role in the combination of proton and hydride with the release of H(2) or, reversibly, the heterolytic activation of H(2) by Lewis pairs (CF(3))(3)BPH(3-n)(Me)(n) have been theoretically investigated at the MP2 and DFT levels. It is found that the B-H...H-P bonds behave similarly to those in neutral pairs and ion-molecule complexes in most respects, such as the linearity of the H...H-P moiety, the characteristics of the electron transfer and rearrangement, and the topological properties of the DHB critical point, except that in certain cases, a blue-shifting of the H-bond vibrational frequency is observed. In [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)], the proton shifting within the complexes leads to the formation of the dihydrogen complex B(CF(3))(3)(eta(2)-H(2)), which is followed by a subsequent H(2) release. The stability of B(CF(3))(3)(eta(2)-H(2)) (D(e)/D(0) = 10.8/6.0 kcal/mol) makes the proton-hydride combination proceed in a fashion similar to the protonation reactions in transition-metal hydrides rather than those in group 13 hydrides EH(4)(-) (E = B, Al, Ga). As for the H(2)-splitting reaction R(3)BPR'(3) + H(2) --> [R(3)BH(-)][HPR'(3)(+)], classical Lewis pair (CLP) (CF(3))(3)BPH(3) exhibits a high barrier and results in an unstable ion pair product [(CF(3))(3)BH(-)][HPH(3)(+)] compared with the "frustrated Lewis pair" (FLP) (C(6)F(5))(3)BP(tBu)(3). A detailed analysis of the mechanistic aspects of H(2) activation by (CF(3))(3)BPH(3) and (C(6)F(5))(3)BP(tBu)(3), supported by another CLP (CF(3))(3)BP(tBu)(3) which has a binding energy comparable to (CF(3))(3)BPH(3) but a reaction exothermicity comparable to (C(6)F(5))(3)BP(tBu)(3), allows us to suggest that the low stability of FLP (C(6)F(5))(3)BP(tBu)(3) is the determining factor for the low reaction barrier. The relative stability and other properties of the ion pair products [R(3)BH(-)][HPR'(3)(+)] have also been analyzed. Results strongly support the view proposed by Rokob et al. [ Rokob , T. A. ; Hamza , A. ; Stirling , A. ; Soos , T. ; Papai , I. Angew. Chem., Int. Ed. 2008 , 47 , 2435 ] that the frustration energy lowers the energy barrier and increases the exothermicity of the reaction.