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.

The reaction of the isostructural anions of group 13 hydrides EH4- (E = B, Al, Ga) with proton donors of different strength (CH3OH, CF3CH2OH, and CF3OH) was studied with different theoretical methods [DFT/B3LYP and second-order Møller-Plesset (MP2) using the 6-311++G(d,p) basis set]. The results show the general mechanism of the reaction: the dihydrogen-bonded (DHB) adduct (EH...HO) formation leads through the activation barrier to the next concerted step of H2 elimination and alkoxo product formation. The structures, interaction energies (calculated by different approaches including the energy decomposition analysis), vibrational E-H modes, and electron-density distributions were analyzed for all of the DHB adducts. The transition state (TS) is the dihydrogen complex stabilized by a hydrogen bond with the anion [EH3(eta2-H2)...OR-]. The single exception is the reaction of BH4- with CF3OH exhibiting two TSs separated by a shallow minimum of the BH3(eta2-H2)...OR- intermediate. The structures and energies of all of the species were calculated, leading to the establishment of the potential energy profiles for the reaction. A comparison is made with the mechanism of the proton-transfer reaction to transition-metal hydrides. The solvent influence on the stability of all of the species along the reaction pathway was accounted for by means of polarizable conductor calculation model calculations in tetrahydrofuran (THF). Although in THF the DHB intermediates, the TSs, and the products are destabilized with respect to the separated reactants, the energy barriers for the proton transfer are only slightly affected by the solvent. The dependence of the energies of the DHB complexes, TSs, and products as well as the energy barriers for the H2 release on the central atom and the proton donor strength is also discussed.

Deprotonation of the MnI NHC‐phosphine complex fac‐[MnBr(CO)3(κ2P,C‐Ph2PCH2NHC)] (2) under a H2 atmosphere readily gives the hydride fac‐[MnH(CO)3(κ2P,C‐Ph2PCH2NHC)] (3) via the intermediacy of the highly reactive 18‐e NHC‐phosphinomethanide complex fac‐[Mn(CO)3(κ3P,C,C‐Ph2PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16‐e mangana‐substituted phosphonium ylide complex fac‐[Mn(CO)3(κ2P,C‐Ph2P=CHNHC)] (6 b) as key intermediate able to activate H2 via a non‐classical mode of metal‐ligand cooperation implying a formal λ5‐P–λ3‐P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre‐catalysts for ketone hydrogenation in the MnI series reported to date (TON up to 6200).

The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph(3)P)(2)Cu(η(2)-BH(4)) interaction with different proton donors [MeOH, CH(2)FCH(2)OH, CF(3)CH(2)OH, (CF(3))(2)CHOH, (CF(3))(3)CHOH, p-NO(2)C(6)H(4)OH, p-NO(2)C(6)H(4)N═NC(6)H(4)OH, p-NO(2)C(6)H(4)NH(2)] using the combination of experimental (IR, 190-300 K) and quantum-chemical (DFT/M06) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thoroughly analyzed by means of quantum-chemical calculations. Bifurcate complexes involving both bridging and terminal hydride hydrogen atoms become thermodynamically preferred for strong proton donors. Their formation was found to be a prerequisite for the subsequent proton transfer and dimerization to occur. Reaction kinetics was studied at variable temperature, showing that proton transfer is the rate-determining step. This result is in agreement with the computed potential energy profile of (Ph(3)P)(2)Cu(η(2)-BH(4)) dimerization, yielding [{(Ph(3)P)(2)Cu}(2)(μ,η(4)-BH(4))](+).

The ring-opening metathesis polymerization (ROMP) of functional norbornenes, i.e., exo,endo-5-norbornene-2-carbonitrile (3), methyl (exo,endo-5-norbornene-2-carboxylate) (4), norborn-5-ene-2-yl acetate (5), 2-propyl exo,endo-5-norbornene-2-carboxylate (6), norborn-5-ene-2-carboxylic acid (7), exo,endo-5-norbornene-2-methanol (8), exo,endo-5-norbornene-2-ylmethyl bromide (9), exo,endo-5-norbornene-2-ylmethylamine (10), exo,endo-5-norbornene-2-triethoxysilane (11), dimethyl exo,endo-5-norbornene-2-yl phosphonate (12), exo,endo-5-norbornene-2-ylcarboxyethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide (13), exo,endo-5-norbornene-2-ylcarboxyethyl-3-ethylimidazolium tetrafluoroborate (14), 7-oxanorborn-5-ene-2,3-dicarboxylic anhydride (15), was performed in a variety of pure ionic liquids (ILs) in the absence of any cosolvent. Both imidazolium, i.e., [1-butyl-3-methylIM] + X -and [1-butyl-2,3-dimethylIM] + X -(IM + ) imidazolium, X -) PF 6 -, CF 3 SO 3 -, (CF 3 SO 2 ) 2 N -, CF 3 COO -, NO 3 -, BF 4 -, Br -), and phosphonium-based ILs, i.e., [P + (C 6 H 13 ) 3 (C 14 H 29 )]X -(X -) PF 6 -, BF 4 -, Cl -), were used. In this context, the principal compatibility of an IL with a series of rutheniumbased catalysts, i.e. RuCl 2 Py 2 (IMesH 2 )(CHPh) (1) and RuCl 2 (IMesH 2 )(2-(2-PrO-C 6 H 4 ) (2) (Py ) pyridine, IMesH 2 ) 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazolin-2-ylidene) was shown. The influence of temperature, concentration of both the initiator and the monomer and nature of the IL on ROMP was investigated. ROMP of norbornene derivatives in ionic solvents proceeded with high speed and offered access to high molecular weight polymers (M w up to 1 500 000 g/mol) in high yields. Most important, ILs allowed for the synthesis of polymers from monomers that are hardly polymerizable in organic solvents, e.g. of 15. The use of 3 in [1-methyl-3-butylIM] + PF 6 -as the reaction medium along with 2-(2-propoxy)styrene allowed for the recycling of the ruthenium catalyst. The interaction of catalyst 2 with different ILs, which allowed for an explanation of the influence of the IL's nature upon the polymerization process, was investigated by IR-and UV-vis spectroscopy. These investigations will facilitate the choice of the optimum IL in future investigations.

The dihydrogen bonding (DHB)

Structural, spectroscopic, and electronic features of weak hydrogen-bonded complexes of CpM(CO)(3)H (M = Mo (1a), W (1b)) hydrides with organic bases (phosphine oxides R(3)PO (R = n-C(8)H(17), NMe(2)), amines NMe(3), NEt(3), and pyridine) are determined experimentally (variable temperature IR) and computationally (DFT/M05). The intermediacy of these complexes in reversible proton transfer is shown, and the thermodynamic parameters (DeltaH degrees , DeltaS degrees ) of each reaction step are determined in hexane. Assignment of the product ion pair structure is made with the help of the frequency calculations. The solvent effects were studied experimentally using IR spectroscopy in CH(2)Cl(2), THF, and CH(3)CN and computationally using conductor-like polarizable continuum model (CPCM) calculations. This complementary approach reveals the particular importance of specific solvation for the hydrogen-bond formation step. The strength of the hydrogen bond between hydrides 1 and the model bases is similar to that of the M-H...X hydrogen bond between 1 and THF (X = O) or CH(3)CN (X = N) or between CH(2)Cl(2) and the same bases. The latter competitive weak interactions lower the activities of both the hydrides and the bases in the proton transfer reaction. In this way, these secondary effects shift the proton transfer equilibrium and lead to the counterintuitive hampering of proton transfer upon solvent change from hexane to moderately polar CH(2)Cl(2) or THF.

The exotic “Asian Lantern” heterometallic cage silsesquioxane [(PhSiO1.5)20(FeO1.5)6(NaO0.5)8(n-BuOH)9.6(C7H8)] (I) was obtained and characterized by X-ray diffraction, EXAFS, topological analyses and DFT calculation.