Acidity dihydrogen complex

In the heterolytic splitting of dihydrogen, the acidic dihydrogen complex [M](ti2-H2) transfers a proton to the base (eq 2-4 of the introduction). The proton can then remain associated with the hydride in a 1.7-2.0 A contact in the ion-pair -[MJH—HB+. This may be an intramolecular interaction or an intermolecular interaction. In the following sections, our work on intramolecular hydridic-pro-tonic bonds will be reviewed first, followed by that on such intermolecular non-classical bonds. 

The fact that H2 loss from MH/HA is reversible means that addition of to a metal complex to give an acidic dihydrogen complex should be possible. Ionic hydrogenation requires that this dihydrogen complex be acidic enough to protonate the substrate and that the corresponding hydride be capable of transferring H to the protonated substrate (eq 2). 

Typical acidic dihydrogen complexes together with their pK values are shown... 

Table 2. Acidic dihydrogen complexes with their values...

Following this observation, a general approach for the synthesis of pincer-type methylene arenium compounds was developed (Scheme 3.4). Upon reaction of the methyl rhodium (or iridium) complexes 5 with a slight excess of triflic acid, dihydrogen (not methane ) was evolved to form the methylene arenium complexes 4a.11 Thus, the methylene arenium form is clearly preferred over the benzylic M(III) form, in which the positive charge is localized at the metal center. 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

The fact that metal hydrides can be acidic may seem paradoxical in view of the nomenclature that insists that all complexes with a M-H bond be referred to as hydrides regardless of whether their reactivity is hydridic or not. Not only can some metal hydrides donate a proton, but some can be remarkably acidic. Some cationic dihydrogen complexes are sufficiently acidic to protonate Et20 [8], and some dicationic ruthenium complexes have an acidity comparable to or exceeding that of HOTf [9],... 

Morris et al. carried out extensive studies [20] of the acidity of metal hydrides in tetrahydrofuran (THF), including metal hydrides of very low acidity as well as dihydrogen complexes that are reactive with CH3CN. The dielectric constant of THF is low compared to that of CH3CN, so ion-pairing issues must be taken into account [21], though these measurements in THF provide useful comparisons to data in CH3CN and other solvents. 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Similar kinetic and thermodynamic data, obtained for transformations of OsH2(PP3) to the dihydrogen complex [OsH(H2)(PP3)] [40] and for Cp Ru(dppe)H to the [Cp Ru(dppe)(H2)] [41] in the presence of various acids, have resulted in the same energy profile of the reactions. 

When the initial hydride CpRuH(CO)(PH3), It, is placed near H3O+, a proton transfer occurs without the energy barrier to form the dihydrogen complex [CpRu(H2)(CO)(PH3)]+, in good agreement with the data of Orlova and Scheiner [42]. Dihydrogen-bonded intermediates appear on the protonation pathway if the hydride It interacts with the weaker acids TFA and PFTB. However, the... 

Dihydrogen complexes, osmium, 37 3(X)-301 Dihydroxo-bridged complexes acid-base equilibria, 32 108-110 quantitative considerations, 32 115-118 binuclear, 32 66-67 crystallographic data, 32 61, 63 stability complexes, 32 103-104... 

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