Ligands dihydrogen complexes

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. 

Schemes 3.2 and 3.3 show intermediates containing dihydrogen ligands with the H-H bond intact. It has only been appreciated since the discovery of the first dihydrogen complexes by Kubas and coworkers in 1984 [14] that such complexes are key intermediates in catalytic cycles [11, 13, 14].

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 ... 

Structure 3.2 Structure proposed for the binuclear Ru dihydrogen complex in ref. 8, where the dihydrogen ligand is binding to two ruthenium atoms. 

Proton transfer to negatively charged hydrogen atoms has attracted the attention of many chemists over the last two decades. This process plays an important role in many chemical and biochemical phenomena that occnr in the gas phase, in solution, and in the solid state [1-3], For example, direct proton attack on hydride ligands generates transition metal dihydrogen complexes which are then involved in various catalytic transformations [4] ... 

On the other hand, it is well known that a dihydrogen complex can be generated through direct proton attack on a hydride ligand or initial protonation of a metal center, leading to a new classical dihydride, [MH2]", which then converts to the dihydrogen complex [M(ti -H2)]. The latter is a thermodynamic product of the protonation reaction shown in Scheme 10.4 [16,17]. 

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