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Kompleksy diwodorowe metali grup przejściowych. Cz.2 Reaktywność i własności katalityczne

Borkowski A.F.

Wiadomości Chemiczne

1999

[Z] 53, 3-4

191-217

The second part of the paper presents the reactivity of dihydrogen complexes beginning with the ability of the coordinated H2 ligand to undergo heterolytic splitting (i.e. their acidic properties). Generally, the low energy of the M-(n2-H2) bond in dihydrogen complexes results in a facile loss of the H2 ligand and is usually accompanied by substitution, substitution followed by an intramolecular reaction, ligand exchange, or elimination followed by dimerization. Dihydrogen complexes (most often Ir, Os, Ru) show catalytic activities in isotope H/D exchange reactions in the homogeneous hydrogenation of alkene, alkynes, arenes, ketones, in alkene silyation as well as in hydrogen transfer reactions between solvent (usually an alcohol) and an unsaturated substrate. In the presence of a dihydrogen acceptor (e.g. t-Bu-CH=CH2) dihydrogen complexes are involved in the stoichiometric dehydrogenationsof alkanes to alkenes. Only one example of a molecular hydrogen complex reducing nitrogen or sulfur containing heteroaromatic rings has been reported. Molecular hydrogen complexes may undergo different reactions depending on their role in the catalytic cycle. Under hydrogenation conditions dihydrogen ligand can be easily substituted by unsaturated organic molecules. In this way H2 molecule stabilizes unsaturated catalysts without bothering the coordination of the substrate . Some dihydrogen complexes are regarded as intermediates for the homolytic activation of dihydrogen . In other complexes the H2 molecule acts as a reagent capable of hydrogenating substrate via an intramolecular acid/base reaction. Similarly, the heterolytic splitting of H2 ligand in [Ru(OEP)(thf)2(H2)] by the strong base (KOH) or in [Ir(bq)H(H2)(PPh3)2]+ by an alkohol promote the H/D isotopic exchange reactions. The proposed mechanisms for the hydrogenation of alkenes (alkynes) catalysed by dihydrogen complexes are typical for the catalytic cycles of many other homogeneous hydrogenation processes. These include formation of a vacant coordination site (usually by loss of a H2 molecule, but also by dissociation of one of the 'arms' of the phosphine ligand followed by coordination of a substrate, its insertion into a M-H bond, hydrogenolysis of an alkyl (allyl) moiety followed by product elimination.

Kompleksy diwodorowe metali grup przejściowych. Cz.1 Metody otrzymywania i badania struktury

Borkowski A.F.

Wiadomości Chemiczne

1999

[Z] 53, 3-4

165-189

In the first part of this review preparative methods and structural studies of s-bonded dihydrogen complexes of the transition group metals have been described. The former includes the most common procedures of direct reaction of coordinatively unsaturated complexes with hydrogen gas and protonation of metal hydride species. The Mˇ-(n2-H2) bonding in these complexes is best described as a s (HH) electron density donation to an empty metal d orbital of s symmetry s(HH)Žds(M) augmented by a synergistic back-donation from filled metal d orbitals (dp(M) Žs*(HH)). From among methods of determining solid state structure, the most accurate are the neutron diffraction studies that have been successfully applied to 11 complexes. Progress in broader application of this method is slow due the need for preparation of large crystals and the shortage in the world of neutron scattering and diffraction facilities. In solution, the most reliable data are acquired by the NMR methods Large J(HD) coupling constant values of 12 to 34 Hz and short minimal spin-lattice relaxation time values of ~~3 ms1.6A (classical dihydrides).