Dihydrogen bridges

There are more noncovalent interactions which cannot all be introduced here. Forces between multipoles have been expertly reviewed recently [12]. Also, weak interactions exist between nitrogen and halogen atoms [13], and dihydrogen bridges [14] can be formed between metal hydrides and hydrogen bond donors. Finally, close packing in crystals is an important force in crystallization and crystal engineering. The present introductory chapter will not discuss these, but rather focus on the most important ones mentioned above. 

In a recent study on the intramolecular dihydrogen bridges of 2-cyclopropyl ethanol derivatives [64] (for the molecular scheme see Fig. 3), calculations predicted that the open conformations are more stable than the closed ones and the observed interactions are probably only H- -H van der Waals contacts although the analysis of the parameters derived from the Bader theory shows that such O-H- - -H-C contacts may be classified as H-bonds . More convincing is the... 

The reduction of porphycenes, e.g. 1, can be achieved by catalytic hydrogenation22-25 or chemical reduction with alkali metals.22 With alkali metals one of the C —C double bond bridges in 1 is reduced to yield a 19,20-dihydroporphycene2. On catalytic hydrogenation of porphycene (1), 2,3-dihydro porphycene (3) is formed which can be compared to chlorin, the dihydrogenated form of porphyrin. 

Herberhold reported activation of water with Re2(CO)io. The reaction proceeded at 200°C to give a tetranuclear complex [Re(CO)3( X -OH)]4 (23) in quantitative yield, and evolution of dihydrogen and CO was observed (Eq. 6.13) [21]. Complex 23 has a pseudo-cubane structure without metal-metal bonds in which the Re(CO)3 groups are linked by triply-bridging OH ligands. Also in this case, no presumed intermediate hydrido(hydroxo) species was detected. 

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

The first step of the reaction of Al with the H2 molecule is the coordination of dihydrogen to Al. The product of this step was interpreted as the complex [P2N2]Zr( i.-t 2-N2H)(p-H)Zr[P2N2] on the basis of H and ISN NMR data [9]. However, low-temperature X-ray diffraction data [9] were interpreted in terms of a complex having a side-on bridging H2 unit. Our calculations show that this coordination does not produce the dihydrogen... 

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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