Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation

Reaction pathways during CO<SUB >2</SUB > hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)<SUB >2</SUB >H<SUB >2</SUB >] (dmpe=Me<SUB >2</SUB >PCH<SUB >2</SUB >CH<SUB >2</SUB >PMe<SUB >2</SUB >) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120&nbsp;bar in toluene at 300&nbsp;K. CO<SUB >2</SUB > and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, <I >cis</I >-[Ru(dmpe)<SUB >2</SUB >(OCHO)<SUB >2</SUB >] and <I >trans</I >-[Ru(dmpe)<SUB >2</SUB >H(OCHO)], were formed at low CO<SUB >2</SUB > pressure (&lt;5&nbsp;bar). The latter occurred exclusively when formic acid reacted with the complex. A RuH&sdot;&sdot;&sdot; HOCHO dihydrogen-bonded complex of the <I >trans</I > form was identified at H<SUB >2</SUB > partial pressure higher than about 50&nbsp;bar. The <I >trans</I > form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the <I >trans</I > route were confirmed. The H<SUB >2</SUB > insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H<SUB >2</SUB > insertion is attributed to the elongated Ru&minus;O bond. The H<SUB >2</SUB > insertion and the subsequent formation of formic acid proceed via Ru(&eta;<SUP >2</SUP >-H<SUB >2</SUB >)-like complexes, in which apparently formate ion and Ru<SUP >+</SUP > or Ru(&eta;<SUP >2</SUP >-H<SUB >2</SUB >)<SUP >+</SUP > interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuH&sdot;&sdot;&sdot;HOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.


Published in:
CHEMISTRY-A EUROPEAN JOURNAL, 13, 14, 3886-3899
Year:
2007
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 Record created 2007-06-13, last modified 2018-03-17


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