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Title: Hydrogen bond mediated reactions using ruthenium complexes
Authors: Fung, Wai-kit
Keywords: Hong Kong Polytechnic University -- Dissertations
Ruthenium compounds
Hydrogen bonding
Issue Date: 2005
Publisher: The Hong Kong Polytechnic University
Abstract: The indenyl ruthenium hydride complex (η⁵-C₉H₇)Ru(dppm)H was found to be active in catalyzing the hydration of nitriles to amides. The chloro analogue (η⁵-C₉H₇)Ru(dppm)Cl was, however, found to be inactive. Density functional theory calculations at the B3LYP level provide explanations for the effectiveness of the hydride complex and the ineffectiveness of the chloro complex in the catalysis. The result shows that the presence of Ru-H...H-OH dihydrogen-bonding interaction in the transition state lowers the reaction barrier in the case of(η⁵-C₉H₇)Ru(dppm)H, but in the chloro system, the corresponding transition state does not contain such interaction and the reaction barrier is much higher. Similar dihydrogen bond-mediating effect is believed to be responsible for the catalytic activity of the hydrotris(pyrazolyl)borato (Tp) ruthenium complex TpRu(PPh₃)(CH₃CN)H in nitrile hydration. The chloro analogue TpRu(PPh₃)(CH₃CN)Cl shows no catalytic activity. In our previous studies of the reactivity of some aminocyclopentadienyl ruthenium complexes, we learned that the amino sidearms of these complexes are able to heterolytically cleave the η²-H₂ ligands to generate intramolecularly dihydrogen-bonded Ru-H...H-N species. To further our investigation on the activities of the aminocyclopentadienyl ruthenium complexes in chemical reactions such as hydrogenation of CO₂ and hydration of nitriles, we synthesized the aminoindenylruthenium complexes, (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H and [η⁵:η¹-C₉H₆(CH₂)₂NMe₂]Ru(dppm)]BF₄, and compared their activities with those of the corresponding aminocyclopentadienyl ruthenium complexes in these reactions. We have found that the aminoindenylruthenium complexes are in general more active than the aminocyclopentadienyl ruthenium complexes.
Furthermore, we have found that the aminoindenylruthenium hydride complex (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H is more active in catalyzing the hydration of nitriles than the indenyl ruthenium hydride complex, which has no amino side arm on its indenyl ligand. We believe that the amino side arm on the indenyl ligand might play an important role in the catalysis. To understand the preponderance of the catalytic activity of (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H in nitrile hydration over that of (η⁵-C₉H₇)Ru(dppm)H, density function calculations at the B3LYP level to establish a plausible catalytic pathway were carried out. It is learned that the presence of Ru-H...H-OH dihydrogen bonding and additional InN...H-N=(OH)Me hydrogen bond interaction in the transition states lower the reaction barriers in the case of (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H, but in the (η⁵-C₉H₇)Ru(dppm)H-catalyzed reaction, the transition states do not involve additional hydrogen bonding and thus the reaction barriers are much higher. In addition to the hydration reaction, the aminoindenylruthenium hydride complex is also capable of catalyzing the hydrogenation of alkenes and selectively reducing the C=C bond in an α,β-unsaturated ketone; the reactivities of the (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H and (η⁵-C₉H₇)Ru(dppm)H in these reactions have been studied and compared. The complex (η⁵-C₉H₆(CH₂)₂NMe₂)Ru(dppm)H shows higher catalytic activity in the hydrogenation of the C=C bond of α,β-unsaturated ketones than (η⁵-C₉H₇)Ru(dppm)H. Presence of the amino side arm appears to facilitate the hydrogenation reaction, and we propose that the hydrogen bonding-interaction in the transition state lowers the reaction barrier in this case.
Description: xx, 205 leaves : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577P ABCT 2005 Fung
Rights: All rights reserved.
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