Hydrogen bond mediated reactions using ruthenium complexes

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Hydrogen bond mediated reactions using ruthenium complexes

 

Author: Fung, Wai-kit
Title: Hydrogen bond mediated reactions using ruthenium complexes
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations
Ruthenium compounds
Hydrogen bonding
Catalysts
Department: Dept. of Applied Biology and Chemical Technology
Pages: xx, 205 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1809951
URI: http://theses.lib.polyu.edu.hk/handle/200/1760
Abstract: The indenyl ruthenium hydride complex (n5-C9H7)Ru(dppm)H was found to be active in catalyzing the hydration of nitriles to amides. The chloro analogue (n5-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(n5-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 n2-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, (n5-C9H6(CH2)2NMe2)Ru(dppm)H and [n5:n1-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 (n5-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 (n5-C9H6(CH2)2NMe2)Ru(dppm)H in nitrile hydration over that of (n5-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 (n5-C9H6(CH2)2NMe2)Ru(dppm)H, but in the (n5-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 a,b-unsaturated ketone; the reactivities of the (n5-C9H6(CH2)2NMe2)Ru(dppm)H and (n5-C9H7)Ru(dppm)H in these reactions have been studied and compared. The complex (n5-C9H6(CH2)2NMe2)Ru(dppm)H shows higher catalytic activity in the hydrogenation of the C=C bond of a,b-unsaturated ketones than (n5-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.

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