Bimetallic Complexes for Enhancing Catalyst Efficiency: Probing the Relationship between Activity and Intermetallic Distance

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• 作者：Marina G. Timerbulatova ; Mark R. D. Gatus ; Khuong Q. Vuong ; Mohan Bhadbhade ; Andr茅s G. Algarra ; Stuart A. Macgregor ; Barbara A. Messerle
• 刊名：Organometallics
• 出版年：2013
• 出版时间：September 23, 2013
• 年：2013
• 卷：32
• 期：18
• 页码：5071-5081
• 全文大小：531K
• 年卷期：v.32,no.18(September 23, 2013)
• ISSN：1520-6041

文摘

A series of new homoditopic ligands (14鈥?b>17) containing two bis(pyrazol-1-yl)methane moieties connected to either flexible (1,6-bis(bis(pyrazol-1-yl)methyl)hexane, L_6C (14); 1,7-bis(bis(pyrazol-1-yl)methyl)heptane, L_7C (15)) or rigid scaffolds (4,5-bis(bis(pyrazol-1-yl)methyl)-9,9-dimethylxanthene, L_Xan (16); 4,6-bis(bis(pyrazol-1-yl)methyl)dibenzofuran, L_Dib (17)) were synthesized. A series of bimetallic rhodium(I) complexes [Rh₂(CO)₄(L_X)][BAr^F₄]₂ (X = Xan (8), Dib (9), Fc ((1,1鈥?bis(bis(pyrazol-1-yl)methyl)ferrocene) (10)), 6C (11), 7C (12)) and [Rh₂(COD)₂(L_X)][BAr^F₄]₂ (COD = 1,5-cyclooctadiene, X = 6C (21), 7C (22)) as well as the monometallic complexes [Rh(CO)₂(L_Ph)][BAr^F₄] (7, L_Ph = 伪,伪-bis(pyrazol-1-yl)toluene) and [Rh(COD)(L_Ph)][BAr^F₄] (20) were synthesized. The solid-state structures of 8, 10, 16, 17, and 21 were determined using single-crystal X-ray diffraction analysis. The catalytic activity of complexes 7鈥?b>12 was established for the dihydroalkoxylation of the alkynediols 2-(5-hydroxypent-1-ynyl)benzyl alcohol (I) and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (II). The rigid bimetallic scaffolds L_Xan and L_Dib were found to yield the most active catalysts, 8 and 9, respectively, with 9 achieving a reaction rate 5鈥? times faster than the monometallic complex 7 for the dihydroalkoxylation of I. Density functional theory calculations were used to examine the intermetallic Rh路路路Rh distances in 8 and 9, and these were compared with those of three other related bimetallic catalysts reported previously. The calculations showed all these species to be very flexible at minimal energetic cost, both in terms of the Rh路路路Rh distance and in being able to access a range of different conformations. No clear correlation between Rh路路路Rh distance and catalytic activity was established here, which suggests that the observed experimental correlation between catalyst structure and activity may derive from the structures of key reaction intermediates.