In Situ Formation of Ruthenium Catalysts for the Homogeneous Hydrogenation of Carbon Dioxide

Abstract

A total of 44 different phosphines were tested, in combination with [RuCl(2)(C(6)H(6))](2) and three other Ru(II) precursors, for their ability to form active catalysts for the hydrogenation of CO(2) to formic acid. Half (22) of the ligands formed catalysts of significant activity, and only 6 resulted in very high rates of production of formic acid. These were PMe(3), PPhMe(2), dppm, dppe, and cis- and trans-Ph(2)PCH=CHPPh(2). The in situ catalysts prepared from [RuCl(2)(C(6)H(6))](2) and any of these 6 phosphine ligands were found to be at least as efficient as the isolated catalyst RuCl(O(2)CMe)(PMe(3))(4). There was no correlation between the basicity of monophosphines (PR(3)) and the activity of the catalysts formed from them. However, weakly basic diphosphines formed highly active catalysts only if their bite angles were small, while more strongly basic diphosphines had the opposite trend. In situ (31)P NMR spectroscopy showed that trans-Ru(H)(2)(dppm)(2), trans-RuCl(2)(dppm)(2), trans-RuHCl(dppm)(2), cis-Ru(H)(O(2)CH)(dppm)(2), and cis-Ru(O(2)CH)(2)(dppm)(2) are produced as the major metal-containing species in reactions of dppm with [RuCl(2)(C(6)H(6))](2) under catalytic conditions at 50 degrees C.

Recent advances in catalytic hydrogenation of carbon dioxide

“…263 Tai et al compared the effectiveness of Ru(II) catalysts with various phosphines or other ligands through in situ catalyst formation. 277 There is no correlation found between the basicity of monophosphines (PR 3 ) and the activity of the catalysts. Among the diphosphines, a rather unusual interplay of electronic and bite angle effects was observed (Fig.…”

Towards a Rational Design of Ruthenium CO2 Hydrogenation Catalysts by Ab Initio Metadynamics

“…However, the general trend indicates that the activity for formic acid formation progressively decreases in the sequence, dppe > PMe 3 > dmpe > X. [12,13,15] Actually, the activity data for the catalyst with the ligand X has not been measured, but it is expected to be almost zero, as it happens when one methyl group of dppe is substituted by hydrogen. [15] The binding-energy vs. activity relationship confirms that, in spite of its bulkier structure, the dppe complex is one of the most efficient catalysts because of the weak binding of the formate group.…”

“…[12,13,15] Actually, the activity data for the catalyst with the ligand X has not been measured, but it is expected to be almost zero, as it happens when one methyl group of dppe is substituted by hydrogen. [15] The binding-energy vs. activity relationship confirms that, in spite of its bulkier structure, the dppe complex is one of the most efficient catalysts because of the weak binding of the formate group. Note that the superior activity of the dppe complex suggested by our calculations fully agrees with the earlier experimental observations where the dppe complex was found most active among various Ru-bidentate complexes (e.g.…”

“…The formation of formic acid by CO 2 hydrogenation has been identified as the first reaction step in all the three different reaction systems. [12] Hence, the elucidation of the formic acid formation pathways is necessary to understand several aspects of the reaction process, like the detailed reaction mechanisms, the effects of different ligands [12,15] , the role of additives (water, alcohols, and amines) [12,16,17] , and the influence of the reactants' pressure [12,16,18] . Some detailed reaction mechanisms have been proposed to date, but discrepancies between the mechanisms and experimental observations exist, [19,20] such as the one between the theoretical assignment of CO 2 insertion to Ru hydride complex as the rate-limiting step and experimentally observed facile CO 2 insertion [21,22] .…”

Homogenous catalytic hydrogenation of bicarbonate with water soluble aryl phosphine ligands

“…Jessop et al [30] examined the correlations between the properties of a series of phosphine ligands and the catalytic activity of ruthenium catalysts in MeOH/N i Pr 3 in the hydrogenation reaction of CO 2 to FA. However the results were not conclusive; in the case of monodentate phosphine ligands, electronic factors (i.e., basicity and Hammett constant) were not affecting the activity.…”