Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

Kubas, Gregory J.

doi:10.1007/s10562-005-7440-3

Cited by 109 publications

(55 citation statements)

References 193 publications

(132 reference statements)

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“…[69] The following step is the formation of a Ru(η 2 -H 2 ) complex (9, Figure 7), where the molecular hydrogen coordinates to the Ru center, giving a global CN(H) approximately equal to 3. This class of η 2 -H 2 complexes has been increasingly reported [53,54] , and their role turned out to be important as intermediates in various hydrogenation reactions. [55][56][57][58] This complex is identified as an energetic minimum and not as a transition-state.…”

Section: Co 2 Insertion / Dissociationmentioning

confidence: 98%

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

Urakawa

Iannuzzi

Hutter

et al. 2007

Chemistry A European J

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Complete reaction pathways relevant to CO2 hydrogenation by using a homogeneous ruthenium dihydride catalyst ([Ru(dmpe)2H2], dmpe=Me2PCH2CH2PMe2) have been investigated by ab initio metadynamics. This approach has allowed reaction intermediates to be identified and free‐energy profiles to be calculated, which provide new insights into the experimentally observed reaction pathway. Our simulations indicate that CO2 insertion, which leads to the formation of formate complexes, proceeds by a concerted insertion mechanism. It is a rapid and direct process with a relatively low activation barrier, which is in agreement with experimental observations. Subsequent H2 insertion into the formateRu complex, which leads to the formation of formic acid, instead occurs via an intermediate [Ru(η2‐H2)] complex in which the molecular hydrogen coordinates to the ruthenium center and interacts weakly with the formate group. This step has been identified as the rate‐limiting step. The reaction completes by hydrogen transfer from the [Ru(η2‐H2)] complex to the formate oxygen atom, which forms a dihydrogen‐bonded RuH⋅⋅⋅HO(CHO) complex. The activation energy for the H2 insertion step is lower for the trans isomer than for the cis isomer. A simple measure of the catalytic activity was proposed based on the structure of the transition state of the identified rate‐limiting step. From this measure, the relationship between catalysts with different ligands and their experimental catalytic activities can be explained.

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Section: Co 2 Insertion / Dissociationmentioning

confidence: 98%

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

Urakawa

Iannuzzi

Hutter

et al. 2007

Chemistry A European J

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“…[98] In 2005, Kubas reviewed examples of catalytic processes involving s complexes, [99] and assembled evidence for their role both as precursors and as intermediates in catalytic cycles. Here, we pick out a few examples of major relevance to this study.…”

Section: Examples Of Dihydrogen and Silane Complexes As Intermediatesmentioning

confidence: 99%

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

2007

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Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.

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“…Such h 2 -H 2 complexes, for example, found by Kubas, [34,35] have been increasingly reported, and the role of such complexes in catalytic hydrogenation is an interesting branch of fundamental research. The H À H distance of 10 of 0.85 belongs to the category of "true H 2 complexes" by the definition of Kubas.…”

mentioning

confidence: 98%

“…The H À H distance of 10 of 0.85 belongs to the category of "true H 2 complexes" by the definition of Kubas. [35] The existence of 10 and the relative stability are suggested by ab initio metadynamics when both H 2 and formic acid concentrations are sufficiently high. [31] The bond characteristics of the formate part of 10 are not different from those of 9 and TS 9-10 .…”

mentioning

confidence: 99%

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High‐Pressure Spectroscopic Investigation

Urakawa

Jutz

Laurenczy

et al. 2007

Chemistry A European J

119

103

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Reaction pathways during CO(2) hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)(2)H(2)] (dmpe=Me(2)PCH(2)CH(2)PMe(2)) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO(2) and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)(2)(OCHO)(2)] and trans-[Ru(dmpe)(2)H(OCHO)], were formed at low CO(2) pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuHHOCHO dihydrogen-bonded complex of the trans form was identified at H(2) partial pressure higher than about 50 bar. The trans 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 trans route were confirmed. The H(2) insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H(2) insertion is attributed to the elongated Ru--O bond. The H(2) insertion and the subsequent formation of formic acid proceed via Ru(eta(2)-H(2))-like complexes, in which apparently formate ion and Ru(+) or Ru(eta(2)-H(2))(+) 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 (RuHHOCHO) 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.

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Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

Cited by 109 publications

References 193 publications

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

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

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High‐Pressure Spectroscopic Investigation

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Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

Cited by 109 publications

References 193 publications

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

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

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High‐Pressure Spectroscopic Investigation

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

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