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Alkane selective functionalization

#3082


Alkane Selective Functionalization. Synthesis of Metallocene-Based Pincer Complexes of a New Generation, Catalysts for Alkane Dehydrogenation

Tech Area / Field

  • CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry
  • CHE-SYN/Basic and Synthetic Chemistry/Chemistry
  • ENV-OTH/Other/Environment
  • NNE-FCN/Fuel Conversion/Non-Nuclear Energy

Status
8 Project completed

Registration date
20.07.2004

Completion date
18.11.2009

Senior Project Manager
Mitina L M

Leading Institute
INEOS (Organo-Element Compounds), Russia, Moscow

Supporting institutes

  • Tbilisi State University, Georgia, Tbilisi

Collaborators

  • Indiana University / Department of Chemistry, USA, IN, Bloomington\nThe University of Montana, Chemistry Department, USA, MT, Missoula

Project summary

The project is directed to the creation of a new generation homogeneous catalysts for selective alkane functionalization, dehydrogenation to give alkenes. It is highly exploratory but, if successful, will lead to the future application of homogeneous catalysts in highly selective “green” industrial processes.
Alkanes are the world’s most abundant organic resource, yet methods for their direct conversion to useful chemicals remain extremely limited. The alkane selective dehydrogenation is potential functionalization since it gives major organic feedstock such as terminal alkenes (-olefins) and, simultaneously, ecologically irreproachable energy carrier, dihydrogen. The dehydrogenation of alkanes by soluble organometallic complexes is of increasing practical interest, since, in general, the yields and selectivity afforded by heterogeneous alkane dehydrogenation catalysts are very low. The development of catalysts for the alkane selective dehydrogenation is an important goal of organometallic chemistry.
To date iridium dihydrido pincer complexes IrH2[2,6-(R2PCH2)2C6H3] (R=Pri, But) are the most efficient among all known homogeneous catalytic systems for alkane dehydrogenation. However, these complexes cannot find practical application due to some circumstances: the rates of dehydrogenation are still insufficient, and product inhibition and isomerization of primarily formed 1-alkenes into internal alkenes take place. It is obvious that a search for approaches to the design of novel pincer complexes is a topical problem. One way of solving this problem is to modify substantially the pincer ligand including changes in its electronic and steric properties.
The advantage of the metallocene-based pincer complexes (B) recently discovered by us (A.A. Koridze, A.M. Sheloumov, S.A. Kuklin, V.Yu. Lagunova, I.I. Petukhova, F.M. Dolgushin, M.G. Ezernitskaya, P.V. Petrovskii, A.A. Macharashvili, R.V. Chedia, Russ. Chem. Bull., Int. Ed. 2002, 51, 1077) compared to the known benzene-based pincer systems (A) is that stereochemistry and electron density at the catalytic center can be altered in a wider range thus allowing fine tuning of pincer complexes for the desired application in a broad spectrum of catalytic reactions.

Two applying groups (Moscow and Tbilisi) discovered a new generation pincer complexes that have several remarkable features attractive from the viewpoint of their use in catalysis. First, in metallocene-based pincer complexes the central atom of the metallocene moiety is located in the vicinity (-position) of the catalytic center, the chelated metal atom M; this arrangement can facilitate tuning of the electron effects on atom M by varying metallocene central atom (Fe, Ru, and Os). Second, the iron atom of the ferrocene unit can be involved into very rapid and reversible redox reaction thus providing the additional possibility to tune the electron density at the catalytic center. Third, the sandwich nature of ferrocene allows one to design planar-chiral molecules, and the presence of the Fe(C5H5) fragment provides additional possibility for fine tuning of the steric environment at the atom M; it may prevent isomerization of 1-alkenes, initially formed in iridium-complex-catalyzed dehydrogenation of linear alkanes, into less valuable internal alkenes. It should be also noted, that the iron group metallocenes possess high thermal stability.
According to our preliminary data, the ferrocene-based pincer complex IrH2[{2,5-(But2PCH2)2C5H2}Fe(C5H5)] reveals unprecedented catalytic activity for transfer dehydrogenation of cyclooctane with norbornylene as a hydrogen acceptor to form cyclooctene and norbornane. At 150C the initial dehydrogenation of cyclooctane by the ferrocene-based complex proceeds at a rate of 330 turnovers h-1 while the reactivity of its benzene-based analog IrH2[2,6-(But2PCH2)2C6H3] at this low temperature is found to be only of 82 turnovers h-1 (C.M. Jensen, J. Chem. Soc., Chem. Commun. 1999, 2443).
There is great interest in the development of “acceptorless” systems in which alkanes are transformed to alkenes upon direct elimination of H2. We propose to develop the efficient metallocene-based catalysts for acceptorless dehydrogenation of alkanes.
A co-operation between the two teams-applicants from Moscow and Tbilisi has been organized with an intention of rational design of metallocene-based pincer complexes as catalysts for alkane selective dehydrogenation. We believe that special attention should be paid to: (a) steric accessibility of substrates, alkanes, for catalytic center, (b) electronic effects of pincer ligands including redox state of the iron atom of the ferrocene unit, which favor triggering inpidual steps of the catalytic cycle, and (c) steric and electronic effects, which may prevent catalyst deactivation by tying up coordination sites with product, alkene, or dinitrogen.
We put ourselves the following exploratory tasks:
1. To develop methods for the synthesis of the pincer ligands precursors, P,CH,P, 1,3-bis(diorganylphosphinomethyl)ferrocenes and -ruthenocenes containing various alkyl, cycloalkyl, and aryl groups at the P atoms.
2. To develop methods for the synthesis of metallocene-based P, C, P pincer complexes of iridium, rhodium, and (as model compounds) palladium and platinum.
3. To study chemical properties of pincer complexes such as reactions with small molecules (H2, CO, CO2, N2, ethylene, and acetylene) and electrochemical redox reactions.
4. To study systematically the structure/reactivity relationship for alkane dehydrogenation reaction catalysed by metallocene-based pincer complexes.
After preliminary testing, the most promising pincer complexes will be selected for the synthesis of anchoring (dendrimeric) pincer systems for practical application.
The ultimate goal of two research groups is creation of catalytic systems exhibiting high activity and selectivity. Being based on conceptually novel bimetallic pincer complexes, this approach is very interesting from both the basic and applied points of view.


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