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Highlights in Chemical Science

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Catalytic complexity

23 April 2007

UK chemists have isolated an unusual type of solvent complex that might play a crucial role as an intermediate in catalytic reactions.

Andrew Weller and colleagues at the University in Bath took an iridium catalyst, which is used to hydrogenate alkenes, and reacted it with hydrogen using dichloroethane (DCE) as the solvent.

The researchers isolated a dihydride iridium complex stabilised by a solvent molecule. Crystallographic and spectroscopic studies revealed that the solvent molecule binds to the iridium centre through its two chlorine atoms.

According to team member Gemma Moxham, well-characterised transition metal complexes of dichloromethane (DCM) are known but this is the first example of a DCE complex. 'Complexes such as this are often proposed as intermediates in catalytic cycles, but are not often stable enough to isolate and fully characterise,' she said.


Structure of an iridium complex stabilised by a dichloroethane molecule

Dichloroethane forms a stronger bond with iridium than dichloromethane


Weller and his team compared the iridium-DCE complex with the same DCM system. They found that DCE forms a stronger bond with the iridium centre, meaning the iridium-DCE intermediate is much less likely to decompose during the catalytic process. The flipside to this, they discovered, is that the catalytic rate is slower in alkene hydrogenation.

Sylviane Sabo-Etienne, an expert in organometallic catalysis at the laboratory of coordination chemistry (LCC) in Toulouse, France, underlined this dichotomy. 'Such a solvent might favour the isolation of key catalytic intermediates or increase catalyst stability, a factor which might of course be detrimental to catalyst activity,' she said.

But Moxham believes it is important to strike the right balance between catalytic activity and catalyst longevity. 'This observation will be extremely useful to catalyst chemists when designing new catalytic systems, particularly in their choice of solvent,' she said.

Stephen Wilkes

Link to journal article

[Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF4]: a well characterised transition metal dichloroethane complex
Gemma L. Moxham, Simon K. Brayshaw and Andrew S. Weller, Dalton Trans., 2007, 1759
DOI: 10.1039/b702399e