US researchers achieve stable, long-lived sigma-methane complex without breaking the C-H bonds
US chemists have characterised the first stable sigma-methane complex - in which methane binds to a metal without breaking its C?-H bonds. The complex provides insight into the activation of unreactive alkanes and the possibility of expanding the use of methane to eventually replace oil as a feedstock for the chemical industry.
Maurice Brookhart, Cynthia Schauer and colleagues at the University of North Carolina and University of Washington, US, devised new ways of preparing rhodium-methane complexes that overcame many of the difficulties encountered by previous studies, and found exactly the right combination of metal and ligands to stabilise the normally fleeting compounds.
’We cheated a little bit,’ says Schauer, explaining that the team used a strong acid to protonate a rhodium methyl (Rh-CH3) complex and form the methane ligand within the coordination sphere of the metal centre, rather than trying to introduce methane from the solution. ’Methane is a very weak ligand,’ says Schauer, ’and it would need to displace something else to coordinate to the metal, but [our complex] spits out the methane [when it is protonated], so we don’t need to displace anything.’
’We’ve been waiting for this since 1975,’ says Robin Perutz from the University of York in the UK, and one of the researchers who first found evidence for such sigma-methane complexes. ’There has been a lot of indirect evidence since, but observing the real thing has eluded people.’
’Perhaps with hindsight people were looking in the wrong place,’ Perutz adds, ’this is a positively charged complex, whereas most people were looking for neutral complexes.’ He describes that the group’s protonation strategy as ’a real breakthrough’ as it gets over the problem that methane is not very soluble in conventional solvents, even at very high pressure, so it is difficult to get a useful concentration in solution.
Such sigma-methane complexes, says Schauer, have long been proposed as intermediates in the coordination and activation of methane in catalytic processes that convert it into more industrially useful chemicals such as methanol or synthesis gas (a mixture of hydrogen and carbon monoxide). Understanding how methane interacts with metals - and how that leads to reactivity of the C-H bonds - could lead to new catalysts that work under much less extreme conditions than the current Fischer-Tropsch or Mobil-type processes for converting methane to more complex chemicals.
Key to that understanding is the ability to study the structure of the complexes, and this is the first time a complex has been made that lasted long enough in solution to do that, explains Bill Jones from the University of Rochester, US. ’They have been detected transiently in laser flash studies, but those lasted for less than a microsecond,’ says Jones. The lifetime of several hours achieved by Brookhart’s group is a key advance that allowed detailed nuclear magnetic resonance and isotope labelling studies, and thus fresh insights into the structure and chemistry of the sigma-methane complex.
W Bernskoetter et al, Science, 2009, DOI: 10.1126/science.1177485