Carbenes have been used to trap and stabilise rare and elusive forms of phosphorus

Researchers in the US and Germany have shown how a rare and highly reactive form of phosphorus can be captured and crystallised, making it stable even at room temperature. 

A team led by Guy Bertrand from the University of California Riverside, with colleagues from Philipps-Universit?t Marburg, used carbenes - compounds in which a carbon atom has a pair of ’unused’ electrons - to trap the rare diphosphorus molecule P2. The team selectively oxidised the complexes to remove either one electron or two, giving rise to a P2 cation possessing an unpaired electron - a radical cation - or a P22+ dication. 

Phosphorus atoms are usually not happy in pairs - they prefer to combine with more phosphorus atoms to form species such as P4. But in 2008, Gregory Robinson’s group at the University of Georgia in Athens, US, showed how two molecules of N-heterocyclic carbene, NHC, could clamp P2 and stabilise it by donating electrons to it from the electron-rich carbene centre. 

p2 fragments-375

Source: © Nature Chemistry

Molecular orbitals of NHC-stabilised P2 fragments with 0,1 and 2 electrons removed (A-C respectively)

Bertrand’s team has now shown that P2 can be similarly stabilised by a pair of different carbene molecules, cyclic(alkyl)(amino) carbene, or CAAC. Furthermore, both the NHC complex and the CAAC complex can be oxidised. ’If you remove one electron from these complexes, they are left with an overall positive charge, and so become cationic,’ says team member Gernot Frenking, who carried out the theoretical calculations in the study. ’However, an unpaired electron remains so the complex is at the same time a radical.’ 

In addition, a further electron can be removed from the NHC complex, but not from the CAAC complex. Removal of a second electron results in a dication.

’Although the P2 needs electrons from the carbene to stabilise it, when you remove one or even two electrons from the complex it remains stable as a salt,’ says Frenking. ’This was truly unexpected. It was surprising that we could take away one electron and even more surprising that we could take away two and retain stability.’ 

Theoretical calculations showed that the electrons were being removed from the P2 centre of the complex but that the subsequent charge deficiency was being compensated for by an increased charge donation from the carbene, which continues to stabilise the P2

’Carbenes have been shown to do a lot of unusual chemistry and this is another example,’ says Frenking. ’It opens the door to a new field of chemistry, and we can now look for other systems that carbene can stabilise and other donors thatcan stabilise P2 in this way.’ 

Petr Kilian, who researches radicals at St Andrew’s University in the UK, says, ’This is an interesting piece of work which extends the current range of radical-stabilising systems. Experimentally it is a considerable achievement.’ 

Simon Hadlington