13-May-2009 - Royal Society of Chemistry (RSC)

Full steam ahead for paddlewheels

Non-interconvertible, conformational isomers in a paddlewheel dimetallic framework have been synthesised by scientists in the US and China.

Conformational isomers are molecules with the same atomic constitution and configurations, but they differ by their arrangement in space. The atoms in these isomers, usually interconvert through rapid rotation about their bonds. However, when this rotation is restricted by low temperatures the individual isomers can be isolated.

Mike Doyle and co-workers at the University of Maryland, and collaborators at Shanghai Jiao Tong University have reported an unprecedented and unexpected barrier between the conformational barriers of isomers. Significantly, two conformational isomers of bisphenyldirhodium(III) carboxamidates, a ‘biplanar’ structure and ‘propeller’ structure, have been found to be non-interconvertible in the solid state even at high temperatures.

This discovery occurred during the preparation and characterisation of the chemically stable paddlewheel dirhodium(III) compounds which have been reported previously.1 However a persistent minor second product was also observed which led Doyle and his team to investigate the system in more detail using crystallographic and theoretical methods.

It was found that on the bisphenyldirhodium(III) core structure the seven-membered ring caprolactamate represents a transition structure between the two conformational isomers, Where the lower homolog exists in the biplanar conformation and the next higher homolog exists in the propeller conformation.

‘Computational analysis suggests that the energy difference between the two conformers is very low and that the barrier between them should allow rapid interconversion at room temperature,’ says Doyle.

Studies are underway to further investigate these systems and uncover valuable information regarding the reaction mechanisms.

References 1) J. M. Nichols. J. Wolf, Peter Zavalij, B. Varughese, M. P Doyle, J. Am. Chem. Soc., 2007, 129, 3504

Original article: Doyle et. al.; "Conformational isomers of extraordinary stability: carboxamidate-bridged dimetalloorganic compounds"; Chem. Commun. 2009

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