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Revisiting a palladium cluster leads to an intricate surprise

Chemists find that a palladium cyclopropenium complex reported in 1964 has a more complex structure than originally thought

by Stephen K. Ritter
September 18, 2017 | A version of this story appeared in Volume 95, Issue 37

In 1964, a team of Russian chemists reported an experiment in which they treated a palladium salt with triphenylcyclopropenium chloride and ethylene. They proposed a product structure consisting of a Pd3Cl4 chain capped by two cyclopropenyl ligands. When Christian Jandl, Karl Öfele, and Alexander Pöthig of Technical University of Munich recently noticed that some reactions of cyclopropenium ions with transition metals result in ring opening and formation of four-membered metallacycles, they became interested in the actual structure of the 1964 molecule and wanted to know more about cyclopropenium’s reactivity. The Munich team repeated the originally reported synthesis and found out with the benefit of today’s analytical tools that the actual molecule is a more complex Pd6Cl8 cluster capped by triphenylpropyl ligands (Organometallics 2017, DOI: 10.1021/acs.organomet.7b00525). The researchers believe the ring-opened ligand forms in a two-step process involving the partial reduction of Pd(II) to Pd(0) by ethylene followed by oxidative addition of cyclopropenium to palladium. They devised an alternative synthesis of the cluster using Pd(II) and Pd(0) salts with triphenylcyclopropenium chloride to confirm the ring-opening event. Pöthig and coworkers say this molecule represents the first isolated organometallic palladium cluster compound, although that was not recognized in 1964, and one of the first transition-metal clusters in general—the concept of metal clusters wasn’t fully defined until 1966 by MIT’s F. Albert Cotton.


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