Reaction of PdCl2 with Diphenylacetylene Revisited: Effect of Solvents and Ball Milling on Cyclization and Crystal Structures of Dimeric Cyclobutenylpalladium Chloride Complexes

In an attempt to establish the structures of the product formed in the reaction of PdCl2 and diphenylacetylene in polar organic solvents, the reactions were carried out in alcohols, acetonitrile and tetrahydrofuran at room temperature. The reactions in protic solvents such as primary and secondary alcohols resulted in the cyclic dimerization of diphenylacetylene to yield the corresponding dimeric endo-4-alkoxy-pi-1,2,3,4-tetraphenylcyclobutenypalladium(II) chloride complexes 1-4 in good yields. In these complexes the tetraphenylcyclobutenyl moieties are coordinated to palladium atoms in eta(3)-mode and are bridged by two chlorides as confirmed by the single crystal X-ray analysis of complexes 1, 2 and 4. In all these complexes, the coordination of cyclobutenyl moieties to palladium atoms are similar but a marked difference was observed for the Pd2Cl2 core which is planar in complex 1 but puckered in complexes 2 and 4 resulting in shorter PdPd distances. The same reaction under solvent free ball milling condition yielded hexaphenylbenzene by the cyclic trimerization while that in the presence of ethanol and n-butanol produced complexes 1 and 3, respectively, as major and hexaphenylbenzene as minor product. An efficient catalytic synthesis of hexaphenylbenzene using PdCl2 was developed using ball milling condition which has several advantages over the existing catalytic systems.