Pyrazole-bridged NHC ligands and their dimetallic (allyl)palladium complexes

A set of compartmental pyrazole-based ligands with appended NHC donors has been synthesized and isolated as [H4L]Cl-3 and [H3L](PF6)(2) salts. Dinuclear (allyl)palladium complexes of these ligands are conveniently accessible via the in situ prepared silver species. Three complexes [(allyl)(2)Pd2L]PF6 and one derivative [(methallyl)(2)Pd2L]PF6 have been characterized crystallographically, which revealed that the metal ions are positioned in close proximity [d(Pd center dot center dot center dot Pd) = 3.97-4.05 angstrom], with two possible mutual orientations of the (meth)allyl ligands within the dimetallic pocket. NMR spectroscopy shows slow interconversion of these isomers (k = 0.05-0.4 s(-1)), where the (meth)allyl ligands are detached trans to the carbene during this eta(3)-eta(1)-eta(3) dynamic process. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).