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 [H 4 L]Cl 3 and [H 3 L](PF 6 ) 2 salts. Dinuclear (allyl)palladium complexes of these ligands are conveniently accessible via the in situ prepared silver species. Three complexes [(allyl) 2 Pd 2 L]PF 6 and one derivative [(methallyl) 2 Pd 2 L]PF 6 have been characterized crystallographically, which revealed that the metal ions are positioned in close proximity [ d (Pd ··· Pd) = 3.97–4.05 Å], 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 η 3 ‐η 1 ‐η 3 dynamic process.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)