Dinuclear and tetranuclear palladium(II) complexes of a thiolato-functionalized, benzannulated N-heterocyclic carbene ligand and their activities toward suzuki-miyaura coupling

Abstract

The thiolato-bridged dimeric Pd(II) benzimidazolin-2-ylidene complex 1, with a [Pd2S2] core, was conveniently prepared by the reaction of the thiol-functionalized benzimidazolium salt C and Pd(OAc) 2. More straightforwardly, 1 can also be synthesized by direct treatment of the thioester-functionalized benzimidazolium salt B with Pd(OAc)2 in wet DMSO under in situ hydrolysis of the thioester function. A subsequent salt metathesis reaction of 1 with AgO 2CCF3 afforded the mixed dicarboxylato/NHC analogue 2 in quantitative yield, leaving the sulfur bridges of the [Pd2S 2] core unaffected despite the use of the soft Ag(I) ions. Treatment of 1 with Me3OBF4 resulted in an unexpected bromido abstraction of 1 leading to an unusual rearrangement/dimerization reaction to give the tetranuclear NHC complex 3, which features a [Pd4S 4] macrocylic square with sulfur corners. These reactions demonstrate the structural diversity of the thiolato-functionalized N-heterocyclic carbene complexes and may offer access to metallo-NHC-based supramolecular architectures. A comparative catalytic study revealed the superiority of NHC/thiolato complex 2 over complexes 1 and 3 in aqueous Suzuki-Miyaura couplings at very low catalyst loading. © 2010 American Chemical Society.