Catalysis with Carbon-Rich Gold(I) and Palladium(II) 1,2,3-Triazole-based Complexes

Abstract

Homogeneous catalysis, where the catalyst and the substrates are in the same phase, is well established due in part to advancements in organometallic chemistry. Ligand design has played an important role in the development of active catalysts and there is an ongoing industrial and environmental need to design catalysts with high turnover.

N-heterocyclic carbene (NHC)-based complexes are powerful catalysts as the ligands possess strong donor properties and the metal-NHC coordination bonds are kinetically stable. Mesoionic carbenes (MICs), in particular 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes, are a recent addition to this class, and possess stronger donor properties. Moreover, the 1,2,3-triazole precursors are synthesised using copper(I) catalyzed azide-alkyne cycloaddition (CuAAC), and so functionalization of the ligands and therefore design and optimization of the resulting catalysts can be achieved in a very straightforward fashion.

Having previously demonstrated that gold(I) triazolylidene complexes are catalytically active, a systematic study was conducted whereby six complexes varying in electronic and steric properties were synthesised, and tested in three gold(I) catalyzed reactions: the 1,6-enyne rearrangement, the alcohol addition to 3,3-cyclopropenes, and the allylic etherification of unactivated alcohols. Improvements in both the enyne rearrangement and the allylic etherification reactions were made solely due to the ancillary 1,2,3-triazolylidene ligand.

The most active triazolylidene catalyst was the most sterically bulky of the range tested, indicating that stability of the metal centre was important. Therefore, Au(I) complexes bearing larger triazolylidene ligands may result in more active catalysts. With this in mind, six 1,2,3-triazoles bearing the large polyphenylene moieties hexaphenylbenzene (HPB) and hexa-peri¬-hexabenzocoronene (HBC) were synthesised using an optimized CuAAC “click” chemistry approach.

With six novel 1,2,3-triazoles, and taking into account the small number of publications describing the coordination chemistry of HPB and HBC containing ligands, investigations into the palladium(II) coordination chemistry of two of the 1,2,3-triazoles was carried out. The [PdCl2L2] complexes, alongside a phenyl analogue and the commercially available [PdCl2(PPh3)2] were tested in the Suzuki-Miyaura reaction with three different arylhalide substrates. All complexes catalyzed the reaction with the moderately activated 4′-bromoacetophenone substrate, however, less activated substrates 3,5 dimethoxybromobenzene and 4′-chloroacetophenone revealed the HBC and PPh3 catalysts as the most active. Investigations into the donor strength of the ligands reveal that the high activity of the HBC containing complex is unknown, and possible contributions to this activity are discussed.

Overall conclusions to the findings of the research are made, and suggestions for improving the catalysts with preliminary findings are described.