Synthesis of N,O,N bis-pyrazole ligands through Ullmann Coupling

Hemilabile pincer-type tridentate ligands have been widely explored in homogeneous catalysis, with special attention devoted to PCP, PNP and POP ligands. For the latter group, xanthene and p-tolyl ether (PTE) can be highlighted as scaffolds of choice for rigid or more flexible derivatives, respectively.

However, despite of the undeniable high performance of phosphine ligands - especially in cross-coupling reactions – a lot of effort has been put in the search of equally efficient nitrogen based surrogates, which usually display advantages such as higher stability and lower cost and toxicity.

The pyrazole nucleous has emerged as a useful coordination unit in nitrogen-based ligands. However, to the best of our knowledge, NON-pirazolyl ligands based on PTE or xanthene scaffolds were not yet explored.

In this context, we synthesized xanthene and p-tolylether bis-pyrazole ligands through Ullmann reactions, which lead to the desired products in moderate yields. Application studies on Suzuki couplings are ongoing.