A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation

Pre-coordination to a transition metal by the terminal donor groups of a tri-dentate ligand is a common strategy to stabilise elusive groups, to achieve unprecedented bond activation and to develop novel modes of metal-ligand-cooperation for catalysis. In the current manuscript, we demonstrate that the oxidative addition of a central E-H-bond after pre-coordination to the metal centre is disfavoured for metals with d(10) electron configuration. For exemplary pincer ligands and metals with d(10) electron configuration, quantum chemical calculations suggest a second barrier, which is associated with the rearrangement of the saw-horse structure, obtained after oxidative addition, to the expected square planar geometry for the resulting d(8) electron configuration. In the case of PBP-type ligands with a central L2BH2-group (L = R3P) the reaction with Pt-0 precursors proceeds via an alternative pathway of activation, which involves the backside attack of a nucleophile to the boron atom, which facilitates the nucleophilic attack of the Pt-0 centre and formation of a boryl complex (LBH2). As the corresponding reaction with a Pt-II precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a L2BH donor, our results show that ligand-stabilized borylenes (L2BH) can in principle be converted to boryls (LBH2) via boronium salts (L2BH2+).

Automated summary

Pre-coordination with transition metals using tri-dentate ligands is a popular method for stabilizing elusive groups, achieving bond activation, and developing metal-ligand-cooperation for catalysis. This study demonstrates that the oxidative addition of a central E-H bond is unfavorable for metals with d(10) electron configuration. Quantum chemical calculations reveal a second barrier for pincer ligands and d(10) metals, which involves rearrangement from a saw-horse structure to a square planar geometry. The reaction with Pt-0 precursors using PBP-type ligands proceeds via an alternative pathway involving nucleophilic attack and the formation of boryl complexes.