Modifying Phosphorus(III) Substituents to Activate Remote Ligand-Centered Reactivity in Triaminoborane Ligands

Triaminoborane-bridged diphosphorus ligands called TBDPhos can undergo cooperative ligand-centered reactions at the 1,8,10,9-triazaboradecalin (TBD) backbone while bound to metals, but the factors that control this reactivity are not well understood. Previous studies showed that TBDPhos reactivity is highly sensitive to phosphorus substituents, ancillary ligands, and metal coordination environment despite these changes being significantly removed from the reactive N–B bond. Here we describe how exchanging phenyl and ethyl substituents attached to phosphorus with methoxy turns on ligand-centered reactions in otherwise unreactive (TBDPhos)­Pt­(S₂C₆H₄) and (TBDPhos)­Mo­(CO)₄ complexes. The synthesis and characterization of the complexes are described alongside comparative reactivity studies using MeOH as a simple but effective test substrate. Complementary density functional theory (DFT) calculations on an expanded series of (TBDPhos)­Pt­(S₂C₆H₄) complexes corroborate the experimental results and show that both size and electronic properties of the phosphorus substituents can significantly influence the Gibbs free energy of reaction at the remote ligand site. These results demonstrate how seemingly ancillary ligand modifications can levy significant control over ligand-centered reactivity in TBDPhos complexes, which may be relevant when considering the design of other chemically reactive ligands with Lewis acidic functional groups.