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.