Martin's Spirophosphorane: A Platform for the Design of [OPO] Pincer Ligands and Their Group 4 Complexes

Martin's well-known spirophosphorane (1) has been transformed into a series of [OPO] pincer-type ligand motifs via three distinct pathways. In the first pathway, upon deprotonation of 1, treatment with suitably chosen lithium alkyl, aryl, or amido reagents induced a double ring expansion, affording an open-chain λ³-phosphane. The second pathway proceeded through reductive cleavage of the bicyclic scaffold, yielding a dimeric, formally trianionic [OPO] framework that, upon controlled monoprotonation, furnished a dianionic [OPO] dilithium salt incorporating a P-H unit. Complexation of this ligand via salt metathesis with Ti(NMe₂)₂Cl₂ induced one dimethylamino substituent at the titanium center to act as an intramolecular Brønsted base, effecting deprotonation of the P-H group and thereby generating a titanium complex featuring a formally trianionic [OPO] pincer ligand. The third pathway demonstrated that spirophosphorane 1 could act directly as a ligand precursor. Reaction with homoleptic group 4 alkyl or amido complexes triggered-upon deprotonation-an intramolecular migration of an alkyl or amido moiety to the phosphorus, initiating a double ring expansion and giving the corresponding [OPO]-coordinated complexes. The mechanistic details of these transformations were elucidated by DFT analysis for the reaction of ZrBn₄ with 1.