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.