Stepwise formation of iridium(III) complexes with monocyclometalating and dicyclometalating phosphorus chelates

With the motivation of assembling cyclometalated complexes without nitrogen-containing heterocycle, we report here the design and systematic synthesis of a class of Ir(III) metal complexes functionalized with facially coordinated phosphite (or phosphonite) dicyclometalate tripod, together with a variety of phosphine, chelating diphosphine, or even monocyclometalate phosphite ancillaries. Thus, treatment of [IrCl ₃(tht) ₃] with stoichiometric amount of triphenylphosphite (or diphenyl phenylphosphonite), two equiv of PPh ₃, and in presence of NaOAc as cyclometalation promoter, gives formation of respective tripodal dicyclometalating complexes [Ir(tpit)(PPh ₃) ₂Cl] (2a), [Ir(dppit)(PPh ₃) ₂Cl] (2b), and [Ir(dppit)(PMe ₂Ph) ₂Cl] (2c) in high yields, where tpitH ₂ = triphenylphosphite and dppitH ₂ = diphenyl phenylphosphonite. The reaction sequence that afforded these complexes is established. Of particular interest is isolation of an intermediate [Ir(tpitH)(PPh ₃) ₂Cl ₂] (1a) with monocyclometalated phosphite, together with the formation of [Ir(tpit)(tpitH)(PPh ₃)] (3a) with all tripodal, bidentate, and monodentate phosphorus donors coexisting on the coordination sphere, upon treatment of 2a with a second equiv of triphenylphosphite. Spectroscopic studies were performed to explore the photophysical properties. For all titled Ir(III) complexes, virtually no emission can be observed in either solution at room temperature or 77 K CH ₂Cl ₂ matrix. Time-dependent DFT calculation indicates that the lowest energy triplet manifold involves substantial amount of metal centered ³MC dd contribution. Due to its repulsive potential energy surface (PES) that touches the PES of ground state, the ³MC dd state executes predominant nonradiative deactivation process.