Deboronation-Induced Turn-on Phosphorescent Sensing of Fluorides by Iridium(III) Cyclometalates with o‑Carborane

Heteroleptic tris-cyclometalated Ir­(III) complexes bearing an o-carborane at the 4- or 5-position in the phenyl ring of the ppy ligand (closo-1 and -2) were prepared and characterized. The X-ray crystal structure of closo-1 reveals the fac arrangement of the three C∧N chelates around the Ir atom. Treatment of closo complexes with fluoride anions led to selective deboronation of the closo-carborane cage, producing the corresponding nido-carborane-substituted complexes (nido-1 and -2). Whereas closo-1 and -2 were almost nonemissive in THF, nido-1 and -2 were highly phosphorescent (ΦPL = 0.94–0.95). Theoretical studies suggested that, while the emission quenching in closo-1 can be ascribed to the substantial involvement of o-carborane in the 3MLCT excited state, the intraligand charge transfer (3ILCT) state from the nido-carborane to pyridyl moieties is responsible for the efficient phosphorescence in nido-1. The addition of fluoride to the buffered THF/H2O solution (1/1, v/v, pH 7) of closo-1 and -2 under mild heating led to strong emission intensity, allowing the turn-on phosphorescence detection of fluoride in aqueous medium at the ppb level.