Gold(III) Photooxidants. Photophysical, Photochemical Properties, and Crystal Structure of a Luminescent Cyclometalated Gold(III) Complex of 2,9-Diphenyl-1,10-Phenanthroline

A luminescent cyclometalated gold(III) complex [Au(C∧N∧N-dpp)Cl](X) (1a, X = C7H7SO3; 1b, X = ClO4; where C∧N∧N = C-anion of 2,9-diphenyl-1,10-phenanthroline) was prepared and characterized by X-ray crystal analysis. Crystal data for [Au(C24H15N2)Cl](C7H 7SO3)·1/2CH3OH: mol wt 950.97, triclinic, space group P1 (No. 2), a = 9.690(3) Å, b = 10.799(2) Å, c = 15.934(3) Å, α = 74.20(1)°, β = 85.59(2)°, γ = 68.72(2)°, V = 1494.6 Å3, Z = 2. In the solid state, two [Au(C∧N∧N-dpp)Cl]+ cations stack on each other with an intermolecular Au⋯Au separation of 3.6 Å. Complexes 1a and 1b are emissive in fluid solutions with quantum yields of ∼10-4 and lifetimes of 0.4-0.7 μs. The emission spectra of 1a and 1b are quite insensitive to solvents (521-670 nm) and display vibrational progression of ∼1400 cm-1. The small spectral red-shift (1400 cm-1) of the emission of 1a from that of free ligand (dpp) suggests a metal-perturbed intraligand emissive state. The estimated excited state reduction potential E°*(AuIII/II) of 2.2 V (vs NHE) suggests that [Au(C∧N∧N-dpp)Cl]+is a strong photo-oxidant. This photooxidizing property has been demonstrated with the formation of the 1,4-dimethoxybenzene cation radical (DMB•+) upon UV-vis irradiation of an acetonitrile solution of [Au(C∧N∧N-dpp)Cl]+ and DMB and visible light (λ = 406 nm) induced reduction of the gold(III) complex by tetrahydrofuran.