Orthopalladated imidazolones and thiazolones: synthesis, photophysical properties and photochemical reactivity

Abstract

The reaction of Pd(OAc)₂ with (Z)-5-arylidene-4-(4H)-imidazolones (2a–e) and (Z)-4-arylidene-5(4H)-thiazolones (3a–e) in trifluoroacetic acid results in the corresponding orthopalladated dinuclear complexes (4a–e, imidazolones; 11a–d, thiazolones) with trifluoroacetate bridges through regioselective C–H activation at the ortho position of the 4-arylidene group. Compound 4e, which contains an imidazolone substituted at 2- and 4-positions of the arylidene ring with methoxide groups and exhibits strong push–pull charge transfer, is an excellent precursor for the synthesis of fluorescent complexes with green yellowish emission and remarkable quantum yields. Breaking the bridging system with pyridine yields the mononuclear complex 5e (Φ_F = 5%), while metathesis of trifluoroacetate ligands with chloride leads to the dinuclear complex 6e, also a precursor of fluorescent complexes by breaking the chloride bridging system with pyridine (7e, Φ_F = 7%), or by substitution of chloride ligands with pyridine (8e, Φ_F = 15%) or acetylacetonate (9e, Φ_F = 2%). In addition to notable photophysical properties, dinuclear complexes 4 and 11 also exhibit significant photochemical reactivity. Thus, irradiation of orthopalladates 4a–c and 11a–c in CH₂Cl₂ with blue light (465 nm) proceeds via [2 + 2] photocycloaddition of the CC double bonds of imidazolone and thiazolone ligands, yielding the corresponding cyclobutane-bridging diaminotruxillic derivatives 10a–c and 12a–c, respectively.