Efficient Substitutional Photochemistry of a Third-Row Transition Metal β-Diketonate Complex

Abstract

Ultraviolet irradiation of Pt(acac) 2 (acac - is the anion of acetylacetone) in nonaqueous solvents results in competing photochemical processes whose relative importance depends upon the solution environment. In CH 3 CN containing a strong protonic acid, the complex undergoes clean, irreversible substitution of an acac - ligand by solvent to yield Pt(acac)(CH 3 CN) 2 + and free H(acac). The quantum yield for this process is high (∼0.3) and independent of the irradiation wavelength, presence of dissolved oxygen, incident light intensity, acid concentration ((1−50) × 10 - 3 M), and temperature. The primary photochemical step has been identified as heterolytic cleavage of a Pt−O bond to yield an intermediate, I 1, formulated as a Pt(II) complex containing a monodentate O-bonded acac - ligand and a coordinated solvent molecule. Protonation of this monodentate ligand labilizes the remaining bond to the metal and facilitates loss of H(acac). In nonacidified CH 3 CN, I 1 undergoes rechelation of the dangling acac - ligand to regenerate Pt(acac) 2 in competition with secondary photolysis to form a complicated mixture of products. Photolysis in CH 2 Cl 2 and CH 3 OH results in inefficient redox decomposition of Pt(acac) 2 to yield Pt metal and free H(acac). The role of solvent in determining the relative importance of photosubstitution vs photoredox decomposition is discussed. In addition, the mechanism by which Pt(acac) 2 functions as a photoinitiator for the anionic polymerization of a 2-cyanoacrylate is reconsidered in light of the present study.