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 CH3CN containing a strong protonic acid, the complex undergoes clean, irreversible substitution of an acac- ligand by solvent to yield Pt(acac)(CH3CN)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 CH3CN, 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 CH2Cl2 and CH3OH 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.

Recent advances on ferrocene-based photoinitiating systems

“…[201][202][203] Typically, these strategies are based on the photooxidation of a metal complex when opposed to the appropriate electron-deficient partner. Using this strategy, a wide range of structures have been examined, including inorganic complexes (trans-Cr(NH3)2(NCS)4 -1 , Pt(acac)2 (with acac standing for acetylacetonate)), [197,204] organometallic complexes (Cr(CO)5L and W(CO)5L with L standing for pyridine and related derivatives), [196] porphyrins, [205,206] pyridinium [199] and phosphonium [200] salts, violet leuconitrile and malachite green leucohydroxide. [207] Even if the mechanism of generation of active initiating species differs with the different aforementioned photoinitiators, as common point, an anion is released during photoexcitation so that the chain growth can rapidly propagate by addition of repeating monomer units, according to the mechanism proposed in the Scheme 1.…”

Evaluation of Heteroleptic Pt (II) β-Diketonate Complexes as Precatalysts for the Photoactivated Curing of Silicone Resins

“…Whereas, as it is sketched in the Introduction, the mechanism of the hydrosilylation reaction involves platinum (0) as a catalytically competent species and is well established, passing through oxidative addition of Si-H, olefin insertion and reductive elimination (Chalk-Harrod mechanism [27]), the photoactivation mechanism of β-diketonate platinum (II) precatalysts as those shown herein is yet to be determined with precision. Studies by Lewis et al, [18], as well as by Lavallee et al, [19] point towards two possible mechanisms, namely: direct photodecomposition to platinum(0) or β-diketonate photoisomerization (from O,O -dicoordinated to O-or C-monocoordinated) or photodissociation, both liberating coordination sites on the metal center and facilitating substitution reactions [28], which, depending on the reaction conditions and on the nature of the entering ligands, may ultimately lead to chemical reduction to platinum (0). The prevalence of either mechanism is considered to depend on the reaction conditions, hence further studies are needed in order to ascertain which is the most probable mechanism under our curing conditions.…”

“…In particular, we started by exploiting a route for the preparation of heteroleptic platinum (II) acetylacetonate complexes with DMSO (DMSO = dimethylsulfoxide), which was described by De Pascali et al, (Scheme 1) [20]. Several classes of compounds have been investigated in this respect, and two of them clearly emerge in this application, namely cyclopentadienyl trialkylplatinum (IV) compounds [16,17] or bis-β-diketonate platinum (II) complexes [8,9,18,19]. The latter class of compounds appears particularly interesting, since it can be potentially much more extensively optimized by changing the nature of the substituents.…”

“…The latter class of compounds appears particularly interesting, since it can be potentially much more extensively optimized by changing the nature of the substituents. In this connection, we have lately embarked on a systematic study aimed at evaluating the reactivity of platinum (II) compounds of general formula (O,O′-β-diketonate)PtLX, since it has been proposed that the photoactivation process of bis-β-diketonate platinum (II) complexes goes through the removal of a β-diketonate ligand or at least through its isomerization from O,O′-chelating bidentate to C-monodentate [8,18,19]. The results of this study will be presented herein.…”

“…Incidentally, complex 4 can also be prepared upon nucleophilic attack by PPh 3 on Pt(acac) 2 [22]. Several classes of compounds have been investigated in this respect, and two of them clearly emerge in this application, namely cyclopentadienyl trialkylplatinum (IV) compounds [16,17] or bis-β-diketonate platinum (II) complexes [8,9,18,19]. The latter class of compounds appears particularly interesting, since it can be potentially much more extensively optimized by changing the nature of the substituents.…”