Photolytic Activation of Late-Transition-Metal–Carbon Bonds and Their Reactivity toward Oxygen

Abstract

The photolytic activation of palladium(II) and platinum(II) complexes [M(BPI)(R)] (R = alkyl, aryl) featuring the 1,3-bis(2-pyridylimino)isoindole (BPI) ligand has been investigated in various solvents. In the absence of oxygen, the formation of chloro complexes [M(BPI)Cl] is observed in chlorinated solvents, most likely due to the photolytic degradation of the solvent and formation of HCl. The reactivity of the complexes toward oxygen has been studied both experimentally and computationally. Excitation by UV irradiation (365 nm) of the metal complexes [Pt(BPI)Me] and [Pd(BPI)Me] leads to distortion of the square-planar coordination geometry in the excited triplet state and a change in the electronic structure of the complexes that allows the interaction with oxygen. TD-DFT computational studies suggest that, in the case of palladium, the Pd(III) superoxide intermediate [Pd(BPI)(κ 1 -O 2 )Me] is formed and, in the case of platinum, the Pt(IV) peroxide intermediate [Pt(BPI)(κ 2 -O 2 )Me]. For alkyl complexes where metal−carbon bonds are sufficiently weak, the photoactivation leads to the insertion of oxygen into the metal−carbon bond to generate alkylperoxo complexes: for example [Pd(BPI)OOMe], which has been isolated and structurally characterized. For stronger M−C(aryl) bonds, the reaction of [Pt(BPI)Ph] with O 2 and light results in a Pt(IV) complex, tentatively assigned as the peroxo complex [Pt(BPI)(κ 2 -O 2 )Ph], which in chlorinated solvents reacts further to give [Pt(BPI)Cl 2 Ph], which has been isolated and characterized by scXRD. In addition to the facilitation of oxygen insertion reactions, UV irradiation can also affect the reactivity of other components in the reaction mixture, such as the solvent or other reaction products, which can result in further reactions. Labeling studies using [Pt(BPI)(CD 3 )] in chloroform have shown that photolytic reactions with oxygen involve degradation of the solvent.

Monitoring Light‐Driven Oxygen Insertion Reactions into Metal Carbon Bonds by LED‐NMR Spectroscopy

“…The formation of [Pt(BPI)OOMe] reaches a maximum after approximately 12 minutes under these conditions due to further reactions, either to a mixture of the hydroxo complex [Pt(BPI)OH] and formaldehyde, or to the chloro complex [Pt(BPI)Cl] and methyl hydroperoxide. The formation of the chloro complex is attributed to the formation of DCl (generated from CDCl 3 during irradiation), as shown in our previous report [6d] . The hydroxo complex [Pt(BPI)OH] converts over time in CDCl 3 to [Pt(BPI)Cl], presumable also due to the in situ generated DCl.…”

“…Spectra were recorded at 1 minute time intervals and the reaction progress was determined by integration of the signals for the individual components in the reaction mixture (see Figure 2 and Figure S8 for the individual spectra). Assignments of the individual reaction components have been reported previously [6d] …”

“…In the case of [M(BPI)Me] complexes, the light‐induced oxygen insertion reactions lead initially to the formation of methylperoxo complexes [M(BPI)OOMe], which undergo further reactions to a range of products (Figure 1). [6d] Here we report our results using LED‐NMR spectroscopy to study these light‐driven oxygen insertion reactions of Pd and Pt methyl complexes [Pt(BPI)Me] and [Pd(BPI)Me] in CDCl 3 in real time.…”

Synthesis and Characterization of Bis(pyridylimino)isoindolide Alkali Metal Complexes in Three Redox States

“…[28] Alkali metal complexes of BPI were synthesized before in its redox-neutral state and one lithium complex [29] was even crystallographically characterized. [30][31][32][33][34] But despite some hints of the possible reductions of BPI ligands [18,19] information on the accessibility and coordination chemistry of the reduced BPI species is, to the best of our knowledge, so far unprecedented. The lack of fundamental knowledge of these useful species prompted us to carry out a detailed study on the isolation and complete characterization of BPI-derived alkali metal complexes featuring mono-, di-and trianionic ligand entities, which benchmarks the isolated reduced redox levels of this ligand scaffold (Figure 1).…”

Metal–Ligand–Anion Cooperation in C–H Bond Formation at Platinum(II)

“…Using the mechanistic understanding of this metal–ligand–anion cooperative (MLAC) reaction, a change in the counteranion allowed for benzene C–H activation. These studies of C–H coupling/activation with (BPI)Pt complexes are of particular interest, as (BPI)­Pt­(CH 3 ) complexes can be aerobically functionalized. , …”