Stability and C–H Bond Activation Reactions of Palladium(I) and Platinum(I) Metalloradicals: Carbon-to-Metal H-Atom Transfer and an Organometallic Radical Rebound Mechanism

One-electron oxidation of palladium(0) and platinum(0) bis(phosphine) complexes enables isolation of a homologous series of linear d⁹ metalloradicals of the form [M(PR₃)₂]⁺ (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BAr^F₄]⁻ (Ar^F = 3,5-(CF₃)₂C₆H₃) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd₃ > PtBu₃, especially in the case of [Pt(PtBu₃)₂]⁺, which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PtBu₂CMe₂CH₂)(PtBu₃)]⁺ and [Pt(PtBu₃)₂H]⁺ upon dissolution at room temperature. Cyclometalation of [Pt(PtBu₃)₂]⁺ can also be induced by reaction with the 2,4,6-tri-tert-butylphenoxyl radical in DFB, and a common radical rebound mechanism involving carbon-to-metal H-atom transfer and formation of an intermediate platinum(III) hydride complex, [Pt(PtBu₂CMe₂CH₂)H(PtBu₃)]⁺, has been substantiated by computational analysis. Radical C–H bond oxidative addition is correlated with the resulting M^II–H bond dissociation energy (M = Pt > Pd), and reactions of the metalloradicals with 9,10-dihydroanthracene in DFB at room temperature provide experimental evidence for the proposed C–H bond activation manifold in the case of platinum, although conversion into platinum(II) hydride derivatives is considerably faster for [Pt(PtBu₃)₂]⁺ (t_1/2 = 1.2 h) than [Pt(PAd₃)₂]⁺ (t_1/2 ∼ 40 days).