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

Abstract

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 = <i>t</i>Bu, 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 > P<i>t</i>Bu₃, especially in the case of [Pt(P<i>t</i>Bu₃)₂]⁺, which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(P<i>t</i>Bu₂CMe₂CH₂)(P<i>t</i>Bu₃)]⁺ and [Pt(P<i>t</i>Bu₃)₂H]⁺ upon dissolution at room temperature. Cyclometalation of [Pt(P<i>t</i>Bu₃)₂]⁺ can also be induced by reaction with the 2,4,6-tri-<i>tert-</i>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(P<i>t</i>Bu₂CMe₂CH₂)H(P<i>t</i>Bu₃)]⁺, 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(P<i>t</i>Bu₃)₂]⁺ (<i>t</i>_1/2 = 1.2 h) than [Pt(PAd₃)₂]⁺ (<i>t</i>_1/2 ∼ 40 days).