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 d 9 metalloradicals of the form [M(PR 3 ) 2 ] + (M = Pd, Pt; R = t Bu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BAr F 4 ] - (Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd 3 > P t Bu 3 , especially in the case of [Pt(P t Bu 3 ) 2 ] + , which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(P t Bu 2 CMe 2 CH 2 )(P t Bu 3 )] + and [Pt(P t Bu 3 ) 2 H] + upon dissolution at room temperature. Cyclometalation of [Pt(P t Bu 3 ) 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(P t Bu 2 CMe 2 CH 2 )H(P t Bu 3 )] + , 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 t Bu 3 ) 2 ] + ( t 1/2 = 1.2 h) than [Pt(PAd 3 ) 2 ] + ( t 1/2 ∼ 40 days).