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 [BArF4]− (ArF = 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 MII–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]+ (t1/2 = 1.2 h) than
[Pt(PAd3)2]+ (t1/2 ∼ 40 days).