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 d 9 metalloradicals of the form [M(PR 3 ) 2 ] + (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2difluorobenzene (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 > PtBu 3 , especially in the case of [Pt(PtBu 3 ) 2 ] + , which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PtBu 2 CMe 2 CH 2 )(PtBu 3 )] + and [Pt(PtBu 3 ) 2 H] + upon dissolution at room temperature. Cyclometalation of [Pt(PtBu 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(PtBu 2 CMe 2 CH 2 )H(PtBu 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 9,10dihydroanthracene 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(PtBu 3 ) 2 ] + (t 1/2 = 1.2 h) than [Pt(PAd 3 ) 2 ] + (t 1/2 ∼ 40 days).

Trendbericht Anorganik 2024: Nebengruppen, Bioanorganik und Koordinationschemie

“…Der Radikalcharakter in [Pt(P t Bu 3 ) 2 ] + führt zur C-H-Aktivierung unter einem für schwere d-Blockelemente unüblichen H-Atom-Transfermechanismus. 3) [Pd I (P t Bu 3 ) 2 ] + lässt sich über N-Donorliganden stabilisieren und dann zur C-C-Kreuzkupplung verwenden. Dabei finden sich erste Beweise für einen Pd I /Pd III -Zyklus, der eine Alternative zu den klassischen Pd 0 /Pd IIoder Pd II /Pd IV -Mechanismen darstellt.…”

T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes

“…As part of work in our group investigating the chemistry of palladium(I) and platinum(I) metalloradicals, 8 we have recently prepared the set of well-defined and persistent d 9 -metalloradicals [M(PR 3 ) 2 ][BAr F 4 ] (M = Pd, Pt; R = t Bu, Ad) by one-electron oxidation of the corresponding zerovalent precursors [M(PR 3 ) 2 ] using [Fc][BAr F 4 ] (Fc = FeCp 2 ) in 1,2-difluorobenzene (DFB). 9 Inspired by literature precedents for the addition of NO to second and third row late transition metal metalloradicals, 10 , 11 we set about examining reactions of [M(PR 3 ) 2 ][BAr F 4 ] with NO in DFB ( Figure 1 ). Rapid reactions resulting in quantitative spectroscopic formation of diamagnetic {MNO} 10 nitrosyl derivatives 1 – 4 were observed in all cases upon addition of NO at RT (M = Pd, dark red; Pt, dark green).…”

Reversible Single-Electron-Transfer to Oxygen in a Stable N-Heterocyclic Carbene Palladium(I) Metalloradical

“…A new breakthrough came in 2016, when the Chaplin and Ozerov groups characterized independently the stable palladium­(I) two-coordinate complex [Pd­(P t Bu 3 ) 2 ] + together with its acetonitrile three-coordinate adduct [Pd­(P t Bu 3 ) 2 (NCMe)] + . , More recently, Deng et al and Mirica et al reported tricoordinate amido and tetracoordinate dithiapyridinophane palladium­(I) complexes, respectively. Beyond these synthetic advances, studies on the reactivity of these stable Pd­(I) radicals are still very scarce − despite their importance for a better understanding of palladium-catalyzed radical reactions.…”

“… 6 , 7 More recently, Deng et al 8 and Mirica et al 9 reported tricoordinate amido and tetracoordinate dithiapyridinophane palladium(I) complexes, respectively. Beyond these synthetic advances, studies on the reactivity of these stable Pd(I) radicals are still very scarce 10 − 12 despite their importance for a better understanding of palladium-catalyzed radical reactions.…”