Consecutive C–H and O₂ Activation at a Pt(II) Center To Produce Pt(IV) Aryls

Abstract

Aerobic C−H functionalization of benzene and a series of electron-rich and electron-poor arenes has been demonstrated using a Pt(II) aqua complex supported by a sulfonated CNN-chelating pincer ligand in wet 2,2,2-trifluoroethanol (TFE) solutions at 20 °C and ambient pressure of air or O 2 . The reaction results in Pt(IV) aryl hydroxo complexes and a Pt(IV) trifluoroethoxo complex as two major products, in a 1:1 molar ratio, along with minor Pt(II) products of the arene and pincer ligand oxidative C−C coupling.

CH3–X Reductive Elimination Reactivity of PtIVMe Complexes Supported by a Sulfonated CNN Pincer Ligand (X = OH, CF3CO2, PhNMe2+)

“…The latter typically requires that the solvent can only weakly coordinate to a Pt­(II) center . Some solvents known to be compatible with C–H activation at a Pt­(II) center are TFE − and aqueous acetic and trifluoroacetic (TFA) acids . In terms of the solubility of 6 – 8 , it is marginal in both TFE and 2/1 acetic acid/water mixtures.…”

“…3,4 Importantly, at least in the case of 1, the derived [LPt II Ph] − species were shown to react cleanly with O 2 to form a derived Pt(IV) aryl complex, LPt IV Ph(OH). Accordingly, a consecutive arene CH and O 2 activation at a Pt(II) center leading to the isolable Pt(IV) aryl hydroxo complexes LPt IV Ar(OH) (4) 5 was demonstrated using complex 1 and a series of arenes (Scheme 2).…”

“…Although conceptually important, the system in Scheme 2 has a number of limitations. (a) The TFE derivative of 1, LPt II (ROH) (R = CH 2 CF 3 ), which is always present in the reaction mixtures, is oxidized concurrently to produce LPt IV (OR)(OH) (5), along with 4 (Scheme 2). Complexes 4 and 5 always form in a 1:1 ratio, unless sacrificial reducing agents such as p-hydroquinone are present that favor selective which makes them more electron rich/less electrophilic and, expectedly, less reactive than 9.…”

“…Previously, while pursuing the development of Pt-based systems for aerobic oxidative CH functionalization (eq ), we have designed and prepared the novel Pt­(II) aqua complexes 1 – 3 supported by sulfonated CNN pincer ligands (Chart ) that are ranked as some of the most active catalysts for H/D exchange of various arenes ArH in wet TFE- d (TFE = 2,2,2-trifluoroethanol) solutions. , Importantly, at least in the case of 1 , the derived [LPt II Ph] − species were shown to react cleanly with O 2 to form a derived Pt­(IV) aryl complex, LPt IV Ph­(OH). Accordingly, a consecutive arene CH and O 2 activation at a Pt­(II) center leading to the isolable Pt­(IV) aryl hydroxo complexes LPt IV Ar­(OH) ( 4 ) was demonstrated using complex 1 and a series of arenes (Scheme ).…”

Catalytic Deuteration of C(sp2)–H Bonds of Substituted (Hetero)arenes in a Pt(II) CNN-Pincer Complex/2,2,2-Trifluoroethanol-d1 System: Effect of Substituents on the Reaction Rate and Selectivity

“…Our subsequent study of reactivity of complex 1 led us to the discovery of Pt-mediated stoichiometric CH oxidation with O 2 of various arenes ArH and complex 1 (eq ). The reaction produces arene-derived Pt­(IV) aryl hydroxo complexes LPt IV Ar­(OH) and can involve both electron-rich o - and m -dialkylbenzenes ArH (alkyl = Me, t -Bu), as well as electron-poor o -dichloro- and o -difluorobenzenes. Interestingly, except for o -F 2 C 6 H 4 , these oxidation reactions result in >90% selective formation of a single isomer of the derived LPt IV Ar­(OH) compounds …”