C–H and O₂ Activation at a Pt(II) Center Enabled by a Novel Sulfonated CNN Pincer Ligand

Abstract

A novel sulfonated CNN pincer ligand has been designed to support CH and O2 activation at a Pt­(II) center. The derived cycloplatinated aqua complex 7 was found to be one of the most active reported homogeneous Pt catalysts for H/D exchange between studied arenes (benzene, benzene-d 6, toluene-d 8, p-xylene, and mesitylene) and 2,2,2-trifluoroethanol (TFE) or 2,2,2-trifluoroethanol-d; the TON for C6D6 as a substrate is >250 after 48 h at 80 °C. The reaction is very selective; no benzylic CH bond activation was observed. The per-CH-bond reactivity diminishes in the series benzene (19) > toluene (p-CH:m-CH:o-CH = 1:0.9:0.2) > xylene (2.9) > mesitylene (1.1). The complex 7 reacts slowly in TFE solutions under ambient light but not in the dark with O2 to selectively produce a Pt­(IV) trifluoroethoxo derivative. The H/D exchange reaction kinetics and results of the DFT study suggest that complex 7, and not its TFE derivatives, is the major species responsible for the arene CH bond activation. The reaction deuterium kinetic isotope effect, k H/k D = 1.7, the reaction selectivity, and reaction kinetics modeling suggest that the CH bond cleavage step is rate-determining.

Novel Sulfonated CNN Pincer Ligands for Facile C−H Activation at a Pt(II) Center

“…The pincer ligand coordination mode in the major LPt(H 2 O) species present in TFE solutions of 2b was not easy to elucidate experimentally, unlike 2a which was previously shown to have a meridional CNO ( CNO L) coordination in solution . Therefore, the configuration of 2b was probed using DFT‐calculations.…”

“…The new sulfonated CNN pincer ligands 1b and 1c have been prepared in a metal‐free zwitterionic N‐protonated form, H( 1b ) and H( 1c ), respectively. A modification of the previously developed procedure was used . Reaction of H( 1b ) or H( 1c ) with K 2 PtCl 4 in aqueous acetic acid was used to prepare NMR‐pure samples of the chloro complexes 3b and 3c , respectively, in 70–77 % isolated yield.…”

“…Comparing the X‐ray structure of 3b with that of its phenylpyridine analog, 3a , it is worth noting that the metal coordination unit in 3a is more distorted than in 3b , as it is seen from the greater deviation Δ of the pyridine nitrogen atom from the metal coordination plane defined by the Pt and two Pt‐bonded atoms, C and N, Δ=0.47 Å for 3a and 0.42–0.44 Å for 3b (Figure b).…”

“…The 1 H NMR spectra of 2b dissolved in TFE are more complex, as compared to spectra of its chloro analog 3b . Similar to the parent aqua complex 2a , there are two types of “LPt” species in TFE solutions of 2b . One of the components produces eleven sharp signals in the aromatic region corresponding to eleven aromatic protons.…”

“…While efficient at O 2 activation, the (dpms)Pt(II) scaffold demonstrated a substantial inhibition of hydrocarbon C−H activation in protic media which could, nevertheless, be observed for olefins bearing allylic C−H bonds . To solve the problem of low reactivity of Pt(II) complexes derived from sulfonated dipyridine ligands toward C−H bond donors, we have introduced a new type of facially chelating ligand, sulfonated CNN‐pincers such as 1a (Chart 1) . The new ligand 1a was shown to support C−H activation in aromatic hydrocarbons such as benzene and (poly)methylbenzenes, while retaining the ability to enable O 2 activation at the coordinated Pt(II) center.…”