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

Abstract

Two novel sulfonated CNN‐pincer ligands 1b and 1c and the corresponding chloro and aqua complexes K[CNNLPtCl] and CNNLPt(H2O), 3b–3c and 2b–2c, were prepared and fully characterized including single crystal X‐ray diffraction. Along with the previously described complexes 2a and 3a, the derivatives of a CNN pincer ligand 1a, these complexes form a family of structurally similar compounds where the pincer core rigidity increases in the series 2a (3a)<2b (3b)<2c (3c), as deduced from their XRD data. The increased ligand rigidity affects the aqua ligand dissociation energy of the CNNLPt(H2O) complexes, as it follows from DFT calculations and as is reflected in the increased reactivity of the aqua complexes 2a, 2b and 2c in processes that involve aqua ligand loss. Among these processes the formation of the presumed dinuclear complexes CNNL2Pt2 and, importantly, catalytic C−D bond cleavage in C6D6 were studied in 2,2,2‐trifluoroethanol solutions. The C−D bond cleavage reactivity was quantified as the rate of H/D exchange between C6D6 and CF3CH2OH at 80 °C.

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

“…The catalytic H/D exchange between aromatic substrates H n Sub and TFE- d 1 (eq ) used as a deuterium source was carried out at 80 °C using, typically, 0.1–0.5 mol % of complex 1 as a catalyst. , In a typical experiment 0.050 mL of H n Sub (or ∼50 mg for solids) was dissolved in 0.450 mL of wet TFE- d 1 containing 0.150 M D 2 O and 6–12 μmol of catalyst 1 . The water additive was shown earlier to slow down the catalyst deactivation caused by the formation of poorly soluble polynuclear species .…”

“…A catalyst deactivation due to the formation of a light yellow precipitate that was not identified was observed for benzoic acid after about 6 h of reaction. Accordingly, the H/D exchange reactivity of this substrate was only analyzed for the first 6 h. For a few other substrates such as chloro- and bromobenzene formation of a red precipitate, presumably the dinuclear product of the decomposition of catalyst 1 decomposition, , was observed, but only by the end of the monitored reaction period (∼24 h). Oxidative addition of the bromobenzene C–Br bond at a Pt­(II) center may also be involved to some extent.…”

“…The reaction mechanism with benzene as a substrate was studied earlier by means of kinetics, mechanistic tests, the DFT calculations. , It was found that the initial rate of the reaction in Scheme b is first order in both the substrate CH bond concentration, [C–H], and the catalyst 1 concentration, [ 1 ], (eq ): The proposed reaction sequence responsible for the substrate/TFE- d 1 H/D exchange, slightly modified in comparison to the original proposal, is presented in Scheme . It involves an arene coordination step a to form the σ-arene complex CNN 1 ArX with a CNN-coordinated pincer ligand, an isomerization step b to form an isomer CNO 1 ArX with a CNO-coordinated pincer ligand, a rate-limiting CH bond oxidative addition to a Pt­(II) center at step c , via the transition state TS Ar‑X , and a redox isomerization of the resulting Pt­(IV) hydrido complex 1 (Ar)(X) at step d via the lower-energy transition state TS 3 (Scheme ) to form the Pt­(II) aryl 1-X Ar and a facile H/D exchange of the latter with TFE- d 1 or D 2 O.…”

“…On the practical side, various homogeneous transition-metal complexes are now used as catalysts in H/D exchange reactions . Recently, we have reported the preparation of Pt­(II) complexes 1 – 3 supported by our novel CNN-pincer ligands (Chart ) that appeared to be some of the most active platinum-based homogeneous catalysts for H/D exchange between C­(sp 2 )–H bonds of arenes (H n Sub) and 2,2,2-trifluoroethanol- d 1 (TFE) (eq ), , in comparison to other reported Pt-based systems: , For instance, reaction with 10 vol %benzene as a substrate and complex 1 as a catalyst can be observed at 20 °C; the catalyst’s turnover frequency reaches 46 h –1 at 80 °C . 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 ).…”

“…Hence, a better understanding of the reactivity of complex 1 in reaction with respect to various arene C­(sp 2 )–H bonds, as well as the effect of arene substitution on such reactivity, may be important for future development of Pt-mediated CH functionalization chemistry. When complexes 2 and 3 are considered as the catalysts of the H/D exchange reaction (eq ), it is worth mentioning that they are more reactive than 1 but they also lose their activity much more rapidly at 80 °C (complex 2 ) or already above 20 °C (complex 3 ) due to the formation of poorly soluble polynuclear platinum species. Hence, in this work we focus on the H/D exchange reaction () catalyzed by complex 1 .…”