A helping hand: The dibenzobarrelene‐based PC sp 3P pincer complex 1 was designed as a bifunctional catalyst for the acceptorless dehydrogenation of alcohols. The mechanism of the H2 release involves interaction between the hydride ligand and the acidic sidearm in the intermediate 2 (red O, yellow Cl, green P, blue Ir). The feasibility of the complete catalytic cycle was studied using a stoichiometric model and the reaction was subsequently realized in a catalytic fashion.
Pincer-type complexes occupy a very important position in chemistry and chemistry-related disciplines. In this article we have selected several spectacular examples of pincer systems and demonstrate that reversible structural changes in the ligand backbone may have a dramatic impact on the reactivity of their metal complexes. This perspective aims to focus on some recent exciting developments in the chemistry of "cooperating" PN(sp 3 )P and PC(sp 3 )P pincer ligands relevant to catalytic applications.
Reversible storage of hydrogen in the form of stable and relatively harmless chemical substances such as formic acid (FA) is one of the cornerstones of a fossil-fuel-free economy. Recently, Ru(III)-PC(sp 3 )P (where PC(sp 3 )P = modular dibenzobarrelene-based pincer ligand possessing a pendant functional group) complex 1 has been reported as a mild and Eselective catalyst in semihydrogenation of alkynes with stoichiometric neat formic acid. Discovery of the additive-free protocol for dehydrogenation of FA launched further studies aiming at the rational design of highly efficient catalysts for this reaction operating under neutral conditions. We now report the results of our investigation on a series of bifunctionl PC(sp 3 )P complexes equipped with different outer-sphere auxiliaries, that allowed us to identify an amine-functionalized Ir(III)-PC(sp 3 )P complex 3, as a clean and efficient catalyst for the FA dehydrogenation. The catalyst is suitable for fuel-cell applications demonstrating a TON up to 5 × 10 5 and TOF up to 2 × 10 4 h −1 (3.8 × 10 5 and 1.2 × 10 4 h −1 with no additives). In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide rationale for the design of improved next-generation catalysts.
Homo‐ and cross‐coupling of alcohols resulting in the formation of the corresponding ketones in very good to excellent yield was realized through the one‐pot sequence of dehydrogenation/aldol condensation/hydrogenation accompanied by the release of molecular hydrogen. The dehydrogenation and hydrogenation steps are catalyzed by the previously reported ruthernium‐ and iridium‐based ligand‐metal cooperating catalysts.magnified image
General Considerations. All manipulations were performed using standard Schlenk techniques under dry N 2 or Ar. All reagents were purchased from the usual suppliers and used without further purification.All reagents were weighed and handled in air. Flash column chromatography was performed with Merck ultra pure silica gel (230-400 mesh). All catalytic reactions were carried out under N 2 . Yields refer to isolated compounds greater than 95% purity as determined proton Nuclear Magnetic Resonance spectroscopy ( 1 H-NMR) analysis. The CAS numbers of the known compound were listed. Spectroscopy data of the known compounds matches with the data reported in the corresponding reference. 1 H-, 13 Cand 31 P-NMR spectra were recorded on a Bruker 400 or 500 MHz instruments with chemical shifts reported in ppm relative to the residual deuterated solvent or the internal standard tetramethylsilane. Diffraction data were collected with a Bruker APEX CCD instrument (MoKα radiation (λ = 0.71073Å)).Crystals were mounted onto glass fibers using epoxy. Single crystal reflection data were collected on a Bruker APEX CCD X-ray diffraction system controlled by a Pentium-based PC running the SMART software package. 1 The integration of data frames and refinement of cell structure were done by the SAINT+ program package. 2 Refinement of the structure 1 on F2 was carried out by the SHELXTL software package. 3
Spectral data
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