Transition Metal Complexes with Pincer Ligands

JUAN PELLEGRINO

Simposio; Humboldt Colloquium: "Shaping the Future of German-Argentinian Scientific Cooperation ?The Role of Curiosity-Driven Research"; 2018

An interesting approach is the use of redox ?noninnocent? ligands as reservoirs of electrons for bond-making and bond breaking reactions at coordinatively unsaturated metals, with the possibility to store electrons in the ligand, followed by metal-centered reactions In a recent paper, following a previous work inspired on the idea of a possible ?synergistic effect? from the combination of robust bulky pincer ligands with the non-innocence of the NO ligand, we reported the one-electron reduction of the four-coordinate complex [Rh(PCPtBu)(NO)][BF4] (1+). The novel paramagnetic complex [Rh(PCPtBu)(NO)]? (1?) was well characterized in solution and its reactivity with organic halides was studied, resulting in C-X bond activation This bond activation reactivity reflects the ability of the nitrosyl ligand to act as an electron reservoir.With the aim to explore the activation of carbon halogen bonds, the reaction of 1? with different aryl halides was studied. In the case of the reaction of 1? with 2-iodofluorobenzene in benzene, quantitative conversion to diamagnetic Rh(PCPtBu)(NO)I was observed within minutes, while the organic product 2-fluorobiphenyl was detected by 19F NMR and GC-MS (Scheme 1). The conversion of 1? to Rh(PCPtBu)(NO)X (X = Cl or I) as the only rhodium complex was observed with different RX substrates, while a complex with coordination of the aryl moiety (R) was not observed as a stable final product in any case. It is possible that the five-coordinate complexes Rh(PCP)(NO)Aryl are never formed due to the fast reaction of the aryl radicals with the solvent, giving for example, the 2-fluorobiphenyl product, as observed in the reaction of 1?with 2-iodofluorobenzene in benzene. We are currently performing the reaction between halogenated substrates and other paramagnetic complexes trying to find a system that favor a defined organic product (example R2 or cross-coupling product between different RX) under catalytic conditions.We are also exploring the combination of pincer ligands and NO to give complexes that catalyze the hydrogen evolution reaction (HER). Reduction of acids to molecular hydrogen as a means of storing energy is catalyzed by platinum, but its low abundance and high cost are problematic. Thus, a key scientific challenge is the invention of new catalysts for the reductive conversion of water to hydrogen. We hypothesized that the redox-active ligand NO will allow the access to reduced complexes which would be able to reduce protons to hydrogen. On the other hand, the pincer ligand could serve as a second sphere proton relay that facilitates the proton-coupled electron transfer process. In Scheme 2 we proposed a possible catalytic cycle for the HER with a ruthenium pincer complex with NO.