Unusual rearrangement of PNP ligand based Ru complexes - ICS84 The 84th Annual Meeting of the Israel Chemical Society

Naziha Tarannam ¹ Shubham Deolka ² Robert R. Fayzullin ³ Eugene Khaskin ² Sebastian Kozuch ¹

¹Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel
²Department of Chemistry, Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan
³Department of Chemistry, Arbuzov Institute of Organic and Physical Chemistry, Kazan, Russia

Pincer complexes are well known for their versatile use in catalytic transformations including dehydrogenation of alkenes and dehydrogenative coupling of alcohols. A Ru based pincer complex, RPNP-Ru (R = ⁱPr, ^tBu) which catalyzes dehydrogenation of alcohol undergoes a very unusual reaction upon deprotonation with KO^tBu and loses its catalytic activity. The complex rearranges by an sp²-sp³ C-C bond cleavage and subsequent C-C bond formation on the picolyl arm with an increment in length of the pincer ligand backbone by a –CH₂unit. Experimentally, it was observed that the ⁱPr substituted complex undergoes this unexpected reaction when the reaction mixture was set to a temperature of 120^oC for 3 days; whereas the ^tBu substituted complex rearranges in about 5 minutes at room temperature. Our DFT studies revealed that the reaction proceeds through several hydrogen-transfer steps along with an intramolecular isomerization process that involves a very unique spyrocyclic cyclopropane transition state which forms by simultaneous aromatization and de-aromatization of the two pyridine rings. The formation of spyrocyclic transition state depends on the ligand-arm pyridine coordination and ligand sterics. The traces of tertbutoxide present on the reaction medium is driving the reaction by significantly lowering the relatively high energy barriers for hydrogen transfer steps.