A Proton-Switchable Bifunctional Ruthenium Complex That Catalyzes Nitrile Hydroboration

Abstract

A new bifunctional pincer ligand framework bearing pendent proton-responsive hydroxyl groups was prepared and metalated with Ru(II) and subsequently isolated in four discrete protonation states. Stoichiometric reactions with H2 and HBPin showed facile E-H (E = H or BPin) activation across a Ru(II)-O bond, providing access to unusual Ru-H species with strong interactions with neighboring proton and boron atoms. These complexes were found to promote the catalytic hydroboration of ketones and nitriles under mild conditions, and the activity was highly dependent on the ligand's protonation state. Mechanistic experiments revealed a crucial role of the pendent hydroxyl groups for catalytic activity.

A PNNH Pincer Ligand Allows Access to Monovalent Iron

“…There are several examples of the borylation of complexed ligands as well as the borylation of organic substrates using iron and cobalt pincer catalysts . In a ruthenium‐based complex, the addition of HBpin is redox‐neutral at the metal and occurs on the ligand driven by the formation of a strong B−O bond and the evolution of dihydrogen.…”

“…There are several examples of the borylation of complexed ligands as well as the borylation of organic substrates using iron and cobalt pincer catalysts . In a ruthenium‐based complex, the addition of HBpin is redox‐neutral at the metal and occurs on the ligand driven by the formation of a strong B−O bond and the evolution of dihydrogen. Chirik and co‐workers reported the oxidative addition of HBpin to a Co I complex with the formation of a Co III intermediate, then subsequent borylation of the ligand′s pyridine ring .…”

“…Although the slow rate of the reaction indicates that it is not mechanistically facile, it is nevertheless a relatively clean conversion. Moreover, it generates a product with reduced metal and a Lewis acidic boryl functionality near to carbonyl ligands that has the potential to be utilized in catalysis as a pendant Lewis base in the secondary coordination sphere …”

Ruthenium NNN complexes with a 2‐hydroxypyridylmethylene fragment for transfer hydrogenation of ketones

“…A metal–ligand cooperative complex might contain a ligand with a proton‐sensitive site, because the ligand's structure could be changed by a base. Following this strategy, many transition metal complexes with 2‐hydroxypyridyl fragment have been developed and applied in organic synthesis …”

Transfer Hydrogenation of Unsaturated Substrates by Half-sandwich Ruthenium Catalysts using Ammonium Formate as Reducing Reagent

“…[2,[12][13][14] Recently, there has been a significant progress in developing milder methods for reduction of nitriles and N-heterocycles based on catalytic hydrosilylation [12,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] and hydroboration. [7,[30][31][32][33][34][35][36][37][38] In some cases these techniques allow for unprecedented chemoselectivity [21,25,26,39] and overall practicability, but still their utility is limited by the necessity to use expensive silanes and boranes and due to generation of wasteful by-products. [40][41][42] Hydrogenation of nitriles [43][44][45][46][47][48][49][50] and heterocycles [9,[51][52]…”