Osmium Catalysts for Acceptorless and Base-Free Dehydrogenation of Alcohols and Amines: Unusual Coordination Modes of a BPI Anion

Abstract

A novel type of catalyst precursors for the dehydrogenation of hydrogen carriers based on organic liquids has been discovered. Complexes OsH6(P i Pr3)2 (1) and OsH(OH)(CO)(P i Pr3)2 (2) react with 1,3-bis(6'-methyl-2'-pyridylimino)isoindoline (HBMePI) to give OsH3{κ 2-Npy,Nimine-(BMePI)}(P i Pr3)2 (3) and OsH{κ 2-Npy,Nimine-(BMePI)}(CO)(P i Pr3)2 (4). The unprecedented  2-Npy,Nimine coordination mode of BMePI is thermodynamically preferred with Os(IV) and Os(II) metal fragments and allows to prepare BMePI-based dinuclear metal cations. Treatment of OsH2Cl2(P i Pr3)2 (5) with 0.5 equiv of HBMePI in the presence of KO t Bu affords the chloride salt of the bis(osmium(IV)) dinuclear cation [{OsH3(P i Pr3)2}2{-( 2-Npy,Nimine)2-BMePI}] + (6). Related homoleptic bis(osmium(II)) complexes have been also synthetized. Complex 4 reacts with the bis(solvento) [OsH(CO){ 1-O-[OCMe2]2}(P i Pr3)2]BF4 to give [{OsH(CO)(P i Pr3)2}2{-( 2-Npy,Nimine)2-BMePI}]BF4 (7), whereas the addition of 0.5 equiv of HBMePI to {OsCl( 6-C6H6)}2(-Cl)2 (8) affords [{OsCl( 6-C6H6)}2{-(κ 2-Npy,Nimine)2-BMePI}]Cl (9). The reactions of 4 with 8 and {OsCl( 6-p-cymene)}2(-Cl)2 (10) lead to the heteroleptic cations [(P i Pr3)2(CO)HOs{-( 2-Npy,Nimine)2-BMePI}OsCl( 6-arene)] + (arene = C6H6 (11), p-cymene (12)). The electronic structrure and electrochemical properties of the dinuclear complexes were also studied. Complexes 3 and 4 are efficient catalyst precursors for the acceptorless and base-free dehydrogenation of secondary and primary alcohols and cyclic and lineal amines. The primary alcohols afford aldehydes. The amount of H2 released per gram of heterocycle depends upon: the presence of a methyl group adjacent to the nitrogen atom, the position of the nitrogen atom in the heterocycle, and the size of the heterocycle.

N–H and C–H Bond Activations of an Isoindoline Promoted by Iridium- and Osmium-Polyhydride Complexes: A Noninnocent Bridge Ligand for Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

“…The reactions were performed in toluene, at 100 °C, using a substrate concentration of 0.255 M and a catalyst concentration of 1.8 × 10 –2 ( 3 ) or 9.0 × 10 –3 ( 4 and 5 ) M: i.e., 7 mol % of the metal in all cases. Table collects the alcohols studied and the yield of carbonyl compound formed as a function of the catalyst, after 24 h. The previously reported conversions obtained with the mononuclear osmium catalyst OsH 3 {κ 2 -N py , N imine (BMePHI)}­(P i Pr 3 ) 2 are also included for comparison …”

“…Because it vacates three basic nitrogen atoms, the mononuclear species bearing the BMePHI ligand coordinated in this fashion can be employed to generate homoleptic and heteroleptic binuclear compounds, where the polydentate ligand acts as a μ-(κ 2 - N py , N imine ) 2 bridge. Frontier orbitals and electrochemical studies showed that the metal fragments behave independently in a sequential manner …”

Sigma-bond activation reactions induced by unsaturated Os(IV)-hydride complexes

“…Dihydride-dichloride 1 also activates the N-H bond of 1,3-bis(6'-methyl-2'pyridylimino)isoindoline (HBMePI), which is a sterically demanding version of 1,3bis(2-pyrilimino)isoindoline (HBPI). 69 The activation generates the isoindolinate anion BMePI and HCl. The latter is removed from the reaction medium with K t BuO.…”

“…The reason for this novel preference is thermodynamic, since it gives rise to the most stable structure from all the possible options. 69 Accordingly, treatment of 2-propanol solutions of 1 with 0.5 equiv of HBMePI, in the presence of 0.5 equiv of K t BuO, at room temperature gives the salt The iridium center of 122 catalyzes the dehydrogenation of secondary alcohols to ketones, primary alcohols to aldehydes or esters, and diols to lactones.…”

Base-Free and Acceptorless Dehydrogenation of Alcohols Catalyzed by an Iridium Complex Stabilized by a N,N,N-Osmaligand

“…Thus, in contrast to 3, it oxidizes 1-diphenylmethanol better than 1phenylethanol (77% versus 60%). 18 Alcohols with two alkyl groups, such as 2-octanol and 1-cyclohexylethanol (runs 8 and 9) are dehydrogenated worse than diphenylmethanol. 2-Octanone and cyclohexyl methyl ketone are obtained in 40% and 50% yield, respectively.…”