Mononuclear Indoline Complexes. 2. Synthesis, Structure, and Reactivity of [(Cymene)Ru(η¹-N-indoline)(CH₃CN)₂](OTf)₂

Abstract

The complex [(cymene)Ru(η 1 -indoline)(CH 3 CN) 2 ](OTf) 2 , 1, has been synthesized by the reaction of [(cymene)Ru(OTf) 2 ] x with indoline in acid solution and characterized by an X-ray diffraction study. Bond distances for the indoline ligand are compared to those reported recently for other η 1 -and η 6 -indoline complexes. Complex 1 rearranged to [(cymene)Ru(η 6 -indoline)](OTf) 2 when heated in dichloromethane solution. The titration of an aqueous solution of 1 has been carried out, and the pK a of the coordinated ligand was determined to be 5.2. The value is similar to that reported for the indolinium ion. Ligand-exchange reactions of 1 have been studied. Deprotonation of the η 1 -indoline ligand in 1 results in dehydrogenation of the ligand to form free indole and ruthenium hydride products. The deprotonation of 1 in the presence of an alkene has been found to result in a hydrogentransfer reaction to form alkane. Characteristics of the hydrogen-transfer reactions are discussed.

Hydrogenation of Indole by Phosphine-Modified Rhodium and Ruthenium Catalysts

“…The fact that no rhodium complex with IN, INE, or INH + was detected in the catalytic mixture reflects the scarce ligating properties of these N-heterocycles. ,,,− Q, which is a fairly good nitrogen nucleophile, behaves differently: its hydrogenation to THQ in the presence of 1 and TfOH proceeds, in fact, via isolable Q adducts as either catalytically active species ([Rh(H) 2 (η 1 - N -Q)(triphos)] + ) or the resting state ([Rh(η 1 - N -Q) 2 (η 2 -triphos)] + ) (Scheme ) 5 …”

“…In homogeneous phase, IN is much more difficult to reduce than Q. ,, Only [RuHCl(PPh 3 ) 3 ] 14 and [RuH(CO)(MeCN)(PPh 3 ) 2 ]BF 4 15 have been reported to hydrogenate IN to INE, but the catalytic activity was very low (TOF ≤ 1). The η 1 (N) coordination, which is critical for selective nitrogen ring reduction in Q, is virtually unknown for IN, which prefers to bind metal centers using the carbocyclic ring. − …”

“…Homogeneous model studies have been very useful to elucidate the structure/reactivity relationships between molecular metal catalysts and N-heterocycles. − Most of the work done, however, has been focused on N-heterocycles containing a basic nitrogen donor such as pyridine and quinoline (Q) as they are more amenable than pyrrole or indole (IN) to form stable metal complexes. ,− Indeed, unlike Q (p K a = 4.9), the nitrogen lone pair in IN (p K a = −3.6) is delocalized over the five-membered ring and is not available for binding a metal center unless an intramolecular hydrogen transfer is induced to form 3 H -indolenine (Scheme , a − c ). − As a result, the most common bonding mode of IN to single metal sites is the η 6 mode through the carbocyclic ring (Scheme , d ) 1 …”

Modelling the Hydrodenitrogenation of Aromatic N-Heterocycles in the Homogeneous Phase

“…[31,35b,37] Of particular relevance are the studies carried out by Rakowski DuBois on the coordination, acid/base properties and activation of indoline by (p-cymene)Ru II fragments. [32a,37] The complex [(p-cymene)Ru(η 1 -indoli-ne)(CH 3 CN) 2 )](OTf) 2 converts to [(p-cymene)Ru(η 6 -indoline)](OTf) 2 upon gentle heating in CH 2 Cl 2 and can be deprotonated to give the corresponding (η 1 -indolinyl) complex [37] (Scheme 14). The reactivity of the latter compound is described in a following section.…”

“…Dehydrogenation of indoline to indole by thermolysis of metal clusters containing C-H inserted indoline has been observed, [37] while indoline has been employed as a hydrogen source for the transfer hydrogenation of olefins in the presence of [RhCl(PPh 3 ) 3 ]. [57] A possible pathway for indoline dehydrogenation is shown in Scheme 31.…”