Iridium(I) Complexes with Anionic N-Heterocyclic Carbene Ligands as Catalysts for the Hydrogenation of Alkenes in Nonpolar Media

Abstract

A series of lithium complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) was prepared in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction of the resulting lithium-carbene adducts with [M(COD)Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)], in which the metal atoms exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)Rh(CO)2] and the ate complex (NEt4)[(WCA-NHC)Rh(CO)2Cl] were prepared, with the latter allowing an assessment of the donor ability of the ligand by IR spectroscopy. The zwitterionic iridium-COD complexes were tested as catalysts for the homogeneous hydrogenation of alkenes, which can be performed in the presence of nonpolar solvents or in the neat alkene substrate. Thereby, the most active complex showed excellent stability and activity in hydrogenation of alkenes at low catalyst loadings (down to 10 ppm).

London Dispersion Interactions in Pnictogen Cations [ECl2]+ and [E=E]2+ (E=P, As, Sb) Supported by Anionic N‐Heterocyclic Carbenes

“…It is noteworthy that the carbene ligands in all compounds display an inclination that generates short contacts between the pnictogen and the ipso ‐carbon atoms of the Dipp substituents adjacent to the borate moieties ( 2 a : 3.047(1), 2 b : 3.042(3); 3 : 3.080(2), 4 : 3.179(2) Å), which are significantly smaller than the sum of the van der Waals radii (P⋅⋅⋅C 3.50, As⋅⋅⋅C 3.55, Sb⋅⋅⋅C 3.76 Å) . This structural motif is reminiscent of the arene‐metal interaction found in coordinatively unsaturated WCA‐NHC transition metal complexes . It is also consistent with the observation of short arene‐element contacts (“Menshutkin interaction”) in heavier terphenyldichloropnictogen compounds (Ter)ECl 2 (E=As, Sb, Bi) …”

“…Contributions of our group have been based on the consideration that frustrated Lewis pairs, composed of bulky imidazolin‐2‐ylidenes and boranes such as B(C 6 F 5 ) 3 , would form abnormal carbene adducts, in which the weakly coordinating anionic (WCA) borate unit is attached to the 4‐position of the N ‐heterocycle . The corresponding anionic carbenes were termed WCA‐NHCs and introduced as ancillary ligands in transition metal complexes and homogeneous catalysts . Thereby, the additional negative charge provided neutral analogues of cationic metal NHC complexes, for example, well‐established Au I and Ir I pre‐catalysts .…”

A bimetallic iridium(ii) catalyst: [{Ir(IDipp)(H)}2][BF4]2 (IDipp = 1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene))

“…[4] The lack of examples of iridium(II) catalysts sharply contrasts with the widespread application of rhodium(II) complexes in catalysis, especially remarkable are the dinuclear catalysts reported by Doyle and coworkers. [5] The reactivity of bimetallic iridium complexes with substrates such as alkynes, molecular hydrogen, halogens, or halocarbons very often diverges from that of their mononuclear analogues.…”

“…An alternative pathway that would involve the deprotonation of the alkyne by the hydrido ligand3 has been discarded as no reaction was observed when 3 was treated with more acidic substrates, such as HBF 4 or MeOD.…”

Reaktionen auf die Vollsperrung der Schiersteiner Brücke auf der A643

“…Diese zwitterionischen Ir‐Verbindungen verfügen über eine stabilisierende Wechselwirkung zwischen einem Arylsubstituenten des NHC‐Liganden und dem Metallzentrum (π‐face donation) (Abbildung 19). Sie ermöglichen die effiziente Hydrierung unterschiedlich substituierter Olefine in Abwesenheit chlorierter Lösungsmittel 1_20…”