16-Electron pentadienyl- and cyclopentadienyl-ruthenium half-sandwich complexes with bis(imidazol-2-imine) ligands and their use in catalytic transfer hydrogenation

Abstract

Bis(η(5)-2,4-dimethylpentadienyl)ruthenium(II), [(η(5)-C7H11)2Ru] (1, “open ruthenocene”), which has become accessible in high yield and large quantities via an isoprene-derived diallyl ruthenium(IV) complex, can be converted into the protonated open ruthenocene 2 by treatment with HBF4 and subsequently into the protonated half-open ruthenocene 3 by reaction with cyclopentadiene. The electronic structure of 3 was studied by DFT methods, revealing that the CH-agostic complex [(η(5)-C5H5)Ru{(1-4η)-C7H12-η(2)-C(5),H(5)}]BF4 (A) represents the global minimum, which is 3.7 kcal mol(−1) lower in energy than the hydride complex [(η(5)-C5H5)RuH(η(5)-C7H11)]BF4 (B). 2 and 3 were treated with the ligands N,N′-bis(1,3,4,5-tetramethylimidazolin-2-ylidene)-1,2-ethanediamine (BL(Me)) and N,N′-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BL(iPr)) to afford the cationic 16-electron pentadienyl and cyclopentadienyl complexes [(η(5)-C7H11)Ru(BL(R))]BF4 (4a, R = Me; 4b, R = iPr) and [(η(5)-C5H5)Ru(BL(R))]BF4 (5a, R = Me; 5b, R = iPr). All complexes catalyse the transfer hydrogenation of acetophenone in isopropanol, and the most active complex 4a in this reaction was employed for the hydrogenation of a broader range of aliphatic and aromatic ketones.

Transition Metal Complexes Containing C2‐Symmetric Bis(imidazolin‐2‐imine) Ligands Derived from a 1‐Alkyl‐3‐arylimidazolin‐2‐ylidene

“…As expected, the ρ values increase upon metal coordination from 0.916 and 0.917 in the free BL Dipp, Me ligand to ca. 0.95–0.97 (Table 1), indicating a substantial charge delocalization, albeit to a smaller extent than found for half‐sandwich cyclopentadienyl and arene ruthenium complexes, where significantly stronger metal–nitrogen interaction is observed 3a,4c,4g. Indeed, the ligands adopt the anticipated C 2 ‐symmetric conformation shown in Scheme (box); however, the M –N–C–N torsion angles are comparatively small and range from 21.5° in 6 to 46.5° in 8 , thus deviating substantially from a perpendicular orientation of the imidazole rings relative to the M N 2 planes.…”

“…Nevertheless, coordination to other complex fragments with larger ancillary ligands and the potential to form stronger metal–nitrogen bonds might afford C 2 ‐symmetric complexes of higher conformational stability. This could be expected for half‐sandwich cyclopentadienyl and arene ruthenium complexes of the type [(η 5 ‐Cp)Ru(BL Dipp, Me )] + and [(η 5 ‐arene)Ru(BL Dipp, Me )] 2+ 3a,4c,4g. In addition, the use of chiral counterions in these systems would also facilitate chiral resolution, which is essential for the use of such complexes in asymmetric hydrogen transfer catalysis 4f…”

“…The introduction of unsymmetric imidazolin‐2‐ylidene moieties with nitrogen substituents of distinctly different size, however, might afford BL R, R' ligands ( R = BIG ≠ R ′ = small) that are able to form chiral C 2 ‐symmetric complexes by adopting a conformationally stable anti ‐orientation of the R and R ′ pairs of substituents as illustrated in Scheme (box). In principle, this would allow the synthesis of chiral complexes for applications in asymmetric catalysis such as transfer hydrogenation 4c,4f,4g. A similar concept was developed for related α‐diimine ligands and their respective nickel(II) and palladium(II) complexes, which serve as catalysts for the production of highly branched polyethylene 6…”

Hydrogen Isotope Exchange with Iridium(I) Complexes Supported by Phosphine‐Imidazolin‐2‐imine P,N Ligands

“…Ligands with this functional group generally act as efficient electron‐donor ligands towards transition metals because of an effective charge delocalization within the guanidine CN 3 unit, which leads to enhanced basicity and nucleophilicity . Bidentate ethylene‐ or ferrocene‐bridged, and tridentate pyridine‐bridged bis(imidazolin‐2‐imine) ligands have already been used in organometallic and coordination chemistry.…”

Solid state structure of (pentamethylcyclopentadienyl)(2,4-dimethylpentadienyl)iron, Fe(C5Me5)(2,4-C7H11), and its incorporation into silica aerogels

“…Ruthenium has been found to be quite useful and/or promising for commercial electronics applications, such as for random access memory storage, semiconductors, copper interconnects, capacitors, or transistors [2b-f], and its incorporations via the commercially available Ru(2,4-C7H11)2 (1; 2,4-C7H11 = dimethylpentadienyl; M = Ru) have been shown to be ideal for many of these processes. Ru(2,4-C7H11)2 has also been found to yield other complexes of chemical interest, such as a highly active catalyst for phase transfer hydrogenation of ketones [3]. Half-open metallocenes, such as M(C5H5)(2,4-C7H11) and related complexes, have also been useful for a number of the above applications, and have been especially helpful in evaluating the relative bonding favorabilities of pentadienyl vs. cyclopentadienyl ligands with various metals [1b].…”