16-Electron pentadienyl- and cyclopentadienyl-ruthenium half-sandwich complexes with bis(imidazol-2-imine) ligands and their use in catalytic transfer hydrogenationElectronic supplementary information (ESI) available: Selected NMR spectra and computational details. CCDC 1054781-1054784. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C5DT01080B

Bis(η 5 -2,4-dimethylpentadienyl)ruthenium( ii ), [(η 5 -C 7 H 11 ) 2 Ru] ( 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 HBF 4 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 -C 5 H 5 )Ru{(1-4η)-C 7 H 12 -η 2 - C 5 , H 5 }]BF 4 ( A ) represents the global minimum, which is 3.7 kcal mol −1 lower in energy than the hydride complex [(η 5 -C 5 H 5 )RuH(η 5 -C 7 H 11 )]BF 4 ( 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 -C 7 H 11 )Ru(BL R )]BF 4 ( 4a , R = Me; 4b , R = iPr) and [(η 5 -C 5 H 5 )Ru(BL R )]BF 4 ( 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. Ligand exchange from "protonated open ruthenocene" afforded 16-electron ruthenium half-sandwich complexes as efficient transfer hydrogenation catalysts.