Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe 3 ) 2 -3,4-(CH 2 ) 4 (η 5 -C 4 COH)]Fe(CO) 2 H (3) and aldehydes were characterized by 1 H NMR, 13 C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe 3 ) 2 -3,4-(CH 2 ) 4 (η 5 -C 4 C=O)]Fe (CO) 2 (HOCH 2 C 6 H 5 ) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh 3 . Dissociation of the alcohol ligand in the presence of H 2 leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD 2 )C 6 H 4 CHO (11-d 2 ), which possesses a potential intramolecular alcohol trapping agent, afforded two alcohol complexes, one with the newly formed alcohol coordinated to iron and the other with the trapping alcohol coordinated. The intramolecular trapping experiments support a mechanism involving concerted transfer of a proton from OH and hydride from Fe of 3 to aldehydes. The kinetics and mechanism of the hydrogenation of benzaldehyde catalyzed by 3 are presented.Alcohol complexes play important roles in transition metal catalyzed reactions. They serve as catalyst precursors by dissociating the alcohol ligand and creating vacant coordination sites for substrate binding. They have been proposed as essential intermediates in aerobic alcohol oxidations, i silane alcoholysis, ii and ionic hydrogenation of carbonyl groups. iii Alcohol complexes have been suggested as possible intermediates in the hydrogenation of ketones and aldehydes catalyzed by Shvo's diruthenium hydride complex 1-S iv and its tolyl analog 1. However, all attempts to prepare or spectroscopically observe these ruthenium alcohol complexes have been unsuccessful. v We succeeded in synthesizing related cationic hydroxycyclopentadienyl ruthenium alcohol complexes, vi which dissociate alcohol rapidly at low temperature. Very few alcohol complexes have been crystallographically characterized; iiia,iiie,vii all of the structurally characterized alcohol complexes are cationic compounds.Ligand-metal bifunctional hydrogenation catalysis is dramatically changing the face of reduction chemistry. viii These transition metal catalysts contain electronically coupled casey@chem.wisc.edu. Supporting Information Available: General experimental information, experimental procedures, synthesis of alcohol complexes, rates of substitution of coordinated alcohols by PhCN, rates of hydrogenation of aldehydes catalyzed by 3, X-ray crystallographic data for {2,5-(SiMe 3 ) 2 -3,4-[(CH 2 ) 4 ](η 4 -C 4 CO)}Fe(CO) 2 (HOCH 2 Ph) (6-H). This material is available free of cha...
