Charles P. Casey

The U.S. Department of Energy's Office of Scientific and Technical Information

The bis(trimethylsilyl)-substituted hydroxycyclopentadienyl ruthenium hydride [2,5-(SiMe₃)₂-3,4-(CH₂OCH₂)(η⁵-C₄COH)]Ru(CO)₂H (10) is an efficient catalyst for hydrogenation of aldehydes and ketones. Because 10 transfers hydrogen rapidly to aldehydes and ketones and because it does not form an inactive bridging hydride during reaction, hydrogenation of aldehydes and ketones can be performed at room temperature under relatively low hydrogen pressure (3 atm); this is a significant improvement in comparison with previously developed Shvo type catalysts. Kinetic and ²H NMR spectroscopic studies of the stoichiometric reduction of aldehydes and ketones by 10 established a two-step process for the hydrogen transfer: (1) rapid and reversible hydrogen bond formation between OH of 10 and the oxygen of the aldehyde or ketone followed by (2) slow transfer of both proton and hydride from 10 to the aldehyde or ketone. The stoichiometric and catalytic activities of complex 10 are compared to those of other Shvo type ruthenium hydrides and related iron hydrides.

The ruthenium hydride of (Ar₄CpOH)Ru(CO)₂H exchanges cleanly and rapidly with D₂ at room temperature to generate the ruthenium deuteride. A chain mechanism is proposed to explain the much more rapid exchange of RuH/D₂ than RuCO exchange with ¹³CO.

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe₃)₂-3,4-(CH₂)₄(η⁵-C₄COH)]Fe(CO)₂H (3) and aldehydes were characterized by ¹H NMR, ¹³C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe₃)₂-3,4-(CH₂)₄(η⁴-C₄C═O)]Fe(CO)₂(HOCH₂C₆H₅) (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 PPh₃. Dissociation of the alcohol ligand in the presence of H₂ 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-(HOCD₂)C₆H₄CHO, 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.

PMe₃ adds selectively to the central carbon of the η³-propargyl complex [C₅Me₅(CO)₂Re(η³-CH₂C≡CCMe₃)][BF₄] (1- t-Bu) to form the metallacyclobutene [C₅Me₅(CO)₂Re(CH₂C(PMe₃)═CCMe₃)][BF₄] (7). The rate of rearrangement of the metallacyclobutene 7 to η²-alkyne complex [C₅Me₅(CO)₂Re(η²-Me₃PCH₂C≡CCMe₃)][BF₄] (8) is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via η³-propargyl complex 1- t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe₃ with the labeled metallacyclobutene 7-d₉. This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe₃ to the central carbon of η³-propargyl complex 1- t-Bu to give 7 compared to addition to a terminal carbon to give 8. The addition of PPh₃ to 1- t-Bu gives the metallacyclobutene [C₅Me₅(CO)₂Re(CH₂C(PPh₃)═CCMe₃)][BF₄] (11). Low-temperature ¹H NMR spectra provide evidence for an equilibrium between metallacyclobutene 11 and η³-propargyl complex 1- t-Bu (K_eq ≈ 44 M⁻¹ at −46 °C and ΔG°(0 °C) = −1.2 ± 0.2 kcal mol⁻¹).

The catalytic hydrogenation of benzaldehyde and acetophenone with the Shvo hydrogenation catalysts were monitored by in situ IR spectroscopy in both toluene and THF. The disappearance of organic carbonyl compound and the concentrations of the ruthenium species present throughout the hydrogenation reaction were observed. The dependence of the hydrogenation rate on substrate, H2 pressure, total ruthenium concentration, and solvent were measured. In toluene, bridging diruthenium hydride 1 was the only observable ruthenium species until nearly all of the substrate was consumed. In THF, both 1 and some monoruthenium hydride 2 were observed during the course of the hydrogenation. A full kinetic model of the hydrogenation based on rate constants for individual steps in the catalysis was developed. This kinetic model simulates the rate of carbonyl compound hydrogenation and of the amounts of ruthenium species 1 and 2 present during hydrogenations.

Reduction of imines by [2,5-Ph₂-3,4-Tol₂(η⁵-C₄COH)]Ru(CO)₂H (1) produces kinetically stable ruthenium amine complexes. Reduction of an imine possessing an intramolecular amine was studied to distinguish between inner sphere and outer sphere mechanisms. 1,4-Bn¹⁵NH(c-C₆H₁₀)NBn (12) was reduced by 1 in toluene-d₈ to give 85% of [2,5-Ph₂-3,4-Tol₂(η⁴-C₄CO)](CO)₂RuNHBn(c-C₆H₁₀)¹⁵NHBn (16-RuN, ¹⁵N), resulting from coordination of the newly formed amine to the ruthenium center, and 15% of trapping product [2,5-Ph₂-3,4-Tol₂(η⁴-C₄CO)](CO)₂Ru¹⁵NHBn(c-C₆H₁₀)NHBn (16-Ru¹⁵N, N), resulting from coordination of the intramolecular trapping amine. These results provide support for an outer sphere transfer of hydrogen to the imine to generate a coordinatively unsaturated intermediate, which can be trapped by the intramolecular amine. An opposing mechanism, requiring coordination of the imine nitrogen to ruthenium prior to hydrogen transfer, cannot readily explain the observation of the trapping product 16-Ru ¹⁵N,N.