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Catalytic Activity of Ruthenium ( II ) Complexes Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones
| Content Provider | Semantic Scholar |
|---|---|
| Author | Chai, Huining Liu, Tingting Wang, Qingfu Yu, Zhengkun |
| Copyright Year | 2015 |
| Abstract | Airand moisture-stable ruthenium(II) complexes bearing a multisubstituted pyrazolyl-imidazolyl-pyridine ligand were synthesized and structurally characterized by NMR and X-ray single-crystal crystallographic analyses. The substituents on the imidazolyl moiety of the NNN ligand exhibited a remarkable impact on the catalytic activity of the corresponding Ru(II) complexes for transfer hydrogenation of ketones in refluxing 2propanol, following the order NHTs > Me > H > NO2, to tune the catalytic activity. The highest final TOF value of 345 600 h−1 was reached by means of 0.05 mol % of the Ru(II)-NHTs-substituted NNN complex as the catalyst. The corresponding structurally confirmed RuH complexes are proposed as the catalytically active species. ■ INTRODUCTION Transition-metal-catalyzed transfer hydrogenation (TH) of unsaturated CO and CN bonds using 2-propanol or formic acid as the hydrogen source has been well explored as a robust protocol. The Noyori Ru(II) complexes containing a monotosylated 1,2-diamine or amino alcohol ligand have been used as the most powerful catalysts in the asymmetric transfer hydrogenation of ketones and imines. Baratta et al. reported the versatile ruthenium(II) 2-aminomethylpyridine (ampy) complexes, which had demonstrated efficient catalytic activity in (asymmetric) transfer hydrogenation of ketones. The complexes bearing a ligand with an NH functionality usually exhibit high catalytic activity in the transfer hydrogenation reactions. Although various types of ligands and their transition-metal complexes featuring no NH functionality have also been developed for the same purpose, development of highly active catalytic systems is still strongly desired. Recently, polydentate nitrogen-containing ligands such as 2,6-bis(immino)pyridines, 2,2′:6′,2′′-terpyridines (terpy), and 2,6-bis(oxazolinyl)pyridines (pybox) have attracted more and more attention for their tunable properties and potential applications in homogeneous catalysis and organic synthesis. These kinds of ligands usually feature two symmetrical coordinating arms. However, unsymmetrical polydentate ligands have also been applied due to the higher catalytic activity of their transition-metal complexes attributed to the hemilabile properties of the ligands. In general, NNN ligands can be conveniently prepared and structurally modified, and their corresponding transition-metal complexes are usually reactive and airand moisture-stable, suggesting that they might be used as efficient homogeneous catalysts. During our ongoing study on Ru(II)-NNN complex catalysts for transfer hydrogenation of ketones, we have found that Ru(II) complex catalysts bearing an unsymmetrical pyridylbased NNN ligand can exhibit very high catalytic activity. Ru(II) complexes bearing a symmetrical 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine ligand (A) or a unsymmetrical bis(trifluoromethyl)pyrazolyl-pyridyl-based ligand (B) were synthesized and showed good catalytic activity in the transfer hydrogenation of ketones in refluxing 2-propanol, and the latter performed more efficiently than the former. Both cationic and neutral Ru(II) complexes C and D demonstrated very good catalytic activity for transfer hydrogenation of ketones under the same conditions. Intrigued by the catalytic activity difference between complexes A and B and the structural features of complexes C and D, we reasonably envisioned that tuning the electronic property of the ligand in complexes of type C or D may alter the catalytic activity of the corresponding complexes for transfer hydrogenation of ketones (Chart 1). Herein, we report the synthesis of Ru(II) complexes 4−6 bearing a pyridyl-based NNN ligand containing a coordinative disubstituted imidazolyl moiety and their catalytic behaviors in the TH reactions of ketones. Received: August 24, 2015 Published: October 20, 2015 Article pubs.acs.org/Organometallics © 2015 American Chemical Society 5278 DOI: 10.1021/acs.organomet.5b00727 Organometallics 2015, 34, 5278−5284 ■ RESULTS AND DISCUSSION Synthesis of Ligands and Their Ru(II) Complexes. Condensation of 6-(3,5-dimethyl-1H-pyrazol-1-yl)picolinimidate (1) with 4,5-dimethylbenzene-1,2-diamine (2a), N,N′-(4,5-diamino-1,2-phenylene)bis(4-methylbenzenesulfonamide) (2b), and 4,5-dinitrobenzene-1,2-diamine (2c) in the presence of glacial acetic acid afforded 3a (97%), 3b (86%), and 3c (32%), respectively. Ligands 3a and 3b were efficiently prepared, but 3c was obtained in only a low yield by extending the reaction time. Reacting equimolar amounts of ligand 3 with RuCl2(PPh3)3 in refluxing 2-propanol gave Ru(II) complexes 4a−c in 88−90% yields. Ionic complex 4a was further converted to the corresponding neutral complex, that is, complex 5a, by means of K2CO3 base to remove one molecule of HCl from 4a (Scheme 1). Treatment of 4a with K2CO3 in 2-propanol afforded RuH complex 6a in 75% yield, suggesting that consecutive dehydrochlorination/reduction occurred (eq 1). It is noted that, starting from 5a under the same conditions, complex 6a was also obtained in a similar yield. Characterization of Ru(II) Complexes 4−6. The NMR analyses of complexes 4−6 are consistent with their compositions. In the H NMR spectra, the proton resonances of the NH functionality in ligand 3a and its Ru(II) complex 4a were shown as singlets at 12.31 and 15.20 ppm, respectively, while these signals disappeared in those of neutral complexes 5a and 6a, revealing formation of a Ru−N bond in the complexes under basic conditions. The P NMR resonances of complexes 4a−c and 5a appeared at 20.4, 23.5, 19.0, and 22.4 ppm, respectively, suggesting the presence of two equivalent PPh3 ligands positioned trans to each other in the complexes. Complex 5a showed a 2.0 ppm increment in its P NMR spectrum as compared to that of 4a, which is in accordance with our previous observation and suggests a change of the coordination pattern of the benzimidazolyl nitrogen atom in 5a through extrusion of HCl from 4a. The H and P NMR spectra of 6a exhibited a triplet at −6.84 ppm for Ru−H and a doublet at 43.3 ppm for its two PPh3 ligands, respectively. The solid-state molecular structures of complexes 4b and 6a were confirmed by X-ray crystallographic studies (Figures 1 and 2). In the solid state, the central ruthenium atom of complex 4b is situated in a distorted octahedral environment surrounded by tridentate NNN ligand 3b, two trans PPh3, and a chloro ligand Chart 1. Ruthenium(II)-NNN Complex Catalysts Scheme 1. Synthesis of Ligands and Complexes 4 and 5a Legend: (i) HOAc, 118 °C, 4−12 h, 0.1 MPa N2; (ii) Ru(PPh3)3Cl2, iPrOH, 82 °C, 4 h, 0.1 MPa N2; (iii) K2CO3, CH2Cl2, 40 °C, 4 h, 0.1 MPa N2. Figure 1. Molecular structure of complex 4b. Thermal ellipsoids are set at 30% probability. Two chloroform molecules are omitted for |
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| Language | English |
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| Resource Type | Article |
