Preparation And Properties Of Ruthenium-hydroxypyridyl Cooperative Catalysts For Transfer Hydrogenation Of Ketones

Metal-ligand cooperative complexes show important applications in bond activation and catalysis.The catalytic activity and selectivity can be increased when the deprotonated ligands directly cooperate with metal center by experiencing themselves invertible structural changes in substrate activation and product formation.A major method of designing metal-ligand cooperative catalysts is to introduce a proton-responsive ligand into the metal center.For example,transition-metal complexes with 2-hydroxypyridyl（2-HOPy）are one important group of metal-ligand cooperative catalysts,and have been applied to many catalytic reactions.In the presence of a base,2-hydroxypyridyl could be deprotonated to a pyridonate form with a C=O bond,which directly affects the first coordination sphere of the metal center.As a result,a series of metal-ligand cooperative catalysts with 2-hydroxypyridylmethylene fragment have been designed and synthesized,which show highly catalytic activities in transfer hydrogenation（TH）of ketones andβ-alkylation of secondary alcohols with primary alcohols.The main research contents are as follows:The NNN-type ligand（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₄N）（II-4）and ruthenium complex[Ru Cl₂（PPh₃）（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₄N）]（II-5）with a 2-hydroxypyr-idylmethylene have been designed and synthesized.Acetophenone could be reduced to 1-phenylethanol by using 0.5 mol%complex II-5 as catalyst in refluxing isopropyl alcohol solution within 10 min,achieving 97%yield and the final turnover frequency of 1.16×10³ h^-1.Compared with the reported ruthenium NNN complex[RuCl₂（PPh₃）（HOC₅H₃N-C₅H₃N-C₅H₃NOH）]without a methylene,the reaction time is decreased from 24 h to 10 min.The relationship between ligand and catalytic activity is investigated from two different views,including the OH position and the numbers.The NNN-type ligands（C₅H₄N-CH₂-C₅H₃N-C₅H₃N-OH）（III-3）and（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-OH）（III-7）,and ruthenium complexes[Ru Cl₂（PPh₃）（C₅H₄N-CH₂-C₅H₃N-C₅H₃N-OH）]（III-11）and[Ru Cl₂（PPh₃）（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-OH）]（III-12）have been designed and synthesized.The catalytic transfer hydrogenation activities of bifunctional metal-ligand complexes II-5,III-11 and III-12 have been tested.The order of reaction activity is complexes II-5,III-11,and III-12.In order to investigate the relationship between ligand and catalytic activity,in the presence of CO,the dichloride ruthenium（II）complexes II-5,III-11 and III-12 have been treated with NH₄PF₆ in methanol or CH₂Cl₂,to give ionic products[RuCl（CO）（PPh₃）（R₁-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-R₂）][PF₆]（R₁=OH,R₂=H,III-16;R₁=H,R₂=OH,III-17;R₁=R₂=OH,III-18）（III-16^III-18）,respectively.By comparing theν_COdata of the deprotonated complexes III-16^III-18,it can be seen that the ligand field strengths for the deprotonated forms are in the order of III-3,II-4 and III-7.The catalytic activities of III-16^III-18 in the transfer hydrogenation of ketones are consistent with the order of their ligand fields,This suggests that in these cases the electron densities in the Ru centers might play a dominant role for their activities.By contrast,complex III-11 shows better catalytic activity than that of III-12,but it is not as active as II-5,which means that for complexes II-5,III-11 and III-12,the hydroxy position is also important for the activity.The NNN-type ligands（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₇N₂）（IV-2）and（HO-C₅H₃N-CH₂-C₅H₃N-C₆H₆N₃）（IV-4）,and ruthenium complexes[Ru Cl₂（PPh₃）（HO-C₅H₃N-CH₂-C₅H₃N-C₅H₇N₂）]（IV-5）and[RuCl₂（PPh₃）（HO-C₅H₃N-CH₂-C₅H₃N-C₆H₆N₃）]（IV-6）have been designed and synthesized by replacing the pyridyl ring with pyrazolyl or benzotriazolyl on the premise that reserves crucial 2-hydroxy pyridylmethylene groups.The catalytic activity could be increased by replacing the pyridyl ring with pyrazolyl.Acetophenone could be reduced to 1-phenylethanol by using 0.2 mol%complex IV-5 as catalyst in refluxing isopropyl alcohol solution within 20 min,resulting in 96%yield and the final turnover frequency of 1.44×10³h^-1.[Ru H（CO）（PPh₃）₂（R₁-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-R₂）][Cl]（R₁=OH,R₂=H,V-1;R₁=H,R₂=OH,V-2;R₁=R₂=OH,V-3）（V-1^V-3）have been synthesized when ligands II-4,III-3 and III-7 are treated with Ru H（CO）（Cl）（PPh₃）₃.In the presence of the base,the remarkable differences have arisen in reactivity of the ruthenium hydride complexes by altering OH groups arrangement of ligand.For example,the selective deprotonation of the OH groups in the non-coordinate2-hydroxypyridyl ring for V-3 have been discovered,followed by the substitution of one PPh₃,affording to a trident NNN ruthenium hydride complex[Ru H（CO）（PPh₃）（O-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-OH）]（V-7）.Even though the excess of t-Bu OK was used,the other OH and the CH₂ groups are unreactive.Complex V-1exhibits similar reactivity with t-Bu OK as V-3,and afforded[Ru H（CO）（PPh₃）（O-C₅H₃N-CH₂-C₅H₃N-C₅H₄N）]（V-4）within 10 min.The results indicate the OH group of PyCH₂PyOH is more acidic than that of PyPyOH.The reactivity of V-2 with t-Bu OK is slightly different from that of V-1 and V-3,and two products[Ru H（CO）（PPh₃）₂（C₅H₄N-CH₂-C₅H₃N-C₅H₃N-O）]（V-5）and[Ru H（CO）（PPh₃）（C₅H₄N-CH₂-C₅H₃N-C₅H₃N-O）]（V-6）are obtained in 11%and 56%yield,respectively,after 24 hours in refluxing isopropanol.Although exhibit in of low catalytic activity in transfer hydrogenation of ketones,these ruthenium hydride complexes show high catalytic activities inβ-alkylation of secondary alcohols with primary alcohols.The yield ofβ-alkylation of benzyl alcohol and 1-phenyethanol is 94%by using 0.5 mol%complex V-2 or V-5 as catalyst within 60 min.The result demonstrates that the complex V-5 could be regarded as the intermediate of V-2 inβ-alkylation of secondary alcohols with primary alcohols.