Effect of Electronic Factor in Ru-phosphine-diamine Complexes on Selective Hydrogenation of CC and...

Chinese Journal of Chemistry, 2009, 27, 937—943 Full Paper

* E-mail: [email protected]; Tel./Fax: 0086-028-85412904 Received June 6, 2008; revised December 17, 2008; accepted February 17, 2009. Project supported by the National Natural Science Foundation of China (Nos. 20371032, 20271035).

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of Electronic Factor in Ru-phosphine-diamine Complexes on Selective Hydrogenation of C＝C and C＝O Bonds

ZHANG, Yu(张愚) YU, Xiaojun(余晓军) YU, Changbin(余长斌) XIA, Yuqing(夏雨青) LI, Ruixiang*(李瑞祥) CHEN, Hua(陈华) LI, Xianjun(李贤均)

Key Laboratory of Green Chemistry and Technology, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China

A series of ruthenium complexes bearing different phosphines and diamines were synthesized and their compo-nents and structures were characterized by NMR spectra and elemental analyses. The catalytic properties of these complexes for the hydrogenation of benzylideneacetone and the mixture of acetophenone and styrene were investi-gated. The results showed that the basicity increase of phosphine or diamine dramatically facilitates the hydrogena-tion activity and selectivity to C＝O double bond. On the contrary, the basicity decrease of phosphine or diamine not only slows down the catalytic activity, but also significantly suppresses the hydrogenation selectivity to C＝O double bond. Based on the effect of electron factors of these complexes on the hydrogenation activity and selectiv-ity of benzylideneacetone and the mixture of styrene and acetophenone, the activation mechanism of dihydrogen in ruthenium-phosphine-diamine system was proposed.

Keywords ruthenium complex, rhosphine, riamine, benzylideneacetone, hydrogenation

Introduction

Noyori’s work, the ternary system composing of [RuCl2(phosphine)3], 1,2-diamine, and a strong base is a great breakthrough in the selective hydrogenation of C＝O double bond in α,β-unsaturated aldehydes or ketones, and the enanotioselective hydrogenations of simple ke-tones.1-4 Generally, the hydrogenation selectivity to C＝O function conjugated with C＝C double bond is up to 100% in this system. To explain the excellent catalytic activity and selectivity, Noyori4,5 proposed the metal- ligand bifunctional catalysis mechanism. Work by Mor-ris et al. further supported the mechanism6-11 and indi-cated that the rate-limiting step is the heterolytic split-ting of η2-H2 in the catalytic recycling.8 Bergens et al.12 studied the interaction between H2 and ruthenium- phosphine-diamine complex by means of NMR tech-nology in situ and demonstrated that the coordination of H2 with ruthenium-phosphine-diamine complex was easy and rapid even if the reaction temperature was de-creased to －60 ℃.

In the past decade, many studies in this field have been focused on synthesizing some new diphosphine ligands and their complexes as the catalysts to obtain the highly enantioselective hydrogenation products.13-22 In general, a ruthenium complex bearing a bulky phosphine and diamine exhibits excellent catalytic ac-tivity and enantioselectivity. However, the reports on the effect of electron factors of the phosphines and dia-mines on the catalytic properties of ruthenium com-plexes are rare. Noyori et al.23 discovered that the transfer

hydrogenation activity of a ruthenium complex for some simple ketones dropped with increasing the electron- withdrawing ability of a substituent in diamine. They further reported that the ratio of syn to anti alcohol formed in the selective hydrogenation of 3-phenyl-2- butanone could be significantly improved if the ruthe-nium complex contained the electron-donating groups in triarylphosphine.24 Xiao et al.25 reported that the strong basicity of the diphosphine in a ruthenium-diphosphine- diamine complex was not favorable to improve the hy-drogenation activity for some simple ketones. However, the work by Mikami et al.26 showed that the aromatic diamine 3,3'-dimethyl-2,2'-diamino-1,1'-binaphthyl (DM- DABN) was a poison for the BINAP-Ru catalyst. A density functional calculation indicated that the low ac-tivity of the BINAP-Ru/DM-DABN complex was ow-ing to the electron delocalization from ruthenium center to the diamine. Our research group investigated the ef-fect of electron factor of the phosphine in a Ru-phosphine-diamine complex on the asymmetric hy-drogenation activity of acetophenone and discovered that the hydrogenation rate of acetophenone could be promoted by increasing the basicity of the phosphine.27

However, no research work is devoted to explore the activation of the dihydrogen coordinated to ruthenium in this ternary system. In order to obtain the exact in-formation about the electron effect of phosphine or dia-mine on the catalytic property of Ru-phosphine-diamine complex, a series of the complexes bearing phosphines or diamines of different substitutents (Scheme 1) were

938 Chin. J. Chem., 2009, Vol. 27, No. 5 ZHANG et al.


Scheme 1 Structures of phosphines and diamines

synthesized and their catalytic behavior for the selective hydrogenation of benzylideneacetone and the mixture of styrene and acetophenone was investigated in detail.

Experimental

Materials

All manipulations were carried out under an argon atmosphere using standard Schlenk technique. Solvents were dried over appropriate drying agents and distilled under argon prior to use. Reagent-grade PPh3 (Aldrich), and RuCl3•xH2O (Institute of Kunming Noble Metals, China) and cyclohexane-1,2-diamine (Acros), o-phenyl-enediamine (Acros), 4-(trifluoromethyl)-1,2-phenylene-diamine (Acros), styrene (Acros), acetophenone (Acros), 1,2-diphenylethylenediamine (Chengdu Institute of Or-ganic Chemistry, Chinese Academy of Sciences, China), and benzylideneacetone (Acros) were used as received. Other materials MOTPP [tris(4-methoxyphenyl)phos-phine],28 TFTPP (tris[4-(trifluoromethyl)phenyl]phos-phine),29 BDPX (1,2-bis[(diphenylphosphino)methyl]-benzene),30 BISBI (2,2'-bis[(diphenylphosphino)methyl]-biphenyl),31 RuCl2(BDPX)(PPh3) and RuCl2(BISBI)-(PPh3),

31 RuCl2(MOTPP)3,32 RuCl2(TFTPP)3,

32 RuCl2-(MOTPP)2[(S,S)-DPEN] (DPEN＝1,2-diphenylethane- 1,2-diamine),27 RuCl2(PPh3)2[(S,S)-DPEN],27 RuCl2- (TFTPP)2[(S,S)-DPEN]27 were prepared according to the reported methods.

Analytical methods 1H NMR and 31P{1H} NMR spectra were recorded

on a Bruker ARX 300 spectrometer at room temperature, 300.13 MHz for 1H and 121.49 MHz for 31P. The chemical shifts of 31P NMR were relative to 85% H3PO4 as an external standard, 1H relative to TMS as an inter-nal standard, with downfield shifts as positive values. Elemental analyses were performed in Shanghai Insti-tute of Organic Chemistry, Chinese Academy of Sci-ences.

Catalytic hydrogenation

An appropriate amount of catalyst and ben-zylideneacetone or a mixture of styrene and acetophe-none were introduced into a stainless steel autoclave (60 mL) equipped with a magnetic stirrer. The autoclave

was flushed with hydrogen (99.99%) five times, filled with the hydrogen to the desired pressure, and then heated at a specified temperature. The hydrogenation products were analyzed by GC-9790 with an FID de-tector and a PEG-20M capillary column (30 m×0.25 mm, 0.25 μm film) at 160 ℃.

Preparation of complexes

RuCl2(MOTPP)2(DAN): Complex RuCl2(MOTPP)3 (0.123 g, 0.1 mmol) and DAN (0.012 g, 0.11 mmol) were dissolved in 10 mL of CH2Cl2. The mixture solution was stirred at room temperature for 3 h while the color of the solution changed from brown to yellow. At the end of reaction, some insoluble substance was removed by fil-tration. The filtrate was concentrated to about 2 mL under vacuum, and 10 mL of diethyl ether (or n-hexane) was added into the concentrated solution. The mixture was put in a refrigerator overnight to generate some orange crys-tals, which were filtrated, washed with diethyl ether twice, and dried under vacuum to give RuCl2(MOTPP)2(DAN) as orange crystals (0.054 g, 55% yield). 1H NMR (C6D6) δ: 7.98—6.44 (m, 28H, PhH), 4.87 (s, 4H, 2NH2), 3.29 (s, 18H, CH3O); 31P{H} NMR (C6D6) δ: 43.5 (s). Anal. calcd for C48H50Cl2N2O6P2Ru: C 58.54, H 5.12, N 2.84; found C 58.26, H 5.08, N 3.13. The other complexes were synthesized with the similar procedures.

RuCl2(TPP)2(DAN): Reddish brown crystals (0.045 g, 56% yield) were obtained by reacting RuCl2(TPP)3-

(0.095 g, 0.1 mmol) with DAN (0.012 g, 0.11 mmol). 1H NMR (C6D6) δ: 7.95—6.37 (m, 34H, PhH), 4.69 (s, 4H, 2NH2);

31P{H} NMR (C6D6) δ: 46.6 (s). Anal. calcd for C42H38Cl2N2P2Ru•1/4CH2Cl2: C 61.44, H 4.67, N 3.39; found C 61.42, H 4.82, N 3.28.

RuCl2(TFTPP)2(DAN): Yellowish brown crystals (0.065 g, 54% yield) were obtained by reacting RuCl2- (TFTPP)3 (0.157 g, 0.1 mmol) with DAN (0.012 g, 0.11 mmol). 1H NMR (C6D6) δ: 7.64—6.39 (m, 28H, PhH), 4.47 (s, 4H, 2 NH2);

31P NMR (C6D6) δ: 46.4 (s). Anal. calcd for C48H32Cl2F18N2P2Ru: C 47.54, H 2.66, N 2.31; found C 47.87, H 2.72, N 2.54.

RuCl2(BDPX)(CYDA): Yellowish brown crystals (0.050 g, 66% yield) were obtained by reacting RuCl2-(BDPX)(PPh3) (0.091 g, 0.1 mmol) with CYDA (0.012 g, 0.11 mmol). 1H NMR (CDCl3) δ: 8.06—6.36 (m, 24H, PhH), 5.12 (br s, 4H, 2NH2), 3.64 (s, 4H,

Ruthenium complex Chin. J. Chem., 2009 Vol. 27 No. 5 939


2P-CH2), 1.42—0.83 (m, 10H, C6H10); 31P NMR (CDCl3)

δ: 39.26 (s), 39.65 (s). Anal. calcd for C38H42Cl2N2P2Ru: C 60.00, H 5.57, N 3.68; found C 59.87, H 5.42, N 3.71.

RuCl2(BDPX)(DAN): Reddish brown crystals (0.052 g, 69% yield) were obtained by reacting RuCl2(BDPX)(PPh3) (0.091 g, 0.1 mmol) with DAN (0.012 g, 0.11 mmol). 1H NMR (CDCl3) δ: 8.36—6.38 (m, 28H, PhH), 5.14 (s, 2H, NHH), 4.59 (s, 2H, NHH), 3.48 (s, 4H, 2P-CH2);

31P{H} NMR (CDCl3) δ: 40.7 (s). Anal. calcd for C38H36Cl2N2P2Ru: C 60.48, H 4.81, N 3.71; found C 60.83, H 4.92, N 3.80.

RuCl2(BDPX)(TFDA): Reddish brown crystals (0.035 g, 43% yield) were obtained by reacting RuCl2(BDPX)(PPh3) (0.091 g, 0.1 mmol) with TFDA (0.019 g, 0.11 mmol). 1H NMR (CDCl3) δ: 8.32—6.43 (m, 27H, PhH), 5.50 (s, 2H, NHH), 4.75 (s, 2H, NHH), 3.37 (s, 4H, 2P-CH2);

31P NMR (CDCl3) δ: 42.68 (d), 41.82 (d, Jp-p＝45.4 Hz). Anal. calcd for C39H35Cl2F3N2-

P2Ru: C 56.94, H 4.29, N 3.41; found C 57.20, H 4.44, N 3.49.

RuCl2(BISBI)(CYDA): Yellowish brown crystals (0.059 g, 71% yield) were obtained by reacting RuCl2- (BISBI)(PPh3) (0.098 g, 0.1 mmol) with CYDA (0.012 g, 0.11 mmol). 1H NMR (CDCl3) δ: 7.93—6.69 (m, 28H, PhH), 2.54 (s, 2H, NHH), 2.44 (s, 2H, NHH), 3.89 (q, 4H, 2P-CH2), 0.5—1.3 (m, 10H, 2CH2);

31P NMR (CDCl3) δ: 47.5 (br s), 45.6 (br s). Anal. calcd for C44H46Cl2N2P2Ru: C 63.16, H 5.54, N 3.35; found C 62.80, H 5.52, N 3.45.

RuCl2(BISBI)(DAN): Reddish brown crystals (0.058 g, 70% yield) were obtained by reacting RuCl2(BISBI)- (PPh3) (0.098 g, 0.1 mmol) with DAN (0.012 g, 0.11 mmol). 1H NMR (CDCl3) δ: 7.91—6.56 (m, 32H, PhH), 4.68 (s, 4H, 2NH2), 3.89 (m, 4H, 2P-CH2);

31P NMR (CDCl3) δ: 39.0 (s). Anal. calcd for C44H40Cl2N2P2Ru: C 63.62, H 4.85, N 3.37; found C 63.81, H 4.84, N 3.34.

RuCl2(BISBI)(TFDA): Reddish brown crystals (0.041 g, 45% yield) were obtained by reacting RuCl2(BISBI)(PPh3) (0.098 g, 0.1 mmol) with TFDA (0.019 g, 0.11 mmol). 31P NMR (C6D6) showed three groups of peaks. The intensity ratio of one group of stronger peaks to the two other groups of weaker peaks was about 3 to 1. The stronger one exhibited two dou-blets centered at δ 46.82 and 46.20 with Jp-p＝35.5 Hz. The two groups of weaker peaks were centered between δ 48.60 and 45.37 with the indeterminable coupling con-stants because of the overlapping for each other. Anal. calcd for C45H39Cl2F3N2P2Ru: C 60.14, H 4.37, N 3.12; found C 59.86, H 4.41, N 3.15.

RuCl2(MOTPP)(CYDA): Yellowish brown crystals (0.048 g, 44% yield) were obtained by reacting RuCl2- (MOTPP)3 (0.123 g, 0.1 mmol) with CYDA (0.012 g, 0.11 mmol). 1H NMR (CDCl3) δ: 7.81—6.63 (m, 30H, PhH), 2.67 (s, 4H, 2NH2), 1.76—0.87 (m, 10H, C6H10); 31P NMR (CDCl3) δ: 40.45 (s). Anal. calcd for C49H60Cl2N2O6P2Ru•CH2Cl2: C 55.00, H 5.72, N 2.57; found C 55.25, H 6.18, N 2.61.

Results and discussion

Structures of complexes

All complexes are highly air sensitive in solution. The color of solution would eventually change from reddish brown to green or purple if they were exposed to air for a short time. Elemental analyses indicated that each complex contained two chlorine atoms, one dia-mine, and two monophosphines or one diphosphine. Except complex RuCl2(BDPX)(CYDA), RuCl2(BDPX)- (TFDA), RuCl2(BISBI)(CYDA) and RuCl2(BISBI)- (TFDA), the others showed a singlet in 31P NMR spectra. The singlet indicates that the two phosphorus atoms are of the same chemical environment and suggests that the two chlorine atoms are in the trans positions, the two phosphorus atoms are in the cis positions, and the two nitrogen atoms are in the cis positions too. The structure is shown in Scheme 2.

Scheme 2 Structures of complexes

For complex RuCl2(BISBI)(CYDA), two broaden singlets centered at δ 47.5 and 45.6 in 31P NMR spec-trum were observed and the intensity ratio of the two broad singlets was about 1∶1, which indicated that the complex displayed a structural transformation in CDCl3. It should be reasonable to attribute a structural trans-formation between the chair form and boat form of cyclohexyl in 1,2-cyclohexylenediamine. Similarly, the structural transformation caused the complex RuCl2- (BDPX)(CYDA) to show also two singlets at δ 39.3 and 39.7 in 31P NMR spectrum. For complex RuCl2(BISBI)- (TFDA), the three groups of peaks were observed in its 31P NMR spectrum, in which a strong group of peaks was about 80% and the other two weak groups of peaks were about 20%. Theoretically, the complex could have four isomers as shown in Scheme 3. However, the large coupling constant peaks, which represent the presence of phosphorus atoms in trans positions, were not ob-served in the 31P NMR spectrum, so there was no isomer D in Scheme 3. The three groups of peaks should be attributed to A, B, and C. According to the single crystal structure of analogous complex RuCl2(BISBI)(DPEN),27 in which two chlorine atoms were in trans positions, two phosphorus atoms in cis positions, and two nitrogen atoms in cis positions, we could conclude that the major isomer was A, while isomers B and C were the minors. However, it was difficult to determine which one group of peaks was from B or C due to the similarity in struc-ture. For complex RuCl2(BDPX)(TFDA), two doublets with a coupling constant of 45.4 Hz were shown at δ 42.7 and 41.8 in its 31P NMR spectrum and no other peaks were observed. The two doublets indicated that the two phosphorus atoms in BDPX were nonequivalent and no other isomers were present in the complex, so the struc-



ture of RuCl2(BDPX)(TFDA) could be assigned to be the same as A in Scheme 3. Unfortunately, a pure complex RuCl2 (TFTPP)2(TFDA) was not isolated as both TFTPP and TFDA were of the relatively poor coordination abil-ity.

Scheme 3 Four possible structures of complex RuCl2(BISBI)- (TFDA)

Catalytic properties of the complexes

The catalytic properties of these complexes for the hy-drogenation of benzylideneacetone were investigated and the hydrogenation routes of the substrate are described in Scheme 4. The results listed in Table 1 showed that the electronic factor of phosphines greatly influenced the catalytic activities and selectivities of the corresponing ruthenium complexes. Complex RuCl2(MOTPP)2(DAN) and RuCl2(MOTPP)2(DPEN) containing the electron-

Scheme 4 Hydrogenation routes of benzylideneacetone

donating phosphine exhibited the higher activities and selectivities for the hydrogenation of C＝O double bond than complex RuCl2(TFTPP)2(DAN) and RuCl2- (TFTPP)2(DPEN) containing the electron-withdrawing phosphine. It was in agreement with our previous work in which a complex bearing the electron-donating phosphine showed much higher activity than a complex bearing the electron-withdrawing phosphine for the asymmetric hydrogenation of acetophenone.27

However, how will the electron factor of diamine in the complex affect the catalytic activity and selectivity for the hydrogenation of benzylideneacetone? For the sake of clarifying this question, other six complexes containing the different diphosphines and diamines were synthesized and used for the hydrogenation of ben-zylideneacetone. Compared with CYDA, the basicity of nitrogen atoms in DAN is weak due to the delocaliza-tion of lone-pair electrons of nitrogen atoms onto phenyl ring of DAN. Furthermore, the basicity of nitrogen at-oms in TFDA is lower than that of DAN due to the ex-istence of a strongly electron-withdrawing group in TFDA. The results listed in Table 1 showed that the catalytic activities and selectivities of these complexes increased with increasing the basicity of diamines. The stronger the basicity of a diamine is, the higher the catalytic activity and selectivity of its corresponding complex are. Among the complexes containing DAN and TFDA, RuCl2(BDPX)(DAN) and RuCl2(BISBI)- (DAN) showed the higher catalytic activities and selec-tivities than RuCl2(BDPX)(TFDA) and RuCl2(BISBI)- (TFDA), respectively. The complex containing a strong basicity diamine CYDA exhibited the highest activity and selectivity for the hydrogenation of C＝O bond among all the complexes. Therefore, the electron factors of diamines played the same role as that of phosphines

Table 1 Effect of ruthenium-phosphine-diamine complexes on the hydrogenation of benzylideneacetonea

Selectivity/% Entry Complex Time/h Temp./℃ Conv./%

A B C

1 RuCl2(MOTPP)2(DAN) 2 30 26.6 27.8 63.6 8.6

2 RuCl2(TPP)2(DAN) 2 30 20.2 21.9 78.1 0

3 Ru Cl2(TFTPP)2(DAN) 2 30 8.0 19.8 80.2 0

4 RuCl2(MOTPP)2(DPEN) 2 30 99.3 96.8 0.4 2.8

5 RuCl2(TPP)2(DPEN) 2 30 98.5 95.1 4.8 0.2

6 RuCl2(TFTPP)2(DPEN) 2 30 1.5 0 100 0

7 RuCl2(BDPX)(CYDA) 2 50 99.0 85.5 0 14.5

8 RuCl2(BDPX)(DAN) 2 50 42.8 71.6 12.4 16.0

9 RuCl2(BDPX)(TFDA) 2 50 27.6 29.3 43.2 27.5

10 RuCl2(BISBI)(CYDA) 2 50 98.8 96.1 3.9 0

11 RuCl2(BISBI)(DAN) 2 50 45.6 79.5 8.3 12.2

12 RuCl2(BISBI)(TFDA) 2 50 33.1 19.6 72.6 7.8

13 RuCl2(MOTPP)2(CYDA) 2 30 99.5 98.3 0.4 1.3

14 RuCl2(TFTPP)3＋TFDA 2 50 74.4 17.8 56.0 26.2 a Reaction condition: Temp.＝50 ℃, time＝2 h, p(H2)＝2. 0 MPa, i-PrOH as solvent (5 mL), substrate/catalyst/KOH＝1000∶1∶40.



in the hydrogenation activity and selectivity of ben-zylideneacetone.

Furthermore, when complex RuCl2(MOTPP)2- (CYDA), which contains both the electron-donating monophosphine and the strong basicity diamine, was used as the catalyst, it gave a much higher catalytic activ-ity and selectivity than its analogous RuCl2(MOTPP)2-

(DAN), RuCl2(TPP)2(DAN), and RuCl2(TFTPP)2(DAN) under the similar conditions. For complex RuCl2- (TFTPP)2(TFDA), which contains electron-withdrawing phosphine and diamine and was formed in situ, no cata-lytic activity was observed at the hydrogenation tem-perature of 30 ℃. In the condition of extended reaction time and elevated reaction temperature, it still showed a poor selectivity to the hydrogenation of C＝O double bond. These results indicated clearly that the hydrogena-tion pathways strongly depended on the electronic fac-tors of ligands. Hence a competitive hydrogenation for the mixture of acetophenone and styrene was investi-gated and the hydrogenation results are shown in Table 2. All hydrogenation results are well consistent with those of benzylideneacetone. When complex RuCl2-

(MOTPP)2(CYDA) was used as a catalyst, the highest activity and selectivity for the hydrogenation of aceto-phenone were gaven. For complex RuCl2(TFTPP)2- (TFDA) formed in situ, as expected, the lowest activity and selectivity for acetophenone were shown.

Catalytic reaction mechanism

According to the metal-ligand bifunctional mecha-nism (outer sphere mechanism), the key step to deter-mine the hydrogenation selectivity of C＝O double bond is a bifunctional addition of a nucleophilic hydride ligand on ruthenium and an acidic hydrogen on nitrogen

to the carbon and oxygen atoms of ketone group, re-spectively, as shown in Scheme 5. The work by Morris et al. has proved that the η2-H2 ligand in e (Scheme 5) retains a high degree of free H2 character and the het-erolytic splitting across a ruthenium-amido bond of η

2-H2 ligand is the turnover limiting step in the catalytic recycling.8,11 However, how the electronic factor of ru-thenium complexes is to influence the heterolytic split-ting of η2-H2 is not clearly described up to date. Ac-cording to the activation mechanism of H2 in a ruthe-nium complex, the σ electron of H2 firstly donates to the empty d orbital of the ruthenium atom in the transition state e, then the d electrons in ruthenium back-donate to the σ* orbital of η2-H2. If a phosphine containing the electron-donating groups coordinates to the ruthenium, it will cause the increase of the electron density of the coordinated ruthenium center and the electron-rich metal center will benefit the d electrons to back-donate to the σ* orbital of η2-H2.

33 A ruthenium complex bear-ing the electron-withdrawing phosphine makes the ru-thenium become an electron-poor metal center. In the view of coordination chemistry, the electron-poor metal center is favorable for H2 to coordinate with the ruthe-nium and makes the coordinated dihydrogen more acidic. Which one is the major activation reason among the coordination of dihydrogen and the back-donation of d electrons to σ* orbital of η2-H2? The fact that the ruthe-nium complex bearing the electron-withdrawing phosphine or diamine gives a low catalytic activity and selectivity for the hydrogenation of C＝O double bond demonstrates that the coordination of H2 with the electron-poor ruthenium and the strong acidity of η2-H2 are not the main reason to cause the heterolytic splitting

Table 2 Effect of different groups in systems on the hydrogenation of the mixture of styrene and acetophenonea

Conv./% Entry Complex Time/h Temp./℃

Styrene Acetophenone

1 RuCl2(MOTPP)2(DAN) 2 40 10.8 51.8

2 RuCl2(TPP)2(DAN) 2 40 50.3 21.4

3 RuCl2(TFTPP)2(DAN) 2 50 46.4 59.9

4 RuCl2(MOTPP)2(DPEN) 1.5 30 29.6 93.7

5 RuCl2(TPP)2(DPEN) 1 30 4.7 92.9

6 RuCl2(TFTPP)2(DPEN) 1.5 50 44.6 19.4

7 RuCl2(BDPX)(CYDA) 1 30 11.5 96.1

8 RuCl2(BDPX)(DAN) 2 40 49.3 80.7

9 RuCl2(BDPX)(TFDA) 2 40 13.1 9.1

10 RuCl2(BISBI)(CYDA) 1 30 6.4 97.2

11 RuCl2(BISBI)(DAN) 2 40 43.6 86.5

12 RuCl2(BISBI)(TFDA) 2 40 18.2 27.1

13 RuCl2(MOTPP)2(CYDA) 1.5 30 3.1 96.6

14 RuCl2(TFTPP)3＋TFDA 2 50 36.1 7.3 a Reaction condition: p(H2)＝2 MPa, i-PrOH as solvent (5 mL), styrene 2.5 mL, acetophenone 2.5 mL, substrate/catalyst/KOH＝1000∶1∶40.



Scheme 5 Hydrogenation mechanism in Ru-phosphine-diamine system

of η2-H2 in the catalytic recycling because the hetero-lytic splitting of η2-H2 is the key step to improve the catalytic activity and selectivity of C＝O bond.8,11 Therefore, the probable reason to promote the hetero-lytic splitting of η2-H2 results from the back-donation of d electrons to the σ* orbital of η2-H2. The stronger ba-sicity in nitrogen atoms not only promotes the d electron of ruthenium to back-donate to the σ* orbital of η2-H2, but also facilitates the binding between the acidic hy-drogen and the basic nitrogen atom in intermediate e. The both factors are advantageous to the heterolytic splitting of η2-H2 and improve the catalytic activity and selectivity for C＝O double bond. Our experiment sup-ports Mikami’s calculation result.26

However, when the complex contains the electron- withdrawing phosphine or diamine, why does the hy-drogenation of C＝C bond in benzylideneacetone be-come a major reaction and must the hydrogenation be carried out at the higher temperature than room tem-perature? It is well known that a ruthenium complex can activate dihydrogen in two forms of the heterolytic splitting and homolytic splitting. For a electron-poor metal center, it is easier to be stabilized by two strong σ electron-donating hydrides formed by the homolytic splitting of dihydrogen than one hydride generated by the heterolytic splitting.33 A reasonable explanation should be that the presence of the electron-withdrawing phosphine or diamine inhibits greatly the back-donation of the d electrons of a metal center to the σ* orbital of η

2-H2, making the heterolytic splitting of η2-H2 difficult. On the contrary, it makes the homolytic splitting of dihy-drogen become a major activation form in order to stabi-lize the electron-poor complex. The active species gen-erated by the homolytic splitting of dihydrogen could cause the hydrogenation to occur in an inner sphere mechanism (insertion mechanism), which leads to that the hydrogenation of C＝C double bond is a major route. In other words, the hydrogenation catalyzed by the sys-tem of ruthenium-phosphine-diamine is a competition mechanism between the inner sphere mechanism and outer sphere mechanism (metal-ligand bifunctional mechanism) and the electron factors of ligands could influence importantly the competition advantage of the two mechanisms. The ruthenium-phosphine-diamine

complex bearing the electron-withdrawing ligand is preference for activating η2-H2 by the homolytic split-ting, so the hydrogenation occurs mainly in C＝C bond. If a complex contains the electron-donating ligand, η

2-H2 is activated by the heterolytic splitting and the hydrogenation mainly occurs in C＝O double bond.

Conclusion

A series of ruthenium-phosphine-diamine complexes were synthesized and characterized. These complexes bearing the electron-donating phosphines or the strong basicity diamines showed the higher activities and se-lectivities for the hydrogenation of C＝O double bond. The main reasons are that ruthenium center bearing the electron-donating phosphine and the strong basicity diamine promotes the back-donation of d electron in the ruthenium center to the σ* orbital of η2-H2 and acceler-ates the heterolytic splitting of η2-H2. The presence of the weak basicity diamine and phosphine causes the homolytic splitting of η2-H2, so that the hydrogenation progresses in the inner sphere mechanism and the cata-lytic activity and selectivity for C＝O hydrogenation are obviously inhibited.

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