Inorganic Chemistry Communications 38 (2013) 28–32

Contents lists available at ScienceDirect

Inorganic Chemistry Communications

j ourna l homepage: www.e lsev ie r .com/ locate / inoche

Synthesis of ruthenium (II) complexes containing adihydroperimidine-derived phosphine ligand and theirapplication in transfer hydrogenation of ketones

Qi Fu, Lei Zhang, Tao Yi, Mingjun Zou, Xiaoyan Wang, Haiyan Fu, Ruixiang Li ⁎, Hua ChenKey Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China

⁎ Corresponding author. Fax: +86 28 85412904.E-mail address: liruixiang@scu.edu.cn (R. Li).

1387-7003/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.inoche.2013.10.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 July 2013Accepted 9 October 2013Available online 17 October 2013

Keywords:RutheniumPhosphineTransfer hydrogenationKetone

The new ruthenium (II) phosphine complexes (η6-C6H6)RuCl2[μ- (PPh2CH2N)2CH2(C10H6)]RuCl2(η6-C6H6) (1)and RuCl2(PPh3)[(PPh2CH2N)2CH2(C10H6)] (2) were synthesized and characterized by 1H NMR, 31P {1H} NMRand elemental analysis. Moreover, the structure of ruthenium (II) phosphine complex 1 was confirmed by X-ray crystallography.With complex 1 as the catalyst, the transfer hydrogenation of ketones reactedwell, affordingthe corresponding alcohols in good yields under mild conditions.

© 2013 Elsevier B.V. All rights reserved.

The transfer hydrogenation reaction of ketone has received con-siderable attention in recent years [1]. Compared with commonreduction processes, this process is safe, highly selective, economicand environmentally friendly with few by-products [2]. Since thepioneering work of Meerwein [3,4], a series of transfer hydrogenationcatalysts have been developed. Among Rh [5], Ir [6], Fe [7] and Ru [8]complexes as the transfer hydrogenation catalysts, Ru complexes havebeen proved to be the most active catalysts. With Ru (II) as the centralmetal, a series of ligands including diamines [9], tridentate nitrogenligands [10], amino alcohols [11], aminophosphines [12] and N-heterocyclic carbenes [13] have been developed and their rutheniumcomplexes show excellent activities. However, the ruthenium–

phosphine complexes have been less studied [14]. Although the Ru(II) complexes bearing phosphines, such as the [Ru-η6-benzene-phosphine (TPP, MOTPP, TFTPP)] [15], [RuCl2(η6-arene)-phosphine][16] and [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] complexes [17]have shown excellent catalytic natures [15–24], new ruthenium–

phosphine complexes are still in high demand. As we know, smallstructural variations among these ligandsmay cause significant changesin their coordination behavior and catalytic activity. Considering theimportance of design new ruthenium–phosphine complexes, we syn-thesized two novel Ru complexes containing a dihydroperimidine-derived phosphine ligand [25] and investigated their catalytic propertiesin transfer hydrogenation of ketone. We choose i-PrOH as the hydrogendonor for it is easy to handle and relatively non-toxic [26,27].

All synthetic reactions were performed with standard Schlenk tech-nique under nitrogen atmosphere. Solvents, when necessary, were dried

ghts reserved.

over drying agents and distilled under nitrogen prior to use. [RuCl2(η6-C6H6)]2 [28], RuCl2(PPh3)3 [29] and 1,3-bis[(diphenylphosphanyl)methyl]-2,3-dihydro-1H-perimidine [25] were prepared accordingto reported methods.

1H and 31P {1H} NMR spectra of all complexes were recorded onBruker Avance II 400MHz spectrometer. The chemical shifts of 31P{1H}NMR were relative to 85% H3PO4 as external standard, and 1H NMRrelative to TMS as internal standard.

The single crystal of complex1wasobtained in themixture solvents ofCH3OH and Et2O. Its structure was determined with a Rigaku Saturn 724imaging-plate diffractometer at 113Kwith graphite-monochromatedMoKα radiation (λ=0.71073Å). The structure of complex 1 was solved byMercury 3.0 from the XRD machine. Elemental analysis was performedwith a Carl Erba 1106. The hydrogenation products were analyzed usinga GC Agilent 6890 N equipped with an FID (Flame Ionization Detector)detector and EC-WAX capillary column (30m×0.25mm, 0.25μm film).

1,3-Bis[(diphenylphosphanyl)methyl]-2,3-dihydro-1H-perimidine(0.11 g, 0.2 mmol) and [RuCl2(η6-C6H6)]2 (0.10 g, 0.2 mmol) weresuspended inMeOH (20ml) and stirred at room temperature overnight.The red–orange solid was formed during reaction. Then the solid wasfiltered, washed with MeOH and dried under vacuum to give red–orange powder of complex 1 (0.15 g, 70% yield). 1H NMR (CDCl3): δ4.41 (d, 4H, PCH2, JPH= 7 Hz), 5.83 (s, 2H, NCH2N), 5.31 (s, Ru-C6H6)7.26–7.63 (m, 24H, C6H5 and naphCH). 31P {1H} NMR (161.98MHz): δ21.3 (s). Anal. Calcd. for C49H44Cl4N2P2Ru2: C, 55.12; H, 4.13; N, 2.63.Found: C, 54.6; H, 4.05; N, 2.80.

1,3-Bis[(diphenylphosphanyl)methyl]-2,3-dihydro-1H-perimidine(0.11 g, 0.2 mmol) and RuCl2(PPh3)3 (0.19 g, 0.2 mmol) weresuspended in THF (20ml) under N2 atmosphere and the mixture wasstirred for 20 h. The solid substances were slowly dissolved and the

Scheme 1. Preparation of complex 1.

29Q. Fu et al. / Inorganic Chemistry Communications 38 (2013) 28–32

color of the solution changed from brown to orange–red. Then, thesolution was concentrated to about 2ml under vacuum. When diethylether (10 ml) was added, the red precipitate was formed. Theprecipitate was filtered by a celite pad, washed with diethyl etherand dried under vacuum to afford the red powder of complex 2(0.22 g, 87% yield). 1H NMR (CDCl3): δ 3.47 (d, 4H, PCH2, 2JPH =7 Hz), 5.45 (d, 2H, NCH2N, 2JHH = 7.9 Hz), 6.26 (dd, 2H, naphCH),6.77(m,PPh3), 7.01–7.27 (m, 24H, C6H5 and naphCH). 31P {1H} NMR(161.98MHz): δ −17.7 (ws), 54.3 (t). Anal. Calcd. for C55H47N2P3Cl2-Ru: C, 66.00; H, 4.70; N, 2.8. Found: C, 65.87; H, 4.85; N, 3.22.

A typical procedure for the transfer hydrogenation of acetophenonewas as follows: under nitrogen atmosphere, the Ru complex (5.0μmol),acetophenone (0.5mmol) and i-PrOH (2.0ml) were introduced into aSchlenk tube. After the solution was refluxed for 10 min, the solutionof i-PrOK in i-PrOH (0.1 ml) was added to initiate the transferhydrogenation of acetophenone. The reaction process was monitoredby GC analysis.

The complex 1was prepared by treating [RuCl2(η6-C6H6)]2 with 1,3-bis[(diphenylphosphanyl)methyl]-2,3-dihydro-1H-perimidine in MeOH

Fig. 1. ORTEP representation (ellipsoids at a

at room temperature (in Scheme 1). The formed complex 1 in solid statewas stable to air andmoisture. A singlet at 21.3ppm in 31P NMR revealedthe existence of phosphine and the two phosphorus atoms had the samechemical environment in the complex 1. The singlet at 5.83 ppm in 1HNMR was attributed to the protons of –CH2 between the two nitrogenatoms; the doublet peaks at 4.41ppm were attributed to the protons ofthe –CH2 between the phosphine and the nitrogen atoms; the singletpeak at 5.31 ppm was attributed to the benzene ring coordinated to Ruatom, the ratio of integrated area for the protons was 2:4:12, whichindicated that one ligand coordinated with two ruthenium atoms incomplex 1. Increasing the molar ratio of ligand to Ru and reactiontemperature did not influence the composition and structure of thecomplex 1. The elemental analysis also revealed the binuclear structureof complex 1, which was further confirmed by single crystal X-raydiffraction (Fig. 1).

The crystal structure was consistent with the results of NMR andelemental analysis. The data of the crystal were listed in Tables 1 and2. According to the crystal structure, the Ru atom is coordinated byone phosphine atom, one η6-benzene and two chloride atoms. The

50% level) and labeling for complex 1.

Table 1Crystal data of complex 1.

Formula C49H44Cl4N2P2Ru2

Formula weight(g/mol) 1066.74Temperature/K 293.15Crystal system triclinicSpace group P-1a/Å 7.9848(3)b/Å 14.4057(5)c/Å 19.6366(7)α/° 83.007(3)β/° 80.923(3)γ/° 86.116(3)Volume/Å3 2211.13(14)Z 2ρcalcmg/mm3 1.602m/mm−1 1.035F(000) 1076Crystal size/mm3 0.40× 0.25 × 0.152θ range for data collection 5.7 to 50°Index ranges −9≤ h≤ 9,−17≤ k≤ 17,

−23≤ l≤ 23Reflections collected 16,678Independent reflections 7789[R(int)= 0.0259]Data/restraints/parameters 7789/0/532Goodness-of-fit on F2 1.027Final R indexes [I N=2σ (I)] R1= 0.0337, wR2=0.0695Final R indexes [all data] R1= 0.0483, wR2=0.0754Largest diff. peak/hole/e Å−3 0.643/−0.436

Table 2Selected bond lengths (Å) of complex 1.

Bonds Bond lengths Bonds Bond lengths

Ru1–Cl1 2.4178(10) Ru2–Cl3 2.3982(9)Ru1–Cl2 2.3845(9) Ru2–Cl4 2.4173(9)Ru1– P1 2.3241(9) Ru2–P2 2.3489(9)Ru1– C25 2.219(4) Ru2–C44 2.159(4)Ru1– C26 2.156(4) Ru2–C45 2.149(4)Ru1– C27 2.165(4) Ru2–C46 2.168(4)Ru1– C28 2.154(4) Ru2–C47 2.179(4)Ru1– C29 2.164(4) Ru2–C48 2.233(4)Ru1– C30 2.226(4) Ru2–C49 2.225(4)

30 Q. Fu et al. / Inorganic Chemistry Communications 38 (2013) 28–32

molecule possesses C2 symmetry axial. Although all carbons in thecoordinated benzene ring on each ruthenium are almost the same,there is a little difference in the length of Ru–C bond. The bond lengthof Ru(1)–C(25) (2.219 Å) and Ru(1)–C(30) (2.226 Å) are longer thanRu(1)–C(26) (2.156Å) Ru(1)–C(27) (2.165Å) due to the trans influence

Scheme 2. Preparatio

of phosphine. Similarly, Ru(2)–C(48) (2.233 Å) and Ru(2)–C(49)(2.225 Å) are longer than Ru(2)–C(44) (2.159 Å) and Ru(2)–C(45)(2.149 Å). The average bond length (2.181 Å) of Ru–C in complex 1 isalmost same that (2.188 Å) as the reported analogous complex[(η6-C6H6)RuCl2(μ-BDNA)RuCl2(η6-C6H6)] [31]. Also, Ru(1)–Cl(1)(2.4178 Å) is little longer than Ru(1)–Cl(2) (2.3845 Å) while the bondlength of Ru(2)–Cl(4) (2.4173 Å) is longer than that of Ru(2)–Cl(3)(2.3982Å) due to the trans influence of electron-rich phosphine aroundruthenium. The lengths (2.324 Å and 2.349 Å) of Ru–P bonds are littlelonger than that (2.29 Å, 2.31 Å) in C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2 [17], but they are shorter than that (2.36Å) in [RuCl2(η6-benzene)PPh3] [16]. The results proved that the lengths of Ru–P bondsin complex 1 were in the normal range. The final stoichiometriccomposition of the complex 1 was C45H44Cl4N2P2Ru2, being consistentwith the results of NMR and elemental analysis..According to singlecrystal X-ray diffraction, it can be concluded that the two rutheniumcenters are almost equivalent.

The structure of complex 2 (in Scheme 2) was confirmed by NMRspectrum and elemental analysis. The singlet at δ 5.45 ppm wasattributed to the protons of the –CH2 between the two nitrogenatoms; the doublet at 3.45 ppm was attributed to the protons of the –

CH2 between the phosphorus atom and the nitrogen atom; the ratio ofintegrated area for the protons was 1: 2, the multiples 6.24–7.28 ppmwere attributed to the protons of aromatic ring. A broad singlet at −17.7 ppm in 31P NMR was attributed to the coordinated diphosphinedue to the quickly structural transformation. A triplet at 54.3 ppm in31P NMRwas attributed to the coordinated PPh3 and the small couplingconstant of 19.8 Hz indicated that triphenylphosphine was in cisposition to diphosphine. Furthermore, the result of elemental analysisalso confirmed the composition of complex 2.

With complexes 1 and 2 as the catalyst, the transfer hydrogenationof acetophenone was performed. The base is pivotal for the catalyticreaction which could proceed in the presence of strong base such asi-PrOK or i-PrONa while no conversion was observed without base.According to the reported results [30], the base facilitated the formationof a ruthenium alkoxide by abstracting a proton from the alcohol,followed by a β-elimination to provide a ruthenium hydride. Whencomplex 1 was applied in the reaction, the optimal molar ratio of thecatalyst to the base was found to be 1:5, while it was found to be 1:10for complex 2. It was possible that complex 2 required more base topromote the formation of the ruthenium hydride. As shown inTable 3, with 1mol% of catalyst loading could afford the desired productin 97% and 85% yields within only 0.5 h, respectively (entries 1 and 3).Even decreasing the catalyst loading to 0.1 mol%, moderate yieldscould still be achieved (entries 2 and 4). The complexes 1 and 2 allexhibited high activity in transfer hydrogenation of acetophenone. It

n of complex 2.

Table 3Complexes 1 and 2 catalyzed transfer hydrogenation of acetophenone with i-PrOH assolvent.a.

Entry Complex S:C Time (h) Yield (%) b

1 1 100:1 0.5 972 1 1000:1 2 853 2 100:1 2 (0.5) 85.5 (33.3)4 2 1000:1 2 50.3

aReaction conditions: 82 °C, i-PrOH (2ml),b Determined by GC (three independent catalytic experiments).

Table 4Transfer hydrogenation of ketones catalyzed by complex 1 in i-PrOH.a.

Entry Ketone Time (h) Yield (%)b

1 8 90

2 8 88

3 8 93

4 0.5 92

5 2 71

6 2 79

7 0.5 98

8 2 90

9 2 92

10 0.5 95

11 2 87

12 2 91

31Q. Fu et al. / Inorganic Chemistry Communications 38 (2013) 28–32

should be noted that complex 1 showed a higher beginning activitythan complex 2, which indicated that complex 1 was an effectivecatalyst in transfer hydrogenation of acetophenone. The high efficiencyis possible due to the presence of η6-benzene-ruthenium and electron-donating phosphine. The catalytic activity of complex 1was also higherthan the reported analogous ruthenium phosphine complexes, such as{Ru[η3-2,6-(Cy2PCH2)2C6H3]} [19], [Ru(CO)2(triphos-κ2p)Cl2] [20] and[C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] [21]. This high catalyticactivity might contribute to the presence of N–H bond in active speciesgenerated by complex 1, which could stabilize a seven membered ringtransition state formed by hydrogen bonding with the oxygen atom ofacetophenone [2, 56, 78, 89].

Compared with ruthenium phosphine complex 2, complex 1showed much higher initial activity, though the final conversions ofboth complexes were almost the same (Fig. 2). However, 2 h later, thecomplex 1 had almost no catalytic activity while complex 2 still hadhigh catalytic activity. This might attribute to that the half-sandwichedRu complex was unstable while complex 2 was stable enough in thetransfer hydrogenation process [17].

The catalytic performance of complex 1was further investigated for aseries of ketones and the results were shown in Table 4. According to theresults in Table 4, complex 1 exhibited excellently catalytic activities intransfer hydrogenation of substituted ketones. For o-chloroaceto-phenone a conversion of 98% was achieved within 0.5 h (entry 4). Thereactions were remarkably tolerant of functional groups regardless ofthe electronic property and steric hindrance of the substituents on theacetophenones. Among the substrates investigated, the acetophenoneswith electron-donating groups exhibited higher reactive activity thanacetophenones with electron-donating substituents (entries 1–6 vs 7–14). Additionally, o-methoxyacetophenone underwent the hydro-genation well, affording the desired alcohol in 92% yield within a shorttime (0.5h), owing to the auxiliary coordination ability of ortho positiongroup. Similarly, o-chloroacetophenone reacted more quickly than

Fig. 2. The relationship between the yield of phenylethanol and reaction time. Reactionconditions: acetophenone/i-PrOK/catalyst ratio 1000: 5: 1 for complex 1 and 1000: 10: 1for complex 2, 82 °C; i-PrOH (2ml).

13 4 90

14 2 92

15 2 48

16 2 96

17 2 94

18 0.5 90

aReaction conditions: S/C= 100, i-PrOK (25 μmol), catalyst (5 μmol), i-PrOH(2ml), 82 °C.bGC yield of the alcohol product.

32 Q. Fu et al. / Inorganic Chemistry Communications 38 (2013) 28–32

m-chloroacetophenone and p-chloroacetophenone (entries 4 vs 5–6).Furthermore, α-tetralone and 1-acetonaphthone gave 90–92% yields in2–4 h (entries 13–14). Luckily, alkyl ketones especially linear ketones(entries 16–17), underwent efficient hydrogenation in 94–96% yieldwithin 2–4 h. The results are higher than [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] [18–24], which could only give 34% yield under 1mol %catalyst loading in 10h.

In summary, two new ruthenium (II) complexes based on adiphosphine ligand (1,3-bis[(diphenylphosphanyl)methyl]-2,3-dihydro-1H-perimidine) were synthesized for the first time. Moreover, thestructure of ruthenium (II) phosphine complex 1 was confirmed byX-ray crystallography and characterized by 1H NMR, 31P {1H} NMR andelemental analysis. With complexes 1 and 2 as catalysts, the transferhydrogenation of ketones could get good yields under mild conditions.

We are grateful for the financial support from the National NaturalScience Foundation of China (nos 20371032).

References

[1] G. Brieger, T.J. Nestrick, Catalytic transfer hydrogenation, Chem. Rev. 74 (1974)567–580.

[2] J.S. Samec, J.E. Backvall, P.G. Andersson, P. Brandt, Mechanistic aspects of transitionmetal-catalyzed hydrogen transfer reactions, Chem. Soc. Rev. 35 (2006) 237–248.

[3] H. Meerwein, Liebig. Ein neues Verfahren zur Reduktion von Aldehyden undKetonen, Ann. Chem. 444 (1925) 221–238.

[4] N. Ponndorf, Der reversible Austausch der Oxydationsstufen zwischen Aldehydenoder Ketonen einerseits und primären oder sekundären Alkoholen anderseits,Angew. Chem. 39 (1926) 138–142.

[5] P. Gamez, F. Fache, M. Lemaire, Asymmetric catalytic reduction of carbonylcompounds using C2 symmetric diamines as chiral ligands, Tetrahedron Asymmetry6 (1995) 705–718.

[6] R. Malacea, R. Poli, E. Manoury, Asymmetric hydrosilylation, transfer hydrogenationand hydrogenation of ketones catalyzed by iridium complexes, Coord. Chem. Rev.254 (2010) 729–752.

[7] R.H. Morris, Asymmetric hydrogenation, transfer hydrogenation and hydrosilylationof ketones catalyzed by iron complexes, Chem. Soc. Rev. 38 (2009) 2282–2291.

[8] S.E. Clapham, A. Hadzovic, R.H. Morris, Mechanisms of the H2-hydrogenation andtransfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes,Coord. Chem. Rev. 248 (2004) 2201–2237.

[9] J. Bayardon, D. Sinou, O. Holczknecht, L. Mercs, G. Pozzi, Fluorous derivatives of(1R,2R)-diaminocyclohexane as chiral ligands for metal-catalyzed asymmetricreactions, Tetrahedron Asymmetry 16 (2005) 2319–2327.

[10] W. Ye, M. Zhao, W. Du, Q. Jiang, K. Wu, P. Wu, Z. Yu, Highly active ruthenium (II)complex catalysts bearing an unsymmetrical NNN ligand in the (asymmetric)transfer hydrogenation of ketones, Chem. Eur. J. 17 (2011) 4737–4741.

[11] K. Everaere, A.Mortreux, J.-F. Carpentier, Ruthenium(II)-catalyzed asymmetric transferhydrogenation of carbonyl compounds with 2-propanol and ephedrine-type ligands,Adv. Synth. Catal. 345 (2003) 67–77.

[12] M. Ito, A. Osaku, S. Kitahara, M. Hirakawa, T. Ikariya, Rapid racemization of chiralnon-racemic sec-alcohols catalyzed by (η5-C5(CH3)5)Ru complexes bearing tertiaryphosphine–primary amine chelate ligands, Tetrahedron Lett. 44 (2003) 7521–7523.

[13] F.E. Fernández, M.C. Puerta, P. Valerga, Half-sandwich ruthenium(II) picolyl-NHCcomplexes: synthesis, characterization, and catalytic activity in transfer hydrogenationreactions, Organometallics 30 (2011) 5793–5802.

[14] S.I. Murahashi, Ruthenium in Organic Synthesis, Wiley, Weinheim, 2004. 32–39.[15] L. Wang, Q. Yang, H.-Y. Fu, H. Chen, M.-L. Yuan, R.-X. Li, Ru-η6-benzene-phosphine

complex-catalyzed transfer hydrogenation of ketones, Appl. Organomet. Chem. 25(2011) 626–631.

[16] A. Grabulosa, A. Mannu, E. Alberico, S. Denurra, S. Gladiali, G. Muller, Neutral η6 –

arene ruthenium complexes with monodentate P-donor ligands: activation inthe transfer hydrogenation reaction, J. Mol. Catal. A Chem. 696 (2012)4221–4228.

[17] R. Cerón-Camacho, V. Gómez-Benítez, R. Le Lagadec, D. Morales-Morales, R.A.Toscano, Ketone transfer hydrogenation reactions catalyzed by a phosphiniteruthenium PCP complex: the X-ray crystal structure of [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2], J. Mol. Catal. A Chem. 247 (2006) 124–129.

[18] K.Y. Ghebreyessus, J.H. Nelson, Conformationally rigid diphosphine arene-ruthenium(II) complexes as catalysts for transfer hydrogenation of ketones, J.Organomet. Chem. 669 (2003) 48–56.

[19] D. Amoroso, A. Jabri, G.P.A. Yap, D.G. Gusev, E.N. dos Santos, D.E. Fogg, Thefirst Ru(η3-PCP) complexes of the electron-rich pincer ligand 1,3-bis((dicyclohexylphosphino)methyl)benzene: structure and mechanism in transferhydrogenation catalysis, Organometallics 23 (2004) 4047–4054.

[20] B.J. Sarmah, D.K. Dutta, Chlorocarbonyl ruthenium(II) complexes of tripodal triphos{MeC(CH2 PPh2)3}: synthesis, characterization and catalytic applications intransfer hydrogenation of carbonyl compounds, J. Organomet. Chem. 695 (2010)781–785.

[21] O.R. Allen, L.D. Field, A.M. Magill, K.Q. Vuong, M.M. Bhadbhade, S.J. Dalgarno,Ruthenium complexes of CP3: a new carbon-centered polydentate podand ligand,Organometallics 30 (2011) 6433–6440.

[22] M. Aydemir, A. Baysal, Ruthenium-catalyzed transfer hydrogenation of aromaticketones with aminophosphine or bis(phosphino)amine ligands derived fromisopropyl substituted anilines, Polyhedron 29 (2010) 1219–1224.

[23] M. Aydemir, A. Baysal, B. Gümgüm, A modular design of metal catalysts for thetransfer hydrogenation of aromatic ketones, Appl. Organomet. Chem. 26 (2012)1–8.

[24] M. Aydemir, N. Meric, A. Baysal, Applications of transition metal complexescontaining 3,3’ -bis(diphenylphosphinoam ine)-2,2’-bipyridine ligand to transferhydrogenation of ketones, J. Organomet. Chem. 720 (2012) 38–45.

[25] A.F. Hill, C.M.A. McQueen, Dihydroperimidine-derived N-heterocyclic pincercarbene complexes via double C–H activation, Organometallics 31 (2012)8051–8054.

[26] G. Zassinovich, G. Mestroni, S. Gladiali, Asymmetric hydrogen transfer reactionspromoted by homogeneous transition metal catalysts, Chem. Rev. 92 (1992)1051–1069.

[27] S.E. Clapham, A. Hadzovic, R.H. Morris, Mechanisms of the H2-hydrogenation andtransfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes,Coord. Chem. Rev. 248 (2004) 2201–2237.

[28] G. Winkhaus, H. Singer, Ruthen(II)-komplexe mit zweizähnigem cycloheptatrienund benzol, J. Organomet. Chem. 7 (1967) 487–491.

[29] P.S. Hallman, T.A. Stephenson, G. Wilkinson, Inorg. Synth. 26 (1990) 238–239.[30] S.E. Clapham, A. Hadzovic, R.H. Morris, Mechanisms of the H2-hydrogenation and

transfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes,Coord. Chem. Rev. 248 (2004) 2201–2237.

[31] C.-B. Yu, Y.-Q. Xia, X.-H. Tian, X.-G. Zou, G.-Y. Fan, R.-X. Li, X.-J. Li, K.-C. Tin, N.-B.Wong, Syntheses and crystal structures of binuclear ruthenium complexes bearing1,8’-bis (diphenylphosphinomethyl) naphthalene, J. Organomet. Chem. 691(2006) 499–506.