Pincer CNN Ruthenium(II) Complexes with Oxygen-Containing

May 10, 2013 - in high yield starting from [RuCl(CNN)(dppb)] (1) via substitution of the chloride with sodium ... The corresponding pincer complexes [...
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Organometallics 2009, 28, 4421–4430 DOI: 10.1021/om900274r

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Pincer CNN Ruthenium(II) Complexes with Oxygen-Containing Ligands (O2CR, OAr, OR, OSiR3, O3SCF3): Synthesis, Structure, and Catalytic Activity in Fast Transfer Hydrogenation Walter Baratta,*,† Maurizio Ballico,† Alessandro Del Zotto,† Eberhardt Herdtweck,‡ Santo Magnolia,† Riccardo Peloso,† Katia Siega,† Micaela Toniutti,† Ennio Zangrando,§ and Pierluigi Rigo† †

Dipartimento di Scienze e Tecnologie Chimiche, Universit a di Udine, Via Cotonificio 108, I-33100 Udine, Italy, ‡Department Chemie, Lehrstuhl f€ ur Anorganische Chemie, Technische Universit€ at M€ unchen, Lichtenbergstrasse 4, 85747 Garching, Germany, and §Dipartimento di Scienze Chimiche, Universit a di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy Received April 10, 2009

The pincer complexes [RuX(CNN)(dppb)] (5-11: X=carboxylate, phenoxide, alkoxide, silanolate, triflate; HCNN=1-[6-(40 -methylphenyl)pyridin-2-yl]methanamine; dppb=Ph2P(CH2)4PPh2), containing the Ru-NH2 functionality and a monodentate oxygen donor ligand, have been prepared in high yield starting from [RuCl(CNN)(dppb)] (1) via substitution of the chloride with sodium or thallium compounds or protonation of the alkoxide [Ru(OiPr)(CNN)(dppb)] 3 n(iPrOH) (3). In the solid state the formate 5 shows intermolecular NH 3 3 3 O hydrogen bonds, whereas the acetate 6 displays an intramolecular interaction, as inferred from X-ray studies. Addition of phenol and alcohol compounds to the phenoxide and alkoxide complexes leads to fast ligand exchange through hydrogen bond interactions. The corresponding pincer complexes [RuX(CNN0 )(dppb)] (12, 13b: X = phenoxide, alkoxide; HCNN0 = N,N-dimethyl-1-[6-(40 -methylphenyl)pyridin-2-yl]methanamine), containing the Ru-NMe2 moiety, have been obtained from [RuCl(CNN0 )(dppb)] (4) and sodium phenoxide or alkoxide. All the NH2 complexes 5-11 are highly active catalysts in the transfer hydrogenation of ketones in 2-propanol with NaOiPr, affording complete conversion in a few minutes and achieving TOF values of up to 3.8  106 h-1 with 0.005 mol % of catalyst. Introduction The catalytic hydrogenation (HY)1 with H2 and transfer hydrogenation (TH)2 with 2-propanol or formic acid are fundamental transformations of current academic and industrial relevance for the preparation of alcohols from carbonyl compounds through environmentally benign reaction conditions. In the late 1990s, Noyori and co-workers made a significant breakthrough in both reactions by the *To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) The Handbook of Homogeneous Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Vols. 1-3. (b) Asymmetric Catalysis on Industrial Scale; Blaser, H.-U., Schmidt, E., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (c) Transition Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany, 2004; p 29. (d) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer, M. Adv. Synth. Catal. 2003, 345, 103. (e) Ohkuma, T.; Kitamura, M.; Noyori, R. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; Wiley-VCH: New York, 2000; Chapter 1 . (2) (a) Baratta, W.; Rigo, P. Eur. J. Inorg. Chem. 2008, 4041. (b) Wang, C.; Wu, X.; Xiao, J. Chem. Asian J. 2008, 3, 1750. (c) Morris, D. J.; Wills, M. Chim. Oggi 2007, 25, 11. (d) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (e) Samec, J. S. M.; B€ackvall, J. E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (f ) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. (g) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393. (h) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201. (3) (a) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1998, 37, 1703. (b) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285. r 2009 American Chemical Society

development of highly efficient systems characterized by appropriate designed chiral ligands containing the NH2 function (bifunctional catalysis).3 The high activity of [(η6arene)RuH(H2NCHPhCHPhNTs)]3b in the TH is due to the concerted delivery of a Ru-H hydride and an amine N-H proton to the ketone substrate, affording a 16-electron ruthenium amide species and the alcohol, through an outersphere mechanism. Notably, DFT calculations indicate that the reaction of the ketone with the amino Ru hydride leads to an amino Ru alkoxide, which is regarded as a catalyst reservoir, stabilized by an intramolecular NH 3 3 3 O hydrogen bond.4 A realistic modeling of the TH in solution with arene Ru amine species has been proposed by Handgraaf and Meijer to lead to a concerted solvent-mediated mechanism with the substrate appearing as alkoxide-like intermediate.5 Interestingly, Hamilton and Bergens provided evidence of the formation of the ruthenium alkoxide [RuH(OR)(BINAP)(dpen)], from [Ru(H)2(BINAP)(dpen)] and ketone, as a putative step in the catalytic HY of carbonyl compounds.6 (4) (a) Alonso, D. A.; Brandt, P.; Nordin, S. J. M.; Andersson, P. G. J. Am. Chem. Soc. 1999, 121, 9580. (b) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466. (c) Petra, D. G. I.; Reek, J. N. H.; Handgraaf, J. W.; Meijer, E. J.; Dierkes, P.; Kamer, P. C. J.; Brussee, J.; Schoemaker, H. E.; van Leeuwen, P. W. N. M. Chem. Eur. J. 2000, 6, 2818. (5) Handgraaf, J. W.; Meijer, E. J. J. Am. Chem. Soc. 2007, 129, 3099. (6) Hamilton, R. J.; Bergens, S. H. J. Am. Chem. Soc. 2008, 130, 11979. Published on Web 07/08/2009

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Recently, we found that the phosphane ruthenium complexes cis-[RuCl2(PP)(Pyme)]7 (PP=diphosphane), based on 1-(pyridin-2-yl)methanamine (Pyme), are among the most efficient asymmetric TH catalysts.3b,8 A more productive system, obtained using 1-[6-(40 -methylphenyl)pyridin-2-yl]methanamine (HCNN), is the terdentate CNN complex [RuCl(CNN)(dppb)]9 (1: dppb = Ph2P(CH2)4PPh2) which promotes the TH of ketones and aldehydes in basic 2-propanol with extremely high TOF10 and TON values (Figure 1).11 The chiral version of this complex, namely [RuCl(CNN*)(PP)] (HCNN*=1-substituted 1-(6-arylpyridin-2-yl)methanamine; PP = Josiphos), afforded high rate, productivity, and enantioselectivity in the TH and also HY of alkyl-aryl ketones.12 Surprisingly, the analogous osmium complexes [OsCl2(PP)(Pyme)] and [OsCl(CNN*)(PP)] were proven to show similar or even higher activity in the asymmetric HY and TH of ketones.13 The identical sense and degree of enantioselectivity for both HY and TH reactions suggests the involvement of the same metal hydride intermediates.14 Evidence has been provided that the ruthenium hydride 2 in the presence of acetone rapidly equilibrates with the alcohol adduct isopropoxide 3 ( -30 °C, whereas the derivatives [RuH(O2CH)(PPh3)2(NN)] (NN = H2NCMe2CMe2NH2, Pyme)21 are stable at room temperature. The CNN pincer formate complex [Ru(O2CH)(CNN)(dppb)] (5) has been prepared by reaction of 1 with HCOOTl in THF at 35 °C and isolated in 76% yield (eq 1).

The 1H NMR signal of one NH2 proton is significantly shifted downfield at δ 6.16 (CD2Cl2), consistent with an intramolecular NH 3 3 3 O hydrogen bond interaction with the formate ligand. In C6D6 the HCO2 proton appears as a doublet at δ 8.88 with 4J(PH) = 8.6 Hz. The carbonyl (17) (a) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35, 44. (b) Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. In Alkoxo and Aryloxo Derivatives of Metals; Academic Press: New York, 2001. (c) Bryndza, H. E.; Tam, W. Chem. Rev. 1988, 88, 1163. (18) (a) Yin, X.; Moss, J. R. Coord. Chem. Rev. 1999, 181, 27. (b) Gibson, D. H. Coord. Chem. Rev. 1999, 185-186, 335. (c) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 344. (d) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259. (e) Aresta, M.; Quaranta, E.; Tommasi, I. New J. Chem. 1994, 18, 133. (f ) Whittlesey, M. K.; Perutz, R. N.; Moore, M. H. Organometallics 1996, 15, 5166. (g) Burling, S.; Kociok-K€ohn, G.; Mahon, M. F.; Whittlesey, M. K.; Williams, J. M. J. Organometallics 2005, 24, 5868. (19) (a) Espinet, P.; Albeniz, A. C. In Fundamentals of Molecular Catalysis; Kurosawa, H., Yamamoto, A., Eds.; Elsevier: Amsterdam, 2003; Current Methods in Inorganic Chemistry Vol. 3, Chapter 6, p 332. (b) Eisenberg, R.; Hendriksen, D. E. Adv. Catal. 1979, 28, 79. (20) Koike, T.; Ikariya, T. Adv. Synth. Catal. 2004, 346, 37. (21) (a) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104. (b) Abdur-Rashid, K.; Abbel, R.; Hadzovic, A.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2005, 44, 2483.

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Figure 1

Figure 2. Diamond23 ball and stick plot of compound 5 in the solid state. The dotted lines indicate the possible inter- and intramolecular hydrogen bonds.

stretching of 5 appears at 1611 cm-1, in agreement with the presence of a monodentate formate ligand.18a,22 It is worth noting that compound 5 is apparently stable in C6D6 at room temperature and no significant formation of the hydride 2 by β-hydrogen elimination was observed, even at 60 °C by NMR. Conversely, in CD2Cl2 compound 5 converts partially into 1 in a few days, possibly through the hydride 2, which is chlorinated by the solvent.9b The molecular structure of 5 was confirmed by X-ray analysis carried out on a single crystal (Figure 2). The selected bond distances and angles are reported in Table 1. Unexpectedly, the atom O2 is not involved in an intramolecular hydrogen bond interaction with a N-H proton, but it points toward a N-H proton of another molecule of the complex via an intermolecular hydrogen bond (N2 3 3 3 O20 = 2.869(3) A˚), building up a one-dimensional infinite chain parallel to the crystallographic b axis. The methylene carbon C6 and the amino nitrogen N2 are displaced by -0.135(3) and þ0.341(2) A˚, respectively, from the best-fit plane through the terdentate ligand. This arrangement leads one (22) (a) Field, L. D.; Lawrenz, E. T.; Shaw, W. J.; Turner, P. Inorg. Chem. 2000, 39, 5632. (b) Albeniz, M. J.; Esteruelas, M. A.; Lledos, A.; Maseras, F.; O~ nate, E.; Oro, L. A.; Sola, E.; Zeier, B. J. Chem. Soc., Dalton Trans. 1997, 181.

N-H bond to be almost parallel to the Ru-O1 bond (H2N2-Ru-O1 dihedral angle of about 8°, with a H2 3 3 3 O1 distance of 2.41 A˚), suggesting a possible weak intramolecular hydrogen bond interaction between N2-H2 and O1.24 In order to prepare ruthenium derivatives displaying a Ru-O bond, we found that the isopropoxide species [Ru(OiPr)(CNN)(dppb)] 3 n(iPrOH) (3), obtained from the chloride 1 and NaOiPr (1.5 equiv) in 2-propanol, is a suitable intermediate which undergoes fast protonation with acidic H-O compounds. An excess of NaOiPr was used to achieve the complete chloride displacement of 1 and to preserve complex 3 during the filtration on Celite to remove NaCl. The acetate derivative [Ru(O2CCH3)(CNN)(dppb)] (6) was easily prepared in 88% yield, by reaction of 3 with acetic acid in excess, also to eliminate the remaining NaOiPr (Scheme 1). The two resonances of the NH2 group are at δ 8.40 and 1.41, as inferred from a 1H-1H COSY experiment, the lowfield resonance being consistent with an intramolecular NH 3 3 3 O hydrogen bond interaction with the acetate ligand. (23) Brandenburg, K.; Diamond, Version 3.1d; Crystal Impact GbR, Bonn, Germany, 2006. (24) (a) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (b) Brammer, L. Dalton Trans. 2003, 3145. (c) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton, E. A.; Orpen, A. G. Chem. Commun. 1998, 653.

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Table 1. Selected Bond Distances (A˚) and Angles (deg) of Complexes 5 and 6 formate 5

acetate 6 3 1.5C6H6

Ru-O1 Ru-N1 Ru-N2 Ru-C7 Ru-P1 Ru-P2

2.170(2) 2.050(2) 2.218(2) 2.060(3) 2.2907(9) 2.2396(9)

2.162(2) 2.058(2) 2.236(3) 2.060(3) 2.2879(11) 2.2496(10)

O1-Ru-N1 O1-Ru-N2 O1-Ru-C7 O1-Ru-P1 O1-Ru-P2 N1-Ru-N2 N1-Ru-C7 N1-Ru-P1 N1-Ru-P2 N2-Ru-C7 N2-Ru-P1 N2-Ru-P2 C7-Ru-P1 C7-Ru-P2 P1-Ru-P2

87.29(8) 77.35(8) 92.48(10) 85.84(6) 179.34(6) 76.60(9) 79.64(11) 171.11(7) 92.52(7) 154.49(11) 96.37(6) 103.22(6) 106.28(10) 86.86(9) 94.41(3)

85.32(9) 85.66(10) 83.23(10) 87.81(7) 168.19(7) 76.99(11) 79.70(12) 170.77(7) 92.40(7) 154.88(12) 96.37(8) 105.14(8) 105.61(10) 84.96(8) 95.56(4)

The carbonyl stretch of 6 appears at 1713 cm-1, which is at higher wavelength with respect to that of the formate 5 (Δν=102 cm-1). The molecular structure of 6 obtained from an X-ray analysis is shown in Figure 3, and the selected bond distances and angles are reported in Table 1. In contrast to the case for the formate 5, the solid-state study of 6 revealed the presence of an intramolecular hydrogen bond interaction of the acetate with an axial N-H proton, leading to a six-membered cycle (N2 3 3 3 O2= 2.817(5) A˚). Also in 6 one N-H bond is almost parallel to the Ru-O1 bond (H1a-N2-Ru-O1 dihedral angle of about -4°, with an H 3 3 3 O distance of 2.61 A˚). Intramolecular hydrogen bonds have been observed for acetate complexes containing the NH2 function.20,25 Attempts to prepare the formate 5 by protonation of 3 with formic acid in 2-propanol failed, resulting in a mixture of different species, including 5, and decomposition of HCO2H. The data in solution and in the solid state for the formate 5 and acetate 6 indicate that the carboxylate ligands interact with the N-H functionality by hydrogen bonds. Both Ru-O and CdO oxygen atoms are involved in the interaction, with the latter moiety leading to both inter- and intramolecular hydrogen bonds in the solid state. Phenoxide and Alkoxide Complexes. The phenoxide complexes [Ru(OAr)(CNN)(dppb)] can be quickly prepared in high yield by protonation of 3 with phenols. Treatment of 3 with 1 equiv of 4-nitrophenol in a 2-propanol/toluene (1:1) mixture at room temperature leads to the complex 7 in 87% yield (Scheme 1). In the 1H NMR spectrum the meta and ortho protons of the phenoxide appear as doublets at δ 8.00 and 5.92 (3J(HH)=9.3 Hz), the latter resonance being shifted upfield with respect to the free 4-nitrophenol (Δδ 1.04), whereas one NH2 proton appears at δ 3.50. Similarly to the synthesis of 7, treatment of 3 with 4-fluoro3-methylphenol leads to the complex 8 in 91% yield. In the 19 F NMR spectrum the coordinated phenoxide shows a resonance at δ -138.9, shifted upfield compared to the free phenol at δ -132.6. When a solution of 8 with 1 equiv (25) (a) Kuznetsov, V. F.; Yap, G. P. A.; Alper, H. Organometallics 2001, 20, 1300. (b) Wong, W.-K.; Lai, K.-K.; Tse, M.-S.; Tse, M.-C.; Gao, J.-X.; Wong, W.-T.; Chan, S. Polyhedron 1994, 13, 2751.

Figure 3. Diamond ball and stick plot of compound 6 in the solid state. The dotted line indicates a possible intramolecular hydrogen bond.

of 4-fluoro-3-methylphenol is heated, the two 19F NMR signals broaden (Δν1/2=208, 385 Hz, for δ -138.9, -132.6, respectively, at 60 °C). The 19F NMR spectra show that during addition of 4-fluoro-3-methylphenol the former resonance decreases and with 10 equiv of the phenol disappears, leading to a broad signal at δ -130.0, close to that of the free phenol. In the 31P NMR spectrum the doublets at δ 60.4 and 38.6 (2J(PP) = 36.9 Hz) broaden and shift to δ 59.9 and 43.4, respectively. These data are in agreement with a dynamic process involving a rapid exchange of the phenoxide with the phenol, possibly through a hydrogen bond interaction, as reported for other phenoxide complexes.17a,26 In addition to the protonation reaction, a second important feature of the isopropoxide 3 is the fast and reversible β-hydrogen elimination that leads to the hydride 2 and acetone.15a Thus, the alkoxide 9 can be promptly prepared in 92% yield at room temperature by reaction of 3 with 1 equiv of 3,30 -dinitrobenzophenone, in 2-propanol/ toluene (Scheme 1). It is worth noting that the presence of electron-withdrawing groups in the alkoxide ligand stabilizes 9 with respect to β-hydrogen elimination. In the 1H NMR spectrum the doublet at δ 5.20 is for the OCH group, showing a coupling constant with a phosphorus atom (4J(HP)=3.1 Hz), indicating that the alkoxide is directly bound to the metal center. The broad signal at δ 4.69 is for one proton of the NH2 group, consistent with a NH 3 3 3 O hydrogen bond that stabilizes the alkoxide 9. It is worth noting that these Ru-O bond containing complexes are moisture sensitive and therefore must be stored under an inert atmosphere. Moreover, these compounds are characterized by a certain degree of intramolecular N-H 3 3 3 O hydrogen bonding, but no proton transfer leading to the formation of the corresponding 16-electron Ru amide species was observed. (26) (a) Clapham, S. E.; Guo, R.; Zimmer-De Iuliis, M.; Rasool, N.; Lough, A. J.; Morris, R. H. Organometallics 2006, 25, 5477. (b) Koike, T.; Ikariya, T. Organometallics 2005, 24, 724. (c) Kapteijn, G. M.; Dervisi, A.; Grove, D. M.; Kooijman, H.; Lakin, M. T.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1995, 117, 10939. (d) Osakada, K.; Ohshiro, K.; Yamamoto, A. Organometallics 1991, 10, 404. (e) Kim, Y. J.; Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc. 1990, 112, 1096. (f ) Kegley, S. E.; Schaverien, C. J.; Freudenberger, J. H.; Bergman, R. G.; Nolan, S. P.; Hoff, C. D. J. Am. Chem. Soc. 1987, 109, 6563.

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Scheme 1

Silanolate and Triflate Complexes. Silicone greases, which are widely used in the laboratory for lubricating glassware and for operations that require low pressure, are polysiloxanes of the chemical formula [R2SiO]n (R=alkyl, aryl). Interestingly, we observed that the alkoxide 3 slowly reacts with silicone grease, leading to a series of new complexes, displaying in the 31P{1H} NMR spectra doublets centered at about δ 60 and 40 with 2J(PP)=36-37 Hz and 1H NMR singlets close to δ 0. These species can be reconverted to 3 using an excess of sodium isopropoxide in alcohol. It is worth noting that treatment of polysiloxanes with strong bases affords silanolates27 and that silanols R3SiOH display a higher acidity28 than the corresponding alcohols and therefore can react with the alkoxide ligands. A number of examples of reactions in which silicone grease became involved as a reactant (e.g., “R2SiO” insertion into the M-X bond) has been reviewed.29 On account of these results and the implications in catalysis, we decided to prepare a pincer ruthenium complex displaying a Ru-OSi bond.30 The ruthenium silanolate [Ru(OSiMe3)(CNN)(dppb)] (10)  cı´ k, J.; Rankin, S. E.; Kirchner, S. J.; McCormick, A. V. (27) Sef J. Non-Cryst. Solids 1999, 258, 187. (28) Damrauer, R.; Simon, R.; Krempp, M. J. Am. Chem. Soc. 1991, 113, 4431. (29) Haiduc, I. Organometallics 2004, 23, 3. (30) (a) Marciniec, B.; Maciejewski, H. Coord. Chem. Rev. 2001, 223, 301. (b) Johnson, T. J.; Folting, K.; Streib, W. E.; Martin, J. D.; Huffman, J. C.; Jackson, S. A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995, 34, 488. (c) Poulton, J. T.; Folting, K.; Streib, W. E.; Caulton, K. G. Inorg. Chem. 1992, 31, 3190.

was easily prepared in high yield (74%) by treatment of 1 with NaOSiMe3 in toluene at room temperature in 1 h (eq 2).

Complex 10 displays two 31P NMR doublets at δ 59.3 and 39.8 (2J(PP)=36.8 Hz). In the 1H NMR spectrum one NH2 proton appears at δ 4.50, which is consistent with a NH 3 3 3 O hydrogen bond interaction, whereas the OSiMe3 signal is shifted upfield at δ -0.17. Interestingly, treatment of the silanolate 10 with 1 equiv of NaOiPr in 2-propanol leads promptly to the quantitative formation of 3. These results suggest that when silicon grease is employed during the catalytic TH, the use of an excess of base has a beneficial effect, inhibiting the formation of pincer Ru silanolates. The triflate ligand can be regarded as one of the less coordinating oxygen donor ligands, affording complexes characterized by an ionic or highly polarized covalent M-O bond,31 (31) (a) Lawrance, G. A. Chem. Rev. 1986, 86, 17. (b) Beck, W.; S€unkel, K. Chem. Rev. 1988, 88, 1405.

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and several ruthenium derivatives have been reported.32 The pincer triflate complex [Ru(O3SCF3)(CNN)(dppb)] (11) is formed in 80% yield by treatment of 1 with 1 equiv of CF3SO3Tl in CH2Cl2 at room temperature (2 h) (eq 3).

Attempt to prepare 11 using CF3SO3Ag failed, leading to a blue solution containing uncharacterized species, possibly as a result of a redox process. Complex 11 is thermally stable and highly soluble in different organic solvents (CH2Cl2, iPrOH, benzene). The broad resonance at δ 4.21 (toluene-d8) is for one NH2 proton involved in an intramolecular N-H 3 3 3 O triflate hydrogen bond interaction.33 The 19F{1H} NMR spectrum of 11 shows a broad singlet at δ -73.5, shifted downfield with respect to the free triflate anion (δ ∼-80.0).34 Upon heating, the two 31P doublets (δ 68.3 and 43.0) broaden and coalesce at 100 °C to an averaged peak at δ 51.5. At this temperature the 1H NMR spectrum shows a triplet at δ 3.52 (J(HH) = 4.8 Hz) for the two NCH2 protons, suggesting the existence of a fast equilibrium of 3 with a fivecoordinate cationic species resulting from the dissociation of the triflate. Below 0 °C, the NMR measurements reveal the presence of an equilibrium between two six-coordinate triflate complexes. At -60 °C, the 31P{1H} NMR spectrum shows two doublets at δ 64.1 and 42.8 (2J(PP)=41.2 Hz) in addition to two doublets at δ 68.4 and 43.6 (2J(PP)=42.2 Hz) in a 1:2 molar ratio, whereas the 19F{1H} NMR spectrum displays two singlets at δ -73.6 and -72.8 (1:2 ratio). This has tentatively been ascribed to the formation at low temperature of two diastereomer complexes which may arise from a N-H 3 3 3 O triflate hydrogen bond33 with a chiral sulfur atom. Notably, the formation of diastereomer complexes [Ru(OR)(CNN)(PP)] with alkoxide ligands has previously been established by 31P and 19F NMR measurements.15a Complex 11, showing a highly polarized Ru-O bond, promptly reacts with 1 equiv of NaOiPr in 2-propanol at room temperature, leading to the alcohol adduct 3. NMe2 Phenoxide and Alkoxide Ru Complexes. The reactivity of the NMe2 derivative 4 toward alkaline alkoxide and phenoxide was also investigated, and the properties of the resulting Ru-O bond containing complexes were compared to those of the related NH2 derivatives. Treatment of 4 with KOiPr in toluene/2-propanol leads to the intermediate [Ru(OiPr)(CNN0 )(dppb)] (13a:9b HCNN0 = N,N-dimethyl(32) (a) Geldbach, T. J.; R€ uegger, H.; Pregosin, P. S. Magn. Reson. Chem. 2003, 41, 703. (b) Belli Dell'Amico, D.; Calderazzo, F.; Grazzini, A.; Labella, L.; Marchetti, F. Inorg. Chim. Acta 2002, 334, 411. (c) Bickley, J. F.; Higgins, S. J.; Stuart, C. A.; Steiner, A. Inorg. Chem. Commun. 2000, 3, 211. (d) Feher, F. J.; Baldwin, R. K.; Ziller, J. W. Acta Crystallogr. 2000, C56, 633. (e) Mahon, M. F.; Whittlesey, M. K.; Wood, P. T. Organometallics 1999, 18, 4068. (f ) Blosser, P. W.; Gallucci, J. C.; Wojcicki, A. Inorg. Chem. 1992, 31, 2376. (33) (a) Pavlik, S.; Mereiter, K.; Puchberger, M.; Kirchner, K. Organometallics 2005, 24, 3561. (b) Fossey, J. S.; Matsubara, R.; Vital, P.; Kobayashi, S. Org. Biomol. Chem. 2005, 3, 2910. (c) Arnold, D. I.; Cotton, F. A.; Matonic, J. H.; Murillo, C. A. Polyhedron 1997, 16, 1837. (d) Feazell, R. P.; Carson, C. E.; Klausmeyer, K. K. Eur. J. Inorg. Chem. 2005, 3287. (34) Jones, V. A.; Sriprang, S.; Thornton-Pett, M.; Kee, T. P. J. Organomet. Chem. 1998, 567, 199.

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1-[6-(40 -methylphenyl)pyridin-2-yl]methanamine), which reacts with 4-fluoro-3-methylphenol, affording the phenoxide 12 in 82% yield (Scheme 2). In the 1H NMR spectrum of 12, the CH2N protons appear as two doublets, whereas the two N-methyl groups give a relatively sharp singlet at δ 1.97. This can be ascribed to the hemilabile terdentate ligand containing the NMe2 group, which undergoes a pyramidal inversion at the nitrogen atom, through decoordination from the Ru metal center, favored by the presence of the ortho-metalated aryl group that exerts a strong trans influence.35 Despite the fact the N-alkyl substituents should increase the σ-donating properties of the N atom, it is generally accepted that the metal-nitrogen bond is intrinsically weaker for tertiary amine ligands compared to the primary or secondary ligands.36 The 31P{1H} NMR spectrum of 12 shows two doublets at δ 52.4 and 39.7 and a small singlet at δ 33.2, the last signal being consistent with the presence of a five-coordinate species. The presence of a NMe2 function, instead of NH2, excludes the formation of a 16-electron amide species. Addition of 4-fluoro3-methylphenol results in a slight broadening of the 31P NMR doublets and an increase of the singlet, which progressively shifts downfield (δ 35.4 with ArOH/12=5). The 19 F NMR spectrum shows two broad signals at δ -139.6 (for 12) and -138.0, with the latter resonance shifting to the value for the free phenol (δ -132.6). These data are consistent with the presence of an equilibrium between 12 and a cationic fivecoordinate species, with the phenoxide stabilized by a hydrogen bond with the free phenol (Scheme 2).24a,37 The alkoxide derivative 13b, lacking β-hydrogens, was isolated in 74% yield by reaction of KOtBu with 4 in toluene/tBuOH (Scheme 2). In the 1H NMR spectrum of 13b in C6D6, the protons of the CH2N group appear as two doublets, whereas the two N-methyl groups give a relatively sharp singlet at δ 2.00. The 31P{1H} NMR spectrum of 13b displays two doublets at δ 48.3 and 35.4 and a small singlet at δ 32.1. Similarly to 12, addition of tBuOH to 13b results in a slight broadening of the doublets and an increase of the singlet, which shifts downfield (up to δ 34.6). As for 12, these data are consistent with an equilibrium involving the complex 13b and a cationic five-coordinate species in which the alkoxide interacts with the alcohol. Attempts to isolate the related isopropoxide [Ru(OiPr)(CNN0 )(dppb)] (13a) failed, on account of the slow conversion to the hydride [RuH(CNN0 )(dppb)], possibly through a β-hydrogen elimination reaction.9b These data suggest that the NMe2 six-coordinate alkoxide and phenoxide ruthenium complexes react with ROH and ArOH, respectively, leading to an equilibrium reaction in which five-coordinate species are formed. When a NH2 function is present on the ruthenium center, the formation of five-coordinate species is prevented by stabilization of the labile six-coordinate alkoxide and phenoxide species through NH 3 3 3 O hydrogen bonds. Thus, for complexes displaying the X-M-N-H motif, the intramolecular H 3 3 3 X interaction may play a significant role in the stabilization of these species. (35) (a) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In Principles and Application of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (36) Meyerstein, D. Coord. Chem. Rev. 1999, 185-186, 141. (37) Buzzeo, M. C.; Zakharov, L. N.; Rheingold, A. L.; Doerrer, L. H. J. Mol. Struct. 2003, 657, 19.

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Scheme 2

Table 2. Catalytic Transfer Hydrogenation of MeCOPh (0.1 M) in 2-Propanol with Pincer Ru Complexes (Ru=0.005 mol %, NaOiPr=2 mol %) complex

T (°C)

Conv. %a

Time (min)

TOF (h-1)

5 6 7 8 9 9a 9b 10 11 1 9b 11

82 82 82 82 82 82 82 82 82 40 40 40

97 97 98 96 98 97 98 98 98 94 92 92

2 min 2 min 5 min 5 min 5 min 5 min 5 min 5 min 5 min 2h 2h 2h

1.4  106 1.8  106 1.1  106 6.0  105 8.7  105 8.0  105 1.0  106 9.4  105 9.3  105 2.3  104 1.8  104 2.1  104

a

The conversion was determined by GC analysis.

TH of Ketones Catalyzed by Pincer Ru Complexes. All the NH2 pincer complexes containing a Ru-O bond have been found to be exceptionally active catalysts for TH of acetophenone in 2-propanol and in the presence of NaOiPr (2 mol %) (eq 4).

Complete conversion has been achieved within 5 min using 0.005 mol % of the pincer complexes at 82 °C (Table 2). The formate (5) and acetate derivatives (6) display turnover frequencies10 of 1.4106 and 1.8106 h-1, respectively. These values are higher than that of the chloride 1 (TOF= 1.1  106 h-1)9a and indicate that these carboxylate species are among the most efficient TH systems.11 The phenoxides 7 and 8 give TOF values of 1.1106 and 6.0105 h-1, whereas the rate for the alkoxides 9a9a and 9b15a ranges from 8.0  105 to 1.0  106 h-1 (Figure 4).

The silanolate (10) and the labile triflate complexes (11) afford similar values (TOF = 9.4  105 and 9.3  105 h-1, respectively), close to those of the alkoxide complexes. The catalytic and stoichiometric studies on 10 may suggest that the use of an excess of NaOiPr has a positive effect on catalysis, preventing the formation of Ru silanolate complexes from accidental reaction with silicon grease. It is worth noting that the compounds 5, 6, 10, and 11 should be considered better catalytic precursors, with respect to the moisture-sensitive phenoxide and alkoxide compounds 7-9. The highly active acetate 6 has been proven to efficiently catalyze the quantitative reduction of different substrates, namely 3-bromoacetophenone, 2-acetonaphthone, and cyclohexanone in less than 1 min and with TOFs in the range of (1.7-3.8)  106 h-1, the latter being the highest value reported to date (Table 3). In addition, the substrate 5-hexen-2-one has quickly and quantitatively been reduced to the corresponding alcohol without hydrogenation of the CdC bond. The phenoxide (12) and the alkoxide complexes (13b) displaying the NMe2 group are poor catalysts in 2-propanol, leading to incomplete conversion of acetophenone (25 and 30% after 2 h, respectively), likely through a inner-sphere mechanism involving NMe2 displacement,2e,9a,9b indicating that the presence of the NH2 functionality is crucial for obtaining high performance. In order to investigate the influence of the ligand X of [RuX(CNN)(dppb)] in the catalysis, the activity of 1 was compared with that of the more labile alkoxide (9b) and triflate complexes (11) at 40 °C. Compound 1 shows a TOF=2.3104 h-1, a value slightly higher than those found for 9b and 11 (TOF=1.8  104 and 2.1  104 h-1), indicating that the displacement of X by OiPr in 2-propanol is a rapid step even at low temperature. This result is in agreement with our kinetic studies, which showed that the chloride 1, the hydride 2, and the isopropoxide 3 display much the same activity in basic media, C-H bond cleavage being the ratedetermining step.15b It is worth noting that the Ru-O pincer

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species. Apparently, the carboxylate, silanolate, and triflate compounds appear to be superior catalytic precursors, with respect to the moisture-sensitive alkoxide and phenoxide complexes.

Experimental Section

Figure 4 Table 3. Catalytic Transfer Hydrogenation of Ketones (0.1 M) with Complex 6 (0.005 mol %, NaOiPr=2 mol %) at 82 °C

a

The conversion was determined by GC analysis.

complexes 5-11 show no catalytic activity without base and an excess of NaOiPr is crucial to achieve fast TH. These catalytic results suggest that, in basic 2-propanol solutions, the derivatives 5-11 are catalytic precursors and give rapid displacement of the oxygen-containing ligand, affording the catalytically active hydride (2) and isopropoxide species (3), in agreement with the previously proposed inner-outer sphere mechanism involving 2-propanol.15

Concluding Remarks In summary, we have isolated and characterized a series of pincer complexes containing a Ru-O bond of formula [RuX(CNN)(dppb)] (X=carboxylate, phenoxide, alkoxide, silanolate, triflate). The solid-state structures and the studies in solution revealed that the six-coordinate NH2 pincer complexes are stabilized by intramolecular and intermolecular N-H 3 3 3 O bond interactions. Reaction of the phenoxide and alkoxide complexes with phenol and alcohol compounds leads to the fast exchange of the ligands. Conversely, the phenoxide and alkoxide NMe2 pincer complexes, which are not stabilized by intramolecular hydrogen bonds, react with H-O acidic compounds, leading to five-coordinate species. While the NMe2 derivatives display poor catalytic activity in transfer hydrogenation, all the NH2 pincer complexes are highly active catalysts, with TOF values in the range of 6.0 105 to 3.8  106 h-1 for the reduction of ketones in basic 2-propanol. These data suggest that the pincer ruthenium complexes with a Ru-O bond can be used as efficient catalytic precursors, on account of their fast conversion into the catalytically active Ru-H/Ru-OiPr

All reactions were carried out under an argon atmosphere using standard Schlenk techniques. The solvents were carefully dried by standard methods and distilled under argon before use. The ruthenium complexes 1, 4, 9a,9b and 9b15a were prepared according to literature procedures, whereas all other chemicals were purchased from Aldrich and used without further purification. NMR measurements were recorded on a Bruker AC 200 spectrometer. 2D COSY experiments were performed using the standard pulse sequences from the Bruker library. Chemical shifts, in ppm, are relative to TMS for 1H and 13C{1H}, whereas 85% H3PO4 was used for 31P{1H} and CFCl3 for 19F{1H}. Infrared measurements were obtained using a Bruker Vector 22 FT-IR spectrometer. Elemental analyses (C, H, N) were carried out with a Carlo Erba 1106 elemental analyzer. Synthesis of 5. Complex 1 (340 mg, 0.447 mmol) was dissolved in THF (20 mL), and HCOOTl (400 mg, 1.604 mmol) was added. The resulting suspension was stirred overnight at 35 °C and filtered on Celite, and the solid on the frit was washed with CH2Cl2 (45 mL). The filtrate was concentrated (3 mL), and pentane was added (10 mL). The orange precipitate was filtered, washed with pentane (22 mL), and dried under reduced pressure. Yield: 260 mg (76%). Anal. Calcd for C42H42N2O2P2Ru: C, 65.44; H, 5.50; N, 3.64. Found: C, 65.01; H, 5.70; N, 3.32. 1H NMR (200.1 MHz, CD2Cl2, 20 °C): δ 8.02-7.92 (m, 3H; aromatic protons and HCO2), 7.58 (t, J(HH)=7.6 Hz, 3H; aromatic protons), 7.52-7.10 (m, 14H; aromatic protons), 7.03 (d, J(HH)=8.1 Hz, 1H; aromatic proton), 6.79 (t, J(HH)= 7.9 Hz, 2H; aromatic protons), 6.63 (d, J(HH) = 6.6 Hz, 2H; aromatic protons), 6.16 (td, J(HH) = 11.3, 5.3 Hz, 1H; NH2), 6.02 (t, J(HH) = 8.0 Hz, 2H; aromatic protons), 4.13 (dd, 2 J(HH) = 15.1 Hz, 3J(HH) = 4.4 Hz, 1H; CH2N), 3.75-3.55 (m, 1H; CH2N), 3.20-2.80 (m, 2H; PCH2), 2.20 (s, 3H; CH3), 2.50-0.75 (m, 7H; CH2 and NH2). 13C{1H} NMR (50.3 MHz, CD2Cl2, 20 °C): δ 182.8 (dd, 2J(C,P)=16.3, 8.1 Hz; CRu), 171.0 (s; HCO2), 163.7 (s; NCC), 158.4 (s; NCCH2), 148.9-115.0 (m; aromatic carbon atoms), 52.8 (d, 3J(C,P)=3.0 Hz; CH2N), 31.8 (d, 1J(C,P)=24.0 Hz; CH2P), 31.3 (d, 1J(C,P)=32.1 Hz; CH2P), 26.5 (s; CH2), 22.1 (s; CH2), 21.7 (s, CH3). 31P{1H} NMR (81.0 MHz, CD2Cl2, 20 °C): δ 60.0 (d, 2J(PP)=38.3 Hz), 42.7 (d, 2J(PP)=38.3 Hz). IR (Nujol): ν 1611 cm-1 (asymmetric ν(OCO)). Synthesis of 6. To a suspension of 1 (218 mg, 0.287 mmol) in toluene (4.8 mL) was added 4.8 mL of a 2-propanol solution of NaOiPr (0.1 M, 0.480 mmol), and the mixture was stirred at 60 °C for 2 h. The suspension was kept at -20 °C for 6 h and filtered on Celite. Acetic acid (35.0 μL, 0.612 mmol) was added to the filtrate, and the solution was stirred for 30 min at room temperature, concentrated to 5 mL, and kept at -20 °C for 2 h. After filtration on Celite, the solution was concentrated (1 mL) and addition of pentane (5 mL) afforded an orange precipitate, which was filtered and dried under reduced pressure. Yield: 197 mg (88%). Anal. Calcd for C43H44N2O2P2Ru: C, 65.89; H, 5.66; N, 3.57. Found: C, 65.58; H, 5.82; N, 3.39. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.40 (br s, 1H; NH2), 8.31 (t, J(HH)=8.6 Hz, 2H; aromatic protons), 7.89 (s, 1H; aromatic proton), 7.74 (t, J(HH)=7.8 Hz, 2H; aromatic protons), 7.466.97 (m, 11H; aromatic protons), 6.93 (d, J(HH)=7.7 Hz, 1H; aromatic proton), 6.72 (t, J(HH)=7.8 Hz, 2H; aromatic protons), 6.65 (t, J(HH) = 8.0 Hz, 2H; aromatic protons), 6.47 (t, J(HH)= 7.3 Hz, 2H; aromatic protons), 6.16 (d, J(HH) = 7.2 Hz, 1H; aromatic proton), 6.07 (t, J(HH)=8.1 Hz, 2H; aromatic protons), 3.87 (dd, 2J(HH)=15.5 Hz, 3J(HH)=4.3 Hz, 1H; CH2N), 3.24

Article (m, 1H; CH2N), 3.05 (m, 1H; CH2P), 2.85 (m, 1H; CH2P), 2.34 (s, 3H; CH3), 2.10-1.50 (m, 5H; CH2), 1.72 (s, 3H; CH3CO), 1.41 (br s, 1H; NH2), 0.84 (m, 1H; CH2). 13C{1H} NMR (50.3 MHz, C6D6, 20 °C): δ 183.4 (dd, 2J(C,P)=16.0, 8.3 Hz; CRu), 181.5 (s, CORu), 163.9 (s; NCC), 159.3 (s; NCCH2), 148.8115.4 (m; aromatic carbon atoms), 52.7 (d, 3J(C,P) = 2.8 Hz, CH2N), 31.6 (d, 1J(C,P) = 24.6 Hz; CH2P), 31.3 (d, 1J(C,P) = 31.9 Hz; CH2P), 26.2 (s; CH2), 25.9 (d, 4J(C,P) = 3.7 Hz; CH3CO), 22.2 (s; CH2), 22.1 (s, CH3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 60.8 (d, 2J(PP) = 38.3 Hz), 44.6 (d, 2J(PP) = 38.3 Hz). IR (Nujol): ν 1713 cm-1 (asymmetric ν(OCO)). Synthesis of 7. To a suspension of complex 1 (105 mg, 0.138 mmol) in toluene (2.1 mL) was added 2.1 mL of a 2-propanol solution of NaOiPr (0.1 M, 0.21 mmol), and the mixture was stirred at 60 °C for 2 h. The mixture was kept at -20 °C overnight, affording the precipitation of NaCl, which was eliminated by filtration on Celite. The compound 4-nitrophenol (21 mg, 0.151 mmol) was added, and the red solution was stirred for 1 h at room temperature. The solvent was eliminated, toluene (2 mL) was added, and the mixture was kept at -20 °C for 2 h. After filtration on Celite, the resulting solution was concentrated (1 mL), and addition of pentane (10 mL) afforded a redorange product which was filtered and dried under reduced pressure. Yield: 104 mg (87%). Anal. Calcd for C47H45N3O3P2Ru: C, 65.42; H, 5.26; N, 4.87. Found: C, 65.33; H, 5.37; N, 4.82. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.16 (ddd, J(HH)= 9.7, 7.7, 2.0 Hz, 2H; aromatic protons), 8.0 (d, 3J(HH)=9.3 Hz, 2H; aromatic protons), 7.73 (m, 3H; aromatic protons), 7.456.90 (m, 13H; aromatic protons), 6.64 (t, J(HH)=7.4 Hz, 1H; aromatic proton), 6.45 (t, J(HH)=8.6 Hz, 2H; aromatic protons), 6.37 (t, J(HH)=8.8 Hz, 2H; aromatic protons), 6.00 (t, J(HH)= 8.1 Hz, 2H; aromatic protons), 5.92 (d, 3J(HH)=9.3 Hz, 2H; aromatic protons), 5.79 (d, J(HH)=7.5 Hz, 1H; aromatic proton), 3.50 (m, 1H; NH2), 3.35-2.80 (m, 4H; CH2N and CH2P), 2.27 (s, 3H; CH3), 2.30-0.80 (m, 7H; CH2, NH2). 13C{1H} NMR (50.3 MHz, C6D6, 20 °C): δ 182.6 (dd, 2J(C,P)=17.1, 7.5 Hz; CRu), 177.9 (s; C-O), 163.9 (s; NCC), 156.6 (s; NCCH2), 149.4-115.4 (m; aromatic carbon atoms), 51.8 (d, 3J(C,P) = 2.8 Hz; CH2N), 31.0 (d, 1J(C,P) = 25.1 Hz; CH2P), 30.9 (d, 1 J(C,P) = 32.2 Hz; CH2P), 26.6 (s; CH2), 22.1 (d, 2J(C,P) = 1.5 Hz; CH2), 22.0 (s, CH3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 62.1 (d, 2J(PP)=38.5 Hz), 41.6 (d, 2J(PP)=38.5 Hz). IR (Nujol): ν 1495 (asymmetric ν(NO2)), 1342 cm-1 (symmetric ν(NO2)). Synthesis of 8. To a suspension of 1 (148 mg, 0.195 mmol) in toluene (3 mL) was added 3 mL of a 2-propanol solution of KOiPr (0.1 M, 0.300 mmol), and the mixture was stirred at 60 °C for 2 h. The suspension was kept at -20 °C for 2 h and KCl was eliminated by filtration on Celite. 4-Fluoro-3-methylphenol (31 μL, 0.279 mmol) was added, and the solution was stirred for 30 min at room temperature. The solvent was eliminated, and the residue was dissolved in toluene (2 mL). Evaporation of the solvent afforded an orange solid which was dried under reduced pressure. Yield: 150 mg (91%). Anal. Calcd for C48H47FN2OP2Ru: C, 67.83; H, 5.57; N, 3.30. Found: C, 68.01; H, 5.39; N, 3.12. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.61 (t, J(HH)= 8.1 Hz, 2H; aromatic protons), 8.07 (t, J(HH) = 8.1 Hz, 2H aromatic protons), 7.85 (s, 1H; aromatic proton), 7.54-6.40 (m, 21H; aromatic protons), 6.15 (t, J(HH)=7.9 Hz, 2H; aromatic protons), 5.90 (d, J(HH)=7.0 Hz, 1H; aromatic proton), 4.47 (broad s, 1H; NH2), 3.45-2.80 (m, 5H; CH2N, CH2P), 2.30 (s, 3H; CH3), 2.05 (d, 4J(HF)=1.5 Hz, 3H; FCCCH3), 2.241.01 (m, 6H; NH2, CH2). 13C{1H} NMR (50.3 MHz, C6D6, 20 °C): δ 185.4 (dd, 2J(C,P)=16.0, 7.8 Hz; CRu), 164.0 (s; NCC), 156.4 (s; NCCH2), 152.5 (d, 1J(CF)=220.6 Hz; C-F), 148.7114.9 (m; aromatic carbon atoms), 52.0 (d; 3J(C,P) = 2.8 Hz, CH2N), 31.4 (d, 1J(C,P) = 30.0 Hz; CH2P), 30.6 (d, 1J(C,P) = 25.1 Hz; CH2P), 26.9 (s; CH2), 22.2 (s; CH2), 22.0 (s; CH3), 15.0 (d, 3J(CF) = 1.6 Hz; FCCCH3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 60.4 (d, 2J(PP) = 36.9 Hz), 38.6

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(d, 2J(PP)=36.9 Hz). 19F{1H} NMR (188.3 MHz, C6D6, 20 °C): δ -138.9 (s). Synthesis of 9. The preparation of 9 was carried out in a way similar to that described for 8, using compound 1 (100 mg, 0.131 mmol), 2.0 mL of a 2-propanol solution of NaOiPr (0.1 M, 0.200 mmol), and 3,30 -dinitrobenzophenone (40 mg, 0.147 mmol), resulting in a dark yellow product. Yield: 121 mg (92%). Anal. Calcd for C54H50N4O5P2Ru: C, 64.99; H, 5.05; N, 5.61. Found: C, 64.93; H, 5.01; N, 5.58. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.47 (s, 1H; aromatic proton). 7.93 (m, 5H; aromatic protons), 7.57 (td, J(HH)=7.9, 1.8 Hz; 2H; aromatic protons), 7.45 (dd, J(HH)=7.9, 1.8 Hz, 1H; aromatic proton), 7.40-6.85 (m, 14H; aromatic protons), 6.78 (td, J(HH)=7.2, 1.3 Hz, 2H; aromatic protons), 6.59 (t, J(HH)=7.8 Hz, 2H; aromatic protons), 6.40 (m, 2H; aromatic protons), 6.23 (m, 2H; aromatic protons), 5.88 (t, J(HH)=8.0 Hz, 2H; aromatic protons), 5.53 (d, J(HH)=6.8 Hz, 1H; aromatic proton), 5.20 (d, J(HP)=3.1 Hz, 1H; OCH), 4.69 (br s, 1H; NH2), 3.19 (m, 2H; CH2N, CH2P), 2.84 (m, 2H; CH2N, CH2P), 2.35 (s, 3H; CH3), 2.03 (m, 2H; CH2), 1.85-1.15 (m, 5H; CH2, NH2). 13C{1H} NMR (50.3 MHz, C6D6, 20 °C): δ 186.9 (m; CRu), 163.9 (s; NCC), 157.2 (s; NCCH2), 157.1 (s; C-NO2), 154.8 (d, J(C,P)=3.0 Hz; C-NO2), 149.1-115.0 (m; aromatic carbon atoms), 80.3 (s, OCH), 52.2 (d, 3J(C,P) = 2.7 Hz; CH2N), 32.5 (m; CH2P), 31.8 (m; CH2P), 26.9 (s; CH2), 23.0 (s; CH2), 22.3 (s, CH3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 57.3 (d, 2J(PP) = 34.9 Hz), 40.4 (d, 2J(PP) = 34.9 Hz). IR (Nujol): ν 1526 (asymmetric ν(NO2)), 1352 cm-1 (symmetric ν(NO2)). Synthesis of 10. The complex 1 (100 mg, 0.132 mmol) and NaOSiMe3 (19 mg, 0.169 mmol) were suspended in toluene (2 mL), and the mixture was stirred at room temperature for 1 h. The suspension was kept at -20 °C overnight and filtered on Celite, and the solution was concentrated (1 mL). Addition of pentane (5 mL) afforded an orange precipitate, which was filtered, washed with pentane (2  2 mL), and dried under reduced pressure. Yield: 80 mg (74%). Anal. Calcd for C44H50N2OP2RuSi: C, 64.92; H, 6.19; N, 3.44. Found: C, 65.20; H, 5.93; N, 3.47. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.43 (t, J(HH)=8.0 Hz, 2H; aromatic protons), 7.94 (t, J(HH)=7.6 Hz, 2H; aromatic protons), 7.84 (s, 1H; aromatic proton), 7.41-6.58 (m, 16H; aromatic protons), 6.50 (t, J(HH)=6.2 Hz, 2H; aromatic protons), 6.02 (m, 3H, aromatic protons), 4.50 (broad s, 1H; NH2), 3.32 (m, 2H; CH2), 2.95 (m, 2H; CH2), 2.30 (s, 3H; CH3), 2.12-0.87 (m, 7H; CH2 and NH2), -0.17 (s, 9H; SiMe3). 13 C{1H} NMR (50.3 MHz, C6D6, 20 °C): δ 185.0 (dd, 2J(C,P)= 15.9, 8.4 Hz; CRu), 164.4 (s; NCC), 156.9 (s; NCCH2), 149.3115.1 (m; aromatic carbon atoms), 51.8 (d, 3J(C,P) = 2.7 Hz; CH2N), 31.3 (d, 1J(C,P) = 23.6 Hz; CH2P), 30.8 (d, 1J(C,P) = 29.5 Hz; CH2P), 27.0 (s; CH2), 22.3 (s; CH2), 22.1 (s; CH3), 4.8 (s; SiMe3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 59.3 (d, 2J(PP)=36.5 Hz), 39.8 (d, 2J(PP) = 36.5 Hz). Synthesis of 11. Complex 1 (154 mg, 0.203 mmol) was dissolved in dichloromethane (5 mL), and CF3SO3Tl (72 mg, 0.204 mmol) was added. The orange suspension was stirred at room temperature for 2 h and filtered on Celite to eliminate TlCl, and the filtrate was reduced to 1 mL. Addition of pentane afforded a yellow precipitate, which was filtered, washed with pentane, and dried under reduced pressure. Yield: 142 mg (80%). Anal. Calcd for C42H41F3N2O3P2RuS: C, 57.73; H, 4.73; N, 3.21. Found: C, 57.65; H, 4.89; N, 3.01. 1H NMR (200.1 MHz, CD2Cl2, 20 °C): δ 7.91 (m, 2H; aromatic protons), 7.60-7.20 (m, 16H; aromatic protons), 7.06 (d, J(HH)=8.0 Hz, 2H; aromatic protons), 6.83 (d, J(HH)=7.7 Hz, 2H; aromatic protons), 6.73 (d, J(HH)=7.5 Hz, 2H; aromatic protons), 6.06 (m, 2H; aromatic protons), 4.18 (m, 1H; NH2), 3.81 (m, 1H; CH2N), 3.61 (m, 1H; CH2N), 3.05 (m, 2H; PCH2), 2.20 (s, 3H; CH3), 2.30-0.80 (m, 7H; NH2 and CH2). 13C{1H} NMR (50.3 MHz, CD2Cl2, 20 °C): δ 178.1 (t, 2J(C,P)=12.6 Hz; CRu), 164.8 (s; NCC), 158.7 (s; NCCH2), 149.7-116.4 (m; aromatic carbon atoms), 119.8 (q, 1J(CF) = 320.0 Hz; CF3), 52.9

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(s; CH2N), 31.9 (s; CH2P), 31.2 (s; CH2P), 26.8 (s; CH2), 22.7 (s; CH2), 21.7 (s; CH3). 31P{1H} NMR (81.0 MHz, CD2Cl2, 20 °C): δ 65.4 (br s), 42.5 (d, 2J(PP) = 30.7 Hz). 19F NMR (188.3 MHz, CD2Cl2, 20 °C): δ -77.6 (s). 1H NMR (200.1 MHz, toluene-d8, 20 °C): δ 8.07 (m, 2H; aromatic protons), 7.807.65 (m, 3H; aromatic protons), 7.37 (t, J(HH) = 6.9 Hz, 2H; aromatic protons), 7.26-7.05 (m, 5H; aromatic protons), 7.02 (m, 1H; aromatic proton), 6.98 (m, 4H; aromatic protons), 6.87 (d, J(HH)=7.6 Hz, 1H; aromatic proton), 6.78 (d, J(HH)=4.3 Hz, 2H; aromatic protons), 6.65 (t, J(HH) = 7.0 Hz, 1H; aromatic proton), 6.44 (t, J(HH)=6.8 Hz, 2H; aromatic protons), 6.20 (m, 1H; aromatic proton), 5.99 (t, J(HH) = 8.3 Hz, 2H; aromatic protons), 4.21 (m, 1H; NH2), 3.61 (m, 1H; CH2N), 3.21 (m, 1H; CH2N), 2.95 (m, 2H; CH2P), 2.26 (s, 3H; CH3), 2.30-0.70 (m, 7H; NH2 and CH2). 13C{1H} NMR (50.3 MHz, toluene-d8, 20 °C): δ 178.2 (dd, 2J(C,P)=18.1, 7.0 Hz; CRu), 164.4 (s; NCC), 158.5 (s; NCCH2), 149.5-116.3 (m; aromatic carbon atoms and CF3), 52.6 (d, 3J(C,P)=1.8 Hz; CH2N), 31.7 (d, 1J(C,P)= 35.4 Hz; CH2P), 30.8 (d, 1J(C,P) = 32.1 Hz; CH2P), 26.4 (s; CH2), 22.0 (s; CH2), 21.9 (s, CH3). 31P{1H} NMR (81.0 MHz, toluene-d8, 20 °C): δ 68.3 (d, 2J(PP) = 42.5 Hz), 43.0 (d, 2 J(PP) = 42.5 Hz). 19F NMR (188.3 MHz, toluene-d8, 20 °C): δ -73.5 (s). Synthesis of 12. The preparation of 12 was carried out in a way similar to that described for 8, using compound 4 (100 mg, 0.127 mmol) instead of 1, 1.9 mL of a 2-propanol solution of KOiPr (0.1 M, 0.190 mmol), and 4-fluoro-3-methylphenol (16 μL, 0.144 mmol). Yield: 91 mg (82%). Anal. Calcd for C50H51FN2OP2Ru: C, 68.40; H, 5.85; N, 3.19. Found: C, 68.15; H, 5.69; N, 3.27. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.23 (t, J(HH)= 9.0 Hz, 2H; aromatic protons), 8.19 (t, J(HH) = 8.8 Hz, 2H; aromatic protons), 7.84-6.35 (m, 21H; aromatic protons), 6.33 (d, J(HH) = 7.3 Hz, 1H; aromatic proton), 6.24 (t, J(HH) = 8.0 Hz, 2H; aromatic protons), 6.08 (m, 1H; aromatic proton), 3.78 (d, J(HH)=14.7 Hz, 1H; CH2N), 3.24 (d, J(HH)=14.7 Hz, 1H; CH2N), 2.17 (s, 3H; CH3), 2.11 (br s, 3H; FCCCH3), 1.97 (br s, 6H; NMe2), 2.98-0.77 (m, 8H; CH2). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 52.4 (d, 2J(PP)=34.2 Hz), 39.7 (d, 2J(PP)=34.2 Hz). 19F{1H} NMR (188.3 MHz, C6D6, 20 °C): δ -139.6 (s). Synthesis of 13b. The complex 4 (110 mg, 0.140 mmol) was suspended in 2.0 mL of a toluene/tBuOH mixture (1:1 by volume), and KOtBu (24 mg, 0.214 mmol) was added. The mixture was stirred for 2 h at room temperature, giving a dark red solution, which was kept at -20 °C overnight and filtered on Celite. The solvent was eliminated, toluene (2.0 mL) was added, and the resulting solution was concentrated (1 mL). Addition of pentane (5.0 mL) afforded a dark red product, which was filtered and dried under reduced pressure. Yield: 86 mg (74%). Anal. Calcd for C47H54N2OP2Ru: C, 68.35; H, 6.59; N, 3.39. Found: C, 68.23; H, 6.41; N, 3.48. 1H NMR (200.1 MHz, C6D6, 20 °C): δ 8.66 (m, 2H; aromatic protons), 8.38 (t, J(HH) = 7.8 Hz, 2H; aromatic protons), 7.97-5.87 (m, 22H; aromatic protons), 3.95 (d, J(HH)=13.7 Hz, 1H; CH2N), 2.83 (d, J(HH) = 13.7 Hz, 1H; CH2N), 2.60-0.76 (m, 8H; CH2), 2.20 (s, 3H; CH3), 2.00 (br s, 6H; NMe2), 1.35 (s, 9H; CMe3). 31P{1H} NMR (81.0 MHz, C6D6, 20 °C): δ 48.3 (d, 2J(PP)=31.1 Hz), 35.4 (d, 2J(PP)=31.1 Hz). Typical Procedure for the Catalytic Transfer Hydrogenation. The ruthenium complex (2.5 μmol) was dissolved in 5 mL of 2-propanol. The ketone (2.0 mmol) was dissolved in 2-propanol (final volume 19.4 mL), and the solution was heated to reflux under argon. By addition of NaOiPr (0.1 M, 0.4 mL, 0.04 mmol) and the solution containing the ruthenium complex (0.2 mL, 0.10 μmol), the reduction of the ketone starts immediately and the yield was determined by GC analysis (complex 0.005 mol %, NaOiPr 2 mol %).

Baratta et al. Table 4. Crystallographic Data for Complexes 5 and 6 formate 5 empirical formula formula wt T (K) cryst syst Space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z θ range data collection (deg) μ (mm-1) F(000) Dcalcd (g cm-3) no. of rflns collected no. of indep rflns/Rint no. of obsd rflns (I > 2σ(I )) no. of params R1/wR2 (I > 2σ(I )) R1/wR2 (all data) GOF (on F 2) largest diff peak and hole (e A˚-3)

C42H42N2O2P2Ru 769.79 153(1) monoclinic P21/n 11.2659(5) 11.5008(14) 27.682(2) 90 94.279(5) 90 3576.7(5) 4 2.83-25.36 0.567 1592 1.430 63 721 6553/0.077 4329 443 0.0331/0.0636 0.0701/0.0706 0.928 þ0.69/-0.64

acetate 6 3 1.5C6H6 C52H53N2O2P2Ru 900.97 293(2) triclinic P1 11.815(4) 13.204(4) 17.016(4) 84.26(3) 72.34(2) 66.90(3) 2326.2(12) 2 2.09-28.28 0.447 938 1.286 35 379 10 080/0.051 5902 538 0.0415/0.0966 0.0735/0.1053 0.875 þ0.32/-0.24

Crystallographic Studies of Compounds 5 and 6. Clear orange crystals were mounted on a glass fiber in both cases. X-ray data of compound 5 were collected on an Oxford Xcalibur system and those of 6 3 1.5C6H6 on a Nonius DIP-1030H system by using Mo KR radiation (λ=0.710 73 A˚). Data were processed and corrected for Lorentz and polarization and for absorption effects with the Crysalis suite of programs38 and the Denzo/ Scalepack packages,39 respectively. Both of the structures were solved by direct methods followed by difference Fourier calculations. Hydrogen atoms were placed in ideal positions (riding model). All calculations were performed with the WinGX program system.40 Crystal and refinement data are summarized in Table 4.

Acknowledgment. This work was supported by the Ministero dell’Universita e della Ricerca (MIUR) and the Regione Friuli Venezia Giulia. We thank JohnsonMatthey/Alfa Aesar for a generous loan of ruthenium and Mr. P. Polese for carrying out the elemental analyses. Supporting Information Available: CIF files giving crystal and data collection parameters, atomic coordinates, bond lengths, bond angles, and thermal displacement parameters for 5 and 6. This material is available free of charge via the Internet at http:// pubs.acs.org. (38) CrysAlis Data Collection Software and Data Processing Software for Oxford Xcalibur diffractometer, Version 1.171; Oxford Diffraction Ltd., Oxfordshire, U.K., 2005. (39) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data Collected in Oscillation Mode. In Macromolecular Crystallography, Part A; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Methods in Enzymology Vol. 276, pp 307-326. (40) (a) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. SIR92. J. Appl. Crystallogr. 1994, 27, 435. (b) Sheldrick, G. M. SHELXL-97; University of G€ottingen, G€ottingen, Germany, 1998. (c) Spek, A. L.; PLATON; Utrecht University, Utrecht, The Netherlands, 2008. (d) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.