Communication pubs.acs.org/Organometallics
Transfer Hydrogenation in Water via a Ruthenium Catalyst with OH Groups near the Metal Center on a bipy Scaffold Ismael Nieto,† Michelle S. Livings,† John B. Sacci, III,† Lauren E. Reuther,† Matthias Zeller,‡ and Elizabeth T. Papish*,† †
Department of Chemistry, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States Department of Chemistry, Youngstown University, One University Plaza, Youngstown, Ohio 44555, United States
‡
S Supporting Information *
ABSTRACT: The new ligand 6,6′-dihydroxy-2,2′-bipyridyl (dhbp) was synthesized via its tautomer, and this provides an efficient route to novel metal complexes of dhbp. In ruthenium complexes of dhbp, these OH groups enhance water solubility and may play a role in aqueous transfer hydrogenation with formate/formic acid as the hydrogen source. A series of cationic catalysts, [(η 6-arene)Ru(N,N)Cl]Cl (arene = cymene, C6Me6; N,N = bipyridyl with OH, OMe, or H at the 6- and 6′-positions), were synthesized, fully characterized, and tested for transfer hydrogenation activity in various polar protic media. In aqueous media (90/10 water/ methanol), Ru complexes of dhbp outperform the other catalysts tested (all at 1 mol %), and high percentage conversion of aromatic ketones to the corresponding alcohols is observed in 6 h. The OH groups appear to be essential for use of water as a green solvent and can potentially allow for metal−ligand bifunctional catalysis.
T
he design of water-soluble organometallic complexes is of fundamental importance for catalysis.1,2 Water is an ideal “green” solvent3 that is nonflammable, reduces waste, and can simplify purification and allow for pH-dependent selectivity. 1,2,4 Hydrogenation reactions that are catalytic and avoid the use of stoichiometric reagents are fundamentally greener and benefit the chemical industry.5 This is true for catalysis with H2 gas and transfer hydrogenation,6−9 which uses a hydrogen source. Several catalysts perform hydrogenation of polar double bonds in water,10−16 but efforts to combine water solubility with scaffolds conducive to metal−ligand bifunctional catalysis have been limited.14,15,17,18 We were inspired to modify bipy for water solubility in part due to water-soluble organometallic transfer hydrogenation catalysts, including [(arene)Ru(bipy)(OH2)]2+ and [(arene)Ru(2,2′-dipyridylamine)Cl]+.4,19 Transfer hydrogenation in water has typically used sodium formate and formic acid as the H2 source, generating CO2 as a byproduct. Himeda has achieved direct hydrogenation of CO2 in water with H2(g) catalyzed by the complex in Figure 1a.20 In Himeda’s studies, CO2 hydrogenation was carried out in the presence base; thus, initially the OH groups were deprotonated, and this catalyst (Figure 1a, right) has much higher activity than the protonated form (Figure 1a, left).20 The OH/O− groups in para positions on this bipy derivative serve three purposes: (1) to increase water solubility, (2) to make the ligand more electron rich, and (3) to enable a pH switch for the catalysis.20 From a design perspective, placement of the water solubilizing groups near the metal center opens up the possibility of hydrogenation reactions that utilize metal−ligand © 2011 American Chemical Society
Figure 1. Structure of Himeda (a) and Casey/Shvo (b) catalysts.
bifunctional catalysis. Metal−ligand bifunctional catalysis can offer large rate enhancements through a cooperative approach to H2 activation and transfer, with the metal providing a hydride and the ligand providing a proton to substrate. 21 Pioneering work in this field by Noyori et al. shows that nearenzymatic rate enhancements are possible and are promoted by a cyclic transition state with simultaneous hydride and proton transfer.21,22 The complex shown in Figure 1b, designed by Shvo23,24 and mechanistically studied by Casey and coworkers,25,26 offers this type of catalysis. Here, Ru−H donates a hydride and OH of the modified Cp ligand donates a proton to substrate, and therefore over the course of each catalytic cycle the nature of the η 5-C5Ph4OH ligand changes in terms of both its charge and number of electrons donated to the metal. 25 The ability to donate or accept protons and ligand flexibility are hallmarks of metal−ligand bifunctional catalysis. Received: July 15, 2011 Published: November 11, 2011 6339
dx.doi.org/10.1021/om200638p | Organometallics 2011, 30, 6339−6342
Organometallics
Communication
We aimed to make a new ligand that would be generally useful for both organometallic catalysis and bioinorganic modeling studies, by incorporating the possibility of metal− ligand bifunctional catalysis into a water-soluble ligand. The ligand 6,6′-dihydroxy-2,2′-bipyridyl (dhbp; Scheme 1) satisfies Scheme 1. Synthesis of dmeobp and dhbp Ligands and Complexation to Rutheniuma
Figure 2. Molecular structure of [(η 6-p-cym)RuCl(dhbp)]Cl (1). The anion is omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1−N1 = 2.125(1), Ru1−N2 = 2.116(1), Ru1−Cl1 = 2.3899(4), Ru1−C(cymene) = 2.198(1) (avg), Ru1−O1(nonbonded) = 3.222(1), Ru1−O2(nonbonded) = 3.200(1); N1−Ru1−N2 = 76.52(5), Cl1−Ru1−N1 = 86.76(3), Cl1−Ru1−N2 = 86.24(3).
However, these OH groups do form strong hydrogen bonds (with MeOH solvent O1···O3 = 2.610(2) Å) and ion−dipole interactions (with noncoordinated chloride, O2···Cl2 = 2.988(1) Å) which lead to structured packing arrangements (see Supporting Information). The strength of the above hydrogen bonds/ion−dipole interactions is also indicated by nearly linear O−H···O and O−H···Cl− angles (∼174°).35 Thus, it seems plausible that the OH groups of dhbp can potentially hydrogen bond with substrates. In order to have a library of potential catalysts with varied features, we prepared analogues of 1 that feature OMe or H in place of the OH groups, and one analogue uses a C6Me6 ring in place of cymene (Scheme 1). [(η 6-p-cym)RuCl(dmeobp)]Cl (2; 50% yield) should be electronically similar to 1 but lacks protic groups. [(η 6-C6Me6)RuCl(dhbp)]Cl (3; 74% yield) has the arene favored by Himeda, which is a stronger donor and is bulkier than cymene.20 Finally, [(η 6-p-cym)RuCl(bipy)]Cl (4; 89% yield) is the unsubstituted analogue described in the literature.36 Complexes 2−4 were characterized by 1H and 13C NMR, IR, MS, and/or elemental analysis, and spectra for 4 were consistent with those of the PF6 salt in the literature.36 Of these structures, only 3 recrystallized readily from methanol as red-orange blocks (see the Supporting Information). The crystal structure of 3 is similar to that of 1, with a pianostool geometry observed and the dhbp tautomer of the ligand bound κ 2 through the nitrogens. This structure has OH···[Cl]−···OH ion−dipole interactions (wherein the OH groups come from dhbp ligands of neighboring Ru complexes, and the Cl− is from the counteranion). As seen with 1, the O− Cl distances (2.924(1) and 2.959(1) Å) and the O−H−Cl angles (172−174°) indicate strong ion−dipole interactions in 3 (see the Supporting Information for more details). Complex 1 was prepared first and was thus used in initial tests aimed at finding optimum conditions for transfer hydrogenation catalysis. Transfer hydrogenation of ketones with 1 mol % of 1 was initially tested in isopropanol with KOH (Table 1). Here isopropanol is the hydrogen source (acetone is a byproduct), and KOH presumably enhances catalysis by promoting formation of intermediates (possibly [Ru]-H from the [Ru]-Cl precatalyst). Over 24 h, most of the ketones fully converted to the corresponding alcohol as determined by 1H
a
Legend: (i) NiBr2(PPh3)2, Zn, nBu4NBr4, DMF; (ii) HBr/HOAc; (iii, iv) [(η 6-p-cym)RuCl2]2, DMF; (v) [(η 6-C6Me6)RuCl2]2, DMF.
these criteria and has been reported in the patent literature as the dilactam tautomer.27 However, metal complexes of this ligand have typically been made as decomposition products starting from bipy or related ligands.28,29 As this work was being published, we learned of a route to Rh complexes of dhbp, but this procedure requires protection and deprotection steps. 30,31 Thus, our procedure is the first to make metal complexes of dhbp directly in one step from the dilactam tautomer. The synthesis of dhbp (Scheme 1) began with 6-methoxy-2chloropyridine coupling to itself via nickel-catalyzed32 Negishi cross-coupling to form 6,6′-dimethoxy-2,2′-bipyridine (dmeobp).27 Isolated dmeobp was then hydrolyzed in refluxing HBr and acetic acid to form the highly insoluble (in aqueous and organic solvents) dilactam. Both bipyridine derivatives were characterized by 1H and 13C NMR, IR, and MS and were consistent with the NMR and MS data in the patent.27 Expecting the desired tautomerization to occur in situ , [(η 6p-cymene)RuCl2]2 was treated with the dilactam, and after heating at 60 °C in DMF the desired [(η 6-p-cym)RuCl(dhbp)] Cl complex (1; Scheme 1) was formed. It is apparent that heat and very polar solvents are needed to solubilize dhbp, which hydrogen bonds to itself. Complex 1 was isolated in high yield (96%) as a yellow-orange solid, is stable to air and moisture, and was characterized by 1H and 13C NMR, IR, MS, elemental analysis, and X-ray crystallography (below). Recrystallization of 1 in ethyl acetate with 5% methanol gave blocks suitable for a single-crystal structure determination (Figure 2). A piano-stool geometry is observed with bond lengths (Ru−Cl, Ru−N, and Ru−C6(centroid)) and angles (see Figure 2 caption) that are similar to those of structurally related ruthenium bipyridyl analogues.33,34 Complex 1 contains the dhbp tautomer (CN distance of ∼1.34 Å), and bond angles in dhbp are subtly influenced by the chelate ring’s geometric requirements, with sp2-hybridized atoms having bond angles of 115 to 126°. The OH groups in dhbp are intact and do not participate in metal binding (O···Ru = ∼3.2 Å). 6340
dx.doi.org/10.1021/om200638p | Organometallics 2011, 30, 6339−6342
Organometallics
Communication
Table 1. Transfer Hydrogenation of Select Ketones by 1 in Isopropanola 24 h
acetophenone 4-methylacetophenone 4-bromoacetophenone 4-methoxyacetophenone 2-methoxyacetophenone 2-acetonaphthone
Table 3. Transfer Hydrogenation of Select Ketones by 1 in 10/90 MeOH/DI H2Oa
6h
18 h
conversn (%)
TOF (h−1)
conversn (%)
TOF (h−1)
100 100 84 31 100 94
4.17 4.17 3.5 1.29 4.17 3.92
44 28 64 4 24 70
7.33 4.72 10.67 0.67 3.94 17.42
acetophenone 4-methylacetophenone 4-bromoacetophenone 4-methoxyacetophenone 2-methoxyacetophenone 2-acetonaphthone
a Reaction conditions: temperature 85 °C. See the Supporting Information for experimental conditions. TOF is the turnover frequency.
1
5
2 3 4
5 1 1
NaO2CH/ HO2CH NaO2CH NaO2CH NaO2CH
5
1
NaO2CH
base
solvent
89
H2 O H2 O 50/50 MeOH/ H2O 10/90 MeOH/ H2O
89 31 96
conversn (%)
TOF (h−1)
95 74 99 71 71 99
5.25 4.11 5.55 3.94 3.94 5.50
97 66 97 65 53 81
16.17 11.00 16.17 10.78 8.83 13.50
Table 4. Transfer Hydrogenation of Acetophenone by 1−4 and [p-cymRuCl2]2 in 10/90 MeOH/H2O (v/v)a conversn (%) base KOH NaO2CH NaO2CH
conversn (%)
H2O
TOF (h−1)
10/90 MeOH/H2O versus isopropanol (compare 6 h data in Tables 1 and 3). Complex 1 is clearly a good catalyst for use in water, but a comparison with similar complexes (2−4) is needed to establish whether aqueous catalysis of hydrogenation is facilitated by the OH groups. Hydrogenation of acetophenone was studied with 1 mol % of a variety of catalysts in several different solvents (Table 4). As shown in Table 4, complexes
Table 2. Transfer Hydrogenation of Acetophenone by 1 in Various Mediaa trial
conversn (%)
a Reaction conditions: temperature 90 °C. See the Supporting Information for further experimental details.
NMR, with insufficient conversion after 6 h. A control experiment involving the hydrogenation of acetophenone in the presence of KOH with no catalyst showed 60−70% conversion in 24 h, as shown in the literature.37 Thus, catalyst 1 performs poorly in isopropanol (perhaps due to limited solubility), and other conditions were needed. A report by Renaud showed high conversion for the transfer hydrogenation of acetophenone in water (86% in 24 h) by an arene ruthenium dipyridylamine complex.19 Similarly, transfer hydrogenation of acetophenone by 1 in more polar media gave promising results (Table 2). Trials 1 and 2 were performed
amt of cat. (mol %)
6h
NaO2CH
solvent i
PrOH H2O 50/50 MeOH/ H2O 10/90 MeOH/ H2O
1
2
3
4
[pcymRuCl2]2
100 29x 96x
100y 13y 19y
100z 15z 39z
98 5 12
100 13 13
95x
22y
50z
22
15
a
Reaction conditions: time 24 h unless otherwise stated (x, 18 h; y, 20 h; z, 21 h), temperature 85−90 °C. See the Supporting Information for further experimental details.
97
a
Reaction conditions: time 16−17 h, temperature 90 °C. See the Supporting Information for further experimental details.
1−4 were all highly effective catalysts in isopropanol, and these complexes all performed similarly to [(η 6-p-cym)RuCl2]2. However, in pure water all Ru complexes performed poorly, with 29% conversion or less. The most interesting feature from this study was that full conversion in MeOH/H2O solutions was seen only with 1, and the other complexes all gave significantly lower percent conversion. Surprisingly, the change in arene from cymene to C6Me6 (from 1 to 3) is quite detrimental, but 3, with OH groups, is our second best catalyst in this solvent system. Thus, protic OH groups in 1 appear necessary for transfer hydrogenation in aqueous solutions, especially given the poor performance of 2, the analogue with OMe groups. Future investigations will address whether the placement of these OH groups is important. Complex 1, our best-performing catalyst in mostly water, appears to be stable and soluble under the reaction conditions in Table 4 (entry 4), as verified by variable-temperature 1H NMR (see the Supporting Information for more details). Preliminary experiments on the acid−base chemistry of complex 1 suggest that the OH groups are deprotonated under typical catalysis conditions (pH ∼8.1 with 1 M NaO2CH in 90/10 water/methanol). A titration experiment shows that 1
with the same amount of catalyst as used by Renaud but differed in the presence of formic acid, and these trials showed that while formic acid was used by Renaud, it is not beneficial with 1; use of just NaOOCH is sufficient, presumably since water can function as a proton source. KOH was not used here since formate or formic acid is needed to provide a hydride source in water (isopropanol or isopropoxide is the hydride source in Table 1). Trials 3−5 show that the quantity of catalyst can be decreased to 1 mol %, with good results as long as methanol is a cosolvent. A control experiment identical to trial 5 but lacking catalyst showed no conversion. Thus, solutions that are mostly water performed far better than water alone or isopropanol. With these new optimized conditions, transfer hydrogenation catalyzed by 1 (1 mol %) was tested on a series of ketones in 10/90 MeOH/H2O (v/v), shown in Table 3. After 18 h, all of the ketones tested show greater than 70% conversion by 1H NMR, and the data show that between 6 and 18 h there was not a huge gain in conversion. Overall, transfer hydrogenation of ketones as catalyzed by 1 shows significant improvement in 6341
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Communication
(6) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393. (7) Samec, J. S. M.; Backvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (8) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201. (9) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. (10) Mebi, C. A.; Nair, R. P.; Frost, B. J. Organometallics 2006, 26, 429. (11) Csabai, P.; Joó, F. Organometallics 2004, 23, 5640. (12) Geldbach, T. J.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J. Organometallics 2005, 25, 733. (13) Akbayeva, D. N.; Gonsalvi, L.; Oberhauser, W.; Peruzzini, M.; Vizza, F.; Bruggeller, P.; Romerosa, A.; Sava, G.; Bergamo, A. Chem. Commun. 2003, 264. (14) Soltani, O.; Ariger, M. A.; Vazquez-Villa, H.; Carreira, E. M. Org. Lett. 2010, 12, 2893. (15) Wu, J.; Wang, F.; Ma, Y.; Cui, X.; Cun, L.; Zhu, J.; Deng, J.; Yu, B. Chem. Commun. 2006, 1766. (16) Ogo, S.; Makihara, N.; Watanabe, Y. Organometallics 1999, 18, 5470. (17) Mao, J.; Wan, B.; Wu, F.; Lu, S. Tetrahedron Lett. 2005, 46, 7341. (18) Li, X.; Wu, X.; Chen, W.; Hancock, F. E.; King, F.; Xiao, J. Org. Lett. 2004, 6, 3321. (19) Romain, C.; Gaillard, S.; Elmkaddem, M. K.; Toupet, L. c.; Fischmeister, C. d.; Thomas, C. M.; Renaud, J.-L. Organometallics 2010, 29, 1992. (20) Himeda, Y. Eur. J. Inorg. Chem. 2007, 2007, 3927. (21) Noyori, R.; Sandoval, C. A.; Muñiz, K.; Ohkuma, T. Philos. Trans. R. Soc. A: Math., Phys. Eng. Sci. 2005, 363, 901. (22) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466. (23) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem. Soc. 1986, 108, 7400. (24) Menashe, N.; Salant, E.; Shvo, Y. J. Organomet. Chem. 1996, 514, 97. (25) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090. (26) Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100. (27) Dubreuil, D. M.; Pipelier, M. G.; Pradere, J. P.; Bakkali, H.; Lepape, P.; Delaunay, T.; Tabatchnik, A. (CNRS, France). Pyridazine and pyrrole compounds, processes for obtaining them and uses. World Patent 012440A2, 2008. (28) Kovacs, J.; Mokhir, A. Inorg. Chem. 2008, 47, 1880. (29) Zhang, J.-P.; Lin, Y.-Y.; Weng, Y.-Q.; Chen, X.-M. Inorg. Chim. Acta 2006, 359, 3666. (30) Conifer, C. M.; Law, D. J.; Sunley, G. J.; Haynes, A.; Wells, J. R.; White, A. J. P.; Britovsek, G. J. P. Eur. J. Inorg. Chem. 2011, 2011, 3511. (31) Conifer, C. M.; Taylor, R. A.; Law, D. J.; Sunley, G. J.; White, A. J. P.; Britovsek, G. J. P. Dalton Trans. 2011, 40, 1031. (32) Travnicek, Z.; Machala, V.; Szuecova, L.; Malon, M.; Marek, J. Transition Met. Chem. 2004, 29, 352. (33) Dykeman, R. R.; Luska, K. L.; Thibault, M. E.; Jones, M. D.; Schlaf, M.; Khanfar, M.; Taylor, N. J.; Britten, J. F.; Harrington, L. J. Mol. Catal. A: Chem. 2007, 277, 233. (34) Polson, M. I. J. Acta Crystallogr., Sect. E: Struct. Rep. Online 2008, E64, m256. (35) Jeffrey, G. A. Crystallogr. Rev. 2003, 9, 135. (36) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33, 4453. (37) Zuidema, D. R.; Wert, K. J.; Williams, S. L.; Chill, S. T.; Holte, K. L.; Kokes, N. K.; Mebane, R. C. Synth. Commun. 2010, 40, 1187. (38) Based on the pKa of 2-pyridone: Williams, R. http://research. chem.psu.edu/brpgroup/pKa_compilation.pdf (accessed 10/6/11).
is doubly deprotonated by pH ∼7.4 in water, and this result is similar to that seen by Himeda (Figure 1a). The OH groups of dhbp along are expected to be acidic (pKa < 11.6)38 and that acidity is enhanced by metal coordination and formation of a cationic complex (expt. estimated pKa ≈ 5). The above stability suggests that complex 1 can be recycled. Extraction of the residual substrate and product (with ether) to isolate the catalyst 1 in the aqueous phase leads to an aqueous solution that can be reused upon adding more substrate, but only with substantial drops in activity. The first use of 1 gave 92% conversion of acetophenone to the corresponding alcohol, and reuse of 1 gave subsequent percent conversions of 33% and 0% (conditions as for Table 4 entry 4, also see the Supporting Information). Given the substantial water solubility of 1, most of it should have remained in the aqueous phase, but some catalyst removal is possible. We are currently investigating more efficient methods for recycling without loss of activity. In sum, dhbp is a novel ligand that is well suited for catalyzing transfer hydrogenation in aqueous solutions. The protic OH groups can hydrogen bond and may allow for metal−ligand bifunctional catalysis. These OH groups are electron donating, especially if they are deprotonated in situ. We aim to further investigate the mechanism and the scope of hydrogenation catalysis with 1 and related complexes. Furthermore, this new ligand offers hydrogen bonds near the metal center in a geometry not well explored previously and should be useful for both aqueous organometallic catalysis and bioinorganic modeling.
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ASSOCIATED CONTENT
S Supporting Information *
Text and tables giving experimental details and characterization data and CIF files giving crystallographic data for 1 ([(η 6-pcym)RuCl(dhbp)]Cl) and 3 ([(η 6-C6Me6)RuCl(dhbp)]Cl). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ACKNOWLEDGMENTS We are grateful for financial support from the Petroleum Research Fund, administered by the American Chemical Society (Grant 48295-AC3), NSF CAREER (Grant CHE0846383), and Drexel University. The diffractometer was funded by NSF grant 0087210, by Ohio Board of Regents Grant CAP-491, and by YSU. Additional thanks should be given to Tim Wade (Drexel University) and John Dykins (University of Delaware) for their mass spectroscopic analysis of the compounds mentioned in this report. Finally, we thank the members of the Papish research group for assistance and suggestions.
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REFERENCES
(1) Aqueous-Phase Organometallic Catalysis: Concepts and Applications, 2nd ed.; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (2) Lindström, U. M. Chem. Rev. 2002, 102, 2751. (3) Wu, X.; Xiao, J. Wiley-VCH: Weinheim, Germany, 2010; Vol. 5, p 105. (4) Ogo, S.; Abura, T.; Watanabe, Y. Organometallics 2002, 21, 2964. (5) Sinou, D. Adv. Synth. Catal. 2002, 344, 221. 6342
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