Transmetalation of a Primary Amino-Functionalized N-Heterocyclic

Organometallics 2009, 28, 6755–6761 DOI: 10.1021/om9007746

6755

Transmetalation of a Primary Amino-Functionalized N-Heterocyclic Carbene Ligand from an Axially Chiral Square-Planar Nickel(II) Complex to a Ruthenium(II) Precatalyst for the Transfer Hydrogenation of Ketones Wylie W. N. O, Alan J. Lough, and Robert H. Morris* Davenport Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Received September 6, 2009

The first homoleptic nickel(II) complex with primary amino-functionalized N-heterocyclic carbene (C-NH2) ligands ([Ni(m-CH2NH2)2](PF6)2, 2) was prepared under mild conditions by the reduction of a nitrile-functionalized imidazolium salt. This axially chiral, square-planar nickel(II) complex was characterized by NMR spectroscopy and an X-ray diffraction study. Enantiopure Δ-TRISPHAT was used as an NMR chiral shift reagent to observe the diastereotopic ion pairs by 1H NMR in acetonitrile-d3. A novel transmetalation reaction moved the primary amino-functionalized N-heterocyclic carbene ligand from the nickel(II) complex 2 to the [Ru(p-cymene)Cl]2 dimer, yielding the complex [Ru(p-cymene)(m-CH2NH2)Cl]PF6, (3), the first ruthenium(II) complex with such a chelating C-NH2 ligand. Complex 3 is a precatalyst for the transfer hydrogenation of acetophenone to 1-phenylethanol in basic 2-propanol at 75 °C with a turnover frequency of up to 880 h-1 and conversion of 96%.

Introduction The use of donor-functionalized N-heterocyclic carbenes (NHC) as ligands in the design of homogeneous catalysts has received significant attention, as they provide coordination versatility and metal-ligand bifunctionality that will enhance catalytic activity.1 Among those, both secondary and tertiary amino-functionalized NHC ligands are found to be important building blocks of active transition metal catalysts that are used for cross-coupling and hydrosilylation reactions (Figure 1).2,3 We are interested in the use of N-heterocyclic carbene ligands in the hydrogenation of polar bonds including those of ketones, imines, and nitriles. In *To whom correspondence should be addressed. E-mail: rmorris@ chem.utoronto.ca. (1) (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732–1744. (b) Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2781–2800. (c) Corberan, R.; Mas-Marza, E.; Peris, E. Eur. J. Inorg. Chem. 2009, 1700–1716. (2) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R.; Houghton, J.; Kariuki, B. M.; Simonovic, S. Dalton Trans. 2004, 3528–3535. (3) (a) Douthwaite, R. E.; Houghton, J.; Kariuki, B. M. Chem. Commun. 2004, 698–699. (b) Houghton, J.; Dyson, G.; Douthwaite, R. E.; Whitwood, A. C.; Kariuki, B. M. Dalton Trans. 2007, 3065–3073. (c) Jimenez, M. V.; Perez-Torrente, J. J.; Bartolome, M. I.; Gierz, V.; Lahoz, F. J.; Oro, L. A. Organometallics 2008, 27, 224–234. (d) Jong, H.; Patrick, B. O.; Fryzuk, M. D. Can. J. Chem. 2008, 86, 803–810. (4) (a) Guo, R.; Lough, A. J.; Morris, R. H.; Song, D. T. Organometallics 2004, 23, 5524–5529. (b) Abdur-Rashid, K.; Guo, R. W.; Lough, A. J.; Morris, R. H.; Song, D. T. Adv. Synth. Catal. 2005, 347, 571–579. (c) Guo, R.; Morris, R. H.; Song, D. J. Am. Chem. Soc. 2005, 127, 516–517. (d) Ito, M.; Ikariya, T. Chem. Commun. 2007, 5134–5142. (e) Saudan, L. A.; Saudan, C. M.; Debieux, C.; Wyss, P. Angew. Chem., Int. Ed. 2007, 46, 7473–7476. (f ) Jia, W. L.; Chen, X. H.; Guo, R. W.; Sui-Seng, C.; Amoroso, D.; Lough, A. J.; Abdur-Rashid, K. Dalton Trans. 2009, 8301–8307. r 2009 American Chemical Society

Figure 1. Examples of transition metal complexes bearing amino-functionalized N-heterocyclic carbene ligands reported by Douthwaite and Oro. See refs 2 and 3.

particular, primary amino-functionalized NHC ligands (C-NH2) and their transition metal complexes containing such chelating C-NH2 ligands are targets since they might have the very high activity for polar bond hydrogenation observed for ruthenium(II) complexes containing chelating phosphino-amino ligands.4 Although the syntheses of these imidazolium salt precursors were thwarted by their low yields and problems with functional group compatibility,5 we show here that nitrile-functionalized N-heterocyclic carbene ligands6 provide useful synthons for primary aminofunctionalized NHC ligands by the reduction of a nitrilefunctionalized imidazolium salt under mild conditions. Once attached to nickel(II), this new type of primary aminofunctionalized NHC ligand can be moved to ruthenium(II) in an efficient transmetalation reaction to yield a precatalyst for ketone transfer hydrogenation.

(5) Busetto, L.; Cassani, M. C.; Femoni, C.; Macchioni, A.; Mazzoni, R.; Zuccaccia, D. J. Organomet. Chem. 2008, 693, 2579–2591. (6) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics 2009, 28, 853–862. Published on Web 11/18/2009

pubs.acs.org/Organometallics

6756

Organometallics, Vol. 28, No. 23, 2009

Scheme 1. Synthesis of a Homoleptic Primary Amino-Functionalized N-Heterocyclic Carbene Complex of Nickel(II) (2)

Results and Discussion Reduction of a Nitrile-Functionalized Imidazolium Salt to the First Homoleptic Primary Amino-Functionalized N-Heterocyclic Carbene Complex of Nickel(II). Caddick et al. and Khurana and Kukreja have reported the use of nickel(II) chloride and sodium borohydride in alcoholic solvents to reduce both aromatic and aliphatic nitriles to primary and secondary amines or to protected primary amines in high yields under mild conditions.7,8 We attempted to synthesize amino-functionalized imidazolium salts by the reduction of the corresponding nitriles by use of such an established procedure.7 The reaction of a stoichiometric amount of hydrated nickel(II) chloride and imidazolium salt 1 (m-CN-BF4) (Scheme 1) with an excess of sodium borohydride at low temperature produced an intractable mixture of products; nevertheless all of the imidazolium salt 1 was consumed. An aqueous workup of the reaction mixture in air, followed by counteranion metathesis with NH4PF6 in water, afforded a yellow solid, which was characterized as the novel homoleptic primary amino-functionalized N-heterocyclic carbene complex of nickel(II) ([Ni(m-CH2NH2)2](PF6)2, 2) in about 20% yield based on 1. The reaction conditions were further optimized by using anhydrous nickel(II) chloride to improve the purity of the product. The yield was increased to 30% by carrying out the reduction under a hydrogen atmosphere and using large volumes of methanol to dissolve the imidazolium salt, which otherwise has limited solubility (Scheme 1). To our knowledge, there are only two primary amino-functionalized N-heterocyclic carbene complexes, those of silver bromide and palladium dichloride reported by Douthwaite and co-workers.2 Only a few crystals of the palladium(II) complexes containing a chelating C-NH2 ligand were synthesized by slow hydrolysis of an imine linkage. Williams and co-workers reported the use of primary amino-functionalized imidazolium salts in enantioselective copper-catalyzed conjugate addition reactions.9 None of these copper complexes, however, were fully characterized or isolated. Complex 2 has been structurally characterized by the use of X-ray diffraction (Figure 2). The complex crystallizes in the orthorhombic chiral space group P212121 with four units residing in the unit cell. The structure shows a square-planar geometry about the metal center, with C(1)-Ni(1)-N(5) and C(1)-Ni(1)-C(12) bond angles of 91.8° and 88.1°, respectively. The Ni-Ccarbene bond distances are typical (7) Caddick, S.; Haynes, A. K. D.; Judd, D. B.; Williams, M. R. V. Tetrahedron Lett. 2000, 41, 3513–3516. (8) (a) Caddick, S.; Judd, D. B.; Lewis, A. K. D.; Reich, M. T.; Williams, M. R. V. Tetrahedron 2003, 59, 5417–5423. (b) Khurana, J. M.; Kukreja, G. Synth. Commun. 2002, 32, 1265–1269. (9) Moore, T.; Merzouk, M.; Williams, N. Synlett 2008, 21–24. (10) (a) Winston, S.; Stylianides, N.; Tulloch, A. A. D.; Wright, J. A.; Danopoulos, A. A. Polyhedron 2004, 23, 2813–2820. (b) Wang, X.; Liu, S.; Jin, G. X. Organometallics 2004, 23, 6002–6007. (c) Xi, Z.; Zhang, X.; Chen, W.; Fu, S.; Wang, D. Organometallics 2007, 26, 6636–6642.

O et al.

of analogous cationic nickel(II) NHC systems.10 The Ni-Namine bond distances are slightly longer than those of a nickel(II)-(2-methyl-1,2-propanediamine) complex (1.913 A˚)11 because of the higher trans influence of the carbene ligand. The amino protons on the carbene ligands were hydrogen bonded to the fluorine atoms of the hexafluorophosphate anions at distances between 2.18 and 2.60 A˚ (see Supporting Information). The phenyl rings are twisted with respect to the imidazolidene ring at dihedral angles of 52.76° and 55.01° to facilitate chelation. Furthermore, the identity of the title compound was established by the observation of a sharp singlet in the 13C{H} NMR spectrum in acetonitrile-d3 solution at 159.0 ppm assigned to carbene carbon Ni-Ccarbene, which is in the expected range for analogous cationic nickel(II) NHC systems.10 The identity of the complex was further established by the disappearance of a broad resonance at 2.80 ppm upon addition of D2O to a solution of 2, as a result of complete deuteration of the coordinated amino groups of the carbene ligands. Solution NMR Studies of the Axial Chirality of the Homoleptic Primary Amino-Functionalized N-Heterocyclic Carbene Complex of Nickel(II). The solid state structure of complex 2 reveals that the metal complex is axially chiral about the two chelating ligands, analogous to a 1,10 -binaphthyl linkage, but where the chiral and C2 rotational axes pass through the metal center, lying in the same plane as the carbene carbon and amine nitrogen atoms. In solution, the racemic mixture of complex 2 shows two diastereotopic protons on the methylene linker at 2.53 and 3.56 ppm, as a result of the chirality established on the 3-methylimidazolidene rings. Chiral square-planar complexes with no stereogenic centers on the coordinating ligands are rare. Mills and Quibell have reported the first optically resolved squareplanar complex of platinum(II) with meso-1,2-diphenylethylenediamine (dpen) and 1,1-dimethylethylenediamine ligands, but these have carbon stereocenters.12 Thereafter, there were reports of chiral square-planar complexes of pyridines without stereogenic centers, some of which have been optically resolved.13 Attempts to resolve the two enantiomers of 2, for example, by sodium L-tartrate failed, as indicated by circulardichroism (CD) spectra of the isolated solids. The use of enantiopure Δ-TRISPHAT as a NMR chiral shift reagent allows the observation of diastereotopic ion pairs if the nickel(II) cation is configurationally stable on the time scale of the NMR experiment.14 In this case the lifetime of the diastereomers must be greater than the inverse of the chemical shift difference (in Hz) between resonances of the diastereomers. The addition of 2 equiv of [Bu4N][ΔTRISPHAT] to an acetonitrile-d3 solution of complex 2 caused diamagnetic shifting and doubling of the resonances of the methyl and imidazolidene ring protons in the 1H NMR spectrum. The integration of signals remained in a ratio of 1:1 as expected starting with a configurationally stable (11) (a) Garcı´ a-Granda, S.; Beurskens, P. T.; Behm, H. J. J.; G omezBeltran, F. Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1987, 43, 236–238. (b) García-Granda, S.; Díaz, M. R.; Gomez-Beltran, F. Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1990, 46, 598–600. (12) Mills, W. H.; Quibell, T. H. H. J. Chem. Soc. 1935, 839–846. (13) (a) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.; Stoeckli-Evans, H. Inorg. Chem. 1996, 35, 4889–4895. (b) Biagini, M. C.; Ferrari, M.; Lanfranchi, M.; Marchio, L.; Pellinghelli, M. A. Dalton Trans. 1999, 1575–1580. (14) (a) Lacour, J.; Ginglinger, C.; Favarger, F.; Torche-Haldimann, S. Chem. Commun. 1997, 2285–2286. (b) Lacour, J.; Frantz, R. Org. Biomol. Chem. 2005, 3, 15-19, and references therein.

Article


6757

Figure 2. ORTEP diagram of 2 ([Ni(m-CH2NH2)2](PF6)2) depicted with thermal ellipsoids at 50% probability. The counteranions and most of the hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Ni(1)-C(1), 1.870(6); Ni(1)-C(12), 1.883(6); Ni(1)-N(5), 1.963(5); Ni(1)-N(6), 1.937(5); C(1)-Ni(1)-N(6), 175.4(3); C(1)-Ni(1)-N(5), 91.8(3); C(1)Ni(1)-C(12), 88.1(3); N(1)-C(1)-N(2), 104.6(6).

Figure 3. Selected sections of the 1H NMR spectra of complex 2 in acetonitrile-d3 (400 MHz, 298 K) and the assignments of the imidazolidene ring (left) and methyl protons (right) in the presence of (a) 0 equiv, (b) 1 equiv, (c) 2 equiv, and (d) 3 equiv of [Bu4N][Δ-TRISPHAT].

racemic mixture (Figure 3). The spectra are consistent with an ion pair structure with the Δ-TRISPHAT anion located near the 3-methylimidazolidene rings. Such stable ion pair structures have been reported by Macchioni and co-workers.15 No diamagnetic shifting and doubling of the resonances (15) (a) Macchioni, A. Chem. Rev. 2005, 105, 2039-2073, and reference therein. (b) Macchioni, A.; Ciancaleoni, G.; Zuccaccia, C. Zuccaccia, D. Chem. Soc. Rev. 2008, 37, 479–489.

were observed in dimethyl sulfoxide-d6 solution, which disfavors ion pairing. The doubling of peaks, however, is not consistent with mixed PF6/Δ-TRISPHAT ion pair structures because the integrations of the doubled peaks remained unchanged as the concentration of [Bu4N][Δ-TRISPHAT] increased. Indeed there will be a fast exchange of ion pairs in solution, leading to an average of the NMR properties of these mixed anion structures. However, diastereotopic diamagnetic shielding

6758


O et al.

Scheme 2. Synthesis of a Ruthenium(II) Complex Bearing a Primary Amino-Functionalized N-Heterocyclic Carbene Ligand (3) by the Transmetalation Reaction Involving Complex 2 and [Ru(p-cymene)Cl2]2

contributed by Δ-TRISPHAT in these structures clearly increased as the [Bu4N][Δ-TRISPHAT] concentration increased, as expected (Figure 3). Transmetalation Reaction of a Primary Amino-Functionalized N-Heterocyclic Carbene from Nickel(II) to Ruthenium(II). The transmetalation reaction to move the chelating C-NH2 ligand from complex 2 to the dimer [Ru(p-cymene)Cl]2 in refluxing acetonitrile solution afforded a deep green solution after 2.5 h (Scheme 2). Subsequent extraction with tetrahydrofuran (THF) or dichloromethane and recrystallization afforded a yellow-colored solid, which was characterized as [Ru(p-cymene)(m-CH2NH2)Cl]PF6 (3) by NMR spectroscopy and an X-ray diffraction study (Figure 4). Complex 3 is an air-stable solid, but it slowly decomposes in solution under prolonged exposure to air. To our knowledge, this is the first ruthenium(II) complex bearing a primary amino-functionalized N-heterocyclic carbene ligand. Complex 3 crystallizes in the orthorhombic chiral space group Pbca, with four pairs of enantiomers residing in the unit cell, as the metal complex is chiral about the ruthenium(II) center. The structure has a piano-stool geometry about the metal center with a planar arene ring coordinated to the metal center in an η6 fashion. The Ru-Ccarbene bond distance is typical of analogous ruthenium(II) complexes of the form [Ru(η6-arene)(NHC)D2]nþ (D = halides or mixed neutral/ halide ligands, n = 0 or þ1).16,17 The Ru-Namine bond distance is comparable to known bifunctional catalysts for polar bond hydrogenation with the general formula [Ru(η6arene)(diamine)X] (X = halides).18-20 Similar to complex 2, the amino protons on the carbene ligand were hydrogen bonded to the fluorine atom of the hexafluorophosphate anion and the oxygen atom of the solvent molecule at distances between 2.06 and 2.36 A˚ (see Supporting Information). The phenyl ring is twisted with respect to the imidazolidene ring at a dihedral angle of 54.59°, and the seven-membered ring with -Ru(1)-C(1)-N(2)-C(5)C(10)-C(11)-N(3)- linkage is nonplanar. In dichloromethane-d2 solution, the carbene carbon Ru-Ccarbene was observed as a singlet at 175.1 ppm in the (16) Herrmann, W. A.; Kocher, C.; Goossen, L. J.; Artus, G. R. J. Chem.-Eur. J. 1996, 2, 1627–1636. (17) (a) Geldbach, T. J.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J. Organometallics 2006, 25, 733–742. (b) Ozdemir, I.; Demir, S.; Cetinkaya, B.; Toupet, L.; Castarlenas, R.; Fischmeister, C.; Dixneuf, P. H. Eur. J. Inorg. Chem. 2007, 2862–2869. (18) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562–7563. (19) (a) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916–4917. (b) Hannedouche, J.; Clarkson, G. J.; Wills, M. J. Am. Chem. Soc. 2004, 126, 986–987. (c) Hayes, A. M.; Morris, D. J.; Clarkson, G. J.; Wills, M. J. Am. Chem. Soc. 2005, 127, 7318–7319. (20) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285–288.

Figure 4. ORTEP diagram of [Ru(p-cymene)(m-CH2NH2)Cl]PF6 (3) depicted with thermal ellipsoids at 50% probability. The counteranion, solvent molecule, and most of the hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Ru(1)-C(1), 2.092(5); Ru(1)-N(3), 2.146(4); Ru(1)-Cl(1), 2.4180(13); Ru(1)-C(16), 2.221(5); Ru(1)-C(17), 2.181(5); C(1)-Ru(1)-N(3), 91.98(17); C(1)-Ru(1)-Cl(1), 88.24(13); C(1)-Ru(1)-C(15), 160.13(19); C(1)-Ru(1)-C(12), 89.50(19); Cl(1)-Ru(1)-N(3), 81.81(11); N(1)-C(1)-N(2), 104.8(4). 13

C{H} NMR spectrum, which is in the expected range for analogous ruthenium(II) complexes of the form [Ru(η6-arene)(NHC)D2]nþ.16,21,22 Complete deuteration of the amino protons of the chelating carbene ligand by D2O was observed by the disappearance of two broad multiplets at 5.12 and 3.68 ppm in the 1H NMR spectrum. The use of enantiopure Δ-TRISPHAT is again useful to observe the diastereotopic ion pairs originating from the presence of two enantiomers in solution. Thus the addition of 1 equiv of [Bu4N][Δ-TRISPHAT] to a dichloromethaned2 solution of complex 3 caused diamagnetic shifting and doubling of the resonances of the imidazolidene ring protons and complete splitting of the C(2) proton of the η6-arene ring in the 1H NMR spectrum, while the relative integration of the split peaks remained in a ratio of 1:1 even when the concentration of [Bu4N][Δ-TRISPHAT] increased (Figure 5). Of note, an increase in the concentration of [Bu4N][Δ-TRISPHAT] caused a diamagnetic shifting of the NH proton, which is not observed in the same experiment with complex 2. A loss of the hydrogen bonding between the amino protons and the hexafluorophosphate anion and an increase in the shielding effect of the amino protons by the ring-current induced by the arene rings of the Δ-TRISPHAT anion might be responsible for such an observation. However, the observed diamagnetic shifting and doubling of the resonances of the protons of the 3-methylimidazolidene ring, the amino group, and parts of the arene ligand indicate that the Δ-TRISPHAT anion takes up various positions of ion pairing and rapidly moves between these locations to produce averaged NMR properties. The methyl protons of (21) Csabai, P.; Joo, F. Organometallics 2004, 23, 5640–5643. (22) Arnold, P. L.; Scarisbrick, A. C. Organometallics 2004, 23, 2519– 2521.

Article


6759

Figure 6. Catalytic transfer hydrogenation of acetophenone to 1-phenylethanol in the presence of 3, potassium tert-butoxide, and 2-propanol (6 mL) at 75 °C (C/B/S = 1:8:200). Table 1. Selected Crystal Data, Data Collection, and Refinement Parameters for 2 and 3a 2 empirical formula

1

Figure 5. Selected sections of the H NMR spectra of complex 3 in dichloromethane-d2 (400 MHz, 298 K) and the assignments of the imidazolidene ring (left), p-cymene ring (middle), and the isopropyl-methyl protons (right) in the presence of (a) 0 equiv, (b) 1 equiv, (c) 2 equiv, and (d) 3 equiv of [Bu4N][Δ-TRISPHAT].

the isopropyl group of the η6-arene ligand are least affected by the addition of up to 3 equiv of [Bu4N][Δ-TRISPHAT] (Figure 5). Similar to the case of complex 2, the extent of the diastereotopic diamagnetic shielding of the protons in 3 caused by Δ-TRISPHAT increased as the anion concentration increased, and this dominated over effects caused by anion exchange between the various ion pairs and solvated anions. Studies of Complex 3 as a Precatalyst for the Transfer Hydrogenation of Acetophenone. The tests of activity of complex 3 as a precatalyst for the transfer hydrogenation of acetophenone in 2-propanol are listed in Table 2. The ruthenium(II) complex 3 is not an active catalyst for the transfer hydrogenation of acetophenone at room temperature in the presence of potassium tert-butoxide and 2-propanol (Table 2, entry 1, catalyst/base/substrate = C/B/S = 1:8:200). At 75 °C, however, complex 3 catalyzed hydrogenation of acetophenone to 1-phenylethanol to 96% conversion in 3 h at the same C/B/S ratio (Table 2, entry 2, Figure 6). The substrate:catalyst loading could be increased to 1200 to achieve 82% conversion in 3 h and a maximum turnover frequency (TOF) of up to 880 h-1 (Table 1, entries 5 and 6). The nickel(II) complex 2, under the same conditions, showed no conversion of acetophenone to 1-phenylethanol, even after 18 h of reaction (Table 2, entry 11). The system with simply the dimer [Ru(p-cymene)Cl2]2 in basic 2-propanol gave a 91% conversion to 1-phenylethanol after 3 h with a higher TOF compared to that of complex 3 (Table 2, entry 12). However, the 1H NMR spectrum of complex 3 in solution did not show any trace of [Ru(p-cymene)Cl2]2 nor complex 2; thus the aforementioned catalytic activity is

fw lattice type space group T, K a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z Fcalc, Mg m-3 μ(Mo, KR), mm-1 F(000) cryst size, mm3 range θ collected, deg reflns collected/unique abs cor max. and min. transmn coeff params refined goodness of fit R1 (I > 2σ(I)) wR2 (all data) peak and hole, e A˚-3

3

C22H26F12N6NiP2

C21H27ClF6N3RuP 3 C4H8O 723.14 675.05 orthorhombic orthorhombic Pbca P212121 150 150 10.1340(2) 17.2032(3) 16.4739(6) 14.6600(8) 17.1019(6) 22.3736(10) 90 90 90 90 90 90 2855.10(15) 5642.6(4) 4 8 1.682 1.589 0.896 0.771 1464 2752 0.20 0.18 0.10 0.24 0.14 0.04 2.64 to 27.50 2.58 to 27.49 14 234/6387 33 763/6411 semiempirical from equivalents 0.955 and 0.847 0.993 and 0.805

390 344 1.029 1.000 0.0664 0.0559 0.1851 0.1602 0.920 and -0.487 1.345 and -0.857 P P P a Definition of R indices: R1 = (Fo - Fc)/ (Fo) ; wR2 = [ [w(Fo2 2 2 P 2 2 1/2 Fc ) ]/ [w(Fo ) ]] .

intrinsic to 3. Of note, ruthenium(II) complexes bearing η6-arene ligands and N-heterocyclic carbene21,23 or phosphine donors24 required refluxing temperatures and long reaction times for full conversion of acetophenone to 1-phenylethanol. Complex 3, on the other hand, gave more than a 90% conversion to 1-phenylethanol in 1 h under refluxing temperature (Table 1, entry 9). When additional acetophenone (C/S = 1:200) was added after 3 h to the 2-propanol reaction mixture (C/B/S = 1:8:200), the catalyst (23) (a) Fekete, M.; Joo, F. Collect. Czech. Chem. Commun. 2007, 72, 1037–1045. (b) Yigit, M.; Yigit, B.; Ozdemir, I.; Cetinkaya, E.; Cetinkaya, B. Appl. Organomet. Chem. 2006, 20, 322–327. (24) Chiu, P. L.; Lee, H. M. Organometallics 2005, 24, 1692–1702.

6760


O et al.

Table 2. Catalytic Transfer Hydrogenation of Acetophenone to 1-Phenylethanol in 2-Propanol entry/ complex

C/B/Sa ratio

base

temperature (°C)

conversionb / 1 h (%)



TOF (h-1)

1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 10/3 11/2 12/[Ru]c

1:8:200 1:8:200 1:4:200 1:16:200 1:8:600 1:8:1200 1:8:200 1:8:200 1:8:200 1:0:200 1:8:200 1:8:200

KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu NaOiPr KOH KOtBu none KOtBu KOtBu

25 75 75 75 75 75 75 75 85 75 75 75

0 87 65 62 82 66 67 71 94 0 0 81

0 94 87 85 89 78 89 88 96 0 0 88

0 96 96 95 90 82 95 94 0 0 91

0 273 156 132 674 883 151 187 360 0 0 412

a

C/B/S: catalyst/base/substrate. All reactions were carried out in 2-propanol (6 mL). b Conversions were determined by GC and were reported as an average of two runs. c [Ru] = [Ru(p-cymene)Cl]2.

nol in 97% ee and 98% yield at 28 °C after 10 h (C/B/S = 1:2:200).18,20 The related active catalysts, the amido complex Ru(p-cymene){(S,S)-TsNCHPhCHPhNH} and the hydrideamine complex RuH(p-cymene){(S,S)-TsNCHPhCHPhNH2}, showed the same catalytic activity without the use of base.20,25 These are referred to as bifunctional catalysts since both the metal and the amido ligand work together in the formation of the active hydride-amine species from 2-propanol and the transfer of the hydride from the ruthenium(II) center and proton from the amino group to the carbonyl group of the ketone.25 Further work is required to prove that the mechanism of catalysis of complex 3 also involves the bifunctional action of the amido group and ruthenium(II) center.

Conclusion

Figure 7. Addition of acetophenone (200 mg, C/S = 1:200) after 180 min of transfer hydrogenation of acetophenone in the presence of 3, potassium tert-butoxide, and 2-propanol (6 mL) at 75 °C (C/B/S = 1:8:200).

showed similar activity and reached an overall conversion to 1-phenylethanol of 93% (Figure 7). The catalytic activity, therefore, must be due to the presence of the same active species, but not any other decomposed products. This also suggested that the active catalyst is not poisoned by the presence of 1-phenylethanol that is generated, nor decomposed by exposure to the basic medium and reaction temperature. The effect of the amount of base present during catalysis was also investigated. It was found that the optimum activity of complex 3 occurred with a C/B ratio of 1:8, although a higher or lower C/B ratio gave similar conversions after 3 h, but with lower TOF (Table 2, entries 2-4). The use of sodium isopropoxide and potassium hydroxide in the same C/B ratio also gave similar conversions after 3 h, but with lower TOF compared to the use of potassium tert-butoxide (Table 2, entries 2, 7, and 8). In the absence of base, complex 3 was not active for the hydrogenation of acetophenone (Table 2, entry 10). The use of potassium hydroxide is informative. In this case the water byproduct did not affect the catalytic activity. Of note, Noyori and co-workers have shown that the ruthenium(II) complex Ru(p-cymene)(S,S)TsNCHPhCHPhNH2)Cl ((S,S)-TsNCHPhCHPhNH2 = (1S, 2S)-N-p-toluenesulfonyl-1,2-diphenylethylenediamine) in the presence of potassium hydroxide and 2-propanol catalyzed the transfer hydrogenation of acetophenone to 1-phenyletha-

In summary, we have reported the facile synthesis of a primary amino-functionalized N-heterocyclic carbene complex by reduction under mild conditions of a nitrile-functionalized imidazolium salt. The resulting nickel(II) complex (2) is an axially chiral square-planar complex, and the use of Δ-TRISPHAT as a NMR chiral shift reagent is particularly useful to observe the diastereotopic ion pairs. A transmetalation reaction that moved the chelating C-NH2 ligand from complex 2 to the [Ru(p-cymene)Cl]2 dimer afforded the first ruthenium(II) complex with a primary amino-functionalized N-heterocyclic carbene ligand, [Ru(p-cymene)(m-CH2NH2)Cl]PF6, (3). This catalyzed the transfer hydrogenation of acetophenone in basic 2-propanol at 75 °C in 3 h to give 1-phenylethanol with a TOF of up to 880 h-1. The catalytic activity was not affected by the choice of the strong base nor the presence of small amounts of water.

Experimental Section General Considerations. All of the preparations and manipulations, except where otherwise stated, were carried out under a nitrogen, argon, or hydrogen atmosphere using standard Schlenk-line and glovebox techniques. Dry and oxygen-free solvents were always used unless otherwise stated. Methanol was stirred over magnesium turnings and iodine chips overnight under an argon atmosphere, refluxed for 2-3 h, and distilled prior to use. The synthesis of 1-(2-cyanophenyl)-3-methylimidazolium tetrafluoroborate (m-CN-BF4, 1) has been reported (25) (a) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393–406. (b) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300– 1308.

Article previously.6 The synthesis of [Ru(p-cymene)Cl2]226 was reported in the literature. All other reagents and solvents were purchased from commercial sources and were used as received. Deuterated solvents were purchased from Cambridge Isotope Laboratories and were used as received. NMR spectra were recorded on a Varian 400 spectrometer operating at 400 MHz for 1H, 100 MHz for 13C, and 376 MHz for 19F. The 1H and 13C{1H} NMR were measured relative to partially deuterated solvent peaks but are reported relative to tetramethylsilane (TMS). All 19F chemical shifts were measured relative to trichlorofluoromethane as an external reference. All infrared spectra were recorded on a Nicolet 550 Magna-IR spectrometer. All UV-vis spectra were recorded on a Hewlett-Packard Agilent 8453 UV-vis spectrophotometer. The elemental analysis was performed at the Department of Chemistry, University of Toronto, on a Perkin-Elmer 2400 CHN elemental analyzer. Single-crystal X-ray diffraction data were collected using a Nonius Kappa-CCD diffractometer with Mo KR radiation (λ = 0.71073 A˚). The CCD data were integrated and scaled using the Denzo-SMN package. The structures were solved and refined using SHELXTL V6.1. Refinement was by full-matrix least-squares on F2 using all data. Details are listed in Table 1. Synthesis of Bis[1-(2-aminomethylphenyl)-3-methylimidazol2-ylidene]nickel(II) Hexafluorophosphate ([Ni(m-CH2NH2)2]PF6, 2). A Schlenk flask was charged with anhydrous nickel(II) chloride (239 mg, 1.8 mmol) and the imidazolium salt 1 (500 mg, 1.8 mmol). A warm methanol solution (48 mL) was added to the solid mixture under a hydrogen atmosphere, and the solution was stirred until all the imidazolium salt dissolved. A fresh, cold methanolic solution (12 mL) of sodium borohydride (489 mg, 12.9 mmol, previously prepared by dissolving sodium borohydride in methanol at 0 °C) was added via a syringe and a needle slowly to the orange slurry containing anhydrous nickel(II) chloride and the imidazolium salt at -78 °C, and vigorous effervescence occurred. The dark black slurry was stirred for 1 h at -78 °C, slowly warmed to room temperature, and stirred overnight. After the reaction had gone to completion, the solvent was removed in vacuo. The residue was extracted with commerical grade dichloromethane (20 mL) in air and filtered through a pad of Celite. If the filtrate was not clear, water (2 mL) was added to the dichlormethane solution and filtered through a pad of Celite again to remove all the black residue. The organic layer was then extracted with water (5 8 mL). The orange aqueous solution was filtered through a plug of cotton wool, and the clear solution was added to a saturated aqueous solution (1 mL) of ammonium hexafluorophosphate (363 mg, 2.2 mmol). The yellow-orange precipitate was then collected, rinsed with water (5 mL), and dried in vacuo. Yield: 200 mg, 30%. Suitable crystals for X-ray difrraction studies were obtained by slow diffusion of diethyl ether solution into a saturated solition of 2 in acetonitrile. 1H NMR (CD3CN, δ): 7.78 (dt, JHH = 1.19, 7.77 Hz, 5-CH of Ph, 1H), 7.75 (dd, JHH = 1.73, 7.77 Hz, 6-CH of Ph, 1H), 7.69 (dt, JHH = 1.73, 7.40 Hz, 4-CH of Ph, 1H), 7.62 (dd, JHH = 1.19, 7.40 Hz, 3-CH of Ph, 1H), 7.36 (d, JHH = 1.79 Hz, 5-CH of imid., 1H), 7.21 (d, JHH = 1.79, 4-CH of imid., 1H), 3.56 (dd, JHH = 3.89, 12.13 Hz, CH2, 1H), 3.44 (s, CH3, 3H), 2.80 (m, br, NH2, 2H), 2.53 (dt, JHH = 4.37, 12.22 Hz, CH2, 1H). 19 F NMR (CD3CN, δ): -72.4 (s), -74.3 (s). 13C{1H} NMR (CD3CN, δ): 159.0 (Ni-Ccarbene), 139.1 (CPh), 133.9 (CPh), 133.0 (CPh), 131.6 (CPh), 130.5(CPh), 126.5 (Cimid.), 126.2 (CPh), 125.2 (Cimid.), 44.1 (CH2), 37.8 (CH3). IR (KBr, cm-1): 3347, 3275 (ν(NH) stretch). UV-vis (acetonitrile; λmax (nm), ε (M-1 cm-1)): 380, 364. MS (ESI, methanol/water; m/z): 431.1 [M Hþ]þ. Anal. Calcd for C22H26F12N6NiP2: C, 36.54; H, 3.62; N, 11.62. Found: C, 36.78; H, 3.71; N, 12.69. Synthesis of [1-(2-Aminomethylphenyl)-3-methylimidazol-2ylidene]chloro-(η6-p-cymene)ruthenium(II) Hexafluorophosphate (26) Bennett, M. A.; Smith, A. K. Dalton Trans. 1974, 233–241.


6761

([Ru(p-cymene)(m-CH2NH2)Cl]PF6, 3). A Schlenk flask was charged with 2 (135 mg, 0.19 mmol) and [Ru(p-cymene)Cl]2 dimer (114 mg, 0.19 mmol). Dry acetonitrile (16 mL) was added to the reaction mixture, and it was refluxed under an argon atmosphere for 2.5 h to give a green solution. The solvent was then evacuated. The residue was extracted with tetrahydrofuran or dichloromethane (8 mL) and filtered through a pad of Celite. The volume of solvent was reduced (2 mL), and addition of diethyl ether (10 mL) to the tetrahydrofuran or dichloromethane solution yielded a yellow precipitate, which was collected and dried in vacuo. Yield: 135 mg, 60%. Suitable crystals for an X-ray diffraction studies were obtained by slow evaporation of the filtrate solution in tetrahydrofuran and diethyl ether under a nitrogen atmosphere. 1H NMR (CD2Cl2, δ): 7.69 (m, 3-CH of Ph, 1H), 7.60 (m, 4-CH and 5-CH of Ph, 2H), 7.48 (m, 6-CH of Ph, 1H), 7.37 (d, JHH = 1.92 Hz, 5-CH of imid., 1H), 7.35 (d, JHH = 1.92, 4-CH of imid., 1H), 5.50 (d, JHH = 5.80 Hz, 2-Ar-CH of p-cymene, 1H), 5.29 (d, JHH = 5.74 Hz, 6-Ar-CH of p-cymene, 1H), 5.15 (d, JHH = 5.74 Hz, 5-Ar-CH of p-cymene, 1H), 5.12 (m, br, NH2, 1H), 4.74 (d, JHH = 5.80 Hz, 3-Ar-CH of p-cymene, 1H), 4.07 (s, CH3, 3H), 3.96 (m, CH2, 1H), 3.68 (m, br, NH2, 1H), 2.86 (t, JHH = 11.19 Hz, CH2, 1H), 2.55 (sept, JHH = 7.01 Hz, CH of (CH3)2CH of p-cymene, 1H), 1.67 (s, CH3 of p-cymene, 3H), 1.13 (dd, JHH = 1.29, 7.01 Hz, CH3 of (CH3)2CH of p-cymene, 1H). 19F NMR (CD2Cl2, δ): -71.3 (s), -73.2 (s). 13C{1H} NMR (CD2Cl2, δ): 175.1 (Ru-Ccarbene), 138.8 (CPh), 132.8 (CPh), 131.6 (CPh), 130.4 (CPh), 130.0 (CPh), 125.9 (Cimid.), 125.6 (Cimid.), 124.5 (C Ph), 111.6 (CAr-p-cymene), 101.1 (CAr-p-cymene), 86.6 (CAr-p-cymene), 85.4 (CAr-p-cymene), 84.3 (CAr-p-cymene), 82.2 (CAr-p-cymene), 46.6 (CH2), 39.8 (CH3), 31.1 (CH of (CH3)2CH of p-cymene), 23.6 (CH3 of p-cymene), 21.0, (CH3 of (CH3)2CH of p-cymene), 18.7 (CH3 of (CH3)2CH of p-cymene). IR (KBr, cm-1): 3326, 3279 (ν(NH) stretch). UV-vis (acetonitrile; λmax (nm), ε (M-1 cm-1)): 391, 360. MS (ESI, methanol/water; m/z): 458.1 [M]þ. HRMS (ESI, methanol/water; m/z): calcd for C21H27N3ClRuþ [M]þ 458.0931, found 458.0915. Several attempts at elemental analyses failed to give an acceptable carbon content, while hydrogen and nitrogen content are in the acceptable range. Typical results: Anal. Calcd for C21H27F6N3ClPRu: C, 41.83; H, 4.51; N, 6.97. Found: C, 38.22; H, 4.41; N, 6.47. General Procedure for Transfer Hydrogenation Studies. A solution of acetophenone (200 mg, 1.7 mmol) in 2-propanol (6 mL) was added via a syringe and needle to a Schlenk flask charged with a mixture of 3 (5 mg, 0.0083 mmol) and potassium tert-butoxide (7 mg, 0.062 mmol) at 75 °C under an argon atmosphere. The solution became homogeneous upon stirring. Samples were taken from the reaction mixture periodically by a syringe and needle and were quenched by exposure to air. The samples were analyzed by gas chromatography (GC) using a Perkin-Elmer Autosystem XL chromatograph equipped with a chiral column (CP chirasil-Dex CB 25 m 2.5 mm). Hydrogen was used as a mobile phase at a column pressure of 5 psi with a split flow rate of 50 mL/min. The injector temperature was 250 °C, FID temperature was 275 °C, and the oven temperature was 130 °C. Retention times (tR/min) for acetophenone, 4.64; (R)-1-phenylethanol, 7.82; (S)-1phenylethanol, 8.27. All conversions were reported as an average of two GC runs. The reported conversions were reproducible.

Acknowledgment. The NSERC Canada is thanked for a Discovery Grant to R.H.M. and a postgraduate scholarship to W.W.N.O. Supporting Information Available: X-ray structural data as crystallographic file (CIF) for complexes 2 and 3 and schematics showing hydrogen bonding of complexes 2 and 3 (PDF). These materials are available free of charge via the Internet at http:// pubs.acs.org.

Transmetalation of a Primary Amino-Functionalized N-Heterocyclic

Recommend Documents