Methylene-Linked Bis-NHC Half-Sandwich Ruthenium Complexes: Binding of Small Molecules and Catalysis toward Ketone Transfer Hydrogenation

: The complex [Cp * RuCl(COD)] reacts with L H 2 Cl 2 ( L = bis(3-methylimidazol-2-ylidene)) and LiBu n in tetrahydrofuran at 65 ° C furnishing the bis-carbene derivative [Cp * RuCl( L )] ( 2 ). This compound reacts with NaBPh 4 in MeOH under dinitrogen to yield the labile dinitrogen-bridged complex [{Cp * Ru( L )} 2 ( μ - N 2 )][BPh 4 ] 2 ( 4 ). The dinitrogen ligand in 4 is readily replaced by a series of donor molecules leading to the corresponding cationic complexes [Cp * Ru(X)( L )][BPh 4 ] (X = MeCN 3 , H 2 6 , C 2 H 4 8a , CH 2 CHCOOMe 8b , CHPh 9 ). Attempts to recrystallize 4 from MeNO 2 /EtOH solutions led to the isolation of the nitrosyl derivative [Cp * Ru(NO)( L )][BPh 4 ] 2 ( 5 ), which was structurally characterized. The allenylidene complex [Cp * Ru  C  C  CPh 2 ( L )][BPh 4 ] ( 10 ) was also obtained, and it was prepared by reaction of 2 with HC  CC(OH)Ph 2 and NaBPh 4 in MeOH at 60 ° C. Complexes 3 , 4 , and 6 are e ﬃ cient catalyst precursors for the transfer hydrogenation of a broad range of ketones. The dihydrogen complex 6 has proven particularly e ﬀ ective,


■ INTRODUCTION
−3 This methodology has as an advantage the use of alcohols, 4 water, 5 or formic acid 6 instead of molecular hydrogen or metal hydrides as sources of the hydrogen atoms transferred in the reaction.Catalytic TH using transition metal complexes has received a great deal of attention due to its selectivity, efficiency, safety, broad scope, and compatibility with Green Chemistry principles.Homogeneous complexes based on transition metals such as Ir, 1,7 Ru, 1,8 or Rh 1,9 are often used as catalysts for TH. 1 However, the development of more efficient and selective transfer hydrogenation catalysts are still in demand.Among them, ruthenium complexes possess higher activity, selectivity, and cheaper cost in comparison to those of rhodium or iridium. 10HC ligands have been successfully used as ancillary ligands in the design and synthesis of homogeneous catalysts. 11These ligands are considered an alternative to phosphine ligands due to their stronger sigma-donor properties that confer a greater stability to the corresponding metallic complexes, and facilitate the modulation of their stereoelectronic properties. 12Chelation is a strategy used to stabilize the M−NHC bond and provide more robust complexes with different topological properties, namely steric hindrance, bite angle, or fluxional behavior. 13In contrast to their monodentate counterparts, the chemistry of transition-metal complexes based on bis(NHC) ligands have been far less explored. 1411a,16−18 However, the synthesis and catalytic application of their related hydrocarbon chain-linked bis(NHC) complexes is scarce in the literature and their potential synthetic applicability still needs to be studied.To the best of our knowledge, only a few reports have been described about the use of hydrocarbon chain-linked bis(NHC) complexes of ruthenium in TH of ketones 19 and hydrogenation of olefins. 20ollowing the recent works carried out in our research group in the preparation of TpRu complexes bearing the methylene-linked bis(NHC) ligand bis(3-methylimidazol-2-ylidene)methane (L), 21 we focused our attention in the preparation of homologous pentamethylcyclopentadienyl (Cp*) ruthenium complexes with the same ligand in order to compare the effect of the replacement of the supporting ligand Tp by Cp* on the reactivity of the metal center.We have now found that this modification enhances reactivity, and that several of the new complexes prepared in this way are efficient catalyst precursors for TH reactions of carbonyl groups.
■ RESULTS AND DISCUSSION Synthesis of Complexes and Their Interaction with Small Molecules. We have successfully used this methodology for the preparation of Ru 18,21,23 and Ni 24 complexes.In particular, we prepared the complex [TpRuCl(L)] by reaction of [TpRuCl(COD)] with L•Ag 2 Cl 2 in dichloroethane at 120 °C over 20 h. 21In an attempt to synthesize the homologous complex bearing Cp* instead of Tp, we carried out the reaction of [Cp*RuCl-(COD)] with L•Ag 2 Cl 2 in dichloroethane at 130 °C over 18 h.A cherry red microcrystalline product was obtained from this reaction.NMR spectroscopy showed that the compound was paramagnetic.Recrystallization from dichloromethane/petroleum ether yielded dark cherry red crystals.The X-ray structure analysis identified this product as the Ru III derivative [Cp*RuCl(L)]Cl (1).An ORTEP view of the complex cation [Cp*RuCl(L)] + in 1 is shown in Figure 1, together with the most relevant bond distances and angles.

Chart 1. Preparation of Complexes 1 and 2
Organometallics signals with a coupling constant of 11.6 Hz corresponding to the methylene bridge protons, which become diastereotopic upon bidentate coordination of the ligand to the Ru atom.The protons of the double bond of the imidazolylidene rings appear as two broad singlets near 7 ppm.Similar NMR features have been observed on the complex [TpRuCl(L)]. 21The 13 C{ 1 H} NMR signal of the equivalent carbene carbon atoms of 2 was not observed.It is assumed that it is too broad even at high temperature and it merges with the baseline.All the resonances for the other carbon atoms of the ligands L and Cp* appear in the expected positions.located as expected for Ru−NHC compounds. 12,13,17,23,25,26The temperature dependence of the NMR spectra is interpreted in terms of the rapid exchange of the chloride ligand with the THF-d 8 donor molecule (Chart 2).
In fact, the chloride ligand in 2 is easily replaced by a range of donor molecules in the presence of a suitable chloride scavenger.Thus, 2 reacts with acetonitrile and NaBPh 4 in MeOH at room temperature furnishing the complex [Cp*Ru-(MeCN)(L)][BPh 4 ] (3).Recrystallization from acetonitrile/ methanol yielded amber crystals suitable for X-ray structure analysis.An ORTEP view of the complex cation [Cp*Ru-(MeCN)(L)] + in 3 is shown in Figure 2, together with the most relevant bond distances and angles.
The complex cation in 3 has a three-legged piano stool structure.−27 The average dihedral angle between the plane defined by the atoms C(10)−Ru(1)−C(15) and the imidazol rings is 35.3°,still above the average values for the same angle in other complexes containing the same ligand L, but very similar to the value found for compound 1.The Ru−C distances for the NHC ligand of 2.029(4) and 2.035(4) Å are both of the same order and consistent with Ru−C separations expected for σ-bonds.The acetonitrile ligand is almost linearly assembled to ruthenium.The Ru(1)−N(5) and N(5)−C(20) bond lengths of 2.051(3) and 1.134(4) Å respectively compare well with the values of 2.043(5) and 1.128(7) Å found in [Cp*Ru(MeCN)-(κ 2 -C,N-picolyl-Me I)][BAr′ 4 ] (picolyl-Me I = 3-methyl-1-(2picolyl)imidazol-2-ylidene). 17All other dimensions in the structure are in the expected ranges and are unexceptional.Complex 3 is far more stable toward atmospheric oxygen than 2. It can be handled as a solid in the air without visible decomposition or oxidation.It shows a medium intensity IR band at 2252 cm −1 corresponding to ν(CN) in the acetonitrile ligand.Its 1 H NMR spectrum in acetonitrile-d 3  shows the two characteristic AB doublet signals for the methylene bridge protons as expected.In this case, the 13 C{ 1 H} NMR signal of the equivalent carbene carbon atoms of 3 appear clearly on the spectrum at 193.0 ppm, i.e., in the expected range for Ru−NHC compounds.Compound 3 is a convenient and efficient catalyst precursor for hydrogen transfer reactions to ketones, as discussed below.
Complex 2 reacts with NaBPh 4 in methanol under N 2 yielding a yellow microcrystalline precipitate which exhibits a strong band at 2050 cm −1 in its Raman spectrum, attributable to ν(NN) in a bridging dinitrogen ligand attached to two Ru atoms.As expected, this band is inactive and does not show on the IR spectrum.This observation is consistent with the formation of the binuclear complex [{Cp*Ru(L)} 2 (μ-N 2 )]-[BPh 4 ] 2 (4).The value for the Raman ν(NN) band compare well with data in the literature for bridging dinitrogen complexes of ruthenium. 23,29,30The position of this band in 4 appears shifted ca.50 cm −1 to lower wavenumbers with respect to the ν(NN) in the homologous complex [{TpRu(L)} 2 (μ-N 2 )][BAr′ 4 ] 2 (2106 cm −1 ), 21 consistent with a slight increase of the relaxation of the coordinated NN in 4 in comparison with the latter.At variance with [{TpRu(L)} 2 (μ-N 2 )][BAr′ 4 ] 2 , which contains a dinitrogen ligand strongly bound to ruthenium, compound 4 is very labile and the compound is very reactive.Whereas the reaction of [{TpRu(L)} 2 (μ-N 2 )][BAr′ 4 ] 2 with CO or acetonitrile is very sluggish, 4 reacts readily with MeCN furnishing complex 3.It also reacts with chlorinated solvents such as dichloromethane and chloroform yielding dark red solutions with display complex NMR spectra.We found nitromethane as the best solvent to be used in this system due to its polarity and poor coordinating abilities.Thus, clean 1 H and 13 C{ 1 H} NMR spectra were obtained in nitromethane-d 3 .One singlet for Cp* and two AB doublet signals for the methylene bridge protons are the most characteristic features of the 1 H NMR spectrum.The equivalent carbene carbon atoms appear as one singlet at 185.1 ppm on the 13 C{ 1 H} NMR spectrum.We attempted recrystallization of 4 in a mixture nitromethane/ethanol. Dark orange crystals were obtained.These crystals showed a strong band at 1813 cm −1 on the IR spectrum.This band was absent in the compound prior to recrystallization.X-ray structure  31 and in the Ru II nitrosyl complexes with pyridine-functionalized N- 32 The formation of the nitrosyl complex 5 at the expense of the dinitrogen derivative 4 is completely unexpected.The crystals of 5 were isolated in low yield based on ruthenium, and the origin of the nitrosyl ligand is unclear.We can only speculate that it comes from nitromethane, but we are unsure about the chemical reactions leading to the ultimate products, as there are no precedents for this.One reviewer tentatively suggested the possibility of formation of the nitrosyl complex 5 by the transfer hydrogenation between nitromethane and EtOH during recrystallization.In fact, we assume that other metalcontaining products must be formed in the process, but we failed to identify any other species.We monitored by NMR a solution of 4 in nitromethane-d 3 under dinitrogen over several days.We were able to detect gradually increasing signals for the nitrosyl compound 5, mixed with many other resonances and broad features corresponding to unidentified species.We did not detect the formation of any hydride containing species in the overall process, which is not clean nor simple, and the nitrosyl species 5 was the only fully characterized compound in this most intriguing reaction sequence.
Compound 4 also acts as a catalyst precursor for ketone hydrogen transfer reactions (vide infra).The dinitrogen ligand in 4 is readily replaced by a number of donor molecules furnishing the corresponding complexes.These reactions are summarized in Scheme 1.
Thus, 4 reacts with H 2 in nitromethane affording the labile dihydrogen complex [Cp*Ru(H 2 )(L)][BPh 4 ] (6).This compound was isolated by reaction of 2 with NaBPh 4 in methanol under a dihydrogen atmosphere in the form of an off white solid.All subsequent manipulations were performed under an atmosphere of argon unless otherwise stated.The 1 H NMR spectrum of 6 in nitromethane-d 3 under dihydrogen shows a broad resonance at −7.81 ppm attributable to the protons of the coordinated dihydrogen.As it is characteristic of dihydrogen ligands, 33 this resonance exhibits a short longitudinal relaxation time T 1 of 21.4 ms at 248 K (500 MHz).This was the shortest measured T 1 , as the minimum value could not be determined due to freezing of the solvent.In any case, the value is fully consistent with the presence of a coordinated dihydrogen molecule.In general, dihydrogen complexes stabilized with NHC-ligands are not very common. 34We were not able to prepare the homologous complex [TpRu(H 2 )(L)] + , 21 an observation that emphasizes the difference between the {Cp*Ru(L)} and {TpRu(L)} moieties.The dihydrogen complex 6 has also been successfully used as catalyst precursor for ketone hydrogen transfer  reactions.The dihydrogen ligand in 6 is readily deprotonated by strong bases yielding the corresponding neutral monohydride complex.Thus, the reaction of 6 with KOBu t in tetrahydrofuran under dihydrogen afforded [Cp*RuH(L)] (7) in high yield as a yellow microcrystalline, air-sensitive solid (Chart 3).
A medium band at 1780 cm −1 in the IR spectrum is attributable to ν(RuH), whereas one singlet at −11.65 ppm in the 1 H NMR spectrum corresponds to the hydride proton.Since 7 it is formed in strongly basic conditions, it is assumed that this compound must be involved in the catalytic ketone hydrogen transfer reactions described below, and it possibly represents a key intermediate in the overall catalytic cycle.
Compound 4 also reacts with olefins CH 2 CHR (R = H, COOMe) to yield the corresponding ).These compounds were not isolated.They were generated in solution either by bubbling ethylene or adding an excess of methyl acrylate to a nitromethane-d 3 solution of 4 under argon, and were characterized in solution by NMR spectroscopy.The resonances for the proton and carbon atoms of coordinated ethylene in 8a appear at 2.14 and 47.2 ppm on the respective 1 H and 13 C{ 1 H} NMR spectra.In the case of 8b, only one diastereoisomer is observed as indicated by one single Cp* signal in the 1 H NMR spectrum, but the presence of the COOMe substituent on the η 2 -alkene ligand renders the imidazolylidene rings inequivalent.Thus, two resonances are observed for the methyl substituents at the nitrogen, and four separate resonances for the H and C atoms in the positions 3 and 4 of the NHC rings.Most notably, two carbene carbon atom signals at 180.8 and 183.9 ppm are present on the 13 C{ 1 H} NMR spectrum.The two AB doublet signals for the methylene bridge protons are maintained.
Compound 4 reacts with freshly prepared solutions of phenyldiazomethane in tetrahydrofuran under argon yielding the carbene complex [Cp*RuCHPh(L)][BPh 4 ] (9) (Scheme 1).This compound can be also prepared starting from the dihydrogen derivative 6 in a similar fashion.We have used a similar synthetic approach in the past for the preparation of the carbene complexes [Cp*RuCHPh(κ 2 -P,N-i Pr 2 PNHPy)][BAr′ 4 ] 35 (Py = C 5 H 4 N) and [TpRu CHPh(Cl)(PMe i Pr 2 )]. 36The 1 H NMR spectrum of compound 9 is characterized by the presence of the carbene proton resonance at 15.7 ppm.This resonance is correlated with a signal at 287.2 ppm in the 13 C{ 1 H} spectrum as shown by a gHSQC 2D NMR experiment.The carbene carbon atoms of the L ligand appear at 184.0 ppm, i.e., in the expected range.Compound 9 is remarkably stable, and unreactive toward ethylene in nitromethane solution.Alkylidene complexes can be protonated by strong acids, and thus we prepared the dicationic η 2 -benzyl derivative [Cp*Ru(κ 2 -P,N-P i Pr 2 NHPy)-(η 2 -CH 2 C 6 H 5 )][CF 3 SO 3 ] 2 by reaction of [Cp*RuCHPh(κ 2 -P,N-i Pr 2 PNHPy)][BAr′ 4 ] with an excess of trifluoromethanesulfonic acid in dichloromethane. 35Addition of an excess of HBF 4 •OEt 2 to 9 in nitromethane at room temperature led to an immediate color change of the solution from dark red to green.However, the 1 H NMR did not show resonances for any recognizable species.It seems that the products undergo decomposition under these reaction conditions, and the system was not further investigated.
We have been studying in depth the reactivity of allenylidene ligands bound to a ruthenium center supported by Nheterocyclic carbene ligands in the general context of the activation of propargyl alcohols by the Ru−NHC fragment. 21hus, the reactivity toward the addition of nucleophiles to the allenylidene ligand attached to the fragment {[TpRu(L)] + } has been studied in detail, and it was compared to that of related systems containing tertiary phosphines as ancillary ligands.We found that nucleophilic additions to the allenylidene ligand may take place alternatively at the C α or C β atoms depending on the R substituents present on the propargyl alcohol, and on the nature of the incoming nucleophiles.We wanted to see the effect of replacing Tp by the stronger donor Cp* ligand on the reactivity of the allenylidene complex.Hence, we prepared the allenylidene derivative [Cp*RuCCCPh 2 (L)][BPh 4 ] (10) by reaction of 2 with HCCC(OH)Ph 2 and NaBPh 4 in MeOH at 60 °C (Chart 4).
The allenylidene complex 10 shows a strong ν(CCC) band in the IR spectrum at 1880 cm −1 .The signals observed at δ 274.9, 225.6, and 139.8 ppm in the 13 C{ 1 H} NMR spectrum are respectively characteristic for the C α , C β , and C γ atoms of the allenylidene ligand, whereas the resonance for the carbene carbon atoms of the L ligand appear at 178.0 ppm.The signal for the C α atom in 10 is shifted upfield ca.30 ppm compared to the value observed for the homologous complex [TpRu CCCPh 2 (L)][BPh 4 ] (305.0 ppm). 21Likewise, the IR ν(CCC) band for the allenylidene ligand in 10 appears also shifted to lower wavenumbers compared to its TpRu counterpart (1913 cm −1 ).The latter exhibits a rich reactivity toward a variety of N-and S-donor nucleophiles such as pyrazole, piperidine, 2-pyridinethiol or 1,3-benzenedithiol leading to vinylcarbene complexes resulting from the addition to the C α atom of the allenylidene ligand. 21At variance with this, complex 10 is apparently much less reactive toward nucleophiles, and did not show any visible reaction with pyrazole at 55 °C in acetone-d 6 after 18 h.Ketone Transfer Hydrogenation.The transfer hydrogenation of ketones was explored using acetophenone (11) as Organometallics model substrate, with 2-propanol as the hydrogen source in the presence of 0.5 mol % of ruthenium complexes 3, 4, and 6 at 80 °C.A catalytic amount of KO i Pr was initially used as base.The results given in Table 1 showed that catalysts 3 and 4 led to the desired product (±)-11a in low yields and TOF values after 2 h (Table 1, entries 1 and 3).With longer reaction times (16 h), yields increased to 75% and 79%, respectively (Table 1, entries 2 and 4).In contrast, complex 6 was found to be the best catalyst toward transfer hydrogenation under the same reaction conditions affording compound (±)-11a in 75% yield after 2 h (Table 1, entry 5).When KOH was used as base instead of KO i Pr, the yield increased up to 94% in the presence of complex 6 (Table 1, entry 6).Reduction of the amount of KOH to 5 mol % gave an identical result (Table 1, entry 7).These new conditions led to a higher catalytic activity after 1 h affording (±)-11a in 81% yield with TOF value of 162 h −1 (Table 1, entry 8).However, the reaction did not take place in the absence of base (Table 1, entry 9).Interestingly, when the catalyst loading of complex 6 was reduced to 0.1 mol %, compound (±)-11a was formed in a similar yield with a TOF value of 455 h −1 after 2 h (Table 1, entry 10).Finally, we confirmed that compound (±)-11a was only obtained in 10% yield in the absence of catalyst (Table 1, entry 11).
On the other hand, the transfer hydrogenation of α,βunsaturated ketones 21−22 was performed to produce the corresponding alkanols (±)-21a and 20a with both CC and CO reduced in 90% and 99%, respectively (Table 2, entries 10−11).
We further extended our study to aldehydes 23−25.Unfortunately, the reaction failed to give the corresponding reduction products.The Guerbet-type alcohol 23a was obtained in 83% yield from octanal (23) (Table 2, entry 12) (Chart 5, a).The Guerbet reaction involves homocoupling of primary alcohols to obtain β-alkylated dimer alcohols where the new C−C bond is generated by an aldol condensation. 37ecently, Ir, Rh, and Ru complexes have been reported to catalyze the Guerbet reaction and produce higher-order alcohols in a more economical and environmentally friendly manner. 38In most cases, these catalytic systems required temperature higher than 100 °C.Compound 23a was also prepared starting from octanol in 80% yield in a similar fashion.Moreover, benzaldehyde derivative 24 performed a cross-coupling with 2-propanol to afford the saturated dimer 24a in 56% yield, whereas 2-furfural (25) gave the unsaturated dimer 25a as major product (Table 2, entries 13−14).These findings indicate that complex 6 could be an efficient catalyst precursor for β-alkylation of secondary alcohols and αalkylation of ketones by borrowing hydrogen reaction (Chart 5, b). 39n comparison with previously reported ruthenium(II) complexes bearing chelating NHCs, our catalytic system based on methylene-linked bis(NHC) ligands exhibits similar efficiency in a wide scope of substrates even with lower catalyst loadings allowing significant shortening of reaction times. 17,18n this sense, compound 6 seems to perform in ketone hydrogen transfer somewhat less efficiently than the picolylcarbene complex [Cp*Ru(MeCN)(κ 2 -C,N-picolyl-Me I)][PF 6 ] (picolyl-Me I = 3-methyl-1-(2-picolyl)imidazol-2-ylidene) previously reported by our research group. 17This compound had already shown an excellent catalytic activity and wider scope compared to other previously reported ruthenium 40 or even iridium 41 pyridyl−NHC complexes.The hydride transfer is a key step in many catalytic transformations, and further studies are in progress aimed to extend the use of these new complexes as catalysts for reactions of N-alkylation of amines with alcohols.

■ CONCLUSION
We have hereby reported the synthesis and characterization of a series of novel pentamethylcyclopentadienylruthenium(II) complexes bearing the methylene linked bis(NHC) ligand bis(3-methylimidazol-2-ylidene)methane.The binuclear dinitrogen-bridged compound 4 displays a rich reactivity toward a range of small molecules.Compound 4 undergoes an unprecedented degradation in nitromethane solution leading to the nitrosyl derivative 5, which has been structurally characterized.Compounds 3, 4, and 6 act as catalyst precursors for the transfer hydrogenation of acetophenone.In particular, the dihydrogen complex 6 has proven to be a very efficient catalyst precursor, reaching a TOF of up to 455 h −1 at catalyst loadings of 0.1% mol.Moreover, 6 exhibits high tolerance of functional groups on the reduction of a broad scope of aryl and aliphatic ketones giving rise the corresponding alcohols in high yields.Additionally, the transfer hydrogenation of aldehydes using 6 as a catalyst afforded Guerbet-type reaction products and the β-alkylation of 2-propanol under mild conditions.The study of further organic transformations mediated by these novel complexes is currently in progress in our laboratory, and will be reported in due course.

■ EXPERIMENTAL SECTION
All synthetic operations were performed under a dry dinitrogen or argon atmosphere following conventional Schlenk techniques.Tetrahydrofuran, diethyl ether, and petroleum ether (boiling point range 40−60 °C) were obtained oxygen-and water-free from a solvent purification apparatus.All other solvents (acetonitrile, dichloromethane, toluene, nitromethane, methanol, 2-propanol) were of anhydrous quality and used as received.All solvents were deoxygenated immediately before use.[Cp*RuCl(COD)] was prepared according to the literature. 42The imidazolium salt bis(3methylimidazolium)methane dichloride ([LH 2 ]Cl 2 ) was prepared following suitable adaptations of published procedures. 25IR spectra were taken in Nujol mulls on a FTIR spectrophotometer.Raman spectrum was recorded at the Instituto de Ciencia de Materiales-CSIC on a dispersive Raman microscope equipped with a He−Ne laser (λ = 532.14nm) using a working power of 0.2 mW in order to avoid overheating and alteration of the sample.Optical rotations were determined with a digital polarimeter.NMR spectra were taken on spectrometers operating at 400 or 500 MHz ( 1 H frequency).Chemical shifts are given in ppm from SiMe 4 ( 1 H and 13 C{ 1 H}). 1 H and 13 C{ 1 H} NMR spectroscopic signal assignments were confirmed by 1 H-gCOSY, gHSQCAD ( 1 H− 13 C), and gHMBCAD ( 1 H− 13 C) experiments.As a general feature, in the spectra of cationic compounds the signals for the [BPh 4 ] − anion are omitted.High resolution mass spectroscopy (HRMS) was performed in a Q-TOF mass spectrometer in the positive ion ESI mode.Silica gel (Merck) was used for column chromatography.TLC was performed on Merck Kiesegel 60 F254, 0.25 mm thick.GC-MS analyses were recorded in a Bruker Scion GC-TQ gas chromatograph coupled to a Bruker TQ mass spectrometer.Microanalyses were performed at the Servicio Central de Ciencia y Tecnologi ́a, Universidad de Cadiz.
[Cp*RuCl(L)]Cl 1.A Fisher-Porter vessel was charged with a mixture of bis(3-methylimidazolium)methane dichloride (0.25 g, 1 Chart 5. Formation of Products Derived from Guerbet-Type Reactions Organometallics mmol) and Ag 2 O (0.28 g, 1.2 mmol).It was protected from the light by wrapping with aluminum foil.Then, 1,2-dichloroethane (15 mL) was added, and the mixture stirred at room temperature for 18 h.After this time, a solution of [Cp*RuCl(COD)] (0.38 g, 1 mmol) in 1,2-dichloroethane (15 mL) was added.The resulting mixture was stirred for further 18 h at 130 °C.A purple suspension was obtained.It was filtered through Celite, and the Celite washed with two portions of dichloromethane.The solvent of the filtered solution was removed in vacuo.The resulting purple microcrystalline product was washed with petroleum ether and dried in vacuo.Recrystallization from dichloromethane/petroleum ether afforded crystals suitable for X-ray structure analysis.Yield (based upon Ru): 0.21 g, 43%.Calcd for C 19 H 27 N 4 Cl 2 Ru: C, 47.21; H, 5.63; N, 11.59.Found: C, 47.29; H, 5.71, N, 11.30.The product is paramagnetic.
[Cp*RuCCCPh 2 (L)][BPh 4 ] 10.To a mixture of 2 (0.15 g, 0.33 mmol), NaBPh 4 (0.3 g, excess) and HCCC(OH)Ph 2 (0.1 g, 0.48 mmol) MeOH (15 mL) was added.An orange precipitate was immediately formed.The mixture was stirred overnight at 60 °C.During this time, the color of the mixture changed to dark red.The mixture was cooled to room temperature and then to −20 °C.The dark red crystalline precipitate was filtered off, washed with ethanol and petroleum ether, and dried in vacuo.Yield: 0. General Procedure for the Catalytic Transfer Hydrogenation of Carbonyl Compounds.To a solution of the corresponding ruthenium catalysts 3, 4, or 6 (0.005 mmol) and KOH (0.05 mmol) in degassed isopropanol (1 mL) under argon, the corresponding ketones 11−22 or aldehydes 23−25 (1 mmol) were added.The reaction mixture was stirred at 80 °C for 2 h and then evaluated by TLC and GC-MS.Solvent was evaporated under reduced pressure and the crude was purified by silica gel column using petroleum ether and ethyl acetate mixtures to afford compounds 11a−25a.Full characterization data for each of the isolated organic products are given in the Supporting Information.
Crystal Structure Analysis.Crystals suitable for X-ray structural determination were mounted on glass fibers and then transferred to a Bruker Smart CCD three-circle diffractometer with a sealed-tube source and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at the Servicio Central de Ciencia y Tecnologi ́a de la Universidad de Cadiz (1) or Servicios Tećnicos de Investigacioń de la Universidad de Alicante (3 and 5).In each case, three (3 and 5) at 298 K or four at 100 K (1) sets of frames were recorded over a hemisphere of the reciprocal space by ω scans with δ(ω) = 0.30°and exposure of 10 to 20 s per frame.In the case of 3 and 5, an additional run at ϕ = 0°of 100 frames was collected to improve redundancy.The diffraction frames were integrated using the program SAINT 43 and the integrated intensities were corrected for Lorentz-polarization effects with SADABS. 44An insignificant crystal decay correction was also applied.The structure of 1 was solved by Patterson method, and direct methods were used for 3 and 5.All structures were refined on F 2 by full-matrix least-squares (SHELX97) 45 using all unique data.All nonhydrogen atoms were refined anisotropically.Hydrogen atoms were placed at idealized positions and refined as rigid atoms.ORTEP was used for plotting. 46CCDC 1483010, 2056271−2056272 contain supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Chart 3 . 6 Chart 4 .
Deprotonation of the Dihydrogen Complex Preparation of the Allenylidene Complex 10

Table 1 .
Optimization of Reaction Conditions for Transfer Turnover frequency (moles of ketone converted to alcohol per mole of catalyst per hour).d 4 mmol of acetophenone in 4 mL of 2propanol.
a Unless noted otherwise, reactions were carried out with 2 mmol of acetophenone in 2 mL of 2-propanol at 80 °C.bDetermined by GC-MS.c