Comparison of the Catalytic Activity of [(η5‑C5H5)Ru(2,2′-bipyridine)(L)]OTf versus [(η5‑C5H5)Ru(6,6′-diamino-2,2′-bipyridine)(L)]OTf (L = labile ligand) in the Hydrogenation ofCyclohexanone. Evidence for the Presence of a Metal−LigandBifunctional Mechanism under Acidic ConditionsDomenico DiMondo,† Michelle E. Thibault,† James Britten,‡ and Marcel Schlaf*,†

†The Guelph-Waterloo Centre for Graduate Work in Chemistry, (GWC)2, Department of Chemistry, University of Guelph, Guelph,Ontario, Canada N1G 2W1‡Department of Chemistry McMaster University, Hamilton, Ontario, Canada L8S 4M1

*S Supporting Information

ABSTRACT: The two title complexes as well as the dimeric complex [Ru(II)(η5-C5H5)(6,6′-diamino-2,2′-bipyridine)]2(OTf)2have been synthesized and characterized by NMR and single-crystal X-ray crystallography. The direct structural comparison ofthe 2,2′-bipyridine and 6,6′-diamino-2,2′-bipyridine complexes suggests that the electronic and steric environments of theruthenium centers in both complexes are essentially equivalent, providing for a unique opportunity to probe the influence of thenoncoordinated amine substituent on the relative reactivity and catalytic activity of the complexes. Opposite to what would beanticipated on the basis of steric effects, the bulkier amine-substituted ligand results in a catalyst showing substantially higheractivity in the hydrogenation of cyclohexanone in acidic medium, which is attributed to the operation of a metal−ligandbifunctional hydrogenation mechanism mediated by the amine substituents in their protonated form acting as proton shuttles.

■ INTRODUCTION

The concept of a metal−ligand bifunctional mediatedheterolytic activation of H2(g) by ruthenium, iridium, andmore recently iron-based complexes has led to the developmentof a wide variety of extremely efficient hydrogenation andtransfer hydrogenation catalysts with wide applicability to CO and CN bond containing substrates.1−5 This concept, inwhich a donor atomtypically but not always an aminefunctionalityacts as proton shuttle in a proton (“H+”)transfer from the donor atom and hydride (“H−”) transferfrom the metal to the unsaturated substrate,6 has also beentermed outer-sphere hydrogenation catalysis.7 A key feature of thisprocess is thatin contrast to classical hydrogenation catalystssuch as the Wilkinson catalystthe substrate does not directlycoordinate to the metal.Arguably the first reported example of a (transfer-)

hydrogenation catalyst operating by an outer-sphere mechanism

was the Shvo system.8−16 The mechanism of the Shvo-typesystems has been extensively studied and discussed by bothBackvall17−21 and Casey,22−32 with the latter extending it to ananalogous iron-based system.33,34 More typical of metal−ligandbifunctional systems are complexes in which the donor atomacting as the proton shuttle is either a directly coordinated orpendant amine function such as those developed byNoyori2,35−38 and Morris39−42 for the asymmetric (transfer-)hydrogenation of ketones and a pincer-type complex43 byMilstein that is active in the hydrogenation of esters.44 Furtherexamples of and mechanistic studies on these types of metal−ligand bifunctional catalysts were reported by Gusev,45,46

Saudan,47 and Bergens.48−51 In these three examples it isfurther postulated that the coordinated nitrogen donor atom

Received: August 29, 2013

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can act as a noninnocent ligand by switching from an amine to anamide, for example, either by deprotonation (for the aliphaticamine ligands) or by dearomatization of the pincer backbone.52

Typically these systems operate under basic or neutralconditions.53

Two examples of complexes with pendant amine functionscapable of heterolytic activation of dihydrogen are shown inScheme 1. While, to our knowledge, the iridium complexshown on the left has not actually been employed as ahydrogenation catalyst, it also exhibits cooperative binding ofketones via the N−H group and the metal, a feature that maybe considered an intermediate or resting state of structurallysimilar systems that are active catalysts. The rutheniumcyclopentadienyl complex shown on the right is active in thehydrogenation of CO2 to formic acid and was reported byLau.54

Further examples of catalytically active complexes withnoncoordinated pendant N−H or O−H functions on theligand backbone that operate under basic or base-activatedconditions and for which a metal−ligand bifunctional hydrogenactivation was inferred by spectroscopic, isotope tracing, ortheoretical means were reported by Sola and Oro,58 Thiel,59

Papish,60 Szymczak,61 and Ikariya.62

In the context of our ongoing research program to develophigh-temperature, acid- and water-stable homogeneous hydro-genation catalysts with potential applications to the selective ortotal deoxygenation of biomass-derived sugars, sugar alcohols,and their condensates,63,64 we recently described the use of thehalf-sandwich complexes [(η6-arene)Ru(X)(N∩N)](OTf)n (X= H2O, H, η

6-arene = p-Me-iPr-C6H4, C6Me6, N∩N = 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen), 6,6′-diamino-bipy (dabipy), 2,9-diamino-phen (daphen), n = 1, 2) and

[Cp(*)Ru(L)(N∩N)](OTf) (L = H2O or CH3CN, N∩N =bipy or phen) as catalysts for the deoxygenation of the sugaralcohol model system 1,2-hexanediol to 1-hexanol andhexane.65 The structures of the catalysts employed are shownin the top row of Chart 1. In this study we postulated that,induced by the operation of a metal−ligand bifunctionalmechanism, the catalysts with the 6,6′-diamino-bipy (dabipy)or 2,9-diamino-phen (daphen) ligands should exhibit a highercatalytic activity.However, the experimental results for the η6-arene systems

clearly established that (a) for these complexes those bearingthe pendant amine function show a lower catalytic activity thanthe parent bipy and phen complexes in the hydrogenation ofcarbonyl substrates to alcohols and (b) in the complexes [(η6-arene)Ru(X)(N∩N)](OTf)n (X = H2O, Cl, N∩N = dabipy,daphen) the amino substituent on the ligand cannot beprotonated in aqueous acidic medium whose acidity isdue tosolvent levelinglimited by that of the acidity of H3O

+ (pKa =−1.74).Together these observations excluded the operation of a

metal−ligand bifunctional mechanism for these η6-arene-basedsystems in aqueous acidic medium with the lower catalyticactivity of the dabipy- and daphen-based catalysts thenattributed to the steric demand of the amino functions onthe ligand, which hinders substrate access to the metal center.Also these catalysts are deactivated by loss of their arene ligandat temperatures above 125 °C.Postulating that the corresponding Cp(*)-based systems

would result in more stable and active catalysts based on thestronger bond between the metal and anionic Cp(*) ligand, wesubsequently synthesized and tested the complexes shown inthe second row of Chart 1.66 The Cp(*) complexes are stable

Scheme 1. Complexes with Noncoordinated Pedant Amines in Their Ligand Framework Capable of Heterolytically ActivatingDihydrogen Gas: (a) Iridium System Reported by Crabtree et al.;55−57 (b) Ruthenium Cyclopentadienyl Complex with aTethered Pendant Amine Function Described by Lau54

Chart 1. Structures of Previously Synthesized and Tested Alcohol Deoxygenation Catalysts Based on the η6/η5-Half-Sandwich-bipy/phen Ruthenium Complex Motif

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and active up to 200 °C in aqueous acidic medium and effect atotal deoxygenation of glycerol to propene. For the syntheti-cally more easily accessed Cp* systems, we were however notable to prepare the complexes with the pendant amine [Ru(η5-C5Me5)(N∩N)(L)](OTf) (N∩N = dabipy, daphen, L = H2O,CH3CN). Simple molecular models indicate that this is due tothe steric interaction of the methyl groups of the Cp* ring withthe amine functions of the dabipy or daphen ligand preventingits coordination, and thus all attempts to isolate a definedreaction product failed.We have now developed a synthetic protocol for the

synthesis of the Cp complex [Ru(η5-C5H5)(N∩N)(NCCH3)]-(OTf) (N∩N = dabipy (2)) shown on the right of Chart 2.

The availability and close structural and electronicequivalence of the pair of catalysts 1 and 2 then provide forwhat we believe to be a unique and unprecedented opportunityto test for the effect of a pendant amine functionality on catalystactivity in the hydrogenation of CO bonds under acidicconditions. Using cyclohexanone as a model substrate we herepresent the comparative evaluation and reactivity study of thesetwo catalysts that we interpret to provide evidence for theenhancement of the relative catalytic activity of 2 over 1 by theaction of the pendant amine group as a proton shuttle in ametal−ligand bifunctional mechanism.

■ RESULTSCatalyst Synthesis. The synthesis of the catalysts was

carried out via reaction of the chelates with [Ru(η5-C5H5)-

(CH3CN)3](PF6) using modified literature protocols.66,67 Asthe hexafluorophosphate salt is not hydrolytically stable andtherefore unsuitable for catalysis in aqueous acidic medium asintended here, the procedure was modified to generate thecorresponding triflate (−OTf = CF3SO3

−) complexes [Ru(η5-C5H5)(bipy)(CH3CN)](OTf) (1) and the new complex[Ru(η5-C5H5)(dabipy)(CH3CN)](OTf) (2). The syntheticsequence is summarized in Scheme 2.68 As previously reportedcomplex 1 was isolated as a crystalline solid in 93% yield, andits single-crystal X-ray structure obtained.66 The dabipycomplex was unexpectedly isolated as a ∼1:1 mixture (by 1HNMR, 70% isolated yield) of the anticipated complex 2 and itsdimer [Ru(η5-C5H5)(dabipy)]2(OTf)2 (3), in which the labileacetonitrile ligand has been displaced by an intermolecularinteraction with one of the free amine substituents. Thus 2could not be obtained as a pure solid, but is generated as thesingle species (by NMR, vide inf ra for a detailed discussion ofthe NMR spectra of 2 and 3) by dissolving the mixture of 2 and3 or pure 3 in CH3CN. The isolation of pure dimer [Ru(II)(η

5-C5H5)(dabipy)]2(OTf)2 (3) from the dimer/monomer mixturewas achieved by heating the mixture of 2 or 3 under dynamicoil pump vacuum (<200 mTorr) at 110 °C for 48 h. Crystals of3 suitable for single-crystal X-ray analysis were obtained by slowdiffusion of THF into a saturated solution of 2 in acetonitrile,which together with the above procedure suggests that in thesolid state dimer 3 rather than monomer 2 is thethermodynamic sink of the system.

X-ray Crystallographic Data and Structural Comparisons.Figure 1 shows ORTEPs of [Ru(II)(η5-C5H5)(bipy)-(CH3CN)](OTf) (1) and [Ru(II)(η5-C5H5)(dabipy)]2(OTf)2(3).With the carbon atom of the Cp ring that defines the plane

CCp−Ru bisecting the Npy−Ru−Npy angle as a reference pointand denoted C1 in both structures, Table 1 gives thecrystallographic information and directly compares thecharacteristic bond length and angles around the rutheniumcenters. The Ru−N distances to the pyridine ring nitrogens in 3are only slightly longer than in 1, corresponding to a slightlysmaller bond angle N11−Ru−N22; however the symmetrybreakage in 3 caused by the bridging amine substituent (N44)has no effect; that is, at the 3σ level the bond lengths Ru−N11and Ru−N22 are still equivalent. Also the distance from theruthenium center to the centroid of the Cp ring in the piano-stool arrangement is the same in both structures. The angle

Chart 2. Structures of the Two Complexes [Ru(η5-C5H5)(N−N)(CH3CN)](OTf) [N−N = 2,2′-Bipyridine (1)and N−N = 6,6′-Diamino-2,2′-bipyridine (2)] Evaluated inDirect Comparison as Catalysts in This Study

Scheme 2. Reaction Pathway for the Synthesis of [Ru(II)(η5-C5H5)(N−N)(CH3CN)](OTf) Complexes (N−N = 2,2′-Bipyridine, 6,6′-Diamino-2,2′-bipyridine)

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Ru−N44−C21 is the ideal tetrahedral angle indicating that thebridging interaction in the dimer does not suffer from stericstrain. Together these structural parameters suggest that thesteric and electronic environments of the ruthenium center arevery similar if not in fact almost identical in both complexes.NMR Spectra. The 1H NMR spectra of complex 1 have

been previously reported.66 Figure 2 shows the 1H NMRspectra of 2 in CD3CN and 3 in the noncoordinating solventCD2Cl2, respectively. The acetonitrile complex 2 is quantita-tively regenerated from 3 by dissolving the latter in CD3CN.The signals for the Cp protons in 2 appear at 4.36 and at

4.52 ppm in 3, compared to 4.41 ppm in 1. In complex 2 thepeak for the free amine substituent appears at 6.08 ppm. Forcomplex 3 the peak for the free pendant amine (b) is present atδ = 6.32 ppm, while the protons on the coordinated amine (i)are diastereotopic and appear at δ = 8.33 and 8.65 ppm. Thelatter are seen at higher chemical shifts due to the loss of

shielding around the nitrogen through donation of electrondensity to ruthenium. On the basis of the similarity in shiftsbetween the monomer and dimer for the aromatic region in thespectrum of 2 and its coupling pattern, the triplet at δ = 7.52ppm was assigned to d. The remaining protons were thenmapped out using 2D COSY and HSQC experiments andassigned as shown in Figure 2.69

Reactivity of [Ru(II)(η5-C5H5)(bipy)(CH3CN)](OTf) (1) withHOTf Acid. Previous work on the catalytic activity of 1 hadshown that the presence of triflic acid in hydrogenationreactions was necessary to avoid deactivation and decom-position of 1.66 This is caused by the coordination ofethylamine generated by hydrogenation of the acetonitrileligand, which is prevented by its protonation in acidic medium.In the course of the current study (i.e., during the actualhydrogenation study, vide inf ra), we observed that the additionof acid to carbonyl solutions of 1, 2, and 3 further stabilizedthese solutions against decomposition in air (as evidenced by acolor change from dark red to brown or black) for extendedperiods of time. Therefore reactions of 1 with triflic acid in bothacetonitrile-d3 and acetone-d6 were studied in detail.

Reactions in Acetonitrile-d3. After addition of 1equivalent of acid, the 1H NMR spectrum of 1 (Figure 3)showed the formation of new well-defined species. Bycomparison with the initial spectrum of 1 in acetonitrile, themost noticeable change is the loss of symmetry in the Cpligand, suggesting protonation to the neutral η4-cyclopenta-diene (CpH). However, the 1H NMR of a Ru(η4-C5H6) speciesshould contain only two alkene peaks due to its symmetry andtwo peaks for the respective exo and endo protons of the−CH2− group.70 This does not agree with the 1H NMRspectrum of the new species, which displays four distinct CHchemical shifts for the CpH ligand. In addition, the 13C NMRspectra show 15 inequivalent carbons, i.e., total loss ofsymmetry in the complex. Analysis of the 13C, 1H, andHSQC spectra, instead, suggested the Ru(η2-C5H6) speciesshown in Figure 3.71

Further evidence for the formation of the [Ru(II)(η2-C5H6)(bipy)(CH3CN)2](OTf)2 complex comes from acomparison to the known literature η2-Cp ruthenium complex

Figure 1. ORTEPs of [Ru(II)(η5-C5H5)(bipy)(CH3CN)](OTf) (1) and [Ru(η5-C5H5)(dabipy)]2(OTf)2 (3) as determined by single-crystal X-raycrystallography. Ellipsoids are drawn at the 50% probability level. Hydrogens, the OTf− counterion, and one equivalent of THF contained in the unitcell of 3 are omitted for clarity.

Table 1. Crystallographic Information and Comparison ofthe Characteristic Bond Lengths and Angles in [Ru(II)(η5-C5H5)(bipy)(CH3CN)](OTf) (1) and [Ru(II)(η5-C5H5)(dabipy)]2(OTf)2 (3)

parameter 1a 3

d(Ru−N11) [Å] 2.095(3) 2.123(5)d(Ru−N22) [Å] 2.094(3) 2.126(4)d(Ru−Cpcentroid) [Å] 1.791(3) 1.790(3)d(Ru−N6) [Å] 2.071(3) n/ad(Ru−N44) [Å] n/a 2.239(5)∠(Ru−N44−C21) [deg] n/a 109.4(3)∠(N11−Ru−N22) [deg] 76.36(10) 76.10(2)∠(N11−Ru−N6) [deg] 87.39(10) n/a∠(N22−Ru−N6) [deg] 85.76(11) n/a∠(N11−Ru−N44) [deg] n/a 87.9(2)∠(N22−Ru−N44) [deg] n/a 84.7(2)∠(C1−Ru−N11) [deg] 110.33(12) 103.1(2)∠(C1−Ru−N22) [deg] 108.88(13) 105.2(2)∠(C1−Ru−N6) [deg] 158.95(12) n/a∠(C1−Ru−N44) [deg] n/a 166.5(2)

aAs previously reported.66

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Ru(η2-C5H6)(NH3)5](OTf)2, which shows similar chemicalshifts and coupling constants for the Cp protons, in particularthe diastereotopic CH2 exo/endo protons of the CH2 group ofthe protonated ring.72 Once the acid is added to the solution of1, and the new species Ru(II)(η2-C5H6)(bipy)(CH3CN)2]-(OTf) is generated, it survives in solution for approximately 6 hat ambient temperature in air before decomposing withformation of Ru(0). On the basis of these results, theestablished stabilizing effect of added acid when using 1 asthe procatalyst in hydrogenation reactions appears inconsistent,

but can be rationalized by the decomposition pathwayproposed in Scheme 3. Acetonitrile is hydrogenated toethylamine, which then can coordinate to the rutheniumcenter. This lowers the pKa of the amine proton and positions itto be transferred to the Cp ring, ultimately causing loss of theCp ligand and decomposition of the catalyst. In the presence ofacid the ethylamine generated is protonated to ethylammonium and can therefore not act as a ligand toward theruthenium center, thus blocking this pathway under catalysisreaction conditions.

Figure 2. 1H NMR spectrum (400 MHz) of 2 formed by dissolving either the 2/3 mixture or 3 in CD3CN (top) and the spectrum of 3 in CD2Cl2(bottom) as a noncoordinating solvent.

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Reactions in Acetone-d6. Addition of 3 equivalents oftriflic acid to a solution of 1 in acetone-d6 resulted in adeepening of the red color of the solution and the changes ofthe 1H NMR spectrum appearance shown in Figure 4. On thebasis of comparison with the reference spectrum of 1 (bottomof Figure 4) the new minor species is a modification of 1 withloss of symmetry in the bipy but not the Cp ligand.Further evidence for the loss of symmetry around the bipy

ligand is seen in the 13C jmod spectrum of the acidic solution,

which displays 10 separate aromatic carbon peaks for the bipyligand of the minor species. The loss of symmetry in the bipyligand of 1 must then result from an exchange of the labileacetonitrile ligand for an asymmetric ligand, which in turn canoriginate only from the acetone solvent, but due to itssymmetry, acetone itself cannot be the ligand in question. Inorder to elucidate the nature of this ligand, we carried out acontrol NMR experiment, which replicated the concentrationof HOTf in acetone-d6 in the absence of 1.73 The 13C NMR

Figure 3. 1H NMR spectrum of 1 in acetonitrile-d3 after addition of 1 equivalent of triflic acid.

Scheme 3. Possible Decomposition Pathway of 1 Observed in the Absence of Stabilizing Acid

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spectra from this experiment show the presence of twoadditional carbonyl resonances at 210.4 and 199.0 ppm alongwith several other peaks that are consistent with the presence ofthe acetone condensation products diacetone alcohol andmesityl oxide, the latter giving a characteristic resonance at 6.12ppm in the 1H NMR. These resonances are also present in the13C NMR of the sample corresponding to Figure 4 (spectrum2, top). Either of these condensation products could act as aweakly coordinating ligand, explaining both the observedsymmetry breakage in the bipy resonances of 1 and the overallstabilization of the complex in acetone. The analogous effectthen takes place in the actual substrate cyclohexanone, forwhich the corresponding condensates have been directlyobserved in the reaction mixture (vide inf ra: Scheme 4 andTable 2).Reactivity of [Ru(II)(η5-C5H5)(dabipy)(CH3CN)](OTf) (2)

with HOTf Acid. In contrast to 1 complexes 2 and 3 do notrequire the presence of HOTf to be stabilized againstdecomposition in solution under reducing conditions. Thisbehavior must be due to the presence of the ortho-aminesubstituents, which either prevent the coordination of ethyl-amine by steric repulsion or change the reactivity of the protonson the coordinating amine function through hydrogen bonding,which then modifies the overall reactivity such that thedecomposition pathway proposed in Scheme 3 no longeroperates.The ability of the pendant amine in complex 2 to be

protonated in the reaction solution is a necessary, but notsufficient condition for it to operate as a metal−ligandbifunctional catalyst. Addition of 0.5 equivalent of triflic acidto a solution of complex 2 in acetonitrile-d3 resulted in the

formation of a new species, as revealed by the comparison ofthe spectra shown in Figure 5. The spectrum after addition ofacid shows two sets of Cp and aromatic CH peaks, with 2remaining as the major species and 2-NH3

+ as the minorspecies, with the loss of the amine peak attributed to rapidproton exchange causing line-broadening into the baseline.

Figure 4. (1) 1H NMR spectrum of 1 in acetone-d6. (2)1H NMR spectrum of 1 in acetone-d6 after addition of triflic acid.

Scheme 4. Metal/Acid-Catalyzed Hydrogenation/Dehydration/Dimerization Reaction Cascade Leading fromCyclohexanone to the Products Listed in Table 2

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These results suggest that the pKa of catalyst 2 is in fact highenough to allow the pendant amine to act as a proton shuttle.As stated above, this is in contrast to the analogous dicationicη6-arene systems described by us earlier, whose amine functioncould not be protonated under similar reaction conditions.65

The increase in the pKa of 2 stems from the anionic nature ofthe Cp ligand, which creates a [Ru]+ complex containing lesspositive charge than the structurally analogous [Ru]2+ η6-arenesystems, allowing the protonation of the amine ligand in the Cpsystems. Furthermore, the ability to protonate the pendantamineagain generating a di- or (by double protonation ofeach amine function) possibly a tricationic speciesmay alsocontribute to the added stability observed for acidic solutions of2 in air.pKa of [Ru(η5-C5H5)(dabipy)(CH3CN)](OTf) in Aqueous

Solution. Triflic acid is the strongest hydrolytically stablenonoxidizing acid known and is fully dissociated in polarsolvents such as water and DMSO.74 In aqueous solution, theconcentration of H3O

+ (pKa = −1.74) formed is therefore equalto the concentration of triflic acid added; that is, [H3O

+] =[HOTf], and the acid strength is solvent leveled to that ofH3O

+. Treating the protonated species 2-NH3+ as a weak acid

[HA], and the unprotonated species 2 as its conjugate weak

base [A−], an acid titration experiment was carried out trackingthe relative concentrations of the species involved by 1H NMR.From this study the pKa of the protonated amine wasdetermined to be ∼0.55, establishing that 2 is in fact protonatedby H3O

+ in aqueous acidic medium.75

[Ru(II)(η5-C5H5)(dabipy)(CH3CN)](OTf) (2) vs [Ru(II)(η5-C5H5)(dabipy)]2(OTf)2 (3) as the Procatalyst. The fact thatcomplex 2 cannot be isolated as a pure solid forces the use ofdimer 3 as the procatalyst for the catalytic reactions, meaningthat the two procatalysts (1 and 3) to be compared would differin more than only the intended difference, i.e., the presence ofthe amine substituents on the dabipy ligand in 2. Establishingthe principal validity of a direct comparison of complexes 1 and3 as procatalysts therefore required further considerations, inparticular whether or not the dimer 3 is broken up intomonomeric species when dissolved in the carbonyl substrates.Figure 6 (a, bottom) shows the 1H NMR spectrum of the samesample of 3 shown in Figure 2 after evaporation to dryness ofthe CD2Cl2 solution in vacuo76 and redissolution in acetone-d6resulting in the formation of several unidentified newcomplexes.As Figure 6b shows, in acetone-d6 2 is quantitatively

regenerated from 3 in situ upon addition of 2 equivalents ofacetonitrile; that is, under these conditions a direct comparisonof 1 and 2 is assured in solution. In order to explore whetherthere is any difference between using 3 with and without thepresence of 2 equivalents of acetonitrile as the procatalyst in thecarbonyl hydrogenations, both these reactions were realizedusing cyclohexanone as the test substrate.

Comparative Catalytic Hydrogenation of Cyclohexanone.The relative catalytic activity of the complexes 1, 2 (obtained insitu from the solid dimer 3 and 2 equivalents of acetonitrile),and 3 was compared through hydrogenation reactions of neatcyclohexanone at 175 °C, 900 psi H2(g) pressure (cold), at anequal concentration of catalysts ([carbonyl substrate]/[ruthe-nium] = 2000:1) and auxiliary acid (2 equivalents of triflic acid/ruthenium). The requirement for the presence of acid whenusing 1 as the procatalyst also determined the selection ofcyclohexanone, as it is comparatively stable against aldolcondensation reactions in the presence of acid at roomtemperature. This is a critical criterion since it allowed forthe preparation of a stable reaction solution without sidereactions of the substrate under ambient conditions. Attemptsto screen other more reactive carbonyl compounds, such asaliphatic aldehydes, led to immediate condensation reactionsafter addition of triflic acid. Decane was added as an internalstandard for the quantitative analysis of the reaction mixture byGC calibrated against authentic compounds, where readilyavailable (cyclohexanone, cyclohexanol, cyclohexane). Otherproducts were identified by GC-MS by matching their M+

peaks and their fragmentation pattern against the MS databasessupplied with the instrument software and quantified byestimating their FID response factors normalized to theircarbon number by using the effective carbon number concept,77

which resulted in overall mass balances of 97−98%. Theprogress of the hydrogenation reactions was monitored throughthe pressure drop in the reactor at temperature with pressurereadings taken every hour for the first 7 h and a final readingafter 10 h.Figure 7 shows the difference in the rate of H2 uptake

(equivalent to the pressure drop in the reactor) for thereactions using 1, 2 (generated in situ from 3 by addition of 2equivalents of acetonitrile), and 3 as the procatalyst for the

Table 2. Product Distribution and Mass Balance of theCylcohexanone Hydrogenation Reaction Catalyzed by 1 and3 at t = 10 ha

aFor dimeric products normalized to cyclohexanone equivalents.

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Figure 5. (1) 1H NMR spectrum of 2 in acetonitrile-d3. (2)1H NMR spectrum of 2 in acetonitrile-d3 after addition of 0.5 equivalent of triflic acid.

Figure 6. 1H NMR spectra of 3 in acetone-d6 (a) and after addition of 2 equivalents of acetonitrile (b).

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hydrogenation of cyclohexanone. Table 2 lists the products andrelative percentage yields (by GC/GC-MS) of the observedproducts and gives the overall mass balance of the reactions.The dominant reaction product in all cases is the initial productcyclohexanol. Due to the acidic nature of the reaction mixture,caused by both the addition of free triflic acid and thepostulated heterolytic activation of hydrogen gas (seeDiscussion section), cyclohexane is also formed by loss ofwater followed by hydrogenation; that is, the catalysts are alsocapable of hydrogenating carbon−carbon double bonds. At thesame time the direct hydrogenation of cyclohexanone competeswith acid-catalyzed aldol condensation side reactions, leading tothe set of dimeric products listed in Table 2. A comparison ofthe relative product yields listed in Table 2 shows that the moreactive the catalyst, the fewer the dimeric products that areformed, which are however also further dehydrated andhydrogenated. Therefore a maximum amount of the initialaldol condensation and dehydration product 2-cyclohexenylcy-clohexanone (26%) remains with the least active catalyst (1),while 2 and 3 generate more deoxygenated and hydrogenatedproducts by both the direct cyclohexanone → cyclohexanol →cyclohexene → cyclohexane route and the indirect routestarting from the aldol condensation product. Scheme 4illustrates the pathways leading to the set of products listedin Table 2. The generation of water by this reaction cascadealso means that the strongest acid present in the reactionmixture is H3O

+, which on the basis of the pKa of the aminesubstituent is acidic enough to protonate the pendant aminefunction in the dabipy complexes.The complexity of the overall hydrogenation/dehydration/

dimerization reaction cascade under acidic conditions, whichresults in the parallel and simultaneous operation of multiplehydrogenation pathways, as well as the requirement to conductthe reactions in a 316SS autoclave at elevated temperature andpressure, makes an actual kinetic analysis of the hydrogenuptake data very challenging. However, taking the rate ofpressure drop and total amounts of hydrogen consumed for allreactions as a quantitative measure of relative activity, it isevident from the plots in Figure 7 that both reactions using 3 as

the procatalyst show a faster rate of hydrogen uptake andultimately much higher total consumption of hydrogen than thereaction catalyzed by 1. Table 3 lists the TON realized for all

three catalysts as defined by the equivalents of hydrogenconsumed (calculated from the amounts of hydrogenatedproducts quantified by GC) and the relative initial rates ofhydrogen uptake in Δpsi/min.78 The reaction with 2 as theprocatalyst, i.e., 3 in the presence of 2 equivalents ofacetonitrile, is slower and shows a 12% lower pressure dropof hydrogen (474 vs 540 psi at 10 h), corresponding to a lowerconversion to hydrogenated products, which does however notcorrelate with the TON calculated from the GC yields, which infact suggests a slightly higher yield of hydrogenated productsfor the reaction catalyzed by procatalyst 2. These differencescan be attributed to the approximations made in thedetermination of the GC yields of the secondary productslisted in Table 2, which by comparison to the accuratelymeasured pressure drops (±0.13% psi by digital pressuretransducer) must have total associated errors of ca. ±7%.79 Thefinal pressure drop for the reaction catalyzed by 1 was 336 psi,i.e., 29% or 38% less than that for procatalyst 2 or 3,respectively. The presence of acetonitrile thus does have animpact on the observed activity of 3; however in both reactionsit still substantially exceeds that of the nonaminated parentcomplex 1, and no starting material remains for the tworeactions catalyzed by 2 or 3.

Figure 7. Hydrogen pressure drop (in psi) at 175 °C as a function of time in the hydrogenation of cyclohexanone by 1, 3, and 3 + 2 equivalents ofCH3CN.

Table 3. TON and Relative Initial Rates Achieved byCatalysts 1, 2 (3 + 2 equiv of CH3CN), and 3

catalyst

initial rate ofH2 uptake[Δpsi/min]

H2 uptake as totalpressure drop at175 °C [psi]

TON (at t = 10 h) [equivof H2 consumed/Ru calcd

from GC yields]

1 0.97 336 12202 (3 + 2equiv ofCH3CN)

1.6 474 1900 ± 60 (±7%)

3 3.26 540 1840 ± 60 (±7%)

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■ DISCUSSION

In contrast to the dicationic η6-arene 6,6′-diamino-bipyridineand 2,9-diamino-phenanthroline complexes previously de-scribed,65 the monocationic η5-Cp systems presented herecan be protonated on their amine function in aqueous acidicmedium and show an enhanced catalytic activity even thoughthe steric bulk of the amino substituents wouldon the basisof our observations with the η6-arene systemsdemand theopposite effect. All other parameters being equal, this thereforepoints to a specific role of the amine substituentsin particularin their protonated formin the rate-determining step of thehydrogenation reaction. While the experimental conditionsrequired (T = 175 °C, p(H2) ≥ 900 psi, 316 SS reactor)prevent a direct observation of metal hydride species in situ orthe catalyst system as a whole in operando, in analogy to thestructurally and electronically similar catalysts [(η6-arene)Ru-(X)(N∩N)](OTf)n and more specifically [(Cp*Ru(CO)2)2(μ-H)](OTf) and the related thermally unstable acidic dihydrogencomplex [Cp*Ru(CO)2(η-H2)](OTf)2,

80−83 we postulate thatall three procatalysts 1, 2, and 3 in solution form complexes ofthe type [CpRu(N∩N)(L)]+ with L being a labile ligand(substrate, water, product alcohol, etc.) that at elevatedtemperature and under hydrogen pressure can be displacedby dihydrogen to form the corresponding dihydrogen complex[CpRu(N∩N)(η2-H2)]

+ as a reactive intermediate. Thiscomplex then heterolytically activates hydrogen gas to givethe hydride complex [CpRu(N∩N)(H)] and a proton, whichfor the reactive intermediates derived from 2 or 3 can be takenup by triflate, water, solvent, substrate, product, or the pendantamine substituents to give [CpRu(6-amino-6′-ammonium-2,2′-bipyridine)(H)]+ (2-H2) as the active catalyst. The observedenhanced catalytic activity of 2/3 vs 1 then also demands thatthe rate-determining step must be associated with anelementary reaction involving the protonated amine function.This could be in either the heterolytic activation of dihydrogen,the transfer of H+/H− to the unsaturated substrate, possibly in aconcerted fashion, or the release of an alkoxide ligand byprotonation from the ammonium substituent. In any of thesescenarios the close spatial proximity of the amine function tothe dihydrogen, hydride, or alkoxide ligand on ruthenium,respectively, is then key to this phenomenon. In summary, thissuggests the existence of a metal−ligand bifunctional hydro-genation mechanism as proposed in Scheme 5 as the cause ofthe relatively higher catalytic activity of 2/3 vs 1. The higheractivity of 3 vs 2, i.e., without the addition of 2 equivalents ofacetonitrile, which under the reaction conditions are convertedto CH3CH2NH3

+,66 is a function of the overall acidity of thereaction medium, as in this case the added HOTf is notconsumed by the protonation of CH3CH2NH2, but insteadpushes the acid−base equilibrium further toward theprotonation of the pendant amine functions, underlining theimportance of a proton transfer in the catalytic cycle.

■ CONCLUSION

The introduction of pendant amine functions in the ortho-positions of the bipy ligands in the complex [(η5-C5H5)Ru-(2,2′-bipyridine)(CH3CN)]OTf to give [(η5-C5H5)Ru(6,6′-diamino-2,2′-bipyridine)(CH3CN)]OTf results in an enhance-ment of its catalytic activity in the hydrogenation ofcyclohexanone, which in the catalytic cycle must be rooted inan active proton-transfer role by the amine functions in theirprotonated form. In contrast to the majority of metal−ligand

bifunctional hydrogenation catalysts the system described hereoperates in acidic rather than basic reaction medium, which webelieve to be the first observation of such an effect under suchconditions. The catalyst tolerates water and comparatively hightemperatures, which may allow the application of this type ofsystem to the catalytic deoxygenation of sugar alcohol-derivedsubstrates by iterative acid-catalyzed dehydration and metal-catalyzed reaction cascades in a single reactor.

■ EXPERIMENTAL SECTIONAll manipulations were conducted under an atmosphere of argon usingstandard Schlenk-line techniques or in a glovebox, using freshlydistilled solvents or degassed water saturated with argon. All otherreagents were available commercially and were used without furtherpurification. NMR spectra were recorded on Bruker 300 MHz, 400MHz, or 600 MHz spectrometers. Spectra were calibrated to theresidual protonated solvent signal, and chemical shifts are reportedrelative to tetramethyl silane (=0 ppm). Mass spectroscopic analyseswere performed by the Biological Mass Spectrometry Facility,University of Guelph, Guelph, ON, Canada. X-ray crystallographywas performed by Jim Britten, Department of Chemistry, McMasterUniversity, Hamilton, ON, Canada. Industrial-grade hydrogen gas wasused for all hydrogenation experiments. An Autoclave Engineers (AE)mini-reactor with a 50 mL stainless steel (316SS) reactor vessel fittedwith a Setra C 206 digital pressure transducer (accuracy ±0.13%) wasemployed for all hydrogenations. Control reactions in the absence ofadded catalyst did not give any substrate conversion under the reactionconditions employed.84 Cyclohexanone was purified by passing itthrough a short column of neutral alumina (Brockmann Activity I)immediately before use as the substrate. GC analyses were performedon a Varian 3800 using a 30 m × 0.25 mm × 0.025 mm high-polarityStabilwax-DA column. Quantitative analysis of the reaction solutionswas performed through a three-level calibration against authenticsamples using decane as an internal standard for cyclohexanone,cyclohexanol, and cyclohexane. Quantification for all other productsidentified in the GC traces was approximated using the GC-FIDresponse factor for the parent carbonyl.77 GC-MS analyses wereconducted using a Varian 3800 GC/Saturn 2000 MS running EIdefault mode using a Stabilwax-DA column.

6,6′-Diamino-2,2′-bipyridine was prepared via 6,6′-dibromo-2,2′-bipyridine following our own optimized protocol.85

Safety Note and Considerations Regarding the Synthesis ofthe 6,6′-Dibromo-2,2′-bipyridine Intermediate. 6,6′-Dibromo-2,2′-bipyridine was prepared following the literature procedure

Scheme 5. Proposed Mechanism for the Metal−LigandBifunctional Hydrogenation of Carbonyls by 2-H2

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described by Holm and co-workers shown in Scheme 6, whichemploys an Ullman-type coupling of 2,6-dibromopyridine by n-BuLiand CuCl2.

86 n-BuLi is present in very slight excess to selectivelylithiate one of the bromide sites and promote generation of a pyridyl-copper(I) species, which can then undergo the oxidative addition/reductive elimination needed to form the coupled product.The primary hazard of this reaction results from the addition of O2

to the reaction. The oxygen treatment was thought to be necessary forthe oxidative coupling of the pyridylcopper(I) species. However, anyexcess n-BuLi remaining after lithiation in the presence of the solventdiethyl ether and oxygen can promote formation of peroxides and/orhydroxyperoxides. Though the reaction is performed between −50 and−80 °C to the avoid formation of such byproducts, if peroxides doform, the reaction becomes shock sensitive, and under an atmosphereof O2 and ether vapor, the risk of explosion is high. Explosions haveoccurred during either the bubbling of O2 or the addition of HCl af terthe reaction in both our laboratory and elsewhere.87 It is therefore ouropinion and advice that any future synthesis of 6,6-dibromo-2,2′-bipyridine should be attempted using only the alternative procedurelaid out by Uchida and Gao (Scheme 7).88,89

According to the literature, this reaction also gives 6,6′-dibromo-2,2′-bipyridine in yields comparable to Holm’s procedure, avoiding useof oxygen gas as an oxidant. In addition, the procedure does notinvolve a workup that requires the reaction solution to be diluted withhydrochloric acid.Ruthenocene [(η5-C5H5)2Ru] and [Ru(η5-C5H5)(C10H8)][OTf]

were prepared following the procedure by Kundig and Monnier.67

[Ru(η5-C5H5)(CH3CN)3][OTf] was prepared as a dark orange,viscous oil by modifying their procedure using a dedicated liquid/liquid extractor. This procedure as well as the synthesis of [Ru(η5-C5H5)(bipy)(CH3CN)][OTf] (1) was previously described by us.66

[Ru(η5-C5H5)(dabipy)(CH3CN)](OTf) (2). A 100 mL Schlenk flaskwas charged with 765 mg (1.75 mmol) of [Ru(η5-C5H5)(CH3CN)3]-[OTf] and the oil dissolved in 5 mL of degassed CH2Cl2, under inertconditions. A separate 50 mL Schlenk was charged with 335 mg (1.7mmol) of 6,6′-diamino-2,2′-bipyridine and the ligand dissolved in 20mL of degassed CH2Cl2/CH3CN (9:1). The ligand solution was thenadded by syringe to the solution of [Ru(η5-C5H5)(CH3CN)3][OTf].The reaction was heated to reflux for 1 h while stirring. The light redsolution was cooled to room temperature, and the solvent removed toyield the crude product as a dark red precipitate. The precipitate wasredissolved in cold CH2Cl2 (100 mL), and any unreacted startingmaterial filtered off. The solvent was removed by dynamic vacuum,yielding a dark red powder containing a mixture of the monomer (2)and dimer (3) complexes in approximately 1:1 ratio. Yield: 0.67 g(70%). 1H NMR (400 MHz, dichloromethane-d2): dimer δ 4.52 (s,5H), 6.14 (dd, J = 6.8, 2.8, 1H), 6.33 (br s, 2H), 6.83 (d, overlay oftwo doublets, 1H), 6.96 (d, J = 8 Hz, 1H), 7 (m, 2H), 7.52 (t, overlayof two triplets, 1H), 8.33 (d, J = 8.4 Hz, 1H), 8.64 (d, J = 9.3 1H);monomer δ 1.974 (s, 3H), 4.41 (s, 5H), 6 (br s, 4H), 6.81 (d, overlayof two doublets, 2H), 7.29 (d, J = 7.6 Hz, 2H), 7.57 (t, overlay of twotriplets, 2H). Dissolving this mixture in CD3CN generates themonomer 2 cleanly as determined by 1H NMR (300 MHz,acetonitrile-d3): δ 1.95 (s, 3H), 4.36 (s, 5H), 6.08 (br s, 4H), 6.79(dd, J = 8.1, 1.2 Hz, 2H), 7.37 (dd, J = 7.5, 0.9 Hz, 2H), 7.58 (t, J = 7.7

Hz, 2H). 13C NMR (150 MHz, acetonitrile-d3): δ 70.8 (CH), 110.4(CH), 111.9 (CH), 138.9 (CH), 156.8 (C), 162.1 (C). MS (ESI+TOF): m/e calcd for C15H15N4

101Ru 353.3623, found 353.0293[Cp101Ru(dabipy)]+.

Preparation of Pure [Ru(η5-C5H5)(dabipy)]2(OTf)2 (3) as a Solid. A0.67 g (1.2 mmol) sample of the mixture was added to a 10 mLSchlenk flask under an inert atmosphere. The red solid was thenheated to 110 °C under dynamic vacuum for 48 h. The vessel wascooled to room temperature, resulting in a color change of the solid toa more yellow tint. Yield: 0.6 g (94%). 1H NMR (600 MHz,dichloromethane-d2): δ 4.52 (s, 5H), 6.14 (dd, J = 7.8, 3 Hz, 1H), 6.33(br s, 2H), 6.83 (d, J = 8.4 Hz, 1H), 6.96 (d, J = 7.2 Hz, 1H), 7 (m,2H), 7.53 (t, J = 7.8 Hz, 1H), 8.33 (d, J = 9 Hz, 1H), 8.64 (d, J = 8.4Hz, 1H). 13C NMR (600 MHz, dichloromethane-d2): δ 68.62 (CH),111.19 (CH), 111.93 (CH), 113.61 (CH), 115.61 (CH), 137.69(CH), 138.37 (CH), 154.19 (C), 155.53 (C), 158.04 (C), 161.11 (C).Single crystals of complex 3 suitable for X-ray diffraction were grownfrom a concentrated acetonitrile solution by slow diffusion oftetrahydrofuran as the cosolvent.

[Ru(η5-C5H5)(C10H8N3(NH3+))(CH3CN)](OTf)2. A 0.035 g (64 μmol)

portion of 2 was dissolved in 1 mL of acetonitrile-d3. Then 3 μL (32μmol) of trifluoromethanesulfonic acid was added to the solution andtransferred to a J. Young NMR tube. The solution was shaken for 1min. The product was obtained in situ as a mixture of the protonatedcomplex and the starting material. The broad R-NH3

+ peak in the 1HNMR is not present due to rapid exchange of the proton (400 MHz,acetonitrile-d3): δ 4.26 (s, 5H), 7.19 (d, J = 9.2, 2H), 7.25 (d, J = 7.6,2H), 7.99 (dd, J = 7.2, 7.6, 2H).

pKa of [Ru(η5-C5H5)(dabipy)(CH3CN)](OTf). A 0.045 (82 μmol)amount of 2 was dissolved in a 5 mL solution of D2O containing 15%CD3CN by volume. Then 14 μL (164 μmol) of trifluoromethane-sulfonic acid was added to the solution and transferred to a J. YoungNMR tube. The solution was shaken for 1 min, and 1H NMR spectrawere acquired. The volume of trifluoromethanesulfonic acid in thesample was then increased by an additional of 14 μL, for a total of 28μL (328 μmol), and the 1H NMR spectrum was acquired again. Thisprocess was repeated with the sample for 10, 20, 30, 40, 50, 60, and 66molar equivalents of acid with respect to catalyst. At 66 equivalentsonly the protonated species [Ru(η5-C5H5)(C10H8N3(NH3

+))-(CH3CN)](OTf)2 was present by

1H NMR. Spectra were calibratedto the residual protonated solvent signal for acetonitrile-d3. The broadNH3 peak in the 1H NMR is not present due to rapid exchange of theproton (300 MHz, acetonitrile-d3): δ 1.95 (s, 1H), 4.22 (s, 5H), 7.08(dd, J = 12, 12 Hz, 2H), 7.16 (dd, J = 9.6, 10 Hz, 2H), 7.92 (dd, J = 10,10 Hz, 2H).

[Ru(η2-C5H6)(bipy)(CH3CN)2](OTf)2. A 0.015 g (30 μmol) sample of1 was dissolved in 1 mL of acetonitrile-d3. Then 8 μL (87 μmol) oftrifluoromethanesulfonic acid was added to the solution andtransferred to a J. Young NMR tube. The solution was shaken for 1min or until it became brown in color. [Ru(η2-C5H6)(bipy)-(CD3CN)2](OTf)2 was obtained in situ in quantitative yield asdetermined by 1H NMR. 1H NMR (400 MHz, acetonitrile-d3): δ 1.95(s, 3H), 2.35 (d, J = 24 Hz, 1H), 2.97 (d, J = 24 Hz, 1H), 5.15 (m,1H), 5.67 (d, J = 4 Hz, 1H), 5.93 (m, 2H, overlay of two doublets),7.68 (m, 2H, overlay of two triplets), 8.14 (m, 2H, overlay of twotriplets), 8.36 (m, 2H, overlay of two doublets), 8.83 (m, 2H, overlayof two doublets). 13C NMR (100 MHz, acetonitrile-d3): δ 41.96(CH2), 94.81 (CH), 103.66 (CH), 124.87 (CH), 125.04 (CH), 128.15(CH), 128.25 (CH), 132.13 (CH), 139.93 (CH), 140.02 (CH) 140.20(CH), 154.22 (CH), 154.92 (CH), 157.83 (C), 157.97 (C).

Procedure for the Hydrogenation of Cyclohexanone in a 50 mL316 SS Autoclave. A 1 mmol amount of decane as the internalstandard (ISTD) was dispensed into a 10.0 mL volumetric flask.Considering likely volume contractions upon mixing, the volume of

Scheme 6. Synthesis of 6,6′-Diamino-2,2′-bipyridine

Scheme 7. Alternative Synthesis of 6,6′-Dibromo-2,2′-bipyridine

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the ISTD (195 μL) was ignored for further calculations. Theremaining volume was then filled with cyclohexanone (10 mL =96.56 mmol). Trifluoromethanesulfonic acid (2 molar equivalents withrespect to catalyst, 8.5 μL) was added, and the solution mixed well.The catalyst (0.05 mol % of substrate, ∼25 mg) was weighed andcombined with the carbonyl solution in the mini-reactor vessel andstirred in the sealed reactor for several minutes before removing 0.5mL for initial GC analysis and resealing the reactor. The reactor wasthen evacuated for 2 min using a water aspirator, pressurized to 900 psiwith H2 gas, and allowed to equilibrate for 2 min. The evacuation/pressurization cycle was repeated twice more. Stirring was set at about350 rpm, and the reactor was heated to 175 °C. Pressure readings weretaken at 0, 1, 2, 3, 4, 5, 6, 7, and 10 h from reaching the set operatingtemperature. After 10 h the reactor heating was turned off and thereactor placed in an ice bath for 30 min to condense any volatileproducts. The reactor was vented and opened, and a final sample takenfor GC/GC-MS analysis.

■ ASSOCIATED CONTENT*S Supporting InformationDetails of the pKa determination of 2-NH3

+. Collection ofcompound data sheets with images of NMR and MS spectra.Details of the reaction of 1 with HOTf in CD3CN and acetone-d6. This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: mschlaf@uoguelph.ca.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Natural Science andEngineering Research Council of Canada (NSERC), theCanadian Foundation for Innovation (CFI), Natural ResourcesCanada (NRCAN), and the Ontario Ministry of Agricultureand Food (OMAF). M.E.T. thanks NSERC and the OntarioGraduate Scholarship program for graduate fellowships.

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dx.doi.org/10.1021/om400871v | Organometallics XXXX, XXX, XXX−XXXN