Hydrogen Transfer Activation via Stabilization of Coordinatively

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Hydrogen Transfer Activation via Stabilization of Coordinatively Vacant Sites: Tuning Long-Range π‑System Electronic Interaction between Ru(0) and NHC Pendants Cristiana Cesari,† Rita Mazzoni,*,† Elia Matteucci,† Andrea Baschieri,† Letizia Sambri,† Massimo Mella,*,‡ Andrea Tagliabue,‡ Francesco Luca Basile,† and Carlo Lucarelli*,‡

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†

Dipartimento di Chimica Industriale “Toso Montanari”, Università degli Studi di Bologna, viale Risorgimento 4 40136 Bologna, Italy ‡ Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria-Como, Via Valleggio 11, 22100 Como, Italy S Supporting Information *

ABSTRACT: The insertion of a pyridine substituent on the lateral chain of Nheterocyclic carbene ligands enhances the catalytic activity of [Ru(CO)2(cyclopentadienone)(NHC)] complexes, precursors of Shvo-type catalysts, toward the hydrogenation of 4-fluoroacetophenone in refluxing 2-propanol as hydrogen donor. DFT calculations evidence the role of pyridine in the donor/acceptor properties and complexes reactivity both in the case of imidazolylidene and triazolylidene ligands. Although the NHC-pyridine derived complexes perform somewhat worse than the forerunner Shvo dimer, the slower evolution of the reaction steps is compatible with the FT-ATR spectroscopy time scale and allows to identify in situ intermediates, invisible when using the original Shvo catalyst due to its higher reaction speed. Thus, mechanistic insight has been disclosed by synergic contribution of FT-ATR in situ experiments and DFT calculations performed on the slowest precatalyst (1N) chosen as a model. The proposed reaction mechanism has been based on both energy profile and experimentally identified intermediate.

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triazolylidenes (aNHC).22−25 The value added with the substitution of a CO with an NHC moiety resides in the versatility of these heterocyclic ligands, which can be variously functionalized allowing the fine-tuning of steric and electronic properties.26 Furthermore, a simple protonation allowed the transformation of the cyclopentadienone imidazolylidene ruthenium(0) complex (1) into its corresponding cationic hydroxycyclopentadienyl ruthenium(II) derivative (3) (Chart 1), with a marked impact on its activity. In fact, both Ru(0) (1 and 2) and Ru(II) (3) type complexes were tested as catalysts for transfer hydrogenation of ketones in refluxing 2-propanol. The screening revealed that, while the cationic precursor 3 can be activated under the latter conditions, neutral complexes 1 and 2 where recovered unchanged after 24h and needed an oxidative additive such as CAN (cerium ammonium nitrate) in order to start the substrate conversion.24,25 An useful extension of the results just discussed may come from the coordination of Ru(0) with bidentate ligands comprising an N-heterocyclic carbene (imidazolylidene or triazolylidene) and a nitrogen-containing substituent, since these are prone to exert great influence on donor/acceptor

INTRODUCTION N-Heterocyclic carbenes (NHCs) are among the most popular ancillary ligands due to a combination of unique features, such as the tunability of electronic and steric properties that influence the metal center. Moreover, the synthesis of NHC ligand precursors and the corresponding complexes is rather simple and very versatile,1−4 allowing the rational design of transition metal catalysts and the improvement of catalytic activity.5−12 Regarding the ruthenium−NHC complexes, most of the literature features ruthenium(II) complexes, while lowvalent NHC ruthenium(0) systems are restricted to a few examples based on either [Ru3(CO)12] or [Ru(CO)2(PPh3)3] as precursors.13−16 A class of Ru(0) complexes designed with the aim of combining NHC and cyclopentadienone ligands has been recently communicated (an example in Chart 1).17 Those complexes have been employed as precursors of catalysts similar to the well-known Shvo complex, which is active in a plethora of metal−ligand bifunctional catalytic transformations such as hydrogenation and hydrogen transfer reactions.18−21 As for its preparation, the Shvo catalyst can, for instance, be obtained heating the triscarbonyl complex (called “Shvo precursor” in Chart 1) in 2-propanol (iPrOH). An extension of the mentioned synthetic methodology led to the insertion of another class of NHC ligands: the abnormal © XXXX American Chemical Society

Received: November 20, 2018

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DOI: 10.1021/acs.organomet.8b00850 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Structuresa

Chart 2. Ruthenium Cyclopentadienone NHC and aNHC Pre-Catalysts Compared in This Study

a

Triscarbonyl precursor of Shvo complex, a neutral cyclopentadienone imidazolylidene ruthenium complex 1, a cyclopentadienone triazolylidene ruthenium complex 2, and a cationic hydroxycyclopentadienyl imidazolylidene Ru complex 3.

Section). They appear as yellow powders stable to air and moisture both in solid state and in solution. Spectra of 2Na−c result comparable with those previously reported.24 13C NMR spectra show the diagnostic signal for the Ru−Ccarbene at around 158 ppm, while CO stretching can be easily followed in the terminal CO region where a couple of bands arose around 2006−2009 cm−1 and 1947−1951 cm−1 in CH2Cl2. By comparing the results obtained in the transfer hydrogenation of 4-fluoroacetophenone employing 2-propanol as both solvent and hydrogen donor, we can observe that the insertion of a pyridine in the lateral chain of the ruthenium complexes induces a catalytic activity not shown by 1 or 2. This is true for both NHC type complexes (Table 1: 1 entry 2 vs 1N entry 3) and aNHC type complexes (Table 1: 2 entry 4 vs 2Na−c entries 5−7). As discussed in the following, this descends from a loss of CO from the precatalysts, which irreversibly convert them into more reactive species under 2propanol refluxing condition. Importantly, the enhanced reactivity is due to a higher energy content (22.2 kcal/mol, vide infra). This notwithstanding, the catalysts bearing an NHC in place of one of the CO ligands are still less active than the Shvo precursor (entry 1 vs entries 2−7) due to the strong σ donor properties of pyridine substituted NHC. Although this may represent a detrimental effect in absolute terms, the slowing down of the catalytic cycle could lead to the pooling of intermediates, which may thus be identified even at the IR time scale (vide infra) despite being undetectable in the presence of the more efficient Shvo catalyst (Figure S11). With this aim in mind, we decided to choose the less active catalyst, namely, 1N, as the model complex for an in-depth reactivity analysis and mechanistic investigations. As first step in the latter quest, we mention that CO release from 1N can be induced in an inert solvent (e.g., dry toluene at 110 °C) and leads to the formation of a new chelated complex (named chelated in Figure 2 and Scheme 1) with the pyridine nitrogen coordinated to the vacant site of the ruthenium center. The chelated complex was completely inactive toward the transfer hydrogenation of 4-fluoroacetophenone under the condition of Table 1 (entry 8). Although desirable, no traces of emilability for the pyridine nitrogen have been observed, and the precatalyst was recovered unmodified from the resting state of the reaction.

properties and general reactivity of the complexes.27−31 In this vein, we herein report the role of a pyridine substituent in the lateral chain of N-heterocyclic carbenes that allows the formation of more reactive species, compared to congeners 1 and 2, in refluxing 2-propanol able to perform transfer hydrogenation of 4-fluoroacetophenone without the need of oxidative additive. Reactivity and mechanistic insights obtained via in situ IR and DFT calculations underlined the significant role played by pyridine on donor properties of N-heterocyclic carbenes and the consequent lower energy required for CO release; the latter is the compulsory step for activation. Although the catalytic activity is still negatively affected with respect to the aforementioned Shvo catalyst, the slower reactivity of the pyridine functionalized NHC-Shvo type complexes can be exploited in order to collect in situ IRATR spectra on the less active precatalyst (1N) chosen as a model and, when analyzed in synergy with DFT calculations, to draw an experimentally supported mechanism from the latter.

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RESULTS AND DISCUSSION Preparation, Catalytic Activity and CO Dissociation Energies of Ruthenium Complexes Containing Cyclopentadienone, CO, and NHC Ligands. In order to have a small library of pyridine-functionalized complexes, the new abnormal N-heterocyclic carbene (aNHC) complexes 2Na, 2Nb, and 2Nc have been prepared and compared with the previously reported aNHC 2,24 NHC 125 and 1N25 complexes (Chart 2). Concerning the synthesis of pyridine functionalized complexes 2Na−c they have been prepared following the same procedure employed for the coordination of imidazolylidene (NHC)17 and triazolylidene (aNHC) ligands24 on cyclopentadienone carbonyl ruthenium(0), previously developed by the group. Notably, the triazolium triflates, precursors of the abnormal carbene ligands, have been obtained from a selective monomethylation reaction of the corresponding substituted triazoles, as we discussed in our recent publications.32,33 All the new complexes, 2Na−c, have been characterized by means of IR in the CO region, NMR spectroscopy, and ESI-MS spectrometry (see the Experimental B


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Besides, a detrimental effect is observed (conversion of 9% after 24h, entry 10) performing the reaction in the presence of one equivalent of CAN per ruthenium center as oxidant additive. This behavior is probably ascribable to the untimely formation of the chelated complex, which is the only ruthenium based detectable species in the resting state of the reaction. As a last reactivity test, the title reaction was performed at higher temperature (isobutanol-iBuOH, 108 °C, Table 1, entry 11) to investigate if the lack of conversion observed employing 1 as catalyst (Table 1, entry 2) was mainly connected to the availability of the energy needed for CO release. De facto, 1 was demonstrated to be active in the mentioned condition, the very low conversion being ascribable to the use of a less reactive hydrogen donor such as the isobutanol. This idea is supported by the results obtained employing the more active 1N (Table 1, entry 12 vs entry 3). Theoretical Analysis of the Reaction Pathway. Aiming to a rationalization of the above-described results, DFT calculations highlighted a pyridine-induced reduction of the energy needed for CO dissociation (Table 2). Indeed, we can

Table 1. Catalytic Transfer Hydrogenation of 4Fluoroacetophenonea

conversion (%) entry

[Ru]

1 2 3 4 5 6 7 8 9 10 11 12

Shvo precursor 1 1N 2 2Na 2Nb 2Nc chelated 1 + pyrc 1N + CANd 1 1N

solvent

8h

24 h

PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i PrOH (80 °C) i BuOH (108 °C) i BuOH (108 °C)

66 0 27 0 38 35 12 0 0 0 6 23

93 025 41b 024 78 82 44 0 6 9

i

27

Table 2. CO Dissociation Energies (in kcal/mol) and Ru Natural Charges (in atomic units)

a

General conditions: ruthenium complex (5 mol % Ru), alcohol (3 mL), reflux; conversions determined by 19F NMR spectroscopy. bThis precatalyst, which is active in the neutral form here reported, has been erroneously found inactive in a previous publication.25 cAddition of 10 mol equiv per ruthenium center of pyridine to the reaction mixture. dCerium ammonium nitrate (CAN); 1 mol equiv per ruthenium center.

Of relevance for the mechanism is also the fact that simply adding pyridine (10 equiv per ruthenium center) to a reaction mixture containing 1 leads to a quite low yield (6%) in 1-(4fluorophenyl)ethanol within 24 h (entry 9); this demonstrates that the pyridine needs to be a pendant substituent on the NHC in order to exerts its influence on the complex reactivity.

entry

[Ru]

ΔE (−CO)

Ru charge

1 2 3 4 5 6 7

Shvo precursor 1 1N 2 2Na 2Nb 2Nc

37.0 44.1 37.2 45.7 37.9 35.8 37.5

−0.177 −0.300 −0.144 −0.293 −0.278 −0.288

observe that the energy requirement for the CO dissociation is the highest, as expected, for NHC 1 (entry 2) and aNHC 2

Scheme 1. Reaction Pathway for the Transfer Hydrogenation of 4-Fluoroacetophenone Catalyzed by 1N

C


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Figure 1. Molecular orbitals indicating electron density transfer from the pyridine p-systems to the Ru(0) orbital left empty by the dissociation of CO from 1N.

Figure 2. Energy profile for the activation of 1N, hydrogen abstraction from 2-propanol and hydrogenation of 4-fluoroacetophenone. The energy “0” is defined as the sum of reactant energies (i.e., of 1N, 2-propanol and 4-fluoroacetophenone).

on the NHC and aNHC σ and π donor behavior; this role is made clear by the analysis of the Ru atomic partial charges (see Table 2), which are more than 0.1 units of charge lower for the pyridine-substituted species, and the molecular orbitals obtained from the DFT calculations on the pyridine-containing complexes following CO loss (Figure 1). In the latter, the molecular orbitals implicated in the electron donation from the π system of pyridine to the metal center (i.e., the ones involved in lowering the Ru charge) in 1N are depicted: The p and d empty orbitals on Ru previously involved in the coordination of CO become somewhat “filled” by electrons originally populating mainly the π systems of pyridine. No such sharing

(entry 4), which are in fact catalytically inactive in refluxing i PrOH. Furthermore, the energy requirement resulted similar for all the complexes with a pyridine in the lateral chain of the NHC (entry 3) or aNHC ligands (entries 5−7), and it is comparable to the energy needed in order to remove a CO from the Shvo precursor (entry 1). This latter resemblance is of particular interest since, according to spectrochemical series, the Shvo precursor represents the most favored complex for CO release due to the mutual back-donation of the three carbonyls. Therefore, the easier dissociation of a CO ligand in the case of pyridine functionalization suggests a strong role of the latter D


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we present in Figure 3 the optimized TS structures for the dehydrogenation of 2-propanol catalyzed by 1 and the precursor of the Shvo complex following CO dissociation.

of electrons is instead found in absence of pyridine as substituent on NHC or aNHC. Molecular orbitals suggesting electronic donation from the π system of pyridine and similar to the ones shown in Figure 1 are also found in the case of the active species generated by the loss of a single carbon monoxide from the precursors 2Na−c or the Shvo precursor (see Figure S12). In the latter case, it is obvious that the presence of two simultaneous back-donating ligands helps in stabilizing the valence unsaturated complex. Turning to the overall hydrogenation process, the complete energy profile for the activation and catalytic activity of 1N is reported in Figure 2 (see also the top panel of Figure S13 for 3D structural representations). This involves, at least initially, the inner-sphere O-coordination of the hydrogen donor 2propanol on Ru (“iPrOH-coord”) following the loss of a CO ligand (“active”) from 1N; this process concomitantly releases 15.0 kcal/mol of energy upon formation of the adduct. The latter can successively isomerize into an outer sphere complex able to pass the TS (“HT-TSAc”) for the hydrogen abstraction from 2-propanol leading to the production of acetone; such TS lies only 9.3 kcal/mol above the “active” valence unsaturated species. As kinetic product directly emerging from HT-TSAc, we found a ruthenium hydride, which also bears a proton transferred from the hydroxyl of the alcohol onto the carbonyl group of the substituted cyclopentadienone ligand. The latter proton is involved into a hydrogen bond with the acetone generated by the process (”hydrideAc”). Also noteworthy is the fact that hydrogen abstraction from 2-propanol might take place transferring a proton to the pyridine nitrogen rather than to the carbonyl oxygen on cyclopentadienone. The involved TS lies, however, 13.8 kcal/ mol above the one shown in Figure 2, so the associated process should be far less likely than the one described above. Following the detachment of the formed acetone, the ruthenium hydride can subsequently coordinate the substrate via the proton attached to the cyclopentadienone ligand; the so-formed adduct may subsequently undergo a simultaneous hydride/proton transfer from the metal complex onto the substrate carbonyl exploiting a TS (“HT-TSSub”) nearly isoenergetic with the one involved in the dehydrogenation of 2propanol, “HT-TSAc”. Indeed, the slight difference in relative energy between the two TS structures (2 kcal/mol) may be easily justified as due to the electronic conjugation between the phenyl ring and the carbonyl group, which is expected to reduce the electrophilic character of the latter moiety. From the “HT-TSSub” geometry, 1-(4-fluorophenyl)ethanol O-coordinated to the vacant site on ruthenium is obtained as kinetic product for the reduction process. The latter could regenerate the “active” species upon dissociating the produced alcohol, a process that seems to require roughly 12 kcal/mol to take place. Such an energetic requirement may be, de facto, important for the stability of the catalytic cycle, as the “active” species may isomerize easily into a more stable “chelated” complex with the ruthenium coordinated to the pyridine nitrogen. This requires surmounting a low-lying TS located roughly only 4 kcal/mol above the unsaturated complex (vide infra the results of the IR investigations). We however expect such process, which would lead to termination of the catalytic cycle, to be hampered by the large excess of 2-propanol used as solvent; the coordination of the latter to the vacant site on ruthenium is, in fact, facile due to its barrierless nature. To conclude our analysis of the impact due to the NHCpyridine ligands on the energy profile for the overall process,

Figure 3. Optimized TS structures for the dehydrogenation of 2propanol catalyzed by the precursor of the Shvo complex (left) and 1 (right) following CO loss. Also shown are the energetic locations of the two TS’s from their respective zeros of the energy scale (precursor plus 2-propanol).

Apart from the similarity in atom positions (see Figure S13), the shown results make apparent the substantially higher energy requirement for the dehydrogenation process involving the active species obtained from 1 compared to the case of the complex derived from the Shvo precursor (roughly 12 kcal/ mol lower) and the active species obtained from 1N (3.5 kcal/ mol lower, Figure 2). Thus, while the substitution of a CO moiety in the Shvo precursor with the monodentate NHC ligands raises the energy barrier of the dehydrogenation due to its impact on the electronic structure of Ru(0), the introduction of pyridine as substituent onto the NHC has a clear ameliorating effect. The impact of the latter on the TS, however, is less marked than on its “active” counterpart (the one from 1N is stabilized by 6.9 kcal/mol compared to the one deriving from 1). This result could have been expected as the vacant coordination site on ruthenium in the hydrogen transfer TS is already partially filled by the electrons of the transferring hydride. With respect to the data shown in Table 1 (entries 1− 3), the difference in energy location between the TS’s indicated by the results in Figures 2 and 3 have important mechanistic consequences: helping to rationalize the different conversions shown by the Shvo precursor, 1 and 1N when kept at reflux in iPrOH or iBuOH (the higher the energetic requirement by the catalyst for the hydrogen transfer, the lower (or nil) the conversion after 8 h), they provide strong support to the idea that another important kinetic bottleneck follows the mandatory activation of precursors along the path, as suggested by the DFT reaction profile. FT-IR Investigation of the Catalytic Cycle. The energy profile described in the previous section evidences that the cyclopentadienone is more likely involved in the catalytic cycle as noninnocent ligand than the pyridine moiety and allows one to depict a mechanism for the transfer hydrogenation of 4fluoroacetophenone catalyzed by 1N together with a possible deactivation pathway (Scheme 1). Apart from the modeling results, support for the latter emerges as a consequence of the lower activity of complex 1N with respect to both the Shvo precursor (Table 1 entry 1 vs entry 3) and the deeply studied Shvo complex,34−42,38 for which substrate-containing intermediates have been determined only with low temperature NMR.43 Thus, a few intermediates were identified by us thanks to a comparison between in situ IR experimental vibrational bands and DFT E


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Organometallics

Figure 4. FT-ATR spectra of 1N after subtraction of a reference spectrum (first spectrum recorded at reflux). (a) In 2-propanol (increasing time at reflux (ITR): lines from pink to dark red; time elapsed after heating stop (TEHS): from azure to blue); (b) in 2-propanol (line: pink at 72 °C; red at 75 °C; dark red at 80 °C; dotted at reflux, 85 °C); (c) in toluene (ITR: lines from pink to dark red; TEHS: from azure to blue).

Importantly, for the overall mechanism, the 37.2 kcal/mol needed for the CO release (see Table 2) can be supplied by the environment to the complex only maintaining 1N at 2propanol reflux temperature. Indeed, monitoring the evolution of 2-propanol/1N solutions at various temperatures below boiling point (Figure 4b) no significant changes in the CO region (bands at 2010 and 1954 cm−1) are detected, with an exception made for a small fluctuation due to a concentration change (continuous lines versus dotted line in Figure 4b). This confirms that the CO release from 1N is quite slow at temperature lower that 85 °C. A behavior similar to the one shown in Figure 4a is also detected monitoring the CO release from 1N in pure toluene at 85 °C (see Figure 4c). The release of CO thus appears as an energy-dependent phenomenon little influenced by the nature of a solvent and its interaction with the complex. Additional details on the activation process can be gathered by comparing Figure 4a,c; thus, the different band shapes are probably ascribable to the formation of H-bonds between 1N and solvent. In fact, two quasi-symmetrical bands were observed for the CO vibrational modes in toluene (Figure 4c), which do not undergo any change in shape during the heating−cooling cycle. Conversely, the CO stretching bands were not symmetric when 2-propanol is used as solvent, showing a shoulder at lower wavenumbers (1993, 1943 cm−1, Figure S15). While the two main bands (2010 and 1954 cm−1) represent the CO symmetric and asymmetric stretching when the 2-propanol is H-bonded with CO of cyclopentadienone

calculations (Tables S1−S3). Mechanistic investigations quite rarely exploit in situ IR-ATR while searching for intermediates along homogeneous catalytic cycles probably due to the difficulties in controlling the reaction conditions. Herein, we present results acquired in an ATR cell equipped with a sealed gasket and a glass top that allowed carrying on the reaction under reflux and monitoring variations in the IR-spectrum. Key evidences extracted from the latter are discussed in the following. As a first test, an in situ IR-ATR experiment was performed on a solution of 1N in 2-propanol. The reaction mixture was continuously monitored from room to reflux temperature (at 85 °C). Once the latter was reached, a reference spectrum was acquired. All the subsequent spectra were processed subtracting them such reference. The spectra time evolution is depicted in Figure 4 and shows that the catalyst progressively undergoes CO release (Figure 4a). In fact, the two bands at 2010 and 1954 cm−1, respectively related to the νCOsim and νCOasim of 1N, decreased concomitantly with the growth of one band at 1929 cm−1, which we attributed to the νCO of the chelated form of the complex (Figure 2 and Scheme 1; see also Figure S14 for spectra of the chelated complex). Relevantly, DFT calculations suggested this species as the lowest-lying among the ones that can be formed after CO release. The increase in intensity of the bands related to the CO stretching upon cooling down the mixture (blue lines in Figure 4a) can be explained as a mere effect of temperature rather than due to concentration changes. F


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Figure 5. FT-ATR spectra after subtraction of a reference spectrum (first spectrum recorded at reflux). Monitoring of hydrogen transfer reaction (conditions: 4-fluoroacetophenone (60 μL), 1N 20% mol, 2-propanol 4 mL); ITR: lines from pink to dark red; TEHS: from azure to blue. (a) CO and hydrides regions; (b) detail of the regions 2050−2030 cm−1 and 1940−1900 cm−1 of the active and the chelated complexes.

Figure 6. FT-ATR spectra after subtraction of a reference spectrum (first spectrum recorded at reflux) obtained while monitoring the hydrogen transfer reaction (conditions: 4-fluoroacetophenone (60 μL), 1N 20% mol, 2-propanol 4 mL); ITR: lines from pink to dark red; TEHS: from azure to blue: dashed and dotted purple line: 4-fluoroacetophenone; dotted green line: 1-(4-fluorophenyl)ethanol. (a) Substrate and product main bands in the region 1728−1495 cm−1; (b) enlargement of the 1625−1585 cm−1 region; (c) enlargement of the 1530−1495 cm−1 region; (d) substrate and product main bands in the region 1280−800 cm−1.

ligand, the shoulders could be attributed to the same vibrational mode when a H-bond is formed between 2propanol and pyridine, as suggested by the DFT calculations. Both H-bonded forms of the complex are detectable at low temperature, whereas only the form in which 2-propanol is Hbonded to the cyclopentadienone remains visible after heating. This leads to a progressive erosion of the shoulder, evidenced

by the formation of a pair of negative bands (Figure 4a) at 1993 and 1943 cm−1, respectively. The hydrogenation of 4-fluoroacetophenone was also monitored by FT-ATR spectroscopy using 20% mol of 1N in 2-propanol (Figure 5). A different evolution from what discussed so far was observed for the CO stretching bands (Figure 5a). By heating at reflux the mixture of 1N and 2propanol in the presence of 4-fluoroacetophenone, new bands G


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Organometallics (at 2040, 1922, and 1909 cm−1) of a monocarbonyl species start to grow as soon as the reaction temperature is reached with the concurrent decrease in intensity of the previously mentioned CO stretching bands in 1N (plot color, from pink to red in Figure 5b). In particular, bands at 2040 and 1922 cm−1 are respectively assigned to Ru-hydride and CO stretching in the intermediate hydrideSub (Scheme 1). The absorption attributed to the Ru-hydride motion arises in a region typical of Ru−H stretching and is comparable with that reported in the literature for the Shvo complex;42 this is wellreproduced under the condition here employed (see Figure S16 for further details). The remaining band (1909 cm−1) in the region under study could, instead, be attributed to the CO stretching of the Prod-coord adduct. An additional interesting observation comes from monitoring the region where νCO of the chelated form appears: Deviating from its quick development in absence of the substrate, the band at 1929 cm−1 emerges only when an almost complete conversion of the substrate is reached. In particular, the two bands at 2040 and 1909 cm−1 disappear, and the band at 1922 cm−1 gradually shifts to 1929 cm−1 confirming the formation of the chelated species. The conclusions reached monitoring the variations of the IR bands in the 1900−2050 cm−1 spectral region can be strengthened following the time evolution of both substrate and product bands in other frequency ranges. Figure 6 shows the most informative regions over which we could compare the pure substrate and product spectra (4-fluoroacetophenone, purple line; 1-(4-fluorophenyl)ethanol, green line) with the spectra of the reaction mixture. First, bands attributable to the substrate decrease in intensity over the course of the reaction; these are the CO, aromatic CC, and CH3 bending modes (located respectively at 1692, 1679, and 1598 cm−1 in Figure 6a), the absorptions from the CO, CH3, aromatic CH bending, and the stretching of CF (respectively located at 1267, 1235, and 840 cm−1 in Figure 6d). In parallel, bands attributable to the aromatic CC or CF stretching and aromatic CH bending (at 1508 cm−1, Figure 6c) in the product increase with time, accompanied by the growth of a band at 1521 cm−1 (Figure 6c). This occurs concurrently with the previously discussed growth of the bands at 2040, 1920, and 1909 cm−1, which do not appear in the absence of the substrate (Figure 4a). Comparing the experimental data to the DFT frequencies, the band at 1521 cm−1 may be assigned to the CH3 and CpO---H---Ophenone bending of hydrideSub, which is about to undergo the transfer hydrogenation. While the reactant conversion approaches completeness, the bands at 1508 and 1521 cm−1 merge into a single structure at 1512 cm−1 (assigned to the aromatic CC stretching and CH3 bending of the product), closely following the shift of the band at 1922−1929 cm−1 (formation of chelated) and the disappearance of the two bands at 2040 and 1909 cm−1. Furthermore, the formation of acetone was noticeable at 1710 cm−1 (Figure 6a). The appearance of bands not attributable to substrate or product absorption was observed also in the region around 1600 cm−1 (Figure 6b). During the erosion of the band at 1598 cm−1, we notice the growth of three bands at 1608, 1616, and 1623 cm−1, which may be attributed to 1609 cm−1 to CpO--H---Ophenone, CH3 bending, and aromatic CC stretching; 1616 and 1623 cm−1 to Ru−H---C bending and CpO---H--Ophenone, CH3 bending. Eventually, these two bands gather together in the band at 1610 cm−1 arising from the CC

aromatic stretching of the product. Some residual bands were hard to assign due to superimposition of vibrational modes in the complex. In the Table S4, we detail the assignment of bands not discussed in the main text. Summarizing, the good agreement between computed vibrational frequencies for the intermediates suggested via DFT calculations (see Figure 2), and the unexpectedly detected absorptions appearing in the FT-ATR spectra supports the catalytic cycle proposed in Scheme 1 for the transfer hydrogenation of 4-fluoroacetophenone catalyzed by 1N.

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CONCLUSIONS Ru(0) complexes containing a cyclopentadienone and bearing a pyridine functionalized N-heterocyclic carbene (imidazolylidene−NHC or tryazolilidene−aNHC) ligand (1N, 2Na−c) demonstrated to be more suitable as precatalysts in the transfer hydrogenation of 4-fluoroacetophenone than congeners 1 and 2 lacking N-based substituents. DFT calculations support the experimental observations and attribute the catalytic results to an easier formation of the catalytically active species in the former cases. Such effect was made possible by an electron density donation from the pyridine pendant to the coordination vacancy on Ru(0) generated by the loss of a CO molecule. Albeit less markedly, the TS also involved in the hydrogen transfer are stabilized via the same mechanism. The catalytic intermediates of the transfer hydrogenation of 4-fluoroacetophenone mediated by 1N have been identified by means of in situ FT-ATR spectroscopy supported by DFT calculations; importantly, 1N was chosen as precatalyst because of its lower activity compared with the Shvo catalyst and, hence, able to generate and consume intermediates at a rate compatible with the IR time scale. The compulsory CO release for the activation of the catalyst has been confirmed, performing the reaction in 2-propanol and toluene at different temperatures, showing that 1N is inactive below 85 °C. Once activated, the catalyst quickly deactivates turning into the chelated form in the absence of a substrate. In the presence of 4-fluoroacetophenone, a screening of the diagnostic spectral regions shows the CO release from the precatalyst and allows the identification of hydride containing adduct hydrideSub and Prod-coord intermediate, which precedes the product release. When the conversion is nearly complete, chelated complex appears as an indication of the beginning of the catalyst deactivation. In conclusion, we were able to put forward a robust and comprehensive reaction mechanism for the transfer hydrogenation promoted by 1N catalyst exploiting the reliable and efficient in situ FT-ATR technique in synergy with catalytic results and DFT calculations.

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EXPERIMENTAL SECTION

General Materials and Procedures. Analytical-grade solvents and commercially available reagents were used as received, unless otherwise stated. Chromatographic purifications were performed using aluminum oxide. Solvents were dried and distilled according to standard procedures and stored under nitrogen. 1H, 13C, and 19F NMR spectra were recorded on a Varian Inova 300 MHz or on a Mercury 400 MHz spectrometer. Chemical shifts (δ) are reported in ppm relative to residual solvent signals for 1H and 13C NMR (1H NMR: 7.26 ppm for CDCl3; 13C NMR: 77.0 ppm for CDCl3). 13C NMR spectra were acquired with 1H broadband decoupling mode. Mass spectra were recorded on a micromass LCT spectrometer using electrospray (ES) ionization techniques. Infrared spectra were H


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Organometallics recorded at 298 K on a PerkinElmer Spectrum Two FT-IR spectrophotometer. The conversions were monitored by 19F NMR. Reagents: silver oxide, chloridric, nitric and sulfuric acid, methyl trifluoromethanesulfonate, methyl iodide, pyridine, and cerium ammonium nitrate (CAN) have been employed as purchased by Sigma-Aldrich. 2-(1-Methyl-1H-1,2,3-triazol-4-yl)pyridine (a),43 1phenyl-3-methyl-4-(pyridin-2-yl)-1H-1,2,3-triazol-3-ium trifluoromethanesulfonate (L2),33 2-(1-phenyl-1H-1,2,3-triazol-4-yl) pyridine (b),44 2-(1-benzyl-1H-1,2,3-triazol-4-yl) pyridine (c),44 1-benzyl-3methyl-4-(pyridin-2-yl)-1H-1,2,3-triazol-3-ium trifluoromethanesulfonate (L3),32 and [3,4-(4-MeO-C6H4)2−2,5-Ph2(η4-C4CO)Ru(CO)3]45 (Shvo precursor) have been prepared as previously reported. Synthesis of Ruthenium Complexes 2Na−c. Method 1 (for Iodide Triazolium Salts).

and the solution concentrated under vacuum. The crude was purified on a chromatography column (stationary phase: aluminum oxide, eluent: ethyl acetate). 2Nb. Y = 67%. 1H NMR (400 MHz, CDCl3) δ (ppm) 8.48 (dt, JHH = 1.3 Hz, JHH = 4.7 Hz, 1H, CHpy), 7.27 (d, JHH = 7.8 Hz, 4H, CHaryl), 7.20−7.15 (m, 1H, CHpy), 7.11−7.07 (m, 4H, CHaryl), 7.05− 6.97 (m, 5H, CHaryl + CHpy), 6.92 (t, JHH = 7.6 Hz, 4H, CHaryl), 6.84 (d, JHH = 8.6 Hz, 4H, CHaryl), 6.45 (d, JHH = 9.0 Hz, 4H, CHaryl), 3.73 (s, 3H, −NCH3), 3.60 (s, 6H, −OCH3). 13C NMR (100 MHz, CDCl3) δ (ppm) 202.4 (CO), 167.0 (CO, Cp), 160.5 (Ru−Ctrz), 158.1 (−COCH3), 148.8 (Cipso), 148.2 (Ctrz−py), 145.8 (C), 138.8 (C), 136.4 (CH), 135.0 (C), 133.4 (CH), 129.7 (CH), 129.5 (CH), 128.3 (CH), 127.3 (CH), 127.2 (CH), 127.1 (CH), 124.8 (C), 124.7 (CH), 124.0 (CH), 112.4 (CHary), 103.1 (C2,5−Cp), 80.8 (C3,4−Cp), 54.8 (−OCH3), 37.2 (−NCH3). IR (CH2Cl2): ν(CO): 2006 cm−1, 1947 cm−1; ν(CpO): 1577 cm−1. HR-MS calcd for C47H36N4O5Ru: 838.1729. Found: 838.1731 [M + H]+. 2Nc. Y = 75%. 1H NMR (400 MHz, CDCl3) δ (ppm) 8.58−8.55 (m, 1H, CHpy), 7.70−7.74 (m, 4H, CHaryl), 7.22−7.16 (m, 5H, CHPh), 7.10−7.01 (m, 10H, CHaryl), 6.80−6.75 (m, 2H, CHpy), 6.56 (d, JHH = 8.9, 4H, CHaryl), 6.51−6.48 (m, 1H, CHpy), 5.13 (s, 2H, −CH2−), 3.69 (s, 3H, −NCH3), 3.67 (s, 6H, −OCH3). 13C NMR (100 MHz, CDCl3) δ (ppm) 201.7 (CO), 167.3 (CO, Cp), 159.7 (Ru−Ctrz), 158.4 (−COCH3), 149.3 (Cipso), 147.7 (Ctrz−py), 146.9 (C), 137.1 (CH), 135.2 (C), 134.8 (C), 133.5 (CH), 129.5 (CH), 128.2 (CH), 127.8 (CH), 127.7 (CH), 127.5 (CH), 127.3 (CH), 125.2 (CH), 124.9 (C), 124.1 (CH), 112.7 (CHaryl), 103.4 (C2,5− Cp), 79.8 (C3,4−Cp), 57.0 (−CH2−), 55.0 (−OCH3), 37.2 (−NCH3). IR (CH2Cl2): ν(CO): 2009 cm−1, 1951 cm−1; ν(Cp O): 1577 cm−1. HR-MS calcd for C48H38N4O5Ru: 847.1991. Found: 847.2016 [M + H]+. Synthesis of Ruthenium Chelated Complex.

L1 (0.085 mmol, 1 equiv) was dissolved in dry CH2Cl2 (10 mL) then, Ag2O (0.1 mmol, 1.2 equiv) was added. The mixture stirred under nitrogen atmosphere and in the absence of light. After 2 h, Ru dimer A was added (0.042 mmol, 0.5 equiv), and the mixture was stirred for additional 2 h. After this time, the resulting solid was removed by filtration on a Celite pad, and the solvent was evaporated under reduced pressure. Column chromatography was performed in order to purify the product (stationary phase: aluminum oxide, eluent: ethyl acetate). 2Na. Y = 80%. 1H NMR (400 MHz, CDCl3) δ (ppm) 8.58−8.55 (m, 1H, CHpy), 7.70 (d, JHH = 1.0 Hz, 4H, CHaryl), 7.20−7.17 (m, 2H, CHpy), 7.15−7.03 (m, 10H, CHaryl), 6.58 (d, JHH = 8.6 Hz, 4H, CHaryl), 6.45−6.42 (m, 1H, CHpy), 3.74 (s, 3H, −NCH3), 3.68 (s, 6H, −OCH3), 3.61 (s, 3H, −NCH3). 13C NMR (100 MHz, CDCl3) δ (ppm) 201.8 (CO), 167.6 (CO, Cp), 158.4 (−COCH3), 157.6 (Ru−Ctrz), 149.3 (Cipso), 148.2 (Ctrz−py), 147.1 (C), 137.2 (CH), 135.5 (C), 133.5 (CH), 129.4 (CH), 127.5 (CH), 127.4 (CH), 125.1 (CH), 125.0 (C), 124.1 (CH), 112.7 (CHaryl), 103.5 (C2,5−Cp), 79.5 (C3,4−Cp), 55.0 (−OCH3), 41.3 (−NCH3), 36.9 (−NCH3). IR (CH2Cl2) ν(CO): 2007 cm−1, 1947 cm−1, ν(CpO): 1577 cm−1. HR-MS calcd for C42H34N4O5Ru: 776.1573. Found: 776.1572 [M + H]+. Method 2 (for Triflate Triazolium Salts).

Dicarbonyl-η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)1-(butyl-3-(2-pyridinyl)-midazol-2ylidene)ruthenium (1N) 0.100g (0.125 mmol) was dissolved, under inert atmosphere, in 20 mL of dry toluene. The reaction mixture was stirred under reflux overnight. At the end of the reaction, the crude was purified by column chromatography (stationary phase: aluminum oxide, eluent: ethyl acetate/ methanol 10:1) in order to obtain the pure product as a dark yellow solid identified as carbonyl-η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)1-(butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium by IR, 1H NMR, 13C NMR and ESI-MS. chelated. Y = 85%. 1H NMR (599.7 MHz, CD3CN) δ (ppm) 8.32 (m, 1H, CHpy), 7.90 (m, 2H, CHNHC + CHpy), 7.65 (d, 1H, CHpy), 7.31 (s, 1H, CHNHC), 7.18 (m, 6H CHaryl), 6.13 (t, 1H, CHpy), 6.89 (m, 4H CHaryl), 6.68 (m, 2H, CHaryl), 6.53 (m, 2H, CHaryl), 4.20 (m, 1H, NCH2), 3.82 (m, 1H, NCH2), 3.72(s, 3H, −OCH3), 3.66 (s, 3H, −OCH3), 1.77, 1.27 (m, 4H, −CH2CH2), 0.88 (t, 3H, −CH3). 13C NMR (150.8 MHz, CD3CN) δ (ppm) 194.18 (CO), 183.63 (Ccarbene), 167.84 (CO, Cp), 159.56, 159.00 (−COCH3), 155.11 (Cipso), 153.59 (CHpy), 140.52, 137.98, 134.41 (CHpy), 134.09− 113.20 (Caryl), 124.54, 123.10 (CHNHC),98.04 (C2,5, Cp), 76.76 (C3,4, Cp), 55.73 (−OCH3), 52.21 (NCH2), 33.45, 20.53 (−CH2CH2), 14.02 (−CH3). IR (CH2Cl2): ν(CO): 1916 cm−1, ν(CpO): 1577 cm−1. ESI-MS (m/z) (+) = 776 [M + H]+. General Procedure for Transfer Hydrogenation. Complex (15 μmol, 5 mol %), iPrOH (3 mL), pyridine (10 equiv), or CAN (1 equiv) when needed, and 4-fluoroacetophenone (36 μL, 300 μmol)

L2-3 (0.085 mmol, 1 equiv) was dissolved in dry acetonitrile (5 mL); then, KCl (0.85 mmol, 10 equiv) and Ag2O (0.3 mmol, 3.5 equiv) were added. The mixture was stirred for 24 h under nitrogen atmosphere and in the absence of light. After this time, the formed solid was removed by filtration and washed with CH3CN. The solution was concentrated, and the resulting white solid was dissolved in dry CH2Cl2 (10 mL), then Ru dimer A added (0.042 mmol, 0.5 equiv), and the mixture was stirred under nitrogen atmosphere and in the absence of light. After 1 h, the mixture was filtered on a Celite pad I


Organometallics

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were added to a 10 mL flask under inert atmosphere. The reaction was maintained under reflux while stirring for 24 h. Samples were taken at regular intervals (8 and 24 h). Aliquots (ca. 0.05 mL) were diluted with CDCl3 (0.5 mL) and conversions were determined by 19 F NMR spectroscopy. In Situ FT-ATR Experiments. In situ FT-ATR experiments were carried out using a Bruker Tensor II instrument equipped with an ATR cell (Zinc Selenure crystal). The general procedure for the investigations is the following: First, 80 mg of complex were completely dissolved in 4 mL of 2-propanol or toluene then deposited in the ATR cell and heated from room temperature to the solvent reflux temperature. The spectra were acquired every 30 s. The same procedure was repeated for monitoring the reaction; in this case, 60 μL of 4-fluoroacetophenone were added to the 2-propanol-catalyst solution. Theoretical Modeling. Optimization of the reactants, products, intermediates, and putative transition states (TS’s) for the species involved in the Ru-mediated transfer hydrogenation reaction have been carried out employing DFT electronic structure calculations at the B3LYP/LANL2DZ level, with and without a PCM-based description of the 2-propanol reaction medium. All stationary point structures have been fully characterized by means of frequency calculations; a few relative energies were also tested employing larger basis sets such as 6-31++G(d,p) in conjunction with the LANL effective core potential (ECP) for Ru. This approach has already demonstrated to deliver sufficiently accurate results for the rationalization of simple metal catalyzed reactions involving organic molecules46−48 or the interaction between open-shell metal complexes with small inorganic species.49 Starting geometries for the energy minimization were built basing on chemical intuition or X-ray diffraction (XRD) derived structures.25 TS structures for the hydrogenation/dehydrogenation of ketones/ alcohols were instead set up exploiting the results discussed in ref 20 where an “outersphere” synchronous dihydrogen transfer onto an aldehyde was studied. Alternative pathways involving alcohols or ketones directly coordinated to the Ru center of the complex (inner sphere) were also investigated; we invariably found that hydrogenation via an outer sphere TS is energetically less demanding.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the PRIN 2015 project from Ministero dell’Università e della Ricerca Scientifica e Tecnologica (20154 × 9ATP_003), the University of Bologna and the Università degli Studi dell’Insubria.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00850. 1

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Article

H NMR and 13C NMR spectra of compounds L1, 2Na−c, molecular orbitals of the species generated by the Shvo precursor following CO loss, vibrational assignments of 4-fluoroacetophenone, 1-(4fluorophenyl)ethanol, and 1N in 2-propanol, in situ IR (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.M.). *E-mail: [email protected] (M.M.). *E-mail: [email protected] (C.L.). ORCID

Rita Mazzoni: 0000-0002-8926-9203 Andrea Baschieri: 0000-0002-2108-8190 Letizia Sambri: 0000-0003-1823-9872 Massimo Mella: 0000-0001-7227-9715 Carlo Lucarelli: 0000-0002-5098-0575 Notes

The authors declare no competing financial interest. J


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Hydrogen Transfer Activation via Stabilization of Coordinatively

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