Pyridine(diimine) Molybdenum-Catalyzed Hydrogenation of Arenes

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Pyridine(diimine) Molybdenum-Catalyzed Hydrogenation of Arenes and Hindered Olefins: Insights into Precatalyst Activation and Deactivation Pathways Matthew V. Joannou, Máté J. Bezdek, and Paul J. Chirik ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00924 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Pyridine(diimine) Molybdenum-Catalyzed Hydrogenation of Arenes and Hindered Olefins: Insights into Precatalyst Activation and Deactivation Pathways Matthew V. Joannou, Máté J. Bezdek, Paul J. Chirik* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States Supporting Information Placeholder ABSTRACT: Pyridine(diimine) molybdenum bis(olefin) and bis(alkyl) complexes were synthesized, characterized, and examined for their catalytic activity in the hydrogenation of benzene and a selection of substituted arenes. The bis(alkyl) molybdenum complex (4-tBu-iPrPDI)Mo(CH2SiMe3)2 (iPrPDI = 2,6-(2,6-(C(CH3)2H)2C6H3N=CMe)2C5H3N) exhibited the highest activity for the hydrogenation of benzene, producing cyclohexane in >98% yield at 23 °C under 4 atm of hydrogen after 48 hours. Toluene and ortho-xylene were similarly hydrogenated to their respective cycloalkanes with the latter yielding predominantly (79:21 d.r.) of cis-1,2dimethylcyclohexane. The molybdenum-catalyzed hydrogenation of naphthalene yielded tetralin exclusively and this selectivity was maintained at higher H2 pressure. At 32 atm of H2, more hindered arenes such as monosubstituted benzenes, biphenyl, and m- and p-xylenes underwent hydrogenation with yields ranging between 20 and >98%. (4-tBu-iPrPDI)Mo(CH2SiMe3)2 was also a competent alkene hydrogenation catalyst, supporting stepwise reduction of benzene to cyclohexadiene and cyclohexene during molybdenum-catalyzed arene hydrogenation. Deuterium labelling studies for the molybdenum-catalyzed hydrogenation of benzene produced numerous isotopologues and stereoisomers of cyclohexane, indicating reversible hydride (deuteride) insertion/βH(D) elimination, diene/olefin binding, and allylic C-H(D) activation during the reaction. The resting state of the catalyst under neat conditions was established as the η6-benzene complex (iPrPDI)Mo(η6-benzene). Under catalytic conditions, pyridine underwent C-H activation of the 2-position, and furan underwent formal C-O oxidative addition to yield a “metallapyran”. Both reactions were identified as important catalyst deactivation pathways for the attempted molybdenum-catalyzed hydrogenation of heteroarenes.

KEYWORDS: Hydrogenation, molybdenum, arene, alkene, mechanism

INTRODUCTION The catalytic hydrogenation of benzene and other substituted arenes to cycloalkanes is an important process in both the fine and petrochemical industries (Figure 1, top).1 Most notable is the hydrogenation of benzene to cyclohexane, which is conducted on a >2 million ton/year scale utilizing Group 10 transition metal-based heterogeneous catalysts.2 These catalysts are highly active and robust, yet are ineffective for the stereoselective hydrogenation of substituted arenes.1,3,4 Development of single-site5 and homogeneous catalysts for the selective hydrogenation of arenes is highly desirable, as methods for the stereoselective functionalization of cycloalkanes have become more widespread and utilized in the synthesis of pharmaceuticals.6 Chemo- and stereoselective hydrogenation of substituted arenes or differential reactivity in mixtures of different arenes7 would allow for efficient generation of stereodefined cycloalkanes via welldefined, organometallic species.

One of the first purportedly homogeneous arene hydrogenation catalysts was a tris(phosphite)cobalt allyl developed by Muetterties and co-workers.8 Catalytic hydrogenation of arenes was observed under mild conditions (1-3 atm of H2, 24 °C, 1 mol% catalyst) and tolerated substituted arenes such as xylenes, naphthalene, and mesitylene; in all cases cis cycloalkanes were obtained. Based on extensive mechanistic studies, the origin of the cis selectivity was proposed to result from successive syn additions of a cobalt-allyl-hydride across the C – C bonds of the arene and subsequent diene and olefin reduction products (providing support for homogeneous catalysis1). Rothwell and co-workers observed similar all-cis selectivity for their tantalum9 and niobium-based10 aryloxide hydrogenation catalysts. More recently, Glorius and coworkers disclosed the cis-selective hydrogenation of fluoroarenes, utilizing a cyclic (alkyl)(amino)carbene rhodium precursor.11 The selectivity and efficiency of these catalytic methodologies demonstrate that homogeneous arene hydrogenation can be highly useful in the selective synthesis of

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ligand to induce a haptotropic rearrangement of the η 6benzene ligand and ultimately promote hydrogenation?

Figure 2. Potential for ligand-induced haptotropic rearrangement of η6-arene complex in arene hydrogenation.

Figure 1. Heterogeneous versus Homogeneous Arene Hydrogenation and their applications. Arene coordination to a transition metal, the first step in arene hydrogenation, is known to dramatically alter arene reactivity and allow for both nucleophilic and electrophilic reactions at even the most unreactive substrates.12 Ubiquitous η6-arene complexes display classic electrophilic reactivity at the arene, and have been exploited and broadly utilized in medicinal, organometallic, and synthetic organic chemistry for the functionalization and substitution of arenes.12 The much less common η2-arene binding mode can lead to enhanced or Umpolung13 reactivity at the arene. 14 Taube and co-workers isolated one of the first η2-benzene compounds: [(NH3)5Os(η2-C6H6)][OTf]2.15 They reported that this dicationic osmium arene complex could be hydrogenated with Pd/C under mild conditions to the corresponding cyclohexene complex: [(NH3)5Os(η2-cyclohexene)][OTf]2. While the hydrogenation could not be accomplished without catalytic Pd/C, it nonetheless demonstrated that η2-coordination of benzene to a transition metal center can significantly lower the barrier for hydrogenation as Pd/C itself is not capable of hydrogenating benzene.16 This enhanced reactivity likely stems from ground state destabilization of benzene from loss of aromaticity upon coordination to osmium.3,17 Our laboratory has previously reported that the pyridine(diimine) (PDI) molybdenum(η6-benzene) complex undergoes haptotropic rearrangement of the arene ligand upon addition of two equivalents of ammonia to yield the corresponding bis(ammonia)(η2-benzene) complex (Figure 2, top).18 While this molybdenum compound was studied for NH bond cleavage by proton coupled electron transfer, we reasoned that the η2-benzene ligand might display similar reactivity to [(NH3)5Os(η2-C6H6)][OTf]2. The molybdenum complex, however, is neutral and supported by a potentially redox active pyridine(diimine) ligand, which could modulate π-backdonation from the metal and promote catalytic and complete arene hydrogenation. A significant fundamental question therefore arose – is dihydrogen a sufficiently potent

Here we describe the development of a molybdenumcatalyzed method for arene hydrogenation, utilizing both 1,5cyclooctadiene (1,5-COD) and bis(neosilyl) molybdenum precatalysts. Mechanistic investigations establish (PDI)Mo(η6benzene) complexes as the catalyst resting states. Hydrogenations with heteroarene substrates were unsuccessful, due to catalyst deactivation pathways, which were characterized for pyridine and furan.

RESULTS AND DISCUSSION Catalyst Evaluation and Optimization. Our studies commenced with evaluation of several different pyridine(diimine) (PDI) molybdenum precatalysts for the catalytic hydrogenation of benzene to cyclohexane. We previously reported the robust and scalable synthesis of PDIMo(1,5-COD) complexes, as well as demonstrated the lability of the bis-olefin ligand when exposed to donor ligands such as ethylene, which suggested that these complexes may serve as efficient precatalysts for arene hydrogenation.19 Initial catalyst optimization studies demonstrated that (iPrPDI)Mo(1,5-COD) (iPrPDI = 2,6(2,6-(C(CH3)2H)2C6H3N=CMe)2C5H3N) was the only precatalyst capable of producing cyclohexane in meaningful amounts (25 TON, Entry 1, Table 1) under 4 atm of H2 in neat benzene at 60°C. Reducing the size of the imine substituents with MesPDI and tBuPDI analogues yielded only 98% NMR yield at ambient temperature (Entry 6, Table 1). As with the parent bis(neosilyl) molybdenum compound, (4-tBu-iPrPDI)Mo(CH2SiMe3)2 is paramagnetic and exhibits a 1H


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ACS Catalysis

NMR spectrum with chemical shifts ranging from δ 16 to -11 ppm. The solid-state structure, confirmed by X-ray crystallography (Figure S1), is distorted trigonal bipyramidal, with the neosilyl groups and pyridine occupying the equatorial plane. The Cimine – Nimine and Cipso – Cimine bond lengths of the pyridine(diimine) ligand are consistent with a doubly reduced chelate, indicating that a Mo(IV) oxidation state assignment is most appropriate. A solid state magnetic moment (magnetic susceptibility balance) of 2.5 µB was measured at 23 ºC, consistent with an S = 1 ground state configuration. This measurement is slightly lower than the spin-only value of 2.8 µB, which has been observed previously for PDI molybdenum (IV) compounds19 and is likely a result of spin-orbit coupling in the triplet ground state (likely a pseudo 2e’’ configuration, see SI for a qualitative molecular orbital diagram).

Table 2. Substrate Scope of Molybdenum-Catalyzed Arene Hydrogenation

Table 1. Optimization of the Benzene Hydrogenation with PDI Molybdenum Precatalysts

Other substituted arenes such as m- and p-xylenes, cymene, ethylbenzene, cumene etc. were unreactive under the above conditions, as 98%, >98%, and 53% NMR yield. Biphenyl and diphenylmethane underwent hydrogenation of both arene rings and produced bicyclohexyl and dicyclohexylmethane in >98% and 69% NMR yield, respectively. M-xylene was hydrogenated to 1,3dimethylcyclohexane, predominately the cis isomer (75:25 d.r.) in 40% NMR yield, while hydrogenation of p-xylene produced 1,4-dimethylcyclohexane in only 24% NMR yield (50:50 d.r.). Increasing the pressure to 32 atm in the hydrogenation of naphthalene still produced tetraline exclusively (>98 NMR yield).

Arene and Alkene Substrate Scope. The scope of the molybdenum-catalyzed arene hydrogenation was also investigated and the results are summarized in Table 2. With 5 mol % (4-tBu-iPrPDI)Mo(CH2SiMe3)2 in cyclohexane at 60 °C under 4 atm of H2, toluene was hydrogenated to methylcyclohexane in >98% NMR yield. Under identical conditions, ortho-xylene was hydrogenated to 1,2-dimethylcyclohexane in 81% NMR yield, predominantly to the cis stereoisomer (79:21 d.r.). Naphthalene underwent hydrogenation of only one aromatic ring, selectively producing tetralin in 74% NMR yield. These conditions are mild compared to other heterogeneous, multisite molybdenum catalysts that require elevated temperatures and pressures.21

Because more hindered arenes required increased hydrogen pressure to observe turnover, we reasoned that this differential reactivity could be exploited for the chemoselective hydrogenation of mixtures of arenes. To this end, an equimolar mixture of toluene and ethylbenzene was subjected to 4 atm of H2 along with 5 mol % (4-tBu-iPrPDI)Mo(CH2SiMe3)2 (Scheme 1). Methylcyclohexane was obtained exclusively in >98% NMR yield, along with recovered ethylbenzene, demonstrating that [(iPrPDI)Mo] is capable of selectively hydrogenating arenes with smaller substituents in a mixture. This also highlights the ability of [(iPrPDI)Mo] to differentiate subtle changes in substrate sterics (i.e. methyl vs. ethyl or ovs. m- and p-xylenes), a distinguishing feature from heterogeneous catalysts which typically are selective for more substituted, electron rich arenes.5c While by no means definitive, the pressure and ligand dependence, chemoselectivity, and


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stereoselectivity of arene hydrogenation support single site, soluble pyridine(diimine) molybdenum complexes as the active catalysts. Scheme 1. Selective Hydrogenation of Toluene over Ethylbenzene.

The significant drop in arene hydrogenation reactivity with larger substrates is likely a result of steric crowding around the molybdenum catalyst, as illustrated in Figure 3. Much like (PDI) molybdenum bis-olefin complexes, (iPrPDI)Mo(η6benzene) is Cs symmetric in the solid state, due to the N-aryl groups of the ligand being bowed away from the coordinated benzene.18 This creates an asymmetric pocket around the molybdenum, where the top face of the molybdenum is blocked by the 2,6-diisopropyl N-aryl groups. For toluene, naphthalene, and o-xylene, the arene can coordinate to place the larger substituents in the more “open” face of the molybdenum, which minimizes steric interactions with the isopropyl groups of the ligand (Figure 3, top). For benzenes with larger substituents, as well as 1,3- and 1,4-disubstituted arenes, there is no configuration where the substituents of the arene do not unfavorably interact with the ligand, rendering coordination to the molybdenum much more difficult, ultimately resulting in diminished catalytic activity at 4 atm of H2 (Figure 3, bottom). This reactivity profile has been observed with other transition metal-catalyzed arene hydrogenations, specifically a cobalt-catalyzed methodology developed by Muetterties and co-workers. In their report, multisubstituted and bulky arenes required much longer reaction times or were completely unreactive.8 Less sterically encumbered molybdenum precatalysts (both 1,5-COD and bis(neosilyl) variants from Table 1) were also screened for the hydrogenation of larger arenes, but these also produced the corresponding cycloalkanes in 98% NMR yield in hydrocarbon solvents (hexanes and cyclohexane, respectively). 1,2dimethylcyclohex-1-ene23 was hydrogenated to 1,2dimethylcyclohexane in 82% NMR yield, 77:23 d.r. (cis:trans), at 60 °C. The selectivity of this reaction was almost identical to the hydrogenation of o-xylene, indicating that 1,2dimethylcyclohexane is likely an intermediate in the hydrogenation of o-xylene (vide infra). While molybdenum catalyzed hydrogenation and hydrosilylation of carbonyl compounds is well established,24 the corresponding hydrogenation of alkenes is less developed. There have been several examples of stoichiometric25, as well as transfer hydrogenation of alkenes using cyclopentadienyl- and phosphine-based molybdenum complexes/catalysts.26 The scope of molybdenum-catalyzed hydrogenation utilizing H2 gas is limited to linear olefins and requires elevated temperatures (>100 °C) and pressures (>50 atm of H2).27 To the best of our knowledge, this is the first instance of a mild (23 °C, 4 atm of H2) molybdenum-catalyzed alkene hydrogenation that is operative for even tetra-substituted olefins. Table 3. Substrate Scope of Molybdenum-Catalyzed Alkene Hydrogenation

To illustrate how the chemoselectivity of molybdenumcatalyzed hydrogenations can be controlled through H2 pressure, the hydrogenation of styrene was explored (Scheme 2). With 5 mol % (4-tBu-iPrPDI)Mo(CH2SiMe3)2 and 4 atm of H2, styrene was quantitatively converted to ethylbenzene after 24 hours at 23 °C. When exposed to 32 atm of H2 at 60 °C with 10 mol % (4-tBu-iPrPDI)Mo(CH2SiMe3)2, styrene was completely reduced to ethylcyclohexane in >98% NMR yield. This demonstrates the versatility and pressure-dependent chemoselectivity of (PDI) molybdenum hydrogenation catalysts. Scheme 2. Pressure-Dependent Chemoselective Molybdenum-Catalyzed Hydrogenation of Styrene to Ethylbenzene or Ethylcyclohexane.

Figure 3. Steric interactions between the η6-arene and ligands: explanation for diminished reactivity.

iPr

PDI

While neither cyclic olefins nor dienes were detected during any of the molybdenum-catalyzed arene hydrogenation reactions reported here, it is likely that these compounds are intermediates generated during turnover. To explore this possibility, (4-tBu-iPrPDI)Mo(CH2SiMe3)2 was employed as an alkene hydrogenation catalyst, the results of which are re-


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Deuterium Labelling Studies. To gain further insight into the regio- and stereoselectivity of the molybdenum-catalyzed arene hydrogenation, catalytic deuteration experiments were conducted with benzene, 1,3-cyclohexadiene, and cyclohexene (Scheme 3). Exposure of benzene to 4 atm of D2 under the conditions detailed in Entry 5, Table 1, produced cyclohexane-dn in 10 turnovers after 24 hours. The reaction mixture was analyzed by quantitative 13C NMR spectroscopy and was found to be a complex mixture of different stereoisomers and isotopologues of cyclohexane (Scheme 1). While no single compound can be precisely identified, meaningful information about specific carbon environments can be extracted from the NMR spectrum: both cis and trans relationships between deuteria are present; carbons with two deuteria attached are present; and carbons with no deuterium incorporation are present (Scheme 2). 1,3-cyclohexadiene was also deuterated under the conditions detailed in Table 3 and a similar mixture of stereoisomers and isotopologues to the benzene deuteration was observed. Finally, cyclohexene was exposed to 4 atm of D2 under the conditions detailed in Table 3. Cyclohexane-d2 was produced in >98% NMR yield and found to be only the cis isomer by quantitative 13C NMR spectroscopy.28 Toluene was similarly deuterated under the conditions in Entry 5, Table 1, which produced methylcyclohexane-dn in 34% NMR yield. The methylcyclohexane-dn that was formed was found to also be a complex mixture of stereoisomers and isotopologues, similar to the benzene deuteration. Importantly, no deuterium incorporation into the methyl group of methylcyclohexane-dn was observed (see SI for details). Mass spectrometry was used to determine the composition of the isotopologues formed from the catalytic deuteration. Methylcyclohexane-d6 accounted for only 35% of the product mixture with the balance of the material comprised of isotopologues ranging from d4 to d11 (see SI for details and full list of ratios). Scheme 3. Deuterium Labelling Experiments

the deuterations of benzene and 1,3-cyclohexadiene, but not cyclohexene (Figure 4). Because deuteration of cyclohexene generates a molybdenum cyclohexyl intermediate, this is less likely to undergo isomerization than an allylic intermediate (generated during the benzene and cyclohexadiene deuterations), which explains the observed stereopurity in the deuteration of cyclohexene. Representative scrambling pathways are summarized in Figure 4. These conclusions explain the observed diastereoselectivity in the hydrogenation of both o-xylene and 1,2-dimethylcyclohex-1-ene: since alkene/diene coordination is reversible and thermodynamically controlled, similar selectivity should be observed when any intermediate along the o-xylene hydrogenation pathway is exposed to the reaction conditions. This proposal also explains the >d6 isotopologues observed for the deuteration of toluene, as the scrambling mechanism via allylic C-H activation generates a molybdenum hydride, which likely exchanges to a deuteride under a D2 atmosphere, which can lead to further deuterium incorporation. Investigations into Catalyst Generation and Resting State. To determine the identity of the active molybdenum catalyst and/or the catalyst resting state, several stoichiometric and catalytic experiments were performed on (iPrPDI)Mo(1,5-COD) and (iPrPDI)Mo(η6-benzene). As depicted in Scheme 4 (top), heating a benzene-d6 solution of (iPrPDI)Mo(1,5-COD) to 60 °C for 18 hours under 4 atm of H2 produced (iPrPDI)Mo(η6benzene-d6) in >98% conversion. Cyclooctane, as well as cyclohexane-dn, were observed by 1H NMR spectroscopy, indicating that the NMR solvent had been hydrogenated. To determine whether the η6-benzene complex was still an active arene hydrogenation catalyst, (iPrPDI)Mo(η6-benzene) was heated to 60 °C in benzene-d6 under 4 atm of H2 and monitored over the course of 18 hours (Scheme 4, middle). Cyclohexane-dn steadily grew in over the course of the reaction, accompanied by disappearance of the coordinated benzene resonance of the molybdenum complex (located at δ 3.56 ppm), reaching 87% conversion to (iPrPDI)Mo(η6-benzene-d6) after 18 hours. It is important to note that no such arene exchange was observed without added H2: heating a benzene-d6 solution of (iPrPDI)Mo(η6-benzene) to 60 °C for 24 h under an atmosphere of N2 produced no detectable arene exchange or decomposition of the starting material. This supports the assertion that dihydrogen can induce a haptotropic rearrangement of coordinated benzene to allow for arene hydrogenation in analogy to the isolable ammine derivatives (Figure 2). Under the catalytic conditions described in Table 1, 0.2 mol % of (iPrPDI)Mo(η6-benzene) in benzene produced cyclohexane in 19 TON, again, confirming that the η6-benzene complex is catalytically active. These data suggest that the resting state of the molybdenum catalyst in benzene is (iPrPDI)Mo(η6-benzene).

These data support a pathway involving reversible insertion of the arene into a Mo-H followed by β-H elimination where coordination of benzene, and its hydrogenation intermediates (cyclohexadiene and cyclohexene) is reversible. PDI molybdenum complexes have been shown to undergo facile allylic C-H activation at ambient temperature, which is a likely pathway for H/D scrambling.19 This would account for the numerous stereoisomers and isotopologues observed from


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Figure 4. Summary of H/D scrambling and isomerization mechanisms for molybdenum-catalyzed deuteration of benzene and its intermediates iPr

Scheme 4. Fate of ( PDI)Mo(1,5-COD) Precatalyst and Activity of (iPrPDI)Mo(η6-benzene) as a Hydrogenation Catalyst

To explore why molybdenum bis(neosilyl) precatalysts were more active than their 1,5-COD counterparts, (4-tBuiPr PDI)Mo(CH2SiMe3)2 was exposed to 4 atm of H2 in benzened6 and analyzed by 1H NMR spectroscopy. After 10 minutes, >98% conversion to a C1 symmetric, diamagnetic PDI molybdenum hydride was observed, with a diagnostic hydride signal centered at δ -3.40 ppm.29 The precise structure of this complex has yet to be determined as it is too reactive to isolate and rapidly converts to (4-tBu-iPrPDI)Mo(η6-benzene-d6), reaching >95% conversion after 3 hours at 23 °C. Cyclohexane-dn was observed in the NMR spectrum after 10 minutes of reaction, indicating this molybdenum hydride intermediate is an active arene hydrogenation catalyst. These experiments indicate that bis(neosilyl) precatalysts rapidly form the active

molybdenum catalyst at ambient temperature, unlike the 1,5COD precatalysts. The alkyl groups of (4-tBuiPr PDI)Mo(CH2SiMe3)2 are readily cleaved by H2 at ambient temperature to generate a highly active molybdenum hydride, which then converts to an η6-benzene complex, a much slower arene hydrogenation catalyst (vide supra). By comparison, (iPrPDI)Mo(1,5-COD) has a longer induction period than (4-tBu-iPrPDI)Mo(CH2SiMe3)2, as it takes heating to 60 °C for 18 hours just to hydrogenate the 1,5-COD ligand and activate the catalyst. Attempts to observe a molybdenum hydride in cyclohexane (without added arene or other coordinating ligands) have been unsuccessful. Exposure of any of the molybdenum precatalysts listed in Table 1 to hydrogen in cyclohexane-d12 led to decomposition of the starting material and produced an intractable mixture of compounds as judged by 1H NMR spectroscopy. Interestingly, exposure of (iPrPDI)Mo(η6benzene) to 4 atm of H2 in cyclohexane-d12 produced one equivalent of cyclohexane, but led to decomposition of the molybdenum complex. These results suggest that a single turnover of arene hydrogenation had taken place and that dihydrogen appears to induce a haptotropic rearrangement of the η6-benzene ligand and facilitate benzene hydrogenation. The resulting PDI molybdenum hydride is likely only stable in the presence of other coordinating ligands (i.e. arene or olefin, vide supra). Identification of Catalyst Deactivation Pathways from the Attempted Hydrogenation of Heteroarenes. Several different nitrogen- and oxygen-containing heteroarenes were tested in the molybdenum-catalyzed arene hydrogenation, but under the conditions detailed in Table 2 (both 4 and 32 atm of H2), only unreacted starting material was observed in the crude 1H NMR spectrum of the unpurified mixture. To identify any potential deactivation pathways that may occur with molybdenum precatalysts in the presence of heteroarenes


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and hydrogen, several stoichiometric reactions were conducted. (iPrPDI)Mo(1,5-COD) was mixed with 2.1 equivalents of pyridine and heated to 60 °C in benzene-d6 under 4 atm of H2, and after 18 hours, all the starting material was consumed and cyclooctane was observed. The resulting purple organometallic complex was identified as the 2-pyridinyl molybdenum pyridine-hydride (iPrPDI)Mo(H)(η2-C5H4N)(pyridine), which was obtained in >98% conversion (Scheme 5). The compound was characterized by multinuclear NMR spectroscopy and its structure confirmed by X-ray crystallography. Crystals for the diffraction study were obtained from independent synthesis by sodium amalgam reduction of (iPrPDI)MoCl3 in the presence of pyridine (see SI for details). The ambient temperature 1H NMR spectrum of (iPrPDI)Mo(H)(η2-C5H4N)(pyridine) in benzene-d6 exhibits the expected number of signals for a Cs symmetric compound and also supports a diamagnetic ground state. Most of the aromatic pyridine signals are well resolved and were assigned, ranging from δ 6.73 to 5.72 ppm. The hydride signal appears as a singlet at δ -8.84 ppm, a chemical shift within the range of molybdenum hydrides with other nitrogen- and oxygenbased ligands.30 C-H activation of this type has been previously observed with molybdenum(0) complexes, as Parkin and co-workers observed C-H activation of pyridine in the 2position by Mo(PMe3)6.30 While (PMe3)4Mo(H)(η2-C5H4N) converts to the η6-pyridine analogue upon heating, this was not observed for (iPrPDI)Mo(H)(η2-C5H4N)(pyridine), which is stable at 80 °C under 4 atm of H2, 1 atm of N2, or vacuum. Scheme 5. Isolation of Catalyst Deactivation Species with Pyridine – C-H Activation of Pyridine (2-position)

Single crystals of (iPrPDI)Mo(H)(η2-C5H4N)(py) suitable for Xray diffraction were obtained from slow diffusion of pentane into a concentrated thf solution at -35 °C over a period of 18 hours. The solid-state structure is presented in Figure 5, along with selected bond distances and angles. (iPrPDI)Mo(H)(η2-C5H4N)(py) has a distorted pentagonal bipyramidal geometry, a 7-coordinate molybdenum center with the hydride (which could not be located) and N-bound pyridine ligand occupying the apical positions, and the C-H activated pyridine and iPrPDI ligands occupying the equatorial positions. The Cimine – Nimine and Cipso – Cimine bond lengths of the pyridine(diimine) chelate are consistent with a doubly reduced chelate, indicating that a Mo(IV) oxidation state is most appropriate.

Figure 5. Representation of the solid-state structure of (iPrPDI)Mo(H)(η2-C5H4N)(pyridine) at 30% probability thermal ellipsoids. Hydrogen atoms and co-crystallized thf molecule are omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1 – N1 2.117(3), Mo1 – N2 2.043(3), Mo1 – N3 2.106(3), Mo1 – N4 2.334(3), Mo1 – N5 2.095(3), Mo1 – C39 2.101(3), N1 – C2 1.335(4), N3 – C8 1.350(4), C2 – C3 1.403(5), C7 – C8 1.407(5), N5 – C39 1.329(5); N1 – Mo1 – N3 146.2(1), N1 – Mo1 – N2 73.4(1), N2 – Mo1 – N3 73.3 (1), N1 – Mo1 – N4 94.5(1), N1 – Mo1 – N5 125.7(1), N1 – Mo1 – C39 88.8(1). To determine if there was a similar catalyst deactivation pathway with oxygen-containing heterocycles, (iPrPDI)Mo(CH2SiMe3)2 was treated with 5 equivalents of furan under 1 atm of H2 in benzene-d6 (Scheme 6). After 10 minutes at 23 °C, the starting material was consumed and tetramethylsilane was detected by 1H NMR spectroscopy. A new, maroon molybdenum complex had formed in >98% conversion, which was identified as the C-O bond cleaved species (iPrPDI)Mo[κ2-O(CH)4], a “metallapyran”. Its composition and structure were determined by a combination of multinuclear NMR spectroscopy and X-ray crystallography, and independently prepared from sodium amalgam reduction of (iPrPDI)MoCl3 in the presence of furan (see SI for details). (iPrPDI)Mo[κ2-O(CH)4] was also obtained from addition of furan to a benzene-d6 solution of (iPrPDI)Mo(1,5-COD) in the presence of hydrogen, but this route required heating to 60 °C for 24 hours, and (iPrPDI)Mo(η6-benzene) was also observed.31 This is a unique instance of oxidative addition into the C–O bond of furan by molybdenum. Aluminum(II) 1,3diketimidate complexes have been shown to undergo oxidative addition into the more reactive C–O bond of benzofuran32, while iridium metallapyrans have been synthesized by alkylation with enolates and C-H activation of enal iridium complexes.33 Group 434 and Group 1035 metallapyrans have also been observed when the respective metal-furanyl compounds are exposed to nucleophiles such as phenyl, methyl, or furanyl lithium. This suggests that C-H activation of furan (analogous to pyridine) may occur en route to (iPrPDI)Mo[κ2O(CH)4], where the resulting molybdenum-hydride-furanyl undergoes a hydride-induced, migratory ring enlargement to yield the metallapyran.34a Scheme 6. Isolation of Catalyst Deactivation Species with Furan – C-O Oxidative Addition into a Metallapyran


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es (Å) and angles (deg): Mo1 – N1 2.062(1), Mo1 – N2 2.052(1), Mo1 – N3 2.073(1), Mo1 – O1 1.921(1), Mo1 – C34 2.030(2), N1 – C2 1.347(2), N3 – C8 1.336(2), C2 – C3 1.406(2), C7 – C8 1.414(2); N1 – Mo1 – N3 143.00(5), N1 – Mo1 – N2 72.45(5), N2 – Mo1 – N3 72.51(5), N2 – Mo1 – O1 172.04(5), N2 – Mo1 – C34 96.57(6), O1 – Mo1 – C34 88.71(6).

CONCLUSIONS

The 1H NMR spectrum of (iPrPDI)Mo[κ2-O(CH)4] in benzened6 is consistent with a diamagnetic ground state and Cs symmetric structure. There are 4 distinct resonances corresponding to the metallapyranyl protons, ranging from δ 7.53 to 6.20 ppm. Crystals of (iPrPDI)Mo[κ2-O(CH)4] suitable for X-ray diffraction were obtained by slow evaporation of a concentrated pentane solution at -35 °C over a period of 24 hours. The solid-state structure is presented in Figure 6, along with selected bond lengths and angles. The coordination geometry of (iPrPDI)Mo[κ2-O(CH)4] is best described as a distorted square-based pyramid, which is consistent with the NMR spectroscopic data. The oxygen of the metallapyran is trans to the pyridine nitrogen of PDI and in an equatorial position, while the C-terminus of the metallacycle bound to the metal occupies the apical position of the square-based pyramid. The C–C bond lengths of the metallapyran are statistically distinguishable: the C-terminus C – C bond (C34 – C35) has a length of 1.365(2) Å and the O-terminus C – C bond (C36 – C37) has a length of 1.348(3) Å, indicating double bond character for both. The C – C (C35 – C36) and C – O (C37 – O1) single bonds have lengths only slightly contracted from normal values, 1.425(3) Å and 1.334(2) Å, respectively. The Cimine – Nimine and Cipso – Cimine bond lengths of (iPrPDI)Mo[κ2-O(CH)4] are consistent with a closed-shell, doubly reduced PDI ligand, analogous to (iPrPDI)Mo(H)(η2-C5H4N)(py) and iPr ( PDI)Mo(CH2SiMe3)2, and supports a Mo(IV) oxidation state.

In summary, a molybdenum-catalyzed arene and alkene hydrogenation method has been developed. Benzene, along with several substituted arenes were fully hydrogenated and demonstrate that the barrier to arene hydrogenation can be significantly lowered through coordination to a transition metal. Deuterium labeling studies reveal the formation of multiple isotopologues and stereoisomers, indicating reversible insertion/β-H elimination, substrate coordination, and allylic C-H activation all taking place during the reaction. The catalyst resting state in benzene was determined to be the η6-benzene complex, while attempts to observe a molybdenum hydride in hydrocarbon solvents were unsuccessful. Catalyst deactivation pathways for selected heteroarene substrates were also identified: C-H activation of pyridine and CO bond cleavage of furan were observed and their resulting organomolybdenum complexes characterized.

ASSOCIATED CONTENT Supporting Information Complete experimental details and procedures including metal complex syntheses, hydrogenation procedures, and characterization data for all compounds. The materials may be downloaded free of charge at pubs.acs.org

AUTHOR INFORMATION Corresponding Author * Email for P.J.C. [email protected]

ORCID Paul J. Chirik: 0000-0001-8473-2898 Matthew V. Joannou: 0000-0002-0079-7107 Máté J. Bezdek: 0000-0001-7860-2894 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Dr. Ilia Korobkov for helpful discussions and Dr. Zöe Turner for the X-ray structure of (iPrPDI)Mo(CH2SiMe3)2. We also thank Dr. John Eng for assistance with mass spectrometry. Financial support was provided by the National Science Foundation (NSF) Grant Opportunities for Academic Liaison with Industry (GOALI) grant (CHE-1265988).

REFERENCES

Figure 6. Representation of the solid-state structure of (iPrPDI)Mo[O(CH)4] at 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distanc-

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Dyson, P. J. Arene Hydrogenation by Homogeneous Catalysts: Fact or Fiction? Dalton Trans. 2003, 15, 2964–2974.


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García-Ventura, I.; Flores-Alamo, M.; J. García, J. “Carbon–carbon vs. carbon–oxygen bond activation in 2- and 3-furonitriles with nickel” RSC Adv. 2016, 6, 101259–101266.

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