Synthesis and Reactivity of Pyridine(diimine) Molybdenum Olefin

Oct 30, 2017 - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States. ‡ SABIC Corporate Research & Development, ...
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Synthesis and Reactivity of Pyridine(diimine) Molybdenum Olefin Complexes: Ethylene Dimerization and Alkene Dehydrogenation Matthew V. Joannou,† Máté J. Bezdek,† Khalid Al-Bahily,‡ Ilia Korobkov,‡ and Paul J. Chirik*,† †

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States SABIC Corporate Research & Development, Fundamental Catalysis, Thuwal 23955-6900, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: Reduced pyridine(diimine) molybdenum olefin complexes have been synthesized and structurally characterized. Examples with 1,5-cyclooctadiene, (PDI)Mo(η2:η2-1,5COD) (COD = 1,5-cyclooctadiene) adopt a distortedtrigonal-bipyramidal geometry and are best described as lowspin Mo(II) compounds arising from significant π backdonation to the ligand from a reduced molybdenum center. With the 2,6-diisopropyl N-aryl-substituted variant of the pyridine(diimine) ligand, a molybdenum bis(ethylene) complex was obtained. Reducing the size of the N-aryl substituents to 2,4,6-trimethyl resulted in isolation of (MesPDI)Mo(η4butadiene)(η2-ethylene) following sodium amalgam reduction of the corresponding molybdenum(III) trichloride complex in the presence of excess ethylene. Analysis of the byproducts of the reaction and olefin addition experiments demonstrate that butadiene formation is consistent with a pathway involving ethylene coupling to form 1-butene followed by allylic dehydrogenation to produce butadiene. Excess ethylene serves as the hydrogen acceptor. The dehydrogenation reaction was also compatible with α-olefins, as reduction of either (iPrPDI)MoCl3 or (MesPDI)MoCl3 in the presence of 1-hexene resulted in isolation of (PDI)Mo(η4-1,3-hexadiene)(η2-1-hexene) complexes. An α,ω-diene complex, (iPrPDI)Mo(η2:η2-1,6-heptadiene), was also synthesized and importantly displayed no cycloaddition chemistry, suggesting that first-row metal pyridine(diimine) complexes are thus far unique in promoting cyclobutane synthesis.

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INTRODUCTION The development of vast global natural gas deposits has increased the supply of ethylene and related downstream αolefins such as 1-hexene and 1-octene.1,2 Accordingly, there is increased motivation to convert these building blocks to new carbon architectures that may offer a broad spectrum of applications.3 The [2 + 2] cycloaddition of alkenes and dienes catalyzed by pyridine(diimine) iron and cobalt catalysts is exemplary in this regard, as new cyclobutanes can be readily accessed under convenient thermal conditions from commodity hydrocarbons.4 Mechanistic studies support oxidative coupling of the alkene or alkene-diene components followed by C(sp3)− C(sp3) reductive elimination to form the four-membered ring.5 For alkene-alkene [2 + 2] cycloadditions, spectroscopic and computational studies establish that the redox-active pyridine(diimine) maintains its singly reduced, radical anionic form throughout the catalytic cycle.5,6 As illustrated in Figure 1, a fundamental question therefore ariseswhat role does this electronic structure play in promoting the unusual [2 + 2] cycloaddition reactivity? Replacing iron and cobalt with molybdenum was explored to determine if a second-row transition metal in the same ligand environment would promote similar chemistry. While they have been less explored than first-row pyridine(diimine) metal © XXXX American Chemical Society

complexes, second- and third-row congeners tend to favor the redox-noninnocent two-electron-reduced closed-shell form of the chelate arising from π back-donation, as opposed to the redox-active radical anion form adopted by many first-row metals.7 Molybdenum was selected for these studies, as it is an earth-abundant transition metal and reduced pyridine(diimine) derivatives have been synthesized by our laboratory and exhibit rich reaction chemistry.8 Here we describe the synthesis and characterization of pyridine(diimine) molybdenum 1,5-cyclooctadiene complexes and explore their reactivity with ethylene, α-olefins, and α,ω-dienes. Molybdenum bis(ethylene), ethylene-butadiene, diene-olefin, and α,ω-diene complexes have been synthesized, and a stoichiometric dehydrogenative coupling of ethylene to butadiene has been observed, the rate of which was dependent on the identity of the aryl substituents.

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RESULTS AND DISCUSSION Synthesis of (PDI)Mo(η2:η2-1,5-COD) Complexes. Our studies commenced with the targeted preparation of reduced pyridine(diimine) molybdenum precursors with the goal of studying fundamental reactivity with commodity alkenes and Received: August 27, 2017

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A similar procedure was used to prepare (MesPDI)Mo(1,5COD) (2b; Mes PDI = 2,6-(2,4,6-(trimethyl)C 6 H 3 N CMe)2C5H3N), a compound that was isolated as a blue, diamagnetic solid in 88% yield. In analogy to 2a, the benzene-d6 1 H NMR spectrum of 2b exhibits the number of peaks consistent with a Cs-symmetric molybdenum complex. While the pyridine resonances of the pyridine(diimine) ligand are sharp and well-defined, the signals corresponding to the mesityl groups and 1,5-COD ligand are broadened, again indicating dynamic behavior of the diene or PDI ligand. Notably, the signals for the olefinic protons on 1,5-COD, assigned on the basis of HSQC experiments, appear almost 4 ppm apart from each other, with one resonance appearing at δ 2.60 ppm and the other at δ −1.04 ppm. These disparate chemical shifts are likely due to the different chemical environments of the equatorially and axially coordinated alkenes. Crystals of 2b suitable for X-ray diffraction analysis were grown by slow evaporation of a saturated diethyl ether solution at ambient temperature under an N2 atmosphere. A representation of the solid-state structure is presented in Figure 2 along with selected bond distances and angles. The

Figure 1. First- versus second-row transition-metal pyridine(diimine) metallacycles derived from commodity alkenes.

dienes. Dinitrogen derivatives, analogous to those used in ironand cobalt-catalyzed [2 + 2] cycloaddition, have been prepared previously but the syntheses of these compounds are generally low yielding and purifications are challenging.7 Because 1,5cyclooctadiene (1,5-COD) complexes are ubiquitous and often excellent precursors for catalysis,9 pyridine(diimine) molybdenum derivatives with this diene were explored. Stirring a toluene slurry of (iPrPDI)MoCl3 (1a; iPrPDI = 2,6-(2,6(diisopropyl)C6H3NCMe)2C5H3N) with 5 equiv of 1.0% Na(Hg) and 1,5-COD followed by filtration and recrystallization resulted in isolation of a green, diamagnetic solid identified

Figure 2. Representation of the solid-state structure of 2b with 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1−N1 2.189(2), Mo1−N2 2.028(2), Mo1−N3 2.218(2), N1−C2 1.329(3), N3−C8 1.330(3), C2−C3 1.422(3), C7−C8 1.427(3), C28−C29 1.436(4), C32−C33 1.430(4); N1−Mo1−N2 73.37(8), N1−Mo1−N3 139.42(8), N2−Mo1−N3 72.96(8), N2−Mo1−C28 160.33(9), N2− Mo1−C29 160.89(9), N2−Mo1−C32 92.48(9), N2−Mo1−C33 89.48(9).

Scheme 1. Synthesis of Pyridine(diimine) Molybdenum 1,5COD Complexes 2a,b

geometry of the compound is best described as a distorted trigonal bipyramid, consistent with the solution NMR data. The mesityl groups of the pyridine(diimine) ligand are directed away from the 1,5-COD ligand to accommodate η2, η2 coordination. The C imine−Nimine and Cipso−C imine bond distances are established metrics for determining the redox state of the pyridine(diimine) as a consequence of the transition metal.10 For 2b, Cimine−Nimine distances of 1.329(3) and 1.330(3) Å and Cipso−Cimine values of 1.422(3) and 1.427(3) Å are consistent with a two-electron-reduced form of the chelate. The observed diamagnetism and the appearance of an imine methyl resonance at δ 2.20 ppm suggested that the closed-shell dianionic (“non-innocent”) 11 form is most appropriate. In this view, both 2a and 2b are best described as low-spin Mo(II) complexes arising from strong π backdonation with the pyridine(diimine) ligand from a reduced molybdenum center. One resonance structure at the dianionic extreme of this bonding picture is depicted in all figures and schemes. Notably, no spectroscopic and metrical data have been obtained to support ligand-centered radicals.

as (iPrPDI)Mo(1,5-COD) (2a) in 92% yield (Scheme 1). The 1 H NMR spectrum of 2a in benzene-d6 was consistent with a Cs-symmetric structure in solution with the diastereotopic allylic protons of 1,5-COD appearing as two broad multiplets at δ 2.93 and 2.28 ppm. Resonances associated with the olefinic and isopropyl methyl/methine protons of the complex are similarly broadened (appearing between δ 1.54 and 0.87 ppm), likely a result of dynamic behavior at ambient temperature. In contrast to the 1,5-COD and isopropyl methyl signals, the resonances for the pyridine ring of the pyridine(dimine) chelate are sharp and well-defined. On the basis of the spectroscopic data, the solution geometry of 2a is likely a distorted trigonal bipyramid, where the broadened resonances are a result of either rapid interconversion of the equatorial and axial alkenes of 1,5-COD or restricted rotation of the N-aryl groups. B

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solution. The Cimine−Nimine and Cipso−Cimine bond distances are consistent with a doubly reduced PDI ligand, analogous to 2b. The axial and equatorial ethylene ligands have statistically distinguishable C−C bond distances; the equatorially bound ethylene has a bond length of 1.421(6) Å, and the axially bound ethylene bond length is 1.379(6) Å. The axial ethylene (trans to the pyridine) π* orbital and a π* orbital of PDI are competing for orbital overlap with a filled dxz orbital on molybdenum, resulting in reduced back-donation to ethylene and minimal C−C bond lengthening. The equatorial ethylene ligand (cis to the pyridine) is the only π-accepting ligand in the xy plane and can effectively overlap with a filled dxy orbital on molybdenum, resulting in the observed ethylene C−C bond lengthening (see SI for a more detailed molecular orbital representation). As in the solid-state structure of 2b, the 2,6-diisopropyl aryl groups of 3 are directed away from the ethylene ligands. Synthesis and reactivity of (MesPDI)Mo(η4-butadiene)2 (η -ethylene). The size of the aryl substituents on the pyridine(diimine) ligand were reduced from 2,6-diisopropyl to 2,4,6-trimethyl with the goal of studying the effect on the stability and reactivity of the resulting molybdenum ethylene complex. Reduction of (MesPDI)MoCl3 with excess 1.0% sodium amalgam in the presence of three equivalents of ethylene followed by filtration and recrystallization produced a bright green diamagnetic solid identified as the pyridine(diimine) molybdenum butadiene-ethylene complex, 4 (Scheme 3). The identity of 4 was established by a combination

Synthesis of (iPrPDI)Mo(η2-ethylene)2. With a reliable procedure for the isolation of reduced molybdenum 1,5-COD complexes in hand, the synthesis of analogous bis(ethylene) compounds was explored. Repeating the reduction with 1a and 2.1 equiv of ethylene resulted in a forest green, diamagnetic solid identified as (iPrPDI)Mo(η2-ethylene)2 and isolated in 82% yield (Scheme 2). The benzene-d6 1H NMR spectrum of 3 Scheme 2. Synthesis of Pyridine(diimine) Molybdenum BisEthylene Complex

is consistent with a diamagnetic compound and contains the expected number of signals (4 isopropyl methyl signals and 2 isopropyl methine signals) for a complex with Cs symmetry. Signals for both η2 ethylene ligands are found at δ 2.29, 1.23 (partially overlapping with the isopropyl methyl and methine resonances) and −0.19 ppm, with the latter appearing as a broad singlet integrating to 4 protons. The broad signals are in contrast to the sharp and well-defined peaks for the hydrogens on the PDI chelate. The complex is stable to vacuum and in the presence of a dinitrogen atmosphere for up to six months. The shelf life of 3 can be extended considerably via storage of the solid at −35 °C. Crystals of 3 suitable for X-ray diffraction were obtained by slow evaporation of a saturated diethyl ether solution at ambient temperature. The solid-state structure is presented in Figure 3 along with selected bond distances and angles. The geometry of 3 is best described as distorted trigonal bipyramidal, consistent with the Cs symmetry observed in

Scheme 3. Synthesis of (MesPDI)Mo(η4-butadiene)(η2ethylene) via Reduction

of multinuclear and 2-D NMR studies and X-ray diffraction (Figure 4). When fewer (1 or 2) equivalents of ethylene were used, 4 remained the only observable product, albeit in lower yield. The room temperature benzene-d6 1H NMR spectrum of 4 exhibits the number of resonances expected for a C1 symmetric complex. Six distinct resonances were observed for the η4-butadiene ligand between δ 0.08 (terminal protons) and 5.58 (internal protons) ppm. The η2-ethylene resonances appear as a broad doublet at δ −1.59 ppm.12 The solid-state structure of 4 established a distorted octahedral molybdenum complex with an η4 s-cis-butadiene ligand trans to the central pyridine of the PDI chelate. The s-cis geometry, while prevalent among butadiene ligands,13,14 contrasts the s-trans coordination mode observed with pyridine(diimine)15 and pyridine(dialdimine)16 iron examples. The η2-ethylene ligand occupies the site cis to the butadiene ligand and the bond distance of 1.398(5) Å suggests minimal back bonding from the molybdenum center. By contrast, the metrical parameters of the pyridine(diimine) chelate are consistent with a two-electron reduced, closed shell form of the ligand. The bond lengths of the s-cis-butadiene ligand deviate significantly from free butadiene (see SI for details), indicating significant metallacyclopentene character resulting from π-back-donation from molybdenum. The presence of

Figure 3. Representation of the solid-state structure of 3 at 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1−N1 2.201(2), Mo1−N2 2.018(2), Mo1−N3 2.181(2), N1−C2 1.329(3), N3−C8 1.332(3), C2−C3 1.425(3), C7−C8 1.427(3), C34−C35 1.379(6), C36−C37 1.421(6); N1−Mo1−N2 72.83(8), N1−Mo1−N3 139.16(8), N2−Mo1−N3 73.90(8), N2−Mo1−C34 158.8(1), N2− Mo1−C35 161.4(1), N2−Mo1−C36 86.4(1), N2−Mo1−C37 85.9(1). C

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comparison rare.20 Bercaw and co-workers observed the production of butadiene and ethane from ethylene with catalytic amounts of (η5-C5Me5)2Ti(η2-ethylene) (12 mol % Ti, 86% yield over 10 months). To understand the origin of stoichiometric butadiene formation from ethylene with the pyridine(diimine) compound, additional experiments were conducted. The addition of ethylene to the molybdenum 1,5COD compound 2b was monitored by 1H NMR spectroscopy. After 9 h at 60 °C, complete conversion to 4 occurred and both free 1,5-COD and ethane were observed, with the latter accounting for the hydrogen balance from ethylene coupling (Scheme 5). Scheme 5. Synthesis of 4 by Substitution of 1,5-COD for Ethylene and Analysis of Hydrocarbon Byproducts

Figure 4. Representation of the solid-state structure of 4 at 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1−N1 2.213(2), Mo1−N2 2.028(2), Mo1−N3 2.209(2), N1−C2 1.325(4), N3−C8 1.328(4), C2−C3 1.425(4), C7−C8 1.419(4), C28−C29 1.425(7), C29−C30 1.389(6), C30−C31 1.432(7), C32−C33 1.398(5); N1− Mo1−N2 72.31(9), N1−Mo1−N3 138.78(9), N2−Mo1−N3 72.48(9), N2−Mo1−C28 152.5(1), N2−Mo1−C29 168.2(1), N2− Mo1−C30 143.4 1), N2−Mo1−C31 123.2 (1), N2−Mo1−C32 81.5(1), N2−Mo1−C33 86.6(2).

three π-accepting, potentially redox non-innocent ligands around a reduced molybdenum center renders its spectroscopic oxidation state ambiguous, consistent with Jørgenson’s original definition of term.17 To determine the relative binding affinity of both ethylene and butadiene with 4, separate exchange experiments using isotopically labeled substrates were conducted. Addition of excess (12 equiv) ethylene-d4 to a benzene-d6 solution of 4 resulted in complete exchange of the isotopically labeled olefin after 20 min at ambient temperature (Scheme 4). Heating the

A likely pathway for molybdenum-promoted butadiene formation would involve initial formation of 1-butene arising from oxidative couple of two molecules of ethylene to form a metallacycle, followed by β-H elimination and C−H reductive elimination. Subsequent dehydrogenation of 1-butene with excess ethylene serving as the hydrogen acceptor would yield the observed butadiene complex and ethane product. The plausibility of this sequence was evaluated by determining whether 1-butene was a competent substrate for butadiene formation. Performing the sodium amalgam reduction of (MesPDI)MoCl3 in the presence of 5 equiv of 1-butene yielded a dark green, diamagnetic solid, 5, that exhibited a broad, largely uninformative 1H NMR spectrum (see the Supporting Information for additional characterization details). Addition of 1 atm of ethylene to a benzene-d6 solution of 5 resulted in immediate generation of 4 in >98% conversion. No other hydrocarbon products were observed, supporting the identity of 5 as the dimeric pyridine(diimine) molybdenum butadiene dinitrogen complex (Scheme 6). These observations support the hypothesis that 1-butene is formed from ethylene during the reduction in Scheme 3 and then immediately undergoes dehydrogenation to butadiene. Because the synthesis of 1-butene from ethylene often occurs through metallacyclopentane intermediates,18,21 the synthesis and reactivity of pyridine(diimine) molybdacyclopentanes, specifically what hydrocarbons are released upon their degradation, were of interest. Salt elimination from metal halides and bis(organo)lithium or Grignard reagents has been used as a synthetic route to a variety of metallacycles.21 Using this approach, 1b was treated with bis(thf)magnesacyclopentane in thawing benzene-d6 followed by stirring at ambient temperature for 18 h. The volatile products of the reaction were collected by distillation and analyzed by 1H NMR spectroscopy to reveal formation of 1-butene and ethylene in a 1.5:1 ratio (Scheme 7). This observation supports formation of a pyridine(diimine) molybdacyclopentane that undergoes either cycloreversion to release 2 equiv of ethylene or by β-hydrogen elimination/reductive elimination to produce 1-butene. Analysis of the nonvolatile components of the

Scheme 4. Ethylene-d4 Exchange Experiment with (MesPDI)Mo(η4-butadiene)(η2-ethylene)

sample to 60 °C for 18 h produced no additional changes as judged by 1H NMR spectroscopy, demonstrating negligible exchange or reversible insertion into the butadiene ligand. This behavior is in contrast with that of the analogous iron complex, where reversible C−C bond formation and [2 + 2] product release are observed.18 The equivalent exchange experiment, addition of excess (12 equiv) butadiene-d6 to 4, produced no evidence for substitution by 1H NMR spectroscopy even after heating to 60 °C for 16 h. The reluctance of butadiene to dissociate from (MesPDI)Mo(η4butadiene)(η2-ethylene) further supports the claim that the butadiene ligand on 4 has significant metallacyclopentene character, increasing the barrier for butadiene release and exchange. Investigations into Butadiene Formation from Ethylene. While dimerization of ethylene to 1-butene is wellestablished with a variety of transition-metal catalysts,19 coupling of ethylene to butadiene, while valuable, is by D

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Scheme 8. Proposed Mechanism for the Dehydrogenative Coupling of Ethylene to Butadiene Promoted by [(MesPDI)]

Scheme 7. In Situ Generation of Molybdacyclopentane and Analysis of Its Decomposition Products

inserts ethylene. Both possibilities converge to a molybdenum allyl-ethyl complex that undergoes β-H elimination and C(sp3)−H reductive elimination of ethane. Coordination of another ethylene molecule completes the coordination sphere and generates 4. Dehydrogenation of 1-Hexene. Because reduced pyridine(diimine) compounds dehydrogenate 1-butene to butadiene, the dehydrogenation of higher α-olefins was explored. As shown in Scheme 9, both 1a and 1b were Scheme 9. Synthesis of Pyridine(diimine) Molybdenum Diene-Alkene Complex from 1-Hexene reaction isolated from filtration revealed formation of (MesPDI)MoCl2(thf) (6) in 67% isolated yield. Formation of 6 likely arises from a comproportionation reaction between (MesPDI)MoCl3 and (MesPDI)MoCl, the latter being formed after the reaction of 1b with the magnesacycle and reduction of the molybdenum.22 Unfortunately, treatment of 6 with additional bis(thf)magnesacyclopentane at 60 °C for 18 h produced no reaction as judged by 1H NMR spectroscopy. Without the observation of a discrete metallacycle, however, radical processes to form 1-butene, ethylene, and 6 cannot be excluded.23 On the basis of the experimental observations, a plausible mechanism for the dehydrogenative coupling of ethylene to butadiene is proposed and presented in Scheme 8. Coordination of 2 equiv of ethylene to the molybdenum followed by oxidative coupling yields the metallacyclopentane. A sequence of β-H and C−H reductive elimination from this intermediate generates an η2-1-butene molybdenum complex that undergoes subsequent dehydrogenation. Two pathways are possible for the dehydrogenation step. Pathway A involves concerted allylic C−H activation and ethylene insertion, a mechanism proposed by Hartwig for the nickel-catalyzed hydroarylation of olefins with trifluoromethylarenes. 24 Pathway B is a stepwise alternative where allylic C−H activation results in a molybdenum allyl-hydride, which coordinates and ultimately

reduced with 1.0% Na(Hg) in the presence of 5 equiv of 1hexene to yield the corresponding 1,3-hexadiene/1-hexene derivatives in 85 and 87% isolated yields, respectively. Both 7a (iPrPDI) and 7b (MePDI) were passed through a small plug of alumina to decompose the compounds and liberate the organic ligands to facilitate identification. Subsequent analysis of the filtrate by 1H NMR spectroscopy revealed free pyridine(diimine), 1,3-hexadiene, and 1-hexene, confirming the identity E

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(diimine) molybdenum complexes were competent in [2 + 2] cycloaddtion chemistry similarly to iron and cobalt, the reactivity of α,ω-dienes with pyridine(diimine) molybdenum was explored. 1b was reduced with excess 1.0% Na(Hg) in the presence of 2 equiv of 1,6-heptadiene (an efficient substrate in both Fe- and Co-catalyzed [2 + 2] cycloaddition) to yield a mixture of several bis-olefin complexes which, after the mixture was heated to 80 °C for 18 h, converged to a single species as judged by 1H NMR spectroscopy, (MesPDI)Mo(η2:η2-1,6,heptadiene) (9) (Scheme 10). This compound is diamagnetic,

of the ligands bound to 7a and 7b. Because the dehydrogenation of 1-hexene occurs with both iPrPDI and MesPDI molybdenum compounds, formation of the butadiene/ethylene complex of (iPrPDI)Mo is likely inhibited by slower ethylene dimerization. With larger substituents in the 2,6-positions of the aryl groups, the isopropyl variant of the ligand likely slows ethylene dimerization in comparison to the methyl version at ambient temperature. Attempts to obtain X-ray-quality crystals of either 7a or 7b have been unsuccessful; however, allowing a diethyl ether solution of 7b to stand for 3 days in an N2-filled glovebox produced several single crystals that were identified as [(MesPDI)Mo(η4-1,3-hexadiene)]2(μ-N2) (8). A representation of compound 8, along with select bond distances and angles, is presented in Figure 5. Analogously to 4, the solid-state

Scheme 10. Attempts at Mo-Promoted [2 + 2] Cycloaddition of 1,6-Heptadiene

Figure 5. Representation of the solid-state structure of 8 (which crystallized as the bridging dinitrogen adduct instead of the 1-hexene derivative) with 30% probability thermal ellipsoids. Hydrogen atoms and aryl groups on the top PDI molecule are omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1−N1 2.177(4), Mo1−N2 2.062(4), Mo1−N3 2.157(4), N1−C2 1.329(6), N3−C8 1.338(6), C2−C3 1.418(7), C7−C8 1.418(7), C28−C29 1.414(7), C29−C30 1.469(8), C30−C31 1.355(0), N4−N4 1.145(4), Mo1− C28 2.239(5), Mo1−C29 2.252(5), Mo1−C30 2.314(6), Mo1−C31 2.415(6); N1−Mo1−N2 72.8(1), N1−Mo1−N3 145.1(1), N2− Mo1−N3 72.8(1), Mo1−N4−N4 178.4(3).

and its benzene-d6 1H NMR spectrum exhibited the correct number of signals for a C1-symmetric compound. Unfortunately, further heating of 9 to 80 °C for 48 h led to no metallacycle or cyclobutane product formation, as judged by 1H NMR spectroscopy. These observations suggest, but are by no means conclusive, that the radical form of the pyridine(diimine) ligand in the corresponding iron and cobalt catalysts plays a role in promoting C(sp3)−C(sp3) reductive elimination.4,6,15 In summary, a series of reduced molybdenum pyridine(diimine) olefin complexes have been synthesized and characterized. With a sufficiently open coordination environment about the molybdenum, as in the case of (MesPDI), net coupling of ethylene to butadiene was observed, likely through a metallacycle pathway involving the intermediacy of 1-butene. This allylic dehydrogenation reactivity is applicable to other αolefins, as (PDI)Mo(η4-1,3-hexadiene)(η2-1-hexene) was isolated from reduction of the molybdenum trichloride precursor with 1-hexene. The strong coordination of the diene ligand prevents substitution by free alkene, inhibiting catalytic turnover. Investigations are currently underway to improve substitutional lability with the goal of achieving catalytic chemistry.

structure of [(MesPDI)Mo(η4-1,3-hexadiene)]2(μ-N2) is distorted octahedral with the end-on dinitrogen ligand bridging to a second (MesPDI)Mo center. The bridging dinitrogen N−N bond length of 1.145(4) Å is only slightly elongated from free dinitrogen, indicating minimal reduction by molybdenum. The bond metrics of the diene ligand in [(MesPDI)Mo(η4-1,3hexadiene)]2(μ-N2) are distinctly different from those of 4, which vary by only 0.007 Å. The olefin trans to the pyridine (C28−C29) has a bond length of 1.414(7) Å, while the other olefin of hexadiene (C30−C31) has a bond length of 1.355(0) Å, a value much closer to that of a free olefin. Despite its s-cis coordination, the 1,3-hexadiene in 8 is best described as a bisolefin ligand with varying degrees of back-donation from molybdenum to each olefin (which accounts for the disparate bond distances). Synthesis and Reactivity of α,ω-Diene Molybdenum Complexes. To investigate whether reduced pyridine-

General Considerations. All air- and moisture-sensitive reactions were performed either on a Schlenk line or in an MBraun inertatmosphere drybox containing an atmosphere of purified nitrogen. The ambient temperature in the laboratory was 23 °C. Solvents for airand moisture-sensitive manipulations were dried and deoxygenated using literature procedures.25 Benzene-d6 was distilled from sodium metal and stored over 4 Å molecular sieves. iPrPDI and MesPDI,26 1a and 1b,27 and MoCl3(thf)328 were prepared according to literature procedures. Sodium was purchased from Sigma-Aldrich as a solid in kerosene. Mercury was purchased from Sigma-Aldrich and purified by shaking with sugar and separating via a separatory funnel prior to use. Celite was dried at 150 °C under vacuum for 3 days prior to use. 1,5cyclooctadiene and 1-hexene were purchased from Sigma-Aldrich and dried over LiAlH4, deoxygenated via three freeze−pump−thaw cycles, and then vacuum-distilled and stored in an N2-filled glovebox. Bis(thf)magnesacyclopentane29 was synthesized according to a literature procedure, isolated as a solid, and stored at −35 °C. Ethylene was purchased from Matheson Tri-Gas Inc., passed through a

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Organometallics

Preparation of (iPrPDI)Mo(η2-ethylene)2 (3). In an N2-filled glovebox, a 20 mL scintillation vial was charged with a magnetic stir bar and freshly cut pieces of sodium (84.0 mg, 3.65 mmol). Toluene (10 mL) was then placed in the vial, followed by neat mercury liquid (8.4 g, 41.9 mmol). A gray suspension immediately formed, which was stirred rapidly for 30 min at ambient temperature. (iPrPDI)MoCl3 (500 mg, 0.730 mmol) was then placed in a 250 mL thick-walled glass tube with a ground glass joint side arm along with a magnetic stir bar, followed by 10 mL of toluene. The amalgam solution was placed in the glass tube and rinsed with toluene (4.4 mL), and the tube was sealed with a Teflon valve. The reaction vessel was removed from the glovebox and the solution frozen at −196 °C (liquid nitrogen). While frozen, the headspace of the reaction mixture was removed on a Schlenk line, after which time ethylene (1.53 mmol, 2.8 cm Hg in 1 L) was introduced into the vessel via calibrated gas-bulb addition. The vessel was sealed, thawed, and then stirred rapidly at ambient temperature for 24 h. The reaction vessel was brought back into the glovebox and the reaction decanted over a thick pad of Celite (4 cm) on a glass frit. The Celite was washed with toluene (3 × 5 mL) and the filtrate concentrated in vacuo to afford a forest green solid in 82% yield (384 mg). Anal. Calcd for C37H51MoN3: C, 70.12; H, 8.11; N, 6.63. Found: C, 69.95; H, 7.73, N, 6.65. 1H NMR (300 MHz, C6D6, 23 °C): δ 7.67 (d, 3JHH = 7.7 Hz, 2H), 7.32 (dd, 3JHH = 7.7, 1.5 Hz, 2H), 7.16 (m, 2H), 7.02 (dd, 3JHH = 7.6, 1.5 Hz, 2H), 6.48 (t, 3JHH = 7.6 Hz, 1H), 3.53 (p, 3JHH = 6.7 Hz, 2H), 2.51 (s, 6H), 1.41−1.15 (m, 14H), 0.71 (d, 3JHH = 6.9 Hz, 6H), 0.53 (d, 3JHH = 6.7 Hz, 6H), −0.19 (br s, 4H). 13C NMR (75 MHz, C6D6, 23 °C) δ 157.5, 149.5, 145.6, 140.9, 139.4, 125.6, 124.6, 122.9, 122.5, 112.5, 64.9, 59.5, 30.0, 27.7, 26.6, 24.3, 24.2, 23.2, 18.0. Preparation of (MesPDI)Mo(η4-butadiene)(η2-ethylene) (4). This complex was prepared in a manner identical with that for 3, but with (MesPDI)MoCl3 (250 mg, 0.417 mmol) and ethylene (2.08 mmol, 3.8 cm Hg in 1 L). The product was isolated as a bright green powder in 86% yield (205 mg). 1H NMR (300 MHz, C6D6, 23 °C): δ 7.72 (d, 3JHH = 7.6 Hz, 1H), 7.62 (d, 3JHH = 7.7 Hz, 1H), 6.91 (d, 3JHH = 13.5 Hz, 2H), 6.74 (d, 3JHH = 16.7 Hz, 2H), 6.63 (t, 3JHH = 7.8 Hz, 1H), 5.57 (d, 3JHH = 7.6 Hz, 1H), 4.70 (d, 3JHH = 8.2 Hz, 1H), 2.43 (s, 3H), 2.38 (s, 3H), 2.30 (s, 3H), 2.18 (s, 3H), 2.15 (s, 6H), 1.87 (d, 3 JHH = 7.4 Hz, 1H), 1.78 (d, 3JHH = 8.3 Hz, 1H), 1.55 (s, 3H), 1.15 (s, 3H), 0.35 (d, 3JHH = 8.4 Hz, 1H), 0.09 (d, 3JHH = 10.0 Hz, 1H), −1.59 (d, 3JHH = 98.2 Hz, 2H). 13C NMR (126 MHz, C6D6, 23 °C): δ 159.2, 152.4, 151.1, 147.9, 142.6, 138.0, 134.2, 134.1, 131.7, 130.2, 130.1, 130.0, 129.9, 129.5, 129.3, 128.8, 122.8, 122.3, 116.3, 113.6, 94.0, 66.2, 65.3, 59.1, 54.9, 20.9, 20.1, 19.3, 18.0, 17.4, 17.3, 16.7. Preparation of [(MesPDI)Mo(η4-butadiene)]2(μ-N2) (5). This complex was prepared in a manner identical with that for 4, but with 1butene (2.08 mmol, 3.8 cm Hg in 1L) and (MesPDI)MoCl3 (250 mg, 0.417 mmol). Anal. Calcd for C62H74Mo2N8: C, 66.30; H, 6.64; N, 9.98. Found C, 67.85; H, 6.74; N, 7.47. See the Supporting Information for additional characterization data. Preparation of (MesPDI)MoCl2(thf) (6). In an N2-filled glovebox a 25 mL Schlenk flask with a Teflon valve opening was charged with (MesPDI)MoCl3 (18 mg, 0.030 mmol) and bis(thf)magnesacyclopentane (7.0 mg, 0.030 mmol). The flask was sealed and removed from the glovebox, where it was cooled to −196 °C and evacuated on a Schlenk line. Degassed benzene-d6 (1 mL) was condensed into the reaction flask, after which time the reaction mixture was warmed to ambient temperature with vigorous stirring for 18 h. The volatiles of the reaction were transferred to a J. Young NMR tube in vacuo and analyzed via 1H NMR. The solid residue was brought back into the glovebox, taken up in toluene, and filtered through a plug of Celite. The filtrate was concentrated in vacuo to yield (MesPDI)MoCl2(thf) as a dark green solid in 67% yield (13 mg). The identity of 6 was confirmed by its outright synthesis via reduction of 1b with 1 equiv of sodium naphthenalide in thf (see the Supporting Information for details). Anal. Calcd for C31H39Cl2MoN3O: C, 58.50; H, 6.18; N, 6.60. Found C, 58.11; H, 5.77; N, 6.37. Magnetic susceptibility (Guoy balance, 23 °C): 2.6 μB. 1H NMR (300 MHz, C6D6, 23 °C): δ 115.17 (Δν1/2 = 233 Hz, 1H), 97.87(Δν1/2 = 287 Hz, 4H), 31.58 (Δν1/2 = 291 Hz, 4H), 18.27 (Δν1/2 = 28 Hz, 6H), 16.03

column of 4 Å molecular sieves and Drierite, and stored in a thickwalled Schlenk tube over 4 Å molecular sieves. Butadiene was purchased from Sigma-Aldrich and stored over calcium hydride in a thick-walled Schlenk tube. 1-Butene was purchased from Fisher Scientific and stored in a thick-walled Schlenk tube over LiAlH4. Ethylene-d4 and butadiene-d6 were purchased from Cambridge Isotopes and stored over 4 Å molecular sieves and calcium hydride, respectively. 1 H NMR spectra were recorded on Bruker AVANCE 300 and AVANCE 500 spectrometers operating at 300.13 and 500.62 MHz, respectively. 13C NMR spectra were recorded on a Bruker AVANCE 500 spectrometer operating at 125.89 MHz. All 1H and 13C chemical shifts are reported relative to SiMe4 using the 1H (residual) and 13C chemical shifts of the solvent as a secondary standard. Elemental analyses were performed at Robertson Microlit Laboratories, Inc., Ledgewood, NJ. Multiple attempts to obtain elemental analyses on compounds 2b, 4, and 7b were made; however, the %CHN values were always inaccurate. The %C and %H values were always substantially lower than expected, indicating decomposition of the molybdenum complexes with oxygen or incomplete combustion. Only NMR and X-ray data are supplied for these compounds. Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil in a drybox, transferred to a nylon loop, and then quickly transferred to the goniometer head of a Bruker D8 APEX3 Venture diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073 Å) and a copper X-ray tube (λ = 1.54178 Å). Preliminary data revealed the crystal system. The data collection strategy was optimized for completeness and redundancy using the Bruker COSMO software suite. The space group was identified, and the data were processed using the Bruker SAINT+ program and corrected for absorption using SADABS. The structures were solved using direct methods (SHELXS), completed by subsequent Fourier synthesis, and refined by full-matrix least-squares procedures. Gouy magnetic susceptibility balance measurements were performed with a Johnson Matthey instrument that was calibrated with HgCo(SCN)4. Preparation of (iPrPDI)Mo(η2:η2-1,5-COD) (2a). In an N2-filled glovebox, a 20 mL scintillation vial was charged with a magnetic stir bar and freshly cut pieces of sodium (42.0 mg, 1.83 mmol). Toluene (5 mL) was then placed in the vial, followed by neat mercury liquid (4.20 g, 19.9 mmol). A gray suspension immediately formed, which was stirred rapidly for 30 min at ambient temperature. In a separate vial were placed (iPrPDI)MoCl3 (250 mg, 0.365 mmol) and a magnetic stir bar, followed by toluene (3 mL) and 1,5-cyclooctadiene (224 μL, 1.83 mmol). The sodium amalgam mixture was then transferred to this vial and the mixture washed twice with toluene (2 mL). The reaction mixture was stirred rapidly at 23 °C for 24 h. After it was allowed to stand for 5 min, the reaction mixture was carefully filtered over a thick (3.5 cm) pad of Celite on a glass frit (taking care not to disturb the solids collected at the bottom of the vial). The Celite pad was washed with toluene (3 × 5 mL) and the filtrate concentrated in vacuo to dryness. The excess 1,5-cyclooctadiene was removed by azeotropic removal of pentane from the residue (3 × 5 mL). The residue was scraped from the sides of the flask to yield a green powder in 92% yield (231 mg). Anal. Calcd for C41H55MoN3: C, 71.80; H, 8.08; N, 6.13. Found: C, 71.42; H, 7.50, N, 5.77. 1H NMR (300 MHz, C6D6, 23 °C): δ 7.62 (d, 3JHH = 7.6 Hz, 2H), 7.14 (s, 6H), 6.45 (t, 3JHH = 7.6 Hz, 1H), 2.91 (d, 3JHH = 11.1 Hz, 4H), 2.44 (s, 6H), 2.26 (d, 3JHH = 9.5 Hz, 4H), 1.57−1.21 (m, 4H), 1.21−1.04 (m, 12H), 0.86 (s, 12H). 13C NMR (126 MHz, C6D6, 23 °C): δ 159.8, 150.1, 146.9, 140.7, 128.4, 125.7, 123.8, 110.6, 28.4, 25.1, 23.5, 22.7, 20.6. Preparation of (MesPDI)Mo(η2:η2-1,5-COD) (2b). This complex was prepared in a manner identical with that for 2a with (MesPDI)MoCl3 (250 mg, 0.417 mmol) and 1,5-cyclooctadiene (255 μL, 2.08 mmol). The product was isolated as a blue powder in 88% yield (220 mg). 1H NMR (300 MHz, C6D6, 23 °C): δ 7.70 (d, 3JHH = 7.6 Hz, 2H), 6.91 (s, 2H), 6.64 (s, 2H), 6.55 (t, 3JHH = 7.6 Hz, 1H), 3.39 (br s, 2H), 2.39 (m, 14H), 2.20 (s, 6H), 0.59 (br s, 6H), −1.04 (br s, 2H). 13C NMR (126 MHz, C6D6, 23 °C): δ 157.2, 148.9, 146.5, 133.4, 129.6, 128.4, 122.2, 111.5, 81.6, 77.2, 33.4, 29.3, 20.9, 19.4, 18.2, 17.0. G

DOI: 10.1021/acs.organomet.7b00653 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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(Δν1/2 = 109 Hz, 12H), 8.22 (Δν1/2 = 23 Hz, 3H), 3.98 (Δν1/2 = 20 Hz, 3H), −91.83 (Δν1/2 = 189 Hz, 2H). Preparation of (iPrPDI)Mo(η4−1,3-hexadiene)(η2-1-hexene) (7a). This complex was prepared in a manner identical with that for 2a except with 1-hexene (183 μL, 1.46 mmol). The product was isolated as a green powder in 85% yield (189 mg). Anal. Calcd for C45H65MoN3: C, 72.65; H, 8.81; N, 5.65. Found: C, 72.61; H, 8.67; N, 6.02. 1H NMR (300 MHz, C6D6, 23 °C): δ 7.50 (t, 3JHH = 7.6 Hz, 1H), 7.39−7.28 (m, 2H), 7.14−7.00 (m, 3H), 6.84 (d, 3JHH = 7.5 Hz, 1H), 5.56 (q, 3JHH = 9.3, 8.8 Hz, 1H), 5.39 (t, 3JHH = 7.2 Hz, 1H), 3.42 (t, 3JHH = 7.6 Hz, 1H), 3.27 (s, 3H), 2.94 (s, 3H), 2.86 (dd, J = 12.8, 6.1 Hz, 1H), 2.54 (td, J = 13.0, 6.4 Hz, 1H), 2.29 (dq, 3JHH = 13.9, 7.1, 6.4 Hz, 1H), 1.47 (d, 3JHH = 6.5 Hz, 3H), 1.37 (m, 1H), 1.26 (t, 3JHH = 6.2 Hz, 7H), 1.17−1.03 (m, 1H), 1.00−0.83 (m, 12H), 0.58 (m, 2H), 0.41 (d, 3JHH = 6.6 Hz, 3H), 0.05 (d, 3JHH = 6.7 Hz, 3H), −0.10 (t, 3 JHH = 3.5 Hz, 4H), −0.61 − −0.78 (m, 1H), −2.37 (p, 3JHH = 6.5 Hz, 1H). 13C NMR (126 MHz, C6D6): δ 155.9, 151.0, 150.0, 142.5, 141.9, 141.5, 140.3, 139.3, 137.6, 129.3, 127.6, 127.5, 125.9, 125.7, 125.6, 125.5, 123.6, 123.4, 122.3, 114.9, 114.6, 112.6, 99.3, 93.0, 65.7, 51.2, 32.6, 30.2, 29.4, 27.7, 27.1, 26.6 26.0, 25.4, 25.1, 24.9, 24.4, 23.9, 23.1, 22.5, 21.4, 18.8, 17.8, 15.5. Preparation of (MesPDI)Mo(η4−1,3-hexadiene)(η2-1-hexene) (7b). This complex was prepared in a manner identical with that for 2b, except with 1-hexene (209 μL, 1.67 mmol). The product was isolated as a brown-green powder in 87% yield (196 mg). 1H NMR (300 MHz, C6D6, 23 °C): δ 7.14 (dt, 3JHH = 14.5, 7.4 Hz, 2H), 6.86 (d, 3JHH = 7.6 Hz, 1H), 6.65 (s, 1H), 6.50 (s, 2H), 6.41 (s, 1H), 5.27 (s, 1H), 5.04 (s, 1H), 3.14 (s, 1H), 2.97 (s, 3H), 2.74 (s, 3H), 2.35− 2.14 (m, 2H), 1.99 (s, 3H), 1.92 (s, 3H), 1.69 (s, 3H), 1.62 (s, 4H), 1.38 (s, 3H), 1.04 (d, J = 42.6 Hz, 2H), 0.85−0.70 (m, 1H), 0.62 (t, 3 JHH = 6.9 Hz, 1H), 0.25 (s, 3H), 0.12 (s, 3H), 0.00 (br m, 4H). 13C NMR (126 MHz, C6D6): δ 156.2, 150.1, 149.9, 148.4, 146.0, 139.5, 137.9, 134.0, 133.6, 131.6, 131.2, 129.6, 129.3, 128.8, 128.6, 125.7, 116.7, 115.9, 114.4, 67.8, 65.9, 59.3, 34.5, 30.3, 28.5, 22.8, 21.4, 20.9, 20.9, 19.5, 19.1, 18.5, 17.9, 16.6, 15.8, 15.6, 14.3. Preparation of (MesPDI)Mo(η2:η2-1,6-heptadiene) (9). In an N2filled glovebox, a 20 mL scintillation vial was charged with a magnetic stir bar and freshly cut pieces of sodium (57.0 mg, 2.50 mmol). Toluene (6 mL) was then placed in the vial, followed by neat mercury liquid (5.70 g, 28.4 mmol). A gray suspension immediately formed, which was stirred rapidly for 30 min at ambient temperature. In a separate vial were placed (MesPDI)MoCl3 (300 mg, 0.500 mmol) and a magnetic stir bar, followed by toluene (5 mL) and 1,6-heptadiene (135 μL, 1.00 mmol). The sodium amalgam mixture was then transferred to this vial and washed twice with toluene (4 mL total). The reaction mixture was stirred rapidly at 23 °C for 24 h. After it was allowed to stand for 5 min, the reaction mixture was carefully filtered over a thick (3.5 cm) pad of Celite on a glass frit (taking care not to disturb the solids collected at the bottom of the vial). The Celite pad was washed with toluene (3 × 5 mL) and the filtrate concentrated in vacuo to dryness. The residue was taken up in benzene and transferred to a Schlenk flask equipped with a magnetic stir bar and sealed with a Teflon valve. The reaction mixture was stirred at 80 °C for 18 h, followed by filtration through a filter paper plug and concentration of the filtrate in vacuo. The product was isolated as a bright green powder in 92% yield (270 mg). Anal. Calcd for C34H43MoN3: C, 69.25; H, 7.35; N, 5.65. Found: C, 68.92; H, 7.27; N, 5.86. 1H NMR (300 MHz, C6D6, 23 °C): δ 7.75−7.68 (m, 1H), 7.64 (dd, 3JHH = 7.7, 1.0 Hz, 1H), 6.86−6.72 (m, 4H), 6.65 (t, 3JHH = 7.6 Hz, 1H), 4.72 (td, 3JHH = 8.5, 4.8 Hz, 1H), 3.45 (dd, 3JHH = 6.8, 3.3 Hz, 1H), 2.70 (m, 2H), 2.50 (t, 3 JHH = 4.3 Hz, 1H), 2.31 (s, 3H), 2.31 (s, 3H), 2.19−2.13 (m, 9H), 1.99 (m, 5H), 1.49 (s, 3H), 1.37 (s, 3H), 0.89 (m, 2H), 0.04 (d, 3JHH = 6.2 Hz, 3H). 13C NMR (126 MHz, C6D6): δ 154.7, 151.2, 150.1, 149.5, 140.2, 139.0, 133.9, 133.7, 132.4, 131.2, 129.3, 129.3, 129.1, 129.1, 129.0, 126.9, 125.7, 123.7, 122.7, 118.9, 113.5, 94.2, 84.6, 76.4, 55.7, 54.9, 32.3, 29.5, 23.1, 20.9, 20.9, 20.8, 19.3, 18.3, 18.1, 17.7, 16.7, 16.0, 14.7.

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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.7b00653. Additional experimental data, NMR spectra, and additional figures (PDF) Accession Codes

CCDC 1554612−1554615 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.J.C.: [email protected]. ORCID

Paul J. Chirik: 0000-0001-8473-2898 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SABIC for financial support and Aaron Zhong for solving the X-ray structure of 8.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00653 Organometallics XXXX, XXX, XXX−XXX