Small-Molecule Activation within the Group 6 Complexes (η5-C5Me5

Apr 14, 2016 - Keane , A. J.; Zavalij , P. Y.; Sita , L. R. J. Am. Chem. Soc. 2013 .... Keane , A. J.; Farrell , W. S.; Yonke , B. L.; Zavalij , P. Y...
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Small-Molecule Activation within the Group 6 Complexes (η5‑C5Me5)[N(iPr)C(Me)N(iPr)]M(CO)(L) for M = Mo, W and L = N2, NCMe, η2‑Alkene, SMe2, C3H6O Wesley S. Farrell, Brendan L. Yonke, Jonathan P. Reeds, Peter Y. Zavalij, and Lawrence R. Sita* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: A series of midvalent monocyclopentadienyl monoamidinate (CPAM) group 6 complexes of the general formula Cp*[N(iPr)C(Me)N(iPr)]M(CO)(L) (II), where Cp* = η5-C5Me5 and M = Mo, W, have been prepared, and most of them have been structurally characterized. Treatment of the ditungsten “end-on-bridged” dinitrogen complex {Cp*[N(iPr)C(Me)N(iPr)]W}2(μ-η1:η1-N2) (3) with excess NCMe under a CO atmosphere produced the ditungsten bridging diimido complex {Cp*[N(iPr)C(Me)N(iPr)]W}2[μη1:η1-NC(Me)C(Me)N] (4). Photolysis of Cp*[N(iPr)C(Me)N(iPr)]M(CO)2, where M = Mo (6), W (7), or treatment of Cp*[N(iPr)C(Me)N(iPr)]Mo(CO)(NCMe) (1a) with excess alkene provided Cp*[N(iPr)C(Me)N(iPr)]M(CO)(L) for M = Mo and L = η2-ethene (8), M = W and L = η2-ethene (9), M = Mo and L = η2-norbornene (10), M = W and L = η2-norbornene (11), M = W and L = η2-cyclopentene (12), M = Mo and L = η2-cyclopentene (13), and M = Mo and L = η2styrene (14). When isobutene was employed as the alkene, C−H bond activation occurred to produce Cp*[N(iPr)C(Me)N(iPr)]W(H)(η3-C4H7) (15). Photolysis of 7 in the presence of SMe2 provided Cp*[N(iPr)C(Me)N(iPr)]W[κ-C,OC(O)Me](SMe) (16) through oxidative C−S bond activation of a coordinated SMe2, followed by 1,1-carbonyl migratory insertion into the new W−C bond. Finally, reaction of 1a with propylene oxide (C3H6O) provided the 16-electron complex Cp*[N(iPr)C(Me)N(iPr)]Mo[C(O)CH(Me)CH2O] (19) via similar oxidative C−O bond activation of coordinated C3H6O, followed by 1,1-carbonyl migratory insertion into the Mo−C bond of an intermediate metallaoxetane. Under a CO atmosphere, 19 is converted to the 18-electron complex Cp*[N(iPr)C(Me)N(iPr)]Mo[C(O)CH(Me)CH2O](CO) (20). These results provide additional support for the development of new stoichiometric and catalytic transformations that are mediated by CPAM group 6 metal complexes and that are relevant to the goal of small-molecule fixation.

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INTRODUCTION Identification of early-transition-metal complexes that can “activate” abundant and inexpensive small molecules, such as dinitrogen (N2), for subsequent “fixation” through metalmediated bond-cleaving, atom-functionalization, and productforming reactions is critical for the development of new classes of sustainable and environmentally friendly “green” chemical processes.1 In this regard, over the past decade we have been exploring the ability of the monocyclopentadienyl monoamidinate (CPAM) ligand set within group 4−6 metal complexes of the general formula (η5-C5R5)[N(R1)C(R3)N(R2)]M(X)(Y) (I), to support a remarkably broad array of stoichiometric and catalytic transformations that are relevant to the goal of smallmolecule fixation.2−14 In each of these reports, the formal 18electron M(II,d4) or M(IV,d2) Cp*[N(iPr)C(R)N(iPr)]M(X)(L) (M = Mo, W; R = Me, Ph; Cp* = η5-C5Me5; X = CNR, CO; L = small molecule) complexes IIa−e in Chart 1 have been isolated, structurally characterized, and experimentally confirmed to be intermediates in catalytic small-molecule fixation cycles that proceed through key oxygen atom transfer © XXXX American Chemical Society

(OAT), nitrene group transfer (NGT), or sulfur atom transfer (SAT) processes.5,6,10−12,14 Other structures within this same family of CPAM group 6 metal complexes have been proposed as key intermediates in novel strong bond-breaking or bondmaking reactions, such as IIf for cleaving the N−N double bond of nitrous oxide (N2O).6 Intriguingly, all of the structures of Chart 1 can be viewed as having been derived from coordination of a strong A−B multiple bond of a small molecule (e.g. OCO, RNCO, and RNCO for IIa−c) to a 16-electron, coordinatively unsaturated M(II,d4) complex, Cp*M[N(iPr)C(R)N(iPr)](X) (M = Mo, W; R = Me, Ph; X = CNR, CO) (III), that has been invoked as a transient intermediate in OAT, NGT, and SAT catalytic cycles. Given the growing apparent ubiquity of II and III for small-molecule fixation schemes based on CPAM group 6 metal complexes, a next logical step to take was to conduct a wider survey of what other derivatives of II might be accessible through the Received: February 16, 2016

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DOI: 10.1021/acs.organomet.6b00131 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1

Scheme 1

fragment is observed. Figure 1 provides the solid-state molecular structure and selected geometric parameters of 4 as

coordination of other categories of small molecules to III, or a relevant synthon for this highly reactive species, in a manner that might facilitate novel chemical transformations and ultimately lead to the discovery of other catalytic processes of potential value. Herein, we now report the results of such an investigation in which different synthetic routes to additional examples of monocarbonyl CPAM group 6 metal complexes, Cp*[N(iPr)C(Me)N(iPr)M(CO)(L) (II for R = Me and X = CO), have been established for L = acetonitrile (NCMe), several η2-alkenes, dimethyl sulfide (SMe2), and propylene oxide (C3H6O). Gratifyingly, in each of these new examples of II, small-molecule activation pathways involving either novel bond-breaking or bond-forming reactions have been documented.

Figure 1. Molecular structure (30% thermal ellipsoids) of compound 4. Hydrogen atoms have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): W1−N3 1.7624(15), N3−C19 1.381(2), C19−C19A 1.374(4); W1−N3−C19 173.04(14), N3− C19−C19A 122.7(2).16

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RESULTS AND DISCUSSION Complexes of II in which L = NCMe, N2. We have previously reported that the structurally characterized, formal Mo(II), monocarbonyl, mono(acetonitrile) complexes Cp*[N(iPr)C(R)N(iPr)]Mo(CO)(NCMe), where R = Me (1a), Ph (1b), can serve as effective chemical synthons for III (M = Mo and L = CO) through facile ligand substitution of NCMe by a small molecule serving to displace the Lewis base.10,12 In these previous studies, 1a,b were conveniently prepared in high yield through reaction of the corresponding dimolybdenum “end-onbridged” dinitrogen complexes {Cp*[N(iPr)C(R)N(iPr)]Mo}2(μ-η1:η1-N2), where R = Me (2a), Ph (2b),4,12 with excess amounts of NCMe in benzene solution under an atmosphere of CO (1 atm). With this useful synthetic route established, we next sought to prepare the analogous tungsten complex Cp*[N(iPr)C(Me)N(iPr)]W(CO)(NCMe) through an identical procedure. As summarized in Scheme 1, treatment of the ditungsten end-on-bridged dinitrogen compound {Cp*[N(iPr)C(Me)N(iPr)]W}2(μ-η1:η1-N2) (3)4 with excess NCMe under a CO atmosphere did not produce the desired monocarbonyl mono(acetonitrile) product; rather, the ditungsten “bridging” diimido complex {Cp*[N(iPr)C(Me)N(iPr)]W}2[μ-η1:η1-NC(Me)C(Me)N] (4), in which two acetonitrile ligands have undergone reductive coupling,15 was obtained in high yield instead.16 In benzene-d6 solution, complex 4 displays Cs symmetry, as determined by 1H NMR spectroscopy, in which only a single methyl group resonance for the bridging diimido

determined by a single-crystal X-ray analysis. The W1−N3 bond length of 1.7624(15) Å is in keeping with the corresponding W−N value of 1.753(2) Å obtained for the previously reported mononuclear CPAM W(IV) imido complex Cp*[N( iPr)C(Me)N( iPr)]WNCMe3.10 These structural data, along with a central C19−C19A bond length of 1.374(4) Å, support the depicted assignment of the two coupled NCMe groups being associated with a single diimido moiety that bridges two W(IV) centers, rather than the alternative formalization in which a diiminato [NC(Me)C(Me)N] group that is coupling two W(III) centers through W−N single bonds.15f−h Regarding a mechanism for the formation of 4 from 3, it is important to note that no reaction of 3 with excess amounts of NCMe occurred in benzene solution in the absence of CO. Accordingly, it seemed reasonable to postulate that prior coordination of CO to the W centers of 3 is a prerequisite for weakening W−N2 π back-bonding17 through introduction of a competing W−CO π interaction to the extent that NCMe can now displace the coordinated N2 ligand. As Scheme 1 shows, support for this hypothesis was obtained by exposing 3 to CO (1 atm) in benzene-d6 solution and monitoring the reaction by 1 H NMR until a near-quantitative yield of a single new C1symmetric species was observed to have occurred, prior to the appearance of the known dicarbonyl species Cp*W[N(iPr)CB

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Organometallics (Me)N(iPr)](CO)2.4,5,16 Subsequent isolation of an analytically pure sample of this new compound and determination of its molecular structure through single-crystal X-ray analysis quite surprisingly revealed it to be the ditungsten end-on-bridged dinitrogen 1,4-dicarbonyl complex {Cp*[N(iPr)C(Me)N(iPr)]W(CO)}2(μ-η1:η1-N2) (5). Figure 2 presents the solidstate molecular structure and selected geometric parameters for 5.16

Scheme 2

carbonyl mono(alkene) complexes Cp*[N(iPr)C(Me)N(iPr)]M(CO)(L), where M = Mo and L = η2-ethene (8), M = W and L = η2-ethene (9), M = Mo and L = η2-norbornene (10), M = W and L = η2-norbornene (11), and M = W and L = η2cyclopentene (12), were observed.16 However, the scale-up of this procedure for obtaining larger quantities of materials was found to be practical only for the tungsten derivatives 9 and 11, which could be isolated in modest to high yields as analytically pure, orange crystalline materials. Indeed, although 1H NMR confirmed a high yield of 12, the noncrystalline nature of this compound frustrated all attempts to obtain it in pure form for complete analytical characterization. Furthermore, in contrast to the ready formation of 12, photolysis of 6 in the presence of excess cyclopentene did not lead to any discernible formation of the corresponding molybdenum derivative, as determined by 1 H NMR spectroscopy. Finally, attempts to employ nonsymmetric styrene as the alkene trap for III provided a complex inseparable mixture of compounds for both M = Mo and M = W. Fortunately, as Scheme 3 reveals, an alternative synthetic route to the desired monocarbonyl η2-alkene derivatives of II

Figure 2. Molecular structure (30% thermal ellipsoids) of compound 5. Hydrogen atoms have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): W1−N5 2.0231(15), N5−N6 1.144(3), W1−C37 1.932(2), C37−O1 1.171(2); W1−N5−N6 176.69(9), C37−W1−N5 79.64(7).

Compound 5 is stable in solution and the solid state, and it does not show any propensity to lose CO or N2 under vacuum. A solid-state infrared (IR) spectrum (KBr pellet) for this compound provided νCO 1768 cm−1 for the stretching frequency of the metal-bound carbonyl ligands, and this value is suggestive of a strong degree of π back-donation from filled metal orbitals into the π* orbital of the C−O bond.17 As hypothesized, this strong M−CO bonding interaction further appears to deplete π back-donation into the analogous π* orbital of the bridging N2 group, as evidenced by the N5−N6 bond length of 1.144(3) Å, which is significantly shorter than the corresponding N−N bond length value of 1.277(8) Å that was previously determined for 3.4 Complexes of II in which L = η2-Alkene. Given the various η2-coordination bonding modes involving A−B double bonds of small molecules already displayed by the structures of Chart 1, it seemed reasonable to explore the possibility of preparing a series of η2-alkene derivatives of II for the purpose of investigating the nature of metal−alkene σ donation, π backdonation bonding interactions. As shown in Scheme 2, a few of the desired monocarbonyl mono(alkene) derivatives of II were successfully prepared through simple photolysis of the previously reported dicarbonyl complexes Cp*[N(iPr)C(Me)N(iPr)]M(CO)2, where M = Mo (6), W (7),4,5 in benzene-d6 solutions containing excess amounts of a symmetric alkene lacking accessible β hydrogens that can engage in strong metal−hydrogen agostic interactions18 (e.g., ethene, norbornene, and cyclopentene) within a Pyrex tube using a Rayonet carousel of medium-pressure Hg lamps.16,19 As previously proposed, photolysis of 6 and 7 generates III as a transient intermediate that is then quickly trapped by coordination of the alkene. When the reaction was monitored by 1H NMR, using durene as an internal standard, near-quantitative conversions to the corresponding mono-

Scheme 3

for M = Mo was successfully developed that involved simply adding excess amounts of the alkene to a benzene solution of the monocarbonyl mono(acetonitrile) complex 1a.16,20 In addition to clean and near-quantitative conversions being observed by 1H NMR, this thermal route could now be easily scaled to provide practical quantities of all the CPAM molybdenum η2-alkene derivatives in high yield as orange-red, crystalline materials.16 The clean nature of this alternative route also proved to be essential for the successful synthesis, isolation, and analytical characterization of the cyclopentene and styrene derivatives 13 and 14, respectively (see Scheme 3). C

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Organometallics

Figure 3. Molecular structures (30% thermal ellipsoids) of (a) 8, (b) 10, (c) 13, and (d) 14. Hydrogen atoms have been omitted for the sake of clarity.

second- and third-row group 6 M(II,d4) carbonyl, η2-alkene complexes are extremely rare.19−22 In the present work, all of the Cp*[N(iPr)C(Me)N(iPr)]M(CO)(η2-alkene) derivatives of II are diamagnetic, thermally stable complexes in solution and do not display any evidence of dynamic rotation about a centralized M−alkene bond axis at room temperature by 1H NMR spectroscopy. In keeping with these observations, the metal−carbon and carbon−carbon bond lengths associated with the M−(η2-alkene) bonding interaction are consistent with a metallacyclopropane resonance structure in which a large degree of π back-donation of electron density from the metal into the π* C−C antibonding orbital exists (see Table 1).17 On comparison of second- and third-row metal effects for the same η2-alkene ligand, νCO stretching frequencies consistently indicate a stronger M−CO π back-donation interaction for M = W over M = Mo (cf. νCO 1867 cm−1 for 8 vs 1858 cm−1 for 9 and 1859 cm−1 for 10 vs 1844 cm−1 for 11 in Table 1). Finally, within the series of molybdenum complexes presented in Figure 3 and Table 1, M−CO π back-donation decreases in the order 10 (L = η2-norbornene) > 8 (L = η2-ethene) > 14 (L = η2-styrene), and these data suggest the opposite correlation of 14 > 8 > 10 in the strength of M−(η2-alkene) bonding. Although further reactivity studies are warranted, at the present time, we have not observed any unique reactions involving the metal−carbon bonds of the formal metallacyclopropane

All of the new crystalline compounds presented in Schemes 2 and 3 were subjected to structural analysis by X-ray crystallography and IR spectroscopy in order to assess the relative degree of π back-donation from the metal center to the CO and η2-alkene ligands. Figure 3 presents the solid-state molecular structures of the series of molybdenum derivatives 8, 10, 13, and 14, and Table 1 presents a correlation of M−C and C−C bond lengths for the η2-alkene ligand and the νCO stretching frequencies for the CO group for these compounds along with the tungsten derivatives 9 and 11. To begin, it is important to note that reports of structurally characterized Table 1. Selected Structural Parameters and IR CO Stretching Frequencies for Cp*[N(iPr)C(Me)N(iPr)]M(CO)(η2-alkene) compd

M

d(M−Calkene) (Å)a

d(C−C) (Å)a

νCO (cm−1)c

8 9 10b 11 13b 14

Mo W Mo W Mo Mo

2.2522(16), 2.008(15) 2.236(5), 2.174(5) 2.284(4), 2.224(4) 2.231(2), 2.192(2) 2.278(2), 2.224(4) 2.2340(11), 2.2605(11)

1.433(2) 1.452(8) 1.433(5) 1.478(3) 1.449(3) 1.4452(15)

1867 1858 1859 1844 1861 1880

a

Bond lengths for M−-(η2-alkene) interaction. bParameters selected from one of two or three unique molecules within the asymmetric unit cell.16 cSolid-state IR (KBr). D

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Organometallics moieties of these Cp*[N(iPr)C(Me)N(iPr)]M(CO)(η2-alkene) derivatives of II, such as 1,1-carbonyl migratory insertion. C−H Bond Activation: Synthesis and Characterization of Cp*[N(iPr)C(Me)N(iPr)]W(H)(η3-C4H7) (15). When alkenes bearing β hydrogens (e.g. propene, 1-butene, and cyclohexene) that are accessible for formation of agostic interactions within a M(η2-alkene) complex were employed in the reactions of Schemes 2 and 3, only formation of complex mixtures of multiple unidentifiable products were observed. The use of isobutene as the alkene trap in the photolysis of the tungsten dicarbonyl 7, however, proved to be an important exception. More specifically, as Scheme 4 reveals, photolysis of a solution

2.235(4) Å and carbon−carbon bond lengths of C19−C20 1.411(7) Å and C20−C21 1.409(7) Å. In benzene-d6 solution, the 13C{1H} NMR (125 MHz, benzene-d6) spectrum of 15 also displays 13C resonances at δ 53.2 and 45.7 ppm with 1 13 J( C−183W) = 12 and 19 Hz, respectively, for the terminal carbon atoms of the η3-methallyl ligand.16 The groups of Legzdins and Henderson have previously reported the formation of W(II) η3-allyl complexes through C−H bond activation pathways.23 In the present case, it is reasonable to propose that photolyic loss of CO from a Cp*[N(iPr)C(Me)N(iPr)]W(CO)(η2-isobutene) intermediate provides an open coordination site for formation of a β hydrogen agostic interaction of one of the distal methyl groups of the η2isobutene ligand with the tungsten center, followed by complete C−H activation to provide 15. Complexes of II in Which L = SMe2. One of the goals of exploring new examples of II was to obtain derivatives in which L can be displaced even more easily than NCMe in ligand substitution reactions (vide supra, Scheme 3). To this end, photolysis of toluene solutions of 6 and 7 containing excess amounts of dimethyl sulfide, SMe2, as a Lewis base σ donor, was pursued.24 Although these studies were not productive for 6, in the case of 7, clean formation of the W(IV) methyl sulfido η2-acyl complex Cp*[N(iPr)C(Me)N(iPr)]W[κ-C,O-C(O)Me](SMe) (16) occurred to provide this compound in a modest isolated yield as an orange, crystalline material according to Scheme 5.16 Figure 5 presents the molecular

Scheme 4

of 7 in benzene under an atmosphere of isobutene (15 psi) resulted in clean production of a new C1-symmetric species. The existence of a 1H resonance in the 1H NMR (400 MHz, benzene-d6) spectrum occurring at δ −0.88 ppm with coupling to 183W (1J(1H−183W) = 55 Hz), as well as the lack of a 13C resonance for a carbonyl ligand in the 13C{1H} NMR (125 MHz, benzene-d6) spectrum, suggested that this product, 15, likely resulted from C−H bond activation of an initial η2-alkene complex. Indeed, this hypothesis was confirmed through isolation and analytical characterization of a pure sample of 15, including a single-crystal X-ray analysis, which provided the molecular structure presented in Figure 4 along with selected geometric parameters. As can be seen, compound 15 is a W(IV) η3-methallyl hydride complex with the empirical formula Cp*[N(iPr)C(Me)N(iPr)]W(H)(η3-C4H7), as depicted in Scheme 4. In the solid state, the η3-methallyl fragment is associated with tungsten−carbon bond lengths of W1−C19 2.250(5) Å, W1−C20 2.187(4) Å, and W1−C21

Scheme 5

structure of 16 along with selected geometric parameters as determined by a single-crystal X-ray analysis. The observed η2 coordination of the acyl ligand is supported by the relatively short W1−O1 distance of 2.209(2) Å and the C20−O1 bond length of 1.282(4) Å, which is in keeping with structural data reported for other midvalent η2-acyl complexes of tungsten.25 Additional structural information for 16 was obtained by conducting the photolysis using 13C-labeled (99%) (13CO)2-7 to provide the corresponding isotopically labeled [η2-13C(O)Me]-16, which displayed a strong 13C resonance at δ 276 ppm with 1J(13C−183W) = 72 Hz in the 13C{1H} NMR (125 MHz, benzene-d6) spectrum.25 It is reasonable to assume that formation of 16 proceeds according to Scheme 5, in which initial generation of the desired 17 is followed by formal oxidative addition involving C−S bond activation to produce

Figure 4. Molecular structure (30% thermal ellipsoids) of compound 15. Hydrogen atoms have been omitted for the sake of clarity, with the exception of the W hydride, which is represented by a white sphere. Selected bond lengths (Å) and angles (deg): W1−H1 1.94(5), W1− C19 2.250(5), W1−C20 2.187(4), W1−C21 2.235(4), C19−C20 1.411(7), C20−C21 1.409(7), C20−C22 1.520(6); C19−C20−C21 111.5(4). E

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a 16-electron, Mo(IV) complex, it is not surprising that introduction of CO (10 psi) into a benzene solution of this compound slowly produced the formal 18-electron complex Cp*[N(iPr)C(Me)N(iPr)]Mo(CO)[κ-C,O-C(O)CH(CH3)CH2O] (20), which was isolated as a crystalline material in high yield.16 The molecular structures of 19 and 20 as determined by single-crystal X-ray analyses are presented in Figure 6 along with selected geometric parameters.

Figure 5. Molecular structure (30% thermal ellipsoids) of compound 16. Hydrogen atoms have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): W1−S1 2.4708(8), S1−C19 1.818(4), W1−C20 1.973(3), W1−O1 2.209(2), C20−O1 1.282(4), C20−C21 1.484(5); W1−S1−C19 109.98(12), W1−C20−C21 156.4(3), W1−C20−O1 82.54(19).

the intermediate 18. Subsequent 1,1-carbonyl insertion into the new tungsten-bound methyl group of 18 then provides 16. To the best of our knowledge, only a handful of transition-metal complexes that occur as a result of oxidative C−S bond activation of a thioether have been isolated and structurally characterized.26 Complexes of II in which L = Propylene Oxide (C3H6O). Given the ease with which the proposed intermediate 17 engages in C−S bond activation of a coordinated thioether (see Scheme 5), it was of significant interest to investigate the chemistry of related derivatives of II in which L is coordinated propylene oxide (C3H6O). As summarized in Scheme 6, Scheme 6

Figure 6. Molecular structures (30% thermal ellipsoids) of (a) 19 and (b) 20. Hydrogen atoms have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): for 19, Mo1−C19 2.1468(12), C19−O1 1.2190(15), C19−C20 1.5664(18), C20−C21 1.5151(19), O2−C21 1.4167(15), Mo1−O2 1.9675(9), Mo1−C19− C20 107.38(8), C19−Mo1−O2 79.37(4), Mo1−O2−C21 118.26(7); for 20, Mo1−O2 2.0579(9), O2−C21 1.3987(15), C21−C20 1.5236(19), C19−C20 1.5408(19), Mo1−C19 2.3120(12), C19−O1 1.2042(16), Mo1−C23 1.9504(13), C23−O3 1.1575(16), C19− Mo1−C23 66.26(5), C21−O2−Mo1 115.97(8), O2−Mo1−C19 71.83(4), Mo1−C19−C20 112.25(9).

Although exhaustive reactivity profile studies for 19 have yet to be undertaken, it is of interest to comment here on the thermal stability of this compound. More specifically, upon gentle heating of a benzene solution of 19 (generated in situ), elimination of methacrolein and formation of the known terminal oxo complex Cp*[N(iPr)C(Me)N( iPr)]Mo(O) (21),5 according to Scheme 7, was observed by 1H NMR spectroscopy. Since compound 21 can be converted back to the dicarbonyl 6 under an atmosphere of CO,5 efforts are currently underway to develop a new catalytic cycle for a “green” chemical conversion of propylene oxide to methacrolein.

addition of C3H6O to a toluene solution of 1a at room temperature cleanly produced the new C1-symmetric, ringopened product Cp*[N(iPr)C(Me)N(iPr)]Mo[κ-C,O-C(O)CH(CH3)CH2O] (19), which could be isolated in high yield as a black, crystalline material. It is reasonable to propose that formation of 19 proceeds through oxidative C−O bond activation of coordinated propylene oxide within a derivative of II (M = Mo, L = C3H6O), to form a transient metallaoxetane,27 followed by 1,1-migratory insertion of CO into the new molybdenum−carbon bond.28 Since 19 is formally F

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Organometallics

(6H, d, J = 6.5 Hz), 1.62 (6H, s), 1.94 (30H, s), 3.56 (2H, sp, J = 6.7 Hz), 3.59 (2H, sp, J = 6.5 Hz). IR (KBr): νCO 1768 cm−1. Synthesis of Cp*[N(iPr)C(Me)N(iPr)]Mo(CO)(C2H4) (8). A solution of 1a (0.060 g, 0.14 mmol) in benzene-d6 was prepared in a Pyrex J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated, the tube was charged with ethene (15 psi), and the mixture was shaken vigorously and allowed to react for 16 h, at which point complete consumption of 1a was observed by 1H NMR. Volatiles were removed in vacuo, the crude material was extracted in pentane, and the extract was filtered through a pad of Celite and pumped down to dryness. The resulting red crystals were dissolved in minimal pentane and cooled to −30 °C to furnish 8 as red crystals (0.043 g, 75% yield). Data for 8 are as follows. Anal. Calcd for C21H36N2OMo: C, 58.86; H, 8.43; N, 6.57. Found: C, 58.87; H, 8.47; N, 6.54. 1H NMR (400 MHz, benzene-d6): 0.94 (3H, d, J = 6.9 Hz), 1.00 (3H, d, J = 6.9 Hz), 1.02 (1H, m), 1.10 (3H, d, J = 6.5 Hz), 1.26 (3H, d, J = 6.4 Hz), 1.31 (1H, m), 1.46 (3H, s), 1.64 (15H, s), 2.15 (1H, m), 3.07 (1H, sp, J = 6.9 Hz), 3.32 (1H, m), 3.61 (1H, sp, J = 6.4 Hz). IR (KBr): νCO 1867 cm−1. Synthesis of Cp*[N(iPr)C(Me)N(iPr)]W(CO)(C2H4) (9). A solution of 7 (0.060 g, 0.12 mmol) in 1 mL of benzene-d6 was prepared in a Pyrex J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated, and the tube was charged with ethene (15 psi), at which point the tube was shaken vigorously and the mixture photolyzed for 16 h. The headspace was evacuated and the tube charged with ethene again, and the solution was photolyzed for 9 h longer, at which point complete consumption of 7 was observed by 1H NMR. Volatiles were removed in vacuo, the resulting crude material was extracted in pentane, the extract was filtered through a pad of Celite, and the filtrate was concentrated and cooled to −30 °C to furnish 9 as orange crystals (0.041 g, 67% yield). Data for 9 are as follows. Anal. Calcd for C21H36N2OW: C, 48.85; H, 7.03; N, 5.42. Found: C, 48.91; H, 6.67; N, 5.44. 1H NMR (400 MHz, benzene-d6): 0.95 (3H, d, J = 6.7 Hz), 0.98 (3H, d, J = 6.9 Hz), 0.99 (1H, m), 1.07 (3H, d, J = 6.5 Hz), 1.23 (3H, d, J = 6.3 Hz), 1.27 (1H, m), 1.41 (3H, s), 1.74 (15H, s), 1.84 (1H, m), 3.00 (1H, sp, J = 6.8 Hz), 3.01 (1H, m), 3.48 (1H, sp, J = 6.3 Hz). IR (KBr): νCO 1858 cm−1. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]Mo(CO)(C7H10) (10). A solution of 1a (0.035 g, 0.08 mmol) and norbornene (0.021 g, 0.22 mmol) in benzene-d6 was prepared and transferred to a Pyrex J. Young NMR tube equipped with a Teflon seal. After 2 h, complete consumption of 1a was observed by 1H NMR. Volatiles were removed in vacuo, the crude material was extracted with pentane, the extract was filtered through a pad of Celite, and the filtrate was concentrated and cooled to −30 °C to furnish 10 as red-orange crystals (0.026 g, 66% yield). Data for 10 are as follows. Anal. Calcd for C26H42N2OMo: C, 63.14; H, 8.56; N, 5.66. Found: C, 63.04; H, 8.55; N, 5.53. 1H NMR (400 MHz, benzene-d6): 0.89 (3H, d, J = 7.0 Hz), 0.99 (1H, d, J = 9.5 Hz), 1.04 (3H, d, J = 7.0 Hz), 1.14 (1H, m), 1.21 (3H, d, J = 6.3 Hz), 1.34 (3H, d, J = 6.3 Hz), 1.42 (2H, m), 1.50 (3H, s), 1.66 (15H, s), 1.87 (1H, d, J = 5.6 Hz), 2.04 (2H, m), 2.28 (1H, br), 2.35 (1H, d, J = 5.6 Hz), 3.14 (1H, sp, J = 7.0 Hz), 3.31 (1H, br), 3.70 (1H, sp, J = 6.3 Hz). IR (KBr): νCO 1859 cm−1. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]W(CO)(C7H10) (11). A solution of 7 (0.052 g, 0.10 mmol) and norbornene (0.103 g, 1.10 mmol) in 1 mL of benzene-d6 was prepared and transferred to a Pyrex J. Young NMR tube equipped with a Teflon seal and photolyzed for 20 h, at which point complete consumption of 7 was observed by 1H NMR. Volatiles were removed in vacuo, the crude material was extracted with pentane, and the extract was filtered through a pad of Celite and pumped down to dryness to furnish 11 as orange crystals (0.057 g, 97% yield). Data for 11 are as follows. Anal. Calcd for C26H42N2OW: C, 53.61; H, 7.27; N, 4.81. Found: C, 53.63; H, 7.00; N, 4.73. 1H NMR (400 MHz, benzene-d6): 0.92 (3H, d, J = 6.8 Hz), 1.00 (3H, d, J = 6.8 Hz), 1.11 (1H, d, J = 9.8 Hz), 1.17 (3H, d, J = 6.6 Hz), 1.28 (3H, d, J = 6.6 Hz), 1.36 (1H, m), 1.46 (3H, s), 1.51 (2H, m), 1.75 (15H, s), 1.91 (1H, d, J = 6.5 Hz), 2.21 (2H, m), 2.28 (2H, m), 3.06 (1H, sp, J = 6.8 Hz), 3.24 (1H, br), 3.60 (1H, sp, J = 6.6 Hz). IR (KBr): νCO 1844 cm−1.

Scheme 7

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CONCLUSION Given the previously well established role that derivatives of II play in catalytic OAT, SAT, and NGT processes, the present study serves to expand the range of such CPAM group 6 metal complexes. In each case, unique reaction pathways involving strong bond cleavage and new bond formation involving the coordinated small-molecule ligand L have been documented. These results support ongoing efforts to develop new metalmediated stoichiometric and catalytic transformations that are relevant to the goal of small-molecule fixation.

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EXPERIMENTAL SECTION

General Considerations. All manipulations with air- and moisture-sensitive compounds were carried out under an N2 or Ar atmosphere with standard Schlenk or glovebox techniques. Et2O and THF were dried over Na/benzophenone and distilled under N2 prior to use. Toluene and pentane were dried and deoxygenated by passage over activated alumina and GetterMax 135 catalyst (purchased from Research Catalysts, Inc.) and collected under N2 prior to use. Benzened6, toluene-d8, styrene, and cyclopentene were dried over Na/K alloy and isolated by vacuum transfer prior to use. Acetonitrile and dimethyl sulfide were dried over CaH2 and distilled under N2 prior to use. Norbornene was dried over molten Na and distilled under vacuum prior to use. Gaseous reagents were purchased from Sigma-Aldrich and used as received. Compounds 1a, 3, 6, and 7 were prepared according to reported methods in similar yield and purity.4,10 Celite was ovendried (150 °C for several days) before use in the glovebox. Cooling was performed in the internal freezer of a glovebox maintained at −30 °C. 1H and 13C{1H} NMR spectra were recorded at 400 and 125 MHz, respectively. Photolysis reactions were performed using a Rayonet Photochemical Reactor containing a carousel of medium pressure Hg lamps. Elemental analyses were carried out by Midwest Microlab LLC. Synthesis of {Cp*[N(iPr)C(Me)N(iPr)]W}2[μ-η1:η1-NC(Me)C(Me)N] (4). A solution of 3 (0.038 g, 0.04 mmol) and NCMe (0.5 mL, 0.10 mmol) in 0.6 mL of benzene-d6 was prepared and transferred to a J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated and charged with CO (1 atm), and the mixture was shaken and allowed to react for 2 days, at which point 1H NMR confirmed the consumption of 3. Volatiles were removed in vacuo, and the resulting crude material was dissolved in a minimal amount of pentane and cooled to −30 °C to furnish 4 as green crystals (0.032 g, 80% yield). Data for 4 are as follows. Anal. Calcd for C40H70N6W2: C, 47.89; H, 7.04; N, 8.38. Found: C, 48.06; H, 6.87; N, 8.28. 1H NMR (400 MHz, benzene-d6): 1.01 (12H, d, J = 6.3 Hz), 1.26 (12H, d, J = 6.3 Hz), 1.51 (6H, s), 2.08 (30H, s), 2.60 (6H, s), 3.22 (4H, sp, J = 6.3 Hz). Synthesis of {Cp*[N(iPr)C(Me)N(iPr)]W(CO)}2(μ-η1:η1-N2) (5). A solution of 3 (0.043 g, 0.05 mmol) was prepared in 0.6 mL of benzene-d6 and transferred to a J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated, the tube was charged with CO (8 psi), and the mixture was allowed to react for 15 h to produce a dark green solution. Volatiles were removed in vacuo, and the residue was dissolved in minimal pentane and cooled to −30 °C to furnish 5 as dark green crystals (0.024 g, 53% yield). Data for 5 are as follows. Anal. Calcd for C38H64N6O2W2: C, 45.41; H, 6.42; N, 8.37. Found: C, 45.39; H, 6.22; N, 8.12. 1H NMR (400 MHz, benzene-d6): 1.11 (6H, d, J = 6.5 Hz), 1.17 (6H, d, J = 6.7 Hz), 1.21 (6H, d, J = 6.7 Hz), 1.28 G

DOI: 10.1021/acs.organomet.6b00131 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

was added propylene oxide (73 μL, 1.06 mmol) at room temperature. The resulting solution was stirred overnight, at which point volatiles were removed in vacuo, the crude product was extracted with pentane, and the extract was filtered through a pad of Celite, concentrated under vacuum, and cooled to −30 °C to furnish 19 as black crystals (0.017 g, 72% yield). Data for 19 are as follows. Anal. Calcd for C22H38N2O2Mo: C, 57.64; H, 8.35; N, 6.11. Found: C, 57.29; H, 8.39; N, 5.91. 1H NMR (400 MHz, benzene-d6): 0.99 (3H, d, J = 6.8 Hz), 1.01 (3H, d, J = 6.8 Hz), 1.14 (3H, d, J = 6.5 Hz), 1.35 (3H, d, J = 6.8 Hz), 1.40 (3H, d, J = 6.8 Hz), 1.71 (3H, s), 1.75 (15H, s), 3.45 (1H, dd, J = 10.3 Hz, J = 9.8 Hz), 3.64 (3H, m), 5.19 (1H, dd, J = 9.9 Hz, J = 7.2 Hz). 13C-labeled 19 was prepared by an identical procedure using 13C-labeled 1a. 13C{1H} NMR (125.6 MHz, benzene-d6): 282.35 ppm. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]Mo(CO)[C(O)CH(CH3)CH2O] (20). A solution of 19 (0.034 g, 0.07 mmol) was prepared in 0.6 mL of benzene-d6 and transferred to a J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated and the tube charged with CO (10 psi). The tube was shaken and the mixture allowed to react for 2 days, at which point volatiles were removed in vacuo, and the resulting brown residue was dissolved in a minimal amount of pentane and cooled to −30 °C to furnish 20 as red-orange crystals (0.028 g, 78% yield). Data for 20 are as follows. Anal. Calcd for C23H38N2O3Mo: C, 56.78; H, 7.87; N, 5.76. Found: C, 56.43; H, 7.84; N, 5.80. 1H NMR (400 MHz, benzene-d6): 1.00 (3H, d, J = 6.7 Hz) 1.12 (3H, d, J = 6.7 Hz), 1.15 (3H, d, J = 6.7 Hz), 1.28 (3H, d, J = 6.7 Hz), 1.47 (3H, d, J = 7.0 Hz), 1.60 (15H, s), 1.70 (3H, s), 3.09 (1H, m), 3.66 (1H, sp, J = 6.7 Hz), 3.96 (1H, t, J = 9.9 Hz), 4.04 (1H, sp, J = 6.7 Hz), 5.19 (1H, dd, J = 9.6 Hz, J = 7.2 Hz). IR (KBr): νCO 1875 cm−1.

Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]Mo(CO)(C5H8) (13). A solution of 1a (0.035 g, 0.08 mmol) and cyclopentene (35 μL, 0.38 mmol) in 0.6 mL of benzene-d6 was prepared in a Pyrex J. Young NMR tube equipped with a Teflon seal, and the mixture was allowed to react for 2 h, at which point complete consumption of 1a was observed by 1H NMR. Volatiles were removed in vacuo, the crude material was extracted with pentane, and the extract was filtered through a pad of Celite. The filtrate was concentrated and cooled to −30 °C to furnish 13 as red crystals (0.020 g, 54% yield). Data for 13 are as follows. Anal. Calcd for C24H40N2OMo: C, 61.53; H, 8.60; N, 5.98. Found: C, 61.21; H, 8.31; N, 6.09. 1H NMR (400 MHz, benzene-d6): 0.96 (3H, d, J = 6.8 Hz), 1.05 (3H, d, J = 6.8 Hz), 1.12 (3H, d, J = 6.5 Hz), 1.24 (3H, d, J = 6.4 Hz), 1.47 (3H, s), 1.65 (15H, s), 1.82 (2H, m), 1.98 (1H, m), 2.56 (1H, t, J = 5.4 Hz), 2.83 (2H, m), 3.18 (3H, m), 3.65 (1H, sp, J = 6.5 Hz). IR (KBr): νCO 1861 cm−1. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]Mo(CO)(C8H8) (14). A solution of 1a (0.023 g, 0.05 mmol) and styrene (116 μL, 1.01 mmol) in a small amount of toluene was prepared in a screw-cap vial and stirred for 3 h. Volatiles were removed in vacuo, the crude material was extracted with pentane, and the extract was filtered through a pad of Celite. The filtrate was concentrated and cooled to −30 °C to furnish 14 as orange crystals (0.021 g, 80% yield). Data for 14 are as follows. Anal. Calcd for C27H40N2OMo: C, 64.27; H, 7.99; N, 5.55. Found: C, 64.35; H, 8.00; N, 6.57. 1H NMR (400 MHz, benzene-d6): 0.98 (3H, d, J = 6.8 Hz), 1.00 (3H, d, J = 6.8 Hz), 1.04 (3H, d, J = 6.8 Hz), 1.15 (3H, d, J = 6.8 Hz), 1.54 (3H, s), 1.59 (15H, s), 1.78 (1H, dd, J = 8.0, 3.3 Hz), 1.91 (1H, dd, J = 10.5, 3.3 Hz), 3.14 (1H, sp, J = 6.8 Hz), 3.61 (1H, sp, J = 6.4 Hz), 3.80 (1H, dd, J = 10.3,8.2 Hz), 7.10 (1H, t, J = 7.1 Hz), 7.35 (2H, d, J = 7.3 Hz), 7.41 (2H, t, J = 7.2 Hz). IR (KBr): νCO 1879 cm−1. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]W(H)(η3-C4H7) (15). A solution of 7 (0.047 g, 0.09 mmol) in 0.6 mL of benzene-d6 was prepared and transferred to a Pyrex J. Young NMR tube equipped with a Teflon seal. The headspace was evacuated, the tube was charged with isobutene (15 psi), and the solution was photolyzed for 17 h. Again the headspace was evacuated, the tube was charged with isobutene, and the solution was photolyzed for 20 h. This process was repeated once more, at which point nearly complete consumption of 7 was observed by 1H NMR. Volatiles were removed in vacuo, the crude material was extracted with pentane, and the extract was filtered through a pad of Celite, concentrated, and cooled to −30 °C to furnish 15 as yellow crystals (0.032 g, 68% yield). Data for 15 are as follows. Anal. Calcd for C22H40N2W: C, 51.17; H, 7.81; N, 5.42. Found: C, 50.03; H, 7.57; N, 5.31. 1H NMR (400 MHz, benzene-d6): −0.87 (1H, t, 3JHH = 3.5 Hz, 1JWH = 54.7 Hz), 0.57 (1H, dd, J = 5.08 Hz, J = 1.8 Hz), 1.09 (3H, d, J = 6.9 Hz), 1.14 (3H, d, J = 6.9 Hz), 1.26 (3H, d, J = 6.7 Hz), 1.33 (3H, d, J = 6.9 Hz), 1.36 (1H, br), 1.47 (1H, d, J = 3.2 Hz), 1.59 (3H, s), 1.73 (15H, s), 2.36 (1H, dd, J = 5.2 Hz, J = 5.0 Hz), 3.05 (3H, s), 3.50 (1H, sp, J = 7.0 Hz), 3.68 (1H, sp, J = 6.8 Hz). 13 C{1H} NMR (125 MHz, benzene-d6): 11.2, 21.1, 24.2, 25.2, 26.0, 26.7, 30.5, 45.7, 47.6, 52.8, 53.3, 79.0, 100.4, 165.3. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]W[η2-C(O)CH3](SCH3) (16). A solution of 7 (0.075 g, 0.15 mmol) and dimethyl sulfide (65 μL, 0.89 mmol) in 4 mL of toluene was prepared and transferred to a Pyrex J. Young NMR tube equipped with a Teflon seal. The solution was photolyzed for 24 h, at which point volatiles were removed in vacuo, and the resulting orange-red oil was extracted with pentane and the extract filtered through a pad of Celite and pumped down to dryness. The crude material was dissolved in minimal Et2O and cooled to −30 °C to furnish 16 as an orange crystalline material (0.047 g, 59% yield). Data for 16 are as follows. Anal. Calcd for C21H38N2OSW: C, 45.83; H, 6.96; N, 5.09. Found: C, 45.82; H, 6.82; N, 5.06. 1H NMR (400 MHz, benzene-d6): 0.88 (3H, d, J = 7.2 Hz), 1.15 (3H, d, J = 7.2 Hz), 1.41 (3H, d, J = 6.5 Hz), 1.63 (3H, d, J = 6.5 Hz), 1.74 (3H, s), 1.91 (15H, s), 2.41 (3H, s), 2.43 (3H, s, JWH = 2.8 Hz), 3.03 (1H, sp, J = 6.9 Hz), 4.13 (1H, sp, J = 6.9 Hz). 13C-labeled 16 was prepared by an identical procedure using 13C-labeled 7. 13C{1H} NMR (125.6 MHz, benzene-d6): δCO 275.7, 1JWC = 72 Hz. Synthesis of Cp*[N(iPr)C(CH3)N(iPr)]Mo[C(O)CH(CH3)CH2O] (19). To a solution of 1a (0.023 g, 0.05 mmol) in 2 mL of toluene

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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.6b00131. Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for L.R.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1361716) for financial support of this work. REFERENCES

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DOI: 10.1021/acs.organomet.6b00131 Organometallics XXXX, XXX, XXX−XXX