Catalytic Production of Isocyanates via Orthogonal Atom and Group

Jun 26, 2014 - Catalytic Production of Isocyanates via Orthogonal Atom and Group Transfers Employing a Shared Formal Group 6 M(II)/M(IV) Redox Cycle...
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Catalytic Production of Isocyanates via Orthogonal Atom and Group Transfers Employing a Shared Formal Group 6 M(II)/M(IV) Redox Cycle Brendan L. Yonke, Jonathan P. Reeds, Philip P. Fontaine, 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: Under an atmosphere of CO, the Mo(IV) imido complex Cp*Mo[N(iPr)C(Me)N(iPr)](NSiMe3) (Cp* = η5C5Me5) (1) serves as a catalyst for production of an isocyanate via metal-mediated nitrene group transfer in benzene solution under mild conditions (55 °C, 10 psi) according to RN3 + CO → N2 + RNCO. Mechanistic and structural studies support a catalytic cycle for nitrene group transfer involving formal Mo(II) monocarbonyl and Mo(IV) (κ2-C,N)-isocyanate intermediates. These results complement an earlier finding that catalytic production of isocyanates can alternatively proceed through oxygen-atom transfer and an isomeric Mo(IV) (κ2-C,O)-isocyanate according to N2O + CNR → N2 + RNCO.

C

Scheme 1

atalytic transformations based on inexpensive, earthabundant transition metals and readily available, innocuous commodity chemical feedstocks are attractive for the development of green industrial processes.1 From an academic perspective, this goal provides motivation to investigate different strategies by which soluble early transition metal complexes, [MLn] (Ln = supporting ligand set), of the groups 4, 5, and 6 of the periodic table can engage in the coordination and “fixation” of small molecules possessing X−Y multiple bonds (e.g., N2, O2, N2O, CO, and CO2) in such a fashion that leads to (1) X−Y multiple bond cleavage through compensating formation of strong M−X and M−Y bonding interactions and (2) pathways for atom-economical X and Y transfers to a substrate that serve to complete a catalytic cycle with respect to [MLn].2 Recently, we reported that the group 6 Mo(IV) terminal oxo complex Cp*Mo[N(iPr)C(Me)N(iPr)](O) (Cp* = η5-C5Me5) is a catalyst for the production of isocyanates, RNCO, from the corresponding isocyanides, RNC, using nitrous oxide, N2O, as an oxygen-atom-transfer reagent in hydrocarbon solution under near-ambient conditions (e.g., 25 °C, 10 psi).3,4 As Scheme 1 reveals, the catalytic cycle (path a) proceeds through formation of a κ2-C,O-isocyanato complex of structure A, a conclusion supported by in situ spectroscopic studies and isolation and structural characterization of the catalytically active molybdenum and catalytically inactive tungsten intermediates Cp*M[N(iPr)C(Me)N(iPr)][κ2-C,O-OCHN(tBu)] (M = Mo and W).3,5 Herein, we report successful realization of an orthogonal approach to catalytic isocyanate production that makes use of a group 6 Mo(IV) terminal imido complex supported by the same ligand set to mediate nitrene group transfer using organoazides (RN3) and CO as reagents that likewise occurs under mild conditions (55 °C, 10 psi) according to the alternate cycle (path b) of Scheme 1.6−9 Spectroscopic and structural evidence is further presented to conclusively implicate a κ2-C,N-isocyanato © 2014 American Chemical Society

complex of structure B as the key intermediate (see Scheme 1). These results, including an evaluation of metal- and imido groupdependent structure/activity relationships, should serve to provide new insights for achieving transition-metal-mediated small-molecule fixation under more energy-efficient conditions. Scheme 2 summarizes the synthetic methods that were employed to prepare the desired group 6 M(IV) terminal imido complexes Cp*M[N(iPr)C(Me)N(iPr)](NR) for M = Mo, R = SiMe3 (1); W, SiMe3 (2); Mo, CMe3 (3); and W, CMe3 (4).10 To begin, addition of 2 equiv of trimethylsilylazide, N3SiMe3, to toluene solutions of the dinuclear “end-on-bridged” dinitrogen complexes {Cp*M[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2) for M = Mo (5) and W (6), which can be conveniently prepared from the corresponding mononuclear M(IV) dichlorides 7 and 8, directly provided excellent yields of 1 and 2, respectively.11 For the tert-butyl imido derivatives, addition of 2 equiv of lithium tert-butylamide, tBuNHLi, to a solution of 7 in diethyl Received: May 21, 2014 Published: June 26, 2014 3239

dx.doi.org/10.1021/om500532s | Organometallics 2014, 33, 3239−3242

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conducted on a preparative-scale, the desired imido product 4 could be obtained in a 60% crystalline yield, based on conversion of 9. Compounds 1−4 are each a diamagnetic, dark red crystalline material for which single-crystal X-ray analyses served to confirm the mononuclear solid-state structures that are depicted in Scheme 2, as well as bond lengths and bond angles associated with the terminal M(IV) imido fragments, MNR, that are all within expected values.10,14,15 Indeed, the collection of molecular structures for 1−4 display a very high degree of conformity irrespective of the nature of the metal (Mo vs W) or imido substituent (SiMe3 vs CMe3). For this reason, only the molecular structure for the Mo(IV) derivative 1 is presented in Figure 1a. However, it is of further interest to note that the two tert-butyl imido compounds, 3 and 4, now complete an isostructural series of early transition metal M(IV) terminal imido complexes for Cp*M[N(iPr)C(Me)N(iPr)][N(tBu)] that span across groups 4, 5, and 6 of the periodic table for M = Ti,16 Zr,12 Ta,13 Mo, and W. In sharp contrast to their observed structural similarities, 1−4 displayed profound differences in chemical reactivity upon introduction of CO (10 psi) into a benzene-d6 solution as monitored by 1H NMR (400 MHz) spectroscopy.10 As presented in Scheme 3, in the case of 1, a quantitative yield of

Scheme 2

ether (Et2O) at −30 °C provided a modest yield of 3 as the expected product. On the other hand, similar treatment of 8 surprisingly provided a 75% yield of an orange-red crystalline material, for which analytical and spectroscopic analyses suggested a molecular structure isomeric to the one desired. Most notably, a 1H NMR (400 MHz, benzene-d6, 25 °C) spectrum displayed a singlet at δ 11.00 ppm with well-resolved 183 W (nat. abun., 14.3%, I = 1/2) spin-coupled satellites [1J(183W−1H) = 43.5 Hz], as well as two doublets appearing at δ 2.95 and 2.99 ppm [2J(1H−1H) = 1.7 Hz] that correlate as magnetically inequivalent methylene protons of a deprotonated amidinate group. In related investigations of the same reaction with group 4 and group 5 M(IV) dichloride isostructural analogues, we have isolated and structurally characterized the monoamido complexes Cp*M[κ2-N,N-N(iPr)C(CH2)N(iPr)][NH(tBu)] (M = Zr and Ta) as kinetic products, which in the case of the group 5 metal derivative undergoes subsequent tautomerization in solution to the isomeric, thermodynamically favored M(IV) terminal imido structure.12,13 While it has not yet been possible to establish which of the two limiting resonance forms shown in Scheme 2 might be the best representation of this intermediate (i.e., 9a or 9b), fortunately, it too was observed by NMR spectroscopy to slowly tautomerize in toluene solution at room temperature over a period of several days, and when

Scheme 3

the Mo(II) bis(carbonyl) Cp*Mo[N(iPr)C(Me)N(iPr)](CO)2 (10)11 and 1 equiv of trimethylsilylisocyanate, Me3SiNCO, was obtained after only 2 h at 25 °C, whereas, in the case of 2, a similar yield of Cp*W[N( i Pr)C(Me)N( i Pr)](CO) 2 (11) 10 and Me3SiNCO was achieved only at higher temperatures and longer reaction times (e.g., 80 °C, 48 h). The greater reactivity

Figure 1. Molecular structures (30% thermal ellipsoids) of (a) 1, (b) 13, (c) 14, and (d) 15. Hydrogen atoms have been removed for the sake of clarity except for those on C21 of compound 15.10 3240

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solution at −30 °C of a mixture consisting of a racemate of this C1-symmetric complex and the starting imido 4. Figure 1b and c present the solid-state molecular structures of 13 and 14 as provided by single-crystal X-ray analyses, and for both complexes, these data provide evidence for a modest to strong degree of back-donation of electron density from the tungsten center into the N−C π* orbitals of the κ2-C,N-isocyanato ligand [cf. W1−C20, 1.999(2) Å; W1−N3, 2.1300(17) Å; C20−N3, 1.337(3) Å for 13 vs W1−C20, 1.945(11) Å; W1−N3, 2.091(10) Å; C20−N3, 1.390(14) Å for 14].5 Further support for this conclusion was obtained from solid-state infrared spectra (KBr), which for 13 displayed strong bond-stretching modes at νC−O = 1937 and 1654 cm−1 for the carbonyl and k2-N,C-OCNSiMe3 ligands, respectively, and for 14 at νC−O = 1921, 1936, and 1668 cm−1 for the carbonyl and k2-N,C-OCN(tBu) ligands [cf. νC−O = 1721 cm−1 for (η5-C5H4SiMe3)2Nb(Cl)(κ2-N,C-OCNPh)].17 The greater solution stability of 13 relative to that of 14 with respect to reversible 1,1-CO addition/elimination (cf. Scheme 4) is in keeping with a greater electrophilicity of the transition metal center in 2 over 4 due to a withdrawing of electron density from the imido nitrogen atom in the former by the more electropositive trimethylsilyl group vis-à-vis a tert-butyl substituent.18 Additional insights regarding what factors might contribute to the relative thermodynamic stability of a group 6 M(IV) κ2-C,Nisocyanato complex and the relative energy barrier height for formal reductive elimination to liberate an isocyanate with generation of a M(II) center await the results of further experimental and computational investigations. The ease with which the Mo(IV) imido complex 1 engages in nitrene carbonylation at even subambient temperatures down to −30 °C is quite surprising, and particularly so for being an early transition metal system.9 It was of academic interest, therefore, to determine if these results could provide the basis of the catalytic process proposed in Scheme 5. Toward this goal, it was

displayed by the second-row derivative over the third-row congener was also repeated on going to the tert-butyl imido complexes 3 and 4, with both of these derivatives being much less reactive than the pair of 1 and 2. Notably, with 3, a quantitative yield of 10 and 1 equiv of tert-butyl isocyanate, tBuNCO, was produced after 4 days at 25 °C. In contrast, 4 provided only a trace amount of 11 after several weeks at the same temperature. With the aid of isotopically labeled 13CO (99%) (10 psi) and variable-temperature 13C NMR (125.6 MHz) spectroscopy conducted in a sealed J-Young NMR tube, the difference in apparent reactivity of 1 and 2 toward productive nitrene group transfer could be traced to differences in thermodynamic stability of the corresponding κ2-C,N-isocyanato intermediates Cp*M[N(iPr)C(Me)N(iPr)](CO)(k2-C,N-OCNSiMe3), where M = Mo (12) and W (13) (see Scheme 3).7 For 1 + 13CO, two new 13 C resonances for 12 appearing as doublets at δ 200.2 and 235.6 ppm [2J(13C−13C) = 3.4 Hz] for [Mo](κ2-C,N-O13CNSiMe3) and [Mo](13CO), respectively, could be observed at −25 °C. Upon warming the solution above 0 °C, however, both of these resonances quickly broadened and disappeared into the baseline, along with concomitant generation of 13C-labeled 10 and Me3SiNCO.10 On the other hand, for 2 + 13CO in benzene-d6, diagnostic 13C resonances were similarly observed for 13 at δ 209.7 ppm [1J(183W−13C) = 23 Hz, 2J(13C−13C) = 2.3 Hz] and 228.6 ppm [1J(183W−13C) = 68 Hz, 2J(13C−13C) = 2.3 Hz] for [W](κ2-C,N-O13CNSiMe3) and [W](13CO), respectively, but with these resonances now persisting in 13C NMR spectra recorded at 25 °C. After isolation of this species (vide inf ra), subsequent heating of an NMR sample of 13C-labeled 13 to 80 °C revealed clean conversion to 11 and Me3SiNCO with no other intermediates being detected. Finally, in comparing the differences in imido group reactivity for R = SiMe3 vs CMe3 in the case of 4 + 13CO, variable-temperature NMR revealed that a fast dynamic process observed at room temperature is within the slow exchange limit at −70 °C. At this latter temperature, characteristic 13C resonances were observed that are consistent with those expected for the C1-symmetric complex Cp*W[N(iPr)C(Me)N(iPr)](CO)(k2-C,N-OCNtBu) (14), at δ 195.3 ppm [1J(183W−13C) = 15.5 Hz, 2J(13C−13C) = 1.8 Hz)] for [W](κ2-C,N-O13CNtBu) and at δ 228.4 ppm [1J(183W−13C) = 69.4 Hz, 2J(13C−13C) = 1.8 Hz] for [W](13CO).10 Further, as shown in Scheme 4, the low solution stability of 14 toward

Scheme 5

Scheme 4

undergoing 1,1-elimination of CO to regenerate the imido fragment of 4 stands in sharp contrast to the robust nature of the isomeric k2-C,O-OCNtBu coordination mode present in Cp*W[N(iPr)C(Me)-N(iPr)](CNtBu)(k2-C,O-OCNtBu), which was previously prepared through 1,1-addition of CNtBu to the terminal oxo complex Cp*W[N(iPr)C(Me)N(iPr)](O).3 The room-temperature solution stability of 13 provided an opportunity to prepare this compound on a preparative scale, and indeed, it was isolated in a 75% yield as an analytically pure crystalline material, which was further subjected to single-crystal X-ray crystallography. Quite surprisingly, efforts to isolate 14 were also successful through fortuitous crystallization from

recognized at the outset that formation of the previously determined thermodynamic and kinetic stability of the Mo(II) bis(carbonyl) complex 10 under catalytic reaction conditions could represent a kinetic dead-end pathway if organoazides, N3R, cannot compete with CO for intercepting the proposed coordinatively unsaturated transient species Cp*Mo[N(iPr)C(Me)N(iPr)](CO) of structure C in Scheme 5. Thus, it was gratifying to find that, at 55 °C, a benzene-d6 solution of the imido complex 1 in the presence of 20 equiv of N3SiMe3 and CO (4 psi) successfully generated 4 equiv of OCNSiMe3 after 36 h, as determined by NMR spectroscopy. 3241

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Appl. Chem. 2012, 84, 1685. (c) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84. (2) Lu, C. C.; Meyer, K., Eds. Small-Molecule Activation by Reactive Metal Complexes (Cluster Issue) Eur. J. Inorg. Chem. 2013, 3728. (3) Yonke, B. L.; Reeds, J. P.; Zavalij, P. Y.; Sita, L. R. Angew. Chem., Int. Ed. 2011, 50, 12342. (4) Reeds, J. P.; Yonke, B. L.; Zavailj, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2011, 133, 18602. (5) For a review of the metal coordination of isocyanates, see: Braunstein, P.; Nobel, D. Chem. Rev. 1989, 89, 1927. (6) For a review of the metal coordination chemistry of organoazides and catalytic nitrene group transfer reactions, see: Cenini, S.; Gallo, E.; Caselli, A.; Ragaini, F.; Fantauzzi, S.; Piangiolino, C. Coord. Chem. Rev. 2006, 250, 1234. (7) For nitrene group transfer catalyzed by early transition metal complexes, see: (a) Johnson, J. S.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 2923. (b) Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci. 2011, 2, 166. (8) For nitrene group transfer mediated by late transition metal complexes, see: (a) Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R. J. Am. Chem. Soc. 2011, 133, 771. (b) Mindiola, D. J.; Hillhouse, G. L. Chem. Commun. 2002, 1840. (c) Amisial, L. D.; Dai, X.; Kinney, R. A.; Krishnaswamy, A.; Warren, T. H. Inorg. Chem. 2004, 43, 6537. (d) Cowley, R. E.; Golder, M. R.; Eckert, N. A.; Al-Afyouni, M. H.; Holland, P. L. Organometallics 2013, 32, 5289. (e) Wiese, S.; Aguila, M. J. B.; Kogut, E.; Warren, T. H. Organometallics 2013, 32, 2300. (f) Gouré, E.; Avenier, F.; Dubourdeaux, P.; Sénèque, O.; Albrieux, F.; Lebrun, C.; Clémancey, M.; Maldivi, P.; Latour, J.-M. Angew. Chem., Int. Ed. 2014, 53, 1580. (9) For late transition metal catalyzed production of isocyanates from organoazides and CO, see for Fe: (a) Cowley, R. E.; Eckert, N. A.; Elhaik, J.; Holland, P. L. Chem. Commun. 2009, 1760. For Ni: (b) Laskowski, C. A.; Hillhouse, G. L. Organometallics 2009, 28, 6114. (10) Details are provided in the Supporting Information. (11) Fontaine, P. P.; Yonke, B. L.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2010, 132, 12273. (12) Kissounko, D. A.; Epshteyn, A.; Fettinger, J. C.; Sita, L. R. Organometallics 2006, 25, 1076. (13) Yonke, B. L.; Keane, A. J.; Zavalij, P. Y.; Sita, L. R. Organometallics 2012, 31, 345. (14) For reviews of transition metal imido complexes, see: (a) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31, 123. (b) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239. (c) Eikey, R. A.; Abu-Omar, M. M. Coord. Chem. Rev. 2003, 243, 83. (15) (a) Green, J. C.; Green, M. L. H.; James, J. T.; Konidaris, P. C.; Maunder, G. H.; Mountford, P. Chem. Commun. 1992, 1361. (b) Green, M. L. H.; Konidaris, P. C.; Michaelidou, D. M.; Mountford, P. J. Chem. Soc., Dalton Trans. 1995, 155. (16) Guiducci, A. E.; Boyd, C. L.; Mountford, P. Organometallics 2006, 25, 1167. (17) Antiñolo, A.; Carrillo-Hermosilla, F.; Otero, A.; Fajardo, M.; Garcés, A.; Gómez-Sal, P.; López-Mardomingo, C.; Martín, A.; Miranda, C. J. Chem. Soc., Dalton Trans. 1998, 59. (18) Li, Y.; Banerjee, S.; Odom, A. L. Organometallics 2005, 24, 3272.

While the above preliminary results provide support for the viability of the catalytic cycle of Scheme 5, at least under the specific reaction conditions so far investigated, we sought to prepare a structurally characterized functional model for C that could be used to further substantiate the role of this intermediate in productive regeneration of imido 1. As Scheme 6 reveals, Scheme 6

introduction of a small quantity of CO at ∼1 psi to a toluene solution of the dinitrogen complex 5 containing 20 equiv of acetonitrile, NCMe, provided an excellent yield of the crystalline Mo(II) monocarbonyl, acetonitrile complex Cp*Mo[N(iPr)C(Me)N(iPr)](CO)(NCMe) (15). A single-crystal X-ray analysis of 15 confirmed the structure of this complex as presented in Figure 1d. Finally, as Scheme 6 further reveals, reaction of 15 in benzene-d6 solution with excess equivalents of N 3 SiMe3 quantitatively produced 1 and OCNSiMe3 at 25 °C within 2 h as monitored by 1H NMR spectroscopy.10 In conclusion, the present results provide further evidence of the unique ability of the monocyclopentadienyl, monoamidinate (CPAM) ligand set in early transition metal complexes to support small-molecule (e.g., N2, CO, CO2, and N2O) activation, strong-bond cleavage, and functionalization. Equally exciting in this respect is that, for the group 6 metals, the CPAM ligand environment also appears capable of modulating the strength of metal−oxygen and metal−nitrogen multiple bonds to the extent required for productive formation of new bonds that includes the productive release of products through atom- and group-transfer processes required for catalytic turnover.3,4



ASSOCIATED CONTENT

S Supporting Information *

Experimental details for all compounds and crystallographic analyses for 1−4 and 13−15. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Funding for this work was provided by the Department of Energy, Basic Energy Sciences (grant DE-SC0002217), for which we are grateful.



REFERENCES

(1) (a) Tolman, W. B. Angew. Chem., Int. Ed. 2010, 49, 1018. (b) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41, 3381. (c) Chow, T. W. S.; Chen, G.-Q.; Liu, Y.; Zhou, C.-Y.; Che, C.-M. Pure 3242

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