Rapid Access to an Oxido-Alkylidene Complex of Mo(VI

Communication pubs.acs.org/Organometallics

Rapid Access to an Oxido-Alkylidene Complex of Mo(VI) Christopher J. Varjas, Douglas R. Powell, and Robert K. Thomson* Department of Chemistry and Biochemistry, Stephenson Life Science Research Center, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United States S Supporting Information *

ABSTRACT: The first well-defined Mo oxido-alkylidene complex, [(tBu3PN)2Mo(O)(CHSiMe3)] (4), and its borane adduct, [( t Bu 3 PN) 2 Mo(O−B(C 6 F 5 ) 3 )( CHSiMe3)] (5), have been prepared through alkylation of the bis(phosphinimide) Mo oxido-dichlorido complex [(tBu3PN)2MoOCl2] (3). The reaction between 2 equiv of tBu3PN−SiMe3 (2) and [(DME)MoO2Cl2] (1) generates 3 through selective elimination of (Me3Si)2O. All complexes have been studied in solution by multinuclear NMR spectroscopy and in the solid state by X-ray crystallography. Preliminary ROMP catalysis results are also presented.

A

Unlike imido-alkylidene species that are typically prepared through salt metathesis with a general starting material, such as [(DME)Mo(CHCMe3)(NAr)(OTf)2],7 analogous established routes to oxido-alkylidene complexes of Mo do not exist. The Schrock group first reported a general synthesis of W oxido-alkylidene species of the type [L2W(CHCMe3)(O)Cl2] (L = phosphine) through ligand exchange with a Ta alkylidene derivative2b and have since reported a more convenient approach using a W dioxido dialkyl precursor,3g but a comparable synthesis on Mo is not known. In this contribution, we show that a novel oxido-alkylidene complex of Mo can be prepared rapidly through the use of the trimethylsilyl-protected phosphinimide ligand family (R3P N−SiMe3). The coordination chemistry of phosphinimide ligands has been studied extensively for many metals on the periodic table, including the group 6 metals.8 However, this ligand system has not been applied broadly to the synthesis of reactive complexes for catalytic applications. A notable exception is the use of these ligands with group 4 metals for the generation of highly reactive olefin polymerization catalysts.9 Installation of phosphinimide ligands on metals is most often accomplished through the elimination of Me3Si−X (X = halide) from transition-metal halide starting materials.8a,10 Elimination of (Me3Si)2O from Na2MoO4 has also proven effective,11 and we reasoned that the stronger driving force of Si−O bond formation vs Si−Cl bond formation could be used to selectively install the phosphinimide ligands, as shown in Scheme 1. Here 2 equiv of the phosphinimine proligand tBu3PNSiMe3 (2) is combined with the readily prepared dioxido starting material [(DME)MoO 2 Cl 2 ] (1), 12 generating (Me3Si)2O and the resulting phosphinimide-stabilized oxido-

lkylidene complexes of the group 6 metals have high utility in catalytic olefin metathesis reactions, including crossmetathesis, ring-opening metathesis polymerization (ROMP), and ring-closing metathesis (RCM) applications.1 The earliest group 6 alkylidene complexes generated were oxido-alkylidene species of tungsten.2 However, subsequent alkylidene chemistry of the group 6 metals has been largely dominated by imidoalkylidene complexes,1a−c as they offer a readily tuned steric and electronic environment, and more effectively block bimolecular deactivation pathways owing to greater steric protection in comparison to their isolobal oxido-alkylidene counterparts. A renewed interest in the study of oxidoalkylidene species has been seen recently,3 as these complexes are postulated to be the active moieties in heterogeneous surface-supported W catalysts.4 A recent study by Trunschke and co-workers evaluated Mo oxide dispersed on the surface of mesoporous silica for catalytic olefin metathesis, and it was shown through IR spectroscopy that surface-bound Mo(VI) oxido-alkylidene species are generated in situ for catalytic crossmetathesis reactions.5 Despite the importance of Mo oxido-alkylidene complexes in such metathesis processes, they have not been widely studied. To date, the only Mo oxido-alkylidene complex to be characterized is the phosphonium adduct shown in eq 1,

which is in resonance with a σ-vinyl species.6 The alkylidene fragment in this complex is formed through alkyne insertion into a Mo−P bond, while the oxido ligand is likely derived from hydrolysis by adventitious water. © 2015 American Chemical Society

Received: August 7, 2015 Published: October 13, 2015 4806

DOI: 10.1021/acs.organomet.5b00686 Organometallics 2015, 34, 4806−4809

Communication

Organometallics

be attributed to the greater ionic character of the silox ligands15 vs the more strongly π-donating capacity of the phosphinimide ligands.8a Complex 3 is rapidly converted to the oxido alkylidene species [(tBu3PN)2Mo(O)(CHSiMe3)] (4) in high yield through addition of 2 equiv of LiCH2SiMe3. A characteristic alkylidene resonance for 4 is observed downfield in the 1H NMR spectrum at δ 10.33 and at δ 213.4 in the 13C NMR spectrum,1b,16 with a single phosphinimide environment observed by both 1H and 31P NMR spectroscopy. The 1JCH coupling constant of 120 Hz for the alkylidene moiety is consistent with the syn isomer.17 It is notable that a long-lived bis(alkyl) intermediate is not observed in this reaction. Conversion of bis(alkyl) complexes of the group 6 metals to alkylidenes often requires the addition of a tertiary phosphine or other donor to induce elimination through steric pressure, resulting in a coordinatively saturated adduct.18 The bulky tri-tert-butylphosphinimide ligands presumably induce α-H migration, which results in alkane elimination. This is in marked contrast to the structurally similar silox complex [(tBu3Si−O)2WO(nBu)2], which exhibits surprising stability, resisting α-H migration upon addition of PMe3, and ultimately undergoes thermally induced β-H elimination to eject butane and generate a W(IV) 1-butene complex.15 Confirmation of the syn isomer of oxido alkylidene complex 4 was possible via XRD analysis, as illustrated in Figure 2.

Scheme 1. Synthesis of Phosphinimide-Supported OxidoDichlorido (3) and Oxido-Alkylidene (4) Complexes and Oxido-Alkylidene Borane Adduct (5)

dichlorido complex 3 in excellent yield. This reaction is highly selective, and no Me3Si−Cl elimination is noted. 1H NMR spectroscopic data are consistent with the formation of a C2vsymmetric complex, suggestive of a trans-dichlorido trigonalbipyramidal structure. A doublet resonance is observed for the phosphinimide tert-butyl protons in 3 due to coupling to the 31 P nuclei. The 31P NMR spectrum of 3 also supports the formation of a complex with a single 31P environment, with a resonance at δ 58.64.13 Verification of the trigonal-bipyramidal geometry of 3 was possible through single-crystal X-ray diffraction (XRD), and the structure is shown in Figure 1.14 This complex is reminiscent of the trigonal-bipyramidal silox complexes [(tBu3Si−O)2WOR2] (R = Cl, Me, Et, nPr, nBu), studied by Wolczanski and co-workers.15 However, the silox ligands in those complexes lie in the axial positions, in contrast to the equatorially located phosphinimide ligands in 3. This can

Figure 2. Thermal ellipsoid plot (30% probability) of [(tBu3P N)2Mo(O)(CHSiMe3)] (4). Non-alkylidene hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mo(1)−O(1) 1.700(4), Mo(1)−C(1) 1.920(5), Mo(1)−N(1) 1.887(4), Mo(1)−N(2) 1.877(4), N(1)−P(1) 1.560(4), N(2)−P(2) 1.563(4); N(1)−Mo(1)−N(2) 106.57(18), Mo(1)−N(1)−P(1) 158.0(3), Mo(1)−N(2)−P(2) 169.1(3), O(1)−Mo(1)−C(1) 105.29(18), Mo(1)−C(1)−Si(1) 134.7(3), O(1)−Mo(1)−C(1)− Si(1) −0.73.

Complex 4 presents a highly exposed metal center with readily accessible oxido and alkylidene moieties. The bond length for the alkylidene ligand is typical for hexavalent MoC bond fragments (1.920(5) Å).1b,18a Likewise, the Mo−C−Si bond angle for the alkylidene ligand in 4 (134.7(3)°) is similar to those in related complexes.1b,18a The alkylidene bond angle is considerably larger than 120°, which is likely a result of minimizing steric interactions between the trimethylsilyl methylidene group and the ancillary ligands. Preliminary reactivity studies of 1-octene with 7 mol % of oxido-alkylidene complex 4 showed no metathesis reactivity on heating at 80 °C for 18 h; however, slow ring-opening metathesis polymerization of norbornene was observed, with 18% conversion after 24 h, and only 22% conversion after more than 48 h, when 10 mol % of 4 was used as a catalyst. The open

Figure 1. Thermal ellipsoid plot (30% probability) of [(tBu3P N)2MoOCl2] (3). Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mo(1)−O(1) 1.707(2), Mo(1)−Cl(1) 2.4545(8), Mo(1)−Cl(2) 2.4826(9), Mo(1)−N(1) 1.836(3), Mo(1)−N(2) 1.838(3), N(1)−P(1) 1.609(3), N(2)−P(2) 1.612(3); Cl(1)−Mo(1)−Cl(2) 179.12(3), N(1)−Mo(1)−N(2) 125.71(11), Mo(1)−N(1)−P(1) 162.19(17), Mo(1)−N(2)−P(2) 159.57(17), O(1)−Mo(1)−Cl(1) 90.43(8), O(1)−Mo(1)−N(1) 115.95(11). 4807

DOI: 10.1021/acs.organomet.5b00686 Organometallics 2015, 34, 4806−4809

Communication

Organometallics

seen in other known Mo and W oxido B(C6F5)3 adducts,20 but is substantially greater than that seen for the weakly coordinated W oxido-alkylidene borane adduct reported recently by Schrock and co-workers, where the oxido unit is elongated by 0.04−0.06 Å upon coordination of the borane.3h The very short B(1)−O(1) bond distance of 1.489(9) Å and the nearly perfectly tetrahedral average C−B−C and O−B−C bond angles in 5 (109.8° and 109.2°, respectively) indicate that the B(C6F5)3 moiety is strongly bound to the oxygen. Solution-phase data for borane adduct 5 are consistent with a Cs-symmetric product having a single phosphinimide environment, as evidenced by a single doublet in the 1H NMR spectrum and a single resonance in the 31P NMR spectrum. In addition, the alkylidene proton resonance for 5 is shifted to δ 9.49 in the 1H NMR spectrum, and the 13C resonance is located at δ 232.1, with a 1JCH value of 120 Hz, again consistent with the syn isomer.17 The high symmetry of the spectrum of 5 and the relative broadness of the alkylidene resonance13 suggest that the alkylidene group is freely rotating and rapidly interconverting between the syn and anti isomers. Variabletemperature NMR spectroscopy was not able to freeze out one isomer over the other, indicating a low barrier to interconversion. When 5 is isolated in the absence of olefin, a secondary product is observed in solution. The minor product in solution has two phosphinimide environments in a 1:1 ratio (C1 symmetry), as shown by two resonances in the 31P NMR spectrum and two doublet resonances in the 1H NMR spectrum.13 Microanalysis of crystals of 5, which exhibit both Cs- and C1-symmetrical species on dissolution, is consistent with a formulation of [(tBu3PN)2Mo(O−B(C6F5)3)( CHSiMe3)] for the mixture, indicating that the other species is an isomer of 5. The nature of the minor isomer is a subject of ongoing investigations, but we propose here that the minor isomer in solution has the borane coordinated at an alternate site, potentially the alkylidene carbon. This may also explain the absence of an alkylidene peak for the minor product, given the proximity of the alkylidene proton to both the metal center and the quadrupolar B nucleus. This is reminiscent of a Ta methylidene complex reported by Piers and co-workers,21 where B(C6F5)3 is coordinated to the methylidene carbon, generating a zwitterionic complex susceptible to insertion reactivity. Catalytic ROMP screening using 10 mol % of borane adduct 5 and norbornene as a substrate was markedly more effective than that for the free alkylidene complex 4, with 84% conversion after 17 h at 80 °C, indicating that Lewis acid coordination to the oxo group greatly enhances catalytic reactivity. In conclusion, a new, facile, high-yielding route to molybdenum alkylidene species supported with phosphinimide ligands has been developed, including the first well-defined oxido-alkylidene complex of molybdenum and its corresponding borane adduct. Lewis acid coordination leads to a substantial perturbation of the MoO bond and indicates that the electronic donation of the oxido ligand can be modulated to control the overall electron density at the metal center. Further metathesis reactivity and oxido coordination studies are being carried out to assess the catalytic utility of this new system, including the generation of more electrophilic complexes bearing electron-withdrawing functionalities on the phosphinimide ligands.

coordination sphere provided by the oxido fragment suggests that steric access to the MoC bond is not an issue, indicating that the MoC bond is electronically deactivated toward metathesis. The strongly electron donating nature of the phosphinimide ligands8a and the atypical SiMe3 substituent on the alkylidene group likely both contribute to this lack of reactivity. Osborn first proposed enhancement of the metathesis reactivity of tungsten oxido-alkylidene species through the addition of Lewis acid activators, where the oxido ligand coordinates to the Lewis acid.4b More recently, Schrock and coworkers reported a borane adduct of a W oxido-alkylidene complex that showed enhanced reactivity toward metathesis vs the free oxido-alkylidene parent complex.3h,i Here, upon addition of 1 equiv of B(C6F5)3 to the oxido-alkylidene complex 4 in excess 1-octene, an immediate color change from red to dark red-brown was observed. Monitoring the solution of 4/B(C6F5)3 by 1H NMR spectroscopy showed no crossmetathesis of 1-octene on heating to 80 °C for 18 h. However, the resulting adduct, [(tBu3PN)2Mo(O−B(C6F5)3)( CHSiMe3)] (5), is thermally robust and was readily crystallized from hot benzene. The solid-state structure of 5 was determined by XRD (Figure 3) and clearly shows a C1-symmetric complex, with the

Figure 3. Thermal ellipsoid plot (30% probability) of [(tBu3P N)2Mo(O−B(C6F5)3)(CHSiMe3)] (5). Non-alkylidene hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mo(1)−O(1) 1.843(5), B(1)−O(1) 1.489(9), Mo(1)− C(1) 1.895(7), Mo(1)−N(1) 1.865(6), Mo(1)−N(2) 1.799(6), N(1)−P(1) 1.594(6), N(2)−P(2) 1.628(6); N(1)−Mo(1)−N(2) 111.5(3), Mo(1)−N(1)−P(1) 158.7(4), Mo(1)−N(2)−P(2) 164.5(4), O(1)−Mo(1)−C(1) 108.0(3), B(1)−O(1)−Mo(1) 155.3(5), Mo(1)−C(1)−Si(1) 148.7(4), O(1)−Mo(1)−C(1)−Si(1) 127.7(8).

alkylidene group forced into a configuration between the syn isomer seen for 4 and the anti isomer.19 Unlike syn alkylidene complex 4, which has an oxido to silicon torsion angle of only −0.73°, the analogous torsion angle for 5 is 127.7(8)°. The dramatically skewed alkylidene configuration is accompanied by a small MoC bond length contraction from 1.920(5) Å for 4 to 1.895(7) Å for 5. The alkylidene Mo−C−Si bond angle is more obtuse at 148.7(4)° for borane adduct 5 than for the free oxido-alkylidene complex 4 at 134.7(3)°. However, the biggest difference noted upon Lewis acid coordination is the nearly 0.15 Å elongation of the Mo−O bond from 1.700(4) Å in 4 to 1.843(5) Å in 5. This degree of elongation is similar to that 4808

DOI: 10.1021/acs.organomet.5b00686 Organometallics 2015, 34, 4806−4809

Communication

Organometallics

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Folting, K.; Glasgow, K. C.; Lucas, E.; Streib, W. E. Organometallics 2000, 19, 884−892. (4) (a) Conley, M. P.; Mougel, V.; Peryshkov, D. V.; Forrest, W. P.; Gajan, D.; Lesage, A.; Emsley, L.; Coperet, C.; Schrock, R. R. J. Am. Chem. Soc. 2013, 135, 19068−19070. (b) Kress, J.; Wesolek, M.; Le Ny, J. P.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1981, 1039−40. (c) Kress, J. R. M.; Russell, M. J. M.; Wesolek, M. G.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1980, 431−2. (5) Amakawa, K.; Wrabetz, S.; Kroehnert, J.; Tzolova-Mueller, G.; Schloegl, R.; Trunschke, A. J. Am. Chem. Soc. 2012, 134, 11462− 11473. (6) (a) Fairhurst, S. A.; Hughes, D. L.; Marjani, K.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1998, 1899−1904. (b) Hughes, D. L.; Marjani, K.; Richards, R. L. J. Organomet. Chem. 1995, 505, 127−9. (7) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875−3886. (8) (a) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. Rev. 1999, 182, 19−65. (b) Dehnicke, K.; Weller, F. Coord. Chem. Rev. 1997, 158, 103−169. (9) (a) Stephan, D. W. Adv. Organomet. Chem. 2006, 54, 267−291. (b) Stephan, D. W. Organometallics 2005, 24, 2548−2560. (10) Rentschler, E.; Nusshaer, D.; Weller, F.; Dehnicke, K. Z. Anorg. Allg. Chem. 1993, 619, 999−1003. (11) Arzoumanian, H.; Bakhtchadjian, R.; Agrifoglio, G.; Atencio, R.; Briceno, A. Transition Met. Chem. 2008, 33, 941−951. (12) Rufanov, K. A.; Zarubin, D. N.; Ustynyuk, N. A.; Gourevitch, D. N.; Sundermeyer, J.; Churakov, A. V.; Howard, J. A. K. Polyhedron 2001, 20, 379−385. (13) NMR spectral data for all complexes are available in the Supporting Information. (14) Full crystallographic details, including associated CIF files, are available in the Supporting Information. (15) Rosenfeld, D. C.; Kuiper, D. S.; Lobkovsky, E. B.; Wolczanski, P. T. Polyhedron 2006, 25, 251−258. (16) (a) Poater, A.; Solans-Monfort, X.; Clot, E.; Coperet, C.; Eisenstein, O. Dalton Trans. 2006, 3077−3087. (b) Solans-Monfort, X.; Eisenstein, O. Polyhedron 2006, 25, 339−348. (17) Oskam, J. H.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 11831−45. (18) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145−179. (b) Cockcroft, J. K.; Gibson, V. C.; Howard, J. A. K.; Poole, A. D.; Siemeling, U.; Wilson, C. J. Chem. Soc., Chem. Commun. 1992, 1668− 70. (c) Chan, M. C. W.; Cole, J. M.; Gibson, V. C.; Howard, J. A. K.; Lehmann, C.; Poole, A. D.; Siemeling, U. J. Chem. Soc., Dalton Trans. 1998, 103−112. (d) Dougan, B. A.; Xue, Z.-L. Organometallics 2009, 28, 1295−1302. (19) Schrock, R. R.; Crowe, W. E.; Bazan, G. C.; DiMare, M.; O’Regan, M. B.; Schofield, M. H. Organometallics 1991, 10, 1832− 1843. (20) (a) Barrado, G.; Doerrer, L.; Green, M. L. H.; Leech, M. A. J. Chem. Soc., Dalton Trans. 1999, 1061−1066. (b) Doerrer, L. H.; Galsworthy, J. R.; Green, M. L. H.; Leech, M. A. J. Chem. Soc., Dalton Trans. 1998, 2483−2488. (c) Doerrer, L. H.; Galsworthy, J. R.; Green, M. L. H.; Leech, M. A.; Muller, M. J. Chem. Soc., Dalton Trans. 1998, 3191−3194. (d) Galsworthy, J. R.; Green, J. C.; Green, M. L. H.; Muller, M. J. Chem. Soc., Dalton Trans. 1998, 15−20. (21) Cook, K. S.; Piers, W. E.; Patrick, B. O.; McDonald, R. Can. J. Chem. 2003, 81, 1502−1503.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00686. Full synthetic and characterization details for compounds 3−5 and crystallographic data for complexes 3−5 (PDF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.K.T.: [email protected]. Author Contributions

C.J.V. synthesized and characterized all of the compounds. D.R.P. carried out X-ray diffraction measurements of complexes 3−5. R.K.T. designed and directed the project and wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the University of Oklahoma for providing funds for this work and the National Science Foundation (grant CHE-0130835) for funds used to purchase the X-ray diffractometer and computers for data processing. The authors are grateful to Dr. Susan L. Nimmo and the University of Oklahoma NMR Facility for exceptional assistance with NMR spectroscopy studies.

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00686 Organometallics 2015, 34, 4806−4809