Synthesis and Characterization of Tungsten (VI) Alkylidene

Sep 2, 2010 - Khalil A. Abboud, and Adam S. Veige* ... The synthesis and characterization of trianionic [tBuOCO]3r pincer-supported tungsten alkyliden...
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Organometallics 2010, 29, 4227–4233 DOI: 10.1021/om100189b

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Synthesis and Characterization of Tungsten(VI) Alkylidene Complexes Supported by an [OCO]3- Trianionic Pincer Ligand: Progress towards the [tBuOCO]WtCC(CH3)3 Fragment Subramaniam Kuppuswamy, Andrew J. Peloquin, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611 Received March 10, 2010

The synthesis and characterization of trianionic [tBuOCO]3- pincer-supported tungsten alkylidene and alkylidyne complexes are described. The reaction of an equimolar ratio of (tBuO)3WtCC(CH3)3 (where tBuO = tert-butoxide) with [tBuOCO]H3 (9) and 2,6-diisopropylphenol affords the alkylidene [tBuOCO]WdCHC(CH3)3(O-2,6-C6H3-iPr2) (10), with a five-coordinate tungsten center that adopts a distorted square-pyramidal geometry. Treatment of (Np)3WtCC(CH3)3 (11) with 9 provides an equilibrium mixture of the two isomeric alkylidenes with the general formula [(tBuOCO)WdCHC(CH3)3(μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12kin and 12therm). Single crystals of the two isomers cocrystallize and were amenable to X-ray diffraction studies, which revealed subtle differences in their molecular structures, most notably the orientation of the bridging ligand. The complexes are each comprised of two square-pyramidal tungsten ions linked by the diphenolate form of the OCO ligand. Isomer 12kin could not be isolated independently; however, adding PMe2Ph to the metalation between 11 and 9 provided 12therm exclusively, thus enabling a full set of characterization techniques including 2-D NMR spectroscopy. Salt metathesis between [tBuOCHO]K2(THF)2 (15) and (DME)Cl3WtCC(CH3)3 (16) in diethyl ether produces the alkylidyne (tBuOCHO)WtCC(CH3)3Cl (17) as the major product along with 12kin and other unidentified decomposition products. As a consequence, characterization of 17 was limited to 1H NMR spectroscopy and mass spectrometry.

Introduction Nitrile-alkyne cross-metathesis (NACM)1-3 is a potentially powerful synthetic tool but receives considerably less attention than alkyne metathesis (AM).4-6 NACM is *To whom correspondence should be addressed. E-mail: veige@ chem.ufl.edu. (1) (a) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J. Am. Chem. Soc. 1982, 104, 4291–4293. (b) Tonzetich, Z. J.; Schrock, R. R.; Wampler, K. M.; Bailey, B. C.; Cummins, C. C.; Muller, P. Inorg. Chem. 2008, 47, 1560–1567. (2) (a) Geyer, A. M.; Wiedner, E. S.; Gary, J. B.; Gdula, R. L.; Kuhlmann, N. C.; Johnson, M. J. A.; Dunietz, B. D.; Kampf, J. W. J. Am. Chem. Soc. 2008, 130, 8984–8999. (b) Boyd, J. P.; Schlangen, M.; Grohmann, A.; Schwarz, H. Helv. Chim. Acta 2008, 91, 1430–1434. (c) Geyer, A. M.; Gdula, R. L.; Wiedner, E. S.; Johnson, M. J. A. J. Am. Chem. Soc. 2007, 129, 3800–3801. (d) Bailey, B. C.; Fout, A. R.; Fan, H. J.; Tomaszewski, J.; Huffman, J. C.; Gary, J. B.; Johnson, M. J. A.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 2234–2235. (e) Gdula, R. L.; Johnson, M. J. A. J. Am. Chem. Soc. 2006, 128, 9614–9615. (f) Gdula, R. L.; Johnson, M. J. A.; Ockwig, N. W. Inorg. Chem. 2005, 44, 9140–9142. (3) Bindl, M.; Stade, R.; Heilmann, E. K.; Picot, A.; Goddard, R.; Furstner, A. J. Am. Chem. Soc. 2009, 131, 9468–9470. (4) (a) Pennella, F.; Banks, R. L.; Bailey, G. C. Chem. Commun. 1968, 1548–1549. (b) Wengrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981, 103, 3932–3934. (c) Sancho, J.; Schrock, R. R. J. Mol. Catal. 1982, 15, 75–79. (5) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748–3759. (6) (a) Schrock, R. R.; Czekelius, C. Adv. Synth. Catal. 2007, 349, 55–77. (b) Zhang, W.; Moore, J. S. Adv. Synth. Catal. 2007, 349, 93–120. (c) Furstner, A.; Davies, P. W. Chem. Commun. 2005, 2307–2320. r 2010 American Chemical Society

important because it represents a method for generating novel alkynes from readily accessible nitriles. Mindiola et al. generated the exceptionally reactive alkylidyne [PNP]TitC(CH3)3] (1)7-9 in situ (where PNP = N[2-PiPr2-4methylphenyl]2-), which adds pivalonitrile or adamantylnitrile to form the first isolable azametallocyclobutadienes [PNP]Ti(NCRCtBu) (R= tBu, 2; Ad, 3) (Figure 1 depicts the proposed planar isomer 2).2d Addition of AlMe3 to 2 completes the reaction and liberates tBuCtCtBu. Calculations8 suggest the TitC linkage in 1 distorts out of the ligand plane, making it highly nucleophilic and reactive, enabling CdN, B-O, C-O, C-H, and C-F bond cleavage reactions.10,11 (7) Bailey, B. C.; Fan, H. J.; Baum, E. W.; Huffman, J. C.; Baik, M. H.; Mindiola, D. J. J. Am. Chem. Soc. 2005, 127, 16016–16017. (8) Bailey, B. C.; Fan, H. J.; Huffman, J. C.; Baik, M. H.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 8781–8793. (9) Mindiola, D. J.; Bailey, B. C.; Basuli, F. Eur. J. Inorg. Chem. 2006, 3135–3146. (10) (a) Fout, A. R.; Scott, J.; Miller, D. L.; Bailey, B. C.; Pink, M.; Mindiola, D. J. Organometallics 2009, 28, 331–347. (b) Fout, A. R.; Bailey, B. C.; Tomaszewski, J.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 12640–12641. (c) Bailey, B. C.; Tout, A. R.; Fan, H.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Angew. Chem., Int. Ed. 2007, 46, 8246– 8249. (d) Bailey, B. C.; Huffman, J. C.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 5302–5303. (11) Basuli, F.; Bailey, B. C.; Brown, D.; Tomaszewski, J.; Huffman, J. C.; Baik, M. H.; Mindiola, D. J. J. Am. Chem. Soc. 2004, 126, 10506– 10507. Published on Web 09/02/2010

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activation across the WtCR bond. Herein, we report the synthesis and characterization of the monomeric alkylidene [tBuOCO]WdCHC(CH3)3(OAr) (10) and the dimeric alkylidenes [(tBuOCO)WdCHC(CH3)3(μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12kin and 12therm). These complexes are the first trianionic pincer alkylidenes.

Results and Discussion Figure 1. Azametallocyclobutadienes: isolated 2, transition state 5, and OCO3- pincer version 7.

Alkoxide-supported tungsten alkylidynes1a catalytically turnover the NACM reaction. Making the tungsten ion electrophillic, Johnson et al. used fluorinated alkoxides and reported efficient catalytic examples.2a,c,f In fact, entry to the cycle is possible from the nitride (RO)3WtN (4-WtN) or alkylidyne (RO)3WtCC(CH3)3 (4-WtCtBu). However, in many cases the NACM transformations are irreversible. For example, the reaction of NtMo{OC(CF3)2(CH3)}3 with 3-hexyne leads to slow irreversible formation of EtCtMo{OC(CF3)2(CH3)}3(DME).2e Calculations indicate the transition state (ΔGq = 29.6 kcal/mol) is the trigonal-bipyramidal species 5 (Figure 1), in which the alkoxides comprise two axial and one equatorial position.2a In N-atom exchange between (RO)3WtN and RCtN,12 Chisholm calculated ΔGq = 25.9 kcal/mol as the barrier for [2þ2] cycloaddition,13 which is the rate-determining step and matches experimental values. Associated with the barrier for [2þ2] cyclometalation is ligand reorganization energy (trigonal f t-shaped).14 Engineering catalysts that circumvent this reorganization energy should lead to more active catalysts. Oxo15 and nitride16 complexes of trianionic pincer ligands17-20 are known and suggest that similar compounds bearing M-C multiple bonds are possible. Specifically, this report details our initial approaches at synthesizing the trianionic pincer alkylidyne complex [tBuOCO]WtCC(CH3)3 (6). Motivating this investigation is the possibility that the OCO trianionic pincer ligand is prearranged to add nitriles to form the trianionic pincer azametallocyclobutadiene 7. Initial metalation attempts with preinstalled alkylidyne moieties do not provide [tBuOCO]WtCC(CH3)3 (6). Instead, alkylidenes result from either ligand O-H or C-H bond (12) Chisholm, M. H.; Delbridge, E. E.; Kidwell, A. R.; Quinlan, K. B. Chem. Commun. 2003, 126–127. (13) Burroughs, B. A.; Bursten, B. E.; Chen, S.; Chisholm, M. H.; Kidwell, A. R. Inorg. Chem. 2008, 47, 5377–5385. (14) Chen, S.; Chisholm, M. H.; Davidson, E. R.; English, J. B.; Lichtenberger, D. L. Inorg. Chem. 2009, 48, 828–837. (15) O’Reilly, M.; Falkowski, J. M.; Ramachandran, V.; Pati, M.; Abboud, K. A.; Dalal, N. S.; Gray, T. G.; Veige, A. S. Inorg. Chem. 2009, 48, 10901–10903. (16) (a) Sarkar, S.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130, 16128–16129. (b) Sarkar, S.; Carlson, A. R.; Veige, M. K.; Falkowski, J. M.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130, 1116–1117. (17) Koller, J.; Sarkar, S.; Abboud, K. A.; Veige, A. S. Organometallics 2007, 26, 5438–5441. (18) Sarkar, S.; McGowan, K. P.; Culver, J. A.; Carlson, A. R.; Koller, J.; Peloquin, A. J.; Veige, M. K.; Abboud, K. A.; Veige, A. S. Inorg. Chem. 2010, 49, 5143-5156. (19) (a) Golisz, S. R.; Bercaw, J. E. Macromolecules 2009, 42, 8751– 8762. (b) Agapie, T.; Henling, L. M.; DiPasquale, A. G.; Rheingold, A. L.; Bercaw, J. E. Organometallics 2008, 27, 6245–6256. (c) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27, 6123–6142. (d) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959. (20) Chisholm, M. H.; Huang, J. H.; Huffman, J. C. J. Organomet. Chem. 1997, 528, 221–223.

Synthesis and Characterization of [tBuOCO]WdCHC(CH3)3(O-2,6-C6H3-iPr2) (10). The attachment of an [OCO]3ligand to a metal center requires the formation of two M-O and one M-C bond. The alkylidyne complex (ArO)2WtCC(CH3)3Np (8) (ArO = 2,6-diisopropylphenoxide, Np = neopentyl),21 prepared by Schrock, contains the desired MtCR bond as well as appropriate leaving groups (namely, two aryloxides and one alkyl). Treating 8 with one equivalent of the terphenyldiol [tBuOCO]H3 (9)16b in hot benzene provides 10 as a deep red oil after 2 h. In addition to free phenol resonances (ArOH = 4.32 ppm in C6D6), a 1H NMR spectrum of the oil indicates the presence of the alkylidene complex [tBuOCO]WdCHC(CH3)3(O-2,6-C6H3-iPr2) (10) (Scheme 1) and inseparable impurities. An alternative synthesis involves treating the commercially available alkylidyne (tBuO)3WtCtBu with 9 in benzene. After addition, evaporation of all volatiles including liberated tert-butanol provides a pale red solid, which is then dissolved in toluene and treated with one equivalent of 2,6diisopropyl phenol to provide 10 in 68% yield. The complex consists of the OCO trianionic pincer, the alkylidene moiety, and one phenoxide ligand. The 1H NMR spectrum indicates 10 is Cs symmetric; both the OCO tBu groups are equivalent and resonate at 1.44 ppm. The iPr resonances indicate that rotation around the W-OAr bond is either halted or slow on the NMR time scale, as two distinct environments exist for each group. The methine protons appear as septets at 4.09 and 2.40 ppm, whereas the methyl protons resonate as doublets at 1.42 and 0.69 ppm (JHH = 6.8 Hz). An HMQC experiment reveals a cross-peak for the alkylidene carbon located at 272.2 ppm, which correlates to the alkylidene resonance at 5.55 ppm. Coupling occurs between the alkylidene proton and the 183W isotope with a magnitude of JHW = 8.7 Hz. The alkylidene proton resonance is significantly shifted upfield compared to other known tungsten imido alkylidene compounds.22 X-ray crystallographic structural analysis of a single crystal of 10 confirms the presence of a trianionic pincer ligand bound to a distorted square-pyramidal W(VI) ion. The pincer occupies three basal positions, the alkylidene is apical, and the phenoxide occupies the fourth basal position (Figure 2). The asymmetric unit contains two molecules related by mirror symmetry. Overall, 10 is C1 symmetric in the solid state. The alkylidene tBu group breaks the Cs symmetry and leans toward O1 with a W-C39-C40 angle of 140.2(7)°. A small dihedral angle between O1-W-C39-C40 of 3.6(9)° indicates the tBu bends parallel to the W1-O1 bond axis. However, in solution the complex is Cs symmetric and the alkylidene substituents lie within the mirror plane but do not rotate freely. The alkylidene proton points toward the OCO central aryl ring and the tBu points toward the 2,6-isopropylaryloxide. Irradiation of the alkylidene WdCH proton (21) Tonzetich, Z. J.; Lam, Y. C.; Muller, P.; Schrock, R. R. Organometallics 2007, 26, 475–477. (22) Solans-Monfort, X.; Eisenstein, O. Polyhedron 2006, 25, 339–348.

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Scheme 1. Synthesis of [tBuOCO]WdCH(CH3)3(O-2,6-C6H3-iPr2) (3)

Figure 2. Molecular structure of 10 with one molecule of the asymmetric unit depicted and hydrogen atoms removed for clarity. Selected bond distances [A˚]: W1-O1 1.834(5), W1-O2 1.880(5), W1-O5 1.945(5), W1-C12 2.161(7), W1-C39 1.913(8). Bond angles [deg]: O1-W1-C39 101.9 (3), O1-W1-O5 91.8(2), O1-W1-C12 84.2(3), O2-W1-C39 94.1(3), O2-W1-O5 86.6(2), O2-W1-C12 85.1(2), C39-W1-C12 99.5(3), C39W1-O5 121.8(5), O1-W1-O2 162.0(2), O3-W1-C12 138.3(2), C40-C39-W1 140.2(7).

(5.69 ppm at 65 °C) in a NOE difference experiment did not show any NOE with the isopropyl methine proton H36. The Cs-symmetric orientation persists even at -100 °C (toluened8); the OCO ligand tBu protons remain equivalent. Located above and below the basal plane, the iPr groups have distinct chemical environments, thus explaining their different NMR signals. Considerable distortions are evident in the ligand backbone. The OCO phenyl rings adopt a butterfly twist such that the central ring is severely bent down and the peripheral rings bend upward. Tonks and (23) Tonks, I. A.; Henling, L. M.; Day, M. W.; Bercaw, J. E. Inorg. Chem. 2009, 48, 5096–5105.

Bercaw et al.23 conclude electronically unsaturated dialkoxide complexes with terphenyl backbones will distort (as in 10) to maximize OpπfWdπ overlap, whereas electronically saturated species exhibit a relaxed pseudo-C2 orientation. In the example of 10, the metal is formally a 12 e- complex (not counting π-interactions). To maximize overlap, the backbone distorts to permit additional π-donation from the aryloxide. The distortion imparts considerable strain to the OCO ring system, but the metal core M-X bond lengths are normal: W1-C12 = 2.161(7) A˚, W1-C39 = 1.913(8) A˚, W1-O1 = 1.834(5) A˚, W1-O2 = 1.880(5) A˚, and W1-O5 = 1.945(5) A˚. The spectroscopic and structural findings indicate the alkylidyne functionality is lost upon metalation. It is plausible for this conversion to occur via O-H or Caryl-H activation across the MtCR bond. Deuterium labeling studies performed by Bercaw et al.19d indicate the di-tert-Bu derivative [tBu2OCO]H3 metalates Me3TaCl2 via alcoholysis of one O-H group, but the backbone attaches via Caryl-H bond activation before the second pincer arm O-H. This suggests that after the first arm attaches, the Caryl-H bond is poised to activate. Aryloxo alkylidynes are known to cyclometalate via aryl C-H bond activation across a WtCR bond.24 Even C-C bond cleavage is facile if the Caryl-R bond is jammed into the metal coordination sphere.25 Alternatively, the alkylidene can form from liberated phenol O-H addition across the MtCR. If phenol recombination is problematic, then alcohol-free metal-alkylidyne reagents are one possible remedy. A potential reagent for this purpose is Np3WtCC(CH3)3 (11).26 (24) (a) Lefebvre, F.; Leconte, M.; Pagano, S.; Mutch, A.; Basset, J. M. Polyhedron 1995, 14, 3209–3226. (b) Couturier, J. L.; Paillet, C.; Leconte, M.; Basset, J. M.; Weiss, K. Angew. Chem., Int. Ed. Engl. 1992, 31, 628–631. (25) (a) van der Boom, M. E.; Liou, S. Y.; Ben-David, Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 13415–13421. (b) van der Boom, M. E.; Ben-David, Y.; Milstein, D. Chem. Commun. 1998, 917–918. (c) Liou, S. Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 9774– 9775. (d) Liou, S. Y.; Gozin, M.; Milstein, D. J. Chem. Soc., Chem. Commun. 1995, 1965–1966. (e) Gozin, M.; Aizenberg, M.; Liou, S. Y.; Weisman, A.; Bendavid, Y.; Milstein, D. Nature 1994, 370, 42–44. (f) Gozin, M.; Weisman, A.; Bendavid, Y.; Milstein, D. Nature 1993, 364, 699–701. (26) (a) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; Rocklage, S. M.; Pedersen, S. F. Organometallics 1982, 1, 1645–1651. (b) Clark, D. N.; Schrock, R. R. J. Am. Chem. Soc. 1978, 100, 6774–6776.

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Scheme 2. Equilibrium of Dinuclear Alkylidene Complexes 12kin:12therm as a 40:60 Mixture

Equilibrium Mixture of Isomers [(tBuOCO)WdCHC(CH3)3( μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12kin and 12therm). Np3WtCC(CH3)3 (11) is available by treating W(OMe)3Cl3 with MgNpCl in diethyl ether at -78 °C.26 Treating Np3WtCC(CH3)3 (11) with 9 in C6D6 at 100 °C results in the formation of the red dinuclear species [(tBuOCO)WdCHC(CH3)3(μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12kin and 12therm) (Scheme 2) in a 40:60 ratio after heating for 116 h. In the 1H NMR spectrum two sets of four singlets appear between 0.56 and 1.83 ppm. Each set of four singlets is attributable to the four nonequivalent tBu groups within the two dinuclear complexes. The alkylidene protons appear at 8.74 and 8.66 ppm for 12kin and 12therm, respectively. Initially, 12kin forms (kinetic product) along with unidentified impurities after 20 h, and then 12therm forms to give a 40:60 ratio. Attempts to isolate 12kin alone at early reaction times were unsuccessful. Slow evaporation of an Et2O solution of a mixture of 12kin and 12therm produces single crystals of both suitable for X-ray analysis. In addition to the single crystals however, other solids deposit, ultimately hampering their purification. However, offering a chance to conclusively identify the isomers, each sample was subject to single-crystal X-ray diffraction experiments. Figure 3 depicts the molecular structures of 12kin and 12therm, and Table 1 lists pertinent bond lengths and angles. Each isomer consists of two square-pyramidal W(VI) centers in which the [tBuOCO]3- pincer occupies three basal coordination sites and the dCHtBu alkylidene moiety is apical. The metal ions connect through the fourth basal position via a diphenolate [tBuOCHO]2- bridge. The differences between the two isomers are subtle. The most significant difference appears in the orientation of the bridging ligand. In 12kin, the bridging ligand pendant arms rotate, placing the oxygen atoms proximal to the central ring. In 12therm, the arrangement reverses, with the oxygen atoms distal to the center ring, akin to the orientation of the free ligand,16b and therefore the more thermodynamically favorable state (Figure 3, bottom). It is plausible the structural changes observed between the two isomers may manifest in differences in the

alkylidene moiety. The WdC bond lengths and the WdC-C bond angles of the two alkylidenes in 12kin are 1.885(6) A˚, 1.887(5) A˚, 151.2(6)°, and 143.7(4)°, respectively, and the corresponding values for 12therm are 1.888(6) A˚, 1.883(6) A˚, 143.2(5)°, and 145.8(5)°, respectively. Despite one large alkylidene angle (151.2(6)°) found in 12kin, the structural changes between the isomers have no effect on the alkylidene moiety. The WdC bond lengths of 12kin and 12therm are significantly smaller than those of 10 (1.912(8) A˚, 140.2(7)°) and manifest in larger WdC-C bond angles. In general, an upfield 1H NMR resonance for an alkylidene proton is indicative of a strong C-H agostic interaction.27 This usually results in a large WdC-C angle as the alkylidene moiety bends to accommodate the agostic interaction. However, in the case of 10 versus 12therm, the WdCHC protons resonate at 5.54 and 8.66 ppm, respectively, yet the WdC-C angle is the smallest (140.2(7)°) for 10. It is not clear that an agostic interaction exists, or why the alkylidene protons resonate at such different energies. In the solid state both alkylidene protons have similar chemical environments. Perhaps the downfield signal for the alkylidene proton on 12therm is due to the presence of an arene ring in close proximity under the W ion. The distances between the W’s and the centroid of the arene ring in 12therm are approximately 3.8/3.6 A˚, respectively. Additional structural features for all three complexes indicate the metal center is compensating for electronic unsaturation by distorting the OCO ligand. Again, the distortion in the backbone of the OCO pincer ligand relates to the relative electronic saturation of the metal.23 When electronically saturated, the ligand backbone relaxes to a pseudo-C2-symmetric orientation. Not counting possible π-donation from aryloxide ligands, the metal centers in 10, 12kin, and 12therm are all 12-electron ions (formal oxidation state method). The ligand distorts to a pseudo-Cs-symmetric arrangement to maximize π-donation from the aryloxides. (27) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2005; p 539.

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Figure 3. Molecular structures of 12kin (left) and 12therm (right) with hydrogen atoms removed for clarity. Below are truncated structures depicting only the bridging OCO ligand, which illustrates the subtle difference between the two isomers.

As a consequence, the alkylidene C-H agostic interaction is weaker than would be expected for a 12-electron species. Influence of Added Phosphine. No method was found to separate or selectively crystallize isomers 12kin and 12therm. It is plausible that the target complex [tBuOCO]WtCC(CH3)3 (6) may require a strong σ-donor for stabilization of the fifth coordination site. Adding two equivalents of dimethylphenylphosphine (PMe2Ph) to the metalation reaction between Np3WtCtBu (11) and [tBuOCO]H3 (9) did not result in the formation of 6; however, it did improve the synthesis of 12therm. The reaction proceeds to completion at 100 °C within 48 h according to Scheme 3 (compare to Scheme 2, 100 °C and 116 h). 1H NMR spectroscopy is amenable to monitoring the reaction progress. Complex 12kin forms initially after 8 h with a maximum conversion of 50%, then steadily declines as consumption of all starting materials occurs and 12therm forms as the major product (51% isolated yield). Now pure, a full suite (DQCOSY, gHMQC, gHMBC, NOE)28 of NMR techniques completes the characterization for 12therm. Of note are four singlets attributable to the four nonequivalent tBu groups that resonate at 0.56, 1.32, 1.68, and 1.79 ppm. A gHMQC experiment relates the alkylidene proton at 8.66 ppm to a carbon signal at 280.7 ppm. (28) See Supporting Information.

Table 1. Selected Bond Lengths (A˚) and Angles (deg) for 12kin and 12therm 12kin

12therm Bond Lengths

W1-O1 W1-O2 W1-C55 W1-O3 W1-C12 W2-O6 W2-C60 W2-O5 W2-O4

1.866(4) 1.824(4) 1.885(6) 1.954(3) 2.162(6) 1.844(3) 1.887(5) 1.894(3) 1.951(3)

W1-O1 W1-O2 W1-C55 W1-O5 W1-C12 W2-O4 W2-C60 W2-O3 W2-O6

1.865(4) 1.914(4) 1.888(6) 1.948(4) 2.134(7) 1.844(5) 1.883(6) 1.903(5) 1.947(4)

Angles O3-W1-C12 O1-W1-O2 C55-W1-C12 O3-W1-C55 W1-C55-C56 C48-W2-O4 O6-W2-O5 C48-W2-C60 O4-W2-C60 W2-C60-C61

149.29(17) 162.91(16) 97.4(2) 113.3(2) 151.2(6) 150.52(15) 161.88(14) 96.9(2) 112.46(19) 143.7(4)

O5-W1-C12 O1-W1-O2 C55-W1-C12 O5-W1-C55 W1-C55-C56 C48-W2-O6 O3-W2-O4 C48-W2-C60 O6-W2-C60 W2-C60-C61

151.1(12) 158.91(18) 95.9(3) 112.3(2) 143.2(5) 157.1(2) 157.35(18) 95.3(3) 107.2(2) 145.8(5)

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Scheme 3. Synthesis of [(tBuOCO)WdCHC(CH3)3(μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12therm)

Scheme 4. Formation of [tBuOCHO]WtC(CH3)3Cl (17)

Confirming 12therm as the thermodynamic product, heating pure 12therm to 100 °C for 66 h reestablishes the 40:60 equilibrium with 12kin. It is not clear how PMe2Ph can accelerate the reaction. PMe3 is known to promote the elimination of CMe4 from 11 via R-hydrogen elimination to form the alkyl-alkylidenealkylidyne complex NpWtCtBu(dCHtBu)(PMe3)2 (13).26b However, when added to the metalation between 9 and 11 at 100 °C, PMe3 instead results in decomposition and provides a mixture of unidentified products within 2 h, with only approximately 10% formation of 12therm. Providing insight to the potential role of phosphine, previous reports indicate PMe3 will bind to (Me3SiCH2)3WtCSiMe3 to establish an equilibrium between the alkylidyne (Me3SiCH2)3WtCSiMe3(PMe3) (13) and bisalkylidene (Me3SiCH2)2W(dCHSiMe3)2(PMe3) (14) complexes.29 The 1H NMR spectrum of a C6D6 solution of PMe2Ph and 11 does not indicate any interaction at 25 °C, although at elevated temperatures neopentane forms and multiple unidentified products form. Binding PMe2Ph at 100 °C may expose one of the neopentyl groups to attack from the ligand, therefore facilitating the metalation. Differing in basicity30 (PPh3: pKa value = 2.73; PMe2Ph: pKa value = 6.49) and cone angle (PPh3: 145°; PMe2Ph: 136°),30c PPh3 does not accelerate the reaction. The slower rate of PPh3 implies an association of PMe2Ph with the metal during the phosphinepromoted metalation. Regardless of the role of phosphine, use of 11 as the alkylidyne precursor failed to provide the target fragment [tBuOCO]WtCC(CH3)3 (6). Synthesis and Characterization of [tBuOCHO]WtCC(CH3)3Cl (17). An alternative approach to generate the [tBuOCO]WtCC(CH3)3 (6) fragment requires circumventing the use of all -OH-containing reagents, including those found on the trianionic pincer ligand precursor [tBuOCO]H3 (9). Executing this plan involves treating the known alkylidyne (DME)Cl3WtCC(CH3)3 (15)26a with the dipotassium salt [tBuOCHO]K2(THF)2 (16)15 according to Scheme 4. The (29) (a) Morton, L. A.; Wang, R. T.; Yu, X. H.; Campana, C. F.; Guzei, I. A.; Yap, G. P. A.; Xue, Z. L. Organometallics 2006, 25, 427– 434. (b) Morton, L. A.; Zhang, X. H.; Wang, R. T.; Lin, Z. Y.; Wu, Y. D.; Xue, Z. L. J. Am. Chem. Soc. 2004, 126, 10208–10209. (30) (a) Allman, T.; Goel, R. G. Can. J. Chem. 1982, 60, 716–722. (b) Henderson, W. A.; Streuli, C. A. J. Am. Chem. Soc. 1960, 82, 5791– 5794. (c) Tolman, C. A. Chem. Rev. 1977, 77, 313–348.

diphenolate complex [tBuOCHO]WtCC(CH3)3Cl (17) forms at -80 °C but is not isolable as a single product. In addition to 17, the dinuclear complex 12kin (6%) forms, as well as other inseparable minor impurities. In this case, it is obvious the alkylidyne moiety reacts with the central aryl C-H bond of the OCO ligand to form the alkylidene 12kin; no other proton sources are available. Since 12kin and other impurities are always present, characterization of [tBuOCHO]WtCC(CH3)3Cl (17) is limited to 1H NMR and mass spectrometry, and thus the assignment is tentative. The 1H NMR spectrum of 17 reveals two singlets at 0.74 and 1.69 ppm corresponding to alkylidyne (nine protons) and ligand tBu groups (18 protons). A characteristic broad singlet for the central aryl ring CH proton appears at 9.75 ppm, indicating the ligand is in the diphenolate dianionic form. Evident in the NMR spectrum is a broadened resonance attributable to one Et2O, which either may be bound or is exchanging rapidly. Complementing the 1H NMR assignments, the parent ion was found using mass spectrometry, APCI-MS, [M þ H]þ = 661.2085 (C31H38O2ClW, theoretical = 661.2058). Conclusion. Prepared for the first time, the trianionic pincer alkylidene complex (tBuOCO)WdCHC(CH3)3(O2,6-C6H3-iPr2) (10) forms upon treating the alkylidyne complex (ArO)2WtCC(CH3)3(Np) (8) with the pincer precursor [tBuOCO]H3 (9). Similarly, the alkylidyne moiety is lost when (Np)3WtCC(CH3)3 (11) reacts with 9 to give an equilibrium mixture (Keq(25 °C) = 0.67) of alkylidene dimers [(tBuOCO)WdCHC(CH3)3(μ-tBuOCHO)WdCHC(CH3)3(tBuOCO)] (12kin and 12therm). Addition of PMe2Ph to the metalation reaction between 11 and 9 produces exclusively 12therm. Attempting to eliminate OH bonds from the metalation strategy, treating the (DME)WtCtBuCl3 with the dipotasium salt [tBuOCHO]K2(THF)2 (16) did provide the alkylidyne complex [tBuOCHO]WtC(CH3)3Cl (17); however purification was hampered by intractable impurities, which include 12kin. Important guiding principles emerge from the synthetic approaches above. More importantly, generating [tBuOCO]WtCC(CH3)3 (6) via preformed W-alkylidyne precursors does not work. Under the somewhat harsh conditions required for metalation, the WtCR bond is susceptible to addition from both Caryl-H and O-H bonds.31 Alternative approaches to generating [tBuOCO]WtCC(CH3)3 (6) in situ are ongoing.

Synthetic Procedures General Considerations. Unless specified otherwise, all manipulations were performed under an inert atmosphere using (31) Heppert, J. A.; Dietz, S. D.; Eilerts, N. W.; Henning, R. W.; Morton, M. D.; Takusagawa, F.; Kaul, F. A. Organometallics 1993, 12, 2565–2572.

Article standard Schlenk or glovebox techniques. Glassware was ovendried before use. Pentane, toluene, diethyl ether (Et2O), tetrahydrofuran (THF), and 1,2-dimethoxyethane (DME) were dried using a GlassContours drying column. Benzene-d6 (Cambridge Isotopes) was dried over sodium-benzophenone ketyl, distilled or vacuum transferred, and stored over 4 A˚ molecular sieves. (tBuO)3WtCC(CH3)3 was purchased from Strem Chemicals Inc. and used as received. (MeO)3WCl3,32 (ArO)3WCl3 (Ar = 2,6diisopropylphenyl),33 (ArO)2WtCC(CH3)3Np (8),21 [tBuOCO]H3 (9),16b Np3WtCC(CH3)3 (11),26 [tBuOCHO]K2(THF)2 (15),15 and (DME)Cl3WtCC(CH3)3 (16)26b were prepared according to the literature procedures. NMR spectra were obtained on Varian INOVA 500 MHz, Varian Mercury broad band 300 MHz, or Varian Mercury 300 MHz spectrometers. Chemical shifts are reported in δ (ppm). For 1H and 13C{1H} NMR spectra the solvent resonance was referenced as an internal reference. Elemental analyses were performed at Complete Analysis Laboratory Inc., Parsippany, NJ. Accurate mass was determined by the atmospheric pressure chemical ionization-mass spectrometric (APCIMS) method in diluted dichloromethane solution, and the spectrum was recorded on an Agilent 6210 TOF-MS at the Mass Spectrometry Facility, University of Florida. Synthesis of [tBuOCO]WdCHC(CH3)3(O-2,6-C6H3-iPr2) (10). A glass vial was charged with [tBuOCO]H3 (9) (41 mg, 0.11 mmol) and benzene (0.5 mL) and then frozen (-35 °C). (tBuO)3WtCC(CH3)3 (52 mg, 0.11 mmol) was dissolved in benzene (0.5 mL) and then added to the frozen solution of 9. All volatiles were removed in vacuo and triturated with pentane (3  1 mL) to provide a light red crystalline solid. The solid was redissolved in toluene (2 mL), and then the solution was cooled to -35 °C. 2,6-iPr2-C6H3OH (18.4 μL, 0.11 mmol) was dissolved in toluene (5 mL) and cooled to -35 °C. The solutions were combined at -35 °C, and the resulting mixture was shaken for 1 min. Then all volatiles were removed, but the solution was kept cold. The crude mixture was checked by 1H NMR spectroscopy to determine if additional 2,6-iPr2-C6H3OH was needed to complete the reaction. The product was dried under vacuum and washed with cold pentane (3  1 mL) to provide 10 as a red crystalline solid (60 mg, 68%). 1H NMR (300 MHz, C6D6) δ (ppm): 7.99 (d, J = 7.9 Hz, 2H, Ar-H), 7.80 (dd, J = 7.9 Hz, J = 1.5 Hz, 2H, Ar-H), 7.38 (t, J = 7.9 Hz, 1H, Ar-H), 7.33 (dd, J = 7.8 Hz, J = 1.5 Hz, 2H, Ar-H), 7.13-7.15 (m, 1H, Ar-H), 6.98 (overlapping doublets, J = 7.9 Hz, J = 7.6 Hz, 2H, Ar-H), 6.87-6.89 (m, 2H, Ar-H), 5.55 (t, JHW = 8.7 Hz, 1H, WdCHC(CH3)3), 4.09 (sept, J = 6.7 Hz, 1H, -CH(CH3)2), 2.40 (sept, J = 6.7 Hz, 1H, -CH(CH3)2), 1.44 (s, 18H, -C(CH3)3), 1.42 (d, J = 6.7 Hz, 6H, -CH(CH3)2), 0.85 (s, 9H, WdCHC(CH3)3), 0.70 (d, J = 6.7 Hz, 6H, -CH(CH3)2). 13C{1H} NMR (75.36 Hz, C6D6) δ (ppm): 272.2 (s, WdCHC(CH3)3), 182.7 (s, W-Cpincer), 160.3, 158.5, 140.9, 138.2, 137.2, 137.1, 133.0, 130.0, 126.7, 126.4, 124.4, 123.9, 123.6, 122.3 (aryl), 47.8 (s, WdCHC(CH3)3), 35.6 (s, -C(CH3)3), 33.3 (s, WdCHC(CH3)3), 31.8 (s, -CH(CH3)2), 30.8 (s, -C(CH3)3), 27.5 (s, -CH(CH3)2), 23.9 (s, -CH(CH3)2), 23.6 (s, -CH(CH3)2). Anal. Calcd for C43H54O3W: C, 64.34; H, 6.78. Found: C, 64.42; H, 6.94. Synthesis of [(t BuOCO)WdCHC(CH 3 )3 (μ- t BuOCHO)WdCHC(CH3)3(tBuOCO)] (12therm). In a J. Young NMR tube, Np3WtCC(CH3)3 (11) (76 mg, 0.163 mmol), 9 (60 mg, 0.163 (32) Handy, L. B.; Sharp, K. G.; Brinckma, F. E. Inorg. Chem. 1972, 11, 523–. (33) Listemann, M. L.; Schrock, R. R.; Dewan, J. C.; Kolodziej, R. M. Inorg. Chem. 1988, 27, 264–271.

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mmol), and PMe2Ph (46 μL, 0.326 mmol) were combined in benzene-d6 (0.5 mL). The solution was degassed. The tube was heated at 100 °C, and the progress of the reaction monitored periodically by 1H NMR spectroscopy. In addition to the starting materials, the isomer 12kin was observed with appreciable intensity after 8 h. 1H NMR (300 MHz, C6D6), δ (ppm): 8.74 (s, 2H, WdCHC(CH3)3), 4.64 (s, 1H, Ar-H), 1.83 (s, 18H, -C(CH3)3), 1.65 (s, 18H, -C(CH3)3), 1.40 (s, 18H, -C(CH3)3), 0.61 (s, 18H, WdCHC(CH3)3). Further heating of the reaction mixture for a total of 48 h completed the reaction. The solvent was removed in vacuo to yield a dark brown material. Extraction into hexanes followed by removal of all volatile material produced a red oil. X-ray quality and analytically pure crystalline material was obtained by slow evaporation of an Et2O solution of 12therm (45 mg, 51%, based on [tBuOCO]H3 (9)). 1H NMR (300 MHz, C6D6), δ (ppm): 8.66 (t, JWH = 12.9 Hz, 2H, WdCHC(CH3)3), 7.75 (m, 2H, Ar-H), 7.66 (d, J = 8.2 Hz, 2H, Ar-H), 7.38 (m, 4H, Ar-H), 7.28-7,32 (m, 4H, Ar-H), 7.00-7.07 (m, 3H, Ar-H), 6.91(m, 4H, Ar-H), 6.60 (m, 2H, Ar-H), 6.03-6.08 (m, 4H, Ar-H), 5.14 (t, J = 7.6 Hz, 1H, Ar-H), 1.79 (s, 18H, -C(CH3)3), 1.68 (s, 18H, -C(CH3)3), 1.32 (s, 18H, -C(CH3)3), 0.56 (s, 18H, WdCHC(CH3)3). 13C{1H} NMR (75.36 Hz, C6D6), δ (ppm): 280.7 (s, WdCHC(CH3)3), 184.3 (s, W-Cpincer), 161.4, 158.2, 155.7, 143.1, 140.6, 140.2, 140.0, 137.6, 136.5, 135.7, 135.4, 133.5, 132.8, 130.9, 129.0, 128.9, 127.8, 127.0, 126.9, 126.7, 126.4, 125.9, 124.5, 123.1, 121.8 (aryl), 49.2 (s, WdCHC(CH3)3), 36.3 (s, -C(CH3)3), 35.9 (s, -C(CH3)3), 35.4 (s, -C(CH3)3), 33.4 (s, WdCHC(CH3)3), 32.0 (s, -C(CH3)3), 31.7 (s, -C(CH3)3), 31.3 (s, -C(CH3)3). Anal. Calcd for C88H102O6W2: C, 65.11; H, 6.33. Found: C, 64.98; H, 6.21. Attempted Synthesis of [tBuOCHO]WtCC(CH3)3Cl (17). In a 100 mL round-bottom flask, (DME)Cl3WtCC(CH3)3 (16) (50 mg, 0.11 mmol) in 20 mL of Et2O was added dropwise to [tBuOCHO]K2(THF)2 (15) (66 mg, 0.11 mmol) in 60 mL of Et2O at -80 °C. The solution turned light to dark yellow within 10 min. The solvent was removed in vacuo to produce a red solid. The solid was extracted into cold hexanes and dried under vacuum to yield 17 (65 mg, 59% 17, 17% [tBuOCO]H3 (9), and 6% 12kin, along with other impurities. 1H NMR (300 MHz, C6D6), δ (ppm): 9.75 (bs, 1H, Ar-H), 8.04 (bm, 2H, Ar-H), 7.31-7.36 (m, 4H, Ar-H), 6.39 (m, 3H, Ar-H), 3.27 (bm, 4H, -OCH2CH3), 1.69 (s, 18H, -C(CH3)3), 0.94 (bm, 6H, -OCH2CH3), 0.74 (s, 9H, WtCC(CH3)3). APCI-MS: [M þ H]þ = 661.2085 (C31H38O2ClW, theoretical = 661.2058).

Acknowledgment. A.S.V. thanks UF, NSF CAREER (CHE-0748408), the Camille and Henry Dreyfus Foundation, and The Alfred P. Sloan Foundation for financial support of this work. K.A.A. thanks UF and the NSF CHE-0821346 for funding the purchase of X-ray equipment. Note Added after ASAP Publication. This paper was published on the Web on Sep 2, 2010, with multiple errors in the formula of the group CC(CH3)3. The corrected version was reposted on Sep 13, 2010. Supporting Information Available: Full experimental procedures, NMR spectra, and X-ray crystallographic details. This material is available free of charge via the Internet at http:// pubs.acs.org.