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Borylcarbyne Complexes: [Mo(tCBR2)(CO)2{HB(pzMe2)3}] (BR2 = B(NMe2)2, BO2C6H4; pz = pyrazol-1-yl) Anthony F. Hill,* Rong Shang, and Anthony C. Willis Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
bS Supporting Information ABSTRACT: The sequential treatment of [Mo(tCBr)(CO)2{HB(pzMe2)3}] (pz = pyrazol-1-yl) with n BuLi and ClBR2 (BR2 = B(NMe2)2, O2C6H4) in tetrahydrofuran provides the borylcarbyne complexes [Mo(tCBR2)(CO)2{HB(pzMe2)3}], in contrast to the reactions of many haloborane electrophiles, which result in the formation of the hydroxypentylidyne complex [Mo{tC(CH2)4OH}(CO)2{HB(pzMe2)3}] via solvent ring opening.
A
lkynylboranes, RCtCBR2, are emerging as intriguing lead compounds for electronic and optical applications,1 the key feature being the conjugation of the CtC multiple bond with the empty π symmetry p orbital on trigonal boron, which can thereby serve as a mesomeric acceptor.2 For illustrative purposes, the relevant canonical forms for the hypothetical aminoborylalkyne H2NCtCBH2 are indicated in Chart 1, which also includes a depiction of the frontier orbitals associated with the pseudocumulenic NCCB spine.2c These suggest that the HOMO involves considerable C(π)B(π) overlap, while the HOMO-4 includes N(π)C(π) overlap. Throughout the development of metal carbyne complex chemistry,3 the perceived isolobal analogy between CtC and MtC triple bonds4 has provided a useful framework for inspiring and guiding investigations. In the case of aminocarbyne complexes, it is generally accepted that the positively mesomeric (þM, π-dative) effect of the amino group is responsible for a somewhat curtailed reactivity and reduced MtC bond order, consistent with the 2-azavinylidene resonance contributor (Chart 2a).5 In contrast, carbyne complexes in which the carbyne substituent is negatively mesomeric, i.e., a π-acceptor (M) substituent, are somewhat rare, not least because the majority of synthetic strategies for carbyne ligand installation (i) introduce the carbyne substituent in nucleophilic form (Fischer synthesis),6 (ii) generate the carbyne substituent by electrophilic attack at a nucleophilic precursor ligand (CNR,5 CS,7 dCdCR2),8 or (iii) involve the intermediacy of kinetically stabilized alkyls which are generally devoid of polar functional groups that might interfere with the requisite R-hydrogen abstraction/elimination processes.9 Thus, while aminocarbyne chemistry is a mature field, r 2011 American Chemical Society
the chemistry of carbyne and carbene ligands bearing boron substituents is a short story. These historical caveats associated with the synthetic strategies say nothing, however, about the viability of the target carbyne complexes. There has been sporadic interest in the interaction of carbyne complexes with boron reagents, with Stone leading the vanguard. Just as alkynes are prone to hydroboration, Stone demonstrated that carbynes may be hydroborated, though the products obtained are ultimately consistent with 1,1-addition of the BH bond to the carbyne carbon rather than 1,2-addition across the MtC bond (Scheme 1).10 Intramolecular hydroboration of a carbyne ligand has also been demonstrated on a number of occasions, when dicarbollide coligated carbyne complexes are employed in bridge-assisted metalmetal bond formation with a variety of metal substrates.11 Depending on the combination of carbyne and metal substrate, the products obtained provide a collection of snapshots along a mechanistic trajectory in which the completely reduced carbyne ligand is transferred from the metal to the carbollide cage. A further example of a carbyne ligand serving as a hydrogen sink involves the reaction of the triboronate salt [Bu4N][W(tCC6H3Me2-2,6)(CO)2Br(B3H8)] with butyllithium, which affords mesitylene and [Bu4N]2[B12H10]12 at subambient temperatures.13 Carbyne hydroboration is also implicit in the formation of [W(S2CR)(CO)2{HB(SCH2R)(pz)2}] (R = C6H4Me-4, pz = pyrazol-1-yl) from the reaction of [W(tCR)Br(CO)4] with sulfur and K[H2B(pz)2].14 The addition of boron reagents to other “C1” ligands has included Received: March 24, 2011 Published: May 27, 2011 3237
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Organometallics Chart 1. Positively (NH2) and Negatively (BH2) Mesomeric Alkyne Substituents and the Topology of the HOMO, HOMO-4, and LUMO of Aminoborylethyne, H2NCtCBH22c
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Scheme 1. Hydroboration of Carbyne Ligands (Tol = C6H4Me-4, Xyl = C6H3Me2-2,6, pz = pyrazol-1-yl)1014
Chart 2. (a) þM Amino and (b) M Boryl Carbyne Canonical Descriptions
carbenes,15 CO,16 and CS17 and a putative titanium neopentylidyne equivalent.18 These scattered results, when taken together, presage a fledgling organoborametallic chemistry; however, simple borylcarbynes, LnMtCBR2, remain limited to the single salt [W{tCB(C6F5)2}HCl(dmpe)2][B(C6F5)4], which was obtained by Piers via the successive treatment of [W(tCH)(Cl(dmpe)2] with his eponymous borane and [CPh3][B(C6F5)4] (Scheme 2).19 Both Piers’ borane HB(C6F5)2 and Schrock’s methylidyne [W(tCH)Cl(dmpe)2]20 are somewhat singular in their properties, such that the approach that provided the archetypal borylcarbyne does not promise general applicability. We report here an alternative approach to borylcarbyne complexes which does offer some generality but which is not without its own attendant caveats. Templeton has previously described the multistep synthesis of the thermally unstable parent methylidyne complex [W(tCH)(CO)2{HB(pzMe2)3}] (1a) and its deprotonation in situ to provide the spectroscopically identified lithiocarbyne [W(tCLi)(CO)2{HB(pzMe2)3}] (2a). This reagent has been shown to react with a range of electrophiles “Eþ” to afford variously functionalized carbyne complexes [W(tCE)(CO)2{HB(pzMe2)3}].21
More recently, it has been shown that the same lithiated carbyne complexes [M(tCLi)(CO)2{HB(pzMe2)3}] (M = W (2a), Mo (2b)) are readily obtained via lithiumhalogen exchange reactions of [M(tCBr)(CO)2{HB(pzMe2)3}] (M = W (3a), Mo (3b)) with nBuLi at low temperature.22 The lithiocarbynes 2 therefore appeared to be suitable substrates for the construction of carbyne complexes bearing a variety of boron substituents on the carbyne carbon via simple reactions with boron(III) electrophiles. Treating a solution of 3b with nBuLi at low temperature followed by the addition of B-chlorocatecholborane (ClBO2C6H4) results in the formation of the new red borylcarbyne complex [Mo(tCBO 2 C6 H 4 )(CO)2 {HB(pzMe 2 )3 }] (4) in high yield. The borylcarbyne complex 4 appears to be thermally sensitive and is very prone to hydrolysis to provide the 3238
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Organometallics
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Scheme 2. Piers’ Boryl Carbyne Synthesis (Arf = C6F5)19
Figure 2. Orientation of substituents in [Mo(tCR)(CO)2{HB(pzMe2)3}]: (a) R = BO2C6H4 (4); (b) R = NEt2;29 (c) R = DMAP(þ);29 (d) R = C6H4Me-4.30a
Figure 1. Molecular structure of 4 in the crystal form (40% displacement ellipsoids, hydrogen atoms omitted, phenylene and pyrazolyl groups simplified). Selected bond lengths (Å) and angles (deg): Mo1N11 = 2.321(2), Mo1N13 = 2.218(2), Mo1N15 = 2.214(2), Mo1C1 = 1.804(2), O1B2 = 1.383(3), O2B2 = 1.389(3), C1B2 = 1.534(4); N11Mo1N13 = 83.89(7), N11Mo1N15 = 82.16(7), N13 Mo1N15 = 82.06(7), N13Mo1C1 = 103.60(9), N15Mo1 C1 = 107.55(9), N11Mo1C11 = 89.71(9), C1Mo1C11 = 83.25(11), N11Mo1C12 = 88.85(9), N13Mo1C12 = 93.65(9), C1Mo1C12 = 81.79(11), C11Mo1C12 = 89.6(1), Mo1C1B2 = 163.6(2), C1B2O2 = 123.5(2), C1B2O1 = 125.3(2), O2B2O1 = 111.2(2).
nonclassical vinylidene dimer [Mo2(μ-CdCH2)(CO)4{HB(pzMe2)3}2] (6), reported previously by Templeton.21b,24 Spectroscopic data for 4 are generally comparable to those for other carbyne complexes of the form [M(tCR)(CO)2{HB(pzMe2)3}], which have been comprehensively reviewed.3a The infrared data for 4 (THF: 2008, 1926 cm1) would appear to suggest that the borylcarbyne ligand is a comparatively strong π-acceptor within the carbyne series, with carbonyl absorptions moved to high frequency of those with more conventional substituents, e.g., the benzylidyne complex [Mo(tCPh)(CO)2{HB(pzMe2)3}] (KBr: νCO 1979, 1890 cm1).25a While the trigonal boron has an acceptor orbital of π-symmetry, this is intimately involved in mesomeric interactions with the catecholato π-system and the CB σ-bond will be inductively polarized in favor of the electronegative carbon. However, it should also be noted that there is a significant hypsochromic shift on moving to a less polar solvent (n-hexane: νCO 2013, 1935 cm1), and so the possibility of weak coordination of THF to the boron should not be discounted. The carbyne resonance expected in the 13C{1H} NMR spectrum of 4 proved elusive, due in part to the direct attachment of the quaternary carbon to quadrupolar boron.
For Piers’ borylcarbyne, 13C enrichment was necessary to identify this resonance (δC 265.0 ppm).19 The 11B{1H} NMR spectrum (C6D6) comprises two resonances due to the borylcarbyne (δB 19.9; cf. 40 ppm for Piers’ borylcarbyne19) and the pyrazolylborate bridgehead boron (δB 9.6). The characterization of 4 included a crystallographic study, the results of which are summarized in Figure 1. The geometric features of the “Mo(CO)2{HB(pzMe2)3}” unit are unremarkable and generally conform to the growing library of structural data for complexes of the form [M(tCR)(CO)2L] (L = HB(pz)3, HB(pzMe2)3).3a The carbyne ligand exerts a significant trans influence on the unique pyrazolyl donor (MoN11 = 2.321(2) Å) relative to that exerted by the carbonyl ligands (Mo1N13 = 2.218(2) Å, Mo1N15 = 2.214(2) Å).22b Angles between the carbyne carbon and carbonyl carbons are acute, while those between the carbyne and pyrazolyl nitrogen donors are obtuse. The departure from linearity of the Mo1C1B1 spine (163.6(2)°) is a commonly encountered phenomenon. Such deviations from linearity are usually attributed to crystal packing effects,2 and for 4 this falls within the range observed for alkylidyne ligands with more conventional hydrocarbyl and positively mesomeric substituents (thiolate, selenolate, and amino groups). The Mo1tC1 bond length of 1.804(2) Å also falls within norms for molybdenum carbyne complexes2a and may be compared with the value of 1.812(2) Å observed by Piers, given the respective covalent radii of molybdenum and tungsten.26,27 Structural data are available for a range of alkynylboronate esters of the form RCtCBO2C6H4,1,28 for which the BC bond lengths span the range 1.5131.524 Å. The observed C1B1 bond length of 1.534(4) Å would therefore appear marginally elongated, presumably as a result of the steric bulk associated with the “Mo(CO)2(Tp*)” unit. Piers’ borylcarbyne displays a somewhat contracted CB bond length (1.512(7) Å), despite the steric bulk of the two C6F5 groups. Since these do not conjugate significantly with the empty boron acceptor orbital (CCBCt torsional angles: 57.4, 77.9°), there is little, if any positive mesomeric stabilization to electroneutralize the strong 3239
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Organometallics Scheme 3. Synthesis of Boryl and Hydroxybutyl Carbyne Complexes (Tp* = HB(pzMe2)3)
negatively inductive influence of the C6F5 substituents. Accordingly, the B(C6F5)2 group acts as a potent negatively mesomeric (M) substituent, such that the short CB bond presumably reflects greater t C(π)B(π) overlap. The borylcarbyne ligand has two acceptor orbitals which by virtue of the negatively mesomeric boryl group are nondegenerate and present the converse of the usual situation observed for aminocarbynes. Thus, one of these is conjugated into the O2C6H4 π-system, while the orthogonal partner includes contributions from orbitals of the O2C6H4 σ-framework. The orientation that is adopted by the “BO2C6H4” heterocycle would, however, appear to be that which aligns the carbyne acceptor orbitals with the coordination axes defined by the pseudo-octahedral MoN3C3 donor set. This is also observed for [Mo(tCNEt2)(CO)2{HB(pzMe2)3}] but is in contrast with the geometries of [Mo(tCDMAP)(CO)2{HB(pzMe2)3}]þ and the aryl derivatives [M(tCC6H3Me3-2,4,6)(CO)2{HB(pzMe2)3}] (M = Mo, W) and [W(tCC6H4Me-4)(CO)2{HB(pzMe2)3}] (Figure 2),29,30 where the carbyne substituents lie in a plane straddled by the two carbonyl ligands to minimize interligand steric interactions. The synthetic route employed for the synthesis of 4 might appear to promise broad generality; however, this proved not to be the case. A range of haloboranes was investigated, and these were found either to not provide tractible products or alternatively (and most commonly) to afford mixtures of the binuclear vinylidene 6 and the new carbyne complex 7 independent of the haloborane employed (BF3 3 OEt2, BCl3, BPhCl2, B(BBN)Br, B(NiPr2)2Cl, B(tBu-NCHdCHNtBu)Br, B(NiPr2)Br2, BN(SiMe3)2Br2, BH2OTf 3 PCy3, BH2Br.PCy3). The complex 7 was identified as the hydroxypentylidyne complex [Mo{tC(CH2)4OH)}(CO)2{HB(pzMe2)3}] (7), the formation of which may be rationalized in terms of a Lewis acid (i.e., boron) mediated ring opening of the THF solvent (Scheme 3). Trichloroborazine, (ClBNH)3, simply acted as a Brønsted acid to afford predominantly the vinylidene
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complex 6. We have not yet successfully identified an effective solvent other than tetrahydrofuran for the lithiumhalogen exchange that converts 3b to 2b. Many of the haloboranes employed are capable of coordinating THF, even if only transiently, thereby activating the CO bond of the solvent to nucleophilic ring opening. Reasoning that, to suppress such a reaction, the electrophilicity of the borane needed to be curtailed, we explored the reactions of 2b with less sterically demanding aminoboranes. A clean reaction was observed with ClB(NMe2)2 to afford the aminoborylcarbyne complex [Mo{tCB(NMe2)2}(CO)2{HB(pzMe2)3}] (8) in good yield. Complex 8 is considerably more robust (thermally and kinetically) than 4, and while rapidly hydrolyzed to 6 by moist solvents or air, it appears indefinitely stable under an inert atmosphere. Spectroscopic data for 8 are essentially comparable to those for 4, though it is worth noting that only one methyl resonance is observed in the 1H NMR spectrum, suggesting that (i) rotation about the BN bonds is rapid on the 1H NMR time scale and (ii) rotation about the CB bond is also rapid. In support of this, we note that only a single NCH3 1H resonance was observed for the propynyl analogue MeCtCB(NMe2)2 (CDCl3: δH 2.70; δB 23.8; δC(NCH3) 40.5).31 Indeed, all NMR data associated with the B(NMe2)2 unit correlate rather well with those for compound 8 (C6D6: δH 2.69; δB 25.0; δC(NCH3) 40.4). In conclusion, organoboranes have become increasingly important reagents in organic synthesis, due to their utility in palladium-mediated CC bond forming reactions: e.g., the classical SuzukiMiyaura reaction.32 Such reactions have been extended to alkynylboron reagents,33 and given the facile hydrolysis of 4 and 8 and the implicit reactivity of the tCB linkage, the possibility that borylcarbynes might serve as carbyne transfer reagents is a promising avenue which we are currently exploring.
’ ASSOCIATED CONTENT Supporting Information. A CIF file giving crystallographic data for 4 (CCDC 816151) and text and figures giving synthetic procedures and characterization data for compounds 4, 7, and 8. This material is available free of charge via the Internet at http://pubs.acs.org.
bS
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported Australian Research Council (Nos. DP0556236 and DP1093516). ’ REFERENCES (1) (a) Chrostowska, A.; Maciejczyk, M.; Dargelos, A.; Baylere, P.; Weber, L.; Werner, V.; Eickhoff, D.; Stammler, H.-G.; Neumann, B. Organometallics 2010, 29, 5192. (b) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.; Brockhinke, A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009, 2823. (c) Weber, L. Coord. Chem. Rev. 2008, 252, 1. (2) (a) Bachler, V.; Metzler-Nolte, N. Eur. J. Inorg. Chem. 1998, 733. (b) Radom, L.; Vincent, M. A. Isr. J. Chem. 1980, 19, 305.(c) Generated by SPARTAN at the B3LYP/6-311þG* level of theory. (3) Reviews on carbyne chemistry include: (a) Caldwell, L. M. Adv. Organomet. Chem. 2008, 56, 1. (b) Herndon, J. W. Coord. Chem. Rev. 3240
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(25) (a) Brower, D. C.; Stoll, M.; Templeton, J. L. Organometallics 1989, 8, 2786. (b) Desmond, T.; Lalor, F. J.; Ferguson, G.; Parvez, M. J. Chem. Soc., Chem. Commun. 1984, 75. (26) Within the [Mo(tCR)(CO)2{HB(pzMe2)3}] series of compounds, deviations from linearity as large as 18° have been observed: (a) Caldwell, L. M.; Hill, A. F.; Wagler, J.; Willis, A. C. Dalton Trans. 2008, 3538. (b) Caldwell, L. M.; Hill, A. F.; Rae, A. D.; Willis, A. C. Organometallics 2008, 27, 341. (27) (a) The currently popular values of 1.54 Å (Mo) and 1.62 Å (W) follow from a statistical analysis of data held by the Cambridge Crystallographic Data Centre.27b As such, they are prone to overrepresenting particular classes of prevalent compounds. The alternative values of 1.38 Å (Mo) and 1.37 Å (W) suggested by Pyykk€o27c would seem more consistent with similar Melement bond lengths found for low-valent molybdenum and tungsten analogues. (b) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverría, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832. (c) Pyykk€o, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 186. (28) (a) Gu, Y.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2001, 373. (b) Schulz, H.; Gabbert, G.; Pritzkow, H.; Siebert, W. Chem. Ber 1993, 126, 1593. (c) Goswami, A.; Maier, C.-J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2004, 2635. (d) Goswami, A.; Pritzkow, H.; Rominger, F.; Siebert, W. Eur. J. Inorg. Chem. 2004, 4223. (29) Cordiner, R. L.; Hill, A. F.; Wagler, J. Organometallics 2008, 27, 4532. (30) (a) Wadepohl, H.; Arnold, U.; Pritzkow, H.; Calhorda, M. J.; Veiros, L. F. J. Organomet. Chem. 1999, 587, 233. (b) Anderson, S.; Cook, D. J.; Hill, A. F.; Malget, J. M.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 2552. (31) (a) Feulner, H.; Metzler, N.; N€oth, H. J. Organomet. Chem. 1995, 489, 51. (b) Wrackmeyer, B.; N€oth, H. Chem. Ber. 1977, 110, 1086. (32) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979, 20, 3437. (33) Ishida, N.; Shinmoto, T.; Sawano, S.; Miura, T.; Murakami, M. Bull. Chem. Soc. Jpn. 2010, 83, 1380.
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