Boratocarbyne Complexes: Li[Mo{≡CB(OMe)3 ... - ACS Publications

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Boratocarbyne Complexes: Li[Mo{CB(OMe)3}(CO)2{HB(pzMe2)3}] and [K(18-crown-6)][Mo(CBF2OMe)(CO)2{HB(pzMe2)3}] (pz = pyrazol-1-yl) Anthony F. Hill* and Rong Shang Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia S Supporting Information *

ABSTRACT: The successive treatment of [Mo(CBr)(CO)2{HB(pzMe2)3}] (1; pz = pyrazol-1-yl) with nBuLi and B(OR)3 (R = Me, iPr) affords, via the lithiocarbyne complex [Mo(CLi)(CO)2{HB(pzMe2)3}] (2), the boratocarbynes Li[Mo{ CB(OMe)3}(CO)2{HB(pzMe2)3}] (R = Me, Li[3a]; R = iPr, Li[3b]). The salt Li[3b] readily eliminates LiOiPr to afford the neutral boryl carbyne [Mo{ CB(OiPr)2}(CO)2{HB(pzMe2)3}] (4b). The analogous boryl carbyne [Mo{ CB(OMe)2}(CO)2{HB(pzMe2)3}] (4a) on treatment with K[HF2] and 18-crown-6 (18C6) provides the structurally characterized difluoro(methoxy)boratocarbyne salt [K(18C6)][Mo(CBF2OMe)(CO)2{HB(pzMe2)3}] ([K(18C6)][6]).

W

Scheme 1. Synthesis of Boryl Carbynes3a,6a

e have recently investigated lithium/halogen exchange reactions involving the halocarbyne1 complexes [M( CBr)(CO)2{HB(pzMe2)3}] (M = Mo (1), W; pz = pyrazol-1yl) which result in carbyne umpolung and formation of the THF-solvated lithiocarbyne complexes [M{CLi(THF)n}(CO)2{HB(pzMe2)3}] (M = Mo (2a), W).2 The complex 2 enters into a wide range of reactions with electrophiles, allowing the isolation of carbyne complexes bearing heteroatom C substituents not readily introduced by the more familiar and time-honored strategies for carbyne synthesis.3 Boron is a case in point: while the N-heterocyclic boryl anions [(HCNR)2B]− (R = C6H2Me3, C6H3iPr2) have recently been observed and their reactivity explored by Yamashita,4 exaggerated steric bulk is required for the boron amino substituents (R) so as to afford sufficient kinetic stabilization. This in turn makes these anions inappropriate for conventional Fischer carbyne synthesis protocols5 which require nucleophilic attack at a metal carbonyl. Prior to our own studies one boryl-substituted carbyne had been described by Piers,6 arising from the reaction of Schrock’s parent methylidyne [W(CH)(dmpe)2Cl] with HB(C6F5)2 followed by hydride abstraction (Scheme 1). With the viability of borylcarbynes, albeit one, having been established by Piers and appreciating that organoborates are versatile transmetalating agents, e.g., in the celebrated Suzuki− Miyaura coupling protocol,7 we have considered whether boratocarbyne complexes might be accessible via the expedient intermediacy of the lithiocarbyne complex 2. We have previously shown that the boryl carbyne complexes [Mo{ CB(NMe2)2}(CO)2{HB(pzMe2)3}] and [Mo(CBO2C6H4)(CO)2{HB(pzMe2)3}] may be synthesized via the reaction of 2 with ClB(NMe2)2 or ClBO2C6H4 (Scheme 1) but that the approach fails for many haloboranes, due to competing reactions with the solvent (THF).3a The key to success for this approach appears to lie in the boron electrophile having a modest steric profile, coupled with π-dative (positively © 2012 American Chemical Society

a

Abbreviations: Arf = C6F5; Tp* = hydrotris(3,5-dimethylpyrazol-1yl)borate). Legend: (i) HBRf 2; (ii) [Ph3C][BArf4]; (iii) nBuLi; (iv) ClBR2.

mesomeric) substituents that reduce the electrophilicity of the boron with respect to interaction with the THF solvent. Herein we report that the first boratocarbyne complexes [LnMCBR3]− are indeed available via the lithium/halogen exchange protocol beginning with 2. Received: March 22, 2012 Published: June 20, 2012 4635

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field position of the resonance for Li[3b] is especially noteworthy in that it approaches the region typical of bimetallic carbido species (tabulated in ref 3c). Although spectroscopic data for Li[3b] akin to those for Li[3a] could be extracted from spectra obtained from this mixture, on standing in solution for prolonged periods Li[3b] converts completely to 4b, which in turn eventually decomposes to [Mo2(μ-C2H2)(CO)4{HB(pzMe2)3}2].3 In contrast, alkynyltriisopropoxyborates are thermally stable and may be deployed in palladium crosscoupling reactions.11 In search of a crystalline derivative of a boratocarbyne, we investigated the reaction of the dimethoxyborylcarbyne complex [Mo{CB(OMe)2}(CO)2{HB(pzMe2)3}] (4a)12 with potassium bifluoride. When this procedure is applied to simple organodi(alkoxy)boranes RB(OMe)2 or organotri(alkoxy)borates, it generally affords the corresponding crystalline and hydrolytically robust organotrifluoroborate salts K[RBF3],13 which are enjoying increasing popularity as boronate ester alternatives in modified Suzuki−Miyaura procedures. Treating 4a with excess K[HF2] in THF afforded the salt K[Mo{CBF2(OMe)}(CO)2{HB(pzMe2)3}] (K[5]), and while this was not especially crystalline, large yellow crystals of [K(18C6)][Mo{CBF 2 (OMe)}(CO)2{HB(pzMe 2 ) 3}] ([K(18-C-6)][6]) could be grown after addition of 1,4,7,10,13,15-hexaoxaoctadecane (18C6) to the reaction mixture. The formulation followed from elemental microanalytical and spectroscopic data,14 among which the presence of a singlet resonance at δH 3.94 in the 1H NMR spectrum indicated by signal integration that one methoxy group had been retained. Treating [K(18C6)][6] with excess K[HF2] for 5 days failed to result in discernible substitution of the final methoxy group. The 11B{1H} NMR spectrum comprised two broad resonances (CD2Cl2: δB −3.62 (1JFB not resolved), −10.5) attributable to the fluoroborate and pyrazolylborate boron nuclei, respectively. The 13C{1H} and 1H NMR data obtained from C6D6 solution indicate that the molecule has no element of symmetry, with two disinct CO environments and three pyrazolyl environments being observed. This would seem consistent with the association of the potassium with one fluoride and one methoxide group (vide infra) and accounts for the solubility in this nonpolar solvent as a result of persistent ion pairing. The failure to observe 1H signal coalescence over the temperature range −50 to +50 °C (CDCl3) is however surprising in that one might expect a low-energy fluxional process to allow the potassium to move easily between fluoride bridges. Not surprisingly, the 13C resonance for the carbyne carbon was not identified, due to a combination of its triplet multiplicity and quadrupolar broadening by the 11B nucleus. Two νCO-associated absorptions were observed in the infrared spectrum (THF: 1968, 1876 cm−1) close to those found for Li[3a], indicating a high electron density on the molybdenum center. The characterization of [K(18C6)][6] included a crystallographic study,14c the results of which are summarized in Figure 1. The geometric features of the “Mo(CO)2{HB(pzMe2)3}” unit call for little comment and generally conform to the wealth of structural data for complexes of the form [M(CR)(CO)2L] (L = HB(pz)3, HB(pzMe2)3).1a The crystal structure does however reveal that, in the solid state, the ions pair such that boron and potassium are bridged by one fluoride and one methoxide group, though the terminal and bridging B−F bonds do not differ significantly in length. The angles at boron are

Treating a solution of 1 with nBuLi at low temperature followed by the addition of trimethylborate results in the formation of a new yellow microcrystalline salt formulated as [Li(THF)n][Mo{CB(OMe)3}(CO)2{HB(pzMe2)3}] ([Li(THF)n][3a]; Scheme 2),8 on the basis of spectroscopic data Scheme 2. Synthesis of Borato Carbynesa

a Legend: (i) nBuLi; (ii) ClB(OMe)2; (iii) K[HF2], 18C6; (iv) B(OiPr)3; (v) B(OMe)3; (vi) −LiOiPr.

which also indicate residual, nonstoichiometric, THF presumably solvating the lithium (implicit, hereafter). Consistent with the anionic nature of the complex and the electropositivity of the boron substituent, the IR spectrum comprises two intense νCO absorptions at comparatively low frequency (THF: 1967, 1875 cm−1). Although the 13C resonance for the quaternary carbyne carbon was not reliably identified, due to quadrupolar broadening by the boron nucleus,9 the 11B{1H} NMR spectrum revealed two resonances (CD2Cl2: δB −2.05 (BOMe), −9.53 (Bpz)) in a region typical of four-coordinate boron. Neither satisfactory elemental microanalytical nor crystallographic data were successfully obtained for (solvated) Li[3a], and accordingly the analogous reaction with B(OiPr)3 was investigated in the hopes of obtaining a more crystalline derivative. This reaction, however, resulted in the formation of a mixture of two compounds10the desired boratocarbyne salt Li[Mo{CB(O i Pr) 3 (CO) 2 {HB(pzMe 2 ) 3 }] Li[3b] (δ B −2.55)10 and the neutral borylcarbyne complex [Mo{ CB(OiPr)2}(CO)2{HB(pzMe2)3}] (4b) (δB 15.0);10 cf. the structurally characterized analogue [Mo{CBO2C 6H4}(CO)2{HB(pzMe2)3}] (δB 19.9).3a In the case of Li[3b] and 4b, broadened 13C resonances could be observed for the carbyne carbon at 407.8 and 348.8 ppm, respectively. The low4636

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close to tetrahedral (104.7(3)−114.8(3)°), the largest deviation being the slight opening of the angle between the oxygen and carbyne carbon (C1−B1−O1 = 114.8(3)). The angle at C1 is bent somewhat from linear (170.8(2)°); however, such distortions are common in structures of the form [M( CR)(CO)2L]1a,15 and the Mo1−C1 separation of 1.795(3) Å falls within norms. Of the large number of potassium organotrifluoroborate salts that are known,13 a small number have been structurally characterized,16,17 including examples in which the potassium is ligated by a crown ether17 but still retains one, two, or three essentially electrostatic B···F contacts (typically rKF = 2.61−2.93 Å). Despite their demonstrated synthetic utility,11,18 structural data for alkynylfluoroborates are not yet available19 while those for the CB(μ-F)(μ-O)K connectivity are limited to the KF adduct of Reetz’ ditopic crown ether 1-(C6H4O2B)C6H3(CH2OCH2)6-2,6.20,21 In conclusion, organoboranes have become increasingly important reagents in organic synthesis, due to their utility in palladium-mediated C−C bond forming reactions: e.g., the classical Suzuki−Miyaura reaction.13 Such reactions have been extended to alkynyl boron reagents,20 and given the facile hydrolysis of 4 and the implicit reactivity of the C−B linkage, the possibility that boryl carbynes might serve as carbyne transfer reagents is a promising avenue that we are currently exploring.

ASSOCIATED CONTENT

* Supporting Information S

A CIF file giving crystallographic data for [K(18C6)][6]·C6H6 (CCDC 867550). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Reviews on carbyne chemistry include: (a) Caldwell, L. M. Adv. Organomet. Chem. 2008, 56, 1. (b) Herndon, J. W. Coord. Chem. Rev. 2007, 251, 1158. (c) Herndon, J. W. Coord. Chem. Rev. 2003, 243, 3. (d) Herndon, J. W. Coord. Chem. Rev. 2001, 214, 215. (e) Mayr, A.; Hoffmeister, H. Adv. Organomet. Chem. 1991, 32, 227. (f) Kim, H. P.; Angelici, R. J. Adv. Organomet. Chem. 1987, 27, 51. (g) Schrock, R. R. Chem. Commun. 2005, 2773. (h) Schrock, R. R. Chem. Rev. 2002, 102, 145. (i) Schrock, R. R.; Czekelius, C. Adv. Synth. Catal. 2007, 349, 55. (2) Cordiner, R. L.; Hill, A. F.; Wagler, J. Organometallics 2008, 27, 5177. (3) (a) Hill, A. F.; Shang, R.; Willis, A. C. Organometallics 2011, 30, 3237. (b) Colebatch, A. L.; Hill, A. F.; Shang, R.; Willis, A. C. Organometallics 2010, 29, 6482. (c) Colebatch, A. L.; Cordiner, R. L.; Hill, A. F.; Nguyen, K. T. H. D.; Shang, R.; Willis, A. C. Organometallics 2009, 28, 4394. (d) Cordiner, R. L.; Hill, A. F.; Wagler, J. Organometallics 2008, 27, 4532. (e) Cordiner, R. L.; Hill, A. F.; Shang, R.; Willis, A. C. Organometallics 2011, 30, 139. (f) Hill, A. F.; Sharma, M.; Willis, A. C. Organometallics 2012, 31, 2538. (4) Segawa, Y.; Yamashita, M.; Nozaki, K. Science 2006, 314, 113. (5) Fischer, E. O. Adv. Organomet. Chem. 1976, 14, 1. (6) van der Eide, E. F.; Piers, W. E.; Romero, P. E.; Parvez, M.; McDonald, R. Organometallics 2004, 23, 314. (7) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979, 20, 3437. (8) 3a: to a solution of 1 (0.20 g, 0.37 mmol) in THF (12 mL) at −78 °C was slowly added n-butyllithium (0.37 mmol). After 30 min of stirring, trimethoxyborane (0.04 mL, 0.37 mmol) was added by syringe. After it was stirred for a further 10 min, the mixture was gradually warmed to room temperature; a bright orange solution was obtained and was stirred at room temperature for a further 30 min. Cooling (−18 °C) yielded 3a as a yellow crystalline solid that was isolated by decantation, washed with n-hexane and diethyl ether, and dried in vacuo. Yield: 0.17 g (82%, 0.30 mmol). IR (νCO, cm−1): THF, 1967, 1875; Nujol, 1969, 1881. NMR (CD2Cl2, 25 °C): 1H, δH 2.34, 2.38, 2.40, 2.78 (s br × 4, 18 H, pzCH3), 3.54 (s br, 9 H, OCH3), 5.75 (s br, 1 H, pzH), 5.94 (s br, 2 H, pzH); 13C{1H}, δC 230.6 (br, CO), 151.43, 145.31 (C3,5(pz)), 106.3 (C4(pz)), 51.32, 50.94 (OMe), 15.47, 14.36, 13.04, 12.61 (pzCH3); 11B{1H}, δB −2.05 (BOMe), −9.53 (Bpz). MS-ESI (negative ion): m/z 510.02 [M − 2CO]−, 564.19 [M]−, 607.12 [M + NCMe]−. (9) Similar problems were encountered by Piers,6 though in that case, the synthetic protocol was more suited to 13C enrichment of the sample, allowing the identification of the boratocarbyne resonance at δC 265. (10) Li[3b]/4b: as described for 3a, triisopropoxyborane (0.09 mL, 0.37 mmol) afforded a bright orange solution that was freed of volatiles and extracted with n-pentane (15 mL); the extract was filtered and freed of volatiles to provide a red-brown solid that comprised a mixture of Li[3b] and 4b. Yield: 0.15 g (ca. 64% combined). Data for Li[3b] are as follows. IR (νCO, cm−1): THF, 1971, 1878. NMR (C6D6, 25 °C): 13C{1H}, δC 407.8 (MoC), 233.2 (CO), 151.8, 143.9, 143.0 (C3,5(pz)), 106.1, 105.9 (C4(pz)), 67.89, 25.69 (CHMe2), 17.72, 14.46, 13.04, 12.70 (pzCH3); 11B{1H}, δB −2.55 (BOiPr), −10.3 (Bpz). Data for 4b are as follows. IR (νCO, cm−1): THF, 2000, 1913. NMR (C6D6, 25 °C): 1H, δH 1.18 (d, 12 H, 3JHH = 6 Hz, CHCH3), 2.08, 2.12, 2.37, 2.72 (s × 4, 18 H, pzCH3), 4.94 (hept, 2 H, OCH), 5.40 (s, 1 H, pzH), 5.60 (s, 2 H, pzH); 13C{1H}, δC 348.8 (MoC), 229.0 (CO), 151.3, 150.9, 144.7, 144.4 (C3,5(pz)), 106.6, 106.5 (C4(pz)), 66.54, 24.81 (CHCH3), 16.23, 14.40, 12.62, 12.42 (pzCH3); 11 1 B{ H}, δB 15.0 (BOiPr), −10.2 (Bpz). Attempts to obtain Li[3b] or 4b in pure form were confounded by the spontaneous conversion of Li[3b] to 4b and the slow conversion of 4b to [Mo2(μ-CCH2) (CO)4{HB(pzMe2)3}2].3a (11) Oh, C. H.; Jung, S. H. Tetrahedron Lett. 2000, 41, 8513. (12) The complex 4a was prepared via the reaction of 2 with ClB(OMe)2, as described previously for the synthesis of the complex [Mo(CBO2C6H4)(CO)2{HB(pzMe2)3}].3a

Figure 1. Molecular structure of one molecule of [K(18C6)][6] in a crystal of [K(18C6)][6]·C6H6 (50% displacement ellipsoids; hydrogen atoms and solvate omitted). Selected bond distances (Å) and angles (deg): Mo1−C1 = 1.795(3), Mo1−N11 = 2.406(2), Mo1−N21 = 2.240(2), Mo1−N31 = 2.233(2), O1−B1 = 1.435(4), C1−B1 = 1.643(5), F1−B1 = 1.419(5), F2−B1 = 1.406(4); Mo1−C1−B1 = 170.9(2), N11−Mo1−C2 = 91.47(10), N21−Mo1−C2 = 96.87(10), C1−Mo1−C2 = 81.26(12), C4−O1−B1 = 117.9(3), F1−B1−F2 = 109.0(3), C1−B1−O1 = 114.7(3), ∑(NMoN) = 242.9°.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council (Grant Nos. DP0556236 and DP1093516). 4637

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(13) Darses, S.; Michaud, G.; Genet, J.-P. Eur. J. Org. Chem. 1999, 1875. (14) [K(18C6)][6]: a mixture of 4a (0.053 g, 0.10 mmol), 18-crown6 (0.026 g, 0.10 mmol), and excess K[HF2] (0.03 g, 0.38 mmol) was stirred in THF (3 mL) for 24 h. The orange solution was freed of volatiles, and the residue was recrystallized from benzene as pale yellow needles of a benzene monosolvate. Yield: 0.054 g (58%, 0.058 mmol). IR (νCO, cm−1): THF, 1968, 1876. NMR (C6D6, 25 °C): 1H, δH 2.19, 2.22, 2.61, 3.06, 3.09, 3.16 (s × 6, 18 H, pzCH3), 3.19 (s, 24 H, OCH2), 3.94 (s, 3 H, OCH3), 5.50 5.62, 5.66 (s × 3, 3 H, pzH); 13 C{1H}, δC 230.1, 229.2 (CO), 151.5, 151.4, 150.7, 143.7, 143.4 (C3,5(pz)), 106.0, 105.9 (C4(pz)), 70.18 (OCH2), 37.04 (OCH3), 16.57, 14.60, 12.83, 12.78, 12.50 (pzCH3); 11B{1H}, δB −3.62 (br, BF2), −10.5 (Bpz). ESI-MS (negative ion): m/z 543.7 [M]−, 528.7 [M − CH3]−. Anal. Found: C, 43.90; H, 5.78; N, 10.10. Calcd for C37H55B2F2KMoN6O9: C, 44.09; H, 5.85; N, 9.95. Crystal data for [K(18C6)][6]·C6H6: C37H55B2F2KMoN6O9. Mw = 922.53, orthorhombic, Pbca, a = 19.4593(3) Å, b = 18.6396(2) Å, c = 25.1695(4) Å, V = 9129.3(2) Å3, Z = 8, F000 = 3840, Dcalcd = 1.342 Mg m−3, μ(Mo Kα) = 0.441 mm−1, T = 200(2) K, pale yellow block, 0.16 × 0.16 × 0.10 mm, 10 164 independent reflections, F2 refinement, R1 = 0.026, wR2 = 0.107 for 81 093 absorption-corrected reflections with I > 2σ(I), 2θmax = 60°, 578 parameters, CCDC 867550. (15) Angles as small as 163° have been observed: Caldwell, L. M.; Hill, A. F.; Wagler, J.; Willis, A. C. Dalton Trans. 2008, 3538. (16) (a) Zukerman-Schpector, J.; Guadagnin, R. C.; Stefani, H. A.; Visentin, L. C. Acta Crystallogr., Sect. E: Struct. Rep. Online 2008, 64, m1525. (b) Brauer, D. J.; Burger, H.; Pawelke, G. J. Organomet. Chem. 1982, 238, 267. (c) Chase, P. A.; Henderson, L. D.; Piers, W. E.; Parvez, M.; Clegg, W.; Elsegood, M. R. J. Organometallics 2006, 25, 349. (d) Vieira, A. S.; Fiorante, P. F.; Zukerman-Schpector, J.; Alves, D.; Botteselle, G. V.; Stefani, H. A. Tetrahedron 2008, 64, 7234. (e) Brauer, D. J.; Burger, H.; Pawelke., G. Inorg. Chem. 1977, 16, 2305. (f) Thadani, A. N.; Batey, R. A.; Smil, D. V.; Lough, A. J. Acta Crystallogr., Sect. E: Struct. Rep. Online 2001, 57, m333. (g) Franz, D.; Wagner, M.; Lerner, H.-W.; Bolte, M. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2010, 66, m152. (h) Conole, G.; Clough, A.; Whiting, A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1995, 51, 1056. (17) (a) Groux, L. F.; Weiss, T.; Reddy, D. N.; Chase, P. A.; Piers, W. E.; Ziegler, T.; Parvez, M.; Benet-Buchholz, J. J. Am. Chem. Soc. 2005, 127, 1854. (b) Hudnall, T. W.; Bondi, J. F.; Gabbai, F. P. Main Group Chem. 2006, 5, 319. (c) Fei, Z.; Zhao, D.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. Eur. J. Inorg. Chem. 2005, 860. (d) Boshra, R.; Venkatasubbaiah, K.; Doshi, A.; Lalancette, R. A.; Kakalis, L.; Jakle, F. Inorg. Chem. 2007, 46, 10174. (18) (a) Molander, G. A.; Katona, B. W.; Machrouhi, F. J. Org. Chem. 2002, 67, 8416. (b) Daniels, D. S. B.; Thompson, A. L.; Anderson, E. A. Angew. Chem., Int. Ed. 2011, 50, 11506. (19) Cambridge Crystallographic Data Centre, Conquest® November 2011 release. (20) Reetz, M. T.; Niemeyer, C. M.; Harms, K. Angew. Chem., Int. Ed. 1991, 30, 1472. (21) This is perhaps surprising and might call into question many reagents considered to be K[R-BF3] but which might in fact be mixed alkoxy/fluoro or hydroxy/fluoro borates. While this might seem of little import for subsequent deployment in catalysis, the intimate mechanism of transmetalation from boron to e.g. palladium is by no means firmly established, especially in the presence of extraneous base: i.e., oxyfluoroborates may play an unappreciated role.

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