Synthesis, Structure, and Reactivity of Alkyl-Substituted Half-Sandwich

Organometallics 2009, 28, 6807–6822 DOI: 10.1021/om900754h

6807

Synthesis, Structure, and Reactivity of Alkyl-Substituted Half-Sandwich η5-Pentadienyl Complexes of Cobalt Kai E. O. Ylijoki, Ross D. Witherell, Andrew D. Kirk, Sebastian B€ ocklein,§ † † Verner A. Lofstrand, Robert McDonald, Michael J. Ferguson, and Jeffrey M. Stryker* Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada. §Department Chemie, Ludwig-Maximilians-Universit€ at M€ uchen, Butenandtstr. 5-13, 81377 Munich, Germany. † Department of Chemistry Structure Determination Laboratory, University of Alberta Received August 28, 2009

Alkyl-substituted η5-pentadienyl complexes of cobalt have been reported to undergo [5 þ 2] cycloaddition reactions with alkynes to form substituted η2,η3- and η5-cycloheptadienyl products, providing a new route to the synthesis of substituted cycloheptadienes. A series of cyclopentadienyl and pentamethylcyclopentadienyl cobalt(III) η5-pentadienyl complexes have been prepared, incorporating alkyl and aryl substituents at various positions on the pentadienyl ligand. The crystalline complexes have been completely characterized spectroscopically and, in the solid state, by X-ray crystallography. The alkyl-substituted pentadienyl complexes can be prepared by a range of methodologies, most generally by acid-promoted dehydration of in situ-derived η2- or η4-dienol complexes. Two variations on this classic strategy have been developed, starting from either conjugated (1,3-) or nonconjugated (1,4-) dienyl alcohols. For both cyclopentadienyl and pentamethylcyclopentadienyl ancillary ligands, the substituted η5-pentadienyl complexes are obtained in reasonable to good isolated yields, limited by the extent of substitution on the starting allylic alcohol. The cationic cobalt(III) η5-pentadienyl complexes are indefinitely stable to air and moisture; isolation and purification is accomplished by chromatography on the bench. The substitutional lability of the pentadienyl ligand has been investigated using both neutral and anionic donor ligands (CO, isonitrile, acetonitrile, and halide salts). The results reveal that η5-pentadienyl complexes react by equilibrium dissociation of the most substituted end of the pentadienyl moiety, providing the corresponding η3-coordinated pentadienyl adducts. A comparative study of 11 X-ray crystal structures covering a range of pentadienyl substitution patterns is also reported, revealing long Co-C1 and Co-C5 bonds and, consistently, short Co-C3 bond lengths. The internal bond angles of substituent-bearing carbon atoms are smaller than those observed at unsubstituted carbons, with the compression as much as 10° at the substituted center. As expected, alkyl substituents invariably deviate out of the pentadienyl plane, toward the metal. In unbiased cases, greater deviations are noted for C1 substituents than for C2 substituents, although the deviations at C2 increase significantly for 1,2-disubstituted ligands. In some cases, the carbon-carbon bond distances display distortions toward η2,η3- or η1,η4-hapticity, although no particular correlation between structural distortion and solution reactivity is evident. Introduction Acyclic and endocyclic “open” η5-pentadienyl complexes have been the subject of intensive research for nearly half a century.1 Originally conceptualized as an isoelectronic but nonaromatic analogue of the ubiquitous η5-cyclopentadienyl ligand, acyclic and endocyclic η5-pentadienyl complexes were assimilated rapidly into organic synthesis, principally

as the electrophilic component in metal-directed nucleophilic alkylation reactions.2 In contrast, the development of metal-mediated cycloaddition reactions incorporating the η5-pentadienyl moiety remains relatively undeveloped,3

*Corresponding author. Tel: (780) 492-3891. Fax: (780) 492-8231. E-mail: [email protected]. (1) Reviews: (a) Ernst, R. D. Chem. Rev. 1988, 88, 1255–1291. (b) Ernst, R. D. Comments Inorg. Chem. 1999, 21, 285–325. (c) Stahl, L.; Ernst, R. D. Adv. Organomet. Chem. 2008, 55, 137–199. (2) Lead references: (a) Chaudhury, S.; Li, S.; Bennett, D. W.; Siddiquee, T.; Haworth, D. T.; Donaldson, W. A. Organometallics 2007, 26, 5295–5303. (b) Barmann, H.; Prahlad, V.; Tao, C.; Yun, Y. K.; Wang, Z.; Donaldson, W. A. Tetrahedron 2000, 56, 2283–2295. (c) Pearson, A. J.; Kole, S. L.; Ray, T. J. Am. Chem. Soc. 1984, 106, 6060–6074. See also ref 5c.

(3) (a) Wilson, A. M.; Waldman, T. E.; Rheingold, A. L.; Ernst, R. D. J. Am. Chem. Soc. 1992, 114, 6252–6254. (b) Kreiter, C. G.; Koch, E.-C.; Frank, W.; Reiss, G. Inorg. Chim. Acta 1994, 220, 77–83. (c) Wang, C.; Sheridan, J. B.; Chung, H. J.; Cote, M. L.; Lalancette, R. A.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 8966–8972. (d) Kreiter, C. G.; Fiedler, C.; Frank, W.; Reiss, G. J. Chem. Ber. 1995, 128, 515–518. (e) Kreiter, C. G.; Fiedler, C.; Frank, W.; Reiss, G. J. J. Organomet. Chem. 1995, 490, 133– 141. (f) Kreiter, C. G.; Koch, E.-C.; Frank, W.; Reiss, G. J. Z. Naturforsch. B 1996, 51B, 1473–1485. (g) Chen, W.; Chung, H.-J.; Wang, C.; Sheridan, J. B.; Cote, M. L.; Lalancette, R. A. Organometallics 1996, 15, 3337–3344. (h) Chung, H.-J.; Sheridan, J. B.; Cote, M. L.; Lalancette, R. A. Organometallics 1996, 15, 4575–4585. (i) Tomaszewski, R.; Hyla-Kryspin, I.; Mayne, C. L.; Arif, A. M.; Gleiter, R.; Ernst, R. D. J. Am. Chem. Soc. 1998, 120, 2959– 2960. (j) Basta, R.; Harvey, B. G.; Arif, A. M.; Ernst, R. D. Inorg. Chim. Acta 2004, 357, 3883–3888. (k) Harvey, B. G.; Mayne, C. L.; Arif, A. M.; Tomaszewski, R.; Ernst, R. D. J. Am. Chem. Soc. 2005, 127, 16426–16435.

r 2009 American Chemical Society

Published on Web 11/04/2009

pubs.acs.org/Organometallics

6808

Organometallics, Vol. 28, No. 23, 2009

particularly for reactions involving the late transition metals. We recently reported a cobalt-mediated [5 þ 2] η5-pentadienyl/alkyne cycloaddition reaction that proceeds with complete control of alkyne stoichiometry and ring size,4 leading to the formation of cobalt cycloheptadienyl complexes cleanly and in high yield (Scheme 1). In many cases, the product can be obtained under kinetic control as the η2,η3-cycloheptadienyl isomer 2, a previously unprecedented coordination mode for monocyclic seven-membered ring compounds. Upon gentle heating or, in some cases, upon standing at room temperature, the kinetic isomer undergoes quantitative conversion to the thermodynamically more stable η5-cycloheptadienyl complex 3. Although η5-pentadienyl complex 1 is coordinatively saturated, the cycloaddition proceeds under unusually mild conditions, provided that the pentadienyl ligand bears a terminal substituent. The mechanistic implication is that initial η5fη3 isomerization of the pentadienyl ligand is necessary to open a coordination site for the alkyne, a process clearly sensitive to the substituent array on the pentadienyl ligand. As a consequence of both the synthetic potential of this novel cycloaddition reaction and our preliminary mechanistic observations, we have investigated the synthesis, structure, and reactivity of half-open cobaltocenium complexes bearing ancillary η5-cyclopentadienyl (Cp) or η5-pentamethylcyclopentadienyl (Cp*) ligands and a range of alkyl- and aryl-substituted η5-pentadienyl ligands. As reported here, several strategies for pentadienyl complex synthesis have been delineated, two with substantial generality. The compounds have been characterized in solution spectroscopically and, in many cases, in the solid state by X-ray crystallography, providing a rational basis for the continued development of this novel organic reaction. Acyclic η5-pentadienyl complexes of cobalt, rhodium, and iridium are reasonably abundant and have been prepared by a range of synthetic strategies.5,6 Nonetheless, only a limited number of pentadienyl substitution patterns have been reported, dominated by the unsubstituted pentadienyl and 2,4-dimethylpentadienyl ligands. Potentially the most general synthetic approach is the coordination and protolytic dehydration of conjugated dienyl alcohols, a method first (4) Witherell, R. D.; Ylijoki, K. E. O.; Stryker, J. M. J. Am. Chem. Soc. 2008, 130, 2176–2177. (5) Rh/Ir η5-pentadienyl complexes: (a) White, C.; Thompson, S. J.; Maitlis, P. M. J. Organomet. Chem. 1977, 134, 319–325. (b) Powell, P.; Russell, L. J. J. Chem. Res. 1978, 283, 3652–3672. (c) Powell, P. J. Organomet. Chem. 1979, 165, C43–C46. (d) Powell, P. J. Organomet. Chem. 1981, 206, 239–255. (e) Powell, P. J. Organomet. Chem. 1983, 244, 393– 399. (f) Powell, P.; Stephens, M.; Muller, A. J. Organomet. Chem. 1986, 310, 255–268. (g) Bleeke, J. R.; Donaldson, A. J. Organometallics 1988, 7, 1588–1596. (h) Bleeke, J. R.; Luaders, S. T. Organometallics 1995, 14, 1667–1673. (i) M€ uller, J.; Schiller, C.; Gaede, P. E.; Kempf, M. Z. Anorg. Allg. Chem. 2005, 631, 38–46. 5 (6) Co η -pentadienyl complexes: (a) Bleeke, J. R.; Peng, W.-J. Organometallics 1984, 3, 1422–1426. (b) Wilson, D. R.; Ernst, R. D.; Kralik, M. S. Organometallics 1984, 3, 1442–1444. (c) Bleeke, J. R.; Peng, W.-J. Organometallics 1986, 5, 635–644. (d) Lee, G.-H.; Peng, S.-M.; Liao, M.-Y.; Liu, R.-S. J. Organomet. Chem. 1986, 312, 113–120. (e) Bleeke, J. R.; Donaldson, A. J.; Peng, W.-J. Organometallics 1988, 7, 33–37. (f) Ernst, R. D.; Ma, H.; Sergeson, G.; Zahn, T.; Ziegler, M. L. Organometallics 1987, 6, 848–853. (g) Bennett, M. A.; Nicholls, J. C.; Rahman, A. K. F.; Redhouse, A. D.; Spencer, J. L.; Willis, A. C. J. Chem. Soc., Chem. Commun. 1989, 1328–1330. (h) Nicholls, J. C.; Spencer, J. L. Organometallics 1994, 13, 1781–1787. (i) Cracknell, R. B.; Nicholls, J. C.; Spencer, J. L. Organometallics 1996, 15, 446–448. (j) Butovskii, M. V.; Englert, U.; Herberich, G. E.; Kirchner, K.; Koelle, U. Organometallics 2003, 22, 1989–1991.

Ylijoki et al. Scheme 1

reported by Petit for the preparation of η5-pentadienyl iron complexes7 and subsequently adapted to the synthesis of η5pentadienyl complexes of rhodium and iridium.5b-f Low-6a-e and high-valent6f-j η5-pentadienyl complexes of cobalt have been reported, although Co(III) η5-pentadienyl complexes are relatively rare. Most arise from serendipitous electrophilic carbon-carbon bond activation reactions unsuitable for generalization.6g-j To develop a general synthesis of differentially substituted η5-pentadienyl cobalt(III) complexes, we investigated adapting the dienyl alcohol coordination/protonolysis protocol to cobalt. Here we report two variations on this theme, each of considerable generality, along with an interesting, if less general, oxidative addition/isomerization pathway. Fundamental aspects of pentadienyl reactivity have been determined, and an extensive series of η5-pentadienyl complexes has been characterized by X-ray crystallography, providing valuable insights into structure/reactivity relationships in substituted η5-pentadienyl cobalt(III) complexes.

Results and Discussion Oxidative Addition/Isomerization of 3-Bromo-1,5-hexadiene. One expedient entry into the synthesis of 1-substituted η5-pentadienyl complexes of cobalt can be realized by exploiting stereochemical and structural isomerizations common to substituted η3-allyl complexes of the late transition metals. Thus, 3-bromo-1,5-hexadiene reacts rapidly and quantitatively with the labile cobalt(I) source (C5Me5)Co(CH2dCHSiMe3)28 (4) to give the syn-substituted 1-(2propenyl)-η3-allyl complex 5, along with a minor amount of the conjugated syn-1-(1-propenyl)-η3-allyl isomer 6-Z, which is obtained exclusively as the Z-alkene (Scheme 2). The minor product cannot be suppressed at lower temperature; changing reaction conditions has no appreciable affect on the product ratio. The crude mixture recrystallizes repeatedly as a mixture, without impact on the product ratio. Slow recrystallization affords analytically pure single crystals of the mixture suitable for X-ray crystallography. The structure determination reflects what appears to be the statistically dominant component 5 alone, although it is possible that it is an average of 5 and contaminant 6-Z (Figure 1).9 Upon mild thermolysis in an ionizing medium, complete isomerization to the conjugated syn-1-(1-propenyl)-η3-allyl complex 6 is obtained, albeit as a mixture of E- and Z-alkenes, slightly favoring the E-isomer. Although the (7) (a) Mahler, J. E.; Pettit, R. J. Am. Chem. Soc. 1963, 85, 3955–3959. (b) Mahler, J. E.; Gibson, D. H.; Pettit, R. J. Am. Chem. Soc. 1963, 85, 3959– 3963. (8) (a) Lenges, C. P.; Brookhart, M.; Grant, B. E. J. Organomet. Chem. 1997, 528, 199–203. (b) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 6965–6979. (c) Lenges, C. P.; White, P. S.; Marshall, W. J.; Brookhart, M. Organometallics 2000, 19, 1247–1254. (9) Full details are provided as Supporting Information.

Article

Organometallics, Vol. 28, No. 23, 2009

6809

Figure 2. 1,4-Pentadienol substrates.

Figure 1. X-ray structure diagram of complex 5 with nonhydrogen atoms represented by thermal ellipsoids at the 20% probability level. Final residuals: R1 = 0.0542, wR2 = 0.1422. Selected C-C bond lengths (A˚): C1-C2, 1.472(14); C2-C3, 1.366(12); C3-C4, 1.489(14); C4-C5, 1.485(13); C5-C6, 1.384(15). Scheme 2

mechanistic details of this transformation have not been investigated, analogous structural and geometric isomerizations have been extensively investigated for pentadienyl complexes of rhodium and iridium.5g,h,6c,10 The thermodynamic preference observed for the neutral inner-sphere “η3pentadienyl” halide complex 6 over the isomeric outersphere η5-pentadienyl cation analogous to 7a is also consistent with the energetics of the corresponding rhodium and iridium congeners, which also favor η3-coordination and inner-sphere halide binding.5f-h Conducting the thermolysis in the presence of excess KPF6 leads to counterion exchange and precipitation of KBr, diverting the reaction cleanly to the cationic 1-methyl-η5pentadienyl complex 7a (Scheme 2). The same complex, however, can be isolated in comparable overall yield by alternative experimental procedures of greater scope and convenience (vide infra). As a consequence, no further optimization or extension of this synthetic strategy was pursued. (10) (a) Bleeke, J. R.; Donaldson, A. J. Organometallics 1986, 5, 2401–2405. (b) Bleeke, J. R.; Peng, W.-J. Organometallics 1987, 6, 1576– 1578. (c) Bleeke, J. R.; Boorsma, D.; Chiang, M. Y.; Clayton, T. W.; Haile, T.; Beatty, A. M.; Xie, Y. F. Organometallics 1991, 10, 2391–2398. (d) Bleeke, J. R.; Rohde, A. M.; Boorsma, D. W. Organometallics 1993, 12, 970–974.

Coordination/Protonolysis of Nonconjugated Pentadienyl Alcohols. A considerable range of substituted η5-pentadienyl complexes can be prepared by protolytic dehydration of doubly allylic alcohols in the presence of a labile source of cobalt(I). This one-step synthetic protocol is operationally simple, if mechanistically dense, and relies on readily available organic substrates also applicable in organic Nazarov cyclizations. To this end, doubly allylic alcohols 8a-k11 (Figure 2) were prepared by vinylation of unsaturated aldehydes and investigated for competence in η5-pentadienyl synthesis. A. Cationic Pentadienyl Complexes of (C5Me5)Co. In solution, (C5Me5)Co(C2H4)2 (9) undergoes ligand exchange with 1,3- and 1,5-dienes upon mild heating, providing η4and η2-diene products in near quantitative yields.4,8a In contrast, analogous reactions with 1,4-dienes do not give tractable products, presumably as a consequence of the coordination strain imposed upon chelation.12 At low temperature, protonation of (C5Me5)Co(C2H4)2 provides a cationic cobalt(III) agostic ethyl/ethylene intermediate, which, undergoes rapid ethylene dissociation and ligand exchange, even at low temperature.13 Two experimental procedures for η5-pentadienyl complex synthesis from 1,4-dienyl alcohols have been demonstrated, differing principally in the order of reactant addition. In the first, (C5Me5)Co(C2H4)2 and the dienol are dissolved in deoxygenated acetone at low temperature (typically -78 °C), followed by the addition of HBF4 3 OEt2. This mixture is warmed gradually to room temperature over several hours, followed by removal of the solvent, flash chromatography in the air, and crystallization. This procedure affords η5-pentadienyl complexes 7a-h in modest to very good yields for a range of relatively simple dienyl alcohols. Substrates incorporating more elaborate substitution and functionality proved to be problematic under these conditions, presumably a function of slow exchange and competitive “offmetal” reactions of the organic carbocation. (11) (a) Oppolzer, W.; Snowden, R. L.; Simmons, D. P. Helv. Chim. Acta 1981, 64, 2002–2021. (b) Underiner, T. L.; Goering, H. L. J. Org. Chem. 1991, 56, 2563–2572. (c) Baldwin, J. E.; Adlington, R. M.; Godfrey, C. R. A.; Patel, V. K. Tetrahedron 1993, 49, 7837–7856. (d) Honda, T.; Mizutani, H.; Kanai, K. J. Chem. Soc., Perkin Trans. 1 1996, 1729–1739. (e) Wang, Y.; Schill, B. D.; Arif, A. M.; West, F. G. Org. Lett. 2003, 5, 2747– 2750. (f) Shing, T. K. M.; Zhu, X. Y.; Yeung, Y. Y. Chem.-Eur. J. 2003, 9, 5489–5500. (g) Bertus, P.; Drouin, L.; Laroche, C.; Szymoniak, J. Tetrahedron 2004, 60, 1375–1383. (12) Using sufficiently electron-deficient olefins, thermally stable 1,4diene complexes can be prepared: Holland, R. L.; Bunker, K. D.; Chen, C. H.; DiPasquale, A. G.; Rheingold, A. L.; Baldridge, K. K.; O’Connor, J. M. J. Am. Chem. Soc. 2008, 130, 10093–10095. (13) (a) Brookhart, M.; Green, M. L. H.; Pardy, R. B. A. J. Chem. Soc., Chem. Commun. 1983, 691–693. (b) Brookhart, M.; Lincoln, D. M.; Volpe, A. F.; Schmidt, G. F. Organometallics 1989, 8, 1212–1218. (c) Brookhart, M.; Lincoln, D. M.; Bennett, M. A.; Pelling, S. J. Am. Chem. Soc. 1990, 112, 2691–2694.

6810

Organometallics, Vol. 28, No. 23, 2009

Table 1. (C5Me5)Co(η5-pentadienyl)þ Complex Preparation

a

Anion exchange (step iii, above) omitted.

A more robust procedure was thus developed, starting from the preformed agostic ethyl cation, which transiently sequesters the proton, presumably allowing the alkene exchange to proceed to completion prior to proton transfer and dehydration of η2-coordinated allylic alcohol. In this

Ylijoki et al. Scheme 3

procedure, HBF4 3 OEt2 is added to a solution of (C5Me5)Co(C2H4)2 at low temperature, followed by addition of the dienol and warming to room temperature. The use of (nominally) anhydrous acetone as solvent leads to cleaner products and higher yields than does the use of tetrahydrofuran or dichloromethane. This procedure extends the range of η5-pentadienyl complexes that can be prepared by protolytic exchange, but does not necessarily deliver higher yields. As indicated in Table 1, the highest isolated yields and cleanest conversions are obtained from unsubstituted and terminally substituted substrates 8a, 8b, and 8h. Internal substitution at either C2 or C3 is particularly unfavorable and, generally, the yields of η5-pentadienyl complexes 7 decrease with increasing substitution on the substrate. Trisubstituted dienol 8k, for example, returned no detectable η5- or η3-pentadienyl products, presumably due to inhibition of the associative exchange and competitive generation of the highly stabilized carbocation. In this series of η5-pentadienyl complexes, improved crystallinity was noted for hexafluorophosphate salts over the corresponding tetrafluoroborate complexes. The counterion exchange proceeds quantitatively, with the water-soluble tetrafluoroborate complexes precipitating from aqueous KPF6 solution as the insoluble hexafluorophosphate salts. Two-chamber liquid diffusion of Et2O into CH2Cl2, however, provides analytically pure single crystals for even the less crystalline salts. Isolated η5-cyclopentadienyl complexes 7 are indefinitely stable at room temperature and insensitive to both air and water. In solution, the compounds remain indefinitely stable under moist air, but thermal stability decreases as a function of increasing substitution, presumably from η5fη3fη1 equilibration of the pentadienyl ligand, followed by facile σ-bond homolysis. Decomposition of pure 1-methylpentadienyl complex 7a, for example, is slow at 60 °C, but proceeds to completion in solution over several hours at 100 °C.14 A reasonable mechanistic rationale has emerged from a combination of experimental observations and previous mechanistic investigations (Scheme 3).13 Substitution of the substrate for ethylene in the agostic ethyl cation 10 can occur by an associative or dissociative mechanism, presumably leading to η2-coordination at the least-substituted end of the dienol. Subsequent proton transfer from the agostic (14) Slow conversion (