Synthesis and Reactivity of a Tetragallium Macrocycle

Dec 2, 2008 - Synopsis. The reaction of GaCl3 with 1,8-bis(trimethylstannyl)biphenylene produces the tetragallium macrocycle (C12H6)4Ga4Cl4 (1) in hig...
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Organometallics 2009, 28, 300–305

Synthesis and Reactivity of a Tetragallium Macrocycle Stefan M. Kilyanek, Xiangdong Fang, and Richard F. Jordan* Department of Chemistry, The UniVersity of Chicago, 5735 S. Ellis AVenue, Chicago, Illinois 60637 ReceiVed September 16, 2008

The reaction of GaCl3 with 1,8-bis(trimethylstannyl)biphenylene produces the tetragallium macrocycle (C12H6)4Ga4Cl4 (1) in high yield. Compound 1 contains a cyclic array of four Ga(biphenylene) units that form a 20-membered ring. The Ga atoms are bridged by µ2-Cl atoms so that the Ga atoms are fourcoordinate. The structure adopts a saddle-shaped conformation with approximate D2d symmetry. 1 reacts with Lewis bases (CH3CN and THF) and halide ions (Cl- and Br-) to form (C12H6)2Ga2Cl2L and (C12H6)2Ga2Cl2X- species. Introduction The binding of cations by crown ethers, cryptands, and other multidentate Lewis bases has been studied extensively.1 In contrast, the coordination of anions by multidentate Lewis acids is less well developed, possibly because of the challenges associated with incorporating several Lewis acid units in the same molecule.2,3 Most previous studies have focused on bidentate Lewis acids, including systems based on 1,8-difunctionalized naphthalenes,4-6 and macrocyclic species that incorporate Hg or Sn as Lewis acid sites.7-9 Multidentate Lewis acids based on group 13 elements may exhibit enhanced reactivity and could have applications in anion recognition and sequestering2a-d and as activators for olefin polymerization catalysts.2e Schno¨ckel reported that the reaction of AlCl with 2,3-Me2butadiene produces a cyclic hexa-aluminum species, cyclo-{µ(CH2CMedCMeCH2)}6(µ-Cl)6Al6.10 Uhl showed that R2GaH hydrides react with terminal alkynes to form (GaR)6(CR′)4 heteroadamantane cages that contain three-coordinate Ga cen* Corresponding author. E-mail: [email protected]. (1) Lehn, J. M. Supramolecular Chemistry; VCH Press: Weinheim, 1995. (2) Reviews: (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486. (b) Wuest, J. D. Acc. Chem. Res. 1999, 32, 81. (c) Vaugeois, J.; Simard, M.; Wuest, J. D. Coord. Chem. ReV. 1995, 145, 55. (d) Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res. 1997, 30, 267. (e) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (f) Melaimi, M.; Gabbaı¨, F. P. AdV. Organomet. Chem. 2005, 53, 61. (g) Wilson, P. A.; Hannant, M. H.; Wright, J. A.; Cannon, R. D.; Bochmann, M. Macromol. Symp. 2006, 236, 100. (h) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1. (i) Schmidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609. (3) Computational studies:(a) Jacobson, S.; Pizer, R. J. Am. Chem. Soc. 1993, 115, 11216. (b) Williams, S. D.; Harper, W.; Mamantov, G.; Tortorelli, L. J.; Shankle, G. J. Comput. Chem. 1996, 17, 1696. (4) For a review see: (a) Piers, W.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem. 2000, 2131. (5) Bidentate boron Lewis acids: (a) Hoefelmeyer, J. D.; Schulte, M.; Tschinkl, M.; Gabbaı¨, F. P. Coord. Chem. ReV. 2002, 235, 93. (b) Schulte, M.; Gabbaı¨, F. P. Can. J. Chem. 2002, 80, 1308. (c) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420. (d) Katz, H. E. Organometallics 1987, 6, 1134. (e) Katz, H. E. J. Org. Chem. 1989, 54, 2179. (f) Jia, L.; Yang, X.; Stern, C.; Marks, T. J. Organometallics 1994, 13, 3755. (g) Shriver, D. F.; Biallas, M. J. J. Am. Chem. Soc. 1967, 89, 1078. (h) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R.; Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244. (i) Aldridge, S.; Bresner, C.; Fallis, I. A.; Coles, S. J.; Hurshouse, M. B. Chem. Commun. 2002, 740. (j) Williams, V. C.; Irvine, G. J.; Piers, W. E.; Li, Z.; Collins, S.; Clegg, W.; Elsegood, M. R. J.; Marder, T. B. Organometallics 2000, 19, 1619. (k) Gabbaı¨, F. P.; Schier, A.; Riede, J. Angew. Chem., Int. Ed. 1998, 37, 622. (l) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39, 1312. (m) Schilling, B.; Kaiser, V.; Kaufmann, D. Chem. Ber. 1997, 130, 923.

ters.11 Gabbaı¨ reported that the reaction of GaCl3 with 1,8bis(trimethylstannyl)naphthalene yields several products including the trigallacycle species (C10H6)3Ga3(µ3-O)(µ-Cl).12 A variety of cyclic and polycyclic species in which Al or Ga centers are linked by group 15 element units have also been (6) For other bidentate Lewis acids see: (a) Schulte, M.; Gabbaı¨, F. P. Chem.-Eur. J. 2002, 8, 3802. (b) Gabbaı¨, F. P. Angew. Chem., Int. Ed. 2003, 42, 2218. (c) Dorsey, C. L.; Jewula, P.; Hudnall, T. W.; Hoefelmeyer, J. D.; Taylor, T. J.; Honesty, N. R.; Chiu, C. W.; Schulte, M.; Gabbaı¨, F. P. Dalton Trans. 2008, 33, 4442. (d) Hudnall, T. W.; Kim, Y. M.; Bebbington, M. W. P.; Bourissou, D.; Gabbaı¨, F. P. J. Am. Chem. Soc. 2008, 130, 10890. (e) Beckwith, J. D.; Tschinkl, M.; Picott, A.; Tsunoda, M.; Backman, R.; Gabbaı¨, F. P. Organometallics 2001, 20, 3169. (f) Tschinkl, M.; Bachman, R. E.; Gabbaı¨, F. P. Organometallics 2000, 19, 2633. (g) Tschinkl, M.; Hoefelmeyer, J. D.; Cocker, T. M.; Bachman, R. E.; Gabbaı¨, F. P. Organometallics 2000, 19, 1826. (h) Gabbaı¨, F. P.; Schier, A.; Riede, J.; Hynes, M. J. Chem. Commun. 1998, 897. (i) Tschinkl, M.; Backman, R. E.; Gabbaı¨, F. P. Chem. Commun. 1999, 1367. (j) Tschinkl, M.; Schier, A.; Riede, A.; Gabbaı¨, F. P. Organometallics 1999, 18, 1747. (k) Tschinkl, M.; Schier, A.; Riede, J.; Mehltretter, G.; Gabbaı¨, F. P. Organometallics 1998, 17, 2921. (l) Tschinkl, M.; Schier, A.; Riede, J.; Schmidt, E.; Gabbaı¨, F. P. Organometallics 1997, 16, 4759. (m) Kawachi, A.; Tani, A.; Shimada, J.; Yamamoto, Y. J. Am. Chem. Soc. 2008, 130, 4222. (n) Boshra, R.; Venkatasubbaiah, K.; Doshi, A.; Lalancette, R. A.; Kakalis, L.; Ja¨kle, F. Inorg. Chem. 2007, 46, 10174. (o) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11, 2099. (p) Ebata, K.; Inada, T.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1994, 116, 3595. (q) Asao, N.; Shibato, A.; Itagaki, Y.; Jourdan, F.; Maruoka, K. Tetrahedron Lett. 1998, 39, 3177. (r) Kira, M.; Kwon, E.; Kabuto, C.; Sakamoto, K. Chem. Lett. 1999, 28, 1183. (s) Panisch, R.; Bolte, M.; Muller, T. J. Am. Chem. Soc. 2006, 128, 9676. (t) Khalimon, A. Y.; Lin, Z. H.; Simionescu, R.; Vyboishchikov, S. F.; Nikonov, G. I. Angew. Chem., Int. Ed. 2007, 46, 4531. (u) Wrackmeyer, B.; Milius, W.; Tok, O. L. Chem.-Eur. J. 2003, 9, 4732. (7) Multidentate Lewis acid coordination of organic carbonyls: Vaugeois, J.; Simard, M.; Wuest, J. D. Coord. Chem. ReV. 1995, 145, 55. (8) Multidentate Lewis acids based on Hg: (a) Lee, H.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 2001, 123, 8543. (b) Lee, H.; Diaz, M.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed 2000, 39, 776. (c) Yang, X.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed. 1991, 30, 1507. (d) Yang, X.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1992, 114, 380. (e) Yang, X.; Knobler, C. B.; Zheng, Z.; Hawthorne, M. F. J. Am. Chem. Soc. 1994, 116, 7142. (f) Hawthorne, M. F.; Yang, X.; Zheng, Z. Pure Appl. Chem. 1994, 66, 245. (g) Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res. 1997, 30, 267. (h) Bayer, M. J.; Jalisatgi, S. S.; Smart, B.; Herzog, A.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed. 2004, 43, 1854. (i) Wedge, T. J.; Hawthorne, M. F. Coord. Chem. ReV. 2003, 240, 111. (j) Wuest, J. D. Acc. Chem. Res. 1999, 32, 81. (k) Tikhonova, I. A.; Dolgushin, F. M.; Tugashov, K. I.; Ellert, O. G.; Novotortsev, V. M.; Furin, G. G.; Antipin, M. Yu.; Shur, V. B. J. Organomet. Chem. 2004, 689, 82. (l) Tikhonova, I. A.; Dolgushin, F. M.; Yakovenko, A. A.; Tugashov, K. I.; Petrovskii, P. V.; Furin, G. G.; Shur, V. B. Organometallics 2005, 24, 3395. (m) Nadeau, F.; Slmard, M.; Wuest, J. D. Organometallics 1990, 9, 1311. (n) Wuest, J. D.; Zacharie, B. J. Am. Chem. Soc. 1987, 109, 4714.

10.1021/om800902d CCC: $40.75  2009 American Chemical Society Publication on Web 12/02/2008

Synthesis and ReactiVity of a Tetragallium Macrocycle Scheme 1

Organometallics, Vol. 28, No. 1, 2009 301 Scheme 2

studied.13 Here we describe the synthesis, structure, and reactivity of a tetragallium macrocycle in which the Ga centers are linked by 1,8-biphenylene units.

Results and Discussion Target Design. The objective of the present work was to prepare multigallium species in which the Ga centers are linked by 1,8-biphenylene units (Scheme 1). Gallium substituents at the C1 and C8 positions of an undistorted biphenylene unit (E in Scheme 1) would be separated by ca. 3.87 Å,14 which should favor binding of chloride and other anions to the two Ga centers. For comparison, the Ga---Ga distance in Ga2Cl7- is 3.75 Å.15 In contrast, the C1 and C8 positions of the 1,8-napthalene unit are separated by only ca. 2.49 Å.14 Synthesis and Structure of (C12H6)4Ga4Cl4 (1). The key starting material for this work, 1,8-dibromobiphenylene (5), was prepared by Scheme 2 using modified literature procedures.16-18 Lithium halogen exchange of 5 with nBuLi, followed by reaction with Me3SnCl, afforded 1,8-bis(trimethylstannyl)biphenylene (6, 69%). Compound 6 is stable to air and water but undergoes partial destannylation on silica gel to form 1-(trimethylstannyl)biphenylene.19 The reaction of 6 with 2 equiv of GaCl3 in refluxing toluene followed by cooling to room temperature affords (C12H6)4Ga4Cl4 (1), which crystallizes from solution and (9) Multidentate Lewis acids based on Sn:(a) Blanda, M. T.; Horner, J. H.; Newcomb, M. J. Org. Chem. 1989, 54, 4626. (b) Newcomb, M.; Madonik, A. M.; Blanda, M. T.; Judice, J. K. Organometallics 1987, 6, 145. (c) Newcomb, M.; Horner, J. H.; Blanda, M. T. J. Am. Chem. Soc. 1987, 109, 7878. (d) Blanda, M. T.; Newcomb, M. Tetrahedron Lett. 1989, 30, 3501. (e) Newcomb, M.; Hormer, J. H.; Blanda, M. T.; Squatritto, P. J. J. Am. Chem. Soc. 1989, 111, 6294. (f) Horner, J. H.; Squatritto, P. J.; McGuire, N.; Riebenspies, J. P.; Newcomb, M. Organometallics 1991, 10, 1741. (g) Zobel, B.; Duthie, A.; Dakternieks, D.; Tiekink, E. R. T. Organometallics 2001, 20, 2820. (h) Zobel, B.; Duthie, A.; Dakternieks, D.; Tiekink, E. R. T. Organometallics 2001, 20, 3347. (i) Jurkschat, K.; Kuivila, H. G.; Liu, S.; Zubieta, J. A. Organometallics. 1989, 8, 2755. (j) Schulte, M.; Gabriele, G.; Schurmann, M.; Jurkschat, K. Organometallics 2003, 22, 328. (10) Dohmeier, C.; Mattes, R.; Schno¨ckel, H. J. Chem. Soc., Chem. Commun. 1990, 358. (11) Uhl, W.; Cuypers, L.; Neumuller, B.; Weller, F. Organometallics 2002, 21, 2365. (12) Hoefelmeyer, J. D.; Brode, D. L.; Gabbaı¨, F. P. Organometallics 2001, 20, 5653. (13) (a) von Hanisch, C.; Stahl, S. Angew. Chem., Int. Ed. 2006, 45, 302. (b) Hoskin, A. J.; Stephan, D. W. Angew. Chem., Int. Ed. 2001, 40, 1865. (c) Janik, J. F.; Wells, R. L.; Young, V. G.; Rheingold, A. L.; Guzei, I. A. J. Am. Chem. Soc. 1998, 120, 532. (14) Distances are from DFT structures optimized at the B3LYP/6311++G** level. See Supporting Information for details. For structural and computational studies of biphenylene see: (a) Waser, J.; Lu, C. S. J. Am. Chem. Soc. 1944, 66, 2035. (b) Yokozeki, A., Jr.; Bauer, S. J. Am. Chem. Soc. 1974, 96, 1026. (c) Maksiæ, Z. B.; Kovaæek, D.; Eckert-Maksiæ, M.; Bo¨ckmann, M.; Klessinger, M. J. Phys. Chem. 1995, 99, 6410. (d) Zimmermann, R. J. Mol. Struct. 1996, 377, 35. (e) Verdal, N.; Hudson, B. S. Chem. Phys. Lett. 2007, 434, 241. (15) Chemistry of Aluminum Gallium Indium and Thallium; Downs, A. J., Ed.; Blackie Academic & Professional: Glasgow, 1993; p 135. (16) Du, C.-J. F.; Hart, H.; Ng, K.-K. D. J. Org. Chem. 1986, 51, 3162. (17) Rajca, A.; Safronov, A.; Rajca, S.; Ross, C. R., II.; Stezowski, J. J. J. Am. Chem. Soc. 1996, 118, 7272. (18) Kabir, S. M. H.; Iyoda, M. Synthesis 2000, 13, 1839. (19) Destannylation of 1 to 1-(trimethylstannyl)biphenylene on silica gel was confirmed by 2D-TLC and GC-MS.

was isolated in 53% yield. The reaction of 6 with 1 equiv of GaCl3 affords 1 in 68% isolated yield. The molecular structure of 1 was established by X-ray crystallography and is shown in Figure 1. Compound 1 contains a cyclic array of four Ga-biphenylene units, which form a 20membered ring. Each pair of adjacent Ga atoms is bridged by a µ2-Cl atom, so that the Ga centers have distorted tetrahedral geometry. The structure adopts a saddle-shaped conformation with approximate D2d symmetry. The Ga atoms form a squareplanar array, and the pairs of adjacent Cl atoms lie on opposite sides of the Ga4 plane. The biphenylene rings are tilted 135° (C(1)-C(24A)) and -130° (C(7)-C(18)) out of the Ga4 plane. The Ga---Ga distances (Ga(1)-Ga(2) 4.095 Å, Ga(3)-Ga(2) 4.094 Å) are slightly larger than the expected value of 3.87 Å (Scheme 1). The 1H NMR spectra of 1 in toluene-d8, benzene-d6, C6D5Cl, or CD2Cl2 solution contain either a single set of doublet (1H), triplet (1H), and doublet (1H) resonances, or a second-order multiplet, for the biphenylene groups, consistent with a highly symmetric structure. Additionally, the atmospheric pressure photoionization mass spectrum (APPI-MS) of a CD2Cl2 solution of 1 contains a prominent molecular ion peak. These results imply that the tetranuclear structure of 1 is retained in solution in noncoordinating solvents. The positive ion MALDI mass spectrum (DHB matrix, from toluene solution) of 1 contains a prominent signal for 1 · H+. Reaction of 1 with CH3CN. While 1 might be expected to bind Lewis bases and anions with concomitant cleavage of Ga-Cl-Ga bridges, in fact more complex chemistry leading to digallium products is observed. Dissolution of 1 in refluxing CH3CN followed by cooling to room temperature results in crystallization of the yellow binuclear complex (C12H6)2Ga2Cl2(NCCH3) (7, Scheme 3), which was isolated in 45% yield. The molecular structure of 7 was established by X-ray crystallography and is shown in Figure 2. Compound 7 contains a cyclic array of two Ga-biphenyl units that form a 10-membered ring. The Ga atoms are bridged by a µ2-Cl atom. One Ga contains a terminal chloride and the other contains a CH3CN ligand. The structure adopts a sawhorse conformation with approximate Cs symmetry.

302 Organometallics, Vol. 28, No. 1, 2009

Kilyanek et al. Scheme 3

Figure 1. Two views of the molecular structure of 1 (50% probability ellipsoids). H atoms are omitted. Key bond distances (Å) and angles (deg): Ga(1)-C(19) 1.936(4), Ga(1)-Cl(1A) 2.3465(10),Ga(2)-C(1)1.947(4),Ga(2)-C(7)1.935(4),Ga(2)-Cl(1) 2.3613(10), Ga(2)-Cl(2) 2.3401(10), Ga(3)-C(18) 1.950(4), Ga(3)-Cl(2) 2.3610(10), Ga(1)-Cl(1)-Ga(2) 120.88(4), Ga(2)Cl(2)-Ga(3) 121.14(4), Cl(1)-Ga(1)-Cl(1A) 101.47(5), Cl(2)Ga(2)-Cl(1) 96.30(4), Cl(2A)-Ga(3)-Cl(2) 99.68(5), C(19A)Ga(1)-C(19) 127.1(2), C(19A)-Ga(1)-Cl(1) 108.93(11), C(19)-Ga(1)-Cl(1) 103.84(10), C(7)-Ga(2)-C(1) 128.94(15), C(7)-Ga(2)-Cl(2) 110.11(12), C(1)-Ga(2)-Cl(2) 105.29(11), C(1)-Ga(2)-Cl(1) 108.50(11), C(18)-Ga(3)-C(18A) 130.1(2), C(18)-Ga(3)-Cl(2A) 102.29(11), C(18A)-Ga(3)-Cl(2A) 109.36(11), C(18)-Ga(3)-Cl(2) 109.36(11), C(18A)-Ga(3)-Cl(2) 102.29(11), C(20)-C(19)-Ga(1) 127.6(3), C(24)-C(19)-Ga(1) 118.6(3), C(2)-C(1)-Ga(2) 126.8(3), C(6)-C(1)-Ga(2) 119.5(3), C(12)-C(7)-Ga(2) 126.7(3), C(8)-C(7)-Ga(2) 119.7(3), C(13)-C(18)-Ga(3) 127.6(3), C(17)-C(18)-Ga(3) 118.0(3).

The 1H NMR spectrum of 7 in CD2Cl2 solution contains two doublets (δ 6.98 and 6.83, 2H each) for the H2 hydrogens, a triplet (4H) and a doublet (4H) for the H3 and H4 hydrogens, and a resonance for one bound acetonitrile ligand (δ 2.63, 3H) that is shifted downfield from the free acetonitrile position (δ 1.95) These results are consistent with the Cs-symmetric structure of 7 observed in the solid state. As illustrated in Figures 3 and 4, addition of excess free acetonitrile to a CD2Cl2 solution of 7 causes significant changes in the 1H NMR spectrum. In the presence of 0.01 M free CH3CN, the two H2 resonances are coalesced to one broad resonance at the average chemical shift, and the resonances for free and bound acetonitrile are broadened. As the concentration

of free CH3CN is increased, the H2 resonance sharpens, and the resonances for free and bound acetonitrile coalesce. These results show that 7 undergoes associative exchange with free CH3CN and that this process results in exchange of the two ends of the biphenylene units. The mechanism in Scheme 3 is consistent with these observations. Reaction of 1 with THF. Addition of excess THF to a CH2Cl2 solution of 1 followed by removal of the volatiles and drying under vacuum yields a yellow solid that is formulated as the THF adduct (C12H6)2Ga2Cl2(THF) (8, Scheme 4). The 1 H NMR spectrum of 8 in CD2Cl2 solution is very similar to

Figure 2. Molecular structure of 7 (50% probability ellipsoids). H atoms are omitted. Key bond distances (Å) and angles (deg): C(1)-Ga(1) 1.943(2), C(8)-Ga(2) 1.961(2), C(13)-Ga(1) 1.948(2), C(20)-Ga(2) 1.956(2), Ga(1)-Cl(1) 2.3220(7), Ga(2)-Cl(1) 2.3905(7), Ga(2)-Cl(2) 2.2158(7), Ga(1)-N(1) 2.002(2), N(1)-C(25) 1.112(4), N(1)-Ga(1)-Cl(1) 94.86(7), C(13)-Ga(1)-C(1) 131.08(10), C(20)-Ga(2)-C(8) 130.69(10), Cl(1)-Ga(2)-Cl(2) 100.22(3), Ga(1)-Cl(1)-Ga(2) 107.23(3).

Synthesis and ReactiVity of a Tetragallium Macrocycle

Organometallics, Vol. 28, No. 1, 2009 303 Scheme 5

Figure 3. Aryl region of the 1H NMR spectrum of 7 (CD2Cl2, 23 °C) in the presence of excess free CH3CN. From bottom to top: [CH3CN]free ) 0.002, 0.01, and 0.04 M.

Figure 4. CH3CN region of the 1H NMR spectra of 7 (CD2Cl2, 23 °C) in the presence of excess free CH3CN. From bottom to top: [CH3CN]free ) 0.002, 0.01, and 0.04 M. Scheme 4

that of 7 and contains two low-field doublets (2H each) for the H2 hydrogens and resonances for one bound THF ligand per (C12H6)2Ga2 unit, which are shifted downfield by ca. 0.6 ppm from the free THF positions. When increasing amounts of free THF are added, the H2 resonances broaden and coalesce to a sharp doublet, and the free and bound THF resonances coalesce to one set of resonances at the weighted average of the free and bound THF chemical shifts. This behavior is consistent with an associative THF exchange process analogous to the CH3CN exchange process in Scheme 3. 8 is also formed by dissolution of 7 in THF. Reaction of 1 with Chloride Ion. The reaction of 1 with 2 equiv of [NBu4]Cl in CD2Cl2 solution at 23 °C results in the quantitative formation of the dinuclear chloride adduct [NBu4][(C12H6)2Ga2Cl3] (9, Scheme 5), which was isolated as a white solid by removal of the volatiles from the CH2Cl2

solution. The solubility of 1 in CD2Cl2 is significantly enhanced by the presence of [NBu4]Cl due to the formation of 9. The negative ion ESI mass spectrum of 9 in CH2Cl2 solution contains a prominent molecular ion peak for (C12H6)2Ga2Cl3-. The 1H NMR spectrum of 9 is consistent with a C2V-symmetric structure and the presence of two biphenylene units per NBu4+ cation. The negative ion MALDI mass spectrum of 9 contains a strong signal for the (C12H6)2Ga2Cl3- anion. When substoichiometric amounts of [NBu4]Cl are added to 1, 9 is formed cleanly and unreacted 1 is observed in solution by NMR. Reaction of 1 with Bromide Ion. The reaction of 1 with 2 equiv of [NBu4]Br in CD2Cl2 yields a mixture of (C12H6)2Ga2Cl3-xBrx- (x ) 0-2) anions. The negative ion ESI mass spectrum of the resulting solution contains prominent signals for (C12H6)2Ga2Cl3- (9) and (C12H6)2Ga2Cl2Br- (10) and a smaller signal for (C12H6)2Ga2ClBr2-. The 1H NMR spectrum contains signals for 9 and a second set of resonances for symmetric biphenylene units that overlap with the resonances for 9 and shows that a total of two biphenylene units are present per NBu4+ cation. The second set of biphenylene resonances is assigned to 10; resonances for (C12H6)2Ga2ClBr2- (which was detected in the ESI mass spectrum) could not be distinguished. The formation of 9 suggests that (C12H6)2Ga2X3- species can exchange halides in solution. When substoichiometric amounts of [NBu4]Br are added to 1, a mixture of 9 and 10 is formed and unreacted 1 is observed in solution by NMR. Comparison of 1,8-Biphenylene and 1,8-Napthalene Systems. The clean formation of 1 in Scheme 2 and the reaction of 1 with Lewis bases and chloride to form (C12H6)2Ga2Cl2L and (C12H6)2Ga2Cl3- species (Schemes 4 and 5) contrast with the chemistry observed for the 1,8-naphthalene system.6g,12 Gabbaı¨ reported that 1,8-bis(trimethystannyl)naphthalene reacts with 1 equiv of GaCl3 at -25 °C to form a mixture of stannagallacycle 11 (21%) and digallacycle 12 (which crystallizes as a dimeric SnMe3Cl adduct, 10%), as shown in Scheme 6.12 At 65 °C, this reaction produces 11 in 65% yield. Chloridebridged polygallium species analogous to 1 were not observed, although in the presence of water (C10H6)4Ga3(µ3-O)(µ-Cl) (13) is formed. These results show that exchange of both SnMe3 units by Ga units is possible in the 1,8-napthalene system and suggest that the mixed Sn/Ga compound 11 is thermodynamically favored over the Ga2 species 12. The difference in reactivity between the 1,8-naphthalene and 1,8-biphenylene systems likely arises from the difference in the structures of the backbone units. The short Ga---Ga distance in the 1,8-Ga2-naphthalene framework (Scheme 1) requires an acute Ga-(µ-Cl)-Ga angle (12,

304 Organometallics, Vol. 28, No. 1, 2009 Scheme 6

76.9°; 13, 73.7°) to accommodate a µ-Cl bridge with reasonable Ga-Cl distances. In contrast, the Ga---Ga distance in the 1,8Ga2-biphenylene framework is nearly optimum for the formation of µ-Cl bridges. The Ga-(µ-Cl)-Ga angles in 1 (120.9°, 121.1°) and 7 (107.2°) are in the same range as that for the unconstrained species Ga2Cl7- (109°).20

Conclusion The reaction of GaCl3 with 1,8-bis(trimethylstannyl)biphenylene (6) produces the tetragallium macrocycle (C12H6)4Ga4Cl4 (1) in high yield. 1 reacts with Lewis bases (CH3CN, THF) to form (C12H6)2Ga2Cl2L species, which undergo associative exchange with free L. 1 reacts with halide ions (Cl- and Br-) to form (C12H6)2Ga2Cl2X- species, which can exchange halides in solution. The Ga---Ga distance in the 1,8-Ga2-biphenylene framework is nearly optimum for the formation of unstrained µ-Cl bridges.

Experimental Section General Procedures. All reactions were performed under a nitrogen atmosphere unless otherwise specified. Nitrogen was purified by passage through activated molecular sieves and Q-5 oxygen scavenger. Diethyl ether, tetrahydrofuran, toluene, benzened6, toluene-d8, and THF-d8 were distilled from sodium/benzophenone ketyl. CH3CN was dried over P2O5 and stored over 3 Å molecular sieves. CD3CN was degassed by three freeze-pump-thaw cycles and dried over 3 Å molecular sieves for 24 h. This process was repeated three times. 2,6-Dibromo-1-iodobenzene (3),16 2,2′,6,6′tetrabromobiphenyl (4),17 and 1,8-dibromobiphenylene (5)18 were prepared by modified literature procedures (see Supporting Information). NMR spectra were recorded on Bruker AVANCE DRX-400 or DMX-500 spectrometers in flame-sealed or Teflon-valved NMR tubes at ambient probe temperature unless otherwise specified. 1H and 13C chemical shifts are reported versus SiMe4 and were determined by reference to the residual solvent resonances. 29Sn chemical shifts are referenced to SnMe4. Coupling constants are reported in Hz. MALDI mass spectra were obtained with a PerSeptive Biosystems Voyager DE-PRO instrument. ESI mass spectra were recorded on an Agilent 1100 LC-MSD. A 5 µL sample was injected by flow injection using an autosampler. Purified nitrogen was used as the nebulizing and drying gas. APPI mass spectra were obtained on the Agilent 1100 LC-MSD instrument (20) (a) A larger Ga-(µ-Cl)-Ga angle (138°) was observed in (C6F5)3Ga2(µCl) - King, W.; Scott, B.; Eckert, J.; Kubas, G. Inorg. Chem. 1999, 38, 1069. (b) Smaller Ga-(µ-Cl)-Ga angles (ca. 90°) are observed in double-bridged Ga-(µ-Cl)2-Ga species. Lustig, C.; Mitzel, N. W. Z. Naturforsch. 2004, 59b, 140.

Kilyanek et al. using a Kr lamp that emits photons at 10.0-10.6 eV as the photoionization source. In all cases where assignments are given, the observed isotope patterns closely matched the calculated patterns. The listed m/z value corresponds to the most intense peak in the isotope pattern. Elemental analyses were performed by Midwest Microlab, LCC (Indianapolis, IN). 1,8-Bis(trimethylstannyl)biphenylene (6). A solution of nBuLi (2.5 M in hexane, 5.5 mL, 14 mmol) was added dropwise to a solution of 1,8-dibromobiphenylene (5, 1.83 g, 5.90 mmol) in Et2O (20 mL) at 20 °C. The mixture was stirred for 30 min. A solution of Me3SnCl (3.56 g, 17.8 mmol) in Et2O (50 mL) was added dropwise at 20 °C. The solution was stirred for 12 h. The solution was washed with saturated aqueous [NH4]Cl solution (100 mL). The organic layer was separated and the aqueous layer was extracted with Et2O (3 × 100 mL). The combined organic fractions were dried over MgSO4 and the solvent was removed under vacuum. The resulting yellow solid was recrystallized from boiling pentane to afford 1,8-bis(trimethylstannyl)biphenylene as colorless needles in two crops (total yield 1.94 g, 69%). Mp: 117.5-118.5 °C. 1H NMR (C6D6): δ 6.83 (dd, J ) 8.0 and 1.0, satellites: J(119Sn-H)) 42, 2H, H2 and H7), 6.55 (dd, J ) 8.0 and 6.8, 2H, H3 and H6), 6.45 (dd, J ) 6.8 and 1.0, 2H, H4 and H5), 0.23 (s, 18H, satellites: J(119Sn-H) ) 58, SnMe3). 13C{1H} NMR (CD2Cl2): δ 163.1, 152.1, 136.8, 132.2, 127.7, 116.6, -6.9 (SnMe3). 119Sn{1H} NMR (benzened6): δ -37.3. GC-MS: m/z 478 (M+). Anal. Calcd for C18H24Sn2: C, 45.25; H, 5.06. Found: C, 45.50; H, 5.06. (C12H6)4Ga4Cl4 (1). A suspension of 1,8-bis(trimethylstannyl)biphenylene (6, 0.182 g, 0.380 mmol) and GaCl3 (0.0662 g, 0.376 mmol) in toluene (12 mL) was refluxed for 12 h and then cooled at 20 °C. Yellow crystals formed. The crystals were separated, washed with pentane (2 × 4 mL), and dried under vacuum to afford a yellow powder (0.0672 g, 69%). 1:2 ratio: A suspension of 6 (1.23 g, 2.57 mmol) and GaCl3 (0.911 g, 5.17 mmol) in toluene (10 mL) was refluxed for 12 h. The mixture was cooled at 20 °C to afford yellow crystals, which were isolated, washed with toluene (3 × 10 mL), and dried under vacuum to afford a fine yellow powder (0.348 g, 52%). 1 is sparingly soluble in benzene, toluene, and CH2Cl2 and chlorobenzene at 20 °C. 1H NMR (CD2Cl2): δ 6.85 (dd, JH2-H3 ) 8.1, JH2-H4 ) 1.0, 4H, H2), 6.75 (2nd order multiplet, JH3-H2 ) 8.1, JH3-H4 ) 6.9, 4H, H3), 6.73 (2nd order multiplet, JH4-H3 ) 6.9, JH4-H2 ) 1.0, 4H, H4); J values were determined by simulation. 13C{1H} NMR (CD2Cl2): δ 161.2, 152.6, 134.1, 130.4, 128.5, 119.2. Anal. Calcd for C48H24Cl4Ga4: C, 56.44; H, 2.37. Found: C, 56.70; H, 2.71. MALDI-MS (DHB matrix, toluene): m/z calcd for C48H24Cl4Ga4H+ 1023.8, found 1023.2. APPI-MS (CD2Cl2): m/z calcd for C48H24Cl4Ga4+ 1022.77, found 1022.8. (C12H6)2Ga2Cl2(CH3CN) (7). A mixture of 1 (35 mg, 0.035 mmol) and CD3CN (5 mL) was refluxed until all of the yellow solid disappeared. The solution was cooled to 20 °C, and yellow crystals formed. The crystals were isolated by removing the mother liquor by syringe (17 mg, 45%). X-ray crystallography showed that this material is 7 · CH3CN. Alternatively, a mixture of 1 and CD3CN (1.5 mL) was refluxed until all of the yellow solid disappeared and then evaporated to dryness under vacuum. The pale yellow solid was dried under vacuum at room temperature to yield 7, which contained some free CH3CN. Vacuum drying of this solid at 75 °C yielded material that contained only a trace amount of free CH3CN. Data for 7: 1H NMR (CD2Cl2): δ 6.98 (d, J ) 6.0, 2H, H2), 6.83 (d, J ) 6.8, 2H, H2), 6.71 (m, 4H, H3), 6.61 (m, 4H, H4), 2.63 (s, 3H, NCCH3). 13C{1H} NMR (CD2Cl2): δ 135.2, 132.5, 128.3, 118.5, 3.9; C9-C12 were not observed. Multiple elemental analyses of spectroscopically pure samples gave inconsistent results. Generation of (C12H6)2Ga2Cl2(THF) (8). An NMR tube was charged with 1 (1.9 mg), CD2Cl2 (0.6 mL), and THF (0.4 mL) and agitated for 5 min at 23 °C. NMR analysis at 23 °C revealed that 1 was completely converted to 8. The volatiles were removed from

Synthesis and ReactiVity of a Tetragallium Macrocycle the solution of 8, CD2Cl2, and THF under vacuum, and the resulting pale yellow solid was dried under vacuum for 20 min. CD2Cl2 (0.6 mL) was added, resulting in a pale yellow solution. NMR analysis showed that 8 and 6.9 equiv of free THF were present. Data for 8: 1 H NMR (CD2Cl2, in the presence of 6.9 equiv of free THF): δ 7.00 (d, J ) 7.9, 2H, H2), 6.82 (d, J ) 8.0, 2H, H2), 6.72 (m, 4H, H3), 6.61 (t, J ) 6.4, 4H, H4), 4.32 (m, 4H, THF), 2.18 (m, 4H, THF); free THF resonances at δ 3.73 and 1.84 were also present. 13 C{1H} NMR (CD2Cl2, in the presence of 6.9 equiv of free THF): δ 152.6, 135.1, 132.4, 128.3, 128.1, 118.5, 118.2, 74.6 (THF), 30.1 (THF); free THF resonances at δ 68.7 and 25.9 were also present; C1, C8, C9, C11 were not observed. [NBu4][(C12H6)2Ga2Cl3] (9). An NMR tube was charged with 1 (11.9 mg, 0.0116 mmol) and [NBu4]Cl (6.1 mg, 0.022 mmol), and CD2Cl2 (0.6 mL) was added by vacuum transfer at -196 °C. The tube was warmed to room temperature and a yellow solution formed. The tube was monitored periodically by NMR. Resonances for several new unsymmetrical species grew in and disappeared. The formation of 9 was complete after 10 h. 9 was isolated by slow evaporation of the solvent in a glovebox. 1H NMR (CD2Cl2): δ 6.95 (dd, J ) 7.9 and 0.8, 4H), 6.63 (t, J ) 7, 4H), 6.53 (dd, J ) 6.8 and 0.8, 4H), 2.94 (m, 8H, NCH2), 1.46 (m, 8H, CH2), 1.31 (m, 8H, CH2), 0.94 (t, J ) 7.3, 12H, CH3). 13C{1H} NMR (CD2Cl2): δ 160.4, 151.4, 135.8, 134.68, 127.34, 117.34, 58.9, 24.1, 20.0, 13.8. Negative ion ESI-MS (CD2Cl2): m/z calcd for C24H12Cl3Ga2-, 544.8, found 544.8. Negative ion MALDI-MS (R-cyano-4-hydroxycinnamic acid external calibration matrix, CD2Cl2): m/z calcd for C24H12Cl3Ga2- 544.8, found 544.8. Anal. Calcd for C40H48Cl3Ga2N: C, 60.92; H, 6.13; N, 1.77. Found: C, 60.29; H, 6.08; N, 2.03. Reaction of 1 with [NBu4]Br. An NMR tube was charged with 1 (2.1 mg, 2.0 µmol) and [NBu4]Br (1.3 mg, 4.0 µmol), and CD2Cl2 was added by vacuum transfer at -196 °C. The tube was warmed to room temperature and a pale yellow solution formed. 1H NMR and ESI-MS analyses showed that the major species present were 9 and [NBu4][(C12H6)2Ga2Cl2Br] (10). Data for 10: 1H NMR (CD2Cl2): δ 6.94 (d, J ) 7.9), 6.65 (t, J ) 7.0), 6.52 (d, J ) 6.8). Negative ion ESI-MS (CD2Cl2): m/z calcd for C24H12Cl2BrGa2590.8, found 590.8. ESI-MS also showed that (C12H6)Ga2ClBr2was present as a minor species (m/z calcd for C24H12ClBr2Ga2634.7, found 634.8.). X-ray Crystallography. Crystallographic data are summarized in Table 1. Details are provided in the Supporting Information. Data were collected on a Bruker SMART (7) or Bruker SMART APEX (1) diffractometer using Mo KR radiation (0.71073 Å). Nonhydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients.

Organometallics, Vol. 28, No. 1, 2009 305 Table 1. Summary of X-ray Diffraction Data formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z temp (K) cryst, color, habit GOF on F2 final R indices (I > 2σ(I))a R indices (all data)

1

7 · 2CH3CN

C48H24Cl4Ga4 1021.35 monoclinic C2/c 15.793(3) 22.495(5) 16.567(4) 90 107.125(4) 90 5625(2) 4 100(2) pale yellow, rod 1.061 R1 ) 0.0436 wR2 ) 0.1212 R1 ) 0.0504 wR2 ) 0.1237

C30H21Cl2Ga2N3 633.84 trigonal R3j 40.653(3) 40.653(3) 9.8092(10) 90 90 120 14040(2) 18 173(2) yellow, block 1.070 R1 ) 0.0305 wR2 ) 0.0841 R1 ) 0.0380 wR2 ) 0.0881

a R1 ) ∑|Fo| - |Fc|/∑|Fo|; wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2, where w ) q[σ2(Fo2) + (aP)2 + bP]-1.

Specific comments for each structure are as follows. Single crystals of 1 were grown from chlorobenzene at 23 °C. These crystals contained 1.7 equiv of C6H5Cl per molecule of 1, which were disordered; the C6H5Cl molecules were excluded using the PLATON SQUEEZE software package. Single crystals of 7 were grown from CD3CN. These crystals contain two acetonitrile molecules per molecule of 7, which were modeled as disordered groups, each 0.50 occupied. Each has two separate positions, which are necessarily correlated. These acetonitrile molecules were restrained (24 restraints) to be linear with normal bond distances and were refined with anisotropic displacement parameters.

Acknowledgment. This work was supported by the National Science Foundation (CHE-0516950). Dr. Ian Steele (University of Chicago) and Dr. Victor G. Young, Jr. (XRay Crystallographic Laboratory, 160 Kolthoff Hall, Department of Chemistry, University of Minnesota) are thanked for their assistance with X-ray crystallography. Supporting Information Available: Synthetic procedures and NMR data for 3, 4, and 5, additional NMR data for 1, X-ray crystallographic data for 1 and 7 (cif files), and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. OM800902D