S Cluster Anions Formed

Dec 8, 2009 - Two Novel μ-CO-Containing Butterfly Fe/S Cluster Anions Formed from ... techniques, as well as 3, 9a, and 10a,b by X-ray crystallograph...
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Organometallics 2010, 29, 205–213 DOI: 10.1021/om900899h

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Two Novel μ-CO-Containing Butterfly Fe/S Cluster Anions Formed from Trithiol MeC(CH2SH)3, Fe3(CO)12, and Et3N: Their Reactions with Electrophiles to Produce the Corresponding Neutral Triple- and Double-Butterfly Cluster Complexes Li-Cheng Song,* Jia Cheng, Jing Yan, Chun-Rong Liu, and Qing-Mei Hu Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Received October 14, 2009

Trithiol MeC(CH2SH)3 reacted with Fe3(CO)12 and Et3N followed by treatment of the resulting three-μ-CO-containing trianion {[Fe2(μ-CO)(CO)6]3[(μ-SCH2)3CMe]}3- (A) with electrophile Ph2PCl to give triple-butterfly complex [Fe2(μ-Ph2P)(CO)6]3[(μ-SCH2)3CMe] (3), whereas double-butterfly complexes [Fe2(μ-PhCdNPh)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (4), [Fe2(μ-SdCNHPh)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (5), and [Fe2(μ-C6H11NdCNHC6H11)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (6) were produced by reactions of electrophiles PhC(Cl)dNPh, C6H5NdCdS, and C6H11NdCdNC6H11 (DCC) with the one-μ-CO-containing monoanion {[Fe2(μ-CO)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6]}- (B) generated in situ from the initially formed trianion A. More interestingly, both triple-butterfly complexes [Fe2(μ-RSCdS)(CO)6]3[(μ-SCH2)3CMe] (8a, R = Me; 8b, PhCH2) and double-butterfly complexes [Fe2(μ-RSCdS)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (9a, R = Me; 9b, PhCH2) were obtained by reactions of trianion A and monoanion B with CS2/MeI or CS2/Ph2CH2Br, whereas the μ4-S-containing triple-butterfly complexes [Fe2(μ-RS)(CO)6][Fe2(μ4-S)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (10a, R = Me; 10b, PhCH2) could be obtained by reactions of monoanion B with (μ-S2)Fe2(CO)6/MeI or (μ-S2)Fe2(CO)6/Ph2CH2Br, respectively. All these triple- and double-butterfly complexes were characterized by elemental analysis and spectroscopic techniques, as well as 3, 9a, and 10a,b by X-ray crystallography. In addition, single-butterfly complex Fe2(μ-t-BuS)(μ-C6H11NdCNHC6H11)(CO)6 (7) was prepared by reaction of monoanion [Fe2(μ-CO)(μ-t-BuS)(CO)6]- with DCC in order to confirm the coordination mode of the protonated DCC ligand in double-butterfly complex 6.

Introduction In recent years, the butterfly Fe/S cluster complexes have received great attention because of their unique structures and novel chemical reactivities,1-6 and particularly their widespread uses as biomimetic models for the active site of [FeFe]-hydrogenases.7-12 In 1985 Seyferth and co-workers

first prepared the one-μ-CO-containing single-butterfly Fe/S cluster monoanion [Fe2(μ-CO)(μ-RS)(CO)6]- through reaction of mercaptan RSH with Fe3(CO)12 in the presence of Et3N.13 Since Seyferth’s pioneering work, we have prepared

*To whom correspondence should be addressed. Fax: 0086-2223504853. E-mail: [email protected]. (1) For reviews, see for example: (a) Ogino, H.; Inomata, S.; Tobita, H. Chem. Rev. 1998, 98, 2093. (b) Bruce, M. I. J. Organomet. Chem. 1985, 283, 339. (c) Song, L.-C. Trends Organomet. Chem. 1999, 3, 1. (d) Song, L.-C. Sci. China Ser. B: Chem. 2009, 52, 1. (2) (a) Seyferth, D.; Womack, G. B.; Archer, C. M.; Dewan, J. C. Organometallics 1989, 8, 430. (b) Seyferth, D.; Womack, G. B.; Archer, C. M.; Fackler, J. P.Jr.; Marler, D. O. Organometallics 1989, 8, 443. (c) Seyferth, D.; Hoke, J. B.; Dewan, J. C.; Hofmann, P.; Schnellbach, M. Organometallics 1994, 13, 3452. (d) Seyferth, D.; Ruschke, D. P.; Davis, W. M.; Cowie, M.; Hunter, A. D. Organometallics 1994, 13, 3834. (3) Nametkin, N. S.; Tyurin, V. D.; Kukina, M. A. J. Organomet. Chem. 1978, 149, 355. (4) Bose, K. S.; Sinn, E.; Averill, B. A. Organometallics 1984, 3, 1126. (5) Wang, Z.-X.; Jia, C.-S.; Zhou, Z.-Y.; Zhou, X.-G. J. Organomet. Chem. 2000, 601, 108. (6) Delgado, E.; Hernandez, E.; Rossell, O.; Seco, M.; Puebla, E. G.; Ruiz, C. J. Organomet. Chem. 1993, 455, 177.

(7) For reviews, see for example: (a) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3683. (b) Liu, X.; Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. Rev. 2005, 249, 1641. (c) Song, L.-C. Acc. Chem. Res. 2005, 38, 21. (d) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. Rev. 2007, 107, 4273. (e) Heinekey, D. M. J. Organomet. Chem. 2009, 694, 2671. (8) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008. (9) Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261. (10) Cheah, M. H.; Tard, C.; Borg, S. J.; Liu, X.; Ibrahim, S. K.; Pickett, C. J.; Best, S. P. J. Am. Chem. Soc. 2007, 129, 11085. (11) (a) Eilers, G.; Schwartz, L.; Stein, M.; Zampella, G.; de Gioia, L.; Ott, S.; Lomoth, R. Chem.;Eur. J. 2007, 13, 7075. (b) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2009, 48, 2. (c) Harb, M. K.; Niksch, T.; Windhager, J.; G€orls, H.; Holze, R.; Lockett, L. T.; Okumura, N.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L.; El-khateeb, M.; Weigand, W. Organometallics 2009, 28, 1039. (12) (a) Wang, W.-G.; Wang, H.-Y.; Si, G.; Tung, C.-H.; Wu, L.-Z. Dalton Trans. 2009, 2712. (b) Zhao, Z.; Wang, M.; Dong, W.; Li, P.; Yu, Z.; Sun, L. J. Organomet. Chem. 2009, 694, 2309. (13) Seyferth, D.; Womack, G. B.; Dewan, J. C. Organometallics 1985, 4, 398.

r 2009 American Chemical Society

Published on Web 12/08/2009

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Song et al. Scheme 1

Scheme 2

a variety of novel μ-CO-containing butterfly Fe/S cluster anions,14-17 such as the two-μ-CO-containing double-butterfly (14) Song, L.-C.; Fan, H.-T.; Hu, Q.-M. J. Am. Chem. Soc. 2002, 124, 4566. (15) Song, L.-C.; Cheng, J.; Hu, Q.-M.; Gong, F.-H.; Bian, H.-Z.; Wang, L.-X. Organometallics 2005, 24, 472. (16) Song, L.-C.; Cheng, J.; Yan, J.; Wang, H.-T.; Liu, X.-F.; Hu, Q.-M. Organometallics 2006, 25, 1544. (17) Song, L.-C.; Fang, X.-N.; Li, C.-G.; Yan, J.; Bao, H.-L.; Hu, Q.-M. Organometallics 2008, 27, 3225.

dianion {[Fe2(μ-CO)(CO)6]2(μ-SZS-μ)}2- produced by reaction of dithiol HSZSH (Z = CH2(CH2OCH2)2,3CH2) with Fe3(CO)12 and Et3N,14 as well as the three-μ-CO-containing triple-butterfly trianion {[Fe2(μ-CO)(CO)6]3[(μ-SCH2)3CMe]}3- (A) and the one-μ-CO-containing double-butterfly monoanion {[Fe2(μ-CO)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6]}- (B) generated from reaction of trithiol MeC(CH2SH)3 with Fe3(CO)12 and Et3N.16 Seyferth’s group and we have not only prepared such μ-CO-containing butterfly cluster anions but also carried out studies on their

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Table 1. Selected Bond Lengths (A˚) and Angles (deg) for 3, 7, and 9a 3 Fe(2)-S(1) Fe(1)-P(1) Fe(2)-P(1) Fe(3)-P(2)

2.2511(7) 2.2324(6) 2.2378(7) 2.2281(8)

Fe(1)-S(1) Fe(1)-Fe(2) Fe(3)-S(2) Fe(4)-S(2)

2.2807(8) 2.5836(7) 2.2611(7) 2.2619(8)

P(1)-Fe(1)-S(1) P(1)-Fe(2)-S(1) P(1)-Fe(1)-Fe(2) P(1)-Fe(2)-Fe(1) S(1)-Fe(1)-Fe(2)

78.662(17) 79.18(2) 54.79(2) 54.596(15) 54.705(16)

P(2)-Fe(3)-S(2) P(2)-Fe(4)-S(2) P(2)-Fe(3)-Fe(4) P(2)-Fe(4)-Fe(3) S(2)-Fe(3)-Fe(4)

78.09(3) 77.961(18) 54.86(2) 54.658(19) 55.32(2)

Fe(1)-C(1) Fe(1)-C(11) Fe(2)-S(1) S(1)-C(7)

1.763(7) 2.008(5) 2.2544(17) 1.859(6)

Fe(1)-S(1) Fe(1)-Fe(2) N(1)-C(11) N(2)-C(11)

2.2577(17) 2.5664(12) 1.290(6) 1.365(6)

C(18)-N(1)-Fe(2) C(11)-Fe(1)-S(1) C(11)-Fe(1)-Fe(2) S(1)-Fe(1)-Fe(2) C(1)-Fe(1)-S(1)

129.3(4) 80.24(16) 70.39(17) 55.27(5) 155.3(2)

N(1)-Fe(2)-S(1) N(1)-Fe(2)-Fe(1) S(1)-Fe(2)-Fe(1) Fe(2)-S(1)-Fe(1) C(3)-Fe(1)-C(11)

78.57(13) 72.32(13) 55.40(5) 69.33(5) 170.2(3)

Fe(1)-S(1) Fe(1)-Fe(2) Fe(2)-S(1) Fe(1)-S(2)

2.280(4) 2.495(2) 2.254(3) 2.246(4)

Fe(3)-S(4) Fe(3)-Fe(4) Fe(4)-S(4) Fe(4)-S(3)

2.259(3) 2.629(2) 2.253(3) 2.294(4)

S(2)-Fe(1)-Fe(2) S(2)-Fe(1)-S(1) S(2)-Fe(2)-S(1) S(2)-Fe(2)-Fe(1) S(1)-Fe(2)-Fe(1)

56.40(9) 83.10(12) 83.58(11) 56.19(10) 57.12(10)

Fe(2)-S(1)-Fe(1) Fe(1)-S(2)-Fe(2) Fe(4)-S(4)-Fe(3) C(18)-Fe(3)-S(4) S(3)-C(18)-Fe(3)

66.78(10) 67.41(10) 71.26(9) 85.3(3) 114.6(6)

7

Figure 1. Molecular structure of 3 with 30% probability level ellipsoids.

chemical reactivities and applications in organometallic synthesis. On the basis of our previous communication regarding trianion A and monoanion B,16 this article will further report the detailed formation and chemical reactivities of these two novel anions, as well as the synthesis and characterization of a new series of neutral butterfly Fe/S cluster complexes.

Results and Discussion Synthesis and Characterization of Triple- and DoubleButterfly Complexes 3 and 4. Previously, we reported16 that (i) reaction of trithiol MeC(CH2SH)3 with Fe3(CO)12 and Et3N in 1:3:3 molar ratio in THF at room temperature gave trianion A and monoanion B (B was generated formally via intramolecular loss of two [(CO)Fe(CO)3]- units (possibly as dianion [Fe2(CO)8]2-) from the initially formed trianion A followed by dimerization of the remaining two [(SCH2)Fe(CO)3] moieties)18 and (ii) further treatment of this system in situ with electrophiles MeSCH2Cl and CH2dCHCH2Br afforded double-butterfly complexes 1 and 2 (Scheme 1). Now, we report that the same system prepared from MeC(CH2SH)3, Fe3(CO)12, and Et3N was able to react with electrophiles Ph2PCl and PhC(Cl)dNPh to give triple- and double-butterfly complexes 3 and 4, respectively (Scheme 2). According to our preliminary study regarding this system,16 as well as the well-known reaction modes of μ-CO-containing butterfly Fe/S cluster anions,1c,d we suggest that complex 3 is most likely produced by initially nucleophilic attack of the three negatively charged Fe atoms of trianion A at the P atom of Ph2PCl, followed by nucleophilic attack of the P atom in intermediate M1 at the neighboring Fe atom and subsequent loss of its μ-CO ligand. Similarly, complex 4 could be generated starting from nucleophilic attack of the negatively charged Fe atom of monoanion B at the Cl-attached carbon of PhC(Cl)d NPh, followed by nucleophilic attack of the N atom in (18) Similar intermolecular processes are known, by which the [Et3NH] salts of the monoanions [(μ-RE)(μ-CO)Fe2(CO)6]- (E = S, Se) can be converted to the dimers (μ-RE)2Fe2(CO)6; see for example: (a) Seyferth, D.; Hoke, J. B.; Womack, G. B. Organometallics 1990, 9, 2662. (b) Song, L.-C.; Lu, G.-L.; Hu, Q.-M.; Fan, H.-T.; Chen, J.; Sun, J.; Huang, X.-Y. J. Organomet. Chem. 2001, 627, 255.

9a

intermediate M2 at the neighboring Fe atom and subsequent loss of its μ-CO ligand (Scheme 2). As previously indicated,16 the relative amounts of trianion A and monoanion B are most likely dependent upon the nature of the utilized electrophiles. So, in the present case it might be that the heteroatom P-based electrophile Ph2PCl reacts with trianion A fast enough to directly give complex 3 as major product. But instead, the C-based electrophile PhC(Cl)dNPh reacts faster with monoanion B than with A, which may induce further conversion of A to the thermodynamically more favored B; thus complex 4 is obtained as the major product. Products 3 and 4 are air-stable solids, which were characterized by elemental analysis and spectroscopy. For example, the IR spectra of 3 and 4 showed three or four absorption bands in the region 2069-1982 cm-1 for their terminal carbonyls,19 whereas the IR spectrum of 4 displayed one additional band at 1568 cm-1 for its CdN double bond.20,21 In addition, the 31P NMR spectrum of 3 showed one singlet at 140.86 ppm for its three identical P atoms,22 whereas the 1H NMR spectra of 3 and 4 displayed one signal at 1.15 and 1.25 ppm for their CH3 groups, respectively. To confirm the structure of 3, an X-ray crystallographic study was undertaken. The ORTEP drawing of 3 is shown in (19) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; California University Science Books: Mill Valley, 1987. (20) Seyferth, D.; Hoke, J. B. Organometallics 1988, 7, 524. (21) Song, L.-C.; Lu, G.-L.; Hu, Q.-M.; Qin, X.-D.; Sun, C.-X.; Yang, J.; Sun, J. J. Organomet. Chem. 1998, 571, 55. (22) Song, L.-C.; Cheng, J.; Gong, F.-H.; Hu, Q.-M.; Yan, J. Organometallics 2005, 24, 3764.

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Figure 1, and Table 1 lists its selected bond lengths and angles. The X-ray diffraction analysis indicated that complex 3 contains three Fe2SP butterfly cluster cores in which each of the P atoms is attached to two phenyl groups and each of the Fe atoms is attached to three terminal CO ligands. In addition, the three butterfly Fe2SP cluster cores are connected through their S atoms to C19, C21, and C22 by three equatorial bonds (the nonbonded angles C(19)-S(1) 3 3 3 P(1) = 157.1°, C(21)-S(2) 3 3 3 P(2) = 158.3°, and C(22)-S(3) 3 3 3 P(3) = 159.8°). Actually, such a conformational arrangement in complex 3 is identical with that in triple-butterfly complex [Fe2(μ-Ph2P)(CO)6]3[(μ-SCH2CH2)3N].22 Obviously, this is in order to avoid the strong axialaxial repulsions between the axially bonded bulky phenyl groups at P atoms and the central part of the starlike complex.23,24 Synthesis and Characterization of Double- and SingleButterfly Complexes 5-7. It was further found that the same system prepared from MeC(CH2SH)3, Fe3(CO)12, and Et3N is able to react with electrophiles PhNdCdS and C6H11NdCdNC6H11 (DCC) under similar conditions to produce double-butterfly complexes 5 and 6, respectively (Scheme 3). By analogy with the suggested pathway for (23) Seyferth, D.; Henderson, R. S.; Song, L.-C. Organometallics 1982, 1, 125. (24) Shaver, A.; Fitzpatrick, P. J.; Steliou, K.; Bulter, I. S. J. Am. Chem. Soc. 1979, 101, 1313.

formation of 3 or 4, complexes 5 and 6 might be produced via the following elementary steps: (i) the initial nucleophilic attack of the negatively charged Fe atom of monoanion B at the unsaturated carbon atom of PhNdCdS or C6H11NdCdNC6H11 to form intermediate M3 or M5; (ii) the subsequent nucleophilic attack of the S atom in M3 or the N atom in M5 at the neighboring Fe atom and subsequent loss of its μ-CO ligand to form intermediate M4 or M6; and (iii) the transfer of a proton from [Et3NH] to the negatively charged N atoms in intermediates M4 and M6 to finally produce 5 and 6 (Scheme 3). Single-butterfly complex 7 was similarly prepared by treatment of t-BuSH with Fe3(CO)12 and Et3N in 1:1:1 molar ratio in THF at room temperature followed by treatment of the resulting solution containing monoanion [Fe2(μ-CO)(μ-t-BuS)(CO)6]- with DCC (Scheme 4). It should be noted that preparation of complex 7 was done to obtain a single crystal for X-ray diffraction analysis (usually, single crystals of the single-butterfly clusters are easier to grow than doubleand triple-butterfly ones) to confirm the coordination mode of the protonated DCC ligand in double-butterfly complex 6. Products 5-7 are also air-stable solids, which were characterized by elemental analysis and spectroscopy. For example, the IR spectra of 5-7 showed three or four absorption bands in the region 2068-1960 cm-1 for their terminal carbonyls.19 In addition, the IR spectrum of 5 displayed one additional band at 934 cm-1 for its CdS

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Scheme 4

double bond,25,26 whereas 6 and 7 exhibited one additional band at 1530 or 1537 cm-1 for their CdN double bonds,20,21 respectively. The 1H NMR spectra of 5-7 showed one singlet at 1.25, 1.21, and 1.44 ppm for their CH3 and C(CH3)3 groups, and the singlet at 8.75 ppm and the doublets at 4.75 and 4.70 ppm can be assigned to the NH groups of 5-7, respectively. As expected, the coordination mode of the protonated DCC ligand in complex 6 has been proved by X-ray diffraction analysis of complex 7 (both 6 and 7 are the first butterfly cluster complexes generated by reactions of such μ-COcontaining cluster anions with electrophile DCC). The ORTEP drawing of 7 is shown in Figure 2, whereas Table 1 lists its selected bond lengths and angles. Complex 7 indeed consists of a protonated DCC ligand and a t-BuS ligand, which are bridged between two Fe(CO)3 units to form a single-butterfly cluster. The C(11)-N(1) bond length (1.290 A˚) is obviously shorter than that of C(11)-N(2) (1.365 A˚), consistent with C(11)-N(1) being a double bond and C(11)-N(2) being a single bond. In addition, the two cyclohexyl groups attached to N(1) and N(2) are in chair conformation, whereas the tert-butyl group is connected to the S(1) atom by an equatorial type of bond (the nonbonded angles C(7)-S(1) 3 3 3 N(1) = 166.3°, C(7)-S(1) 3 3 3 C(11) = 164.9°).23,24 Synthesis and Characterization of Triple- and DoubleButterfly Complexes 8a,b and 9a,b. More interestingly, we found that reactions of the same system prepared from MeC(CH2SH)3, Fe3(CO)12, and Et3N with electrophile CS2 followed by treatment with organic halides RX afforded not only triple-butterfly complexes 8a,b, but also double-butterfly complexes 9a,b (Scheme 5). Similar to the previously reported reactions of some μ-CO-containing butterfly anions with electrophiles CS2/RX,2b,27 8a,b were possibly produced through the initial nucleophilic attack of trianion A at CS2 to give intermediate M7 followed by nucleophilic attack of M7 at RX, whereas 9a,b were produced from nucleophilic attack of monoanion B at CS2 and subsequent attack of the resulting intermediate M8 at RX. Complexes 8a,b and 9a,b are air-stable solids, whose structures were characterized by elemental analysis and spectroscopy. The IR spectra of 8a,b and 9a,b displayed (25) Patin, H.; Mignani, G.; Mahe, C.; Le Marouille, J.-Y.; Southern, T. G.; Benoit, A.; Grandjean, D. J. Organomet. Chem. 1980, 197, 315. (26) Song, L.-C.; Gong, F.-H.; Meng, T.; Ge, J.-H.; Cui, L.-N.; Hu, Q.-M. Organometallics 2004, 23, 823. (27) Song, L.-C.; Fan, H.-T.; Hu, Q.-M.; Yang, Z.-Y.; Sun, Y.; Gong, F.-H. Chem.;Eur. J. 2003, 9, 170.

Figure 2. Molecular structure of 7 with 30% probability level ellipsoids.

three absorption bands in the range 2070-1988 cm-1 for their terminal carbonyls19 and one band in the range 1026-1015 cm-1 for their bridged CdS double bonds.2b,27 While the 1H NMR spectra of 8a and 9a showed one singlet at 2.57 or 2.80 ppm for their SCH3 groups, 8b and 9b displayed one singlet at 4.30 or 4.33 ppm for methylene groups in their benzyl groups. The structure of 9a was further confirmed by X-ray crystallography. The ORTEP drawing of 9a is shown in Figure 3, and its selected bond lengths and angles are given in Table 1. Figure 3 shows that it contains a methyl-substituted diiron propanedithiolate (PDT) moiety that is connected through a methylene group to the S(4) atom of an openbutterfly 2Fe2SC cluster core. In the diiron PDT moiety of 9a, the six-membered ring C(13)S(1)Fe(2)S(2)C(14)C(16) exists as a chair conformer and another six-membered ring C(13)S(1)Fe(1)S(2)C(14)C(16) as a boat conformer. The methyl carbon C(15) is axially bonded to C(16), whereas the methylene carbon C(17) is equatorially bound to both C(16) and S(4). This molecule is actually the most favored conformational isomer of 9a, since the bulky diiron PDT moiety and the bulky open-butterfly cluster both lie in equatorial positions.23,24 Synthesis and Characterization of Triple-Butterfly Complexes 10a,b. To further examine the influences of the utilized electrophiles and to prepare the μ4-S-containing triplebutterfly clusters, we carried out reactions of the same

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Scheme 6

Figure 3. Molecular structure of 9a with 30% probability level ellipsoids.

system prepared from MeC(CH2SH)3, Fe3(CO)12, and Et3N with electrophile (μ-S2)Fe2(CO)6 followed by treatment with electrophiles MeI and PhCH2Br. As a result, two μ4-Scontaining triple-butterfly complexes, 10a and 10b, were obtained. According to the similar reported reactions of the μ-CO-containing butterfly anions,27-30 complexes 10a, b could be suggested to be generated starting from monoanion B via the following steps: (i) the nucleophilic attack of the negatively charged Fe atom of B at one S atom of (μS2)Fe2(CO)6 followed by cleavage of its S-S bond and loss of one μ-CO ligand to give intermediate M9 and (ii) the nucleophilic substitution of MeI and PhCH2Br by M9 with (28) Song, L.-C.; Lu, G.-L.; Hu, Q.-M.; Fan, H.-T.; Chen, Y.; Sun, J. Organometallics 1999, 18, 3258. (29) Wang, Z.-X.; Jia, C.-S; Zhou, Z.-Y.; Zhou, X.-G. J. Organomet. Chem. 1999, 580, 201. (30) Song, L.-C.; Wang, J.-Y.; Gong, F.-H.; Cheng, J.; Hu, Q.-M. J. Organomet. Chem. 2004, 689, 930.

elimination of one molecule of [Et3NH]X (X = Cl, Br) (Scheme 6). The air-stable complexes 10a,b were characterized by elemental analysis and IR and 1H NMR spectroscopic methods. The IR spectra of 10a,b displayed three absorption bands in the range 2075-1990 cm-1 for their terminal carbonyls. The 1H NMR spectra of 10a,b showed one singlet at 1.12 and 1.09 ppm for their bridgehead-substituted CH3 groups, whereas the SCH3 group of 10a displayed a singlet at 2.18 ppm and the SCH2 group in the benzyl group of 10b exhibited an AB quartet in the range 3.57-3.72 ppm. Both structures of 10a and 10b were confirmed by X-ray crystallography. Their ORTEP drawings are shown in Figures 4 and 5, whereas Table 2 presents their selected bond lengths and angles. As shown in Figures 4 and 5, 10a,b both contain a methyl-substituted diiron PDT moiety that is connected through a methylene group to double-butterfly cluster core Fe(3)Fe(4)S(3)S(4)Fe(5)Fe(6)S(5) for 10a or Fe(1)Fe(2)S(3)S(1)Fe(3)Fe(4)S(2) for 10b. Although S(4) in 10a and S(1) in 10b are four-coordinated μ4-S atoms, the

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other sulfur atoms in 10a,b are all doubly bridged μ2-S atoms. It is worthy of note that in 10a,b the methyl carbon C(24) and the benzyl carbon C(19) are both equatorially bound to S(5) and S(2), respectively. This is obviously in order to avoid the axial-axial repulsions of the axially bonded methyl or benzyl group with the μ4-S-bonded cluster moieties (the Fe(3)Fe(4)S(3) moiety for 10a and the Fe(1)Fe(2)S(3) moiety for 10b).23,24 Interestingly, complexes 10a,b could be regarded as better structural models than complexes 1 and 2,16 since they contain not only a diiron PDT moiety but also a double-butterfly [4Fe3S] cluster that is very close in composition to the cubic [4Fe4S] cluster present in the natural enzymes.31,32

Conclusions Figure 4. Molecular structure of 10a with 30% probability level ellipsoids.

Figure 5. Molecular structure of 10b with 30% probability level ellipsoids.

We have carried out a systematic study on the formation and chemical reactivities regarding the novel butterfly threeμ-CO-containing trianion A and one-μ-CO-containing monoanion B produced by reaction of trithiol MeC(CH2SH)3 with Fe3(CO)12 and Et3N. It has been found that (i) triple-butterfly complex 3 can be obtained by reaction of trianion A with the heteroatom P-based electrophile Ph2PCl, whereas double-butterfly complex 4 is produced by reaction of monoanion B with the C-based electrophile PhC(Cl)dNPh; (ii) while double-butterfly complex 5 is obtained by reaction of monoanion B with electrophile PhNdCdS, double- and single-butterfly complexes 6 and 7 are prepared by reactions of monoanions B and [Fe2(μ-CO)(μ-t-BuS)(CO)6]- with electrophile DCC, respectively; (iii) both trianion A and monoanion B can react with electrophiles CS2 and RX to afford triple-butterfly complexes 8a,b and doublebutterfly complexes 9a,b, whereas monoanion B reacts with electrophiles (μ-S2)Fe2(CO)6 and RX to produce the μ4-S-containing triple-butterfly complexes 10a,b; and (iv) complexes 4-6, 9a,b, and 10a,b could be regarded as structural models for the active site of [FeFe]-hydroganases since they all contain a diiron PDT moiety. Further studies on chemical reactivities of the novel anions A and B and their wide applications in organometallic and biomimetic synthesis are in progress in our laboratory.

Table 2. Selected Bond Lengths (A˚) and Angles (deg) for 10a and 10b

Experimental Section

10a

General Comments. All reactions were carried out under an atmosphere of prepurified nitrogen by using standard Schlenk and vacuum-line techniques. Tetrahydrofuran (THF) was distilled from sodium/benzophenone ketyl under nitrogen. Et3N, Ph2PCl, PhNdCdS, C6H11NdCdNC6H11 (DCC), t-BuSH, CS2, MeI, and PhCH2Br were available from commercial suppliers and used without further purification. MeC(CH2SH)3,33 Fe3(CO)12,34 PhC(Cl)dNPh,35 and (μ-S2)Fe2(CO)623 were prepared according to literature procedures. Preparative TLC was carried out on glass plates (25  15  0.25 cm) coated with silica gel G (10-40 μm). IR spectra were recorded on a Bruker Vector 22 or a Bio-Rad FTS 135 infrared spectrophotometer. 1H (31P) NMR spectra were obtained with a

Fe(1)-S(1) Fe(1)-S(2) Fe(3)-S(3) Fe(3)-Fe(4)

2.2535(16) 2.2570(14) 2.2859(13) 2.5423(10)

Fe(4)-S(4) Fe(1)-Fe(2) Fe(5)-S(4) Fe(5)-Fe(6)

2.2376(13) 2.5011(11) 2.2423(13) 2.5398(10)

S(1)-Fe(1)-Fe(2) S(2)-Fe(1)-Fe(2) S(4)-Fe(3)-Fe(4) S(3)-Fe(3)-Fe(4) S(4)-Fe(5)-S(5)

56.13(4) 56.53(4) 55.32(4) 55.58(4) 76.79(5)

S(4)-Fe(5)-Fe(6) Fe(2)-S(1)-Fe(1) Fe(4)-S(3)-Fe(3) Fe(6)-S(4)-Fe(5) Fe(4)-S(4)-Fe(3)

55.44(4) 67.51(5) 67.97(4) 69.03(4) 69.12(4)

Fe(1)-S(1) Fe(1)-Fe(2) Fe(2)-S(1) Fe(3)-Fe(4)

2.2419(18) 2.5311(13) 2.2455(17) 2.5436(13)

Fe(5)-S(5) Fe(5)-Fe(6) Fe(3)-S(1) Fe(4)-S(2)

2.2602(19) 2.4962(14) 2.2423(18) 2.2578(19)

S(1)-Fe(1)-S(3) S(1)-Fe(1)-Fe(2) S(3)-Fe(1)-Fe(2) S(1)-Fe(4)-S(2) S(1)-Fe(4)-Fe(3)

76.90(6) 55.73(5) 55.94(5) 77.08(6) 55.42(5)

S(2)-Fe(4)-Fe(3) S(4)-Fe(5)-S(5) S(3)-Fe(2)-Fe(1) S(1)-Fe(3)-S(2) S(4)-Fe(5)-Fe(6)

55.53(5) 83.78(7) 56.43(5) 77.28(6) 56.79(5)

10b

(31) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853. (32) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; FontecillaCamps, J. C. Structure 1999, 7, 13. (33) Kolomyjec, C.; Whelan, J.; Bosnich, B. Inorg. Chem. 1983, 22, 2343. (34) King, R. B. Organometallic Syntheses; Transition-Metal Compounds; Academic Press: New York, 1965; Vol. 1, p 95. (35) Vaughan, W. R.; Carlson, R. D. J. Am. Chem. Soc. 1962, 84, 769.

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Bruker Avance 300 spectrometer. Elemental analyses were performed with an Elementar Vario EL analyzer. Melting points were determined on a Yanaco MP-500 apparatus and are uncorrected. Standard in Situ Preparation of the [Et3NH] Salts of Trianion {[Fe2(μ-CO)(CO)6]3[(μ-SCH2)3CMe]}3- (A) and Monoanion {[Fe2(μ-CO)(μ-SCH2)(CO)6][[Fe2(μ-SCH2)2CMe(CO)6]}- (B). A 100 mL three-necked flask fitted with a magnetic stir-bar, a rubber septum, and a nitrogen inlet tube was charged with trithiol MeC(CH2SH)3 (0.168 g, 1.0 mmol), Fe3(CO)12 (1.512 g, 3.0 mmol), Et3N (0.42 mL, 3.0 mmol), and THF (20 mL). The mixture was stirred at room temperature for 0.5 h to produce a brown-red solution containing the [Et3NH] salts of trianion A and monoanion B, which was employed immediately in the following reactions with electrophiles to prepare the corresponding double- and triple-butterfly Fe/S cluster complexes. The yields of all the products were calculated on the basis of the starting trithiol MeC(CH2SH)3 used for preparation of the standard [Et3NH] salt solution described above. Preparation of [Fe2(μ-Ph2P)(CO)6]3[(μ-SCH2)3CMe] (3). While stirring, the above-prepared standard THF solution was cooled to -40 °C, and then Ph2PCl (0.78 mL, 4.5 mmol) was added. After the mixture was warmed to room temperature, it was stirred at this temperature for 24 h. Solvent was removed under reduced pressure and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (v/v = 1:4) as eluent. From the main orange-red band, complex 3 (0.620 g, 40%) was obtained as a red solid. Mp: 123-124 °C. Anal. Calcd for C59H39Fe6O18P3S3: C, 45.42; H, 2.52. Found: C, 45.70; H, 2.56. IR (KBr disk): νCtO 2060 (s), 2023 (vs), 1982 (vs) cm-1. 1 H NMR (300 MHz, CDCl3, TMS): 1.15 (s, 3H, CH3), 2.63 (s, 6H, 3CH2), 7.26-7.60 (m, 30H, 6C6H5) ppm. 31P NMR (121.5 MHz, CDCl3, 85% H3PO4): 140.86 (s) ppm. Preparation of [Fe2(μ-PhCdNPh)(μ-SCH2)(CO)6][Fe2(μSCH2)2CMe(CO)6] (4). To the above-prepared standard THF solution was added PhC(Cl)dNPh (1.20 g, 5.6 mmol) at room temperature, and then the mixture was stirred at this temperature overnight. The same workup as that of 3 was utilized to give complex 4 (0.210 g, 23%) as a red solid. Mp: 153-154 °C. Anal. Calcd for C30H19Fe4NO12S3: C, 39.81; H, 2.12; N, 1.55. Found: C, 39.75; H, 2.15; N, 1.53. IR (KBr disk): νCtO 2069 (s), 2031 (vs), 2002 (vs), 1985 (vs); νCdN 1568 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.25 (s, 3H, CH3), 2.28-2.72 (m, 6H, 3CH2), 6.54-7.15 (m, 10H, 2C6H5) ppm. Preparation of [Fe2(μ-SdCNHPh)(μ-SCH2)(CO)6][Fe2(μSCH2)2CMe(CO)6] (5). The same procedure was followed as for 4, but C6H5NdCdS (0.72 mL, 6.0 mmol) was used instead of PhC(Cl)dNPh. From the main red band, complex 5 (0.203 g, 24%) was obtained as a red solid. Mp: 59-61 °C. Anal. Calcd for C24H15Fe4NO12S4: C, 33.48; H, 1.76; N, 1.63. Found: C, 33.32; H, 1.78; N, 1.75. IR (KBr disk): νCtO 2068 (s), 2031 (vs), 1979 (vs); νCdS 934 (w) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.25 (s, 3H, CH3), 2.25-2.81 (m, 6H, 3CH2), 7.31-7.39 (m, 5H, C6H5), 8.75 (s, 1H, NH) ppm. Preparation of [Fe2(μ-C6H11NdCNHC6H11)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (6). To the stirred standard THF solution prepared above was added C6H11NdCdNC6H11 (0.824 g, 4.0 mmol), and then the mixture was stirred at 50 °C for 2 h and at room temperature overnight. After the same workup as that of 3, complex 6 (0.258 g, 28%) was obtained as a red solid. Mp: 86-88 °C. Anal. Calcd for C30H32Fe4N2O12S3: C, 38.66; H, 3.46; N, 3.01. Found: C, 38.65; H, 3.51; N, 2.91. IR (KBr disk): νCtO 2045 (s), 2021 (vs), 1980 (vs); νCdN 1530 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.21 (s, 3H, CH3), 1.06-1.92 (m, 20H, 10CH2 of two cyclohexyls), 2.24-2.54 (m, 6H, 3SCH2), 2.77-2.87 (m, 1H, NHCH of one cyclohexyl), 3.47-3.59 (m, 1H, NCH of another cyclohexyl), 4.75 (d, 1H, J = 9.3 Hz, NH) ppm. Preparation of Fe2(μ-t-BuS)(μ-C6H11NdCNHC6H11)(CO)6 (7). A three-necked flask described above was charged with

Song et al. t-BuSH (0.090 g, 1.0 mmol), Fe3(CO)12 (0.504 g, 1.0 mmol), Et3N (0.14 mL, 1.0 mmol), and THF (20 mL). The mixture was stirred at room temperature for 0.5 h to give a brown-red solution containing the [Et3NH] salt of monoanion [Fe2(μCO)(μ-t-BuS)(CO)6]-. To this solution was added DCC (0.412 g, 2.0 mmol), and then the mixture was stirred at 50 °C for 5 h and at room temperature overnight. After solvent was removed at reduced pressure, the residue was subjected to TLC separation using CH2Cl2/petroleum ether (v/v = 1:8) as eluent. From the main red band, complex 7 (0.138 g, 24% based on the starting t-BuSH) was obtained as a red solid. Mp: 108-110 °C. Anal. Calcd for C23H32Fe2N2O6S: C, 47.94; H, 5.60; N, 4.86. Found: C, 47.96; H, 5.62; N, 4.89. IR (KBr disk): νCtO 2054 (s), 2015 (vs), 1978 (vs), 1960 (s); νCdN 1537 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.44 (s, 9H, C4H9), 1.02-1.92 (m, 20H, 10CH2 of two cyclohexyls), 2.77-2.87 (m, 1H, NHCH of one cyclohexyl), 3.48-3.59 (m, 1H, NCH of another cyclohexyl), 4.70 (d, 1H, J = 9.3 Hz, NH) ppm. Preparation of [Fe2(μ-MeSCdS)(CO)6]3[(μ-SCH2)3CMe] (8a) and [Fe2(μ-MeSCdS)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (9a). To the above-prepared standard THF solution was added CS2 (0.36 mL, 6.0 mmol), and then the mixture was stirred at room temperature for 0.5 h. After MeI (0.36 mL, 6.0 mmol) was added, the new mixture continued to be stirred at room temperature for 24 h. Solvent was removed under reduced pressure, and the residue was subjected to TLC separation using CH2Cl2/petroleum ether (v/v = 1:4) as eluent. Two major red bands were developed. Complex 8a (0.312 g, 24%) was obtained from the second major band as a red solid. Mp: 112-114 °C. Anal. Calcd for C29H18Fe6O18S9: C, 27.25; H, 1.42. Found: C, 27.56; H, 1.69. IR (KBr disk): νCtO 2067 (s), 2030 (vs), 1992 (vs); νCdS 1019 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.56 (s, 3H, CH3), 2.57 (s, 9H, 3SCH3), 2.88-3.16 (m, 6H, 3CH2) ppm. Complex 9a (0.312 g, 38%) was obtained from the first major band as a red solid. Mp: 135-137 °C. Anal. Calcd for C19H12Fe4O12S5: C, 27.97; H, 1.48. Found: C, 27.91; H, 1.58. IR (KBr disk): νCtO 2066 (s), 2026 (vs), 1988 (vs); νCdS 1026 (m) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.55 (s, 3H, CH3), 1.77-2.59 (m, 6H, 3CH2), 2.80 (s, 3H, SCH3) ppm. Preparation of [Fe2(μ-PhCH2SCdS)(CO)6]3[(μ-SCH2)3CMe] (8b) and [Fe2(μ-PhCH2SCdS)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (9b). The same procedure was followed as for 8a and 9a, but PhCH2Br (0.72 mL, 6.0 mmol) was used instead of MeI. Similarly, Complex 8b (0.240 g, 16%) was obtained from the second major band as a red solid. Mp: 119-121 °C. Anal. Calcd for C47H30Fe6O18S9: C, 37.48; H, 2.01. Found: C, 37.78; H, 1.97. IR (KBr disk): νCtO 2066 (s), 2035 (vs), 1993 (vs); νCdS 1015 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.57 (s, 3H, CH3), 2.90-3.15(m, 6H, 3SCH2), 4.30 (s, 6H, 3C6H5CH2), 7.15-7.40 (m, 15H, 3C6H5) ppm. Complex 9b (0.270 g, 30%) was obtained from the first major band as a red solid. Mp: 145-147 °C. Anal. Calcd for C25H16Fe4O12S5: C, 33.66; H, 1.81. Found: C, 33.38; H, 2.03. IR (KBr disk): νCtO 2070 (s), 2032 (vs), 1992 (vs); νCdS 1016 (s) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.22 (s, 3H, CH3), 2.21-2.66 (m, 6H, 3SCH2), 4.33 (s, 2H, C6H5CH2), 7.23-7.35 (m, 5H, C6H5) ppm. Preparation of [Fe2(μ-MeS)(CO)6][Fe2(μ4-S)(μ-SCH2) (CO)6][Fe2(μ-SCH2)2CMe(CO)6] (10a). To the above-prepared standard THF solution was added (μ-S2)Fe2(CO)6 (1.032 g, 3.0 mmol), and then the mixture was stirred at room temperature for 1 h. After MeI (0.36 mL, 6.0 mmol) was added, the new mixture was stirred at room temperature for 24 h. Solvent was removed at reduced pressure to give a residue, which was subjected to TLC separation using CH2Cl2/petroleum ether (v/v = 1:4) as eluent. From the major red band, 10a (0.314 g, 29%) was obtained as a red solid. Mp: 84-86 °C. Anal. Calcd for C24H12Fe6O18S5: C, 26.60; H, 1.12. Found: C, 26.58; H, 1.18. IR (KBr disk): νCtO 2074 (s), 2039 (vs), 1990 (vs) cm-1. 1H NMR (300 MHz, CDCl3, TMS): 1.12 (s, 3H, CH3), 2.09-2.48 (m, 6H, 3CH2), 2.18 (s, 3H, SCH3) ppm.

Article

Organometallics, Vol. 29, No. 1, 2010

Table 3. Crystal Data and Structure Refinements for 3, 7, and 9a 3 mol formula

C59H39Fe6O18P3S3 3 0.5C6H14 3 0.25CH2Cl2 mol wt 1624.44 crystsyst triclinic space group P1h a/A˚ 15.037(4) b/A˚ 15.770(5) c/A˚ 18.792(5) R/deg 75.751(5) β/deg 67.156(5) γ/deg 66.433(6) 3742.2(19) V/A˚3 Z 2 -3 1.444 Dc/g 3 cm 1.360 abs coeff/mm-1 F(000) 1646 index ranges -7 e h e17 -16 e k e18 -21 e l e22 no. of reflns 19 530 no. of indep reflns 13 107 0.0646 Rint 50.02 2θmax/deg R 0.0749 0.1543 Rw goodness of fit 0.974 0.889/-0.392 largest diff peak and hole /eA˚-3

7

Table 4. Crystal Data and Structure Refinements for 10a and 10b

9a

C23H32Fe2N2O6S

C19H12Fe4O12S5

576.27 orthorhombic Pbca 18.929(4) 12.633(3) 22.822(4) 90 90 90 5457(2) 8 1.403 1.177 2400 -23 e h e 17 -15 e k e 15 -25 e l e 28 29 278 5625 0.1744 53.06 0.0526 0.1069 1.033 0.476/-0.606

815.99 monoclinic C2/c 24.812(10) 9.725(4) 26.843(11) 90 114.184(7) 90 5909(4) 8 1.835 2.334 3248 -29 e h e 29 -6 e k e 11 -31 e l e 28 14 770 5210 0.0612 50.02 0.0782 0.1806 1.089 1.141/-0.703

Preparation of [Fe2(μ-PhCH2S)(CO)6][Fe2(μ4-S)(μ-SCH2)(CO)6][Fe2(μ-SCH2)2CMe(CO)6] (10b). The same procedure was followed as for 10a, except that PhCH2Br (0.72 mL, 6.0 mmol) was used in place of MeI. From the major red band, 10b (0.275 g, 24%) was obtained as a red solid. Mp: 90-91 °C. Anal. Calcd for C30H16Fe6O18S5: C, 31.07; H, 1.39. Found: C, 30.80; H, 1.69. IR (KBr disk): νCtO 2075 (s), 2038 (vs), 1992 (vs) cm-1. 1 H NMR (300 MHz, CDCl3, TMS): 1.09 (s, 3H, CH3), 2.08-2.44 (m, 6H, 3SCH2), 3.57, 3.61, 3.68, 3.72 (q, AB pattern, J = 13.2 Hz, 2H, C6H5CH2), 7.35 (s, 5H, C6H5) ppm. X-ray Structure Determinations of 3, 7, 9a, and 10a,b. Single crystals of 3, 7, 9a, and 10a,b qualified for X-ray diffraction analysis were grown by slow evaporation of their CH2Cl2/ hexane solutions at about 4 °C. Each single crystal of these complexes was mounted on a Bruker SMART 1000 automated diffractometer. Data were collected at room temperature by using a graphite monochromator with Mo KR radiation (λ = (36) Sheldrick, G. M. SADABS, A Program for Empirical Absorption Correction of Area Detector Data; University of G€ottingen: Germany, 1996.

213

mol formula mol wt cryst syst space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z Dc/g 3 cm-3 abs coeff /mm-1 F(000) index ranges no. of reflns no. of indep reflns Rint 2θmax/deg R Rw goodness of fit largest diff peak and hole/e A˚-3

10a

10b

C24H12Fe6O18S5 1083.74 orthorhombic Fdd2 32.674(4) 55.010(6) 8.6148(10) 90 90 90 15484(3) 16 1.860 2.528 8576 -38 e h e 40 -57 e k e 68 -10 e l e 10 22 512 7873 0.0387 52.86 0.0350 0.0622 1.025 0.483/-0.279

C30H16Fe6O18S5 3 CH2Cl2 1244.75 triclinic P1 9.231(2) 13.084(3) 19.471(4) 79.196(4) 85.815(4) 84.608(4) 2296.2(8) 2 1.800 2.257 1236 -7 e h e 11 -16 e k e 12 -24 e l e 21 13 497 9384 0.0444 52.94 0.0564 0.1218 0.970 0.840/-0.760

0.71073 A˚) in the ω-φ scanning mode. Absorption correction was performed by the SADABS program.36 The structures were solved by direct methods using the SHELXS-97 program37 and refined by full-matrix least-squares techniques (SHELXL-97)38 on F2. Hydrogen atoms were located using the geometric method. Details of crystal data, data collections, and structure refinements are summarized in Tables 3 and 4, respectively.

Acknowledgment. We are grateful to the National Natural Science Foundation of China and the Research Fund for the Doctoral Program of Higher Education of China for financial support. Supporting Information Available: Full tables of crystal data, atomic coordinates and thermal parameters, and bond lengths and angles for 3, 7, 9a, and 10a,b as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. (37) Sheldrick, G. M. SHELXS97, A Program for Crystal Structure Solution; University of G€ottingen: Germany, 1997. (38) Sheldrick, G. M. SHELXL97, A Program for Crystal Structure Refinement; University of G€ottingen: Germany, 1997.