Synthesis and Structures of Low-Valent Alkynyl Tin and Germanium

Jul 13, 2010 - X-ray Crystal Structures. ..... structures in the solution phase, despite the sharp difference between their solid-state .... 506 nm (s...
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Organometallics 2010, 29, 5585–5590 DOI: 10.1021/om100492u

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Synthesis and Structures of Low-Valent Alkynyl Tin and Germanium Complexes Supported by Terphenyl Ligands: Heavier Group 14 Element Enediyne Analogues† Hao Lei, James C. Fettinger, and Philip P. Power* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616 Received May 19, 2010

The reaction of (Ar0 SnCl)2 (Ar0 = -C6H3-2,6-(C6H3-2,6-iPr2)2) with LiCtCR (R = SiMe3 or tBu) afforded the orange-red, alkynyl-substituted, symmetric distannene Ar0 (Me3SiCtC)SnSn(CtCSiMe3)Ar0 (1) or the blue unsymmetric stannylstannylene Ar0 SnSn(CtCtBu)2Ar0 (2), respectively, whose structures were determined by X-ray crystallography. In solution at room temperature, however, both compounds have very similar UV-vis and 1H NMR spectra, consistent with the formation of monomeric stannylene units. Cooling a solution of 1 resulted in a color change to pink and the appearance of a new UV-vis absorption at 506 nm, consistent with the formation of a symmetric dimeric structure at low temperature. The different structures of 1 and 2 in the solid state are probably a result of packing effects. In contrast, the analogous reactions of (Ar0 GeCl)2 with LiCtCR (R=SiMe3 or tBu) resulted in the exclusive formation of digermene derivatives Ar0 (RCtC)GeGe(CtCR)Ar0 (R=SiMe3 (3), tBu (4)), which maintain their dimeric structures in solution. Introduction The coordination chemistry of metal alkynyl (R-CtC-) compounds has attracted considerable interest in part because of the isoelectronic relationship between the alkynyl group and other well-known ligands such as CN-, CO, and N2.1 The linear geometry, structural rigidity, and possible πelectron conjugation have made metal alkynyl species promising candidates as building blocks for nonlinear optical materials, molecular electronics, and luminescent materials.2 While transition metal alkynyl complexes have been intensely investigated, relatively less attention has been focused on their p-block metal derivatives.3 Tetravalent LnM-CtC-R (M = Si, Ge, Sn, Pb) species have been reported to react with organoboranes via 1,1-organoboration to afford organometallic-substituted alkenes.4 Although various low-valent heavier group 14 element complexes (especially analogues of alkene,5 alkyne,5 and carbene5) have been prepared and studied † Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth. *To whom correspondence should be addressed. E-mail: pppower@ ucdavis.edu. (1) Nast, R. Coord. Chem. Rev. 1982, 47, 89–124. (2) (a) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586–2617. (b) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555–563. (3) Zheng, W.; Roesky, H. W. J. Chem. Soc., Dalton Trans. 2002, 2787–2796. (4) (a) Wrackmeyer, B. Coord. Chem. Rev. 1995, 145, 125–156. (b) Wrackmeyer, B.; Milius, W.; Klimkina, E. V.; Bubnov, Y. N. Chem.— Eur. J. 2001, 7, 775–782. (c) Wrackmeyer, B.; Pedall, A.; Milius, W.; Tok, O. L.; Bubnov, Y. N. J. Organomet. Chem. 2002, 649, 232–245. (d) Wrackmeyer, B. Heteroat. Chem. 2006, 17, 188–208. (5) For recent reviews, see: (a) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479–3511. (b) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev. 2000, 210, 251–277. (c) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39–52. (d) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165–4183. (e) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209–223. (f) Rivard, E.; Power, P. P. Inorg. Chem. 2007, 46, 10047–10064. (g) Power, P. P. Organometallics 2007, 26, 4362–4372.

r 2010 American Chemical Society

in recent decades, it is surprising that reports on such lowvalent compounds with alkynyl substituents are very limited.6-9 In 1999, Jutzi and co-workers reported the preparation of ethynyl-substituted three-coordinate germanium(II) monomers MamxGe(CtCR) (Mamx =2,4-di-tert-butyl-6-((dimethylamino)methyl)phenyl; R = H and Ph),6 in which germanium is bound to the alkynyl group, the ipso-carbon, and the nitrogen of the bidentate Mamx ligand. The compounds were obtained by reaction of MamxGeCl with NaCtCH or LiCtCPh, respectively. More recently, the monomeric phenylacetyl Ge(II) complex Ge(CtCPh){N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)} was synthesized by employing the bidentate pyridyl-enamido ligand.7 Interestingly, Driess and co-workers showed that such alkynyl germylene species were accessible through the reaction of ylide-like N-heterocyclic germylene LGe: (L = CH[(CdCH2)CMe][N(aryl)]2, aryl = 2, 6-iPr2C6H3) with phenylacetylene.8 It is noteworthy that all these reported examples of alkynyl germylenes are stabilized by intramolecular lone pair donation from N to a Ge atom. In parallel with our work described here, Tokitoh and co-workers very recently described two 1,2-dialkynyldisilene complexes, which can be viewed as silicon analogues of (E)enediyne due to possible conjugation between SidSi and CtC units.9 To the best of our knowledge, no structurally characterized low-valent alkynyl tin compound, base-stabilized or not, and no germanium(II) alkynyls supported by monodentate co-ligands have been reported. Herein, we describe that the reaction of (Ar0 SnCl)2 with LiCtCSiMe3 results in the alkynyl-substituted symmetric distannene (6) Jutzi, P.; Keitemeyer, S.; Neumann, B.; Stammler, H.-G. Organometallics 1999, 18, 4778–4784. (7) Leung, W.-P.; So, C.-W.; Chong, K.-H.; Kan, K.-W.; Chan, H.-S.; Mak, T. C. W. Organometallics 2006, 25, 2851–2858. (8) Yao, S.; van W€ ullen, C.; Driess, M. Chem. Commun. 2008, 5393–5395. (9) Sato, T.; Mizuhata, Y.; Tokitoh, N. Chem. Commun. 2010, 4402-4404. Published on Web 07/13/2010

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Table 1. Selected Crystallographic Data and Collection Parameters for 1-4

formula fw color habit space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z dcalcd, Mg/m3 θ range, deg μ, mm-1 no. of obsd data, I > 2σ(I) R1 (obsd data) wR2 (all data)

1 3 2C6H6

2 3 C6H12

3

4 3 C7H8

C82H104Si2Sn2 1383.21 orange block P1 12.7099(4) 13.3036(4) 13.6135(4) 63.3794(3) 74.5853(3) 62.9725(3) 1827.50(10) 1 1.257 2.56-27.58 0.758 8125 0.0226 0.0618

C78H104Sn2 1278.99 blue rod P2(1)/n 13.0040(6) 23.6350(11) 22.6519(11) 90 97.0350(10) 90 6909.6(6) 4 1.229 2.58-27.50 0.763 14 587 0.0286 0.0733

C70H92Ge2Si2 1134.80 yellow block P1 12.4748(14) 12.5284(8) 12.7044(8) 115.8480(10) 98.2320(10) 107.5880(10) 1613.5(2) 1 1.168 2.49-25.25 1.007 5093 0.0302 0.0776

C79H100Ge2 1194.77 yellow block P1 12.9836(8) 12.9845(8) 13.0755(8) 64.2466(7) 62.1458(7) 65.1210(7) 1688.49(18) 1 1.175 2.92-27.60 0.932 6995 0.0283 0.0747

Scheme 1. Synthetic Routes of 1-4

Ar0 (Me3SiCtC)SnSn(CtCSiMe3)Ar0 (1), whereas the reaction with LiCtCtBu afforded the unsymmetric stannylstannylene Ar0 SnSn(CtCtBu)2Ar0 (2). In contrast, the reactions of (Ar0 GeCl)2 with LiCtCR (R=SiMe3 or tBu) led to the exclusive formation of the symmetric digermene derivatives Ar0 (RCtC)GeGe(CtCR)Ar0 (R = SiMe3 (3), tBu (4)).

Results and Discussion Synthesis. The reaction of 2 equiv of LiCtCR (R = SiMe3 or tBu) with (Ar0 ECl)2 (E= Sn or Ge) in hexanes at low temperature afforded the alkynyl derivatives 1-4 in good yields (Scheme 1). Complex 1 was isolated as orange-red crystals from a saturated hexane/benzene solution, while blue crystals of 2 were obtained from a hexane solution after cooling at -18 °C. Both 3 and 4 were isolated as yellow crystals, from either hexane or a hexane/toluene mixture. X-ray Crystal Structures. The solid-state structures of all four compounds were determined by X-ray crystallography. Compound 1 3 2C6H6 crystallizes in the triclinic space group P1 (Table 1). It has a trans-pyramidal structure (Figure 1) with the sum of interligand angles around tin of 325.4°. The (10) Stanciu, C.; Richards, A. F.; Power, P. P. J. Am. Chem. Soc. 2004, 126, 4106–4107. (11) (a) St€ urmann, M.; Saak, W.; Klinkhammer, K. W.; Weidenbruch, M. Z. Anorg. Allg. Chem. 1999, 625, 1955–1956. (b) Goldberg, D. E.; Hitchcock, P. B.; Lappert, M. F.; Thomas, K. M.; Thorne, A. J.; Fjeldberg, T.; Haaland, A.; Schilling, B. E. R. J. Chem. Soc., Dalton Trans. 1986, 2387– 2394. (c) Klinkhammer, K. W.; F€assler, T. F.; Gr€utzmacher, H. Angew. Chem., Int. Ed. 1998, 37, 124–126. (d) Klinkhammer, K. W. Polyhedron 2002, 21, 587–598. (e) Klinkhammer, K. W.; Schwarz, W. Angew. Chem., Int. Ed. 1995, 34, 1334–1336.

Figure 1. Solid-state molecular structure of 1 3 2C6H6 (H atoms and solvent molecules are not shown; thermal ellipsoids are shown at 30% probability). Selected bond distances (A˚) and angles (deg): Sn(1)-Sn(1A) 2.85126(19), Sn(1)-C(1) 2.2031(14), Sn(1)-C(31) 2.1426(16), C(31)-C(32) 1.219(2), C(1)-Sn(1)Sn(1A) 123.95(4), C(1)-Sn(1)-C(31) 94.75(5), C(31)-Sn(1)Sn(1A) 106.73(4), Sn(1)-C(31)-C(32) 174.14(14), Si(1)-C(32)C(31) 176.87(15).

angle between the C1 Sn1 C31 plane and the Sn1-Sn1A bond is 49.36(4)°. The Sn-Sn bond length is 2.85126(19) A˚, which is almost 0.1 A˚ longer than the Sn-Sn distance in the analogous benzyl complex (Ar0 SnCH2C6H4-4-tBu)2 (2.7705(8) A˚).10 It is also longer than the range of Sn-Sn distances (2.702-2.833 A˚) in most reported distannene R2SnSnR2 structures;11 the only exception is the sterically very crowded tetrakis(2-tBu-4,5,6trimethylphenyl)distannene,12 which features a Sn-Sn distance of 2.910(1) A˚. The long Sn-Sn length in 1 indicates substantial weakening of the interaction between two Sn atoms, which may be due, at least in part, to p-π conjugation between CtC bonds and the metals. The Sn1-C1 bond length (2.2031(14) A˚) is longer than the Sn1-C31 (alkynyl) distance (2.1426(16) A˚), which is most likely a result of the different hybridization of the ligand-bound carbons, although steric effects may also play a role. The CtC bond length (1.219(2) A˚) (12) Weidenbruch, M.; Kilian, H.; Peters, K.; von Schnering, H. G.; Marsmann, H. Chem. Ber. 1995, 128, 983–985.

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Figure 2. Solid-state molecular structure of 2 3 C6H12 (H atoms and solvent molecules are not shown; thermal ellipsoids are shown at 30% probability). Selected bond distances (A˚) and angles (deg): Sn(1)-Sn(2) 2.9038(2), Sn(1)-C(1) 2.1888(19), Sn(1)-C(31) 2.1021(19), Sn(1)-C(37) 2.129(2), Sn(2)-C(43) 2.2207(18), C(31)-C(32) 1.199(3), C(37)-C(38) 1.199(3), C(1)Sn(1)-Sn(2) 118.56(5), C(1)-Sn(1)-C(31) 109.69(7), C(1)Sn(1)-C(37) 96.23(7), C(31)-Sn(1)-Sn(2) 118.42(5), C(37)Sn(1)-Sn(2) 104.75(5), C(31)-Sn(1)-C(37) 105.63(7), Sn(1)C(31)-C(32) 168.55(18), C(31)-C(32)-C(33) 178.2(2), Sn(1)C(37)-C(38) 177.05(18), C(37)-C(38)-C(39) 176.6(2), Sn(1)Sn(2)-C(43) 103.35(5).

is slightly longer than those found in tetrakis(trimethylsilylalkynyl)tin (1.190(9)-1.202(7) A˚),13 also suggesting some πinteraction between Sn and the alkynyl group. It is noteworthy that the CtC distance in 1 is comparable to those in recently reported 1,2-dialkynyldisilenes Bbt(RCtC)SiSi(CtCR)Bbt (Bbt =2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl, R = SiMe3 (1.213(6) A˚) or Ph (1.214(3) A˚)).9 Complex 2 crystallizes with 1 equiv of methylcyclopentane (a component of solvent hexanes) in the monoclinic space group P21/n (Table 1). The structure consists of an unsymmetric dimer with two metal atoms in þ1 and þ3 oxidation states, respectively (Figure 2). The Sn1 atom is tetracoordinate with bonds to two alkynyl groups (C31 and C37), one terphenyl ligand (C1), and Sn2. On the other hand, the Sn2 atom is bonded to C43 and Sn1 and, hence, is divalent tin. The Sn1-Sn2 distance (2.9038(2) A˚) in 2 is indistinguishable from that (2.9034(13) A˚) in the unusual cyclometalated stannylstannylene obtained upon photolysis of diazomethylstannylene Me3SiC(dN2)(Ar*)Sn: (Ar* = -C6H3-2,6-Trip2, Trip = -C6H2-2,4,6-iPr3)14 and is comparable to that in Ar* SnSnMe2Ar* (2.8909(2) A˚).15 However, it is slightly shorter than the Sn-Sn length in Ar*SnSnPh2Ar* (2.9688(5) A˚),16 possibly due to less bulky alkynyl and aryl substituents in 2. The coordination at Sn1 has a distorted tetrahedral geometry, while Sn2 has a bent coordination with a Sn1-Sn2-C43 angle of 103.35(5)o, which is slightly wider than that in Ar*SnSnMe2Ar* (101.17(5)o),15 but narrower than that in Ar*SnSnPh2Ar* (13) Dallaire, C.; Brook, M. A.; Bain, A. D.; Framton, C. S.; Britten, J. F. Can. J. Chem. 1993, 71, 1676–1683. (14) Setaka, W.; Hirai, K.; Tomioka, H.; Sakamoto, K.; Kira, M. Chem. Commun. 2008, 6558–6560. (15) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39, 5444–5449. (16) Philips, A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 7520–7521.

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Figure 3. Solid-state molecular structure of 3 (H atoms and solvent molecules are not shown; thermal ellipsoids are shown at 30% probability). Selected bond distances (A˚) and angles (deg): Ge(1)-Ge(1A) 2.3224(4), Ge(1)-C(1) 1.9725(19), Ge(1)-C(31) 1.911(2), C(31)-C(32) 1.205(3), C(1)-Ge(1)-Ge(1A) 118.05(6), C(1)-Ge(1)-C(31) 114.21(8), C(31)-Ge(1)-Ge(1A) 109.68(6), Ge(1)-C(31)-C(32) 167.38(18), Si(1)-C(32)-C(31) 176.14(19).

(108.48(6)o).16 It is noteworthy that the CtC bond distances in 2 (1.199(3) A˚) are shorter than that in 1 (1.219(2) A˚) and consistent with those values in tetrakis(trimethylsilylalkynyl)tin (1.190(9)-1.202(7) A˚),13 consistent with little or no π-interaction between the alkynyl groups and the tetravalent tin atom. Both digermene derivatives 3 (Figure 3) and 4 3 C7H8 (Figure 4) crystallize in the triclinic space group P1 (Table 1). The compounds feature a trans-pyramidal structure similar to 1, with a lower degree of pyramidalization at the tetrels (sums of interligand angles around Ge atoms are 341.9° (3) and 342.5° (4)). This can be rationalized in terms of the decreased mixing of the σ* and π orbital of the heavier group 14 element double bond in the Ge derivative relative to Sn.17 The angles between the C1 Ge1 C31 plane and the Ge1-Ge1A bond in 3 and 4 are 41.66(1)° and 41.43(1)° respectively. The almost identical Ge-Ge bond lengths in 3 (2.3224(4) A˚) and 4 (2.3239(3) A˚) lie about midway between the shortest (ca. 2.21 A˚) and longest (ca. 2.42 A˚) bonds observed for R2GeGeR2 species.11b,18 The Ge1-C31 distances in 3 (17) (a) Power, P. P. Chem. Rev. 1999, 99, 3463–3503. (b) Grev, R. S. Adv. Organomet. Chem. 1991, 33, 125–170. (18) (a) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.; Thorne, A. J. J. Chem. Soc., Chem. Commun. 1984, 480–482. (b) Batcheller, S. A.; Tsumuraya, T.; Tempkin, O.; Davis, W. M.; Masamune, S. J. Am. Chem. Soc. 1990, 112, 9394–9395. (c) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 3204–3205. (d) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232– 12233. (e) Sch€afer, H.; Saak, W.; Weidenbruch, M. Organometallics 1999, 18, 3159–3163. (f) Hurni, K. L.; Rupar, P. A.; Payne, N. C.; Baines, K. M. Organometallics 2007, 26, 5569–5575. (g) Snow, J. T.; Murakami, S.; Masamune, S.; Williams, D. J. Tetrahedron Lett. 1984, 25, 4191–4194. (h) Stender, M.; Pu, L.; Power, P. P. Organometallics 2001, 20, 1820–1824. (i) Weidenbruch, M.; St€urmann, M.; Kilian, H.; Pohl, S.; Saak, W. Chem. Ber. 1997, 130, 735–738. (j) Kira, M.; Iwamoto, T.; Maruyama, T.; Kabuto, C.; Sakurai, H. Organometallics 1996, 15, 3767–3769. (k) Pampuch, B.; Saak, W.; Weidenbruch, M. J. Organomet. Chem. 2006, 691, 3540–3544. (l) Tokitoh, N.; Kishikawa, K.; Okazaki, R.; Sasamori, T.; Nakata, N.; Takeda, N. Polyhedron 2002, 21, 563–577.

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Figure 5. Temperature-dependent UV-vis spectra of 1 in hexane. Scheme 2. Equilibrium in Solution of 1

Figure 4. Solid-state molecular structure of 4 3 C7H8 (H atoms and solvent molecules are not shown; thermal ellipsoids are shown at 30% probability). Selected bond distances (A˚) and angles (deg): Ge(1)-Ge(1A) 2.3239(3), Ge(1)-C(1) 1.9745(14), Ge(1)-C(31) 1.9074(15), C(31)-C(32) 1.203(2), C(1)-Ge(1)Ge(1A) 116.06(4), C(1)-Ge(1)-C(31) 115.11(6), C(31)-Ge(1)Ge(1A) 111.33(5), Ge(1)-C(31)-C(32) 165.61(14), C(31)-C(32)C(33) 177.51(17).

(1.911(2) A˚) and 4 (1.9074(15) A˚) are significantly shorter when compared to the values in the reported monomeric Ge(II) alkynyl compounds (1.976(4)-2.004 (4) A˚).6-8 This shortening of the Ge-C bonds can be explained by the possible long-range conjugation between the two alkynyl groups through the germanium-germanium linkage, in which case 3 and 4 can be viewed as germanium analogues of enediynes. However, the C31-C32 distances in 3 (1.205(3) A˚) and 4 (1.203(2) A˚) are typical for a carbon-carbon triple bond. Spectroscopic Studies. The UV-vis spectra of both 1 and 2 are characterized by a moderately strong absorption observed at 444 nm, indicating that the two species have similar structures in the solution phase, despite the sharp difference between their solid-state structures. In addition, a shoulder absorption at around 506 nm is also present in the electronic spectrum of 1 at room temperature. Interestingly, upon cooling in liquid nitrogen, the hexane solution of 1 undergoes a color change from orange to pink. To elucidate this observation, we monitored the changes of the UV-vis spectrum by slowly warming a precooled hexane solution of 1 (Figure 5). At low temperature, only the absorption band at 506 nm could be observed. Upon warming, this absorbance gradually decreases while the signal at 444 nm appears and gradually increases in intensity so that the original room-temperature spectrum could be fully restored together with the orange color. This behavior clearly indicates that two species are present and in equilibrium in the solution of 1, with one isomer dominant at room temperature. In contrast, such an equilibrium was not observed in solutions of 2-4, because only one essentially temperature-independent absorbance (444 nm (2), 438 nm (3), 430 nm (4)) was observed in the visible region. Reversible thermochromic behavior of the electronic spectrum has been reported for both distannnene19 and digermene18j,20 (19) Davidson, P. J.; Harris, D. H.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 1976, 2268–2274. (20) Kishikawa, K.; Tokitoh, N.; Okazaki, R. Chem. Lett. 1998, 27, 239–240.

species. However, various mechanisms have been proposed for the origin of this behavior: (a) an equilibrium between the distannene (or digermene) and the corresponding monomeric stannylene (or germylene);19,20 (b) an equilibrium between the trans-pyramidal structure and another dimeric isomeric form.18j Interestingly, the absorption maxima of 1 and 2 (444 nm) are close to that of monomeric stannylene Ar*SnPh (462 nm),16 while those of 3 (438 nm) and 4 (430 nm) are similar to that of digermene Ar0 HGeGeHAr0 (434 nm),18d supporting the (a) mechanism for the thermochromism of 1. Furthermore, although the structures of 1 and 2 are very different in the solid state, the ready dissociation of 1 and 2 to monomers in solution supports the view that these differences are due to packing effects.21,22 The solution 1H NMR spectra of 1 and 2 have very similar features, showing two equal-intensity doublets for the CH3 protons at the isopropyl groups of the ligands. In contrast, four sets of CH3 protons signifying different chemical environments were observed in the spectra of 3 and 4. This indicates that 1 and 2 exist as monomers in the solution at room temperature, while 3 and 4 maintain their dimeric structures in solution. The 1H NMR spectrum of 1 shows line broadening upon cooling to -60 °C, suggesting the presence of a fluxional process, which is consistent with an equilibrium between the monomeric and symmetric dimeric structures at low temperature (Scheme 2). The 119Sn NMR spectrum of 2 displays a broad resonance at 1224 ppm, which is consistent with the two-coordinate Sn(II) geometry of the monomer. For comparison, the 119Sn NMR spectrum of Ar*SnPh displays a resonance at 1517 ppm,16 Sn(C6H-2-tBu-4,5,6-Me3)2 at 1506 ppm,12 and Sn(C6H2-tBu-4,5,6-Me3){Si(SiMe3)3} at 1401 ppm.11a The resonance in 2 is therefore somewhat upfield in comparison with these bis(aryl)tin(II) species. This is consistent with the upfield shift observed in Sn(CtCH)4 (-356.4 ppm) and Sn(CtCSiMe3)4 (-384.5 ppm) in comparison to SnMe4 (0 ppm) and SnPh4 (-128.8 ppm).13,23 In contrast, a signal at 381 ppm was (21) Peng, Y.; Fischer, R. C.; Merrill, W. A.; Fischer, J.; Pu, L.; Ellis, B. D.; Fettinger, J. C.; Herber, R.; Power, P. P. Chem. Sci., in press. (22) Fischer, R. C.; Pu., L.; Fettinger, J. C.; Brynda, M. A.; Power, P. P. J. Am. Chem. Soc. 2006, 128, 11366–11367. (23) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1999, 38, 203–264.

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observed in the spectrum of 1 at room temperature, possibly due to the presence of a small fraction of the dimeric form in solution. The chemical shift of this signal is in a similar region to those of tetrakis(2,4,6-triisopropylphenyl)distannene (427.3 ppm)24 and {Sn[CH(SiMe3)2]2}2 (740 and 725 ppm).25 The resonance of the monomeric isomer of 1 has not yet been detected possibly due to the fast equilibrium between monomer and dimer in solution, as well as the greater anisotropy of the chemical shift tensors for the divalent tin environment, which shortens the relaxation time of these nuclei and may broaden the signal into the baseline (cf. broad 119Sn NMR signal for 2).26

Conclusion In summary, alkynyl-substituted low-valent tin and germanium derivatives 1-4 were prepared by a salt elimination route. 1, 3, and 4 display symmetric dimeric strucutures, while 2 exists as unsymmetric stannylstannylene in the solid state. The different structure of 2 in the crystal phase is probably a result of packing effects, where relatively small changes at the periphery of ligands in low-valent tin complexes have been shown to result in large structural changes.21,22 Digermenes 3 and 4 maintain their dimeric structures in solution; however, 1 and 2 exist mainly as monomers. Spectroscopic studies also suggest an equilibrium between the monomeric and dimeric forms of 1 in the solution, with the monomer favored at room temperature.

Experimental Section General Procedures. All manipulations were carried out by using modified Schlenk techniques under an atmosphere of N2 or in a Vacuum Atmospheres HE-43 drybox. All solvents were dried over an alumina column, followed by storage over 3 A˚ molecular sieves overnight, and degassed three times (freezepump-thaw) prior to use. The compounds (Ar0 ECl)2 (E = Sn or Ge)27 and LiCtCR (R = SiMe328 or tBu29) were prepared according to literature procedures. 1H NMR data were obtained on a Varian Mercury 300 spectrometer at 293.1 K. 119Sn NMR spectra were recorded on a Varian Inova 600 MHz spectrometer (223.6 MHz) and referenced externally to neat SnnBu4. Melting points were measured in glass capillaries sealed under N2 by using a Mel-Temp II apparatus and are uncorrected. UV-vis data were recorded on a Hitachi-1200 spectrometer. Ar0 (Me3SiCtC)SnSn(CtCSiMe3)Ar0 (1). A 40 mL amount of hexanes was added to a mixture of (Ar0 SnCl)2 (0.308 g, 0.28 mmol) and LiCtCSiMe3 (0.062 g, 0.60 mmol) at ca. -78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. The red solution was filtered and concentrated to incipient crystallization. The solution was stored at ca. -18 °C overnight to afford orange-red crystals of 1. X-ray quality crystals of 1 3 2C6H6 could be obtained by recrystallization from a mixture of hexanes and benzene at ca. -18 °C. Yield: 0.192 g (56%). Mp: 181-183 °C. 1H NMR (300.1 MHz, C7D8, 25 °C): δ 0.15 (s, 18H, Si(CH3)3), 1.06 (d, 3JHH = 6.9 Hz, 24H, CH(24) Masamune, S.; Sita, L. R. J. Am. Chem. Soc. 1985, 107, 6390–6391. (25) Zilm, K. W.; Lawless, G. A.; Merrill, R. M.; Millar, J. M.; Webb, G. G. J. Am. Chem. Soc. 1987, 109, 7236–7238. (26) Eichler, B. E.; Phillips, B. L.; Power, P. P.; Augustine, M. P. Inorg. Chem. 2000, 39, 5450–5453. (27) Pu, L.; Philips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626–11636. (28) Cano, A.; Cuenca, T.; Galakhov, M.; Rodrı´ guez, G. M.; Royo, P.; Cardin, C. J.; Convery, M. A. J. Organomet. Chem. 1995, 493, 17–25. (29) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organometallics 1983, 2, 709–714.

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(CH3)2), 1.29 (d, 3JHH = 6.9 Hz, 24H, CH(CH3)2), 3.08 (sept, JHH = 6.9 Hz, 8H, CH(CH3)2), 6.97-7.26 (m, 18H, aromatic H). 119Sn{1H} NMR (223.6 MHz, C7D8, 25 °C): δ 381. UV-vis (hexane) λmax (ε): 444 nm (2000 mol-1 L cm-1), 506 nm (sh). Ar0 SnSn(CtCtBu)2Ar0 (2). A 40 mL amount of hexanes was added to a mixture of (Ar0 SnCl)2 (0.303 g, 0.28 mmol) and LiCtCtBu (0.058 g, 0.66 mmol) at ca. -78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. The orange-brown solution was filtered and concentrated to incipient crystallization. The solution was stored at ca. -18 °C overnight to afford X-ray quality blue crystals of 2 3 C6H12. Yield: 0.196 g (60%). Mp: 104-106 °C (decomposes to a red oil). 1H NMR (300.1 MHz, C7D8, 25 °C): δ 1.06 (d, 3JHH = 6.9 Hz, 24H, CH(CH3)2), 1.12 (s, 18H, C(CH3)3), 1.35 (d, 3JHH = 6.9 Hz, 24H, CH(CH3)2), 3.15 (sept, 3JHH = 6.9 Hz, 8H, CH(CH3)2), 7.07-7.28 (m, 18H, aromatic H). 119Sn{1H} NMR (223.6 MHz, C7D8, 25 °C): δ 1224. UV-vis (hexane) λmax (ε): 444 nm (3400 mol-1 L cm-1). Ar0 (Me3SiCtC)GeGe(CtCSiMe3)Ar0 (3). A 40 mL portion of hexanes was added to a mixture of (Ar0 GeCl)2 (0.201 g, 0.20 mmol) and LiCtCSiMe3 (0.049 g, 0.47 mmol) at ca. -78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. The resulting yellow solution was filtered and concentrated to incipient crystallization. Storage at ca. -18 °C overnight afforded X-ray quality yellow crystals of 3. Yield: 0.122 g (54%). Mp: 214-215 °C (turns orange at 89 °C). 1H NMR (300.1 MHz, C7D8, 25 °C): δ 0.04 (s, 18H, Si(CH3)3), 0.60 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 0.89 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 1.14 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 1.41 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 2.79 (sept, 3JHH = 6.9 Hz, 8H, CH(CH3)2), 6.92-7.34 (m, 18H, aromatic H). UV-vis (hexane) λmax (ε): 280 nm (820 mol-1 L cm-1), 438 nm (1200 mol-1 L cm-1). Ar0 (tBuCtC)GeGe(CtCtBu)Ar0 (4). A 40 mL sample of hexanes was added to a mixture of (Ar0 GeCl)2 (0.239 g, 0.24 mmol) and LiCtCtBu (0.055 g, 0.62 mmol) at ca. -78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. The yellow solution was filtered and concentrated to incipient crystallization. Overnight storage at ca. -18 °C afforded yellow crystals of 4. X-ray quality crystals of 4 3 C7H8 could be obtained by recrystallization from a mixture of hexanes and toluene at ca. -18 °C. Yield: 0.174 g (67%). Mp: 225-226 °C (turns orange at 118 °C). 1H NMR (300.1 MHz, C7D8, 25 °C): δ 0.56 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 0.87 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 1.02 (s, 18H, C(CH3)3), 1.17 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 1.42 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 2.83 (sept, 3JHH = 6.9 Hz, 8H, CH(CH3)2), 6.89-7.36 (m, 18H, aromatic H). UV-vis (hexane) λmax (ε): 276 nm (3800 mol-1 L cm-1), 430 nm (5800 mol-1 L cm-1). X-ray Crystallographic Studies. Crystals of 1 3 2C6H6, 2 3 C6H12, 3, and 4 3 C7H8 were removed from a Schlenk tube under a stream of nitrogen and immediately covered with a thin layer of hydrocarbon oil. A suitable crystal was selected, attached to a glass fiber on a copper pin, and quickly placed in the cold N2 stream on the diffractometer.30 All data were collected at 90 K on a Bruker SMART Apex II diffractometer with Mo KR radiation (λ = 0.71073 A˚). Absorption corrections were applied using SADABS.31 The crystal structures were solved by direct methods and refined by full-matrix least-squares procedures in SHELXTL.32 All non-H atoms were refined anistropically. All H atoms were placed at calculated positions and included in the refinement using a riding model. A summary of the data collection parameters is provided in Table 1. CCDC-777451 (1 3 2C6H6), 777452 (2 3 C6H12), 777453 (3), and 777454 (4 3 C7H8) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from 3

(30) Hope, H. Prog. Inorg. Chem. 1995, 41, 1–19. (31) SADABS is an empirical absorption correction program that is part of the SAINT Plus NT version 5.0 package: Bruker AXS: Madison, WI, 1998. (32) SHELXTL version 5.1; Bruker AXS: Madison, WI, 1998.

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the Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.

Acknowledgment. We thank the National Science Foundation (CHE-0948417) for financial support.

Lei et al. Supporting Information Available: Copies of the 1H NMR spectra of 1-4 and 119Sn{1H} NMR spectra for 1 and 2. Crystallographic information files (CIFs) for 1-4. These materials are available free of charge via the Internet at http://pubs. acs.org.