5488
Organometallics 1995, 14, 5488-5489
Synthesis and Structural Characterization of {LiN(SiMe3)2MMe3}, (M = Al, Ga): Amido Ligands Isoelectronic to the Alkyl Group -C(SiMe& Mark Niemeyer and Philip P. Power* Department of Chemistry, University of California, Davis, California 9561 6 Received September 29, 1995@ Summary: The synthesis and structural characterization of the lithium salts {LiN(SiMe&MMe3}, (M = A1 (l), Ga (2)) are described. The compounds are essentially isostructural and are associated into infinite chains that involve bridging by the lithium ion to a n aluminum or gallium methyl group of a neighboring molecule. The ligands -C(SiMe& and -N(SiMes)flMe3 are isoelectronic. Yet, there are very few examples of structurally characterized compounds in which the properties of these two ligands can be directly compared (vide infra).1!2 The alkyl group -C(SiMe& has been employed throughout the periodic table and has been shown t o be effective in stabilizing low coordination numbers and unusual oxidation states in a variety of element^.^-^ Its lithium salt LiC(SiMe313 is of key importance in the synthesis of most of these derivatives. When crystallized in the presence of THF, it exists as the "ate" complex [Li(THF)4l[Li{C(SiMe3)3}21,4but as a solvent-freespecies it exists as the dimer { LiC(SiMe3)3}2.5 The structure of the related amide LiN(SiMe& (either unsolvated6 or solvated by various donors7) has been well studied, and it exists as the trimer {LiN(SiMe3)2}3 when uncomplexed.6 The simple addition of trimethy l a l u " to LiN(SiMe3)z affords a species LiN(SiMe3)zAlMe3 which is isoelectronic to LiC(SiMe313. It is therefore of some interest t o examine the relationship between their structures. In this paper the synthesis and structure of LiN(SiMe3)flMe3 and its gallium analogue are now reported. The compounds {LiN(SiMe3)2MMe3}, (M = AI (11, Ga (2)) were isolated8 in high yield as colorless, isomorphous crystals by the addition of trimethylaluminum or -gallium to LiN(SiMe3)s in toluene solution. X-ray crystallographic datag for 1 and 2 show that the structures are composed of infinite chains in which monomeric units of LiN(SiMe312MMe3 (M = Al, Gal are linked by an interaction between the lithium ion and a Abstract published in Advance ACS Abstracts, November 15,1995. (1)Boncella, J . M.; Andersen, R. A. Oganometallics 1986, 4, 205. (2) Eaborn, C.; Hitchcock, P. B.; Izod, K.; Smith, J . D. J . Am. Chem. SOC. 1994, 116, 12071. (3) (a)Al-Juaid, S. S.; Eaborn, C.; Hitchcock, P. B.; McGeary, C. A,; Smith, J . D. J . Chem. SOC., Chem. Commun. 1989, 273. (b) Buttrus, N. H.; Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan, A. C. J . Chem. SOC.,Chem. Commun. 1985, 1380. (4)Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan, A. C. J . Chem. SOC., Chem. Commun. 1983, 827. (5) Hiller, W.; Layh, M.; Uhl, W. Angew. Chem., Int. Ed. Engl. 1991, 30, 324. (6)Mootz, D.; Zinnius, A.; Bottcher, B. Angew. Chem., Int. Ed. Engl. 1969, 8, 378. (7) (a) Lappert, M. F.; Slade, M. J.; Singh, A.; Atwood, J . L.; Rogers, R. D.; Shakir, R. J . Am. Chem. SOC.1983, 105,302. (b) Engelhardt, L. M.; May, A. S.; Raston, C. L.; White, A. H. J . Chem. Soc., Dalton Trans. 1983, 1671. (c) Power, P. P.; Xu, X. J . Chem. SOC., Chem. Commun. 1984, 358. (d) Engelhardt, L. M.; Jolly, B. S.; Punk, P. C.; Raston, C. L.; White, A. H. Austr. J . Chem. 1986, 39, 1337. @
Figure 1. Thermal ellipsoidal plot (30%)of 1 illustrating the coordination of the Li+ ion. methyl group (i.e. C(1a)) from the MMe3 moiety in the neighboring molecule. The structure of 1 is illustrated in Figure 1,and further structural details are provided in Table 1. From these data it can be seen that the two structures are almost identical. Each Li+ ion interacts most strongly with the amido nitrogen and with C(3) and C(1a) methyl groups such that the nitrogen and two carbons define an approximately trigonal planar coordination sphere for the metal. The stronglo interaction between Lif and the C(3) methyl group is consistent with M-C(3) bonds that are longer (by ca. 0.06 A) than (8) All manipulations were carried out under anaerobic and anhydrous conditions. AlzMee in PhMe (2 MI, GaMea, and HN(SiMe& were obtained from commercial suppliers and used as received. LiN(SiMe& was generated in solution by the addition of 1 equiv of 1.6 M n-BuLi in hexane and was purified by sublimation. With stirring at room temperature a 2 M solution of AlMe3 (5.25,mL, 10.5 mmol) in PhMe was added dropwise via a syringe to LiN(SiMe3)z (1.75 g, 10.5 mmol) in toluene (30 mL). Within 5 min colorless crystals began to precipitate. These were partly redissolved by warming with a heat gun. Stirring was then discontinued, and the solution was cooled to room temperature over several hours to give large crystals of the product 1 that were suitable for X-ray crystallographic studies. A further crop of crystals was obtained by cooling in a -30 "C freezer overnight. The synthesis of 2 as colorless crystals was accomplished in a similar manner with use of GaMea (0.78 g, 6.8 mmol) and LiN(SiMe& (1.12 g, 6.8 mmol) and ca. 20 mL total volume of PhMe. Data for 1, with those for 2 in braces, are as follows: Yield 2.11 g (84%){ 1.50 g (79%)}, mp 175-177 "C {129-130 "C}; lH NMR (C6Ds) d 0.21 IO.19) (SiMes, 18H),-0.55 {-0.33) (MMe3,gH);I3C NMR (C6D6)6 6.1 I6.0) (SiMes), 2.6 { -0.9) (MMe3); 7Li NMR ( C ~ D Bd )-2.3 { -2.2); IR (Nujol, cm-') 2175 w, 1923 w, 1862 w, 1616 w, 1402 m, 1294 sh, 1264 sh, 1252 vs, 1208 s, 1189 s, 1108 s, 930-580 vs br, 521 s, 465 s, 403 m, 371 m, 340 m-s; I2210 w, 1920 w, 1858 w, 1625 w, 1397 m, 1290 sh, 1262 sh, 1250 vs, 1204 m-s, 1193 s, 1124 s, 947 vs br, 876-727 vs br, 677 vs, 634 m-s, 609 vs, 527 vs, 482 vs, 402 m, 350 sh, 330 m-s}. (9) Crystal data at 130 K with Cu K a ( I = 1.541 84 A)radiation: 1, CgH27AlNSi2, a = 12.334(3) A, b = 15.563(3) A, c = 16.449(3) A, orthorhombic, space group Pbca, 2 = 8, R = 0.048 for 1622 (I > 20(I)) data; CgHz,GaNSiz,2, a = 12.336(3)A, b = 15.563(3) A, c = 16.434(3) A, orthorhombic, space group Pbca, 2 = 8, R = 0.062 for 1745 ( I > 2 d ) ) data.
0276-7333/95/2314-5488$09.QQ/Q0 1995 American Chemical Society
Organometallics, Vol. 14, No. 12, 1995 5489
Communications Table 1. Selected Distances (si> and Angles (deg) in 1 and 2 compd l(M=Al)
2 (M = Ga)
2.027(7) 2.252(8) 2.157(8) 1.944(3) 1.741(3) 1.749(3) 2.005(4) 1.974(4) 2.032(4)
2.014(12) 2.233(13) 2.205(13) 2.038(4) 1.742(4) 1.727(4) 2.022(6) 1.987(7) 2.044(6)
82.4(2) 102.5(3) 106.5(2) 114.6(2) 117.3(2) 103.0(2) 129.9(4) 96.0(3) 128.5(4)
81.7(3) 101.4(4) 107.9(4) 112.4(2) 117.1(2) 101.9(2) 129.2(6) 97.4(5) 126.6(5)
the M-C(2) bonds which involve terminal methyl groups bound solely to Al or Ga. In addition, the longer M-C(l) distances are in agreement with the role of this methyl group in bridging the LiN(SiMedzMMe3 moieties. The interaction between the Li+ ion and the methyl groups can also be expressed in the form of Li- - -H distances. For this purpose the hydrogens in 1 were located on a difference map and refined isotropically. The Li-H(lAA, AB, and AC) distances are 2.05(41, 2.25(4), and 2.10(4) A, whereas the corresponding distances for Li- - -H(3A and C) are 2.15(4) and 2.11(4) A. The Li-N bond lengths are slightly longer than that observed (Li-N = 2.00 A) in the unsolvated trimer {LiN(SiMe3)2}3.6 The only significant differences between the structures of l and 2 involve the M-N and Li- - -C(3)distances which are respectively ca. 0.09 and 0.05 A longer in the case of the gallium derivative. This difference is probably due to the weaker Lewis base character of GaMeP and the lowered ionic effects in the Ga-N and Ga-C bonding owing to the higher electronegativity of gallium in comparison to aluminum.12 Why are structures of {LiN(SiMe3)&Me3}, (1) and {LiC(SiMe3)3}2(3) different? The answer may lie in the difference in the strengths of the interaction between the Li+ ion and the methyl groups in the two molecules. These are very much stronger in the nitrogen derivative, ca. 2.16 and 2.25 Ain 1 (marginally weaker in 2) versus ca. 2.47 and 2.54 A in 3.5 Significantly, both Li- - -CH3 interactions in 1 involve aluminum methyl rather than silyl methyl groups. Apparently the more electroposi(10) The Li- - -C distances in 1 and 2 are similar to those observed in many typical organolithium structures in which there is a direct interaction between the Lif and a carbanionic center: Setzer, W. N.; Schleyer, P. v. R. Adu. Orgunomet. Chem. 1986, 24, 353. (11)This compound is a monomeric liquid at room temperature whereas trimethylaluminum is associated as dimers. (12) The Allred-Rochow electronegativity values for aluminum and gallium are 1.47 and 1.82, respectively.
tive aluminum center12induces a greater 6- charge on its methyl substituents which causes the stronger Li- - -CH3 interaction. In addition, the fact that the AI-N and Ga-N bonds are dative ones and are probably weaker than the corresponding C-Si bond in 3 may permit greater flexibility in the orientation of MMe3 moieties. As a result of this, the N(l)-Al(Ga)-C(3) angles are at least 10" narrower than the corresponding angles involving C(1) and C(2). By the same token the Li+ ion is displaced toward the C(3) carbons so that the Li-N-M angles are only ca. 82" in each compound. Nonetheless, interaction of Li+ with just one of the methyl groups is insufficient t o saturate the Li+ coordination sphere and a second strong Li- - -CH3 interaction is not possible within a dimeric structure due to the fact that the AlMe3 groups would adopt a transorientation with respect to the LizNz ring plane. In other words, the polymeric structure in 1 occurs in order to preserve two strong Li- - -CH3 interactions involving the aluminum methyl groups. Increased crowding is also probably involved in determining the structure. This is due to the fact that the Li-N and N-Si bonds are shorter than corresponding Li-C and C-Si bonds so that a dimeric arrangement which had a structure similar to 3 would be more sterically encumbered. The polymeric structure seen in 1 and 2 has not been previously observed for amides; however, polymeric structures with a different Li- - -N- - -Li- - -N backbone are known for { LiN(i-Pr)2},13 and {(TMEDA)LiN(iPr)2}2.14 The species (LiCH(SiMe3)2),l5 also has a polymeric backbone, and in this case it is composed of a Li- - -C- - -Li- - -C array. It remains t o be seen if the differences in structure between 1 or 2 and 3 will be observed in other metal derivatives. Only one other pair of compounds is currently available for comparison-the species Yb(N(SiMe3)2AlMe3}2' and Yb{C(SiMe3)3}2.2 The structures of these two compounds bear a very close similarity and have comparable Yb- - -Me interactions in each compound. It is possible in this case that the large size of the Yb2+ ion ensures a sterically more relaxed species in which almost equal Yb- - -Me contacts can occur in each compound. Investigations of other metal -N(SiMe3)2AlMe3 derivatives are in hand. Acknowledgment. We are grateful to the National Science Foundation for financial support. Supporting Information Available: Tables of data collection parameters, complete atom coordinates and U values, distances and angles, and anisotropic thermal parameters (18 pages). Ordering information is given on any current masthead page. OM950771P (13)Barnett, N. D. R.; Mulvey, R. E.; Clegg, W.; ONeil, P. A. J . Am. Chem. SOC.1991,113,8187. (14)Bernstein, M. P.; Romesberg, F. E.; Fuller, D. J.; Harrison, A. T.; Collum, D. B.; Liu, $.-Y Williard, P. G. J . Am. Chem. SOC.1992, 114, 5100. (15)Atwood, J. L.; Fjeldberg, T.; Lappert, M. F.; Luong-Thi, T.; Shakir, R.; Thorne, A. J. J. Chem. SOC.,Chem. Commun. 1984, 1163.
