DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 2603–2605 2603 A sandwich complex of lithium oxide: {Li[BunC(NBut)2]}4?Li2O Tristram Chivers,*,†,a Andrew Downard a and Glenn P. A. Yap b a Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4 b Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 Hydrolysis of {Li[BunC(NBut)2]}2, prepared from LiBun and ButNCNBut in hexanes, produced the nineteen atom cluster {Li[BunC(NBut)2]}4?Li2O; an X-ray structure determination revealed an Li2O molecule encapsulated by two eight-membered Li2N4C2 rings.Novel structural and/or reaction chemistry often results from ligands that provide unusual steric and/or electronic environments at metal centres. To this end N-silylated benzamidinates RC(NSiMe3)2 2 (R = aryl) 1,2 and, more recently, dialkylamidinates RC(NR9)2 2 (where R and R9 are diVerent alkyl groups) have been investigated extensively.1b,3–5 Although a wide range of both main-group and transition-metal complexes of these chelating ligands has been characterized,6 structural information for the lithium derivatives of these important reagents is limited to complexes in which the lithium ions are co-ordinated to Lewis bases such as RCN (R = aryl),7 THF,8 HMPA,9 N,N,N9,N9-tetramethylethylenediamine 9 or N,N,N9,N9,N0- pentamethyldiethylenetriamine.9 The complexes [4-MeC6- H4C(NSiMe3)2Li(THF)]2 8 and [PhC(NPh)2Li(HMPA)]2 9 form dimeric, step-shaped structures whereas chelating Lewis bases give rise to monomeric structures.9 We describe here the unexpected generation and crystal structure of the complex {Li[BunC(NBut)2]}4?Li2O 2 in which a molecule of lithium oxide is trapped between two twisted Li2N4C2 ladders of a dimeric lithium amidinate.Compared to other complexes of Li2O,10–14 complex 2 exhibits some novel features that result from the unique ligand behaviour of the Li2N4C2 ring. Amidinates Li[RC(NR9)2] are readily obtained by the nucleophilic addition of an organolithium reagent (RLi) to a carbodiimide R9NCNR9.3,5 In this work, the addition of a 2.5 M solution of LiBun in hexanes (3.7 mL) to a solution of 1,3-ditert- butylcarbodiimide (9.22 mmol) in hexane (10 mL) under argon at 23 8C produced a transparent, pale yellow solution.Removal of volatile materials in vacuo gave a viscous yellow oil, which was redissolved twice in diethyl ether (ª5 mL). Evaporation of the solvent in vacuo produced {Li[BunC(NBut)2]}n 1 as a fine yellow powder (8.63 mmol, 94%).‡ Recrystallization of 1 from a saturated toluene solution (4 d at 220 8C) produced a few X-ray quality crystals with NMR parameters significantly diVerent from those of 1.An X-ray structural determination revealed that the composition of these crystals is {Li[BunC(NBut)2]}4?Li2O 2 (Fig. 1).§ This nineteen atom cluster has a m6-OLi6 core. Six-fold coordination of O22 by metal cations in molecular compounds is rare and usually involves regular Oh symmetry.12–14 A major diVerence between the structure of 2 and those of other Li2O aggregates 11–14 is that the molecule of Li2O is readily identified in 2 because of the relatively low symmetry of this cluster.† E-Mail: chivers@acs.ucalgary.ca Thus 2 may be viewed as an almost linear Li2O molecule [Li(1)]O]Li(2) 175.8(2)8] sandwiched between two twisted Li2N4C2 ladders. The oxygen atom is tightly co-ordinated to all six lithium atoms, but the mean Li]O distance in the Li2O molecule [1.803(4) Å] is significantly shorter than that in the other Li]O bonds [mean value 1.869(4) Å], cf. 1.89(1) Å in [(cyclo-C5H9)N(H)Li]12?Li2O,13 1.81–1.90(2) Å in [Pri 2(Mes)- SiP]8Li16?Li2O14 (Mes = C6H2Me3-2,4,6). The distortion of the octahedral geometry of the OLi6 unit in 2 is reflected in the Li ? ? ? Li separations which range from 2.169(5) Å across the Li2N4C2 rings to 3.127(5) Å {cf. 2.63–2.67(2) Å in [Pri 2(Mes)- SiP]8Li16?Li2O14}.All six lithium atoms can be viewed as fourco- ordinate, but there is considerable variation in the Li]N bond distances. Those belonging to the Li2O moiety are bonded symmetrically to two nitrogen atoms of different Li2N4C2 rings [|d(Li]N)| 2.106(4) Å] and are also involved in a third, weaker Li ? ? ? N interaction [2.573(4) Å]. This results in a ‘pinching in’ of the Li2N4C2 rings as reflected from the values of |Li]O]Li| 70.9(2)8 and |N]C]N| = 115.3(9)8.The other four lithium atoms are bonded unsymmetrically to two nitrogen atoms of the same Li2N4C2 ring [|d(Li]N)| 2.03(2) and 2.36(2) Å]. As a result there are three four-co-ordinate and one five-co-ordinate nitrogen atom in each Li2N4C2 ring. The mean C]N bond distances are slightly longer for the four-coordinate compared to the five-co-ordinate N atoms [1.346(2) vs. 1.329(2) Å]. ‡ 1 Mp 51–54 8C. 1H NMR (25 8C, 200 MHz, C6D6): d 0.90 (t, 3 H, CH3CH2CH2CH2), 1.32 [s 1 m, 20 H, CH3CH2CH2CH2 and C(CH3)3], 1.85 (m, 2 H, CH3CH2CH2CH2), 2.50 (m, 2 H, CH3CH2CH2CH2). 13C NMR (25 8C, 50.288 MHz, C6D6): d 14.1 (s, CH2CH2CH2CH3), 24.1 (s, CH2CH2CH2CH3), 33.0 (s, CH2CH2CH2CH3), 33.2 (s, CH2CH2- CH2CH3), 33.7 [s, C(CH3)3], 51.6 [s, C(CH3)3], 178.4 [s, C(NBut)2Bun]; (25 8C, 75.432 MHz, solid state): d 14.2 (s, CH2CH2CH2CH3), 24.1 (s, CH2CH2CH2CH3), 34.1 [s br, CH2CH2CH2CH3, CH2CH2CH2CH3 and C(CH3)3], 51.3 [s, C(CH3)3], 175.3 [s, C(NBut)2Bun]. 7Li NMR (25 8C, 155.508 MHz, C6D6, relative to 1 M LiCl in D2O): d 20.62 (s); (25 8C, 116.54 MHz, solid state, relative to LiCl): d 1.46 (s). 2 Mp 132–134 8C (Found: C, 69.40; H, 12.78; N, 12.51. Calc. for C52H108Li6N8O: C, 69.16; H, 12.05; N, 12.41%). 1H NMR (25 8C, 200 MHz, C6D6): d 0.99 (t, 3 H, CH3CH2CH2CH2), 1.46 [s 1 m, 20 H, CH3CH2CH2CH2 and C(CH3)3], 1.95 (m, 2 H, CH3CH2CH2CH2), 2.60 (m, 2 H, CH3CH2CH2CH2). 13C NMR (25 8C, 50.288 MHz, C6D6): 15.5 (s, CH2CH2CH2CH3), 23.5 (s, CH2CH2CH2CH3), 32.9 (CH2CH2- CH2CH3), 34.5 (s, CH2CH2CH2CH3), 34.6 [s, C(CH3)3], 51.9 [s, C(CH3)3], 179.6 [s, C(NBut)2Bun]; (25 8C, 75.432 MHz, solid state): d 14.4 (s, CH2CH2CH2CH3), 24.5 (s, CH2CH2CH2CH3), 35.5 [s br, CH2CH2CH2CH3, CH2CH2CH2CH3 and C(CH3)3], 52.7 [s, C(CH3)3], 180.6 [s, C(NBut)2Bun]. 7Li NMR (25 8C, 155.508 MHz, C6D6, relative to 1 M LiCl in D2O): d 20.82 (s), 21.23 (s); (25 8C, 116.54 MHz, solid state, relative to LiCl): d 2.97 (s). § Crystal data: C52H108Li6N8O, M = 903.10, triclinic, space group P1� , a = 10.137(3), b = 14.205(4), c = 21.961(6) Å, a = 91.7718(5), b = 103.207(5), g = 101.442(5)8, U = 2008(1) Å3, Z = 2, m = 0.58 cm21, T = 213 K, 24 534 reflections collected, 13 710 independent reflections, Rint = 0.0701.The final R(F) and wR(F 2) values were 0.0545 and 0.0821, respectively. CCDC reference number 186/1071. See http:// www.rsc.org/suppdata/dt/1998/2603/ for crystallographic files in .cif format.2604 J. Chem. Soc., Dalton Trans., 1998, Pages 2603–2605 The most obvious explanation for the formation of 2 is the partial hydrolysis of 1 by trace amounts of water present in the solvent or flask used for recrystallization [reactions (1) and (2)].Li[BunC(NBut)2] 1 H2O æÆ LiOH 1 BunC(NBut)[N(H)But] (1) 1 3 5 1 1 LiOH æÆ {Li[C(NBut)2(Bun)]}4?Li2O 1 3 (2) 2 The adventitious presence of water has previously been identi- fied as the source of Li2O in aggregates with lithium amides.12,13 To test this hypothesis a stoichiometric amount of water was added, by syringe, to a 0.46 M solution of 1 in toluene (5 mL) at 23 8C.This produced an oily white solid, which was stirred for 1 h to give an opaque yellow solution. The volume of the solution was reduced by one-half and colourless crystals of 2 were obtained in 30% yield after 3 d at 214 8C. The analytical and spectroscopic characterization of 2 were completed on this product.‡ The observation of two resonances in the 7Li NMR spectrum (in C6D6) at d 20.82 and 21.23 (the latter is of lower relative intensity) suggests a higher average symmetry (D2) for 2 in solution compared to that observed (C2) in the solid state.The 7Li NMR specum of 1 in C6D6 exhibits a singlet at d 20.62. There are significant diVerences in the 13C NMR chemical shifts observed for 1 and 2.‡ In particular, d [C(NBut)2- (Bun)] provides a diagnostic distinction between 1 and 2 both in solution and, especially, in the solid state.The 1H NMR spectrum of the mother-liquor from reaction (1) showed it to consist of a mixture of unreacted 1 and the hydrolysis product 3. Thus hydrolysis of 1 is clearly established as a route to 2. Further support for this conclusion is provided by the observation that the direct reaction of 1 with LiOH in toluene at 23 8C for 48 h produces 2 in 41% yield, but 2 is not formed from the treatment of 1 with Li2O under similar conditions. A conceptual representation of the assembly of the nineteen atom cluster 2 from two Li2N4C2 dimers and a Li2O molecule is Fig. 1 Molecular structure and atomic numbering scheme for complex 2. Thermal ellipsoids are depicted at 30% probability. For clarity only the a-carbon atoms of Bun and But are shown. Selected bond distances (Å) and angles (8): O]Li(1) 1.805(4), O]Li(2) 1.801(4), O]Li(3), 1.880(4), O]Li(4) 1.883(4), O]Li(5) 1.852(4), O]Li(6) 1.862(4), Li(1)]N(1) 2.082(4), Li(1)]N(2) 2.664(4), Li(1)]N(8) 2.092(4), Li(2)]N(4) 2.102(4), Li(2)]N(5) 2.148(4), Li(2)]N(6) 2.481(4), Li(3)]N(1) 2.047(4), Li(3)]N(2) 2.505(4), Li(3)]N(3) 2.293(4), Li(4)]N(2) 2.316(4), Li(4)]N(3) 2.458(4), Li(4)]N(4) 2.050(4), Li(5)]N(6) 2.390(4), Li(5)]N(7) 2.443(4), Li(5)]N(8) 2.062(4), Li(6)]N(5) 2.048(4), Li(6)]N(6) 2.514(5), Li(6)]N(7) 2.376(4); |N]C]N| 115.1 [range 114.6(2)–115.9(2)] shown in Scheme 1, where the source of Li2O is LiOH produced by the hydrolysis of 1.An alternative source of LiOH and, hence, Li2O in the original formation of 2 is the commercial LiBun used for the preparation of 1.15 Indeed the 7Li NMR spectrum of fresh LiBun (2.5 M in hexanes, Aldrich) in C6D6 exhibited a small resonance at d 20.89 in addition to the dominant resonance at d 20.22 (vs. 1 M LiCl in D2O). The intensity of the former relative to that at d 20.22 increased upon addition of water to the solution, but not upon addition of solid LiOH. Although the identity of the d 20.89 species has not been established, we cannot rule out commercial LiBun as a source of Li2O in the formation of 2.Finally, we note that the co-ordination of Li2O does not aVect the use of 2 as a source of the chelating amidinate ligand BunC(NBut)2 2. For example, reaction of 2 (5.82 mmol) with PhBCl2 (5.29 mmol) in toluene (15 mL) produces PhB(Cl)[C(NBut)2Bun] 4 in 82% Yield.¶ The four-membered ring structure of 4 has been confirmed by X-ray crystallography and full details of this structure and those of related fourco- ordinate boron complexes will be reported in a separate publication.16 In summary, complex 2 provides the first demonstration of the ligand behaviour of a dimeric lithium amidinate.The entrapment of other alkali-metal chalcogenides, e.g. Li2S, Na2O, by lithium amidinates is an interesting possibility that will be pursued. Acknowledgements We thank NSERC (Canada) for financial support. References 1 (a) F. T. Edelmann, Coord. Chem. Rev., 1994, 137, 403; (b) J.Barker and M. Kilner, Coord. Chem. Rev., 1994, 133, 219. 2 R. Duchateau, A. Meetsma and J. H. Teuben, Chem. Commun., 1996, 223; R. Duchateau, C. T. Van Wee, A. Meetsma and J. H. Teuben, Organometallics, 1996, 15, 2291. 3 Y. Zhou and D. S. Richeson, Inorg. Chem., 1996, 35, 2448; Y. Zhou and D. S. Richeson, J. Am. Chem. Soc., 1996, 118, 10 850. 4 M. P. Coles and R. F. Jordan, J. Am. Chem. Soc., 1997, 119, 8125; M. P. Coles, D. C. Swenson and R. F. Jordan, Organometallics, 1997, 16, 5183.Scheme 1 Schematic representation of the formation of 2. (i) Dimerisation; (ii) partial hydrolysis ¶ Mp 76–79 8C (Found: C, 67.76; H, 9.97; N, 8.45. Calc. for C19H32BClN2: C, 68.16; H, 9.65; N, 8.37%). 1H NMR (25 8C, 200 MHz, C6D6): d 0.73 (t, 3 H), 1.11 (m, 2 H), 1.17 (s, 18 H), 1.60 (m, 2 H), 2.14 (m, 2 H) 7.2–8.1 (m, 5 H). 11B NMR (25 8C, 64.2 MHz, relative to BF3?OEt2): d 6.8 (s). EI-MS [70 eV (eV ª 1.602 × 10219 J)]: m/z 334 (M1, good agreement between calculated and observed isotopic distribution).J. Chem.Soc., Dalton Trans., 1998, Pages 2603–2605 2605 5 P. Berno, S. Hao, R. Minhas and S. Gambarotta, J. Am. Chem. Soc., 1994, 116, 7417; S. Hao, S. Gambarotta, C. Bensimon and J. J. H. Edema, Inorg. Chim. Acta, 1993, 213, 65. 6 H. H. Karsch, P. A. Schlüter and M. Reisky, Eur. J. Inorg. Chem., 1998, 433; J. Barker, N. C. Blacker, P. R. Phillips, N. W. Alcock, W. Errington and M. G. H. Wallbridge, J. Chem. Soc., Dalton Trans., 1996, 431. 7 T. Gebauer, K. Dehnicke, H. Goesmann and D. Fenske, Z. Naturforsch., Teil B, 1994, 49, 1444; M. S. Eisen and M. Kapon, J. Chem. Soc., Dalton Trans., 1994, 3507. 8 D. Stalke, M. Wedler and F. T. Edelmann, J. Organomet. Chem., 1992, 431, C1. 9 J. Barker, D. Barr, N. D. R. Barnett, W. Clegg, I. Cragg-Hine, M. G. Davidson, R. P. Davies, S. M. Hodgson, J. A. K. Howard, M. Kilner, C. W. Lehmann, I. Lopez-Solera, R. E. Mulvey, P. R. Raithby and R. Snaith, J. Chem. Soc., Dalton Trans., 1997, 951. 10 H. Dietrich and D. Rewicki, J. Organomet. Chem., 1981, 205, 281. 11 H.-J. Gais, J. Vollhardt, H. Günther, D. Moskau, H. J. Lindner and S. Braun, J. Am. Chem. Soc., 1988, 110, 978. 12 S. C. Ball, I. Cragg-Hine, M. G. Davidson, R. P. Davies, M. I. Lopez-Solera, P. R. Raithby, D. Reed, R. Snaith and E. M. Vogl, J. Chem. Soc., Chem. Commun., 1995, 2147. 13 W. Clegg, L. Horsburgh, P. R. Dennison, F. M. Mackenzie and R. E. Mulvey, Chem. Commun., 1996, 1065. 14 M. Driess, H.Pritzkow, S. Martin, S. Rell, D. Fenske and G. Baum, Angew. Chem., Int. Ed. Engl., 1996, 35, 986. 15 C. Lambert, F. Hampel, P. Rague von Schleyer, M. G. Davidson and R. Snaith, J. Organomet. Chem., 1995, 487, 139. 16 P. Blais, T. Chivers, A. Downard and M. Parvez, unpublished work. Received 29th June 1998; Communication 8/04955FJ. Chem. Soc., Dalton Trans., 1998, Pages 2603–2605 2605 5 P. Berno, S. Hao, R. Minhas and S. Gambarotta, J. Am. Chem. Soc., 1994, 116, 7417; S.Hao, S. Gambarotta, C. Bensimon and J. J. H. Edema, Inorg. Chim. Acta, 1993, 213, 65. 6 H. H. Karsch, P. A. Schlüter and M. Reisky, Eur. J. Inorg. Chem., 1998, 433; J. Barker, N. C. Blacker, P. R. Phillips, N. W. Alcock, W. Errington and M. G. H. Wallbridge, J. Chem. Soc., Dalton Trans., 1996, 431. 7 T. Gebauer, K. Dehnicke, H. Goesmann and D. Fenske, Z. Naturforsch., Teil B, 1994, 49, 1444; M. S. Eisen and M. Kapon, J. Chem. Soc., Dalton Trans., 1994, 3507. 8 D. Stalke, M. Wedler and F. T. Edelmann, J. Organomet. Chem., 1992, 431, C1. 9 J. Barker, D. Barr, N. D. R. Barnett, W. Clegg, I. Cragg-Hine, M. G. Davidson, R. P. Davies, S. M. Hodgson, J. A. K. Howard, M. Kilner, C. W. Lehmann, I. Lopez-Solera, R. E. Mulvey, P. R. Raithby and R. Snaith, J. Chem. Soc., Dalton Trans., 1997, 951. 10 H. Dietrich and D. Rewicki, J. Organomet. Chem., 1981, 205, 281. 11 H.-J. Gais, J. Vollhardt, H. Günther, D. Moskau, H. J. Lindner and S. Braun, J. Am. Chem. Soc., 1988, 110, 978. 12 S. C. Ball, I. Cragg-Hine, M. G. Davidson, R. P. Davies, M. I. Lopez-Solera, P. R. Raithby, D. Reed, R. Snaith and E. M. Vogl, J. Chem. Soc., Chem. Commun., 1995, 2147. 13 W. Clegg, L. Horsburgh, P. R. Dennison, F. M. Mackenzie and R. E. Mulvey, Chem. Commun., 1996, 1065. 14 M. Driess, H. Pritzkow, S. Martin, S. Rell, D. Fenske and G. Baum, Angew. Chem., Int. Ed. Engl., 1996, 35, 986. 15 C. Lambert, F. Hampel, P. Rague von Schleyer, M. G. Davidson and R. Snaith, J. Organomet. Chem., 1995, 487, 139. 16 P. Blais, T. Chivers, A. Downard and M. Parvez, unpublished work. Received 29th June 1998; Communication 8/04955F