Alkali metal–1-azaallyl complexes: X-ray crystallographic, NMR spectroscopic and ab initio calculational studies

Abstract

A series of alkali metal–1-azaallyl complexes, [{CH₃CH₂CH₂C(H)C(Bu^t)N(H)Li·HMPA}₂] 1, [{CH₃CH₂CH₂C(H)C(Bu^t)N(H)Na·2HMPA}₂] 2 and [{CH₂C(Bu^t)N(H)Li·HMPA}₂] 3, has been synthesised by treating each appropriate metal alkyl reagent (n-butyllithium, n-butylsodium or methyllithium, respectively) with tert-butyl cyanide in the presence of the Lewis base HMPA [hexamethylphosphoramide, (Me₂N)₃P[double bond, length half m-dash]O]. X-Ray crystallographic studies have established that each structure is dimeric and built around a precisely or approximately centrosymmetric rhomboidal (N–M)₂ ring. However, the nature of the azaallyl–metal bonding differs with 1 and 2 displaying a terminal η¹-N arrangement, while 3 displays a chelating η³-NCC arrangement. ¹H and ¹³C NMR spectroscopic studies suggest that these distinct bonding modes are retained in [²H₈]toluene solution. Long-range (⁴J ) “W” coupling (2.4 Hz) is observed for 3 between the NH and one of the α-CH₂ protons, consistent with the trans orientation of the NH and C [horiz bar, double dot above] C linkages seen in the solid state. The preference for this geometry is confirmed by ab initio MO calculations on models of 3, which examine the energetics of the ketimide–azaallyl isomerism involved in the formation of 1–3.

References

1. P. C. Andrews, D. R. Armstrong, M. MacGregor, R. E. Mulvey and D. Reed, J. Chem. Soc., Chem. Commun., 1989, 1341.
2. D. Stalke, M. Wedler and F. T. Edelmann, J. Organomet. Chem., 1992, 431, C1.
3. I. Cragg-Hine, M. G. Davidson, F. S. Mair, P. R. Raithby and R. Snaith, J. Chem. Soc., Dalton Trans., 1993, 2423.
4. M. S. Eisen and M. Kapon, J. Chem. Soc., Dalton Trans., 1994, 3507.
5. P. B. Hitchcock, M. F. Lappert and D.-S. Liu, J. Chem. Soc., Chem. Commun., 1994, 2637.
6. M. F. Lappert and D.-S. Liu, J. Organomet. Chem., 1995, 500, 203.
7. The monomeric structure of [{(Me₃Si)₂C(C₆H₄Br-4)C(SiMe₃)N}-Li·THF] has also been described in outline. See M. F. Lappert and M. Layh, Tetrahedron Lett., 1998, 39, 4745.
8. For other lithium azaallyl structures see R. I. Papasergio, B. W. Skelton, P. Twiss, A. H. White and C. L. Raston, J. Chem. Soc., Dalton Trans., 1990, 1161.
9. P. B. Hitchcock, M. F. Lappert and M. Layh, J. Chem. Soc., Dalton Trans., 1998, 1619.
10. D. Barr, W. Clegg, R. E. Mulvey, R. Snaith and D. S. Wright, J. Chem. Soc., Chem. Commun., 1987, 716.
11. P. C. Andrews, R. E. Mulvey, W. Clegg and D. Reed, J. Organomet. Chem., 1990, 386, 287.
12. H. Gunther, in NMR Spectroscopy, J. Wiley and Sons, Chichester, 1980, p. 115.
13. Gaussian 94 (Revision A.1), M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. A. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995.
14. W. J. Hehre, R. DitchWeld and J. A. Pople, J. Chem. Phys., 1972, 56, 2257; P. C. Hariharan and J. A. Pople, Theoret. Chim. Acta, 1973, 28, 213; J. D. Dill and J. A. Pople, J. Chem. Phys., 1975, 62, 2921.
15. C. Moller and M. S. Plesset, Phys. Rev., 1934, 46, 618.
16. W. G. Kofron and L. M. Baclawski, J. Org. Chem., 1976, 41, 1879.
17. C. Redshaw, V. C. Gibson, W. Clegg, A. J. Edwards and B. Miles, J. Chem. Soc., Dalton Trans., 1997, 3343.