Dynamic stereochemical rearrangements in ‘chiral-at-ligand’ complexes. Tricarbonylhalogenorhenium(I) and halogenotrimethylplatinum(IV) complexes of 2,6-bis[4-(S)-methyloxazolin-2-yl]pyridine

Abstract

Enantiomerically pure 2,6-bis[4-(S)-methyloxazolin-2-yl]pyridine, L, reacted with the pentacarbonylhalogenorhenium(I) and halogenotrimethylplatinum(IV) metal moieties to afford complexes of general formulae fac-[ReX(CO)₃L] and fac-[PtXMe₃L] (X = Cl, Br or I) in moderate yield. The potentially terdentate ligand acts here in a bidentate bonding mode, and undergoes a dynamic structural rearrangement which exchanges the pendant and co-ordinated oxazole rings. The bidentate bonding mode and absolute configuration of the ligand were confirmed by the crystal structure of the complex [ReCl(CO)₃L]. The chiral centres on the ligand provide an excellent spectroscopic handle on the mechanism of the rearrangement, and two independent fluxional pathways are apparent. The energetics of the rearrangement in the complexes [ReX(CO)₃L] (X = Cl, Br or I) was measured by standard ¹H NMR band-shape analysis; ΔG^‡(average) (298 K) is 74.3 kJ mol^-1 for the ‘tick-tock twist’ mechanism and 84.0 kJ mol^-1 for the ‘rotation’ mechanism. Platinum-195 NMR data are reported for the complexes [PtXMe₃L] (X = Cl, Br or I).

References

1. E. C. Constable, Adv. Inorg. Chem. Radiochem., 1987, 30, 69.
2. E. W. Abel, V. S. Dimitrov, N. J. Long, K. G. Orrell, A. G. Osborne, H. M. Pain, V. Sik, M. B. Hursthouse and M. A. Mazid, J. Chem. Soc., Dalton Trans., 1993, 291.
3. E. W. Abel, V. S. Dimitrov, N. J. Long, K. G. Orrell, A. G. Osborne, H. M. Pain, V. Sik, M. B. Hursthouse and M. A. Mazid, J. Chem. Soc., Dalton Trans., 1993, 597.
4. E. R. Civitello, P. J. Dragovich, T. B. Karpishin, S. G. Novick, G. Bierach, J. F. O'Connel and T. D. Westmoreland, Inorg. Chem., 1993, 32, 237.
5. E. W. Abel, K. G. Orrell, A. G. Osborne, H. M. Pain and V. Sik, J. Chem. Soc., Dalton Trans., 1994, 111.
6. (a) E. W. Abel, K. G. Orrell, A. G. Osborne, H. M. Pain, V. Sik, M. B. Hursthouse and K. M. A. Malik, J. Chem. Soc., Dalton Trans., 1994, 3441; (b) E. W. Abel, A. Gelling, K. G. Orrell, A. G. Osborne and V. Sik, Chem. Commun., 1996, 2329.
7. E. Rotondo, G. Giordano and D. Minniti, J. Chem. Soc., Dalton Trans., 1996, 253.
8. E. W. Abel, K. A. Hylands, M. D. Olsen, K. G. Orrell, A. G. Osborne, V. Sik and G. N. Ward, J. Chem. Soc., Dalton Trans., 1994, 1079.
9. A. Gelling, K. G. Orrell, A. G. Osborne and V. Sik, J. Chem. Soc., Dalton Trans., 1994, 3545.
10. E. W. Abel, D. Ellis and K. G. Orrell, J. Chem. Soc., Dalton Trans., 1992, 2243.
11. E. W. Abel, P. J. Heard, K. G. Orrell, M. B. Hursthouse and M. A. Mazid, J. Chem. Soc., Dalton Trans., 1993, 3795.
12. E. W. Abel, D. Ellis, K. G. Orrell and V. Sik, Polyhedron, 1991, 10, 1063.
13. E. W. Abel, J. C. Domer, D. Ellis, K. G. Orrell, V. Sik, M. B. Hursthouse and M. A. Mazid, J. Chem. Soc., Dalton Trans., 1992, 1073.
14. R. Romeo, A. Grassi and L. M. Scolaro, Inorg. Chem., 1992, 31, 4383.
15. H. Nishiyama, M. Kondo, T. Nakamura and K. Itoh, Organometallics, 1991, 10, 580.
16. T. G. Grant and A. I. Meyers, Tetrahedron, 1994, 50, 2297 and refs. therein.
17. S. P. Schmidt, W. C. Trogler and F. Basolo, Inorg. Synth., 1979, 28, 160.
18. J. C. Baldwin and W. C. Kaska, Inorg. Chem., 1975, 14, 2020.
19. D. H. Goldsworthy, Ph.D. Thesis, University of Exeter, 1980.
20. D. F. Shriver, Manipulation of Air-sensitive Compounds, McGraw-Hill, New York, 1969.
21. D. D. Perrin and W. L. F. Armarego, Purification of Laboratory Chemicals, Pergamon, Oxford, 1988.
22. D. A. Kleier and G. Binsch, DNMR 3, Quantum Chemistry Exchange, Indiana University, 1970.
23. G. Binsch and H. Kessler, Angew. Chem., Int. Ed. Engl., 1980, 19, 411.
24. G. Bodenhausen, H. Kogler and R. R. Ernst, J. Magn. Reson., 1984, 58, 370.
25. J. A. Darr, S. R. Drake, M. B. Hursthouse and K. M. A. Malik, Inorg. Chem., 1993, 32, 5704.
26. G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
27. G. M. Sheldrick, SHELXL 93, Program for Crystal Structure Refinement, University of Göttingen, 1993.
28. J. A. Ibers and W. C. Hamilton(Editors), International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. 4.
29. N. P. C. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983, 39, 158; adapted for FAST geometry by A. I. Karavolv, University of Wales, Cardiff, 1991.
30. R. J. Abraham, J. Fisher and P. Loftus, Introduction to NMR Spectroscopy, Wiley, New York, 1991.
31. D. A. Edwards and J. Marshalsea, J. Organomet. Chem., 1977, 131, 73.
32. J. Emsley, The Elements, Oxford University Press, 2nd edn., 1991.
33. E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem., 1984, 32, 1.