Synthetic, structural and electrochemical studies of cationic chromium(VI) and molybdenum(VI) bis(imido) complexes supported by 1,4,7-triazacyclononane macrocycles

Abstract

Treatment of [Cr(N^tBu)₂Cl₂] or [Mo(N^tBu)₂Cl₂(dme)] with 1,4,7-triazacyclononane (tacn) in THF, followed by metathesis using NaClO₄ in aqueous medium, readily afforded [M(tacn)(N^tBu)₂Cl]ClO₄ (M = Cr 1 or Mo 2); [M(Me₃tacn)(N^tBu)₂Cl]Cl (M = Cr 3 or Mo 4) were similarly prepared using the metal precursors and 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn). Complexes 1–4 are resistant to hydrolysis, but do not mediate imido group transfer reactions. Distinctive UV-vis absorption bands are tentatively assigned to imido-to-metal LMCT transitions. The crystal structures of 1 and 2·CH₃CN show that the 3 nitrogen atoms of tacn are not equidistant from the respective metal centres. A severely distorted imido moiety (imido angle 151.8(4)°) has been observed in the structure of 1 but not 2·CH₃CN, and is attributed to steric and crystal packing factors rather than electronic effects. Metathesis of both chloride groups in 3 using AgOTf (OTf = SO₃CF₃) yielded [Cr(Me₃tacn)(N^tBu)₂(OTf )]OTf 5. The cyclic voltammograms of 1–5 in acetonitrile consist of a quasi-reversible couple (+0.86 to +1.20 V vs. Cp₂Fe^0/+) and an irreversible wave (–1.04 to –2.08 V vs. Cp₂Fe^0/+); the former couple is assigned to an imido-centred oxidation.

References

1. P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem., 1987, 35, 329.
2. R. Bhula, P. Osvath and D. C. Weatherburn, Coord. Chem. Rev., 1988, 91, 89.
3. Mo and W: (a) G. Backes-Dahmann and K. Wieghardt, Inorg. Chem., 1985, 24, 4049; (b) K. Wieghardt, G. Backes-Dahmann, B. Nuber and J. Weiss, Angew. Chem., Int. Ed. Engl., 1985, 24, 777; (c) K. Wieghardt and W. Herrmann, Polyhedron, 1986, 5, 513; (d) K. S. Bürger, G. Haselhorst, S. Stoetzel, T. Weyhermüller, K. Wieghardt and B. Nuber, J. Chem. Soc., Dalton Trans., 1993, 1987; (e) P. S. Roy and K. Wieghardt, Inorg. Chem., 1987, 26, 1885; (f) G. Haselhorst, S. Stoetzel, A. Strassburger, W. Walz, K. Wieghardt and B. Nuber, J. Chem. Soc., Dalton Trans., 1993, 83.
4. Re: (a) K. Wieghardt, C. Pomp and B. Nuber, Inorg. Chem., 1986, 25, 1659; (b) W. A. Herrmann, P. W. Roesky, F. E. Kühn, W. Scherer and F. Kleine, Angew. Chem., Int. Ed. Engl., 1993, 32, 1714.
5. Ru: (a) W. C. Cheng, W. Y. Yu, K. K. Cheung and C. M. Che, J. Chem. Soc., Chem. Commun., 1994, 1063; (b) W. C. Cheng, W. Y. Yu, K. K. Cheung and C. M. Che, J. Chem. Soc., Dalton Trans., 1994, 57; (c) W. H. Fung, W. Y. Yu and C. M. Che, J. Org. Chem., 1998, 63, 2873.
6. V, Cr and Mn nitrido: A. Niemann, B. Bossek, G. Haselhorst, W. Wieghardt and B. Nuder, Inorg. Chem., 1996, 35, 906.
7. (a) P. J. Wilson, A. J. Blake, P. Mountford and M. Schröder, Chem. Commun., 1998, 1007; (b) related 1,3,5-triazacyclohexane complexes: P. J. Wilson, P. A. Cooke, A. J. Blake, P. Mountford and M. Schröder, New J. Chem., 1999, 271.
8. W. H. Fung, W. C. Cheng, S. M. Peng and C. M. Che, Polyhedron, 1995, 14, 1791.
9. J. Y. K. Cheng, K. K. Cheung, M. C. W. Chan, K. Y. Wong and C. M. Che, Inorg. Chim. Acta, 1998, 272, 176.
10. A. A. Danopoulos, W. H. Leung, G. Wilkinson, B. Hussain-Bates and M. B. Hursthouse, Polyhedron, 1990, 9, 2625.
11. P. W. Dyer, V. C. Gibson, J. A. K. Howard, B. Whittle and C. Wilson, J. Chem. Soc., Chem. Commun., 1992, 1666.
12. T. J. Atkins, J. E. Richman and W. F. Oettle, Org. Synth., 1978, 58, 86; K. Wieghardt, K. Schmidt, B. Nuber and J. Weiss, Chem. Ber., 1979, 112, 2220.
13. K. Wieghardt, P. Chaudhuri, B. Nuber and J. Weiss, Inorg. Chem., 1982, 21, 3086.
14. TEXSAN, Crystal Structure Analysis Package, Molecular Structure Corporation, The Woodlands, Texas, 1985 and 1992.
15. W. H. Leung, M. C. Wu, T. C. Lau and W. T. Wong, Inorg. Chem., 1995, 34, 4271.
16. J. Sundermeyer, J. Putterlik, M. Foth, J. S. Field and N. Ramesar, Chem. Ber., 1994, 127, 1201.
17. A. A. Danopoulos, G. Wilkinson, T. K. N. Sweet and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 1995, 2111.
18. M. P. Coles, V. C. Gibson, W. Clegg, M. R. J. Elsegood and P. A. Porrelli, Chem. Commun., 1996, 1963.
19. D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239; W. A. Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds, Wiley Interscience, New York, 1988.
20. J. M. Garrison, D. Ostovic and T. C. Bruice, J. Am. Chem. Soc., 1989, 111, 4960.
21. (a) B. L. Haymore, E. A. Maatta and R. A. D. Wentworth, J. Am. Chem. Soc., 1979, 101, 2063; (b) A. A. Danopoulos, G. Wilkinson, T. Sweet and M. B. Hursthouse, J. Chem. Soc., Chem. Commun., 1993, 495; (c) V. C. Gibson, E. L. Marshall, C. Redshaw, W. Clegg and M. R. J. Elsegood, J. Chem. Soc., Dalton Trans., 1996, 4197.
22. For example, see: G. Hogarth, P. C. Konidaris and G. C. Saunders, J. Organomet. Chem., 1991, 406, 153; M. L. H. Green, G. Hogarth and G. C. Saunders, J. Organomet. Chem., 1991, 421, 233; W. H. Leung, M. C. Wu, K. Y. Wong and Y. Wang, J. Chem. Soc., Dalton Trans., 1994, 1659; H. Shen, R. A. Senter, S. G. Bott and M. G. Richmond, Inorg. Chim. Acta, 1996, 247, 161; H. Schumann, Inorg. Chem., 1996, 35, 1808.
23. S. M. Lee, R. Kowallick, M. Marcaccio, J. A. McCleverty and M. D. Ward, J. Chem. Soc., Dalton Trans., 1998, 3443.