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DOI: 10.1039/A705824A (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1998, 369-374

Comparison of the rates and mechanisms of formation and solvolysis of [Fe(bipy)3]2+ (bipy = 2,2′-bipyridine) and [FeL]2+ [L = 1,4,7-tris(2,2′-bipyridyl-5-ylmethyl)-1,4,7-triazacyclononane] and their stabilities in dimethylformamide solution[hair space]†

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Ana Maria Josceanu and Peter Moore

Abstract

Multiwavelength stopped-flow spectrophotometry has been used to compare the rates and mechanisms of formation of [Fe(bipy)3]2+ (bipy = 2,2′-bipyridine) and [FeL]2+ [L = 1,4,7-tris(2,2′-bipyridyl-5-ylmethyl)-1,4,7-triazacyclononane, containing three co-ordinating bipy groups], in dimethylformamide (dmf[hair space]) solution. Molecular modelling of L, and kinetic studies of its reaction with [Fe(dmf[hair space][hair space])6]2+, are consistent with the presence of two N-configurational isomers (RRR and RRS[hair space]); at 25 °C, the RRR isomer reacts with [Fe(dmf[hair space])6]2+ to give [FeL]2+ with a formation rate constant, 10–5kf = 2.34 ± 0.09 dm3 mol–1 s–1, and the RRS isomer reacts to give an intermediate bis(bipy) complex with 10–5kf = 1.04 ± 0.01 dm3 mol–1 s–1. This bis(bipy) intermediate rearranges to the tris(bipy) chelate with a first-order rate constant of 0.71 ± 0.02 s–1, but is also involved in the formation of a small amount of an [Fe2L2]4+ dimer. The kinetically determined dimer formation constant is (1.2 ± 0.1) × 105 dm3 mol–1. For the reaction of [Fe(dmf[hair space])6]2+ with bipy at 25 °C, rapid formation of [Fe(dmf[hair space])4(bipy)]2+ (10–5kf = 1.4 ± 0.1 dm3 mol–1 s–1) is followed by the much slower formation of [Fe(dmf[hair space])2(bipy)2]2+ (kf = 181 ± 7 dm3 mol–1 s–1) and [Fe(bipy)3]2+ (kf = 24.3 ± 0.9 dm3 mol–1 s–1). The complex [FeL]2+ is significantly more stable than [Fe(bipy)3]2+, as shown by the ready solvolysis of [Fe(bipy)3]2+ at low dilution in dmf, whilst [FeL]2+ is stable to solvolysis under the same conditions. At 25 °C first-order rate constants for the dissociation of bipy from [Fe(bipy)3]2+ and [Fe(dmf[hair space])2(bipy)2]2+ are (7.0 ± 0.3) × 10–3 s–1 and (1.4 ± 0.1) × 10–3 s–1 respectively, showing that both are inert, low-spin, t2g6 systems. This contrasts with the behaviour in water, since [Fe(OH2)2(bipy)2]2+ is known to be high spin and labile. The kinetically determined stepwise formation constants in dmf confirm this conclusion; values vary in the order 10–3K1/dm3 mol–1 = 9.6 ± 0.7 < 10–5K2/dm3 mol–1 = 1.3 ± 0.7 > 10–3K3/dm3 mol–1 = 3.5 ± 0.2 (in water K1 > K2 [double less-than, compressed] K3). Overall formation constants for [Fe(bipy)3]2+ in dmf (log10 β3 = 12.6) and in dimethyl sulfoxide (log10 β3 = 10.7) are significantly smaller than the value in water (log10 β3 = 17.4). Rates of solvolysis of [M(bipy)3]2+ (M = Fe or Ni) were also investigated in dimethyl sulfoxide solution.

References

  1. P. Sheldon, W. Errington, P. Moore, S. Rawle and S. Smith, J. Chem. Soc., Chem. Commun., 1994, 2489 RSC.
  2. J. H. Baxendale and P. George, Trans. Faraday Soc., 1950, 46, 736 RSC; E. C. Constable, Adv. Inorg. Chem., 1989, 34, 1 CrossRef CAS.
  3. P. Sheldon, Ph.D. Thesis, University of Warwick, 1995.
  4. A. M. Josceanu, P. Moore, S. C. Rawle, S. M. Smith and P. Sheldon, Inorg. Chim. Acta, 1995, 240, 159 CrossRef CAS.
  5. C. Cossy, L. Helm and A. E. Merbach, Helv. Chim. Acta, 1987, 70, 1516 CrossRef CAS.
  6. H. P. Bennetto and E. F. Caldin, J. Chem. Soc. A, 1971, 2198 RSC.
  7. M. W. Twigg, Transition Met. Chem., 1993, 18, 18 CrossRef.
  8. C. C. Deb, D. K. Hazia and S. C. Lahiri, Z. Phys. Chem. Leipzig, 1968, 267, 769 Search PubMed.
  9. P. Krumholz, J. Am. Chem. Soc., 1949, 71, 3654 CrossRef CAS.
  10. Stability Constants of Metal Ion Complexes, The Chemical Society, London, Special publ. 17, 1964 and 25, 1971 Search PubMed.
  11. H. Irving and D. H. Mellor, J. Chem. Soc., 1962, 5222 RSC.
  12. R. H. Holyer, C. D. Hubbard, S. F. A. Kettle and R. G. Wilkins, Inorg. Chem., 1965, 4, 929 CrossRef CAS; 1966, 5, 622.
  13. H. P. Bennetto and E. F. Caldin, J. Chem. Soc. A, 1971, 2191; 2207 RSC; H. P. Bennetto, J. Chem. Soc. A, 1971, 2211 RSC.
  14. D. W. Margerum, G. R. Cayley, D. C. Weatherburn and G. K. Pagenkopf, ACS Monogr., 1978, 174, 1 Search PubMed.
  15. F. Basolo, J. C. Hayes and H. M. Neumann, J. Am. Chem. Soc., 1953, 75, 5102 CrossRef CAS.
  16. A. I. Vogel, Textbook of Quantitative Inorganic Analysis, 4th edn., Longman, London and New York, 1978, p. 359 Search PubMed.
  17. F. P. Dwyer and H. A. McKanzie, Proc. R. Soc. N. S. W., 1947, 81, 93 Search PubMed.
  18. GLINT, version 4.01, Applied Photophysics, Leatherhead, UK.
  19. H. Gampp, M. Mäder, M. Meyer and A. D. Zuberbühler, Talanta, 1986, 32, 95, 257 CrossRef; 33, 943.
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