Structure and stability of complexes of macrocyclic ligands bearing 2-hydroxycyclohexyl groups. Structure of the copper(II) complex of 1-(2-hydroxycyclohexyl)-1,4,7,10-tetraazacyclododecane and the strontium(II) complex of 7,16-bis(2-hydroxycyclohexyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadec ane

Abstract

The compound 1-(2-hydroxycyclohexyl)-1,4,7,10-tetraazacyclododecane (L¹) and its complex [CuL¹][ClO₄]₂1 have been prepared, as well as the complex [SrL²(H₂O)][NO₃]₂2 from the previously reported ligand L² (L² = D,L -7,16-bis(2-hydroxycyclohexyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctade cane). X-Ray studies of 1 and 2 gave for 1, triclinic, space group P1̄, a = 8.632(1), b = 14.677(2), c = 17.797(4) Å, α = 86.80(2), β = 78.71(2), γ = 83.32(1)°, Z = 4, R = 0.0606, while 2 gave monoclinic, space group Cc, a = 16.809(6), b = 8.889(2), c = 21.232(6) Å, β = 101.47(2)°, Z = 4, R = 0.0314. The structure of 1 showed Cu–N and Cu–O bond lengths which were in the normal range. The structure shows some steric crowding of the co-ordinated ligand, with short H · · · H contacts between hydrogens on the cyclohexyl group and adjacent hydrogens on the macrocyclic ring. This acts to press the 2-hydroxycyclohexyl group towards the macrocyclic ring, and to have a compressive effect on the metal ion. The occurrence of 2 in space group Cc indicates spontaneous resolution of the complex into crystals with (R,R) or with (S,S) diastereomers of the ligand only. The Sr^II is nine-co-ordinate, with a water molecule occupying a co-ordination site. An extensive hydrogen-bonding network involving hydrogens from the co-ordinated water on Sr^II and nitrate oxygens, and hydrogens from the co-ordinated hydroxyls of the 2-hydroxycyclohexyl groups and nitrate oxygens, appears to be responsible for the spontaneous resolution. Ligands where ethylene bridges between donor atoms have been replaced by cyclohexanediyl bridges tend to show greater selectivity for smaller metal ions. This has been interpreted in terms of greater steric crowding on the outside of the ligand as the metal ion increases in size and decreases the curvature of the ligand. The structure of 2 shows six rather short H · · · H distances (2.05–2.2 Å) between hydrogens on the cyclohexyl group, and on the macrocyclic ring, which are much shorter than similar contacts in complex 1, supporting this suggestion. The protonation constants (log K) of L¹ are 10.65, 9.51 and 4.03, while the formation constants (log K₁) are 13.85 (Zn^II), 14.58 (Cd^II) and 11.40 (Pb^II), all in 0.1 mol dm^-3 NaNO₃ at 25 °C. The effect of the 2-hydroxycyclohexenyl bridge on the stability of complexes is discussed.

References

1. A. E. Martell and R. D. Hancock, Metal Complexes in Aqueous Solutions, Plenum, New York, 1996.
2. R. D. Hancock, Acc. Chem. Res., 1990, 23, 253.
3. D. J. Cram, Angew. Chem., Int. Ed. Engl., 1986, 25, 1039.
4. G. Schwarzenbach, R. Gut and G. Anderegg, Helv. Chim. Acta, 1954, 37, 937.
5. R. M. Izatt, J. S. Bradshaw, S. A. Neilsen, J. D. Lamb, J. J. Christensen and D. Sen, Chem. Rev., 1985, 85, 271.
6. R. M. Izatt, K. Pawlik, J. S. Bradshaw and R. L. Bruening, Chem. Rev., 1991, 91, 6540.
7. R. M. Izatt, K. Pawlik, J. S. Bradshaw and R. L. Bruening, Chem. Rev., 1995, 95, 2529.
8. A. E. Martell and R. M. Smith, Critical Stability Constants, Plenum, New York, 1974–1985, vols. 1–6.
9. W. Zhang, J. L. Loebach, S. R. Wilson and E. N. Jacobsen, J. Am. Chem. Soc., 1990, 112, 280.
10. M. W. Brechbiel and O. A. Gansow, J. Chem. Soc., Perkin Trans. 1, 1992, 1173.
11. A. S. De Sousa, G. J. B. Croft, C. A. Wagner, J. P. Michael and R. D. Hancock, Inorg. Chem., 1991, 30, 3525.
12. A. S. De Sousa and R. D. Hancock, J. Chem. Soc., Chem. Commun., 1995, 415.
13. M. F. Tweedle, D. D. Dischino, E. J. Delaney, J. E. Emswiler, J. S. Prasad and S. K. Srivistava, Inorg. Chem., 1991, 30, 1265.
14. J. J. Yaouanc, N. Le Bris, G. Le Gall, J. C. Clement, H. Handel and H. Des Abbayes, J. Chem. Soc., Chem. Commun., 1991, 206.
15. D. Parker, K. P. Pulukkoddy, T. J. Norman, L. Royle and C. J. Broan, J. Chem. Soc., Perkin Trans. 2, 1993, 605.
16. J. J. Stewart, J. Comput. Chem., 1989, 19, 209 and refs. therein.
17. A. C. North, D. C. Philips and F. Mathews, Acta Crystallogr., Sect. A, 1968, 24, 350.
18. G. Sheldrick, SHELXS 86, Program for Crystal Structure Solution, University of Göttingen, 1986.
19. (a) G. Sheldrick, SHELXL 93, Program for Crystal Structure Refinement, University of Göttingen, 1993; (b) International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. 4.
20. L. J. Zompa, Inorg. Chem., 1978, 17, 2531.
21. P. M. May, K. Murray and D. R. Williams, Talanta, 1985, 32, 483.
22. CACHE, Cache Scientific, Beaverton, OR, 1993.
23. B. P. Hay, J. R. Rustad and C. Hostetler, J. Am. Chem. Soc., 1993, 115, 11158.
24. C. Eaborn, P. B. Hitchcock, J. D. Smith and A. C. Sullivan, J. Organomet. Chem., 1984, C23, 263.
25. I. Bernal and G. B. Kauffman, J. Chem. Educ., 1987, 64, 604; 1989, 66, 293; I. Bernal, J. Chem. Educ., 1992, 69, 468; Inorg. Chim. Acta, 1985, 96, 99; 1985, 101, 175.
26. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.
27. N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984, p. 1413.
28. R. Luckay, J. H. Reibenspies and R. D. Hancock, J. Chem. Soc., Chem. Commun., 1995, 2365.
29. N. L. Allinger, J. Am. Chem. Soc., 1977, 99, 8127.