Metal ion size selectivity of 1-Thia4, 7-diazacyclononane (9-aneN:S), and other tridentate macrocycles. A study by molecular mechanics calculation, structure determination, and formation constant determination of complexes of 9-aneN 2S

Abstract

Several aspects of the coordination chemistry of the tridentate cyclononane type macrocycles are examined using molecular mechanics calculations, crystallography and formation constant determinations. The molecular mechanics calculations show that small metal ions coordinate best to these ligands, such that metal ions with a covalent radius of 1.25 A fit best into 9-aneS3 and 1.40 Ä fit best into 9-aneN3 (9-aneS3 = 1,4,7-trithiacyclononane, 9-aneN3 = 1,4,7-triazacyclononane). For mixed donor members of the series such as 9-aneN:S (1-thia4,7-diaza- cyclononane) the disparity in M-L bond length between the M-N and M-S bond lengths leads to a much higher strain situation than expected from the strain energies of the 9-aneN3 and 9-aneS3 complexes. This accounts for the order of ligand field strength in complexes of these ligands of 9-aneS 3 > 9-aneN 3 > 9-aneN:S. It is concluded that in the absence of the strain effects encountered in mixed donor ligands containing the thioether donor group, the latter group should always be higher in the spectrochemical series than ligands containing the secondary nitrogen donor. The formation constants of 9-aneN:S with Ni(II), Zn(II), Cd(II), Co(II), Fe(II), and Pb(II) are reported. Comparison of these with the formation constants for the 9-aneN:O and 9-aneN3 complexes shows that the macrocyclic effect (the difference in stability between the complex of the macrocycle and of its open chain analogue) is much higher for small metal ions, and small with large metal ions, in agreement with the molecular mechanics calculations which show that the cyclononane macrocycles coordinate best with small metal ions. The crystal structure of the complex [Cu(9-aneN:S)Br:] is reported: monoclinic, space group P2 1/n, with cell dimensions a = 7.603(1), b = 13.167(2), and c -- 10.873(2) Å, and β = 91.94- (1)°, Z = 4. Final conventional R = 0.061. The un-