The apparent paradox that larger metal ions coordinate more strongly to the small macrocycle 12-aneN4 than to the large 14-aneN4 is examined. (12-aneN4 = 1,4,7,10-tetraazacyclododecane; 14-aneN4 = 1,4,8,11-tetraazacyclotetradecane.) Molecular mechanics calculations (MM) show that, in its trans-I conformer, 12-aneN4 has a larger cavity, with best-fit M-N lengths of 2.11 Á, compared with 2.05 Á for 14-aneN4. In addition, the macrocyclic ring of 12-aneN4 is more flexible than that of 14-aneN4, allowing better response to a change of metal ion size. Thus, as explained by using the MM calculations, most metal ions complex most strongly to 12-aneN4, with only small metal ions such as Cu(II) preferring the large 14-aneN4. In order to supplement and check existing formation constants on the tetraaza macrocycles, log K¡ values for the following metal ions were determined: with 12-aneN4, i /Cu(II) 23.29; with 13-aneN4, Cu(II) 24.36; with 14-aneN4, Cu(II) 26.5 (all at 25 °C and in 0.5 M NaNQ3, determined spectroscopically by using the variation in the UV-visible spectrum as a function of pH); with 13-aneN4, Pb(II) 13.48, Cd(II) 12.71; with 14-aneN4, Pb(II) 10.83, Cd(II) 11.23 (all determined by glass-electrode potentiometry in 0.1 M NaN03 at 25 °C).Molecular mechanics (MM) calculations have been carried out1•2 to calculate the hole sizes in tetraaza macrocycles. These calculations have been confined to the case where the metal ion is constrained to lie within the plane of the four nitrogen donors and give best-fit metal to nitrogen bond lengths for these macrocycles as follows:1

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The H+- and Cl--assisted dissociation kinetics and the stabilities of the complexes [Hg(sar)]2+ and [Hg((NH2)2-sar)]2+ (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane and (NH2)2-sar = 1,8-diamino-sar) were determined. The Hg2+ dissociation rates depend on both the proton and the chloride ion concentrations. H+ competes with the metal ion for dissociated amine groups, and Cl- competes with the amine for vacant coordination sites. The rate laws are complicated. For the [Hg(sar)]2+ system (0.1 ≤ [H+] ≤ 1.0 M, 0.01 ≤ [Cl-] ≤ 1.0 M, I = 2.0 M (NaO3SCF3), 25.0 °C) the observed rate law is v - Hg 2+ = (a + b[Cl-])[H+][Hg(sar)2+]/(1 + c[Cl-]), with a = 35(3) M-1 s-1, b = 2.9(4) × 103 M-2 s-1, and c = 33(5) M-1. For the [Hg((NH3)2-sar)]4+ system (0.001 ≤ [H+] ≤ 1.0 M, 0.01 ≤ [Cl-] ≤ 1.0 M, I = 1.0 M (LiClO4), 25.0 °C) the observed rate law is v - Hg 2+ = (a + b[H+] + c[H+]2)[Cl-][Hg((NH3)2-sar)4+])/((1 + d[Cl-])(1 + e[H+])), with a = 0.056(6) M-1 s-1, b = 8(3) M-2 s-1, c = 5(3) M-3 s-1, d = 1.3(4) M-1, and e = 1.1(5) × 102 M-1. Intimate mechanisms for the dissociation reactions are proposed. Using iodide ion or sar ligand as competing ligands and the reported values for the stabilities of HgI3 -and HgI4 2- the stability constants at 25.0 °C were determined for [Hg(sar)]2+ (1028.1(1) M-1), [Hg(sar)I]+ (1029.1(1) M-2), [Hg((NH2)2-sar)I]+ (1028.5(1) M-2), and [Hg(cyclam)I]+ (1030.8(1) M-2) (cyclam = 1,4,8,11-tetraazacyclotetradecane) with [OH-] = 0.1 M, I = 0.5 M (NaClO4) and for [Hg((NH2)2-sar)]2+ (1026.4(3) M-1) with [OD-] = 0.1 M, I = 0.1 M (NaOD).

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