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• W2090911092 abstract "The origin of the Macrocyclic Effect [1] in complexes of cyclam as compared with those of its open-chain analogue 2,3,2-tet is discussed. It is not readily apparent that the presence of four secondary (2°) nitrogens in cyclam, as opposed to two primary (1°) and two 2° in 2,3,2-tet should make much difference, since the pKa's of 2° amines are not much different from those of 1°. Proton basicity in water is a misleading guide to base-strength. for example logKn-(RNH2 with Ag(I) increases strongly as R changes NiLnL = enpnen3diendien2tetren2,3,2-tetcyclamΔU0.612.140.804.046.7011.063.331.33−ΔH, calc:8.997.4628.0012.062.5518.0419.2724.67−ΔH, obs:9.07.828.011.92.5318.319.224.1 in the order R = Me < Et < i-propyl <t-butyl, although the pKa's are almost invariant. This effect is attributable to greater steric hindrance to solvation for the proton. For 1° versus 2° nitrogens, it is found in the gas phase, for example, that (CH3)2NH is a much stronger base than CH3NH2, although their pKa's in water are identical. This effect is partly attributable to quenching of polarizability effects [2], and also steric hindrace to solvation. The E and C equation of Drago and Wayland [3] is used to show that in the absence of steric effects, the CuN bond in [Cu(Hfac)2NRR′2NH. This qualitative suggestion that the MN bond is stronger when N is 2° is supported [4] by empirical force field calculations of the increase in conformational potential energy, U, on complex-formation of Ni(II) with a variety of polyamine complexes. In each case, U has been calculated for each of the species in the reaction below, allowing calculation of ΔU. It was found that consistency could only be obtained by making the 2° NiN bond 1.7 kcal mol−1 more exothermic than the 1°. If a value of 4.8 kcal mol−1 is accorded to the ideal NiN bond to a nitrogen in the absence of steric strain, summation of the appropriate number of contributions from 1° and 2° nitrogens followed by subtraction of ΔU allows one to calculate −ΔH as shown in the Table. All calculations are for the high-spin complexes. From these results, one may assign the macrocyclic enthalpy to, firstly the presence of more 2° nitrogens in cyclam, followed by a smaller ΔU than is found for 2,3,2-tet. An important point in favour of these ideas which suggests stronger NiN bonds, is that 10 Dq for the Ni(II) cyclam complex is higher than for 2,3,2-tet. Contributions of solvation and entropy effects to the macrocyclic effect are discussed. Results below are an kcal mol−1." @default.
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• W2090911092 title "The macrocyclic effect in tetraaza-macrocyclic ligands" @default.
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• W2090911092 doi "https://doi.org/10.1016/s0020-1693(00)92099-1" @default.
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