OPEN-CHAIN POLYAMINE LIGANDS WITH MORE RIGID DOUBLE CONNECTING BRIDGES - STUDY OF THEIR METAL-ION SELECTIVITIES BY MOLECULAR MECHANICS CALCULATION, CRYSTALLOGRAPHY, AND THERMODYNAMICS

Ligands with two N-(2-aminoethyl), N-(3-aminopropyl), or N-acetate pendant groups on piperazine (PIP), homopiperazine (HPIP), or 1,5-diazacyclooctane (DACO) are described. Protonation and formation constants of some of the complexes of these ligands with Cu(II), Ni(II), Zn(II), Cd(II), Pb(II), Ca(II), Sr(II), and Ba(II) are determined. Electronic spectra of the complexes with Cu(II) and low-spin Ni(II) are reported. It is shown that (A) doubly bridged open-chain polyamines have metal ion size selectivity patterns that resemble those of isomeric tetraaza macrocycles and (B) ligand field (LF) strengths of doubly bridged open-chain polyamines are similar to those of isomeric tetraaza macrocycles. Point A suggests that size selectivity is controlled by chelate ring size rather than macrocyclic ring size, and point B indicates that LF strength is controlled by donor atom basicity along the series primary < secondary < tertiary if there are no large differences in steric distortions of the M-N bonds, log values of complexes of doubly bridged open-chain polyamine ligands are lower than those ot tetraaza macrocycles. This is attributed to low levels of preorganization in the doubly bridged open-chain polyamine free ligand, where there is a large increase in strain energy on altering the ligand from its lowest energy conformer to that required for complex formation. The crystal structure of [Ni(BAP-HP)](C1O4)2is reported (BAP-HP = l,4-bis(3-aminopropyl)-l,4-diazacycloheptane). Crystal data: C11H26Cl2N4O8Ni, monoclinic, space group P2/n, a = 9.008 (8) ⇋, b = 14.251 (3) ⇋, and c = 14.490 (4) ⇋, β = 98.09 (5) °, Z = 4, V= 1841.6 ⇋3, dm= 1.710 g-cm−3, dc= 1.702 g·cm−3. The final conventional R factor was 0.0603. Molecular mechanics (MM) calculations showed that the rigidity of doubly bridged open-chain tetraamines in their complexes was slightly less than that of tetraaza macrocycles. © 1990, American Chemical Society. All rights reserved.