ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
An efficient combination of electrospray mass spectrometry (ES-MS), spectrophotometric and 1H-NMR titrations in solution is used to characterize the assembly of the segmental ligand 2-{6-[1-(3,5-dimethoxybenzyl)-1H-benzimidazol-2-yl]pyridin-2-yl}-1, 1′-dimethyl-5,5′-methylene-2′-(5-methylpyridin-2-yl)bis[1H- benzimidazole] (L2) with ZnII and 4f metal ions, LnIII. Ligand L2 reacts with Zn(ClO4)2 in MeCN to give successively [Zn(L2)2]2+, where the metal ion is coordinated by the tridentate binding units of the ligands, and the double-helical head-to-head complex [Zn2(L2)2]4+. When L2 reacts with Ln(ClO4)3 (Ln = La, Eu, Lu), LaIII only leads to a well-defined cylindrical C1-symmetrical homodinuclear head-to-tail complex [La2(L2)3]6+ in solution, while chemical-exchange processes prevent the 1H-NMR characterization of [Eu2(L2)3]6+, and LuIII gives complicated mixtures of complexes. However, stoichiometric amounts of LnIII (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Y, Lu), ZnII, and L2 in a 1:1:3 ratio lead to the selective formation of the C3-symmetrical heterodinuclear complexes [LnZn(L2)3]5+ under thermodynamic control. Detailed NOE studies show that the ligands are wrapped about the C3 axis defined by the metal ions, and the separation of dipolar and contact contributions to the 1H-NMR paramagnetic shifts of the axial complexes [LnZn(L2)3]5+ (Ln = Ce, Pr, Nd, Sm, Eu) in MeCN establishes that ZnII occupies the pseudo-octahedral capping coordination site defined by the three bidentate binding units, while LnIII lies in the resulting ‘facial’ pseudo-tricapped trigonal prismatic site produced by the three remaining tridentate units. Photophysical measurements show that [LnZn(L2)3]5+ (Ln = Eu, Tb) are only weakly luminescent because of quenching processes associated with the C3-cylindrical structure of the complexes. The use of 3d metal ions to control and design isomerically pure ‘facial’ tricapped trigonal prismatic lanthanide building blocks is discussed together with the calculation of a new nephelauxetic parameter associated with heterocyclic N-atoms coordinated to LnIII.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19950780612
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