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Tropocoronand ligand
The tropocoronand ligand (H2TC-m,n) is a macrocyclic ligand in which two aminotroponiminate rings are connected to one another via polymethylene linker chains of length m and n. Double deprotonation of the ligand yields a dianionic macrocylic species that is capable of binding divalent transition metal ions to form neutral complexes (). The 2-aminotroponeimine units are bridged by polymethylene linker chains with all four nitrogen atoms of the tropocoronand ligand bonded to a metal atom.〔Seichi Imajo and Koji Nakanishi. J. Am. Chem. Soc. 1983,105, 2071-2073.〕
==Introduction==
The tropocoronand family of ligands(H2TC-m,n) have proved valuable for studying how changes in macrocycle ring size and flexibility can tune the physical and chemical properties of transition metal ions.〔Franz, K.J.; Doerrer, L.H.; Bernhard Spingler, and Lippard S.J. Inorg Chem. 2001, 40, 3374-3780.〕 Although the size of the binding cavity in most N4 macrocycles, including porphyrins, can vary to only a small degree, a series of tropocoronand ligands (TC-m,n) 2− having the same set of four aromatic nitrogen donor atoms but with a range of macrocycle sizes can be prepared. The hole size can be systematically adjusted by controlling the number of methylene bridges, m and n, in the arms connecting the two aminotroponiminate rings. Moreover, the flexibility of the linker chains allows the seven-membered rings to twist relative to each other as the macrocyclic ring size expands. In mononuclear complexes with large m and n, the rings will rotate to reduce torsional strain within the linker arms. These distortions induce substantial alterations in the energies of the molecular orbitals at the metal center, thus allowing its properties to be tuned. This features of the tropocoronand ligand could be investigated mostly with Group 4 metal centers.〔Michael, J.S, Lippard, S. J. Inorganica Chemica Acta 1997,263, 287-299.〕 They represent a new class of molecules with potential to be modified with a chiral moiety and applied to enantioselective reactions.〔Chenier, P. J.; Halfen, J. A.; Tami L. Autumn E. Rich, R; Splan, K.E.; Yoshioka K; R. Hoye,T. Synthetic Communications, 2001,31(4), 487-503.〕
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