CHELATE RING GEOMETRY, AND THE METAL-ION SELECTIVITY OF MACROCYCLIC LIGANDS, SOME RECENT DEVELOPMENTS

The idea (Hancock, 1992) that the dominant architectural feature in co ntrolling metal ion selectivity in both open-chain and macrocyclic lig ands is the size of the chelate ring is pursued further. It is shown t hat when more than one or two six-membered chelate rings are present i n the complex of a nitrogen donor macrocycle, the steric requirements of the six-membered chelate ring of a M-N bond length of 1.6 Angstrom and N-M-N angle of 109.5 degrees become particularly severe, and can o nly be met by a small tetrahedral metal ion. Thus, the ligand 16-aneN( 4) (1,5,9,13-tetraazacyclohexadecane) forms complexes of low stability with all metal ions studied to date, but a conformer of 16-aneN(4) is identified by MM calculation which is predicted to form complexes of high stability with very small tetrahedral metal ions. The question of the M-O bond length and O-M-O angles that will produce minimum strain in chelate rings containing neutral oxygen donor is addressed. The ob servation (Hay, 1993) that the geometry around an ethereal oxygen coor dinated to a metal ion approximates to trigonal planar rather than tet rahedral leads to ideal M-O-C angles of about 126 degrees, which leads to minimum strain energy with much longer M-L lengths in chelate ring s containing neutral oxygen donors than neutral nitrogen donors. It is suggested that this fact accounts for the general tendency of crown e thers to form their most stable complexes with potassium out of the al kali metal ions, and also accounts for the very small macrocyclic effe ct observed in complexes of macrocycles containing mixed nitrogen and oxygen donor groups. The preferred geometry of four-membered chelate r ings is discussed, and it is shown that higher coordination numbers of metal ions are associated with four membered chelate rings, and that four membered chelate rings may be used to engineer preference for lar ger metal ions. Very rigid reinforced chelate rings are discussed, and it is shown that open-chain ligands with reinforced bridges between t he donor atoms can display all the thermodynamic and kinetic aspects a ssociated with macrocyclic ligands.