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Factors in polypyridyl ligands that control their thermodynamic metal ion selectivity in aqueous solution, and their use in selective fluorescent sensing, are examined. Preorganization of polypyridyl ligands ranging from bidentate to tetradentate by bridging benzo groups, as are present in 1,10-phenanthroline (phen) compared to 2,2′-bipyridyl (bpy), is discussed. The role of solvation is considered in relation to the relative affinity of ligands containing pyridyl groups for divalent and trivalent metal ions in aqueous solution. The effects of steric clashes between H atoms on polypyridyl ligands in decreasing complex stability are evaluated, as well as the effect of chelate ring size on metal ion selectivity. Phen ligands with other donor groups present at the 2 and 9 positions, such as alcohols, amides, carboxylates, and oximes are discussed. The design of pyridyl-based ligands for the separation of Am(III) from lanthanide(III) ions is considered, as well as ligands for the removal of metal ions such as Cu(II) or Zn(II) in neurological diseases such as Alzheimer’s. The design of pyridyl-based fluorescent sensors for selective sensing of metal ions is examined in terms of the role of spin–orbit coupling constants (ζ), paramagnetism, and steric effects in the development of selective fluorescent sensors that operate via chelation enhanced fluorescence (CHEF). It is concluded that for lighter metal ions with smaller ζ values such as Zn(II) and Ca(II), and to a lesser extent Cd(II), that the CHEF effect can be achieved with pyridyl-containing fluorophores that coordinate directly to the metal ion. The way in which steric effects can be used to decrease the CHEF effect in Zn(II) relative to Cd(II) to enable selective sensing of the latter is analyzed. For heavier metal ions such as Hg(II) and Pb(II), because of their large ζ values which quench fluorescence, it is concluded that the fluorophore should be tethered to the metal-binding part of the sensor, and prevented from binding to the metal ion by steric and electronic factors. How Hg(II) can quench the CHEF effect by π-contact with fluorophores such as the anthracenyl group, which at first sight might not seem able to bond with metal ions, is examined.

The complex [CoL6(Cl)][CoCl4]·H2O, where L6 is the reinforced macrocycle 7-amino-7-methyl-1,5,9,13-tetraazabicyclo[11.2.2]heptadecane, has been prepared and its crystal structure determined. The green-black crystals are monoclinic, space group P21/c, with a= 8.778(2), b= 13.344(4), c= 19.380(8)A, β= 98.48(2)°, Z= 4, and a final conventional R= 0.0736. The cobalt is co-ordinated in a planar fashion in the cavity of the macrocycle, with one axial co-ordination site occupied by the pendant primary amine group, and the other by a chloride. The Co–N bond lengths to the tertiary nitrogens of the piperazine bridge of the macrocycle are 2.01(1) and 2.02(1)A, distinctly longer than those [1.97(1)A] to the secondary amines in the cavity of the macrocycle. The Co–N distance to the pendant primary amine is 2.01(1)A, and Co–Cl to the axially co-ordinated chloride trans to the pendant amine group is 2.26(1)A. Molecular mechanics calculations are used to analyse the highly distorted co-ordination sphere around the CoIII, where the N–Co–N angle involving the nitrogens of the piperazine bridge of the macrocycle is only 73.5(4)°. The calculations reproduce the co-ordination geometry around the CoIII well, including the variation in Co–N bond length and the highly distorted N–Co–N angles.

A molecular mechanics (MM) analysis is carried out on complexes of crown ethers CH2(OCH2CH2)nCH2O, 12-crown-4 (n=3), 15-crown-5 (n=4), 18-crown-6 (n=5), 24-crown-8 (n=7), and 30-crown-11 (n=9) to determine the nature of the selectivity shown by these ligands for metal ions on the basis of metal ion size. The MM program used is SYBYL, and M-O bonds are represented using a ‘covalent’ model, i.e. the M-O bonds are modelled with ideal M-O bond lengths and force constants. The previously used technique of calculating strain energy as a function of M-O bond length is used for all the complexes, and also the complexes of the non-macrocyclic polyethylene glycol analogues. It is concluded that the crown ethers fall into three groups with regard to selectivity for metal ions. Group one consists of the smaller macrocycles such as 12-crown-4 and 15-crown-5, where metal ions generally are too large to enter the cavity of the macrocycle, and the metal ion is coordinated lying outside the plane of the donor atoms of the ligand. Here factors that control selectivity are the same as in non-macrocyclic ligands, chiefly the size of the chelate ring. Group 2 contains only 18-crown-6 of the ligands studied here. 18-Crown-6 complexes have three important conformers, one of which, theD 3d , shows sharp size match selectivity, preferring metal ions with M-O bond lengths of about 2.9 A. The other two conformers are adopted by metal ions too small for theD 3d conformer, and are more flexible, exerting little size-match selectivity. These other two conformers are of higher energy than theD 3d conformer for metal ions with M-O bond lengths greater than 2.55 A. Thus, a genuine ‘size match selectivity’ is found for K+ with 18-crown-6. With an ideal M-O bond length of 2.88 A, K+ fits the cavity of theD 3d conformer of 18-crown-6 very closely. The third group consists of very large macrocycles such as 24-crown-8 and 30-crown-10. These enfold the metal ion in extremely folded conformations, but may, as does 30-crown-10, exert considerable selectivity for metal ions on the basis of their size by virtue of the conformation resulting in a set of torsional angles in the ring atoms of the macrocycle which confer considerable rigidity on the ligand.

The idea (Hancock, 1992) that the dominant architectural feature in controlling metal ion selectivity in both open-chain and macro-cyclic ligands is the size of the chelate ring is pursued further. It is shown that when more than one or two six-membered chelate rings are present in 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 A and N-M-N angle of 109.5° become particularly severe, and can only be met by a small tetrahedral metal ion. Thus, the ligand 16-aneN4 (1,5,9,13-tetraazacyclohexadecane) forms complexes of low stability with all metal ions studied to date, but a conformer of 16-aneN4 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 observation (Hay, 1993) that the geometry around a...

The coordinating properties of open-chain ligands containing alcoholic or ethereal oxygen donors are examined. Addition of oxygen donors usually leads to complex stabilisation for large metal ions (Pb2+, Cd2+) and to less favourable effects on complex stability for small metal ions (Cu2+, Ni2+). The formation constants of these metal ions with the set of ligands RN(CH2CHOH·CH3)2 where R is H, CH2CHOH·CH3, CH2CH2OCH3, CH2CH2OCH2CH2OH, and CH2CHOCH2CH2CH2 are reported. The largest stabilisation for each case where R is an O-donor group relative to R = H occurs for Pb2+, the largest metal ion, while Cu2+, the smallest metal ion, shows the smallest stabilisation. The crystal structure of [Ni(HOCH2CH2NHCH2CH2NH2)2] (NO3)2 is reported. The space group is P 1 , with cell constants a = 13.098(3), b = 8.737(4), and c = 7.746(3) A, β = 112.66(3), β = 90.65(3), and γ = 85.03(2), and Z = 2. Disorder of the nitrate anions hindered refinement, with the result that a final conventional R factor of 0.0903 was achieved. The NiN bond lengths average 2.06(1) (secondary nitrogen) and 2.10(2) (primary nitrogen). The NiO bond lengths are rather long, averaging 2.15(1) A, which is used to support the idea that the steric effects are responsible for destabilising the complexes of small metal ions such as Ni(11) when neutral oxygen donors are present.

The formation constant for the Ni(II) complex of cyclam (1,4,8,11-tetraazacyclododecane) was measured by an out-of-cell technique to be 20.1 ± 0.5 in 0.5 M NaCl at 25 °C. This is somewhat less than the value of 22.2 reported by D. Hinz and D. W. Margerum, Inorg. Chem., 13 (1974) 2941. The new log K 1 for Ni(II) with cyclam is more reasonable in relation to the log K 1 values observed for other tetraazamacrocycle complexes of Ni(II), and also other metal ions. Possible reasons for the difference between the value for log K 1 reported here and Margerum's value are discussed.

The chelate, macrocyclic, and cryptate effects are analyzed. It is concluded that for all three effects considerable stabilization is derived from the greater basicity induced in donor atoms as ethylene bridges are added. Further considerations of importance in these effects are (1) desolvation effects, where steric constraints to solvation of the donor atoms in the free ligand lead to increased complex stability, (2) enforced dipole-dipole repulsion in the ligand, which is relieved on complex formation, and (3) structural preorganization of the ligand such that the donor atoms in the free ligand are already correctly oriented for complex formation. Only for the chelate effect is entropy of paramount importance, where it is derived from a cratic effect. It is emphasized that the level of preorganization of macrocycles, and to a lesser extent cryptands, is much lower than commonly realized. Newly emerging types of more highly preorganized ligands are discussed.