Complexation of Metal Ions of Higher Charge by the Highly Preorganized Tetradentate Ligand 2,9-Bis(hydroxymethyl)-1,10-Phenanthroline. A Crystallographic and Thermodynamic Study

The metal ion selectivity for M(III) (M = metal) ions exhibited by the highly preorganized ligand PDALC is investigated (PDALC = 2,9-bis(hydroxymethyl)-1,10-phenanthroline). The structures are reported of [Bi(PDALC)(H2O)(2)(ClO4)(3)] H2O (1), monoclinic, P2(1)/c, a = 12.8140(17), b = 19.242(3), c = 9.2917(12) angstrom, beta = 91.763(2)degrees V = 2289.9(5) angstrom(3), Z = 4, R = 0.0428; [Th(PQALC)(NO3)(4)]center dot 3H(2)O (2), monoclinic, P2(1)/n, a = 7.876(3), b = 22.827(9), c = 12.324(5) angstrom, beta = 94.651(6)degrees, V = 2208.4(15) angstrom(3), Z = 4, R = 0.0669; [Cd(PDALC)(2)](ClO4)(2) (3),), triclinic, P (1) over bar, a = 7.5871(16), b = 13.884(3), c = 14.618(3) angstrom, alpha = 74.081(2)degrees, beta = 88.422(2)degrees, gamma = 78.454(2)degrees, V = 1450.2(5) angstrom(3), Z = 2, R = 0.0267. The Bi in 1 is best regarded as 9-coordinate, with four short bonds to the PDALC, and two short bonds to the coordinated water molecules, with three long bonds to perchlorate oxygens. The Bi-N bonds at 2.35 angstrom are by a considerable margin the shortest Bi-N bonds to 1,10-phenanthroline (phen) type ligands, which is suggested to be due to the Bi adapting to the metal ion size requirements of PDALC. The Th(IV) in 2 is 12-coordinate, with four bonds to PDALC, and the four chelated nitrates, with close to normal bond lengths to the PDALC ligand. The Cd(II) in 3 is 8-coordinate, with Cd-N and Cd-O bonds that are similar to those found in other 8-coordinate Cd(II) complexes. The five known structures of PDALC complexes, including the three reported here, suggest that the M-N bonds to PDALC are quite easily varied in length in response to differing metal ion sizes, but that the M-O bonds are more constrained by the rigid ligand to be close to the ideal value of 2.50 angstrom. The formation constants (log K-1) for M(III) ions with PDALC show that for small metal ions such as Ga(III) and Fe(III), log K-1 is only slightly higher than for phen, suggesting that these metal ions are too small to coordinate to the alcoholic oxygen donors of PDALC. For larger metal ions such as Bi(III), Gd(III), Th(IV), and UO22+, log K-1 for PDALC is higher than log K-1 for phen by more than 5 log units, which stabilization is attributed to the fact that PDALC is preorganized for complexation with large metal ions with an ionic radius of about 1.0 angstrom. The fluorescence of M(III) complexes of PDALC is discussed. PDALC free ligand gives fluorescence typical of phen ligands, with the protonated form giving a broad less intense band, and the non-protonated form of the ligand giving an intense structured set of bands. Large lanthanide ions without partially filled f-subshells, such as La(III), Lu(III), and also Y(III), give a fairly strong CHEF (chelation-enhanced fluorescence) effect, while those with partially filled f-subshells, such as Gd(III), Yb(III), and Tb(III), strongly quench the fluorescence of PDALC. A heavy element such as Bi(III) has strong spin-orbit coupling effects that act to quench the fluorescence of PDALC almost completely, which effect is enhanced by the covalence of the Bi-N bonds.