The hydrothermal synthesis and structures of [UO2(PDA)] (1) and [Th(PDA)2(H2O)2].H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) A, b = 6.6926(4) A, c = 17.3114(12) A, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P1, a = 7.6190(15) A, b = 10.423(2) A, c = 17.367(4) A, alpha = 94.93(3) degrees , beta = 97.57(3) degrees , gamma = 109.26(3) degrees , V = 1278.3(4) A (3), Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U-N = 2.558 and U-O = 2.351 A) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U-O and U-N bond lengths, and O-U-N and N-U-N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th-N = 2.694 and Th-O = 2.430 A) as well as two water molecules (Th-O = 2.473 and 2.532 A) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th(4+) in 0.1 M NaClO4 at 25 degrees C indicate that log K1 for the Th(4+)/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 A such as Th(IV) would have remarkably high log K1 values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K1 for Th(IV) with PDA.

The selectivity of the rigid ligand PDA (1,10-phenanthroline-2,9-dicarboxylic acid) for some M(III) (M = metal) ions is presented. The structure of [Fe(PDA(H)(1/2))(H(2)O)(3)] (ClO(4))(2).3H(2)O.(1)/(2)H(5)O(2) (1) is reported: triclinic, P1, a = 7.9022(16) A, b = 12.389(3) A, c = 13.031(3) A, alpha = 82.55(3) degrees , beta = 88.41(3) degrees , gamma = 78.27(3) degrees , V = 1238.6(4) A(3), Z = 2, R = 0.0489. The coordination geometry around the Fe(III) is close to a regular pentagonal bipyramid, with Fe-N lengths averaging 2.20 A, which is normal for a 1,10-phenanthroline type of ligand coordinated to seven-coordinate Fe(III). The Fe-O bonds to the carboxylate oxygens average 2.157 A, which is rather long compared to the average Fe-O length of 2.035 A to carboxylates in seven-coordinate Fe(III) complexes. The structure of 1 supports the idea that the Fe(III) is too small for ideal coordination in the cleft of PDA, and the structure shows that the Fe(III) adapts to this by inducing numerous small distortions in the structure of the PDA ligand. The log K(1) values for PDA at 25 degrees C in 0.1 M NaClO(4) were determined by UV spectroscopy with Al(III) (log K(1) = 6.9), Ga(III) (log K(1) = 9.7), In(III) (log K(1) = 19.7), Fe(III) (log K(1) = 20.0), and Bi(III) (log K(1) = 26.2). The low values of log K(1) for PDA with Al(III) and Ga(III) are because these ions are too small for the cleft in PDA, which requires a large metal ion with an ionic radius (r(+)) of 1.0 A. In(III) and Fe(III) (r(+) = 0.86 and 0.72 A for a coordination number (CN) of 7) are somewhat too small for the cleft in PDA but may adapt by increasing the coordination number, which increases the metal ion size, and have high log K(1) values. Very large log K(1) values are found, as expected, for Bi(III) (r(+) = 1.17 A, CN = 8), which fits the cleft quite well. Fluorescence studies show that Y(III) produces the largest CHEF (chelation enhanced fluorescence) effects, followed by La(III) and Lu(III), in the PDA complexes. Metal ions with nonfilled d or f subshells produce very large quenching of the fluorescence, as do heavy-metal ions such as In(III) and Bi(III), which have large spin-orbit coupling effects. The Al(III)/PDA complex produced an intense broad band at longer wavelength than the pi*-pi emissions of the PDA ligand, which is at a maximum at pH 6, and the possibility that this might reflect an exciplex, where one PDA ligand in the Al(III) complex pi-stacks with the excited state of a second PDA ligand, is discussed.

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Recent experimental evidence has pointed to the possible presence of a short, strong hydrogen bond in the enzyme-substrate transition states in some biochemical reactions. To date, most experimental measures of these short, strong hydrogen bonds have monitored their equilibrium properties. In this work we show that kinetic measurements can also be used to detect the presence of short, strong hydrogen bonds. In particular, we find nontrivial differences among rate constant ratios of protonated to deuterated hydrogen bonds between strong and weak hydrogen bonds for proton transfer between donor-acceptor sites. We quantify this kinetic isotope effect by performing dynamical calculations of these rate constants by computing reactive flux through a dividing surface. This reactive flux is computed by evolving trajectories on an effective quantum mechanical potential energy surface.