The idea that M···C π contacts between diamagnetic heavy metal ions such as Pb(II), Ag(I), Pd(II), or Hg(II) and the anthracenyl fluorophore of adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) are responsible for quenching the fluorescence of the complexes of these metal ions with adpa is explored crystallographically. The structures of [Pb(adpa)(NO3)2] (1), [Ag(adpa)NO3] (2), [Pd(adpa)NO3]NO3 (3), [Zn(adpa)(NO3)2] (4), and [Cd(adpa)Br2] (5) are reported. The π contacts with the fluorophore are for 1 are a Pb···C π contact of 3.178 Å; for 2, an Ag···C π contact of 3.016 Å; and for 3, a Pd···C π contact of 2.954 Å on the axial site of the Pd(II) ion. The Zn(II) ion in 4 has no Zn···C π contact, with the anthracenyl fluorophore rotated completely away from the Zn(II) ion. These structures confirm that in the Pb(II), Ag(I), and Pd(II) complexes of adpa, which experience strong quenching of fluorescence, there are strong M···C π contacts, as expected if it is the π contacts that quench fluorescence. In contrast, for the Zn(II) adpa complex, which forms no π contact, there is a strong increase in fluorescence intensity. The structure of 5 shows a long Cd···C π contact at 3.369 Å, in contrast to a previously reported structure with two coordinated nitrates where the Cd···C π contact is 3.097 Å. The long Cd···C π contact in [Cd(adpa)Br2] suggests how coordination of Br(-), as well as other more covalently bound ligands such as Cl(-), SCN(-), and S2O3(2-), cause an increase in fluorescence intensity, reported for the Cd(II)adpa complex in 50% CH3OH/H2O. Coordination of covalently bound ligands to the Cd(II) weakens the Cd···C π contact and so enhances fluorescence, whereas more ionically bound ligands such as SO4(2-), NO3(-), or H2O produce a strong Cd···C π contact and weakened fluorescence. Complexes of the Cd(II)/adpa type may form the basis for a new type of anion/small molecule sensor. The tendency of metal ions to form π contacts with aromatic groups is analyzed in terms of the frequency of occurrence of π contacted structures in the literature, as well as by DFT calculations on the adpa complexes.

Poly(acrylamidoxime) adsorbents are often invoked in discussions of mining uranium from seawater. While the amidoxime-uranyl chelation mode has been established, a number of essential binding constants remain unclear. This is largely due to the wide range of conflicting pK(a) values that have been reported for the amidoxime functional group. To resolve this existing controversy we investigated the pK(a) values of the amidoxime functional group using a combination of experimental and computational methods. Experimentally, we used spectroscopic titrations to measure the pK(a) values of representative amidoximes, acetamidoxime, and benzamidoxime. Computationally, we report on the performance of several protocols for predicting the pK(a) values of aqueous oxoacids. Calculations carried out at the MP2 or M06-2X levels of theory combined with solvent effects calculated using the SMD model provide the best overall performance, with a root-mean-square deviation of 0.46 pK(a) units and 0.45 pK(a) units, respectively. Finally, we employ our two best methods to predict the pK(a) values of promising, uncharacterized amidoxime ligands, which provides a convenient means for screening suitable amidoxime monomers for future generations of poly(acrylamidoxime) adsorbents.

The Cd(II) complex of adpa (N-(9-anthracenylmethyl)-N,N-di-(picolyl)amine) in MeOH-H2O has increased fluorescence intensity with [Cl(-)], a new type of anion sensor. The structure of [Cd(adpa)(NO3)2] has a proposed fluorescence-quenching π-contact between Cd and the fluorophore, while the Cl in [Cd(adpa)Cl2] disrupts the π-contact, restoring fluorescence.

The formation constants of the UO 2 2+ cation with the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) are reported. These are of interest in light of their proposed use as the functional groups of extractants for uranium in seawater. The formation constants of bzam with UO 2 2+ were measured by monitoring the absorbance of the π→π* transitions in the UV spectrum of the bzam ligand in the presence of 1:1 UO 2 2+ as a function of pH. This yielded log K 1 = 12.4 for UO 2 2+ with bzam, and log K = 6.9 for the equilibrium UO 2 (bzam) + + OH-= UO 2 (bzam)OH at 25 o C and ionic strength zero. The bzam complexes were also studied monitoring the fluorescence of the UO 2 2+ system. Analysis of the intense fluorescence that occurs in 5 x 10-6 M UO 2 2+ solutions between pH 5 and 9 suggested that this was due to the [(UO 2) 3 O(OH) 3 ] + trimer. Monomeric species such as UO 2 2+ and [UO 2 (OH) 4 ] 2-, and dimers such as [(UO 2)(OH) 2 ] 2+ , fluoresce only weakly. Titration of such solutions with bzam supported the above log K values measured by absorbance, and with higher bzam concentrations yielded log  2 = 22.3. The acetam ligand does not have any absorbance, so that complexformation was monitored by fluorescence only. Formation constants measured by fluorescence may differ from those measured by other techniques such as absorbance. The agreement obtained between log K values measured by absorbance and fluorescence for the bzam complex of UO 2 2+ supported the log K values measured for the acetam complexes by florescence alone were reliable: log K 1 = 13.6, log  2 = 23.7, and log K UO 2 (acetam) + + OH-= UO 2 (acetam)OH = 6.8. The high log K values found for the bzam and acetam complexes of UO 2 2+ were analyzed using DFT calculations. These log K values are related to the ability of polymer-based extractants bearing bzam or acetam type functional groups to extract UO 2 2+ at a concentration of 1.3 x 10-8 M and in the competing 0.0025 M CO 3 2present in the oceans.

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