Michael B. Andrews

Synthesis, Structures, and Luminescent Properties of Uranyl Terpyridine Aromatic Carboxylate Coordination Polymers

Six novel uranyl terpyridine aromatic carboxylate coordination polymers, [UO(2)(C(6)H(2)O(4)S)(C(15)H(11)N(3))] (1), [UO(2)(C(6)H(2)O(4)S)(C(15)H(10)N(3)Cl)]·H(2)O (2), [UO(2)(C(8)H(4)O(4))(C(15)H(11)N(3))] (3), [UO(2)(C(8)H(4)O(4))(C(15)H(10)N(3)Cl)] (4), [UO(2)(C(12)H(6)O(4))(C(15)H(11)N(3))] (5), and [UO(2)(C(12)H(6)O(4))(C(15)H(10)N(3)Cl)] (6), were synthesized under solvothermal conditions and characterized by single-crystal and powder X-ray diffraction and luminescence and UV-vis spectroscopy. Compounds 1, 2, and 5 crystallize as molecular uranyl dimers, whereas compounds 3, 4, and 6 contain ladder motifs of uranyl centers. Fluorescence spectra of 1-4 show characteristic UO(2)(2+) emission, wherein bathochromic and hypsochromic shifts are noted as a function of organic species. In contrast, uranyl emission from 5 and 6 is quenched by the naphthalene dicarboxylic acid linker molecules.

Investigation of in Situ Oxalate Formation from 2,3-Pyrazinedicarboxylate under Hydrothermal Conditions Using Nuclear Magnetic Resonance Spectroscopy

We have investigated the assembly of a two-dimensional coordination polymer, Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2), that has been prepared from the hydrothermal reaction of Nd(NO(3))(3)·6H(2)O and 2,3-pyrazinedicarboxylic acid (H(2)pzdc). In situ oxalate formation as observed in this system has been been investigated using (1)H and (13)C nuclear magnetic resonance spectroscopy, and a pathway for C(2)O(4)(2-) anion formation under hydrothermal conditions has been elucidated. The oxalate ligands found in Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2) result from the oxidation of H(2)pzdc, which proceeds through intermediates, such as 2-pyrazinecarboxylic acid (2-pzca), 2-hydroxyacetamide, 3-amino-2-hydroxy-3-oxopropanoic acid, 2-hydroxymalonic acid, 2-oxoacetic acid (glyoxylic acid), and glycolic acid. The species are generated through a ring-opening that occurs via cleavage of the C-N bond of the pyrazine ring, followed by hydrolysis/oxidation of the resulting species.

Uranyl Hybrid Material Derived from In Situ Ligand Synthesis: Formation, Structure, and an Unusual Phase Transformation†

Interest in the synthesis of uranyl (UO2 ) organic hybrid materials has resulted in a family of compounds with a broad range of structural topologies which have been studied for their interesting luminescent properties as well as potential applications in the fields of photocatalysis and photoelectric conversion. In an effort to expand the structural chemistry of uranyl hybrid materials we have explored the use of in situ ligand synthesis (ISLS). In these systems the ligands are formed during the hydroor solvothermal synthesis of the hybrid material. While ISLS was initially observed as a serendipitous phenomenon, recent efforts to harness these reactions have emerged. As such, many well-known organic reactions have been explored with the intention of promoting an organic reaction in the presence of inorganic building units. A common theme of ISLS is the conversion of poorly coordinating compounds into ligands with greater affinities for the particular metal being used. For instance, carboxylic acids have been formed by a variety of reactions. An organic reaction that appears to be underutilized is the oxidation of alkyl halides to form carboxylic acids. In an effort to assess the potential applications of this reaction in the development of metal–organic hybrid materials, we have studied the behavior of 4-(bromomethyl)benzoic acid (BMBA) with the uranyl cation under hydrothermal conditions. The oxidation of the alkyl halide to a carboxylic acid was found to have a strong pH dependence (Scheme 1), and

Metal–organic hybrids involving the [UO2Cl3(NO3)]2− tecton and the role of halogen polarizability

The reaction of uranyl nitrate hexahydrate with 4-halopyridinium ions (X = Cl, Br, I) in high chloride media has resulted in three novel compounds, each of which is based around the [UO2Cl3(NO3)]2− tecton; (C5H5NCl)2[UO2Cl3(NO3)], (C5H5NBr)2[UO2Cl3(NO3)] and (C5H5NI)3[UO2Cl3(NO3)](NO3). In each material the halogen is involved in various forms of intermolecular interaction, including hydrogen bonding, halogen–halogen interactions and halogen–nitrate interactions. Presented is a discussion of the role of the halogen, in terms of Lewis acidity, in directing the overall structure.

In situ oxalate formation during hydrothermal synthesis of uranyl hybrid materials

The hydrothermal reaction of the uranyl cation with pyridine-, pyrazine- and pyrimidine-based organic compounds has resulted in four new and four known uranyl-organic hybrid materials that have been characterized by single-crystal and powder X-ray diffraction. Three of these materials contain the oxalate anion, formed as a result of in situ reactivity of the organic precursors. Trends in ligand reactivity have been observed and various factors that may influence oxalate formation have been explored, including time, temperature, pH and identity of the uranyl counterion. A mechanism for oxalate formation has been proposed and various methods of inhibiting in situ reactivity have been suggested.

Supramolecular Interactions in PuO2Cl42– and PuCl62– Complexes with Protonated Pyridines: Synthesis, Crystal Structures, and Raman Spectroscopy

The synthesis, crystal structures, and Raman spectra of seven plutonium chloride compounds are presented. The materials are based upon Pu(VI)O2Cl4(2-) and Pu(IV)Cl6(2-) anions that are charge balanced by protonated pyridinium cations. The single crystal X-ray structures show a variety of donor-acceptor interactions between the plutonium perhalo anions and the cationic pyridine groups. Complementary Raman spectra show that these interactions can be probed through the symmetric vibrational mode of the plutonyl moiety. Unlike previously reported studies in similar uranyl(VI) systems, the facile redox chemistry of plutonium in aqueous solution has demonstrated the feasibility of using not only the An(VI)O2Cl4(2-) anion with approximate D4h symmetry but also the approximately Oh An(IV)Cl6(2-) anion in order to manipulate both the structure and dimensionality of such hybrid materials.

Responsive, di-metallic lanthanide complexes of a piperazine-bridged bis-macrocyclic ligand: modulation of visible luminescence and proton relaxivity

The synthesis of a new functionalised bis-macrocyclic ligand (L1) is described together with the corresponding Ln(III) complexes, Ln(2)- (Ln = Gd(III), Eu(III)). Phosphorescence measurements on Gd(2)- at 77 K allowed the ligand-centred triplet state ((3)pi-pi*) to be estimated at ca. 28500 cm(-1). Steady state and time-resolved measurements confirmed emission from the f-centred excited state ((5)D(0)) for Eu(2)-. (1)H NMRD profiles revealed the longitudinal proton relaxivity (r(1)) of Gd(2)- to be 8.3 mM(-1)s(-1)(30 MHz, 25 degrees C). The interaction of Cu(II) and Hg(II) with the lanthanide complexes was probed using luminescence and relaxivity measurements. Addition of Cu(II) (10 eq.) resulted in quenching of the Eu(III) emission, but no increase in r(1) of the Gd(III) dimer. Addition of Hg(II) (10 eq.) caused changes to the hypersensitive emission bands of Eu(III) together with an increase in r(1) of Gd(2)- to be 10.3 mM(-1)s(-1)(30 MHz, 25 degrees C) suggesting a net increase in hydration at the Gd(III) centres.

Coumarin-based luminescent ligand that forms helicates with dicationic metal ions.

The potentially hexadentate ligand L, which contains two terminal coumarin fluorophores, forms dinuclear double-stranded helicates with dicationic metal ions, giving species of the generic form [M(2)L(2)](4+). In solution the free ligand was fluorescent with emission attributed to the coumarin fluorophores (lambda(em) = 437 nm). The luminescent properties of the corresponding dimetallic helicates complexes were examined and revealed that the Zn(2+) complex demonstrates enhanced emission when compared to the parent ligand, whereas Co(2+), Cu(2+), Cd(2+) and Hg(2+) induce varying degrees of fluorescence quenching. In particular, comparative luminescence measurements at 77 K and room temperature showed that the quenching mechanism for [Cu(2)L(2)](4+) can be attributed to a photoinduced electron transfer. ESI-MS selectivity studies showed that in the presence of a mixture of metal dications no preference for any one metal ion was observed.