Novel binding of beryllium to dicarboxyimidazole-based model compounds and polymers

The ligand 4,5-dicarboxyimidazole (H(2)DCI) and its methyl derivative 1-methyl-4,5-dicarboxyimidazole (H(2)MDCI) have been shown to bind to Be(II) forming a zwitterionic species that has been structurally characterized. A new dicarboxyimidazole-based polymer has been prepared and its Be-binding properties have been studied using NMR ((1)H and (9)Be) and fluorescence spectroscopy; it represents a rare example of beryllium binding to a polymer. Models of the mononuclear and polymeric Be(II)-binding sites have been studied using density functional theory (DFT), and the (9)Be NMR chemical shifts of these model materials have been calculated for the purpose of direct comparison to experimentally observed values. Differences in the binding modes of the mononuclear and polymeric species are discussed.     

The design and synthesis of porphyrin/oligiothiophene hybrid monomers

In an effort to build effective photovoltaic cells based on porphyrin-functionalised polythiophenes we have focused on synthetic routes to three monomer types. By controlling the geometric structure of the monomer, oxidation of these materials should produce polymers with different architectural structures, and as a result, different opto-electronic properties. Employing Wittig protocols allowed access to monomers in which the porphyrin moiety is connected to the beta-position of the thiophene via an alkene linkage. In addition, monomers were constructed using porphyrin condensation methods to afford alpha-thiophene meso-substituted porphryins. Another set of monomers was also prepared via porphyrin condensation routes, but instead utilising beta-formylthiophenes. By utilising different formyloligothiophenes we were able to generate a series of monomers that can be used to control the loading of the porphyrin in the polythiophene matrix.     

Thermal stereomutations and stereochemically elucidated [1,3]-carbon sigmatropic shifts of 1-(E)-propenyl-2-methylcyclobutanes giving 3,4-dimethylcyclohexenes

The thermal stereomutations and [1,3] carbon sigmatropic shifts shown by (+)-(1S,2S)-trans-1-(E)-propenyl-2-methylcyclobutane and by (-)-(1S,2R)-cis-1-(E)-propenyl-2-methylcyclobutane in the gas phase at 275 degrees C leading to 3,4-dimethylcyclohexenes have been followed. The reaction-time-dependent data for concentrations and enantiomeric excess values for substrates and [1,3] shift products have been deconvoluted to afford rate constants for the discrete isomerization processes. Both trans and cis substrates react through four stereochemically distinct [1,3] carbon shift paths. For one enantiomer of the trans reactant the relative rate constants are k(si) = 58%, k(ar) = 5%, k(sr) = 33%, and k(ai) = 4%. For a single enantiomer of the cis reactant, k'(si) = 18%, k'(ar) = 11%, k'(sr) = 51%, and k'(ai) = 20%. A trans starting material reacts through orbital symmetry allowed suprafacial,inversion and antarafacial,retention paths to give trans-3,4-dimethylcyclohexenes 63% of the time. A cis isomer reacts to give the more stable trans-3,4-dimethylcyclohexenes through orbital symmetry-forbidden suprafacial,retention and antarafacial,inversionpaths 71% of the time. The [1,3] carbon sigmatropic shifts are not controlled by orbital symmetry constraints. They seem more plausible rationalized as proceeding through diradical intermediates having some conformational flexibility after formation and before encountering an exit channel. The distribution of stereochemical outcomes may well be conditioned by dynamic effects. The thermal stereomutations of the 1-(E)-propenyl-2-methylcyclobutanes take place primarily through one-center epimerizations. For the trans substrate, the relative importance of the three distinction rate constants are k(2) = 48%, k(1) = 34%, and k(12) = 18%. For the cis isomer, k'(2) = 44%, k'(1) = 32%, and k'(12) = 24%. These patterns are reminiscent of ones determined for stereomutations in 1,2-disubstitued cyclopropanes.     

Technetium(III), Technetium(II), and Technetium(I) Complexes with Pyridine Ligands. Can Pyridine Coordination Stabilize the Low Oxidation States of Technetium?

The substitution chemistry of TcCl(3)(PPh(3))(2)(CH(3)CN) is rather facile relative to the analogous rhenium complex, since both the chloride and phosphine ligands are easily substituted for various pyridine ligands. Consequently a series of Tc(III) complexes with amine, pyridine, and polypyridyl ligands were prepared and characterized by (1)H NMR and cyclic voltammetry. In addition, the zinc reduction of TcCl(4)(py)(2) in the presence of pyridine results in TcCl(2)(py)(4). Structural and spectroscopic data indicate that this Tc(II) complex exhibits strong metal-pyridine interactions characteristic of low-valent amine complexes of Re(II) and Os(II). For example, a decrease of 0.04 and 0.06 ? is observed for the trans-Tc-N bond length in TcCl(2)(py)(4 )relative to mer-TcCl(3)(pic)(3) and [TcCl(2)(py)(3)(PPh(3))](+), respectively. This ability of pyridine to function both as a strong sigma-donor and moderate pi-acid ligand has resulted in the isolation of technetium complexes in various oxidation states with similar ligand environments. As a result, a structural comparison of [TcCl(2)(py)(3)(PPh(3))](+), TcCl(2)(py)(4), TcCl(tpy)(py)(2), and other known Tc(III) and Tc(II) pyridine complexes is presented. Crystals of [TcCl(2)(py)(3)(PPh(3))]PF(6) are triclinic, with space group P&onemacr;, Z = 2, and lattice parameters a = 12.677(4) ?, b = 13.064(4) ?, c = 13.103(5) ?, alpha = 110.14(3) degrees, beta = 101.12(3) degrees, gamma = 96.61 degrees, V = 1959 ?(3), and R = 0.0615 (R(w) = 0.1148). Crystals of TcCl(2)(py)(4) are tetragonal, with space group I4(1)/acd, Z = 8, and lattice parameters a = 15.641(4) ?, c = 16.845(6) ?, V = 4121 ?(3), and R = 0.0373 (R(w) = 0.0290). Crystals of TcCl(tpy)(py)(2) are orthorhombic, with space group C222(1), Z = 4, and lattice parameters a = 9.359(3) ?, b = 16.088(6) ?, c = 18.367(4) ?, V = 2765 ?(3), and R = 0.0499 (R(w) = 0.0599).     

Metallation effects on the thermal interconversion of atropisomers of di(orthomethylarene)-substituted porphyrins

A new series of meso-substituted diaryl free-base and metalloporphyrins have been prepared. Each arene has been substituted with both a methyl group in the ortho position and a formyl group in the meta position. Rotation of the arene units is prevented at room temperature due to the steric restrictions imposed by the flanking methyl groups at the porphyrin beta-pyrrolic positions on the methyl groups at the ortho position on the meso-substituted arene unit. This allowed the alpha alpha and alpha beta atropisomers of this porphyrin to be separated and characterised. X-Ray crystallographic determination of the structure of the free-base porphyrin revealed a very flat porphyrin core. Metallation resulted in the isolation and characterisation of the nickel, zinc and copper derivatives. The assignments of the alpha alpha and alpha beta isomers are confirmed by X-ray crystallographic determination of the structures of the Cu(II) analogues. The copper alpha alpha structure exhibits a very twisted porphyrin core, the copper alpha beta structure is also distorted, but to a lesser degree. The activation energy for rotation has been calculated for each of the 2H, Ni and Zn derivatives. The energy required to rotate the arene ring increases in the order Ni < Zn approximately 2H. No significant difference in the free energy of rotation was observed between experiments carried out with the alpha alpha and small alpha beta isomers.