Coordination and metalation bifunctionality of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin: toward a mixed-valence two-dimensional coordination network

We investigated the coordination self-assembly and metalation reaction of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin (2HTPyP) on a Au(111) surface by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. 2HTPyP was found to interact with Cu through both the peripheral pyridyl groups and the porphyrin core. Pairs of pyridyl groups from neighboring molecules coordinate Cu(0) atoms, which leads to the formation of a supramolecular metal-organic coordination network. The network formation occurs at room temperature; annealing at 450 K enhances the process. The interaction of Cu with the porphyrin core is more complex. At room temperature, formation of an initial complex Cu(0)-2HTPyP is observed. Annealing at 450 K activates an intramolecular redox reaction, by which the coordinated Cu(0) is oxidized to Cu(II) and the complex Cu(II)TPyP is formed. The coordination network consists then of Cu(II) complexes linked by Cu(0) atoms; that is, it represents a mixed-valence two-dimensional coordination network consisting of an ordered array of Cu(II) and Cu(0) centers. Above 520 K, the network degrades and the Cu atoms in the linking positions diffuse into the substrate, while the Cu(II)TPyP complexes form a close-packed structure that is stabilized by weak intermolecular interactions. Density functional theory investigations show that the reaction with Cu(0) proceeds via formation of an initial complex between metal atom and porphyrin followed by formation of Cu(II) porphyrin within the course of the reaction. The activation barrier of the rate limiting step was found to be 24-37 kcal mol(-1) depending on the method used. In addition, linear coordination of a Cu atom by two CuTPyP molecules is favorable according to gas-phase calculations.     

Direct synthesis of a metalloporphyrin complex on a surface

We demonstrate that well-defined monolayers of a metal complex on a surface can be prepared by direct vapor deposition of the metal atoms on monolayers of the ligand. In particular, ordered monolayers of adsorbed tetraphenylporphyrin (2H-TPP) on a silver surface were exposed to cobalt vapors, resulting in the complexation of the metal by the porphyrin. The formation of the metal complexes was monitored by means of X-ray photoelectron spectroscopy (XPS), which reveals that this metalation reaction leads to a chemical equivalence of all four nitrogen atoms. The described in situ metalation provides a convenient way to produce adsorbed monolayers of more reactive (e.g., air- or solvent-sensitive) or thermally unstable metalloporphyrins that are difficult to evaporate or even to obtain as pure compounds at room temperature.     

A planar water tetramer with tetrahedrally coordinated water embedded in a hydrogen bonding network of [Tc4(CO)12-(mu3-OH)4.4H2O

A virtually planar water tetramer in which the water molecules are virtually tetrahedrally coordinated could be realized in the solid in a three-dimensional network of [Tc4(CO)12-(mu3-OH)4.4H2O]. The network could be produced by cocrystallization of the new cubane-like cluster [Tc(CO)3-(mu3-OH)]4 and water as a complementary component. The amphiphilic behavior of cluster and water results in a highly ordered three-dimensional network. The complementary components, the water tetramer and the cubic cluster, independently of one another form two interpenetrating tetragonal lattice networks held together exclusively by hydrogen bonds.     

Lanthanide labeling offers fast NMR approach to 3D structure determinations of protein-protein complexes

A novel nuclear magnetic resonance (NMR) strategy based on labeling with lanthanides achieves rapid determinations of accurate three-dimensional (3D) structures of protein-protein complexes. The method employs pseudocontact shifts (PCS) induced by a site-specifically bound lanthanide ion to anchor the coordinate system of the magnetic susceptibility tensor in the molecular frames of the two molecules. Simple superposition of the tensors detected in the two protein molecules brings them together in a 3D model of the protein-protein complex. The method is demonstrated with the 30 kDa complex between two subunits of Escherichia coli polymerase III, comprising the N-terminal domain of the exonuclease subunit epsilon and the subunit theta. The 3D structures of the individual molecules were docked based on a limited number of PCS observed in 2D 15N-heteronuclear single quantum coherence spectra. Degeneracies in the mutual orientation of the protein structures were resolved by the use of two different lanthanide ions, Dy3+ and Er3+.     

Crystal structure and theoretical investigation of the configurations of 1,3-dichloro-1,3-diazetidine-2,4-dione and 1,3-bis(trimethylsilyl)-1,3-diazetidine-2,4-dione

Crystal structures of 1,3-dichloro-1,3-diazetidine-2,4-dione ( 1 ) and the hitherto unknown compound 1,3-bis(trimethylsilyl)-1,3-diazetidine-2,4-dione ( 2 ) have been determined by X--ray crystallography: 1 : (CINCO)₂, M_r = 154.94, T = 295 K, orthorhombic, Pbca, a = 7.699(1), b = 6.706(1), c = 10.598(2) Å, V = 547.2(2) ų, Z = 4, d_x = 1.881 g/cm³, μ = 10.9 cm⁻¹, R = 3.14%, R_w = 2.82% (660 observations, 38 parameters). 2 : [(CH₃)₃SiNCO]₂, M_r = 230.41, T = 100 K, monoclinic, I2/a, a = 20.257(2), b = 6.416(1), c = 21.260(3) Å, β = 101.29(1)°, V = 2709.7(6) ų, Z = 8, d_x = 1.130 g/cm³, μ = 2.4 cm⁻¹, R = 4.86%, R_w = 4.39% (2375 observations, 151 parameters). In both compounds, the symmetry of the (XNCO)₂ framework (X = Cl, Si) was determined to be nearly C_2h with trans configuration of the exocyclic X atoms. Extreme values were observed for the angles between the ring plane and the exocyclic N–X bonds: 32.5(1)° in 1 and 2.5(2)° and 0.8(2)° in 2 , respectively. Quantum chemical procedures at various levels of theory (ab initio SCF and semi-empirical PM3) applied to 1 revealed the possible appearance of two isomers, a lower energy trans form and a higher energy cis form (approx. 2.4 kcal/mol above trans) differing mainly in the spatial arrangement of the chlorine atoms. The calculations excluded a planar heavy-atom configuration by missing a local energy minimum.