Photoinduced Electron Transfer in Pentaammineruthenium(II) Complexes of 1-(4-Cyanophenyl)imidazole

A new bridging ligand, 1-(4-cyanophenyl)imidazole (CPI) has been prepared, as well as its N-methylated derivative 1-methyl-3-(4-cyanophenyl)imidazolium iodide (CPI-Me(+)I(-)). The mononuclear and binuclear complexes [(NH(3))(5)Ru-CPI-Me](3+) and [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](4+) have been obtained. Free CPI is planar, according to theoretical calculations (MMX and MNDO), and its luminescence properties suggest the occurence of a twisted internal charge transfer (TICT) state. The comparison of the two ruthenium complexes reveals the spectral and electrochemical features of coordination by the cyanophenyl or by the imidazole groups. Controlled oxidation of the binuclear complex [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](4+) yields the mixed valence species [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](5+) in which the ruthenium coordinated to the cyanophenyl group is ruthenium(II) while the ruthenium linked to imidazole is ruthenium(III). An intervalence band is observed at 640 nm (epsilon = 188), from which the effective metal-metal coupling through the bridging ligand is determined as 0.032 eV. This value is satisfactorily reproduced by a theoretical calculation using the effective Hamiltonian theory. Finally the binuclear complex exhibits a weak luminescence when excited either on the ligand band near 260 nm or on the metal-to-ligand charge transfer band near 410 nm. The CPI ligand is the first example of a TICT-forming species with appreciable coupling between metallic sites and can be considered as a first step toward a molecular switch.     

A General QSAR Model for Predicting the Toxicity of Organic Chemicals to Luminescent Bacteria (Microtox® test)

Abstract A large data set of Microtox? toxicity results was used to derive a general QSAR model. Chemicals were described by means of a modified autocorrelation method. The autocorrelation vectors were generated from atomic contributions encoding the hydrophobicity and molar refractivity of the molecules. A three-layer backpropagation neural network was used to design the model. The obtained results were compared with those obtained from a principal components regression analysis.     

Homo- and heterobimetallic niobium(v) and tantalum(v) peroxo-tartrate complexes and their use as molecular precursors for Nb-Ta mixed oxides

New water-soluble bimetallic peroxo-tartrato complexes of niobium(V) and/or tantalum(V) have been prepared, characterized from the structural and spectroscopic point of view, and used as molecular precursors for Nb-Ta mixed oxides. Two new homometallic complexes, (gu)5[Nb2(O2)4(tart)(Htart)] x 4H2O (1a) and (gu)6[Ta2(O2)4(tart)2] x 4H2O (2a), and the corresponding heterometallic complex, (gu)5[NbTa(O2)4(tart)(Htart)] x 4H2O (3), have been obtained. The crystal structures of the homometallic compounds, (gu)5[Nb2(O2)4(tart)(Htart)] x 6H2O x 1H2O2 (1b) and (gu)6[Ta2(O2)4(tart)2] x 6H2O (2b), have been determined, showing, for both cases, two 8-fold-coordinated metal atoms, each surrounded by oxygen atoms belonging to two bidentate peroxides, two monodentate carboxylato, and two alkoxo groups from both bridging tartrato ligands. The coordination polyhedron around each metal atom is a dodecahedron. The thermal treatment of complexes 1a, 2a, and 3 in air at 700 or 800 degrees C, depending of the Ta content, provided Nb2O5, Ta2O5, and the solid solution TaNbO5, respectively. The thermal treatment of a 1:1 Nb/Ta molar ratio mixture of 1a and 2a has also been studied. BET and SEM measurements have been carried out and reveal these oxides possess relatively high specific surface areas and display a porous character. Comparison between the use of homo- and heterometallic precursors is discussed.     

Spectroscopic and structural characterizations of novel water-soluble tetraperoxo and diperoxo[polyaminocarboxylato bis(N-oxido)]tantalate(V) complexes

New water-soluble homoleptic peroxo complexes and heteroleptic peroxo-polyaminocarboxylato (PAC) complexes of tantalum(V) have been prepared. In the case of the peroxo-PAC complexes, the synthesis in the presence of excess H2O2 leads to the oxidation of the nitrogen atoms of the ligand into N-oxides. The compounds correspond to the general formula (gu)3[Ta(O2)2(LO2)] x xH2O (gu = guanidinium, L = edta or pdta) in which H4LO2 refers to the bis(N-oxide) derivative of the PAC ligand. The TaV complexes have been characterized on the basis of elemental and thermal analysis and by IR and 13C and 15N NMR spectroscopy. These last two spectroscopic methods have been used to suggest the coordination mode of the PAC ligand in the complexes. ESI mass spectrometry measurements have also been carried out for the peroxo-PAC compounds. The crystal structures of the homoleptic tetraperoxotantalate, (gu)3[Ta(O2)4] (1), and the heteroleptic complex, (gu)3[Ta(O2)2(edtaO2)] x 2.32H2O x 0.68H2O2 (2b), have been determined, showing, for both cases, an 8-fold-coordinated Ta atom surrounded either by four bidentate peroxides or by two peroxides and one tetradentate edtaO2 ligand.     

The Number of Cylindrical Shells

Given a set P of n points in three dimensions, a cylindrical shell (or zone cylinder) is formed by two circular cylinders with the same axis such that all points of P are between the two cylinders. We prove that the number of cylindrical shells enclosing P passing through combinatorially different subsets of P has size (n ³) and O(n ⁴) (the previously known bound was O(n ⁵)). As a consequence, the minimum enclosing shell can be found in O(n ⁴) time.