Host-guest complexes of cucubit[8]uril with phenanthrolines and some methyl derivatives

Cucurbituril a molecular container (or host) has a rigid hollow interior cavity which is large enough to accommodate, one or more, smaller molecules (or guests). The cavity is accessible through two carbonyl portal openings. Molecules or guests enter the …     

Kinetic studies of some substituted hexarhodium carbonyl clusters

Reactions of the halides X- (X- = chloride, bromide or iodide) with the substituted cluster Rh6(CO)15(PPh3) in oxygen-free chloroform lead to [Rh5(CO)14(PPh3)]-, Rh(CO)2(PPh3)2X and [Rh(CO)2X2]- in the molar ratios 2:1: approximately 13. Oxidation by the solvent is assumed to lead to most of the Rh(I) product, and the stoichiometry for reactions with I- can be defined as 4Rh6(CO)15(PPh3) + 27I- + 12CHCl3 --> 2[Rh5(CO)14(PPh3)]- + Rh(CO)2(PPh3)2I + 13[Rh(CO)2I2]- + 6C2H2Cl4 + 4CO + 12Cl-. This can be rationalized quite simply with the aid of a few generally justifiable assumptions. Rate constants for reactions with bromide increase to a limiting value with increasing [Br-] in a way that shows that breaking of one Rh-Rh bond, with an unusual closo to nido structural change, is rate determining. This opening of the cluster might be spontaneous or solvent induced. To complete the reaction, the bromide has to compete with the reverse nido to closo change. The same closo to nido change is also a major rate determining step for reactions with P(OPh)3 in oxygen-free solutions, and for reactions with bromide in oxygenated solutions in the presence of trifluoroacetic and some other acids. The limiting rates increase slightly with increasing basicity of the ligands P(p-XC6H4)3 along the series X = F3C, Cl, F, H and MeO. Activation parameters for these reactions are reported.     

Topotactic oxidation of TiGaPO-1, a pyridine-templated titanium gallophosphate with a new octahedral-tetrahedral 3-D framework structure containing Ti(III)/Ti(IV)

The first 3-D open-framework TiGaPO complex, constructed from Ti(III)O(6), Ti(IV)O(6), GaO(4), and PO(4) polyhedra, contains pyridinium cations in a 1-D pore network and can be oxidized in air at 543 K with retention of the original framework structure.     

Synthesis of some benzimidazole- and benzothiazole-derived ligand systems and their precursory diacids

Procedures are described for the preparation of various bidentate and linear tetradentate benzimidazoles and benzothiazoles incorporating units such as pyridyl and thioether, and for the preparation of certain thioether dicarboxylic acids precursory to them. Condensations of ortho-functinal anilines with carboxylic acids were carried out in polyphosphoric acid or refluxing HCl solution. Syntheses are reported for: [HO₂C(CH₂)2S(CH₂)₂]₂X (X = O, S), 1,9-bis(benzimidazol-2-yl)-2,5,8-trithianonane, 1,11-bis(N-methylbenzimidazol-2-yl)-3,6,9-trithiaundecane, 1,11-bis(2-benzimidazol-2-yl)-6-oxo-3,9-dithiaundecane, 2-(2-pyridyl)benzothiazole, 2,6-bis(benzothiazol-2-yl)pyridine, 2-(2-pyridyl)-N-methylbenzimidazole, 2-(2-pyridylmethyl)benzimidazole and 2-(N-methyl-2-piperidyl)benzimidazole. The compounds were characterized, where appropriate, by their mass, uv and ¹H-nmr spectra.     

Pincer-porphyrin hybrids

[reaction: see text] Starting from tetrakis(3,5-bis(bromomethyl)phenyl)porphyrin, pincer-porphyrin hybrid molecules (tetrakis(ECE-pincer)porphyrin; E = N, P, S) based on a tetraphenylporphyrin skeleton have been prepared in high yields. These multi-ligand site compounds could be selectively metalated at their peripheries, which was shown by X-ray crystallography.     

Preparation and Characterization of Compositions Based on PbO-MgO-Nb2O5 Using the Sol-Gel Method

Lead Magnesium Niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), is an important relaxor ferroelectric material. A significant problem exists, however, in the preparation of this material: it is very difficult to prepare pure phase, without the presence of a pyrochlore impurity phase which degrades the dielectric properties. Depending on the processing conditions, the amount of pyrochlore phase varies.Considering the ternary diagram PbO-MgO-Nb₂O₅, different compositions have been prepared by a simple sol-gel method at room temperature using Pb(CH₃COO)₂, Mg(CH₃COO)₂, Nb(OC₂H₅)₅ as precursors. After the heat treatment, the samples obtained were analyzed by XRD and EPMA with the purpose of studying the compositions formed.From the analysis of these results, it seems to be that the appearance of pyrochlore can be related to the reactivity of the MgO and/or the presence of other phases of the binary system PbO-Nb₂O₅. An excess of Pb and Mg is necessary for compositions to be formed near the PMN. The control of the amount of these two elements is very important because an excess of MgO would lead to rich compositions in Mg as a secondary phase.     

Gas-phase reactivity of heterobinuclear oxometalate anions [CrMoO6(OR)]-, [CrWO6(OR)]-, and [MoWO6(OR)]- (R = H, nBu)

Heterobinuclear oxometalate anions based upon [CrMoO7]2-, [CrWO7]2-, and [MoWO7]2- were generated and transferred to the gas phase by the electrospray process from acetonitrile solutions containing two of the salts (Bu4N)2[MO4] (M = Cr, Mo, W). Their reactivities were examined and compared with those of the related homobinuclear anions based upon [M2O7]2- (M = Cr, Mo, W). Particular emphasis was placed upon reactions relevant to gas-phase catalytic cycles described previously for oxidation of alcohols by [Mo2O6(OH)]- (Waters, T.; O'Hair, R. A. J.; Wedd, A. G. J. Am. Chem. Soc. 2003, 125, 3384-3396). The protonated anions [MM'O6(OH)]- each reacted with methanol with loss of water to form [MM'O6(OCH3)]- at a rate that was intermediate between those of [M2O6(OH)]- and [M'2O6(OH)]-. The butylated anions [MM'O6(OBu)]- were generated by collisional activation of the ion-pairs {Bu4N+ [MM'O7]2-}-. Collisional activation of [MM'O6(OBu)]- resulted in either the loss of butanal (redox reaction) or the loss of butene (elimination reaction), with the detailed nature of the observations depending on the nature of both M and M'. Selective 18O labeling indicated that the butoxo ligands of [CrMoO6(OBu)]- and [CrWO6(OBu)]- were located on molybdenum and tungsten, respectively. This structural insight allowed a more detailed comparison of reactivity with the homobinuclear species, and highlighted the importance of the neighboring metal center in these reactions.