摘要:

High-nuclearity lantthanide complexes, having unique structures and potentially important applications, have been prepared via limited hydrolysis of lanthanide ions controlled by α-amino acids as supporting ligands. These novel complexes, synthesized under physiological or higher-pH conditions, exist as lanthanide-hydroxo clusters, reflecting the subtle balance between the hydrolysis of the metal ions and the multidentate supporting coordination of the ligands. A cuboid [Ln4(μ3-OH)4]8+unit with four lanthanide ions and four triply bridging hydroxo ligands occupying the alternate vertices of a distorted cube has been identified as a prevalent structural motif in these complexes, wherein lanthanide complexation may be achieved via only the carboxylate group or simultaneous coordination of both the carboxylate and the amino moieties of an amino acid ligand. Bridging ligands with more than one carboxylate group may support open framework structures utilizing the [Ln4(μ3-OH)4]8+unit as building blocks. In addition, “wheel-like” structures composed of four or five vertex-sharing [Ln4(μ3-OH)4]8+unit can be realized by the halide-templated synthesis. It has been further demonstrated that octahedral [Ln6(μ6-O)(μ3-OH)8]8+core-containing clusters with an interstitial μ6-O ligand can be self-assembled via direct hydrolysis of lanthanide nitrates and perchlorates in the absence of any organic supporting ligands. Using a structurally unprecedented lanthanide-EDTA complex, the significant influence of pH conditions on the reaction outcome is illustrated.

ISSN:0260-3594    

DOI:10.1080/02603590008050862

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Recent advances in NMR spectroscopy methods for studying zeolites and their catalyzed reactions are briefly reviewed. In-situ NMR spectroscopy, with its controlled atmosphere conditions, is used to characterize the active surface sites of zeolites (highly acidic micro-porous molecular sieves) and to elucidate the hydrocarbon reactions they catalyze. The strong Bronsted acid sites of these catalysts, SiO(H)Al, are accessible to adsorbed molecules via well defined pore systems (0.3 to 1.2 nm). Substantial progress has been made in: obtaining high resolution NMR spectra under conditions (pressure, temperature, reagent flow) similar to real reactor conditions, developing new NMR methods for characterizing quadrupolar nuclei in solids, understanding acidity in solids and evidencing the details (intermediates, pathways, kinetics) of zeolite catalyzed chemical reactions. Some of the practical aspects and limitations in performing these studies are mentioned along with future prospects. Given its high versatility and adeptness for determining: the local structure around selected nuclei (2H,17O,27Al,29Si), and the structure and dynamics of organic molecules (1H,13C), NMR spectroscopy is destined to attain evermore important and far-reaching application in this field of study.

ISSN:0260-3594    

DOI:10.1080/02603590008050863

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The superexchange interaction in cyano-bridged transition metal dimers is analysed by a valence bond configuration interaction model in combination with fourth order perturbation theory. Ferro- and antiferromagnetic contributions to the exchange splitting are expressed in terms of metal-ligand hybridization matrix elements, metal-to-ligand, lig-and-to-metal, metal-to-metal charge transfer energies, and intra-atomic exhange integrals. The formalism is simplified and we arrive at a two-parameter model, with which it is possible to estimate magnetic ordering temperatures of cubic stoichiometric Prussian Blue like phases containing first row transition metals. The relevance and applicability of our theoretical results is demonstrated on critical temperatures reported in the literature. The model accounts for both ferri- and ferromagnetic ordering temperatures. It is found that cyanide is an efficient mediator of exchange due to the participation of the π and π* orbitals to equal extent.

ISSN:0260-3594    

DOI:10.1080/02603590008050864

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Growing interest in site-specifically labeled metallo-oligodeoxynucleotides for diagnostic, therapeutic and mechanistic applications provides the impetus for the development of new synthetic methods that are user-friendly and amenable to large-scale production. This review describes advances in the field since the early 1980′s and highlights recent solution-phase and automated solid-phase coupling procedures.

ISSN:0260-3594    

DOI:10.1080/02603590008050865

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor