Showing papers in "Chemical Reviews in 1998"

A Survey of Applications of Polyoxometalates

Abstract: The field of polyoxometalates (POMs), although a mature field, continues to attract significant attention. The number of publications and patents continues to grow. New researchers are entering the field. The scientific communities of fast-growing new economic/technology powers such as Peoples Republic of China and India are becoming important contributors of the total number of worldwide publications on POMs. Figure 1 depicts the growth of the POM literature per year since 1966. In 1996, according to Chemical Abstracts, nearly 600 refereed publications and over 120 patents were issued in reference to the POM chemistry and technology. Figure 2 shows the countries where the most research activity exists based on publications, and Figure 3 lists the 10 largest patent assignee countries in the world. Japan issues 40% of the worldwide patent literature followed by the USA with ∼17%. The applications of POMs are based on combinations of so-called “value-adding properties” which are summarized in Table 1. From the above-listed properties, the applications of POMs are centered primarily on their redox properties, photochemical response, ionic charge, conductivity, and ionic weights. The majority of the patent and applied literature is devoted to the applications of the Keggin type heteropolyacids (HPA) and their salts. Primarily H3PMo12O40, H3PW12O40, H4SiMo12O40, and H4SiW12O40 are used as the main examples for many applications. Their popularity can be attributed to a large extent to the enormous volume of literature over several decades that describes their fundamental chemistry and to their commercial availability, which makes them convenient starting materials. Approximately two-thirds of the applied chemistry/technology literature describes applications that are based on these POMs. Dimitris E. Katsoulis was born in Athens, Greece in 1955. He received a B.S. degree in Chemistry from University of Athens in 1977. He subsequently joined the research group of Prof. Michael T. Pope at Georgetown University and obtained a Ph.D. degree in 1985. He continued with a postdoctoral assignment at Georgetown University, and in 1988 he joined the Science and Technology function of Dow Corporation in Midland Michigan. He is currently an Associate Research Scientist in the Rigid Materials Science Expertise Center (Central R&D) in Dow Corning. His research interests include hybrid materials, with focus on siloxane-polyoxometalate compositions, sol−gel chemistry, silsesquioxanes, gel systems, polymer matrix composites, and nanocomposites. 359 Chem. Rev. 1998, 98, 359−387

Metal-Initiated Amination of Alkenes and Alkynes.

Abstract: The catalytic production of organic molecules is one of the most important applications of organometallic chemistry. For this purpose the distinct reaction chemistry of organic ligands covalently bound to transition metals is exploited. Most organometallic chemistry has focused on the formation of carboncarbon or carbon-hydrogen bonds. The platinum group metals, in particular Pd and Rh, have been the most commonly used elements insfrequently commercializedscatalytic processes that include hydrogenation, hydroformylation and others. On the other hand, carbon-oxygen and carbon-nitrogen bonds are found in the majority of organic molecules and are of particular importance in physiologically active substances. However, catalytic organometallic reactions that lead to the formation of carbonheteroatom bonds are less common.1,2 The catalytic construction of carbon-nitrogen bonds in amines is particularly rare.3-10 Clearly, efficient catalytic routes to nitrogen based molecules are of great interest.11 Especially useful are catalytic hydroaminations of olefins and alkynes which avoid production of byproducts, like salts, generally observed in metal-catalyzed aminations of C-X derivatives (X ) e.g., halogen). However, known aminations of olefins often require stoichiometric use of transition metals and general methods for carrying out aminations catalytically are not yet available.12,13 Most of the present enantioselective syntheses of molecules bearing an amine functionality use classical stoichiometric reactions with chiral auxiliaries or utilize enantiomerically pure starting material.14-16 Hydroamination of alkenes and alkynes, which constitutes the formal addition of a N-H bond across a carbon-carbon multiple bond (Scheme 1), is a transformation of seemingly fundamental simplicity and would appear to offer the most attractive route to numerous classes of organo-nitrogen molecules such as alkylated amines, enamines or imines. Organic chemists have developed various synthetic approaches for the amination of olefins.17-19 Direct addition of nucleophiles H-NR2 to activated alkenes is of general importance for the synthesis of compounds with nitrogen atoms â to groups such as keto, ester, nitrile, sulfoxide, or nitro.13,20-23 These additions usually lead to the anti-Markovnikov products. On the other hand aliphatic olefins as well as most aromatic olefins are often aminated to give the Markovnikov product. One possibility to reverse the reactivity of aliphatic olefins is the use of electrophilic nitrogen radicals which have been used to obtain anti-Markovnikov products.24 In the past much work has been done on the activation of alkenes with stoichiometric amounts of metal.24 Reactions are mostly promoted by complexes of titanium,25 iron,26 zirconium,27 palladium28-31 and mercury.32,33 However, catalytic additions of amines H-NR2 to nonactivated double or triple bonds are still rare. Two basic approaches have been employed to catalytically effect aminations and involve either alkene/alkyne or amine activation routes (Scheme 2).34,140 Alkene activation is generally accomplished with late-transition-metal catalysts, which render coordinated olefins more susceptible to attack by † Dedicated to Dipl. Chem. Martin Eichberger (deceased 11/20/ 1997). 675 Chem. Rev. 1998, 98, 675−703

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives.

Abstract: Cyclodextrins are extremely attractive components of artificial enzymes and other biomimetic materials. They are readily available, they bind hydrophobic substrates into their cavities in water solution, and they have two rims of hydroxyl groups (Figure 1) that can either react with substrates themselves or be used to attach other catalytic and functional groups. Of course, they have disadvantages. For one, unless they are extensively modified their complexes with substrates can be rather flexible and, perhaps, with unpredictable preferred geometry. They are also unstable to strong acid. Thus for some purposes such synthetic cavity species as calixerenes1 or synthetic macrocycles2-4 may have advantages. However, one of the chief advantages of cyclodextrins is highly attractivesthey are readily available, so it is possible to avoid the synthesis of a binding group and go directly to studies of what can be achieved with their use. Afterward, the lessons learned may be applied to other systems with advantage. This review will cover all the literature on reactions in which cyclodextrins bind substrates and then either catalyze their reactions or mimic a step in an enzymatic catalytic sequence. However, it will not describe work in which cyclodextrins simply change the course of a reaction without playing an obvious catalytic role involving substrate binding. For example, there are systems in which the main function of the cyclodextrin seems to be to complex a metal ion and keep it in solution.5-11 There are other studies in which binding into a cyclodextrin simply alters the selectivity of attack by an external reagent in some way12-24 or causes solubilization to facilitate phase transfer catalysis.12,25,26 Presumably such other areas are described elsewhere in this volume. While much work on artificial enzymes using cyclodextrins has been done in the author’s laboratory, and will be described, every effort is made to describe all the relevant work in the field. Several reviews of this subject already exist and should be consulted for further information.2,27-70 The readily Ronald Breslow, born in 1931 in Rahway, NJ, completed his B.A. in chemistry in1952, his M.A. in medical science in 1953, and his Ph.D. in chemistry in 1955 with R. B. Woodward, all at Harvard University. After a postdoctoral year with Alexander Todd in Cambridge, he came to Columbia University where he is now University Professor and Professor of Chemistry. His work on enzyme models, on novel conjugated aromatic and antiaromatic molecules, on electrochemical and hydrophobic methods in mechanistic chemistry, and on anticancer cytodifferentiating agents has been recognized by a number of awards, including the U.S. National Medal of Science. In 1996, he served as President of the American Chemical Society.

Oxidative Strand Scission of Nucleic Acids: Routes Initiated by Hydrogen Abstraction from the Sugar Moiety.

Abstract: ion from the Sugar Moiety Wendy Knapp Pogozelski† and Thomas D. Tullius*,‡ Department of Chemistry, State University of New York at Geneseo, Geneseo, New York 14454, and Department of Chemistry, Boston University, Boston, Massachusetts 02215 Received August 27, 1997 (Revised Manuscript Received February 26, 1998)

Polyoxometalate-Based Molecular Materials

Abstract: Molecule-based materials with active physical properties, in particular electrical, magnetic, and optical, are a focus of contemporary materials chemistry research. Certainly, one reason for this interest has been the realization that these materials can exhibit cooperative properties typically associated with the inorganic network solids, as for example metallic conduction or even superconductivity,1 ferromagnetism,2 and nonlinear optical properties.3 With respect to the electrical properties, many important achievements were obtained in the 1970s with the discovery of the first molecule-based metal in 1972,4 namely the π-electron donor-acceptor complex [TTF][TCNQ] (TTF ) tetrathiafulvalene, TCNQ ) tetracyano-p-quinodimethane),5 and the report of the first molecule-based superconductors in 1979 based on the Bechgaard salts [TMTSF]2X (X ) PF6, AsF6; TMTSF ) tetramethyltetraselenafulvalene).6 The recognition of molecule-based ferromagnetic compounds is more recent and has emerged only in the past decade with the discovery of the electron-transfer salt [Fe(C5Me5)2][TCNE] (TCNE ) tetracyanoethylene) in 1985.7 Since these pioneering studies, the two areassmolecule-based metals and magnetsshave witnessed rapid development, and many new molecules have been designed which, if assembled in the appropriate manner in the solid, will enable researchers to improve the physical properties by increasing superconducting and ferromagnetic critical temperatures. A current development in the general area of molecule-based materials is to design, from a wise choice of the constituent molecules, new materials that combine properties not normally associated with a single material. Some intriguing applications of this concept would be to couple conductivity or optical phenomena with magnetic properties. In fact, this challenging goal was proposed8 in the mid-1980s but has only recently begun to be explored. Efforts in this direction nucleated with the design of hybrid materials formed by two molecular networks, such as anion/cation salts or host/guest solids, where each network furnishes distinct physical properties. In these new types of materials, each network contributes distinct physical properties to the solid. Examples include hybrid molecule-based materials, combining inorganic metal complexes that act as structural or magnetic components with an organic π-electron donor or acceptor molecule that furnishes the pathway for electronic conductivity.9 Polyoxometalates have been found to be extremely versatile inorganic building blocks for the construction of the aforementioned functionally active solids.10 The present article highlights recent results and provides a perspective of the use of polyoxometalates in the construction of molecule-based materials. We present herein the different classes of polyoxometalate-based hybrid materials that are of considerable interest due to their electrical or/and magnetic properties. These are (i) organic/inorganic hybrid salts in which the electron donors are organic molecules of the TTF type, (ii) organometallic/inorganic salts in which the electron donor is the decamethylferrocene complex, and (iii) organic/inorganic films in 273 Chem. Rev. 1998, 98, 273−296