1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

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Chemical Redox Agents for Organometallic Chemistry

The development in the field of coordination polymers or metal-organic coordination networks, MOCNs (metal-organic frameworks, MOFs) is assessed in terms of property investigations in the areas of catalysis, chirality, conductivity, luminescence, magnetism, spin-transition (spin-crossover), non-linear optics (NLO) and porosity or zeolitic behavior upon which potential applications could be based.

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Engineering coordination polymers towards applications

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C–C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of...

Advances in Synthetic Applications of Hypervalent Iodine Compounds

Ammonia–borane, H3NBH3, is an intriguing molecule for chemical hydrogen storage applications. With both protic N–H and hydridic B–H bonds, three H atoms per main group element, and a low molecular weight, H3NBH3 has the potential to meet the stringent gravimetric and volumetric hydrogen storage capacity targets needed for transportation applications. Furthermore, devising an energy-efficient chemical process to regenerate H3NBH3 from dehydrogenated BNHx material is an important step towards realization of a sustainable transportation fuel. In this perspective we discuss current progress in catalysis research to control the rate and extent of hydrogen release and preliminary efforts at regeneration of H3NBH3.

Ammonia-borane: the hydrogen source par excellence?

http://www.chemie.uni-muenchen.de/ac/kluefers/homepage/L/bac/ca_2004.pdf

Synthetic analogues relevant to the structure and function of zinc enzymes.

Zinc is known to be functional in almost 300 enzymes representing all subgroups of the enzyme classification.1 Although it is present in organisms only about half as much as its famous congener iron, it competes with the latter in terms of versatility. In comparison, the divalent metals magnesium and calcium, being 1-2 orders of magnitude more abundant, rarely share the biochemical abilities and rarely interfere with the biological functions of zinc, and the homologues of zinc, cadmium and mercury, just manage to exist as highly poisonous impurities. The question “Why?” suggests itself. What makes zinc, the “boring element without properties”, so unique? Like all “Why?”, questions this one is difficult to answer. Definitely the redox inertness and the hard-soft properties of zinc play an important role. Unlike all other firstrow transition metals, zinc cannot get caught in a redox trap, i.e., change its properties by changing its oxidation state. Unlike calcium and magnesium, zinc likes to be coordinated by the soft donor functions in a protein environment, i.e., cysteine thiolate and histidine imidazole. Unlike cadmium and mercury, it does not hold on irreversibly to these donors. The intermediate nature of zinc in this respect, not really hard and not really soft, certainly makes it suitable to be bound to and to act in all available donor environments in living organisms. The two aforementioned properties illustrate the availability of zinc for biological purposes. When it comes to usefulness, other properties must be named. The first of these is a combination of high (kinetic) lability and low (thermodynamic) stability, which characterizes the complexes of zinc more than those of any other transition metal. Unfortunately this combination, which is the essence of catalysis, is an unpleasant symptom for a preparative chemist trying to build enzyme models. In our opinion two other properties are the most important ones in zinc enzymes, specifically for the largest part of them effecting group transfer. They are the high nucleophilicity of “relevant” anions (OH, OR, SR, OPO2X) when bound to tetrahedral zinc and the high coordinative flexibility in the ligand sphere of the metal. In an enzyme environment, the Zn-OH function which exists at neutral pH is an equivalent of free OH-, and all zinc-catalyzed group-transfer reactions involve a rapid change of coordination numbers and coordination geometries at the zinc center.1 This Account is meant to provide some information from zinc coordination chemistry related to an understanding of these latter two phenemena. Both involve atomic interactions and atomic motions in the immediate environment of the metal ion, i.e., the inorganic part of zinc-catalyzed organic or biochemical reactions. We are well aware that the properties of “biological zinc” are a consequence of its “biological environment” and that the catalytic processes can only be understood by a full analysis of all environmental effects, i.e., the interactions of the substrate with the catalyst, solvent, enzyme interior, or cofactors. But the catalytic bond breaking and bond making take place when the reagents are bound to zinc. With this in mind we put up for discussion some observations from coordination chemistry and some generalizations drawn thereof. On the basis of the fact that most zinc enzymes contain the catalytic zinc tetrahedrally coordinated by three protein side chains and the reagents (water, hydroxide, or substrate) as depicted below, we developed the “Freiburg enzyme model”, which reliably reproduces this bonding situation.2 This enzyme model mimics the protein environment by attachment of the metal to three heterocyclic nitrogen donors of a tris(pyrazolyl)borate ligand and by encapsulation of the metal by the three substituents R attached at the 3-positions of the pyrazole rings. Throughout this Account, the graphical symbol annotated as Tp*Zn will be used to represent the “enzyme” part of this model. The Tp*Zn-X complexes have been found versatile in many respects. They allow a wide variation of substituents X. They are suitable for studying the properties of single Zn-X units in a tetrahedral and hydrophobic environment, in this context the nucleophilicity of X. Due to their reasonably inert nature, they offer themselves for mechanistic studies, and the large amount of structural data gathered for them provides a basis for an evaluation of mechanistic pathways. Our contributions to pyrazolylborate-zinc chemistry started with invaluable help from the inventor of the ligands, Jerry Trofimenko. They grew complementary, in competition and in cooperation with the contributions by our colleagues G. Parkin, W. Klaui, W. Tolman, and the late N. Kitajima. Heinrich Vahrenkamp received his Dr. degree in 1967 at the University of Munchen, working under the direction of H. Noth in boron chemistry. After picking up cluster chemistry and X-ray crystallography from L. F. Dahl at the University of Wisconsin, he worked at the Universities of Marburg and Munchen on organometallic Lewis bases, becoming Privatdozent in 1972. In 1973 he was called to the chair of inorganic chemistry at the University of Freiburg where he has worked until today. After more than 20 years of research on polynuclear organometallic compounds, the main interest of his research group has now shifted to the coordination chemistry of zinc related to an understanding of its biological functions. Acc. Chem. Res. 1999, 32, 589-596

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Transitions, Transition States, Transition State Analogues: Zinc Pyrazolylborate Chemistry Related to Zinc Enzymes

A mononuclear zinc hydroxide complex stabilized by a highly substituted tris(pyrazolyl)hydroborato ligand: analogies with the enzyme carbonic anhydrase

The types of sulfur bonding—as sulfane or sulfide—encountered in the molecules of maingroup elements are almost unknown in the chemistry of metal complexes, where the sulfur atoms function instead as two-electron donors by bridging two metal atoms, as four-electron donors by bridging three or four metal atoms, or as six-electron donors by incorporation between four metal atoms. In such complexes, the metal-metal bond can be modified over a wide range by chemical or electrochemical variation of the number of electrons present. The readiness with which polynuclear complexes containing metals and sulfur undergo redox reactions is also utilized by Nature in the active sites of some redox proteins.

Sulfur Atoms as Ligands in Metal Complexes

In this perspective some results from the work by the author's research group on the coordination chemistry of zinc are presented. They are selected so as to highlight the principles which, in the opinion of the author, make zinc unique in its ability to function as the catalytic center for an ever-increasing number of biological processes. In essence, the “non-properties” of zinc are the basis of its success: no redox chemistry, no ligand field effects, no typical coordination numbers or geometries, no stability or inertness of its complexes, no typical “hard” or “soft” characteristics. The chemistry presented here is centered around the “Freiburg Enzyme Model”, the pyrazolylborate-ligated zinc-hydroxide complex. The topics discussed in detail include the zinc–water combination, the non-similarity between the Zn–OH and the Zn–SH, Zn–OR or Zn–SR complexes, and the functionalization of CO2 by zinc-bound nucleophiles. The value of structure correlation analysis for the elucidation of mechanistic details is demonstrated, and the first examples of catalysis by the TpZn–OH complexes are presented.

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Why does nature use zinc--a personal view.

The macrocycle 1,4,7-triazacyclononane (C 6 H 15 N 3 ; L) reacts with Zn(II) ions in methanolic solution forming the 2:1 species [L 2 Zn] 2+ . The crystal structure of [L 2 Zn](PF 6 ) 2 (1) has been determined. Crystal data: space group C2/c, a=9.719 (2) A, b=16.312 (3) A, c=15.017 (3) A, β=92.04 (3) o , V=2379.3 (8) A 3 , and Z=4.

Mono- and dinuclear zinc(II) complexes of biological relevance. Crystal structures of [L2Zn](PF6)2, [L'Zn(O2CPh)2(H2O)], [L'2Zn2(.mu.-OH)2](ClO4)2, and [L'2Zn2(.mu.-OH)(.mu.-CH3CO2)2](ClO4).cntdot.H2O (L = 1,4,7-triazacyclononane, L' = 1,4,7-trimethyl-1,4,7-triazacyclononane)

Heinrich Vahrenkamp

Papers

Transitions, Transition States, Transition State Analogues: Zinc Pyrazolylborate Chemistry Related to Zinc Enzymes

A mononuclear zinc hydroxide complex stabilized by a highly substituted tris(pyrazolyl)hydroborato ligand: analogies with the enzyme carbonic anhydrase

Sulfur Atoms as Ligands in Metal Complexes

Why does nature use zinc--a personal view.

Mono- and dinuclear zinc(II) complexes of biological relevance. Crystal structures of [L2Zn](PF6)2, [L'Zn(O2CPh)2(H2O)], [L'2Zn2(.mu.-OH)2](ClO4)2, and [L'2Zn2(.mu.-OH)(.mu.-CH3CO2)2](ClO4).cntdot.H2O (L = 1,4,7-triazacyclononane, L' = 1,4,7-trimethyl-1,4,7-triazacyclononane)