Topic
Ligand
About: Ligand is a research topic. Over the lifetime, 67732 publications have been published within this topic receiving 1359684 citations. The topic is also known as: complexing agent & ligands.
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TL;DR: The hypothesis that iron-catalyzed formation of hydroxyl radical from superoxide anion radical (O-.2) and H2O2 requires the availability of at least one iron coordination site that is open or occupied by a readily dissociable ligand such as water is investigated.
853 citations
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TL;DR: Examples of MLC in which both the metal and the ligand are chemically modified during bond activation and 2) Bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligands is not directly bound to the metal.
Abstract: Metal-ligand cooperation (MLC) has become an important concept in catalysis by transition metal complexes both in synthetic and biological systems. MLC implies that both the metal and the ligand are directly involved in bond activation processes, by contrast to "classical" transition metal catalysis where the ligand (e.g. phosphine) acts as a spectator, while all key transformations occur at the metal center. In this Review, we will discuss examples of MLC in which 1) both the metal and the ligand are chemically modified during bond activation and 2) bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligand is not directly bound to the metal (e.g. via tautomerization). The role of MLC in enabling effective catalysis as well as in catalyst deactivation reactions will be discussed.
846 citations
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TL;DR: The role of the lone pair of electrons of Pb(II) in determining the coordination geometry is analyzed from crystallographic studies and ab initio molecular orbital optimizations in this article, where factors that contribute to the disposition of ligands around the lead with geometries that are: (1) holodirected, in which the bonds to ligand atoms are distributed throughout the surface of an encompassing globe, and (2) hemidirected (i.e., there is an identifiable void in the distribution of bonds to the ligands).
Abstract: The role of the lone pair of electrons of Pb(II) in determining the coordination geometry is analyzed from crystallographic studies and ab initio molecular orbital optimizations. Of particular interest are factors that contribute to the disposition of ligands around the lead with geometries that are (1) holodirected, in which the bonds to ligand atoms are distributed throughout the surface of an encompassing globe, and (2) hemidirected, in which the bonds to ligand atoms are directed throughout only part of an encompassing globe, i.e., there is an identifiable void in the distribution of bonds to the ligands. The preferred coordination numbers for lead were found to be 4 for Pb(IV) and 4 and 6 for Pb(II). All Pb(IV) structures in the CSD have a holodirected coordination geometry. Pb(II) compounds are hemidirected for low coordination numbers (2−5) and holodirected for high coordination numbers (9, 10), but for intermediate coordination numbers (6−8), examples of either type of stereochemistry are found. A...
845 citations
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TL;DR: The definition and scope of hemilabile ligands are recalled, the main classes of ligands containing one or more oxazoline moieties are presented, with an emphasis on hybrid ligands, and why the combination of these two facets of ligand design appears particularly promising are explained.
Abstract: Ligand design is becoming an increasingly important part of the synthetic activity in chemistry. This is of course because of the subtle control that ligands exert on the metal center to which they are coordinated. Ligands which contain significantly different chemical functionalities, such as hard and soft donors, are often called hybrid ligands and find increasing use in molecular chemistry. Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions remain very difficult, not only because of the considerable diversity encountered within the Periodic Table-different metal centers will behave differently towards the same ligand and different ligands can completely modify the chemistry of a given metal-but also because of the small energy differences involved. New systems may-even through serendipity-allow the emergence of useful concepts that can gain general acceptance and help design molecular structures orientated towards a given property. The concept of ligand hemilability, which finds numerous illustrations with hybrid ligands, has gained increased acceptance and been found to be very useful in explaining the properties of metal complexes and in designing new systems for molecular activation, homogeneous catalysis, functional materials, or small-molecule sensing. In the field of homogeneous enantioselective catalysis, in which steric and/or electronic control of a metal-mediated process must occur in such a way that one stereoisomer is preferentially formed, ligands containing one or more chiral oxazoline units have been found to be very valuable for a wide range of metal-catalyzed reactions. The incorporation of oxazoline moieties in multifunctional ligands of increasing complexity makes such ligands good candidates to display hemilabile properties, which until recently, had not been documented in oxazoline chemistry. Herein, we briefly recall the definition and scope of hemilabile ligands, present the main classes of ligands containing one or more oxazoline moieties, with an emphasis on hybrid ligands, and finally explain why the combination of these two facets of ligand design appears particularly promising.
839 citations
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TL;DR: The history of “ligand effects” in catalysis, a range of reactions for which a notable effect has been observed, and some of the established examples of bite angle effects involve diphosphine ligands.
Abstract: phinoethane) seemed mainly to stabilize intermediates, and often the catalytic reactions were slower when dppe was used instead of the most common monodentate triphenylphosphine. We will briefly review the history of “ligand effects” in catalysis before discussing a range of reactions for which a notable effect has been observed. It has taken quite some time before the positive effect that bidentates can have on selectivities and rates of catalytic reactions was fully recognized. Most of the established examples of bite angle effects involve diphosphine ligands. Therefore, many important catalysts containing a chelate ligand such as bipyridine and diimine will fall outside the scope of this review. The connecting bridge in these bidentates does play a dominant role in the performance of these catalysts, but systematic studies have not been published. The effects of phosphine ligands in catalysis have been known for quite some time. One of the first reports involves the use of triphenylphosphine in the “Reppe” chemistry, the reactions of alkynes, alcohols, and carbon monoxide.1 It was found that formation of acrylic esters was much more efficient using NiBr2(PPh3)2 than NiBr2 without ligand. In the commercial system, though, a phosphine-free catalyst is used. While the reaction was not yet understood mechanistically, the use of phosphines in catalysis attracted the attention of the petrochemical industry worldwide. An early example of a phosphine ligand modified catalytic process is the Shell process for alkene hydroformylation using a cobalt catalyst containing a trialkylphoshine.2 The reaction requires higher temperatures, but it leads to more linear product as compared to the unmodified catalyst. The general mechanism of the hydroformylation reaction has been known for a long time.3 Hydrocyanation as used by Du Pont is another early example of an industrially applied catalytic reaction employing ligands.4 It is a nickel-catalyzed reaction in which aryl phosphite ligands are used for the production of adiponitrile. The development of this process has played a key role in the introduction of the now very common study of “ligand effects” in the field of homogeneous catalysis by organometallic complexes.5 While several industries were working on new homogeneous catalysts, important contributions to the new field were made in academia in the early 1960s with the appearance of the first phosphinemodified hydrogenation catalysts. An early example of a phosphine-free ruthenium catalyst was published by Halpern.6 Triphenylphosphine-modified platinumtin catalysts for the hydrogenation of alkenes were reported by Cramer from Du Pont in 1963.7 In the same year Breslow (Hercules) included a few phosFigure 1. Bite angle: The ligand-metal-ligand angle of bidentate ligands. 2741 Chem. Rev. 2000, 100, 2741−2769
833 citations