Metal complexes coordinated ligand reactions

Reactions of transition-metal complex-coordinated ligands with protonic acids can yield N—H bond-containing products. Coordinated nitrosyl groups react with HCl ... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

For example, the substituted aniline Ar-NH2 (Ar = />-CH3OC6H4) reacts with the ruthenium nitrosyl complex Ru(bpy)2(Cl)(NO)2+ (bpy = 2,2 -bipyridine) to give a complex of the diazo ligand, namely Ru(bpy)2(Cl)(NNAr)2+ (57). Upon employing the 15N labeled nitrosyl complex Ru(bpy)2Cl(15NO)2+ this reaction resulted in the 15N coordinated product, Ru(bpy)2Cl(15NNAr)2+, demonstrating that the reaction occurs within the metal complex coordination sphere. When the reactions were conducted in non-protic solvents, these nucleophile-nitrosyl adducts could be isolated. 

The chapter by C. J. Swan and D. L. Trimm, which also emphasizes the effect on catalytic activity of the precise form of a metal complex, shows too that, depending on the metal with which it is associated, the same ligand can act either as a catalyst or inhibitor. The model reaction studied was the liquid-phase oxidation of ethanethiol in alkaline solution, catalyzed by various metal complexes. The rate-determining step appears to be the transfer of electrons from the thiyl anion to the metal cation, and it is shown that some kind of coordination between the metal and the thiol must occur as a prerequisite to the electron transfer reaction (8, 9). In systems where thiyl entities replace the original ligands, quantitative yields of disulfide are obtained. Where no such displacement occurs, however, the oxidation rates vary widely for different metal complexes, and the reaction results in the production not only of disulfide but also of overoxidation and hydrolysis products of the disulfide. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

The most important classes of bimolecular reactions of transition metal complexes are ligand substitutions, reactions of the coordinated ligands and inner and outer sphere oxidation-reduction reactions28. ... 

The CS2 complexes are prepared by the reaction of CS2 with transition metal complexes. The ligand in the M-rf-CSz and M-tj CSj coordination modes acts as a good 77-acceptor and a poor cr-donor as shown by the lower CS stretching frequencies found in the complexes relative to that of free CS2. 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

Abstraction reactions are elimination reactions in which the coordination number of the metal does not change. In general, they involve removal of a substituent from a ligand, often by the action of an external reagent, such as a Lewis acid. Two types of abstractions, a and 3 abstractions, are illustrated in Figure 14-12 they involve, respectively, removal of substituents from the a and P positions (with respect to the metal) of coordinating ligands. a-Abstraction has been encountered previously, in the synthesis of carbyne complexes discussed in Section 13-6-3. 

Both of these pathways are disjunctive with respect to the complex ML and may be either adjunctive or disjunctive with respect to the reaction of the intermediate free M or L with the complex M L. The complete dissociation of both initial complexes is an unlikely pathway unless both are weak as may be the case for alkaline earth metal complexes. Double-exchange reactions of transition metal complexes often involve coordination chain mechanisms (for example, further reaction of L, produced by a ligand-initiated pathway, with the initial complex ML) and the overall rates may be strongly dependent on trace concentrations of reactants that promote or terminate coordination chain reactions (Margerum et a ., 1978). 

Transition metal complexes can promote reactions by organizing and binding substrates. We have already seen this in terms of metal-directed reactions. Another important function is the supply of a coordinated nucleophile for the reaction, which is incorporated in the product. We have already seen a coordinated nucleophile at work in the reaction discussed above of Co— OH with NO+ nucleophiles, which are electron-rich entities, are best represented in coordination chemistry by coordinated hydroxide ion, formed by proton loss from a water molecule this is a common ligand in metal complexes. Normally, water dissociates only to a very limited extent, via... 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

Several efficient oxidation reactions with molecular oxygen were developed using transition-metal complexes coordinated by variuos ligands in combination with apprOTriate reductants. Recently, it was found that cyclic ketones such as 2-methylcyclohexanone and acetals of aldehyde such as propionaldehyde diethyl acetal were effectively employed in aerobic epoxidation of olefins catalyzed by cobalt(II) complexes. In the latter case, ethyl propionate and ethanol were just detected in nearly stoichiometric manner as coproducts (Scheme 12), therefore the reaction system is kept under neutral conditions during the epoxidation. 

This chapter is concerned with coordinated ligand reactions, and notably enhanced reactivity resulting from coordination. It is divided into three sections. The first two deal with coordination complexes of cobalt(III) and other metal centers, while the last section discusses ligand reactivity of organometallic compounds. The emphasis is on stoichiometric reactions catalytic processes are covered in Chapter 14. 

An important group of reactions of coordinated ligands which does involve the metal-ligand bond are the so-called insertion reactions. They are often of importance in the catalysis of organic reactions by transition metal complexes, although insertion reactions have a history dating back to the mid-19th century in the reaction... 

Polymeric metal complexes with polyester macroligands have been generated by initiation from hydroxyl functionalized ligand and metal complex reagents. Ligand initiators lead to macroligands which can be combined with metal precursors in coordination reactions to produce PMCs (51). Metalloinitiators, on the other hand, produce PMCs directly (2). 

Ligand Exchange Reactions of Inert-Metal Complexes—Coordination Numbers 4 and 5... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

In a complexation reaction, the reaction unit is an electron pair. For the metal, the number of reaction units is the number of coordination sites available for binding ligands. For the ligand, the number of reaction units is equivalent to the number of electron pairs that can be donated to the metal. One of the most important analytical complexation reactions is that between the ligand ethylenediaminetetracetic acid (EDTA), which can donate 6 electron pairs and 6 coordinate metal ions, such as Cu thus... 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

This reaction requires a metal complex that is coordinatively unsaturated and provides another way for bonding of reactant ligands to a metal.Insertion ... 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

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