Metal-Ion Coordination

The directionality of metal-ion coordination can be considered in two ways -from the point of view of a coordinating ligand such as a carboxylate or imidazole group in a protein side chain, and from the point of view of the metal ion, that is, how directional the bonds in its coordination sphere are. These two aspects of metal binding are discussed here. They are relevant to the manner in which metal ions mediate intermolecular as well as intramolecular interactions. 

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From the equilibrium constant and the apparent rate constant, the rate constant for reaction of the metal-ion coordinated dienophile can be calculated using equation 2.2 (derived in... 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

Creating regular arrangement of nucleobases through metal ion coordination and H-bond formation 98PAC977. 

In chelation complexes (sometimes called inner complexes when uncharged) the central metal ion coordinates with a polyfunctional organic base to form a stable ring compound, e.g. copper(II) acetylacetonate or iron(III) cupferrate ... 

Electrochemical effects of metal ion coordination to non-innocent biologically important molecules. M. J. Clarke, Comments Inorg. Chem., 1984, 3,133-151 (57). 

Two-dimensional protein layer orientation could be also effected by metal-ion coordination Monolayer of iminodiacetate-Cu(II) lipid was successfully employed as substrate for oriented immobilization of proteins naturally displaying histidine residues on their surface [37]. Affmity-resin-displaying Ni(II) complexes could also be successfully employed for oriented protein immobilization [38]. 

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

Natural a-amino acids provide a moderately effective N,OJ chelating donor set derived from carboxylate and amino groups, respectively. Several side-chain donor atoms may also be involved in metal ion coordination, especially those of Cys (thiol sulfur) and His (imidazole nitrogen(s)). (Abbreviations for amino acid residues are those recommended by IUPAC-IUB.1702)... 

Meso substitution of porphyrines to give tetraazaporphyrines, so-called porphyrazines, modulates the electronic character of the macrocycle. While porphyrazines have received considerably less attention than porphyrines over many years, this has changed due to the development of efficient syntheses of soluble derivatives.1 6-1809 Also, various porphyrazines (and phtalocyanines) with peripheral groups for metal ion coordination have been prepared and used for the construction of multimetallic complexes.1806 Ni porphyrazines (695) typically show absorptions spectra with a strong Q band at around 615nm. 

The LPDE system is applied to several reactions in which the metal ions coordinate to the lone pairs of heteroatoms, thereby activating the substrate. Initially, the effectiveness was shown in Diels Alder reactions (Scheme 1). In a highly concentrated (5.0 M) LPDE solution, Diels- Alder reactions proceeded smoothly.6-7 Generally, a catalytic amount of LiC104 is not effective in this reaction. In some cases, a catalytic amount of an additional Bronsted acid, such as camphorsulphonic acid (CSA), gives better results.8 An interesting double activation of carbonyl moieties by using dilithium compounds has been reported (compound... 

Many ligands of this category offer the prospect of inducing axial metal-ion coordination even for those cases where the pendant arms incorporate weak donor functions. Coordination will be enhanced simply because the donors are held near to the metal and hence their effective ... 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

Fig. 1 (a) Chemical structures of fluorescent ligands metal ion coordination sites are indicated in blue, (b) Absorption (solid) and fluorescence spectra (dotted) of 1 (black) and its Zn2+ complex (red) in water at pH 7 whereas uncomplexed 1 is virtually nonfluorescent (fluorescence quantum yield of 1-Zn2+ amounts to 0.34 [31]... 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

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