Coordinative binding

Since amines, unlike alcohols, do not react with dialkylzincs but may coordinatively bind to the zinc atom, the dilithio salt of 2,5-diisopropylpiperazine is a more potent catalyst than the piperazine itself, presumably due to the greater nucleophilic power of the dianion10. Dimethylzinc reacts rather slowly with aryl aldehydes and although the product is obtained in high enantiomeric excess, the chemical yield is low10. 

Silver(III), with a ds electronic configuration, forms only a limited number of stable compounds because of the inaccessibility of a suitable ligand framework to coordinatively bind the unusual, higher valent central metal while, at the same time, resisting intramolecular electron transfer. They are thermodynamically and kinetically unstable. 

This result shows that is reasonable to neglect other type of interactions between aluminium ions and the polymer for instance coordination binding with amide groups,... 

In fact this "unhydrolyzed" polyacrylamide sample is slightly charged and its low polyectrolyte character is confirmed by a slight difference of red values at pH 7 and 5, for salt free solutions. A really neutral polymer should be necessary to differentiate low effects of electrostatic interactions from non ionic interactions. coordination binding at low pH and hydrogen bonds at pH 7. Nevertheless, at this pH, the adsorption of the chain on Al(0H)3 aggregates can probably be considered as the main origin of the loss of viscosity. 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Reaction coordinate Binding AE (kcalmol-1) CT Qw(e) Bond length (A) Bond order ... 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

The sample kinetic data listed in Table 5.2 shows that the size of rate enhancement critically depends on the substrate-metal ion combination, and is markedly influenced by the solvent. The largest effect is displayed by 2-AcO-21 C6, which reacts with EtOBaBr half a million times faster than with EtONMe4. The conclusion was reached [6] that the huge rate enhancements observed in the ethanolysis reactions are a consequence of the fact that not only cation-anion electrostatic binding but also coordinative binding to the polyether chain in the metal-bound transition states are much more efficient in EtOH than in MeOH. 

Identical strands correspond to homotopic arrangements, whereas non-symmetrical strands represent heterotopic helicates, which can form isomers by different orientations of the coordinating binding units (Head to Head or Head to Tail)... 

Since Lewis bases such as pyridine and imidazole have high affinities for the axial coordination site in Por and Pc chelates, the complexes are well retained on immobilized ligands, for example, imidazole covalently linked to SiCL (104-106). Moreover, this axial coordination often improves the catalytic activity and selectivity, as in the Mn porphyrin-catalyzed epoxi-dation with H2O2 (7,107). However, immobilization by coordinative binding can be quite sensitive to solvent effects and competitive binding of ligands. 

An obvious way to immobilize MTO would be by coordinative binding on a PVP resin. However, in our experience, peroxidized MTO is not efficiently retained on PVP. Two alternative approaches have been proposed. First, MTO was supported on Nb2Os, and its activity was tested in the epoxidation of stilbenes and of styrenes (382). However, competitive metathesis and pronounced hydrolysis of the epoxides to the diols were observed. A second approach was suggested by Neumann and Wang (383). In a strategy similar to that for H5PV2M010O40 (370), they adsorbed MTO in a surface layer of... 

OSO4 can coordinatively bind to PVP and other polymers with, for example, l,4-diazabicyclo[2,2,2]octyl groups. This binding makes the compound much less volatile and hence safer. Cainelli et al. (388) noted that OSO4 on PVP can be used with several oxidants. With H2O2, much of the ketol overoxidation product is formed. Better results are obtained with r-BuOOH for dihydroxylation of hindered olefins or with trimethylamine N-oxide for most other olefins. Herrmann et al. (389) used a similar material however, when OSO4 was immobilized on PVP in the presence of tetrahydrofuran solvent, the Os was reduced to the +VI state, as evidenced by XPS data (389). 

There is a wider general interest in understanding the oxidation of cysteine thiolates in proteins since they are involved in redox-sensing reactions [99], Therefore, such oxidation reactions of thiols induced by Ru coordination may also play a more general role in the pharmacological activity of Ru-arene complexes by coupling Ru coordinative binding to redox processes both outside and inside cells. 

Electrochemical NO sensors based on platinized or electrocatalyst-modified electrodes often in combination with a permselective and charged membrane for interference elimination were proposed. Although the catalytic mechanism is still unknown, it can be assumed that NO is co or dinative ly bound to the metal center of porphyrin or phthalocyanine moieties immobilized at the electrode surface. The coordinative binding obviously stabilizes the transition state for NO oxidation under formation of NO+. Typically, sub-pM concentrations of NO can be quantified using NO sensors enabling the detection of NO release from individual cells. 

At the same time, a conclusive and sufficiently reliable answer is frequently required. We may be interested in, for example, the question of the possibility of dissociative adsorption, or the problem of the existence of some chemisorption structures as in the discussion (see below) on the coordinative binding of water molecules by silicon atoms, etc. Ab initio calculations are required in these cases. They are needed as well to check some principal conclusions based on semiempirical schemes. Also, they are useful in providing the basis for proper choice and improving the parametrization of semiempirical methods. Therefore the nonempirical approach is finding ever-increasing application to the surface problems. 

Indirect evidence in favor of the coordinative binding is the existence of the hexacoordinated organosilicon compounds. However, a counterargument is that ammonia, possessing a greater ability for coordinative binding than... 

If the 9-position is blocked, the other imidazole nitrogen, N-7, is coordinated. Binding appears somewhat less likely through N-l than through N-7 there is also binding with both N-l and N-7. 

CD spectrum including formation of a band at 255 nm (+), a shoulder at about 270 nm (+), a weaker signal at 300 nm (-), and a broad signal centered at 330 nm (+). The CD spectrum became saturated after addition of one equivalent of Pb + per phytochelatin molecule. The authors concluded from these spectral data that the bound Pb + adopts a two-coordinate binding geometry when bound to this phytochelatin. ... 

A dynamic model of the catalytic reactions that lead to strand cleavage has been proposed based on structural considerations (22). Assembly is thought to involve coordinate binding of transposase, its DNA substrate(s), and two divalent metal ions. On transposase DNA binding, the two metal ions (H and T for hydrolysis and transfer, respectively Fig. lal) find then-appropriate positions in the active site and are poised for catalysis. Metal ion H orients and activates the water molecule (depicted as OH) for nucleophilic attack. Metal ions T (which... 

The potassium salt of the podand ligand tris[3-(2-pyridyl)-pyrazol-l-yl]borate O Tp) reacts with thorium tetra(nitrate) to generate the complex (P Tp)Th(N03)3. The crystal structure of the complex reveals that the metal center is 12-coordinate, binding to the six nitrogen atoms of the podand ligand, and to two of the oxygen atoms of each nitrate. The molecule has three-fold symmetry, and the nitrates are located between the bidentate arms of the podand. 

The first literature example of a mercury based receptor (32) for anions was analogous to the chelating boron receptor discussed earlier. The crystal structure indicated that two molecules of Compound 32 asociate with one chloride ion, which sits in a four-coordinate binding site. Solution studies, however, gave results indicative of 1 l binding for halide anions (94, 95). This simple... 

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