Ligand-directed labilization

Philips BL, Casey WH and Craweord SN (1997) Solvent exchange in AIF (H20)Iz% (aq) complexes Ligand-directed labilization of water as an analogue for ligand-induced dissolution of oxide minerals. Geochim Cosmochim Acta 61 3041-3049. 

Stable Ligands Can Change Reactivities of Other Sites in the Inner-Coordination Sphere. Some ligands affect the reactivity of other metal-oxygen bonds in the inner-coordination sphere, and particularly those trans to the substitution. These are ligand-directed labilizations. Experiments on dissolved complexes can help identify the important ligands and to elucidate the mechanisms of rate enhancement. 

The simplest such ligand-directed labilization is deprotonation of a hydration water (ti -OH2) to form a hydroxyl (ri -OH sites) in a dissolved monomer. This deprotonation is effectively a ligand substitution because an equilibrium concentration of such hydroxyl ligands is maintained dynamically by rapid proton exchange with the solvent. It is distinct from other substitutions, however, because no metal-oxygen bond is broken. The rates of exchange of protons are so fast that the waters convert to hydroxyls virtually instantaneously. 

Mechanisms of Dissolution. Little is really known about the molecular mechanisms of oxide-mineral dissolution because the surface-speciation information is so difficult to acquire. Speculations about the mechanism generally fall into two groups. The first model is noncatalytic and treats adsorbate effects as due to ligand-directed labilization in the case of adsorbed ligands and induction in the case of protons. 

Deprotonation of the terminal ti -OH2 site weakens the bridging hydroxo- or oxo-groups and accelerates the rates of dissociation (Table 2). The effect is not trivial. This ligand-directed labilization by a stable hydroxyl is manifested in accelerated rates of polymerization, anation, water exchanges, and mineral dissolution. Rates of water exchange from the inner co-ordination sphere of Cr(H20)g to the bulk solution, for example, are a factor of 75 times slower than around the depro-tonated Cr(H2 0)5 (aq) complex. Similar increases are observed for other metals [e.g., 16, 18]. 

Replacement of a hydration water with other stable ligands can also make the distal oxygens more reactive. Substitution of fluoride ion for a hydration water in Al(H20)g to form the AlFe(H20)5 complex, for example, causes the rate of exchange of each of the other water molecules in the inner co-ordination sphere to increase by a factor of sslO [19]. The effect is progressive so that rates continue to increase with each additional substitution. This pathway is marked 3 in Fig. 4 and is a type of ligand-directed labilization ... 

Ligand-directed labilization is well documented for ammonia substitution into... 

All of the important classes of ligand-directed labilization are represented at mineral surfaces, including the deprotonation of a terminal ti -OH2 site. One therefore expects a similar increase in dissolution of oxide minerals as the surface sites become deprotonated. For a mineral siuface, these deprotonations form negative surface charge [cf Eqs (5) and (10)] and the rates correspondingly increase with pH or charge. The actual location of the deprotonations is unknown, but a likely scenario is that ri -OH2 sites on monomolecular steps deprotonate with increases in pH. 

The ubiquitous chelates of 1,10-phenanthroline were studied some time ago for their antiviral activity [17]. The inhibitory effect of various chelates on the multiplication of influenza virus (Melbourne strain) in chick chorioallantoic membrane in vitro was studied, and the most effective complex was shown to be the [Ru(acac) (3,5,6,8-Me4-l,10-phen)2] cation, which inhibited multiplication at concentrations of 6 X 10 M(—log M = 6.2). Other chelate complexes were active at concentrations of 10 to 10 M. A structure—activity study of metal chelates showed, however, that for a given chelate ligand, more labile complexes such as those of Cd or Cu were more active than their inert counterparts, the order for doubly-charged (2+) chelates being Cd(II) > Cu(II) > Zn(II) > Mn(II) > Fe(II) > Co(II) > Ni(II) > Ru(II) [18]. This correlation coincided with that found for some antibacterial effects (Chapter 9). Further studies showed that the virostatic activity may be manifested by either direct inactivation of the virus (possibly through dissociation of the more labile chelates) or by direct action on the host cell (for inert complexes). The latter effect is indicated by the fact that the trend in virostatic activity is similar to that in antitumour activity [19] (see also Chapter 6) and the fact that, of the various 1,10-phenanthrolines studied, the tetramethyl derivatives most easily penetrate cells [20]. 

In the case of other systems in which one or both of the reactants is labile, no such generalization can be made. The rates of these reactions are uninformative, and rate constants for outer-sphere reactions range from 10 to 10 sec b No information about mechanism is directly obtained from the rate constant or the rate equation. If the reaction involves two inert centers, and there is no evidence for the transfer of ligands in the redox reaction, it is probably an outer-sphere process. 

There is a large amount of data available concerning the thermodynamic effects of ligands on other coordination sites (i. e., the thermodynamic cis- and iraws-effects). However, very little is known about the effects of ligands on the kinetic lability of other coordination sites. In fact, very little work has been carried out, directly with Bi2-derivatives, or with models of B12, on the kinetics of ligand substitution at the cobalt center. Of particular biochemical interest would be studies on the rate of displacement of coordinated benzimidazole by various ligands. Such work has not been reported at present. If the benzimidazole is replaced during enzymatic catalysis so that the lower axial position is occupied by some other Lewis base, one would expect this displacement, and the reverse step, to be very facile. This appears to be qualitatively true in that when water displaces benzimidazole as the benzimidazole is pro-... 

Bipyridines were efficiently used in supramolecular chemistry [104], Since the molecule is symmetric no directed coupling procedure is possible. In addition, 2,2 6/,2//-terpyridine ligands can lead to several metal complexes, usually bis-complexes having octahedral coordination geometries [105,106], Lifetimes of the metal-polymeric ligand depend to a great extent on the metal ion used. Highly labile complexes as well as inert metal complexes have been reported. The latter case is very important since the complexes can be treated as conventional polymers, while the supramolecular interaction remains present as a dormant switch. 

It should be noted that the basic reactions used to prepare phthalocyanine derivatives today are fundamentally those developed by Linstead and coworkers in the 1930s [52-54]. Due to the large number of substituted phthalocyanines described in the literature, space limitations mean that a detailed review of synthetic aspects cannot be provided here. The following discussion is concerned with the synthesis of lanthanide phthalocyanines via (i) template tetramerization of phthalonitrile with lanthanide salts, (ii) direct metalation of the metal-free ligands by the salts or (iii) metal exchange of a labile metal ion or ions for a lanthanide. 

In addition to the discussed cyclotetramerizations, direct metallation of the metal-free ligands or metal exchange of a labile metal ion or ions for one held more robustly, the desired complexes may be prepared by the direct substitution, exchange or modification of substituents on preformed phthalocyanine derivatives. However, a review of works carried out on these types of transformations lies out of the scope of this chapter. 

It was found by Trost that the low reactivity could be circumvented by the employment of labile ligands, such as the propionitrile in the Mo(CO)3(EtCN)3 precatalyst [57]. Instead of directly transferring this procedure to microwave heating applications, a useful and easily handled microwave procedure was developed for rapid and selective molybdenum-catalyzed allylic alkylations under noninert conditions (Eq. 11.39) [12]. The former, more sensitive, two-step reaction was fine-tuned into a robust one-step procedure employing the inexpensive and stable precatalyst Mo(CO)6, used in low concentrations. The alkylations were conducted in air and resulted in complete conversions, high yields, and an impressive enantiomeric excess (98%) in only 5-6 min. Despite the daunting temperatures, up to 250°C with THF... 

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