Hydroxo complexes, inert

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

Hydroxo complexes containing cis-bidentate tertiary diphosphines, such as Pt(OH)R(dppe) (R = CH3, C6H9) or Pt(OH)(CH3)(dppp) (dppp = l,3-bis(diphenylphosphino)propane) are inert towards methanol at room temperature. It seems likely that the equilibrium shown in Equation 13 strongly favors the hydroxo complex, but that rapid decomposition of the methoxide in the trans series... 

At 104 mM chloride ion, typical of the plasma, for both cA-DDP and its trans-isomer, the species distribution as a function of pH is similar to that depicted earlier with the dichloro and chloro-hydroxo complexes being the dominant species at pH 7.4 [18]. Both dominant species are relatively inert kinet-ically. This reference also shows a plot of mole fraction vs. chloride-ion concentration at pH 7.0. Only at lower chloride-ion concentrations do the more reactive species containing an aqua ligand appear to a significant extent. 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

In 1995, Beletskaya [34] and Collum [35] reported independently the application of alkyltrichlorostannanes instead of tetraorganotin compounds, overcoming the disadvantage of three inert anchoring groups ( atom economy ) and technologically more important, because of their lower toxicity and availability via economic direct synthesis from tin(II) compounds [36], Furthermore, the hydrolysis of the tin-halide bond in water results in higher water-solubility, activation of the C—Sn bond toward electrophiles (e.g., in transmetallation) and less toxic by-products. The reaction may be accomplished via intermediate anionic hydroxo complexes [37], produced in situ in aqueous alkaline solution, and proceeds in most cases in 3 h at 90-100°C (Eq. 12). 

Dissolution of the bronze colored crystals of 4 (X = Cl, Br) in water yields a mixture of the diamagnetic platinum(II) and platinum(IV) materials outlined in Eq. 42. Recent data (559) suggest a more complex equilibrium is present than in Eq. 42. Preliminary evidence suggests that the rra s-Pt(CN)4X(OH)2- ion is formed upon dissolution of 4, Fig. 22 (364a, 559). This is consistent with the formation of this hydroxo complex in the aquation and subsequent hydrolysis reaction of I/ a j-Pt(CN)4X2 (X = Cl, Br) (86) and the note that Pt(II) complexes catalyze the substitution reactions of formally inert Pt(IV) complexes (34). Addition of 0.1 Af KX suppresses these interfering reactions. Fig. 22. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

The kinetics in the basic media are summarised in the table. Since the rate is not affected by the buffer under due conditions, the reaction mechanism seems to involve essentially that of hydroxo complexes except for thallium, and the attack of hydroxide ions upon the complex appears to play an important role. The fact that the rate of the indium complex is proportional to the square of the hydroxide ion concentration tells that indium is the most inert among the three. Such an inertness of indium complexes may be reflected in the small value of its apparent dissociation constant of the coordinated water molecule, K. ... 

Tracer 0 studies have established that base hydrolysis of coordinated acetyl phosphate in the complex [(NH3)5Co-OP03COCH3] [ifcoH = 0.53 M s at 25°C and I = l.OM (NaC104)] occurs by exclusive carbon-oxygen bond fission. The hydrolysis of the acetyl phenyl phosphate monoanion is significantly catalyzed by the exchange-inert hydroxo complex [(NH3)sCoOH] ( MOH = 2.9 X 10 M s at 25 C) which operates by... 

Cisplatin diaqua is very reactive, but the deprotonated hydroxo forms are usually considered to be relatively inert, therefore the acidity of the coordinated water molecules in aqua complexes can be directly relevant to their reactivity with target molecules. The pKa values of some Pt-aqua complexes are listed in Table II. 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

The active Pt(IV) compounds are octahedrally coordinated and possess axial bound chloride or—to improve the solubility—hydroxo ligands, i.e., two Y ligands in the trans orientation. These compounds are far more inert than the corresponding Pt(II) compounds that lack these axial ligands. Most likely the Pt(IV) compexes are reduced in vivo to the corresponding Pt(II) complexes, which are in fact the active species (17-19). They can therefore be considered as a type of prodrug that requires in vivo activation (substitution and reduction) to the square-planar Pt(II) compounds to exhibit antineoplastic activity. This hypothesis is supported by the observation that platinum(IV) compounds are unable to react with DNA under ambient conditions (19), and that appreciable amounts of Pt(II) derivatives can be detected in the urine of Pt(IV)-treated patients(fS). 

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