8-Hydroxyquinoline, metal complexes

Research progress of 8-hydroxyquinoline metal complexes in electroluminescent devices 06CJO783. 

JIN 13b] Jin X., Yu X., Zhang W. et al., Synthesis and photovoltaic properties of main chain polymeric metal complexes containing 8-hydroxyquinoline metal complexes conjugating alkyl fluorene or alkoxy benzene by C=N bridge for dye-sensitized solar cells , Polymer Composites, vol. 34, no. 10, pp. 1629-1639, 2013. 

Hydroxyquinoline (oxine, 8-quinolinol) [148-24-3] M 145.2, m 71-73 , 75-76 , 76 , b 267 pKj 4.91, pK 9.81. Crystd from hot EtOH, acetone, pet ether (b 60-80 ) or water. Crude oxine can be purified by pptn of copper oxinate, followed by liberation of free oxine with H2S or by steam distn after acidification with H2SO4. Stored in the dark. Forms metal complexes. [Manske et al. Can J Research 27F 359 1949 Phillips Chem Rev 56 271 1956.]... 

Soroka K, Vithanage RS, Phillips DA et al (1987) Fluorescence properties of metal complexes of 8-hydroxyquinoline-5-sulfonic acid and chromatographic applications. Anal Chem 59 629-636... 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

General Order of Rate Constants. The rate constants of electrophilic reactions of aromatic ligands and their metal complexes fall in the order fo, > kML > kffL. The difference between these rate constants becomes greater as the activity of the attacking reagent decreases. When L is a phenolate, HL is the phenol when L is an amine, HL is the corresponding ammonium derivative. The possible synthetic applications of this sequence can be appreciated from the fact that 8-hydroxyquinoline is usually sulfonated with 15 to 30% oleum, while its copper (II) complex can be readily sulfonated in 70% sulfuric add (5). 

These equations show the general theoretical basis for the empirical order of rate constants given earlier for electrophilic attack on an aromatic ligand L, its metal complex ML, and its protonated form HL, one finds kt > n > hl. Conflicting reports in the literature state that coordination can both accelerate electrophilic aromatic substitution (30) and slow it down enormously (2). In the first case the rates of nitration of the diprotonated form of 0-phenanthroline and its Co(III) and Fe(III) complexes were compared. Here coordination prevents protonation in the mixed acid medium used for nitration and kML > h2l. In the second case the phenolate form of 8-hydroxyquinoline-5-sulfonic acid and its metal chelates were compared. The complexes underwent iodination much more slowly, if at all, and kL > kML ... 

Considerable activity has been concerned with polymeric attachment of the 8-hydroxyquinoline heterocycle because of its excellent metal complexing ability. A comprehensive examination of attachment of the group to a variety of polymers has revealed that the overall capacity for a variety transition metals depends not only on the 8-hydroxyquinoline content but also on rather ill-defined factors introduced by the polymer modified (73MI11105). 

Metal complexes, analogous to 8-hydroxyquinoline,54,151 of 4-hydroxy-1,5-naphthyridine,54 8-hydroxy-1,6-naphthyridine,54 151 and 8-hydroxy-1,7-naphthyridine54 with Cu(II), Ni(II), Fe(II), and Fe(III) have been prepared. The hydroxynaphthyridines chelate less readily than 8-hydroxyquinoline since they are weaker bases.151... 

Terminally metallizable dyes (30) are obtained by the interaction of a diazonium salt and a coupling component containing a chelating system, for example salicylic acid, catechol, salicyl-aldoxime or 8-hydroxyquinoline, and their coordination chemistry is typical of these compounds. Such dyes were rarely used as preformed metal complexes but were usually applied to cotton and then converted to their copper complexes on the fibre to improve their fastness to wet treatments. A typical example is the blue dyestuff (31). 

Table 19 Rate Constants, Activation Parameters and Formation Constants for the Metal Complexes of Methyl 8-Hydroxyquinoline-2-carboxylate ...

Although they do not appear to have found application, lanthanide metal complexes of 8-hydroxyquinoline, 2-picolinic acid and catechol have been found to exhibit fungicidal properties.155... 

Additionally to 8-hydroxyquinoline, forming complex compounds of the type 445, convenient ligands for the synthesis of five-member metal-cycles are arylidene-o-aminophenols 610. Their interaction (3.46) with metal acetates in methanol results in complexes 611 [53-56] ... 

In North America, Chapman Chemical Co. is promoting different metal complexes of 8-hydroxyquinoline (9), but the acceptance of these formulations is slow. 

With regard to stabilization, perhaps the most thoroughly investigated aspect of metallo-organic chemistry is the evaluation of stability constants. For the present discussion, a stability constant can be considered as analogous to the reciprocal of the dissociation constants of acids and bases. Thus, if L represents the 8-hydroxyquinoline anion and ML2 the metal complex, then ... 

Curves which are of the same form as curve B have been obtained for the diazo-coupling (52) and iodination (, 7,13) of 8-hydroxyquinoline-5-sulfonic acid and its metal complexes and for the oxidation of glucose and its boric acid complex with ferricyanide (50). 

An alternative and often facile route to appropriately functionalised ICPs, that avoids the synthetic problems outlined in (ii) above, is the use of sulfonated species containing the desired molecular recognition/receptor site as the dopant anion for the conducting polymer chains. For example, calixarene-containing polypyrroles [34] and polyanilines [35] for selective metal ion detection have recently been prepared via the use of sulfonated calixarenes as dopant anions. We have similarly found that the incorporation of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants in polypyrroles provides a simple route to metal ion-selective ICPs [36]. 

Examples of both types can be seen in the hydrolysis of carboxylic esters of 8-hydroxyquinoline. The hydrolysis of 8-acetoxyquinoline (44) is promoted by metal ions, but as the formation constants for metal complex formation are rather low, the system is not very amenable to detailed kinetic analysis. The derivative 8-acetoxyquinoline-2-carboxylic acid (45) with an additional bonding site complexes strongly to a variety of metal ions and detailed kinetic studies are possible. In the pH range 5-12 the hydrolysis of the stubstrate (HA) follows the rate law, rate = fco[A ]+koH[A ][OH ], where A is the anion, with ko= 1-67 x 10 s and koH = 0-84 s at 25 °C. The 1 1 metal complexes of the ester MA" (M = Zn and Cu")... 

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