Sodium hexachloroiridate

The quantity of sodium hexachloroiridate(IV), obtained from the complete consumption of 1.7 g. (8.85 mmoles) iridium metal in the manner described elsewhere,4 or 5.0 g. of the commercial hexahydrate is dissolved in 100 ml. of ethanol and the solution immediately filtered. A solution of 1.4 g. (9.4 mmoles) of sodium iodide in 50 ml. ethanol is then added to the solution. The fast reduction of the iridate(IV) ion, [IrCl6]2-, to iridate-(III), [IrCh]3-, by iodide ions is a well-known reaction 5 when it is carried out under these conditions, the sodium hexachloro-iridate(III) precipitates almost quantitatively and can easily be filtered, washed with ethanol, and dried at 100°. 

Hence, the first clearcut evidence for the involvement of enol radical cations in ketone oxidation reactions was provided by Henry [109] and Littler [110,112]. From kinetic results and product studies it was concluded that in the oxidation of cyclohexanone using the outer-sphere one-electron oxidants, tris-substituted 2,2 -bipyridyl or 1,10-phenanthroline complexes of iron(III) and ruthenium(III) or sodium hexachloroiridate(IV) (IrCI), the cyclohexenol radical cation (65" ) is formed, which rapidly deprotonates to the a-carbonyl radical 66. An upper limit for the deuterium isotope effect in the oxidation step (k /kjy < 2) suggests that electron transfer from the enol to the metal complex occurs prior to the loss of the proton [109]. In the reaction with the ruthenium(III) salt, four main products were formed 2-hydroxycyclohexanone (67), cyclohexenone, cyclopen tanecarboxylic acid and 1,2-cyclohexanedione, whereas oxidation with IrCl afforded 2-chlorocyclohexanone in almost quantitative yield. Similarly, enol radical cations can be invoked in the oxidation reactions of aliphatic ketones with the substitution inert dodecatungstocobaltate(III), CoW,20 o complex [169]. Unfortunately, these results have never been linked to the general concept of inversion of stability order of enol/ketone systems (Sect. 2) and thus have never received wide attention. 

Ammonium hexachloroiridate(IV) can be prepared by the metathesis of sodium hexachloroiridate(IV) with ammonium chloride or by the addition of ammonium chloride to either a solution of iridium(IV) hydroxide in hydrochloric acid or of iridium(IV) chloride. Although many workers have prepared sodium hexachloroiridate(IV) by chlorinating mixtures of sodium chloride and iridium, few exact data are available concerning optimum temperatures, reaction times, and yields. 

Among the hexachloroiridates, the compounds of tripositive iridium are more stable than those of the tetrapositive metal. Wohler and Balz report that in a stream of chlorine, sodium hexachloroiridate(IV) be ns to decompose into hexachloroiridate(III) at 570°. At 750° the product is primarily sodium hexachloroiridate(III), which in turn decomposes into metallic iridium above 900°. The chlorine content of the residue was found to be dependent upon the temperature. That the reaction proceeds stepwise has been shown in a more recent study by Puche. Typical results of chlorination studies of mi.xtures of sodium chloride (4.00 g.) and iridium (2.00 g.) are as follows ... 

Temperatures of 450 to 560°, i.e., below the dissociation temperature of sodium hexachloroiridate(IV), were found to be unsatisfactory because of the slow rate of conversion. A temperature of about 625° seems optimum. The product in this case consists mainly of sodium hexachloroiridate(IV) as well as hexachloroiridate(III), unchanged sodium chloride, and unchanged iridium. Sodium hexachloroiridate-... 

The phosphine combines with cobalt(II) dihalides to give the 5-coordinate derivatives, LXXVIII, but these do not undergo ionization in organic solvents. Rhodium (III) trihalides give normal 6-coordinate compounds, LXXIX, whereas sodium hexachloroiridate(IV), Na2lrCl6,... 

The use of N2H4 in the synthesis of sodium hexachloroiridate(III), Na3[IrCl6l, from Na2[IrCl6] has been investigated, Eq. 9.48 64... 

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