Tetraphenylarsonium Cyanate and Cyanide

The tetraphenylarsonium cation is useful in coordination chemistry because of its large size. Tetraphenylarsonium cyanate dihydrate was prepared byNor-buryand Sinhain 1968. The preparation involved precipitation of the cyanate from an aqueous solution, but the method suffers from the disadvantage of low yields. Tetraphenylarsonium cyanide has been prepared from methanol solution with fairly good yields, but the product is isolated as the monohydrate. The ion-exchange method developed for the tetraalkylammonium salts can be readily applied to the preparation of anhydrous tetraphenylarsonium cyanate and cyanide, thus eliminating many of the problems initially involved in the synthesis of these componds. 

A 2- by 60-cm column terminating in a Teflon stopcock is packed with 50 g of IRA-400 resin and then charged with 50 g (0.61 mole) of potassium cyanate dissolved in 450 ml of 50 % aqueous methanol. A flow rate of 2 drops/ sec is employed during the charging process. After the charging is complete, the column is rendered anhydrous by rinsing it with 550 ml of anhydrous methanol, using an increased flow rate. 

A solution of tetraphenylarsonium bromide is prepared by dissolving 5 g (0.011 mole) of the bromide in 20 ml of anhydrous methanol. This solution is placed on the column and eluted to a total volume of 220 ml. Again, the flow rate from the column should not exceed 2 drops/sec. The eluate is evaporated to dryness under dynamic vacuum (10 torr) at ambient temperature. The yield, based on tetraphenylarsonium bromide, is 4.31 g (94%). If necessary, recrystallization is accomplished by dissolving the crude product in 20 ml of acetonitrile, followed by precipitation of the pure product by the addition of 75 ml of ethyl acetate. The cyanate is isolated by filtration, and the product is dried under dynamic vacuum at ambient temperature for 24 hr. Both recrystallization and filtration steps should be performed in a dry-box. Anal. Calcd. for C25H2oAsNO C, 70.59 H, 4.75 N, 3.29 As, 17.61. Found C, 70.3 H, 4.7 N, 3.2 As, 17.6. 

Anhydrous tetraphenylarsonium cyanate is a white solid which is hygroscopic and must be stored in a dry inert atmosphere. It is soluble in ethanol, methanol, and acetonitrile. The compound melts, with decomposition, at ca. 224°. The infrared spectrum, taken as a Nujol mull, exhibits the following bands due to the cyanate fundamental absorptions (incm-i) v(As) at 2140 (s) and d at 622 (s). This spectrum agrees well with a recent spectral study of this compound. Again, as with tetraethylammonium cyanate, Fermi resonance is exhibited in the spectrum of this compound at 1280 (m) and 1192 (m) cm-i. 

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