Water-soluble organometallic compounds. 9. Catalytic hydrogenation and selective isomerization of olefins by water-soluble analogues of Vaska's complex
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Water-soluble analogues of Vaska's complex, trans-[IrCl(CO)(PPh3)2], have been prepared using the water-soluble phosphine ligands TPPMS and (1,3,5-triaza-7-phosphaadamantane) PTA. The structural parameters in trans-[IrCl(CO)(TPPMS)], where the sodium cations are encapsulated with kryptofix-221, closely resemble those found in the parent complex, as revealed by X-ray crystallography. 13C and 31P NMR of the PTA derivative demonstrate the trans arrangement for phosphine ligands in this derivative as well. The oxygen adduct [(O2)-IrCl(CO)(TPPMS)2] has been isolated and identified by infrared spectroscopy (CO = 2012 cm-1 and O2 = 854 cm-1) and 31P NMR ( 12.8 ppm). The solution behavior of trans-[IrCl-(CO)(TPPMS)2] (1) in water is markedly different from that of Vaska's complex in organic solvent; that is, reactions with O2 and H2 are irreversible due to formation of the strongly hydrated proton and chloride ions produced during these processes. Importantly, complex 1 has been shown to be an active catalyst for the hydrogenation of olefinic double bonds in short-chain unsaturated acids in aqueous solution. Included in these studies were crotonic, maleic, fumaric, and -acetamidocinnamic acids. The turnover frequency for the hydrogenation of maleic acid in water was significantly greater employing 1 as a catalyst than the comparable process involving Vaska's complex in dimethylacetamide at a much higher temperature. In addition complex 1 was demonstrated to be an effective catalyst for both hydrogenation and isomerization of unsaturated fatty acids in soybean lecithin. More significantly was the observation that cis-trans isomerization was selective over hydrogenation in these liposomes; for example, oleic acid was isomerized to elaidic acid with little hydrogenation.