pH-Selective Hydrogenation of Water-Soluble Carbonyl Compounds and Alkenes with [Cp*Ir<SUP>III</SUP>(H<INF>2</INF>O)<INF>3</INF>]<SUP>2+</SUP> (Cp* = η<SUP>5</SUP>-C<INF>5</INF>Me<INF>5</INF>) as a Catalyst Precursor in Very Acidic Media

The organometallic aqua complex [Cp*Ir^III(H<INF>2</INF>O)<INF>3</INF>]²⁺ (<BO>1</BO>) acts as a catalyst precursor for hydrogenation of water-soluble carbonyl compounds and alkenes in water (5−20 mM of <BO>1</BO>, 100 mM of the substrates) in a pH range of about −1 to 4 under H<INF>2</INF> at a pressure of 0.1−0.7 MPa at 25 °C. The solution pH was adjusted by using 0.1−10 M HClO<INF>4</INF>/H<INF>2</INF>O and 0.1 M NaOH/H<INF>2</INF>O. The aqueous hydrogenation shows unique pH-selectivity to be governed by the following three factors:  (i) pH-dependent structural change of the catalyst precursor <BO>1</BO>, which is deprotonated to form a catalytically inactive dinuclear complex, [(Cp*Ir^III)<INF>2</INF>(μ-OH)<INF>3</INF>]⁺ (<BO>2</BO>), above pH 4, (ii) stability of a putative iridium hydride active catalyst depending upon pH, and (iii) difference in the proton affinity of the substrates, i.e., Lewis basicity of the carbonyl oxygen atoms of the carbonyl compounds and the C&dbd;C moieties of the alkenes. Turnover frequencies of the hydrogenations are also discussed on the basis of Lewis acidity of the carbocations that accept a hydride ion from the active catalyst.