Abstract
New aqua-soluble rhodium(I) [Rh(CO)(PTA)4]Cl (1) (PTA = 1,3,5-triaza-7-phosphaadamantane) and rhodium(III) [RhCl2(PTA)4]Cl (2) complexes have been synthesized via the reaction of [{Rh(CO)2(μ-Cl)}2] or RhCl3·3H2O, respectively, with stoichiometric amounts of PTA in ethanol. Compound 1 is also obtained upon reduction of 2 in an H2/CO atmosphere. They have been characterized by IR, (1)H and (31)P{H} NMR spectroscopies, elemental and single crystal X-ray diffraction analyses. While compound 1 shows distorted square-pyramid geometry (τ5 = 0.09) with a P3C-type basal plane, compound 2 is octahedral with the chloro ligands in the cis position. The hydride rhodium(I) complex [RhH(PTA)4] (3) is formed upon the addition of NaBH4 to an aqueous solution of 1 or 2. Compounds 1-3 (in the case of 2 upon reduction by H2) act as homogeneous catalysts, or catalyst precursors, in the isomerisation and condensation of allyl alcohol at room temperature and in an aqueous medium. The product selectivity is easily controlled by changing the concentration of the base in the reaction mixture, thus resulting in the exclusive formation of either 3-hydroxy-2-methylpentanal (HP) or 2-methyl-2-pentenal (MP) in quantitative yields.
Highlights
IntroductionRh and Ru complexes with PTA and its N-alkylated derivatives have been extensively studied, finding their wide application as efficient and versatile catalysts for the hydrogenation, hydroformylation and isomerisation of unsaturated organic substrates in aqueous media
The study reports a suitable way for the syntheses of new water-soluble rhodium complexes of the types [Rh(CO)(PTA)4]Cl (1), [RhCl2(PTA)4]Cl (2) and [RhH(PTA)4] (3), which is based on the reaction of [{Rh(CO)2(μ-Cl)}2] or RhCl3 with 1,3,5-triaza7-phosphaadamantane (PTA)
The solubility and stability in water of the obtained complexes encouraged their applications in aqueous catalysis
Summary
Rh and Ru complexes with PTA and its N-alkylated derivatives have been extensively studied, finding their wide application as efficient and versatile catalysts for the hydrogenation, hydroformylation and isomerisation of unsaturated organic substrates in aqueous media.. In the case of hydrogenation reactions, it was shown that rhodium complexes bearing more than three PTA ligands in the coordination sphere are much less active than those having only two PTA moieties.. In the presence of an excess of PTA, the latter complex is completely inactive due to the formation of the coordinatively saturated Rhcompound [RhCl(PTA-H)3(PTA)]Cl3.6 On the other hand, rhodium(I) complexes of the type [RhH(PTA-R)4]I4 (R = Me, Et) that bear the N-alkylated PTA cationic derivatives (PTA-R)+ instead of neutral PTA are efficient catalysts for CvC bond reduction. Rhodium(I) acetylacetonato complexes with PTA, P(CH2CH2CN) and P(m-C6H4SO3Na) ligands have been studied in the hydrogenation of allyl alcohol to n-propanol (Scheme 1). In the presence of these catalysts, the alcohol undergoes isomerisation to enol before its hydrogenation to n-propanol.
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