The effect of pH on the reactions of catalytically important Rh-I complexes in aqueous solution: Reaction of [RhCl(tppms)(3)] and trans-[RhCl(CO)(tppms)(2)] with hydrogen (TPPMS = mono-sulfonated triphenylphosphine)

Hydrolysis and hydrogenation of [RhCl(tppms)(3)] (1) and trans-[RhCl(CO)(tp pms(2)] (2) was studied in aqueous solutions in a wide pH range (2 < pH < 1 1) in the presence of excess TPPMS (3 -diphenylphosphinyl-benzenesulfonic a cid sodium salt). In acidic solutions hydrogenation of 1 yields a mixture o f cis-mer- and cis;fac-[RhCIH2(tppms)(3)] (3a,b) while in strongly basic so lutions [RhH(H2O)(tppms)(3)] (4) is obtained, the midpoint of the equilibri um between these hydride species being at pH 8.2. The paper gives the first successful H-1 and P-31 NMR spectroscopic characterization of a water solu ble rhodium(I)-monohydride (4) bearing only monodentate phosphine ligands. Hydrolysis of 2 is negligible below pH 9 and its hydrogenation results in f ormation of [Rh(CO)H(tppms)(3)] (5), which is an analogue to the well known and industrially used hydroformylation catalyst [Rh(CO)H(tppts)(3)] (6) (T PPTS = 3,3',3"-phosphinetriyltris(benzenesul-fonic acid) trisodium salt). I t was shown by pH-potentiometric measurements that formation of 5 is strong ly pH dependent in the PH 5-9 range; this gives an explanation for the obse rved but previously unexplained pH dependence of several hydroformylation r eactions. Conversely, the effect of pH on the rate of hydrogenation of male ic and fumaric acid catalyzed by 1 in the 2< pH < 7 range can be adequately described by considering solely the changes in the ionization state of the se substrates. All these results warrant the use of buffered (pH-controlled ) solutions for aqueous organometallic catalysis.