Attila Cs. Bényei

Solution pH: A Selectivity Switch in Aqueous Organometallic Catalysis—Hydrogenation of Unsaturated Aldehydes Catalyzed by Sulfonatophenylphosphane–Ru Complexes

The equilibribrium distribution of water-soluble ruthenium hydrides [HRuCl(tppms)3 ] and [H2 Ru(tppms)4 ] (tppms = (3-sulfonatophenyl)diphenylphosphane) in the reaction with H2 is governed by the pH value. As a consequence, the selectivity in the hydrogenation of cinnamaldehyde for reaction at C=C or C=O can be completely inverted by changing the pH value (see drawing below).

The effects of pH on the molecular distribution of water soluble ruthenium(II) hydrides and its consequences on the selectivity of the catalytic hydrogenation of unsaturated aldehydes

Abstract The effect of pH on the formation and equilibrium distribution of the water soluble ruthenium hydrides [HRuCl(TPPMS) 2 ] 2 , [HRuCl(TPPMS) 3 ] and [H 2 Ru(TPPMS) 4 ] (TPPMS=(3-sulfonatophenyl)diphenylphosphine sodium salt) was studied in aqueous solution by pH-potentiometric and 1 H and 31 P NMR methods. Depending on the pH, [RuCl 2 (TPPMS) 2 ] 2 and its hydrido-derivatives hydrolyse extensively, giving rise to formation of hydroxo-ruthenium complexes. It was established that at pH≤3.3 the dominant ruthenium(II) species was [HRuCl(TPPMS) 3 ], while at pH≥7 it was [H 2 Ru(TPPMS) 4 ]. While [HRuCl(TPPMS) 3 ] catalyzed the slow, selective hydrogenation of the CC bond in trans -cinnamaldehyde, [H 2 Ru(TPPMS) 4 ] was found an active and selective catalyst for CO reduction. Consequently, the selectivity of the hydrogenation of trans -cinnamaldehyde could be completely inverted by minor changes in the solution pH, shifting the equilibrium between [HRuCl(TPPMS) 3 ] and [H 2 Ru(TPPMS) 4 ].

Aqueous organometallic chemistry: the mechanism of catalytic hydrogenations with chlorotris(1,3,5-triaza-7-phosphaadamantane) rhodium(I)

Abstract The water-soluble phosphine complex of Rh(I), RhCl(PTA) 3 ( 1 ) was shown to be an active catalyst for the hydrogenation of various olefinic and oxo-acids, as well as of allyl alcohol and 4-sulfostyrene in aqueous solution under mild conditions. Detailed kinetic investigations were carried out with crotonic acid and allyl alcohol as substrates. The rate of hydrogenation of both compounds showed a sharp maximum as a function of pH at 4.7. Hydrogenation of itaconic, crotonic and α-acetamidocinnamic acid in D 2 O led to 45–100% deuteration of the products with 25–100% stereoselectivity towards the α-carbon atom. These results, together with those of pH-static hydrogenation of complex 1 , suggest that water strongly assists the dehydrochlorination of 1 to yield the catalytically active monohydrido species HRh(PTA) 3 ( 2 ). Nevertheless, depending on the substrate and the pH of the solution the dihydridic pathway may remain partially operative.

Der pH‐Wert: ein Selektivitätsschalter für die Metallkomplex‐Katalyse in wäßrigen Lösungen – die Sulfonatophenylphosphan‐Ru‐ Komplex‐katalysierte Hydrierung ungesättigter Aldehyde

Die Gleichgewichtsverteilung der wasserloslichen Rutheniumhydride [HRuCl(tppms)3] und [H2Ru(tppms)4] (tppms = (3-Sulfonatophenyl)diphenylphosphan) bei der Umsetzung mit H2 hangt vom pH-Wert ab. Damit kann die Selektivitat der Hydrierung von Zimtaldehyd durch Verandern des pH-Wertes zugunsten der Reduktion an der C = O- oder an der C = C-Gruppe eingestellt werden (siehe unten).

The effect of phosphonium salt formation on the kinetics of homogeneous hydrogenations in water utilizing a rhodium meta-sulfonatophenyl-diphenylphosphine complex

Abstract Reaction of mono-sulfonated triphenylphosphine in water with certain activated olefins (for example maleic acid or fumaric acid) yields the corresponding phosphonium salt under very mild conditions. This interaction promotes the formation of the catalytically active species in the hydrogenation of the olefin catalyzed by the rhodium complex of the phosphine.

Transfer hydrodehalogenation of organic halides catalyzed by water soluble ruthenium(II) phosphine complexes

Abstract C–X bonds in carbon tetrachloride, chloroform and 1-hexyl halogenides were transformed into C–H bonds in a transfer hydrodehalogenation reaction catalyzed by water soluble ruthenium(II) phosphine complexes. A turnover frequency of up to 1000 h−1 can be achieved when an aqueous solution of sodium formate is the hydrogen donor and RuCl2(TPPMS)2 (1) the catalyst (TPPMS=m-sulfophenyl diphenylphosphine sodium salt).

Synthesis and X-ray diffraction structures of novel half-sandwich Os(ii)-and Ru(ii)-hydroxamate complexes

Novel water soluble half-sandwich complexes of the general formulae [M(η6-p-cym)(ha)]2(CF3SO3)2, [M(η6-p-cym)(ha)Cl] or [M(η6-p-cym)(ha)(py)]X (M = Os, Ru; ha = hydroxamate; py = pyridine; X = Cl− or CF3SO3−), incorporating metal-containing entities and hydroxamates both with potential anti-proliferative features, were prepared and characterized by elemental analysis, spectroscopy (NMR, IR) and ESI mass spectrometry. The X-ray crystal structure of [Ru(η6-p-cym)(μ-meaha)]2(CF3SO3)2 (5), [Os(η6-p-cym)(meaha)Cl] (6), [Ru(η6-p-cym)(phebha)Cl], (9), [Ru(η6-p-cym)(bha)(py)](CF3SO3) (12) and [Ru(η6-p-cym)(phebha)(py)](CF3SO3) (14), 6 is the first published structure of an organometallic Os(II)-hydroxamate reported. The effect of size differences of the metal ions, the steric demand of the RC and RN substituents at the hydroxamate group and the type of the monodentate ligand co-present in the stoichiometry, along with the binding architecture of the half-sandwich metal(II) hydroxamate complexes are discussed. A novel dinuclear, dihydroxo bridged complex [Os(η6-p-cym)(py)(μ-OH)]2(CF3SO3)2 (16) is prepared and characterized by X-ray crystallography. Unexpected formation of a dinuclear oxo bridged OsII/OsVI complex [{Os(η6-p-cym)(meaha)}(μ-O){Os(O)(meaha)2}]Cl (17) occurs, and the crystal and molecular structure has been determined by X-ray method. Complexes 1, 5–8, 10 and 14 were tested for their in vitro cytotoxicity, using human-derived ovarian cancer cell lines (A2780 and A2780 cisR), and showed no anti-proliferative effect in the concentration range (0–200 μM) studied.

Interaction of [Ru(η6-p-cym)(H2O) 3]2+ with citrate and tricarballate ions in aqueous solution; X-ray crystal structure of novel half-sandwich Ru(II)-citrato complexes

The interaction between [Ru(η(6)-p-cym)(H(2)O)(3)](2+) and an important low molecular weight serum component, citric acid (citrH(3)), was studied with the aid of combined pH-potentiometric, (1)H NMR, (13)C NMR and electrospray ionization mass spectrometry (ESI-MS) methods in aqueous solution. For comparative purposes propane-1,2,3-tricarboxylic acid (tricarballylic acid, tricH(3)) having no alcoholic-OH group in position 2 was also investigated. Stoichiometries, stability constants and the most plausible solution structures of the complexes formed in the systems were determined. Depending upon the pH, citrate was found to coordinate to the metal ion via [COO(-), COO(-), OH] or [COO(-), COO(-), O(-)] fashion yielding mononuclear complexes with high stability. As a consequence at physiological pH the hydrolysis of the metal ion is completely hindered even at 1:1 metal to ligand ratio. Crucial role of the alcoholic/alcoholate function of the citric acid in [Ru(η(6)-p-cym)(H(2)O)(3)](2+) binding is reflected in the low stability of the species formed with tricarballylic acid. The X-ray crystal structures of [Ru(η(6)-p-cym)(citrH)]·H(2)O·CH(3)OH and 2[Ru(η(6)-p-cym)(citrH)]·3H(2)O being the first published structures of an organometallic Ru(II)-citrate and both featuring a [COO(-), COO(-), OH] coordinated ligand, are also reported.

AQUEOUS ORGANOMETALLIC CHEMISTRY. EFFECTS OF A REACTIVE SOLVENT

In aqueous organometallic chemistry water is often considered solely as a solvent. However, there are several examples when such an assumption is not justified [1]. Water is a strongly coordinating, polar solvent, capable of assisting reactions yielding polar or charged (ionic) products. H2O itself can oxidatively add to a low valent transition metal complex with formation of hydrido-hydroxometal complex compounds. Hydroxometal species are also formed by replacement of anionic ligands (e.g. halides) by OH- (metal ion hydrolysis equilibria). Dissolved salts strongly influence reaction mechanisms involving polar or charged transition states and intermediates. In addition acid/base catalytic reactions of the substrates or products effect rates and selectivities. All these reactions are influenced by the pH which should be regarded of paramount importance in aqueous organometallic catalysis. Presence of a separate water phase in aqueous/organic biphasic reactions may have important consequences on the reaction mechanism due to the separation of reactants, products and intermediates (physical or solubility effects). This paper focusses on a few selected examples of such effects from our research on reactions of or catalyzed by platinum metal complexes with the water soluble phosphines: m-sulfonatophenyldiphenylphosphine (TPPMS) and 1,3,5-triaza-7-phosphaadamantane (PTA).