Luca Gonsalvi

Experimental Evidence of Phosphine Oxide Generation in Solution and Trapping by Ruthenium Complexes

Phosphorus oxides, oxyacids, and their esters are important chemicals for industry. Apart from playing a role in most organisms in ruling their energy transformations, they find wide and diverse applications, such as fertilizers, pesticides, herbicides, lubricants, flame retardants, additives for special plastics and materials, and drugs for different diseases. Little attention has however been paid to lower-oxidation-state species, such as PO, HPO, and P2O, for which synthetic isolation procedures and even direct evidence of their existence are scarce. One of the most elusive species in this regard is phosphine oxide, H3PO (I ; Scheme 1). This molecule was first observed by reacting atomic oxygen with PH3 using a discharge–flow system equipped with molecularbeam sampling mass spectrometry. Alternatively, red-light photolysis of co-deposited PH3/O3 onto an argon matrix at 12–18 K was used to generate and trap I in a very diluted concentration together with its tautomer phosphinous acid H2P(OH) (II), which was identified by FTIR. [4] Finally, Ault and Kayser observed the formation of H3PO in argon matrices after photochemical irradiation of a mixture of VOCl3, CrO2Cl2, and PH3. [5] Both molecules have been studied by theoretical methods. Application of an adequate phosphorus basis set has recently shown that, in contrast with previous ab initio studies, I is more stable than II by only about 1 kcalmol 1 in the gas phase. In contrast, computational analysis in aqueous solution showed that upon solvation I is largely preferred by about 10 kcal mol 1 owing to stronger hydrogen bonding with the highly polar P!O bond. The possible involvement of H3PO in the oxidative polymerization of phosphine to give polyhydride phosphorus PxHy polymers has been also proposed. Herein we show that the previously unknown P I species H3PO (I) can be easily generated in solution by electrochemical methods, and we provide evidence of its solution stability, its characterization by conventional NMR spectroscopy, and its trapping as a ligand in the coordination sphere of hydrosoluble ruthenium complexes after tautomerization to II. The electrochemical generation of H3PO was performed in a single electrochemical cell with a lead cathode and a sacrificial zinc anode using P4 melted in a slightly acidic water/ ethanol solution (2:1 volume ratio, water acidified with HCl, 2m) at 60 8C (Supporting Information, Figures S1, S2). The overall electrochemical process may be divided in two parts. In the first step, the electrochemical generation of PH3 on the lead cathode takes place as previously described, 11] while in the second step, mild oxidation of PH3 to H3PO occurs at the anodic surface of the zinc electrode. In agreement with cyclic voltammetry experiments showing an irreversible oxidation wave, PH3 is electrochemically active in the anodic potential range + 0.80–1.25 V (vs. Ag/AgNO3, 0.01m in CHCN3) and can be therefore oxidized in acidic water/ethanol 2:1 solution to H3PO (Supporting Information, Figure S3). Scheme 2 shows the overall electrochemical process resulting in the cathodic reduction of P4 to PH3 and anodic oxidation of PH3 to H3PO (E = + 1.24 V vs. Ag/AgNO3, 0.01m in CHCN3). Different working conditions were investigated to optimize the production of H3PO. The best performance was Scheme 1. Phosphine oxide (I) and its tautomer, phosphinous acid (II).

Anchoring rhodium(I) on thiourea-functionalized silica xerogels and silsesquioxanes part II. Matrix effects on the selectivity in the hydroformylation of styrene

Abstract Three thiourea-functionalized siloxane materials, 5SiO2 · SiO3/2(CH2)3NHC(S)NHPh (XGphtu), SiO3/2(CH2)3NHC(S)NHPh (XGphtu*) and p-{SiO3/2(CH2)3NHC(S)NH}2C6H4 (XGphenditu*) were prepared. They are able to anchor Rh(I) species giving supported complexes that are very active recoverable catalysts for the hydroformylation of styrene. Some of these materials show regioselectivity variation when used in consecutive catalytic runs. The recovered catalysts have been investigated by XPS and EDX and the change in regioselectivity has been ascribed to matrix effects. In fact, the surface rhodium leaching apparently forces the catalytic process to move in the inside of the materials causing the substrate to experience the inner matrix environment. Furthermore, the non-siloxanized thioureas PhNHC(S)NHPh (Phtu) and p-{PrNHC(S)NH}2C6H4 (Phenditu), which give discrete molecular Rh(I) complexes, were studied as models for the surface binding functions. The structure of [Rh(cod)Cl(Phtu)] (cod = 1,5-cyclooctadiene) has been determined by X-ray diffraction methods.

Facile heterogeneous catalytic hydrogenations of CN and CO bonds in neat water: anchoring of water-soluble metal complexes onto ion-exchange resins

Rare examples are known of non-covalent immobilization of water-soluble transition metal complexes onto solid supports and their application in catalytic hydrogenations in neat water. In the present work we report on the synthesis of a novel Ir(I) complex bearing the water soluble monodentate cage phosphine PTA (PTA = 1,3,5-triaza-7-phosphaadamantane) and its immobilisation onto commercial ion-exchange resins by a straightforward heterogenization method. The materials obtained were used as catalysts for the hydrogenations of cyclic imines and α-keto esters under mild conditions using water as a green reaction medium, avoiding as much as possible the use of toxic and/or hazardous solvents.

An active, stable and recyclable Ru(II) tetraphosphine-based catalytic system for hydrogen production by selective formic acid dehydrogenation

Well-defined and in situ made Ru(II) complexes of the linear tetraphosphine ligand meso-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos-1, P4) proved to be effective catalysts for selective formic acid dehydrogenation with good yields and TONs in the presence of an added amine, both under batch and continuous feed experimental conditions. The mechanism was studied by solution NMR and DFT calculations, highlighting the role of the trans-dihydrido complex [Ru(H)2(meso-P4)] as the active species in catalysis.

Transfer Hydrogenation of Ketones Catalyzed by Surface-Active Ruthenium and Rhodium Complexes in Water

An improved, high-yield, one-pot synthetic procedure for water-soluble ligands functionalized with trialkyl ammonium side groups H2 N(CH2 )2 NHSO2 -p-C6 H4 CH2 [NMe2 (Cn H2n+1 )](+) ([HL(n) ](+) ; n=8, 16) was developed. The corresponding new surface-active complexes [(p-cymene)RuCl(L(n) )] and [Cp*RhCl(L(n) )] (Cp*=η(5) -C5 Me5 ) were prepared and characterized. For n=16 micelles are formed in water at concentrations as low as 0.6 mM, as demonstrated by surface-tension measurements. The complexes were used for catalytic transfer hydrogenation of ketones with formate in water. Highly active catalyst systems were obtained in the case of complexes bearing C16 tails due to their ability to be adsorbed at the water/substrate interface. The scope of these catalyst systems in aqueous solutions was extended from partially water soluble aryl alkyl ketones (acetophenone, butyrophenone) to hydrophobic dialkyl ketones (2-dodecanone).

Mechanistic Studies on the Interaction of [(κ3-P,P,P-NP3)IrH3] [NP3 = N(CH2CH2PPh2)3] with HBF4 and Fluorinated Alcohols by Combined NMR, IR, and DFT Techniques

The novel iridium(III) hydride [(kappa(3)-P,P,P-NP(3))IrH(3)] [NP(3) = N(CH(2)CH(2)PPh(2))(3)] was synthesized and characterized by spectroscopic methods and X-ray crystallography. Its reactivity with strong (HBF(4)) and medium-strength [the fluorinated alcohols 1,1,1-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)] proton donors was investigated through low-temperature IR and multinuclear NMR spectroscopy. In the case of the weak acid TFE, the only species observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol, while with the stronger acid HBF(4), the proton transfer was complete, giving rise to a new intermediate [(kappa(3)-P,P,P-NP(3))IrH(4)](+). With a medium-strength acid like HFIP, two different sets of signals for the intermediate species were observed besides dihydrogen bond formation. In all cases, the final reaction product at ambient temperature was found to be the stable dihydride [(kappa(4)-NP(3))IrH(2)](+), after slow molecular dihydrogen release. The nature of the short-living species was investigated with the help of density functional theory calculations at the M05-2X//6-31++G(df,pd) level of theory.

Counterion-dependent deuteration of pentamethylcyclopentadiene in water-soluble cationic Rh(III) complexes assisted by PTA

[Cp(*)RhCl(PTA)(2)]X (PTA = 7-phospha-1,3,5-triazaadamantane) undergoes an H/D exchange process between the methyl groups of Cp(*) and D(2)O whose rate depends on the coordinating ability of the counterion X(-). Kinetic studies and DFT calculations indicate that deuteration involves the abstraction of a Me-Cp(*) proton by a coordinated OH(-); the formation of the latter seems to be facilitated by the presence of the N-basic centers of PTA.

Electro-oxidation of Ruthenium Cyclopentadienyl PTA Complexes in DMF ESI-MS, Cyclic Voltammetry, and Online Electrochemistry/ESI-MS Studies

Halogen complexes of ruthenium cyclopentadienyl [CpRu(PTA) 2 X]; [CpRu(PTA)(PPh 3 )X]; [CpRu(PPh 3 ) 2 Cl], and [CpRu(mPTA)(PPh 3 )X] + (Cp = C 5 H 5 ; PTA = l,3,5-triaza-7-phosphaadamantane; mPTA + = [1-methyl-1,3,5-triaza-7-phosphaadamantane] + ; X = Cl - , I - ) were investigated by electrospray mass spectrometry (ESI-MS), in flow-cell cyclic voltammetry, by microelectrodes, and by combined online electrochemistry and electrospray mass spectrometry (EC/ESI-MS) in dimethyl formamide solution. Coordination changes and the structures of the initial compounds and the products of the electro-oxidation of the Ru(II) complexes were traced by in situ EC/MS n experiments which revealed their fragmentation pathways. ESI-MS collision-induced dissociation fragmentations of the initial reactants and the oxidation products were explained by soft acid-hard base considerations taking into account the different nature of Ru(II)-Ru(IV) centers. The electrochemical studies show that it is possible to tune the formal potentials for the oxidation of [CpRuL 2 X] complexes by over 300 mV by proper selection of the ligands. The increase of the redox potential by the different ligands follows the order PTA < PPh 3 < mPTA + . We demonstrate a similarity between the propensity of the ligand to fragment out in the gas phase and its relationship to the formal potential of the complex.

New Wind in Old Sails: Novel Applications of Triphos-based Transition Metal Complexes as Homogeneous Catalysts for Small Molecules and Renewables Activation.

Recent developments in the coordination chemistry and applications of Ru-triphos [triphos = 1,1,1-tris-(diphenylphosphinomethyl)ethane] systems are reviewed, highlighting their role as active and selective homogenous catalysts for small molecule activation, biomass conversions and in carbon dioxide utilization-related processes.

Aqueous Phase Reactions Catalysed by Transition Metal Complexes of 7-Phospha-1,3,5-triazaadamantane (PTA) and Derivatives

The possibility to run selective catalytic transformations in water has fascinated generations of chemists working in the field of homogenous catalysis. One of the most common approaches has been so far to translate organic phase transition metal complex catalyzed processes into water phase by replacing ancillary ligands such as phosphines with their water soluble analogs. A class of neutral, stable, easy-to-handle and functionally versatile monodentate phosphines is represented by 7-phospha-1,3,5-triazacyclo-[3.3.1.1]decane (PTA) whose application has witnessed a true renaissance in the first decade of the present century after some interest starting from its discovery in 1974. This chapter summarizes the most relevant applications of transition metal complexes of PTA in catalysis, from C=C and C=O bond hydrogenation, to olefin hydroformylation and various C–C and C-element bond forming reactions.