Beatriz Royo

[MoO2Cl2] as catalyst for hydrosilylation of aldehydes and ketones

The high valent molybdenum-dioxo complex [MoO2Cl2] catalyzes the addition of dimethylphenylsilane to aldehydes and ketones to afford the corresponding dimethylphenylsilyl ethers in quantitative yield.

Highly Effective Water Oxidation Catalysis with Iridium Complexes through the Use of NaIO4

Exceptional water oxidation (WO) turnover frequencies (TOF=17,000 h(-1)), and turnover numbers (TONs) close to 400,000, the largest ever reported for a metal-catalyzed WO reaction, have been found by using [Cp*Ir(III)(NHC)Cl2] (in which NHC=3-methyl-1-(1-phenylethyl)-imidazoline-2-ylidene) as the pre-catalyst and NaIO4 as oxidant in water at 40 °C. The apparent TOF for [Cp*Ir(III)(NHC)X2] (1X, in which X stands for I (1I), Cl (1Cl), or triflate anion (1OTf)) and [(Cp*-NHCMe)Ir(III)I2] (2) complexes, is kept constant during almost all of the O2 evolution reaction when using NaIO4 as oxidant. The TOF was found to be dependent on the ligand and on the anion (TOF ranging from ≈600 to ≈1100 h(-1) at 25 °C). Degradation of the complexes by oxidation of the organic ligands upon reaction with NaIO4 has been investigated. (1)H NMR, ESI-MS, and dynamic light-scattering measurements (DLS) of the reaction medium indicated that the complex undergoes rapid degradation, even at low equivalents of oxidant, but this process takes place without formation of nanoparticles. Remarkably, three-month-old solution samples of oxidized pre-catalysts remain equally as active as freshly prepared solutions. A UV/Vis feature band at λmax =405 nm is observed in catalytic reaction solutions only when O2 evolves, which may be attributed to a resting state iridium speciation, most probably Ir-oxo species with an oxidation state higher than IV.

Dioxomolybdenum(VI) complexes as catalysts for the hydrosilylation of aldehydes and ketones

The dioxomolybdenum(VI) complexes [MoO2Cl2] (1), [MoO2(acac)2] (2), [MoO2(S2CNEt2)2] (3), [CpMoO2Cl] (4), [MoO2(mes)2] (5) and the polymeric organotin-oxomolybdates [(R3Sn)2MoO4] [R = n-Bu (6), t-Bu (7), Me (8)] were examined as catalysts for the hydrosilylation of aldehydes and ketones with dimethylphenylsilane. Of these, [MoO2Cl2] (1) was the most efficient catalyst, affording quantitative yields of the corresponding silylated ethers at room temperature in acetonitrile. Complexes 2, 4-8 also catalyzed the same reaction but required heating at 80 degrees C and longer reaction times compared with 1. Compound 3 is inactive. The wide scope of molybdenum oxide-mediated hydrosilylation was established with a variety of aldehydes and ketones. Counter intuitively, the activity of is 1 highest in NCMe. In the absence of a carbonyl substrate, [MoO2Cl2(NCBu(t))] (10) reacts with HSiMe2Ph affording [MoO(OSiMe2Ph)Cl2]2 (11) which has been fully characterized by NMR and IR spectroscopy, elemental analyses and mass spectrometry. Addition of radical scavengers strongly slows down the [MoO2Cl2]-based hydrosilylation suggesting the intermediacy of oxygen-centered radicals.

An overview of chiral molybdenum complexes applied in enantioselective catalysis

The aim of this contribution is to highlight the attractive applications of molybdenum in asymmetric catalysis. Even if molybdenum plays several biological roles mainly in metalloproteins, it has been less employed in catalysis in comparison with other transition metals. This perspective focuses on molybdenum complexes linked to chiral ligands applied in enantioselective processes. The versatility of molybdenum in terms of oxidation states and coordination geometries triggers its capability to catalyse different kind of processes such as carbon–carbon bond formation, olefin metathesis or alkene epoxidation among the most relevant transformations.

Cyclopentadienyl–Silsesquioxane Titanium Complexes: Highly Active Catalysts for Epoxidation of Alkenes with Aqueous Hydrogen Peroxide

Titanium complexes bearing an unprecedented tridentate cyclopentadienyl-silsesquioxanate ligand provide a new class of efficient and selective catalysts for epoxidation of olefins with aqueous hydrogen peroxide under homogeneous conditions.

Dichloro(methyl)silyl-substituted cyclopentadienyl titanium complexes

We are grateful to the DGICYT (Project PB-92-0178-C) and CAM (I + D 0034/94) for financial support. B.R. acknowledges Universidad de Alcala de Henares for support provided (Project 042/95)

SYNTHESIS OF BIS-CYCLOPENTADIENYL, BIS-INDENYL AND MIXED-RING INDENYL HALIDES OF TUNGSTEN

A convenient route to the tungstenocene derivatives Cp 2 WX 2 from CpW(η 3 -C 5 H 5 )(CO) 2 via [Cp 2 W(CO) 2 ] 2+ or [Cp 2 W(NCMe)(CO)] 2+ is described. Similar preparations, starting from CpW(η 3 -C 9 H 7 )(CO) 2 , lead to the mixed-ring indenyl analogues of tungstenocene, IndCpWI 2 [Ind=(η 5 -C 9 H 7 )]. A stepwise route to bis-indenyl derivatives of tungsten is reported, based on the novel W(IV) complex IndWCl 3 (CO) 2 which is synthesized by the reaction of IndW(η 3 -C 3 H 5 )(CO) 2 with HCl gas in dichloromethane. Treatment of IndWCl 3 (CO) 2 with KInd in THF gives IndW(η 3 -Ind)(CO) 2 in high yield. This complex is a convenient starting material for the preparation of some bis-indenyl analogues of tungstenocene, Ind 2 WI 2 . The molecular structures of [IndW(η 4 -C 5 H 6 )(CO) 2 ]BF 4 ( 7a ) and [IndCpWH(CO)]BF 4 ( 7b ) were characterized by single-crystal X-ray diffraction. The crystals belong to the triclinic and monoclinic space group P and P 2 1 / c with a =729.98(3) and 715.0(3) pm; b =783.06(3) and 1484.0(6) pm; c =1419.20(4) and 1389.2(6) pm; α =100.210(2) and 90°; β =104.557(2) and 99.18(3)°; γ =93.441(2) and 90°, respectively. The final refinements of 7a and 7b converged at R 1 =0.0236 and 0.0680; wR 2 =0.0581 and 0.1378, respectively.

The effect of trimethylsilyl substituents on the ring-slippage of bis-indenyl-molybdenocene derivatives

Abstract A group of new trimethylsilyl indenyl derivatives of molybdenum is reported, starting from the allyl compound TMSIndMo(η3-C3H5)(CO)2 [TMSInd=η3-SiMe3C9H6]. The novel allyl Mo(II) complex, which is synthesized by the reaction of Mo(η3-C3H5)(CO)2(NCMe)2Cl with Li(TMSInd) in THF, is the precursor to trimethylsilyl indenyl derivatives. Treatment of TMSIndMo(η3-C3H5)(CO)2 with HCl and addition of LiInd or LiTMSInd gives the ring-slipped complexes (η5-TMSInd)Mo(η3-Ind)(CO)2 or (η5-TMSInd)Mo(η3-TMSInd)(CO)2 in high yield. The presence of the trimethylsilyl substituent changes substantially the flexibility of the indenyl groups. These new substituted indenyl complexes were characterized by experimental techniques and ab initio calculations, and examined by cyclic voltammetry.

Pentamethylcyclopentadienyltitanium-(III) and -(IV) carboxylates. Crystal structures of [Ti(η-C5Me5)(O2CPh)3] and [{Ti(η-C5Me5)(O2CPh)2}2]

The compound [Ti(η-C5Me5)Me3] reacts with 3 equivalents of carboxylic acids HO2CR (R = Me, Ph, or p-MeOC6H4) to give the corresponding tris(carboxylates)[Ti(η-C5Me5)(O2CR)3]. The X-ray structure of the complex with R = Ph shows the three benzoate ligands acting as chelates, the geometry around Ti is a distorted pentagonal bipyramid and the metal attains the 18-electron configuration. Methyl carboxylate complexes [Ti(η-C5Me5)Me3-n(O2CR)n](n= 1 or 2) could not be isolated, but exposure of their solutions to sunlight gave the dimers [{Ti(η-C5Me5)(O2CR)2}2]. The X-ray structure of the acetato complex reveals four O2CMe groups bridging two Ti(C5Me5) fragments.

Novel carbohydrate-substituted cyclopentadienyls of titanium, molybdenum, manganese and iron

Abstract The synthesis of the carbohydrate-substituted cyclopentadiene 1 is obtained by reaction of 3-( O - tert -butyldimethylsilyl)-1,2- O -isopropylidene-5- O -( p -tolylsulfonyl)-α- d -xylofuranose and cyclopentadienyl sodium in dimethylformamide. Thallation of 1 and reaction with FeCl 2 or CpTiCl 3 affords the symmetric ferrocene Cp s 2 Fe ( 3 ) and the mixed titanocene Cp s CpTiCl 2 ( 4 ), respectively. The manganese monocyclopentadienyl Cp s Mn(CO) 3 ( 5 ) is isolated by reaction of the thallium salt of 1 with MnBr(CO) 5 . Direct reaction of 1 with the molybdenum tricarbonyl Mo(CO) 3 (NCMe) 3 followed by treatment with iodoform led to Cp s Mo(CO) 3 I ( 6 ). All new compounds were characterised by elemental analysis, IR and NMR spectroscopy.