Christian Lorber

Cis-dioxomolybdenum(VI) complexes as new catalysts for the Meyer-Schuster rearrangement

Abstract We describe a new catalytic system for the isomerisation of propargylic alcohols into α,β-ethylenic carbonyl derivatives (Meyer-Schuster rearrangement) 1 , based on the combination of dioxomolybdenum(VI) catalysts and sulfoxides.

Selective and Environmentally Benign Aerobic Catalytic Oxidation of Alcohols by a Molybdenum-Copper System

The oxidation of activated primary and secondary alcohols to the corresponding aldehydes and ketones can be carried out with molecular oxygen, in the presence of the bimetallic molybdenum–copper system MoO2(acac)2–Cu(NO3)2 as catalyst.

Synthesis and Reactivity Studies of Model Complexes for Molybdopterin-Dependent Enzymes

The molybdenum cofactor (Moco)-containing enzymes are divided into three classes that are named after prototypical members of each family, viz. sulfite oxidase, DMSO reductase and xanthine oxidase. Functional or structural models have been prepared for these three prototypical enzymes: (i) The complex [MoO2(mnt)2]2- (mnt2- = 1,2-dicyanoethylenedithiolate) has been found to be able to oxidize hydrogen sulfite to HSO4- and is thus a functional model of sulfite oxidase. Kinetic and computational studies indicate that the reaction proceeds via attack of the substrate at one of the oxo ligands of the complex, rather than at the metal. (ii) The coordination geometries of the mono-oxo [Mo(VI)(O-Ser)(S2)2] entity (S2 = dithiolene moiety of molybdopterin) found in the crystal structure of R. sphaeroides DMSO reductase and the corresponding des-oxo Mo(IV) unit have been reproduced in the complexes [M(VI)O(OSiR3)(bdt)2] and [M(VI)O(OSiR3)(bdt)2] (M = Mo,W; bdt = benzene dithiolate). (iii) A facile route has been developed for the preparation of complexes containing a cis-Mo(VI)OS molybdenum oxo, sulfido moiety similar to that detected in the oxidized form of xanthine oxidase.

[ONNO]-type amine bis(phenolate)-based vanadium catalysts for ethylene homo- and copolymerization

The synthesis and solution and solid-state structural characterization of a family of amine bis(phenolate) [ONNO]-vanadium complexes is reviewed. These compounds have oxidation states ranging from vanadium(II) to vanadium(V), and were evaluated as olefin polymerization catalysts. In association with EtAlCl2 cocatalyst, we studied the homopolymerization of ethylene, propene, and 1-hexene, as well as the copolymerization of ethylene with α-olefins (1-hexene, 1-octene) and cycloolefins (norbornene, cyclopentene). Some of these catalysts were shown to produce copolymers with a good activity and comonomer content.

[Cp2V] Migration along an Octatetrayne Chain: From the Monometallic Complex [Cp2V(3-4η-tBuC≡C−C2−C≡CC≡CtBu)] to the Dimetallic Complex [(Cp2V)2(1-2η:7-8η-tBuC2−C≡CC≡C−C2tBu)]

The oxidative addition of one equivalent of [Cp2V] (4) to the tetrayne ligand tBuC≡CC≡CC≡CC≡CtBu (5) gives the monometallic complex [Cp2V(3-4η-tBuC≡CC2−C≡CC≡CtBu)] (7). Compound 7 reacts further with a second equivalent of [Cp2V] to give the dimetallic complex [(Cp2V)2(1-2η:7-8η-tBuC2−C≡CC≡CC2tBu)] (8), which involves a shift of the first coordinated [Cp2V] unit from the internal C3–C4 to the external C1–C2 positions on the alkynyl ligand. Compound 8 is also directly obtained by the addition of two equivalents of [Cp2V] to 5. Reversibly, reaction of 8 with 5 leads to 7. This exchange reaction between 7 and 8 by adding successively 5 and 4 has been monitored by EPR spectroscopy. By contrast, the oxidative addition of one or two equivalents of [Cp2V] to the tetrayne ligand PhC≡CC≡CC≡CC≡CPh (6) gives the homodimetallic complex [(Cp2V)2(1-2η:7-8η-PhC2−C≡CC≡CC2Ph)] (9). Both monometallic and dimetallic complexes 7, 8, and 9 have been characterized by X-ray diffraction. Magnetic moment measurements for 8 and 9 from 300 to 4 K indicated a weak antiferromagnetic J exchange coupling of −12.5 and −4.1 cm−1, respectively.

Imido-bridged homo- and heterobimetallic complexes.

Transamination reactions of primary amines with group 4 and 5 amido precursors M(NMe(2))(4) have been studied to prepare homo- and heterobimetallic complexes [(Me(2)N)(2)M(1)(μ-NR(1))(μ-NR(2))M(2)(NMe(2))(2)(NHMe(2))(x)] (x = 0, 1) with two identical or distinct bridging imido ligands.

Isomerization of 2-methyl-3-butyn-2-ol into prenal: mechanistic observations on tricomponent catalyst systems of the type Ti(OR)4–CuCl–R′CO2H. Crystal structure of [CuCl(Me2C(OH)CCH)]4

Abstract The mechanism of the title catalytic system was investigated, in particular by synthesizing several compounds that model the postulated metal–oxo–propargyloxo intermediates, such as [Ti(μ-O)(OBu t )(O 2 CTol)] 2 (Tol= p -MeC 6 H 4 ), Ti(OCMe 2 CCH) 4 and [CuCl(Me 2 C(OH)CCH)] 4 . The molecular structure of [CuCl(Me 2 C(OH)CCH)] 4 was determined by X-ray crystallography and heterobimetallic Ti–Cu complexes comprising the 2-methyl-3-butyn-2-oxo fragment are reported.

Kinetics of oxygen-atom transfer reactions involving molybdenum dithiolene complexes†

Oxo transfer reactions from [MoO2(mnt)2]2– [mnt2– = 2,3-disulfanylmaleonitrile dianion (1,2-dicyanoethylenedithiolate)] to the abiological oxygen acceptors PPh3–xEtx (x = 0–3) have been studied spectrophotometrically in acetonitrile. The pseudo-first-order rate constants depend linearly on the concentration of the oxo acceptors. The sequence of reactivity is PEt3 > PEt2Ph > PEtPh2 > PPh3, reflecting the basicities of the phosphines, while the steric influence of the substituents on the phosphorus atom appears to be of minor importance. The activation entropies for the reaction with PEt3 and PPh3 are negative. These observations are in agreement with the proposal that in the intermediate the entering phosphine binds to the molybdenum complex through the oxygen ligand. Oxidation of HSO3– by [MoO2(mnt)2]2– has also been studied in acetonitrile and in acetonitrile–water mixtures. The reaction exhibits Michaelis–Menten behaviour. In CH3CN–H2O, the rate of the reaction increases as the water concentration decreases; the reacting species appears to be a product from the reaction of the molybdenum starting complex with water. (Less)

Synthesis and crystal structure of highly soluble ansa-titano- and zirconocene dichloride complexes [Me2Si(η5-C5H2(SiMe3)2]2MCl2 (M=Ti, Zr)

Abstract The synthesis and X-ray characterization of ansa-metallocene dichloride titanium and zirconium complexes of the type [Me2Si(η5-C5H2(SiMe3)2)2]MCl2 (M=Zr (1), Ti (2)) are reported. The complexes have been tested for ethylene polymerization.

Synthesis and X-ray crystal structure of [VCl3((-)-sparteine)] and [FeCl2((-)-sparteine)]

Abstract Novel complexes of vanadium(III), chromium(III) and iron(II) containing the alkaloid (–)-sparteine as a bidentate ligand have been prepared. Two of them, 〚VCl3((–)-sparteine)〛 and 〚FeCl2((–)-sparteine)〛, were characterised by X-ray diffraction. Known cobalt(III) and nickel(II) analogue complexes were prepared using the same procedure.