Complexes of the general formula [MoO2X2L2] (X=Cl, Br, Me; L2=bipy, bpym) have been prepared and fully characterized, including X‐ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO2Cl2(4,4′‐bis(hexyl)‐2,2′‐bipyridine)] has been synthesized. The reaction of the complexes with tert‐butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoVI η1‐alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo−X bonds nor the Mo−N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X‐O‐N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo‐bound peroxo oxygen atom, epoxide and tert‐butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.

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Reaction of chromium() acetate with HBF 4 in nitrile solutions led to air-sensitive, paramagnetic complexes of the general formula [Cr(NCR) 4 ][BF 4 ] 2 (R = Me, Bu t or Ph). In contrast to an earlier report of the synthesis of [Cr(NCMe) 4 ][BF 4 ] 2 by electrochemical methods, these compounds are blue. The crystal structure of [Cr(NCMe) 4 ][BF 4 ] 2 has been determined. The chromium() cation is surrounded by nitrile ligands in a squareplanar manner. The BF 4 Ϫ counter ions are located at the apical positions of an elongated octahedron. Magnetic susceptibility measurements yielded µ eff = 4.9 µ B at 300 K which is characteristic for a d 4 high-spin complex. The paramagnetic complexes [Cr(NCR) 4 ][BF 4 ] 2 are easy to prepare and can be used for the high-yield synthesis of several other chromium derivatives in low oxidation states like [Cr(py) 4 ][BF 4 ] 2 , [Cr(C 5 H 5 ) 2 ], [Cr(dppe) 2 ][BF 4 ] 2 and [Cr(NO)(NCMe) 5 ][BF 4 ] 2 .

The dioxomolybdenum(VI) complex [MoO 2 Cl 2 (L-L)], containing the bidentate 1,4-diazabutadiene ligand RN= C(Ph)−C(Ph)=NR [R = (CH 2 ) 3 Si(OEt) 3 ], was immobilised in the ordered mesoporous silica MCM-41 by carrying out a grafting reaction in dichloromethane. The grafted material (3.3 wt.-% Mo) was tested as a catalyst for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) at 55°C. Selectivity to the epoxide was very high and the observed kinetic profile was similar to that of the complex [MoO 2 Cl 2 (L-L)] in the homogeneous phase. On recycling several times, some activity was lost from the first to second runs, but thereafter stabilised. Tethered complexes of this type were also prepared by a stepwise approach. The mesoporous silicas MCM-41 and MCM-48 were first treated with a toluene solu-

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The dioxomolybdenum(VI) complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}] has been prepared in good yield by reaction of the solvent adduct MoO 2 Cl 2 (THF) 2 with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH 3 MgCl gives the dimethyl derivative [MoO 2 (CH 3) 2 {p-tolyl(CH 3 DAB)}]. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases, the initial activity is ca. 175 mol mol À1 Mo h À1 and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and (R)-(þ)-limonene, with TBHP. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor-oxygen donor complex formation is first-order in TBHP and in the metal complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}]. A specific rate of 3.2 mol À1 dm 3 s À1 was found for catalyst formation at 25 C. Activation parameters for this reaction have also been measured (DH 6 ¼ ¼ 48 AE 3 kJ mol À1 , DS 6 ¼ ¼ À112 AE 10 J mol À1 K À1).

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The dioxomolybdenum(VI) fragment MoO2X2 has been confined within the ordered mesopores of pure siliceous hexagonal MCM‐41 and cubic MCM‐48 molecular sieves either by direct grafting (solvent impregnation) with MoO2X2(THF)2 or by using a spacer ligand [L = NC(CH2)2Si(OEt)3]. The materials have been characterised by elemental analysis, powder X‐ray diffraction, N2 adsorption, IR spectroscopy, and magic‐angle spinning NMR spectroscopy (13C, 29Si). All catalysts, homogeneous and heterogenised, are active in the epoxidation of cyclooctene with tert‐butyl hydroperoxide.