The complexes [Mo(O)2(QR)2] [R = cyclohexyl (1), ethylcyclopentyl (2), hexyl (3), and neopentyl (4)] have been obtained in good yields by treatment of [Mo(O)2(acac)2] with 2 equivalents of acylpyrazolone compounds HQR [HQR = 3‐methyl‐1‐phenyl‐4‐alkylcarbonyl‐5‐pyrazolone; R = cyclohexyl (HQCy), ethylcyclopentyl (HQEtCp), hexyl (HQHe), neopentyl (HQnPe)]. They were isolated as yellow crystalline solids and characterized spectroscopically [IR, 1H and 13C(1H) NMR] and structurally (X‐ray for 2 and 3). The deoxygenation of selected epoxide substrates to alkenes by employing compounds 1 and 3 as catalysts and PPh3 as the oxygen acceptor showed good activities in toluene. The use of the ionic liquid [C4mim]PF6 as solvent gave lower yields, but the resulting catalytic system could be conveniently recycled. The [Mo(O)2(QR)2] derivatives 1 and 3 were also found to be moderately active catalysts for the deoxydehydration of vicinal diols.

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[Mo(O)(O(2))(2)(L)(2)] compounds (L = pz, pyrazole; dmpz, 3,5-dimethylpyrazole) were reacted stoichiometrically, in the absence of an oxidant, with cis-cyclooctene in an ionic liquid medium where selective formation of the corresponding epoxide was observed. However, this oxo-transfer reaction was not observed for some other olefins, suggesting that alternative reaction pathways exist for these epoxidation processes. Subsequently, DFT studies investigating the oxodiperoxomolybdenum catalysed epoxidation model reaction for ethylene with hydrogen peroxide oxidant were performed. The well known Sharpless mechanism was first analysed for the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst and a low energy reaction pathway was found, which fits well with the observed experimental results for cis-cyclooctene. The structural parameters of the computed dioxoperoxo intermediate [Mo(O)(2)(O(2))(dmpz)(2)] in the Sharpless mechanism compare well with those found for the same moiety within the [Mo(4)O(16)(dmpz)(6)] complex, for which the full X-ray report is presented here. A second mechanism for the model epoxidation reaction was theoretically investigated in order to clarify why some olefins, which do not react stoichiometrically in the absence of an oxidant, showed low level conversions in catalytic conditions. A Thiel-type mechanism, in which the oxidant activation occurs prior to the oxo-transfer step, was considered. The olefin attack of the hydroperoxide ligand formed upon activation of hydrogen peroxide with the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst was not possible to model. The presence of two dmpz ligands coordinated to the molybdenum centre prevented the olefin attack for steric reasons. However, a low energy reaction pathway was identified for the [Mo(O)(O(2))(2)(dmpz)] catalyst, which can be formed from [Mo(O)(2)(O(2))(dmpz)(2)] by ligand dissociation. Both mechanisms, Sharpless- and Thiel-type, were found to display comparable energy barriers and both are accessible alternative pathways in the oxodiperoxomolybdenum catalysed olefin epoxidation. Additionally, the molecular structures of [Mo(O)(O(2))(2)(H(2)O)(pz)] and [Hdmpz](4)[Mo(8)O(22)(O(2))(4)(dmpz)(2)]·2H(2)O and the full X-ray report of [Mo(O)(O(2))(2)(pz)(2)] are also presented.

The aerobic aqueous solution syntheses and structures of monomeric and polymeric MnII complexes containing the oxydiacetate ligand [O(CH2COO)22− = oda] are reported. The magnetic properties of the polymeric products have also been investigated. The initially obtained species [{Mn(oda)(H2O)}·H2O]n (1) has been reacted with bidentate or tridentate N‐donor ligands such as o‐phenanthroline (phen), 2,2′‐bipyridine (bipy) and 2,2′:6,2′′‐terpyridine (terpy) to give the complexes [{Mn(oda)(phen)}·4H2O]n (2), [Mn(oda)(bipy)(H2O)]·2H2O (3) and [Mn(oda)(terpy)]·2H2O (4), respectively. Species 1−4 are the first Mn‐oda structures determined by X‐ray methods. In all cases, the oda acts as a tridentate ligand toward one metal and adopts the typical planar arrangement. However, while the 3 and 4 are monomers, 2 and 1 are polymers as oda exploits both atoms of its carboxylate groups to coordinate other metals; 1 consists of a three‐dimensional diamond‐type network, while 2 is one‐dimensional, consisting of an extended chain of {Mn(oda)(phen)} subunits. For both compounds, magnetic susceptibility measurements down to 2 K showed only weak antiferromagnetic interactions between high‐spin MnII ions. Despite the expectedly similar coordinating capabilities of N‐donor ligands, the bipy complex 3 is a monomer with the six‐coordination completed by one water ligand. The tridentate nature of simultaneously present oda and terpy ligands excludes the coordination of water in complex 4, the geometry of which is distorted from the octahedron, most likely due to a peculiar packing effect. Finally, the known five‐coordinate complex [MnCl2(terpy)] (5), a side product of our synthetic experiments, has also been structurally characterized. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The copper catalysed aerobic oxidation of selected alcohol substrates in supercritical carbon dioxide (scCO(2)), employing a range of simple copper(II) catalyst compounds, is here described. The copper acetate complex of polydimethylsiloxane (PDMS) functionalised pyridine (1), compound 2, has previously been synthesised and characterised by us and its solubility in scCO(2) demonstrated. Due to this solubility we anticipated that the selective aerobic oxidation of alcohols to aldehydes could be homogeneously catalysed by this compound in scCO(2) in combination with the co-catalyst 2,2,6,6-tetramethylpiperidin-1-yloxy free radical (TEMPO). Our initial results showed that complete oxidation of 4-nitrobenzyl alcohol was achieved within 4 h of reaction. However, the activities of analogous copper derivatives containing simpler pyridine substituents, [Cu(AcO)(2)(py)](2) and [Cu(AcO)(2)(4VP)](2) (4VP = 4-vinylpyridine), were shown to be similar, in spite of their negligible solubility in scCO(2). When we repeated the reactions in highly non-polar hexane rather than scCO(2) similar observations were made. In both cases, as 2 is soluble and the pyridine analogues are not, a much higher reaction rate was anticipated for 2 as it is the only compound capable of homogeneous catalysis. However, in some cases slightly better activities were observed for [Cu(AcO)(2)(py)](2) rather than for the PDMS functionalised analogue, 2. Thus, despite poor catalyst solubility typically being very inhibitory in this type of catalytic process, in this system solubilisation of the catalyst is not necessary. In continuation the activity of silica supported copper complexes was therefore investigated. Employing such catalysts the 4-nitrobenzyl and benzyl alcohol substrates were completely oxidised to the corresponding aldehydes in scCO(2), this time employing lower catalyst loadings. Other types of alcohol substrate showed more limited conversions however. To conclude, alcohol oxidation in the non-conventional green solvent scCO(2), with the benign terminal oxidant, dioxygen, and simple, cheap, easily prepared metal catalyst compounds was demonstrated. This is the first copper-TEMPO catalysed alcohol oxidation system in scCO(2) to be described.

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The bis(imido) complex MoCl(2)(Nmes)(2)(dme) (1) (mes = 2,4,6-trimethylphenyl, dme = 1,2-dimethoxyethane) has been used as the starting material for the preparation of the compounds (L(OEt))Mo(Nmes)(2)Cl (2) (L(OEt) = (eta-C(5)H(5))Co{P(O)(OEt)(2)}(3)) and Mo(Nmes)(2)(acac)(2) (5), as well as for the synthesis of the mixed oxo-imido MoCl(2)(Nmes)(O)(dme) (6), the latter reaction involving conproportionation of 1 and MoCl(2)(O)(2)(dme). Similarly, the paramagnetic mono(imido) species Mo(Nmes)Cl(3)(dme) (11) can also be obtained from 1 by interaction with MoCl(4)(THF)(2) (THF = tetrahydrofuran) in refluxing dme. Compounds 6 and 11 are suitable sources for the synthesis of other diamagnetic mixed oxo-imido and paramagnetic mono(imido) compounds, respectively. Three of the newly synthesized complexes, namely, (L(OEt))Mo(Nmes)(2)Cl (2), (L(OEt))Mo(Nmes)Cl(2) (12), and MoCl(3)(Nmes)(depe) (14) (depe = Et(2)PCH(2)CH(2)PEt(2)) have been characterized by X-ray crystallography.

Palladium complexes of general formula [PdCl2(L)2], 3a−c, were synthesized by reaction of [PdCl2(COD)] (COD = 1,5-cyclooctadiene) with the appropriate functionalized phosphine L = [P(C6H5) n ((C6H4CH2CH2Si(CH2CH2CH2SiEtMe2)3)3 - n )] (n = 2, 2a; 1, 2b; 0, 2c, respectively), which integrated carbosilane dendritic wedges attached to the para-carbon atom of the phenyl group. As a result of the inclusion of the carbosilane dendrons, the solubility of the corresponding palladium complexes 3 in supercritical carbon dioxide (scCO2) was increased in comparison with the palladium complex containing a PPh3 ligand, which showed negligible solubility. The employment of these complexes as precatalysts in a selected Heck coupling reaction in scCO2 was investigated. Good activities were found for 3a−c in such a medium due to the acquired solubility, in contrast with the palladium complex containing PPh3, which showed no activity.