No abstract

Aldehydes react with W 2 (OCH 2 t Bu) 6 (py) 2 to give W 2 (µ-CHR)(O)(OCH 2 t Bu) 6 (py) compounds, where R ) alkyl or aryl, which, in the presence of excess pyridine, yield W 2 (µ-CR)(µ-O)-(OCH 2 t Bu) 5 (py) 2 compounds and neopentanol. A relative order of rates of reductive cleavage of the CdO bond is found to be aryl > alkyl, and for p-X-C 6 H 4 CHO, the rates follow the order X ) CF 3 > H > F > Me > OMe. Four complexes have been structurally characterized by single-crystal X-ray crystallography: W3, and W 2 (µ-C(p-MeOC 6 H 4 ))-(OCH 2 t Bu) 5 (py) 2 , 4. Factors influencing the reductive cleavage of the CdO bond are discussed in light of earlier studies of related reactions, and most notably comparisons are made with the development of a molecular alternative to the McMurry reagent.

In hydrocarbon solutions W 2 (OCH 2 t Bu) 6 (py) 2 , 1, and 1,3-butadiene and isoprene reversibly form adducts W 2 (OCH 2 t Bu) 6 (diene)(py), compounds 2 (diene ) 1,3-butadiene) and 3 (diene ) isoprene). The structures of 1, 2, and 3 are reported. For 1 there is a central O 3 NWt WNO 3 core with W-W ) 2.334(1) Å, and each W atom is in a distorted square-based pyramidal environment with the WtW bond in the apical site. In 2 and 3 there is a common W 2 O 6 C 4 N core wherein the diene adopts a novel mode of coordination, µ,η 1 ,η 4 . One W atom is coordinated to five oxygen atoms and the bridging carbon atom of the µ-diene ligand, while the other is bonded to three oxygen atoms, a pyridine nitrogen, and four carbon atoms of the diene. The structure may be viewed as being derived from a confacial bioctahedron with two bridging OR ligands and one bridging alkyl. In this view the diene can be considered as a µ-metalated π (η 3 -)allyl or a 2-ligand. The W-W distances are 2.471(1) Å in 2 and 2.464(1) Å in 3, consistent with a (WdW) 8+ core. The isoprene structure 3 reveals a disorder involving primarily the location of the isoprene C-Me group over two sites. The variabletemperature 1 H and 13 C{ 1 H} NMR spectra reveal the dynamic equilibria involving 1 + the diene and 2 and 3. The spectra of 2 and 3 are entirely reconcilable with the solid-state structures, and for the deuterated complexes W 2 (OCD 2 t Bu) 6 (diene)(py) all the 1 H signals of the diene are assigned by a combination of COSY and HETCOR spectra. For 3 there exists a mixture of the two isomers, differing with respect to the position of the isoprene C-Me group as seen in the solid-state structure. These interconvert by a reversible dissociative pathway. Though other 1,3-dienes have not been found to form adducts, we have found selective 1,2-hydrogenation to give 3-enes in the presence of W 2 (OCH 2 t Bu) 6 (py) 2 . This hydrogenation is not restricted to dienes but is also observed for cis-cyclic olefins. The addition of D 2 to norbornene reveals the stereospecific cis-addition at the endo sites. The addition of D 2 to 1,3-butadiene and isoprene gives CH 2 DCHDCHdCH 2 and CH 2 DCHDCMedCH 2 , respectively. These results are discussed in terms of other substrate activations by W 2 (OR) 6 compounds and other hydrogenation reactions.

By the use of Nicholson's method, the heterogeneous electron-transfer rate constants (ks) for the oxidation of a series of M2(O2CR)4 complexes have been determined in benzonitrile, where the metal M = Mo, W, Ru, or Rh and R = alkyl or aryl. For R = tBu, the values of ks follow the order M = Mo > W > Ru > Rh. No simple influence of R on ks was observed, although added ligands that are known to reversibly bind to the dinuclear center were shown to influence the E1/2 values in order of their basicity and to suppress the rate of electron transfer. The reported data are compared with those obtained for Cp2Fe0/+, Cp2*Fe0/+, and Ru(bpy)2(2)+/3+ and with earlier work on dirhenium multiply bonded compounds.

No abstract