Preparation and Characterization of 1,3-Butadiene and Isoprene Complexes, W₂(OCH₂^tBu)₆(diene)(py), and Studies of the Selective Hydrogenation of 1,3-Dienes

In hydrocarbon solutions W-2((OCH2Bu)-Bu-t)(6)(py)(2), 1, and 1,3-butadiene and isoprene reversibly form adducts W-2((OCH2Bu)-Bu-t)(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 O3NW=WNO3 core with W-W = 2.334(1) Angstrom and each W atom is in a distorted square-based pyramidal environment with the W=W bond in the apical site. In 2 and 3 there is a common W2O6C4N core wherein the diene adopts a novel mode of coordination, mu,eta(1),eta(4). One W atom is coordinated to five oxygen atoms and the bridging carbon atom of the mu-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 mu-metalated pi (eta(3)-)allyl or a 2- ligand. The W-W distances are 2.471(1) Angstrom in 2 and 2.464(1) Angstrom in 3, consistent with a (W=W)(8+) core. The isoprene structure 3 reveals a disorder involving primarily the location of the isoprene C-Me group over two sites. The variable-temperature H-1 and C-13{H-1} 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((OCD2Bu)-Bu-t)(6)(diene)(py) all the H-1 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((OCH2Bu)-Bu-t)(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 CH2DCHDCH=CH2 and CH2DCHDCMe=CH2, respectively. These results are discussed in terms of other substrate activations by W-2(OR)(6) compounds and other hydrogenation reactions.