Synthesis, Structure, and Properties of Homoleptic l-Oxa-l,3-diene Molybdenum and Tungsten Complexes

Z. Naturforsch. 50b, 315-325 (1995); received September 7, 1994 Oxadiene Complex, Molybdenum, Tungsten, X-Ray, Selective Hydrogenation Homoleptic tris(l-oxa-l,3-diene)molybdenum and tungsten complexes are formed as yel­ low, crystalline compounds in 54-80% isolated yield on treatment of the corresponding /3-unsubstituted a,/3-unsaturated ketones with W(CO)3(CH3CH2CN)3 or (y6-C6H6)Mo(CO)3 in refluxing hexane. Reaction of these oxadienes with (^6-C6H 5CH3)2Mo in tetrahydrofuran or /7-hexane provides a new high yield route to homoleptic molybdenum complexes that are otherwise difficult to prepare. X-ray crystal structure analyses of two air-stable, isomeric tungsten complexes reveal a significant contribution of a cr2,?/2-bonding mode in the coordi­ nated oxadiene moieties. Under reaction conditions that allow the platinum catalyzed hydro­ genation of phenyl substituents in these ligands, the coordinated oxadiene fragments are inert, thus demonstrating an unprecedented chemoselectivity in the hydrogenation of arylsubstituted enones. On the other hand, the complexes are labile enough to catalyze the cyclotrimerization of alkynes such as methyl propiolate.