Synthesis and characterization of [W( η5-C 5H 5) 2(CH 2CH 3)( μ-O)W( η5-C 5H 5)(O)(CH 2CH 3)]PF 6, a mixed valence oxo alkyl dimer in which d 0 and d 2 tungsten centers are connected by an asymmetric oxo bridge

Abstract

Reaction of the ethylene hydride complex [W( η 5-C 5H 5) 2(CH 2=CH 2)H]PF 6( 4PF 6) with mild oxygen atom sources such as dimethyl sulfoxide or propylene oxide gives the terminal oxo ethyl complex [W( η 5-C 5H 5) 2(O)(CH 2CH 3)]PF 6 ( 3PF 6). Reaction of this with PMe 2Ph gives a strongly colored compound which crystallizes in the monoclinic space group C2/ c with a = 19.483(4)Å, b = 22.397(6)Å, c = 14.289(3)Å, β = 133.59(1)°. A single crystal X-ray diffraction study ( R F = 5.75%, R w = 5.31%) has established that the compound contains the cationic dimer [W( η 5-C 5H 5) 2(CH 2CH 3)( μ-O)W( η 5-C 5H 5)(O)(CH 2CH 3)] + ( 1 +). Metric parameters support formulation of 1 + as a mixed valence dimer of W(IV) and W(VI) in which a mono-cyclopentadienyl tungsten oxo alkyl [W( η 5-C 5H 5)(O) 2(CH 2CH 3)] is coordinated through an oxo ligand to a cationic tungstenocene alkyl “[W( η 5-C 5H 5) 2(CH 2CH 3)] +”. The oxo bridge is asymmetric, with a double bond (1.78(1)Å) to the mono-cyclopentadienyl tungsten center and a single bond (2.03(1)Å) to the bis-cyclopentadienyl tungsten, while the terminal oxo ligand has a triple bond to tungsten (1.68(1)Å). The strong absorbance of 1 + at 583 nm is assigned to a metal to metal charge transfer transition. Formation of 1 + is proposed to involve initial nucleophilic addition of PMe 2Ph to a cyclopentadienyl ligand in 3 + to give a transient cyclopentadiene ligand which is displaced by the oxo ligand of a second equivalent of 3 +. This sequence is supported by 1H and 31P NMR studies which establish that there is a single P containing product formulated as the cyclopentadiene-substituted phosphonium salt [P(C 5H 5)Me 2Ph] + ( 5 +), as confirmed by independent synthesis and spectroscopic characterization of 5 + by addition of C 5H 5I to PMe 2Ph. Photolysis of 1 + results in dissociation of the monocyclopentadienyl oxo alkyl fragment [W( η 5-C 5H 5)(O) 2(CH 2CH 3)]( 2), which has been independently characterized, and formation of the alkene hydride [W( η 5-C 5H 5) 2(CH 2=CH 2)H] +.