Preparation, X-ray Structure, and Reactivity of an Olefin-Carbene-Osmium Complex: α-Alkenylphosphine to α-Allylphosphine Transformation via an Osmaphosphabicyclopentane Intermediate

Abstract

The isopropenyldi(isopropyl)phosphine complex (1) reacts at room temperature with phenyldiazomethane to give the olefin-carbene derivative Os(η5-C5H5)Cl(CHPh){PiPr2[C(CH3)CH2]} (2), which has been characterized by X-ray diffraction analysis. In toluene at 70 °C, complex 2 evolves into the osmaphosphabicyclopentane compound (3). Treatment of 3 with TlPF6 at room temperature leads to a 1:1 mixture of the α-allylphosphine complex (4) and its α-alkenyl-γ-(η3-benzyl)phosphine isomer (5). The heating of the mixture in tetrahydrofuran at 66 °C gives rise to the isomerization of 5 into 4, which has been also characterized by X-ray diffraction analysis. The hydride ligand of 4 is fairly acidic. Its deprotonation with NaOCH3 leads to an equilibrium mixture of the neutral osmium(II) α-allylphosphine (6), α-alkenyl-γ-(η3-benzyl)phosphine (7), and α-alkenyl-γ-carbenephosphine (8) isomers. Complex 2 also reacts with TlPF6. The reaction affords the hydride-carbyne derivative [OsH(η5-C5H5)(⋮CPh){PiPr2[C(CH3)CH2]}]PF6 (9), which evolves into 4. The isomerization is a first-order process with activation parameters of ΔH⧧ = 23 ± 3 kcal·mol-1 and ΔS⧧ = −4 ± 4 cal·K-1·mol-1. The hydride ligand of 9 is also fairly acidic. Its deprotonation with NaOCH3 leads to the neutral carbyne Os(η5-C5H5)(⋮CPh){PiPr2[C(CH3)CH2]} (10), which reacts with methanol to give the hydride-alkoxycarbene derivative OsH(η5-C5H5){C(OMe)Ph}{PiPr2[C(CH3)CH2]} (11). The carbene carbon atom of 2 has amphiphilic character, reacting with electrophiles and nucleophiles. The reaction with HBF4 leads to the hydride-benzylcyclopentadienyl derivative (14) via η1-benzyl intermediates, whereas the reaction with CH3Li gives the hydride-styrene compound OsH(η5-C5H5)(η2-CH2CHPh){PiPr2[C(CH3)CH2]} (15).