It has previously been demonstrated that both [(C5Me5)Ir(PMe3)(CH=CH2)H] and [(C5Me5)Ir(PMe3)(H2C=CH2)] are formed when [(C5Me5)Ir(PMe3)] is thermolytically generated in the presence of ethylene. At higher temperatures, the vinyl hydride is converted to the 2 -ethylene adduct. Density functional theory has now been used to investigate this reaction, using the B3LYP functional, two types of basis sets (LanL2DZ and TZV*) and two models of the [( C5R5)Ir(PR3)] species (R = H and CH3). The study consists of full optimizations of local minima, first order saddle points, and minimum energy crossing points (MECP). The experimental results are best accounted for by considering both singlet and triplet spin surfaces. The relative energies of singlet [(C5R5)Ir(PR3)(CH3)H], [(C5R5)Ir(PR3)(CH=CH2)H], and [(C5R5)Ir(PR3)(H2C=CH2)] are in good agreement with experiment, as is the calculated barrier for the conversion from the vinyl hydride to the 2 -alkene complex. However, the singlet surface alone fails to explain the experimentally observed product ratio, or the intermediate inferred from experimental isotope effect studies. Locating the MECP between singlet and triplet surfaces indicates that the thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5R5)Ir(PR3)]. The weak van der Waals adduct of triplet [(C5R5)Ir(PR3)] and ethylene is proposed to be the key intermediate in the overall reaction.The interchanging of the available ethylene CH bonds in this triplet -complex accounts for the observed kinetic isotope effects, and partitioning between alkene -complexation and CH bond activation may also occur from this common intermediate. The possible role of steric factors and molecular dynamics are also discussed.
Convenient syntheses for Cp, Cp*, and related cyclopentadienyl derivatives ( 4 Cp = C 5 HiPr 4 ; CpЈЈЈ = C 5 H 2 tBu 3 -1,2,4) of formula [(Ring) 2 Mo 2 O 5 ] are described. Compound [Cp 2 Mo 2 O 5 ] was produced in good yields by the rapid oxidation of red [CpMoO 2 ] 4 with PhIO in CH 2 Cl 2 . [Cp* 2 Mo 2 O 5 ] was obtained by CH 3 COOH acidification of aqueous solutions of [Cp*MoO 3 ] − Na + , the latter being generated in a single step from [Cp*MoCl 4 ] and more than 5 equiv. of aqueous NaOH in air. Minor quantities of [Cp*MoO 2 ] 2 were also isolated from this reaction, although the formation of this by- [a]
Electronically and coordinatively unsaturated [Cp*W(NO)(L)] complexes have been postulated as intermediates in several related systems. Model [CpW(NO)(L)] compounds (L PH 3 , CO, CH 2 , H 2 CCH 2 , HCCH) have been investigated theoretically by means of density functional theory computational techniques. The structural parameters calculated for saturated [CpW(NO)(PH 3 )(L)] complexes are in good agreement with the solid-state molecular structures determined crystallographically for the corresponding [Cp*W(NO)(PMe 3 )(L)] compounds. The 16-electron, singlet [CpW(NO)(L)] species have geometries comparable to those of the same fragment in thephosphine adducts and include a highly pyramidal conformation at W. The energy of the triplet spin state is calculated to be close to or even lower than that of the singlet state for these unsaturated compounds, and depends largely on the p-bonding capabilities of L (DE sÀt DE t À DE s À 3.3 kcal mol À1 (PH 3 ), 2.8 (CO), 2.4 (CH 2 ), 6.3 (H 2 CCH 2 ), À 2.3 (HCCH)). The optimization of partially constrained structures in both spin states allows for a conformational analysis of the [CpW(NO)(L)] species.The inversion of the conformation of the pyramidal singlet [CpW(NO)(L)] complexes via the planar-at-W triplet species (two-state pathway) is calculated to be competitive with the equivalent process solely along the singlet spin hypersurface. Rotation of the W ± CH 2 bond in the singlet carbene species is also found to proceed more readily via a two-state pathway. The preferred alkyne conformation, the unusually stable triplet states, and the strong W-to-L p-donation observed in these systems may all be rationalized by the relatively high energies of the occupied orbitals of the formally W 0 compounds.
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