Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

The complex [Cp*RuCl(iPr2PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C6H4CF3) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru≡C≡CHR(iPr2PNHPy)]Cl (R = COOMe (2a- Cl), C6H4CF3 (2b-Cl)), whereas reaction of 1 with NaBPh4 in MeOH followed by addition of HC≡CR (R = COOMe, C6H4CF3) yields the metastable π-alkyne complexes [Cp*Ru(η2-HC≡CR)(iPr2PNHPy)][BPh4] (R = COOMe (3a-BPh4), C6H4CF3 (3b-BPh4)). The transformation of 3a-BPh4/3b-BPh4 into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HC≡CR (R = COOMe, C6H4CF3) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η2-HC≡CR)(iPr2PNHPy)]Cl (R = COOMe (3a-Cl), C6H4CF3 (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh4]− anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanolassisted hydrogen migrations. Calculations suggest participation of a hydrido−alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles

REACTIONS OF {eta}{sup 5}-CYCLOPENTADIENYLCOBALT(III) ALKYLS WITH COBALT(I) PHOSPHINES AND IRON CARBONYLS. EVIDENCE FOR DIRECT {eta}{sup 5} -CYCLOPENTADIENYL AND TRIMETHYLPHOSPHINE GROUP TRANSFER BETWEEN METAL CENTERS

We have found that {eta}{sup 5}-methylcyclopentadienyl- (triphenylphosphine)dimethylcobalt(III) (1) undergoes intermolecular cyclopentadienyl ligand exchange with {eta}{sup 5}-cyclopentadienylbistriphenylphosphinecobalt(I). The unsubstituted cyclopentadienyl(triphenylphosphine)dimethylco-balt(III) undergoes exchange of phosphine for carbon monoxide with both Fe(CO){sub 4} and Fe(CO){sub 5} by two different mechanisms. The first involves electrophilic displacement of coordinated phosphine by unsaturated Fe(CO){sub 4} and the second takes place by electrophilic displacement of CO from Fe(CO){sub 5} by the unsaturated CpCoMe{sub 2} fragment (generated by phosphine dissociation from the saturated starting material)

Phosphine Substitution in n5-Cyclopentadienyl-Bis-Triphenylphosphinecobalt (I): Evidence for a Dissociative Mechanism

The substitution of trimethylphosphine for triphenylphosphine in {eta}{sup 5}-cyclopentadienyl-bis-triphenylphosphinecobalt(I), 1, to form {eta}{sup 5}-cyclopentadienyltrimethylphosphinetriphenylphosphinecobalt(I) was studied at -60°C in an NMR spectrometer. Kinetic measurements show the process to be first order in 1 and zero order in PMe{sub 3}; added PPh{sub 3} strongly inhibits the reaction rate. This information indicates the reaction proceeds by rapid reversible phosphine dissociation through the unsaturated CpCo(PPh{sub 3}) intermediate. The rate for generation of that intermediate, k{sub 1}, is 1.15 x 10{sup -3} sec{sup -1} while the ratio of rate constant k{sub 2} (for conversion of intermediate to products) to k{sub -1} (return to starting materials) is 4 at -60°C. Possible structures for CpCo(L) are discussed in light of recent indications that the linear structure has an open-shell electronic configuration

Carbon Dioxide Activation by "Non-nucleophilic" Lead Alkoxides

A series of terminal lead alkoxides have been synthesized utilizing the bulky beta-diketiminate ligand [{N(2,6-(Pr2C6H3)-Pr-i)-C(Me)}(2)CH](-) (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)PbOR reacts with methyl iodide only under forcing conditions yet reacts readily, but reversibly, with carbon dioxide. The degree of reversibility is strongly dependent upon minor changes in the R group. For instance, when R = isopropyl, the reversibility is only observed when the resulting alkyl carbonate is treated with other heterocumulenes; however, when R = tert-butyl, the reversibility is apparent upon any application of reduced pressure to the corresponding alkyl carbonate. The differences in the reversibility of carbon dioxide insertion are attributed to the ground-state energy differences of lead alkoxides. The mechanism of carbon dioxide insertion is discussed