Mechanisms of addition of primary aromatic amines and cyclohexylamine to tricarbonyl(cycloheptatrienyl)tungsten cation

X-substituted anilines (X = H, 2-Me, 4-Me or 2-Cl) and cyclohexylamine are shown to add to the tropylium ring of cation [(η)-C₇H₇)W(CO)₃]⁺ to give the corresponding ring adducts of tricarbonyl(cyclohepta-1,3,5-triene)tungsten. Kinetic studies have demonstrated that the anilines form the triene ring adducts via the rapid pre-equilibrium formation of a π-complex, which rearranges in a rate-determining fashion to form a cationic triene intermediate [(1–6-η-C₆H₇.NH₂R)-W(CO)₃]⁺ (R = C₆H₅, 2-MeC₆H₄, 4-MeC₆H₄ or 2-ClC₆H₄); from this the final product is rapidly formed via amine- or solvent-assisted proton loss. With the aliphatic cyclohexylamine, the cationic triene intermediate is formed directly, followed by competing rate-determining solvent- and amine-assisted proton removal.     

Kinetics of nucleophilic attack on coordinated organic moieties : XII. Addition of triphenylphosphine to [(C8H11)Co(C5H5)]BF4

A stopped-flow investigation of the reversible addition of Ph₃P to [(C₈H₁₁)Co(C₅H₅)]⁺ indicates the rate law, k_obs = k₁[Ph₃P] + k_?1. The low Δ2 of 21.0 ± 1.2 kJ mol^?1 and the negative ΔS2 of ?114 ± 5 J K^?1 mol^?1 are consistent with rapid addition to the enyl ligand. The higher Δ2 of 86.2 ± 5.1 kJ mol^?1 and the positive ΔS2 of +60 ± 17 J K^?1 mol^?1are as expected for the reverse dissociation. Preliminary studies show that the related complex [(C₇H₉)Co(C₅H₅)]⁺ is at least 65 times more electrophilic towards Ph₃P.     

Use of chiral capillary electrophoresis and circular dichroism for the determination of absolute values of Δε for diimine transition metal complexes

Circular dichroism (CD) alone cannot be used to derive absolute values of Δε for optically active transition metal (TM) complexes in the frequent instances in which pure, resolved standards are unavailable. In the past, this has led to discrepancies in the literature regarding the most appropriate value for use in assessing optical purity. We have recently shown that capillary electrophoresis (CE), employing a chiral selecting agent in the buffer, is a highly effective means to fully resolve the Λ and Δ enantiomers of [M(diimine)₃]ⁿ⁺ complexes. In this extension to that work, CE peak areas for optically active (but not optically pure) TM complexes are used in combination with CD spectra that are independently obtained for the same samples to extrapolate absolute circular dichroism values, Δε, for the pure Δ and Λ isomers. Examples are provided for a variety of tris- and bis-diimine systems, including different coordinated metal (e.g., Cr, Ni, Ru, Co) and ligand (e.g., bipyridine, phenanthroline, oxalato, cyano, etc.) combinations rendering outer sphere complex charges of 0, +1, +2 and +3. Based on the absolute values for Δε presented in this report, accurate measurement of optical purity for a number of [M(diimine)₃]ⁿ⁺ and cis-[M(diimine)₂X₂]ⁿ⁺ systems is now possible via routine CD analysis.     

Cyclic voltammetry of silyl- and stannyl-tricobalt cluster compounds

The electrochemical behavior of the clusters PhM′Co₃(CO)_n (M′ = Si, n = 11 and M′ = Sn, n = 12) has been examined via cyclic voltammetry in CH₂Cl₂ solution. In both cases the radical anions PhM′Co₃(CO)_n? are unstable, rearranging to Co(CO)₄ and other products. However, for the silicon compound, where there is one formal cobalt—cobalt bond, the radical anion is sufficiently stable to allow for detection of anion reoxidation at medium scan speeds. The half-life of PhSiCo₃(CO)₁₁? has been calculated as 2.3 s at 25°C.