Emilio Bustelo

Hydrido(3‐hydroxyalkynyl) Complexes as Intermediates in the Activation of Propargyl Alcohol Derivatives by [Cp*RuCl(dippe)] [Cp* = C5Me5, dippe = 1,2‐bis(diisopropylphosphanyl)ethane]

The reaction of propargyl alcohol derivatives with the complex [Cp*RuCl(dippe)] [dippe = 1,2-bis(diisopropylphosphane)ethane] and NaBPh4 in MeOH yields hydrido(3-hydroxyalkynyl) compounds [Cp*Ru(H){C≡CC(OH)RR′}(dippe)][BPh4] [R, R′ = Ph, Ph (1a); H, Ph (1b); H, Me (1c)]. These represent intermediates in the formation of 3-hydroxyvinylidene species [Cp*Ru{=C=CHC(OH)RR′}(dippe)][BPh4] [R, R′ = Ph, Ph (2a); H, Ph (2b); H, Me (2c)], into which they irreversibly rearrange both in solution and in the solid state. Solution kinetic studies have been carried out on this isomerization process. Ulterior dehydration processes are feasible, resulting in the formation of allenylidene [Cp*Ru(=C=C=CRR′)(dippe)][BPh4] [R, R′ = Ph, Ph (3a); H, Ph (3b)] or vinylvinylidene [Cp*Ru{=C=CHCH(=CH2)}(dippe)][BPh4] (4) species. The X-ray crystal structure of the novel secondary allenylidene complex [Cp*Ru(=C=C=CHPh)(dippe)][BPh4] is presented.

Cycloaddition of alkynes to diimino Mo3S4 cubane-type clusters: a combined experimental and theoretical approach

A heterocyclic ligand 4,4′-di-tert-butyl-2,2′-bipyridine (dbbpy) has been coordinated to the Mo3S4 cluster unit affording the complex [Mo3S4Cl3(dbbpy)3]+ ([1]+) in a one-step ligand-exchange protocol from [Mo3S4(tu)8(H2O)]Cl4·4H2O (tu = thiourea). The new cluster was isolated as [1]PF6 and [1]Cl salts in high yields and the crystal structure of the latter determined by X-ray analysis. The synthetic procedure was extended to tungsten to afford [W3S4Cl3(dbbpy)3]+ ([2]+). Kinetic and NMR studies show that [1]+ reacts with several alkynes to form dithiolene species via concerted [3+2] cycloaddition reactions whereas [2]+ remains inert under similar conditions. The different rates for the reactions of [1]+ are rationalised by computational (DFT) calculations, which show that the more electron-withdrawing the substituents of the alkyne the faster the reaction. The inertness of [2]+ is due to the endergonicity of its reactions, which feature ΔGr values systematically 5–7 kcal mol−1 more positive than for those of [1]+.

On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M3S4 Clusters with Alkynes: Insights from Experiment and Theory

Whereas the cluster [Mo3 S4 (acac)3 (py)3 ](+) ([1](+) , acac=acetylacetonate, py=pyridine) reacts with a variety of alkynes, the cluster [W3 S4 (acac)3 (py)3 ](+) ([2](+) ) remains unaffected under the same conditions. The reactions of cluster [1](+) show polyphasic kinetics, and in all cases clusters bearing a bridging dithiolene moiety are formed in the first step through the concerted [3+2] cycloaddition between the C≡C atoms of the alkyne and a Mo(μ-S)2 moiety of the cluster. A computational study has been conducted to analyze the effect of the metal on these concerted [3+2] cycloaddition reactions. The calculations suggest that the reactions of cluster [2](+) with alkynes feature ΔG(≠) values only slightly larger than its molybdenum analogue, however, the differences in the reaction free energies between both metal clusters and the same alkyne reach up to approximately 10 kcal mol(-1) , therefore indicating that the differences in the reactivity are essentially thermodynamic. The activation strain model (ASM) has been used to get more insights into the critical effect of the metal center in these cycloadditions, and the results reveal that the change in reactivity is entirely explained on the basis of the differences in the interaction energies Eint between the cluster and the alkyne. Further decomposition of the Eint values through the localized molecular orbital-energy decomposition analysis (LMO-EDA) indicates that substitution of the Mo atoms in cluster [1](+) by W induces changes in the electronic structure of the cluster that result in weaker intra- and inter-fragment orbital interactions.

Kinetic Analysis and Mechanism of the Hydrolytic Degradation of Squaramides and Squaramic Acids

The hydrolytic degradation of squaramides and squaramic acids, the product of partial hydrolysis of squaramides, has been evaluated by UV spectroscopy at 37 °C in the pH range 3-10. Under these conditions, the compounds are kinetically stable over long time periods (>100 days). At pH >10, the hydrolysis of the squaramate anions shows first-order dependence on both squaramate and OH-. At the same temperature and [OH-], the hydrolysis of squaramides usually displays biphasic spectral changes (A → B → C kinetic model) with formation of squaramates as detectable reaction intermediates. The measured rates for the first step (k1 ≈ 10-4 M-1 s-1) are 2-3 orders of magnitude faster than those for the second step (k2 ≈ 10-6 M-1 s-1). Experiments at different temperatures provide activation parameters with values of ΔH⧧ ≈ 9-18 kcal mol-1 and ΔS⧧ ≈ -5 to -30 cal K-1 mol-1. DFT calculations show that the mechanism for the alkaline hydrolysis of squaramic acids is quite similar to that of amides.

Analysis of the Solid-State Rearrangement of Hydrido-Alkynyl Ruthenium Complexes to their Vinylidene Tautomers

The solid-state tautomerization of the hydrido-alkynyl derivatives [Cp*RuH(C≡CR)-(dippe)][BPh4] (Cp* = C5Me5; R = SiMe3, Ph, H; dippe = l,2-bis-(diisopropylphosphino)-ethane) to their vinylidene isomers [Cp*Ru=C=CHR(dippe)][BPh4] was studied by IR spectroscopy. Characteristic isothermic α vs. t curves for each individual rearrangement process were recorded. Their shape, and hence the isomerization mechanism, depends strongly on the nature of the substituent R. The kinetic analysis of the above curves using the Avrami-Ewfeev provided some mechanistic information about the isomerization process in the solid.