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.

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic Mo3CuS4 Clusters with 4,4'-Di-tert-butyl-2,2'-bipyridine.

Treatment of the triangular [Mo3S4Cl3(dbbpy)3]Cl cluster ([1]Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo3(CuCl)S4Cl3(dbbpy)3][CuCl2] ([2][CuCl2]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This species, which contains two distinct types of Cu(I), is the first example of a diimine-functionalized heterometallic M3M'S4 cluster. Kinetics studies on both the formation of the cubane from the parent trinuclear cluster and its dissociation after treatment with halides, supported by NMR, electrospray ionization mass spectrometry, cyclic voltammetry, and density functional theory calculations, are provided. On the one hand, the results indicate that addition of Cu(I) to [1]+ is so fast that its kinetics can be monitored only by cryo-stopped flow at -85 °C. On the other hand, the release of the CuCl unit in [2]+ is also a fast process, which is unexpectedly assisted by the CuCl2- counteranion in a process triggered by halide (X-) anions. The whole set of results provide a detailed picture of the assembly-disassembly processes in this kind of cluster. Interconversion between trinuclear M3S4 clusters and their heterometallic M3M'S4 derivatives can be a fast process occurring readily under the conditions employed during reactivity and catalytic studies, so their occurrence is a possibility that must be taken into account in future studies.

On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M3 S4 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.

An opened route to 1,3-dimethylenecyclobutanes via sequential ruthenium-catalyzed [2 + 2] cycloaddition of allenyl boronate and palladium Suzuki coupling.

The regioselective head-to-head [2 + 2] cyclodimerization of allenyl boronate catalyzed by the ruthenium catalyst [Cp*RuCl(COD)] leads to a novel diboronated 1,3-dimethylenecyclobutane. Consecutive palladium-catalyzed C-C couplings open a route to novel disubstituted 1,3-dimethylenecyclobutane species. The X-ray crystalline structure of the phenyl-substituted 1,3-dimethylenecyclobutane is provided.

First X-ray characterization and theoretical study of pi-alkyne, alkynyl-hydride, and vinylidene isomers for the same transition metal fragment [Cp*Ru(PEt3)2]+.

The reaction of the chloro-complex [CpRuCl(PEt(3))(2)] with acetylene gas in methanol gave the pi-alkyne complex [CpRu(eta(2)-HCtbd1;CH)(PEt(3))(2)][BPh(4)] (1), which has been structurally characterized by X-ray analysis. The alkyne complex undergoes spontaneous isomerization even at low temperature, yielding the metastable alkynyl-hydride complex [CpRu(H)(Ctbd1;CH)(PEt(3))(2)][BPh(4)] (2), as the result of the oxidative addition of the alkyne C-H bond. This compound has also been structurally characterized despite it tautomerizes spontaneously into the stable primary vinylidene [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3). This species has been alternatively prepared by a two-step deprotonation/protonation synthesis from the pi-alkyne complex. Moreover, the reaction of the initial chloro-complex with monosubstituted alkynes HCtbd1;CR (R = SiMe(3), Ph, COOMe, (t)Bu) has been studied without detection of pi-alkyne intermediates. Instead of this, alkynyl-hydride complexes were obtained in good yields. They also rearrange to the corresponding substituted vinylidenes. In the case of R = SiMe(3), the isomerization takes place followed by desilylation, yielding the primary vinylidene complex. X-ray crystal structures of the vinylidene complexes [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3) and [CpRu(=C=CHCOOMe)(PEt(3))(2)][BPh(4)] (10) have also been determined. Both, full ab initio and quantum mechanics/molecular mechanics (QM/MM) calculations were carried out, respectively, on the model system [CpRu(C(2)H(2))(PH(3))(2)](+) (A) and the real complex [CpRu(C(2)H(2))(PEt(3))(2)](+) (B) to analyze the steric and electronic influence of ligands on the structures and relative energies of the three C(2)H(2) isomers. QM/MM calculations have been employed to evaluate the role of the steric effects of real ligands, whereas full ab initio energy calculations on the optimized QM/MM model have allowed recovering the electronic effects of ligands. Additional pure quantum mechanics calculations on [CpRu(C(2)H(2))(PH(3))(2)](+) (C) and [CpRu(C(2)H(2))(PMe(3))(2)](+) (D) model systems have been performed to analyze in more detail the effects of different ligands. Calculations have shown that the steric effects induced by the presence of bulky substituents in phosphine ligand are responsible for experimentally observed alkyne distortion and for relative destabilization of the alkyne isomer. Moreover, increasing the phosphine basicity and sigma donor capabilities of ligands causes a relative stabilization of an alkynyl-hydride isomer. The combination of both steric and electronic effects, makes alkyne and alkynyl-hydride isomers to be close in energy, leading to the isolation of both complexes.