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 (<i>k</i>₁ ≈ 10^–4 M^–1 s^–1) are 2–3 orders of magnitude faster than those for the second step (<i>k</i>₂ ≈ 10^–6 M^–1 s^–1). Experiments at different temperatures provide activation parameters with values of Δ<i>H</i>^⧧ ≈ 9–18 kcal mol^–1 and Δ<i>S</i>^⧧ ≈ −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-<i>tert</i>-butyl-2,2′-bipyridine

Treatment of the triangular [Mo₃S₄Cl₃(dbbpy)₃]Cl cluster ([<b>1</b>]­Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo₃(CuCl)­S₄Cl₃(dbbpy)₃]­[CuCl₂] ([<b>2</b>]­[CuCl₂]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4′-di-<i>tert</i>-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 [<b>1</b>]⁺ 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 [<b>2</b>]⁺ 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

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic Mo₃CuS₄ Clusters with 4,4′-Di-<i>tert</i>-butyl-2,2′-bipyridine

Treatment of the triangular [Mo₃S₄Cl₃(dbbpy)₃]Cl cluster ([<b>1</b>]­Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo₃(CuCl)­S₄Cl₃(dbbpy)₃]­[CuCl₂] ([<b>2</b>]­[CuCl₂]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4′-di-<i>tert</i>-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 [<b>1</b>]⁺ 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 [<b>2</b>]⁺ 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