Macrobicyclic decomposition

The kinetics and mechanism of synthesis and decomposition of macrocyclic compounds are regarded as one of the most important aspects in the chemistry of these compounds. The majority of papers concern metal ions complexing with preliminarily synthesized macrocyclic ligands and metal ion substitutions by other metal ions in the preliminarily prepared complexes. Template synthesis, the most promising approach to the directed preparation of macrocyclic compounds with desired structures [17], plays a still more decisive role in the chemistry of macrobicyclic complexes with encapsulated metal ion. However, the literature contains only scarce data on the kinetics and the mechanism of the template synthesis of macrocyclic compounds because of the difficulties encountered in experimental determinations of kinetic and thermodynamic parameters, such as low product yields, nonaqueous media, high temperatures, and side reactions. 

In this respect, much interest may be focused on the photoreduction cycle of water decomposition to produce hydrogen [384] this is represented by Scheme 123, in which S is a sensitizer and R is an electron-transfer agent (ETA), Q is an electron donor. The macrobicyclic complexes in this cycle act as ETAs. Such a cycle, in which [Ru(bpy)3]2 cation is used as a sensitizer and cation as an ETA, has already been implemented. 

The first model implies the rupture of the coordination bonds. This is inconsistent with the inertness of the macrobicyclic complex because its decomposition causes the rupture not only of M-N bonds, but also of C-C and C-H bonds. Therefore, this model predicts a relatively long lifetime of the state for the [Cr(sep)]3+ cation. In fact, the E states for [Cr(en)s] and [Cr(sep)] + cations have very similar lifetimes (of the order of 10 ps) in DMF at 0°C and similar spectral characteristics (both of them show an intense band at 15 151 cm-i and possess a quantitatively similar low-intensity vibronic structure). The macrobicyclic ligand ensures that ligand dissociation will have a large activation barrier, even in metal-centred electronic exited states. Neither the first model nor the second one adequately accounts for the photolytic similarities of [Cr(sep)]3+, [Cr(en)3] + and [Cr(NH3)e] cations. The third model seems the most realistic alternative [159]. 

Macrocyclic complexes of transition metals are widely used as both oxidising and reducing agents as well as catalysts in many redox processes [17, 50-56], In particular, macrobicyclic complexes such as [Co(L824)] and related compounds appear to be effective electron carriers in the photocatalytic decomposition of water [57-63],... 

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