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Interlocked Systems in Catalysis and Switching
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
Metallacycles are observed as reaction intermediates in many organometallic reactions. Ligand modification allows synthesis of metallacycles of different nuclearities. Modifications of ligands on a metallacycle recognise substrate. Metallacycles are able to adopt porous structures which provide advantages over conventional catalysts. For example, the metallacycle 4.13a composed of a diazapyrenium ligand coordinated to platinum(II) is a reusable catalyst for the nucleophilic substitution reaction of 3,5-dinitro-bromobenzene with sodium azide (Figure 4.13). The substitution reaction is 1,000 times faster than an uncatalysed reaction. The nitrate salt of the ligand has poor catalytic activity. The addition of the nonreactive aromatic compound pyrene to the reaction mixture of 3,5-dinitro-bromobenzene with sodium azide in the presence of catalytic amounts (10 mol %) of the platinum complex drastically decreases the reaction rate. Beyond the porous nature of the complex, the interior portion of the pores being filled with π-deficient aromatic molecules makes ways for pyrene, thereby decreasing reactivity. The catalytic ability of the complex increases by association of the azide to the diazapyrenium cationic subunit 4.13b.
Understanding oxidative addition in organometallics: a closer look
Published in Journal of Coordination Chemistry, 2022
Nabakrushna Behera, Sipun Sethi
However, an unsaturated substrate which allows its multiple bonds to participate in the oxidative addition process involves a slightly different approach. Unlike substrate molecules which use their single σ-bond in the oxidative addition for forming two separate anionic ligands, the unsaturated substrate sacrifices only its π-bond toward the cleavage leaving the σ-bond intact. Accordingly, it gets converted to a dianionic ligand after receiving two-electrons from the metal center and then coordinates to the same metal. Hence, the formation of a three-membered metallacycle is observed. For instance, the addition of molecular oxygen to 1 does not cleave the O = O bond (Scheme 7) [5, 17]. In the product 18, the oxygen molecule acts as if it is linked to the metal center in cis location as O22− ion. It is important to mention that although the reaction of 1 with molecular oxygen is found to be reversible which mimics the action of hemoglobin, the binding of O2 in both systems is unusual; with 1 it is bound side-on and end-on with hemoglobin. Reaction of Buckminsterfullerene C60 with 1 results in an analogous product, 19, as in the case of O2 (Scheme 7) [5, 18]. These are all classical type ‘A’ oxidative addition reactions.