Abstract
Guanidino groups and guanidinate substituents are used to stabilise positive charges, both through mesomeric and through inductive effects. The interplay between several guanidino groups is employed to create redox-active compounds. One realisation concept leads to guanidino-functionalised aromatic compounds (GFAs), which comprise aromatic systems substituted with several guanidino groups and constitute a relatively new class of strong organic electron donors as well as redox-active ligands. Some properties of these compounds and applications are presented, e.g. photochemical reductive C–C coupling, redox switches and stabilisation of polyanions. In addition, we allude to dinuclear copper complexes of bridging GFA ligands with several oxidation states of copper and the GFA ligand. In a second concept, two guanidinyl groups are connected, leading to bisguanidines which are generally termed urea azines. They could be oxidised in two separated one-electron steps. GFAs and bisguanidines are compared with other organic electron donors, and a relationship is established between the (gas-phase) adiabatic ionisation energy and the redox potential in solution. Lewis acid-base adducts between boranes and bicyclic guanidines could be subjected to dehydrocoupling reactions, leading to new sp3–sp3-hybridised diboranes of special reactivity. Their coordination chemistry and oxidative insertion reactions into the B–B bond are discussed. The electron-rich bridging guanidinate substituents allow for the synthesis of unprecedented cationic boron hydride compounds.
Published Version
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