Coordination chemistry of carboxamide ‘Nx’ ligands to metal ions for bio-inspired catalysis

Abstract

Deprotonated carboxamidate ‘N’ donors have been extensively explored as ligands to support metal ions with unusual higher oxidation states. Their ability to serve as both σ and π-electron donor, being resistant to oxidation and provide exceptional hydrolytic stability to a coordinated metal ion has altogether enhanced the broad applicability of carboxamidate ligands in bio-mimetic catalysis. Although the biological precedents with such class of ligands are very less (Nitrile hydratases, Bleomycin, nickel superoxide dismutases, etc.), the synthetic modelling of various metalloenzymes employing carboxamidate ligands have emerged rapidly in the past few decades. The present review accounts for the brief discussion on the unique electronic structure and coordination chemistry these ligands exhibit and exclusively outlines the significant developments in bio-mimetic oxidation chemistry with special reference to high valent metal oxidants and their reactivity towards organic substrates. The notable achievements obtained by using these classes of ligands stretches back to 1848 for the development of the popular biuret reaction followed by the exploration of bio-mimetic metal-peptide complexes, pyridylcarboxamides for stabilization high valent metal-oxo species of late first row transition metal ions. The design of urea based tripodalcarboxamide ligands by Borovik et al. to install hydrogen bonding interactions to stabilize oxo/hydroxo-metal complexes also renders significant development in the bio-inspired catalysis. To the best of our knowledge, the most important outcome of research employing carboxamide ligands has been the discovery of tetraamido macrocyclic ligands (TAMLs) by Collins and co-workers who managed to make seven generations of the Fe-TAML catalyst that find interesting applications.