Trinuclear copper complexes as biological mimics: Ligand designs and reactivities

Tris-copper centers are present at the active site of multicopper oxidases (MCO) which couple the four electron reduction of molecular oxygen to water with the oxidation of substrates. Modelling these sites with small molecular complexes has thus attracted the interest of many groups in the (bio)inorganic community over the past three decades and still appears a challenge. These enzymes and their model complexes presently enjoy a renewed interest as potential non-precious metal catalyst for the Oxygen Reduction Reaction. Moreover recent work has revealed that tris-copper centers can catalyze methane oxidation. Therefore the elaboration of tris-copper system constitutes an important and timely issue. The aim of the present review is to analyze the various attempts at preparing tris-copper complexes in terms of strategy of ligand design. Of course the challenge to be met is to force three independent binding sites to converge and react in concert. Three main approaches have been developed to anchor these binding sites based on the use of (i) a node either (a) a single atom node (tren-based systems and related), or (b) an hexa-atom node (mesityl-based systems and derivatives), (ii) macrocyclic systems, and (iii) combination of mono- and dinuclear sites. The structures of the different systems will be described and analyzed accordingly. Then the various reactivities exhibited by these systems will be presented so as to evaluate how the ligand design influences the reactivity and to discern promising future directions. (C) 2018 Elsevier B.V. All rights reserved.