Functional metal complexes based on bridging “imino”-quinonoid ligands

• The [NR] for [O] isoelectronic substitution is a sound strategy for generating new kinds of bridging quinone ligands.
• Structural data of the metal complexes display general trends of bond localization and delocalization.
• The metal complexes display intriguing properties like NIR electrochromism and switchable molecular magnetic properties.
• Such metal complexes can also be used for bond activation reactions and catalysis.

2,5-Dihydroxy-1,4-benzoquinone (dhbq) has been extensively used as a ligand for generating mono and polynuclear complexes. In recent years, new ligand classes have been developed where one or more of the “O” groups of the dhbq unit have been replaced with isoelectronic “NR” groups. The resulting ligands have been used for synthesizing dinuclear assemblies that have been utilized for investigating metal–metal interaction, generation of electrochromic NIR dyes, studies of unusual chemical reactivity, for small molecule activation and for homogenously catalysis. Additionally, these ligand platforms have also been used for studying spin–spin coupling and for the generation of switchable molecular magnetic materials. In the following, we report on the recent developments on the generation of functional metal complexes with this ligand class, with a particular focus on the ligands with [O, N, O, N] donor sets based on a 2,5-di-[(substituted)-anilino]-1,4-benzoquinone platform.

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