Versatile Electronic and Structural Properties of Ruthenium-Mediated 1,4-Diaza-1,3-butadiene Ligands: A Review

Ruthenium α-diimine complexes are widely known for their unique low-lying excited states and potent light absorption capabilities in the visible and NIR regions. A few specially constructed Ru α-diimine complexes have also shown enormous potential in the biomedical field. Several Ru-diimine complexes are well known for their catalytic activity. In this context, the simplest α-diimine system is 1,4-diaza-1,3-butadiene, abbreviated as (DAB/DAD), which received the greatest attention from the research communities. DAB ligands are redox-active. DAB ligands typically function as effective electron donors by employing lone pairs of nitrogen atoms and the π-electrons of the C=N bonds while coordinating the metal ion. The structural, electronic, and photophysical properties of the metal-mediated (DAB) complexes largely depend on the substitution of the ligand backbone as well as metal precursors. A versatile ‘redox noninnocent’ 1,4-diaza-1,3-butadiene motif can stabilise different oxidation states of ruthenium metal depending on the reaction conditions and the presence of co-ligands. The comparative studies of the structural and electrical characteristics of diverse ruthenium-DAB compounds are intriguing, which opens up a new route for researchers to utilise them in a variety of application domains. It is challenging and fascinating to determine the precise electronic structure of redox noninnocent diimine complexes in the presence of a ‘redox active’ metal like ruthenium. In this concise review, we provided a brief overview of the structural and electrical features of various Ru DAB complexes that solely comprise (–N=CH–CH=N–) fragments in the skeleton.