Special Collection: Non‐Innocent Ligands in Sustainable Catalysis

Non-innocent ligands in metal complexes pave the way for more energy-efficient and sustainable catalytic processes by involving both the metal and the ligand in bond activation processes. This issue features selected examples illustrating the role of the ligand in enabling (or disabling) catalytic bond cleavage/formation processes via metal-ligand and metal-metal cooperation. Homogeneous catalysis plays a vital role in synthetic chemistry, with applications in various industries and academia. Efforts are being made to enhance the sustainability of catalytic processes by improving noble metal catalyst efficiency or transitioning to base metal alternatives. The concept of non-innocent ligands has emerged as a promising approach in the past two decades, where catalytic performance is enhanced by the active participation of the ligand in the catalytic process. The present special collection “Non-innocent Ligands in Sustainable Catalysis”, highlights recent progress in the development and application non-innocent/cooperative ligands. As such, it consists of 15 research articles, 13 published in ChemCatChem and 2 published in the European Journal of Inorganic Chemistry. This issue illustrates a variety of approaches to design non-innocent ligand systems for selective bond activation and functionalization. For example, an ubiquitous 2-hydroxypyridine motif was employed in metal-ligand cooperative alcohol dehydrogenation in Ru-catalyzed methylation with MeOH, polymer-supported iridium catalysts for alcohol dehydrogenation, and iridium catalysts for ring opening of epoxides with alcohols. The phosphino-2-pyridone-supported iridium complex featuring a strained chelate ring has been shown to induce metal-ligand cooperative H2 cleavage across a metal-N bond. A reversible 1,2-addition across the Ru=C double bond in solution and the solid state was demonstrated in ruthenium complexes supported by an iminophosphinoyl system, which was also reactive in alcohol dehydrogenation via metal-ligand cooperation and catalytic transfer hydrogenation. In an alternative approach to alcohol dehydrogenation, non-innocent ligands containing azo groups enabled catalytic radical-promoted alkylation strategies with alcohols using non-precious metals nickel and zinc, bypassing the need to involve metal hydride intermediates. The effect of subtle changes in ligand structure on catalytic activity is illustrated by the inhibition of cross-coupling reactivity in pyridinophane nickel complexes featuring anagostic axial interactions. The importance of fine steric tuning has been highlighted in the study of picolylphosphine iron complexes for ring-opening metathesis polymerization, molybdenum-catalyzed alkene isomerization, and in-situ generation of active zinc catalysts for hydroboration with small NHC ligands. The effect of octahedral distortion on the H2 release step in Co polypyridyl catalysts for water reduction has been studied theoretically. In a different approach, a synergistic effect between two metals was targeted in the design on binuclear diruthenium complexes for water oxidation and CO2 hydrogenation. An interesting example of the synergistic action between two different metals in Guerbet upgrading of ethanol to butanol was investigated in detail, where a combination of a manganese(I) MACHO-type pincer complex and a ruthenium(II) PNP-pincer complex showed enhanced catalytic activity as compared to a single metal alone. It is our hope that readers will gain a comprehensive understanding of recent developments in non-innocent ligand research through perusing this collection, and will be further inspired to explore this exciting field of contemporary research.