The Shrinking World of Innocent Ligands: Conventionaland Non‐Conventional Redox‐Active Ligands

Abstract

This essay for EurJIC's cluster issue on cooperative and redox non‐innocent ligands introduces the reader to redox‐active ligands, which range from the small archetypical NO+/•/– and O20/•–/2–systems via the classical 1,4‐dihetero‐1,3‐diene chelates (e.g. α‐diimine, dithiolene, or o‐quinone redox series) to π‐conjugated macrocycles. The increased attention paid recently to the redox activity of ligands in coordination chemistry has now prompted wider successful searches, resulting in the establishing of less‐conventional examples such as cyanide, carbon monoxide, thioethers, or acetylacetonate derivatives as non‐innocently behaving ligands. By considering situations with significantly covalent metal–ligand bonding, the cases of metal–oxo, metal–hydrido, and organometallic compounds will also be addressed, with a perspective on how pervasive non‐innocent ligand behavior is. The materials and reactivity potential of redox‐active ligands will be pointed out.

Synthesis and Characterization of (pyNO−)2GaCl: A Redox-Active Gallium Complex

“…We also attempted the synthesis of the corresponding indium complex under identical reaction conditions. However, unlike the aluminum and gallium systems which cleanly provide monometallic complexes, there is a ligand redistribution in the indium compound and after crystallization the zwitterionic [(pyNO − ) 2 In][(pyNO − ) 2 InCl 2 ] (2) complex is isolated. Compound 2 was crystalized from slow diffusion of pentane into a concentrated THF solution at −25 • C, and it is not clear if the crystal sample obtained is representative of the bulk sample.…”

“…The synthesis of metal complexes of redox-active and non-innocent ligands is a research area that has been expanding in recent years and has resulted in new classes of complexes for metals from throughout the periodic table [1][2][3][4]. Of specific interest to us is the ability to modulate the reactivity of redox-active ligands through coordination of Lewis acids.…”

“…Metal complexes of these ligands have been prepared for several f-block [11][12][13][14][15] and transition [16][17][18] metals. Inspired by these compounds we prepared and fully characterized aluminum complexes of the type ( R pyNO − ) 2 AlCl ( R pyNO − = N-tert-butyl-N-(2-(5-R-pyridyl))nitroxyl, R = H, CH 3 , CF 3 ) [7] and {(µ-R pyNO − )Al(CH 3 ) 2 } 2 (R = H, CH 3 ) [8] and explored their electrochemical behavior. The results of this study suggested that coordination to the Lewis-acidic aluminum ion affected the redox energetics of the ligand to stabilize the reduced pyNO − form.…”

“…In light of these results, we have become interested in the preparation of redox-active ligand complexes of other group 13 metals, namely gallium and indium, to access how the identity of the metal translates to differences in the physical properties of the complexes. In this current contribution, we report the synthesis of the pyridyl-nitroxide gallium complex (pyNO − ) 2 GaCl and present its characterization by various physical methods. We then discuss the results in the context of the corresponding aluminum complex.…”

On the Electronic Structure ofmer,trans‐[RuCl3(1H‐indazole)2(NO)], a Hypothetical Metabolite of the Antitumor Drug Candidate KP1019: An Experimental and DFT Study

“…Furthermore, NO acts as a non-innocent ligand in coordination chemistry, [20,21] which renders the description of the exact electronic structure of the metal-NO entity difficult. Almost four decades ago, Enemark and Feltham [22] offered a useful approach for describing the electronic structure of metal-nitrosyl species as {M(NO)} n [n being the total number of electrons in the d (metal) and π* (NO) orbitals].…”