Versatile Coordination of Azocarboxamides: Redox‐Triggered Change of the Chelating Binding Pocket in Ruthenium Complexes

Abstract

Azocarboxamides occupy a special place among azo ligands owing to their versatility for metal coordination. Herein ruthenium complexes with two different azocarboxamide ligands that differ in the presence (or not) of a coordinating pyridyl heterocycle are presented. By making full use of the O,N(amide), N(azo), and N(pyridyl) coordinating sites, the first diruthenium complex that is bridged by an azo ligand containing two different binding pockets was obtained. Moreover, it was conclusively proven that, in the mononuclear complexes, oxidation at the ruthenium center leads to a complete change of coordination at the chelating binding pocket. The complexes were characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Additionally, the mechanism of the aforementioned redox-triggered change in the chelating binding pocket and the electronic structures of all the complexes were investigated by a combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations. This is first instance in which a redox-driven change in the complete chelating binding pocket has been observed in a ruthenium complex as well as with azo-based ligands. These results thus show the potential of these versatile azocarboxamide ligands to act as redox-driven switches with possible relevance to electrocatalysis.

Iridium Azocarboxamide Complexes: Variable Coordination Modes, C–H Activation, Transfer Hydrogenation Catalysis, and Mechanistic Insights

“…The irreversible reduction process is likely a ligand-centered process considering the free radical EPR signal without hyperfine coupling with a g value of 2.038 at 298 K (Figure S2) obtained for the one-electron-reduced form of complex Ir-1 . Moreover, as reported in previous publications from our group, the free ligand ( L1 ) exhibits an irreversible reduction process at −1.33 V, which indeed supports a ligand-centered reduction process in the complexes. − The irreversible reduction and oxidation processes indicate the involvement of an electron-transfer/chemical reaction (EC) mechanism with the elimination of chloride ion for the reduction and possible ligand rearrangement for the oxidation processes.…”

“…While other azo-containing ligands such as pap (phenylazopyridine) and azobispyridine have been extensively studied in coordination chemistry as redox-active bridging ligands, − azocarboxamides, a special class of azo-containing ligands, have rarely been used in coordination chemistry and for the study of redox as well as catalytic properties. A recent report on ruthenium azocarboxamide half-sandwich complexes from our group illustrated the redox-triggered change in the chelating binding pocket and the electronic properties of the complexes . We have also investigated the antitumor activity of ruthenium azocarboxamide complexes .…”

“…Azocarboxamides, a special class of multifaceted redox-active azo chelating ligands have garnered considerable attention by virtue of its diverse coordination modes, and redox activity. − The azo group (−NN−) in the azocarboxamide ligand is surrounded by a carbonyl oxygen of the amide group, which can potentially be used as a donor atom along with the nitrogen atom of the azo group (N ∧ O-chelated ring, Chart ). Furthermore, in the presence of a base, the azocarboxamide ligand can also bind to a metal center via the deprotonated nitrogen atom of the amide together with the nitrogen atom of the azo group (N ∧ N-chelated ring). , Interestingly, it can also provide an additional binding pocket to the metal center by activating the C–H bond from one of the phenyl rings.…”

“…This depends on the functionalization of the phenyl ring and the electronic properties of the metal center and its ancillary ligands, leading to the formation of the stable five-membered cyclometalated complex (N ∧ C-chelated ring). While examples of the first two coordination modes are already available in the literature for ruthenium complexes, − the latter cyclometalated coordination mode is not reported for azocarboxamide ligands.…”

Photocatalytic Properties and Electronic Effects of Incorporation Bis(acetylacetonato) Ruthenium(II/III) Complexes with Bidentate Nitrogen and Sulfur Ancillaries

“…[1][2][3][4] Among them, tris(acetylacetonato)-ruthenium(III) is a versatile precursor and is usually used to synthesize the diacetonitrile-bis(β-diketonato)ruthenium(II) complex [Ru II-(acac) 2 (CH 3 CN) 2 ] to further form the type d 6 low-spin species [Ru(acac) 2 (L) x ], where x is 1 for a bidentate ligand or 2 for a monodentate ligand. [5][6][7] The two easily leaving acetonitrile ligands could be substituted by more electron-donating ligands, such as pyridine, [8] o-aminoquinone, [9] substituted diphenylphosphine [10] and DMSO (DMSO = dimethyl sulfoxide), [11] which could make the ruthenium species electron-rich and more active. [12,13] Hydrogen is a promising clean and renewable energy source, which can alleviate the decreasing energy crisis of fossil fuel resources and solve environmental pollution.…”