Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin-Spin Interaction

Potent pincer-zinc catalyzed homogeneous α-alkylation and Friedländer quinoline synthesis reaction of secondary alcohols/ketones with primary alcohols

Herein, we describe an air- and moisture-stable, homogeneous zinc catalyst stabilised using an electron deficient N^N^N pincer-type ligand. This ternary, penta-coordinated neutral molecular catalyst [Zn(N^N^N)Cl2] selectively produces α-alkylated ketone derivatives (14 examples) through a one-pot acceptorless dehydrogenative coupling (ADC) reaction between secondary and primary alcohols using the borrowing hydrogen (BH) approach in good to excellent isolated yields (up to 93%). It is worth noting that this catalyst also provides an eco-friendly route for the synthesis of quinoline derivatives (30 examples) using 2-aminobenzyl alcohols as alkylating agents via successive dehydrogenative coupling and N-annulation reactions. This cost effective, easy to synthesize and environmentally benign catalyst shows excellent stability in catalytic cycles under open-air conditions, as evident from its high turnover number (∼104), and is activated by using a catalytic amount of base under milder conditions.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes

The redox non-innocent behavior of the diaryl-azo-oxime ligand L^NOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(L^NO-)Cl₂(PPh₃)₂] 4 and [Ir(L^NO-)₂Cl(PPh₃)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(L^NO-)Cl(CO₂H)(PPh₃)₂] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO₂H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Polymerisation of styrene using pincer type amine functionalized azo aromatic complexes of Co(II) as catalysts

In the present report, three mononuclear azo-aromatic complexes of Co(II), 1-3, and an imine-based Co(II) complex, 4, were synthesized through a reaction of respective amine-functionalized pincer-like ligands, HL1-4, with CoCl₂·6H₂O in the ligand-to-metal ratio of 1 : 1. All the complexes, 1-4, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory (DFT) calculations. Complexes 1-4 were explored for the catalytic styrene polymerisation reaction separately in the presence of modified methyl aluminoxane (MMAO). All the complexes, 1-4, are indeed active for the polymerisation of styrene under mild conditions at room temperature upon activation with MMAO. Among the azo-aromatic complexes 1-3, complex 3 is the most efficient. The activity of the imine complex 4 is poor compared to those of the azo-aromatic complexes 1-3. The weight average molecular weight (M_w) of the isolated polystyrene ranges from 32.9 to 144.0 kg mol^-1, with a polydispersity index (Đ) in the range of 1.1-1.8. Microstructural analysis of the isolated polymer from complexes 1-4 was carried out by ¹³C NMR spectroscopy, infrared spectroscopy, and powder X-ray diffraction studies. Their thermal properties were scrutinized by differential scanning calorimetry and thermogravimetric analysis. These studies have shown the atactic and amorphous nature of the polymers. The mechanical strength of the polymers was measured by a nanoindentation technique which has shown the good plastic/soft nature of the polymers.

Polyaromatic hydrocarbon derivatized azo-oximes of cobalt(iii) for the ligand-redox controlled electrocatalytic oxygen reduction reaction

Two new polyaromatic hydrocarbon (PAH) derivatized cobalt(iii) azo-oxime complexes were synthesized and their activity in electrocatalytic oxygen reduction reaction (ORR) were explored.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Redox-active diaminoazobenzene complexes of rhodium(iii): synthesis, structure and spectroscopic characterization

Coordination diversity of an aromatic diamine with Rh(iii) is presented together with the elucidation of the molecular and electronic structures, electron transfer, and electronic transitions.