Ruthenium Azobis(benzothiazole): Electronic Structure and Impact of Substituents on the Electrocatalytic Single-Site Water Oxidation Process

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [Ru^II(Ph-trpy)(Cl)(L)]ClO₄, [1]ClO₄-[3]ClO₄ (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2'-azobis(benzothiazole) ([1]ClO₄); L2: 2,2'-azobis(6-methylbenzothiazole) ([2]ClO₄); L3: 2,2'-azobis(6-chlorobenzothiazole) ([3]ClO₄)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [1]ClO₄-[3]ClO₄ established selective stabilization of (i) the unperturbed azo (N═N)⁰ function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L trans to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of [2]ClO₄ (CH---O) via the involvement of ClO₄^- anions. The detailed electrochemical studies revealed metal-based oxidation of [Ru^II(Ph-trpy)(Cl)(L)]⁺ (1⁺-3⁺) to [Ru^III(Ph-trpy)(Cl)(L)]²⁺ (1²⁺-3²⁺); however, the electronic form of the first reduced state (1-3) could be better represented by its mixed Ru^II(Ph-trpy)(Cl)(L^•-)/Ru^III(Ph-trpy)(Cl)(L^2-) state. Both native (1⁺-3⁺) and reduced (1-3) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, [1]ClO₄-[3]ClO₄ delivered an electrochemical OER (oxygen evolution reaction) process in alkaline medium on immobilizing them to a carbon cloth support, which divulged an amplified water oxidation feature for [2]ClO₄ due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [2]ClO₄ could also be rationalized by its lowest Tafel slope (85 mV dec^-1) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of [2]ClO₄ with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.

Photochemical oxidation of water catalysed by cyclometalated Ir(iii) complexes bearing Schiff-base ligands

Synthesis, characterization and photochemical oxidation of water catalysed by cyclometalated Ir(iii) complexes bearing Schiff-base ligands in the presence of Na₂S₂O₈ and [Ru(bpy)₃]²⁺ as a PS has been demonstrated.

Heterogeneous catalysis of water oxidation supported by a novel metallamacrocycle

Metallamacrocycles 1 and 2 were constructed, and 1 was further explored as a precatalyst for water oxidation, giving a good efficiency.

Unprecedented Strong Panchromic Absorption from Proton-Switchable Iridium(III) Azoimidazolate Complexes

Two new heteroleptic iridium(III) complexes bearing an aryldiazoimidazole ligand are reported. These complexes differ structurally with respect to the protonation state of the imidazole ring, but can be independently accessed by varying the synthetic conditions. Their structures have been unequivocally confirmed by X-ray crystal structure analysis, with surprising differences in the structural parameters of the two complexes. The strongly absorbing nature of the free diazoimidazole ligand is enhanced in these iridium complexes, with the protonated cationic complex demonstrating extraordinarily strong panchromic absorption up to 700 nm. The absorption profile of the deprotonated neutral complex is blueshifted by about 100 nm and thus the interconversion between the two complexes as a function of the acidity/basicity of the environment can be readily monitored by absorption spectroscopy. Theoretical calculations revealed the origins of these markedly different absorption properties. Finally, the protonated analogue has been targeted as an acceptor material for organic photovoltaic (OPV) applications, and preliminary results are reported.

Two star-shaped tetranuclear Ru(II) complexes containing uncoordinated imidazole groups: synthesis, characterization, photophysical and pH sensing properties

Tetrapodal ligands H4L(1) and H4L(2) containing imidazole groups have been synthesized by the reaction of 1,10-phenanthroline-5,6-dione with 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene and 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]benzene, respectively, in presence of NH4OAc. Two star-shaped complexes [{Ru(bpy)2}4(μ4-H4L(1))](PF6)8 and [{Ru(bpy)2}4(μ4-H4L(2))](PF6)8 (bpy = 2,2'-bipyridine) have been prepared by refluxing Ru(bpy)2Cl2 ·2H2O and each ligand in ethylene glycol. The deprotonated complexes [{Ru(bpy)2}4(μ4-L(1))](PF6)4 and [{Ru(bpy)2}4(μ4-L(2))](PF6)4 have been obtained by the reaction of sodium methoxide with [{Ru(bpy)2}4(μ4-H4L(1))](PF6)8 and [{Ru(bpy)2}4(μ4-H4L(2))](PF6)8, respectively, in methanol. The pH effects on the UV-vis light absorption and emission spectra of both complexes have been studied, and ground- and excited-state ionization constants of both complexes have been derived. The photophysical properties of both complexes are strongly dependent on the solution pH. They act as proton-induced off-on-off luminescent sensors through two successive deprotonation processes of imidazole groups, with a maximum on-off ratio of 8 in buffer solution at room temperature. Theoretical calculations for the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LOMO) orbitals of bridging ligand are also presented for plausible explanations of the fluorescence changes.

A new copper species based on an azo-compound utilized as a homogeneous catalyst for water oxidation

A new azo-complex [(L)Cu(II)(NO3)] [L = (E)-3-(pyridin-2-yldiazenyl)naphthalen-2-ol (HL)], was prepared via a one-pot synthetic method at 60 °C and was structurally characterized by IR, EA, PXRD and single crystal X-ray diffraction. In addition, TGA studies indicated that the complex was stable in air. The redox properties were determined by cyclic voltammetry, which revealed that the complex could be utilized as a catalyst for water oxidation under mild conditions. Subsequently, the complex was employed as a catalyst to take part in water oxidation reaction in the presence of a Ce(IV) salt utilized as an oxidant at pH 11 in PBS (Phosphate Buffered Saline) solution. The results suggested that the catalyst exhibited a high stability and activity toward water oxidation reaction under these conditions with an initial TOF of 4.0 kPa h(-1). Calculation methodology was performed to study the mechanism of the reaction, which revealed that in this catalytic process, the initial oxidation of Cu(II) to Cu(III) occurred by the formation of an intermediate "Cu(III)-O-O-Cu(III)". The formation of this intermediate, resulted in a release of oxygen and closing of the catalytic cycle.