Recent advancement on the redox properties of a selection of transition metal complexes of the azoaromatic ligands: bidentate L1 [2-(arylazo)pyridine] and tridentate HL2 [2-(aminoarylphenylazo)pyridine] are described and compared. Due to the presence of a low lying azo-centered π*-orbital, these azoaromatic ligands may exist in multiple valent states. The coordination chemistry of the L1 ligands was thoroughly studied during the 1980s. These complexes undergo facile reduction in solution at low accessible potentials. One electron reduced azo-complexes, though known for a long time to occur in solution, have only recently been isolated in a crystalline state. New synthetic protocols for the synthesis of metal-bound azo-radical complexes have been developed. Low-valent metal complexes such as metal carbonyls have been found to be excellent starting materials for this purpose. In a few selected cases, syntheses of these complexes were also achieved from very high valent metal oxides using triphenylphosphine as both a reducing and oxo-abstracting agent. Issues related to the ambiguities of the electronic structures in the azo-metal complexes have been discussed considering bond parameters, redox and spectral properties. Unusual redox events such as RIET (Redox-Induced Electron Transfer) phenomena in a few systems have been elaborated and compared with the known example. Novel examples of NN bond cleavage reactions via four-electron reduction and subsequent C–N bond formation in metal-bound coordinated ligands have been noted.

Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives

The reaction of cis-trans-cis-RuCl2(RaaiR′)2 [RaaiR′ = 1-alkyl-2-(arylazo)imidazole, p-R–C6H4–NN–C3H2–NN-1-R′, where R = H (a), Me (b), Cl (c) and R′ = Me (1), Et (2) and CH2Ph (3)] either with 2,2′-bipyridine (bpy) and AgNO3 followed by NaClO4 or [Ag(bpy)2](ClO4) in boiling acetone led to red-brown [Ru(bpy)(RaaiR′)2](ClO4)2. The complex may exist in three isomeric structures. The structure of [Ru(bpy)(MeaaiMe)2](ClO4)2
was confirmed by X-ray diffraction study and suggests that only one isomer is crystallised. Electronic spectra exhibit a strong MLCT band at 520 ± 5 nm along with two weak transitions at longer wavelength. The complexes emit at 77 K and quantum yields vary from 0.011–0.025. The 1H NMR spectral measurements of the complexes suggest the existence of two isomers in 1 ∶ 0.3 concentration ratio. The methylene, –CH2–, in RaaiEt of [Ru(bpy)(RaaiEt)2]2+ gives a complex AB type multiplet while the RaaiCH2Ph moiety of [Ru(bpy)(RaaiCH2Ph)2]2+ shows AB type quartets. Cyclic voltammograms show a Ru(III)/Ru(II) redox couple along with three successive ligand reductions. The plot of difference in potential of first oxidation and reduction versus energy of main MLCT band (CT) is linear. Electrochemical
parametrization of the Ru(III)/Ru(II) redox couple determines ligand potential EL(L). Using EL(L) the potential of the present complexes has been predicted and is linearly related to the observed potential.

Heteroleptic tris-chelates of ruthenium(II)-bipyridine-arylazoimidazoles: Synthesis, spectral characterisation and electrochemical properties. Single crystal X-ray structure of [(2,2′-bipyridine)-bis-{1-methyl-2-(p-tolylazo)imidazole}ruthenium(II)] perchlorate

Ruthenium(III) mediated C–H activation of azonaphthol: Synthesis, structural characterization and transfer hydrogenation of ketones

Reaction of Cu(ClO4)2 6H2O with 1-alkyl-2-(arylazo)imidazole [RaaiR′ where R = H (a), Me (b); R′
				= Me (1), Et (2)] and NaN3/NH4SCN (1 ∶ 2 ∶ 2 mole ratio) form mononuclear complexes [Cu(RaaiR′)2(N3)2]
				(3,4) and [Cu(RaaiR′)2(SCN)2] DMF (5,6) The complexes are characterized by elemental, spectral, electrochemical and other physicochemical results The structures of both the complexes are confirmed by the single crystal X-ray diffraction studies of [Cu(MeaaiEt)2(N3)2]
				(4b) and [Cu(HaaiMe)2(SCN)2] DMF (5a) The complexes are reduced to [Cu(RaaiR′)2]+ by ascorbic acid However, air oxidation of [Cu(MeaaiEt)2](ClO4) in presence of N3−/SCN− produces [Cu(MeaaiEt)2(N3/NCS)2]

Copper coordination compounds of chelating imidazole-azo-aryl ligand. The molecular structures of bis[1-ethyl-2-(p-tolylazo)imidazole]-bis-(azido)copper(II) and bis[1-methyl-2-(phenylazo)imidazole]-bis(thiocyanato)copper(II)

Syntheses, spectral and structural studies of Schiff base complexes of η5-pentamethylcyclopentadienyl rhodium and iridium

Synthesis, spectral characterization and electrochemical studies of mixed-ligand complexes of platinum(II) with 2-(arylazo)pyridines and catechols. Single-crystal X-ray structure of dichloro{2-(phenylazo) pyridine}platinum(II)

Ruthenium(II) complexes of α-diimines: synthesis, spectral characterisation, electrochemical properties and single-crystal X-ray structure of bis(2,2′-bipyridine){1-benzyl-2-( p -tollylazo)imidazole}ruthenium(II) perchlorate

Alexander M.Z. Slawin

Papers

Synthesis, spectral characterization and electrochemical studies of mixed-ligand complexes of platinum(II) with 2-(arylazo)pyridines and catechols. Single-crystal X-ray structure of dichloro{2-(phenylazo) pyridine}platinum(II)

Ruthenium(II) complexes of α-diimines: synthesis, spectral characterisation, electrochemical properties and single-crystal X-ray structure of bis(2,2′-bipyridine){1-benzyl-2-( p -tollylazo)imidazole}ruthenium(II) perchlorate