Chayan Das

Osmium(II) complexes of 2-[(arylamido)phenylazo]pyridines. New examples of deamination reactions—X-ray structure and redox properties

Reaction of [NH4]2[OsBr6] with 2-[(arylamino)phenylazo]pyridine, NH4C5NNC6H4N(H)C6H4(R) [R = H (HLa) or CH3 (HLb)], in the presence of dilute NEt3 affords multiple products. Five compounds of molecular formulas [Os(HL)(L)Br], 1, [Os(L)(pap)Br], 2, two isomers of [Os(pap)2Br2], 3 and 5, and [Os(HL)(pap)Br2], 4, where L and pap stand for the conjugate base of HL and 2-(phenylazo)pyridine, respectively, have been separated on a preparative TLC plate. The X-ray structures of the new and representative complexes 1a, 2a and 4a have been solved to characterise them. The complexes 3 and 5 were characterised by comparing their spectral properties with those of the known and analysed samples of isomeric [Os(pap)2Br2]. Except for complex 1, the rest are formed due to cleavage of an otherwise unreactive C(phenyl)–N(amine) bond which is promoted by the metal ion. The bidentate-tridentate combination of HL and L in 1 is due to electronic factors. Structural data of the compounds have revealed very strong metal-ligand interactions. Osmium(II)-ligand interactions with the neutral azo ligands, viz. HL or pap, are stronger than those with the anionic L ligand. All of the complexes display resolved 1H NMR spectra. However, the spectral pattern is complex due to serious overlap of the resonances. There are multiple electronic transitions in the range 1200–220 nm. The lowest energy transition (HOMO → LUMO) is presumably due to metal-to-ligand charge transfer (MLCT). These complexes undergo multiple and successive one-electron redox processes. The lowest potential anodic response, in each case, has been assigned to the OsII/OsIII couple. E1/2 of this response in 1 and 2 is similar and occurs near 0.45 V. Low oxidation potentials of the above couples allowed the generation of [1a]+ and [2a]+ in solution by exhaustive constant potential coulometry. The trivalent osmium complexes showed rhombic EPR spectra at 77 K. Distortion parameters using the observed g values have been computed.

Ruthenium complexes of quinone related N-aryl-1,2-diimines. Metal mediated synthesis, X-ray structure and chemical reaction

A family of ruthenium N-aryl-1,2-diiminoarene chelate complexes, [RuCl2(bpy)(diim)], 1 (bpy=2,2′-bipyridine, diim=N-aryl-1,2-diiminoarene) were isolated from the reaction of [RuCl3(bpy)] and a primary aromatic amine (ArNH2) in the presence of aerial oxygen. The diim ligand was formed in situ due to oxidative dimerisation of ArNH2. The metal complex, in this reaction, acts as a mediator. The dichloro complexes were characterized based on their physicochemical data. The X-ray structure of one member of the family was determined and the results used to authenticate the formation of the product from an unusual oxidative fusion of aromatic monoamines. The X-ray structure of 1c (substituent R on the aromatic ring=Cl) revealed a cis geometry with respect to two coordinated chlorides. The bond length trends within the chelate as well as the aromatic rings are consistent with a diimine oxidation state of the ligand bound to ruthenium in its bivalent oxidation state. These complexes showed multiple transitions in the near-IR and visible regions. The highly intense transition at around 540 nm has been assigned to a transition involving two heavily mixed metal–ligand orbitals. The ruthenium(III)/ruthenium(II) couple for the dihalo complexes appeared at a high anodic potential (range 0.50 to 0.90 V), which for differently substituted compounds depend on the Hammett ∑σp parameters of the substitution on the diim ligand. The syntheses and characterization of a few heteroleptic tris-chelates, [Ru(bpy)(diim)L](ClO4)2·nH2O (L=a bidentate N,N-donor), involving the reaction of 1 with two moles of the silver reagent, [AgL2](ClO4), are reported. The tris-chelates showed intense charge transfer in their electronic spectra and multiple cathodic voltammetric responses, which have been assigned as reductions of coordinated ligands.

Transition Metal Promoted Oxidative C–N Fusion Reactions of Aromatic Amines and Their Coordination Chemistry

Some unusual examples of metal promoted oxidative dimerization and polymerization, via C–N bond formation, of aromatic mono- as well as di-amines are described. Successful isolation and thorough characterization of the products follow these reactions. Electrochemical and spectral studies of the complexes are reported and compared with related systems. While the heavier transition metal ions like ruthenium(III) and osmium(IV) mediate ortho-dimerization of primary amines, ArNH2, Fe(III), on the other hand, promotes polymerization. Similar fusion reaction of 1,3-diaminobenzene with the mediator complex, [Ru(acac)3] (acac=acetyl acetonate), led to the formation of diruthenium complexes.

Synthesis, structure and redox properties of isomeric [RuCl2(L)2] (L=N-aryl-1,2-arylenediimine) complexes formed by the oxidative dimerization of coordinated aromatic amines

Abstract The di-arylamino complex [RuCl2(ArNH2)2L] (ArNH2=aromatic monoamine and L=N-aryl-1,2-diimino arene) reacts spontaneously with aqueous H2O2 to produce two isomers of [RuCl2(L)2] via oxidative dimerization of the two coordinated aromatic amines. The geometries of two isomers are cis with respect to two chlorides and are trans, cis(tc) and cis, cis(cc) in the following sequence: NH(imine), HN(imine) and ArN(imine), ArN(imine). Of the above two, the tc-[RuCl2(L)2] forms good X-ray quality crystals. The structure of a representative has been solved by X-ray diffraction. The geometry of the cc-[RuCl2(L)2] is, however, revealed by the spectral data. The tc-isomer possesses a C2-symmetry axis and the two-coordinated diimine ligands are magnetically equivalent. As a result, a single NH resonance in tc-[RuCl2(L)2] appeared at approximately δ 12.40. By contrast, the cc-isomer is unsymmetrical and a complex pattern of 1H NMR was observed in this case. Thus, there were two NH resonances observed in the range approximately δ 14.11–11.88. A semi-empirical extended Huckel MO calculations on a representative example showed strong metal–ligand overlap. A highly intense electronic transition that appeared in the visible range spectra of the complexes was assigned to a transition involving two molecular orbitals with considerable metal and ligand contributions. Both the isomers of the complex showed a high potential anodic response (0.8 V) due to the RuIII/RuII couple. The trivalent congeners [RuCl2(L)2]+ were generated in solution which showed characteristic rhombic EPR for the low spin d5 ions. The distortion parameters have been computed using the observed g values. The axial distortion (Δ) in each case is found to be stronger than the rhombic (V) distortion.

Isolation and characterization of iridium(III) and iridium(V) complexes of 2-(arylazo)pyridine and studies of amine fusion reactions at the coordinated diazo-ligand

The reaction of IrCl3.3H2O with 2-(arylazo)pyridine (HL1) in boiling methanol has afforded [Ir(III)Cl2(L1)(HL1)](1) and [Ir(V)Cl4(HL1)]Cl (2). In complex , one of the two ligands [L1]- is orthometallated via coordination of an ortho-carbon of the aryl ring of [L1]- and one of the two azo nitrogens to form a five-membered chelate. X-Ray crystal structures of the two representative complexes, viz. 1a and 2a, have been solved. Notably, the Ir-N length (2.140(3) A)trans to the Ir-C bond in 1a is appreciably longer than the other three Ir-N lengths present in the same molecule. The N-N lengths in these two compounds lie close to that observed in the uncoordinated ligand. Thorough NMR studies were made to authenticate the carbon-bonded structure of compound 1a. In its 13C NMR spectrum, the resonance near delta 148 is assigned to the carbon bonded to the iridium metal center. UV-visible spectra along with the redox properties of these complexes are reported. The iridium(V) complex, 2 showed a reversible response near 1.40 V, presumably due to the iridium(V)-iridium(VI) couple. Several reductive responses at cathodic potentials, due to ligand reductions, were also observed. Metal promoted aromatic ring amination reactions at the coordinated HL1 ligand in complexes 1 and 2 were investigated. The products were characterized using X-ray diffraction.