Chittaranjan Sinha

Chemistry of azopyrimidines. Part II. Synthesis, spectra, electrochemistry and X-ray crystal structures of isomeric dichloro bis[2-(arylazo)pyrimidine] complexes of ruthenium(II) ☆

Abstract 2-(Arylazo)pyrimidines (aapm, 3) are new N,N′- chelating ligands in the azoimine family and were reacted with RuCl3 in ethanol under refluxing conditions. Three isomers of the composition Ru(aapm)2Cl2 have been chromatographically separated and are established as having trans–cis–cis (tcc), cis–trans–cis (ctc) and cis–cis–cis (ccc) configurations with reference to the order of coordination pairs as Cl; N(pyrimidine), N and N(azo), N′. Two of the three isomeric structures have been confirmed by X-ray crystallography. In both of these structures, the Ru–N(azo) distances are relatively shorter than those of Ru–N(pyrimidine), indicating stronger bonding in the former and the presence of a Ru-(aapm) π-interaction that is localised in the Ru-azo fragment. The isomer configuration is supported by IR and 1H NMR data. The complexes exhibit t2(Ru)→π*(aapm) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple in the green complexes [tcc-Ru(aapm)2Cl2] at 1.1–1.2 V and in the blue complexes [ctc- and ccc-Ru(aapm)2Cl2] at 1.2–1.4 V versus saturated calomel electrode (SCE) and two successive azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated with νCT [t2(Ru)→π*(aapm)].

Chemistry of azoimidazoles: synthesis, spectral characterization and redox properties ofbis(N(1)-alkyl-2-(arylazo)imidazole) copper (I) and silver(I) complexes

Abstract N(1)-Alkyl-2(arylazo)imidazoles p-C6H4N NC3H2H N(1)R′, RaaiR′ R′ =Me(2), Et (3);R=H(a), Me(b), OMe(c), Cl(d), NO2(e) yield catonicbis-related with both copper(I) (4,5) and silver(I) (6,7). These were isolated as perchlorate salts and the composition is supported by elemental analyses. The complexes are 1:1 electrolytes in methanol. The N N stretch in copper(I) complexes shows a large shift to lower frequency (ca 1335 cm−1) from the free ligand value (ca 1400 cm−1) due to d(Cu) → π* (RaaiR′) back bonding. This effect is much less pronounced in silver(I) complexes indicating a negligible dπ-π* (RaaiR′) interaction. The complexes show highly resolved symmetrical1H NMR spectra. The copper(I) complexes exhibit intense MLCT transitions in the visible region. In methanol the Cu(RaaiR′)2+ couple has E1/2 ca 0.45V vs SCE at 298 K. The high positive potential may be due to the distortion in the geometry of Cu(RaaiR)2+.

A new resin containing benzimidazolylazo group and its use in the separation of heavy metals

A new resin incorporating benzimidazolylazo group into a matrix of polystyrene divinylbenzene has been prepared. The exchange capacity of the resin for the ions mercury(II), silver(I) and palladium(II) as a function of pH has been determined. The resin exhibits no affinity for alkali or alkaline earth metals. It is highly selective for Hg(II), Ag(I) and Pd(II). In column operation, it has been observed that Hg(II), Ag(I) and Pd(II) in trace quantities can be selectively separated from geological, medicinal and environmental samples.

Mono- and binuclear cobalt(II)-azido complexes of arylazoimidazole: synthesis, spectral characterization, electrochemistry and crystal structure

Abstract Upon setting up proper reaction conditions 1-alkyl-2-(arylazo)imidazoles (RaaiR′ where R=H (a), Me (b); R′=Me (1/3/5), Et (2/4/6) may react with Co(OAc)2·4H2O and NaN3 in methanolic solution to give two classes of azido complexes: mononuclear bis-chelated [Co(RaaiR′)2(N3)2] (3,4) and binuclear μ-(1,1) azido bridged mono-chelated [Co(RaaiR′)(μ-(1)N3)(μ-(1,1)N3]2 (5,6) complexes. They are characterized by UV–Vis, IR spectra, room temperature magnetism and they have been structurally confirmed by X-ray diffraction study of [Co(MeaaiMe)2(N3)2] (3b) and [Co{(μ-1)N3}(MeaaiMe)]2 (5b). The mononuclear complex (3b) is distorted octahedral whereas the binuclear azide bridged complex (5b) has the distorted square pyramidal geometry around the cobalt(II) centre. Redox properties of the complexes are examined by cyclic voltammetry and show a high potential Co(III)/Co(II) couple along with ligand reductions. EHMO calculation and comparison between the two types of complexes explain the spectral and redox properties of the complexes.

Cyclopalladation versus hydroxylation. A case of pH dependence

Alkyl-2-(naphthyl--azo)imidazoles (NaiR, 2 )( C 10H7NNC3H2N21R; R= Me (a), Et (b), PhCH2 (c)) have been reacted with Na2PdCl4 in MeOH or MeCN solutions of Pd(MeCN)2Cl2 to synthesise Pd(NaiR)Cl2 (3). The reaction of Pd(OAc)2 in boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex Pd(NaiRH)Cl (4). The ligand, NaiR, acts as a N,N-bidentate chelator while NaiRH acts as a tridentate N,N,C-cyclometallat- ing ligand. The infrared spectra of 3 exhibit two PdCl stretches correspond to a cis-PdCl2 geometry, and a single (PdCl) band in 4 suggests one PdCl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEtH)Cl (4b) and the metallation takes place at C(8)-position. The solution of Pd(NaiR)Cl2 (3) is also irreversibly transformed into Pd(NaiRH)Cl (4) when the pH is adjusted to 4.5-6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li2CO3 etc.). At higher pH values (8-10) the reaction shows the chelative hydroxylation at the C(2)-site to synthesise Pd(NaiRO)Cl (5). The structure of the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl (5b). The reaction of Pd(NaiR)Cl2 in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment with Tollens reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds have been characterised by elemental analyses, IR, UV-vis and 1 H-NMR data. The solution spectral behaviour has been interpreted by EHMO calculations.

Synthesis, spectral characterisation, redox studies of isomeric dichloro-bis-[1-alkyl-2-(naphthyl-(α/β)-azo)imidazole]ruthenium(II). Single crystal X-ray structure of blue-green dichloro-bis-[1-ethyl-2-(naphthyl-α-azo)imidazole] ruthenium(II)

Abstract Ruthenium(II) complexes with 1-alkyl-2-(naphthyl-(α/β)-azo)imidazoles (α/β-NaiR), where R=Me, Et and Bz have been synthesised. Two isomers, blue-green, trans–cis–cis and blue, cis–trans–cis of the composition Ru(NaiR)2Cl2, are chromatographically separated. With reference to the pairs Cl, N(imidazole) and N(azo), the blue-green isomer is assigned to a trans–cis–cis configuration and the blue, isomer is assigned to a cis–trans–cis configuration. They are characterized by IR and 1H NMR data. The structure of the blue-green complex, Ru(α-NaiEt)2Cl2 has been determined by X-ray crystallography and the trans–cis–cis configuration has been confirmed. The complexes exhibit t2(Ru)→π* (ligand) MLCT transitions in the visible region. Cyclic voltammetry shows a Ru(III)/Ru(II) couple for the blue-green isomer at 0.4–0.5 V and the blue complexes at 0.5–0.6 V versus SCE, as well as multiple azo reductions.

Naphthylazoimidazole and mercury(II) complexes. Single crystal X-ray structure of 1-ethyl-2-(naphthyl-α-azo)imidazolium hexaflurophosphate

Abstract 1-Alkyl-2-(naphthyl-(α/β)-azo)imidazole [α-NaiR (2)/β-NaiR (3); R=Me (a), Et (b), CH2Ph (c)] are N,N′-bidentate chelating ligands. The structure of 1-ethyl-2-(naphthyl-α-azo)imidazolium hexaflurophosphate has been confirmed by single crystal X-ray structure study and shows supramolecular geometry through non-covalent interactions (C–H⋯F, C–H⋯π, π⋯π). Mercury(II) complexes of the formula Hg(NaiR)Cl2 (4/5) have been synthesised and characterised by spectral (IR, UV–Vis and 1H and 13C NMR) studies. α-NaiR has been used to synthesise organomercury derivatives, Hg(α-NaiR–H)Cl (6) and the site of mercuration has been supported by charge density calculation using a semi-empirical PM3 technique.

Chemistry of azopyrimidines: Synthesis, spectral characterization, electrochemistry and X-ray crystal structure of bis[2-(arylazo)pyrimidine] complexes of copper(I)

Abstract 2-(Arylazo)pyrimidines (aapm, 3) have been synthesized by condensing nitrosoaromatics with 2-aminopyrimidine. They yield cationic bis-chelated complexes with copper(I), Cu(aapm)2+ and are isolated as perchlorate salts. The complexes are 1:1 electrolytes in MeOH and exhibit intense MLCT transitions in the visible region. The NN stretch in copper(I) complexes shows a large shift to lower frequency (approx. 1315 cm−1) from the free ligand value (approx. 1425 cm−1) due to d(Cu)→π*(aapm) back bonding. The complexes show highly resolved symmetrical 1H NMR spectra. In MeOH the Cu(aapm)22+/Cu(aapm)2+ couple appears at E1/2 at approx. 0.7 V versus SCE at 298 K. The structure has been confirmed by X-ray crystallography.

Facile regiospecific aromatic hydroxylation in palladium azopyridines and structural characterization of phenolato product

Abstract The reaction of Na2PdCl4 with 2-(arylazo)pyridines (A) in ethanol affords yellow complexes of composition [PdACl2] in which the PdCl2 fragment has acis configuration [ν(Pd Cl): 350, 365 cm−1]. Upon treating [PdACl2] with dilute sodium hydroxide in air the pendent aryl group is selectively hydroxylated at theortho position, affording the phenolato complex [PdBCl] in high yields [B− is deprotonated 2-(2′-hydroxyarylazo)pyridine]. A possible reaction pathway is proposed by analogy with the hydroxylation of certain organic compounds by OH−/O2. The crystal and molecular structure of one [PdBCl] complex is reported. In the highly planar complex, the Pd N(azo) length is significantly shorter than the Pd N(pyridine) length. A single Pd Cl stretch at 365 cm−1 characterizes [PdBCl] which, unliked [PdACl2], has a structured intense absorption in the visible region near 670 nm.

Copper(I)–azoimidazoles: a comparative account on the structure and electronic properties of copper(I) complexes of 1-methyl-2-(phenylazo)imidazole and 1-alkyl-2-(naphthyl-(α/β)-azo)imidazoles

Abstract Bis-[1-alkyl-2-(naphthyl-(α/β)azo)imidazole]copper(I) perchlorate derivatives, [Cu(α-NaiR)2(ClO4)] and Cu(β-NaiR)2](ClO4), have been characterised by spectral and electrochemical studies. The single crystal X-ray structure of bis-[1-ethyl-2-(naphthyl-α-azo)imidazole]copper(I) perchlorate shows strong bonding with two imidazole-N atoms, two azo-N donors interact weakly and the structure is described as having a [2+2] distorted linear geometry. A structural comparison has been done with the X-ray structure of bis-[1-methyl-2-(phenylazo)imidazole]copper(I) perchlorate which is Td symmetric. Solution electronic spectra and redox properties are compared and have been correlated with EHMO calculation.