Prasanta Kumar Santra

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)].

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.

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.

Synthesis, spectral and electrochemical studies of 2-(arylazo)heterocycle complexes of zinc(II). Single-crystal X-ray structure of [Zn(papm)Cl2·CH3OH] (papm=2-(phenylazo)pyrimidine)

Abstract 2-(Arylazo)pyridines (aap) (RC6H4NNC5H4N, 1) and 2-(arylazo)pyrimidines (aapm) (R-C6H4NNC4H3N2, 2); R=H (a), o-Me (b), m-Me (c), p-Me (d), p-Cl (e) are used to synthesise title compounds. The complexes are characterised by elemental analyses, IR, UV–Vis and 1H NMR spectral data. Single-crystal X-ray structure of dichloro-{2-(phenylazo)pyrimidine}zinc(II)·methanol suggests that the complex is a distorted trigonal bipyramidal symmetric around Zn(II), and Cl(1), Cl(2), N(4) (N (azo)) make the trigonal plane. Zn(II) moves downwards by 0.12 A from the centre of gravity of the plane. Two axial positions are occupied by N(1) (N(pyrimidine)) and methanol-O. Molecular packing shows one-dimensional infinite chain via hydrogen bonding. The EHMO calculation has been carried out to explain the electronic behaviour of the complexes and the results are compared with that of copper(I) complex.

Synthesis, spectral characterization and X-ray crystal structures of mercury(II)-azoimine compounds

Abstract 1-Ethyl-2-(phenylazo)imidazole (PaiEt, 2) and 2-(arylazo)pyrimidines (Raapm) (RH (3a), p-Me (3b), p-Cl (3c)) are used to synthesize mercury(II) compounds. The complexes are characterized by elemental analysis, IR, UV–Vis, 1H NMR spectral data and single crystal X-ray structures of di[chloro-{1-ethyl-2-(phenylazo)imidazole}(μ-chloro)-mercury(II)] (4) and di[chloro-{2-(phenylazo)pyrimidine} (μ-chloro)-mercury(II)] (5a). The first complex is a centro-symmetric dimer with two highly asymmetric chloro-bridges which make the rhombohedral plane. The second complex forms an unsymmetric trapezohedral geometrical structure.

Reaction of Ru(PPh3)3Cl2 with 2-(arylazo)pyrimidines. Spectral and redox characterisation of the products.: Part V. Single crystal X-ray structure of Ru(PPh3)2(papm)Cl2 (papm=2-(phenylazo)pyrimidine)

Abstract Reaction of 2-(arylazo)pyrimidine (aapm) with Ru(PPh3)3Cl2 in CH2Cl2 solution affords [Ru(PPh3)2(aapm)Cl2] (2) while the reaction under refluxing conditions in EtOH isolates [Ru(PPh3)2(aapm)2](ClO4)2·H2O (3/4). Single crystal X-ray diffraction study of dichloro-bis(triphenylphosphine){2-(phenylazo)pyrimidine}ruthenium(II) has assigned a cis-Ru(PPh3)2 motif to the complex. Isomers of [Ru(PPh3)2(aapm)2](ClO4)2 have been characterised by 1H NMR data and they exist in cis–trans–cis and cis–cis–cis configurations in which coordination is considered with reference to three pairs of sequence of P, P (PPh3 abbreviated as P), N, N (N is N(pyrimidine)) and N′, N′ (N′ is N(azo)). The complexes exhibit MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple is in the range 0.8–1.2 V vs. SCE and [Ru(PPh3)2(aapm)2](ClO4)2 exhibits a higher potential value than that of [Ru(PPh3)2(aapm)Cl2].

Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II) Complexes

2-(Arylazo)pyrimidine (1) reacts with palladium chloride and gives Pd(aapm)Cl2 (2) which undergoes a palladium(II)-mediated novel carbon−nitrogen bond formation reaction of the aromatic amines to the ortho-C−H function of the pendant aryl ring in the coordinated 2-(arylazo)pyrimidine. The reactant, Pd(papm)Cl2 (2a) and the product, Pd(papm-N-C6H4-OCH3-p)Cl (5a) have been characterised by X-ray crystal structure. The coupled products exhibit low energy transitions (NIR region) unlike the parent complex, Pd(aapm)Cl2. The cyclic voltammogram shows an oxidation couple at positive potential to SCE. The EHMO calculation suggests that the spectral feature in Pd(aapm)Cl2 (2) is due to the MLCT [d(Pd)⇄π*(azo)] whereas in Pd(aapm-N-C6H4-X-p)Cl (3−6) the transition at the NIR region is due to the intravalence charge transfer (IVCT) transition.

Silver(I) helicates with 2-(arylazo)pyrimidines. Single crystal X-ray structure of {2-(phenylazo)pyrimidine}silver(I) nitrate

Abstract 2-(Arylazo)pyrimidines (aapm) react with AgNO3 in methanol and crystallises in a single strand right-handed helical coordination polymer, [Ag(aapm)(NO3)]n. The X-ray structure determination of Ag[2-(phenylazo)pyrimidine] (papm) (NO3) (abbreviated, Ag(papm)(NO3)) suggests the triangular coordination around Ag(I) which is made up of chelated azoimine, –NN–CN–, function from one ligand and the third donor centre is coming from meta-related pyrimidine-N of the adjacent papm of the second Ag(papm)-unit. The coordination polymer is propagated through pyrimidine-N→Ag bonding.

Osmium /azopyrimidine chemistry. Part VII: synthesis, structural characterisation and electrochemistry

Abstract Reaction of 2-(arylazo)pyrimidines (RC6H4NNC4H3N2, abbreviated aapm; R=H (papm), o-Me (o-tapm), m-Me (m-tapm), p-Me (p-tapm), p-Cl (p-Clpapm)) with (NH4)2[OsCl6] in 2-methoxyethanol gives two isomers of composition OsCl2(aapm)2. They are structurally characterised by 1H NMR spectra and established as the cis–trans–cis (ctc) and cis–cis–cis (ccc) isomer. With reference to the coordination pairs of Cl; N(pyrimidine) (N) and N(azo) (N′), the structure of ctc-OsCl2(p-tapm)2 is confirmed by an X-ray diffraction study. The complexes exhibit multiple MLCT transitions in the visible to near-IR region. Redox studies show an Os(III)/Os(II) couple at 1.2–1.3 V and Os(IV)/Os(III) couple at ∼2 V versus SCE. EHMO calculations and comparison with analogous ruthenium(II) complexes explain the spectral and redox properties of the complexes.