Kamalaksha Nag

Metal complexes of sulphur-nitrogen chelating Agents. I. 2-aminocyclo-pentene-l-dithiocarboxylic acid complexes of Ni(II), Pd(II) and Pt(II).

Abstract The Ni(II), Pd(II) and Pt(II) chelates of 2-amino- cyclopentene-l-dithiocarboxylic acid (ACDA) are described. With each metal ions, two differently colored products having identical composition M(ACDA)2 are obtained when reactions are carried out with the acid itself and its ammonium salt. PMR spectra of ACDA indicates strong hydrogen bonding between one amino proton and thiocarbonyl sulphur atom. The nature of bonding in the complexes have been investigated from their i.r, pmr and electronic spectra, magnetic moment, thermal analysis and X-ray diffraction data. In all the chelates bonding with metal ions occur through amino nitrogen and deprotonated thiolsulphur. The stability constant of Ni-ACDA complex has been evaluated spectrophotometrically. Ni(ACDA)2 forms a 1:2 adduct with pyridine. The color difference in the chelates probably arises due to geometrical isomerism.

Dinuclear metal complexes. Part 4. Electrochemical studies of macrocyclic dicopper(II) complexes. Investigation of the effect of solvents, donor groups, and steric constraints on the stability of mixed-valence copper(II)–copper(I) complexes

Two series of macrocyclic dicopper(II) complexes of stoicheiometry [Cu2L][ClO4]2·2H2O have been synthesized and characterized. Compounds belonging to series (1) are derived from the 6,12,18,24-substituted (H,H,Me,Me; H,H,Me,Ph; Me,Me,Me,Ph; Me,Me,Ph,Ph) macrocycie 7,11;19,23-dimetheno-9,21-dimethyl-1,5,13,17-tetra-azacyclotetracosa-5,7,9,12,17,19,21,24-octaene-25,26-diol (H2L). Complexes belonging to series (2) are related to those of series (1) by replacement of two of the –CHN– linkages by –CH2–NH– groups and the segment containing two azomethine linkages having the 6,12-substituents H,H; Me,Me; Me,Ph; Ph,Ph. The electrochemistry of these compounds has been investigated in MeCN and CH2Cl2 solvents using platinum and glassy carbon electrodes. In all cases cyclic voltammograms show sequential one-electron transfers at two different potentials, and from these data the conproportionation constant (Kc) and free energy of the mixed-valent species (ΔGc) have been evaluated. Electrochemical results have been analysed in terms of the effect due to solvent, donor groups, and steric constraints. Values of Kc for all of the compounds are greater in MeCN than those in CH2Cl2, and the Kc value for any compound belonging to series (1) is greater than its analogue in series (2). Factors affecting stability and intramolecular electron-transfer rates of CuIICuIL+ species are discussed.

Synthesis and structural characterization of lanthanide(III) nitrate complexes of a tetraiminodiphenol macrocycle in the solid state and in solution

The lanthanide(III) complexes [Ln(LH2)(NO3)3] 1-11(La-Er), 15(Y) and [Ln(LH2)(NO3)2(H2O)](NO3) 12-14 (Tm-Lu) of the tetraiminodiphenolate macrocycle L2- have been prepared by the transmetallation reaction between [Pb(LH2)(NO3)2] and Ln(NO3)3.nH2O. In these compounds, the uncoordinated imino nitrogens are protonated and are hydrogen bonded to the phenolate oxygens. The X-ray crystal structures of the La (1), Ho (10) and Lu (14) compounds have been determined. Compounds 1 and 10, in which all the three nitrates are bound in bidentate fashion, are isostructural with distorted bicapped square antiprism geometry for the metal centre. In [Lu(LH2)(NO3)2(H2O)](NO3) 14, of the two metal bound nitrates one is bidentate and the other is unidentate, while the metal centre obtains a distorted square antiprism coordination environment. Proton NMR spectra of the paramagnetic lanthanide complexes have been studied in detail. Contributions of contact and pseudo-contact shifts to the lanthanide induced isotropic shifts (LIS) of the macrocycle protons have been separated and good agreement has been obtained between the calculated LIS values and the experimentally observed values. Analysis of the NMR data has led us to conclude that all the complexes in dimethyl sulfoxide solution attain similar configurations. The absorption and emission spectral characteristic of several compounds have been investigated. The complexes of samarium (5) and europium (6) on photoexcitation at 400 nm exhibit well-resolved luminescence spectra at 77 K both in the solid state and a methanol-ethanol (1 : 4) glassy matrix. For the terbium (8) and dysprosium (9) complexes, however, the observed luminescence peaks are less resolved and weak in intensity.

Doubly cyclopalladated complexes of N,N,N′,N′-tetraethylbenzene-1,3-bis(methylamine)

N,N,N′,N′-Tetraethylbenzene-1,3-bis(methylamine)(L) undergoes regiospecific double cyclopalladation with Li2[PdCl4] at the 4,6 positions of the benzene ring to produce the chloro-bridged polymeric complex [Pd2(L – 2H)Cl2]. Reactions of this complex with PPh3, methylpyridines, acetylacetone, ethylacetoacetate, monothiodibenzoylmethane and trifluoro(thenoyl)acetone have been studied and the products characterized by 1H and 13C NMR spectroscopy. Treatment of [Pd2(L – 2H)(PPh3)2Cl2] with MeCO2H and LiCl produces at room temperature the nitrogen-protonated species [Pd2L(PPh3)2Cl4], but trans-[Pd2(PPh3)2Cl4] and the metal-free ligand (L) are formed at 80 °C. The complex [Pd2(L – 2H)Cl2] undergoes exchange reaction with ligands capable of forming monocyclopalladated complexes. The crystal structure of [Pd2(L – 2H)(PPh3)2Cl2] has been determined: monoclinic, space group C2/c, a= 20.676(4), b= 12.436(3), c= 19.445(4)A, β= 104.29(3)° and Z= 4; refinement led to R= 0.035 and R′= 0.043 using 3533 unique reflections with I > 2σ(I).

Organodipalladium(II) cyclometallates of the schiff base N,N′-diethyl-2,6-dialdiminobenzene

Abstract Syntheses and reactions of dipalladiobenzene derivatives have been reported. Li 2 [PdCl 4 ] reacts with N,N′-diethyl-2,6- dialdiminobenzene (H 2 L) to form the cyclometallated compound, [Pd 4 L 2 Cl 4 ], in which metallation occurs at the 3,5-carbon atoms of the aromatic ring. The tetrachloro compound can be converted to a dinuclear precursor complex, [Pd 2 L(MeCN) 2 (ClO 4 ) 2 ], which reacts with pyridine, 4-methylpyridine, pyrazole and imidazole (B) to produce complexes of the type [Pd 2 L(B) 4 ](ClO 4 ) 2 , and with 2,2′- bipyridine to produce [Pd 2 L(bpy) 2 ](ClO 4 ) 2 . A tetranuclear neutral complex [Pd 4 L 2 (pz) 4 ] is formed when the precursor complex interacts with pyrazole and triethylamine. A series of neutral chelate compounds [Pd 2 L(AB) 2 ] has been obtained by reacting the precursor with sodium salts of acetylacetone, ethylacetoacetate, N-methylsalicylaldimine (HAB) and N,N′- dimethyldithiocarbamic acid (HAB). The compounds have been characterized from their 1H NMR spectra.

Metal complexes of sulphur-nitrogen chelating agents. III. 2-aminocyclo-pentene-l-dithiocarboxylic acid complexes of Cr(III), Fe(III), Co(III), Fe(II) and Co(II)

Abstract With 2-aminocyclopentene-l-dithiocarboxylic acid (ACDA) complexes of the type M(ACDA) 2 · nH 2 O (Co(II), n = 3; Fe(II), n = 2), Cr(ACDA) 3 · 4H 2 O and M(ACDA) 3 (Fe(III) and Co(III)) have been prepared. The compounds have been characterized from their infrared and electronic spectra, magnetic moment and thermal analysis. The Co(II) complex is of square planar configuration. The i.r. spectra suggest that in the Fe(II) and Fe(III) complexes bonding take place from (N,S − ) donor sites of the ligand and in the Co(II), Co(III) and Cr(III) complexes disulphur chelation occur. The electronic spectra of the latter two complexes support this view. The Fe(II) complex is probably polymeric in nature.

Homo- and heterobimetallic complexes of ruthenium(II) and osmium(II) with pyrazole-3,5-dicarboxylic acid and 2,2'-bipyridine as co-ligands. Synthesis, structure, stereochemistry, spectroscopy and redox activities

Mononuclear, homodinuclear and heterodinuclear ruthenium(II) and osmium(II) complexes of composition [(bpy) 2 Ru(H 2 pzdc)](ClO 4 )·3H 2 O ( 1 ), [(bpy) 2 Os(H 2 pzdc)](ClO 4 )·3H 2 O ( 2 ), [(bpy) 2 Ru(pzdc)Ru(bpy) 2 ](ClO 4 )·H 2 O ( 3 ), [(bpy) 2 Os(pzdc)Os(bpy) 2 ](ClO 4 )·H 2 O ( 4 ) and [(bpy) 2 Ru(pzdc)Os(bpy) 2 ](ClO 4 )·H 2 O ( 5 ) have been synthesized with pyrazole-3,5-dicarboxylic acid (H 3 pzdc) and 2,2′-bipyridine (bpy). Complexes 3 – 5 isolated solely in the homochiral forms have been characterized by their 1 H and { 1 H 1 H} COSY NMR spectra. The crystal structures determined for 3 and 5 have revealed that the two compounds are isostructural and the diruthenium compound 3 is present in the rac (ΛΛ/ΔΔ) form, while the osmiumruthenium compound 5 is present as the enantiomeric pair (ΛΛ and ΔΔ). The metal-centered oxidation and bipyridine-centred reduction in 1 – 5 take place reversibly. As compared with the diruthenium complex 3 , the stepwise oxidation of the metal centers in the diosmium complex 4 take place at less positive potentials by 0.45 V. The equilibrium constants for the comproportionation reaction M II M II +M III M III ⇌2M II M III are found to be almost equal (ca. 6×10 2 ) for the diruthenium and diosmium species. From pH-dependent redox potential measurements the acid dissociation constants for the mononuclear complexes 1 and 2 have been determined. The luminescence spectral behavior of the complexes have been investigated in methanol–ethanol (1:4) solution (at 300 K) and in glassy state (77 K) and the quantum yields for the ruthenium(II) complexes are reported.

Metal complexes of sulphur–nitrogen chelating agents. Part 11. Synthesis, characterisation, and thermodynamics of nucleophilic substitution reactions of monohalogeno nickel(II) complexes of tridentate ligands of the type SNN in solution and the solid state

Square-planar nickel(II) complexes [NiLX](X = Cl, Br, I, or SCN) have been obtained through deprotonation of the ligands methyl 2-(β-aminoethylamino)cyclopent-1-enedithiocarboxylate (HL1), methyl 2-(β-aminoisopropylamino)cyclopent-1-enedithiocarboxylate (HL2), and methyl 2-(β-diethylaminoethylamino)cyclopent-1-enedithiocarboxylate (HL3). Diamagnetic complexes of composition [NiL(MeCN)][ClO4](L = L1–L3) and [Ni2L2(HL)][ClO4]2(L = L1 or L2) have also been synthesised. The equilibrium constants for the nucleophilic substitution reaction [NiL2X]+ B [NiL2(B)]++ X–(X = Cl, Br, or I; B = pyridine and its methyl-substituted derivatives) have been determined spectrophotometrically in the temperature range 25–50 °C. A linear correlation has been obtained between the ΔH⊖ and ΔS⊖ values. The enthalpy changes (ΔH2) and activation energies (Ea*) for the thermal dissociation [NiL2(B)]X(s)→[NiL2X](s)+ B(g) have been determined. A linear correlation has been found between the (ΔS2) and Ea* values.

Formation of oxo-bridged tetrairon(III) complexes mediated by oxygen activation. Structure, spectroscopy, magnetism and electrochemistry

The reaction involving 2,6-diformyl-4-methylphenol, 1,3-diaminopropane, iron(II) perchlorate and sodium acetate in the ratio (2 : 2 : 2 : 3) in methanol produces the acetate-bridged macrocyclic diiron complex [FeII2L(μ-O2CCH3)(CH3OH)2](ClO4) (1). The direct reaction between the perchlorate salt of the macrocyclic ligand [LH4](ClO4)2 and iron(II) perchlorate in the presence of a mixture RCO2H and triethylamine also produces in solution the diiron complex [FeII2L(μ-O2CR)]+ (where R = H, CH3, C2H5, (CH3)3C and C6H5), which on exposure to air readily transforms to the corresponding oxo-bridged tetrairon(III) complex [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 (2–6). The X-ray crystal structures of the acetate (3) and benzoate-bridged (6) complexes have been determined. Variable temperature (4–300 K) magnetic susceptibility measurements carried out for 3 and 6, have been analyzed in terms of molecular field approximation. The dominant antiferromagnetic exchange interaction (J) occurring through the oxo-bridged diiron(III) is −115 cm−1 for 3 and −109 cm−1 for 6, while the magnitude of spin exchange coupling occurring through the phenoxide bridges (J′) is −4 cm−1 for both 3 and 6. Complexes 2–6 exhibit identical proton NMR spectra in CD3CN, indicating that the capped carboxylate ligand gets detached from the metal centres. The Fe⋯H distances estimated from relaxation time (T1 and T2) measurements have been found to agree well with the crystallographic distances. Complexes 2–6 exhibit identical electrochemical behaviour and undergo reduction in three steps involving a first two-electron transfer reaction to [{FeIILFeIII}2(μ-O)2]2+, followed by two one-electron two-proton coupled reactions to [FeIILFeII]2+.

Linear dependence of spin exchange coupling constant on bridge angle in phenoxy-bridged dinickel(II) complexes

A magneto-structural relationship showing linear dependence of antiferromagnetic exchange coupling constant (–J) on Ni–O–Ni bridge angle as well as the intramolecular Ni ⋯ Ni distance has been obtained with a series of phenoxy-bridged macrocyclic dinickel(II) complexes with centrosymmetric structures.