Ramprasad Das

Sterically promoted N2 bound tetrazolate complex formation in the 1,3-dipolar cycloaddition of p-OMeC6H4CN to the coordinated azide in the palladium complex [Pd(N-N-S)(N3)]. X-ray crystal structure of [Pd(N-N-S)(N4C-p-OMeC6H4]

Abstract The 1,3-dipolar cycloaddition of p-OMeC6H4CN to the coordinated azide in the [PdL(N3)] complex (HL=methyl 2-(((2-diethylamino)ethyl)amino)cyclopent-1-enedithiocarboxylate) has been studied, the tetrazolate complex [PdL(N4C-p-OMeC6H4) (1) thus produced has been characterized from its IR and 1H NMR spectra, and X-ray crystal structure. Complex 1 crystallizes in the triclinic space group P 1 with a=8.922(1), b=17.041(1), c=8.043(1) A, α=90.26(1), β=101.68(1), γ=82.81(1)°, V=1187.8(4) A3 and Dcalc=1.546 g cm−3 for Z=2. The structure was solved by the heavy-atom technique and refined by the block-diagonal least squares method to converge at R=0.057 and Rw=0.051 for 3669 observed (¦Fo¦>3σ¦Fo¦) reflections. The tetrazole in 1 is bound to Pd through its N2 nitrogen atom.

Iso- and mixed-valent phenoxy bridged binuclear macrocyclic complexes of cobalt, iron and manganese

Abstract The binuclear complexes of a tetraaminodiphenol macrocyclic ligand (H2L) of cobalt(II) and manganese(II) with the composition [M2LX2]βH2O (X = Cl or Br) and of iron(II) with the composition [Fe2L(imidazole)2(CH3OH)2](ClO4)2·2CH3OH have been synthesized and spectroscopically characterized. Cyclic voltammetric measurements of the dicobalt(II) complexes have indicated that the generation of CoIII-CoII and CoIII-CoIII species take place quasi-reversibly or irreversibly at about 0.38 and 0.90 V vs SCE, respectively. The dimanganese(II) complexes undergo quasi-reversible one-electron oxidation at about 0.32 V, but further oxidation does not occur up to 1.2 V. The iron(II) complex irreversibly gets oxidized at ca 1 V. The dicobalt(II) and dimanganese(II) complexes can be oxidized with bromine and the mixed-valence complexes [MIIIMIILCl2Br(CH3OH)]·solvent have been isolated. Variable-temperature magnetic measurements of [CoIIICoIILCl2Br (CH3OH)]·0.5CH2Cl2, [Mn2IILBr2(CH3OH)2]·H2O and [Fe2IIL(imidazole)2(CH3OH)2] (ClO4)2·2CH3OH have indicated that the mixed-valence cobalt complex behaves as a S = 3 2 system, while both the diiron(II) and dimanganese(II) complexes exhibit antiferromagnetic exchange coupling with J = −6.3 and −6.1 cm−1, respectively.

Phenoxo-bridged dinickel(II) complexes of a macrocyclic ligand: synthesis, stereochemical equilibria and structure

Dinuclear nickel(II) complexes of a tetraaminediphenol macrocyclic ligand (H2L) of composition [Ni2L(B)2][ClO4]2(B = NH3, imidazole, pyridine, or pyrazine) have been synthesised and characterized. With pyrazole (Hpz)[Ni2L(pz)(H2O)2][ClO4] and [Ni2L(pz)(Hpz)(H2O)][ClO4] complexes have been obtained in which pz– appears to act as a bridging ligand. The equilibrium constants for the reaction [Ni2L(MeOH)4]2++ 2B ⇌[Ni2L(B)2]2++ 4MeOH [B = pyridine, 2-methylpyridine (2Me-py), 3Me-py, 4Me-py, imidazole, or pyrazine] in methanol solutions have been determined spectrophotometrically at room temperature. The X-ray structure of [Ni2L(py)2][ClO4]2 has been determined: monoclinic, space group P21/n, with a= 9.251(1), b= 11.457(2), c= 18.555(2)A, β= 100.21(1)°, and Z= 2. The structure was determined by direct methods and refined to R= 0.053 and R′= 0.039. In the complex cation the two nickel(II) centres are in square-pyramidal configuration with N2O2 in-plane donor sets, involving two phenoxide bridges and two trans-axially disposed pyridines. The distance between the two nickel centres is 3.206(5)A.

Synthesis, characterization and electrochemistry of copper(II), nickel(II), cobalt(II) and zinc(II) complexes of a binucleating ligand with [N2S]2O donors

Abstract A heptadentate ligand (H 3 L) with a phenoxo group bridging two metal centres has been synthesized by reacting 4-methyl-2,6-diformylphenol with methyl 2-((2-aminoethyl)amino)cyclopent-l-enedithiocarboxylate. Mono- and binuclear complexes of copper(II), nickel(II), cobalt(II) and zinc(II) have been synthesized and characterized by their UV—vis, IR, 1 H and 13 C NMR and EPR spectra. The electrochemistry of the complexes has been studied and the factors affecting electron-transfer reactions of the binuclear complexes vis-a-vis corresponding mononuclear complexes have been examined.

Macrocyclic oxovanadium(IV) complexes: structure, magnetochemistry and electrochemistry

The oxovanadium(IV) complexes, [VO(L)]·H2O 1, [VO(H2L)(SO4)]·3H2O 2, [(VO)2L(µ-SO4)]·MeOH·3H2O 3 and [Zn(L)VO(SO4)]·4H2O 4 have been synthesised from the dinucleating tetraaminodiphenol macrocyclic ligand H2L. The ESR and IR spectra of 1 have indicated stacking of the molecules due to ⋯ VO ⋯ VO ⋯ interactions. In complexes 2 and 4 the unidentate sulfate is bound to the octahedral vanadium and two of the secondary amino groups in 2 are protonated. Both of these complexes undergo oxidation to produce oxovanadium(V) species with E½= 0.40 V (2), 0.455 V (4)vs. saturated calomel electrode (SCE). Fairly strong antiferromagnetic exchange interaction has been found in 3 from variable-temperature susceptibility measurements, J=–128 cm–1. The crystal structure of 3 has been determined; tetragonal, space group P4122, a=b= 12.252(2), c= 40.819(5)A and Z= 8; refinement led to R= 0.053 and R′= 0.055 using 1825 unique reflections with I > 3σ(I). The two octahedral vanadium centres in the complex, separated by 3.077 A, are bridged by sulfate and have syn oxo configuration.

Formation of mixed spin-state macrocyclic dinickel(II) complexes through variation of hole size

Dinuclear nickel(II) complexes of two tetraaminodiphenol macrocyclic ligands in which one (H2L1) has two –NH(CH2)3NH– units and the other (H2L2) contains one –NH(CH2)3NH– and one –NH(CH2)2NH– moiety have been studied. In the inclusion complex [Ni2L1(H2O)4][ClO4]2·4NH2CONH21, both metal centres are in octahedral configuration. On reaction with pyridine (py), complex 1 transforms into the square-pyramidal complex [Ni2L1(py)2][ClO4]2. The macrocycle H2L2 forms the mixed spin-state complex [Ni2L2(H2O)2]I22 in which one metal centre is square planar and the other octahedral. On reaction with pyridine, complex 2 produces the square-planar–square-pyramidal complex [Ni2L2(py)]I24; the low-spin Ni2+ of both 2 and 4 is accommodated by the smaller N2O2 cavity provided by the phenolic oxygens and –NH(CH2)2NH– nitrogens. The redox properties of complexes 1 and 2 have been studied by cyclic voltammetry and their crystal structures determined. Complex 1: a= 7.530(1), b= 26.499(3), c= 11.434(1)A, β= 96.89(1)°, Z= 2, R= 0.051 and R′= 0.059. Complex 2: a= 11.386(1), b= 20.094(2), c= 13.158(1)A, β= 102.16(1)°, Z= 4, R= 0.050, R′= 0.059.

Carboxylate bridging of amino acids in dinuclear macrocyclic nickel(II) complexes

The dinuclear nickel(II) complex [Ni2L(MeOH)2(ClO4)2]·2NEt3HClO4, derived from a tetraazadiphenol macrocyclic ligand H2L, on reaction with glycine, β-alanine and glycylglycine affords the µ-carboxylato complexes of composition [Ni2L(–O2C–NH3+)(H2O)2][ClO4]2·nH2O. The structure of [Ni2L(µ-O2CCH2NH3)(H2O)2][ClO4]2·2H2O has been determined: monoclinic, space group Cc, with a= 17.465(2), b= 21.834(2), c= 11.480(1)A, β= 115.27(2)° and Z= 4. The structure was solved by direct methods and refined to R= 0.047 and R′= 0.064.

Design and properties of phenoxo-bridged binuclear, trinuclear and tetranuclear macrocyclic complexes

Multinuclear complexes derived from binucleating and tetranucleating macro-cyclic ligands of the types tetraaminodiphenol and octaaminotetraphenol are reported. The effect of electronic interactions between metal centres held in close proximity by the macro-cyclic ligands on chemical reactivities, redox and magnetic properties of the complexes is discussed. Systematic changes in the structure and stereochemistry of the metal complexes brought about by variation of the auxiliary axial ligands are examined. A magneto-structural relation for the binuclear nickel (II) complexes implicating linear dependence of spin-exchange integral (J) on Ni-O(phenoxide)-Ni bridge angle or Ni...Ni separation is discussed.