Kausik K. Nanda

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.

Homo- and hetero-dinuclear metal complexes of bridging ligands containing phenol and azole moieties. Structure, spectroscopy, electrochemistry and magnetochemistry

A series of phenoxo-bridged homo- and hetero-dinuclear complexes of 4-methyl-2,6-bis(5-methylpyrazol-3-yl)-phenol (H3L1) have been synthesized and characterized. Variable-temperature magnetic susceptibility measurements carried out for the homo-dinuclear complexes [Cu2(H2L1)2][ClO4]21, [Cu2(H2L1)(acac)2][ClO4]2, [Cu2(H2L1)(µ-OH)Cl2]3, [Ni2(H2L1)2(H2O)4][ClO4]24 and [Fe2(H2L1)4(µ-OH)2]·4H2O 7 have shown that in all the cases the dinuclear metal centres are antiferromagnetically coupled with the following values of the exchange coupling constant J: –242 (1), –55 (2), –170 (3), –24 (4) and –11 cm–1(7). The ESR spectra of the heterodinuclear complexes [ZnCu(H2L1)2][ClO4]2·2H2O 5 and [ZnMn(H2L1)2Cl2]6 have been examined. The 1H NMR spectra of the dicopper(II) complexes 1 and 2 are reasonably sharp and the isotropic shifts of 1 are relatively less compared to those of 2. The crystal structures of 7 and 2·CHCl3 have been determined. That of 7 comprises two edge-shared sterically non-equivalent FeO4N2 distorted octahedra. The structure of 2·CHCl3 consists of two nearly square-planar copper(II) centres supported by a phenoxide bridge. The cryomagnetic behaviour of two phenoxo-bridged dimeric iron(III) complexes derived from a bis(iminomethyl)triphenol and a bis(aminomethyl)triphenol have also been studied.

MACROCYCLIC DIMERIC VANADIUM(IV) AND HETERODINUCLEAR VANADIUM(IV)-NICKEL(II) COMPLEXES. STRUCTURE, MAGNETIC, ELECTRONIC AND REDOX PROPERTIES

The oxovanadium(IV) complexes [{VO(H2L1)}2(µ-SO4)][NO3]2, [(VO)L1Ni(H2O)2(SO4)]·H2O and [(VO)L1Ni(µ-SO4)(H2O)]·2H2O have been synthesized from the dinucleating tetraaminodiphenol macrocycle H2L1. The crystal structure of the first, in which two unco-ordinated amino nitrogens are protonated, has been determined. The two distorted-octahedral vanadium centres in the complex, separated by 6.741(4)A, are bridged by sulfate and have an anti-oxo configuration. In the solid state the magnetic moment of this complex per vanadium decreases from 1.71 to 1.64 µB on lowering the temperature from 299 to 5 K, indicating very weak intra-/or inter-molecular exchange interactions. Its ESR spectra in fluid solution and glass, however, showed the absence of VO ⋯ VO electron-spin interaction. This complex undergoes stepwise oxidation to produce VOIVVOV and VOVVOV species with E1/2(1)= 0.445 V and E½(2)= 0.60 V vs. Ag–AgCl electrode; the comproportionation constant Kc= 4.2 × 102. The other two complexes have identical composition but differ in their IR and UV/VIS spectra and electrochemistry. In the first the oxovanadium(IV) and nickel(II) centres are revesibly oxidized at 0.436 and 0.756 V, respectively, while in the second irreversible oxidation of both metal centres takes place at 1.05 and 1.25 V. Both behave ferromagnetically and their exchange coupling constants J are 10(1) and 6(1) cm–1, respectively.

Structural characterization of the doubly cyclopalladated complex [Pd2L(−2H)(BTA)2] derived from N,N′-diethylbenzene-l,3-dicarbaldimine (L) and benzoyltrifluoroacetone (HBTA)

Doubly cyclopalladated mixed chelates of the type [Pd2L(-2H)(β-diket)2] derived from N,N′-diethylbenzene-l,3- dicarbaldimine (L) and the unsymmetrical β-diketones (Hβ-diket) benzoyltrifluoroacetone (HBTA), benzoylacetone (HBA) and thenoyltrifluoroacetone (HTTA) have been synthesized and characterized. The crystal structure of [Pd2L(−2H)(BTA)2] is reported. The complex C32H26N2O4F6Pd2 crystallizes in the triclinic space group P with a=10.872(1), b=11.915(1), c=13.525(1) A, α= 101.74(1), β=112.78(1), γ=97.77(1)°, V=1537.5(7) A3 and Z=2. The structure was solved by direct methods and refined by block-diagonal least-squares method to R=0.034 and Rw=0.037 based on 3535 observed reflections with I>3σ(I).

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.

Macrocyclic CuII4 and CuII2MII complexes. Structure of a heterotrinuclear CuII2ZnII complex exhibiting a helical twist

Three heterotrinuclear complexes [Cu2Zn(HL)][CIO4]3·3H2O 1, [Cu2Mg(HL)][CIO4]3·2H2O 2 and [Cu2Mg(HL)][NO3]3·3H2O 3 and two tetranuclear complexes [Cu4L][CIO4]4·4H2O 4 and [Cu4L(µ3-OH)][CIO4]3·MeOH 5 have been synthesized from a 38-membered macrocycle (H4L) having eight amine and four phenol functionalities. The compounds have been characterized on the basis of IR, UV/VIS and ESR spectroscopy and magnetic susceptibility measurements. Their redox chemistry has been studied by cyclic voltammetry and differential pulse voltammetry. The crystal structure of 1 has been determined. The two copper centers have N2O2 square-planar co-ordination environments and the zinc has a trigonal-bipyramidal geometry with N3O2 donors; one of the copper(II) ions is bridged to the zinc(II) ion by two phenolate oxygens. The cation in 1 has a helically twisted configuration.

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.