Synthesis, characterization, and anion selectivity of copper(II) complexes with a tetradentate Schiff base ligand

A new mononuclear Cu(II) complex, [CuL(ClO4)(2)] (1) has been derived from symmetrical tetradentate di-Schiff base, N,N'-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by X-ray crystallography. The copper atom assumes a tetragonally distorted octahedral geometry with two perchlorate oxygens coordinated very weakly in the axial positions. Reactions of I with sodium azide, ammonium thiocyanate or sodium nitrite solution yielded compounds [CuL(N-3)]ClO4 (2), [CuL(SCN)ClO4 (3) or [CuL(NO2)]-ClO4 (4), respectively, all of which have been characterized by X-ray analysis. The geometries of the penta-coordinated copper(H) in complexes 2-4 are intermediate between square pyramid and trigonal bipyramid (tbp) having the Addition parameters (tau) 0.47, 0.45 and 0.58, respectively. In complex 4, the nitrite ion is coordinated as a chelating ligand and essentially both the 0 atoms of the nitrite occupy one axial site. Complex 1 shows distinct preference for the anion in the order SCN- > N-3(-) > NO2- in forming the complexes 24 when treated with a SCN-/N-3(-)/NO2- mixture. Electrochemical electron transfer study reveals (CuCuI)-Cu-II reduction in acetonitrile solution. (c) 2006 Elsevier B.V.. All rights reserved.

Bis- and tris-(methylphosphonic) acid derivatives of a 14-membered tetraazamacrocycle containing pyridine: synthesis, protonation and complexation studies

Two N-methylphosphonic acid derivatives of a 14-membered tetraazamacrocycle containing pyridine have been synthesized, H4L1 and H6L2. The protonation constants of these compounds and the stability constants of complexes of both ligands with Ni2+, Cu2+ and Zn2+ were determined by potentiometric methods at 298 K and ionic strength 0.10 mol dm(-3) in NMe4NO3. The high overall basicity of both compounds is ascribed to the presence of the phosphonate arms. H-1 and P-31 NMR spectroscopic titrations were performed to elucidate the sequence of protonation, which were complemented by conformational analysis studies. The complexes of these ligands have stability constants of the order of or higher than those formed with ligands having the same macrocyclic backbone but acetate arms. At pH = 7 the highest pM values were found for solutions containing the compound with three acetate groups, followed immediately by those of H6L2, however, as expected, the increasing pH favours the complexes of ligands containing phosphonate groups. The single-crystal structure of Na-2[Cu(HL1)]NO3.8H(2)O has shown that the coordination geometry around the copper atom is a distorted square pyramid. Three nitrogen atoms of the macrocyclic backbone and one oxygen atom from one methylphosphonate arm define the basal plane, and the apical coordination is accomplished via the nitrogen atom trans to the pyridine ring of the macrocycle. To achieve this geometric arrangement, the macrocycle adopts a folded conformation. This structure seems consistent with Uv-vis-NIR spectroscopy for the Ni2+ and the Cu2+ complexes and with the EPR for the latter.

A very rare hydrogen-bridged hexanuclear CuII complex containing a triangular Cu3O core capped by an unusual triply coordinated perchlorate anion

An unusual hexanuclear Cu-II complex, [{[Cu(NHDEPO)](3)(mu(3)-O)(O3ClO)}(2)(mu-H)]center dot 7ClO(4)center dot 4H(2)O (1) was prepared starting from Cu(ClO4)(2)center dot 6H(2)O and the oxime-based Schiff base ligand NHDEPO (= 3-[3-(diethylamino)propylimino]butan-2-one oxime). Structural characterization of the complex reveals that it consists of two triangular Cu3O units, the copper ions being at the corners of an equilateral triangle, separated by an O center dot center dot center dot O distance of 2,447(5) angstrom, held together solely by a proton. In each triangle, the copper atoms are in square-pyramid environments. The equatorial plane consists of the bridging oxygen of the central OH-(O2-) group together with three atoms (N, N, O) of the Schiff base. All Unusual triply coordinated perchlorate ion (mu(3)-kappa O:kappa O':kappa O '') interacts in axial position with the three copper ions, Variable-temperature (2-300 K) magnetic susceptibility measurements show that complex 1 is antiferromagnetically Coupled (J = -148 cm(1-)). The EPR data at low temperature clearly indicates the presence of spin frustration phenomenon in the complex.

Cis–trans isomerism in diphenoxido bridged dicopper complexes: role of crystallized water to stabilize the cis isomer, variation in magnetic properties and conversion of both into a trinuclear species

The trans-[Cu2L2Cl2] (1), and cis-[Cu2L2Cl2]·H2O (2) isomers of a diphenoxido bridged Cu2O2 core have been synthesized using a tridentate reduced Schiff base ligand 2-[(2-dimethylamino-ethylamino)-methyl]-phenol. The geometry around Cu(II) is intermediate between square pyramid and trigonal bipyramid (Addison parameter, tau = 0.463) in 1 but nearly square pyramidal (tau = 0.049) in 2. The chloride ions are coordinated to Cu(II) and are trans oriented in 1 but cis oriented in 2. Both isomers have been optimized using density functional theory (DFT) calculations and it is found that the trans isomer is 7.2 kcal mol(-1) more favorable than the cis isomer. However, the hydrogen bonding interaction of crystallized water molecule with chloride ions compensates for the energy difference and stabilizes the cis isomer. Both complexes have been converted to a very rare phenoxido-azido bridged trinuclear species, [Cu3L2(mu(1,1)-N-3)(2)(H2O)(2)(ClO4)(2)] (3) which has also been characterized structurally. All the complexes are antiferromagnetically coupled but the magnitude of the coupling constants are significantly different (J = -156.60, -652.31, and -31.54 cm(-1) for 1, 2, and 3 respectively). Density functional theory (DFT) calculations have also been performed to gain further insight into the qualitative theoretical interpretation on the overall magnetic behavior of the complexes.

Synthesis, crystal structures and magnetic properties of two bis(μ-phenoxido)dicopper(II) complexes derived from reduced Schiff base ligands

Two phenoxido bridged dinuclear Cu(II) complexes, [Cu-2(L-1)(2)(NCNCN)(2)] (1) and [Cu-2(L-2)(2)(NCNCN)(2)]center dot 2H(2)O (2) have been synthesized using the tridentate reduced Schiff-base ligands 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL1) and 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL2), respectively. The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. Both the complexes present a diphenoxido bridging Cu2O2 core. The geometries around metal atoms are intermediate between trigonal bipyramid and square pyramid with the Addison parameters (tau) = 0.57 and 0.49 for 1 and 2, respectively. Within the core the Cu-O-Cu angles are 99.15 degrees and 103.51 degrees and average Cu-O bond distances are 2.036 and 1.978 angstrom for compounds 1 and 2, respectively. These differences have marked effect on the magnetic properties of two compounds. Although both are antiferromagnetically coupled, the coupling constants (J = -184.3 and -478.4 cm (1) for 1 and 2, respectively) differ appreciably.

The Jua Kali and the Mali-Mali: the informal/formal nexus amongst Kenyan micro enterprises

This paper presents a preliminary exploration of the informal/formal economy nexus and entrepreneurial processes amongst a sample of Kenyan roadside vendors who mostly operate in the informal economy. Using semi-structured interviews, data was collected from sixty street vendors across Kenya. In particular the paper focuses on the relationship between the informal and formal economy and the factors that promote formality amongst micro and small enterprises in developing countries. The paper presents a conceptualization of a potential segmentation of the informal economy, considering the implications of this in terms of base of the pyramid initiatives and the promotion of development through enterprise.

Imitation, entrepreneurship and the informal/formal economy nexus: an African perspective

This paper explores a segmentation of micro and small enterprises (MSEs) in developing countries within the formal/informal economy nexus that has wide-ranging implications for the targeting of base-of-the-pyramid initiatives and entrepreneurship theory. This proposed segmentation emerges from the analysis of a sample of Kenyan MSEs utilising current and prior business models; the antecedent influences shaping the business model; barriers to entry associated with knowledge, capital and skills; the degree of innovation or imitation evident in the business model linked to the nature of opportunity recognition; and their relationship with the formal institutional business environment.