Showing papers by "Paula Brandão published in 2011"

A Trinuclear Copper(II) Cryptate and Its μ3‐CO3 Cascade Complex: Thermodynamics, Structural and Magnetic Properties

Abstract: The 2,4,6-triethylbenzene-capped hexaamine macrobicycle with pyridyl spacers (pyr) was able to coordinate three copper(II) ions within its cavity. Potentiometric studies performed at 298.2 K in MeOH/H(2)O (50:50 v/v) and at ionic strength 0.10 mol dm(-3) in KNO(3) revealed that trinuclear species predominate in solution from pH 5.0, the hydroxo complexes being the main species, which start forming at unusual very low pH values. The single-crystal X-ray determination of the trinuclear complex showed that the three copper centres have square-planar geometry, arranged in an almost equilateral triangle, and have carbonate bridging the three metal centres. The presence of carbonate resulted from fixation of atmospheric CO(2). The present study represents the first μ(3)-CO(3)-bridged trinuclear copper(II) complex located in the interior of a macrobicyclic cavity. The magnetic data of [Cu(3)(pyr)(μ(3)-CO(3))]·(ClO(4))(4) showed ferromagnetic intramolecular interactions [J=3.80 cm(-1), based on the Hamiltonian H=-J(S(1)S(2)+S(2)S(3)+S(1)S(3))] yielding a spin quartet, S=3/2, ground state. Density functional calculations on the experimental geometry of the trinuclear complex showed that the ferromagnetic nature of the magnetic coupling can be attributed to the syn-anti conformation of the carbonato bridge, and a magneto-structural correlation, based on the different conformations (syn-anti, syn-syn and anti-anti), is presented. The interesting properties observed, namely the lowering of the pK(a) of coordinated water molecules to unusual values and the good fit of the carbonate anion between the copper centres, derive from the special architecture of pyr.

Recognition of oxalate by a copper(II) polyaza macrobicyclic complex.

Abstract: A new polyamine macrobicyclic compound was synthesised through a [1+1] "tripod-tripod coupling" strategy and using a Schiff base condensation reaction, followed by sodium borohydride reduction. The resulting compound is a heteroditopic cage (btpN(7)) in which one of the head units is appropriate for the coordination of copper(II), whereas the other head is available for additional hydrogen-bonding and electrostatic interactions with substrates. The acid-base behaviour of the new compound, the stability constants of its complex with the Cu(2+) ion and the association constants of the copper(II) cryptate with oxalate (oxa(2-)), malonate (mal(2-)), succinate (suc(2-)), maleate (male(2-)) and fumarate (fum(2-)) were determined by potentiometry at 298.2 K in aqueous solution and at an ionic strength of 0.10 mol dm(-3) in KNO(3). These studies revealed a clear preference of the receptor [CuH(h)btpN(7)H(2)O]((2+h)+) for oxa(2-) over the other dicarboxylate substrates. This arises from co-operativity between metal-anion coordination and electrostatic and hydrogen-bonding interactions, in accordance with the ideal size of this dicarboxylate, which allow it to take full advantage of the potential binding sites of the receptor. A qualitative indicator-displacement study, in agreement with the potentiometric studies, demonstrated that the copper cryptate receptor can be used as a selective visual sensor for oxalate.

Properties of Metal Complexes of a New Dioxadiaza Macrocycle Containing a Dibenzofuran Unit and Acetate Pendant Arms

Abstract: A new dioxadiaza macrocycle containing a rigid dibenzofuran group (DBF) and bearing two acetate pendant arms, ac2[17](DBF)N2O2, has been synthesized in good yield by condensation of the parent macrocycle with potassium bromoacetate in basic aqueous solution. The protonation constants of ac2[17](DBF)N2O2 and the stability constants of its complexes with Mn2+, Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ ions were determined at 298.2 K in aqueous solutions and at ionic strength 0.10 mol dm–3 in NMe4NO3. The stability constants revealed that ac2[17](DBF)N2O2 has a higher affinity for larger metal ions, Cd2+ and Pb2+, showing a clear preference for cadmium. Spectroscopic data in solution and X-ray crystal structure determinations revealed that the higher affinity for Cd2+ and Pb2+ is due to the better fit of these metal ions into the macrocyclic cavity, derived from its ring size and rigidity and the involvement of all the donor atoms of the ligand in the coordination to these metal centres. Considering the negative effect of these heavy metal ions on human health and the environment, this study represents an important step in the development of selective chelators for these two pollutants.

A second monoclinic polymorph of N-(diethyl-amino-thio-carbon-yl)-N'-phenyl-benzamidine.

Abstract: The asymmetric unit of the title compound, C18H21N3S, contains five mol­ecules. The equivalent bond distances in the five mol­ecules are in excellent agreement, the r.m.s. fit being within 0.007 A. The five mol­ecules are linked into a chain consisting of alternating pseudo-enanti­omers by N—H⋯S hydrogen bonds supplemented by weak C—H⋯π inter­actions. The action of a glide plane links the asymmetric unit into an extended chain. A polymorph of the title compound with one mol­ecule in the asymmetric unit was reported by Braun et al. [Cryst. Res. Technol. (1988), 23, 35–39].