Equilibrium studies on the protonation and Cu(II) complexation by an hexaaza macrocycle containing p -xylyl spacers. The crystal structure of the hexaprotonated ligand and the kinetics of decomposition of the Cu(II) complexes
Abstract The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-1(26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25°C by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylenetriamine subunits (BMXD). The crystal structure of a salt of H6L6+ containing Br− and NO3 − as anions has been solved by X-ray diffraction procedures and reveals that the macrocycle adopts an almost planar configuration. The protonated amine groups are involved in a complex network of hydrogen bonds with the anions and water molecules, with two anions being placed close to the cavity of the macrocycle. Despite the lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3+ complexes is several log units higher than that corresponding to the analogous BMXD complexes, which suggests the possibility of a different coordination mode for both closely related ligands. Actually, the whole set of stability constants for the mono and binuclear Cu(II)–L complexes can be rationalised by considering that the macrocycle acts as tetradentate in the mononuclear species. In contrast, the kinetic parameters for decomposition of the mono and binuclear Cu(II)–L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes.
LIGAND SUBSTITUTION REACTIONS
Publisher Summary This chapter discusses recent progress in the area of kinetics and mechanisms of ligand substitution. The chapter describes classical complexes and a selection of bioinorganic reactions reflecting the growing importance of this bridging area, a few references of relevance to inorganic pharmacology, and organometallic systems. A rapidly developing area where mechanistic information is currently minimal but could usefully be greatly increased is that of self-assembly and supramolecular chemistry. The chapter focuses on catenanes, rotaxanes, helicates, and knots, to provide a qualitative view of the development of mechanistic investigation and understanding in this field. Substitution at copper(I) is an important feature of supramolecular chemistry, especially in relation to catenane formation. Mechanistic developments should be imminent in relation to the ligand-replacement reactions that are an integral part of the entry of copper into cells and of its subsequent central role in metalloprotein mediation of such processes as iron uptake and energy generation through electron transfer.
Host-guest interaction between the dinuclear copper(II) complex of the hexaaza macrocyclic ligand 3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]triaconta-1(29),11(30),12,14,25,27-hexaene and malonate, maleate, fumarate, pyrophosphate and orthophosphate anions
Abstract The hexaaza macrocyclic ligand 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1]triaconta-1(29),11(30),12,14,25,27-hexaene (L), forms strong dinuclear complexes, as well as several protonated and hydroxo chelates, with Cu(II) ions. The host-guest interactions between the dinuclear copper(II) complexes of L and malonate, maleate, fumarate, pyrophosphate and orthophosphate anions were investigated by potentiometric conventional pH-metry as well as by spectrophotometric titrations in 2:1:1 molar ratio for Cu(II), L and guest. Several ternary complexes are found in aqueous solution as a result of coordinate bonding, hydrogen bond formation and Columbic attraction between the host and guest. The stability constants for all the species found are reported. The dinuclear copper(II) complex of L shows stronger complexation with the maleate anion than with malonate and fumarate anions. It is suggested that the presence of the double bond in maleate offers a more favourable geometry for coordination to the Cu 2 L host. Complexation with pyrophosphate and orthophosphate anions show little difference in stability. The big, flexible, pyrophosphate anion probably coordinates outside the macrocycle.