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.
Dinuclear copper and zinc complexes of a hexaazamacrocycle containing p-xylyl spacers and bridging anions: theoretical and spectroscopic studies
The hexaazamacrocycle 7,22-dimethyl-3,7,11,18,22,26-hexaazatricyclo[26.2.2.213,16]tetratriaconta-1(30),13,15,28,31,33-hexaene (Me2[30]pbz2N6) was synthesized and characterised by single crystal X-ray diffraction. The macrocycle adopts a conformation with the two aromatic rings almost parallel at a distance of ca. 4.24 A, but displaced relative to each other by ca. 1.51 A. The protonation constants of this compound and the stability constants of its complexes with Cu2+ and Zn2+, were determined in water–methanol (9 ∶ 1 v/v) at 25 °C with ionic strength 0.10 mol dm−3 in KCl. The potentiometric and spectroscopic studies (NMR of zinc, cadmium and lead complexes, and EPR of the copper complexes) indicate the formation of only dinuclear complexes. The association constants of the dinuclear copper complex with anions (thiocyanate, terephthalate and glyphosate) and neutral molecules (1,4-benzenedimethanol, p-xylylenediamine and terephthalic acid) were determined at 20 °C in methanol. The structural preferences of this ligand and of its dinuclear copper(II) complex with a variety of bridging ligands were evaluated theoretically by molecular mechanics calculations (MM) and molecular dynamics (MD) using quenching techniques.
Hydrolytically-sensitive hexaimino and hydrolytically-inert octaamino-cryptand hosts for dicopper
The hexa-Schiff-base cryptand N[(CH2)2NCH(C6H4-m)CHN(CH2)2]3N L1 has been found to encapsulate a pair of AgI or CuI cations without hydrolytic attack on the imine bonds; a problem which is evident when the [CuII2(OH)]3+ assembly is incorporated. The structure of the dicopper complex of a singly ring-opened ligand, derived from L1, has been determined. The octaamine cryptand L2(= L1+ 12H) more easily accommodates a [Cu2(µ-X)]3+ assembly where X = imidazolate, N3– or OH–. Collinear disposition of the bridge with the Nbr–Nbr axis is confirmed in the first case by an X-ray crystallographic structure determination of the imidazolate-bridged cryptate, and inferred for the second case by spectroscopic and magnetic data, while, for the third case, magnetic susceptibility data indicate severe bending of the Cu–O(H)–Cu assembly.