Metal complexes of an oxatriaza macrocycle containing pyridine: Thermodynamic stability and structural studies

A new 14-membered oxatriaza macrocycle containing pyridine has been synthes ized, 7-oxa-3,11,17-triazabicyclo[11.3.1]heptadeca-1(17),13,15-triene, L-1. The protonation constants of this compound and stability constants of its complexes with the Mn2+ to Zn2+, Cd2+, and Pb2+ were determined by potentio metric methods at 25 degreesC and ionic strength 0.10 mol dm(-3) in KNO3. L -1 presents two high values of protonation constants, and the third is very low. Its overall basicity is low because the three basic centres are in cl ose proximity. Only mononuclear complexes were found, and the stability con stants with all the metal ions studied are of the same order as those of th e corresponding complexes of the oxatriaza macrocycle L-2 (1-oxa-4,8,12-tri azacyclotetradecane), but lower than those of tetraaza macrocycles of simil ar or lower cavity size. The electronic spectra together with the values of magnetic moments of the cobalt(II) and nickel(II) complexes of L-1 suggest that five co-ordinate species are formed in aqueous solution. The EPR spec troscopy of frozen solutions of the copper(II) complex of L-1 has shown onl y one species characteristic of rhombic symmetry with elongation of the axi al bonds and a d(x2-y2) ground state, and the analysis of the EPR parameter s suggests the presence of a bis complex containing two macrocyclic units. The single crystal structure of the complex [(CuLCl)-Cl-1]ClO4 1 was determ ined. The complex crystallises in the triclinic system, space group P (1) o ver bar, a = 7.4973(9), b = 9.649(2), c = 12.712(2) Angstrom, beta = 111.02 (2), beta = 96.65(1), gamma = 90-11(1)degrees, Z = 2, D-calcd = 1.691 g cm( -3). Final R and R ' values of 0.0578 and 0.1454 for 2603 reflections with I >2 sigma (I) and 0.0782 and 0.1619 for all data were obtained. The comple x displays a distorted square pyramidal co-ordination sphere, the three nit rogen atoms of the macrocycle and one chlorine atom determining the basal p lane and the apical position occupied by the oxygen atom of the macrocyle. The metal centre is 0.275(2) A away from the N3Cl plane towards to the apic al ligand giving rise to a Cu - O bond length of 2.247(4) Angstrom. To achi eve this geometric arrangement the oxatriaza macrocycle folds about the lin e defined by the nitrogen atoms contiguous to the pyridine ring leading to a dihedral angle of 72.2(2)degrees. The single crystal presents a 1-D centr osymmetric supramolecular structure formed by two chains of cations and ani ons linked by hydrogen bonding via N-H . . .O and C-Hdelta+. . .Odelta-. in termolecular interactions.