Synthesis, solution studies and structural characterisation of complexes of a mixed oxa-aza macrocycle bearing nitrile pendant arms

The mixed oxa-aza macrocycle [15]aneN3O2 with cyanomethyl pendant arms on all three N-centres, l,4,7-tris(cyanomethyl)-l,4,7 triaza-10,13-dioxacyclopentadecane (L), and its complexes with Cu(II), Ni(II), Zn(II), Cd(II) and Pb(II) have been prepared. The protonation constants of the ligand and the thermodynamic stabilities of the 1:1 metal:ligand complexes with Cu(II), Zn(II), Cd(II) and Pb(II) have been investigated by means of potentiometric measurements in aqueous solutions. These metal ions form only mononuclear complexes with unusually low stability constants, compared with those found for other cyclic or acyclic triamine compounds due to the relatively poor σ-donating properties of tertiary nitrogens and to the stiffening of the macrocyclic framework imposed by the cyanomethyl side arms, which are unable to co-ordinate to the metal ion sitting within the macrocyclic cavity. The single crystal X-ray structures of 1:1 complexes with Cu(II), Ni(II), Cd(II) and Pb(II) have been determined. The structures of [Cu(L)(CH3CN)](ClO4)2 and [Ni(L)(CH3CN)](ClO4)2 show slightly distorted octahedral co-ordination geometries with the metal cation bound to all the donor atoms of the macrocyclic ring and to a MeCN molecule. The structure of [Cd(L)(NO3)2] confirms binding of all the donor atoms of the ring, but in this case the metal ion is also co-ordinated to two nitrate anions, one of them in a bidentate fashion. In the case of Cu(II), Ni(II) and Cd(II) complexes, the nitrile arms do not take part in co-ordination to the metal ion and are directed away from its co-ordination sphere. Interestingly, the structure of the Pb(II) complex [Pb(L)(ClO4)]22+ confirms the formation of a binuclear species with binding of a nitrile pendant arm from an adjacent molecule. Two ClO4- anions bridge within the binuclear units in a bidentate fashion. NMR spectroscopic studies on the Zn(II), Cd(II) and Pb(II) complexes have revealed a rigidity in the complexes which is lost on increasing temperature. © 2002 Elsevier Science B.V. All rights reserved.