Comparative study of the interaction of ‘lop-sided’ 14- to 17-membered tetraaza macrocycles containing fused dibenzo substituents with nickel(II)

Abstract

The effect of macrocyclic and chelate-ring size on the complexation behaviour of a series of dibenzo tetraaza macrocycles incorporating 14- to 17-membered inner rings has been investigated. Change in ring size along this ‘lop-sided%’ series has been achieved solely by varying the number of methylene carbons that link adjacent benzyl nitrogen atoms. Several solid complexes of type NiLX₂·xH₂O (L = macrocyclic ligand; x= 0 or 1; X = Cl, NCS or NO₃) with these rings have been isolated. The results of five X-ray crystallographic analyses, as well as molecular mechanics studies, have allowed comparison of the effect of change of ring size on the structures of the 14- to 17-membered ring complexes with X = NCS. All complexes have similar six-co-ordinate trans-isothiocyanato geometries in which the four donors of the macrocycle occupy the equatorial plane. For this series an inverse correlation between in-plane and axial bond lengths around the nickel atoms occurs for the 14- to 16-membered ring complexes; the correlation is less apparent in the case of the 17-membered species. The X-ray and molecular mechanics studies indicate that, as the number of methylene carbons linking adjacent benzylamino nitrogens increases, the corresponding bite angle at the metal increases, with the opposite bite angle decreasing in a (partial) compensatory manner. As a consequence, there is only a minor variation in the planarity of the macrocyclic donor plane along the series. The crystal structure of the 16-membered ring complex of nickel(II) nitrate is also reported, as is that of the 17-membered ring complex of nickel(II) chloride. The former contains an equal mixture of two (trans-octahedral) complexes of type [NiL(NO₃)₂] and [NiL(NO₃)(H₂O)]NO₃. Cyclic voltammetry indicates that formation of the respective nickel(III) species is facilitated by the smaller-ring ligands; the oxidation of the 17-membered ring complex is non-reversible and probably ligand based.