Macrocyclic ligand ring size effects. Part 4. Comparative study of the interaction of 'lop-sided' 14- to 17-membered tetraaza macrocycles containing fused dibenzo substituents with nickel(II)

The effect of macrocyclic and chelate-ring size on the complexation behavior of dibenzo tetraaza macrocycles I (n = 2-5) incorporating 14- to 17-membered inner rings were studied. Change in ring size along this 'lop-sided' series was achieved solely by varying the no. of methylene carbons that link adjacent benzyl N atoms. Several solid complexes NiLX2•xH2O (L = macrocyclic ligand; x = 0 or 1; X = Cl, NCS or NO3) with these rings were isolated. The results of five x-ray crystallog. analyses, as well as mol. 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-coordinate 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 Ni 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 mol. mechanics studies indicate that, as the no. 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 Ni(II) nitrate is also reported, as is that of the 17-membered ring complex of Ni(II) chloride. The former contains an equal mixt. of two (trans-octahedral) complexes [NiL(NO3)2] and [NiL(NO3)(H2O)]NO3. Cyclic voltammetry indicates that formation of the resp. Ni(III) species is facilitated by the smaller-ring ligands; the oxidn. of the 17-membered ring complex is irreversible and probably ligand based.