MACROCYCLIC LIGAND RING SIZE EFFECTS .4. COMPARATIVE-STUDY OF THE INTERACTION OF LOP-SIDED 14-MEMBERED TO 17-MEMBERED TETRAAZA MACROCYCLES CONTAINING FUSED DIBENZO SUBSTITUENTS WITH NICKEL(II)

The effect of macrocyclic and chelate-ring size on the complexation be haviour 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 NiLX(2) . xH(2)O (L = macrocyclic liga nd; x = 0 or 1; X = Cl, NCS or NO3) with these rings have been isolate d. The results of five X-ray crystallographic analyses, as well as mol ecular mechanics studies, have allowed comparison of the effect of cha nge of ring size on the structures of the 14- to 17-membered ring comp lexes with X = NCS. All complexes have similar six-coordinate trans-is othiocyanato geometries in which the four donors of the macrocycle occ upy the equatorial plane. Far this series an inverse correlation betwe en in-plane and axial bond lengths around the nickel atoms occurs for the 14- to 16-membered ring complexes; the correlation is less apparen t in the case of the 17-membered species. The X-ray and molecular mech anics studies indicate that, as the number of methylene carbons linkin g adjacent benzylamino nitrogens increases, the corresponding bite ang le at the metal increases, with the opposite bite angle decreasing in a (partial) compensatory manner. As a consequence, there is only a min or variation in the planarity of the macrocyclic donor plane along the series. The crystal structure of the 16-membered ring complex of nick el(II) nitrate is also reported, as is that of the 17-membered ring co mplex of nickel(II) chloride. The former contains an equal mixture of two (trans-octahedral) complexes of type [NIL(NO3)(2)] and [NIL(NO3)(H 2O)]NO3. Cyclic voltammetry indicates that formation of the respective nickel(III) species is facilitated by the smaller-ring ligands; the o xidation of the 17-membered ring complex is non-reversible and probabl y ligand based.