Ni II and Zn II complexes of the hexa-dentate macrocyclic ligand cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetra-decane-6,13-diamine The title pendent-arm macrocyclic hexaamine ligand binds stereospeci®cally in a hexadentate manner, and we report here its isomorphous Ni II and Zn II complexes (both as perchlorate salts), namely (cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetra-decane-6,13-diamine-6 N)nickel(II) diperchlorate, [Ni-(C 12 H 30 N 6)](ClO 4) 2 , and (cis-6,13-dimethyl-1,4,8,11-tetraaza-cyclotetradecane-6,13-diamine-6 N)zinc(II) diperchlorate, [Zn(C 12 H 30 N 6)](ClO 4) 2. Distortion of the NÐMÐN valence angles from their ideal octahedral values becomes more pronounced with increasing metal-ion size and the present results are compared with other structures of this ligand. Comment The coordination chemistry of the isomeric pendent-arm macrocycles trans-and cis-6,13-dimethyl-1,4,8,11-tetraaza-cyclotetradecane-6,13-diamine (L 1 and L 2 , respectively) has revealed a number of interesting variations in the structural and physical properties of their complexes. Complexes of the hexadentate-coordinated trans isomer (Curtis et al., 1987; Bernhardt et al., 1989, 1990, 1991; Borzel et al., 1998) are more common than the corresponding cis complexes (Bernhardt et al., 1992, 1993, 1997; Lye et al., 1994). It is an interesting feature that L 1 and L 2 can only bind in one con®guration when coordinated as hexadentates, so the structure of the organic ligand determines the isomerism of the resulting complexes. The hexadentate trans isomers coordinate such that the metal is coplanar with the four secondary amines, and the pendent amines bind in trans coordination sites. By contrast, in the corresponding cis isomers, the macrocycle adopts a folded conformation and the pendent amines coordinate in cis sites. Molecular-mechanics calculations (Bernhardt & Comba, 1991) predicted that the cis isomer L 2 , bound as a hexadentate, would be able to complex both small and large metal ions, whereas the hexadentate-coordinated trans isomer could only accommodate metals up to a certain size, until one or both axial MÐN bonds were broken as a consequence of strain in the complex. These computational results have been borne out by subsequent experimental data. The two complexes [NiL 2 ](ClO 4) 2 , (I), and [ZnL 2 ](ClO 4) 2 , (II), are isomorphous and the absolute con®guration was determined in each case. The Ni II and Zn II crystals studied here, both grown from racemic solutions, were found to be enantiomorphs. Views of the two complex cations are shown in Figs. 1 and 2. The folded conformation of the macrocycle is evident, with the coordinated four secondary amines having the same absolute con®guration i.e. RRRR (SSSS). Each complex cation has (non-crystallographic) C 2 symmetry; the principal axis bisects the N5ÐMÐN6 angle, and the MÐN bond lengths separate into three distinct pairs (Tables 1 and 3). Both macrocyclic ®ve-membered chelate rings adopt the same conformation (or !!), where the CÐC bond in each ring is oblique to the C ...