Facially Coordinating Triamine Ligands with a Cyclic Backbone: Some Structure−Stability Correlations

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeLa) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me3tach) was studied. The structure of the free ligands (HxMeLa)x+ and HxMe3tachx+ (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H2Me3tach)(p-O3S−C6H4−CH3)2 showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me3tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pKa-values of H3Me3tach3+ (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H3MeLa)(BiCl6)·2H2O and (H3MeLa)(ClO4)Cl2 exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeLa)2]3+ (cis: 13+, trans: 23+), trans-[Fe(MeLa)2]3+ (33+), [(MeLa)ClCd(μ2-Cl)]2 (4), trans-[Cu(MeLa)2]2+ (52+), and [Cu(HMeLa)Br3] (6) were characterized by single crystal X-ray analysis of 1(ClO4)3·H2O, 2Br3·H2O, 3(ClO4)3·0.8MeCN·0.2MeOH, 4, 5Br2·0.5MeOH, and 6·H2O. Formation constants and redox potentials of MeLa complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [MII(MeLa)]2+-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (La), it is only slightly enhanced. In analogy to La, MeLa exhibited a pronounced tendency for forming protonated species such as [MII(HMeLa)]3+ or [MII(MeLa)(HMeLa)]3+ (see 6 as an example). In contrast to MeLa, Me3tach forms [MIIL]2+ complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an “all-or-nothing” process. Molecular mechanics calculations showed that the low stability of La and MeLa complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me3tach and tacn (tacn =1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.