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

Abstract

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeL(a)) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (H(x)MeL(a))(x+) and H(x)Me(3)tach(x+) (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(p-O(3)S-C(6)H(4)-CH(3))(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me(3)tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H(3)MeL(a))(BiCl(6))·2H(2)O and (H(3)MeL(a))(ClO(4))Cl(2) exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeL(a))(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeL(a))(2)](3+) (3(3+)), [(MeL(a))ClCd(μ(2)-Cl)](2) (4), trans-[Cu(MeL(a))(2)](2+) (5(2+)), and [Cu(HMeL(a))Br(3)] (6) were characterized by single crystal X-ray analysis of 1(ClO(4))(3)·H(2)O, 2Br(3)·H(2)O, 3(ClO(4))(3)·0.8MeCN·0.2MeOH, 4, 5Br(2)·0.5MeOH, and 6·H(2)O. Formation constants and redox potentials of MeL(a) complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M(II)(MeL(a))](2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (L(a)), it is only slightly enhanced. In analogy to L(a), MeL(a) exhibited a pronounced tendency for forming protonated species such as [M(II)(HMeL(a))](3+) or [M(II)(MeL(a))(HMeL(a))](3+) (see 6 as an example). In contrast to MeL(a), Me(3)tach forms [M(II)L](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 L(a) and MeL(a) 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 Me(3)tach 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.

Crystallographic and solution NMR structural analyses of four hexacoordinated gallium(iii) complexes based on ligands derived from 6-amino-perhydro-1,4-diazepine

“…This is a feature consistently observed for metal complexes featuring a tridentate bound DAZA core, and has been considered to reflect the flexibility associated with the exocyclic nitrogen group. [25][26][27] The Ga-N bond distances are considerably elongated compared to the M-O bonds, consistent with the difference in donor-acceptor strength of the Ga-N and Ga-O bonds (Table 4). There is significant variation in the Ga-O bond lengths which is a consequence of a number of factors, including the asymmetry of the ring (Fig.…”

Di-μ-chlorido-bis(chlorido{8-[2-(dimethylamino)ethylamino]quinoline}cadmium) ethanol monosolvate

“…The two pendant Cl À ions are oriented roughly perpendicular to, but on opposite sides, of the plane of the (Cd 2 Cl 2 ) core in both the hydrate and ethanol solvate forms. Similar perpendicular arrangement of the pendant Cl À ions is observed in the Cl-(Cd 2 Cl 2 )-Cl fragments of other complexes with different ligands (Neis et al, 2010;Marsh 1999;Pauly et al, 2000). An alternative co-planar arrangement is also possible (Cannas et al, 1980).…”

Redetermination of 1,3-diammonio-1,2,3-trideoxy-cis-inositol dichloride

“…The crystal structure of 1,3-diammonio-1,2,3-trideoxy-cis-inositol sulfate has been reported by Neis et al (2012). The importance of intramolecular hydrogen bonding in 1,3,5-trisubstituted cyclohexane derivatives has been described by Gencheva et al (2000), Saaidi et al (2008) and Neis et al (2010), and the implication of increased 1,3-diaxial repulsion on the conformation of a cyclohexane ring has been discussed by Fritsche-Lang et al (1985), Kramer et al (1998) and Kuppert et al (2006). For the synthesis, see: Merten et al (2012).…”

“…As already noted by Palm (1967), the cyclohexane rings of both cations adopt a chair conformation, having one hydroxy and two ammonium groups in equatorial and two hydroxy groups in axial position. triaxial arrangement, a different behaviour has frequently been noted (Gencheva et al, 2000;Saaidi et al, 2008;Neis et al, 2010). If a third syn-axial substituent is present, it appears that formation of such intramolecular hydrogen bonds is sometimes even a prerequisite for the adoption of a stable cyclohexane chair.…”