The Coordination Chemistry of 1,4‐Diazepan‐6‐amine

The new, tridentate, facially coordinating ligand 1,4‐diazepan‐6‐amine (daza) has been prepared from ethane‐1,2‐diamine and 2,3‐dibromo‐1‐propanol in a seven‐step procedure with an overall yield of 22 %. The conformation of the free ligand has been elucidated by pH‐ and temperature‐dependent 1H NMR spectroscopy and by a single‐crystal X‐ray structure analysis of H3dazaCl3·H2O. A twisted chair with a predominantly equatorial orientation of the primary amino group has been established for daza and its protonation products Hxdazax+ (1 ≤ x ≤ 3). The formation constants of [M(daza)]2+ and [M(daza)2]2+ have been determined in aqueous solutions for M = NiII, CuII, ZnII, CdII, and CoII by potentiometric and spectrophotometric measurements, and a remarkably high stability has been found for the bis complexes ML2 in comparison to the mono complexes ML. This effect is discussed in terms of the particular steric requirements of the daza ligand. [Cu(daza)Cl2], [Ni(daza)2]Cl2·3.2H2O and [Zn(daza)2]SO4·5H2O have been characterized by single‐crystal X‐ray analyses. Aerial oxidation of Co2+ in the presence of daza results in the formation of the inert[Co(daza)2]3+, which was isolated as a mixture of the cisand trans isomers. These two isomers were separated by chromatographic methods and identified by NMR spectroscopy and single‐crystal X‐ray structure analysis of cis‐[Co(daza)2][ZnBr4]Br·H2O and trans‐[Co(daza)2]3[ZnBr4]2Br5·4H2O. The redox potentials of [Ni(daza)2]3+/2+ (1.04 V vs. NHE) and [Co(daza)2]3+/2+ (–0.21 V vs. NHE) were determined by cyclic voltammetry. The values are slightly more positive than for corresponding complexes with related cyclic triamines. This effect is again discussed in terms of the particular steric requirements of these ligands.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)