Isolation of a thermodynamically unfavorable [Ni(DACO)(2)](2+) complex andits isomer: syntheses, characterization, crystal structures and theoretical study (DACO=1, 5 diazacyclooctane)

Two Ni-II complexes of 1,5-diazacyclooctane (DACO), [Ni(DACO)(2)]Br-2 (I) a nd [Ni(DACO)(2)](BrClO4)-Cl-. (II) have been newly synthesized and characte rized. Single crystal X-ray diffraction analysis of DACO and both Ni-II com plexes reveals that DACO takes boat/chair conformation in the solid state a nd its Ni-II complexes. In complex 1, Ni-II ion is at the center of symmetr y, which is four-coordinated by nitrogen donors of DACO. However, in comple x II, an unexpected coordination mode of [M(DACO)(2)](2+) (M = Cu-II and Ni -II) was found, in which two DACO ligands are related to each other by a mi rror plane and the coordination sphere of Ni-II is a distorted planar geome try. Furthermore, complexes I and II form quite different packing patterns (macrocycle or chain) through hydrogen bonds, which may be a key role to st abilize the crystals. The results of theoretical calculation indicate that complex I has thermodynamic stability, while II has chemical stability. The refore, both of them have the probability to be obtained from different rea ction processes or conditions. (C) 2001 Elsevier Science B.V. All rights re served.