Isolation of a thermodynamically unfavorable [Ni(DACO)2]2+ complex and its 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) and [Ni(DACO)(2)](BrClO4)-Cl-. (II) have been newly synthesized and characterized. Single crystal X-ray diffraction analysis of DACO and both Ni-II complexes reveals that DACO takes boat/chair conformation in the solid state and its Ni-II complexes. In complex 1, Ni-II ion is at the center of symmetry, which is four-coordinated by nitrogen donors of DACO. However, in complex 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 mirror plane and the coordination sphere of Ni-II is a distorted planar geometry. Furthermore, complexes I and II form quite different packing patterns (macrocycle or chain) through hydrogen bonds, which may be a key role to stabilize the crystals. The results of theoretical calculation indicate that complex I has thermodynamic stability, while II has chemical stability. Therefore, both of them have the probability to be obtained from different reaction processes or conditions. (C) 2001 Elsevier Science B.V. All rights reserved.