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 NiII complexes of 1,5-diazacyclooctane (DACO), [Ni(DACO)2]Br2 (I) and [Ni(DACO)2]Br·ClO4 (II) have been newly synthesized and characterized. Single crystal X-ray diffraction analysis of DACO and both NiII complexes reveals that DACO takes boat/chair conformation in the solid state and its NiII complexes. In complex I, NiII 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=CuII and NiII) was found, in which two DACO ligands are related to each other by a mirror plane and the coordination sphere of NiII 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.