Syntheses, properties and X-ray crystal structures of the octahedral nickel(II) complexes of 2,5,9,12-tetramethyl-1,4,8,11-tetraazacyclotetradecane

• Published in: Polyhedron 1998-01, Vol.17 (9), p.1415-1422
• Main Authors: Choi, Ki-Young, Choi, Suk Nam, Suh, Il-Hwan
• Format: Article
• Language: English
• Subjects: octahedral nickel(II) complexes; tetramethyl tetraaza macrocycle
• ISSN: 0277-5387
• DOI: 10.1016/S0277-5387(97)00425-7

The syntheses and structural characterization of NiLX 2 (X = H 2O ( 1), N 3 − ( 2) and NCS − ( 3); L = 2,5,9,12-tetramethyl-1,4,8,11-tetrazacyclotetradecane) complexes are reported. The crystal structures of NiLX 2 complexes have an axially elongated octahedral geometry with two axial ligands and adopt the trans-III conformation. The equilibrium of 1 NiL 2+ + 2H 2O ⇌ NiL(H 2O) 2 2+ has been studied in aqueous solution over a temperature range, yielding ΔH° = −7.1 ±0.1 kJ mol − and ΔS° = − 18.7 ± 0.3 J K −1 mol −1. Electronic absorption spectra of NiLX 2 complexes show that two absorption bands ( 3 B 1g → 3 E g c and 3 B 1g → 3 E g b ) reflect the normal spectrochemical series for axial ligands but one absorption band ( 3 B 1g → 3B 2g) depends in reciprocal fashion in the range 300–800 nm. Cyclic voltammetry of NiLX 2 complexes exhibit two one-electron waves corresponding to Ni(II)/Ni(III) and Ni(II)/Ni(I) processes. The redox potentials of the complexes are influenced significantly by the nature of the axial ligands.