Multifunctional mononuclear bisthienylethene-cobalt(ii) complexes: structures, slow magnetic relaxation and photochromic behavior

Abstract

Based on two new bisthienylethenes containing N,O-donor binding sites, 2-(2-hydroxy-5-bromo-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (BrLH) and 2-(2-hydroxy-5-diethylphosphono-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (PLH), multifunctional mononuclear complexes Co(BrL)₂·3CH₃OH (1) and Co(PL)₂·2CH₃OH (2) have been synthesized and characterized by crystallographic analysis. In the molecular structures of 1 and 2, the Co(II) ion adopts a distorted tetrahedral coordination geometry, and is coordinated by two nonequivalent bisthienylethene molecules (BrL⁻ in 1, PL⁻ in 2), showing non-photoactive parallel and photoactive antiparallel conformations, respectively. Compounds 1 and 2 show a distinct distortion of Co(II) coordination geometry, with bond angles of N–Co–N = 112.71(12)° and O–Co–O = 99.87(11)° for 1 and N–Co–N = 119.93(12)° and O–Co–O = 107.31(13)° for 2. Thus, 1 and 2 revealed different magnetic behaviors, which are demonstrated by the χ_MT vs. T plots, and the frequency dependence of the χ^′_M and χ^′′_M signals at low temperature. Besides the field-induced slow magnetic relaxation, both 1 and 2 also showed photochromic behavior. Upon irradiation with 360 nm light for 1 and 343 nm light for 2, their CH₂Cl₂–CH₃CN solutions could change color from being nearly colorless to blue purple. It was demonstrated that the substituent groups of Br atom and –PO(OEt)₂ in 1 and 2, respectively, could significantly influence their crystal structures, magnetic relaxations and photochromic properties.