Structural, magnetic, and electronic properties of phenolic oxime complexes of Cu and Ni

Square planar complexes of the type Ni(L(1))(2), Ni(L(2))(2), Cu(L(1))(2), and Cu(L(2))(2), where L(1)H = 2-hydroxy-5-t-octylacetophenone oxime and L(2)H = 2-hydroxy-5-n-propylacetophenone oxime, have been prepared and characterized by single-crystal X-ray diffraction, cyclic voltammetry, UV/vis spectroscopy, field-effect-transistor measurements, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, and, in the case of the paramagnetic species, electron paramagnetic resonance (EPR) and magnetic susceptibility. Variation of alkyl groups on the ligand from t-octyl to n-propyl enabled electronic isolation of the complexes in the crystal structures of M(L(1))(2) contrasting with π-stacking interactions for M(L(2))(2) (M = Ni, Cu). This was evidenced by a one-dimensional antiferromagnetic chain for Cu(L(2))(2) but ideal paramagnetic behavior for Cu(L(1))(2) down to 1.8 K. Despite isostructural single crystal structures for M(L(2))(2), thin-film X-ray diffraction and scanning electron microscopy (SEM) revealed different morphologies depending on the metal and the deposition method (vapor or solution). The Cu complexes displayed limited electronic interaction between the central metal and the delocalized ligands, with more mixing in the case of Ni(II), as shown by electrochemistry and UV/vis spectroscopy. The complexes M(L(2))(2) showed poor charge transport in a field-effect transistor (FET) device despite the ability to form π-stacking structures, and this provides design insights for metal complexes to be used in conductive thin-film devices.