Spin-crossover, mesomorphic and thermoelectrical properties of cobalt(ii) complexes with alkylated N3-Schiff bases

Abstract

Three new cobalt(II) complexes, [Co(L¹²)₂](BF₄)₂ (1), [Co(L¹⁴)₂](BF₄)₂·H₂O (2) and [Co(L¹⁶)₂](BF₄)₂·H₂O (3), where L^12–16 are N₃-Schiff bases appended with linear C_12–16 carbon chains at the nitrogen atoms, were obtained in good yields by facile one-pot reactions. The single crystal X-ray structure of complex 1 shows a tetragonally compressed CoN₆ coordination geometry. The melting temperatures of 1–3 were lower than 373 K, while their decomposition temperatures were above 473 K. All complexes have high-spin Co(II) centres at 300 K and exhibit a columnar mesophase above 383 K. Complexes 1 and 3 showed normal thermal spin-crossover behaviour with weak hysteresis loops at about 320 K. Hence, these complexes showed uncoupled phase transitions (class iiia). The values for the Seebeck coefficient (S_e) of the cobalt redox couples formed from 1 and 2 were 1.89 ± 0.02 mV K⁻¹ and 1.92 ± 0.08 mV K⁻¹, respectively, identifying them as potential thermoelectrochemical materials.