Spin-crossover, mesomorphic and thermoelectrical properties of cobalt(II) complexes with alkylated N-3-Schiff bases

Three new cobalt(II) complexes, [Co(L-12)(2)](BF4)(2) (1), [Co(L-14)(2)](BF4)(2)center dot H2O (2) and [Co(L-16)(2)](BF4)(2)center dot H2O (3), where L12-16 are N-3-Schiff bases appended with linear C12-16 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