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

Abstract

2 O (3), where L [12][13][14][15][16] are N 3 -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 CoN 6 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 a normal thermal spin-crossover behaviour with weak hysteresis loops at about 320 K. Hence, these complexes showed uncoupled phase transitions (class iiia). 1 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.

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2′:6′, 2′′‐Terpyridines

“…That a complex of this type leads to liquid crystal formation is of some interest, not least because of the known spin‐crossover behaviour associated with Co II and Co III complexes and N 6 ligand donor sets, including terpyridine . An example of a related liquid–crystalline Co II complex with an N 6 donor set derived from pyridine‐2,6‐diimine ligands has been reported recently and exhibited spin‐crossover properties; other examples of possible or actual liquid–crystalline complexes of cobalt with spin‐crossover properties have been reported . The data for the X‐ray structure of 20 ‐1 were recorded at 110 K and then again at room temperature so that bond lengths could be compared.…”

“…That ac omplex of this type leads to liquid crystal formation is of somei nterest, not least because of the known spin-crossover behaviour associated with Co II and Co III complexes and N 6 ligand donors ets, including terpyridine. [17] An example of ar elated liquid-crystalline Co II complex with an N 6 donor set derived from pyridine-2,6-diimine ligandsh as been reported recently and exhibited spin-crossover properties; [18] other examples of possible or actual liquid-crystallinec omplexes of cobalt with spin-crossover properties have been reported. [19] The data for the X-ray structure of 20-1 werer ecorded at 110K and then again at room temperature [26] so that bond lengths could be compared.D espite the much larger estimated standard deviations in the room-temperature structure, it was evident from the CoÀNb ond lengths that there was no spin-crossover behaviour in the range 298-110 K. Nonetheless,b yt uning with the ligand,s uch complexes in due courser epresent an avenue for developmenti nc ombining liquid crystal and magnetic properties.…”

Ultra-high Seebeck coefficient of a thermal sensor through entropic optimisation of ligand length of Fe(ii) spin-crossover (SCO) materials

“…From the TGA, the major mass losses for each complex agreed with the values calculated from the loss of two BF 3 and two ligands. 2 Besides that, complex 1 suffered initial mass losses of 0.8% (temperature ranged from 357 K to 513 K) due to the evaporation of lattice H 2 O. Thus, the thermal stability of complex 1 (T dec ¼ 515 K) is higher than complex 3 (T dec ¼ 463 K) due to a higher HS of complex 1.…”

“…The thermal sensor studied in this work has the same operating principle as the TEG, but the ultrahigh Seebeck (S e ) coefficient is achieved using the SCO complex by primarily exploiting its intrinsic phase and magnetic transitions, namely the liquid crystal (LC) effect and spin Seebeck effect (SSE). [2][3][4][5] The sensitivity of the SCO complex to a thermal stimulus has driven entropy changes, which subsequently drives a Seebeck voltage. The entropy change (DS) and the transport properties of the charge carrier affect the magnitude and sign of S e .…”

“…Fig. 6 Seebeck coefficient measurement of 0.002 M (a) [Fe(L 12 ) 2 ](BF 4 ) 2 (1), (b) [Fe(L 14 ) 2 ](BF 4 ) 2(2), and (c) [Fe(L16 ) 2 ](BF 4 ) 2(3). The maximised entropy of the molecular reaction of Fe(II) SCO complexes is achieved through six factors: (i) spin state transition, (ii) liquid crystal physical changes, (iii) spin Seebeck effect, (iv) kosmotropic and chaotropic effect, (v) fastener effect and (vi) thermal heat absorbance are labelled in the figure.…”