Investigation of SCO Property - Structural Relationships in a Family of Mononuclear Fe(II) Complexes

Six new mononuclear Fe(ii) complexes of the [FeL(NCE)(2)] family have been synthesized: L = N,N'-dimethyl-N,N'-bis-(quinoline-2-ylmethyl)ethane-1,2-diamine (L-2Me(quinoline)), E = S (1), Se (2), BH3 (3) and L = N,N'-dibenzyl-bis-(pyridin-2-ylmethyl) ethane-1,2-diamine (L-Dibenzyl), E = S (4a and 4b), Se (5), and BH3 (6). Introduction of quinolyl groups in 1-3 results in elongated Fe-N(quinolyl) distances and severe distortion of the coordination environment. The quinolyl and benzyl groups in the tetradentate ligand hamper a dense crystal packing and only weak van der Waals interactions were found for all these complexes. 1, 2, 3, 4b and 5 are all stabilized in a high spin (HS) state over all temperatures. 4a, a polymorph of 4b, undergoes a spin crossover (SCO) around 81 K. 6 is also SCO-active and the conversion curve is centred at 183 K. A comparison of 23 analogous [FeL(NCE)(2)] complexes reported so far shows that both the ligand field strength of the NCE- co-ligands and the Fe-NC(E) angle play a determinant role in T-1/2. The replacement of the tetradentate ligand (confined to those of the same type as L-2Me(quinoline) and L-2Me(quinoline)) does not affect the first coordination sphere notably, except in the case of a highly steric ligand L-2Me(quinoline), and hence has a slight effect on the temperature of SCO.