Investigation of SCO property–structural relationships in a family of mononuclear Fe(ii) complexes

Abstract

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 (^2MeL_quinoline), E = S (1), Se (2), BH₃ (3) and L = N,N‘-dibenzyl-bis-(pyridin-2-ylmethyl) ethane-1,2-diamine (^DibenzylL), 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 ^2MeL_quinoline and ^2MeL_quinoline) does not affect the first coordination sphere notably, except in the case of a highly steric ligand ^2MeL_quinoline, and hence has a slight effect on the temperature of SCO.