Hysteretic thermal spin-crossover in heteroleptic Fe(II) complexes using alkyl chain substituted 2,2’-dipyridylamine ligands

The alkyl chain carrying ligands N,N-di(pyridin-2-yl)butanamide (LC 4) and N,N-di(pyridin-2-yl)decanamide (LC 10) were combined with NCS - co-ligands to form the neutral heteroleptic Fe(ii) complexes trans-[Fe II(LC 4) 2(NCS) 2] (1C 4) and trans-[Fe II(LC 10) 2(NCS) 2] (1C 10). Variable temperature crystallographic studies revealed that 1C 4 is in the orthorhombic space group Pna2 1 between 85-200 K whereas 1C 10 is in the monoclinic space group P2 1/c between 85-140 K. The average Fe-N bond lengths suggest that at 85 K 1C 4 contains LS Fe(ii) centres; however, the ca. 0.18 Å increase in the average Fe-N bond lengths between 85 and 120 K suggests a spin-transition to the HS state occurs within this temperature interval. 1C 10 contains LS Fe(ii) centres between 85 and 105 K. Upon warming from 105 to 140 K the average Fe-N bond lengths increase by ca. 0.19 Å, which suggests a spin-transition to the HS state. Solid-state magnetic susceptibility measurements showed that 1C 4 undergoes semi-abrupt spin-crossover with T 1/2 = 127.5 K and a thermal hysteresis of ca. 13 K whereas, 1C 10 undergoes an abrupt spin-crossover with T 1/2 = 119.0 K, and is also accompanied by thermal hysteresis of ca. 4 K. The crystallographic and magnetic data show that the length of the complex's alkyl chain substituents can have a large impact on the structure of the crystal lattice as well as a subtle effect on the T 1/2 value for thermal spin-crossover.