Hysteretic Thermal Spin-Crossover and Symmetry Breaking 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 (LC4) and N,N-di(pyridin-2-yl)decanamide (LC10) were combined with NCS- co-ligands to form the neutral heteroleptic Fe(II) complexes trans-[FeII(LC4)2(NCS)2] (1C4) and trans [FeII(LC10)2(NCS)2] (1C10). Variable temperature crystallographic studies revealed that 1C4 is in the orthorhombic space group Pna21 between 85-200 K whereas 1C10 is in the monoclinic space group P21/c between 85-105 K before undergoing a crystallographic phase transition to the triclinic space group P1􀴤 by 140 K. The average Fe-N bond lengths suggest that at 85 K 1C4 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 occurs within this temperature interval and the HS state is predominant beyond this. 1C10 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 that a spin-transition to the HS accompanies the P21/c to P1􀴤 crystallographic phase transition. Solid-state magnetic susceptibility measurements showed that 1C4 undergoes semi-abrupt spin-crossover with T1/2 = 127.5 K and a thermal hysteresis of ca. 13 K whereas, 1C10 undergoes an abrupt spin-crossover with T1/2 = 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 T1/2 value for thermal spin-crossover.