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.