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

Abstract

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-[Fe^II(LC₄)₂(NCS)₂] (1C₄) and trans-[Fe^II(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 85–140 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 to the HS state occurs within this temperature interval. 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 a spin-transition to the HS state. Solid-state magnetic susceptibility measurements showed that 1C₄ undergoes semi-abrupt spin-crossover with T_1/2 = 127.5 K and a thermal hysteresis of ca. 13 K whereas, 1C₁₀ 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.