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 (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)] (1C(4)) and trans-[Fe-II(LC10)(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 angstrom 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 angstrom, 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.