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submitted on 17.08.2019 and posted on 19.08.2019by Blaise Geoghegan, Wasinee Phonsri, Peter Horton, James Orton, Simon Coles, Keith Murray, Peter Cragg, Marcus Dymond, Ian Gass
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.