Understanding the two-step spin-transition phenomenon in Iron(II) 1D chain materials

Three analogous one dimensional (1D) polymeric iron(II) spin crossover (SCO) materials containing the new ligand 4,6-bis(2',2''-pyridyl)pyrazine (bdpp) have been comprehensively characterised magnetically (thermal and light-induced) and structurally. Within this series are two polymorphs of the formula [Fe(NCS)(2)(bdpp)], 1 and 2 a, which differ magnetically in that phase 1 undergoes a full two-step SCO (T(1/2(1))=135 K and T(1/2(2))=90 K) whereas phase 2 a remains high spin (HS) over all temperatures. The central distinction between these two materials lies in the presence of intermolecular pi-pi interactions generated by the crystal packing in 1, which are absent in 2 a. The isostructural selenocyanate analogue of 2 a, [Fe(NCSe)(2)(bdpp)], 2 b, undergoes a full two-step SCO (T(1/2(1))=200 K and T(1/2(2))=125 K). Structural analyses of 1 and 2 b at a range of temperatures provide deep insight into their two-step SCO nature. Structural analysis of 1 at 25 K (1(LS-LS)), 123 K (1(LS-HS)) and 250 K (1(HS-HS)) reveals two distinct iron(II) centres at each temperature, with ordered, alternating HS and LS (low spin) sites at the intermediate plateau (IP) temperatures. In contrast, structural analysis of 2 b at 90 K (2 b(LS)), 150 K (2 b(LS/HS)) and 250 K (2 b(HS)) reveals one unique iron(II) centre at each temperature with an "averaged" LS/HS character at the IP temperature. Weak planes of diffuse scattering in the single-crystal X-ray diffraction patterns were observed for this phase at 90 and 150 K, indicating that 1D long range ordering of alternating HS/LS iron(II) centres occurs along the 1D coordination chains, but that there is no correlation between chains. The lack of observable diffuse scattering at 250 K suggests that the onset of the 1D structural ordering in the chain direction corresponds to the first step of the SCO and that this structural transition is electronically driven. The photomagnetic properties of both 1 and 2 b have been investigated and show approximately 62 and 53 % photo-excitation of a HS metastable state at low temperatures and T(LIESST) values of 55 and 49 K, respectively. Relaxation studies on the HS fraction in 2 b fitted well to a stretched exponential model with kinetic parameters indicative of weak cooperativity.