A two-step spin transition and order-disorder phenomena in the mononuclear compound [Fe(Hpy-DAPP)](BF4)2

This paper reports the synthesis and detailed characterization of the mononuclear complex [Fe-II(Hpy-DAPP)](BF4)(2) (Hpy-DAPP = {bis [N-(2-pyridylmethyl)-3-aminopropyl] (2-pyridylmethyl)amine)), which manifests a two-step spin crossover accompanied by an ordering and a conformational transition in the ligand. The magnetic susceptibility measurements and Mossbauer spectra reveal that the two steps of the spin transition are separated by an inflexion point at about 130 K at which 50% of the complex undergoes a spin conversion. The high-temperature step centered at 181 K is gradual, whereas the low-temperature one displays a thermal hysteresis with two transition temperatures (T-c down arrow = 119 and Tc up arrow = 123 K). The single-crystal X-ray structure has been determined for the HS (298 K) and LS (90 K) forms, as well as for the mixed form with an HS:LS ratio of 1:1 (130 K). The structural studies show that the crystal lattice contains a single Fe-II crystallographic site over the whole temperature range. At room temperature, the most important peculiarity of the HS complex structure is the disorder of the N3 amino aliphatic atom, which is in the first coordination sphere of Fe-II, over two positions. The strong involvement of the N3 atoms in H-bonding with two neighboring counterions is considered as a possible origin of this disorder. At 130 K, the disorder is increased by the distribution of the N1 and C9 atoms over two positions, leading to the conformational disorder (chair and twist-boat conformations with a 1:1 ratio) in the six-membered N2C7C8C9NlFe chelate cycle. At 90 K, the disorder in the positions of the N1, N3, and C9 atoms disappears and the N2C7C8C9NlFe metallacycle adopts a unique twist-boat conformation. The results of structural studies suggest that the two-step spin-transition behavior of the complex [Fe(Hpy-DAPP)](BF4)(2) is induced by two different geometries of the [FeN6] coordination core generated by the disorder in the ligand. The HS -> LS transition is also associated with the conformational change in the chelate cycle. Both crystallographic data analysis and electronic structure calculations allow us to relate the disordered geometries of the [FeN6] coordination core to their HS and LS character.