Synthesis, structures and spin crossover properties of infinite 3D frameworks of iron(ii) containing organodinitrile bridging ligands

Abstract

Four new Fe(II) coordination polymers of formulation [Fe(L)₃][MCl₄]₂, M = Fe^III, In^III have been synthesized and structurally and magnetically characterized. They contain organodinitrile bridging ligands where L is 1,1′-azobiscyclopentanecarbonitrile (ACCN) or 2,2′-azobisisobutyronitrile (AIBN). The compounds are [Fe(AIBN)₃][InCl₄]₂ (1), [Fe(AIBN)₃][FeCl₄]₂ (2), [Fe(ACCN)₃][InCl₄]₂ (3), and [Fe(ACCN)₃][FeCl₄]₂ (4). Compounds 1 and 2 were first reported many years ago, elsewhere, but not then structurally characterized. All four compounds adopt an “open framework” α-Po type 3D lattice structure with the [M^IIICl₄]⁻ anions held in the channels thereof. There are two crystallographically independent, distorted octahedral Fe^IIN₆ sites in 1 and 2, but only one Fe(II) site in 3 and 4. These are the first open framework systems containing covalently linked dinitrile bridging ligands and join a related class of [Fe(L)₃]²⁺ frameworks known with ditriazole and ditetrazole linkers and exhibiting spin crossover behaviour. Indeed, compounds 1 and 2 display a ‘half’ spin transition in their magnetic susceptibility vs. temperature plots, this being due to the fact that only half of the Fe(II) d⁶ centres in the framework changed spin from HS → LS as the temperature decreased, as also evidenced by the structural details around the Fe(II) centres, determined below T_1/2 (118 and 170 K for 1 and 2, respectively). Mössbauer spectra confirmed these spin changes. The weak cooperativity between the SCO centres led to a lack of thermal hysteresis. When the ACCN ligand was used instead of the AIBN ligand, in compounds 3 and 4, only one crystallographically unique Fe(II) centre was observed in an otherwise similar framework. No SCO transition was observed in 3 and 4 as evidenced by temperature independent moment data, backed up by single crystal X-ray crystallographic analysis at 123 K. Thus the HS Fe(II) ions remain in this spin state at all temperatures, presumably because the ligand field, at the nitrile N-donors, is weaker in ACCN than in AIBN. In addition, the crystal structure of a minor phase of an aqua complex, [Fe(AIBN)₂(H₂O)₂][FeCl₄]₂ (2a), obtained from the reaction that produced 2, showed a 2D layer motif, with trans-Fe(N)₄(H₂O)₂ ligation.