Structural and Magnetic Investigations of the Mixed‐Valence Fe II,III Two‐Dimensional Layer Complex, [Fe 2 II Fe 2 III (HCOO) 10 (C 6 H 7 N) 6 ] n

The structure of the complex, [Fe 2 II Fe 2 III (HCOO) 10 (C 6 H 7 N) 6 ] n , ( 1 ) exhibits a neutral two‐dimensional layer network of alternating iron( II ) and iron( III ) ions, bridged equatorially by formate groups. All iron atoms are octahedrally coordinated, with iron( III ) coordinating axially to one γ‐picoline and one formate group, while the iron( II ) centers interact axially with two γ‐picoline groups, above and below the layer plane. The complex crystallizes in the triclinic space group P1 at all studied temperatures [at 120 K, the cell dimensions are: a =10.228(1), b =12.071(1), c =12.072(1) Å, α =89.801(2), β =71.149(2), γ =73.371(2)°]. An intralayer antiferromagnetic exchange interaction of J =−2.8 cm −1 between iron( II ) and iron( III ) was observed in the magnetic studies. Decreasing the temperature to close to 20 K causes a magnetic‐ordering phenomenon to occur and a low‐temperature phase with a long‐range antiferromagnetic spin orientation appears. The magnetic phase transition was confirmed by Mössbauer spectroscopic studies at temperatures above and below the critical temperature. Structural information of 1 from synchrotron X‐ray diffraction data collected at room temperature and 16 K suggests that the antiferromagnetic ordering is caused by an enhanced π–π interaction between χ ‐picoline groups from adjacent layers.