Acetonitrile‐Facilitated Reductive Dimerization of TCNE to Octacyanobutanediide, [C 4 (CN) 8 ] 2− , by Iron(II) Chloride

The redox properties of MCl 2 (M=Mn, Fe, Co) acetonitrile solvates were electrochemically and spectroscopically characterized. The three voltammogram waves at 0.86, 0.48, and 0.21 V versus SCE for FeCl 2 dissolved in MeCN are assigned as one‐electron reduction potentials for [Fe II Cl x (NCMe) 4− x ] 2− x (1≤ x ≤3), respectively, and only [Fe II Cl 3 (NCMe)] − {and [Fe II Cl 4 ] 2− ( ${E{{{\rm o}\hfill \atop 1/2\hfill}}}$ =0.04 V vs. SCE), if present} has a potential sufficient for TCNE reduction. The presence in solution of [Fe II (NCMe) 6 ] 2+ was also spectroscopically confirmed. The reduction potentials of M II Cl 2 (M=Mn, Co) acetonitrile adducts were inadequate to reduce TCNE. FeCl 2 reacts with TCNE in MeCN to form [Fe II {C 4 (CN) 8 }(NCMe) 2 ] ( 4 ), which is composed of layers of octahedral Fe II ions equatorially bonded to the four terminal N atoms of the μ 4 ‐[C 4 (CN) 8 ] 2− ion, with trans ‐MeCN groups and with 15‐membered rings. Formation of 4 requires the simultaneous presence of [TCNE] .− and [Fe II (NCMe) 6 ] 2+ in solution, which is in contrast to when [(NEt 4 ) 2 ][Fe II Cl 4 ] was used to reduce TCNE. A reaction scheme is proposed. Paramagnetic [Fe II {C 4 (CN) 8 }(NCMe) 2 ] exhibits substantial antiferromagnetic coupling ( θ =−13.3 K), and orders as an antiferromagnet below 7.8 K. The irreversibility of zero‐field and field‐cooled magnetization suggests that the material exists below T c in two magnetic states relatively close in energy, that is, the purely antiferromagnetic ground state and a slightly canted antiferromagnetic metastable state.