Electron-transfer salts of 1,2,3,4,5-pentamethylferrocene, FeII(C5Me5)(C5H5).: Structure and magnetic properties of two 1:1 and two 2:3 Fe(C5Me5)(C5H5) electron-transfer salts of tetracyanoethylene

The reaction of Fe-II(C5Me5)(C5H5), FeCpCp*, with percyano accepters, A [A = C-4(CN)(6) (hexacyanobuyadiene), TCNQF(4) (perfluoro-7,7,8,8-tetracyano-p-quinodimethane), and DDQ (2,3-dichloro-5,6-dicyanobenzoquinone)], results in formation of 1:1 charge-transfer salts of [(FeCpCp)-Cp-III*](.+)[A](.-) composition. With A = TCNQ (7,7,8,8-tetracyano-p-quinodimethane) a 1:2 electron-transfer salt with FeCpCp* forms. With A = TCNE (tetracyanoethylene) a pair of 1:1 salts as well as a pair of 2:3 salts of [FeCpCp*](2)[TCNE](3).S (S = CH2Cl2, THF) have been isolated and characterized by single-crystal X-ray diffraction. [FeCpCp*][TCNE] consists of parallel 1-D (...)D(.+)A(.-)D(.+)A(.-)D(.+)A(.-...) chains, while [FeCpCp*][TCNE](MeCN)-Me-. has a herringbone array of D(.+)A(2)(2-)D(.+) dimers separated by solvent molecules. Although each [TCNE](-) is disordered, the diamagnetic [TCNE](2)(2-) dimer is structurally different from those observed earlier with an intradimer separation of 2.79 Angstrom. The [TCNE](-) in the 2:3 [FeCpCp*](2)[TCNE](3)S-. exists as an eclipsed diamagnetic [TCNE](2)(2-) dimer with an intradimer ethylene C . . .C separation of 2.833 and 2.903 Angstrom for the CH2Cl2- and THF-containing materials, respectively. The bond distances and angles for all the cations are essentially equivalent, and th distances are essentially equivalent to those previously reported for [FeCp*(2)](.+) and [FeCp2](.+) cations. The average Fe-C5H5-ring and Fe-C5Me5-ring centroid distances are 1.71 and 1.69 Angstrom, respectively, which are 0.05 Angstrom longer than reported for (FeCpCp)-Cp-II*. The one-electron reduction potential for (FeCpCp)-Cp-II* is 0.11 V (vs SCE). The 5 K EPR of [FeCpCp*](.+)[BF4](-) exhibits an axially symmetric powder pattern with g(//) = 4.36 and g(perpendicular to) = 1.24, and the EPR parameters are essentially identical to those reported for ferrocenium and decamethylferrocenium. The high-temperature magnetic susceptibility for polycrystalline samples of these complexes can be fit by the Curie-Weiss law, chi = C/(T - theta), with low theta values and mu (eff) values from 2.08 to 3.43 mu (B), suggesting that the polycrystalline samples measured had varying degrees of orientation. [FeCpCp*][TCNE] exhibits the highest effective moment of 3.43 mu (B)/Fe and weak ferromagnetic coupling, as evidenced from the theta of 3.3 K; however, unexpectedly, it does not magnetically order above 2 K. The formation of the four phases comprising FeCpCp* and TCNE emphasizes the diversity of materials that may form and the present inability to predict neither solid-state compositions nor structures types.