Characterization of the charge-transfer reaction between decamethylferrocene and 7,7,8,8-tetracyano-p-quinodimethane (1:1). The ⁵⁷Fe Mössbauer spectra and structures of the paramagnetic dimeric and the metamagnetic one-dimensional salts and the molecular and electronic structures of [TCNQ]ⁿ (n = 0, -1, -2)

The charge-transfer reaction of decamethylferrocene, Fe(C₅Me₅)₂, Fc*, and 7,7,8,8-tetracyano-p-quinodimethane, TCNQ, has been characterized. Three major reaction products of varying stoichiometry, conductivity, and magnetism are formed: a 1-D metamagnetic 1:1 salt, [Fc*]^•+[TCNQ]^•-; a paramagnetic [Fc*]₂^•+[TCNQ]₂^2-; 1:1 dimeric salt, [Fc*]₂^•+[TCNQ]₂^2-; and a conducting 1:2 salt of [Fc*]^•+[TCNQ]₂^- composition. The crystal and molecular structures of the 1:1 paramagnetic dimeric and metamagnetic one-dimensional salts were solved. The ions in the dimeric phase crystallize in the centrosymmetric monoclinic space group P2₁/c with a = 9.708 (1) Å, b = 12.211 (2) Å, c = 23.585 (4) Å, β = 95.01 (1)°, Z = 4, and R = 0.058 for 3665 independent reflections. The structure consists of discrete stacks of DAAD dimeric (D = Fc*; A = TCNQ) units. One-dimensional stacking of ions is not observed. The decamethylferrocenium cations have average Fe-C distances of 2.096 (7) Å, longer than the 2.050 Å reported for neutral decamethylferrocene. The C₅Me₅ rings appear eclipsed; however, as a result of the disorder only one ring was partially resolvable. The C₅Me₅ rings are essentially parallel to the TCNQ moieties and separated by 3.554 Å. The TCNQ anions form a tight a_g dimer (separated by a short distance of 3.147 Å) and are slipped along the short TCNQ axis. Bonding arises through filling of the bonding a_g dimer orbital. The magnetic susceptibility of powder samples of [Fc*]₂^•+[TCNQ₂]₂ ^2- obeys the Curie expression above 1.5 K suggesting very little magnetic coupling between the intradimer S = 1/2 Fe(III)'s which are separated by ∼ 14 Å. The [TCNQ]₂^2- dimer is strongly antiferromagnetically coupled. The zero-field Mössbauer spectra of the dimeric phase above 1.4 K are typical of S = 1/2 [Fc*]^•+ and exhibit rapidly relaxing paramagnetic behavior. However, relatively small external fields (<5 kG) are all that is necessary to induce slow paramagnetic relaxation and a substantial internal hyperfine field (∼350 kG) at the decamethylferrocenium ions. Ions in the linear chain metamagnetic phase crystallize in the centrosymmetric triclinic space group P1. The unit cell at -106°C is a = 8.635 (4) Å, b = 9.384 (6) Å, c = 10.635 (9) Å; α = 116.76 (5)°, β = 112.58 (5)°; γ = 72.49 (4)°, V = 701.8 ų, Z = 1, and R_w = 0.060 for 1156 reflections. The poorer quality of this structure is due to the inability to grow suitable crystals of this kinetically favored phase with respect to the thermodynamically favored dimeric phase. The structure consists of discrete one-dimensional chains comprised of alternating radical cation donors, D, and anion acceptors, A, i.e., ⋯DADA⋯. The decamethylferrocenium cation has a staggered conformation with an average Fe-C distance of 2.090 Å. This is equivalent to that observed for [Fc*]^•+ in the dimeric phase. The C₅Me₅ rings are staggered and are canted with respect to the [TCNQ]^•- moieties by 3.9°. The distance between the TCNQ and the C₅Me₅ rings is 3.43 Å. The [TCNQ]^•- anion represents the first structural characterization of an isolated [TCNQ]^•- in the solid state. The one-dimensional phase exhibits metamagnetic properties with a Néel temperature of 2.55 K. The intrachain Fe^III-Fe^III separation is 10.549 Å. Below ∼3.5 K the Mössbauer spectra of this phase undergo progressive magnetic hyperfine splitting for the decamethylferrocenium singlet (δ_Fe≡O (293 K) = 0.34 mm/s) leading to a highly broadened six transition Zeeman pattern at 2.7 K consistent with slow paramagnetic relaxation. Between 2.7 and 2.5 K the latter spectrum undergoes a further abrupt transition to a narrow line width ten transition pattern corresponding to the overlap of two Zeeman patterns with a 2:1 intensity ratio. At 1.60 K the internal hyperfine fields, H_INT, corresponding to the patterns are 390 and 432 kG. Ab initio molecular orbital calculations with the STO-3G basis set were carried out for [TCNQ]ⁿ, n = 0, -1, -2. The calculated structure and scaled vibrational frequencies are in excellent agreement with the observed values. The charge distributions are given together with the spin distribution for [TCNQ]^•-. To confirm the results of the MO calculations, [TCNQ]^2- (as the [Co(C₅Me₅)₂]⁺ (2:1) salt) was structurally characterized for the first time. The ions crystallize in the centrosymmetric orthorhombic space group Pbca with a = 15.465 (6) Å, b = 23.557 (9) A•, c = 26.794 (11) A•, Z = 8, R_w = 0.085. The structures of C₅ and D_5d decamethylcobaltocenium cations have been reported for the first time. The better determined D_5d cation possesses average Co-C, C-C, and C-Me distances of 2.048, 1.42 and 1.506 Å, respectively. The average C₅ ring-Co(1) centroid distance is 1.653 Å. These values are comparable to that reported for isoelectronic Fe(C₅Me₅)₂. The structure of [TCNQ]^2- has also been determined for the first time. The dianion is planar within the experimental error and possesses D_2h local symmetry. The C₆ ring is benzene-like with an average ring C-C distance of 1.41 Å. The average C=C(CN)₂, C-CN, and C≡N distances are 1.44, 1.42, and 1.15 Å, respectively.