Ferromagnetically coupled linear electron-transfer complexes. Structural and magnetic characterization of [Cr(.eta.6-C6MexH6-x)2][TCNE] (x = 0,3,6) and S = 0 [TCNE]22-

Abstract

The reaction of Cr0(C6MexH6-x)2 (x = 0, 3, 6), D, with TCNE, A, results in formation of 1:1 electron-transfer salts of [CrI(C6MexH6-x)2][TCNE] (x = 0, 3, 6) composition. The x = 0 and 3 complexes have been structurally characterized. The [CrI(C6H6)2]•+ salt belongs to the centrosymmetric P21/m space group [a = 10.347 (4) Å, b = 12.423 (10) Å, c = 12.763 (4) Å, β = 111.33 (3)°, Z = 4, T = 23°C, V = 1528 (3) Å3, Ru = 0.042, and Rw = 0.044]. The [TCNE]•- anion has average C-C, C-CN, and C≡N bonds of 1.436, 1.414, and 1.140 Å, respectively, and the NC-C-CN, NC-C-C, and N≡C-C angles average 118.6, 120.2, and 178.4°, respectively. The solid-state structure consists of linear chains of ⋯D•+A22-D•+D •+A22-D•+⋯ with the anion existing as dimeric dianions, [TCNE]22-. The [TCNE]22- is centrosymmetric, and the intradimer C⋯C bond distance is 2.904 Å. The S = 0 [TCNE]22- dimer is nonplanar and bent 10° via a b3u out-of-plane distortion. One habit of the [Cr(C6Me3H3)2]•+ salt crystallizes in the P1 space group [a = 10.475 (5) Å, b = 11.427 (4) Å, c = 8.419 (6) Å, α = 64.20 (3)°, β = 78.59 (4)°, γ = 81.56 (3)°, Z = 2, T = 23°C, V = 1266 (9) Å3, Ru = 0.075, and Rw = 0.081]. This is a poorer quality structure with average C-C, C-CN, and C=N bonds of 1.45,1.40, and 1.10 Å, respectively, while the NC-C-CN, NC-C-C, and N≡C-C angles average 133, 114, and 162°, respectively. The solid-state structure also consists of linear chains of ⋯D•+A22-D•+D •+A22-D•+⋯ with the intradimer separation of 3.09 Å for the [TCNE]22-. The [TCNE]22- dimer exhibits three characteristic C≡N vibrations in the infrared region at 2160 s, 2169 s, and 2190 m cm-1. The [CrI(C6H6)2]•+ and this [CrI(C6Me3H3)2] •+ structure have eclipsed and staggered conformations, respectively. The average Cr-C and C-C separations are 2.143 and 1.395 Å, respectively, for [CrI(C6H6)2]•+, and the average Cr-C, C-C, and C-Me separations for [CrI(C6Me3H3)2] •+ are 2.16, 1.40, and 1.51 Å, respectively. The Cr-C6(ring) centroids are equivalent at 1.625 (5) Å. The magnetic susceptibility between 2 and 320 K for these [TCNE]22- complexes can be fit by the Curie-Weiss law, χ= C/(T - θ), for one independent per spin repeat unit. In solution two signals with g = 2.001 and 1.896 characteristic of [TCNE]•- and [Cr(η-arene)2]•+ are observed; thus, [TCNE]•- and not [TCNE]22- is present in solution. For the solid phase the spin is assigned to the cation; with the dimer being diamagnetic. A second structure type is suggested for the [CrI(C6MexH6-x)2] •+ (x = 3, 6) salts as they exhibit the characteristic v(C≡N) vibrations for an isolated [TCNE]•-. Their high-temperature susceptibility can be fit by the Curie-Weiss law with θ = ∼+11.4 K and μeff = ∼2.4 μB consistent with ferromagnetic coupling; thus, we propose that these complexes possess a ⋯D•+A•-D•+A •-⋯ chain structure. The magnetization saturated faster than accountable by the Brillouin function for independent spins; however, bulk 3-D ordering is not evident. The configurational mixing of a charge-transfer excited state with the ground-state model predicts antiferromagnetic coupling should be stabilized for this compound as both the donor and acceptor possess an 2A1g ground state. Charge-transfer excitation from the next highest occupied molecular orbital, however, is consistent with the stabilization of ferromagnetic coupling for this system. Thus, other molecular/organic systems with 2A ground states might be suitable components for a bulk ferromagnet. © 1989 American Chemical Society.