MIXED-VALENCE BIFERROCENES - PRONOUNCED ANION DEPENDENCE OF VALENCE DETRAPPING AND EFFECTS OF AN ASYMMETRIC CRYSTAL-LATTICE

The effect of changing the anion from I3- to either PF6- or SbF6- upon the rate of intramolecular electron transfer in salts of the mixed-valence 1′, 1‴-dibenzylbiferrocenium cation is investigated. Single-crystal X-ray structures are presented for 1′, 1‴-dibenzylbiferrocene (1), 1′, 1‴-dibenzylbiferrocenium hexafluorophosphate (2), and 1′, 1‴-dibenzylbiferrocenium hexafluoroantimonate (3). Complex 1 crystallizes at 296 K in the monoclinic space group P21/c with a unit cell of a = 5.846 (1) Å, b = 25.878 (7) Å, c = 8.354 (2) Å, and β = 93.73 (2)° with Z = 2. Refinement was carried out with 1862 (5.0σ) observed reflections to give R = 0.0353 and Rw = 0.0383. A center of symmetry (planar fulvalenide ligand) and a trans conformation characterize the Fe2II molecule in 1. The PF6- salt 2 crystallizes at 296 K in the monoclinic space group C2/c with a unit cell of a = 14.640 (2) Å, b = 12.938 (2) Å, c = 15.463 (3) Å and β = 93.70 (1)° with Z = 4. Refinement was carried out with 2139 (5.0σ) observed reflections to give R = 0.0483 and Rw = 0.0545. The PF6- anion was found to be disordered in two positions, one with an occupancy of 64% and the other 36%. Since the mixed-valence 1′, 1‴-dibenzylbiferrocenium cation sits on a center of symmetry, the dihedral angle for the fulvalenide ligand is 0°. The two crystallographically equivalent metallocene moieties in the cation of 2 have dimensions intermediate between those of FeII and FeIII metallocenes. The packing in complex 2 is that of layers of mixed-valence cations with the small PF6- anions sandwiched between the layers. 1′, 1‴-Dibenzylbiferrocenium hexafluoroantimonate (3) crystallizes in the monoclinic space group P21, which at 198 K has a unit cell of a = 10.850 (2) Å, b = 11.874 (3) Å, c = 11.990 (6) Å, and β = 103.03 (2)° with Z = 2 and at 298 K has a unit cell of a = 10.899 (2) Å, b = 11.969 (3) Å, c = 12.145 (5) Å, and β = 102.88 (2)° with Z = 2. The refinements were carried out with 2568 (2.58σ) and 1955 (2.58σ) observed reflections at 198 and 298 K, respectively, to give R = 0.034 and Rw = 0.042 at 198 K and R = 0.039 and Rw = 0.047 at 298 K. The SbF6- anion in complex 3 was not found to be disordered at either temperature; however, at 298 K the thermal parameters for this anion are large. The mixed-valence cation has a trans conformation where the fulvalenide dihedral angle is 6.81° at 298 K and 7.01° at 198 K. The dimensions of the two halves of the mixed-valence cation in complex 3 are different. The positioning of the SbF6- anion relative to the nearest cation is not symmetric. At 298 K one Fe.dots. Sb distance is 5.471 (3) Å, whereas the other one is 5.833 (4) Å. For the mixed-valence cation of SbF6- salt 3 there is a driving force to make one iron ion FeIII and the other FeII due to the positioning of the SbF6- anion. The 57Fe Mössbauer spectrum of PF6- complex 2 shows that the mixed-valence cation in this salt begins to become valence detrapped above ∼100 K, and by 170 K the cation is valence detrapped on the Mössbauer time scale. This is contrasted to the case of 1′, 1‴-dibenzylbiferrocenium triiodide which was reported to valence detrap at 270 K. For the SbF6- complex 3 Mössbauer data show an onset of valence detrapping in the 120-150 K range, and above ∼200-250 K this complex is completely valence detrapped. An analysis of the temperature dependence of the spectral areas of the Mössbauer signals indicates that the PF6- and SbF6- anions are probably converting from static to dynamic in the temperature region where the mixed-valence cations in complexes 2 and 3 are becoming valence detrapped. The room-temperature IR spectrum for SbF6- complex 3 shows ⊥ C-H bending vibrational bands for both FeII and FeIII metallocene units. Thus, there is at all temperatures a potential-energy barrier for electron transfer. At 7 K complex 3 gives an axial EPR spectrum with g‖ = 3.35 and g⊥ = 1.86. This signal broadens with increasing temperature and disappears above ∼150 K. A similar axial signal is seen at low temperatures for complex 2, but upon heating 2 this signal is replaced by a single derivative at g = 2.00, which persists up to at least 295 K. The origin of this difference in EPR characteristics between 2 and 3 is discussed. The nature of the appreciable anion influence on rate of intramolecular electron transfer is described as is the mechanism by which the mixed-valence cation in SbF6- complex 3 valence detraps even though it is sitting in such an asymmetric lattice site. The potential significance of these environmental effects on rates of electron transfer is highlighted in reference to solution redox processes. © 1990, American Chemical Society. All rights reserved.