EXTREME ENVIRONMENTAL SENSITIVITY OF THE RATE OF INTRAMOLECULAR ELECTRON-TRANSFER IN MIXED-VALENCE 1',1'''-DIBENZYLBIFERROCENIUM TRIIODIDE

The extreme sensitivity of the rate of intramolecular electron transfer for the mixed-valence 1',1'''-dibenzylbiferrocenium cation to different environments is explored. Two different crystalline morphologies of 1',1'''-dibenzylbiferrocenium triiodide (1) could be identified by their different crystal habits (needle and plate-like crystals). They are structurally characterized by single-crystal and powder X-ray diffraction techniques. Thc single-crystal X-ray structure of a parallelepiped (needle) crystal of 1 at 298 K shows that this polymorph crystallizes in the triclinic space group P1BAR. The unit cell parameters are a = 9.900 (1) angstrom, b = 9.962 (1) angstrom, c = 10.193 (2) angstrom, alpha, = 112.97 (2)degrees, beta = 114.50 (1)degrees, and gamma = 97.80 (1)degrees with Z = 1. The structure is also reported for the same crystal at 135 K. An analysis of the full sphere of data again indicates P1BAR as the appropriate space group with unit cell constants a = 9.782 (2) angstrom, b = 9.930 (1) angstrom, c = 10.108 (2) angstrom, alpha = 113.08 (2)degrees, beta = 114.08 (1)degrees, and gamma = 98.06 (1)degrees with Z = 1. Refinements were carried out with 2990 (6-sigma) and 3315 (6-sigma) observed reflections at 298 and 135 K, respectively, to give R = 0.026 and R(w) = 0.037 at 298 K and R = 0.025 and R(w) = 0.038 at 135 K. Both the mixed-valence 1',1'''-dibenzylbiferrocenium cation and triiodide anion are centrosymmetric at both temperatures. The two crystallographically equivalent metallocene moieties of the cation have dimensions intermediate between those of Fe(II) and Fe(III) metallocenes. The packing of 1 consists of weakly interacting 1',1'''-dibenzylbiferrocenium cations forming chains which are surrounded by and separated from other similar chains by the triiodide anions. The 296 K single-crystal X-ray structure of the plate-like crystals of complex 1 show that this polymorph crystallizes in the monoclinic space group P2(1)/n. The unit cell parameters are a = 22.286 (9) angstrom, b = 19.413 (10) angstrom, c = 22.840 (5) angstrom, and beta = 98.55 (3) degrees with Z = 12. Refinement was carried out with 4486 (6-sigma) observed reflections to give R = 0.089 and R(w) = 0.131. There are three different 1',1'''-dibenzylbiferrocenium cations and three different I3-anions in the asymmetric unit. There is not crystallographically imposed symmetry for any of the cations or anions. The temperature dependencies of the ''Fe Mossbauer spectra of the P1BAR and P2(1)/n polymorphs of complex 1 differ widely. A sample of needle crystallites has a Mossbauer spectrum characteristic of a valence detrapped mixed-valence species down to a temperature of 25 K. On the other hand, a sample of the P2(1)/n plate-like crystals exhibits a Mossbauer spectrum characteristic of a valence-trapped species which remains trapped even at 300 K. It was surprising to rind that mild grinding of the needle crystallites produces a valence-trapped Mossbauer signal in addition to the valence detrapped signal. Appreciable amounts of thc trapped species persist for the ground P1BAR needles even up to 300 K. At 7 K, the EPR spectrum of a recrystallized microcrystalline sample of 1 consists of the superposition of three signals: two axial powder patterns each with its own g parallel-to and g perpendicular-to, signals and one isotropic g = 2.16 signal. The g tensor anisotropies of the two axial signals are DELTA-g = 1.29 and 0.76. The axial pattern with DELTA-g = 0.76 is assigned to the P1BAR form, whereas, the DELTA-g = 1.29 pattern is assigned to the P2(1)/n form. The two axial EPR patterns broaden with increasing temperature and the isotropic g = 2 signal gains in relative intensity, so that by 150 K it is the only signal observed. This Lorentzian g = 2.16 signal persists to high temperatures. In contrast, the variable-temperature EPR spectrum of an ethanol glass of 1 shows a single axial signal (DELTA-g = 1.82) which broadens with increasing temperature and becomes unobservable above approximately 77 K. The EPR properties of mixed-valence 1',1'''-dibenzylbiferrocenium cation are modified by the local environments. Not only does the EPR signal change from the P1BAR to the P2(1)/n crystal forms but also there is even a sample history dependence of the g values for the two crystal forms. The g = 2.16 signal is the result of intermolecular interactions.